Posted by David Sherman on Saturday, February 19, 2005 at 4:07PM :
In Reply to: Another stupid electrical question. posted by Robert on Saturday, February 19, 2005 at 2:58PM :
There are two sets of coils in the starter motor, the rotor (the part that spins) and the stator (the part attached to the frame). The same current goes through both of them, so if you reverse the current, they both get reversed and the net effect is to still turn in the right direction. Reversible DC motors either have both coils brought out to separate wires so you can reverse one coil without reversing the other, or the stator has a permanent magnet rather than a coil so that it keeps the same polarity when the rotor is reversed.