Phasor Diagrams & Equivalent Circuit Of a Transformer On-No Load And On Load Condition




The resulting equivalent circuit as shown in Fig. 16 is known as the exact equivalent circuit. This circuit can be used for the analysis of the behaviour of the transformers. As the no-load current is less than 1% of the load current a simplified circuit known as `approximate' equivalent circuit (see Fig. 16(b)) is usually used, which may be further simplified to the one shown in Fig. 16(c).






On similar lines to the ideal transformer the phasor diagram of operation can be drawn for a practical transformer also. The positions of the current and induced emf phasor are not known uniquely if we start from the phasor V1. Hence it is assumed that the phasor  is known. The E1 and E2 phasor are then uniquely known. Now, the magnetizing and loss components of the urrents can be easily represented. Once I0 is known, the drop that takes

place in the primary resistance and series reactance can be obtained which when added to E1 gives uniquely the position of V1 which satisfies all other parameters. This is represented
in Fig. 17(a) as phasor diagram on no-load.



Next we proceed to draw the phasor diagram corresponding to a loaded transformer. The position of the E2 vector is known from the flux phasor. Magnitude of I2 and the load power to be known. But the angle θ2 is defined with respect to the terminal voltage V2 and not E2. By trial and error the position of I2 and V2 are determined. V2 should also satisfy the Kirchoff's equation for the secondary. Rest of the construction of the phasor diagram then becomes routine. The equivalent primary current I02 is added vectorially to I0 to yield I1. I1(r1+jxl1)is added to E1 to yield V1. This is shown in fig. 17(b) as phasor diagram for a loaded transformer.


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