A wound rotor induction motor (WRIM) is a type of induction motor where the rotor windings are connected through slip rings to external resistances. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound rotor induction motors can be started with low inrush current by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.
Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns. The induced voltage is then higher and the current lower than for a squirrel-cage rotor. During start-up, a typical rotor has three poles connected to the slip ring. Each pole is wired in series with a variable power resistor. When the motor reaches full speed, the rotor poles are switched to short circuit. During start-up, the resistors reduce the field strength at the stator. As a result, the inrush current is reduced. Another important advantage over squirrel-cage motors is higher starting torque.
A wound rotor induction motor can be used in several forms of adjustable-speed drive. 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. Today, speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.