Use and function of slip ring and brush in induction motor

The slip ring and brush in the induction motor are mainly used in the winding rotor induction motor, rather than the cage induction motor. In the winding rotor induction motor, the use and function of the slip ring and brush mainly include the following aspects:

Slip ring

The slip ring is a metal ring fixed to the motor shaft, usually made of copper. The number of slip rings depends on the design of the motor and is generally the same as the number of phases in the rotor windings. The main functions of the slip ring are as follows:

• Transfer power: The slip ring allows an external resistor or controller to make an electrical connection to the rotor windings through a connection to an external circuit, thereby changing the resistance of the rotor windings.

• Mechanical rotation: The slip ring rotates with the rotor of the motor to ensure that good contact with the brush is maintained while the rotor is rotating.

Electric brush

Brushes are carbon - or metal-graphite components installed in the motor housing, which are in contact with the slip ring and transmit current. The main functions of the brush are as follows:

• Conductive connection: The brush maintains contact with the slip ring, forming a conductive path that allows the external circuit to establish an electrical connection with the rotor winding.

• Wear compensation: Due to friction between the brush and the slip ring, the brush is designed as a replaceable part to compensate for wear and ensure good contact over a long period of time.

The working principle of winding rotor induction motor

The rotor winding of the winding type rotor induction motor can be connected with the external circuit, through the slip ring and brush, can be connected to the external resistor or speed regulation device. The purpose of this is mainly to improve the starting performance or achieve speed control:

•  Improved starting performance: During starting, external resistors connected by slip rings and brushes can increase the resistance of the rotor windings, thereby increasing the starting torque and reducing the starting current. Once the motor accelerates to a high enough speed, the external resistance can be shorted or gradually reduced to restore the normal operating state of the motor.

• Speed control: By adjusting the external resistance of the access rotor winding, the running speed of the motor can be changed. This method is called rotor resistance speed regulation.

Advantage

• Increase in starting torque: The starting torque can be significantly increased by increasing the rotor resistance.

• Reduced starting current: The starting current can be effectively controlled to reduce the impact on the grid.

• Speed control ability: A certain degree of speed control can be achieved through external resistance.

Shortcoming

• Increased complexity:Compared with the cage induction motor, the winding rotor induction motor has added components such as slip rings and brushes, making the motor structure more complex.

• Maintenance needs: slip rings and brushes need to be checked and replaced regularly, increasing maintenance costs.

• Efficiency loss: Increasing the rotor resistance will result in a certain efficiency loss.

Application scenario

Wound-rotor induction motors are commonly used in applications where large starting torques are required or speed regulation is required, such as in industrial applications such as heavy-duty starting equipment, cranes, and winches.

Sum up

The slip ring and brush play a key role in connecting the rotor winding with the external circuit in the winding rotor induction motor, through which the motor's starting performance can be optimized and speed control can be achieved.