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I have a couple of questions regarding re-wiring a three phase transformer.

Say you have a three phase TX with a ratio of 415:22 and you want to use it as a single phase step-up TX (excite it with a variac or something), this is a thought-experiment.

If you excited only the middle leg of the many turn side (leaving the other two outer 415 coils open), and series-ed up the outer two legs of the 22 coils, as in the RHS of the picture, you would have Φ flux equally through each secondary, so it'd be a 415:44. (Because you left the middle 22 coil open). However, if you also series-ed up the middle leg secondary in reverse (compared to the outer two, as in the LHS of the picture), because it has 2xΦ flux as the outer two, I assume it would have double the voltage. So what would the resulting turns ratio be? 415:66? Or 415:88?

My second question is about the mutual coupling coefficient, between all the coils on a TX core, single or three phase, does this 'M' have to equal a total maximum of '1' between all the coils? That is to say, even if you have multiple identical coils on the same core, coupled well, between any two the coupling is only at most 0.5? For example on a three phase core, the middle coil will link to the other two 0.5 each.

And a side question about this, is that if you were actually looking at the coupling of one limb to another, given that the coils are not wound one on top of the other on the same limb. The flux has to travel through the core to it. How much less will the coupling coefficient be, compared to if the coils were both wound over each-other (say, bifilar) on the same limb? (How much increased leakage flux, very generally speaking.)

Thanks very much

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# Mutual inductive coupling

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