Thermal Circuit.pdf

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10/28/2010 9:52 AM M.V.Ramana Rao EMD

Thermal Circuit

The process of energy transfer in the case of transformers and

electro-mechanical energy conversion in the ease of rotatingelectrical machines involves currents in the conductors, and

fluxes in the ferromagnetic parts.

Therefore there are I2R losses in windings and core losses in the

ferromagnetic cores. In addition losses occur in tank walls, endplates and covers on account of leakage flux.

The losses appear as heat and therefore the temperature of every

affected part of the machine rises above the ambient medium which

is normally the surrounding air.

The heated parts of an electrical machine dissipate heat into their

surroundings by conduction, and convection assisted by radiation.


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Conduction mode of dissipation of heat is important in the case

of solid parts of machine like copper, iron and insulation.

Consider two points in an electric circuit having potentials V1 andV2 the current flowing between them is

Similarly, we can write the equation for heat flow for conduction

between two surfaces separated by a heat conducting medium, as:

where R is the electrical resistance

of the conducting medium between them.


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The thermal resistance, like electrical resistance,

can be written as

Thermal Resistance.

The thermal resistance is defined as the thermal resistance whichcauses a drop of 10 C per watt of heat flow.

Equation permits heat conduction problems to be solved by

methods of calculation similar to those used in electric circuits.


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Considering above Eqn we find that a

material having a high value of thermal

resistivity will dissipate less amount ofheat or alternatively for the dissipation

of same heat the temperature rise will

be higher.


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Temperature rise with time (Heating and Cooling curves)

The temperature of a machine rises when it is run under steady

load conditions starting from cold conditions. The temperature atfirst increases at a rate determined by power wasted.

As the temperature rises, the active parts of the machine dissipate

heat partly by conduction, partly by radiation, and in most cases,

largely by means of air cooling.

The higher the temperature rise, the greater would be the effect of

these methods of cooling. Therefore, as the temperature rises, its

rate of increase falls of owing to better heat dissipating conditions.

As shown later, the temperature-time curve is exponential in nature.


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Thus the end windings have to transfer to the air, not only the heat

produced in them but also a part of the heat produced in the slot

portion of the winding.

The temperature of any part of a machine, not only depends on

the heat produced in itself but also on heat produced in other parts.

This is because there is always a heat flow from one part to

another for example, the heat produced in the part of the winding

embedded in the slot flows partially through the insulation to the

laminations partially to the end windings.


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However, it is worthwhile taking theory of heating of

homogeneous bodies as the basis for analyzing the process of

machine heating. The results obtained from such a theory are

applicable to a certain degree, to the different parts of machine as a

whole.

Electrical machines are not homogeneous bodies. Their parts are

made up of different materials like copper, iron and insulation.

These materials have different thermal resistivities and due to this,

it is rather difficult to calculate the temperature of a part of a

machine.


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Heating curve

Back


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The heat energy dissipated by the body into the ambient medium due

to radiation, conduction and convection,

If in the process of heating, the temperature of the surface rises by over the

ambient medium, at the instant considered.

def

( Heat energy stored in the body)

(Heat energy dissipated by the body)

As the heat developed in the machine is equal to the heat stored in

the parts plus the heat dissipated.

(Heat energy developed in the body)


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Solving the differential equation

where Kis the constant of integration.

The value of Kis found by applying the boundary condition,


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Substituting this value of K in Eqn.

When the machine attains final

steady temperature rise.


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The term has the dimensions of time and is called the

heating time constantTh.


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If the machine starts from cold conditions,

Above relation is the equation of temperature rise with time.The temperature rise- time curve is exponential in nature as shown

in Fig.

Heating curve.


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Cooling Curve

The value of Kis obtained by putting boundary conditions,


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If a machine is shut down, no heat is produced and so its final

steady temperature rises when cooling is zero

It is clear from above Eqn. that the cooling curve is also

exponential in nature as shown in Fig. above Eqn. is applicable to

machines which are shut down.

Cooling curve.

Thus we can define the cooling

time constant as the time takenby the machine for its temperature

rise to fall to 0.368 0f its initial

value.


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Problem :

?

Loss in strips

?


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To evaluate total heat dissipating surface area we required length of Each strip l

There are 8 strips in parallel

Total heat dissipating surface

75


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Suppose tis the time required to reach 99 per cent of the final steady temperature.

Required volume of 8 strips

t= 170.4 s


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Problem :

When Heating :- Since the transformer starts from cold conditions

therefore its temperature rise is given by Eqn.

1


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Problem :


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QUANTITY OF COOLING MEDIUM (COOLANT)

The quantity of cooling medium required to absorb losses of

machines can be calculated with the help of expressions derived

below:

Air


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Volume of air under actual working conditions is therefore


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The values for day air are

Substituting these values in above Eqn.


