Air Compressor has Trouble Starting Under Load?

[clubairth]

I have a big 7.5HP-80 Gallon compressor. It's 220V and I have been running it for the last 2 years on a dedicated 8 gauge wire/50 Amp breaker with no problems.

It's a Husky TF291204 which is a re-branded CH unit. Been working fine and I use it a lot as I have a large blast box and use SATA HVLP paint guns. All of these are air hogs.

Purchased the compressor used 2 years ago with a new motor and many missing parts. The shop I bought it from had a bunch of clueless experts that screwed it up so bad nothing would work. The boss told them to just buy a new compressor and stop messing with the old one. They had already bought a new 7.5 HP motor from Grainger and that was a big plus for me!

Got the manual and replaced everything missing including the pressure switch, check valve and unloader assembly. Unit worked great and pumps up to 175 psi and shuts off with a big hiss from the unloader.

Problem is it's now having trouble starting under load. Starts but struggles to get up to speed like it's having trouble switching from the start to the run mode in the motor?

Sounded like it was trying to start against pressure which points to the check valve or unloader. Further if I shut the ball valve to the air system off and drain the tank it starts up fine. If it has been running then it starts under load OK like when I am blasting as the compressor only shuts off for a short time before kicking back on.

When the compressor shuts off and the hissing from the unloader stops. I have unhooked the outlet pipe from the pump and there is no pressure so I think that shows the check valve is holding and the unloader has drained the trapped pressure off?

Can the start circuit on the motor be causing these problems?

Here is a diagram of my compressor;

.
.
.

 

[The Cobbler]

I have a craftsman that does the same thing, the pressure was building too fast in the line from the pump to the tank , and before the motor got up to speed, it would pop the breaker. I installed a cold start valve that allows pressure to bleed off . I have a thread on it here that I can dig up later. a larger volume in the pump line would have done the same thing.
some folks have had that problem with bad start caps too.

cold start valve http://www.ebay.com/itm/New-air-com...hash=item2ec433a01b:m:mWN4ZeBRrSdOlNdcTxbV2xg

 

[redmondjp]

Another common failure point for this type of issue is something wrong in the motor starting circuit, either with the motor starting capacitor(s) or the centrifugal switch inside one end of the motor that cuts out the starting circuit at full speed.

You want to get this type of issue fixed sooner rather than later, as you are cooking your motor windings in the mean time which will dramatically shorten the motor's life.

And last but not least - check your belt tension. As reported in many other threads here, a loose belt has ended up causing this issue as well.

 

[clubairth]

The cold start valve is interesting and kind of points to my problem. But the factory did not put it in and the unit was running fine before this for 2 years without one?

I have checked and there is no back pressure against the pump. I have cracked the outlet tube fitting some so it leaks when the pump is running so it can't store any pressure between the pump and the tank and it does not.

I am leaning towards the starting circuit as the problem but again the motor was brand new 2 years ago and I am only a DIY hobby guy so it can't have that many hours on it.

One other possible clue? It seems worse the colder it is. I did read where the pump lube can create a bigger than normal load when cold. I changed the pump oil when I rebuilt it and used synthetic compressor oil as Husky recommends. The oil site glass shows the proper level and the oil is a bit dark in color.

I don't let motor labor (It sounds terrible when it's trying to start). If it does not start I shut it off, isolate the tank, drain the tank to about 40-60 psi switch it back on and it will usually take right off. Sometimes I have to bleed it to a lower pressure before it will catch.

I am afraid of cooking something in the motor like you said. I leave the compressor off unless I am working in the shop.
.
.
.

 

[Davefr]

Remove the belt and try turning the pump by hand after the pressure relief valve has farted. How does it feel. (same as when the tank is empty??)

Now turn the motor over by hand. How does it feel?

Test the capacitors.

Put an amp clamp on the motor when it's running and compare it to nameplate.

 

[American Locomotive]

If the unloader is indeed working (which it seems to be since you can take the line off the pump), then I suspect you either have bad start-capacitors inside the motor, or you have an electrical fault.

If you're handy with a multimeter, I'd start at the electrical side of things. If you don't have one, or you're not comfortable with electricity, I'd go ahead and replace the start capacitors.

 

[jl4c]

If the unloader is indeed working (which it seems to be since you can take the line off the pump), then I suspect you either have bad start-capacitors inside the motor, or you have an electrical fault.

