Appendix 3: The transformer circle diagram

By means of the circle diagram the loci of the ends of the phasors representing terminal voltages and currents in single and polyphase transformers may be located at all power factors and all loads.

The amounts of, and the phasor relations between, the primary and secondary voltages and currents under any conditions of load and power factor may be determined and the regulation may be obtained graphically.

Referring all quantities to the secondary side, and working on a ‘per phase’ basis:

Let N1 and N2 be the number of primary and secondary turns respectively.

V”1 primary terminal voltage, reversed in time phase and multiplied by the ratio N2/N1
V2 secondary terminal voltage
I”1 total primary full-load ...

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