Wound rotor motor

During the start-up a typical rotor has 3 poles connected to the slip ring.

When the motor reaches full speed the rotor poles are switched to short circuit.

Another important advantage over squirrel-cage motors is higher starting torque.

Certain types of variable-speed drives recover slip-frequency power from the rotor circuit and feed it back to the supply, allowing wide speed range with high energy efficiency.

Doubly-fed electric machines use the slip rings to supply external power to the rotor circuit, allowing wide-range speed control.