Electrical Engineering ⇒ Topic : Equivalent Circuit

Parker
 
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 noload 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 noload .............. (1) ............ (2) Where The above quantities, the core loss component of the noload current (/_{c}) and the magnetising component of the noload current (I_{m}) are shown in the phasor diagram of Fig. In Fig.(a), the shunt branch of the impedance consisting of R_{0} 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 noload current I_{O} and the transformer core loss are generally a function of the supply voltage and therefore the shunt branch consisting of R_{0} and X_{m}. is connected across the supply source.  
 
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