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5-2Contents

12.1 Purpose 12/319

12.2 Unfavourable operating conditions 12/319

12.3 Fault conditions 12/324

12.4 Protection 12/32712.4.1 Protection against unfavourable operating

conditions 12/32712.4.2 Protection against fault conditions 12/33212.4.2(a) Applications of different coordinations of

components 12/33512.4.3 Protection against stalling and locked rotor 12/33512.4.4 Protection against voltage unbalance or negative

phase sequence 12/33512.4.5 Protection against single phasing (SPP) 12/33512.4.6 Protection against voltage surges 12/337

12.5 Single-device motor protection relays 12/33812.5.1 Electromagnetic relays 12/33812.5.2 Solid-state relays 12/33912.5.3 Microprocessor-based relays 12/341

12.6 Summary of total motor protection 12/343

12.7 Motor protection by thermistors 12/34612.7.1 Embedded temperature detectors (ETDs) 12/349

12.8 Monitoring of a motor’s actual operating conditions 12/35012.8.1 Motor winding temperature detection (by PTC

thermistors and RTDs) 12/35112.8.2 Bearing temperature detection (by PTC

thermistors or RTDs) 12/35112.8.3 Coolant circuit water pressure and temperature

(moisture) detection 12/35112.8.4 Detection of moisture condensation in the

windings (by space heaters) 12/35112.8.5 Vibration probes 12/35112.8.6 Use of speed switch or tacho-generator 12/352

12.9 Switchgears for LV motors 12/352

12.10 Selection of main components 12/35312.10.1 Switches and contactors 12/35412.10.2 Breakers (ACBs or MCCBs) 12/35512.10.3 Over-current relay (OCR) 12/35512.10.4 HRC fuses 12/35512.10.5 Selection of cables 12/355

12Protection ofelectric motorsand selection ofcomponents

12/317

12.11 Fuse-free system 12/35612.11.1 Motor protection circuit breakers (MPCBs) 12/35712.11.2 Component ratings 12/35712.11.3 Soft starters 12/35712.11.4 Intelligent starters 12/357

12.12 Switchgears for MV motors 12/357

Relevant Standards 12/364

List of formulae used 12/365

Further Reading 12/365

Source material 12/365

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12.1 Purpose

An electric motor must be adequately protected againstall unfavourable operating conditions and internal orexternal faults. We have classified these conditions intothree categories to identify the most suitable protection:

1 Unfavourable operating conditions2 Fault conditions3 System disturbances and switching surges (for MV

motors)

12.2 Unfavourable operatingconditions

Operating conditions that may over-load a machine andraise its temperature beyond permissible limits may becalled unfavourable. This over-heating, however, will begradual (exponential), unlike rapid (adiabatic) heatingas caused during a locked rotor condition. The machinenow follows its own thermal curve and therefore aconventional thermal protection device can be used toprotect it from such conditions. These conditions mayarise due to one or more of the following:

(i) Over-loading Due to excessive mechanical loading.(ii) Under-voltage Low voltage results in forced over-

loading due to higher slip losses and higher currentinput to sustain the same load requirement. Anunstable sub-distribution network, a number ofsmall LV loads on a long and already over-loadedLV distribution system, or inadequate cable sizesmay cause an excessive fall in the receiving endvoltage. See also Section 23.3, where we analysethe effect of low power factor on the terminalvoltage.

The effect of small voltage drops (Section 1.6.2)is taken care of by the standard over-currentprotection used in the motor’s switching circuit.But for installations where the voltage available atthe motor terminals may fall below the permissiblelevel, say, below 90% of the rated voltage, over-heating or even stalling may occur if the voltagefalls below 85%. In such a condition, dependingupon the severity of the voltage fluctuation andthe load requirement, a separate under-voltage relaymay also be used.

The no-volt coil of the contactor or the under-voltage trip coil of the breaker, used for the motorswitchings, are usually designed to pick up at 85%of the rated voltage. These coils drop out at a voltagebetween 35% and 65% of the rated voltage andwould not protect the motor against under-voltages.In normal service conditions the system voltage isnot likely to fall to such a low level, particularlyduring running.

Thus, the protection will not prevent the closingof the contactor or the breaker when the supplyvoltage is a85% or more, nor will it trip the motoruntil the voltage falls to a low of a65% of the rated

voltage. In both cases, therefore, separate under-voltage protection will be essential. This problem,however, is a theoretical one, as an industrial powersystem would seldom fluctuate so widely.aNoteSometimes when there are perennial wide voltage fluctuationsat certain locations/installations the manufacturers of thecontactors on demand from users may design their holdingcoils for even lower pick-up and higher drop out voltagesthan noted to save the feeders from unwanted trips, the usermaking extra capacity provision in the motor or getting itredesigned for special voltages to sustain the wide voltagefluctuations.

(iii) Reverse rotation This may occur due to a wrongphase sequence. While the motors are suitable foreither direction of rotation, the load may be suitablefor one direction only and hence the necessity forthis protection. A reverse rotation means a reverserotating field and is prevented by a negative phasesequence, i.e. a voltage unbalance or single-phasingprotection. Moreover, this protection is also of littlesignificance, as once the motor is commissionedwith the required direction of rotation, it israre that the sequence of the power supply wouldreverse.

(iv) Protection from harmonic effects Motors areinfluenced less with the presence of harmonics.This is due to the benign effects of harmonics oninductive loads, on the one hand, and the motorproviding no path to the third harmonic quantitieson the other, as it is normally connected in delta.In MV motors, however, which are normally starconnected, the neutral may be left floating to provideno path to the third harmonics.

Higher harmonics increase the harmonicreactance and have a damping effect (Section23.5.2(b)). A motor circuit, LV or MV, possesses ahigh inductive impedance due to inter-connectingcables and its own inductance, and provides a self-damping effect to the system’s harmonics. Thereis thus no need, generally, to provide protectionagainst harmonics specifically, except for high no-load iron losses.

If, however, high contents of harmonics exist,as when the machine is being fed through a staticpower inverter (Section 6.13), they will producemagnetic fields, rotating in space, proportional tothe individual harmonic frequencies. These fieldsmay be clockwise or anti-clockwise, depending uponwhether the harmonic is positive or negative. Thefields produce different torques, which may also beclockwise or anti-clockwise. The net effect of allthis is a pulsating torque which is not a desirablefeature and calls for suppression of such harmonics.In a six-pulse thyristor circuit, for instance,the harmonic disorder is –5, +7, –11, +13, –17 and+19 etc. giving rise to clockwise and anti-clockwisefields.

To reduce the no-load iron losses caused bysuch harmonics the machine core may be formedof thinner low-loss laminates (see also Section1.6.2(A-iv)). When the machine has already been

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manufactured and there is a need to suppress theseharmonics, filter circuits may be employed alongthe lines discussed in Section 23.9.

Excessive harmonics may also make theprotective devices behave erratically or render theminoperative. Filter circuits would suppress theharmonics and eliminate these effects.

NoteThe protective devices which measure r.m.s. values of currentalso detect the harmonic contents and provide automaticprotection for the machine against harmonic disorders. Butin installations that contain high harmonics and that mayinfluence the performance of machines operating on such asystem, it is advisable to suppress the harmonics rather thanallow a machine to trip at higher harmonics. Drives generatinghigh harmonics, such as static drives, are provided withharmonic suppressors as normal practice.

(v) Voltage unbalance (negative phase sequence)This causes negative sequence components andresults in excessive heating of the motor windings.An unbalanced voltage may occur due to unevenlydistributed single-phase loads.

A voltage unbalance may also be due to unequalphase impedances of the feeding HV line and maybe a result of unequal spacing between the horizontaland vertical formation of conductors or asymmetricalconductor spacings. These effects cause an unequalinduced magnetic field and hence an unequalimpedance of each conductor. The implication ofsuch a system can be studied by assuming it to becomposed of two balanced systems (Figure 20.1),one positive and the other negative. The negativesystem having the reverse rotating field and tendingto rotate the rotor in the reverse direction. Eachfield produces a balanced current system, the phasorsum of which will decide the actual current themotor windings will draw. The main effect of anegative sequence current is thus to increase ironlosses and reduce the output of the motor. In such acondition, as the current drawn by the motorincreases, the torque and the power developed reduce,and the motor operates at a higher slip. All theselosses appear in the rotor circuit as slip losses. The

rotor operates at a higher slip and becomes relativelymore heated than the stator and more vulnerable todamage.

According to IEC 60034-1, a negative sequencecomponent up to 1% of the positive sequencecomponent of the system voltage, over a long periodor 1.5% for a short period, not exceeding a fewminutes, and with the voltage of the zero sequencecomponent not exceeding 1%, may be considereda balanced system.

A motor is able to sustain a negative sequencevoltage up to 2% for short durations, while asustained unbalance may deteriorate its insulationand affect its life. For higher unbalances, a deratingof the motor output must be applied according toIEC 60892 or to Figure 12.1.

A voltage unbalance may affect motorperformance in the following ways:– It would cause over-heating and the effective

output may have to be reduced to avoid this.See Figure 12.1.

– It would reduce Tst, Tpo and Tr etc. by a Vu2 (Vu

= unbalanced voltage). An unbalance up to 1%will not affect motor performance below thepermissible limits. See also MG-1-14.34 andFigure 12.2. The effect of unbalance on torquemay, however, be considered insignificant. Forexample, even at an unbalance of 5%, thenegative torque will be = (0.05)2 ¥ Tr or 0.25%of Tr, which is insignificant.

– Due to smaller output and torque, the slip wouldrise and add to the rotor’s losses (mainly ironlosses) (see Figure 12.3). The voltage unbalancecan be calculated as follows:

Voltage unbalance

=

Max. voltage variation fromthe average voltage

Average voltage 100%¥ (12.1)

Figure 12.2 Effect of negative sequence voltage on motortorque

Negative torque

Effective torqueRated torque

Pos

itive

tor

que

Neg

ativ

e to

rque

0

+Tr

–Tr

Speed

Nr

Figure 12.1 Derating in motor output due to voltage unbalance

Der

atio

n

1.0

0.9

0.8

0.70 1 2 3 4 5

% Voltage unbalance

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NoteThe negative sequence voltage is caused by an unbalance in themagnitude of voltages in the three phases, rather than in the phaseangle.

Example 12.1

Consider three line voltages 390 V, 400 V and 416 V. Then

the average voltage = 390 + 400 + 4163

= 402 V

and the maximum variation from the average voltage

= 416 – 402, i.e. 14 V

\ unbalance = 14402

100 = 3.48%¥

NoteA supply system having a voltage unbalance of more than5% is not recommended for an industrial application, whichmay have a number of electric motors connected on it. Ruraldistribution, however, is an exception due to excessive LVloads on the same network (Section 7.6), but such loads aremostly individual and not of the industrial type.

• Stator currents

The machine offers very low impedance to negativesequence voltages. As a result, the percentage increasein the stator current is almost the same as the startingcurrent on DOL switching, i.e. six to ten times the ratedcurrent.

To understand this, consider the equivalent motor circuit

diagram of Figure 1.15 with a normal positive sequencevoltage Vr. The unbalanced voltage Vu will produce anegative phase sequence and rotate the magnetizing fieldin the opposite direction at almost twice the supplyfrequency. The frequency of the negative phase sequencevoltage and current in the rotor circuit will thus become(2 – S) ¥ f and the slip (2 – S). In such a condition, theequivalent motor circuit diagram (Figure 12.4(a)) willassume the impedance parameters, as shown in Figure12.4(b). Thus

IV

R RS

SR X S ssX

rr

1 2 2

2

1 22

=

+ + (1 – )

+ [ + ]¢ ¥ ¢ÈÎÍ

˘˚̇

¥ ¢

(12.2)and

IV

R RS

SR X S ssX

uu

1 2 2

2

1 22

=

+ + ( – 1)(2 – )

+ [ + (2 – ) ]¢ ¥ ¢ÈÎÍ

˘˚̇

¥ ¢

(12.3)Iu = negative phase sequence current component.

(a) If the current during start (when S = 1) = IstThen Ist in the former case,

IV

R R X ssXst

r

1 22

1 22

= ( + ) + ( + )¢ ¢

Since ( + ) >> ( + )1 2 1 2X ssX R R¢ ¢

Figure 12.3 Heating effect caused by an unbalanced voltagesystem

300

275

250

225

200196

175

156

148

125

100

% I

2 R

copp

er lo

ss

0 10 2025

3040

50 60 70

% Negative sequence current

1

2

4

3

150

Maximum hot – phase heating (positive and negativesequence currents in phase).

Minimum hot – phase heating.

Average stator heating per phase.

Rotor heating.

2

3

4

1

Heating at 125%Balanced load

¢ ◊RS

S2(1– )

Figure 12.4(a) Equivalent circuit diagram with a balancedsupply voltage (at slip S)

V�

R1 X1

In�

Im

Data at slip S

S X.ss 2¢¢R2

¢Im

Figure 12.4(b) Equivalent circuit diagram with an unbalancedsupply voltage (at slip = 2 – S )

In�

Im

or ( + )uV V�

Vu = Component of unbalanced voltage

(2 – S )S.SSX2R1 X1

¢ ◊RS

S21– (2 – )

(2 – )

¢ ◊RS

S2( –1)(2 – )

¢R2

¢Im

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\ I

VX ssXst

r

1 2

( + )�

¢

(b) Now consider the current Iu during normal runningspeed, i.e. when s � 0. Then in the latter case,

I

V

R R R X ssXu

u

1 2 22

1 22

[ + – /2] + ( + 2 )

�¢ ¢ ¢

Applying the same rule of approximation as above,

since ( + 2 ) >> ( + /2)1 2 1 2X ssX R R¢ ¢

\ I

VX ssXu

u

1 2

( + 2 )�

¢and this relationship produces almost the same amountof current as during a start.

Corollary

Impedance of a motor during a normal running conditionto a negative phase sequence voltage will be almost thesame as that of the impedance of a motor during a start,in a balanced supply system. Thus, the current effectduring normal running of a negative sequence voltagewill be the same as that of the starting current effect ona balanced supply system.

The effect of voltage unbalance is, therefore, morepronounced than the percentage of unbalance itself. Forinstance, a voltage unbalance of 3% may cause a currentunbalance of 18–30%, which is detrimental to the life ofthe motor. The effective current in the stator windingswould depend upon the relative positions of the positiveand the negative sequence components. In one of thewindings they may be in phase, producing the maximumcurrent and the associated heating effect, and in the othertwo they may be 120∞ apart. In Figure 12.5 we havedrawn the maximum and the minimum effective currentswhich the stator windings may experience on anunbalanced supply system.

• Stator maximum heat

This occurs when the positive and negative sequencecomponents fall in phase in which case the equivalentstator current will become

I I Ieq r u(max.) = +

and the maximum heat generated, Heq(max.) µ(Ir + Iu)2

(Figure 12.3, curve 1)

or Heq (max.) µ ¥ ( + + 2 )r2

u2

r uI I I I (12.4)

Example 12.2Consider a negative sequence component of 40% of therated current. Then the maximum heat generated as in equation(12.4)

Heq (max.) µ (12 + 0.42 + 2 ¥ 1 ¥ 0.4)

or µ (1 + 0.16 + 0.8)

i.e. 1.96 or 196%

• Stator minimum heat

This occurs in the other two windings, which are 120∞phase apart (Figure 12.5). Hence

I I I I Ieq r2

u2

r u(min.) = + + 2 cos 60¥ ∞

= + + r2

u2

r uI I I I¥

and the minimum heat generated,

H I I I Ieq r2

u2

r u(min.) ( + + )µ ¥

(Figure 12.3, curve 2) (12.5)

Example 12.3The minimum heat generated in the above case

Heq (min.) µ (12 + 0.42 + 1 ¥ 0.4)

or µ (1 + 0.16 + 0.4)

i.e. 1.56 or 156%

• Stator average heat

In practice the temperature attained by the stator windingswill be significantly below the maximum or even theminimum heats as determined above due to the heat sinkeffect. The heat will flow from the hotter phase to thecooler phases/area (see curve 3 of Figure 12.3). But forprotection of the motor windings against negative sequencecomponents, the average heat curve is of no relevance,for it will take a considerable time for the heat to stabilizeat this curve, and that may be much more than the thermalwithstand capacity of the most affected winding.

It is therefore essential to provide adequate protectionfor the motor to disconnect it from the mains quicklybefore any damage is caused to the most affected winding.The protection is based on the maximum heat that maybe generated in the motor windings in the event of anegative sequence component in the system.Figure 12.5 Equivalent stator currents during unbalanced voltage

l eq(min)

B

I u

YIu

–ve

BI u

R

Iu

R

I r+ve

I r

I r

Y

Ieq(max)

= I r + I u

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• Magnitudes of negative sequence componentsfor protection

The additional heat generated by a negative sequencecomponent may vary from six to ten times the theoreticalheat produced by that amount of a positive sequencecomponent, as analysed above. It has, however, been foundthat it is more relevant to consider a factor of 6 to representthe effect of such a component. The heat generated canthen be rewritten as

H I Ieq r2

u2 ( + 6 )µ (12.6)

and the current on unbalance,

I I Ieq r2

u2 ( + 6 )µ (12.7)

The factor 6 is a design parameter for all future references.In fact, this empirical factor has been established overmany years of experience and field data collected on thebehaviour and performance of a motor in such anunfavourable operating condition.

The heat derived from this equation may be less thanthe minimum heat (Equation (12.5)) or even more thanthe maximum heat (Equation (12.4)) depending uponthe severity and the phase disposition of the negativecomponent with reference to the positive component.This can be illustrated by the following examples.

Example 12.4Referring to Examples 12.2 and 12.3 above, the heat producedaccording to the above empirical formula is as follows:

Heq µ (12 + 6 ¥ 0.42)

i.e. 1.96 or 196%

which is the same as that derived in Example 12.2.

CorollaryOne can thus easily obtain the significance of the factor 6 torepresent the status of the most affected winding of the motorin the event of a voltage unbalance resulting in a negativesequence current component. For more clarity, considerEquations (12.4) and (12.6) to ascertain the similarity in boththese equations. Since both must represent the maximumheating effect

\ I I I I I Ir2

u2

r2

u2

r u + 6 = + + 2 ◊ ¥

or 5 = 2 u2

r uI I I¥

or I u = 2/5 ¥ I r, i.e. 0.4 I r

Thus these two methods will yield the same result for a negativesequence component of 40%. If the negative sequence currentI u is lower than 40%, the heat produced from Equation (12.6)will be lower than the minimum heat obtained from Equation(12.5). In contrast, for a negative sequence component ofmore than 40%, the situation is likely to be reversed, sincethe heat produced as in Equation (12.6) will be higher thanthe maximum heat produced by Equation (12.4). See thefollowing example for more clarity.

Example 12.5Consider a negative sequence component of 15% and 50%respectively.

(a) For a 15% negative sequence component:

From equation (12.5)

Heq (min.) µ (12 + 0.152 + 1 ¥ 0.15)

i.e. 1.1725 or 117.25%

and from Equation (12.6)

Heq µ (12 + 6 ¥ 0.152)

i.e. 1.135 or 113.5%

which is even less than the minimum heat obtained fromEquation (12.5).

(b) For a 50% negative sequence component:

From Equation (12.4)

Heq (max.) µ (12 + 0.502 + 2 ¥ 1 ¥ 0.5)

i.e. 2.25 or 225%

and from Equation (12.6)

Heq µ (12 + 6 ¥ 0.52)

i.e. 2.5 or 250%

which is even more than the maximum heat obtained fromEquation (12.4). Equation (12.6) is thus more appropriate fora protective device and represents the effect of a negativesequence component in a motor winding more precisely.

