Current Imbalance at VFD Input

Discussion in 'Power Electronics' started by chetanb, Oct 10, 2016.

  1. chetanb

    Thread Starter New Member

    Sep 13, 2014
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    0
    Hello ,

    We have a motor which is running on VFD . Full load current shown on the VFD display is 473 , 470,467 A
    Whereas , at the supply breaker , the current measured in 430,435,526A .
    We have checked the VFD components , but all seem to be ok.
    What can be the issue which leads to the current imbalance at the source ?
    Thanks in advance.
     
  2. tsan

    Member

    Sep 6, 2014
    41
    4
    If the supply unit is a diode rectifier, current unbalance can be caused by supply voltage unbalance or it can be a measurement error.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,997
    3,756
    I assume you have a 200 HP electric motor running 240V and enough current to vaporize some copper wires nearly instantly and you are asking an on-line forum for advice? Hire an electrician trained in industrial applications.
     
    shortbus likes this.
  4. chetanb

    Thread Starter New Member

    Sep 13, 2014
    8
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    Hello ,
    The motor is running on 415 V . I have seen motors running on 500 A and above current before , but never encountered such issue.
    Just wanted to understand what factors may lead to this problem.And since , VFD output current is stable I am not doubting motor or the VFD.
     
  5. chetanb

    Thread Starter New Member

    Sep 13, 2014
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    Hello , The supply is 415V and supply voltage has 0.5 % imbalance which is acceptable . Also , the measurements were done using a fluke power analyzer and also confirmed using other meter as well. So no issues in measurement also.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    At that supply current into the VFD I would suspect the bridge consists of individual stud mount rectifiers?
    It is possible you could have an open rectifier on the bridge.
    Max.
     
  7. tsan

    Member

    Sep 6, 2014
    41
    4
    Again I assume supply unit is a diode rectifier(not igbt supply and not some half or full controlled thyristor rectifier):
    Motor runs steady so is dc voltage steady too? it should be when supply voltage unbalance is only 0,5 %. Steady dc also means all rectifier diodes are ok.
    On steady dc reason for supply current difference has to come from supply side. It could be that supply cables, ac input choke on vfd or supply transformer has some damage. I guess you have checked those already. Can your fluke make harmonic measurement? If you have a compensation capacitor or tuned filters for example for 5th and 7th harmonic then perhaps there is some problem there. Is this the only vfd on the network? Is this the only equipment with supply current unbalance? Could it be ground fault?
     
  8. chetanb

    Thread Starter New Member

    Sep 13, 2014
    8
    0
    Hello ,
    I checked the rectifiers and I got a constant DC Voltage of 640V . There are 2 VFD's and both are showing the same phenomenon.
    It leads to suspecting the power supply but then the power does not show any voltage imbalance .Also the 3rd , 5th harmonics and THD is less than 3%
     
  9. chetanb

    Thread Starter New Member

    Sep 13, 2014
    8
    0
    Hello ,

    DC is constant at 640V .
    There are 2 VFD's and both are showing the same phenomenon.
    It leads to suspecting the power supply but then the power does not show any voltage imbalance .Also the 3rd , 5th harmonics and THD is less than 3%. I am not sure if there are specific tests to carried on supply transformer.It is a 2000 kVA transformer.
     
  10. tsan

    Member

    Sep 6, 2014
    41
    4
    If this 6 pulse bridge? Check current harmonics, not voltage harmonics. 5th and 7th current harmonic has to be much more than 3%.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
    1,103
    How is the current being sensed? Shunts? Hall devices? .....?
     
  12. chetanb

    Thread Starter New Member

    Sep 13, 2014
    8
    0
    Hello ,

    VFD has its own CT for current sensing and I am using Fluke CT's with power analyzer to check the current at the supply end.
     
  13. chetanb

    Thread Starter New Member

    Sep 13, 2014
    8
    0
    Hello ,
    Since this motor is for a compressor , current harmonics % will be high right ? There will always be change in current .
    THD current goes as high as 72%
     
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