GENERAL CONSIDERATIONS 275 



While synchronous motors are almost invariably used with 

 frequency changers, induction motors may be used if proper 

 arrangements are provided for adjusting the slip so as to insure 

 a satisfactory parallel operation. This adjustment, of course, 

 means the introduction of a permanent resistance and a corre- 

 sponding loss, and is, therefore, undesirable unless other advan- 

 tages of greater importance can be obtained. Where only one 

 set is required speed adjustment is not necessary, and the motor 

 may be designed with a slip which will just be sufficient to bring 

 the generator frequency to the right value. 



Generators. The frequency of synchronous generators in 

 alternations per minute is equal to the number of poles times the 

 revolutions per minute, and the periodicity or cycles per second 

 is shown by the following equation : 



Number of poles X rev. per min. 



Cycles 



120 



Due to the fact that there is a natural relation between the 

 windings of electrical apparatus which varies inversely as the 

 square of the frequency, the higher the frequency the greater is, 

 in general, the peripheral velocity at the same revolutions per 

 minute. Increase in peripheral velocity means a larger diameter 

 with a smaller length and a better natural ventilation. The 

 higher periodicity in definite pole machines is also preferable 

 in that the load of the rim of the spider is better distributed and 

 smaller in amount at the point of attachment of poles. 



The induced e.m.f. is directly proportional to the frequency 

 and, due to the lower core loss with lower frequencies, the effi- 

 ciency is naturally better at 25 than at 60 cycles. The cost is 

 also increased by the frequency there being a natural tendency 

 for 25-cycle apparatus to be heavier than 60-cycle. As a general 

 rule, the labor item is higher on the higher frequency machines, 

 and the material item higher with the lower frequencies. 



Parallel operation is more satisfactory at low frequencies, so 

 far as the variation in angular velocity is concerned. Due to 

 other factors, the conditions for parallel operation depend more 

 upon the relations between natural and impressed frequencies, 

 rather than upon the absolute value of either. 



Transformers. The frequency has a very important bearing 

 both on the design and operation of transformers. With trans- 