• Rotor power

The negative sequence voltage sets up a reverse rotatingfield and the slip of the rotor becomes ‘2 – S’, comparedto the positive sequence slip S. The motor will thus operateunder the cumulative influence of these two slips, wherepower output P can be expressed by (see also Section2.3).

P I RS

SI R

SS

= 3 (1 – )

– (1 – )(2 – )rr

22 ru

22¥ ¥ ¥ ¥Ê

ˈ¯ (12.8)

whereIrr = positive sequence current in the rotor circuit, andIru = negative sequence current in the rotor circuit

• Rotor heat

The unbalanced voltage will produce an additional rotorcurrent at nearly twice the supply frequency. For example,for a 2% slip, i.e. a slip of 1 Hz, the negative sequencestator current, due to an unbalanced supply voltage, willinduce a rotor current at a frequency of (2f – 1) = 99 Hzfor a 50 Hz system. These high-frequency currents willproduce significant skin effects in the rotor bars andcause high eddy current and hysteresis losses (Section1.6.2(A-iv)). Total rotor heat may be represented by

µ( + 3 )rr2

ru2I I (12.9)

(refer to curve 4 of Figure 12.3). And cumulative rotorcurrent

I I Irr total rr2

ru2 = ( + 3 ) (12.10)

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Example 12.6For Example 12.2, the rotor heat at 40% stator negativesequence current

µ (12 + 3 ¥ 0.42)

i.e. 1.48 or 148%

Refer to curve 4 of Figure 12.3.

12.3 Fault conditions

These are conditions in which over-heating of the machinemay not trace back to its own thermal curves as in thefirst case. The temperature rise may now be adiabatic(linear) and not exponential and hence rapid. Now anormal thermal protection device may not be able torespond as in the previous case. Some conditions causingover-heating may not necessarily be fault conditions.Nevertheless, they may require fast tripping, and henceare classified in this category for more clarity. Suchconditions may be one or more of the following:

1 A fault condition, such as a short-circuit between phases.2 A ground fault condition.3 A prolonged starting time.4 A stalling or locked rotor condition: An under-voltage,

an excessive load torque or a mechanical jamming ofthe driven equipment may cause this. It may lock therotor during a start, due to an inadequate startingtorque. Such a situation is known as stalling andresults in a near-locked rotor condition (see Figure12.6). At reduced voltage start the motor may stall atspeed Na and may not pick-up beyond this. If the

motor was already running and the motor torque takesthe shape of curve B due to a voltage fluctuation, themotor may not stall but may operate at a higher slipS2. Although, this is not a stalling condition it maycause severe over-loading. At high slips, the currentIr traces back the starting current curve as on DOL asillustrated, and may assume a very high value.

In certain cases, for example in large motors whereTpo is normally not very high, during a severe drop involtage Tpo may fall to a value less than the loadrequirement and result in a stalling or even a lockedrotor condition (see Figure 12.7).

5 Frequent starts: It is not a fault condition, but rapidheating of motor’s stator and rotor due to frequentstarts will be no less severe than a fault condition,hence considered in this category.

6 Protection against single phasing.

In all these conditions protection against 2, 3, 4 and 5 isgenerally applicable to large LV motors, say 100 h.p.and above, and all MV motors. This protection is normallyprovided by a single-device motor protection relay,discussed in Section 12.5.

Single phasingThis is a condition of a severe unbalance. Until the 1970sthis had been the most frequent cause of motor failureduring operation. About 80% of installed small andmedium-sized motors, say, up to 100 h.p. experiencedburning due to single phasing because of the absence ofadequate single-phasing protection. With the introductionof single-phasing protection in the 1970s, as a built-infeature with thermal over-current relays (OCRs), thiscause of motor failure has significantly diminished in allthe later installations.

The following are the possible causes of single phasing:

• Immobilization of one of the phases during operationby the melting of a faulty joint such as poor cabletermination

• Blowing of one of the fuses during a start or a run(modern practice is to adopt a fuse free system (Section12.11))

Figure 12.6 Stalling condition during start and run

Torq

ue

Tst

Tr

Reduced voltagestarting or a lowvoltage condition

a

Na

Stallingcondition

Speed N r2S 2

N r1S 1

I r

I athigherslip

Cur

rent

Slip

C

B

A

D

D Current

A Motor torque

B Reduced voltage torque µ V 2

C Load torque

Ia

Ia = Current during stallingReduced voltage torqueLoad torque

Motor torque

Tst

Torq

ue

Tr

A

B

Nr1Na Speed

A low voltagecondition

Figure 12.7 Stalling condition during run

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• Defective contacts or• A cable fault, immobilizing one of the phases.

Effects of single phasing

1 The Tpo of an induction motor, say, up to 100 h.p., isnormally more than 150–200% of Tr. Therefore whenthe motor is operating at only one half to two thirdsof Tr and experiences single phasing during the run, itmay still be running without stalling, although at ahigher slip. It will now be subject to a rapid burnoutwithout adequate protection. The motor will now drawmuch higher currents in the healthy phases, to supple-ment the lost phase, as well as to compensate for thehigher slip losses. It may even stall if it was operatingat more than 70% of Tr at the time of single phasing.

2 If the motor is switched on, in a single phasingcondition, it will not rotate in the absence of a rotatingfield, similar to a single-phase motor without a startwinding.

3 If the motor stalls during pick-up, it will come to astandstill as a result of a locked rotor. The motor will

now be vulnerable to rapid burnout as the heat will belocalized and rapid in one part of the rotor and maydamage it without appreciably raising its overalltemperature.

4 Since single phasing is a condition of severe unbalance,it causes varying proportions of dangerous currentsin the motor windings, as discussed below.

Heat generated in a star-connected stator winding

In the event of single phasing, in star-connected windings,one of the phases of each positive and negative sequencecomponents will counter-balance each other, to producezero current in the open phase (Figure 12.8(a)). Themagnitude of these components in the two healthy phasesare equal, i.e. Ir = Iu, and the maximum heat of the statoras in Equation (12.4)

µ ¥ ¥ ( + + 2 )r2

r2

r rI I I I

i.e µ 4 r2I

and the maximum current Ieq (max.) = 2IrThe minimum heat as in Equation (12.5)

µ ¥( + + )r2

r2

r rI I I I

i.e µ3 r2I

and the minimum current Ieq (min.) = 3 .rI See Figure12.8(b).

Heat generated in a delta-connected statorwinding

In delta-connected windings we cannot derive the phasecurrents during single phasing by the above simplehypotheses. Now two of the phase windings are connectedin series, while the third forms the other path of the

Figure 12.8 Stator and line currents on single phasing for astar-connected winding

I B = 0

R

Y

I R = I r

IY = I r

(a) Winding diagram

I Ieq(min) r= ◊3

Y ¢

I b =

0

+ve

–ve

I r Y ¢Y I r

B

R = Ir

R ¢ = Ir

I eq(max) = 2Ir

(b) Phasor diagram

Figure 12.9 Stator and line currents on single phasing for adelta-connected winding

R Y B

F1–3 Fuse

I r I y

Lineopened

OCR

Y

Z

X

Motor winding in deltaI r = I y, I b = 0

B

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parallel circuit, as illustrated in Figure 12.9. The currentsin the immobilized phase as well as the healthy supplylines now vary disproportionately with load, the increasein the lone winding being more pronounced. A generalidea of the magnitude of phase and line currents soproduced is given in the curves in Figure 12.10 bymeasuring these currents by creating a single-phasingcondition.

Heat generated in the rotor

Referring to Equation (12.9) the rotor heat can beexpressed byµ ( + 3 )rr

2ru2I I

In single phasing, Iru = Irr, the rotor heat will be

µ ( + 3 )rr2

rr2I I

or µ 4 rr2I

i.e. four times the normal heat of the rotor, and the rotorcurrent = 2Irr.

If single phasing occurs at 50% of the full load, therotor current will be 100%. See curve 4 of Figure 12.10.

Conclusion

1 Star-connected stators(a) The theoretical heats of the motor as derived earlier

for star-connected stators and rotors are almostthe same. But even if single phasing does not leadto a stalled or a locked rotor condition, it maycause the motor to operate at a much higher slipdue to a lower torque, Tr. The rotor is thereforesubjected to a faster temperature rise compared tothe stator due to excessive eddy current lossesand its smaller volume compared to that of thestator. In single phasing, therefore, although thestator and rotor current curves may appear similar,their relative heats will be substantially different.The rotor in such motors is thus more critical andmust be protected specifically.

(b) Three-phase LV motors are built in delta connectionexcept for very small sizes, say, up to 1 or 2 h.p.,but all MV motors are generally wound in starconnection (see also Section 4.2.1). Generally,

in LV motors the stator and in MV motors the

rotor are more vulnerable to a fast burnout in

the event of single phasing.

2 Delta-connected stators(a) In delta-connected windings, the lone winding X,

(Figure 12.9) for motor loads 50% and above carriesa current higher than the rated full load currentand also higher than that of the rotor, and becomesmore vulnerable to damage compared to the rotor.This difference is more significant at loads closerto the rated load (Figure 12.10).

(b) A study of Figure 12.10 will suggest that in theevent of single phasing, protection should be suchthat it traverses the replica of the heating curve ofphase X.

(c) It also suggests that the heat generated in the rotorcircuit, due to voltage unbalance or single phasing,is less than the maximum stator heat. The factorof 6 considered in Equation (12.6) is adequate totake account of it.

(d) For loads of less than 50% of the rated motorcurrent, this protection is not required as the currentin phase X will not exceed 100% of the full loadcurrent during a single phasing.

Corollary

Since an inter-turn fault also causes unbalance, itis protected automatically when a negative sequenceprotection is provided depending upon its sensitivityand the setting.

3 Rotor circuit(a) In a rotor circuit the rotor current and the heating

effects, due to single phasing, remain the samefor both star- or delta-connected stators due to thesame (2f – S) � 100 Hz rotor currents on a 50 Hzsystem.

(b) A stator thermal withstand curve cannot beconsidered a true reflection of the rotor thermalconditions. In a delta-connected motor (mostlyLV motors), the stator would heat-up more rapidlythan the rotor, and normally protection of the statormay also be regarded as protection for the rotor.But not so in the following cases:

Figure 12.10 Current magnitudes in different phases of adelta-connected stator winding and rotor during single phasing

Lone

win

ding

Rotor

curre

ntsLi

necu

rrent

s

Healthy ph

as

es

1007550 60250

% Motor current under healthy condition

300

250

200

150

135%

100110

50

0

% M

otor

cur

rent

on

sing

le p

hasi

ng

3Y/Z

4

2R/Y

1

X

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• Prolonged starting time• Stalling or a locked rotor condition• Frequent starts, and• All MV motors wound in star.

In all the above conditions, the rotor would heat-up much more rapidly than the stator due to itslow thermal time constant (t), and its smallervolume compared to that of the stator, on the onehand, and high-frequency eddy current losses athigh slips, due to the skin effect, on the other.True motor protection will therefore requireseparate protection of the rotor. Since it is notpossible to monitor the rotor’s temperature, itsprotection is provided through the stator only.Separate protection is therefore recommendedthrough the stator against these conditions for largeLV and all MV motors.

Notes1 It is for this reason that rotors, as standard practice, are designed

to withstand a much higher temperature of the order of 400–450∞C in LV motors and 300–350∞C in MV motors, comparedto a too-low temperature of the stator. This temperature issuch that for almost all motor operating conditions meticulousprotection of the stator would also protect the rotor. It is alsoobserved that rotor failures are therefore rare compared tostator failures.

2 Nevertheless, whenever the rotor is more critical, despite ahigher rotor operating temperature, rotor thermal curves areprovided by the manufacturer for facilitating protection forthe rotor also through the stator.

3 The rotor design, its cooling system or the motor size itselfmay have to be changed substantially for motors to be installedin fire hazard environments to limit their temperature rise inadverse operating conditions within safe limits (Table 7.6).

12.4 Protection

12.4.1 Protection against unfavourable operatingconditions

Protective devices and their selection

We will now discuss protective devices, and their selection,that will be essential to safeguard a motor against allunfavourable operating and fault conditions. A machinemay be provided with a modest or elaborate protectivescheme, depending upon its size, application and voltagerating. This enables savings on cost, where possible, andprovides a more elaborate protection where more safetyis necessary. Accordingly we have sub-divided theprotection as follows:

1 Protection of small and medium-sized LV motors, upto 300 h.p.

2 Protection of large LV motors, say, 300 h.p. and above.This is to be decided by the user, based on the loadrequirement and critical nature of the drive

3 Protection of MV motors

1. Small and medium-sized LV motors

Protection against over-loadingThis can be achieved by an over-current relay. The basic

requirement of this relay is its selectivity and ability todiscriminate between normal and abnormal operatingconditions. Three types of such relays are in use: thermal,electromagnetic and static. Thermal relays are employedfor motors of up to medium size and electromagneticand static relays for large LV and all MV motors, asdiscussed in Section 12.5.

The thermal relays in general use are of two types, i.e.

• Bimetallic, and• Fusible alloy or eutectic alloy

Because of their spread between hot and coldcharacteristics, these relays allow a tripping time of lessthan the starting time when a hot motor stalls, so a separatestalling protection is normally not necessary. They detectthe r.m.s. value of the current and thus account for theeffects of harmonics, present in the current, drawn bythe motor. They also take into account the heating, dueto previous running of the motor as they are also heatedalong with the motor. This feature is known as thermalmemory. These relays thus possess tripping characteristicsalmost matching the thermal withstand capacity of themotor.

Bi-metallic* thermal relays (Figure 12.11)Each phase has a heater in series with the circuit. One ormore bi-metallic strips are mounted above these heaters,which act as latches for the tripping mechanism or togive an alarm signal if desired. The heaters may be heateddirectly for small motors or through current transformers(CTs) for medium-sized motors. Bending of the bimetallicstrips by heating, pushes a common trip bar in the directionof tripping to actuate a micro-switch to trip the relay orcontactor. The rate of heating determines the rate ofmovement and hence the tripping time, and provides aninverse time characteristic. The power consumption ofthe bimetal heating strips varies from 2 to 2.5 watts/phase, i.e. a total of nearly 7.5 watts.

The latest practice of manufacturers is to introduce avery sensitive differential system in the trippingmechanism to achieve protection even against single-phasing and severe voltage unbalances. In the relays withsingle-phasing protection a double-slide mechanism isprovided. Under single phasing or a severe voltageunbalance, the two slides of the relay undergo a differentialdeflection. One slide senses the movement of the bimetalthat has deflected to the maximum, while the other senses

*Any bi-metal combination, having large differences in theircoefficients of linear expansion, such as a bimetal of brass andsteel is used for such applications. One end of the strip is fixed andthe other is left free for natural movement. When heated, brassexpands more than the steel and bends towards the steel as shown,giving the desired movement to actuate a tripping lever.

SteelBrass

Movement on heatingFixed atone end

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the minimum. These slides are linked so that thecumulative effect of their movement actuates a micro-switch to trip the relay. Figure 12.12 illustrates the trippingmechanism of an over-current-cum-single-phasing thermalrelay. Because of differential movement it possesses dualcharacteristics, as shown in Figures 12.13(a) and (b),one for an ordinary over-current protection during three-phase normal operation (Figure 12.13(a)) and the otherwith differential movement for over-current protectionduring a single phasing or severe unbalance (Figure12.13(b)). For instance, for a setting at the rated current(100% Ir) under normal conditions the relay would stayinoperative (Figure 12.13(a)), while during a singlephasing it will actuate in about 200 seconds (Figure12.13(b)) and provide positive protection against singlephasing.

Characteristics of a bi-metallic thermal relayThe thermal characteristics are almost the same as thoseof an induction motor. This feature makes them suitablefor protecting motor by making a judicious choice of theright range of the required duty. (See Figure 12.11 for atypical thermal over-load relay and Figure 12.13 for itsthermal characteristics.) Ambient temperaturecompensation is achieved through an additional strip inthe over-load relay, which operates the tripping lever inthe other direction than the main relay to achieve adifferential effect and is so arranged that it is independentof the main relay.

Operation of the relay may not necessarily start at thepreset value due to certain allowable tolerances. As inIEC 60947-4-1, the relay must not trip within two hoursat 105% of FLC but it must trip within the next twohours when the current rises to 120% of FLC. Also, itshould trip in two hours in the event of single phasingwhen the line current in the healthy phases is 115%, but

it should not trip in less than two hours during a healthycondition, when two of the phases carry 100%, while thethird carries 90% of FLC (a case of voltage unbalance).The curves of Figure 12.10 illustrate the likely operatingcurrents in different phases of a delta-wound motor onsingle phasing or voltage unbalance. A good thermalrelay should be able to detect these operating conditionsand provide the required protection. The thermal curveof a relay is thus in the form of a band as shown inFigure 12.14.

With the introduction of single-phasing detection andprotection feature in the conventional thermal relays thetripping current–time (I2 versus t) characteristics of therelay traverse almost the same thermal curve as may beprevailing in the most vulnerable phase of the motorwinding during a single phasing, i.e. according to curve‘X’ of Figure 12.10. The characteristic curve of the relayis chosen so that it falls just below the motor thermalcurve and has an adequate band formation, somewhatsimilar to the curves of Figure 12.14.

Relays for heavy dutySuch relays may be required for motors driving heavyduty loads with large inertias or for motors that employreduced voltage starting and require longer to accelerate.Consequently, a relay which can allow this prolongedstarting period without causing a trip during the startwill be desirable. CT-operated relays can be used forsuch duties. They comprise three saturated currenttransformers (CTs) associated with the ordinary bi-metalover-current relay (Figure 12.15). These saturated currenttransformers linearly transform the motor line or phasecurrents up to a maximum of twice the CT primary current.

Above this ratio, the cores of the CTs become saturatedand prevent the secondary circuit reflecting the startingcurrent in the primary and thus prevent the relay from

Figure 12.11 Thermal over-current relay (Courtesy: L&T)

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tripping during a permissible prolonged start. For example,a CT of 150/5A will have a saturation at approximately300 A, irrespective of the magnitude of the starting current.For schematics of such relays refer to Figure 13.54.

Over-current setting of relaysThese can be adjusted by varying the contact traverse.The mechanism’s design is such that an increase or adifference in the line currents, due to voltage unbalanceor single phasing, drives the mechanism towards thetripping lever. These relays operate at 100% of theirsetting and are therefore set at

Relay setting (% of FLC)

= (Operating current %) (CT ratio) (Relay rating)

(typical)r¥¥

I

NoteAll thermal relays are available in two types of trip contacts, self(auto) reset and hand (manual) reset. In self reset relays, after a tripthe relay resets automatically, as soon as the bimetallic elementcools down and regains its original shape. In a hand-reset relay,after every trip the operator has to reset the relay manually beforea restart of the motor. The latter type thus provides an opportunityto the operator to investigate the causes of a trip and correct these,if possible, before a restart. In the former case, the motor, withoutbeing subjected to an investigation, may restart on its own (when it isso wired) as soon as the bimetal cools. Since it may not be feasible toachieve bimetallic elements having identical cooling characteristicsas those of the motor, it is likely that the bi-metal may cool fasterthan the motor and allow the motor to restart, without allowingadequate time to cool. In fact, the motor windings have a considerablyhigher thermal time constant than the bimetal relays and cool moreslowly than the relays. Whereas the relay will permit rapid repeatedswitchings, these may not be warranted. Hand-reset relays are thuspreferred, wherever possible, to give the operator an opportunity toinvestigate the causes of tripping before a restart.

Eutectic alloy relaysThese relays also possess characteristics similar to thoseof a bimetallic relay and closely match the motor heatingand cooling curves. They are basically made of a lowmelting eutectic alloy which has defined meltingproperties. The alloy, with specific proportions ofconstituent metals such as tin, nickel and silver, can bemade for different but specific melting temperatures. Thisproperty of the alloy is used in detecting the motor’soperating conditions.

