Theory of Electrical Machines EXIT Course Title: Theory of Electrical Machines Course Code: EE 359...

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Theory of Electrical MachinesTheory of Electrical Machines EXIT

Course Title: Theory of Electrical Machines

Course Code: EE 359

Topic : D C Machines

Branch: ME Semester: V

Text Books:1.S. Chapman, Electric Machinery Fundamentals, 4th Ed., McGraw-Hill, 2003.2.R. K. Rajput, Electrical Machines, 3rd Ed., Laxmi Publications (P) Ltd., 2003

D.C Machine1.Introduction

2.Construction

3.Principle of Operation

4.Types of DC Machines

5.Characteristics

6.Applications

DC MACHINESDC MACHINES

INTODUCTIONINTODUCTION

A DC machine is an electro-mechanical energy conversion device. It can convert Mechanical power into Electrical Power. When output electrical power is DC , it is called DC Generator. When it converts DC electrical power into mechanical power , it is known as DC Motor.

MAIN CONSTRUCTIONAL FEATURES

3. FIELD or EXCITING COILS

1. BODY OR MAGNETIC FRAME OR YOKE

2. POLE CORE AND POLE SHOES

4. ARMATURE CORE

5. ARMATURE WINDING

6. COMMUTATOR

MAIN CONSTRUCTIONAL FEATURESMAIN CONSTRUCTIONAL FEATURES

9. BEARINGS

7. BRUSHES

8. END HOUSINGS

10. SHAFT

Body / Yoke

Field Winding

Commutator Armature

PulleyBrush

Brushholder

Field Core

Bearing

End Housing

YOKEARMATURE

COMMUTATOR

BRUSHFIELD POLE

& COIL

The outer cylindrical frame to which main poles and inter poles are fixed and by means of the machine is fixed to the foundation is called YOKE.

1. MAGNETIC FRAME or YOKE :

It serves two purposes:

a) It provides

mechanical protection

to the inner parts of the

machines.

b) It provides a low

reluctance path for the

magnetic flux.

The yoke is made of

cast iron for smaller …

machines and cast steel

or fabricated rolled

steel for larger

machines.

The pole core and pole shoes are fixed to the yoke by bolts. They serves the following purpose :

a) They support the field or exciting coils.

2. POLE CORE AND POLE SHOES :

POLE CORE

b) They distribute the magnetic flux on the armature periphery more uniformly.

c) The pole shoes have

larger X- section, so,

the reluctance of the

magnetic path is

reduced. The pole core

and pole shoes are

made of laminated

steel assembled by

riveting together under

hydraulic pressure.

Field coils or exciting coils are used to magnetise the pole core. Enameled copper wire is used for the construction of these coils.When direct

3. FIELD or EXCITING COILS :

current is passed through these coils/ winding, it sets up the magnetic field which magnetise the pole core to the reqd. flux.

3. FIELD or EXCITING COILS :

Armature is a rotating part of the DC

machine, reversal of flux takes place,

so hysteresis losses are produced.

To minimise this loss, silicon steel is

used for the construction.

4. ARMATURE CORE:

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The rotating armature cuts the main

magnetic field , therefore an e.m.f is

induced in the armature core.This e.m.f

circulates eddy currents in the core

which results in eddy current loss in it.

4. ARMATURE CORE:

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The armature core is laminated to reduce

the eddy current loss.Armature core serves the following purposes:a) It houses the conductors in the slots.b) It provides an easy path for magnetic flux

4. ARMATURE CORE:

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The no. of conductors in form of coils placed in the slots of the armature and suitably inter connected are called winding .

5. ARMATURE WINDING ;

ARMATURE WINDING

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This is the

armature winding

where conversion

of power takes

place i.e. in case

of generator ,

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mechanical power

is converted into

electrical power

and in case of a

motor, electrical

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power is

converted into

mechanical

power.

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Depending upon the types of inter connection. of coils , the winding can be classified into two types;i) Lap Winding;

The conductors/coils are connected in such a way that no of parallel paths are equal to no. of poles.

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If machine has ‘P’ no. of poles and ‘Z’ no. of conductors, then there will be ‘P’ no. of parallel paths.And each path will have ‘Z/P’ no of conductors in series. Also the no. of brushes are equal to no. of parallel paths.

