What makes some motors not reversible ?

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.

John Nagle

A light bulb just went off when you wrote that! I had been thinking strictly of motors with magnets.

Thanks!

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 reverse direction.

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 reversing polarity.

-- Gordon

Well, guess I'll just ask before I buy "Do It Go the Other Way?" :-) JCD

Some motors have brushes offset for smoother electrical opperation going in one direction. These motors may be reversable, but spark if you do.

Windscreen wiper motors are like that - even after turning one from a 3 pole motor to 2 pole I got enough RF to blow my receiver... DAMN!