DC motors with field coils. Changing the input polarity
reverses both the field and armature, so there's no direction
change. These are rare today, since permanent magnets are so good.
Pole shading for AC motors.
There are yet other reasons you may see a DC motor in the surplus market
that isn't reversible. A common reason is that it's specifically
designed to inhibit reverse direction, because reversing might damage
whatever it's attached to. So the motor may have a pawl or clutch bulit
onto it, or it could have a diode connected to it in a way that prevents
Depends, speaking on a pure motor basis, permanent magnet motors are
generally reversable, but motors that generate their own magnetic field
with a coil generally need to be required to reverse direction.
Secondly, on gear motors, the down stream gear design may not be able to
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