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Out of which half of the brushes will be positive and remaining will be negative.ii) Wave Winding;

The conductors are so connected that they are divided into two parallel paths only , irrespective of the no. of poles. If machines has ‘Z’ no. of …

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conductors, there will be only two parallel paths and each will be having ‘Z/2’ no. of conductors connected in series with only two brushes.

Click here to study detailed contents of winding

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6. COMMUTATOR

It is the most important part of a DC machine and serves the following purpose :- i) It connects …

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6. COMMUTATOR

the rotating armature conductors to the stationary external circuit through the brushes.

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ii) It converts altering current induced in the armature conductors into unidirectional …..

COMMUTATOR

6. COMMUTATOR

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6. COMMUTATOR

current in the external load circuit in generating action and it converts alternating torque into unidirectional

COMMUTATOR

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6. COMMUTATOR COPPER SEGMENTRISER

END RING

ADJUSTING NUT

METAL SLEEVESHAFT

MICA INSULATION

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6. COMMUTATOR

torque produced in the armature in motoring action. The commutator is of cylindrical shape and is made of wedge shaped hard drawn copper segments.The segments are insulated from each ….

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6. COMMUTATOR

other by a thin sheet of mica.The segments are held together by means of two V-shaped rings that fit into the V-grooves cut into the segments. Each armature coil is connected to the commutator segment through riser.

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7. BRUSHES

Brushes are made of high grade carbon.They form the connecting link between armature winding and the external circuit. The brushes are held in particular position around the commutator by brush holders.

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8. END HOUSINGS

They are attached to the ends of main frame and support bearing . The front housing supports …..

END HOUSING

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8. END HOUSINGS

the bearing and the brush assembly whereas rear housing supports the bearing only.

END HOUSING

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9. BEARINGS

The function of the bearing is to reduce

friction between the rotating and

stationary parts of the machines.These

are fitted in the end housings.

Generally, high carbon steel is used for

the construction of the bearings.

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10. SHAFT

The function of

shaft is to transfer

mechanical power

to the machine or

from the machine . SHAFT

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Shaft is made of

mild steel with

maximum breaking

strength. All the

rotating parts likeSHAFT

10. SHAFT

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Armature

core, ,commutator,

cooling fan etc. are keyed

to the shaft.

10. SHAFT

Principle of OperationPrinciple of Operation

Fig. 1

DC MACHINES

FIELDPOLES N

SFIELD

WINDING

ARMATURE CONDUCTORS

ARMATURE

MAIN FIELD AXIS

BRUSH AXIS + _

DC MACHINES

The field windings are shown as

excited from external source. The

polarity of electro-magnetic field will

depend upon the direction of field

current as shown in the fig.

Fig. 2

DC MACHINES

MECHANIC

( MOTOR )

vDCSUPPLY

APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS

DC MACHINES

The armature carries conductors in

side the slots.Two brushes are placed

at the right angle to the main field axis.

The brushes are stationary whereas

armature is free to rotate.

APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS

DC MACHINES

When the armature is rotated in the

magnetic field, an e.m.f will be induced

in the armature conductors.The

direction of the induced e.m.f can be

found by applying Fleming’s Right

Hand Rule.

DC MACHINES

The direction of induced e.m.f will depend upon the direction of rotation of armature , if polarity of field poles to be kept unchanged.When load is connected across the armature terminals , the current will flow through the armature circuit.

DC MACHINES

The direction of current will be same as that of induced e.m.f. The armature will now be considered as electro-magnet and its polarity is shown in the fig. 2 .The electro-magnetic torque Te will be developed in the anti-clock wise direction as shown in the fig. no. 1 and 2.

DC MACHINES

The magnitude of Te will depend on the

strength of the field poles and armature field which further depends upon the currents flowing through the respective windings. As the external load on the generator is increased, the magnitude of Te increases.

DC MACHINES

As Te acts in the opposite direction to

the applied mechanical torque, more

torque will be required through the

prime mover to maintain the speed of

armature .

DC MACHINES

The direction of currents in the upper

conductors in the armature are

indicated by ‘dots’ and conductors in

lower half of armature are indicated by

‘cross’ .

