Air Compressor Problem - Motor Stalls

So the original post is incorrect then? WHich is right, this or the other one? Is motor current high when the motor stalls?

I measured about 5 amps at startup (0 psi) rising to close to 6 when it stalls. When it stops turning, the current actually drops back closer to 5 amps, then increases as the motor starts turning again.

On this compressor, the check valve/unloader is completely separate from the pressure switch. The unloader is similar to this one:

The voltage to the motor is never interrupted during the stop/start cycle. Above a certain psi (about 95), the motor starts bogging down and eventually stops. This occurs even if I bypass the pressure switch and attach the cord directly to the motor leads. Then, after the unloader vents the pressure, the motor will restart.

Input current does not swing sky-high, though, nor does the breaker trip, nor is there a hell of a noise. It definitely comes to a dead stop, though.

Yes, those're the symptoms.

So it stalls first, then the unloader bleeds off pressure at the compressor and allows it to restart?

I'm puzzled, but at this point I would disable the switch and unloader and see if the compressor runs up to 120 as it used to.

You said the motor current is only 5 or 6 amps when it stalls? Locked-rotor current for a 1/3HP motor should be much more than that, at least 30 amps (I haven't looked it up). You're sure it's getting full line voltage at that point? I don't see how it could be.

Alan

The URL above is 'dead'...

This is probably what you are trying to point to:

So the motor actually stalls, and there are no sounds (humming, etc.) from it?

A most interesting set of symptoms... Motor stalls to locked rotor, no significant current draw, odd...

How 'hot' is the motor? Is it possible the thermal cutout is tripping (at too low a temperature, perhaps?)

Well, he did say (in another post) he's bypassed the pressure switch for a test run, and the symptoms persist. I think he's got a high resistance point in the wiring too, but I'm not sure where.

Maybe monitor voltage at the motor terminals might give a clue.

As Alan says, locked rotor should result in very high current/power draws. It is very noticable, and unless the compressor is on a circuit that is vastly overfused result in a blown fuse/tripped breaker almost immediately.

Can you (the OP) post specicifications from the motor's name plate, and the compressor assembly? FWIW, no compressor has a fractional HP motor, most are motors rated at 1 to 3 HP on small compressors.

Don't assume. What is the capacitance? Some small split phase motors only have a run capacitor. The fact that the current only rises slightly indicates a bad capacitor or a poor connection to one winding.

--
Greed is the root of all eBay.

I don't think GE is making the motors anymore, and thats why you can't get info. Either test the cap or replace it. Its probably a run cap.

greg

The main problem is that the motor is being overloaded. As mentioned already, induction motors don't get tired or exhibit lower power output with age.

You don't state some of the most essential info.. namely, the brand name and model number of the air compressor, and the size and type of air pump. The complete info listed on the motor label should also be included.

We can't see it from where we are.

Forget the capacitor.. the motor is starting normally, so the cap isn't the problem.

A mechanical failure in the motor may be contributing to the overload. The motor bearings may be worn out, they're likely to be sleeve bearings in a

1/3 HP motor, and not replaceable, but all hope is not lost if the motor shaft isn't scored. FWIW, sleeve bearings can wear to the extent that the rotor begins to rub on the stator segments (lots of friction and excess heat). If the bearing surfaces aren't completely wiped out, re-orienting the end bells of the motor may provide better bearing surfaces. If the bearings are the sleeve type, and haven't been oiled regularly, it's likely they would be worn or damaged.

I can't positively explain why the motor isn't already destroyed, or why the circuit breaker isn't tripping.

It's very likely that the overloading of the motor is related to air pressure from the tank resisting movement of the piston in the pump, if the pump is a piston-type pump. Normally on small air compressors, tank pressure is isolated from the piston by a check valve at the tank fitting. Additionally, (in a piston air pump) there are likely to be reed valves in the head of the air pump that would separate the cylinder from the outlet fitting (the tubing constitutes a manifold in air compressor nomenclaure, if the unit doesn't have a separate part designated as a manifold). If the air intake is restricted (or choked off from a clogged filter as many small air compressors only have a piece of wool felt filters), this may contribute to more load on the motor.

Many piston air pumps have oil lubrication in the pump's case. If the oil becomes very dirty or the level is inadequate, bearings, the pison and cylinder can be damaged, resulting in excess friction until a full failure takes place.

It's possible that the motor's thermal protector is responding to high current, and if this is the case in this particular situation, it's the reason the circui breaker hasn't tripped, and likely saved the motor from destruction. It's fairly obvious that the restarting actions described are the result of a self-resetting protective device.

With the power cord unplugged, and the belt removed from the air pump (if it's a piston-type pump), the pumping action can be checked by placing a finger over the outlet fitting and briskly turning the pump pulley by hand. The pump's basic operations of intake and compress/ouput can be observed with this fairly simple test.

Some of the same conditions included above may pertain to diaphram-type air pumps, although many diaphram types are direct drive, where the motor and pump are a single unit, which may make checking the pump's basic operation a little more difficult.

-- Cheers, WB .............

snippage

Additional comment. The five amps draw is perhaps normal for this unit, as it appears to be a 240 volt setup. I was thinking it was 120 volt (US) but I appear to be wrong there...

I can't read the markings on the cap without taking the motor apart. But, thanks to everybody's help, it looks like that's the next step. I'll definitely replace the cap and otherwise clean, lubricate, and tighten as appropriate.

Thanks!

Sorry, yes, that's the one.

Yes, it actually stalls. There's not super loud humming. There may be some motor noise, but it's hard to hear over the sound of the unloader.

Well, it does get warm. It gets warm when running with the belt off, too. When it goes into it's stall cycle, it's definitely warmer than that. I can put my Mk I hand on it for about 8 seconds, which says it's hot, but not super hot.

Yes, that's correct.

It's a mechanical unloader, integrated with the check valve, so disabling it would be difficult. However, it's pretty obvious that the unloader only opens after the motor stalls when the check valve closes.

There's definitely no current spike when it stalls, nor does it trip the breaker, dim the lights, etc. Is it getting full voltage? It is at the point the motor leads connect to the pressure switch. I can't easily measure any closer than that because the leads run into the motor and there's no easy way to get to them, especially when it's running. But, it looks like I need to disassemble the motor and check things out inside, so that'll be the next step.

Thanks!

Yes, bypassing the pressure switch doesn't make any diffence. You may well be right about a high resistance point.

I can't easily get to the internal terminals. The closest I can measure is at the pressure switch, and the voltage looks fine there.

It's on a standard outlet circuit with a 20A breaker. It hasn't tripped. Nor have there been any of the signs that you mention for high current draw.

1/3 HP is what's on the nameplate. These days, it would probably be rated at 1 or 2 "marketing HP" :-)

The compressor itself has long lost it's label. But, it's a single cylinder reciprocating, belt driven compressor with about a 20 gallon tank, a 3 inch pulley on the motor and a 9 inch on the compressor.

The motor was made by GE, and is labeled:

Model: 5KC42JG391AX Volts: 115/230 Ph: 1 Amps: 6.0/3.0 HP: 1/3 HZ: 60 RPM: 1725

It's running on 120 volts.

Is that ther right Part #? Does it have some dashes inbetween the numbers and letter like most Ge motors do?

Is it wired for 120?

No dashes. That's what's printed on the motor nameplate.

Yes.

...

I had a somewhat similar set of symptoms with my compressor. Spent a while trying to figure out what was wrong.

The problem turned out to be that wasps had plugged up the air input holes with mud. Cleaned out the mud and away she went

David

Darn mud daubers! Not the issue with mine, though.

Thanks!