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
The above quantities, the core loss component of the no-load current (/c) and the magnetising component of the no-load current (Im) are shown in the phasor diagram of Fig. In Fig.(a), the shunt branch of the impedance consisting of R0 and Xm. 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 IO and the transformer core loss are generally a function of the supply voltage and therefore the shunt branch consisting of R0 and Xm. is connected across the supply source.
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