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Ideal Transformer

The use of equivalent circuits enables an engineer to calculate many transformer problems with comparative ease. It is always necessary to multiply properties in the referred winding by the proper ratio. This has led to the interposition of a transformer of the right turns ratio somewhere in the equivalent circuit, usually across the load. The transformer thus used must introduce no additional losses or voltage drops in the circuit. It is called an ideal transformer,(1) and it has negligibly small winding resistances, leakage flux, core loss, magnetizing current, and winding capacitances. Some power and audio transformers very nearly approach the ideal transformer at some frequencies. For example, in a typical 50-kva plate transformer, the winding resistance IR drops total 1 per cent and the leakage reactance IX drops 3 per cent of rated voltage, the core loss 0.6 per cent of output power and magnetizing current 2 per cent of rated primary current. When the term ideal transformer is used, it should be borne in mind that negligibly small is not zero. Particularly in electronic work, where frequency may vary, a limiting frequency may be reached at which the transformer is no longer ideal. Moreover, even if the limiting frequency is very low, it is never zero. There must be voltage variation if transformation is to take place. The assumptions of equations 5 to 7 were the same as for an ideal transformer.

(1) See Magnetic Circuits and Transformers, M.I.T. Electrical Engineering Staff, John Wiley & Sons, New York, 1943, p. 269.

Last Update: 2011-02-17