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Thread: 600volt to 480 Wye Autotransformer

  1. #1
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    600volt to 480 Wye Autotransformer

    If an 10KVA Autotransformer with 600volt Primary has an Wye 480volt Secondary, connecting 480volt Ungrounded Delta to the Secondary would produce 600volt on the Primary but what happens if: 1) the Common Wye is Grounded? 2) if the Common Wye is left unconnected? Apparently someone connected it to an 600volt Machine Lathe but it sounds dangerous to me?

  2. #2
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    Do not ground a "primary" wye point when there is a delta secondary. Any voltage imbalance on the 480 side can cause enormous circulating currents in the delta and the wye neutral.

  3. #3
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    This is not the normal situation where a delta-wye isolation transformer is being reverse connected. Rather powerplay is discussing a wye _autotransformer_. As an autotransformer there isn't a separate primary and secondary winding, and it can be operated to buck or boost the supply.

    One question: is this connected to an _ungrounded_ supply, or simply the three phase legs of an otherwise grounded supply?

    Normally this wye autotransformer would be connected to a wye supply and the neutral point of the autotransformer would be left floating. The wye of the autotransformer would be at approximately neutral voltage, but not exactly. Connecting the neutral of the supply to the neutral of the autotransformer could cause significant circulating currents.

    Connected to an _ungrounded_ 480V supply, the result would be an ungrounded 600V output. Not particularly more or less dangerous than any other ungrounded system.

    Connecting the autotransformer neutral to ground wouldn't give you a good 'solidly grounded' system, since a wye transformer has relatively high impedance to the neutral. You need a slightly different transformer to operate as a 'grounding transformer'.

    One type is a 'zig-zag' wye transformer, and I suppose it would be possible to create a combined zig-zag grounding transformer and boost autotransformer.

    Another approach is to have a 'tertiary' delta winding, no connected to any load, but one which permits circulating currents in the event of a ground fault.

    -Jon

  4. #4
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    Quote Originally Posted by winnie View Post
    This is not the normal situation where a delta-wye isolation transformer is being reverse connected. Rather powerplay is discussing a wye _autotransformer_. As an autotransformer there isn't a separate primary and secondary winding, and it can be operated to buck or boost the supply.

    One question: is this connected to an _ungrounded_ supply, or simply the three phase legs of an otherwise grounded supply?


    -Jon

    It is an 480volt Ungrounded Delta System.

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