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Transformers & Phase Difference

  1. Feb 2, 2014 #1
    Hi, I'm trying to figure out what phase differences take place with an ideal transformer. This is what I have figured out so far:

    If you apply an AC current to the primary coil then you will get an alternating flux in the primary and secondary coils simultaneously therefore the fluxes are in phase. The induced emf at the secondary will be 90 degrees out of phase to its flux which follows on from Faraday's Law. I'm trying to figure out the phase differences of the back emf's. Will the back emf in the primary coil also be 90 degrees out of phase and therefore be in phase with the emf of the secondary? I'm not sure about the back emf of the secondary? Is there anything that is 180 degrees out of phase?

    Thank you for any help given.
  2. jcsd
  3. Feb 2, 2014 #2
    I was under the impression that transformers were a 180 degree phase shift, as the currents match up but the sign gets reversed. The primary is just an inductor if you take away the secondary though, so the current trails voltage by a quarter cycle, which is a -90 degree phase shift. The secondary is the same way, however, there's something in Faraday's law which changes the sign so that it's 180 rather than 0. It's been a while though, so someone double check me.
  4. Feb 5, 2014 #3

    Philip Wood

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    Gold Member

    Yes. But this is as long as we confine ourselves to emfs arising from rate of change of primary current. The fun starts when we also have a varying current in the secondary because we have connected a load across it. We then get another emf induced in the primary (mutual induction) and another in the secondary (self induction). This alters things quite a lot. When certain reasonable approximations are made, the secondary current does indeed turn out to be pi out of phase with the applied emf. The maths is surprisingly straightforward.
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