The functions of speed relay depends on the magnitude of speed as the signal to match the contact and to realize the reverse connection braking to the motor. So the speed relay is also called reverse connection braking relay. 

The speed relay shaft and the motor shaft are connected together, on the shaft there is cylindrical permanent magnet, the outside of which is provided without ring that can turn to a certain angle, this out ring is embedded with the squirrel-cage winding. 

Working Principles of Speed Relay: When the speed relay is driven by motor, its permanent magnet flux cuts the squirrel-cage winding of outer ring and inside it induces the electric potential and current. This current and the permanent magnet flux interact to produce the force on the squirrel-cage winding and to rotate the outer ring. The top piece at the holder fixed together with the outer ring makes the dynamic-closing contact close and the dynamic-disconnecting contact disconnect. The rotating direction of the outer ring of the speed relay is determined by the motor; therefore the top piece can stir the contact leftwards, and also can stir the contact rightwards to make it act. When the speed of the speed relay shaft is lower than a certain rotating speed, the top piece will recover to its place and will be in the middle position.