# Doubt about voltage and magnetic flux in a transformer

#### JLuis_GT

Joined Jan 18, 2016
3
In the section of the book related to transformers (http://www.allaboutcircuits.com/tex...chpt-9/mutual-inductance-and-basic-operation/), in the next image

It's not clear to me if the plotted voltage is the source voltage or the induced for the flux one (which neutralize the source voltage). I think that it's the induced one, but the footnote says "Magnetic flux lags applied voltage by 90º", which sounds like source voltage. But, if it's the source voltage, how the induced voltage in the secondary coil could have the same phase and magnitude than this? I would expect same magnitude (1:1 transformer) but opposite phase, right?

Excellent resource are you doing here, by the way. Thanks a lot for it.

#### crutschow

Joined Mar 14, 2008
29,825
The source voltage and the induced voltage both have the same phase.
So the voltage in your plot could be either.

#### JLuis_GT

Joined Jan 18, 2016
3
The source voltage and the induced voltage both have the same phase.
So the voltage in your plot could be either.
Hi

I had the impression that the induced voltage should be opposed to the source voltage in order to respect the KVL? Should not look symmetrical one to another respect to horizontal axis?

#### crutschow

Joined Mar 14, 2008
29,825
I had the impression that the induced voltage should be opposed to the source voltage in order to respect the KVL? Should not look symmetrical one to another respect to horizontal axis?
The input and output voltages are of the same polarity for windings wound in the same direction on a core.
The difference, of course, is that the current direction is opposite in the input and output windings, since the input has power going in and the output has power going out.