A flow of current through a conductor will set up a magnetic field around it. This magnetic field has a north and south pole, just like a permanent magnet. If you reverse to direction of flow (exchange + and - poles) the North and South pole will also be changed.
Motors work by the magnetic fields either attracting or repelling each other. (opposites attract, same poles repel)So reversing the flow, will change the poles and what used to attract, will now repel. So the motor will rotate in the opposite direction.
This will be so, for motors that have a permanent magnet construction. If field and rotor are electromagnetic, and fed by the same source, both fields will change and have a net result of no change in direction.
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