Stray losses on transformers

The object of this paper is to obtain a convenient method of evaluating the actual losses in the copper of a transformer and in figuring the regulation. The effects of various factors on the copper loss of transformers are discussed. It is pointed out that they are, as a rule, negligible in well designed transformers. It is shown theoretically that the copper loss and regulation of a mutual inductance may be derived from its short circuit loss and impedance voltage. The transformer differs from the mutual inductance, due to the fact that the primary and secondary inductances are functions of the induction in the iron. It might be inferred therefore that the short circuit losses, as obtained in the ordinary way, might differ from the equivalent quantities obtained under full load condition. The equation derived in Section 2 furnishes a method of testing to obtain this equivalent short circuit loss under full load conditions. To ascertain if the values of short circuit loss and impedance, obtained in this way, differ from those obtained in the ordinary way, tests were made by both methods yielding results which, after proper corrections were made, checked very closely. Results of tests are tabulated. Practical formulas for figuring copper loss and regulation are given. Recommendations are made as to tests and formulas to be used for obtaining true copper loss and regulation.