We know, T α (s E22 R2)/(R22 +(s X2)2)
For low slip region (s X2)2 << R2 and can be neglected and for constant supply voltage is also constant.
... T α ( s R2)/R22 α s/R22
Thus if the rotor resistance is increased, the torque produced decreases. But when the load on the motor is same, motor has to supply same torque as load demands. So motor reacts by increasing its slip to compensate decreases in T due to R2 and maintains the load torque constant. So due tot additional rotor resistance R2, motor slip increases i.e. the speed of the motor decreases. Thus by increasing the rotor resistance R2, speeds below normal value can be achieved. Another advantage of this method is that the starting torque of the motor increases proportional to rotor resistance.
The Fig. 1 shows the torque-speed curves for rotor resistance control.
But this method has following disadvantages :
1. The large speed changes are not possible. This is because for large speed change, large resistance is required to be introduced in rotor which causes large rotor copper loss due to reduce the efficiency.
2. The method can not be used for the squirrel cage induction motors.
3. The speeds above the normal values can not be obtained.
4. Large power losses occur due to large loss.
5. Sufficient cooling arrangements are required which make the external rheostats bulky be expensive.
6. Due to large power losses, efficiency is low.
Thus the method is rarely used in the practice.
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