I have a 7.5hp single phase 220v motor that has five capacitors on it (3 tall cylinder shaped black and 2 large oval shaped metal), that runs my air compressor. The compressor was running for quite a while and the circuit breaker in the panel(40amp) blew when the compressor tried to start up.
I checked the capacitors and they look good. I had a friend check it and he said wasn't case shorted. The motor stars to roll over and then gets a very loud hum and the breaker pops. What would be the problem? Is it worth fixing?

 

Check the compressor make shure it turns freely.
Does the motor have a centrifical switch? is so check for operation

 

7.5 HP: Is it single stage or two stage?

Check to be sure the unloader valve is working. If you don't know how to do that, just bleed the storage cylinder to almost zero pressure and see if it starts.

John

 

Sounds to me like it might be a problem with the starting capacitors, or a problem with the centrifugal switch.

How did you check the capacitors? Simply a visual inspection, or do you have a capacitance meter?

Even if they check OK with a capacitance meter, they could still be shorting out or have far too much leakage current when HV is applied.

 

The compressor is a 2 stage and is not hooked to the tank currently. It spins freely(no Load at startup) and I've tried to spin the compressor while trying to start the motor and that doesn't help. I checked the capacitors according to procedures from the internet(discharging and the coneect the meter and whatch the needle swing. Where would this centrifugal switch be??

 

The centrifugal switch is inside one end of the motor; almost always the end that has the inspection plate with the connections inside.

If you have to disconnect any wiring, make sure to draw a diagram of exactly how it's connected before you disconnect anything.

The contact points may be burned or stuck open. If they're burned, a piece of very fine wet/dry sandpaper works well for dressing the contacts. Don't use garnet or flint paper, or emery cloth; as non-conductive particles will become embedded in the contacts causing them to quickly burn again. Just remove the high spots. Close the points on the wet/dry sandpaper and draw the paper through.

 

I'll have to try that later when I get home. I'll let you know results.
Thanks.