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Additive and Subtractive Polarity

  1. Jan 24, 2010 #1
    In the case of step-down transformers:

    The polarity between the primary and secondary transformer is subtracting when supplying test voltage to the primary transformer in the case where the H1 is matching the X1.

    Now if the polarity markings are wired but switched on the transformers to be additive. Does that mean the step down transformer has become a step up transformer?
  2. jcsd
  3. Jan 24, 2010 #2


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    On a normal transformer, polarity won't affect the step-up or step-down action of the transformer.

    The ratio of output voltage / input voltage depends on the ratio of secondary to primary turns in the transformer

    On an AUTO transformer like this:

    The polarity is normally marked but for safety reasons if the input is the mains power supply. It is necessary to get the active and neutral the right way around.
    However getting them wrong will not affect the output voltage as long as the turns ratio stays the same.

    If you did have two independent windings as in the first case above, but connected one end of the secondary to the top end of the primary, then it would make a difference which end of the secondary you did this with.

    The secondary would then add to, or subtract from, the primary (if you took the output from the other end of the secondary) depending on whether it was in phase with it or out of phase.
    Last edited: Jan 24, 2010
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