Diagnosing single phase 5HP motor

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fastline

Senior Member
Location
midwest usa
I borrowed one of these oiless 5HP compressors that I really need for a one day job but of course it has issues......:rant:

Plugged it into the genny and could hear it pulling way too much power. Inrush test was 46A, constant current is 30A, obviously kicked it out after a few seconds.

Here are the specs
240V, 1PH, 14A FLA, 3450rpm, compressor direct coupled to motor, spins free as a bird.

Things checked
Motor and compressor freewheel by hand without issue, start and run cap test good, motor windings not showing conduction to ground, motor running RPM is 2500, end bell of motor removed and centrifugal switches look and test decent(5-7ohms but should work?), Verified 245V no load to compressor, the load pulls it down to 210V but genny only rated to 5000W or 21A I guess


IMO, this points to a good motor and a problem with the load on it? The load is SO simple that I would be surprised if that is it but the tests point that way. I verified also that the compressor is kicking out plenty of compressed air and seems to function just fine. I have fixed compressors in the past when a check valve is failing causing compressed air to stay in the pump but I have pulled the air line so it is not doing anything. Nothing changes the current.

Is it possible that I just don't have enough generator on it? I noticed the start cap was a bit warm but at 2500rpm, would that thing still be in circuit? That would sure answer the question.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If I had to guess (and I do) I'd say the centrifugal switch is broken and not opening, so the start winding and cap is staying in the circuit and it is not switching out to put everything onto the Run winding.
 

Fnewman

Senior Member
Location
Dublin, GA
Occupation
Sr. Electrical Engineering Manager at Larson Engineering
I agree with both - just probably hasn't been on long enough yet to blow the cap. Keep trying, it probably will.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Ya but when starting caps stay in the circuit they blow up.
I noticed the start cap was a bit warm but at 2500rpm, would that thing still be in circuit? That would sure answer the question.
I agree with both - just probably hasn't been on long enough yet to blow the cap. Keep trying, it probably will.
Persistence will pay off in identifying the issue...
 

fastline

Senior Member
Location
midwest usa
Well it has been confirmed that the compressor is just fine and power is the problem. I had 46A inrush on the genny and 76A on house power. Running amps calmed down to 9.3A. I think we are just right on the edge of not being able to get enough kick on that motor to get the start circuit to kick out. Thinking about adding a cap.
 

kwired

Electron manager
Location
NE Nebraska
Well it has been confirmed that the compressor is just fine and power is the problem. I had 46A inrush on the genny and 76A on house power. Running amps calmed down to 9.3A. I think we are just right on the edge of not being able to get enough kick on that motor to get the start circuit to kick out. Thinking about adding a cap.
Adding a capacitor would just make it demand a little more current wouldn't it? You need a generator that can deliver more surge current.
 

fastline

Senior Member
Location
midwest usa
Well, I think for this one application, I might ramp up my genny engine rpm to increase capacity a bit and see if I can get it to pull in. That should increase the peak capacity of the genny but also increase the frequency which I guess could further increase demand. We shall see.
 

kwired

Electron manager
Location
NE Nebraska
Well, I think for this one application, I might ramp up my genny engine rpm to increase capacity a bit and see if I can get it to pull in. That should increase the peak capacity of the genny but also increase the frequency which I guess could further increase demand. We shall see.
Your generator may be able to provide the needed surge of power to start that motor, but the prime mover may not be able to deliver enough power to the generator.
 
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