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Originally Posted by David Guzman
Ok now im confused because firt John V says that the motors will burn before the 30 amp breaker trips. However Al says that their stall current is over 90 amps. I don't know if im understanding correctly but if the motor burns before it trips the 30 amp circut breaker then that would mean that it would burn before it gets to its stall current?
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David,
Mike refers to the current trip point in the breaker but we also must add into the variable mix the fact that your speed controller is not at full up all the time. That means that there are current pulses being fed to the motor that could be at full stall current but the average is actually less. What makes the situation worse is the fast rise times at the switch points do produce some significant back EMF when the motor is heavily loaded. These add to the heat in the motor due to eddy currents and arcing brushes that the circuit breaker will not react to. As to the circuit breakers, they are usually out in the open getting cooled off by moving air and the large diamter wire will draw heat away from the breakers as well. However, when these self resetting breakers start to trip under heavy loads they do get extremely hot and reset times can be under a second, so fast that the breakers actually buzz. When this occurs, do not allow anyone to get near the breaker panels until they have had time to cool. The temperatures are beyond the point of producing second cegree burns.
Other things that are beyond our control that affect whether a motor winding burns are the uniformity of the varnish coating and bending and nicking of the wire as it is wound on the armature. A wire that is bent and/or pulled tightly around a sharp corner will become the weak point in the winding, having the highest resistance and therefore the highest heat. Add these variables to the mix and winding failure is inevitable at the weak point(s).
Again, running this motor at normal design speeds will let the fan move air through the motor minimizing the heating factors.