I know this is kind of an odd question for this forum. But I feel we have a lot of great minds on here whom might be able to make sense of this for me.
We have a powerflex 4 drive that started giving the F5 fault (overvoltage). Long story short replacing the motor stopped us from getting the fault.
But everything I read about overvoltage faults is related to deceleration and/or braking be it dynamic or regenerative.
It only faulted out once it got the start/run signal. Which would lead me to believe this happens on the acceleration side.
I just haven’t be able to find a solid theory to explain this.
I would appreciate any help or light anyone can shed on this for me.
My mind is driving me crazy trying to explain this.
Thank you in advance!
AC Motors’ stators are big coils. Imagine a flaky connection in your motor, or a crack developing in the winding. Such that thermal stress opens it. Imaging a high current pulse from the drive getting interrupted by such a crack opening. What happens to the flyback from the current in the coils?
Ground fault in the motor, I’d say. If you have a phase going to ground the voltage between the other winding and ground and you have a floating ground (delta power bus for example) or a high resistance ground, then the voltage on the other legs will actually spike up in reference to ground.
This can happen on either side of the inverter. An old place I worked at in a previous life had a delta bus. If a tank heater on one end of the plant would short to ground (being in a liquid) it would fault an induction heater at the other end of the plant for DC bus voltage. Technically, it would have had an effect on everything on that substation if it was using ground as a reference.
Delta. good for submarines, not always so good for factories.
@JordanCClark but those drives have a ground fault. Why would it trigger the overvoltage vs the ground fault?
@pturmel I’m not familiar with term flyback. I did some quick googling. Not sure if the motor has one inside, i’d have to crack it open. I assume if there is one in the VFD it is okay, because it was not replaced. unless it is wearing out. I’ll have to research more on how to find and test that for this.
@JordanCClark if im not mistaken the bus is in the vfd which was not replaced. the other drives in the same panel experience no issues.
It’s not a thing. It is a phenomenon that happens when a circuit containing an inductor is broken when the inductor is carrying an non-zero current. At the very least you get arcing at the break. It takes high voltage to initiate the arc. The sum of the voltage drops around the circuit still has to be zero, so that high voltage gets reflected elsewhere in the circuit.
Note that in real life, the arc doesn’t have to jump the break. It can jump to adjacent coils or to the motor frame. That greatly complicates the mesh of possibilities and expands the freakiness quotient.
I was talking about your incoming power. But, nonetheless, if your ground is floating, regardless of the type your incoming power is, will affect the DC bus in the VFD. It’s just more noticeable in delta power systems.
This makes more sense to me now. Probably not worth the investigation. But this settles my mind and makes the “troubleshooting” easier.
Thank you both for your help and understanding
I was going to say that if you have an insulation tester you can use on the motor, that may give you some clues too
Psst, pick this up during the black Friday sale. It’s got a good mix of concepts presented in an accessible way. Forums are cool but books are better .
@JordanCClark thanks just ordered one.
@bmusson Do you have this book? And does it cover this specific thread/question?
There’s nothing in the book *specifically *about ground faults causing voltage spikes, but I think the sections on inductance and AC motor operating principles would help with getting a general intuition for why it happens.