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A common way to build a 2-speed motor is to put in another brush. Imagine the commutator (a complete circle) with power applied to one side through a brush and the ground brush diametrically opposite. The voltage is dropped around both sides, which represent the armature windings. Move one of the brushes and the efficiency of the motor changes, with one side of the circle shorter than the other - the speed will be lower. But voltage is still dropped uniformly around both. Put the first brush back in the picture and power both at once and part of the armature is essentially shorted. I'm not sure what happens to all the current flow, but I suppose the shorted segment has a high current flow and there would likely be more arcing at the brushes. Or something like that. Like they said, probably doesn't instantly kill the motor, but might reduce its life a lot if the condition were continuous - and you wouldn't get the speed that you expected.
Gary Casey
Now I'm confused. I thought there was one motor that you either power in
high or low mode.
What really goes on in there?
Colyn
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