Baghouse fan on the roof. 15hp, 460VAC, plain TEFC motor from 2012, on a PF40 that's been in the MCC as long as the motor has. Motor leads are THHN in conduit, about 60 m up to the roof, no shield on them.
Second rewind in twelve months and the rewind shop says the same thing both times, turn to turn short in the first coils. The drive never faults, runs at 78% FLA all day. I checked the lugs at both ends, the fan bearings and the sheave alignment and there was nothing to find.
I'm about to put a 20hp in and be done with it, but one of the lads reckons I'd only be buying the same problem twice. Any help would be appreciated before I order it.
Don't buy the motor yet. Two questions. Does the nameplate say inverter duty, or give an insulation voltage rating? And what's the carrier frequency set to on the drive?
60 m unshielded on a PWM drive is the whole story I think, and a 20hp with the same insulation will go the same way.
Nameplate says nothing about inverter duty. Standard TEFC, class F insulation, that's all it gives you. Carrier is on default on the 40, 4 kHz, and nobody has been in there as far as I know. Maybe worth a look at the rest of that group.
We had this exact thing on a roof fan. Dropped the carrier to 2 kHz and the motor lasted longer, not forever. It still went eventually.
Every PWM edge goes down the cable as a pulse and reflects at the motor terminals. Past a certain length, and 60 m is well past it on 460V, the reflection lands on the next edge and the motor sees close to twice the bus voltage, 1200 volts and more, on every edge. The first turns take the hit, which your rewind shop found twice.
Lowering the carrier cuts the hits per second. Same size hit. That's vinnie86's result.
Do at least two of: shielded VFD cable bonded both ends, an output reactor or dv/dt filter at the drive, inverter duty motor next time.
Decent article on the drive side of it: https://plctr.com/utilizing-plc-for-motor-control-and-drive-systems/
Went with a dv/dt filter at the drive output and pulled shielded VFD cable through the existing conduit, shield bonded both ends. Drive locked out and bus bled down before anyone touched the output terminals.
Rewind shop's guy came up with a scope and a differential probe. Peaks at the motor before, 1200 and change. After, about 700. Motor stays, its new. Next rewind gets specified inverter duty.
So it was the reflected wave on the long unshielded run all along, and the filter and the cable took care of it.
Three other roof fans on the same drives with 25 m runs have never had a rewind. Under the length where it matters, I guess.
Thanks dan, and vinnie86 for the carrier note.
The scope reading is the part nobody does. Good to have a number.