Such relays are in the form of a small tube insidewhich is a loosely fitted rotatable shaft, held by a verythin film of this alloy. The alloy senses the motortemperature through a heater connected in series withthe motor terminals and surrounding this tube. When themotor current exceeds the predetermined value, the alloymelts and enables the shaft to rotate and actuate thelever of the tripping mechanism.

Such relays are satisfactory in performance andmay be adopted for all industrial controls. Some of thefeatures of these relays for use on motor starters aregiven below:

1 The motor can be switched ON only after the alloysolidifies again and hence prevents the motor froman immediate switching after a trip, thereby givingthe motor adequate time to cool.

2 An inadvertent or deliberate high setting of the relay(which is quite common to prevent frequent trippings)than recommended is not possible on these relaysdue to their very narrow operating range. Prima facie,it may appear to be a disadvantage with such a relay,as a number of alloy ‘strip sets’, with differentoperating ranges, may have to be stocked for everymotor. But they provide more precise protection forthe machine.

NoteThe eutectic relays have an advantage by setting the pointermore closely in the field, based on the actual measurement ofthe load current.

Slider(I)Lever

aR

Microswitch

BYR

Slider(II) a a a

R

R

b b b b

R

R

R

R

Tripping action of the relay

a

1 During cold state

2 At rated current

b Movement of lever during a singlephasing in phase R

a Movement of lever during an overloading

3 During symmetrical 3 phase overload

Figure 12.12 Tripping mechanism of an overcurrent-cum-single-phasing thermal relay

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3 Since the operation of the eutectic alloy relays dependsupon the magnitude of heating, which is a functionof current and time, these relays also give an inversecurrent–time characteristics.

4 Due to their very narrow operating range, these relayshave limitation in their application on drives withfluctuating loads such as cranes, hoists or polechanging motors, or loads with intermittent dutieswith frequent variations in the load current. In viewof a generally wide voltage fluctuation on an LVdistribution network, even a definite duty motor mayhave cognisable current variations, leading tounwanted trippings of the machine.

NoteIn view of these limitations, such relays did not find adequateacceptance under Indian conditions. As a result, their manufacturehas been discontinued.

2. Large LV motors

Large motors call for a more judicious selection of relays.Unlike small motors, one cannot take for granted thatthe thermal characteristics of the relay will be the sameas that of the motor, and arbitrarily select any thermalrelay. To make use of the optimum capacity of a motor

and to yet protect it from all possible unfavourableoperating conditions it is essential that the motor and therelay’s thermal characteristics are matched closely.

Motors designed according to IEC 60034-1 are notmeant for continuous over-load running unless specificallydesigned for this. They should be closely protected withthe available devices. On the one hand, the protectionshould be discriminating, to allow for starting currentsurge and yet detect an over-loading, unbalance, short-circuit or a ground fault before these cause damage tothe motor. On the other, it should ensure a full-loadoperation of the motor.

A thermal relay cannot be set reliably to remaininoperative at 100% of the full load current and thenoperate instantly as soon as it exceeds this. A good thermalrelay can be set to operate between 110% and 115% ofthe Ir, or even more if that is desirable, provided that thethermal capacity of the motor can permit this. To ascertainthis, availability of the motor thermal withstand curve isessential. Accordingly, the relay can be set for the optimumutilization of the motor by setting it for

Relay setting (% of FLC)

= Motor maximum operating current (%)

1.1 or 1.15 (typical)

Figure 12.13 Typical characteristics of a thermal over-current relay

MinimumMaximum

Tim

e in

sec

onds

0.7 0.9 1 1.2 1.5 2 3 4 5 6 7 8 9 10

100% I r 1.05x I r

(a) Tripping under a healthy condition.

104

86

4

2

103

86

4

2

102

86

4

2

1086

4

2

MinimumAverageMaximum

104

86

4

2

103

86

4

2

102

86

4

2

1086

4

2

Tim

e in

sec

onds

0.7 0.9 1 1.15 1.5 2 3 4 5 6 7 8 9 10

100% I rx I r

(b) Tripping under a single phasing condition.

200 sec.

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Additional protection through a supplementaryIDMT relaySince thermal relays, with numerous characteristics andadjustable settings to match every individual motor, arenot feasible, the nearest characteristic relay available inthat range must be chosen. If it is considered necessaryto ensure adequate protection at each point of the motorcurve, this relay may be additionally supplemented throughan inverse definite minimum time (IDMT) relay, havinga definite time or inverse to very inverse time

Figure 12.14 Operating band of a thermal over-current relay

Tim

e (S

econ

ds) Maximum over-load

characteristics

Minimum

10

Ir IstCurrent (Amps.)

Motor characteristics

Motor characteristics

Figure 12.15 CT-operated thermal overcurrent relay(Courtesy: Siemens)

OCR

CTs

Figure 12.16 Supplementing a thermal relay with an IDMTrelay for complete motor protection

Bimetallic relaycharacteristicsMotor thermalcurve

Tim

e

1 2 3

1

3

2

IDMT relay

t ir < t m < t r

t m t i r

t r

CurrentI r I st

AIDMT protectionOCR protection

characteristics, whichever may best suit the motor’sunprotected region on the thermal curve, as illustrated inFigure 12.16. As can be observed, the closest relay chosenfor this motor does not protect it during a start due to ahigher tripping time than the motor thermal withstandtime (tr > tm), while during a run, beyond the operatingregion ‘A’, it lies closely below the motor curve asrequired. During a start, therefore, it has beensupplemented by an IDMT relay, whose startingcharacteristic lies closely below the motor thermalwithstand curve (tm > tir) and provides the required startingprotection. Hence with the use of these two relays, themotor can be fully protected.

NoteIt is also possible that in the operating region, beyond point ‘A‘,the curve of the thermal relay had fallen far below the motor thermalcurve and had overprotected the motor. In other words, it wouldhave under-utilized its capacity, in which case, it will be necessaryto call for a re-selection of the thermal relay that would permitoptimum utilization of the machine and, if necessary, giving itsupport through an IDMT relay to cover the under-protected region.Such a combination of an OCR and an IDMT relay is satisfactoryfor detecting a system fault, over-loading or a stalling conditionbut it cannot guarantee total protection. This combination does nottrace a replica of the motor heating and cooling curves. It cansimply detect the motor line currents and not the conditions thatmay prevail within the windings, such as those as caused by anunbalance or a single phasing. Nor can they accurately assess therotor’s heat caused by prolonged starting time or frequent starts.These relays, at best, can be employed with instantaneous definiteminimum time to inverse and very inverse I2–t characteristics tomatch the machine’s requirement as closely as possible. In view ofthis, it will be worthwhile to have a single device protection againstover-load and stalling which may occur due to under-voltage,unbalance, single phasing or a ground fault. Such a protection is

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possible through a single device motor protection relay, discussedin Section 12.5.

3. MV motors

These call for a closer protection, which is possible througha single point motor protection relay (MPR). Since asingle MPR provides protections against unfavourableoperating as well as fault conditions, we discuss thisrelay separately in Section 12.5.

12.4.2 Protection against fault conditions

Protection against short-circuits(use of HRC fuses)

It is always desirable to protect a power circuit againstshort-circuits separately. HRC fuses are the immediateanswer to such a requirement for small and medium-sized LV motors. They have a delayed action characteristicunder an over-load and are instantaneous against a short-circuit condition and thus inherit the quality ofdiscrimination. They reduce the electromagnetic andthermal stresses and enhance the withstand capacity ofthe protected equipment for higher fault levels. Quiteoften, they are used on the receiving end side of a supplysystem to enhance the short-circuit withstand capacityof all the equipment connected in the circuit and installedafter the HRC fuses. Figures 12.17–12.19 illustrate howthe HRC fuses, by quickly isolating the circuit on a faultwell below the prospective fault level, reduce theelectromagnetic and thermal stresses on the connectedequipment. If the same fault had been cleared by anordinary short-circuit relay it could reach its momentarypeak value, which can rise to 2.2 times the r.m.s value offault current in LV and 2.5 times in HV circuits (Table13.11) as it had taken more than one-half of a cycle toclear the fault. For instance, a fuse of 320 A, ofcharacteristics shown in Figure 12.18, will cut off a faultof 30 kA symmetrical, with a crest (prospective) valueof around 63 kA at only 15 kA, which is well below itsprospective value.

Selection and co-ordination of fuse rating

The selection should be such as to provide properdiscrimination at the various levels of a multi-distributionnetwork. Our discussions generally take account of therecommendations in IEC 60947-4–1 regarding co-ordination between the short-circuit protection and themain components such as switching devices [switch,breaker, MCCB (moulded case circuit breaker) or MCB(miniature circuit breaker) and contactor] and the over-load relay. These recommendations permit damage ofcomponents on fault to varying degrees as noted below:

• Type 1: Under short-circuit conditions the contactoror the starter will cause no damage to the operator orthe installation but may not remain suitable for furtherservice without repairs or replacement of some of itsparts. In other words, contact welding of the contactoris allowed and burnout of OCR is acceptable. In eithercase replacement of components may be necessary.

• Type 2: On the other hand, Type 2 degree of protectionlimits the extent of damage in the case of a short-circuit. Now under short-circuit conditions, the

Figure 12.17 Cut-off feature of an HRC fuse

Commencementof short-circuit

Symmetricalshort-circuitcurrent

Waveform of ashort-circuit currentwith d.c. component

ttt1ts

isc

ts = Pre-arcing timet1 = Arcing timet t = Total cut-off time

i sc(peak) = Prospective peak value of a fault currenti sc = Peak cut-off current by fuse

isc (peak)

Figure 12.18 Typical current cut-off characteristics for LV HRC fusesat prospective current up to 80 kA

6

10

16

5032/25

6380

100125

160200

224

320250

355400

425500

630

Current cut-offlevel

Pea

k cu

t-of

f cu

rren

t i s

c (k

A)

100.080.0

60.0

40.0

25.0

20.0

<1510.0

8.0

6.0

4.0

2.0

1.00.8

0.6

0.4

0.2

0.11 2 4 6 8 10 20 40 60 80

Symmetrical prospectiveshort-circuit current Isc(rms)(kA)

30

Illustration:A 320A fuse will cut-off an I sc of 30 kA (prospective peakvalue up to 2.1 ¥ 30 kA) at less than 15 kA (peak).

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contactor or the starter will present no risk to theoperator or the installation and will remain suitablefor further service. In other words, no damage to thecontactor or the OCR is permitted. It may, however, beinterpreted that contact welding may be permissible tothe extent that the contactor can be put to service onceagain after a brief period by separating the contactswith the help of a tool such as a screw-driver butwithout calling for replacement of any part.

Type 2 co-ordination is more prevalent and commonlyused for all industrial applications. Below, we concentrateon co-ordination Type 2, permitting the least damageand longer service life. This co-ordination can safelywithstand normal fluctuations in system parameters andoperating conditions during normal working. It is alwaysadvisable to verify the authenticity of the co-ordinationin a laboratory. For procedure, to establish the type ofco-ordination, refer to IEC 62271-200. To achieve therequired precise co-ordination we discuss a few typicalcases below.

Discrimination between fuse to fuseRefer to the normal distribution network of Figure 12.20.Selection of the fuse ratings must be made on the followingbasis:(a) Only fuses nearest to the fault should operate. For

instance, for a fault at location C, the only fuses atlocation C should operate and not those at B or A.

(b) To ensure the above, the total arc energy, I2 · tt, of the

lower fuses at C should be less than the pre-arcingenergy, I2 · ts of the upper fuses at locations B or A.In other words, the current–time (I2 – t) characteristicsof fuses at C should lie below that of fuses at B, and

Figure 12.19 HRC fuse characteristics6A

10A

16A20A 25A

32A 50A63A

80A100A

125A160A

200A

630A

500A425A

400A355A

250A320A

224A

50

20

10

5

2

1

20

10

5

2

1

0.5

0.2

0.1

0.05

0.02

0.01

0.005

0.002

0.001

Min

utes

Pre

-arc

ing

time

Sec

onds

10 20 50 100 200 500 1000 2000 5000 10 000 20 000 50 000Prospective current in amps (r.m.s)

A

B

C

F

Figure 12.20 Co-ordination of fuse ratings and theircharacteristics

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that of fuses at B should lie below the fuses at A, etc.,throughout their operating range.

Co-ordination of fuses with an over-currentrelay or any other over-current protective deviceThe selection of the fuses should be such that:(a) They do not operate during a start.(b) They do not operate against over-loads as these are

taken care of by the over-current relay or any otherover-current protective device.

(c) They should isolate the supply to the motor in theevent of a fault sufficiently quickly before the faultcauses damage to the connected equipment by burningand welding of contacts of the contactor or by causingpermanent damage or deformation to the bi-metalelements of the OCR. This is possible by ensuring thelet-through energy (I2 · t) of the fuses under faultconditions to be less than the corresponding let-throughenergy of the OCR (Figure 12.21).

Co-ordination of fuses with a breakerWhen the fault level of the system is more than thebreaking (rupturing) capacity of the associated breaker(usually MCB or MCCB) or when the fault-makingcapacity of the breaker falls short of the momentary peakvalue of the fault current of the system (Table 13.11) thatfuses can be used before the breaker to provide back-upprotection and supplement breaker’s breaking capacity,to make it capable of performing switching operationssuccessfully on a fault. Co-ordination between the fusesand the built-in over-load and short-circuit releases ofthe breaker can be carried out on the following basis:

(a) Over-loads on the system should be cleared by trippingthe breaker through the in-built releases only and notthrough the fuses.

(b Short-circuit currents up to the breaking capacity of

the breaker should also be cleared by tripping thebreaker through the in-built releases and not throughthe fuses.

(c) Short-circuit currents in excess of the breakingcapacity of the breaker alone should be cleared bythe operation of the fuses.

To achieve the above the characteristic of the fusesshould lie well above the characteristic of the over-currentand short-circuit releases of the breaker for the lowerregion of currents, such that only the breaker operates.However, it should lie well below the characteristic ofthe breaker in the higher region of currents to ensure thatthe fuses operate sufficiently quickly and long beforethe in-built releases. The breaker is thus prevented fromoperating at currents that are in excess of its breakingcapacity. Figure 12.22 illustrates such a co-ordination.

Note1 Back-up protection to supplement an interrupting device to make-

up its rupturing capacity and make it capable of meeting thesystem fault level is a concept of LV systems only to sometimesreduce cost. Interrupting devices for an HV system normallypossess adequate rupturing capacity to meet system needs easily.Moreover, HRC fuses beyond certain voltages (>11 kV) andcurrent ratings (>1000 A) are generally not used.

Where interrupting devices have a limitation in their rupturingcapacity (which may sometimes occur on an EHV system), thesystem fault level can be altered accordingly, so that the availableinterrupting devices can still be employed (Section 13.4.1(5)).

On an LV system too back-up protection is not a recommendedmethod for an ACB or an OCB as they both would possess atripping time of more than a cycle (Table 19.1) compared to thecurrent limiting properties of the HRC fuses. Current limitingproperties of HRC fuses make them operate much faster (< 1/2cycle) than a breaker, during a fault condition, even when thebreaker could safely clear the fault. In such cases, therefore,when breakers are required for a higher rupturing capacity thana standard breaker possesses, then a breaker of a higher rating,which may have the required fault level, may be chosen with alower setting of the OCR. Back-up protection may, however, be

Figure 12.22 Co-ordination of fuses with a breaker

HRC fuse

Breakermin. setting

Breakermax. setting

Fau

lt co

nditi

onfo

r th

e fu

ses

Breaker’sunprotected

region

Fault level ofthe system

Breaking capacityof the breaker

Trip

ping

tim

e

I r

On faults exceeding Isc of breaker, I 2t (fuses) < I 2t (breaker)

Current

Figure 12.21 Co-ordination of fuses with an OCR

On fault I 2t (fuses) < I 2t (OCR)tst – Safe thermal withstand time of motor

during a full voltage start

Trip

ping

tim

e

CurrentIst

Ir

Faul

t con

ditio

nfo

r th

e fu

ses

HRC fuse

OCR min.setting

OCR max.setting

tst

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applied in an MCB or an MCCB, when they possess a rupturingcapacity less than required. Since the MCB and the MCCB can bothbe current limiting the characteristics of the fuses and the breakerscan be co-ordinated such that faults that are in excess of the rupturingcapacity of the breakers alone are handled by the fuses.

In such cases it may often be possible to meet the requirementby selecting a higher frame size of MCB or MCCB, which maypossess a higher rupturing capacity also. If not, and to save oncost, one may provide HRC fuses for back-up protection.

2 To make a proper selection of HRC fuses it is essential that thecurrent–time characteristic curves for the releases of the breakerand the fuses are available from their manufacturers.

3 With the availability of new generation compact and energysaving high rupturing capacity MCCBs and ACBs the applicationof back-up protection is now a practice of the past. The oldinstallations too, using such a practice may retrofit theirinstallations with space and energy saving new generationswitchgear components and assemblies.

Co-ordination of fuses with a switch or a contactorSince both these devices possess a certain level of makingand breaking capacities, the same criteria will apply asfor the breaker noted earlier. Rating of fuses shall not bemore than the switch or contactor rating unless fuse isplaying a short-circuit protective device (SCPD) for therelay.

Co-ordination of fuses with a transformerConsider a distribution HV/LV transformer. If the fusesare provided on both HV and LV sides, the fuses on theHV side must protect a fault within the transformerwhile the fuses on the LV side must clear over-currentand fault conditions on the LV side. Thus, for a fault onthe LV side, only the LV side fuses must operate and notthe HV side, similar to the requirements discussed above.

If the transformer is HV/HV, the same requirementmust prevail, i.e. for a fault on the downstream (secondaryside) only the downstream fuses must operate and notthe fuses on the upstream (primary side).

NoteIt is, however, possible to eliminate the use of HRC fuses in LVsystems at least, with the availability of more advanced technologyin an MCCB or an MPCB (motor protection circuit breaker). SeeSection 12.11 for a fuse-free system.

12.4.2 (a) Applications of different coordinationsof components

The level of coordination is a matter of starter application.Table 12.0(1) provides brief guide lines of the likelyapplications of the above-mentioned coordinations.

12.4.3 Protection against stalling andlocked rotor

Motors which do not possess a sufficient gap betweentheir hot withstand and starting curves generally call forsuch a protection. Large LV motors and all MV motorsare recommended to have a separate protection againstsuch a condition. A locked rotor protection relay basicallyis an over-current relay, having an adjustable definitetime delay to trip the motor when it exceeds its permissiblestarting time, but before the safe stall withstand time.This feature is available in a motor protection relaydiscussed later. Where, however, such a relay (MPR) is

not provided, a high-set IDMT over-current relay can bechosen to match the upper range of the motor thermalwithstand curve (Figure 12.16).

12.4.4 Protection against voltage unbalance ornegative phase sequence

Such a condition also generates negative sequencecomponents. For small and medium-sized motors, say,up to 100 h.p., no separate protection for such a conditionis normally essential, when the over-current relaypossesses a built-in single-phasing protection or thetripping circuit is provided with a single-phasing preventer.If a separate protection for this is considered desirable,one may use a negative phase sequence relay like the oneshown in Figure 12.23(b) or similar static or PLC-basedrelays. For larger motors, however, one should employ arelay like a motor protection relay, which covers in oneunit all the protection as described in Section 12.5.

NoteOne should employ only current sensing relays as far as possiblefor such applications as a negative sequence current has a severeeffect on motor windings due to a much lower negative sequenceimpedance of the motor (Section 12.2(v)) than a correspondingnegative sequence voltage.

Such a relay is connected on the supply side as shown in Figure12.23(a). The arrangement is such that unless the relay closes, themotor switching circuit will not energize and the motor will notstart. The contact closes only when the supply voltage is normaland the phase sequence positive. Even for under-voltage conditions,the torque developed by the relay may not be adequate to close thecircuit. Such relays are, therefore, effective against

(i) Negative sequence voltages when torque developed by its coilis negative.

