Induction motor is the most frequently used AC motor in industry and home appliances. It works on the principle of electromagnetic induction. When supply is been connected with the stator winding, current flows through it inducing a rotating magnetic field. This rotating magnetic field interacts with the rotor winding resulting in the induction of current through the rotor winding. The rotor winding is been arranged in such a fashion that it becomes short-circuited. Due to the flow of current through the rotor winding, a lagging flux will induce in rotor winding, resulting in torque in the rotor winding. Usually induction motor has squirrel cage rotor windings.
Due to the low resistance of rotor winding, the starting current of the squirrel cage motor is very high resulting in a poor starting torque. To reduce the starting current and to improve the starting torque, it is important to somehow add some external resistance to the rotor winding at the start of the motor and then reduce it as motor pickup speed and finally remove it when motor attains its maximum speed.
This task of adding external resistance to the rotor winding of the induction motor is been achieved by means of slip rings. One end of slip rings is been connected with the rotor winding with the help of brushes while the other end of the slip rings is connected to the rheostat resistors.
The figure below shows the schematic of the induction motor with slip rings and external resistances. This adjustable resistance overcomes the limitation of starting high current and poor initial torque issues of induction motor. However, maintenance requirements and cost of the motor increases due to added components and brushes. Nevertheless, this enables the induction motor to be used in very high starting torque requirement applications like weight lifting using ropes or chains in cranes and lifts.