# Air compressor motor



## FaultCurrent (May 13, 2014)

Electrolytic capacitors have a wide tolerance. Per your info they are good. Check if your motor windings show any signs of overheating and if the centrifugal switch is good. Check the motor voltage when trying to start.

Just a guess but I suspect the compressor has a mechanical problem. Probably has a bad bearing, unloader, scuffed pistons, or something making it periodically hard to start.


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## retiredsparktech (Mar 8, 2011)

te12co2w said:


> In January I replaced the one starting capacitor and the two run capacitor on a 7.5 hp, 230v single phase motor. Baldor motor. Cat# L151OT. Motor name plate says 31 run amps. Got a call yesterday on it. Not starting. Loud hum. Voltage ok. Over 200amps when it is trying to start. I checked the capacitor values again. Each one is just slightly out of tolerance. E.G. start capacitor should read 30mfd with 5% tolerance. It reads 33mfd. Both run capacitors should be 216-259. One of them reads 280 and the other reads 277.
> I would have no problem replacing them again, but I don't want to just keep doing this. This motor drives an air compressor with a m# 340-33. And then 170661-L. No brand name that I could see. I haven't tried to look for the compressor yet.
> It is belt driven, (3 belts). I tried to move the motor to loosen the belts just a little but there doesn't seem to be any adjustment there. Belts seem pretty tight. Maybe 1/4" deflection. Does that seem too tight?
> When I finally gave up last night, the mechanic working there told me that this compressor came from a location that had 3 phase power and the original motor was a 3 phase motor. I have no idea if the motor is sized correctly for this compressor.
> ...


Are you sure you have your cap values correct?
Ordinarily the start cap values are real high and the run cap values are a lot lower. Is there a current relay in the start circuit?
I only have experience with single phase motors up to 3HP. 
BTW, the cap values should be OK.


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## micromind (Aug 11, 2007)

There's a very good possibility that the motor is in the process of burning up. The winding insulation has been overheated to the point that adjacent turns of the coils are shorted to each other. The result will be lower torque and higher current.


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## Wirenuting (Sep 12, 2010)

I is hard to start or stalled, check the unloading valve. With air pressure on the heads, she will want to fight hard to roll.


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## sparkywannabee (Jan 29, 2013)

micromind said:


> There's a very good possibility that the motor is in the process of burning up. The winding insulation has been overheated to the point that adjacent turns of the coils are shorted to each other. The result will be lower torque and higher current.


This is exactly what I found in an air comp some dude at work brought in for me to take a look at. I looked up the model# online and found comp originally came with a 4 HP motor, somebody had replaced it with 3 HP Harbor Freight junk. One way or another, you have to find out what HP motor the comp had originally, some genius before you might have replaced a 10 hp with a 7.5. The comp is engineered with the motor for a reason.


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## former farmer (Feb 27, 2013)

te12co2w said:


> Motor starts and run fine when I could finally get the belts off, but it draws 33 amps instead of the rated 31.
> Any clues? Thanks.


33 amps with no load?


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## retiredsparktech (Mar 8, 2011)

sparkywannabee said:


> This is exactly what I found in an air comp some dude at work brought in for me to take a look at. I looked up the model# online and found comp originally came with a 4 HP motor, somebody had replaced it with 3 HP Harbor Freight junk. One way or another, you have to find out what HP motor the comp had originally, some genius before you might have replaced a 10 hp with a 7.5. The comp is engineered with the motor for a reason.


What they should do, is buy a new single phase compressor, equipped with the proper motor. I'm not sure how large they come in single phase, maybe 10HP?
In a body shop, some air is better, than none at all!


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## Galt (Sep 11, 2013)

You have your start and run capacitors mixed up. First check if you can turn pulley by hand if the unloader isn't working it will turn very hard. If it turns fairly easy check the start switch. You have to take motor apart to do this which will allow a visual inspection of windings.


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## micromind (Aug 11, 2007)

retiredsparktech said:


> What they should do, is buy a new single phase compressor, equipped with the proper motor. I'm not sure how large they come in single phase, maybe 10HP?
> In a body shop, some air is better, than none at all!


10HP is the largest single phase motor normally available though Baldor makes a 15HP, and Franklin makes 15HP submersible pump motors.


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## retiredsparktech (Mar 8, 2011)

micromind said:


> 10HP is the largest single phase motor normally available though Baldor makes a 15HP, and Franklin makes 15HP submersible pump motors.


I always thought Baldor made a good motor.
They seem to be the only firm to build single phase motors, that large. 
What speed, 1800 or 3600 RPM?


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## te12co2w (Jun 3, 2007)

Thanks for all the replies. I did indeed have the start and run capacitors mixed up. I called baldor and the ones on there are correct for that motor. I think I have eliminated most of the mechanical issues that might cause this unless they are intermittent. Unloader is working, compressor turns by hand with no problem. Yes 33 amps with no load. I did not take the motor apart to check start switch. When I mentioned that the owner started adding up money and got scared and didn't want me to do any more. This is 1800 rpm. 
I won't be doing anything else to this motor. The owner is going to let it go until it burns down or seizes up. I found the compressor mfg, and they said this originally would have had a 7.5 hp 3 phase motor, so I think we are all right size wise.
I checked Grainger's price for an new exact replacement for this motor. $1669.00. Owner says he can get one for 1/2 that price so I didn't look any further. Thanks again


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## The_Modifier (Oct 24, 2009)

te12co2w said:


> Owner says he can get one for 1/2 that price


And we're all shocked to hear that statement. :laughing:


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## oliquir (Jan 13, 2011)

with no load you should have under 10 amps, if not motor have bad cap or have burned coils


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