Electrically released brakes are commonly used with motors or brake motors to rapidly slow down a system or prevent a system from moving altogether. These brakes are designed to stop or hold when power is removed from the brake (de-energized) and allow motor motion when the brake is powered (energized).
Electrically released brakes utilize spring arrangements to force stationary friction plates against disks that are attached to the motor. When power is applied to the brake coil, the resulting magnetic force compresses the springs, fully disengaging the friction plate from the disk and allowing the motor side plate to turn freely.
It is important to note that with electrically released brakes, brake coils are rated for specific voltages. Voltage options include 115, 230 and 460-volt AC, and 12, 24, 90 and 230-volt DC. Many brake coils require DC voltage and utilize a rectifier arrangement to convert supplied AC voltage to the coil’s DC volt requirements. To fully release the brake when powered, the full voltage must be present on the brake wires. This becomes most important during use with an AC drive-controlled motor.
AC Drive Usage with Electrically Released Brakes
AC drives are widely used to power motors and have the advantages of allowing operators to better control acceleration, manage speed and regulate deceleration. When motors begin to accelerate upon startup, motor voltage increases from zero to full operating voltage and speed. As motors reach operating speed but operate below motor base speed, the average voltage is lower than the full rated voltage. Finally, as motors decelerate to a stop, motor voltage decreases to zero.
If the AC drive power is connected to both motor wires and brake wires, brakes will not fully release until the AC drive reaches a voltage high enough to disengage the friction disk. This means that the motor will be held at a stop until voltage reaches a high enough level to release the brake. Because of this, the AC drive may trip while the motor is held at a stop, or the brake may drag when the voltage reaches a high enough level to begin the release. Sometimes, the brake friction disk may drag if the motor is not running at full speed and the brake is not at full rated voltage. During controlled deceleration, the voltage supplied to the brake will reduce until it drags, stopping the motor before motor reaches zero volts. As a result, connecting AC drive power to motor and brake wires can reduce brake life.
When a brake is used with a motor controlled by an AC Drive, the brake should be powered by a separate full voltage power source. I have personally encountered OEM equipment that uses AC drives connected to both motor and brake leads. If acceleration is quick, the motor runs at full speed, and deceleration is quick or coasts to a stop, this setup may offer good performance in certain applications. Ideally though, a separate circuit with full voltage should power the brake, fully removing power from the brake during motor uptime, allowing for the best brake performance and service life.
Finally, a word about brake motors. Many motor manufacturers offer two types of brake motors. One version has contains a brake internally connected to motor leads. As such, on cannot use separate power sources for brakes and motors. Only across-the-line or full-voltage motor control should be used with these motors.
The second version of brake motors offers separate brake wires for connecting the brake coil to a full voltage source. When using an AC Drive with a brake motor, select a brake motor with separate brake wires. Then use a separate full voltage power source, brake contactor and proper circuit protection to control the brake. Also, make sure the brake is powered & released when the AC drive is running including accelerating, at speed, and decelerating the motor.
For more information on electrical brakes, please contact your Kaman representative or specialist. He or she can help you select, supply, size and troubleshoot brakes, motors, and AC drives.
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