(ii) Voltages far too low to produce an adequate torque to close therelay. This is possible during a start only, as during a run therelay contacts are already established and the coil does notdetect a fall in the voltage.

(iii) Single phasing during a start, as this will also produce a negativetorque to close the relay.

12.4.5 Protection against single phasing (SPP)

An ordinary thermal relay senses only the line currentsand is not suitable for detecting a single-phasing condition.

Table 12.0(1) Applications of different coordinations

S. Type of coordination Requirement Application1

no

1 1 (IEC 60947-4-1) Standard starters Air conditioningplants,

2 2 (IEC 60947-4-1) High performance Lifts, escalatorsstarters

3 Total coordination2 Very high Toxic fume(IEC 60947-6-2) performance starters extraction fans

1It will largely depend on the user based on economics and criticalityof the application. For individual applications like pump and fanmotors usually type-1 and for process plants type-2 coordination ispreferred.2Total coordination will ensure no danger to operator and theinstallation. There will be no damage to the starter (no contactwelding permitted). An immediate restarting will be possible withoutinspection of the starter.

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Referring to the curves of Figure 12.10, an ordinary relayset at 110% of FLC will not trip in the event of a singlephasing when the motor is operating under-loaded, say, atonly 60% or less of the rated current, while in the lonephase X, the current would be as high as 135% of FLC.

For single-phasing protection, ‘single-phasingpreventors’ are available. Although it is essential to protecteven a small motor against single phasing, there is littlepoint in employing these preventers unless they formpart of the basic starter (OCR with built-in SPP feature)as an economic consideration.

As discussed earlier, most of the leading manufacturersof switchgear components produce thermal over-currentrelays with a built-in feature of single-phasing prevention.The use of a separate single-phasing preventor device isthus not necessary up to medium-sized motors, wherethis feature is available in the relays. For large-motorsand for critical installations, a separate unit for singlephase protection may be required for prompt single-phasing alarm and/or trip. Interestingly, where star deltaswitching is employed, the over-load relays, which arenow connected in phases of the motor windings,automatically sense an abnormal condition in any or allthe phases, and provide a single-phasing protection. Seethe power circuit diagram in Figure 13.56 for more clarity.

A separate single-phasing protection device is availablein two versions:

1 Voltage sensing, and2 Current sensing

Voltage-sensing preventers have limitations and are not

reliable since they offer protection up to the sensingterminals only. Protection beyond these terminals up tothe motor terminals is not possible. In the event of aphase failure beyond this point, the voltage-sensingequipment will not detect this. (See the schematic ofFigure 12.24.) Current-sensing preventers are thereforerecommended for more reliable detection of a faultanywhere within the system up to the motor terminals.Moreover, voltage-sensing preventers may act erratically,when the motor is generating high back e.m.f. on singlephasing, and also when the power factor improvementcapacitors are connected across the motor terminals. In thiscase high back e.m.f. can be produced across the voltage-sensing relays, which can make its operation uncertain.

The current-sensing solid-state type relays consist ofa filter circuit to sense the negative sequence current.The output of this filter is proportional to the negativesequence component of the current. The output is fed toa sensor, which detects the level of negative sequencecomponent of current and trips the starter circuit whenthis level exceeds the set limit. (See Figure 12.25.)Normally such preventers are designed up to 30 A for

Static negative sequence relay (Courtesy: English Electric)

Figure 12.23(b)

Figure 12.23(a) Schematic for a negative phase sequence(NPS) relay

R Y B

Sw

F1–3

NPS

Negative phasesequence voltage

relay

NPS

C OnPB

C Holding coil

N

C

M

OffPB

PLC or micro-processor-based negative sequence relay(Courtesy: Alstom)

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motors up to 20 h.p. For larger motors, the output currentcan be sensed through CTs of 5 A secondary (Figure12.26). 5 A secondary is chosen to make detection easy.

At a preset value, the relay operates and opens thecontrol circuit to trip the starter unit. To avoid nuisancetripping, due to surges and momentary line disturbances,

a time delay of 4 to 7 seconds is normally introducedinto the tripping scheme.

12.4.6 Protection against voltage surges(for systems 3.3 kV and above)

Voltage surges may be of two types, external or internal(Section 17.5). A motor will require protection againstboth for absolute safety. For external surges, lightningarresters are provided as standard practice at the powerreceiving end as illustrated in Figure 12.27, to protectthe electrical installation as a whole. This lightning arresterwill limit line surges due to external causes, within safelimits as in Table 13.2 for series I or Tables 14.1 and 14.2for series II voltage systems. In all likelihood it will alsoprotect the main insulation of the rotating machines. Theinsulation level of a motor is much less than other electricalequipment such as transformers and switchgears connectedon the system. The line side lightning arrester is selectedto protect the basic equipment only, but a motor normallyinstalled away from the arrester will be subject to onlyan attenuated lightning surge by the time it reaches themotor, and hence would be safe.

For internal causes, when considered necessary,particularly when multiple reflections are expected atthe motor terminals due to the long lengths of cablebetween the starter and the motor, an additional surgearrester may be provided at the motor terminals or asnear to it as possible, in association with a surge capacitor,as shown in Figure 12.27. This will account for theprotective distance (Section 18.6.2) and also limit themagnitude and steepness of the switching surges to protectthe turn insulation. This subject is dealt with in greater

Figure 12.24 Power and control scheme for a voltage-operated single-phasing preventer

R Y B N

Sw

C

Singlephasing

preventer

CF

NL

ResetPB

SPP

StopPB

StartPB

C

O/C

C

C

O/C &• SP trip

On Off

F1–

3

OCR

h 3h 1 h 2M

• Single phasing

R Y B N

Sw

F 1–3

NL

CF

OCR

Singlephasing

preventer

C

M

T 1 T 2

O/C

StopPB

StartPB

h 1 h 2

On

C

C C

ResetPB

SPP

h 3

OffO/C &• SP trip

Figure 12.26 Power and control scheme for a current-operatedsingle-phasing preventer (use of CTs for higher ratings)

• Single phasing

Figure 12.25 Power and control scheme for a current-operatedsingle-phasing preventer (direct sensing up to � 30 A)

C C

C

R Y B N

Sw

F 1–3CF

NL

Singlephasing

preventer

OCR

M

SPP

O/CResetPB

StopPB

StartPB

Ch1 h2 h3

On OffO/C &• SP trip

• Single phasing

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detail in Sections 17.7 and 17.8. For latest developmentsin vacuum interrupters see Section 19.5.6.

With the availability of modern very low choppingcurrent interrupters the use of an arrester may not beoveremphasized and a surge capacitor alone may beadequate (Section 17.10.3).

12.5 Single-device motor protectionrelays

These relays are programmed to provide all possibleprotections necessary for a machine in one unit such asover-current, unbalance and single phasing, locked rotor,prolonged starting time and multiple starts, short-circuitand ground faults etc. They take account of the heatingeffect caused by negative sequence components, whichmay arise due to system unbalance, a fault in the motorwindings or a single-phasing condition. They are alsotemperature compensated and can be set to follow closelythe changes occurring in the operating temperature ofthe machine, so that the machine will trip only when itheats up beyond the permissible limits. It thus ensuresthe optimum utilization of its capacity. Moreover, theovershoot of the thermal element is kept low (of theorder of 2% or so) and the relay can be set between thestarting I2 – t and the ‘hot’ I2 – t characteristics, withoutthe risk of operation on a hot start. They thus possess adefinite advantage over a conventional thermal over-current relay. They incorporate circuits which can be setto trace the motor’s internal conditions during operationand hence provide a thermal replica protection to themachine through signal, blinker and alarm facilities,

available with them. These relays are recommended forlarge LV motors (say, 300 h.p. and above) dependingupon their application and all MV motors, where moreprecise and accurate protection is rather essential, besidesrequiring the optimum capacity utilization of the machine.A good relay will normally incorporate the followingfeatures:

1 It measures r.m.s. values to take account of harmonicspresent in the supply system. Ir.m.s. =

I I I12

32

52 + + + º where I1, I3, I5 are the different

harmonic components.2 It stays immune under permissible operating conditions.3 It gives an alarm or an indication of a likely

unfavourable operating condition well in advance.4 It trips quickly on a fault condition and relieves the motor

from the prolonged stresses of the fault, which maycause excessive thermal and electrodynamic stresses.

5 It simulates the motor’s cooling-down condition, for atleast 30–60 minutes, during a temporary power failure.

6 It monitors and displays the starting conditions suchas time of start and number of starts etc.

7 It measures the values of V1, I� and temperature, qetc., and displays/monitors the cause of the last trip.

8 Trip indication will memorize the operating conditionsof tripping with causes of the trip until the fault isacknowledged.

9 All trips with causes and starting parameters areprogrammed for an accurate diagnosis of the causesof a trip. It helps to identify remedial action necessaryin the operating conditions of the load for a healthyfunctioning of the machine.

10 With these relays, there is no need to use HRC fusesor thermal OCRs. The power circuit is thus devoid ofany heat generation in these components and providesenergy conservation.

These relays are available in various versions such as,

• Electromagnetic: These are quickly outdated butwe discuss these relays briefly below to give anidea of the theory and basic operating principles ofsuch relays. The same principle of application isthen transformed into a static or microprocessor-based relay

• Solid state: based on discrete ICs and• Solid-state microprocessor-based: These are more

sensitive and accurate. They can be made digital tobe connected to a computer for remote monitoringand control of the process that the motor is driving.Also see Table 13.17.

12.5.1 Electromagnetic relays

In this case the relay is in the form of a bridge circuit andthermal detection is achieved through various methodsother than a bi-metallic heater element discussed below.

Heat sink method

This is achieved through a heat sink thermistor, Th1. Thethermistor has two heaters H1 and H2 (one heated by thepositive and the other by the negative sequence

Figure 12.27 Typical scheme for surge protection of a rotatingmachine

Transformer

Line side lightningarrester to limitthe amplitude ofexternal surges

Overhead line

Main switchgear bus

FOW withhigh VT

Tamed wavelow VT

To protect theturn insulationfrom internalsurges

Surge arresterto limit theamplitude

M

G

Cable

Surge capacitorto tame the

steepness

VT

VT

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component). They form one arm of a sensitive Wheatstonebridge. A temperature-compensated thermistor, Th2 formsthe other arm of the bridge (Figure 12.28). The heat sinkthermistor is heated directly or indirectly by the linecurrents. The heating changes its resistance, which isused to provide a signal by the relay, for either trippingor an alarm. Heater, H2, of the negative filter network isso designed that it produces six times the heat that wouldbe produced by heater H1 for the same amount of current.Heaters H1 and H2, thus detect a heating effect equivalentto ( + 6 ),r

2u2I I which would be the maximum heat

produced in the stator or the rotor during an unbalanceor a single phasing, as discussed in Sections 12.2(v) and12.3. They also overcome the deficiency of theconventional thermal over-current or IDMT* relay byproviding longer periods on over-loads yet tripping quicklyon short-circuits. These inherent features of heat sinkrelays make them ideal for motor protection and areused by various manufacturers of motor protection relays.These relays also possess a property to discriminatebetween start and stalling conditions due to their low‘overshoot’ and can therefore detect a prolonged startingor stalling condition without a trip.

Operation of heat sink thermistor relay

This relay will operate whenever there is an unbalancein the bridge circuit due to a change in the resistance of

*IDMT: instantaneous definite minimum time.

Figure 12.28 A typical scheme of a heat sink circuit

R Y B

CT1

CT2

TB

TY

C

R 6

R1

B

R

Y

Instantaneousunbalance unit

Instantaneousovercurrent unit

1:1 Isolatingtransformer

H 2

R 4

R 5

R 8

R 3

H 1

Ballastresistor

AuxiliaryD.C. supply

+ –

M

R 2Th2

P2

P1Th 1

Movementcoil

H1: Heater energized by positive filter.H2: Heater energized by negative filter.Th1: Main thermistor.Th2: Ambient temperature compensated thermistor.R2; R3; R8 etc.: To give the bridge linear resistance/temp. characteristic.

the main thermistor, because of the combined heating ofheaters H1 and H2, as a result of over-load, voltageunbalance, inter-phase fault or a single phasing. Theoperating time will depend upon the rate of heating, i.e.the amount of over-load or unbalance, thus giving aninverse time characteristic.

The following protections may be possible:

1 Over-load2 High set instantaneous over-current through the

positive sequence network. An initial delay of a fewcycles is introduced to avoid a trip during a start,whereas it will trip instantly on a phase fault, cablefault or a short-circuit.

3 Instantaneous unbalance current.4 Prolonged starting and stalling protection. When the

starting current of the motor does not fall within apredefined starting time or up to the thermal withstandtime of the machine, the relay will trip dependingupon the setting of its stalling detection unit.

5 Instantaneous ground fault by providing a zerosequence relay in the residual circuit of the CTs.

Therefore a motor protection relay, which may be a‘normal time’ or a ‘long time’, will be able to providethrough one unit a comprehensive motor protection,generally requiring no more protection, except for asurge protection, which may be needed for an MV motor.

12.5.2 Solid-state relays

There are no moving parts in these relays (Figures 12.29(a)

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and 12.29(b)). They employ static technology, i.e.transistorized integrated circuits to achieve a thermalreplica protection. To detect the presence of a negativesequence component, filter circuits are used to separatepositive and negative components Ir and Iu of the inputcurrent in each phase. These currents are then used toproduce voltages proportional to these currents. Thesevoltages are then fed to a squaring circuit to give a heatingeffect in the motor windings proportional to I Ir

2u2 and .

Figure 12.30(a) is a typical schematic diagram of such arelay and a brief operating description is given below.

The three line currents through the secondary of thethree CTs, as shown in Figure 12.30(b), are fed to asequence filter network which separates the positive andnegative sequence components of the line currents drawnby the motor. These currents are then fed to separatepotentiometers, at different settings, through theinstantaneous operating elements Ir and Iu. Thepotentiometers provide two output voltages V� and Vu,corresponding to the positive and negative sequence

Figure 12.29(a) (Courtesy: English Electric)

Solid state motor protection relay Relay withdrawn from its chassis

Figure 12.29(b) PLC or micro-processor-based motor protectionrelay (Courtesy: Alstom)

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current components respectively. V� and Vu are fed intothe squaring circuits to give k V1

2 ◊ � and k V2 u2 ◊ effects.

These values are then added to give an output voltageeffect of k V k V1

22 u

2 + 6 ◊ ◊� to the integrator. A feedbackcircuit across the integrator causes the output voltage

from the integrator circuit to rise exponentially fromzero to a voltage which is equivalent to about 105%(typical) of the relay setting current. The output voltagefrom the integrator is fed to a level detector, whichenergizes an output unit when the set voltage is reached.This operates electrically separate contacts that can beused to trip or give an alarm to the motor power circuit.Similar squaring circuit or its modified form is used byvarious manufacturers of motor protection relays.

12.5.3 Microprocessor-based relays

A static relay as discussed above is capable of providingmore functions and more accurate operations and canalso memorize historical data and monitor a process moreclosely. This relay can also be provided with amicroprocessor. A microprocessor-based relay consistsof PC boards, a processor board and other electroniccircuits. A typical relay is shown in Figure 12.29(b).These relays can also be made digital to be connected toa central control system for close monitoring and controlof a process. Now they become intelligent equipmentdevices (IEDs) and can have much wider application,such as better communication and information feedbackfacilities, to optimize a process and maximize productivity.With the use of communication protocols and media,IEDs can be made to communicate with a remote controlstation for monitoring, control and automation of a powersystem or a process line (Section 24.11.5). For comparisonof relays see Table 13.17.

The following are the normal protections that may beavailable in both – a solid-state or a microprocessor-based relay, making them a single-device protection:Figure 12.30(a) Power diagram

R Y B

CT1

CT2

CT3

M

B Y

R

S1

S2

S1

S2

S1

S2

7

8

9

10

11

12

RV 6

Instantaneous Ig

Y

R

R

B

T1

T2

Figure 12.30(b) Typical schematic illustrating the squaring technique

a

b d

c e

f g h j

i

k

RYBR

R

N

V1

Vu

Ig Iu I1 Th

Ig Iu ThI1

Notea – Sequence filterb – I1 time delayc – Iu time delayd – Squaring circuite – Squaring circuitf – Summation circuitg – Integratorh – Level detector and outputi – Power supplyj – Output (Th)k – Ig Instantaneous

1

2

3

4

5

6

11

12

10987

k Vu22

k V

k Vu

12

22

+6

�

¸̋˛

(� 60 ms(3 cycles for a50 Hz system))

k V1 2

�

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1 Prolonged starting protection. If the temperature riseof the machine is more than 50% of the permittedrise (qm) during the first start, the relay will lock-outto allow a pause and prevent a consecutive or a quickrestart until the machine cools sufficiently and thetotal temperature rise qc or qh does not exceed qm(Equations (3.2) and (3.4)) during the next start. Thestarting time is fed to the memory of the relay tomonitor the total starting time.

Likely settings – q versus tripping time, or timeof start versus tripping time – whichever occurs first.Likely features – an advance alarm and an indicationbefore a trip.

2 Stalling or locked rotor protection. This is also detectedby the prolonged starting time as well as over-heatingof the machine. It is possible that the machine wasalready under operation and hot when it had stalled.Under such a condition, the rotor operates at a highfrequency and is more vulnerable to damage. Sinceit is not possible to create a replica of the rotor, separateprotection, other than a prolonged starting time, istherefore essential to monitor both q and startingtime and hence this protection.

Likely settings – q versus tripping time or time ofstart versus tripping time. A problem may arise inproviding an accurate time setting when tst is largedue to high-inertia drives or reduced switchingvoltages, when it may approach the safe stall time.

NoteIn both the above cases, which are almost similar, so far as theswitching heats of the stator or the rotor are concerned, theover-current protection (noted at serial no. 4) is redundant, asits time constant is much higher (of the order of several minutes)compared to the temperature rise, particularly of the rotor,which is linear and much more rapid under such conditions.Therefore, such protection saves the machine from excessivethermal stresses.

3 Repeat start protection (Figure 12.31). The relay nowdetects both the summated starting times andtemperature rises qc and qh of the windings. It alsodetects the cooling time effect between the two startsif it existed, to protect the machine against excessivestresses by a lock-out feature against repeated startswhen the summated starting time exceeds the presettime. The total time of start is set according to thethermal capacity of the machine.Likely settings – summated starting time versustripping time, or q versus tripping time.

4 Over-current protection. To provide a thermal replicaprotection, the relay is set according to motor’s heatingand cooling (I2 – t) curves supplied by the motormanufacturer. If these curves are not available, theycan be established with the help of motor heatingand cooling time constants, as in Equations (3.2) and(3.4). A brief procedure to establish the motor thermalcurves when they are not available is explained inSection 3.6.

5 System unbalance protection. As discussed earlier,an asymmetry in the supply voltage causes.

• High I2R losses in the stator, and• Eddy current losses in the rotor.

Thus, besides voltage unbalance it can also detect aninter-turn fault, which leads to a current unbalance. Asmall amount of unbalance is already detected by thethermal element of the over-current protection but asevere unbalance, such as during a single phasing,would require quicker protection and hence, this protec-tion. The relay may be set for an Iu of around 3% or so.Likely setting: Iu versus tripping time.

6 Short-circuit protection. To provide an instantaneoustripping on a short-circuit delay of, say, one or two cyclesmay be introduced into the tripping circuit to bypassany transient currents and avoid an unwanted trip.Likely setting: Isc versus tripping time.

