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Variac VA

Discussion in 'General Electronics Discussion' started by Sinewave, Oct 21, 2017.

  1. Sinewave

    Sinewave

    126
    2
    Feb 15, 2013
    Hi, I've got a Varic which I'm told is 2.2kva. There is no data plate on it. What do you think is a reasonably accurate way of finding out what it's rating really is?
     
  2. kellys_eye

    kellys_eye

    4,281
    1,144
    Jun 25, 2010
    Measure its physical dimensions and get back to us.
     
  3. Sinewave

    Sinewave

    126
    2
    Feb 15, 2013
    Measurements are approx as I don't want my steel rule coming into contact with the windings.

    Outside diamater 130mm
    Height 95mm

    and these are the measurements of the actual windings.

    It's an open variac, would the weight help? It is rather heavy and needs two very strong and steady hands to move it!

    Thanks
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,214
    2,695
    Jan 21, 2010
    That seems quite small. Yeah the weight might help.

    I have a 750VA variac and that is about the same dimensions. If I have time I'll open the box and measure the dimensions
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,214
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    Jan 21, 2010
    Here is a picture of mine. It's about 70mm tall, and 120mm diameter.

    IMG_20171022_093312.jpg

    And here's the front panel. (Not that I think it's useful)

    IMG_20171022_093828.jpg
     
  6. Sinewave

    Sinewave

    126
    2
    Feb 15, 2013
    I'll measure it again, maybe I got it wrong.

    I was happily running it tonight (or last night now) at 4amps for a while and for a few moments ran it at 9 amps. It didn't even appear warm. Though it was sold to me in good faith as a 2kva (or 2.2 can't remember), I just wanted to be sure.

    At work we have one rated at 10 amps ( I know transformers are better rated in VA) and the one at work isn't any bigger.
     
  7. Sinewave

    Sinewave

    126
    2
    Feb 15, 2013
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It appears to have a much heavier gauge wire than mine. Mine is more like this.
     
  9. duke37

    duke37

    5,256
    726
    Jan 9, 2011
    The current limit will be more to do with the brush connection rather than the wire diameter. In this respect, current limit is a better measure than VA. VA will depend on set output voltage.
     
  10. Sinewave

    Sinewave

    126
    2
    Feb 15, 2013
    Today I opened our variac at work. It is labelled as 10 amps. The wire gauge appears the same as mind. The transformer measures 160mm by 85mm high, which is a little wider than mine, but not as tall, which I think seems consistent with a difference of 2 amps in rating between them.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,214
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    Jan 21, 2010
    It's worth noting that for any load neither the VA or the current limit should be exceeded.

    Ok, you can exceed them a little for a short duration.

    But if you input voltage is 240V and your output is 20V, don't even think for a moment that you can use the VA rating to determine the max current. As @duke37 has said, the brush is a limiting factor, but also the windings do not use a gauge of wire that will be up to the task of the current at low voltages for a constant VA.
     
  12. Sinewave

    Sinewave

    126
    2
    Feb 15, 2013
    You can use the input voltage to determine the max current, but not the variable output voltage. The non variables (max VA) aren't going to move as the voltage varies.

    2kva at 20v would assume 166va, or 2kva at 20v in a calculation would be 100 amps, which I don't think anyone in their right mind would ever think can magically appear from an 8 amp/2kva variac.
     
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