A slip ring motor or wound-rotor motor is a type of induction motor

Induction motor

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...

. The rotor

Rotor

Rotor may refer to:*A rotating part of a mechanical device, for example in an electric motor, generator, alternator or pump.In engineering:...

 windings are connected to a slip ring

Slip ring

A slip ring is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels, or electrical rotary joints, are commonly found in electric motors, electrical generators for AC...

, 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.

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

Inrush current

Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases...

 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 Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.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.