Electrical-Engineering-portal.com-Motor Protection Depending on Size and Voltage Level

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Motor protection depending on size and voltage level (on photo 3-phase asynchronous motorLeroySomer P280 S-8, 55 kW)

http://electrical- engineering- portal.com/motor- protection- depending- on- siz e- and- voltage- level May 2, 2013

Motor protection depending on size and voltage level

Edvard

Motor

protections vary widely depending on the size of the motor and voltage level involved, thus onlythemore common ones are discussed in this technical article.

Protection Index

1. Motor Instantaneous Over-current Protection

Instantaneous over-current is usually the result of fault conditions (phase to phase, phase toground), in which current f low will greatly exceed normal values. Damage due to windingoverheating and burning damage associated with large fault currents can occur without this typeof protect ion.

These types of faults can be rapidly detected by a dif ferent ial protection scheme using CoreBalance CTs as will be discussed later and cleared before major damage results. In these
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situat ions, fast act ing electromagnetic relays will be used to t rip the af fected motor.

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2. Motor Timed Over-Current Protection

Continuous operat ion of an electric motorat currents marginally above its rated value can result inthermal damageto the motor.

The insulat ion can be degraded, result ing in reduced motor life through eventual internal motorfaults. Typically, an electric motor has a service factor rating listed on its nameplate. This numberrepresents the cont inuous allowable load limit that can be maintained without sustaining damageto the motor. For example, a typical electric motor is designed to withstand a continuous overloadof about 15% without sustaining damage and has a service factor = 115%.

Continuous operation at or above this value will result in thermal damage.To protect againstmotor damage, we mustensure that this condition is not reached, hence we must trip the mot orbefore the overload limit (service factor) is reached.

The relay most commonly used for this purpose is the induction disc relay. In this relay (Figure 1),

the current in two coils produces opposing magnetic fluxes, which create a torque on a disc. Asthe mot or current increases, so does the torque on the disc.

When the torque overcomes the spring torque, the disc begins to rotate. When the movingcontact meets the stat ionary contact on t he disc, the trip will operate.
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Figure 1 - Induction Disc Relay

Tap sett ings and t imecharacteristic adjustments canbe made to alter the t ime delayof the relay. The major benefitof the induction disk timed overcurrent relay is that the speedof rotation is proportional to

the motor current.

Hence major over-currentconditions will t rip the supplybreaker almostinstantaneously, while currentsjust above rated load will causeoperation after severalseconds (or minutes).

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3. Thermal OverLoad

Another common type of relayused for timed overloadprotect ion is a thermaloverload relay. In this type ofrelay, the motor current or afract ion of t he current througha currentt ransformer isconnected to an in-line heater.Figure 2shows a simplif iedthermal overload relay. The

heater (heated by I2R action)is used to heat a bimetallicstrip, which causes thedisplacement of a relaycontact. A bimetallic stripconsists of two differentmaterials bonded together,each having diff erent thermal expansion properties.

As the materials are heated, one side will lengthen more than the other, causing bending.

Normal operating currents or short durat ion overload condit ions, will not cause the bimetallicelement tobend enough to change the relay contact positions.

Excessive currents will cause increased heat ing of the bimetallic strip, which will cause relaycontacts to open and/or close, tripping the motor.
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Figure 2 - Thermal Overload Relay

The thermal overload relay hasan inherent reaction time, sincethe heater and bimetallicelement take time to heat. Caremust be taken to match thecurrent heating characterist icsof the motor or else the motor

could be damaged during thelocked rotor start ing condit ions.

This type of relay can be usedfor direct protection againstexcessive motor currentcaused by electrical faultsandmotor overloads. Also, it is oftenused in combination with t hetimed over-current protection.

Thermal overload relays using in-

line heaters and bimetallic strips,provide an alarm in the case ofcont inuous overload. Thisprovides an opportunity for theoperator to correct the problem before it reaches trip level magnitude.

As we have stated, thermal over-load trips can occur during repetit ive starts on a motoror duringmotor over-loading. Thermal overload trips will seal-in to prevent the motor contactor from closingThis lock-out will require manual reset before the mot or can be re-started. The operator oratt endant will have to physically conf irm that t he motor has had suff icient t ime to cool down andthat the cause for the overload has been removed. If the operator is confident t hat t here is not a

permanent fault on the mot or the relay can be reset.

Note however, that if an instantaneous over-current t rip has occurred, no attempt at closing themotor contactor should be made. An instantaneous trip will only occur if t here is a fault in themotor or supply cable and this must be corrected before any attempt to reset t he relay.

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4. Motor Ground Fault Protection

In the detection of ground faults, as with the detection of instantaneous over-currents, it is

extremely important that the fault be detected and cleared quickly to prevent equipment damage.Insulation damaged by heat (f rom extended overload operation), brit t leness of insulation (due toaging), wet insulat ion or mechanically damaged insulation can cause ground faults.

