Rotor resistance starter

•    Slip ring induction motors are practically always started with full line voltage applied across the stator terminals. 

•    The value of starting current can be adjusted by introducing a variable resistance in the rotor circuit through slip rings

•    The external resistance connected in the rotor circuit is in the form of a rheostat connected in star.

•    The resistance gradually cut out of the rotor circuit as the motor picks up speed and finally the slip rings are short-circuited.

•    By increasing the rotor resistance, the rotor current is reduced at starting since impedance increases.

•    The protective devices, no-volt relay and over-load relay can be provided in this starter also.

The figure shows a rotor resistance starter with protective devices.

•    When the on button is pressed the no-volt relay energizes and closes the line contactor.

•    The full voltage is applied to the stator terminals.

•     When the on button is released, the relay gets supply through the contactor.

•    The rotor external resistance is cut step by step. Finally the slip rings are short-circuited. 

•    The brushes may be lifted from slip-rings surfaces when the motor runs at normal speed or during normal working conditions.

•    This is to avoid wear and tear and also to reduce the frictional losses.


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