Hello Gentlemen,
I need some help with an AC motor on a Bridgeport mill clone. The motor is about 17 years old, with only about 25 hours on it. Tried to turn it on but I only get a hum and then the circuit breaker trips after 20-30 seconds. It's a 220V, 60 Hz, 3 HP, single phase motor. Last used about 4 months ago. I pulled the starting cap - marked as 800 mcF, 120 V rating. I tested it at 770 mcF with ESR less than 1.6. Despite this testing at meeting spec, I replaced it with new 800 mcF, 250 V cap. Still no go... The motor turns freely, but won't spin up - I can put the mill in neutral with no load on the motor and turn the rotor freely, but it won't "fire up". Since I can turn freely, I don't think the bearings are gone or it's a binding issue. I tested the voltage at the motor and get a good 220V. The motor is wired for forward and reverse, but in either switch position all I get is the hum (60 cycle?).
My knowledge of AC motors is shallow at best... Any ideas for troubleshooting and/or repair would be greatly appreciated.
Dr Joe.