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Thread: Standard VFD and transformer sizing practices?

  1. #21
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    Quote Originally Posted by Besoeker View Post
    The existing transformer survived unless I'm mistaken. And 125kVA would seem more than adequate.
    The motor is 100hp/75kW. Again, it survived unless I'm mistaken.
    So, I don't see any justification for upgrading either.

    The drive worked for five years before it failed (from what cause we don't know) so one might assume it was fit for purpose.
    So again, I don't see any need for an increase in rating.

    Unless something has changed on the process side that we don't know about - then all bets are off.
    You see no reason
    I see a possible reason based on partial information
    need drive mfg/pn

    The moor is 108 HP unless a typo
    afaik the drive has not failed?
    drive is running at 140 A (at the limit for many 100 hp drives)
    I'm surprised you did not ask how long it had been in operation?

    As I said the xfmr may be fine if designed for this use
    why do you like selectively chopping my posts up to distort their context?
    so you can 'correct' me

    load/xfmr = 140/150 = > 93% ignoring losses
    not sure 'more' than adequate
    if based on motor fla >95% 120 kva/125 kva
    Last edited by Ingenieur; 10-07-17 at 01:27 PM.



  2. #22
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    Quote Originally Posted by Ingenieur View Post
    I'm surprised you did not ask how long it had been in operation?
    From the OP:

    The only data I can find on this set up is that its been going since 2012 trouble free
    Si hoc legere scis nimium eruditionis habes.

  3. #23
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    Quote Originally Posted by Besoeker View Post
    From the OP:
    I asked him that
    surprised YOU did not

    I believe in basics
    i, v, load, operational methods, etc
    data on the actual devices used
    not esoteric treatise on xfmrs until we have that data

    with the limited data
    drive looks marginal at best
    xfmr if designed fro vfd use ok but close to rating

    need to know
    Any life safety implications
    revenue impact upon failure
    in addition to tech data to make a vetted due dilligence opinion
    otherwise idle speculation


    Quote Originally Posted by Ingenieur View Post
    How long has it been operating in its current configuration?
    Quote Originally Posted by Ingenieur View Post
    has the xfmr temp ever been measured?
    drive output i measured?
    xfmr output i measured?
    drive input i measured?
    xfmr config: y:y, d:d, etc? pu Z?

    for all we know this set-up may run to an expected service time without issues

    hmmmm



  4. #24
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    Quote Originally Posted by Ingenieur View Post
    I asked him that
    surprised you did not

    I believe in basics
    i, v, load, operational methods, etc
    data on the actual devices used
    not esoteric treatise on xfmrs until we have that data

    with the limited data
    drive looks marginal at best
    xfmr if designed fro vfd use ok but close to rating

    need to know
    Any life safety implications
    revenue impact upon failure
    in addition to tech data to make a vetted due dilligence opinion
    otherwise idle speculation



    [/B]
    hmmmm
    OK.
    Si hoc legere scis nimium eruditionis habes.

  5. #25
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    The motor is 108hp.
    The drive failed from overheating, at least that's what the fault code said. Then it wouldn't run longer than 5 mins without over heating again. I cleaned it out and still couldn't get it to run any amount of time. An educated guess would be that it failed from running at the upper limit of the drive for so long. I had another 100 hp drive that I put in to get it going again but now looking to get a replacement of adequate size.
    I'm looking at this 110kw drive, so about 150hp. My only concern is being too large for the 125kva xfmr and end up frying that.
    There is another 90kw drive, so about 120hp. That matches the existing 125kva xfmr pretty good, but I feel it is still on the small size for the motor...

  6. #26
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    Quote Originally Posted by paul_arc View Post
    The motor is 108hp.
    The drive failed from overheating, at least that's what the fault code said. Then it wouldn't run longer than 5 mins without over heating again. .
    Yet it ran without problems for five years?
    Fans?
    Filters?
    Different operating regime?
    Si hoc legere scis nimium eruditionis habes.

  7. #27
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    Put the transformer in the water as well to keep it cooler....

    (Can you put it in a can filled with xfmr oil, and immerse that in the water.....?)

  8. #28
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    Quote Originally Posted by Open Neutral View Post
    Put the transformer in the water as well to keep it cooler....

    (Can you put it in a can filled with xfmr oil, and immerse that in the water.....?)
    WHY??? The xfmr never failed!

    The vfd only lasted 5 yrs.... should be 20yrs... so OVERSIZE it!!! Dah?

    Sent from my SM-G900V using Tapatalk

  9. #29
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    As I see it you seem to think that there is a possibility that too large of a drive can somehow put the transformer at risk. Not so. The drive is just a source of power, the motor and pump
    Is the load. The transformer simply sits in between. So long as the pump does not demand more load, nothing changes.

    But that's where what I brought up earlier comes into play. If the VFD was automatically limiting the speed to 51Hz in order to maintain itself at the 140A Current Limit, using a drive with a higher current limit may allow the pump to run at the full 60Hz (worst case scenario). Again assuming a centrifugal pump, 51/60 = .85, so you are increasing the speed by 117.6% and that small change in speed on a centrifugal pump will demand roughly 163% more HP from the VFD. If allowed to do that, that then means 162% more power flowing through that transformer too.

    Simple solution would be to simply buy the bigger VFD, but program the current limit to still be 140A, since you know the transformer has been ok with that all this time. So the only thing that changes really is the thermal capacity of the VFD. The new one would not be running at its upper limits.
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  10. #30
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    Quote Originally Posted by Jraef View Post
    As I see it you seem to think that there is a possibility that too large of a drive can somehow put the transformer at risk. Not so. The drive is just a source of power, the motor and pump
    Is the load. The transformer simply sits in between. So long as the pump does not demand more load, nothing changes.

    But that's where what I brought up earlier comes into play. If the VFD was automatically limiting the speed to 51Hz in order to maintain itself at the 140A Current Limit, using a drive with a higher current limit may allow the pump to run at the full 60Hz (worst case scenario). Again assuming a centrifugal pump, 51/60 = .85, so you are increasing the speed by 117.6% and that small change in speed on a centrifugal pump will demand roughly 163% more HP from the VFD. If allowed to do that, that then means 162% more power flowing through that transformer too.

    Simple solution would be to simply buy the bigger VFD, but program the current limit to still be 140A, since you know the transformer has been ok with that all this time. So the only thing that changes really is the thermal capacity of the VFD. The new one would not be running at its upper limits.

    Yes, I could set the current limit so it doesn't exceed the transformer rating but a lot of times the operators like to get in there and pretend to be an electrician. Most of the time its just to adjust the speed for their needs. It's usually only a 1 or 2 hz change one way or the other. We get a lot of power flickers here and the equipment will drop out, when the operators try starting it back up without an electrician on site, they really get in the drives and screw them up some times.

    I guess the part i'm concerned with is giving them the capability of really blowing something up. Because the pumps get stuck sometimes when it goes down which requires hard starting. Changing voltage boost, sync delay, current limit, accel time, and what ever else they get into while trying to get it started again.

    Looking at the current limit of this drive, it is set at 185A@ 480v, which is 120% of the motor name plate. That exceeds the transformer already, but i'm not sure if the drive would even be able to hit that number since its only a 100hp.
    I guess that's all i'm thinking about, limiting the power so that someone who doesn't know cant blow the transformer.

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