T&L Publications

I have been told that you cannot reverse the direction of a SINGLE PHASE 120v AC MOTOR. (as compared to a three phase motor), and yet I can flip a switch on my ceiling fan (summer/winter) and it will change direction. Can you change the direction of a single phase AC motor?, and how do you do it, and how is it done on a ceiling fan? Or am I just receiving bad information??

I know that a single phase motor can be reversed if it is built that way. Reversable motors have two stator windings and you reverse one to make it turn the other way, I believe.

Only certain motors can be reversed.

All of our industrial 240 motors were wired for this trick but single phase 120 rarely have this ability because of the way they have fixed windings.

Fan motors are wired to the switch flip flop to deliver this trick of direction of which coil first.

It depends on the type of motor.

Many motors that run on single phase power use some method to generate a phase shifted magnetic field that will provide starting torque and define a direction of rotation.

One form of such a motor uses two field windings at right angles to each other. One field winding is connected directly to the power source. The other winding is fed through a series capacitor which, along witn the winding inductance, shifts the current in the second winding by about 90 degrees. This produces a rotating field and the motor effectively acts as a two phase motor. This can be reversed by either reversing the connections to one winding or by changing the phase shifting capacitor to the other winding. Sometimes the capacitor and additional winding are disconnected once the motor starts.

The shaded pole motor splits the poles into two sections. One section has a shading coil wound on it. This winding (Often one turn) is shorted. The induced current in the shorted winding is shifted in phase from the field in the other half of the pole. This creates the torque needed for starting and running. If a multiple turn coil is would on each half of the pole pieces, this type of motor can be reversed by shorting one or the other of the shading windings.

I had a single phase 120V AC gearhead motor that I used to aim my sat dish. It was unidirectional but I was able to get it running in either direction by a simple disassembly and rewire. Just find the two sets of windings like Russ referred to and separate their connections so that you can reverse the connections of one winding. This reverses the direction of the motor.

You can wire one winding to a 4PDT relay for reverse mode. I used 4 Triacs for direction control with a 5th Triac for main power in.

Bob

This leeson controller that I just sold, happily reverses 120 volt motors, as well as brakes, and controlls speed.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... &rd=1&rd=1

I used it on my 120 volt drill press, to back out of stuff that became caught
It has internall jumpers for practically any voltage motor.