In a rotor resistance starter, a star connected variable resistance is connected in the rotor circuit through slip-rings. The full voltage is applied to the stator windings. The connection arrangement of the rotor resistance starter is shown in the figure.
At the instant of starting, the handle of variable resistance (rheostat) is set to ‘OFF’ position. This inserts maximum resistance in series with each phase of the rotor circuit. This reduces the starting current and at the same time starting torque is increased due to external rotor resistance.
As the motor accelerates, the external resistance is gradually removed from the rotor circuit. When the motor attains rated speed, the handle is switched in the ‘ON’ position, this removes the whole external resistance from the rotor circuit.
Slip Ring induction motors are mainly used for driving high inertia loads or the loads which require a starting torque across a full speed range. Hence, by correctly selecting the starting resistors inserted into the rotor circuit, the maximum torque can be obtained from the motor at a relatively low starting current. Thus, the magnitude of starting torque is controlled by the value starting resistance.
The starting resistors need to be designed to allow for the starting duty cycle without overheating or too large change in the value of the resistance when hot.
Following are the chief advantages of the rotor resistance starter −
The disadvantages of the rotor resistance starter are given as follows −