Using a larger kVA transformer with a smaller kVA RPC
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    Default Using a larger kVA transformer with a smaller kVA RPC

    Is it 'ok'/will it work if a rotary phase converter is connected to a transformer which has a larger kVA rating? ie: Could one hook up a 5kVA RPC to a 15kVA transformer so long as the load put on the transformer does not exceed the max output of the RPC? Clearly not an ideal setup, just curious if it could be used for testing equipment and such.

    Thanks!

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    Yep, not a problem.

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    Quote Originally Posted by huleo View Post
    Yep, not a problem.
    Thanks Huleo! It seems reasonable enough to me, but I wanted to make sure I did not tear a hole in the space-time continuum when I turned it on.

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    Not all of us get as lucky, but a select few win "magic smoke" with the electrical purchases. You will know if you are one of the lucky ones soon enough!

    transformer should be no biggy.

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    You say clearly not the ideal situation. Actually I think it is a very good situation. Transformer stays cool, voltage drops/wasted power minimized.

    That said the only negative I can think of is that the combined current draw at startup (stalled idler motor and magnetizing current of transformer) will be increased. If your circuit breaker is marginal, this could cause it to trip.

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    Quote Originally Posted by Bill Simmons View Post
    You say clearly not the ideal situation. Actually I think it is a very good situation. Transformer stays cool, voltage drops/wasted power minimized.

    That said the only negative I can think of is that the combined current draw at startup (stalled idler motor and magnetizing current of transformer) will be increased. If your circuit breaker is marginal, this could cause it to trip.
    Thanks! I will keep that in mind.

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    Quote: combined current draw at startup (stalled idler motor and magnetizing current of transformer) will be increased.
    :Unquote

    That's why I have mine set up (9 kva transformer, 5.5 kw idler) with separate contactors to power up the transformer and to connect motor to it, separate pushbuttons so they come on in sequence (with the transformer contactor interlocked through a normally-closed aux. contact set on the motor contactor, so the motor has to be disconnected to power up the transformer). Circuit breakers are available with different profiles, in Europe a type D is best for big transformers, to tolerate the initial inrush current (the magnetic part of the contact breaker is less sensitive) without increasing the trip current in normal use (the thermal part is thermally massier for a slightly slower response).

    Dave H. (the other one)

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    Quote Originally Posted by Bill Simmons View Post
    You say clearly not the ideal situation. Actually I think it is a very good situation. Transformer stays cool, voltage drops/wasted power minimized.

    That said the only negative I can think of is that the combined current draw at startup (stalled idler motor and magnetizing current of transformer) will be increased. If your circuit breaker is marginal, this could cause it to trip.
    Kind of a trivial point, but even though voltage drop and transformer temperature will be down, wasted power may be up.

    Transformers are generally designed to be most efficient at their most common load, which is something like 60% for residential, and I assume higher for industrial. The most efficient point of any transformer happens to be where the core losses from eddy currents are equal to the resistive losses in the windings. So if you have a 15kVA transformer that is 90% efficient and intended for 100% load, then 5% of that 15kVA or 750 actual watts will be dissipated in the core as heat any time the primary side is powered. Eddy current losses depend solely on voltage and frequency, but not load (It acually decreases with increasing load, but only very slightly).

    Without knowing more about the transformer I can't tell for sure what the actual power dissipation is. But oversizing transformers by increasing kVA capacity is definitely bad for efficiency. If you wanted to get better efficiency by using a larger transformer you would make the transformer using the same core as the 15kVA version, but using more turns of thinner wire so that magnetic flux goes down and resistive losses go up to meet at the same point.

    Not really useful here, but maybe someone will be happy to learn a bit more.

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    Point taken about eddy current losses. I think that your calculation is may be off by about 40%. I read (must be true, I saw it on the internet)that peak efficiency of transformer is when magnetic losses equal IR losses. If we assume that to be 60 percent load as you stated, this would result in a lower number than you calculated.

    Regardless of the actual number, the losses are something to be mindful of. I am too lazy to try and figure the actual total losses between having a 5kva and 15kva transformer when loaded and it would only be relevant for one load setting any way.

    Fortunately the transformer is only energized when the RPC is running. If it was always energized in a standby mode this would be a far more significant issue.

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    Quote Originally Posted by Hopefuldave View Post
    Quote: combined current draw at startup (stalled idler motor and magnetizing current of transformer) will be increased.
    :Unquote

    That's why I have mine set up (9 kva transformer, 5.5 kw idler) with separate contactors to power up the transformer and to connect motor to it, separate pushbuttons so they come on in sequence (with the transformer contactor interlocked through a normally-closed aux. contact set on the motor contactor, so the motor has to be disconnected to power up the transformer). Circuit breakers are available with different profiles, in Europe a type D is best for big transformers, to tolerate the initial inrush current (the magnetic part of the contact breaker is less sensitive) without increasing the trip current in normal use (the thermal part is thermally massier for a slightly slower response).

    Dave H. (the other one)
    Dave - That sounds like a pretty slick setup. Can you post a picture or wiring diagram? I am trying to picture how it works. You have main power/building power, rotary phase converter, transformer, and load motor(s). You push a button and the RPC gets power, then another button to connect the transformer to the RPC? I am a noob! thx again


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