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Thread: Standard VFD and transformer sizing practices?

  1. #11
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    Quote Originally Posted by mike_kilroy View Post
    We typically use an AUTO transformer rather than iso
    I've done that too but, I think that in this case, the ratio is too great to make that a practical proposition.
    Si hoc legere scis nimium eruditionis habes.

  2. #12
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    Quote Originally Posted by Besoeker View Post
    I've done that too but, I think that in this case, the ratio is too great to make that a practical proposition.
    True, the autofmr would not save any money as typical compared to iso xfmr, but we don't do it for that reason; we use auto on output so it works just as well at 10hz as 60 or 100hz to pass thru the VFD output.

  3. #13
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    Quote Originally Posted by mike_kilroy View Post
    True, the autofmr would not save any money as typical compared to iso xfmr, but we don't do it for that reason; we use auto on output so it works just as well at 10hz as 60 or 100hz to pass thru the VFD output.
    A double wound will do that too. It's a matter of a constant V/Hz ratio.
    Si hoc legere scis nimium eruditionis habes.

  4. #14
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    How long has it been operating in its current configuration?
    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
    Last edited by Ingenieur; 10-07-17 at 11:31 AM.



  5. #15
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    Quote Originally Posted by Jraef View Post
    Over sizing by 50% is a "poor man's" work around to the problem of the complexity of running a motor through a transformer on the load side of a VFD rather than spend the money (and lead time) to have a custom transformer built to handle it properly. Besoeker's mention of building it with a special air gap is a common solution, I've seen others, but none of them cheap or fast.

    The problem from the VFD's standpoint is that the drive must be sized not only for the motor, but also for the losses between the drive and the load, which will now include the transformer and cable losses. So "fudging it" by over sizing the transformer to handle the added heat means paying for it in the VFD sizing anyway.

    In your case you already have this transformer and for all we know, it was custom made for this application, I mean it's not as though a bizarre ratio like that exists off the shelf somewhere anyway. So assuming someone knew what they were doing with that, and assuming that part of it was working fine, to my mind all you need do is increase the size of the VFD. The fact that the first one eventually failed was possibly related to it being constantly operated at maximum capacity. Over sizing would be almost imperative in this situation. Over sizing a VFD for an application hurts nothing but the budget, up to a point. Many VFDs will not like being connected to a load smaller than 50% of its rating just because of sensor range tolerance issues. So I would not go larger than a 200HP drive. But how big, I don't know.

    If you have any empirical data on how the last drive was operating, than might be helpful in deciding, but if it was in a constant state of current limit at its maximum output, you may not know what it could have seen if it were possible to operate at full speed. But one possible indirect (albeit crude) way of estimating would be that if for example it was in a constant state of current limit at 125A, and to achieve that the output frequency was limited to, let's say 48Hz, that's 80% speed. If it's a simpler constant torque load, like it would be with a positive displacement pump, you can simply see it as a 125% increase (the inverse of 80%) in power (kW) to get 100% speed from it. If the load is a centrifugal (aka quadratic) pump it gets a little more complicated as to the load at 100% speed because the power requirement will increase at the cube of the speed change, so 195% power would be inferred (125% cubed). Then from whatever power increase you see, to get the drive output current required you will have to know (or reasonably guess) the power factor of the motor at those speeds.

    Yours is not a simple challenge to be sure...
    The transformer is a K-rated transformer. The last running data before the drive went out was the motor running at 51hz, 140A. So the transformer and drive are both pretty much maxed out.
    It runs at a constant speed, doesn't go up or down automatically, only manually.

    I can go with a 125hp drive to be a close match to the transformer but if it doesn't hurt anything, then i'd rather go with a 150hp drive.

    It looks like this 100hp drive and transformer was installed in 2012. So a 5 year run time pretty much maxed out before having any problems.

  6. #16
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    Quote Originally Posted by Ingenieur View Post
    How long has it been operating in its current configuration?
    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
    The only data I can find on this set up is that its been going since 2012 trouble free and running at 51hz, 140A @ 480v. So not that much data but some at least.

  7. #17
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    Quote Originally Posted by mike_kilroy View Post
    I agree that you can put in a larger VFD so it does not have to work as hard and should then last longer than the last one did (how long DID it last?).

    We typically use an AUTO transformer rather than iso - get same output voltage conversion, added L filtering, and non of the additional transformer heating due to running at lower than its design frequency; but in our uses, we typically need to run from about 10hz to 100hz. Use them for both increase and decrease output voltages for various reasons. Not as common as it used to be though. All were designed for the application, or oversized if standard transformers as Jraef stated. Easy analogy for oversizing: is you would not expect a 60hz transformer to run loaded at 50hz, let alone the added harmonic load.

    Since yours is a pump, I agree you probably run at near base speed; and as commented previously, the transformer still works fine. I doubt you would run a new 25-50% oversized VFD at higher load than present. If it made you feel better, slide thermal switches between core and each coil, put them in series, and feed to your control or VFD to shut down if it gets too hot. Then just move on to the next project.
    That is a good idea about the thermal switch, but unfortunately they would rather the equipment burn up than causing a shutdown because of heat. There are however pressure switches installed to shut the drive down if there is an unsafe condition down the hole.

  8. #18
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    Quote Originally Posted by paul_arc View Post
    The only data I can find on this set up is that its been going since 2012 trouble free and running at 51hz, 140A @ 480v. So not that much data but some at least.
    you are at the ragged edge
    upsizing would be prudent
    the cost delta 125 hp to 150 is small (imo depending on i out rating 125 is ok)
    the xfmr will the real cost, if designed for this app 150 kva should do it
    if designed for this application the existing 125 kva may be ok
    125000/(480 x sqrt3) = 150 A > 140 A, tight but ok
    measure the xfmr operaring temp
    Last edited by Ingenieur; 10-07-17 at 12:41 PM.



  9. #19
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    Quote Originally Posted by Ingenieur View Post
    the xfmr will the real cost, if designed for this app 150 kva should do it
    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.
    Si hoc legere scis nimium eruditionis habes.

  10. #20
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    Quote Originally Posted by mike_kilroy View Post
    I agree that you can put in a larger VFD so it does not have to work as hard and should then last longer than the last one did (how long DID it last?).

    We typically use an AUTO transformer rather than iso - get same output voltage conversion, added L filtering, and non of the additional transformer heating due to running at lower than its design frequency; but in our uses, we typically need to run from about 10hz to 100hz. Use them for both increase and decrease output voltages for various reasons. Not as common as it used to be though. All were designed for the application, or oversized if standard transformers as Jraef stated. Easy analogy for oversizing: is you would not expect a 60hz transformer to run loaded at 50hz, let alone the added harmonic load.

    Since yours is a pump, I agree you probably run at near base speed; and as commented previously, the transformer still works fine. I doubt you would run a new 25-50% oversized VFD at higher load than present. If it made you feel better, slide thermal switches between core and each coil, put them in series, and feed to your control or VFD to shut down if it gets too hot. Then just move on to the next project.
    ......



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