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Transformer connections and VA rating

Discussion in 'General Electronics Discussion' started by kong, Mar 16, 2015.

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  1. kong

    kong

    122
    2
    Sep 26, 2010
    If I have two transformers with 120v primaries, and I place the primaries in parallel and in phase and feed them 120v and I connect the secondaries in series, the secondary voltages will add, and the current will be the same, correct?

    If I have two transformers with 120v primaries, and I put them in series and feed them 240v, with the secondaries still in series, will I have essentially the exact same output?

    Say the parallel 120v primary scenario draws 11 amps open circuit.

    Would the series 240v scenario draw 5.5A?
    Or am I looking at this wrong? To get double the overall power out, would I need to feed both primaries 240v in parallel, or can I feed the primaries in series 240v? Sorry if this is confusing, I'm confused.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,818
    Jan 21, 2010
    An initial read suggests you have it correct. Half the current at double the voltage is the same power, so yes, your input currents are relatively correct (relatively, in that if it is 11A at 120V you would expect half that with the alternate wiring at double the voltage).

    There are some potential issues with placing the secondaries in parallel, but far less so with placing them in series. If you get 0V output (or a very low output) then just reverse one of the secondaries (or primaries). Connecting the secondaries in parallel can cause smoke to come out if you get the phasing incorrect.

    Do not feed the transformers double their rated voltage. What you are suggesting will give you double the power of a single transformer by giving you the rated current of the transformers at double their output voltage.

    I am assuming that both transformers are identical. If they are not then the output current should be limited to that of the lowest rated secondary current of either of the transformers.
     
  3. kong

    kong

    122
    2
    Sep 26, 2010
    Ok, this makes sense. If I don't know the exact parameters of the transformer, can I use a variac at say 12v ac output to mimic the 120v input, and measure the secondary current/voltage this way? All I would have to do is multiply by a factor of 10 to get the actual parameters.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,818
    Jan 21, 2010
    That will work for voltage but not so well for current
     
  5. kong

    kong

    122
    2
    Sep 26, 2010
    I guess for current I could use Ohm's law....get the approx. voltage through the variac method....then measure the secondary resistance and divide the voltage by the resistance to get the available current.
     
  6. kong

    kong

    122
    2
    Sep 26, 2010
    Disregard that last post, I wasn't thinking clearly. If I know the input voltage and I measure the input current with a meter, I can calculate the total VA (and ultimately wattage if I had a watt meter). If I use the variac method to determine the secondary voltage, then I can calculate the output current since I already know what the secondary voltage is and the total system VA. VA/Es=Is
     
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