7.5HP Compressor motor keeps tripping breaker

[RedBKM]

I'm not sure what is going on with my compressor. When I turn on the breaker the motor spins up for around 2 seconds and the breaker trips. I checked the two start caps and one run cap. They measured OK (250uF and 50uF) but I changed them because they were leaking.

The caps made no difference. The compressor is not stuck and pumps a trace of air for the few seconds it runs.

The motor is a Baldor 7.5HP 250V single phase. It uses a starter similar too this one. The adjustable limit has been set to 40A since new.
https://www.ebay.com/itm/7-5-HP-Single-1-Phase-Magnetic-Starter-Motor-Control-New/280639333310?epid=27006211979&hash=item41576863be:g:UMAAAOSwzQFaAeIm

 

[EOC_Jason]

Use a clamp-on amp meter to see what the current draw is. That can help you determine if the motor is pulling too much current, or maybe you have a weak breaker, or maybe a loose connection somewhere between the breaker & motor?

 

[RedBKM]

I snugged all the connectors and didn't find any suspects.

I've heard of HOM series breakers going bad. Will they quit working completely or just trip to early?

 

[Bellaireroad]

Sounds like your unloader valve is defunct


Sent from my iPhone using Tapatalk Pro

 

[bpjr]

Swap the breaker with another and see what happens.

 

[larry4406]

Sounds like your unloader valve is defunct


Sent from my iPhone using Tapatalk Pro

My thoughts as well

 

[Packard V8]

Some troubleshooting steps:

1. Remove the drive belts and start the motor without load. Does it start, spin up and keep running?

2. Replace the drive belts. Loosen the output line fitting, so the compressor starts without load. Does it spin up and keep running?

3. Tighten the output line fitting and start the motor and compressor on an empty tank. Does it start, spin up, fill the tank and shut off?

4. Bleed off pressure until the pressure switch turns on the motor. What happens then?

jack vines

 

[GETRIDAONE]

Excellent Advise !!!

 

[Dingleburry]

I snugged all the connectors and didn't find any suspects.

I've heard of HOM series breakers going bad. Will they quit working completely or just trip to early?

Ive had contactor overloads go bad, tripping early, not tripping at all, or not resetting.

I believe those overloads have the same basic operating principals as breakers. (Bi metal strips?) I could be full of **** though.

Id start with a clamp on amp meter. If you dont have one see if something is putting excessive load on the motor, bearings, pump etc. Take off belts and run motor. Could be bad windings, could be something grounding out, excessive load, bad bearings, somethings seized etc. Check pump and morot. I also believe if you have a weak start cap itll pull high amps cause it dosent have the required torque to start. Id also check for voltages at the motor. See if your single phasing it. As far as i onow single phasing it will sping pretty slow though.

 

[larry_g]

other things not mentioned,

Is the centrifugal switch dropping out the start cap? You'll see this on motor only running. Or is the CF staying open?

lg
no neat sig line

 

[Dingleburry]

other things not mentioned,

Is the centrifugal switch dropping out the start cap? You'll see this on motor only running. Or is the CF staying open?

lg
no neat sig line

I dont think it would trip in 2 seconds if the switch wasnt opening, and if stuck open it wouldnt get rotation at all.

But deff something to check!

 

[Bert_]

If the switch stuck closed it will spin but draw high amps. Is there a click when the motor spins down?

What size breaker do you have on it? Needs about a 60A breaker.

 

[EOC_Jason]

Yeah the centrifugal switch being stuck and the check valve (clogged / stuck) could be likely culprits...

Pulling the belt off and diagnosing one step a time would be the easiest.

 

[BudgetRacing]

I dont think it would trip in 2 seconds if the switch wasnt opening, and if stuck open it wouldnt get rotation at all.

But deff something to check!

I had this on a bunch of motors on commercial door operators. They had a bad batch with switches that wouldn't switch over. About 1-3 seconds and it would trip the breaker.

 

[RedBKM]

OK I pulled the belts off and the breaker tripped the same. That rules out the compressor pump.

I replaced the HOM breaker with a new 60A and it still trips. I thought I'd get lucky but not this time.

While in the breaker panel I clamped on and measured a max of 257A on each line. That can't be good.

I pulled the end cap off the motor and I don't see anything strange. If I push the plate on the switch it moves the "arms". I'm not sure how much tension this should require.

With the motor reassembled I measure 0.4 ohms across the leads. I guess that's normal.

The motor will coast to a stop after tripping and doesn't feel stuck or sticky. It runs 3450 rpm so it takes a while to stop.

 

[sberry]

Its broke.

 

[EOC_Jason]

Only thing I can think is the centrifugal switch is sticking, but you said it moved okay?

Might just be time to bite the bullet...

 

[RedBKM]

Does 0.4 ohms indicate a shorted winding? I know it shouldn't be an open.

Yes, the switch moves with a push and doesn't seem sticky. There was some rust on the shaft so I cleaned it off and applied a light coat of grease.

 

[Dingleburry]

Thats way too low. Ohms law.
I = E / R
240v / .4 ohms = 600 amps. Thats DC mind you.
5592 watts (745 watts per hp) says @ 240v should be about 10.3 ohms. But this is all in dc. Not ac. But i think it should be fairly close.

 

[larry_g]

Check your resistance from the terminals/windings to the case or ground. It should be open.

lg
no neat sig line

 

[RedBKM]

Getting closer...

Both leads to the motor case are shorted at 0.4 ohms.

 

[RedBKM]

Forgot to mention that's with caps removed, armature assembly removed, end caps removed and wiring disconnected.

Both leads still shorted to each other and ground! Daggone it.

 

[66cj225]

Check the line voltage into the shop, it's air conditioning season. Not everything goes well for the power co either.

