8. Braking of Electric Machines

Why Braking To stop the electric motor. To sustain the speed of the drive system at the desired value. To prevent the motor from over-speeding. To hold the motor shaft at a certain position regardless of the applied load torque.

1. Regenerative Braking When the load torque causes the machine to run at a speed > no-load speed, but without changing its direction of rotation, the motor is in regenerative braking.

w w M G T T  Speed Power Flow Power Flow Torque  Load Load m m Fm Fm

2. Dynamic Braking When the motor is running, kinetic energy is stored in its rotating mass. If the kinetic energy of the motor is dissipated in a resistive element, the motor stops.

IB IA MOTOR B RB Source A

3. Counter Current Braking If the the airgap field reverses its rotation, the rotor shaft must also reverse its rotation.