On the Generation of an Extra-Low-Frequency E. M. F., and the Measurements of Mechanical Constants of Electrical Meters

Abstract

Alternating E. M. F. at extra-low frequency, say from a fraction of acycle to several cycles per second, can be derived from a separately excited D. C. generator, when the brushes are slowly revolved by some mechanical means around the commutator. The frequency is given by the number of revolutions per second of the brushes multiplied by the number of pairs of poles of the generator. The magnitude of the E. M. F. may be varied by the field excitation.
One of the various applications of the extra-low-frequency E. M. F. is the determination of mechanical constants of electrical measuring instruments. The method is based on the fact that the amplitude characteristic of the forced vibration of the moving system reveals resonance for certain frequency of the impressed E. M. F.
Test results are given for the instruments of the moving coil type, the moving iron type, the electrodynamometer type and the hot-wire type.
It is ascertained that measuring instruments of industrial use generally have natural frequencies less than two to three cycles per second, and that many of them have small value of bluntness, i. e., insufficient damping effect.
Hysteresis phenomena are observed for instruments of permanent magnet type, and even jumping phenomena for those with very small bluntness.
This method of study was first suggested to the author by Prof. H. Yagi, to whom the author's sincere thanks are due. (Sendai, june 1923)