Phase difference in a transformer

[MalachiK]

Today I took two coils and connected one of them (coil A) to an AC power supply. I held the second coil (coil B) close to the first one and then connected them both to a dual trace CRO. As expected, an emf was induced in coil B and the traces on the scope were 90 degrees out of phase. As I understand it, this is because the induced emf in B depends on the rate of change of flux produced by A. So far so good.

Next, I connected up an iron core transformer and looked at the primary and secondary coils on the scope. This time, both of the traces were in phase.

As far as I can see, the only difference between these two situations is the iron core. I have some vague ideas about what is going on, but then again it's just as likely that I miss read the scope.

Would someone be kind enough to point me in the direction of an explanation of what's going on here?

 

[sophiecentaur]

It could be the (choice of) triggering on the scope, if it's an analogue you can find it triggers on both traces. With two big amplitude traces (Iron core would achieve this), it may be triggering on each one at the same part of the cycle - giving the impression that they are in phase. On an old fashioned scope you can choose either alternate traces or to 'chop' between the traces. That will / should show the true phase relationship.

 

[MalachiK]

Thanks. I thought it might be something like that.