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Electrical Machines Lab
Experiment-No.

ROTOR RESISTANCE SPEED CONTROL OF WOUND ROTOR

INDUCTION MOTOR

AIM: To vary the speed of the wound rotor induction motor using rotor rheostat
control.

Theory

The main advantage of wound rotor induction motor over the squirrel cage motor is
that  it  offers  one  more  degree  of  freedom  to  the  Engineers  for  starting  &  speed
control. Wound rotor Induction Motor is used where the load requirements are high
starting  toque  &  variable  speed,  or  where  the  motor  is  to  be  started  under  heavy
load.  Typical  applications  of  these  motors  are  crane  &  hoist  control.  Resister
controllers  in  the  rotor  circuit  are  used  to  achieve  smooth  start  &  speed  control.
Resent  investigations  have  shown  that  certain  desired  torque  speed  characteristics
can  be  achieved  by  insertion  of  relatively  simple  passive  frequency  sensitive
networks. Careful selection of network parameters leads to highly reduced starting
current  &  &  improved  torque/  current  ratio.  To  understand  the  industrial  systems
incorporating  the  wound  rotor  Induction  Motors  it  is  therefore  necessary  to  study
the effect of rotor impedance on the performance of Induction Motors.

In this method of speed control of three phase induction motor external resistance
are added on rotor side. The equation of torque for three phase induction motor is

The  three  phase  induction  motor  operates  in  low  slip  region.  In  low  slip  region
term (sX)2 becomes very  small as compared to R2. So, it can be  neglected. And
also E2 is constant. So the equation of torque after simplification becomes,

Now  if  we  increase  rotor  resistance,  R2  torque  decreases  but  to  supply  the  same
load torque must remain constant. So, we increase slip, which will further results in
decrease  in  rotor  speed.  Thus  by  adding  additional  resistance  in  rotor  circuit  we
can decrease the speed of three phase induction motor. The main advantage of this
method is that with addition of external resistance starting torque increases but this
method  of  speed  control  of  three  phase  induction  motor  also  suffers  from  some
disadvantages:

1. The speed above the normal value is not possible.
2. Large speed change requires large value of resistance and if such large value of

resistance is added in the circuit it will cause large copper loss and hence
reduction in efficiency.

3. Presence of resistance causes more losses.
4. This method cannot be used for squirrel cage induction motor.

From the

figure 1 it is clear how does the speed vary with the change of rotor

resistance.

The model used to simulate this experiment

Figure 2 simulation the wound rotor induction motor

Parameters of components
1- A synchronous machine block:
a- Configuration:

Preset model:  No
Mechanical input:  Torque Tm
Rotor type:  wound rotor
Reference frame:   Rotor
Mass units:   SI
b- Parameters:

Pn (VA):     2850
Vn (Vrms L-L):     380
f(Hz):   50
Another data are remain constant
c- Advanced:
Sample time: -1

3 phase source:
This block consists of three A.C. Voltage sources, as shown below:

1-  Phase A:
2-  V= K/ (sqrt (3)*sqrt(2))
3-  f= 50Hz
4-  Phase= 0 degree
5-  Sample time= -1
6-  Phase B:
7-  V= K/ (sqrt (3)*sqrt(2))
8-  f= 50Hz
9-  Phase= -120 degree
10-

Sample time= -1

11-

Phase C:

12-

V= K/ (sqrt (3)*sqrt(2))

13-

f= 50Hz

14-

Phase= -240 degree

15-

Sample time= -1

PROCEDURE:

1  - Open File ---> New---> Model.
2  - Open Simulink Library and browse the components.
3  - Connect the components as shown in the above circuit, figure 2.
4  - Simulate the circuit to start motor with rotor resistance starter.
5  – observe the waveforms of speed, torque, stator current and rotor current.

6 - The rotor resistance is varied and corresponding values of speed, voltage ,

currents and torque are noted down.

Rotor
Resistance

Voltage

(V)

Stator

current (A)

Rotor

current(A)

Speed

(rpm)

Torque

(N.m)

Report

1- Plot the relation between the rotor resistances with speed.
2- As you added resistance externally to the rotor circuit, what happened to

rotor speed? Why?

3- As rotor speed decreased, what happened to the torque output of the motor?

Why?

4- As rotor speed decreased, what happened to the current being induced into

the rotor circuit?