Electrical Machines-I

UNIT III

DC Motors

Prepared by,

Mr.A.Venkadesan,

Assistant Professor,

EEE Department,

SRM University

MOTORS Function:

Motors are the electrical machines which convert electrical energy into mechanical energy.

Electrical Energy

Mechanical Energy

MOTORS

Motors

• Working Principle

– Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.

– The direction of induced emf can be found using Fleming’s left Hand Rule

dcmotor 4

Direction of force- Fleming’s left Hand Rule Or motor Rule

Illustration of Working Principle of DC Motor

No torque

Case I

Illustration of Working Principle of DC Motor

No torque

Case II

Illustration of Working Principle of DC Motor

Case III

Torque, motor rotates in anticlockwise direction

Types of Motor

DC Motors

Separately Excited

Self Excited

Shunt Series Compound

Long

Differential Cumulative

Short

Differential Cumulative

Shunt Motor and back emf

b

φpN ZE = ×

60 A

V>Eb

Back emf opposes the applied

voltage

N is small, Eb is less, Ia increases

As N increases, Eb increases,

Ia decreases

Eb-acts as self governor

Back emf

• Back EMF – it makes the motor to draw required amount of current from the supply.

Power Relationship

Condition for maximum power

Problem

• The field and armature resistance of 220V shunt machine are Rf=88ohm, Ra=0.05ohm.Calculate armature power developed when working

• (i)As a generator, delivering power of 22 kw

• (ii) As a motor, taking power of 22 kw.

Torque Equation

done by this force in one rev,

2

done 2 2

Time Taken 60 / 60

2,

60

2;

60

m

m e b a

Torque F r

Work

w F r

Work F r NPower F r

N

NPower P T

NP P E I T

Torque Equation

2

2

60

60

1

2

: ; tan

: saturation; Ia ; T

: saturation; is constant; T

b a

b

a

a

a

a

a

NE I T

ZNPE

A

PT ZI

A

T K I

Shunt T I cons t

Series before I

After I

Problem

• A dc shunt motor having Ra=0.24 ohm takes an armature current of 80 A at 300 V. It has 8 poles and 800 lap connected conductors. Flux per pole is 0.042wb. Calculate N and gross torque developed by armature (Ta).

Power stages in motor

Motor

input

Eb Ia

Motor

Output

Cu-Loss

Friction &

Iron Loss

Losses

• Rotational losses = friction + windage + iron loss

• Armature power-Rotational loss=Shaft power

• wTa-Pr=wTsh

• wTsh=wTa-Pr

• wTsh= Eb Ia-Pr

• Tsh = (Eb Ia-Pr /w)=Pout /w

Condition for maximum efficiency

• Constant loss = Variable loss

Problems

• A 4 pole 220 V lap connected DC shunt motor delivers 12 KW. N=1000 rpm, Ia=60 A, Ish = 2A, Z=500, Ra=0.12 ohm. Find

• Total torque

• Flux per pole

• Rotational losses

• Efficiency

• Assume 1V/brush for contact drop.

Characteristics of motor

• Electrical (Ia, Ta)

• Mechanical (Ia,N) and (Ta, N)

Shunt motor (Ta,Ia)

Shunt motor – (Ia,N)

Shunt motor-(Ta,N)

Series motor

Series motor

V is constant

Series motor cannot be

Started with no load.

Series motor

Compound motors

Compound motors

Compound motors

Applications

Applications

Starters

• Need

– Starters are used to reduce initial high current

Why motor draws high current during starting?

b a a

ba

a

E V I R

V EI

R

At start Back emf is zero, Ia is large

Two point starter

Speed control

a aV I RN

Speed control for shunt motor

• Field control

• Armature control-IaRa drop

• Applied voltage control

– Method 1

– Method 2 - Ward Leonard method

Shunt motor-Field control

•At start, field rheostat is kept at

Minimum position,

Field resistance is less

Field current is more, flux is more.

•Motor runs at rated speed

•Field resistance increases, Field

current decreases, flux decreases and

speed increases.

•Suitable for above the rated speed.

Shunt motor-Armature Control

Armature resistance is varied.

IaRa drop is varied.

If IaRa drop increases, speed decreases

If IaRa drop decreases, speed increases

Suitable for below the rated speed.

Applied voltage control method – method 1

Applied voltage control method – method 2

Speed control of series motor

• Field control

– Field divertor – above rated speed

– Armature divertor – Below Rated Speed

– Trapped Field Control

– Paralleling of field coils

• Variable Resistance in series with motor

Series Motor-field divertor

The series winding are shunted by a

variable resistance known as field diverter.

Any desired amount of current can be

passed through the diverter by adjusting its

resistance.

Hence the flux can

be decreased and consequently, the speed

of the motor increased.

Series Motor-Armature Divertor

A diverter across the armature can be

used for giving speeds lower than the

normal speed.

For a given constant load torque, if Ia

is reduced due to armature diverter, the

flux must increase (∵Ta=ɸIa ).

This results in an increase in current

taken from the supply (which increases

the flux and a fall in speed (Nα 1/ɸ).

The variation in speed can be

controlled by varying the diverter

resistance.

Trapped Field Control

Series Motor-Paralleling Field coils

Varying resistance in series with the armature

Thyristor based control

• Normally the three phase AC voltage is converted into pulsating dc voltage using three phase diode rectifier.

• The output DC voltage is filtered using the filter and it is given to DC motor.

• The filtered dc voltage obtained from the diode rectifier is fixed in magnitude.

Thyristor based control

• To get variable output dc voltage, thyristor (SCR) is used instead of diodes.

Thyristor

The gate signal applied to the gate is

varied (firing angle is varied)

to get variable output DC voltage.

Rectifier based on thyristor

Thyristor based rectifier fed DC motor

Thyristor (Rectifier)

With filter

Three phase

AC Supply M

Problem

Ia2=40A, N2=2930 RPM