Here is a reply post I did on another forum when someone noticed every blade motor failure resulted on the blade not braking:
Somehow I missed this thread… a rotating DC motor act as a DC generator and a short circuit is sometimes applied to "brake" the "motor/generator". Driving a generator into a "short circuit" develops an infinite current. Usually a small circuit with mosfets is designed to handle the back emf, high currents and work in a timely fashion. I suspect that as these motor brushes wear and have less contact on the rotating commutator, shorting the back emf will have a much lesser effect on stopping the motor.
As an example you can take a piece of wire and short the voice coil on a high excursion subwoofer. Try to manually (but carefully) move the cone up and down. The loudspeaker will be quite a bit more difficult to move once the motor (loudspeaker) is shorted. The motor can no longer generate its own voltage. This is one of the contributing factors that make an amplifiers damping factor important when dealing with these types of speakers. Or short the coil on an electric meter movement. A lot of analog meters (e.g. RF power meters) have a built in feature that allow you to short the coil to protect the meter movement needle from damage when transporting the test equipment.
I have always understood D.C. motors to have more power and the brushes last longer if they are broke-in for a certain period of time to shape the brushes before arcing (causing pits on the brush) takes place. You run the motor at a significantly reduced voltage (slower speed) for one-two hours or so depending on the style of motor. Something to look into…
Edited by samelung on Aug. 06 2009, 10:44 PM
Steve Amelung
