How do slip rings and carbon brushes work in a wound-rotor induction motor?

In a wound-rotor induction motor, slip rings and carbon brushes serve as the interface between the rotating rotor windings and the external electrical circuit. Unlike standard squirrel-cage motors, wound-rotor motors feature insulated three-phase windings within the rotor, which are connected to three insulated conducting rings, known as slip rings, mounted on the rotor shaft.1

Carbon brushes are positioned to maintain contact with these rotating slip rings, providing a path for external electrical connections. This configuration allows for the following operational adjustments:1

• Starting Torque: By connecting external resistors to the slip rings via the brushes, the motor can achieve high starting torque without drawing excessive starting current.1
• Speed Control: Varying the external resistance allows for a degree of speed control for certain mechanical loads.1
• Slip Recovery: Alternatively, an electronic rectifier-inverter system can be connected to the slip rings to extract power and feed it back into the supply system, providing efficient speed regulation.1

This design enables secondary circuit adjustments that are not possible in conventional induction motors.2