A question regarding transformer...

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Joined Mar 3, 2006

I have a question regarding the description of how the transformer works in "Lessons In Electric Circuits -- Volume II Chapter 9 TRANSFORMERS".

The chapter explains how the current on the secondary side does not make any additional magnetic flux. Here is the paragraph that has the explanation:

"No current will exist in the secondary coil, since it is open-circuited. However, if we connect a load resistor to it, an alternating current will go through the coil, in phase with the induced voltage (because the voltage across a resistor and the current through it are always in phase with each other).

At first, one might expect this secondary coil current to cause additional magnetic flux in the core. In fact, it does not. If more flux were induced in the core, it would cause more voltage to be induced voltage in the primary coil (remember that e = dΦ/dt). This cannot happen, because the primary coil's induced voltage must remain at the same magnitude and phase in order to balance with the applied voltage, in accordance with Kirchhoff's voltage law. Consequently, the magnetic flux in the core cannot be affected by secondary coil current. However, what does change is the amount of mmf in the magnetic circuit."

I am thinking that if the voltage that applied to the primary is a square wave. The current appeared on the secondary side will be a square wave too (if the load on the secondary is a resistor).

Let's assume this current DOES introduce additional flux in the core. Since it is square wave (not sine shape), it will just OFFSET, NOT CHANGE the shape, or dΦ/dt, of the exciting flux. In other words, the additional flux caused by the secondary current should be allowed.

If that is true, then, how does the secondary load current force the primary to generate a current to ballance the secondary current as stated in the paragraph?

I have been having this question for quite a while. I really hope the author can explain it to me. I really appreciate it!