Motors & Drives Information

Know the various ways to electromagnetically bring a motor to a stop, to maintain control of stopping times and load movement

ANY MOTOR MUST START BEFORE IT CAN run-we all know that. Starting duty-accelerating some driven machine to operating speed-can have more influence on motor design than the running performance. In many applications, stopping the drive-bringing the load to a stop, and perhaps holding it there-can be equally important.

Although many braking methods exist, one of the most common is the friction brake. It has the advantage of providing both holding torque and decelerating torque. Of either disc or shoe construction, like automotive brakes, the friction motor brake is most often flange-mounted directly on the motor, which has an additional shaft extension to which braking torque is transmitted. The assembly is often sold as a single "brakemotor" unit. Although they operate on the same principles, some brake models are of "through-- shaft" design for installation elsewhere in the power train.

Such brakes are termed spring-set, electrically released. While the motor is energized and driving the load, the brake is held in the released or off position by an electromagnet or solenoid, to which power is supplied by the motor circuit or control equipment. Since bringing the motor to a stop requires that it be de-energized, the brake is automatically set by mechanical spring action whenever the motor power is interrupted.

With increasing motor (and brake) size, the forces involved become too great for such simple mechanisms. Shoe-type construction predominates, and the electromagnetic release may be augmented by pneumatic or hydraulic pressure. But the principle remains unchanged; when the motor is de-energized, the brake is automatically set. For simplicity, we'll go into detail here concerning only the disc-type electromagnetic design, most often applied to motors in the range of 1 to 150 hp.

Questions to ask

Choosing the proper friction brake involves these questions: 1. What is to be stopped? 2. How often must it be stopped? 3. How quickly must it stop?

In addition, the electrical release circuit must suit the available voltage and control scheme, the brake enclosure must suit the environment, and the brake mounting must suit the motor frame or drive configuration.

# posted by Medical Information : 3:54 AM