Braking in induction motors refers to quickly bringing the speed of the motor to zero. Braking can be categorized into two broad categories viz. mechanical braking and electrical braking.
Mechanical braking involves stopping the shaft by means of a braking shoe. When the braking is to be done, the supply to the motor is cut off and the brake is applied to bring the motor to a halt.
Mechanical braking used in cranes and hoists. It is also used in elevators when the elevator has to stop at a specific floor of the building.
Electrical braking involves stopping the motor using electrical means. Most electrical braking systems have a mechanical brake to hold the shaft in position once the machine has been stopped.
There are two main types of Electrical braking.
- Dynamic braking
- Regenerative braking
Plugging involves reversing the supply in two of the phases. For instance, R and Y can be interchanged. This leads to a torque being developed in the opposite direction to the rotation of the motor. This causes the motor to stop at once. Once the motor stops, the reverse supply is cut off (to prevent the motor from running in the opposite direction). The rotor is secured by a mechanical brake.
Dynamic Braking can be classified into DC injection braking, AC dynamic Braking and Capacitor Braking.
AC dynamic Braking
In AC dynamic braking, the supply to one of the phases is cut off. Thus the motor runs as a single phase motor. This induces negative phase sequence components in the supply and the motor stops. Another method is to give the remove one phase and give the same phase to two terminals. For instance, two terminals will have 'Y' phase and one will have 'B' phase.
DC injection braking
In DC injection braking, a separate rectifier circuit produces a dc supply. When the brake is to be applied, the ac supply to the stator is disconnected and a dc supply is given to two of the phases. The dc voltage in the stator sets up its own magnetic field. The conductors of the rotor which is rotating will cut the magnetic field. As the conductors are short circuited, a high current is produced. This causes a braking torque to be produced in the rotor. The current produced in the rotor is dissipated as heat. This system can be used only when the rotor can withstand the heat which will be produced when the brake is applied.
Here the AC supply to the stator terminals is cut off and the terminals are connected to a three phase capacitor bank. The capacitors will excite the induction generator. This sets up a magnetic field which will cut the rotor bars. The rotor energy is thus converted into heat and the motor is stopped.
In Regenerative braking, the supply frequency to the stator is reduced. This is possible with VFDs where the frequency can be varied. When the supply frequency is reduced, the synchronous speed of the motor is reduced. When the synchronous speed falls below the rotor speed, the induction motor works as an induction generator and power is supplied back to the terminals. The energy in the rotor is thus recovered. Due to the loss of energy, the rotor slows down and stops.