Screw conveyor on the batching line, 3hp 460VAC, line fed through a contactor from the PLC. It jammed on a lump Tuesday, stalled, and by the time anyone noticed the motor was smoking. Nameplate FLA is 4.8 A.
Panel was built last year by an outside builder. Breaker, contactor, motor, and that's the lot. I've been through it twice and I still can't find an overload block anywhere in there. Bench checked the breaker and it's fine, 15 A, and the motor's away at the rewind shop.
My guess is the motor was undersized for the screw and the breaker should have tripped long before it cooked. Is a 15 A breaker enough protection for a 4.8 A motor, or do I want a smaller one in there? Could someone explain what's meant to catch a stall like that?
Breaker is short circuit protection. It's sized for the starting inrush, 15 A on a 4.8 A motor is normal. A stalled 3 HP pulls maybe 30 A. On the breaker's thermal curve 30 A takes a minute or more to trip. The winding doesn't have a minute.
The overload is a separate device. Is there anything at all between the contactor and the motor terminals? Photo of the contactor with the cover off.
Photo attached. Bare contactor with an aux block on top, then straight to the terminal strip and out to the motor. Nothing under it.
Rang the builder. He said the drive protects the motors. The only drive in that panel is the mixer's PF525 and this motor isn't on it, so I'm not sure what he's on about.
Draw it out. Breaker, contactor, overload relay, motor. The overload sits between the contactor and the motor, its heaters carry the motor current, and its NC contact sits in series with the contactor coil. Motor stalls, current climbs, overload trips, coil drops, contactor opens. That's how a stalled motor gets saved.
A drive protects the motor on its own output terminals and nothing else. Pretty sure your builder looked at the one drive in the panel and decided the whole panel was covered.
What ron said. Every line-fed motor gets its own overload, no exceptions. For yours: electronic overload that clips under the contactor, class 10 trip, dial set at nameplate 4.8 A, NC contact in the coil circuit. Lock out the panel before you're in it, that contactor line side is live with the door open.
While you're at it, the builder's other panels. If he skipped one, he skipped them all.
There's a writeup on the direct-on-line side against the drive side here: https://plctr.com/utilizing-plc-for-motor-control-and-drive-systems/
Fitted an overload under the contactor, dial at 4.8, NC contact wired in series with the coil. Tested it with the trip button, contactor drops. Rewound motor is back in and running.
There was never an overload in that panel at all, the builder looked at the one drive and called the whole thing covered. One relay under the contactor, wired the way ron drew it, and the motor would still be here.
He built two other panels here. Opened one yesterday, same thing, no overload on the two line-fed motors. The third one I haven't got to yet.
Thanks boris and ron.
Open the third one before it teaches you the same lesson.