Circuit symbol diagram of speed relay

Photocoupler

In automatic control, it is sometimes necessary to switch on and off certain circuits according to the speed of the motor, such as the reverse brake of the squirrel cage motor. When the motor speed drops to a very low level, the current should be cut off immediately to prevent the motor from being reversed. Start to start. This action requires a speed relay to control the completion.

Figure 1a is a schematic diagram of a speed relay. The shaft of the speed relay is connected to the motor shaft. A cylindrical permanent magnet is fixed on the rotating shaft of the speed relay; the outer sleeve of the magnet is sleeved with an outer ring which can be deflected at a certain angle in the positive and negative directions; a squirrel cage winding is embedded on the circumference of the outer ring. When the motor rotates, the squirrel-cage winding of the outer ring cuts the magnetic lines of the permanent magnet to generate an induced current, and generates a torque that causes the outer ring to rotate an angle with the direction of rotation of the motor. At this time, the top block fixed on the outer ring bracket faces the moving contact to make a group of contacts move. If the motor is reversed, the top block toggles the other set of contacts. When the motor speed drops to about 100r/min, the top block returns and the contacts are reset because the electromagnetic force of the squirrel cage winding is insufficient. Because the contact of the relay is related to the speed of the motor, it is called the speed relay, and because the speed relay is used for the reverse braking of the motor, it is also called the reverse brake relay. Figure 1b shows the symbol of the speed relay.