7 Ground leakage current protection. A separate groundfault protection should normally not be required inview of the protection already available against anunbalance. But since the setting of the unbalancedelement may not be sensitive enough to detect a smallground leakage current, a separate ground leakageprotection is recommended. This can be achieved by

Figure 12.31 Heating and cooling curves of an intermittent dutymotor

2

3

4

1

I r

Time

One cycle

During an overload condition, the relay maybe set to give alarm if normal conditionsdo not restore by the end of the cycle.

0 t1 t2 t3 t4

Load

I�2

I �3

I �1

Note The thermal data are marked for loads I l1 and I l2

qe2

qm

qc1 = qa

qe1orqi1

0 t1 t2 t3 t4

qc1 = q o11

3

42

q h2

= q

e2 –

qa

Normal heating curveat rated load I r

Tem

pera

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ris

e ‘q

’

Time

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detecting the ground leakage current, Ig, through theground circuit or the residual current (Sections 21.2.2and 21.4).Likely setting: Ig versus tripping time.The setting may be kept on the higher side to avoidnuisance tripping due to:(a) Difference in CT outputs which may also cause

unbalanced currents and initiate operation of therelay, particularly during a start when the currentis high.

(b) Flow of zero sequence cable capacitive leakagecurrents during an external ground fault, and

(c) Momentary ground transient currents whiledischarging a switching surge through a surgearrester if this has been provided at the motorterminals.

8 Lock-out or blocking feature. Such a feature isnecessary for complete safety of the machine afterevery trip for any of the reasons discussed above. Itis imperative that the relay blocks and prevents thenext switching, unless the fault is acknowledged,the reason for the trip is analyzed and normaloperating conditions are restored. The settings ofthe relay for all such unfavourable operating or faultconditions are made depending upon the functionsand the setting ranges available with the relay. Sinceall the protections are based on r.m.s. values, asensitive relay will also detect the harmonic quantitiespresent in the system and provide more accurateprotection for the machine.

It may often be asked why separate settings arenecessary for different types of protections when it ispossible to set the relay for a replica protection. It istrue that the relay will monitor thermally what isoccurring within the machine as long as there are onlynormal to unfavourable operating conditions. But thisis not so, when a fault condition occurs. It is possiblethat on a fault the temperature rise is not consistentwith the assumed thermal replica due to high timeconstants. For instance, for a fault of a few seconds thetemperature rise of the whole machine will almost benegligible (Equation (3.2)). This would delay the trip,whereas the heat may be localized and very high at theaffected parts and may escape undetected as well aselectrodynamic forces, which may also cause damageto the machine. Similarly, on a single phasing, monitoringof the stator temperature alone is not sufficient, as therotor would heat up much faster (in Y-connected (usuallyMV) stators) due to double-frequency eddy currentheating and less weight compared to the stator. Similarly,it may take much longer to trip under a stalled condition.A severe unbalance, as may be caused by an internalfault, may also result in heavy negative phase sequencerotor currents and require protection similar to short-circuit protection. A short-circuit protection will not detecta single phasing. Hence the necessity to provide separateprotections for different operating and fault conditionsto achieve optimum utilization of the machine, with theleast risk of damage. The relay must discriminate betweenan unfavourable operating condition and a fault condition.While the former may permit a delay tripping, the latter

will need a more discrete and quick tripping to save themachine from more severe damage.

Under-load protection

It is also possible to provide this protection in such relays.This will provide very vital system process information.A sudden drop in load may be the result of a fallout ofthe load due to disengagement of the coupling, breakageof a belt or a tool, etc. It can therefore help monitor thesystem process line more accurately.

12.6 Summary of total motorprotection

In view of the effect of various unfavourable operatingconditions on a motor’s performance, one should bemeticulous when selecting the protection to safeguard themachine against these conditions. See also Section 3.6where we have provided a brief procedure to reproducethe motor thermal curves q versus I and vice versa, forthe relay to provide a replica protection to a motor and tohave closer settings of its various protective features. Thefollowing is a brief summary of our discussions so far.

As discussed in Section 12.2 and explained in Figure12.3, an ordinary over-current relay will detect betweencurves 1 and 2, as against the average heating curve 3and is thus oversensitive to insignificant amounts ofunbalance, not actually causing harm to the motor. Forinstance, an unbalance in voltage of, say, 3% may causean unbalance in the current (i.e. an apparent over-loading)of, say, 18% in one particular phase and may be detectedby an ordinary bi-metallic thermal over-load relay, whilethe stator does not heat up accordingly. Even the rotor,which is more sensitive to an unbalance is heated

corresponding to a current of I Ir2

r2 + 3(0.18 ) or

1.047Ir, i.e. an over-loading of nearly 5%. Similarly, duringsingle phasing the relay should be able to detect twicethe full load current in a star-connected winding, or asshown in curve X of Figure12.10, for a delta-connectedwinding, and not corresponding to the stator current of÷3 · Ir for a star-connected winding and curve R/Y ofFigure 12.10, for a delta-connected winding. An ordinaryrelay will take longer to operate corresponding to theline current, whereas some of the internal circuits willbe subject to much higher currents during this period.

For meticulous protection, therefore, it is advisableto use a motor protection relay for large LV and all MVmotors. The following motor details and workingconditions are essential to know before making a properselection of a protective scheme:

1 Type of motor – squirrel cage or slip-ring2 Rating – kW (CMR)3 Rated voltage and current4 Type of starting5 Starting current versus time characteristics6 Locked rotor current and corresponding ‘hot’ thermal

withstand time7 Motor thermal withstand characteristic curves

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8 Number of starts or reversals, if required, and theirfrequency

9 Ambient temperature and10 Maximum fault current

Having discussed the effect of the above parameterson the motor’s performance, we will now illustrate byway of an example a general case to broadly suggest aprocedure that can be followed to select the protectivescheme for a motor. For more detailed selection of themotor protection relay, reference may be made to therelay manufacturer.

Example 12.7For the purpose of protection, consider the squirrel cage motorof Example 7.1 for one hot start for which this motor is suitable.

Step I: Assume the motor starting current versus timecharacteristics as shown in Figure 12.32, and divide theaccelerating torque curve into three sections, A, B and C asshown, to ascertain the magnitude of the starting current andacceleration time for the different sections.

Section A

(a) I a = 750% of I r

I b = 730% of I r

\ I av

2 2 = 7.5 + 7.3

2= 740% of I r

(b) Ta = 82% of T r

or = 450 974980

0.82¥ ¥

= 367 mkg

and starting time t As = 1866 333.3375 367

¥¥

� 4.52 seconds

Assume the safe time to be 5.2 seconds (considering a safetyfactor of nearly 15%)

Section B

(a) I b = 730% of I r

I c = 630% of I r

\ I av

2 2

= 7.3 + 6.30

2

� 682% of I r

(b) T a = 48%

or 215 mkg

and starting time ts B = 7.71 seconds.Consider the safe time as 8.8 seconds.

Section C

(a) I c = 630% of I r

I r = 100% of I r

Figure 12.32 Determining the duration of starting current

Current

Motor torque

Load torque

333.3 333.3 313.4

Tr

% T

orqu

e

A B

Ta = 80%Ta = 48%Ta = 82%

ab

5.2 sec 8.8 sec. 5.0 sec

19.0 sec.

20

630

750730

I st

600

450

300

150

100 I r

20

Speed (rpm)

% C

urre

nt

N r = 980

S

c

C

220

200

165

150

130

110100

85

70

48

A B C

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= (104.72 + 36.47 )2 2

= 110.9 A

All further calculations must be based on this current.

Step II: Selection of HRC fuses

I r = 123.2 A

I st = 750% of I r

i.e. 924 A

t s = 19.0 seconds (5.2 + 8.8 + 5.0 seconds)

Consider four equally spread starts/hour. From the selectionchart in Figure 12.34, select the characteristics B–B anddetermine the fuse rating as 350 A on the ordinatecorresponding to a starting current of 924 A on the abscissa.

Step III: Selection of bimetallic over-current relayConsider the available setting range as

(a) Over-current: 80–125%(b) Instantaneous unit: 800–1400%

The thermal curve of the motor does not show any significantover-load capacity and therefore the relay must be set asclose to the full-load current as possible, say at a setting of110%. Then

Over-current relay tapping = 1.1 123.21.05 150

¥¥

(relay rating: 1 A)

= 86% of 1 A

Figure 12.33 Plotting of starting current and thermal curves

Instantaneoussetting

Motor hot thermal curve

Motor protection relay(allows better utilization)

Bimetallic relay

Time (seconds)0 5.2 19 38 40

8.8

Two consecutivehot starts

Cur

rent

(%

)

450

682

750740

815

I r 100

I st

5.0

I c = 28.43A

104.72Af

36.47A

28.43A

64.9A

Ir = 123.2A

110.9A

cos f = 0.85sin f = 0.527

\ I av

2 2 = 6.3 + 1

2

= 450% of I r

(b) Ta = 80% or

= 358 mkg.

and starting time ts C = 4.36 seconds.Consider the safe time as 5.0 seconds.Plot the starting current and thermal curve of the motor as inFigure 12.33.Considering h = 94% and p.f. = 0.85, then

I r = 4503 2.64 0.85 0.94¥ ¥ ¥

Amp. (Max.) at 80% of 3.3 kV

= 123.2 A

Use a CT of ratio 150/1 A.

NoteIf a power capacitor is connected after the relay, say acrossthe motor terminals, to improve the p.f. of the machine, thenonly the corrected value of the current must be used. Forexample, if a capacitor bank of 130 kVAr is used for anindividual correction of the machine then

I c = 1303 2.64¥

= 28.43 A

where, I c = capacitor current

Since I r at 0.85 p.f. = 123.2 A

\ I r (active) = 123.2 ¥ 0.85

= 104.72 A

and I r (reactive) = 123.2 ¥ 0.527

= 64.9 A

\ Net reactive current = 64.9 – 28.43

= 36.47 A

and I r (corrected) that the relay will detect

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NoteThe factor 1.05 (typical) is the pick-up current of the relay.

Say the relay is set at 85%. Then it will operate at 85/86 ¥110, i.e. at 108.7% of I r, which should be appropriate and theinstantaneous setting

= 1.087 ¥ 750 = 815.25%

say, 815%

This setting is only a calculation. The exact thermal curves ofthe motor and the relay should be available to closely matchtheir characteristics at every point. An ideal relay in this casewould be one which, without tripping, will permit twoconsecutive hot starts, i.e. its characteristic should lie abovethe motor starting curve and close below the thermal curveas shown in Figure 12.33.

Static thermal relay (discrete ICs or microprocessor-based)For medium and large motors, 300 h.p. and above, this typeof protective relay should be preferred to achieve optimumutilization of the motor’s capacity. Consider the available settingranges in the vicinity of

(a) Thermal over-load unit: 70–130% of the CT rated current.(b) Instantaneous (I r) unit: 600–1200% of the thermal unit

setting.(c) Instantaneous (I u) unit: 200–600% of the thermal unit

setting.(d) Ground fault (I o) unit: 20% of the rated current(e) Stalling protection unit: 150–600% of the CT rated current,

2.5–25 seconds

Settings(i) The over-load unit setting: as worked out above, at 85%,

i.e. for 108.7% of I r.(ii) An instantaneous setting of 750% of the relay setting

should be appropriate, which can protect currentsexceeding

7.5 ¥ 1.087 ¥ 123.2 A

or 1004 A

(iii) Unbalanced setting. If set at 110%, can operate unbalancedcurrents to the extent of

110 = 100 + 6 2 2u2I (from Equation (12.6))

or I u

2 2 = 110 – 100

6

= 18.7%, i.e. a voltage unbalance of nearly 3%

(iv) Stalling protection unit setting. The current unit is to beset at 200–300%, and the time delay unit a little abovethe starting time but less than the safe withstand stall time.

(v) Single-phasing setting. If set at 110%, can operate single-phasing currents to the extent of

110 = + 62u2

u2I I

or I u � 41.58%

NoteThe above exercise is merely an approach to the selection ofthe most appropriate relay and its protective settings for aparticular machine. The exact selection of the relay and itssetting will depend upon the type of relay, its sensitivity andprotective features available.

12.7 Motor protection bythermistors

A thermistor is a thermally sensitive, semiconductor solid-state device, which can only sense and not monitor (cannotread) the temperature of a sensitive part of equipmentwhere it is located. It can operate precisely and consistentlyat the preset value. The response time is low and is of theorder of 5–10 seconds. Since it is only a temperaturesensor, it does not indicate the temperature of the windingsor where it is located but only its preset condition.

This is a later introduction in the sensing of temperaturecompared to the more conventional types of temperaturedevices available in an embedded temperature detector(ETD), such as a thermocouple or a resistance temperaturedetector (RTD) described later. Thermistors can be oneof the following types:

Figure 12.34 Fuse selection chart for 6.6 kV system for a motor with run-up time not exceeding 60 seconds

Fus

e ra

ting

(A)

100 200 300 400 500 700 1000 1500 2000 3000 3500

Motor starting current (A)924

C

630500

400

350

315

250

200

160

125

100

80

6350

No. of startsper hour 8 4 2

BA C

A

B

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Figure 12.35 Characteristic of an NTC thermistor

60/7.16

70/4.93

80/3.46

90/2.47

100/1.80

110/1.33120/1.00

130/0.76140/0.59

150/0.46160/0.36

The

rmis

tor

resi

stan

ce k

W

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.050 60 70 80 90 100 110 120 130 140 150 160 170

Response temp ∞C

(i) NTC – having a negative temperature coefficient, and(ii) PTC – having a positive temperature coefficient.

The resistance–temperature characteristics of both thesetypes are shown in Figures 12.35 and 12.36 respectively.One can note an exponential variation in case of an NTCthermistor. Thermistor resistance decreases with anincrease in temperature whereas in a PTC thermistor theresistance remains constant up to a critical temperatureand then undergoes a very steep and instantaneous changeat a predefined temperature, known as the Curie point. Itis this feature of a sudden change in the resistance of aPTC thermistor that makes it suitable for detecting andforecasting the motor’s winding temperature at the mostvulnerable hot spots when embedded in the windings.However, they can be installed only in the stator windings,as it is not possible to pick up the rotor signals whenprovided on the rotor side. To take out leads from therotor circuit would mean providing slip-rings on the rotorshaft, which is not recommended.

The resistance values of a PTC thermistor are given as:

(a) The resistance values in the range of –20∞C to(Tp – 20)∞C should not exceed 250 W, with all valuesof the measuring d.c. voltages up to 2.5 V.

(b) At (Tp – 5)∞C – not more than 550 W with a d.c.voltage of not more than 2.5 V.

(c) At (Tp + 5)∞C – not less than 1330 W with a d.c.voltage of not more than 2.5 V.

(d) At (Tp + 15)∞C – not less than 4000 W with a d.c.voltage of not more than 7.5 V.

where Tp is the tripping reference temperature or Curiepoint as indicated in Table 12.1.Figure 12.36 Characteristic of a PTC thermistor

Res

ista

nce

Switching point(Curie point)

Temperature

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A typical characteristic curve of a PTC thermistorhaving a Curie point of 120∞C is shown in Figure 12.37.

The current through the thermistor circuit reducesdrastically and instantly as soon as the critical temperatureis attained as the resistance rises manifold. This featureis utilized to actuate the protective relay used in thetripping circuit, to protect the motor from over-heating.Figure 12.38, illustrates a typical PTC thermistor protectivecircuit. It is this feature that has made PTC thermistorsmore useful and adaptable universally, compared to theNTC type. It is interesting to note that in a PTC thermistor,the switching point at which the resistance rises suddenlycan be adjusted, and the device can be designed for anytemperature to suit a particular application or class ofinsulation. They are normally available in the range of110, 120, 130, 140 and 150∞C and can withstand atemperature up to 200∞C, such as required during

impregnation and curing of the stator windings. Dependingupon the reference temperature (Curie point), thethermistors may be made of oxides of cobalt, manganese,nickel, barium and titanium. For motor protection, theyare chosen according to the class of insulation used inthe winding. For example, for a class E motor the switchingpoint can be chosen at 120∞C and for class B at 130∞C(refer to Table 9.1). These are tripping temperatures. Fora pre-warning by an alarm or annunciation they can beset at a slightly lower temperature so that an audible orvisual indication is available before the motor trips togive an opportunity to the operator to modify the operatingconditions, if possible, to save an avoidable trip.

Since the thermistor circuit will trip the protective circuitas soon as the thermistor current reduces drastically, itprovides an inherent feature to trip the protective circuiteven when the thermistor circuit becomes damaged or open-circuited accidentally, extending a feature of ‘fail to safety’.

A thermistor is very small and can be easily placedinside the stator overhangs, bearings or similar locations,wherever a control over the critical temperature is desired.It is not provided in the rotor circuit (particularly squirrelcage rotors), as noted earlier. This device is embedded inthe windings before impregnation, for obvious reasons.For exact temperature monitoring, the thermistor is alwayskept in contact with the winding wire. The number of

Table 12.1 Curie points for a few PTC thermistors

Winding temperature (∞C)

Insulation class ææÆ E B F

Temperature reference

Steady overload condition 155 165 190Stalled condition 215 225 250

Recommended reference temperature(Curie point) for thermistors (∞C)

Drop-off (tripping) Tp 130 140 160Warning Tp 110 120 140

Figure 12.38 A PTC thermistor protective circuit

R

Y

B

N

Sw

F1–

3

M

PTCthermistors

I

0

h

b

12

R

N

Tripping unit

14

11

2

5 6

4

1

Figure 12.37 Typical characteristics of a PTC thermistor

Typical curve

Ideal curve as per IEC – 60034–11

c

b

a

Res

ista

nce

(W)

10 000

5000

20 000

4000

1000

550

1330

250200

100

50

10

Curie point for ideal curve

Curie point for typical curve

– 40 –20 0 40 80 140 160100

Tp + 15Tp + 5TpTp – 5

Tp – 20

Temperature (∞C) 120

135

1

1

2

2

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thermistors will depend upon the number of statorwindings and the specific requirement of warning ortripping or both. Likely locations where a thermistor canbe placed in a motor are illustrated in Figure 12.39(a).Figure 12.39(b) shows the wiring diagram.

Such a device can actually predict the heating-upconditions of a motor winding, at their most vulnerablelocations. It does not only provide total motor protectionbut also ensure its optimum capacity utilization. Theconventional methods of a motor’s protection through anover-load relay, a single-phasing preventor, a reverse powerrelay or negative sequence voltage protection all detect thelikely heating-up conditions of the motor windings underactual operating conditions, whereas a thermistor can sensethe actual winding heating-up condition. A thermistor may

Figure 12.39(a) General safety devices and their locations in a motor

5

3

4

2 1 2 6

PTC thermistors in theend windings.

Temperature sensors inthe bearings.

Temperature monitorin the windings.

Pulse transmitters

Vibration probes

Speed responsiveswitches

Tachogenerator (TG)

1

3

2

7

6

5

4

Figure 12.39(b) Wiring diagram

Thermistors and RTDs orthermocouples are

embedded in all the threephases of the stator

windings

R

Y

B

Thermistor terminalsfor alarm or trip

RTD or thermo-couplesterminals for monitoring

and protection

prove highly advantageous for the protection of motors thatare operating on a power system that contains manyharmonics, and the actual heating of the motor windingsmay be more than the apparent heating, due to distortionsin the sinusoidal waveform (Section 23.8). A thermistordetects this situation easily by sensing the actual heat. It istherefore, possible to employ such a single device for motorprotection to make protection simple, compact, much moreeconomical and even more accurate. It also extends anopportunity to an optimum utilization of the motor’s capacity.The only likely shortcoming to the operator or the workingpersonnel is the total absence of an indication of the actualfault or the unfavourable operating conditions. The causeof a trip is now only guesswork, which is not desirable, andhence the necessity for an elaborate protective scheme,discussed earlier. But thermistors are very useful forpredicting unexpected hot spots in a motor during actualrunning, which other devices may not be able to do. Theyare therefore employed extensively in large and criticalmotors as temperature sensors. They are also being usedas thermistor relays to trip a motor by converting theresistance change into a current change of the control circuit.Figure 12.40 shows such a relay.

