Motor strains above 90 psi, tries to stall over 100 psi

by GT
(Birmingham, AL)

Leroy Some motor plate

Leroy Some motor plate

I picked up an AMP 120 gal compressor with a 10 hp Leroy Somer motor from an industrial company that upgraded to a much larger unit when they started media blasting.

We swapped the starter coil to work on 230, and all works well. It fires right up and works like a champ quickly building pressure until around 90 psi. The motor starts dragging, and around 110 psi, the motor starts to get hot & tries to stall. I removed both air filters, and injected a bit of WD-40 while she was filing the tank.

The pressure regulator is set for 140, but cannot achieve that high of a pressure.

The unloader seems to be operating correctly, and the belt tension looks good.

Oil level is fine. I have it wired directly to the box on a 3-leg, 20 amp breaker.

I have had suggestions to replace the regulator/switch & to upgrade to a 30 amp breaker. The 20 amp has not tripped yet (we have manually shut it down when the motor strains)and the pressure has yet to come close to the 140 shut off pressure.

At this point, it's out of my realm of knowledge.

Any advice is welcome.
First GT, I ain't an electrician.

Yet, a bit of research suggests that a 10 HP motor typically wants 33 amps to run properly.

So yeah, your symptoms suggest the motor isn't getting enough power to overcome the increasing load, and if it were my compressor, I'd probably want it powered from at least a 40 amp breaker.

Assuming no valve problems impeding flow from the pumps, then I'd go with the breaker first.

Other opinions (particularly from electricians) are welcome.


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Jun 06, 2013
by: Gregg Pfaff

You run a great site and are a wealth of information. there can be so many reasons causing these symptoms that more heads are better than one. Process of elimination troubleshooting is all we can do from a distance not knowing the entire situation. Some multi-voltage motors are very confusing when wiring for different voltages and if he has it not quite right could also cause lack of power, so he needs to check and recheck all that is related to the power. hard to say what the condition of the motor windings are also.

Thank you for providing this site :)
You are welcome, and thank YOU for your insights. Please feel free to comment on any other postings for which you feel you have information. This helps everybody.


Jun 06, 2013
Voltage drop
by: Gregg Pfaff

no matter what amp breaker you have if the wire is too light of gauge you'll have voltage drop while running and the amps will increase which is puzzling you haven't tripped the breaker. Check the voltage at the compressor before starting it then while running it. if it drops to more than 10% below name plate that's your reason. could be too light of wire, bad connections, too low of supply voltage.
Gregg, much obliged for your contribution. Good to hear from someone that knows what they are talking about, unlike yours truly! :-)


Jun 06, 2013
Motor Strainer
by: David Halves

Did you rewire the motor for 230 volt?& Bills
correct #8 wire & 40amp breaker,check belts.

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Wire compressor to a 3-pronged dryer outlet.. How?

by Mark B
(Sacramento CA)

A while back, I got a horizontal 33gallon 220V Craftsman air compressor from a friend for a great price. Only problem was that his ex-wife cut the plug off the end of the power cord.

My garage only has one 220V outlet (for the clothes dryer).. so I picked up a 3-pronged plug (to match my dryer plug) to put on the end of the power cord of the compressor. There were 3 wires on the power cord - black, white, and green. I wired the green one to the center prong, and the black and white each to the angled flat-bladed prongs on the plug. I plugged it into the outlet (after unplugging the dryer), and the compressor started for a split second and stopped. Tripped the breakers (2x30amp).. I reset the breakers and tried again.. no luck. So, we plugged the dryer back into the outlet, reset the breakers and now the dryer won't even turn on.

So, I'm guessing I toasted the 2 breakers on the 220 circuit to the garage. Is this something I can replace myself, or do I pay an arm and a leg to have a professional electrician come out?

Also - any idea why the compressor wouldn't work on that circuit? Did I wire the plug incorrectly?

Bill says...

Not being an electrician I can't give you electrical advice. However, I would likely have attached the white return wire to the center prong, had it been me doing it.

Professional comments are very welcome here!

If you fried the breaker by grounding the power (which might have happened putting a coloured wire to the ground plug) then likely the breaker needs replacing.

If you aren't comfortable doing it, don't. While you might get lucky and only get a nasty shock from a 120 VAC circuit, if you make a mistake with 220 Volts, you'll be talking to your angels instantly thereafter, in person!

What's your life worth versus paying someone else that is qualified to take the risk?



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