Appendix 1: Transformer equivalent circuit
The calculations of a combined electrical system or circuit comprising transformers, transmission and distribution lines are often simplified by the use of the equivalent circuit diagram. The characteristics of a loaded transformer also can often be indicated more clearly by the same means. Figure A1.1 shows the more general form of diagram of connections, and Fig. A1.2 the corresponding phasor diagram for a loaded transformer.
| V1 | primary terminal voltage |
| E1 | primary induced e.m.f. |
| E2 | secondary induced e.m.f. |
| V2 | secondary terminal voltage |
| I2 | secondary load current |
| I’2 | load component of total primary current |
| I1 | total primary current (including I0 and I’2) |
| I0 | primary no-load current |
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