DC MACHINES

The brush ‘B’ will collect dot currents

and brush ‘A’ will collect cross

currents as the armature continues to

rotate in clockwise direction. In the out

put circuit, across terminals ‘A’ and ‘B’,

current will flow in one direction.

DC MACHINES

The dc machine shown in fig. 2 is

working as Generator. The same

machine will work as motor , if the

armature is provided with electric

supply as shown in fig. 3.

DC MACHINES

( GENERATOR )

ELECTRICALLOAD

DC MACHINES

The armature is connected across a

supply voltage ‘V’ and the field

windings are excited from the same

supply or from any external dc source.

The magnetic polarities due to the

current in armature winding will be as

shown in fig.3.

DC MACHINES

The electro-magnetic torque Te will be

developed in the anti-clockwise

direction as opposite poles of armature

field and main field will attract each

other. The armature will rotate in anti-

clockwise direction due to Te .

DC MACHINES

To reverse the direction of rotation of armature, either the direction of current in the field winding or armature winding will have to be reversed.If the direction of currents in both the windings are reversed, direction of rotation of armature will be unchanged.

DC MACHINES

As the mechanical load on the armature i.e. rotor shaft represented by load torque TL is increased, more and more electro-magnetic torque will be developed by the armature to balance the mechanical torque requirements for which the armature will draw more current from the supply mains.

MAGNETIC FIELD

B P+ _

MAGNETIC FIELD

A P+ _

MAGNETIC FIELD

DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS )

TYPES OF DC TYPES OF DC MachinesMachines

Depending upon the type of excitation

to the field winding. The dc machine

can be classified into three categories:

1.Machines with permanent field,

2.Separately excited

3.Self excited type

TYPES OF EXCITATIONS TYPES OF EXCITATIONS ( DC MOTORS )( DC MOTORS )

Dc motors with permanent magnetic

field, are manufactured for small rating

applications such as toys, cassette tape

recorders etc.large rating dc motors …

TYPES OF EXCITATIONS TYPES OF EXCITATIONS ( DC MOTORS )( DC MOTORS )

are constructed with electro-magnetic

field i.e field winding is placed on the

field core and this winding is supplied

with dc current called excitation.

Depending upon the type of

connections to the field winding for ….

TYPES OF TYPES OF DC MOTORS DC MOTORS

The dc motors can be classified into

two categories ;

1) Separately excited dc motors

2) Self excited dc motors.

SEPARATELY EXCITED DC MOTORSSEPARATELY EXCITED DC MOTORS

MSupply

• The field winding is excited from a

supply which is not connected to the

armature winding. It may be noted

that current flowing through the field

winding is independent of load and

is equal to V / Rf , where Rf is the….

field circuit resistance. The flux

produced is proportional to the field

current i.e. Ø If

SUPPLYERa

SELF EXCITED DC MOTORS ( DC SHUNT MOTORS )SELF EXCITED DC MOTORS ( DC SHUNT MOTORS )

SELF EXCITED DC MOTORS SELF EXCITED DC MOTORS ( DC SHUNT MOTORS )( DC SHUNT MOTORS )

In this type of excitation , armature and

field windings are connected across a

constant source of supply. The field

current If is drawn from the same

source as that of armature current. As

shown in fig.

SELF EXCITED DC MOTORS ( DC SERIES MOTORS )SELF EXCITED DC MOTORS ( DC SERIES MOTORS )

M SUPPLY

The field winding is connected in

series with the armature so that If = Ia =

IL . Therefore field winding is made up

of thick winding wire of less no. of

turns as compared to that of shunt field

winding so that armature current can ...

SELF EXCITED DC MOTORS SELF EXCITED DC MOTORS ( DC SERIES MOTORS )( DC SERIES MOTORS )

flow through it without overheating. In

case of dc series machine , Ø If Ia ..

The relationship between induced

e.m.f. and terminal voltage is as follows

; …..See Fig.

V = E + Ia Ra + Ia Rse

orE = V - Ia ( Ra + Rse )

Ia = Ise = ILand

SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS )SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS )

RshSUPPLY

SELF EXCITED DC MOTORS SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS )( DC COMPOUND MOTORS )

There are two field windings , namely a

shunt field winding and a series field

winding. The shunt field winding is

connected in parallel with the armature

and series field winding is connected

in series with the combination .