Ground fault protection schemes use dif ferential protection to detect and clear the faultedequipment. For mot ors, the common method is to use a Core-Balance CT as illustrated in Figure3. The output o f the core-balance CT will be the dif ference or imbalance of current between thethree phases.

If no ground fault is present, no current imbalance is present; hence no current will f low in theprotection circuit.
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Figure 3 - Three Phase Ground FaultProtection

Figure 4 - Single Phase Ground FaultProtection

If a ground fault develops, a current imbalance willbe present and a current will f low in the protectioncircuit , causing it to operate to t rip the supplybreaker.

Figure 4shows a similar protection scheme, witheach of the windings of the motor protectedindividually (this scheme is not normally installed insmall motors, but may appear in the protection ofvery large motors).

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5. Motor Stall Protection

Stalling or locking the rotor, is a situation in which the

circuits of a mot or are energized but t he rotor is notturning. Motors are particularly susceptible tooverheating during starts, due to high currentscombined with low cooling air f lows (due to t he lowspeed of the motor, cooling fans are delivering onlysmall amounts of air).

This is also why some larger motors have a limit onthe number of att empted motor starts before acooling off period is required. However, stallconditions can occur during normal operation. For

example, mechanical faults such as a seized bearing,heavy loading or some type of foreign object caughtin a pump could be possible causes of motor stalling.

The loss of a single phase while the motor is notrotating or under high load, is another situat ion inwhich a motor may stall.

The typical starting time of a motor is less than ten seconds. As long as this start t ime is notexceeded, no damage to a motor will occur due to overheating from the high currents.Duringoperation, a motor could typically stall for twenty secondsor more without resulting in excessive

insulation deterioration.

We use a stalling relay to protect motors during starts, since a standard thermal relay has toomuch time delay. A stalling relay will allow the motor to draw normal starting currents (which areseveral times normal load current) for a short t ime, but will trip the mot or for excessive t ime at highcurrents.

A stalling relay uses the operating principle of a thermal overload relay, but operates faster than astandard thermal relay.


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Figure 5 - Stalling Relay

A schematic representat ion of a stallingrelay has be been provided in Figure 5for reference.

By passing a portion of the motorcurrent directly through the bimetallicelements in this relay, the heating isimmediate, just as would be experiencedwithin the windings of the motor.

This type of relay is usually operationalonly when the motor current isabove 3times the normal operating currentandis switched out when the current isbelow 2 t imes the normal operatingcurrent. This switching in/out is achievedby the use of an additional relay contact.

When the mot or is operating normally,

the current in this prot ection schemepasses through the resistor andbypasses the bimetallic elements.

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6. Motor Over-Fluxing Protection

As you can recall f rom the module on motor theory, the current drawn by a motor is roughlyproport ional to t he core flux required to produce rotation. Moreover, the flux in the core is roughlyproport ional to the square of the slip speed.

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Obviously over-f luxing is most severe during the locked rotor or stall condition when the slip is atthe maximum. The stall relay previously discussedprotects against this.

However, there is another condit ion where we can enter into a state of over-f luxing the mot or. Ifone of the three phases of the supply has high resistance or is open circuit (due to a blown f use,loose connection, etc.), then the magnetic flux becomes unbalanced and the rotor will begin to slipfurther away from the stator f ield speed.

The rotor (shaf t) speed will decrease while the supply current will increase causing winding over-heating as well as core iron heating. Also intense vibration due to unbalanced magnetic forces cancause damage to the motor windings and bearings.

This open-phase condition is oddly enough called single phasing of the motor, even though twophases are still connected. If the motor continues to operate with an open supply line, the currentin the remaining two healthy leads will exceed twice the current normally seen for a given load. Thiswill result in rapid, uneven heating within the motor and damage to insulation, windings, reducedmachine life and thermal distort ion.

If torque required by the load exceeds the amount of torque produced, the motor will stall. Themotor will draw locked rotor current ratings, which are, on average, 3-6 times full load current.
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Figure 6 - Phase Unbalance and Overload Protection

This will lead to excessive heating of the windings and will cause the insulat ion to be damaged. Ifthe open circuit is present before the mot or start is attempted, it is unlikely that the mot or will beable to start rot ating.

The phase-unbalance relay used to protect against this scenario is similar in design to the stallrelay, but is set for about 20% of the full load current. A rough representat ion of the operation ofthe relay is included in Figures 6and 7for reference only.

If any one of the phases in the motor loses power, the heater will cool down. The bimetallic stripwill turn, causing the unbalance contacts to close and t he motor t o be tripped. This relay will alsoprotect against thermal overload, as the heaters cause the bimetallic strips to close the overloadtrip contact.

You will also see a compensating bimetal element, which will compensate for ambient temperaturechanges, thus preventing unnecessary trips.

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Resource:Science and ReactorFundamentals Electrical i CNSCTechnical Training Group


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Figure 7 - Phase Unbalance and Overload Protection

Electrical-Engineering-portal.com-Motor Protection Depending on Size and Voltage Level

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