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How do I reverse this motor?

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Well Cookin

New Member
Hello. I want to reverse the rotation of an AC motor from an old air compressor. It is a 240v single phase AC motor. Here is a diagram of what I believe the wiring to be:



The heavier wires go to one end of the motor and the other two pairs of wires to the other end. The latter two pairs are labelled U1/U2 and Z1/Z2 respectively. I put a multimeter on all three pairs - all <5 ohms. L = live, N = neutral. Earth is grounded to motor chassis. The squares represent the 4 terminals.

Can anyone help me? I know very little about AC motors :oops: and I need to know where to put the L in order to reverse the rotation (assuming it is that simple...).

Thanks,
Well Cookin'
 

Dean Huster

Well-Known Member
Reversing ac motors

Looks like you have a capacitor-start/capacitor-run motor, although isn't there a centrifugal switch somewhere in this circuit? From what I remember, if you can reverse the winding connections to the start winding (the one with the bigger cap), the motor will reverse. Trouble is, according to your diagram, you can't just jump around terminals to do this. You have to get behind the phenolic to do the job.

Dean
 

Well Cookin

New Member
Thanks Dean. Not sure if there is a centrifugal switch - I haven't opened the main housing. What are the implications if there is such a switch?
Also what do you mean by 'the phenolic'?
 

Phasor

Member
I would have expected the centrifugal switch to be in the place where you have drawn a winding, between U2 and 3.
 

Well Cookin

New Member
You are correct Phasor and Dean. There is indeed a centrifugal switch between U2 and 3. Sorry for making the wrong assumption. Ok so here is the updated diagram:



Can someone give me a brief description of what all this is doing?
Oh and how to reverse it...

I noticed that there are no windings on the armature. It's a heavy metal cylinder on a shaft, inside of which appears to be a grey (magnet like?) substance. Does this mean it is simply a magnetic armature rather than a wound one? Does this have anything to do with it being/not being an induction motor? I'm way out of my depth here. Please help.

Sorry for all the dumb questions - I'm a newbie.

Thanks again,
Well Cookin'
 

nettron1000

New Member
Like as already bin said, thats a capacitor start-capacitor run motor.
To reverse this motor you have to reverse the connections on either the start (auxilary) winding (Z1,Z2) or the run (main) winding ( U1, U2) but not both. Not sure if thats possible in your case.

Heres a link to some info and a wiring diagram.

http://www.lmphotonics.com/single_phase_m.htm
 

Phasor

Member
Well Cookin said:
I noticed that there are no windings on the armature. It's a heavy metal cylinder on a shaft, inside of which appears to be a grey (magnet like?) substance. Does this mean it is simply a magnetic armature rather than a wound one? Does this have anything to do with it being/not being an induction motor?
Yes, it has everything to do with being an induction motor. There are 2 basic types of rotors - induction rotors (as you have) and wound rotors. Each has inherent advantages and disadvantages.

The rotor has no permanent magnetisation. The current (and resulting magnetic field) in the rotor is induced (hence, 'induction motor') by the rotating magnetic field of the stator. The greater the difference in speed between the magnetic fields of the rotor and stator, the greater the induced current.

An induction motor always runs below synchronous speed, as there must be some 'slip' between the rotor and stator fields. If the rotor ran at synchronous speed, the stator field would not cut the rotor bars, and hence there is no induced current, and no torque.

The advantage of an induction motor is lower cost, lower maintenance, and non-sparking (for hazardous areas). However the wound rotor motor is able to have the resistance of its rotor winding varied (by an external resistor/potentiometer) - this is used to produce greater starting torque - maximum torque is obtained when R(rotor) = X(rotor). X(rotor) varies with motor speed, so if the external resistance (R) is varied accordingly, the greatest possible torque can be obtained throughout the speed range.
 

Well Cookin

New Member
Thanks for the good description Phasor.

I reversed the Z1/Z2 winding and the motor turns the other way. Great! :D

Thanks for all your help guys.
 
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