Page 707 - Basic Electrical Engineering

Slip of the motor, and hence the rotor speed can be changed by introducing,
i.e., injecting a voltage of slip frequency directly into the rotor circuit.
Depending upon the phase of the injected EMF with respect to the rotor

EMF, the slip, and hence the speed will change.

Figure 8.18 Speed control by using the consequent pole technique: (a) four-pole formation; (b) two-
pole formation

(d) Speed control by changing rotor-circuit resistance

In slip-ring induction motors, it is possible to add extra resistance in the rotor
circuit. To obtain high starting torque and also to control speed when the

motor is running, the extra resistance connected in the rotor circuit is
changed. Higher the rotor-circuit resistance, lower will be the speed obtained.
However, starting torque will increase with increase of rotor-circuit

resistance.

8.16 DETERMINATION OF EFFICIENCY