A 15Hp ,460 volt, 17.7 amp ABB drive, is tripping out on over current. The motor is running a fan in an AHU. I didn't start the diagnoses, but I got pulled in, because right now nobody can find the problem. Originally, the motor megged greater 2.2GΩs. So they replaced the drive, same problem. The motor was then replaced, same problem. Replaced the wire between drive and the motor, same problem. Replaced the drive again, same problem. The drive does not trip right away, but seems like there is a 7-10 day interval, between trips. Could dirty power do this? All lugs at the drive, and in the peckerhead are tight.Any other ideas as to what to look for?
Has anyone looked at the current? Has the mechanical load on the motor changed or is it somehow changing when the trips occur? I'd put a power logger on it and tell the jackoffs in charge to STOP throwing parts at it for the love of god.
If it's tripping on overucrrent, what are the current values per phase during a trip? What's the normal running current? Is it belt-driven or direct? Is the plenum shared by any other blowers?
Looks like our OP went AWOL on us, too bad. These were the right courses of action, maybe he took a look and discovered, wonder of wonders, that the drive was tripping on over current because.... drum roll.... the motor was drawing too much current too quickly!
If you are still there Al, there is a slight difference in in a VFD between an Over Current trip and an Over Load trip. If the motor was running along and the flow from the fan was too much, the motor draws too much current over a long time and the motor thermal over LOAD protection kicks in, just like it would on any motor controller. You can get too much air flow when there is a problem with the plenum, the air duct, not providing sufficient pack pressure, ie "open channel flow". When that happens, the power required by the motor increases at the cube of the flow, so the motor overloads. This can happen in an HVAC system when someone remodels, cuts into a plenum to get air into a room, and pays no attention to the effect that will have on the fan load.
An over CURRENT trip on a VFD however indicates that for some reason, the motor (or circuit) is very suddenly is demanding more current than the VFD is capable of delivering. In the case of a typical HVAC fan drive, that could be as little as 110% of the VFD current rating for 30 seconds, or 150% for 3 seconds. In this case, the drive is tripping off to protect ITSELF. This can happen in HVAC systems if there is something like icing taking place, a bearing is seizing up, a motor winding is going bad etc. etc. You have eliminated the motor winding possibility and a motor bearing possibility by changing the motor, you eliminated the possibility of it being the drive or cables by changing them. What's left that hasn't changed? The belts and bearings, icing or the possibility that the load design is the problem.
What can happen there is that under some circumstances, the fan is being called to run when other fans sharing that plenum are already running, and they have no automatic dampers to prevent this fan from spinning due to the air draft pulling through it, called "windmilling". If the fan is spinning forward, the cure may be as simple as enabling a feature called "Flying Restart" in the VFD that allows the VFD to detect the motor speed, match itself to it, and then turn on, avoiding the current spike that can take place. But if the fan is BACK spinning, that is a more difficult problem. The automatic backflow dampers are supposed to prevent this, but if they are not there, that might be the best cure. If for some reason they can't do that, you may have to find a way to BRAKE the motor to a stop before trying to run it. Some VFDs can do this, it's called an "Anti-Windmilling Brake" feature. If your drive doesn't have that, but it does have Dynamic Braking as an option, you may need to rig up a work around using that. If that's the case, post back and I can guide you on how to do it, I've done it many times.
Or you may be off on another project because this problem went away, and all this was a complete waste of time, other than to get it off my mind so I can sleep at night... :whistling2:
That's when they got around to calling me.