12.7.1 Embedded temperature detectors (ETDs)

They are basically temperature monitoring devices andcan indicate the temperature on a temperature scale. Theymay be one of the following types.

Thermocouples

These are bimetal elements, consisting of a bimetaljunction which produces a small voltage proportional tothe temperature at the junction. They are thus able todetect the winding temperature conditions when embeddedinside the motor windings. For more details refer to DIN43710.

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Resistance temperature detectors (RTDs)

These are normally of pure platinum wound on a ceramicor glass former and encapsulated in a ceramic or glassshell, having an operating range of –250∞C to +750∞C,with an accuracy of less than 1∞C and a response time ofthe order of 0.2 second. Pure metals possess almost alinear temperature coefficient of resistance r, over a widerange and this characteristic is used in monitoring thetemperature of a particular object. In pure platinum

Rt = R0(1 + rq) W

whereRt = resistance at temperature q ∞CR0 = resistance at 0∞C = 100 W q = end temperature ∞C r = 3.85 ¥ 10–3 per ∞C at 0–100∞C

The exact resistance variations of a Pt – 100, RTD overa range of temperatures are given in Table 12.2, and notvery different from those calculated by the above equationand drawn in Figure 12.41.

Application

All the above sensing and monitoring devices are basicallysupplements to over-load and single-phasing protection.They are worthwhile for critical installations that requiremore accurate sensing or monitoring of the operating

temperatures of the different vital components or likelyhot spots in the motor. They also eliminate any chanceof tripping for all operating conditions that can becontrolled externally. A few applications may be speedvariation, affecting cooling at lower speeds, frequentlyvarying loads, frequent starts, stops, plugging and reversalsetc. In all these cases, the other protections may notdetect the heating conditions of the vital components ofa machine as accurately as a temperature detector.

12.8 Monitoring of a motor’s actualoperating conditions

To warn of an unfavourable operating condition by theuse of an audio-visual alarm or trip or both, schemes can

Figure 12.40 Thermistor motor protection relay (Courtesy: L&T)

Thermistor

Table 12.2 Characteristics of a Pt – 100 RTD

Temperature Resistanceq ∞C W

0 10050 119.40

100 138.50110 142.28115 144.18120 146.06125 147.94130 149.82135 151.70140 153.57145 155.45150 157.32

Source: IEC 60751

Figure 12.41 Characteristic of a Pt – 100 resistance temperaturedetector (RTD)

160

150

140

130

120

110

1000 25 50 75 100 125 150

Temp (q)∞C

Res

ista

nce

(Rt)W

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be introduced in the motor control circuit by means of atemperature detector or other devices to monitor any orall of the following internal conditions of a motor:

• Motor winding temperature• Motor or driven equipment bearing temperature• Coolant circuit inlet water pressure and temperature• Moisture condensation in the windings• Motor speed• Vibration level• Safe stall time• Rotor temperature• Any other similar condition, interlocking with other

feeders or sequential controls etc.

To detect the above, sensing and monitoring devicescan be installed in the motor. Some of these are built-induring the manufacturing stage and others are fitted atsite during installation.

Several types of sensing and monitoring devices areavailable that are embedded at a motor’s vulnerablelocations and the hot spots at the manufacturing stageand final assembly of the motor by the motor manufacturer.All these accessories are normally optional and must berequisitioned to the motor manufacturer at the time of theinitial indent. These devices can be of the following types.

NoteCritical machines can also adapt to microprocessor-based monitoring,control and protective devices as discussed in Section 13.8.

12.8.1 Motor winding temperature detection (byPTC thermistors and RTDs)

MV motors specifically, and large LV motors generally,are recommended to have six such detectors, two in eachphase to sense or monitor the temperature of likely hotspots and provide an audio-visual alarm, annunciationand protection signals. For a stator winding, the preferredlocations for a PTC thermistor and an RTD may beconsidered as below:

• PTC thermistors These are fragile and require usablespace in the slots. They are normally fitted in theoverhangs of the stator windings, as shown in Figure12.39(a). A sudden problem with the motor, causingover-heating, is instantly detected by an audio-visualalarm or a trip. These are preferred for smaller motors.For larger motors, protection through monitoring ispreferred over sensing. Monitoring is possible throughRTDs or thermocouples.

• RTDs or thermocouples These are normallyembedded in the stator windings as illustrated inFigures 12.42 and 12.39(a). The winding temperaturecan now be monitored continually and a temperaturereplica of the machine obtained at any time. Theyare generally preferred to thermistors for large LVand all MV motors.

12.8.2 Bearing temperature detection (by PTCthermistors or RTDs)

Motors are also recommended to have one bearingtemperature detector in each bearing. This can be fitted

within the threaded walls of the bearing that reach up tothe bottom of the bearing shell, i.e. close to where theheat is produced. Each detector may have two sets ofcontacts, each having ‘2-NO’ contacts, rated for 5 A at240 V a.c. or 0.5 A at 220 V d.c. (typical). One set can beset at a lower value to provide an audio-visual alarm andthe other at a higher value to trip the motor.

12.8.3 Coolant circuit water pressure andtemperature (moisture) detection

Water-Cooled motors, type CACW (cooling type ICW37 A 81 or ICW 37A 91) (Section 1.16, Table 1.12)should be fitted with moisture detectors to provide anaudio-visual alarm in the event of a leakage in the watercircuit or a higher coolant temperature.

12.8.4 Detection of moisture condensation in thewindings (by space heaters)

Motors generally above 37.5 kW located in a humidatmosphere or required to be stored idle for long periodsmay be provided with one or two and even more spaceheaters, depending upon the size of the motor, suitable for240 V, 1– f a.c. supply, to maintain the motor’s internaltemperature slightly above the dew point to prevent moisturecondensation or deterioration of the insulation during ashutdown. The heaters are located inside the motor at thelower end of the stator so that they are easily accessibleand their removal and replacement presents no problem.The rating of total heating power may vary from about100 watts to 3500 watts, depending upon the size of themotor. For motors up to 400 kW, one or two space heaters,totalling about 100–250 watts, may be adequate, whereas,for a 10 000 kW motor there may be as many as four to sixspace heaters, totalling about 3500 watts.

12.8.5 Vibration probes

These may be used to detect the hunting of the machineand to provide an audio-visual alarm for unusual runningor for vibrations.

Conductor oftop coil side

Conductor ofbottom coil side

RTD orthermo-couple

Figure 12.42 Location of an RTD or thermo-couple in amotor winding

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Table 12.3 Recommended sizes of cables for various sensingand monitoring devices

(A) LV motors Number of cables and sizes

(i) PTC, thermistors or embedded 12 ¥ 2.5 mm2, 650/1100 Vresistance temperature detectors(RTDs): for 6 sets of copperconductor cables(2 cores for each set)

(ii) PTC, thermistors or resistance 4 ¥ 2.5 mm2, 650/1100 Vbearing temperature detectors(RTDs): for 2 sets

(iii) Moisture detectors: 2 ¥ 2.5 mm2, 650/1100 VFor each detector

(iv) Space heaters: 2 ¥ 2.5 mm2, 650/1100 VFor each space heater

(v) Speed switch: 2 ¥ 2.5 mm2, 650/1100 VFor each switch

(B) MV motorsGenerally the same as for LV motors, except that therecommended size of cables will be 6 mm2

NoteWherever a cable lead connecting the above devices (such as forRTDs) has to pass through a magnetic field, it may be screened withtinned copper-braided wires to nullify the effect of stray fields (EMI(for details see Section 23.18)). The field may distort the readings.

Note1. Switch-fuse unit or a circuit breaker.2. Isolator near the motor is also recommended, when the motor is

away or not visible from the switching station (it is a safetyrequirement)

3. A stay put type stop push button may also be mounted near themotor for maintenance safety

Incoming line

Bus bars

Switch

Fuse

Contactor

Overcurrentrelay

Motor controlcentre

Cable

M M M M

Isolator

Motor P.B.

1

2

3

Figure 12.43(a) Power line diagram of an MCC

12.8.6 Use of speed switch or tacho-generator

This is a speed-sensitive device and is employed to monitorthe starting time, ts, in normal motors or the heating-uptime, tE, in increased-safety motors (Section 7.13.2). Thisis installed to detect a stalling condition for criticalinstallations where a false tripping due to an overprotectionor a malfunction of the locked rotor protection relay is notdesirable, or where the starting time, ts, may exceed theheating-up time, tE. The speed may be set at about 10–30% of Nr, depending upon the speed–torque curves of themotor and the load. The purpose is to detect a speed, Na(Figure 12.6), where a locked rotor condition may occurduring a normal speed-up period. This situation may arisedue to a voltage fluctuation, a sudden excessive torque ora load demand, etc. At the preset speed, the switch contactsopen to energize a timer. If the motor is not able to pick upwithin the motor thermal withstand time, the timer willoperate and actuate the tripping circuit in conjunction withthe locked rotor protection. When a motor is staticallycontrolled, this device is also used for speed correctionthrough the feedback control system (Section 6.6).

All these devices and their likely locations are indicatedin Figure 12.39(a). The sizes of control cables to connectthese devices are indicated in Table 12.3.

*MCC – motor control centre. For details refer to Chapter 13. Figure 12.43(b) MCC for single line diagram of Figure 12.43(a)

12.9 Switchgears for LV motors

The general arrangement of a motor’s protectionswitchgear (MCC*) is shown in Figure 12.43(b). Thistakes into account the protection of the main equipmentsuch as motors and cables and makes provision for

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isolation of the outgoing circuits from the incoming supply.The single line diagram for this switchgear is shown inFigure 12.43(a). The over-current relays are selected, asfar as possible, with thermal characteristics to those ofthe motors. The over-load setting should preferably bewithin 25–75% of the relay range.

For small motors with a number of brands and varyingthermal characteristics the above may not be practical.Moreover, to arrange the thermal curves for each relayand motor and then match them individually for closerprotection may also not be practical. The practice adopteduniversally, therefore, is to select the most appropriate

Table 12.4 Selection table for switches, fuses, relays and cables for different sizes of LV motors

kW HP Approx. Ir /÷3 Switch rating HRC fuses for Over-current relay range** Size of Al conductor cables***FLC at backup protection*415 V (Ir) For For For DOL For Y/D For DOL For Y/D

DOL Y/D For ForDOL Y/D Before After

the the starter starter

A A A A A A A A mm2 mm2 mm2

0.18 0.25 0.7 0.45 16 16 4 – 0.5–1/0.6–1 – 1.5 – –0.25 0.33 0.9 0.52 16 16 4 – 0.5–1/0.6–1 – 1.5 – –0.37 0.5 1.1 0.64 16 16 4 – 1–2/1–1.6 – 1.5 – –0.55 0.75 1.5 0.87 16 16 6 – 1–2/1.5–2.5 – 1.5 – –0.75 1.0 2.0 1.15 16 16 6 – 1–2/1.5–2.5 – 1.5 – –

1.1 1.5 2.6 1.50 16 16 6 – 1.5–3/2.5–4 – 1.5 – –1.5 2 3.7 2.14 16 16 10 – 2–4/2.5–4 – 1.5 – –2.2 3 4.9 2.84 16 16 16 6 3–6/4–6.5 1.5–3/2.5–4 1.5 1.5 1.53.7 5 7.8 4.50 16 16 16 10 6–12/6–10 3–6/4–6.5 1.5 1.5 1.55.5 7.5 11.3 6.50 25 16 25 16 6–12/9–14 4–8/6–10 2.5 1.5 1.5

7.5 10 15.5 8.95 25 25 25 25 10–16/13–21 6–12/6–10 4 2.5 1.59.3 12.5 19.0 11.00 63 25 35 25 12–24/13–21 6–12/9–14 6 4 2.5

11 15 22.0 12.70 63 25 50 25 12–24/20–32 6–12/9–14 6 4 2.515 20 29 16.80 63 63 63 35 16–32/20–32 12–24/13–21 10 6 418.5 25 35 20.20 63 63 63 50 24–45/28–42 12–24/20–32 16 10 6

22 30 40 23.20 63 63 63 50 24–45/30–45 16–32/20–32 16 16 630 40 54 31.20 100 63 100 63 32–63/45–70 24–45/28–42 25 25 1037 50 66 38.20 125 100 125 80 50–90/60–100 24–45/28–42 35 25 1645 60 80 46.20 250 100 160 100 50–90/60–100 32–63/45–70 50 35 2555 75 100 57.70 250 250 200 125 70–110/90–150 32–63/45–70 70 70 25

75 100 133 76.80 250 250 250 160 90–135/90–150 50–90/60–100 95 95 5090 125 165 95.50 250 250 250 200 140–170/120–200 70–110/90–150 150 150 70

110 150 196 113.00 400 250 355 200 180–300 90–135/90–150 185 185 95125 170 220 127.00 400 250 400 250 180–300 120–155/90–150 240 240 120160 220 285 165.00 630 400 500 320 200–400 120–200 400 400 150

180 245 315 182.00 630 400 500 320 280–400 120–200 400 400 185200 270 350 205.00 630 400 630 355 280–400 180–300 2 ¥ 185 2 ¥ 185 240220 300 385 222.00 630 400 630 400 350–500 180–300 2 ¥ 240 2 ¥ 240 240

*The recommended ratings are for general guidance only. For more accurate selection of fuses, it is advisable to check these ratings by comparingtheir I2 – t characteristics with those of the selected OCR (see Section 12.4.1).

**The over-current relay range may vary slightly from one manufacturer to another.***It is advisable to choose cables one or two size higher to save on I2R loss and conserve energy (also see Section 1.19).

Notes1 It is recommended that relays beyond 100 A be CT operated for better accuracy and stability and to avoid a trip during start (Section 12.4.1).2 For drives with longer starting times, relays suitable for heavy-duty starting, i.e. CT operated, must be employed (Section 12.4.1).3 In Y/D starting, the relay is connected in phases. Its rating is therefore considered as Ir / 3 i.e. 58% of Ir. With drives having longer start times,

this relay may trip during a start. Provided that the motor thermal rating permits a current up to 2Ir (approx.) in the star position, for the durationof the start the OCR may be connected on the line side with a range corresponding to Ir. This is 3 times more than the range in the phasesand would take nearly ( 3 ) ,2 i.e. three times longer to trip than when it is used in the phases.

4 The sizes of cables considered above refer to normal operating conditions and a distance of around 10/15 metres between the starter and the motor.5 (i) For slip-ring motors, the selection of components may be made corresponding to a Y/D switching.

(ii) With the short-circuiting and brush-lifting device, the cable on the rotor side, when rated for only half the rated rotor current, will be adequate,as it has to carry the current during start only.

relay, within the required range, from among the rangesand brands available in the market for different ratings ofmotors. Table 12.4 suggests the recommended relay ranges,sizes of switches, fuses and cables for different sizes ofmotors. This selection is only for general guidance. For amore detailed approach, refer to the notes to the table.

12.10 Selection of main components

The selection of main components such as switches,contactors and breakers is made on the basis of their

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• Continuous current rating (CMR): to carry the circuitcurrent continuously.

• Thermal capacity: to perform the required switchingduties and sustain the fault conditions, at least up tothe cut-off time of the short-circuit protective device,say, the HRC fuses.

• Electrical and mechanical life: which is defined bytheir a.c. duty. As in IEC 60947-4-1 for contactors andIEC 60947-3 for switches. It is noted in Table 12.5.

Notes1 The type of duty defines the capacity of a switch or a contactor

by the value of the current and the p.f. of the associated circuit,it can make or break on fault. For values of currents and p.f.for different duties, refer to the relevant standards as notedabove.

2 There are a few more utilization categories. For details refer toIEC 60947-4-1.

In fact, the same contactor or switch can perform differentduties at different ratings and have correspondingelectrical* lives.

For instance, an AC3 duty contactor when performingthe duty of a resistive load can carry a higher load and itsnormal rating can be overrated. Similarly, whenperforming the duty of AC4, it can carry a lower loadand will require derating. Accordingly, its electrical lifewill also be affected. As standard practice, such ratingsare prescribed by the manufacturers in their productcatalogues. Based on the above and the discussions sofar, we have provided in Table 12.4 the recommendedratings of switches, fuses, relays and sizes of aluminiumcables for different motor ratings. These recommendationswill generally provide protection consistent with co-ordination type 2 as in IEC 60947-4-1. From this table aquick selection of the components can be made for anyrating of LV motors.

For MV motors, the protection is specific and must bedetermined on a case-by-case basis. The components soselected and backed-up with over-load and short-circuitprotections will ensure that

• They will make and break, without damage, all currentsfalling even outside the protected zone of a thermal

over-current relay or the built-in over-current (o/c)and short-circuit (s/c) releases of a breaker, but withinthe protected region of the HRC fuses, as illustratedby the hatched portion, of the over-current and short-circuit, I2 – t curves (Figure 12.44).

• They will withstand the let-through energy (I2 · t) andthe peak let-through current of the relays/releases orthe fuses, when making or breaking the circuit on fault,without damage or welding of the interrupting contacts.

The basis of selection of these components is brieflydescribed below.

12.10.1 Switches and contactors

These are selected so that they will sustain without damageto its contacts or to any other part the motor switchinginrush current (Ist) and possess a thermal capacity ofmore than the fuse let-through energy. Figures 12.45 and12.46 show the over-view of an LV switch fuse unit andan LV contactor respectively.

Table 12.5 Duty cycles for a contactor or a switch

Serial no. AC duty Application

For contactors For switchesand motor starters

1 AC1 AC21 Nearly resistive switching such as heaters, resistance furnaces and lighting loads etc.2 AC2 AC22 High-resistance and low-inductance switching such as a slip-ring motor switching3 AC3 AC23 High-inductive switching such as the switching of squirrel cage motors and inductors.

Occasional inching and plugging operations, such as during start-up period4 AC4* –* Stringent inductive switching, such as the switching of a squirrel cage motor with

inching and plugging operations (Section 2.9.1(B))5 AC6b AC23 High capacitive switching such as capacitor banks

*AC4 duty is applicable only for contactors. A switch is neither required nor suitable to perform a duty such as inching or plugging.

*Mechanical life is independent of current and does not depend onload, duty or application. Figure 12.44 Co-ordination of fuses with a breaker

HRC fuse

Breakermin. setting

Breakermax. setting

Fau

lt co

nditi

onfo

r th

e fu

ses

Breaker’sunprotected

region

Fault level ofthe system

Breaking capacityof the breaker

Trip

ping

tim

e

I r

On faults exceeding Isc of breaker, I 2t (fuses) < I 2t (breaker)

Current

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12.10.3 Over-current relay (OCR)

It is necessary to co-ordinate the HRC fuses with theover-current relay to ensure that during a fault the relayis capable of withstanding the let-through energy of thefuses without damage (Figures 12.11 and 12.15).

12.10.4 HRC fuses

The rating of the fuses often decide the rating of theabove components. They are readily available up to 1000 Ato switch all sizes of motors recommended on an LVsystem. The characteristics of the fuses must match withthe thermal withstand I2 – t characteristic of the equipmentor the circuit it has to protect, as discussed earlier. Broadlyspeaking, they must prevent the switch or the contactorfrom breaking currents beyond their thermal capacityand prevent contact welding. The minimum rating of thefuses is chosen as 1.6 to 2 times the full-load current ofthe motor for a DOL start and about 1.25 to 1.50 for aY/D start to ensure that the fuses stay intact withoutinterrupting the motor during a start. One particular brandof HRC fuses has been considered in Table 12.4 in makingthe selection. The ratings may slightly vary with theother brands of fuses, depending upon their I2 – tcharacteristics (Figure 12.19). Figure 12.48 shows thegeneral view of a few HRC fuses.

