VFD Current Imbalance
VFD Current Imbalance
(OP)
We have a client who installed 2 149kW motors and VFDs using regular cable and no reactors.
The cables are typically 250 - 300 meters long (VFD to motor).
We are seeing current difference between phases of 5 - 7%
Assuming the motors are ok (all new) the only thing I can attribute this unbalanced load to is harmonics acting on the cables.
Unfortunately due to the location I am unable to get good information on the cable they used but I assume it's 3 phases with 1 ground conductor.
Does this seem like a reasonable assumption or is there some other explanation for phase differences?
Would adding load reactors help to bring the phases back into line?
Thanks
Roy
The cables are typically 250 - 300 meters long (VFD to motor).
We are seeing current difference between phases of 5 - 7%
Assuming the motors are ok (all new) the only thing I can attribute this unbalanced load to is harmonics acting on the cables.
Unfortunately due to the location I am unable to get good information on the cable they used but I assume it's 3 phases with 1 ground conductor.
Does this seem like a reasonable assumption or is there some other explanation for phase differences?
Would adding load reactors help to bring the phases back into line?
Thanks
Roy





RE: VFD Current Imbalance
What load is on the motors when you see this inbalance?
RE: VFD Current Imbalance
Or are you referring to input current to the drive? If so, it would have nothing to do with the motor current.
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RE: VFD Current Imbalance
I am told the reading is from the display panel on the VFD, "it shows the 3 phase currents and frequency".
The drives are Siemens Micromaster 440 operating on 380V 50 Hz.
It is very hard to verify the information, but I have to assume it's correct since it was collected by our mechanical engineer.
RE: VFD Current Imbalance
RE: VFD Current Imbalance
I did speak to a VFD expert Siemens Canada, he didn't think length was an issue even though it's outside what the manual recommends.
Puting in a drive without reactors wouldn't be my choice but I'm trying to justify installing them after the fact.
My question still is, are the harmonics causing the imbalance and will reactors help?
Thanks
Roy
RE: VFD Current Imbalance
Member Oz is the guru of the MM drives though, he used to work for them in that specific division (I was in another group). Hopefully he will see this thread and chime in.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD Current Imbalance
"So are they reading the values of r0069, 0, 1, and 2 (U, V, W phase currents)?"
WHAT - that would be just too funny, do you think the 3 parameters you mention would be that close together
I have
260, 256, 270 for motor 1
273, 261, 255 for motor 2
The motors are the same size, identical service (parallel trains) but the leads would be a bit longer for one
I will do some more digging
Thanks for the input so far
Roy
RE: VFD Current Imbalance
I thought some drives measure the DC link current but the block diagram gives no clue.
Somehow I will have to find out which parameters they are watching.
Thanks Jraef, you've given me something to do :)
Roy
RE: VFD Current Imbalance
But I have seen some drives that had hall-effect transducers on the outputs to measure individual phase currents. It's generally too expensive and drives the cost of the drive up to make them uncompetitive, but Siemens has never let hat stop them in the past.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD Current Imbalance
RE: VFD Current Imbalance
Your explanation makes perfect sense to me, all I have to do now is figure out what they are looking at. There must be a parameter somewhere that sets the display.
Thanks for the help
Roy
RE: VFD Current Imbalance
Yes, Jeff is correct in his last post.
A usual tip/trick is attached for running through the motor magentising setup.
Also, setting the display is P0005. Decide the r0#### (read value) value you wish to use and go to P0005 and put this numeric value in as the setting. Basically, you 'connect' a read value to this parameter. Parameter P0005 will 'use' this (r) value as the display. This is the technology Siemens call BiCo that allows you to use the parameters connecting as inputs/outputs.
It's a complex explanation but quite simple once you understand it.
RE: VFD Current Imbalance
BICO is one of those tools that is for people that would use it often, I never got the hang of it. I did understand it at one point when I got hands on training, but the printed explanation confused the heck out of me. So the next time I had to use it, I had forgotten it again and the translated German information got me confused again. One would think that for all the years I spent with German companies, I would have learned to think like them, but alas, not so. I guess it takes more time.
But I believe that the only real time current value that you can assign to P0005 is r0027, which is the motor current, but averaged across the 3 output phases.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD Current Imbalance
RE: VFD Current Imbalance
Talking with the process engineer he gave me some further information
On the outside of the cabinet they have a 4 line display which I think must be the AOP version.
Inside on the drive itself is a second single line display showing Hz (a BOP I think)
The observed readings on the AOP were 260, 256, 270, 262 (262 happens to be the average of first 3 numbers)
So I think this panel must be displaying r0027
Looking at the parameter listing for r0027 it gives 3 options Min:- Def:- & Max:-
Are these the first 3 numbers we observed, how are they derived, over time?
The client is limiting the drive frequency based on the highest of the 4 numbers (270), does that make sense to you?
Thanks again
Roy
RE: VFD Current Imbalance
Since I last posted we have been talking with Siemens several times, they were as puzzled as I am, and told me it's not possible to read the individual motor phases and didn't recognise the display from my 3rd hand description.
I also found there are CTs fitted to just two of the input phases connected to a black box which I assume must be the display on the front of the enclosure I previously thought this was an AOP (my mistake)
From previous post above
The observed readings on the
AOPwere 260, 256, 270, 262 (262 happens to be the average of first 3 numbers)The 3 currents we were told are the motor phase currents must be the AC input current, 2 measured, 1 calculated with the 4th being the average (it's exactly (A+B+C)/3) so none of these is the actual VFD output, there is a BOP on the drive itself displaying frequency.
The drives 4-20 mA output goes to the DCS but the electrician in the switchroom doesn't see that, he is watching the display on the enclosure front and limiting the speed by observing the highest phase current, 270 in this case.
The electrical design is not in our scope, all we have is a photograph of the drive drawings but in order to understand the pump performance we have to play detective.
Does any of this make sense?
RE: VFD Current Imbalance
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD Current Imbalance
Thanks for confirming the readout is nonsense.
If only we could convince the client to let the VFD look after the motor.
It's very dificult to get through