compressor help

[fubar008]

sorry if this has been asked, i tried searching but i didn't find an answer.
I have a 7hp air compressor maybe 10 yrs old. It worked fine until I moved it into my new shop last night. When I turn it on it only hums and will not start. I can turn it by hand but I decided to remove the belt and the motor runs fine on it's own. What should I do now? Thanks

 

[jkwilson]

Check your power source.

 

[bad_idea]

Perhaps the motor capacitor is failing? The capacitor gives the motor a bump to get it going (basically). If the motor doesn't get that bump it doesn't have the umph to turn the pump. I'm no sparky, as my terminology might show. Hope this points you in a direction to look.

 

[JJThrasher]

Was the pump kept upright so that the oil is was in the crankcase, not on its side where the oil could have leaked into the cylinders and hydro-locked it? Assuming that the compressor was not damaged during transport I would definitely check the new power supply to it.

 

[fubar008]

It was tipped down to get it off the trailer but not all the way on its side. I checked the oil none ran out. If it was hydro-locked I wouldn't be able to turn it over by hand would I? The new power supply all checks out although i only have a 60amp service but when the guys came to spray foam it ran there trailer with there compressor and proportioner just fine

 

[JJThrasher]

I missed that part of your original post.

 

[fubar008]

I missed that part of your original post.

No problem. This was actually my first thought was that i tipped it down to far, It got real heavy quick l

 

[sld961]

When you say the new power source checks out, what all have you checked? What size wire do you have, and how far is it from the panel? Have you checked voltage at the motor when it's running (or trying to)?

Could be start capacitor, but that would be a coincidence. What has changed between the old and new location?

Do you have an unloader valve? Is it functioning?

 

[cgrutt]

I have a smaller compressor (3HP) but did the same thing. Thought it was bad caps and replaced both but still would not start just hum. Power was fine. Turned out to be bad switch in motor that switches start windings to run windings within motor. Brought motor into repair shop and they diagnosed and repaired.

 

[matt_i]

Good steps so far. As above, typical steps are to verify voltage in new location is same as old. After that, the start cap or the centrifugal switch that engages and disengages it would be suspected next.

Its theoretically possible that there's a problem with the pressure switch or motor starter (obliquely referred to as MAG Starter in compressor lingo only) that has a failing contact and can't transmit the entire voltage under the full load.

 

[Rossco]

Spinning the right way?

 

[fubar008]

Well when I say it checked out I had the electrician that did the electrical in the new shop double check it (I have little experience in electrical work) he said it was good but I have not verified it myself. I do own a meter I can test it tonight. The plug is about 3 feet from the panel installed exactly the same as the old shop with a 20 amp breaker. I do not know the wire size its behind the foam and I have no opened the panel.

Its sounding like the start capacitor? How exactly would I go about testing that?

Thanks

 

[sld961]

20A is too small for a 7hp motor. 20A breaker should have 12 wire. I doubt there it's that much voltage drop in 3ft, but 20A breaker and 12 wire is too small for 7hp.

 

[bobmulry]

Hi,

I think that your electrician gave you bad info.......

7 HP motor on 240 volts = 35 to 40 amps (FLA)......

At a minimum there should be a 50 amp breaker feeding 8 ga. wire......

12 ga. is only rated for 20 amps...........

Pull the panel face and see what size wire is feeding the 20 amp breaker....

It might need a re-do to use your compressor on that circuit or pull a new one with the correct size wire.....

Good luck,
Bob

 

[CNGsaves]

OP . . . post up copy of MOTOR PLATE showing details and 7 HP as you say.

Your "electrician" that put 7HP compressor on 20A breaker should be shot !!
He didn't know what the hell he was doing !!

GJ Sparky's will set you straight once you put up motor plate showing the HP of the compressor motor . . . that's what determines wire size and breaker.

 

[TAB57]

Hello does it have a motor starter? If so should be a reset button on it try that. Does sound like you have a motor starter as you can hear a hum and I think your problem is there.

 

[redmondjp]

Guys, come on now, it probably isn't a true 7HP motor at all - more like 3.7HP - it is probably one of the compressors made before that class-action lawsuit forced them to dial back their stated (peak) horsepower claims. Most of these compressors work just fine on a 240V, 30A circuit (a dryer circuit, in other words, wired with 10GA wire).

First I'd check to see that there really is 240VAC at the compressor - then check what that voltage drops to when the compressor tries to start.

For awhile, due to a miswired extension cord, I was running my 240VAC table saw on 120V and didn't even realize it. Different situation as the motor starts no-load and it took longer than normal for the motor to start, but I felt really dumb when I figured it out. No way that a compressor will start on half-voltage, however, so that is something to check.

 

[fubar008]

Guys, come on now, it probably isn't a true 7HP motor at all - more like 3.7HP - it is probably one of the compressors made before that class-action lawsuit forced them to dial back their stated (peak) horsepower claims. Most of these compressors work just fine on a 240V, 30A circuit (a dryer circuit, in other words, wired with 10GA wire).