12.10.5 Selection of cables

This is also based on the full-load current. A cable generallyhas a much higher thermal capacity than the cut-off timeof the HRC fuses of a corresponding rating. Also refer toI2 – t curves for cables in Chapter 16, Appendix 1, FiguresA16.4(a) and (b) and cable selection criterion as in FigureA16.5. It is, however, advisable to check the fuse let-through energy, which should not be more than the short-time rating of the cable. The following may also be keptin mind while selecting the cables:

• Whether the installation of the cable is in air, a ductor in ground. This will determine the type of cablerequired, i.e. armoured or unarmoured.

• Ambient temperature.• Ist and its duration.• Number of power cables running together and their

configuration. For more details refer to Chapter 16,Appendix 1. The cooling of the cables is affected bythe number of cables and their formation. This detailis also provided in Appendix 1. For more detailsconsult the cable manufacturer.

• Length of the cable from the starter to the motor.This will help to determine the voltage drop from thestarter to the motor terminals during a start. It mustbe limited to only 2–3% of the rated voltage becausethe incoming receiving point voltage itself may alreadybe less than the rated. This is illustrated in FigureA16.3. When the cumulative effect of all such dropsexceeds 6%, it may tend to destabilize the distributionsystem and influence other feeders connected on thesame system.

Figure 12.46 Air break contactor (Courtesy: L&T)

Figure 12.45 Switch disconnector fuse unit (Courtesy: L&T)

12.10.2 Breakers (ACBs or MCCBs) (Figure12.47)

While a fuse-free system is gradually gaining preference,for all ratings, as discussed later, it is recommended thata breaker (ACB or MCCB) be employed for large motorsof at least 300 h.p. and above to ensure better protectionfor the motor.

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• A heavy drive, requiring a prolonged starting time,may require larger fuses. In which case the cablesize may also be increased accordingly.

In some cases, where the nearest rating of the fuse itselfis too high for the rated current, a larger cable isrecommended. The thermal (I2 – t) characteristics of allsuch components will vary from one manufacturer toanother and may not be readily available with a design ora field engineer, while making the selection. The manu-facturers of such components therefore as standard practice,perform this co-ordination for their products and make

such data readily available for the user to make a quickselection. It may be noted that OCR and fuses at least, ofdifferent brands, will require a new co-ordination.

12.11 Fuse-free system

Fuses are prone to cause a single phasing by not operatingall three of them simultaneously. They may also requirea longer downtime to replace. Therefore, the new conceptthat has gained preference is a fuse-free system,particularly on an LV system. It is possible to achievesuch a system through miniature and moulded case circuitbreakers (MCBs and MCCBs) designed especially tomatch the I2 – t characteristics of a motor. They are alsodesigned to be fast acting and current limiting, like HRCfuses. On a fault, they would also trip long before theprospective peak of the fault current, and havecharacteristics similar to Figure 12.17 and limit the let-through energy (I2 · t) of the fault. Although a costlierproposition, they eliminate the above deficiencies of afuse and ensure a three-phase interruption, on the onehand and a prompt reclosing, after a trip on fault, on theother. The contactor, however, may still be essential inthe circuit to permit frequent switching operations. Aseparate OCR may also be essential to closely match thethermal characteristics of the motor, as the characteristicsof the releases of the MCBs or MCCBs may under-protectthe motor. Such an exercise may also be time consuming,as the manufacturers of MCBs and MCCBs may, at best,provide their characteristics and co-ordination with theirbrand of components. The co-ordination of MCBs andMCCBs with components of other brands may have tobe done individually, by the design engineer, more soinitially, until all in the field become acquainted withthis philosophy and characteristics of all brands of MCBsand MCCBs.

Figure 12.48 HRC fuses (Courtesy: L&T)

Figure 12.47

Air circuit breaker (ACB) (Courtesy: Siemens)Moulded case circuit breaker (MCCB)

(Courtesy: GE Power Controls)

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12.11.1 Motor protection circuit breakers(MPCBs)

A few manufacturers have provided an improvised versionof the above in the form of a motor protection circuitbreaker (MPCB). This can match closely the thermalcharacteristics of a motor. In this case one MPCB will besufficient to replace the HRC fuses and the thermal relayand a separate OCR may not be necessary. Since therange of MPCBs is limited presently, so are the motorratings that can be protected. With the use of MPCBs noindividual component matching will be necessary as inthe above case. Figure 12.49 illustrates a typicalcharacteristic of an MPCB. Notice that the basic differencebetween an ordinary OCR and an MPCB is the withstandcapability of MPCB on a fault. An MPCB is now able totake care of fault currents and switching transient currentsand isolate the circuit much more quickly on an actualfault. An ordinary relay cannot withstand fault currentsand requires backup protection through HRC fuses.

The characteristic shown also suggests that the MPCBscan withstand current transients up to 100 times the ratedcurrent (or the current setting) and hence are capable ofswitching a capacitor and protecting them against over-loads.

NotePresently MPCBs are being manufactured only up to 125 A or sobecause of their technical limitations such as,

– Absence of spring mechanism as in contactors, making themincapable of remote operations unless they are incorporatedwith a motor operation and that is a costly affair.

Motor therm

al with-stand curve

MP

CB

characteristic

Motor starting characteristic

Figure 12.49 Co-ordination of MPCB characteristics with themotor characteristics (eliminating the use of HRC fuses)

10 000

1000

100

10

1

0.1

.01

0.0020.001

Ir

I st � 7I r

Current (xIr)I transient up to 2 ¥ I st(� 14 I r–20 I r) during anopen transient condition

Trip

ping

tim

e (s

ec.)

0 1 2 4 6 10 20 40 60 100

– They are incapable of frequent switchings and reversals asrequired for lifts, cranes and similar applications unless backedup with contactors and that too is a costly affair besides defeatingthe purpose of an MPCB.

– Low electrical (� 0.1 m operations) and mechanical lives(� 0.5 m operations) as against average 10 m operations andmore for a normal contactor.

Rigorous research work, however, is underway by differentmanufacturers worldwide to overcome these short-comings andimprovise their electrical and mechanical properties at least, inview of their other inherent advantages noted earlier.

With the availability of state-of-the-art microprocessorbased protection releases, now an MCCB can also bemade digital to act like an IED (intelligent electronicdevice). Using telemetry softwares (communicationprotocols) and media it can be made capable for serialdata transmission for process control and energy savingfrom a remote station also (Section 24.11.5). With theuse of MCCBs and microprocessor based releases controlof any size of LV motor is now compact and easy.

12.11.2 Component ratings

In both the above cases, besides saving on the cost of theswitch and fuses, one can also economize on the cost ofother equipment, such as main power cables, contactorand the internal wiring of the starter (particularly fromthe busbars to the MCB, MCCB or MPCB). The ratingof all such equipment can now be lower and commensuratewith that of the MCB, MCCB or MPCB and, hence, canbe very close to the full load current of the motor andthus economize on cost. In conventional fuse protection,their rating was governed by the rating of the fuses, andthe fuses had to be of higher rating than the rated currentof the motor to remain immune from momentary transientconditions and also to allow for a minimum fusing timeduring the switching operation of the motors.

12.11.3 Soft starters

They make use of static technology. See Chapter 6.

12.11.4 Intelligent starters

A standard starter incorporating a microprocessor basedmotor protection relay becomes an IED (intelligentelectronic device). It can monitor and control the machinelocally via a laptop computer (PC) and a keyboard orremotely with the use of telemetry softwares(communication protocols) and media (Section 24.11.5).Some manufacturers have produced these starters in acompact version using their own compact electronic relays.These relays may have similar features as of amicroprocessor based relay. One such compact starterunit with an isolator, contactor and an electronic relay isshown in Figure 12.49(a). For details consult themanufacturer.

12.12 Switchgears for MV motors

With the availability of 3.3, 6.6 and 11 kV vacuum

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contactors the control of MV motors up to 11 kV systemshas now become easier and economical, compact andeven more reliable. Otherwise, vacuum or SF6 (Sulphurhexafluoride) breakers can always be used. The MVmotor’s switching and protection through a vacuumcontactor and HV HRC fuses provides a replica of an LVsystem. Figure 12.50 shows the over-view of an MVvacuum contactor and Figure 12.51 contactor with HRCfuses. The earlier practice of interrupting an MV circuitusing MV OCB, MOCB or air blast circuit breaker isnow a concept of the past.

Figures 12.52 and 12.53 illustrate typical power andcontrol circuit diagrams respectively, for a 6.6 kV breaker-operated motor starter and Figure 12.54 generalarrangement of a vacuum circuit breaker. The latestpractice now up to 11 kV motors is to use an MV loadbreak, fault-making isolator in conjunction withappropriate type and size of HRC fuses, a vacuumcontactor and a motor protection relay. The contactoralso provides a three-phase simultaneous make and breakof contacts through its ‘no volt’ coil. Figures 12.55 and12.56 illustrate typical power and control circuit diagrams,respectively, for a 6.6 kV vacuum contactor-operatedmotor starter.

The comparatively new vacuum contactors and vacuumbreakers have gained favour in view of their reliabilityand durability. For details see Section 19.5.6, which alsodeals with their switching behaviour and phenomenonof arc re-ignition.

MV isolators

The isolator should be a load-break and fault-make typeFigure 12.49(a) Compact intelligent starter and a relay (Courtesy:Schneider Electric)

Compact electronic relay

1

1

4

3

2

5

1 – Auxillary switch2 – Closing solenoid3 – On/off indicator4 – Mechanical off-push button5 – Operation counter6 – Mechanical latching device7 – Trip coil8 – Metallic base frame

1 – Upper contact terminal2 – Epoxy cast armature assembly3 – Vacuum interrupter4 – Lower contact terminal5 – Epoxy resin moulded body

Figure 12.50 6.6 kV vacuum contactor (Courtesy: Jyoti Ltd.)

Rear view

2

4

3

5

6

7

8

Front view

1

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of switching device. Normally, it is provided with an integralgrounding switch, such that in the isolated position it willautomatically short-circuit and ground all the three phasesof the outgoing links. This is a basic safety requirementfor a gang-operated isolator, mounted on outdoor polesand operated manually from the ground by a disconnectinglever. For an indoor switchgear, however, it is not soessential, since the switchgear itself is adequately protectedfor a ground fault. Generally, all manufacturers providesuch a built-in feature as standard practice, irrespective ofits application. For a capacitor bank also, if installed inthe circuit, such an isolating and grounding switch isessential to ground the capacitor banks on a switch-OFF.Figure 12.57 shows a typical MV isolator.

MV HRC fuses

As discussed in Section 17.7, during a start MV motorsmay encounter severe switching surges on the system.The starting inrush current on such a system can thereforehave a momentary very high peak, similar to and evenmore severe than that shown in Figure 4.5, dependingupon the type of interrupter being used and the surgeprotection scheme, if provided. Unlike LV motors, oneshould therefore take cognisance of the switching surgephenomenon, in addition to the thermal rating demand,to sustain the repeat start surges while selecting the fuserating for the switching of an MV motor.

The following points may be borne in mind whileselecting the correct rating of the fuses:

Figure 12.51 12 kV vacuum contactor with HRC fuses on atrolley (Courtesy: Joyti Ltd.)

Figure 12.52 A simple power circuit diagram for a breaker-operated MV motor starter

R Y B

Q 1

T 1

T2

T3

F34

F35

F36

Shortinglinks

Isolatinglink

CT

inpu

t

An assumedmotor

protectionrelay

Auxiliary supplyfail alarm

Pre trip alarm

Stallingalarm/trip

Remote thermalreset

F31

F32

F33

VSS

P 4

V

G G

P 1

AASS

12 1 2 3 4 5 6 15 20

Isolatinglink

Control supplycontact

Tripcontact

Alarmcontact

M

T 7

T 8

T 9

Shortinglinks

T 4

T5

T6

16 17 18 197

8

9

10

11

• Starting time of the motor• Number of consecutive starts• Number of equally spaced starts per hour• Consider the rating of the fuse at about twice the

rated current for a DOL starting and about 130% foran A/T starting. The rating will depend upon theI2 – t characteristics of the HRC fuses and must ensurethat they remain intact while handling short-duration excessive current during normal operation,such as during a start, and clear a fault conditionquickly.

Current–Time (I2 – t) characteristics

The typical current–time characteristics for 50–630 A,6.6 kV fuses are shown in Figure 12.58. The cut-offcurrent characteristics are given in Figure 12.59.

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Figure 12.53 Typical schematic and control wiring diagram for power circuit of Figure 12.52

TNC

Sw

CF1 CF3

S13 1

S 14A T

NC

S14 2TNC

B

S16

S 16C 95 Q2

Spr

ing

char

ged

h 4C 95

CF 2 CF 4

Anti pumpingrelay

ACBclosing

coil

Q 2

S 16C 95

C86Remote

tripMech.

tripMPR*

trip

Q 2

ACBtripcoil

Lockout

relayC86

h 2

CF 6

S 13

Remote

Local

1-A

Open

Close

2-B

Close

Open

S 14

Trip

Neutral

Close

1-A

Open

Open

Close

2-B

Close

Close

Open

CF5

LocalRemote

B

M

Clo

sing

inte

rlock

Knobposition

Knobposition

On

*MPR: Motor protection relay

110/

220

V D

C c

ontr

ol s

uppl

y

+

2

Q 2C 86

h 3 h 1

O/C tripOff

Undervoltage

trip

Remotetrip

TNC

A

NoteAll thermal operated devices, relays, releases or fuses necessarilydetect the heating effect. Heating curves therefore are basicallyI 2 – t curves but to facilitate assessment of trip time at variouscurrent settings they are represented as I – t curves.

Method of selection

The manufacturers of HRC fuses provide a selection

chart to help a user select the proper type and size offuses. For the fuses covered in Figures 12.58 and 12.59we have also provided a selection chart (Figure 12.60),for motors having a starting time of not more than 15seconds. Similar charts from other manufacturers canalso be obtained for different ratings of fuses and differentstarting times of motors.

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Protection of electric motors and selection of components 12/361

R Y B

IsolatorSw

CCF1-3 CF4-6

VSS

T 7

T 8

T 9

P4V

AP1A

SS

Isolatinglink

An assumedmotor

protectionrelay

T6

T4

T5

T1

T2

T3

M

Isolatinglink

CT

inpu

t

Controlsupplycontact

Tripcontact

Alarmcontact

6.6 kV 3 Ph 50 Hz

G GShorting

links

12 1 2 3 4 5 6 15 20

16 17 18 1978

910

11

Auxiliary supplyfail alarmPre trip alarm

Stallingalarm/tripRemote thermalreset

Figure 12.55 A simple power diagram for a 6.6 kV motor controlwith an isolator, contactor and a motor protection relay

Shortinglinks

7.2–36 kV vacuum circuit breaker (Courtesy: Siemens)

Figure 12.54 General arrangement of the breaker on a trolley

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Figure 12.56 Typical schematic and control wiring diagram for power circuit of Figure 12.55

SwM

otor

pro

tect

ion

rela

y tr

ip c

onta

ct

StopPB

C11

C12

StopPB

(remote)

LocalRemote

StartPB

StartPB

C 1 C11

1 2

A B

Lock outrelay

On Off Trip Controlsupplyhealthy

h 2 h 3 h1 h 4

240V ACControl supply

CF 2CF1

C1

Motorspaceheater

To motor forwinding andbearing tem-perature etc.

Spaceheatercircuit

CF1 CF 2

C1

C1C11

DC switching andprotection control

circuit

C12

Thermistorrelay circuit

110/220 VDC control

supply

Emergencytrip PB

(stay-put)

AC control circuit forauxiliaries

C1

S13

Figure 12.57 HV isolating switch (Courtesy: Siemens)

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Figure 12.59 Typical current cut-off characteristics of 6.6 kV HRC fuses at prospective currents up to 43.8 kA

IllustrationA 100 A fuse will cut-off an I sc of 30 kA (peak value up to2.5 ¥ 30 kA) at less than 6 kA peak

100

10

<65

1.01.0 10 50 100

Prospective currentkA (rms symmetrical)

Max

imum

cut

-off

curr

ent

kA (

peak

)

630A500A

400A350A

250A200A160A125A100A80A63A50A

30 43.8

315A

50

5

Figure 12.58 Typical time–current characteristics of 6.6 kV HRC fuses

Pre

-arc

ing

time

(sec

.)

10 000.0

1000.0

100.0

10.0

1.0

0.1

0.010.005

4hrs.

630A500A400A350A315A250A200A160A125A100A

80A63A50A

1 10 100 1000 10 000 100 000

Symmetrical prospective current (rms) Amperes

20 000.0

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Figure 12.60 Fuse selection chart for a 6.6 kV system for motors with run-up time not exceeding 15 seconds

630500

400

350315

250

200

160

125

100

80

63

50

Fus

e ra

ting

(A)

No. of startsper hour 32

100 200 300 400 500 700 1000 1500 2000 3000 3500Motor starting current (A)

16 8 4 2

IEC

60034-1/2004

60034-11/2004

60072-1/1991

60072-2/1990

60072-3/1994

60470/2000

60644/1979

60751/1995

60892/2000

60947-1/2004

60947-2/2003

60947-3/2001

60947-4-1/2001

60947-5-1/2003

62271-200/2003

TR 61818/2003

–

–

Title

Rotating electrical machinesRating and performance.

Rotating electrical machines. Built-in thermal protection,rules for protection of rotating electrical machines.

Dimensions and output series for rotating electrical machines.Frame number 56 to 400 and Flange number 55 to 1080.

Dimensions and output series for rotating electrical machines.Frame number 355 to 1000 and Flange number 1180 to 2360.

Dimensions and output series for rotating electrical machines.Small built-in motors – Flange numbers BF 10 to BF 50.

A.C. contactors for voltages above 1 kV and up to andincluding 12 kV.

Specification for high voltage fuse-links for motor circuitapplications.

Industrial platinum resistance thermometer sensors.

Effects of unbalanced voltages on the performance of 3f cageinduction motors.

Specification for low voltage switchgear and control gear.General rules.

Low voltage switchgear and controlgear, Circuit breakers.

Specification for LV switchgear and controlgear. Switches,disconnectors, switch-disconnectors and fuse-combinationunits.

Low voltage switchgear and controlgear. Electromechanicalcontactors and motor starters including rheostatic rotorstarters.

Control circuit devices and switching elements –electromechanical control circuit devices.

A.C. metal-enclosed switchgear and controlgear for ratedvoltages above 1 kV and up to and including 52 kV.

Application guide for low voltage fuses.

Low voltage fuses

Contactors for voltage not exceeding 1000 V or 1200 V d.c.

