Nature of energy losses in air capacitors at low frequencies

Abstract

The power factors of a number of types of air capacitors have been observed in the frequency range from 60 to 1,000 cis. The method of measurement permitted evaluation of true, or "absolute," power-factor values as small as 5X10-7• In guard-ring air capacitors the losses occur on the surfaces of the electrodes and give rise to a power factor which is proportional to capacitance (inverse electrode separation). The same type of losses in addition to the losses in the solid insulation , are evident in two-electrode capacitors. For most two-electrode air capacitors of variable capacitance, C, the loss may be represented by the function A+BIC, where A depends upon the electrode material , humidity, and frequency, and B depends upon the insulation material, humidity, and frequency. Of the 10 metals used as electrode materials in guard-ring capacitors, aluminum gave the greatest power factor and silver plate the smallest. The power factors for the other materials, which included brass, monel metal, stainless steel, nickel, and brass plated with gold, rhodium, and chromium, were only slightly different from that for silver-plated brass, but they were appreciably smaller than the values for aluminum. The power factor of a guard-ring capacitor is a function of humidity and shows a maximum value at relative humidities between 40 and 70 percent at low frequencies. The maximum value is apparently due to a variable conductivity of the moisture layer with thickness. The loss which occurs in the electrode surface layer is also a function of the homogeneity of the electric field. The nonuniform field in a three-electrode rotating plate capacitor gives rise to a power factor which decreases as the capacitance is increased. CONTENTS Page I. Introduction __________________________________ ________________ 673 II. Calibration of standard for power factor ___ ___ ____________________ 674 III. Power factors of two-electrode capacitors_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 676 IV. Power factors of three-electrode capacitors ________________________ 682 V. Contribution of a surface layer to power factor _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 687 VI. Effect of a nonhomogeneous field on power factor _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 691 VII. Conclusion ___________________________________ _____________ ____ 694 VIII. References _____________________ ___ _______________ ___ __________ 695