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  1. #1
    Join Date
    May 2001
    Location
    Western Wa
    Posts
    1,842

    VFDs on ungrounded delta systems.

    What say you guys about VFDs, large ones, like 500hp and up, on ungrounded delta 480 volt systems?
    God Bless our Veterans

    God Bless the USA

  2. #2
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Apparently there can be issues. Note this warning from an ABB ACS550 manual:

    Ungrounded.jpg

    I would think you would want the factory engineering folks to bless any proposed install that is not standard 4W Wye. If I were going to install a 500HP+ drive I would not want to be disconnecting any static suppression without engineering approval. Other drives may have different requirements, so the answer would also depend on the exact drive you are using.

    Also, most drive manufacturers will definitely have heartburn over an open delta (two transformer) service.

  3. #3
    Join Date
    May 2001
    Location
    Western Wa
    Posts
    1,842
    This was on a chiller, and the OEM didn't see a problem. The motor winding had spots that looked funny to me, and there was some suspicious arced looking spots on the bearings and thrust. This unit had been torn down for abnormal wear. We sent the stator out to be checked by the best motor shop we could find and they blessed it. I'm still a bit itchy about the ungrounded system. Hope this doesn't all happen again. Sleeve bearings, by the way.

    I guess nobody else could see the rotating red light in my head...
    God Bless our Veterans

    God Bless the USA

  4. #4
    Join Date
    May 2013
    Location
    Utah
    Posts
    57

    Induced Voltage in motor shaft

    I came across an article that detailed induced voltage in the motor shaft from VFD's causing bearing damage.
    This has brought about the advent of something called a shaft grounding collar or similar.
    In the case mentioned, I would expect this would not be allowed.

  5. #5
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    2,071
    Ungrounded systems have a capacitive value against nearby conductors. I can say this with a little authority having read on the subject since your delta anomaly post. They are not safer for staff or equipment. http://www.eaton.com/ecm/idcplg?IdcS...amp;dID=167373

  6. #6
    Join Date
    Apr 2009
    Location
    Mobile, AL.
    Posts
    88
    I hesitate to post, but I deal with these things daily.
    The motor never sees the power from the line. The motor is powered from a DC capacitor bank. The switching transistors generate the pulsed AC from that bank of caps and it does happen to be Y connected when it leaves the VFD. By Y connected I mean there is a ground and neutral and the voltages are not sine wave anymore, but rather pulsed to generate the three phase waveforms.
    The problen with the ungrounded delta is that the varistor noted above has no particular relation to ground since neutral and grouns are not connected.
    The full wave bridge will still charge the capacitor bank which is all that happens from the AC line. The resultant DC is converted to drive the motor.
    Motors both VFD operated and normal can have circulating durrents through the shaft and the path completed through the bearings. The grounding brushes can help keep these cullrents lower. In our applications on large motors we often install insulated ODE bearings along with shaft grounding systems to keep from arcing the bearing.
    In any case you should make certain that you use an inverter duty motor with protection from the high voltage spikes that can be generated especially with long lines to the drive and motor.
    Often line and load reactors need to be installed especially if the lines are "stiff" to increase the reactance in the system.
    With currents in the shaft discharged through the bearings you will find a washboarding of the races and very short bearing life. Also if the motors are not inverter rated you can expect to see the windings fail in the first turns of the windings where crona discharge is most likley to occur.
    My apologies for posting while not "accepted"

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