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.
A to B 478. B to C 260. A to C 250. Read them twice with the contactor pulled in, same both times. Those two don't look right to me.
That's single phasing. C is open somewhere upstream and the 250 you see on it is the motor making it through its own windings. Two phases won't turn a 20hp fan, so it hums, pulls locked rotor current, and the overload does its job after a minute. Bearings and the contactor are innocent.
Lock out, open the disconnect, prove it dead. Pull all three fuses and check them out of the clips on ohms, one will be open. Fit three new, same class and rating, never just the one. Before you close up look for why: a loose lug, a brown clip, a wrong size fuse. Fuses fine, work back to the breaker.
C fuse open. The lugs on the disconnect were fine but the C fuse clip had a burned spot on one side. Replaced all three fuses and the fuse block. Fan runs, current 22 A on all three.
Single phasing all along, one open fuse, and the motor and the contactor were both innocent.
Not sure why it opened though. 22 A running on a 40 A fuse, unless that clip had been arcing away for a while first. Thanks vinnie86, ed.
Clip was heating the fuse for months. Then one hot day.