Reduction of transformer exciting current to sine-wave basis

As a sequel to an earlier investigation and development of a method for the reduction of core-loss measurement to sine-wave basis, this paper describes two methods developed for the reduction of exciting current to sine-wave basis. The first method consists of making two measurements at wave shapes as widely different as possible, setting the voltage in each case by means of the flux voltmeter. The current corresponding to sine-wave voltage is obtained, by extrapolation from the observed values of currents and form factors. Although the method might be considered to some extent empirical, it is found to yield an accuracy within one per cent even under extremely unfavorable conditions. The second method utilizes as before the flux voltmeter for setting the voltage but uses a “crest ammeter” (developed for this purpose) for reading the instantaneous maximum values of the corresponding currents. Measurements are made at 100 per cent, 86.6 per cent and 50 per cent voltages. These data determine the fundamental, third and fifth harmonics of the exciting current corresponding to sine-wave voltage and hence the exciting current itself, because these harmonics are the only important components in determining the effective value of the exciting current. Theory of the crest ammeter is given, and its applicability (in conjunction with the flux voltmeter) to the determination of d-c. saturation curves by means of a-c. tests in magnetic investigations is indicated.