Braking characteristics for ac and dc drives?

Answer 1

There are many electronic ways to brake AC and DC motors the construction of the motors also influence the braking alternatives that can be chosen.
most AC motors can by braked very effectively with a short burst of DC, this locks up the motor very rapidly but may take its toll on the DC supply components , rectifiers contractor's etc. Obviously as its stalling the motor it may also have physical consequences on the mechanical parts involved as well.
An AC car wheel balancer as seen in tyre fitting shops often use DC to slow the motor then also use DC to hold the motor while the operator adds weights.
DC motors can generally be braked by using them as generators, a good example is the regenerative braking systems being designed into modern hybrid vehicles, In a sense they are being used as generators to recharge the battery system.
Until the batteries are fully charged this can be arranged so it is almost as effective as shorting out the motor to magnetically lock it up, there has to be a bit of dynamic impedance matching so that the motor sees a very low impedance,
Some motors such as stepper motors have some inherent braking, if you take a small stepper motor from an old printer you will feel as it rotates it steps to the next position, this can be easily exaggerated by twisting the drive wires together to short out the stepper,
The permanent magnet within the stepper tries to become a generator into the shorted coils and will often lock the motor solid.