Troubleshooting Air Compressor - Runs But Sometimes Pops Circuit Breaker on Startup

[tracerbullet]

I'm hoping to get a little help figuring out how to keep my air compressor from popping the circuit breaker when it starts up. It's a Devilbiss compressor left behind by the previous owner of my house, so I don't know much about its history.

It seems to run well, but sometimes (1 in 4) when it starts up it pops the breaker for the circuit it's connected to. It seems to dim the lights quite a bit too so I shouldn't be surprised.

I pulled off the cover and see a pair of small capacitors, but I'm not sure of a few things. I thought they were for assisting the motor to start up, and one or both were bad, but I don't see any apparent damage to them (bulging, leaking). On a whim I checked voltage to them, figuring they must be receiving it even when the motor is not running, but even with the compressor plugged in and in the "auto" position they aren't seeing anything. Are these "running" caps perhaps and I should be looking elsewhere?

I've googled for an hour or two and am just more confused.

(MO-9045 GE motor, caps are GE 97F9606 and Mallory 540-648MFD if this helps at all).

 

[tracerbullet]

I read about valves, and checked them (I think), they seem good. The check valve screwed into the tank itself does not seem to leak any air when the other two lines (to the pump and the other valve) are disconnected. That other valve does indeed "whoosh" when the compressor shuts off.

In other words, it does not appear that the compressor is trying to start up with a whole lot of air pressure against it right away.

 

[boomerang]

Caps can definitely be bad without bulging or leaking. Maybe this will help.

https://www.youtube.com/watch?v=iRt9WYK1wdQ

Just watched it myself and his methodology is a little too simplistic but as always with youtube there are some others to the right that may be more helpful.

 

[tracerbullet]

Thanks, I did see that and try it, but my multimeters are both digital and they don't quite relay the same info he shows with his analog version. They just sort of "blip", which is something they always seem to do regardless of what you touch. That may be enough indication the caps are live but it's not really what you'd call convincing.

Unless I can figure out something else, I'm hoping I can find a multimeter at work that has a cap testing function on it.

 

[skyking]

look for loose connections as well. anything that looks discolored, etc

 

[dbcooper1]

Is there something else on the circuit that might be in use only sometimes? Sounds like the startup current for the motor is right at the limit of the breaker or the breaker has weakened over time.

 

[boomerang]

Thanks, I did see that and try it, but my multimeters are both digital and they don't quite relay the same info he shows with his analog version. They just sort of "blip", which is something they always seem to do regardless of what you touch. That may be enough indication the caps are live but it's not really what you'd call convincing.

Unless I can figure out something else, I'm hoping I can find a multimeter at work that has a cap testing function on it.

I just had a cap replaced on the compressor on our A/C while the tech was there for another issue. It was extremely rusty and in a Florida climate with us being gone for many months out of the year I had him replace it as more of a preventative maintenance type of thing. Anyway, I watched the guy do it and he had nothing special just a digital multimeter. That's why after watching that video I suggested a related video might have more specific info.

I'd like to know myself the "right" way to test them.

Edit: Just looking around a bit and there are in fact capacitor testers that can test their capacitance value. I think you're about two or three steps ahead of me but I am learning something.

 

[boomerang]

I'm wondering too if the compressor is plugged in to the same location as the previous owner had it. In other words if it runs off 110 is it plugged into a 15A outlet instead of a 20A outlet. If it runs off 220, I am pretty much clueless. I only know so much about this stuff.

 

[jaha2000]

Plugged into extension cord?

 

[gbeirn]

Some Google-fu came up with this: http://www.justanswer.com/electrical/92ln3-ge-motor-compressor-5kcr49sn2137x-works-fine-first.html

My guess is the compressor is fine but if that motor is indeed a 5hp 15a &120v it needs to be on a circuit all its own, preferably 20a. The breaker could be worn out especially if it has been tripping like this for a long time.

If it's older the caps are probably fine since older ones were typically built better than cheaper new Chinese crap. As long as the windings don't look or smell burnt it is probably the circuit is undersized or over used. Especially on startup when more current is needed initially. Good luck!

 

[RagingBITCH]

It's the caps. They provide that initial boost (hold charge) to kick-start everything. If the compressor kicking on blows your circuit, it's that. (Or something in your panel is fubared/overloaded, let's hope it's not that b/c that is a hell of a lot more expensive to troubleshoot/replace than a few capacitors)

Caps could also be overrated for the compressor...the cap on my main unit went out. I replaced it last summer and during my last tune-up, the AC girl said it was actually way overpowered for the unit.

Not sure if you can Google or call to find out the right capacity for the caps. That'd be my best guess at it.

 

[tracerbullet]

Loose wires - good point, it really could be that simple. I'll have to pull things apart a little further tonight and check. it'll also give me an idea of how the power is sent to the motor, caps, etc. which might be helpful.

The rest of the circuit connects to the living room, on the other side of the wall, the only other thing that might be on is a floor lamp with a couple CF bulbs. Good question but I don't think that's it. Nothing else like a toaster or a plasma TV, stereo equipment or anything.

It is just a 110 outlet. I think it's probably plugged into the same spot as the previous owner. But he may have had issues as well, I get the feeling he didn't use it very often. He was in somewhat poor health and some back problems that make me think he wasn't doing a whole lot of wrenching in the garage. Who knows.

Not in an extension cord, direct to the wall. I will assume the outlet is good / correctly wired. House is late 80's.

