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Thread: Buck and Boost

  1. #11
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    Quote Originally Posted by pv_n00b View Post
    That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.
    Quote Originally Posted by Besoeker View Post
    So what do you think the kVA should be?
    Quote Originally Posted by winnie View Post
    Most commonly for buck and boost applications under 600V, the transformer used has separate primary and secondary coils. These coils are then connected in an autotransformer configuration.

    The secondary coil(s) are 12 or 16 or 24V, and often there are 2 coils which can be in series or parallel. These transformers have 600V insulation on both the primary and secondary side. In this case the transformer secondary will be rated to carry the full current needed to give the rated kVA at the secondary voltage. The secondary coil(s) will be wound with different size wire than the primary.

    The other common autotransformer is the variable autotransformer. In this case there is only a single coil, generally made with a single size wire. The kVA rating will be at full output voltage, and at low voltage output the available kVA is less than the rating.

    -Jon
    So, like I said: "essentially" delta v times amps
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #12
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    Quote Originally Posted by electrofelon View Post
    So, like I said: "essentially" delta v times amps
    Um, It was I who said that, wasn't it?

  3. #13
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    Quote Originally Posted by winnie View Post
    Most commonly for buck and boost applications under 600V, the transformer used has separate primary and secondary coils.
    I SLIGHTLY disagree with the terminology primary and secondary. In boost applications that terminology fits mostly, but in buck I don't think it does. There are, as you said, often 2 (usually identical) low voltage coils allowing a 12, 16, 24, 32, or 48 volt change (I've never run into the pair of 24V coils, but there is certainly no reason it wouldn't exist; all it offers is a 48V change.)

    Of course, there is no problem in using a transformer with one primary (even if dual coils) and 1 or 2 secondaries as a buck-boost ...

    George

  4. #14
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    Quote Originally Posted by electrofelon View Post
    So, like I said: "essentially" delta v times amps
    That would be my take also.
    I was questioning pv n00b's disagreement with that.
    Si hoc legere scis nimium eruditionis habes.

  5. #15
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    181013-1446 EDT

    GeorgeB:

    My definition of primary is the pair of terminals to which net positive average power is input. A secondary is any pair of terminals thru which net positive average power is delivered. Thus, in an autotransformer some winding may be part of both the primary and secondary.

    Where this may not be useful is in a telephone transformer where simultaneously there can be uncorrelated power transferred in both directions.

    For a distribution transformer supplying a home with solar panels the role of the primary can shift back and forth between the two physical coils based on the direction of average energy flow. In this case we have synchronous correlated signals.

    .
    Last edited by gar; 10-13-18 at 03:12 PM.

  6. #16
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    Biggsc...

    Because load kVA is 240x17/1000, then minimum AT-Xfmr size is 4.08-kVA! I hope you didn’t purchase an AT based on the ratio provided!
    Remember, an AT changes input-to-output voltages, but primary-to-secondary power transfer must remain the same!

    Regards, Phil Corso

    Ps: if additional detail is required contact me!

  7. #17
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    Quote Originally Posted by Phil Corso View Post
    Biggsc...

    Because load kVA is 240x17/1000, then minimum AT-Xfmr size is 4.08-kVA! I hope you didn’t purchase an AT based on the ratio provided!
    Remember, an AT changes input-to-output voltages, but primary-to-secondary power transfer must remain the same!

    Regards, Phil Corso

    Ps: if additional detail is required contact me!
    The buck-boost transformer configuration delivers the load current times input voltage directly from the source without any involvement of the "primary" section of the single winding. The output power corresponding to the voltage delta times the load current comes from a corresponding input power in which current over turns ratio flows through the primary winding.
    If there is only one tapped winding, then all of the primary winding must be sized for the output current, though the extra heat dissipation will not be present.
    Sent from my XT1585 using Tapatalk

  8. #18
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    Biggscc, GoldDigger, Gar, et al...

    I humbly apologize for my grave error.. I overlooked the basic principle of the AT... power is delivered by both transformer and conduction action. Mea Culpa!

    More tomorrow!

    Phil

  9. #19
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    Come home, Phil, all is forgiven.

    Sent from my XT1585 using Tapatalk

  10. #20
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    Here's the calculator, plug and chug folks: http://www.buckboostcalculator.com/index.cfm

    OP does not say if they are looking for a single phase or 3 phase transformer, the rating of the transformer depends on it.

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