A Slip ring motor is a type of induction motor. The rotor windings are connected to a slip ring, which makes it different from the squirrel-cage rotor. The main advantage arises from higher torque while start-up and coincidentally lower current flow. Today slip ring motor are mostly superseded by induction motors with variable-frequency drive.

Simplified schematic

The working principle is quite the same like in squirrel-cage rotor. The rotor of the slip ring motor has more winding than the squirrel-cage rotor so that induction voltage is higher and the necessary current for the same field strength lower. Instead of a short circuit the rotor has typically 3 poles connected to the slip ring. At start these are wired with a variable power resistors and at full speed switched to short circuit.

While start-up the resistors leads to lesser weakening of the field strength from the stator. As a result the inrush current is reduced. Another important advance is higher start-up torque. The resistors also generate a phase shift in the field and thus the resulting magnetic force onto the rotor has a favorable angle.