Electromagnetic Brake (Fail-Safe Brake)
It is a mechanical brake that closes with spring force and opens when the coil is energised. Because it engages automatically when power is removed, it falls to the safe side in the event of a fault. In lifting mechanisms and those carrying suspended loads it is the only reliable way to hold the load in the air.
It is usually mounted on the rear cover of the motor and a disc on the shaft is clamped by the linings. The coil is supplied with 12 V or 24 V DC; when energised the spring is overcome and the brake opens.
The sequence is critical when using it together with a driver:
- When the motion command arrives, the brake coil is energised first and the brake opens.
- After a short delay, the motor starts being driven.
- When stopping, the speed is brought to zero first, then the brake is closed.
If the brake closes too early the lining rubs against the turning disc and wears out quickly; if it closes too late the load slips somewhat. Because the coil is an inductive load, a suppression diode must be fitted at the relay or output terminal driving it; otherwise the voltage spike at the moment of switch-off stresses the contacts and the electronics.
Do not forget that electronic braking does not replace this brake: dynamic braking produces torque only while the motor is turning and power is present.
The context in which this term is used: DC Motor Driver Selection Guide