Slip ring induction motor explained

Slip ring induction motor - symbol

A slip ring induction motor, also referred to as wound rotor motor, is a variation of the three-phase induction motor, designed to provide high starting torque for loads with high inertia, while requiring low starting current.

The stator of a slip ring motor corresponds to a typical squirrel cage induction motor. However, the rotor is different - it consists of a three-phase winding, with each terminal connected to separate slip rings.
The slip rings are in contact with an external secondary circuit via carbon brushes, in which resistors can be connected in series .

This additional resistance causes the rotor current to run more in phase with the stator current, which leads to a high torque.
In addition to a very high starting torque, you also have a low starting current.

Here you can see how different resistances in series to the rotor affect the torque:

Slip ring induction motor explained

Use of these slip ring motors

Slip ring motors are mainly used in the high power range, i.e. for loads with high moments of inertia and load torques. Particularly for powers from approx. 600 kW and operating voltages from 6000 V, these motors are also found in new plants. Since large converters are disadvantageous in weak power grids, slip-ring induction motors show here their advantages .

Examples would be:  Mills, cranes, stone crushing machines, cable cars, etc..