In a version of the AC induction motor referred to as a wound rotor motor, slip rings are used not for transferring power, but for inserting resistance into the rotor windings. A wound rotor motor uses three slip rings — typically made of copper or a copper alloy - mounted to (but insulated from) the motor shaft. Each slip ring is connected to one of the three phases of rotor windings. The slip ring brushes, made of graphite, are connected to a resistive device, such as a rheostat. As the slip rings turn with the rotor, the brushes maintain constant contact with the rings and transfer the resistance to the rotor windings.
Adding resistance to the rotor windings brings the rotor current more in-phase with the stator current. (Recall that wound rotor motors are a type of asynchronous motor, in which the rotor and stator electrical fields rotate at different speeds) The result is higher torque production with relatively low current. The slip rings are only used at start-up, however, due to their lower efficiency and drop-off of torque at full running speed. As the motor reaches its operating speed, the slip rings are shorted out and the brushes lose contact, so the motor then acts like a standard AC induction (aka “squirrel cage”) motor.
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