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3 phase compressors

I was just thinking can single phase compressors be used on three phase units, using all three phases. Isnt the purpose of a capacitor the same as having a third phase.

Why do you have to change the wires on a three phase unit when the compressor is not turning on or the blower the blower is not spinning in the right direction.


  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Yes, but why

    First of all, let me make a point for the sake of clarity. Three phase power is much more efficient than single phase power. At best, single phase power will operate with a power factor of 1.0, while a three phase power supply can operate with a power factor as high as 1.7.

    So, to answer the first part of your question: Yes you could install a single phase compressor on a system that was designed, wired and fabricated for use with a three phase pump. Of course you would have to alter the wiring and use only two hot legs. The real question is "Why would you want to do this".

    There are many three phase pieces of A/C equipment that are designed to operate with three phase compressors and single phase blower motors. This is very common, mainly because the amperage draw of the blowers is very low when compared to the amperage draw of the compressor.

    Now for the phasing aspect of your question.

    On smaller three phase compressors that do not have a mechanical oil pump, there is no need to phase the compressor as it knows not the difference between the piston going up-down-up and going down-up-down.

    Larger three phase compressors, however, that do have mechanical oil pumps must be phased properly.

    If you take a look at a diagram of a three phase power supply, you will notice that the order in which each of the individual power leges reaches its maximum voltage is different. By changing the order that these individual power supplies reach their peaks, we can alter the direction of rotation of the motor.
  • Empire_2
    Empire_2 Member Posts: 2,343

    The answer to the first Q: is no, and the answer to the second Q: is no. A Run cap only gives the start winding a (Kick) in the **** to start the compressor or motor. Once the cap has discharged its initial charge the run and com windings are where you will get the highest amp reading and very little on the start winding because these 2 windings are essentially the only ones in the circuit. Look at the wiring diagram. L1 to com L2 to Run...

    If you were EVER connect a 1 Phz comp. to a 3 Phz compressor set up, you would see fire works like the 4th of July. Essentially this is because in a 3 Phz load, there is always power flowing through all 3 legs and that includes the start winding which is actually not a start winding, just another part of the motor.. Ever see a start cap on a 3 Phz motor? NO.

    By swapping 2 leg's on a 3Phz load, you will reverse it's rotation due to the fact that you have changed the Cycle or phase that the motor see's. Switching the Phz changes the rotation because the windings see a different pattern (if you will) on all t 3 windings as a whole.

    There is a whole lot more to this and I just touched on some points to help out. Hope this makes some scents.

    Mike T.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Hi Eugene

    I think he wants to use all 3 legs of the 3 Phz service to power the 1 Phz comp......?

    Mike T.
  • Barrington White
    Barrington White Member Posts: 37

    > I think he wants to use all 3 legs of the 3 Phz

    > service to power the 1 Phz comp......?


    > Mike T.

    Yeah I was just thinking if it would work.
  • Barrington White
    Barrington White Member Posts: 37

    Just trying to understand the equiptment alot better.
  • Empire_2
    Empire_2 Member Posts: 2,343

    Most refrigeration books have some detail of what you are after to understand. I think Eugene could probably point out a few that could help.

    Mike T.
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