Series field winding will carry a large

armature current Ia or IL and therefore it

is made of wire of large cross section

and has a few turns only. The

resistance of series field winding is

very small.

The shunt field winding is made up of

wires of small cross section and has

high resistance. Since the resistance of

shunt field winding is high , the current

flowing through it is very small as

compared to that of series field

winding

or armature current Ia . The main

magnetic field flux is produced by the

shunt field current / winding but it is

modified by the field of series winding.

A compound machine therefore

combines the best features of dc

shunt machines and dc series

machines.

Depending up on the connections of

shunt field winding in the combination

of armature and series field winding, dc

compound generators can be named

i) Short shunt compound generators.

ii) Long shunt compound generators.

SHORT SHUNT TYPE ( DC COMPOUND MOTORS )SHORT SHUNT TYPE ( DC COMPOUND MOTORS )

RshSUPPLY

SHORT SHUNT TYPE SHORT SHUNT TYPE ( DC COMPOUND MOTORS )( DC COMPOUND MOTORS )

In this case the shunt field winding is

connected across the armature winding

only as shown in the fig of slide no.

i) SHORT SHUNT DC COMPOUND MOTORS

Ise = IL = Ia + Ish

V = E + Ia Ra+ Ise Rse

= E + Ia Ra+ ( Ia + Ish ) Rse

LONG SHUNT TYPE ( DC COMPOUND MOTORS )LONG SHUNT TYPE ( DC COMPOUND MOTORS )

RshSUPPLY

LONG SHUNT TYPE LONG SHUNT TYPE ( DC COMPOUND MOTORS )( DC COMPOUND MOTORS )

In this case the shunt field winding is

connected across the combination of

armature and series field winding as

shown in the fig. Ise = Ia and IL= Ia + Ish

V = E + Ia Ra+ Ise Rse

= Ish Rsh

ii) LONG SHUNT DC COMPOUND MOTORS

Depending upon the direction of flow of

current through series field, we can

classify dc compound motors into two

categories namely ;

I) Cumulative compound dc motors

II) differential compound dc motors

CUMULATIVE TYPE ( DC COMPOUND MOTORS )CUMULATIVE TYPE ( DC COMPOUND MOTORS )

RshSUPPLY

Ø = Øsh + Øse

CUMULATIVE TYPE CUMULATIVE TYPE ( DC COMPOUND MOTORS )( DC COMPOUND MOTORS )

The direction of current in the series

field winding is such that magnetic field

produced by it is in the direction to that

of shunt field. Total magnitude of the

field is the sum of shunt field and series

field so that Ø = Øsh + Øse. .

DIFFERENTIAL TYPE ( DC COMPOUND MOTORS )DIFFERENTIAL TYPE ( DC COMPOUND MOTORS )

RshSUPPLY

Ø = Øsh - Øse

DIFFERENTIAL TYPE DIFFERENTIAL TYPE ( DC COMPOUND MOTORS )( DC COMPOUND MOTORS )

The direction of current in the series

field winding is such that magnetic field

produced by it is in the opposite

direction to that of shunt field. Total

magnitude of the field is the difference

of shunt field and series field so that

Ø = Øsh - Øse.

FACTORS DETERMINING THE SPEED OF DC MOTOR

FACTORS DETERMINING THE SPEED OF DC MOTORFACTORS DETERMINING THE SPEED OF DC MOTOR

The expression for back e.m.f.

developed in the armature of a dc motor

is given as follows :

P Ø Z N 60 A

E = …..(i)

E = V - Ia Ra …..(ii)

P Ø Z N 60 A

= V - Ia Ra

Comparing expressions (i) and (ii)

K Ø N = V - Ia Ra OR

N = V - Ia Ra

Where K is the constant of

proportionality and equal to PZ / 60 A

Now in the above expression for speed,

the speed can be varied by varying the

applied voltage ‘V’ , field flux Ø and

resistance of the armature .

It is clear that speed is directly

proportional to the supply voltage ‘V’. So

the speed increases with increase in

voltage ‘V’ and vice versa.

