VFD Current Imbalance 2

[roydm]

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

 

[LionelHutz]

With the info given, I'd have to think your assumption that it's not the motors in incorrect.

What load is on the motors when you see this inbalance?

 

[jraef]

How is it that you are reading motor output current from the drive, using the VFD's display or a hand held meter? Most common digital meters are not able to reliably measure current on the output of a VFD without some fairly sophisticated filtering of the harmonics.

Or are you referring to input current to the drive? If so, it would have nothing to do with the motor current.

"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

 

[roydm]

The motors are at full load current (274 Amps at about 95% of full speed, pump application

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.

 

[controlsdude]

300meter (900 ft) distance would require a load reactor or some form of resistor/load reactor combo to eliminate the possiblity of standing waves for this distance. Was this the manufacturer's recommendations to not use load reactors? Some drives are better than others, but thought most would recommend a Load reactor at that distance.

 

[roydm]

I don't know what the manufacturer recommended to client, they wouldn't tell me anyway.
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

 

[jraef]

So are they reading the values of r0069, 0, 1, and 2 (U, V, W phase currents)? If so, this may be the problem. As I understood it, those are not an on-line current monitoring function, those are values used in the Motor Identification procedure, essentially the "Auto-Tune" function for creating the motor model used in the vector control algorithm. They are a reference point from the initial setup test when the drive calculates all the necessary model information. I'm not sure they are meant to be used as individual running phase currents. I might be wrong on that, it's been a few years now. But as far as I know, there was no on-line real time measurement of individual phase currents, only an overall motor RMS current value (r0027).

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

 

[roydm]

Quote by Jraef
"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

 

[roydm]

Reading through the parameter list it certainly makes you think r0069.0, 1 & 2 are the individual phase currents but how would they measure that?
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

 

[jraef]

I believe that they do one circuit at a time during Motor Identification testing, in other words they turn on A-B phases, measure the bus current, then turn it off and repeat for B-C then C-A. That's why I don't think it works as an on-line current monitor.

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

 

[controlsdude]

i had a warning from a drives guy, that if you dont put load reactors on a long lead run, you have the chance of breaking down the insulation on the motor. You must have a nice motor for now, well maybe not for long. The drives guys I remember said that you could just make the motor fail eventually, depending on what your using for motor leads from drive to motor. We were using just THHN in a dry enviroment for leads so probably would of dusted that motor in 5 mins without the reactor. At least thats what he said, I did not try it just ordered the load reactor before I had a problem onsite. I think also, if your load varies alot you will get more standing waves versus a steady state contant speed load like we were running.

 

[roydm]

Jraef,
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

 

[ozmosis]

roydm
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.

 

[jraef]

Thanks for chiming in Patrick, I knew this was more up your alley.

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

 

[khanengr]

try adding harmonic filters, hope they resolve the issues...

 

[roydm]

Thanks again Jraef & Patrick, its starting to make sense

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

 

[roydm]

Update
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 AOP were 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?

 

[jraef]

Yes. If this black box is on the input side and it is only reading 2 phases, it is calculating the 3rd phase current. Perfectly acceptable in normal sinusoidal currents, but unless that black box was SPECIFICALLY designed to be able to deal with harmonic rich currents, it is likely worthless. Given that it is only looking at 2 of the phases, that right there indicates it was not intended for this use. You need a GOOD quality Power Monitor, and a hint that it is capable of properly interpreting the harmonic current levels is that it will include the ability to display harmonic currents or THD separately. Those types of power monitors are very expensive and thus often avoided by OEMs looking to minimally satisfy some vague project specification that failed to bring this issue up. But they will always use 3 separate CTs.

"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

 

[roydm]

Jref,
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