 

[dutchgray]

Needs rewinding or a new motor, whichever is cheapest or easiest.

 

[Dingleburry]

Needs rewinding or a new motor, whichever is cheapest or easiest.

This is your only option. Unless you can find where its grounded out. Chances are slim though.
Sorry to hear. **** *****.

 

[Dagny]

Ohms law cannot be used for inductive loads. Because inductance also lowers current.Large motors can have some very low resistance readings. however there should be a very high resistance to ground.

 

[RedBKM]

My electrician mentioned a shop in Richmond that can rewind it.

How do you get the winding frame out? I see no fasteners or welds inside the case.

 

[sberry]

I still say it's broke.

 

[jl4c]

Ohms law cannot be used for inductive loads. Because inductance also lowers current.Large motors can have some very low resistance readings. however there should be a very high resistance to ground.

Dagny is 100% correct on both counts.

 

[jl4c]

I still say it's broke.

Is this what passes for humor in Michigan nowadays?

 

[TRWham]

My electrician mentioned a shop in Richmond that can rewind it.

How do you get the winding frame out? I see no fasteners or welds inside the case.

Sometimes a screw or bolt through the case into the stator frame, or perhaps it's pressed in and needs to be pressed out. I would take the whole thing to the shop and let them handle it.

 

[redmondjp]

Yup - check with local motor rewind shop - for a 7.5HP motor, it might make sense to rewind vs. replace, due to the cost of a new motor.

Sorry to hear about that. The only other thing I can think of to check would be to find the 'Y' point where the start and run windings are crimped together to see if the insulation on that splice failed - maybe you'll get lucky - that may be on the opposite end of the motor from where the centrifugal switch is.

 

[larry_g]

Before assuming this needs a total rewind check things a bit closer. Get a stong light and LOOK for some bad insulation, rubbed insulation, or other possible shorts that are NOT the windings. If you have a powered test light, clip it onto the case and one of the shorted wires and the light should light. Now with a wood probe, chop stick, start gently moving wires a bit and see if the short disappears. If it does can you now find it? Is the inside of the motor clean or a caked up mess? Clean it if need be so you can see. It costs you nothing to do a bit of looking around before dropping it off at the motor shop.

lg
no neat sig line

 

[RedBKM]

I tore it down completely and it is very clean inside. The motor is only a few years old.

I cut the string wrap and checked connections at both ends and I still see nothing. I used a light on the opposite side to look down the "grooves" and still nothing. It must be melted/fused deep down. The epoxy coating is keeping me from getting any deeper.

I know I will not be buying another Baldor Industrial. It only ran a few minutes on the weekends.

Thanks for the help anyway. It weighs 55 pounds so I can get a few bucks at the scrap yard.

 

[DSLTRK]

I tore it down completely and it is very clean inside. The motor is only a few years old.

I cut the string wrap and checked connections at both ends and I still see nothing. I used a light on the opposite side to look down the "grooves" and still nothing. It must be melted/fused deep down. The epoxy coating is keeping me from getting any deeper.

I know I will not be buying another Baldor Industrial. It only ran a few minutes on the weekends.

Thanks for the help anyway. It weighs 55 pounds so I can get a few bucks at the scrap yard.

Nobody mentioned your source voltage. If you have bad continuity from your sub to the motor, motor will draw more amps. If you have low voltage, the motor will draw more amps.

Did you check the motor with a meggar?

The winding resistance seems ok at .4 ohms, keep in mind it is AC impedance that occurs, not simple DC resistance measurements.

 

[DSLTRK]

Getting closer...

Both leads to the motor case are shorted at 0.4 ohms.

Oh missed that part, not looking good.

 

[mowkep]

We have the same motor on our shop compressor. It was rated for 230 but one of the internal switches wasn't. Maybe the starter switch. They replaced it with a Eaton 40 amp, 50 resistive, 110/120v a.c. 50/60Hz. Catalog no C25DNF340A. Sorry I don't know more but this is what happened to ours

 

[bsaint]

Thats way too low. Ohms law.
I = E / R
240v / .4 ohms = 600 amps. Thats DC mind you.
5592 watts (745 watts per hp) says @ 240v should be about 10.3 ohms. But this is all in dc. Not ac. But i think it should be fairly close.

No its not close. Mind you a big short would also not let him run for 2 seconds.

Motor and transformer windings will always show a short. You need an LCR meter to figure the impedance (ac version of resistance) of a motor winding.

Since you've ruled out the pump, check the capacitors. Bad centrifugal switch would generally mean it never switches over to full speed, I dont think itll trip out.

 

[redmondjp]

No its not close. Mind you a big short would also not let him run for 2 seconds.

Motor and transformer windings will always show a short. You need an LCR meter to figure the impedance (ac version of resistance) of a motor winding.

Since you've ruled out the pump, check the capacitors. Bad centrifugal switch would generally mean it never switches over to full speed, I dont think itll trip out.

You guys are right, but apparently many of you didn't read the fact that there is a 0.4 ohm short TO THE MOTOR CASE! You don't need an LCR meter to detect that.

 

[Dingleburry]

No its not close. Mind you a big short would also not let him run for 2 seconds.

Motor and transformer windings will always show a short. You need an LCR meter to figure the impedance (ac version of resistance) of a motor winding.

Since you've ruled out the pump, check the capacitors. Bad centrifugal switch would generally mean it never switches over to full speed, I dont think itll trip out.

Sorry i assumed it would be fairly close, as in within 50%, i understand all the impedance stuff but i didnt think it was THAT much, i just checked a brand new motor i have here its 208-230/480 3 phase, i connected the leads in 208/230 and the winding resistance was .75ish.
No idea if 3 phase would be different. Id assume not?
I stand corrected!
Also earlier the OP stated he had .4 ohms to ground from windings.