IS

4722/2001

–

1231/1997

1231/1997

996/2002

9046/2002

–

–

–

13947-1/1998

13947-2/1993

13947-3/1998

13947-4-1/1998

13947-5-1/1998

12729/2000

–

13703-1/1993

13947/1993Part 4 Sec 4

BS

BS EN 60034-1/1998

BS EN 60034-11/2004

BS 4999-141/2004

BS 4999-103/2004

BS 5000-11/1989

BS EN 60470/2001

BS EN 60644/1993

BS EN 60751/1996

–

BS EN 60947-1/1998

BS EN 60947-2/2003

BS EN 60947-3/ 2003

BS EN 60947-4-1/1992

BS EN 60947-5-1/1992

BS EN 60298/1996

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

Relevant Standards

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Protection of electric motors and selection of components 12/365

List of formulae used

Voltage unbalance (negative phase sequence)

Voltage unbalance =

Max. voltage variationfrom the average voltage

Average voltage 100%¥

(12.1)

Stator current on an unbalanced voltage

IV

R RS

SR X S ssX

rr

1 2 2

2

1 22

=

+ + (1 – )

+ [ + ]¢ ¥ ¢ÈÎÍ

˘˚̇

¥ ¢

(12.2)

and

IV

R RS

SR X S ssX

uu

1 2 2

2

1 22

=

+ + ( – 1)(2 – )

+ [ + (2 – ) ]¢ ¥ ¢ÈÎÍ

˘˚̇

¥ ¢

(12.3)

Iu = negative phase sequence current componentVu = unbalanced voltage

¢R2 = rotor resistance referred to stator¢X2 = rotor reactance referred to stator

Stator heat on an unbalanced voltage(a) Maximum theoretical heat

H I I I Ieq r2

u2

r u(max.) ( + + 2 )µ ¥ (12.4)

(b) Minimum theoretical heat

H I I I Ieq r2

u2

r u(min.) ( + + )µ ¥ (12.5)

Actual heat generated

H I Ieq r2

u2 ( + 6 )µ (12.6)

Actual current on unbalance

I I Ieq r2

u2 ( + 6 )µ (12.7)

Rotor power during an unbalance

P I RS

SI R

SS

= 3 (1 – )

– (1 – )(2 – )rr

22 ru

22¥ ¥ ¥ ¥Ê

ˈ¯ (12.8)

Irr = positive sequence current in the rotor circuit, and

ANSI-C37.42/1996ANSI-C37.47/1992NEMA/AB-1/2002NEMA/AB-3/2001NEMA/FU-1/1986NEMA/MG-1/2003

Standard on distribution cut out and fuse links.Specifications for distribution fuse disconnecting switches, fuse supports and current limiting fuses.Moulded case circuit breakers and moulded case switches up to 1000 V a.c. Rating more than 5000 A.Moulded case circuit breakers and their application, up to 1000 V a.c. Rating 5000 A and more.Low voltage cartridge fuses.Motors and generators ratings, construction, testing and performance.

Relevant US Standards ANSI/NEMA and IEEE

Notes1 In the table of relevant Standards while the latest editions of the Standards are provided, it is possible that revised editions have become

available or some of them are even withdrawn. With the advances in technology and/or its application, the upgrading of Standards is a continuousprocess by different Standards organizations. It is therefore advisable that for more authentic references, one may consult the relevant organizationsfor the latest version of a Standard.

2 Some of the BS or IS Standards mentioned against IEC may not be identical.3 The year noted against each Standard may also refer to the year it was last reaffirmed and not necessarily the year of publication.

I ru = negative sequence current in the rotor circuit

Total rotor heat

µ ( + 3 )rr2

ru2I I (12.9)

Iru = negative sequence rotor current

Cumulative rotor current

I I Irr total rr2

ru2 = ( + 3 ) (12.10)

Further Reading

1 Beeman, D., Industrial Power Systems Handbook, McGraw-Hill, New York (1955).

2 Dommer, R. and Rotter, N.W., ‘Temperature sensors for thermalover-load protection of machines’, Siemens Circuit, XVII, No.4, October (1982).

3 Ghosh, S.N., ‘Thermistor protection for electric motors’, SiemensCircuit, XI, No. 3, July (1976).

4 Kaufmann, R.H. and Halberg, M.H., System over-voltages,causes and protective measures.

5 Kolfertz, G., ‘Full thermal protection with PTC thermistors ofthree-phase squirrel cage motors’, Siemens Review, 32, No. 12(1965).

6 Lythall, R.T., AC Motor Control (on earth fault protection andthermistor protection).

7 Ramaswamy, R., ‘Vacuum circuit breakers’, Siemens Circuit,XXIII, April (1988).

Source material

Unbalanced Voltages and Single Phasing Protection, M/s MinilecControls Private Ltd.

High Voltage HRC Fuses, Publication No. MFG/47, The EnglishElectric Co. of India Ltd.

Surge Suppressors-Bulletin No. T-109, Jyoti Ltd.Thermistor Motor Protection Relay Catalogue-SP 50125/5482,

Larsen & Toubro.Motor Protection Relay, English Electric Co. of India Ltd, Catalogue

Ref. PR. 05:101:A/6/85.Negative Phase Sequence Relay, M/s English Electric Co. of India

Ltd, Catalogue Ref. PR:01:306:A.Thermal Bimetallic Over-load Relays, Bhartia Cutler Hammer.

Product Catalogue C-305 MC 305/ANA3/6/85.

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Appendix: Rules of thumb for every day use A/367Power requirements for pumps A/369Power requirements for lifts A/369Power requirements for fans A/369

Important formulae A/370

Conversion table A/370

AppendixRules of thumb foreveryday use

A/367

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Appendix A/369

Power requirements for pumps

h.p. = US GPM

4000 F◊ ◊

◊H rh (A.1)

Alternatively,

I GPM 3300

F◊ ◊◊H r

h

whereUS GPM = discharge in US gallons per minute

I GPM (UK) = discharge in imperial gallons perminute

HF = head (in feet)= suction head + static delivery head +

frictional head (loss) in pipes andfittings + velocity head

For determining the frictional head, refer to friction lossin pipes, bends, elbows and reducers and valves asprovided in Tables A.1 and A.2:

r = specific gravity of liquid in g/cm3

h = unit efficiency of pump

or h.p. = LPS

75 m◊ ◊

◊H r

hLPS = discharge in litres per second

Hm = head (in metres)

Friction loss in pipes

Table A.1 provides, for a particular rate of discharge inLPS, the friction loss in pipes for every 1000 m of straightpipe length, reasonably smooth and free from incrustation.

• Friction loss in bends, reducers, elbows etc. areprovided in Table A.2 in equivalent pipe length

• To determine the size of pipe

The economics would depend upon the smoother flow offluid without excessive friction loss. A smaller section ofpipe may not only require a higher h.p. for the same suctionand lifting head due to greater frictional losses, but mayalso cause the pipe to deteriorate quickly as a result of theadditional load on its surface. Losses due to bends andvalves should also be added in the total friction loss.

ExampleConsider a discharge of, say, 20 LPS against a total suctionand delivery head of 150 m through a mains 1000 m long.Considering an average of 25 bends, elbows, tees and reducerfittings in the total length of pipe, then from Table A.1.

Total equivalent length of pipe (assuming that every fitting hasan average 5 m of equivalent pipe length, to account for friction)

= 1000 + 25 ¥ 5

= 1125 m

A 125 mm pipe has a friction loss of nearly 33 m per 1000 mand a 150 mm size of pipe, 13 m per 1000 m.

\ Total frictional head for a 125 mm pipe = 11251000

33¥

= 37.125 m

and for a 150 mm pipe = 11251000

13¥

= 14.625 m

A friction loss of 37.125 m in a total length of 1000 m is quitehigh and will require a larger motor. Therefore, a 150 mmmain pipeline will offer a better and more economical designcompared to a 125 mm pipeline such as the reduced cost ofthe prime mover and lower power consumption during thelife of pumping system, in addition to a longer life span of a150 mm pipe compared to a 125 mm pipe.

NoteEnergy saving : With the ever rising impetus on energysaving (Section 1.19), it is desirable to reduce the frictionallosses in pipes for flow of air, gas or fluid to the bare minimumand enhance longevity of the pipes and fittings to the optimum.It is possible to do so by choosing a slightly bigger diameterpipe as noted above and employing frictionless pipes andfittings as far as possible. Like, using rigid PVC pipes andfittings and where they are not suitable using copper, stainlesssteel or good quality MS pipes.

Power requirements for lifts

(i) For linear motion drivesWhere the weight of the cage and half of the passengersload is balanced by the counter weight

P W V = 0.746 2.75

¥ ¥¥ h (A.2)

where P = kW requiredW = passengers load in kg V = speed of lift in m/s h = unit efficiency of the drive.

(ii) For rotary motion drivesUsing equation (1.8),

P T N = 974

◊◊ h (A.3)

where P = kW required T = load torque in mkgN = speed of drive in r.p.m. h = unit efficiency of the drive

Power requirements for fans

Pp

= LPS 75

◊◊ h (A.4)

whereP = kW required

LPS = quantity of air in litres per second.p = back pressure of air at the outlet in metres of a

column of water.

NoteThese are actual power requirements for various applications. Add10–15% to these figures to select the size of the motor to accountfor unforeseen losses during transmission from motor to load andother frictional losses. Too large a motor will give a poor powerfactor and a poor efficiency, while too small a size will run over-

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A/370 Electrical Power Engineering Reference & Applications Handbook

loaded (Section 1.8). These considerations must be kept in mindwhile selecting the motor rating.

Important formulae

Moment of inertia

GD2 = 4 · g · (mr2)= 4 Wr2 (A.5)

whereW = m · g (mass ¥ gravity)r = radius of gyration

Temperature

Conversion from Fahrenheit (∞F) to Celsius (∞C)

F C – 329

= 5

(A.6)

where

F = ∞FC = ∞CAbsolute zero = kelvin (1 K)

1 K = –273.15∞C

Absolute zero is the theoretical temperature, at whichthe atoms and molecules of a substance have the leastpossible energy. This possibly is the lowest attainabletemperature.

Conversion table

Lengths

1 inch = 25.4 mm1 foot = 30.48 cm1 mile = 1.609 km

1 cm = 0.3937 inch1 metre = 39.37 inches

= 3.28 feet1 km = 3280 feet

= 0.621 mile

Areas1 in2 = 6.4516 cm2

1 ft2 = 0.0929 m2

1 cm2 = 0.155 in2

1 m2 = 10.8 ft2

1 m2 = 1.196 yd2

Volumes/weights*1 Imperial gallon (UK) = 4.546 litres

*1 US gallon = 3.79 litres1 pint = 0.568 litre1 litre = 0.22 Imperial gallons

1 lb = 0.453 kg1 kg = 2.204 lb

1 MT = 0.984 ton1 T = 1.016 MT

1 litre per second = 13.2 gallons per minute1 ft3 of water = 6.23 gallons1 m3 of water = 220 gallons = 35.31 ft3

1 ft3/s = 22,500 gallons/h (GPH)= 375 gallons/minute (GPM)

1 atmosphere = 30≤ of mercury= 14.7 lb/in2

Torque/force1 cm.kg = 0.866 inch pounds1 m.kg = 7.23 foot pounds

Work done

1 kW = 1.36 metric h.p. (PS)= 1.341 h.p.

1 h.p. = 0.746 kW= 1.014 PS (metric h.p.)

1 PS (mhp) = 0.736 kW= 0.986 h.p.

*Unless specified all conversions made earlier relate to Imperialgallons only.

Table A.6 Some useful units

Unit of

1 N = 0.101972 kgf Force

1 kg f = 9.807 N Force

1 kg fm = 9.807 N · m Energy/torque

1 kg f/m2 = 9.807 N/m2 Force

1 J = 1 N.m = 0.10187 kg · fm Energy/torque

1 W = 1 J/s Power

1 Wb = 1 V · s Flux

1 T = 1 Wb/m2 Flux density

1 Hz = 1 Hz (s–1) Frequency

1 Pa (Pascal) = 1 N/m2 Pressure

1 bar = 105 N/m2

� 1 atm.

1 atm. (atmosphere) � 1 kgf/cm2

� 735.6 mm of mercury Pressure� 1.01325 ¥ 105 N/m2

1 bar =105 pascal � 0.9870 atm.

1 torr = 1.01325 10760

N/m

1760

atm.

52¥

=

¸

˝Ô

˛Ô

Pressure

¸

˝Ô

˛Ô

¸˝˛

Pressure

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Appendix A/371Ta

ble

A.1

Hea

d lo

ss in

met

res

due

to f

rict

ion

in p

ipel

ines

per

100

0 m

pip

e le

ngth

Pip

e di

amet

er i

n m

illi

met

res

Dis

ch.

4050

6070

8010

012

515

020

025

030

035

040

045

050

060

070

080

090

010

0012

00l/

s 1.0

3711

4.3

1.2

5016

5.8

2.7

1.4

7322

8.3

3.7

1.6

9228

115.

02.

3

1.8

118

3714

6.2

2.9

2.0

155

4517

7.3

3.7

2.2

162

5221

.59.

04.

4

2.4

205

6425

10.7

5.2

1.6

2.6

235

7529

12.7

6.2

1.8

2.8

275

8733

14.7

7.0

2.2

3.0

320

100

3816

.88.

32.

5

3.5

425

135

5323

113.

3

4.0

560

175

7330

154.

51.

3

4.5

715

225

8838

18.5

5.5

1.7

5.0

870

280

108

4723

6.8

2.2

5.5

330

124

5727

8.3

2.6

0.95

6.0

400

155

6832

9.6

3.2

1.18

6.5

470

183

8038

11.5

3.6

1.4

7.0

540

215

9345

134.

21.

7

7.5

620

240

106

5215

4.7

1.8

8.0

700

280

116

6018

5.5

2.1

8.5

800

310

133

6820

6.2

2.3

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A/372 Electrical Power Engineering Reference & Applications Handbook

9.0

900

360

150

7522

6.8

2.7

9.5

1000

380

170

8325

7.6

2.9

1043

019

094

288.

53.

20.

65

1263

027

013

040

12.3

4.7

1.0

1486

037

018

055

16.5

6.3

1.3

1647

023

072

228.

31.

70.

55

1860

030

090

2810

.52.

20.

68

2072

037

011

033

132.

70.

88

2286

045

013

541

163.

31.

00.

38

2452

016

048

194.

01.

20.

45

2662

018

756

224.

71.

450.

55

2872

022

066

255.

51.

650.

6

3080

025

075

286.

31.

90.

70.

32

3533

010

340

8.5

2.7

0.95

0.45

4045

013

552

113.

31.

280.

580.

28

4556

017

065

144.

31.

60.

730.

37

5070

021

083

175.

52.

00.

90.

450.

23

5585

026

597

226.

52.

41.

10.

550.

28

6010

0032

011

525

7.5

2.8

1.3

0.65

0.35

0.20

6537

014

029

9.0

3.4

1.5

0.75

0.40

0.23

7042

016

335

10.9

4.0

1.8

0.88

0.47

0.27

7548

018

539

124.

62.

11.

00.

530.

30

Tab

le A

.1(C

ontd

.)

Pip

e di

amet

er i

n m

illi

met

r es

Dis

ch.

4050

6070

8010

012

515

020

025

030

035

040

045

050

060

070

080

090

010

0012

00l/

s

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Appendix A/373

8055

022

043

145.

22.

41.

120.

60.

35

8560

023

050

155.

82.

71.

30.

70.

380.

16

9068

027

056

176.

52.

91.

450.

770.

450.

17

9575

030

063

197.

23.

31.

60.

850.

500.

19

100

850

330

7022

8.0

3.7

1.8

0.95

0.55

0.22

120

460

9832

11.2

5.3

2.6

1.4

0.77

0.29

0.13

140

650

135

4215

.57.

03.

51.

851.

050.

420.

17

160

4.5

2.5

1.4

0.54

0.22

0.11

180

5.7

3.2

1.8

0.68

0.30

0.14

200

7.0

3.8

2.2

0.83

0.36

0.18

220

8.5

4.6

2.7

1.0

0.43

0.22

0.12

240

10.0

5.6

3.2

1.2

0.52

0.25

0.15

260

126.

53.

71.

40.

620.

30.

17

280

147.

34.

21.

650.

700.

340.

19

300

15.5

8.4

5.0

1.85

0.8

0.4

0.22

350

2211

.56.

62.

61.

10.

520.

28

400

2815

.08.

53.

31.

50.

70.

380.

22

450

3519

.011

.04.

21.

850.

80.

480.

28

500

4321

.514

.05.

22.

31.

10.

60.

35

550

5328

16.5

6.2

2.8

1.3

0.72

0.42

600

6333

207.

33.

31.

60.

850.

500.

19

650

7340

238.

53.

81.

91.

00.

580.

22

700

8547

2710

.04.

52.

21.

20.

700.

25

750

100

5331

11.5

5.2

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Auth

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ISBN

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642-

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A/374 Electrical Power Engineering Reference & Applications Handbook

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642-

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Appendix A/375

Table A.3 Table of conversions of gallons* per minute into litres per second

GPM l/s GPM l/s GPM l/s GPM l/s

5 0.379 175 13.25 510 38.6 860 65.210 0.758 180 13.65 520 39.4 870 66.012 0.910 185 14.0 530 40.2 880 66.713.2 1.0 190 14.4 540 40.9 890 67.415 1.14 195 14.8 550 41.6 900 68.220 1.52 200 15.16 560 42.4 910 68.925 1.89 210 15.9 570 43.2 920 69.730 2.28 220 16.9 580 43.9 930 70.432 2.5 230 17.4 590 44.7 940 71.235 2.65 240 18.9 600 45.5 950 71.940 3.15 250 18.2 610 46.2 960 72.745 3.41 260 19.7 620 47.0 970 73.450 3.79 270 20.4 630 47.7 980 74.255 4.16 280 21.2 640 48.4 990 75.060 4.55 290 22.0 650 49.2 1000 75.765 4.92 300 22.7 660 50.0 1050 79.070 5.3 310 23.4 670 50.7 1100 83.375 5.68 320 24.2 680 51.4 1150 87.080 6.05 330 25.0 690 52.2 1200 91.085 6.45 340 25.7 700 53.0 1250 94.590 6.82 350 26.4 710 53.7 1300 98.5

100 7.58 360 27.2 720 54.4 1350 102.0105 7.95 370 28.0 730 55.2 1400 106.0110 8.33 380 28.7 740 56.0 1450 110.0115 8.7 390 29.4 750 56.7 1500 113.5120 9.1 400 30.2 760 57.5 1600 121.0125 9.45 410 31.0 770 58.3 1650 125.0130 9.85 420 31.7 780 59.1 1700 129.0135 10.2 430 32.4 790 59.8 1750 132.0140 10.6 440 33.2 800 60.5 1800 136.5145 11.0 450 34.0 810 61.3 1850 140.0150 11.35 460 34.7 820 62.1 1900 144.0155 11.75 470 35.4 830 62.8 1950 148.0160 12.1 480 36.2 840 63.5 2000 151.5165 12.5 490 37.0 850 64.4170 12.9 500 37.9

*Imperial gallons (UK)

Table A.4 Head of water in feet and equivalent pressure inpounds per square inch

Head lb/in2 Head lb/in2 Head lb/in2

(ft) (ft) (ft)

1 0.43 55 23.82 190 82.292 0.87 60 25.99 200 86.623 1.30 65 28.15 225 97.454 1.73 70 30.32 250 108.275 2.17 75 32.48 275 119.106 2.60 80 34.65 300 129.937 3.03 85 36.81 325 140.758 3.40 90 38.98 400 173.249 3.90 95 41.14 500 216.55

10 4.33 100 43.31 600 259.8515 6.50 110 47.64 700 303.1620 8.66 120 51.97 800 346.4725 10.83 130 56.30 900 389.7830 12.99 140 60.63 1000 433.0935 15.16 150 64.9640 17.32 160 69.2945 19.49 170 73.6350 21.65 180 77.96

Table A.5 Pressure in pounds per square inch andequivalent head of water in feet

lb/in2 Head lb/in2 Head lb/in2 Head(ft) (ft) (ft)

1 2.31 45 103.90 150 346.342 4.62 50 115.45 160 369.433 6.93 55 126.99 170 392.524 9.24 60 138.54 180 415.615 11.54 65 150.08 190 438.906 13.85 70 161.63 200 461.787 16.16 75 173.17 225 519.518 18.47 80 184.72 250 577.249 20.78 85 196.26 275 643.03

10 23.09 90 207.81 300 692.6915 34.63 95 219.35 325 750.4120 46.18 100 230.90 350 808.1325 57.72 110 253.98 375 865.8930 69.27 120 277.07 400 922.5835 80.81 130 300.1640 92.36 140 323.25 500 1154.48