Electrical Engineering ⇒ Topic : Equivalent Circuit

When the secondary of a transformer is open circuited, it takes only a very small current. In an open circuit, the losses are mainly due to the iron losses only, which remain constant at all loads. This suggests that a transformer on no-load can be represented by an equivalent circuit as shown in Figure (a)From the figure, it can be seen that

                       FIGURE (a) Equivalent circuit of a transformer on no-load

                                                             ..............   (1)

                                                            ............     (2)

Where                                     

The above quantities, the core loss component of the no-load current (/_c) and the magnetising component of the no-load current (I_m) are shown in the phasor diagram of Fig. In Fig.(a), the shunt branch of the impedance consisting of R₀ and X_m. in parallel cannot be arranged in series because this will not only produce a large voltage drop but also will not be able to maintain the core loss as constant for all loads on the secondary.

The primary no-load current  I_O and the transformer core loss are generally a function of the supply voltage and therefore the shunt branch consisting of R₀ and X_m. is connected across the supply source.