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Posted by Sherman in Idaho [72.47.9.37] on Tuesday, December 18, 2012 at 19:53:37 :
In Reply to: Won't the starter spin backwards? posted by Greg Coffin [4.225.211.13] on Tuesday, December 18, 2012 at 19:21:36 :
In simple terms, the coils in the field (the part on the outside) and the coils in the armature (the part that spins) make magnetic fields that push against each other. So long as the current is going the same way in each coil, they push the same way. "Universal" motors used in vacuum cleaners, plug-in drills, etc swap the direction of the current with every cycle of the power line, and yet they always spin the "right" way.
To make a motor like that spin the other direction, you have to swap either the field or the armature leads, but not both. Little toy motors have permanent magnets for the field, so with them, swapping the wires to the motor does change the direction since of course the permanent magnet doesn't change.
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