Abstract: When an ideal transformer is connected to a pure resistive load，the voltage and the current of the

primary coil are in phase. However，many beginners are confused about this point. As a result，possible errors and

omissions in instantaneous value analyses can lead to a clearly wrong conclusion that there is a phase difference of

π/2 between the voltage and the current of the primary coil. In this paper，the phase relations between the physical

quantities of an ideal transformer are analyzed using vector diagrams. The correct instantaneous value analysis is also

given，so that the main reason of the wrong conclusion is pointed out.

Key words: transformer, phase, magnetic flux, no-load current