Forums : # phase air compressor

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# phase air compressor


Tue Mar 27, 2012 7:36 pm
I bought a used Champion air compressor that has a 3 phase motor. My shop already has a 3 phase saw which is run off a 7 HP rotary phase converter. I have had this setup for about 15 years and, although I hooked it up myself back then I am having trouble figuring it out now.(that's was getting old does to you, I guess) Anyway the compressor had a pressure switch which was removed so I had to buy a new pressure switch(for 3 phase)
My problem is with the wiring.
The wires going into the motor are as follows.
T6,T4 and T5 are connected together with a wire nut. T8 and T2 are twisted together but bare(no wirenut). T0 and T3 are also twisted together as are T1 and T7. Again no wirenuts. Because the wire pairs are bare I presume these 3 pairs were connected to the pressure switch. The pressure switch that I have only has connections for 2 sets of wires and a ground.
Coming out of the rotary phase converter(and going into the saw) is a 220 volt plug that is 4 pronged.
So my questions are;
Is there a way to determine which of the 3 pairs of wires runs through the pressure switch? Will all 3 pairs carry the same current as read with a voltmeter i.e. 220 volts across any 2 pairs of wires.? I would like to take my readings off the plug, if possible.With only 2 pairs of wires running through the pressure switch will this work?
Thanks for you help.
Fred Mc


Fri Apr 06, 2012 9:59 pm
Current and voltage are two entirely different things, btw.
With a proper voltmeter you should read 208v between any combination of the three pairs. This is voltage, not current. If you have a clamp-type current meter ( that clamps around a wire but doesn't actually touch it ) you can measure the current ( amps ) flowing on each leg/phase. Yes - all three should also read the same current. There will be some variation, but not much. Variations are usually greatest in "wild-leg" pseudo three-phase that power companies supply to small businesses. The difference in current is usually due to differences in voltage between the legs - each motor coil is going to draw the same power in watts, but when voltage is lower current must be higher ( watts = volts x amps ) and vise-versa.
Unlike a single-phase motor, which you can switch on/off by making and breaking a single connection ( yes, I know: single connection switching is unsafe on a 240v motor - but it works ), a three-phase motor needs three connections switched or the unswitched coils remain on.
Normally this is done with a contactor ( a really high-current relay ). The pressure switch powers on/off the contactor ( which runs on 208v power between legs, since it only has one coil ), which would then make/break the three connections for the motor.
Your bare wires lead me to believe you actually had a 3-contact pressure switch. Since the new one is a 2-contact plus ground, it's not for 3-phase but for normal household 240v split-phase.
You can pick up a sufficient contactor on eBay for around $20
The device would have 8 connectors at a minimum - 6 for in/out and two for control ( coil ). A device with more connectors usually uses them as a convenient neutral bus -and/or- separate dry contact[s] for wiring in a start/run method.
Just make sure the rating of the contactor ( in amps or kw ) exceeds your draw and the coil voltage is 208v ( usually says 220vac max ) @ 60hz so you can use lines to control it. You will see some that use control voltages of 24v - this is common - but you cannot use one of those unless you have a control transformer in your compressor ( unlikely, since your pressure switch was controlling legs by itself ).
You then can use any old pressure switch - switch turns on/off contactor and contactor runs motor.
Edited Fri Apr 06, 2012 11:54 pm by AnOldMan


Fri Apr 06, 2012 10:11 pm

If you study this image, your "T4, T5, and T6" are the center connection of "neutral" and your "T1, T2, and T3" are the three "line" connections.
Your "T7, T8, and T0" would be the incoming line connections that are switched by the pressure switch or contactor.
This is a "Y" or "WYE" connection. Note that some motors only use this for start, and are DELTA for run...
Note: if, after wiring this all up, your motor runs backwards - just swap any two lines.