[Eldon_Joh]

If you short the motor, all of the stored energy in the system will be dissipated as heat in the motor windings. Not the best way of slowing down the motor.

It is ok to do that if there is a resistance in series to absorb the energy

if there is no resistance in series the motor won't slow down very quickly, and some say you can demagnetize the magnets, of which i am skeptical.

here's an interesting example:

I have a 30 pole permanent magnet 3 phase motor I made from a 36 slot 1/3rd hp induction motor core. it has 1.9 cubic inches of neodymium magnets in it and when run as a generator, it can stall a 1/2 hp 850 rpm induction motor directly coupled. its windings are 1 ohm each, meaning 3 separate 1 ohm windings.

The motor/generator is capable of dumping 600-800 watts or so at the time it stalls the 1/2 hp induction motor, on the order of 6 amps in the coils, wasting 108 watts in the coils. (i cannot remember the actual numbers, its been a few years)

But if you short circuit the generator, it will produce 9 amps of current in the windings and consume only 81*3 or 243 watts of power from the 850 rpm motor. no more, no less.. regardless of the rpm. the minimum rpm needed to generate those 9 amps is only on the order of 150 rpm, and i've shorted it out at 1750 rpm as well. same 9 amps.

if i were to have done a better job winding the machine, i could probably have fit at least 50% more copper in the machine.. which would mean that when shorting out the motor the maximum watts that could be dumped into it.. would be on the order of 150-170 watts.. compared to 800 watts when optimally loaded as a generator.

A much better designed servo will likely have an even sharper cut off where without any resistance, above some rpm they will be nearly effortless to turn.