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Hydrogen

Proceeding as in the case of air :

Hfor hydrogen is 2000 2500 mm.


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Water

Oil


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Problem :

? Q


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Given :

volume of air


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Amount of water :


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Problem :


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Cooling of rotating machines

Bureau of Indian Standards (BIS 63621971) Designation of

Methods of Cooling of Rotating Electrical Machines defines termsconnected with cooling of Rotating Electrical Machines. Some of the

commonly used terms are explained below :

1. Cooling. A process by means of which heat resulting from

losses occurring in a machine is given up to a primary coolant

by increasing its temperature.

The heated primary coolant may be replaced by new coolant at

lower temperature or may be cooled by a secondary coolant in

some form of heat exchanger.

2. Primary Coolant. A medium (liquid or gas) which, by beingat a lower temperature than a part of a machine and in contact

with it, removes heat from that part.


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3. Secondary Coolant. A medium (liquid or gas) which, being at a

lower temperature than the primary coolant, removes the heat from

the primary coolant in a heat exchanger.

4. Heat Exchanger. A component intended to transfer heat from

one coolant to another while keeping the two coolants separate (i.e.

air cooled heat exchanger, water cooled heat exchanger, double

wall, ribbed tubes, etc.).

5. Inner Cooled (Direct Cooled) Winding. A winding which has

either hollow conductors or tubes which form an integral part of the

winding, through which the coolant flows.

7. Closed Circuit Cooling System. A method of cooling in which a

primary coolant is circulated in a dosed circuit through the machine,

and if necessary through a heat exchanger. Heat is transferred from

the primary coolant to the secondary coolant either through the

structural parts or in the heat exchanger.


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8. Dependent Circulating Circuit Component. A separate

component in the coolant circulating circuit which is dependent for

its operation on the operation of the main machine.

9. Independent Circulating Circuit Component. A separatecomponent in the coolant circulating circuit which is independent of

the operation of the main machine.

10. Integral Circulating Circuit Component. A component, in the

coolant circulating circuit which forms part of the machine, and which

can be replaced only by partially dismantling the main machine.

11. Machine Mounted Circulating Circuit Component. A

component in the coolant circulating circuit which is mounted on

machine, and forms part of it, but which-can be replaced without

disturbing the main machine.12. separately Mounted Circulating Circuit Component. A

component in the coolant circulating circuit which is associated with a

machine, but which is not mounted on or integral with it.


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Cooling System

According to BIS 4722-1968 Specification for Rotating Electrical

Machines the cooling systems are classified into three typesdepending upon the origin of cooling

1. Cooling System. The machine, is cooled by natural air currents

set up either by rotating parts or due to temperature differences. The

machine thus is cooled without the use of a fan by the movement ofair and radiation.

2. Natural Cooling. The machine is cooled by cooling air driven by

a fan mounted on the rotor or one driven by it.

3. Separate Cooling. The machine is cooled either by a fan notdriven by its shaft, or it is cooled by a cooling medium other than air

put into motion by means not belonging to the machine.


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Types of Enclosures

The problem of ventilation in rotating electrical machines is closely

linked with the types of enclosures used. The various types of enclosures used are

1. Open Machine. One in which there is no restriction to ventilation

other than that necessitated by good mechanical construction.

2. Open Pedestal Machine (OP). An open machine which has

pedestal bearing supported independently of the machine frame.

3. Open End-Bracket Machine (OEB). An open machine having

end-brackets of which the bearings form an integral part.

4. Protected Machine (P). A machine in which the internal rotating

parts and live parts are protected mechanically from accidental or

inadvertent contact, while ventilation is not materially impeded.


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5. Screen Protected Machine (SP). A protected machine in which

the ventilating openings are not less than 64.5 mm in area. Such

protection may be provided by screens of wire mesh, expanded

metal, perforated metal or other suitable covers. The use of openings smaller than 64.5 mm is not recognized, as such openings

are liable to become closed in service.

6. Drip-Proof Machine (DP). A protected machine in which the

openings for ventilation are so protected as to exclude vertically

falling water or dirt.

7. Splash-Proof Machine (SPLP). A protected machine in which

the ventilating openings are so constructed that drops of liquid or

solid particles falling on or reaching any part of them machine at

any angle between the vertically downward direction and 1000 fromthat direction cannot enter the machine, whether the machine is

running or at rest, by splashing, or otherwise, either directly or by

striking and running along a surface.


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8. Hose-Proof Machine (HSP). A protected machine so enclosed as

to exclude water whether the machine is running or at rest, when

washed by a hose having a 9.5 mm diameter nozzle with a

maximum pressure of 3.5 kg for a period not exceeding 30 seconds,

from a minimum distance of 1.8 metres.


