What are brushes and why do some motors use them?
Brush-also known as a rotating electrical joint, is a device that can transmit power, electrical signals or data between a fixed part and a rotating part. If the component rotation involves the number of revolutions, a spool with sufficient cable length can be used to allow the required number of revolutions, although cable management may be complicated. However, if a part continuously rotates, it is impractical in many cases to use cables to transmit signals between the rotating part and the fixed part.
Brushes in AC motors
In one form of AC induction motor called a wound rotor motor, brushes are not used to transmit power. A wound rotor motor uses three brushes-usually made of copper or copper alloy-mounted on the motor shaft. Each brush is connected to one of the three-phase rotor windings. The brush made of graphite is connected to a resistance device, such as a varistor.
Increasing the resistance of the rotor winding will cause the rotor current to be in phase with the stator current. The result is a high torque at a relatively low current. The brush is used at start, but its efficiency is low, and the torque drops at full speed. When the motor reaches its operating speed, the brushes are short-circuited and the brushes lose contact, so the motor is just like a standard AC induction motor.
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