Braking in AC Motors

• Generator Action (AC Motor)
If an induction motor is forced to run at speeds in excess of the synchronous speed, the load torque exceeds the machine torque and the slip is negative, reversing the rotor induced EMF and rotor current. In this situation the machine will act as a generator with energy being returned to the supply.
If the AC supply voltage to the stator excitation is simply removed, no generation is possible because there can be no induced current in the rotor.
o Regenerative braking
Thus in traction applications, regenerative braking is not possible below synchronous speed in a machine fed with a fixed frequency supply. If however the motor is fed by a variable frequency inverter then regenerative braking is possible by reducing the supply frequency so that the synchronous speed becomes less than the motor speed.
AC motors can be microprocessor controlled to a fine degree and can regenerate current down to almost a stop whereas DC regeneration fades quickly at low speeds.
o Dynamic Braking
Induction motors can be brought rapidly to a stop (and / or reversed) by reversing one pair of leads which has the effect of reversing the rotating wave. This is known as "plugging". The motor can also be stopped quickly by cutting the AC supply and feeding the stator windings instead with a DC (zero frequency) supply. With both of these methods, energy is not returned to the supply but is dissipated as heat in the motor. These techniques are known as dynamic braking.
Regenerative braking
‡Problem in industry: Most AC inverters cannot return the excess power to the power line.There is a diode bridge that takes AC power and charges a capacitor bank.(There are a very few specialized AC inverters that use a bi-directional bridge of transistors to charge the capacitors or return power to the AC line.)The capacitor bank is connected to the motor via a bridge of transistors or gate turn off SCRs.This bridge is bi- directional.So whenever the motor is a generator, the capacitor bank is charged up and potentially overcharged.Some drives turn off to protect the controller.Some drives switch on a resistor to dump the excess energy.Here we are not really regenerative, ‡ If you were to replace the capacitor bank with batteries, like some electric cars, you now have the ability to absorb a lot of energy from the motor, and you can easily be regenerative over the entire speed range of the motor.

(Also see the DC injection braking mechanism from wikipaedia. It basically is breaking the motor by removing the AC supply and applying DC to stator to any two terminals)