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I have a really perplexing problem with an older air compressor. The compressor is an old 2-stage Saylor Beall 80 gallon with a 240v 5HP Dayton electric

motor.

When the compressor is plugged in, the motor starts and begins to turn the compressor pump, but then trips the circuit breaker right away. I have checked

the current with an AC clamp meter, and the motor is drawing a peak of 213 amps on startup with the belts hooked up to the compressor.

I have tried unhooking the compressor from the tank so that the compressor head cannot build up pressure. This did not change my results. I still tripped

the breaker with the tank unhooked.

If I remove the belts so that the motor is not coupled to the compressor head, the motor will start and run fine, but does make a lot of noise like the

bearings may not be in the best shape. When I remove power to the motor, the motor keeps spinning for a LONG time. As the motor spins down, there is a

point at which I hear a click and the speed decreases quite suddenly. It's almost as if there is a brake inside the motor that kicks in at this point.

When I hook the belts back up after running the motor by itself as described above, I can start and run the compressor successfully one time. The

compressor will run until it reaches the cutout pressure and then shut down. After this one successful run, I cannot start the compressor again without

tripping the breaker.

There are three capacitors on top of the electric motor. One is a 25MFD oval cap, and the other two are round 1070MFD. I have tested these with the

capacitance function on my AC clamp meter, and they all test OK.

I would greatly appreciate any help, as I am really stumped with this.

Compressor.jpg

Motor Info.jpg

Capacitors.jpg
 

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Many motors draw that kind of amps (especially at startup), it just depends how big the motor is.

Did you check the motor windings? Maybe 1 is broken and it runs on only two. You can easily check it with a simply multimeter.

You could also measure the insulation resistance (should be i the megaohms) of the motor windings, that's a common failure. But you need a special high-voltage meter for that.
 
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