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Exciting Current, Conductance, and Susceptance

The general subject of exciting current, core-loss current and magnetizing current was taken up in Chapter 5 in connection with reactors. Figure 5-10 showed equivalent circuits for a reactor. When capacitance effects are neglected, as is done in this discussion, a transformer operating at no load performs like a reactor and the only current that flows is in the primary. This no-load primary current is the exciting current. However, if the secondary is closed to a load, such that a current IL flows in the secondary winding, then the primary must carry a component N2IL/N1 = IL' in addition to the exciting current Iexc. The component N2IL/N1 in the primary current is required to overcome the mmf N2IL of the current in the secondary winding. It is a simple matter to modify the equivalent circuit of the ideal transformer to take into account the exciting current. This is done by connecting the parallel circuit, which consists of the exciting admittance yexc1 = g1 - jb1, across the primary of the ideal transformer, as in Fig. 6-6(a). Then






where Iexc1, Imag1 and Ic1 are the exciting current, the magnetizing current and core-loss current, respectively, in the primary winding. Under load, the primary current is the phasor sum expressed by


This addition is shown graphically in the phasor diagram of Fig. 6-6(b).

Figure 6-6. (a) Equivalent circuit of a transformer taking into account exciting current; (b) phasor diagram for induced emfs and currents in equivalent circuit.
Figure 6-7. Mutual flux path and leakage flux paths in (a) shell-type transformer, (b) core-type transformers. The magnetic coupling of the windings in both transformers is loose, resulting in high leakage.

Last Update: 2011-01-06