3 Phase Shop Air Compressor

[Fred Mc]

I know there are some electrical guru's on this board so I would like to ask your help.

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 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.

Regards

Fred Mc
GM PD 4106

[TedsBUSted]

It'd be very unusual for a three-phase compressor motor to be directly switched form a pressure-switch. I would guess that what was "robbed" from the compressor was a motor starter (contactor and overload protection combined) and the pressure switch.

The pressure-switch would typically be connected as a single-pole switch used as part of the control circuit and would only handle a very small current, just enough current to energize a small coil which would then  magnetically "pull in" (close) larger 3-pole relay contacts, to energize the motor. The control circuit is often (but not always) a lower voltage than the equipment itself and is usually powered by a control transformer, which drops line voltage to control voltage.

The motor lead connections of course depend on the supply voltage being connected to.

Ted.

[oldmansax]

What Ted said.....

In addition, 3 phase electrical wiring is all over the map depending on your location & local electrical supply, but still within some basic parameters. Where my old shop was, T1 = 120V, T2= 120V, & T3=240V. My new shop T3=277V.  Usually T1 & T2 will = 120V & T3 will be somewhere between 120V & 300V. On the motor side, the highest voltage is usually fed to T3.

In your case, I would connect to the pairs that are wired together with no wire nuts, & feed the highest voltage I have to the pair that includes T3.

If your pressure switch rating is the same or higher than the motor, it should be OK.

If you can find some wiring diagrams & ratings, you would be on firmer ground.

Also, remember your phase converter has a rating that cannot be exceeded. Secondly, the ones I have worked on had to be setup specifically for the equipment load. That was done by internally wiring  more capacitors or less capacitors in to the circuit.

Clear as mud?   

TOM

[Len Silva]

As Ted said, you don't want to use the pressure switch to carry the motor load and you definitely want to disconnect all three leads when shutting down, which means you need a three phase motor controller and a separate control circuit through the pressure switch.

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p5197.m570.l1313&_nkw=three+phase+motor+starter&_sacat=See-All-Categories

I would also suggest that you remove the belt until you are sure the motor is running in the right direction.  Three phase motors are reversed by switching any two leads.

[Ericbsc]

Most three phase motors are either low, or high voltage (230/440). The way the pairs mate determine this. The switch will break one of the three legs. If you are using a rotary phase converter, the generated leg will be higher voltage than the two legs off of the panel. DO NOT use the generated leg to operate the switch. I run app. 250 hp in several pcs. of equipment including Electronic equipment. The gen leg always goes to the motor not control side. Been running this way for 20 yrs. with no problems. P.S. the switch is nothing more that a well pump switch most of the time. Just my 2 cents.!! Good luck.

[kevink1955]

The switch has to break 2 of the 3 legs otherwise the motor will be single phased. If it was running when you open 1 leg the motor will continue to run and over heat, if it was stoped and you apply power with 1 leg open it will just sit there and hum till the smoke comes out.

It sounds like you have a multi voltage motor and most do NOT have any built in overload protection. It will require a motor starter (contactor) with properly sized overload heaters to protect it. Check the nameplate if it does not say internal over load protected (or something like it) it will need a starter.

Also check the horse power VS what your convertor is rated to run, if the convertor is to small it will never be able to start it and you may let the smoke out of both trying.

[Ericbsc]

YOU ARE CORRECT SIR!! I was thinking 220v single phase!! I have one old recip. Compressor. I has a switch with two legs.

[TomC]

Your motor is not a straight 208v 3 phase-it is a multi voltage motor.  There should be a diagram on the motor, or go to the manufacture and get one.  It sounds like it can run on 208/240/480v depending on how the wires are connected.  If it was a straight 208v motor, it would just have 4 wires plus ground.  Good Luck, TomC

[buswarrior]

This site is pretty neat:

http://www.anoldman.com/strange/re-wiring_a_three_phase_generator

Lots of pictures and explanations that might help to refresh memories?

happy coaching!
buswarrior