A slip ring motor or wound-rotor motor is a type of induction motor. The rotor windings are connected to a slip ring, connected to external resistances, which allows controlling the speed/torque characteristic of the motor. Wound-rotor 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. [1]
The rotor of the slip ring motor has more windings than a squirrel-cage rotor so that induction voltage is higher and the current for the same field strength lower. During the start-up a typical rotor has 3 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 becoming a standard squirrel-cage motor. During start-up the resistors reduce the field strength in the stator. As a result the inrush current is reduced. Another important advantage over squirrel-cage motors is higher start-up torque.
A wound-rotor motor can be used in several forms of adjustable-speed drive. Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.
Doubly fed electric machines use the slip rings to supply external power to the rotor circuit, allowing wide-range speed control.
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.
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