Motors should not coast more than a few shaft revolutions after being deenergized. Critical applications use electromechanical clutch and brake devices to stop loads when motor power is removed. However, when motor braking can be less precise, dynamic braking may be a less-costly solution.
Dc motors are normally braked by disconnecting the armature from the power source. The armature is then short circuited or a current-limiting resistor placed across the armature terminals while the field coils remain energized. In either case, armature current reverses, armature torque reverses, and the motor tries to reverse. The speed in the forward direction rapidly decreases as does the voltage generated in the armature. At the point of reversal or zero speed, generated voltage is zero. The motor stops at this point since current cannot flow and no more reversing torque is generated.
The value of the shunting resistor controls the braking rate. A small resistance allows a large current flow. Since braking torque is proportional to armature current, the motor and load stop in a short time. Some resistance is recommended to limit the severity of braking, especially with gearmotors.
Induction-type motors are generally braked by removing ac power and applying dc. The motor then acts like a dc generator with a short-circuited armature. The electrical output of the generator has high circulating currents in the shorted (squirrel-cage) rotor bars, and rotational energy is dissipated in the form of rotor heat. The motor is quickly brought to a stop.
Dc braking sources vary from batteries and highly filtered power supplies to full-wave and half-wave sources or a charged capacitor. The choice is based on cost and degree of braking required. Generally, pure dc is better than less-continuous dc, but more expensive.