Air Compressor Problem - Motor Stalls

[Alan Douglas]

And all those replies ignore the fact the unloader is operating (at a

wrong, low pressure) which certainly doesn't point an evil finger at
the motor at all...

Hmmm, yes, I think that wasn't in the original post so I missed it.
That would account for gradual deterioration too.

Alan

 

[Sparks Fergusson]

Hmmm, yes, I think that wasn't in the original post so I missed it.
That would account for gradual deterioration too.

The unloader is functioning properly, as far as I can tell. It's a
combination check valve/unloader. So, when the check valve is open, it
allows the high pressure air into the tank. When the check valve
closes, it vents the pump side to atmosphere.

So, then the motor stalls, the check valve closes and the unloader
part releases the pressure. When the presssure falls sufficiently, the
motor restarts and the cycle continues.

 

[Sparks Fergusson]

1. Check the thermal overload. Remove it from the circuit, and find a
way to load it up with a known current until it trips. It should trip
after some time at 1.15 times the nameplate current. It generally
takes a long time, like several minutes, at that current. This will
be around 2x the motor FLA. If the overload trips before you reach
1.15x, then the overload is bad.

I'll try to check it, but the thermal overloads I've dealt with in the
past usually don't reclose quickly. In this case, the motor stalls,
the unloader releases, and the motor restarts inside of about 5
seconds...and then the cycle repeats.

But, it's looking likely that I need to take the motor apart and
inspect the innards (or take it to a motor shop.) So, I'll certainly
check the overload(s).

 

[Sparks Fergusson]

It shouldn't. The unloader is supposed to open when the motor first
starts, to let it come up to speed.

An electric unloader, yes. This is a mechanical one, integrated with
the check valve. It is working as designed.

Since the motor is stalling, you
either have a bad pressure sensor, or the run capacitor has lost some or
most of its capacitance. The pressure sensors deteriorate with age, and
the calibration goes to hell.

The pressure switch is OK. It is not what is stopping the motor. The
contactor never opens and the motor is receiving full voltage through
the stall cycles. The capacitor could certainly be the issue, though.

The capacitors have to pass a lot of current, and aren't made to last
forever. They are AC electrolytics to keep the size and cost within a
range you can afford to build and sell a product. A film capacitor
would be quite large, and can caost more then a new compressor.

Indeed. This motor is probably 15-20 years old, so I can easily
believe the capacitor may be at fault. I'll have to disassemble the
motor and check it as soon as I get a chance.

Some compressors have two capacitors. One in the start circuit, and
the other in series with one of the run windings to provide the required
phase shift.

I only see one capacitor, so I'm assuming it's in the starting
circuit. I don't have a wiring diagram, and GE has no information on
it's website for this motor.

Thanks!

 

[PeterD]

Hmmm, yes, I think that wasn't in the original post so I missed it.
That would account for gradual deterioration too.

Alan

From the original post that started this thread:

 

[PeterD]

The unloader is functioning properly, as far as I can tell. It's a
combination check valve/unloader. So, when the check valve is open, it
allows the high pressure air into the tank. When the check valve
closes, it vents the pump side to atmosphere.

So, then the motor stalls, the check valve closes and the unloader
part releases the pressure. When the presssure falls sufficiently, the
motor restarts and the cycle continues.

Your checkvalve has a small copper tube to the pressure switch, right?
That's the unloader, which is also the small valve on the pressure
switch. Read my original response on adjusting the pressure swtich.

 

[PeterD]

It shouldn't. The unloader is supposed to open when the motor first
starts,

Typical small compressors the unloader opens when the pressure switch
activates and shuts off the compressor. This releases all pressure on
the compressor head, allowing the motor to start when pressure drops
down to the cutin PSI.

to let it come up to speed. Since the motor is stalling, you
either have a bad pressure sensor, or the run capacitor has lost some or
most of its capacitance. The pressure sensors deteriorate with age, and
the calibration goes to hell.

IHMO it has not been said that the motor is stalling! A stalled motor
(called locked rotor) will cause input current to swing sky-high,
usually tripping the breaker, making a hell of a noise, and is very
obvious.

 

[PeterD]

An electric unloader, yes. This is a mechanical one, integrated with
the check valve. It is working as designed.


The pressure switch is OK. It is not what is stopping the motor. The
contactor never opens and the motor is receiving full voltage through
the stall cycles. The capacitor could certainly be the issue, though.

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

 

[Sparks Fergusson]

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.

 

[Sparks Fergusson]

Typical small compressors the unloader opens when the pressure switch
activates and shuts off the compressor. This releases all pressure on
the compressor head, allowing the motor to start when pressure drops
down to the cutin PSI.

On this compressor, the check valve/unloader is completely separate
from the pressure switch. The unloader is similar to this one:
http://www.amazon.com/Control-Devices-CA121A-Unloader-Check/

IHMO it has not been said that the motor is stalling! A stalled motor
(called locked rotor) will cause input current to swing sky-high,
usually tripping the breaker, making a hell of a noise, and is very
obvious.

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.

 

[Sparks Fergusson]

From the original post that started this thread:

Yes, those're the symptoms.

 

[Alan Douglas]

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

 

[PeterD]

On this compressor, the check valve/unloader is completely separate
from the pressure switch. The unloader is similar to this one:
http://www.amazon.com/Control-Devices-CA121A-Unloader-Check/

The URL above is 'dead'...

This is probably what you are trying to point to:

http://www.amazon.com/Control-Devic...ref=sr_1_1?ie=UTF8&s=hi&qid=1268055829&sr=8-1

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.

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

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.

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?)

 

[PeterD]

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

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.

 

[GregS]

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.

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

 

[PeterD]

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.

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... <g>

 

[Sparks Fergusson]

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.

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!

 

[Sparks Fergusson]

This is probably what you are trying to point to:

http://www.amazon.com/Control-Devic...ref=sr_1_1?ie=UTF8&s=hi&qid=1268055829&sr=8-1

Sorry, yes, that's the one.

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

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.

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, 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.

 

[Sparks Fergusson]

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

Yes, that's correct.

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.

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.

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.

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!

 

[Sparks Fergusson]

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.

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

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

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.

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.

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.

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.

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.