64 SYNCHRONOUS ALTERNATORS. [Exp. 



ing field. The inductor alternator, although possessing obvious 

 mechanical advantages, is handicapped by large magnetic leakage 

 and consequent poor regulation unless built in an expensive 

 manner with much material. 



3. Choice of Frequency. In the early applications of alter- 

 nating current, when power transmission was not developed and 

 current was used for lighting only, the common frequencies in 

 America were 125 and 133 J cycles per second, and these fre- 

 quencies were satisfactory for the service. The efficiency of a 

 transformer increases with the frequency (Exp. 5-B), and from 

 this consideration even a higher frequency would be desirable; 

 but as frequency is increased, we have greater inductive drop 

 and poorer regulation in generator, line and transformer. 



The rotary converter, introduced in the early nineties, required 

 a lower frequency. The highest frequency at which it can operate 

 is practically 60 cycles, and 25 cycles is better. With its advent, 

 the higher frequencies were abandoned ; 25 and 60 cycles became 

 standard, the former for power alone, and the latter for lighting 

 and (usually) for combined power and lighting. The induction 

 motor has its best* operation within this same range. 



Below 25 cycles, or thereabouts, the flicker of incandescent 

 lamps of the usual types becomes prohibitive. On account of the 

 high speed of the steam turbine, it is not adapted for driving 

 generators below 25 cycles. The series alternating current 

 motor, which is more economical the lower the frequency, is prac- 

 tically the only apparatus for which a frequency lower than 25 

 cycles is desirable. As the art progresses, it is possible that 

 some new application may be developed which will demand a 

 frequency much higher or lower than the frequencies now recog- 

 nized as standard. 



* (3a). In a discussion on the choice of frequency, A. I. E. E., Vol. 

 XXVI., p. 1400, June, 1907, Dr. Steinmetz stated that the most efficient 

 frequency for the induction motor is 40 cycles, the best frequency for small 

 motors being higher and for large motors lower. He also states that, for 

 converters, 25 cycles is better than either a higher or lower frequency. 



