Motors are generally permitted to slow to a stop when disconnected from the power line, but there may be in- stances when that is not an option or not convenient. There are several methods that can be employed to pro- vide braking for a motor. Some of these are:
• Mechanical brakes
• Dynamic braking
• Plugging
Mechanical Brakes
Mechanical brakes are available in two basic types, drum and disk. Drum brakes use brake shoes to apply pressure against a drum (Figure 43 – 1). A metal cylinder, called the drum, is attached to the motor shaft. Brake shoes are placed around the drum. A spring is used to adjust the amount of pressure the brake shoes exert against the drum to control the amount of braking that takes place when stopping the motor. When the motor is operating, a solenoid is energized to release the pressure of the brake shoes. When the motor is to be stopped, the brakes engage immediately. A circuit of this type is shown in Figure 43 – 2. Mechanical brakes work by converting the kinetic (moving) energy of the load into thermal (heat) energy when the motor is stopped. Mechanical type brakes have an advantage in that they can hold a suspended load. For this reason, mechanical brakes are often used on cranes.
Disc brakes work in a very similar manner to drum brakes. The only real difference is that brake pads are used to exert force against a spinning disc instead of a cylindrical drum. A combination disc brake and mag- netic clutch is shown in Figure 43 – 3.