The speed is inversely proportional to

the field flux Ø . So speed decreases as

the Flux Ø increases and vice versa.

PERFORMANCE AND CHARACTERISTICS OF DC MOTORS

The important characteristics of dc

motors are :

1) Speed - armature current ( Load )

characteristics

2) Torque - armature current ( Load )

characteristics

3) Speed - Torque characteristics

CHARACTERISTICS OF CHARACTERISTICS OF DC MOTORSDC MOTORS

It is very much important to know the

characteristics mentioned above for

different types of dc motors because it

enables the selection of a specific type

of dc motor for specific purpose.

CHARACTERISTICS OF CHARACTERISTICS OF DC MOTORSDC MOTORS

CHARACTERISTICS OF DC SHUNT MOTORSCHARACTERISTICS OF DC SHUNT MOTORS

SUPPLYERa

FULL LOAD

0( Amps)

For a dc motor , we know that ;

N = V - Ia Ra

A dc shunt motor is connected across

the mains having supply voltage ‘V’ .

FULL LOAD

0( Amps)

1. Speed - Armature current (Load ) characteristics

CHARACTERISTICS OF CHARACTERISTICS OF DC SHUNT MOTORSDC SHUNT MOTORS

This supply voltage is assumed to be

constant.The field winding is connected

across the armature as shown in Fig.

The magnetic flux Ø produced by field

current If will be constant as V remains

constant.

Speed - Armature current (Load ) characteristics

But in actual practice, the air gap flux is

slightly reduced due to the effect of

armature reaction. From the expression

for the speed mentioned earlier, it is

evident that as the armature current Ia …...

increases , speed will decrease by a

small amount due to an increase in Ia Ra

drop is very small as compared to V .

The speed verses armature current

characteristics is shown in Fig.

The shunt motor being thus more or less

a constant speed motor , can be used in

the applications such as driving of line

shafts, lathes conveyors etc.

0( Amps)

2. Torque - Armature current (Load ) characteristics

The equation for torque can be written as

follows ; T = kt Ø Ia

If flux Ø is taken as constant, the torque

T becomes directly proportional to

armature current (Load current) Ia . It is a

straight line passing through the origin.

( N-m)

3. Speed - Torque characteristics

And The relationship between speed and torque can be drawn as shown in Fig .

N = V - Ia Ra

The relation between T and Ia and N

and Ia are as under ;T = kt Ø Ia

CHARACTERISTICS OF DC SERIES MOTORSCHARACTERISTICS OF DC SERIES MOTORS

M SUPPLY

CHARACTERISTICS OF CHARACTERISTICS OF DC SERIES MOTORSDC SERIES MOTORS

1. Speed - Load characteristics

0Ia ( Amps)

From the expression ; N = V - Ia Ra

It is seen that the speed N is inversely

proportional to flux Ø

For a dc series motor ,magnetic flux Ø is

proportional to Ia.

Thus , if V is constant, N is inversely

proportional to Ia. The N verses Ia

characteristics is therefore a rectangular

hyperbola as shown in Fig . It is seen

from the characteristics that ….

the speed decreases as the load on the

motor increases. At a very low load , the

speed is dangerously high . Thus if a dc

series motor is allowed to run on very

light load or at No- Load , its speed will

become much higher than its ……..

normal speed which may cause damage

to the motor. For this reason , dc series

motors are never started on No- Load

and are not used in the applications

where there is a chance of Load being

completely removed , when the motor ...

remains connected to the supply. The

load on the dc series motor is connected

through the gears and not through the

belt pulley arrangement.This is because,

in case of failure of belt , the load will be

removed from the motor and thereby the

motor will attain a dangerously high

speed . In case of load connected

through the gears, however in the event

of an accidental release of load, gears

will provide some load on account of the

frictional resistance of the gear teeth.

1. Torque - Load characteristics

0Ia ( Amps)

SATURATION OF SERIES FIELD CORE

The equation for the torque for dc motor

is given by ; T = kt Ø Ia

The magnetic flux for a dc series motor is

proportional to armature current Ia. Thus

the torque T = kt Ia Ia.