First I'd check to see that there really is 240VAC at the compressor - then check what that voltage drops to when the compressor tries to start.

For awhile, due to a miswired extension cord, I was running my 240VAC table saw on 120V and didn't even realize it. Different situation as the motor starts no-load and it took longer than normal for the motor to start, but I felt really dumb when I figured it out. No way that a compressor will start on half-voltage, however, so that is something to check.

I didn't know about this but agree it does not seem like 7hp thats just what it says. It's an old farm hand from tsc the same one that I looked at today in the store that says 3.7. I will go take a picture of the motor plate

 

[fubar008]

It is 12-2 with a 20 amp breaker. I understand now that that needs to be changed but this compressor was used everyday in the old shop with the same wiring set up

 

[bobmulry]

WOW.............

15 amps @ 240 volts = 1 1/4 HP

Talk about BS'ing the consumer....

It looks like they use a 3450 rpm motor and under-drive the compressor to make up for the lack of power..............

Bob

 

[sld961]

WOW.............

15 amps @ 240 volts = 1 1/4 HP

Talk about BS'ing the consumer....

It looks like they use a 3450 rpm motor and under-drive the compressor to make up for the lack of power..............

Bob

I don't think that's right. 15amp at 240v is around 3hp as far as I know. I could be wrong tho.

 

[matt_i]

Round up to 3kW, then 745W per HP puts it right around 4hp if you believe the label rating.

 

[bobmulry]

I don't think that's right. 15amp at 240v is around 3hp.

My bad.....

I ran out of fingers and toes.......

15 amps @ 240 = 2 1/2 HP......

That's still along way from 7 or even 3.7 HP.....

Bob

PS:
15 amps @ 120 volts = 1 1/4 HP

 

[Dagny]

Under HP it says spl that means they can lie to you on some other sticker on the tank or somewhere else. Any one who works on a lot of motors can spot these a mile away. A real 7.5 hp motor will weigh nearly as much as that whole comp.That being said they work quite well for their cost.

 

[fubar008]

I'm glad it was given to me if I ever buy a new one i'll definitely be doing some research before I buy this also explains why this thing has always kinda sucked. my uncle had it for his foam business and it wouldn't keep up so he gave it to me.

Is my 20 amp breaker an 12 wire ok now??

 

[bobmulry]

I'm glad it was given to me if I ever buy a new one i'll definitely be doing some research before I buy this also explains why this thing has always kinda sucked. my uncle had it for his foam business and it wouldn't keep up so he gave it to me.

Is my 20 amp breaker an 12 wire ok now??

Based on 15 amp (FLA) it will work fine...

 

[fubar008]

so assuming the wiring is correct where do I go from here?

 

[MJD1]

so assuming the wiring is correct where do I go from here?

Is there an unloader valve? It is typically brass with a wire ring you pull to release pressure on the compressor head. Pull that then try starting the motor.

 

[sld961]

Is there an unloader valve? It is typically brass with a wire ring you pull to release pressure on the compressor head. Pull that then try starting the motor.

I think you're getting the unloader valve confused with the pressure relief valve. The pressure relief valve is the one with the ring. The unloader valve is generally incorporated into the pressure switch.

 

[fubar008]

I tried pulling the pressure relief valve and starting, no change. It does have an unloader valve. I still think it might have something to do with the start cap but I'm not sure how to go about testing that.

 

[sld961]

I tried pulling the pressure relief valve and starting, no change. It does have an unloader valve. I still think it might have something to do with the start cap but I'm not sure how to go about testing that.

Not sure how to test it. Pull the cover off the capacitor and get the number off of it. Google it. It's probably like $5 for a new one. Buy a new one and put it on, see if that fixes it.

 

[cgrutt]

My Ingersoll Rand has an Emerson Motor that also says SPL as HP., 3450 RPM, 230v & 18 A on motor plate. There is another plate on the tank itself that says 3HP and lists amps at 16.6 but close enough. The motor was doing the same thing, just humming when power was put to it. I changed out both caps (Starting and Running) and installed a new start switch/cut off valve, which didn't fix problem. As stated in earlier post, I took it to a motor shop and it turned out to be a bad switch within the motor itself. It wound up costing about $60 to diagnose and repair.

I'd take it to a repair shop and have them bench test it before spending a lot of money on new caps, etc. I just went through this a couple months ago...

Oh, BTW, I'm running 30A breaker with 10GA wire.

Good luck.

 

[jkwilson]

Before anything else, measure the voltage where the power enters the compressor while it is trying to start.

The fact that it runs without the belt points strongly to inadequate current capacity on the power feed.

 

[VDubJoe]

Just Google testing a capacitor. Very easy. Probably is the problem.

 

[Bandit]

That unit does have a unloader built into the pressure switch .
If that isn't venting the discharge line from the head , you would get the results that you have .
But since you moved it , I would start with the breaker and motor wiring and work your way out .
Bob