Efficiency

Circuit efficiency should be distinguished from transformer efficiency. Magnetization current represents a loss in efficiency, but it may be returned to the circuit after the pulse. Circuit efficiency may be estimated by comparing the area of the actual wave shape across the load to that impressed upon the transformer; it includes the loss in source resistor R₁ (Fig. 233). Except for this loss, the circuit and transformer efficiency are the same when the source is cut off at the end of the pulse. It is important in testing for losses to use the proper circuit.

Core loss can be expressed in watt-seconds per pound per pulse. A convenient way to measure core loss is to use a calorimeter. The transformer is located in the calorimeter, and the necessary connections are made by through-type insulators. Dielectric loss is included in such a measurement. It is appreciable only in high-voltage transformers, and may be separated from the iron loss by first measuring the loss of the complete transformer and then repeating the test with the high-voltage winding removed. At 6,000 gauss and 2 microseconds pulse width, the loss for 2-mil grain-oriented steel is approximately 6,000 watts per pound, or 0.012 watt-second per pound per pulse. For square pulses, core loss varies (a) as B² or E² for constant pulse width and (b) as pulse width, for constant voltage and duty τf, where τ is the pulse width and f is the repetition rate. Dielectric loss is independent of pulse width and varies (a) as the repetition rate, for constant voltage, and (b) as E² for constant repetition rate.

copper loss is usually negligible because of the comparatively few turns required for a given rating if a wire size somewhere near normal for the rms current is used. If the windings are used to carry other current, such as magnetron filament current, the copper loss may be appreciable but this is a circuit loss.

Efficiencies of over 90 per cent are common in pulse transformers, and with high-permeability materials over 95 per cent may be obtained. These figures are for pulse power of 100 kw or more. Maximum efficiency occurs when the iron and dielectric losses are equal.