Figure 1.20 shows the elementary diagram of an ideal transformer with secondary side open circuited. It has no ohmic resistance and leakage reactance. There is no loss in an ideal transformer. In Figure 1.16, alternating voltage (V1) is applied at the primary and hence alternating current flows in the primary. The primary draws the magnetizing current Iμ only because it is purely inductive in nature. Iμ is small in magnitude and lags behind V1 by an angle 90°. The function of Iμ is to magnetize the core, and it produces an alternating flux (Φ), which is proportional to Iμ. The alternating flux (Φ) is linked with both primary and secondary windings and causes self-induced emf (E1) in the primary. This self-induced emf (E1) is equal and opposite to V1 at every instant. This induced emf is known as back emf or counter emf. Due to mutual induction, an emf E2 is produced in the secondary. This emf is known as mutually induced emf. It is anti-phase with V1 and its magnitude is proportional to the rate of flux as well as the number of turns of the secondary windings.
Figure 1.21(a) shows the instantaneous values of applied voltage, induced emfs, flux and magnetizing current by sinusoidal waves, while Figure 1.21(b) shows the vectorial representation of the effective values of the above quantities.