Or T Ia2

1. Torque - Load characteristicsCHARACTERISTICS OF DC SERIES MOTORSCHARACTERISTICS OF DC SERIES MOTORS

The relationship between torque and

armature current , is therefore of the form

of a parabola . With increase in Ia , the

field flux increases linearly but due to

saturation of the core, beyond a certain

magnitude of Ia the increase in flux is

negligible.. Thus T is proportional to the

square of Ia up to the saturation point

beyond which T varies linearly with Ia.

From the torque load characteristics , it

can be observed that a dc series motor ..

started on-load , develops a very high

starting torque.

Hence dc series motors are used in

applications where high starting torque

is required such as in electric trains ,

hoists, trolleys etc.

1. Torque - Speed characteristicsCHARACTERISTICS OF DC SERIES MOTORSCHARACTERISTICS OF DC SERIES MOTORS

From the characteristics shown in slide

no. , it can be seen that for low speeds ,

the torque is high and for high speeds

the torque is very small. This is why dc

series motor is widely used in the … ..

applications where motor is to be started

on bulk loads such as electric loco-

motive.

CHARACTERISTICS OF DC COMPOUND MOTORSCHARACTERISTICS OF DC COMPOUND MOTORS

RshSUPPLY

1. Speed - Load characteristicsCHARACTERISTICS OF DC COMPOUND MOTORSCHARACTERISTICS OF DC COMPOUND MOTORS

Ia ( Amps)

DIFFERENTIALCOMPOUND

CUMULATIVECOMPOUND

In cumulative compound motors , series

field winding is connected in such a way

that magnetic flux produced by it helps

the flux produced by shunt field winding.

Series field is directly proportional to the

load current , ….. . … .

therefore total flux increases with

increase in load current / armature

current due to the series field in addition

to the voltage drop in the armature

winding.The speed of dc motor is

inversely proportional to the . .. . ..

total main flux Ø . Therefore speed drops

more sharply as compared to dc shunt

motor.Refer Fig.

See Fig.

0Ia ( Amps)

SHUNTCUMULATIVECOMPOUND

The torque developed by a cumulative

compound motor increases with sudden

increase in load and at no-load , it has a

definite speed. Cumulative compound

motors are therefore, suitable where

there is sudden application . . . . . . .

of heavy loads like sheers, punches,

rolling mills etc. The speed of differential

compound motors remains more or less

constant. With increase in load but its

torque decreases with load. Since the dc

shunt motor develops a good torque and

its speed does not vary appreciably with

increase in load, differential compound

motors are not preferred over dc shunt

motors and hence are rarely used.

T (N-m)

CUMULATIVECOMPOUND

APPLICATION OF DC MACHINES

(i) Shunt motors are used in situations, such as driving a line shafting etc. where the speed as to be maintained approximately constant between no-load and full-load.

APPLICATIONS OF DC MOTORS

(a) DC SHUNT MOTORS

(ii) In situations where variable load is to be driven at different speeds but at each load, the speed is to be kept constant. Such as driving a lathe.

(a) DC SHUNT MOTORS

DC Series Motors are used in applications such as driving hoists, cranes, trains, etc., as in these cases a large starting torque is required. They are also used where the motor can be permanently coupled to the load, such as

(b) DC SERIES MOTORS

Fans , whose torque increases with speed. Where constancy in speed is not essential, the decrease of speed with increase of load has the advantage that the power absorbed by the motor does not increase as rapidly as the torque .

Series motors acquire very high speed at no-load or at very light load . That is why they should not be used for a belt drive where there is a possibility of the load decreasing to very small value.

DC Compound Motors are used in application where large starting torque are required but where the load may fall to such a small value that a series motor would reach a dangerously high speed.

(c) DC COMPOUND MOTORS

Where the supply voltage may fluctuate , for instance on a traction system, the series winding reduces the fluctuation of armature current partly by its inductance and partly by its influence on the value of

flux and therefore on that of the induced e.m.f.

When the load is of a fluctuating nature , e.g. for driving stamping processes, etc. the shunt excitation prevents the speed …

Becoming excessive on light load, and the decrease of speed with increase of load enables the flywheel, usually fitted to such a machine, to assist the motor in in dealing with the peak load by giving

up some of its kinetic energy.

* THANKS *

Theory of Electrical Machines EXIT Course Title: Theory of Electrical Machines Course Code: EE 359...

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