274 



ELECTRICAL EQUIPMENT 



while, if the load mainly consists of power, 25 cycles is often prefer- 

 able, especially if the load consists of a large number of syn- 

 chronous converters. With a large induction motor load it may, 

 on the other hand, be more advantageous to use 60 cycles on 

 account of the greater number of speeds, which are possible with 

 this frequency. 



In the following discussion the influence of frequency will be 

 treated in connection with frequency changers, generators, trans- 

 formers, transmission lines, induction motors, synchronous con- 

 verters, railroad work and illumination. 



Frequency Changes. Frequency changers are primarily used 

 for effecting a change in frequency. They are either utilized for 

 obtaining a frequency high enough for lighting purposes from a 

 low-frequency system, or, as a means of interchanging power 

 between systems operating at different frequencies. 



The change from 25 to 60 cycles or vice versa requires a set 

 running at 300 R.P.M., which is a serious limitation because this 

 speed is much too low for the economical design of frequency 

 changers of small or moderate size. If an exact ratio is not abso- 

 lutely necessary, as when power is taken from an existing system 

 for lighting and industrial purposes, and the frequency changer 

 is not intended for tying two generating systems together, the 

 available range of speed is greatly increased as shown in the fol- 

 lowing table. 



TABLE XL 



FREQUENCY-CHANGER COMBINATIONS 



