What are the two different capacity ratings on control transformers?
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  1. #1
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    Sep 2010
    Guinea-Bissau, West Africa
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    Default What are the two different capacity ratings on control transformers?

    After some reading, I think I'm ok to use a control transformer as a general purpose one (please correct me if I'm wrong).

    My application from the other thread is to transform 400v to 110v to power my U.S. power tools and tool battery chargers.

    I've purchased this: SIEMENS 4AM5941-8BE40-0C 7.35KVA TRANSFORMER PRI. 190-550 SEC. 110-240V | eBay

    I only need about 1.5kva of 110v capacity, but the nameplate on most of the control transformers I was looking at were all like this one, in that they list a capacity, a slash, and then another about 6x the first. (1.6 / 7.35kva in this case)

    Is the first a continuous rating? If so, what about the second? Maybe "you can go this high for only a few seconds" limit? Perhaps the second is if you're actively cooling, and can keep heat out of it, it can take that much juice?

    This question seems so basic (because this characteristic seems to exist on almost all control transformers) that I assumed I could find an answer on google - but it must be so obvious to everyone else that no one has ever bothered writing down the answer, haha, because I couldn't find anything!

  2. #2
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    Oct 2010
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    I am not completely certain about this, but since you see these dual ratings primarily on control transformers, I think the lower value is the "sealed VA load" and the higher value is the "in-rush VA load" supported by the transformer. This jargon comes from control transformers driving relay/contactor coils. The "sealed" load is drawn by relay coils to hold the contacts closed. The "in-rush" load drawn by relay coils when actually closing the contacts is just like motor startup load: up to 10 times the normal steady load for a very short duration.

    So, continuous load and (very) momentary peak load.

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