Hey guys, dust collector fan here, 20hp, 480VAC, across the line starter with a contactor and an overload block. Press start and the contactor pulls in, the motor hums loud, shaft doesn't move. After about a minute the overload trips. Reset, same again.
Locked it out and turned the shaft by hand first, turns free, no grinding. One of the other lads said it's nearly always the contactor on these, and the old one's contacts were pitted anyway, so I put the spare from stores in. Same hum.
Starter's fine now with the new contactor, so I'm thinking the motor has gone stiff inside somehow, bearings, but you can't feel that by hand? Ran fine Friday. No drive on this one, just the starter. It ran fine Friday, would a bearing really go stiff enough over a weekend to hold a 20hp and still turn by hand?
Turns by hand, so it's not seized. Meter it. Phase to phase at the motor terminals while it's humming, all three pairs. You should be reading 480VAC on every pair and I'm not sure you will be.
Lock out to get the cover off and land the leads while it's dead, meter rated 600V CAT III at least, then arc rated kit and gloves before it goes live again. Somebody else presses start, you read, and you stand to the side of the box. Don't hold the leads on with your hands out over the terminals.