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 I_L flows in the secondary winding, then the primary must carry a component N₂I_L/N₁ = I_L' in addition to the exciting current I_exc. The component N₂I_L/N₁ in the primary current is required to overcome the mmf N₂I_L 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 y_exc1 = g₁ - jb₁, across the primary of the ideal transformer, as in Fig. 6-6(a). Then

[6-25]

and

[6-26]

also

[6-27]

where I_exc1, I_mag1 and I_c1 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

[6-28]

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.