If you're handy with a multimeter, I'd start at the electrical side of things. If you don't have one, or you're not comfortable with electricity, I'd go ahead and replace the start capacitors.

I agree with AL. If the unloader valve is functioning properly and the pump spins freely then you have an electrical fault: either a bad start or run capacitor (or they have a bad connection), or the centrifugal switch in the motor that causes the motor to switch from the start to the run capacitor, or a short in the motor windings themselves.

A good Fluke meter will test your capacitors for you. The capacitors are fairly inexpensive. If you have a bad cap and don't fix it soon, you will burn out your motor in short order.

 

[clubairth]

Good suggestions!
Yes I do have a Fluke that will test capacitors too. I just need to brush up a bit on how to measure them! I am sure they store a LOT of juice!

The last ones I replaced were about 10 years ago in my big screen TV. On smaller capacitors it's common when they fail to swell up and look different than good capacitors.

Should I expect to see the tops swell on these big capacitors?
.
.
.

 

[American Locomotive]

Sometimes, but not always. Remember, depending on the type of motor, it may have both run and start caps.

 

[VDubJoe]

I have a Ch 7.5 about 15 years old. They haven't changed much. It has been a workhorse.
It did the exact same thing. It has 3 capacitors. I checked two of them and 1 was bad. It was only 12 bucks. Replaced it still did the same.
Then there was a 3rd one that I had no clue how to ck it. Had double the wires on it. I took it to a motor shop and he checked it. I forget what it was called. But he finally found one and it was 175.00 bucks. Replaced and everything back to normal.
Good luck.

 

[EOC_Jason]

I think everyone has mentioned possible things to test...

Assuming unloader is working (you say it hisses when it go off), and also check valve is good otherwise it would constantly be hissing air.

Also taking off the belt to verify by hand the motor & pump spin as they should.

1. Belt Tension - If it's too loose it will act a lot like you are talking about.
2. Start / Run Capacitors - Possibly... Especially since you said when it's cold. Caps don't work as well in cold weather but once they warm up they can function normally and can be a pain to troubleshoot in certain scenario's. Yes, sometimes they will swell or even leak, but not always.
3. Pump Oil - Usually synthetic is good for extreme temps, both hot & cold. If it seems hard to spin the pump by hand, maybe switching to a lighter weight synthetic would help?
4. Check your electrical connections are tight.
5. Checking Amperage - If you have an amp meter, clamp a lead and see what happens. That might give some indication as to what is going on.

 

[clubairth]

Good idea's!

I will check amp draw and check how easy the pump and motor are to spin with the belt off.

It's just such a pain to get the belt tight again as they have no adjusting mechanism.
.
.
.

 

[EOC_Jason]

Someone posted a pic somewhere where they made an expander to tension their belt from a couple blocks of wood and some allthread, nuts & washers...

 

[sberry]

How bout this?

 

[clubairth]

I think I found the problem?

I have 3 capacitors.
Here is what they are marked with and what they measured below that.
I have a Fluke 87V that will measure capacitors.

1. 40 µF, +/- 5%, 450 VAC
Measures .39 nF so it's way bad!

2. 35 µF, +/- 5%, 450 VAC
Measures 35 µF which is right on the money.

3. 640-715 µF, no + or - rating?, 330 VAC
Measures 739 µF.

Is the 3rd one far enough out of range to replace? With no + or - I am not sure?

I was going to look on EBay unless someone can suggest somewhere else?

This seems to be the right one?

http://www.ebay.com/itm/Run-Capacitor-40-MFD-uf-370-440-v-vac-volts-Round-AC-Electric-Motor-HVAC-40-uf-/261104075876?hash=item3ccb040464:g:nogAAOSw4HVWFDf0
.
.
.

 

[Davefr]

I think I found the problem?

I have 3 capacitors.
Here is what they are marked with and what they measured below that.
I have a Fluke 87V that will measure capacitors.

1. 40 µF, +/- 5%, 450 VAC
Measures .39 nF so it's way bad!

2. 35 µF, +/- 5%, 450 VAC
Measures 35 µF which is right on the money.

3. 640-715 µF, no + or - rating?, 330 VAC
Measures 739 µF.

Is the 3rd one far enough out of range to replace? With no + or - I am not sure?

I was going to look on EBay unless someone can suggest somewhere else?
.
.
.