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9. Pipe-Ventilated or Duct-Ventilated Machine. A machine in

which there is a continuous supply or fresh ventilating air, the frame

being so arranged that the ventilating air may be conveyed to and/orfrom the machine through pipes or ducts attached to the enclosing

case

(a) A pipe- or duct-ventilated machine may be one of the

followingthree types:

i) With provision for inlet duct only.(ii) With provision for inlet and outlet ducts.

(iii) With provision for outlet duct only.

(b) A pipe-ventilated or duct-ventilated machine may be cooled

by one of the following means

(i) Self-ventilation (PV).(ii) Forced-draught with air supplied by external pressure (PVFD).

(iii) Induced draught with air drawn through the machine by external

means (PVID).


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10. Totally Enclosed Machine (TE). A machine so constructed that

the enclosed air has no connection with the external air but is not

necessarily air-tight.

11. Totally Enclosed Fan-Cooled Machine (TEFC). A totally

enclosed machine with cooling augmented by a fan, driven by the

motor itself, blowing external air over the cooling surface and/or

through the cooling passages, if any, incorporated in the machine.

12. Totally Enclosed Separately Air-Cooled Machine (TESAC). A

totally-enclosed machine with cooling augmented by a separately-

driven fan blowing external air over the cooling surface and/or

through the cooling passages, if any, incorporated in the machine.

13. Totally Enclosed Water or other Liquid-Cooled Machine

(TEWC). A totally enclosed machine with cooling augmented by

water-cooled or other liquid-cooled surfaces embodied in the

machine itself.


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14. A Totally Enclosed Closed Air Circuit Machine. A totally

enclosed machine having special provision for cooling the enclosed

air by passing it through its own cooler, usually external to the

machine. The cooler may be of any recognized form using(i) Air (ii) Water (iii) Other suitable cooling medium.

15. Totally Enclosed Closed Gas Circuit Machine (CGGW). A

totally enclosed machine cooled by gas other than air, the coolinggas being circulated through associated water- cooled gas coolers.

16. Weather-Proof Machine (WP). A machine so constructed that it

can work without further protection from weather conditions

specified by the purchaser.


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17. Watertight Machine (WT). A machine so constructed that it

will withstand, without damage or sign of leakage, complete

immersion in water to a depth of not less than 1 rn, or subjection toan external water pressure of 0.1 kg/cm for a period of one hour. The

test for watertightness shall be made with the machine stationary and

the temperature of the machine shall not exceed the temperature of

the water in which it is immersed.

18. Submersible Machine. A machine capable of working for an

indefinitely long period when -submerged under a specified head of

water.


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INDUCED AND FORCED VENTILATION

Both self ventilation and separate ventilation may be subdivided into

two categories(i) Induced ventilation, (ii) Forced ventilation.


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RADIAL AND AXIAL VENTILATION

The ventilating systems can be classified into three types depending

upon how the air passes over the heated machine parts, as(i) Radial, (ii) Axial, (iii) Combined Radial and Axial.


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On the other hand, if a motor of higher power rating is used, the

motor is under utilized and therefore the economic efficiency of the

installation is reduced and the drive becomes expensive and has

large energy losses.

Over motoring(using a motor of higher rating than is required by

load) leads to higher capital costs and increased losses because of

lower efficiency at reduced load. In ac. drives, motors working at

reduced loads lead to poor power factor leading to uneconomic

loading of supply circuits and apparatus.

In order to select the motor power rating properly, it is not only

necessary to know the load under steady state conditions but also

the loads that are met with under transient conditions. For thispurpose use is made of Load Diagrams (Time sequence graphs)

which show the variation of motor torque, power and load current

as function of time.


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TYPES OF DUTIES AND RATINGS

The following are the types of duty as per IS 4722-1968

Specification for Rotating Electric Machinery

(i) continuous duty

(ii) short time duty

(iii) Intermittent periodic duty

(iv) Intermittent periodic duty with starting

(v) Intermittent periodic duty with starting and

braking

(vi) Continuous duty with intermittent periodicloading

(vii) Continuous duty with starting and braking

(viii) Continuous duty with periodic speed changes.

S1

S2

S3

S4

S5

S6

S7S8


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Continuous Duty (Duty S1) On this duty the duration of load isfor a sufficiently long time such

that all the parts of the motor attain

thermal equilibrium i.e. the motor

attains its maximum final steady

temperature rise.

Examples :

Running fans, pumps and other

equipment which operate for

several hours and even days at a

time.

The continuous rating of a motor may bedefined as the load that may be carried by

the machine for an indefinite time without

The temperature rise of any part

exceeding the maximum permissible

value.Back


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Intermittent Periodic Duty (Duty Type S3)

The simplified load for this type of duty is shown in Fig.

On intermittent duty the periods of

constant load and rest with machine de-

energized alternate. The load periods are too short to allow the motor

to reach its final steady state value while periods of

rest are also too small to allow the motor to cool

down to the ambient temperature.