The motor has a tag with "15A/7.5A" on it, it doesn't actually say what that is but I am wondering if it's startup and run amperages? It could be as simple as a 15A circuit breaker popping when 15A is being pulled. I was really hoping the caps would keep that from happening but... yeah it could be that simple.

I guess at this point I have to check the breaker rating, and pull the caps and find a multimeter capable of checking said caps. While I'm at it I'll look for any loose wiring around the thing as well.

Hope to get to it tonight, check the breaker at least and wiring, and pull the caps to bring them into work. I'll report back with what I find.

 

[jaha2000]

If your on a 15A breaker that's the problem for sure.
Rule of thumb is 80%. That's 12Amps. And if the breaker is original to the house, its probably even less than that.

 

[boomerang]

Loose wires - good point, it really could be that simple. I'll have to pull things apart a little further tonight and check. it'll also give me an idea of how the power is sent to the motor, caps, etc. which might be helpful.

The rest of the circuit connects to the living room, on the other side of the wall, the only other thing that might be on is a floor lamp with a couple CF bulbs. Good question but I don't think that's it. Nothing else like a toaster or a plasma TV, stereo equipment or anything.

It is just a 110 outlet. I think it's probably plugged into the same spot as the previous owner. But he may have had issues as well, I get the feeling he didn't use it very often. He was in somewhat poor health and some back problems that make me think he wasn't doing a whole lot of wrenching in the garage. Who knows.

Not in an extension cord, direct to the wall. I will assume the outlet is good / correctly wired. House is late 80's.

The motor has a tag with "15A/7.5A" on it, it doesn't actually say what that is but I am wondering if it's startup and run amperages? It could be as simple as a 15A circuit breaker popping when 15A is being pulled. I was really hoping the caps would keep that from happening but... yeah it could be that simple.

I guess at this point I have to check the breaker rating, and pull the caps and find a multimeter capable of checking said caps. While I'm at it I'll look for any loose wiring around the thing as well.

Hope to get to it tonight, check the breaker at least and wiring, and pull the caps to bring them into work. I'll report back with what I find.

Got a heavy duty extension cord? The type rated for outdoor use? Plug into an outlet that you know is not being actively shared or better, into a kitchen outlet (attached garage?) that should be on a 20A circuit. See if it trips the breaker. IMO if it pops a 20A it's caps. If it pops a 15A not under any other load, it could be caps. Obviously you would only use the extension cord if you need it to reach a specific outlet. I know you're going to check the caps anyway and I would too under the circumstances. But then again, if they're like $5 each, I might just get replacements because that's the kind of guy I am.

It's possible the guy never used it, just owned it. Maybe he was a picker and couldn't pass up a good deal.

 

[gbeirn]

It's 15a on a 120v circuit and 7.5a on a 220v circuit. Even a few lights shared on the same circuit will be enough to trip it. A dedicated 20a circuit for it or rewiring it for 220v use will solve your problem.

Edit: The motor will draw more than 15a for a brief couple of ms when first starting, that's what the caps help for. a good breaker accounts for this and only trips over a certain threshold of ms.

 

[tracerbullet]

It's 15a on a 120v circuit and 7.5a on a 220v circuit.

That makes perfect sense. And yep it's connected to an outlet w/ a 15A breaker. Funny it's labeled "lites" and not the 20A labeled "garage" which doesn't seem to actually connect to any outlet in the garage. I don't expect to launch into a panel relabel project anytime soon, but it's good I guess to know it's definitely not correct, in part at least.

But yeah, 15A breaker, 15A rated draw. Bad combo. I'll bring the caps to work and hopefully find a good meter to check them.

And I will not be randomly installing a 20A breaker, so no worries there. (If I do, ever, I'll be 100% certain the wires are rated for it, but I'm not optimistic or I think that's what would already be there.)

 

[DrPizza]

Startup current is typically higher than the rated current for a very very brief amount of time.

 

[crashtech]

Could be inrush current. Making sure all wiring and connections are sound, of the proper gauge, and no longer than necessary is a good plan. If the start capacitors are shot, the compressor won't start at all. Before changing to a breaker with a higher current rating, consult an electrician.

 

[edro]

Startup current on an electric motor is 5-10 times FLA (full load amps).
Circuit breakers have a trip curve that allow well over the rated current for a while.
15a load takes around 10 min. to trip a 15a breaker.

The problem here is that your startup current well exceeds the trip curve of the breaker.
Many breakers can handle around 8-10x their rating for 1 sec. (the trip curves are flat up to about a second)
This is usually long enough for motor startups (inductive inrushes).

If the motor has some age, hasn't been used in a while, has older caps, etc, it can take more than 1sec. to get up to speed.
This will cause the 15a breaker to trip.

I know my 5gal takes at least a sec. to start. The lights dim, the motor hums like crazy, then it starts, and that's on a 20a circuit.

You need to use a 20A circuit.

 

[tracerbullet]

Caps tested good. Breaker is 15A and wire is 14 gauge so that's that.

I'll live with it or consider buying a compressor that pulls a little less juice.

Thanks everyone for the input and advice!

 

[Mandres]

Might be a good time to consider adding a sub-panel in the garage. Eventually you're going to want a table saw, welder, etc. and a single 15a circuit isn't going to cut it.

It isn't that hard to do, or expensive to have done.

 

[crashtech]

Every manly garage should have at least a 50A subpanel.

 

[tracerbullet]

I understand but there's like 1,039 other projects to work on as well. I guess it could be a point to just hire someone to do.

Funny how this went from replacing a starter cap DIY to save a couple $'s to hiring an electrician to install a subpanel.