Brush lifting system for wound rotor induction motors

Wound Rotor Induction Motors have the rotor windings connected to an external resistance through a brush/slip ring assembly. When the brushes are in continuous contact with the slip rings during normal operation of the motor, the brushes and the slip rings experience premature wearing. This leads to frequent stops for maintenance and brush replacements. Once the motor reaches the normal operation speed, there is no need to have the external resistance in the circuit and so the brushes-to-slip rings contact can be ceased. Disengaging the brush/slip ring assembly during rated operation avoids wearing of brushes and collector rings. This also allows longer usage time of the machine. In this paper a novel method of brush lifting and shorting the rotor winding is discussed along with key design challenges and reliability requirements.