This type of duty cycle is encountered

in cranes, lifts and certain metal cutting

machine tool drives.

The duty factor is determined on the basis

of a cycle 10 minutes long.

Back


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The Intermittent Rating of a motor applies to an operating condition

during which short time load periods alternate with periods of rest or

no load without the motor reaching the thermal equilibrium andwithout the maximum temperature rising above the maximum

permissible value.

In this duty the current does not significantly affect the temperature

rise. The duty factor for this operation is


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Intermittent Periodic Duty with Starting (Duty Type S4)

This type of duty consists of a sequence of

identical duty cycles each consisting of a

period of starting, a period of operation at

constant load and a rest period, the

operating and rest periods are too short to

obtain thermal equilibrium during one duty

as shown in Fig.

In this duty the stopping of the motor is

obtained either by natural deceleration after

disconnection of the electric supply or by

means of breaking such as mechanical brake

which does not cause additional heating of

windings.The duty factor is given by

Back


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The operating and rest periods aretoo short to obtain thermal

equilibrium during one duty cycle as

shown in Fig. In this duty braking is

rapid and is, carried out by electrical

means.

This type of duty consists of a

sequence of identical duty cycles each

consisting of a period of starting, a

period of operation at constant load, a

period of braking and a rest period.

Intermittent Periodic Duty with Starting and Braking (Duty Type S5)

The duty factor is

Back


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Continuous Duty with Intermittent Periodic Duty (Duty type S6)

The machines with excited windings

have normal no load voltage excitation

during the load period. The operation andno load periods are too short to attain

thermal equilibrium during one cycle as

shown in Fig.



period of operation at constant load and

period of operation at no load.

The duty factor is given by

Back


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Continuous Duty with Starting and Braking (Duty Type S7)

This type of duty consists of

sequence of identical duty cycleseach having a period of starting, a

period of operation at con load and a

period of electric braking.

There is no rest or de-energized

period. The load diagram is shown inFig.

The duty factor for this duty cycle : 1.

Back


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Continuous Duty with Periodic Speed Changes (Duty Type S8)

The operating period is too short to attain thermal

equilibrium during one duty cycle there being no rest

and de-energized period. This duty cycle is shown in

Fig.



period of operation at constant loadcorresponding to a determined speed of

rotation, followed immediately by a period of

operation at another load corresponding to a

different speed of operation.

The various duty factors are


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Methods Used for Determination of Motor Rating for

Variable Load Drives :-

It is clear from the above discussion that it is necessary to use

suitable methods to calculate the proper rating of motors for

variable load drives. The four commonly used methods are

(i) method of average losses(ii) equivalent current method

(iii) equivalent torque method

(iv) equivalent power method.


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Method of Average Losses


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The average losses are given by

The method of average losses

does not take into account the

maximum temperature rise

under variable load conditions.

However, this method is

accurate and reliable for

determining the average

temperature rise of the motor

during one work cycle. No

doubt the motor is subject to

short time


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Equivalent Current Method

This method also assumes that the constant losses are independent of

the load.


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The above method allows the equivalent current values to be

calculated with accuracy sufficient for practical purposes.


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Equivalent Torque and Equivalent Power Methods

The torque is directly proportional to current (assuming constant flux

and constant power factor) and therefore the equivalent torque is

The equation for equivalent power follows directly from above Eqn.

as power is directly proportional to the torque. At constant speed or

where the changes in speed are small, the equivalent power is given

by the following relationship

P bl


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An induction motor has to perform the following duty cycle

75 kW for 10 minutes,

No load for 5 minutes,45 kW for 8 minutes,

No load for 4 minutes,

which is repeated indefinitely. Determine a suitable capacity of a

continuously rated motor to perform the aforesaid duty. Motors of

standard (continuous) ratings of 45, 55. 75 kW are available. Theratio of maximum torque to nominal torque should be less than 1.8.

Problem :

The capacity of continuously rated motor to perform the above duty

cycle can be found by using equivalent power method.

Solution.


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= 51.8 KW

A motor with a standard rating of 55 kW is selected.Since the induction motor is practically a constant speed motor, the

ratio of maximum torque to nominal torque is equal to the ratio

maximum power to nominal power.

which is less than the maximum allowable value of 1.8.

E l D t i th t d t f t f f th


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Example : Determine the rated current of a transformer for the

following duty cycle 500 A for 3 minutes, a sharp increase 1000 A

and constant at this value for 1 minute, gradually decreasing for 2

minutes to 200 A and constant at this value for 2 minutes, graduallyincreasing to 500 A during 2 minutes and the repetition of the cycle.

Solution.

The load diagram

is plotted in Fig.


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The load diagram can be

divided into rectangles and

trapeziums (in this case) with

the help of line segments. Theequivalent current with sides

I1andI2is :

Equivalent current for the entire cycle is :

= 528.835 A


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