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Thread: Buck-boost transformer sizing question

  1. #1
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    Aug 2004
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    Buck-boost transformer sizing question

    In a 3-phase 208v. circuit feeding a 220v. motor, we are installing voltage boosting autotransformers for voltage correction; we know how to do this. The question is how voltage at the motor terminals is affected by varying the motor current. Under light load conditions motor now draws 7.6 amps (208v. 3-ph.) but when heavily loaded approaches nameplate rating of 11 amps. If we size boosting transformers for the 11-amp load, what voltage will we have at 7.6-amps?

    How is this calculated? I know it will be proportional to the current change, but cannot find a formula or explanation in any reference.
    Steve Skolnik

  2. #2
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    Re: Buck-boost transformer sizing question

    Assuming the incoming supply is adequate the voltage will not change from no load to full load.

    Size the transformer to the maximum load you expect, when the load is less the voltage will still remain the same.

    I have to ask why you would bother to boost 208 to 220, typically 208 will run a 220 motor fine.

  3. #3
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    Aug 2004
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    Re: Buck-boost transformer sizing question

    Bob, thanks for your reply to my question. Owner has lost a couple of motors and mfr. has advised him voltage is too low; this is the reason for the boosting transformers.

    I am interested in the WHY of this question, which you did not address. I thought that, since autotransformers handle a portion of the overall circuit current, that the voltage change would be proportional to the current, and that there would be some formula to calculate this proportion. Is this not the case?
    Steve Skolnik

  4. #4
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    Re: Buck-boost transformer sizing question

    Hi Steve

    I have installed many buck boost transformer and have never witnessed the voltage changing in proportion to the load.

    Typically I boost 208 to 230, if I measure 230 with no load I will have 230 with load as long as the transformers are properly sized.

    I took time to look at catalogs in my truck and also some on line sources, I can not find any buck boosts to bring 208 input to 220 output.

    The choices I find with 208 input are 230 or 240 output.

    Bob

    [ August 09, 2004, 04:13 PM: Message edited by: iwire ]

  5. #5
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    Re: Buck-boost transformer sizing question

    Seems odd to have a 220V rated motor. Usually they are 200 or 230V.
    A regular transformer will vary with load, with the enemy being impedance. As you get to full load, the secondary voltage will drop by the percentage of the impedance on the name plate.
    Ron

  6. #6
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    Re: Buck-boost transformer sizing question

    Steve,
    Not exact science but close enough.

    The voltage stability of buck-boost transformer arrangements is not any more load depedent then any other arrangement. While autotransformers only transform a portion of the total circuit current, they do transform 100% of their rated current and voltage.

    Bob,
    Standard buck-boost changes are: 5%, 10%, and 20%, for 120/240-12/24V transformers; and 6.6%, 13.3 and 26.6% for 120/240-16/32V units. The relevant formulas are; %V=(HV-LV)/LV, LV=HV/(1+%V), and HV=LV*(1+%V).
    Just because you can, doesn't mean you should.

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