- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
OK, being that it is an ACH550, that is a drive that ABB sells specifically for the HVAC market, so I'm going to Ass-U-Me that this was purchased as a packaged unit from ABB with a bypass system, most likely what is called a "3 contactor bypass" package. In that system, there is a contactor ahead of the VFD, then there are 2 mechanically interlocked contactors down stream of the VFD, one with an overload relay for use as an Across-the-Line bypass starter. When you are in Bypass, the VFD is totally isolated off of both the line and load connections. This allows the motor to run while the VFD is removed for servicing without anyone re-wiring anything, a very commonly specified feature in HVAC systems because most HVAC techs are not sufficiently trained as electricians.
So was the motor wired wrong? Doesn't sound like it to me, I think your ABB tech is unfamiliar with PW motors and just spewed that out to deflect blame. Sounds to me as though the contactor failed, maybe welded shut, the motor could not be shut down and eventually overloaded until someone noticed and killed the disconnect switch, damaging the motor.
Possible problems that may have contributed:
Either way, it's doubtful you are going to be able to conclusively prove it and I seriously doubt anyone is going to admit being dumb like that.
So was the motor wired wrong? Doesn't sound like it to me, I think your ABB tech is unfamiliar with PW motors and just spewed that out to deflect blame. Sounds to me as though the contactor failed, maybe welded shut, the motor could not be shut down and eventually overloaded until someone noticed and killed the disconnect switch, damaging the motor.
Possible problems that may have contributed:
- ABB contactors have a row of convenient "buttons" right in the middle of the armature (the moving part of the contactor) and a lot of people erroneously looked at that as if it was a "Go" button. So they would get lazy about trying to figure out a control scheme when they wanted to just see the motor run, and stick a finger on that row of buttons and push. Problem is, no way can they push hard enough to overcome the magnetic forces of the contacts trying to separate, so they vibrate and fry REALLY FAST, as in seconds. Then by the time the pusher realizes they have done something wrong and let go, it's too late, the contacts have welded. Happens a LOT with ABB contactors, that's why I rarely use them on smaller systems.
- It could be that your building engineer cycled it on and off several times in rapid succession. Same issue, it can cause the contacts to weld especially if they were a little undersized, as may have happened for a PW motor being started ATL.
Either way, it's doubtful you are going to be able to conclusively prove it and I seriously doubt anyone is going to admit being dumb like that.