sizing a control transformer
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  1. #1
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    Default sizing a control transformer

    I need to build an estop circuit with a transformer that will hold in a 24vac coil on a contactor that has an inrush of 200va, "sealed" 24va, heat dissipation 10w. what VA 240v to 24vac transformer do I get?

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    Is this a long control line as in 20-30 feet from the contactor? If so then the resistance and capacitance of the wires have to be taken into account. A good introduction to calculate the size transformer is in this reference.

    http://dl.icdst.org/pdfs/files3/10b9...774ff2f1d8.pdf

    Just from experience I would say 75VA minimum.

    Tom

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    Pretty short line, the estop button and transformer are in the same 3' x 3' x1' cabinet...Good paper, that..75-100va it is.

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    If it's all in the same cabinet, just use a 240V contactor. Your switch should be rated for it.

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    240 volts at the button is considered lethal voltage and is verboten in many places...as I recall.

    Stuart

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    Might be a regional thing. A lot of simple control equipment here in NZ has 400V on the buttons.

    As long as the button is suitably rated for mains voltage and all wiring is contained in the box, there's nothing wrong with it - 277V light switches exist, too.

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    Quote Originally Posted by SomeoneSomewhere View Post
    Might be a regional thing. A lot of simple control equipment here in NZ has 400V on the buttons.
    US is a little more safety-conscious. They don't run 220v to the vacuum cleaner, either

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    normally if in the same cabinet I would run line voltage as control voltage but this application requires 24 vac


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