Help with AC motor - Page 1
Forum section: electronics / repair

author: DrJoe
author_rank: Contributor
timestamp: August 16, 2017, 03:55:38 am
content: 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.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: August 16, 2017, 05:30:55 am
content: If you have a megger, I would check the motor windings (coils) to ground. If the megger shows less than 1 Mohm to ground then it is bad. Also take a meter set to ohms, make sure there is no power on, and check across each winding for opens. If a winding is open, then the motor is dead. There usually is a diagram in the wiring box of the motor to show the coil arrangement.Also, if it has a reversing switch, I would check the wiring to that to insure that there are no opens/shorts in the connections and the switch is working properly. Generally checkout all the wiring to the switch and from the motor for shorts or opens or any wiring problems.Are there ants in the switch box? Ants are attracted to switches and will pile up and die by the thousands around a switch causing problems. (or spider webs)Does the motor have a starting contactor? If so, check that out to make sure that a contact is not bad and that the wiring is in order.Are you sure it is a 1 phase motor?Happy hunting.

author: Armadillo
author_rank: Super Contributor
timestamp: August 16, 2017, 05:41:31 am
content: Sounds like start winding is open or the centrifugal switch is open. Did you measured the resistance of both of the windings [main winding and start winding?]. Change the centrifugal switch if defective.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 16, 2017, 06:01:36 pm
content: Stuck centrifugal switch is quite likely, especially if it's not run for a long time. If you can get the straw near it, a squirt of WD might solve it. Maybe along with judicious tap from a mallet.(Be aware that WD is inflammable just after spraying!)

author: DrJoe
author_rank: Contributor
timestamp: August 16, 2017, 09:02:14 pm
content: Solved! Thanks so much to the 3 of you that offered help. I found the centrifugal clutch. It was working fine, but I thought the contacts could be dirty. Took a a bit of fine sandpaper (didn't have any contact cleaner) to them and she starts right up. Wasn't getting enough juice to fire her up. Thank goodness as I had also opened up the reversing switch and couldn't make head or tail of her.