#3 should be just fine. Capacitance isn't that precise.

 

[clubairth]

Thanks and good to know!

The EBay ones range from $8 to $40+ so many variations.
.
.
.

 

[EOC_Jason]

Yeah, #3 is okay, that's like +3%, every capacitor is generally rated +/- 5% as the standard.

 

[turbodave]

I think everyone has mentioned possible things to test...

Assuming unloader is working (you say it hisses when it go off), and also check valve is good otherwise it would constantly be hissing air.

Also taking off the belt to verify by hand the motor & pump spin as they should.

1. Belt Tension - If it's too loose it will act a lot like you are talking about.

+1 on checking belt tension, my 5hp Sanborn compressor starting chugging when trying to start under load, worse when cold, and I was convinced it was the unloader, motor or some other issue. Turns out the belt was just slipping. You couldn't hear it, and where my compressor is located it's hard to see the belt, but now every couple years it works loose and a retensioning of the belt solves the problem.

On mine I can get a small bottle jack between the motor and the compressor, this works pretty well to get it tensioned.

 

[Cope]

Grainger sells a plate that you mount between your motor and base that allows you to adjust the belt with a wrench.

 

[hedgehog]

Another common failure point for this type of issue is something wrong in the motor starting circuit, either with the motor starting capacitor(s) or the centrifugal switch inside one end of the motor that cuts out the starting circuit at full speed.

You want to get this type of issue fixed sooner rather than later, as you are cooking your motor windings in the mean time which will dramatically shorten the motor's life.

And last but not least - check your belt tension. As reported in many other threads here, a loose belt has ended up causing this issue as well.

What he said , capacitors are easy to check if you have a multimeter. Check out a recent video on youtube on compressors uploaded by "Ave" he is just as informative as he is funny

 

[American Locomotive]

I think I found the problem?

I have 3 capacitors.
Here is what they are marked with and what they measured below that.
I have a Fluke 87V that will measure capacitors.

1. 40 µF, +/- 5%, 450 VAC
Measures .39 nF so it's way bad!

2. 35 µF, +/- 5%, 450 VAC
Measures 35 µF which is right on the money.

3. 640-715 µF, no + or - rating?, 330 VAC
Measures 739 µF.

Is the 3rd one far enough out of range to replace? With no + or - I am not sure?

I was going to look on EBay unless someone can suggest somewhere else?

This seems to be the right one?

http://www.ebay.com/itm/Run-Capacitor-40-MFD-uf-370-440-v-vac-volts-Round-AC-Electric-Motor-HVAC-40-uf-/261104075876?hash=item3ccb040464:g:nogAAOSw4HVWFDf0
.
.
.

That cap looks good to me as long as the dimensions are correct.

 

[6PTsocket]

About that over value cap. I have never heard of a cap gaining capacitance as it ages so presumably, the compressor has been running OK with that high value cap since you got it.

Sent from my SM-G900V using Tapatalk

 

[clubairth]

You mean cap #3?
It's not very much higher.
.
.
.

 

[truckdriver]

Mine was doing close to the same thing. So I pulled off the belt and then ran the motor. It started but took a long time to get to speed. My compressor came from a body shop and the connections for the caps were covered in primer over spray. Cleaned the connections and it was all good. replacing the start cap will probably fix the issue.

 

[clubairth]

Great thanks for all the help!
I always learn so much from this forum that I did not know but needed too!!

A new Chinese capacitor and it is working as new again.
$8 delivered off EBay.
.
.
.

 

[6PTsocket]

You mean cap #3?
It's not very much higher.
.
.
.

Exactly. The question was whether to replace it, too. In a word, no.

Sent from my SM-G900V using Tapatalk

 

[jl4c]

About that over value cap. I have never heard of a cap gaining capacitance as it ages so presumably, the compressor has been running OK with that high value cap since you got it.

Agreed. Good observation.

 

[jl4c]

Great thanks for all the help!
A new Chinese capacitor and it is working as new again.
$8 delivered off EBay.

Congrats. Hard to beat that.

Incidentally the function of the capacitor is to create a rotating magnetic field inside the motor. The rotor "follows" this rotating magnetic field. When the cap goes bad the field oscillates (it's AC power after all), but doesn't rotate. Hence the motor vibrates instead of turning or has little to no startup torque.