Compressor motor starter/contactor question

[EarlyBroncoGuy]

Picked up a big (80 gallon, 220v 5hp 1 phase) industrial air compressor for free, customer of mine installed a new one and said I could have the old one if I drug it off - so I did.

It's a few years old, but in decent shape. He said it would trip the breaker every now and then, once I got it home and looked it over good I found some cracked insulation on the wiring from the starter/contactor to the motor, so I removed and will replace that. When I checked out the starter/contactor, I get continuity between the terminals (line and load) when I push in on the contacts, which is normal, but I also measured 550 ohms between the two lines (black and white wire terminals, which are each 110v when hooked up). That doesn't seem right, I would think the two lines would be independent of each other, the only continuity between them would be the windings of the motor - but it's not connected.

Am I thinking correctly?

 

[LS6 Tommy]

I can't follow what you're saying. Got a diagram or pics?

Tommy

 

[nehog]

Sounds like you are measuring the coil resistance because the pressure switch is closed.

 

[matt_i]

Imo unless you know about his wiring and breaker and the electrical strength of the connections, then your power system isn't the same as his. I wouldn't mess with measurements, plug the thing in on an appropriately sized circuit and see what it does.

If you have same issues it could be an unloader problem, a mechanical problem in the compressor or a motor issue.

 

[goingtoarizona]

Your 550 ohm measurement is measuring the total resistance between (If I understand correctly) the both sides of the contactor and the motor wiring. The reading you are giving is meaningless. IF you have the specs for the motor windings you could measure that, but eliminate the contactor. I would check the resistance between the windings and the frame to check for a shorted winding.

I would run it and check the amp draw and see if it trips what ever breaker it's rated for (hopefully on the spec plate). From my experience, the reasons it would trip a breaker are a weak breaker, shorted windings, a stuck start switch, bad motor bearings or a bad pump. Good lick with your find.

Let us know what you find when you get it fixed.

 

[Cyberbear]

Give the entire unit a good visual inspection, check proper wire size, check for proper breaker size. If it ran for the other guy, it should run for you as well. These are simple hook ups and any problems should be evident once you've tried to run the unit. If you have doubts, call out a professional electrician to trouble shoot it for you.

 

[redmondjp]

Sounds like you are measuring the coil resistance because the pressure switch is closed.

I'm thinking the exact same thing!

 

[ducatithunder]

550Ohms doesnt sounds right. It should be a few ohms when measured across the winding. I would measure at the motor instead of at the contactor. The motor resistance is what matters not the whole circuit. This will give you a much better reading. Is it a 2 wire motor of 3 wire motor? If its a 3 wire you can take a start winding resistance and run winding resistance. The start will always be greater then the run winding. But it sounds like if you have a black and white wire its only a 2 wire. Take a pic so we can see what your working with?

 

[redmondjp]

550Ohms doesnt sounds right. It should be a few ohms when measured across the winding. I would measure at the motor instead of at the contactor. The motor resistance is what matters not the whole circuit. This will give you a much better reading. Is it a 2 wire motor of 3 wire motor? If its a 3 wire you can take a start winding resistance and run winding resistance. The start will always be greater then the run winding. But it sounds like if you have a black and white wire its only a 2 wire. Take a pic so we can see what your working with?

Go back and read his original post - this is the resistance measured between the two line in conductors with the contactor open. So the motor was not in the circuit at all.

 

[ducatithunder]

Go back and read his original post - this is the resistance measured between the two line in conductors with the contactor open. So the motor was not in the circuit at all.

Thanks. I see that now. I assumed he was getting Line and Load confused. Seems like the rest of the guys where where thinking he was measuring the load side too.

 

[DeltaWye]

A picture is worth 1000 words...