PwrFlex4 & PFC

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ptonsparky

ptonsparky

Tom
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EC - retired
We have a small PF4, 1HP, 480v, that likes to trip on F005. Three motors with PFC will trip the drive when they start. We know the PFCs are guilty, because disconnecting them removes the problem. (Removed one and started that motor with no problem.)

Line reactor the solution?
 
ptonsparky said:
We have a small PF4, 1HP, 480v, that likes to trip on F005. Three motors with PFC will trip the drive when they start. We know the PFCs are guilty, because disconnecting them removes the problem. (Removed one and started that motor with no problem.)

Line reactor the solution?
Click to expand...

I will guess F005 is overcurrent fault?

Do you have 3 motors on it?

I will guess PFC means power factor capacitors

Lots of guessing... you really have capacitors on the output side of a vfd, that is a 100% no no and will cause faults. You need to remove any capacitors on output of vfd(s).

Keep in mind the vfd itself corrects displacement PF to 1.0 by itself so there would not even be any need for keeping old caps on the motor....
 
The VFD prevents any displacement power factor caused by the motor from bring seen at the VFD input. But to some extent the input current will show a distortion power factor instead.

Tapatalk!
 
There is absolutely no need to have PFC caps on a motor fed by a VFD! The VFD corrects the displacement Power Factor as seen by the line, the exact same thing the PFC caps do. Remove them, and do it immediately. It is a race to see whether the harmonics of the VFD causes the PFC caps to fail, or the charging current of the caps causes the VFD to fail.

Yes, the VFD adds some level of distortion power factor, but typical utility PF metering schemes don't see it because it takes place at frequencies other than the fundamental. So standard PFC caps will do nothing but cause resonnance, if you are concerned, you need tuned harmonic filters to address the distortion. Few people worry about it.
 
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There is absolutely no PFC on the motor controlled by the VFD. It is a 1 HP motor.

Three other motors have the PFC caps, required by the POCO. A 75, 60 and 30 HP.

Each will cause the the VFD to fault when it starts. We removed the cap on the 75 and the vfd would stay on when we started the 75.

I used my 43B to catch the min/max voltage as we started the 75, then the 60 and there is a pretty good drop in voltage but very brief and the motors do not struggle to start.

I was expecting the voltage to spike up as the caps came online but that was not apparrent in the min max mode. I did not try to record transients. Might have been a better choice
 
Ah, I misunderstood the PFC issue.

The PFC caps on nearby motors are likely resonating with the caps on the PF4 DC bus and causing a bus over voltage transient, maybe too fast to be recorded but enough to trigger the fault. Protection faults take priority over display values. Resonance issues like this is something that happens every now and then, especially if the PFC caps are on-line even when the motors they are intended to correct are not running. You said it works OK if the larger motors are started first, but that's not the sequence. Does that then mean that the caps are on-line even when these motors are off? If so, the simplest solution may be to move the PFC caps to the LOAD side of the larger motor starters, so the caps are only on line if the motors are on line.

If they are on-load caps, it might be then that the caps, in supplying the high VARs for starting the motors, are causing a current surge on the line, which briefly drops the voltage, then swings it back high again, like a tank circuit. A line reactor might help the drive by slowing down the rise time of that ringing transient.
 
Lots of drives allow adjusting the overvolts fault level; you might consider raising this, even only for test, to see if it is marginally close. You might be able to just raise it 10-20v and mask the issue.
 
Another easy thing to try would be to take some of the PFC caps OFF that big motor; w/o caps, the line voltage should droop on statrt and NOT bounce back up HIGHER than nominal; so sounds like Jraef hit it with resonating; if you can remove some of those caps it may be decent compromise?

Heck for a test, try removing ALL the PFCs and confirm it does not cause F5?
 
Jraef said:
Ah, I misunderstood the PFC issue.

The PFC caps on nearby motors are likely resonating with the caps on the PF4 DC bus and causing a bus over voltage transient, maybe too fast to be recorded but enough to trigger the fault.
Click to expand...
Been here before. The DC bus on a variable frequency drive has BIG capacitors. Usually electrolytic.
Fast changes in voltage just ain't gonna happen.

Just in passing, a colleague and I were on site looking and the capacitor banks on a variable speed drive we made some years ago. There are 192 2,200uF 450V capacitors.
i=Cdv/dt.......
 
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