Rotor Resistance Control is also one of the methods by which we can control the speed of the Induction motor. The speed of the wound induction motor can be controlled by connecting an external resistance in the rotor circuit through slip rings. This method is not applicable to cage rotor induction motor.
As we know that the maximum torque is independent of the rotor resistance, yet the accurate location of the maximum torque Ʈmax is dependent on it. The larger the value of the resistance, larger will be the value of the slip at which the maximum torque occurs.
If the resistance of the motor is increased, then the pull out speed of the motor decreases. But the maximum torque remains constant. Thus, by Rotor Resistance Control method, the speed control is provided by the rated speed to the lower speeds.This method of speed control is very simple. It is possible to have a large starting torque, low starting current and large values of the pullout torque at a small value of slip.
The Major Disadvantages of the rotor resistance control method are that the efficiency is low because of the additional losses present in the resistors connected in the rotor circuit. The efficiency is greatly reduced at low speeds because of the higher value of the slip. This method of speed control is used in Cranes, Ward Leonard drives and other intermittent load applications because of the low cost and high torque capability at the lower speed.
This speed control method can also be used in fans or pump drives, where speed variation over a small range near the maximum or top speed is required.