GENERAL CONSIDERATIONS 277 



is, however, negligible for low frequencies and small conductors, 

 but increases rapidly for higher frequencies and large conductors. 

 With magnetic material it is much higher than with non-magnetic, 

 and its effect should be considered where iron conductors are used 

 and for heavy copper work. 



Induction Motors. The speeds of 25-cycle induction motors 

 for general application are practically limited to 750, 500 and 375 

 R.P.M., while the corresponding speeds for 60-cycle motors would 

 be 1200, 900, 720, 600, 514, 450, and 400 revolutions. Twenty- 

 five-cycle motors could, of course, be wound for two poles, giving a 

 speed of 1500 revolutions, but this is rarely done except in the 

 very small sizes. The objection is that since the flux per pole is 

 twice as large as in the four-pole type, the section of iron back of 

 the slots must be twice as great, for the same rotor diameter. 

 Moreover, the end connections become very long and the machine 

 difficult to wind and consequently the cost is very materially 

 increased. 



The efficiency depends upon a number of features. The lower 

 frequency will, of course, tend to make the iron loss less, but on the 

 other hand, the copper loss will be considerably greater on account 

 of the longer end connections, and, as a rule, the efficiency is found 

 to be somewhat lower for low- than for high-frequency motors. 



The power factor of an induction motor is expressed by the 



ratio T . w ' . . It is affected by the reactance and the mag- 

 Kv.A. input 



netizing current. At constant line voltage the latter remains 

 practically constant, while the former varies with the current. 

 The shape of the power factor curve, that is, the power factor at 

 fractional loads and overloads, therefore, depends upon the rela- 

 tive values of the magnetizing current and the reactance. 



r 



Power factor = cos <t> = -=. 



A motor with a relatively large magnetizing current and a low 

 reactance will, in general, have a low-power factor at fractional 

 loads and a rapidly increasing power factor at higher loads, while 

 a motor with a relatively low magnetizing current and a high reac- 

 tance will have a high-power factor at fractional loads and only a 

 slightly greater power factor at overloads. 



The 25-cycle motor has an inherently lower reactance and 



