278 ELECTRICAL EQUIPMENT 



requires less magnetizing current, for which reason its power 

 factor is considerably higher than for high-frequency motors. 



The starting torque and the maximum torque depend inversely 

 on a function of the reactance, and are, therefore, higher for low 

 frequencies. 



The starting torque of an induction motor is equal to: 



* Z 2 ' 

 the starting current is equal to 



E 

 Z' 



the running torque is equal to 



E 2 sn 



kr 



the maximum torque is equal to 



E 2 



where k = constant ; 



E = applied voltage; 



s = slip; 



r = stator resistance per phase; 

 r\ = rotor resistance per phase; 

 X = total reactance; 

 Z = total impedence ; 



Comparing the weights based on motors of the same capacity 

 and speed, it is found that, on the average, 25-cycle motors will 

 weigh about 15 per cent more than 60-cycle motors. For the 

 smaller sizes there is very little difference in the .cost, but as the 

 sizes increase there is a marked difference in favor of the 60-cycle 

 motors. 



Synchronous Converters. A synchronous converter being in 

 effect a combination in one machine of a synchronous motor and a 

 direct-current generator, the important factors in which the fre- 

 quency is concerned have to do almost entirely with the contin- 

 uous-current side. The continuous-current generator, as a rule, 

 runs at frequencies much below 25 cycles, and at the frequencies 



