248 



ALTERNATING CURRENTS 



open and thereby reducing the value per ampere of the leak- 

 age flux which links the individual conductors. Ordinarily it is 

 not desirable that the slots be entirely open, as this increases the 

 reluctance of the air-gap and more magnetizing current is re- 

 quired. This in turn reduces the power-factor. Also with open 

 slots the tooth losses may become excessive, particularly in large 

 motors. The rotor-slot design is actually a compromise among 

 these conflicting factors. 



Because of the lower reactance accompanying a lower fre- 

 quency, a 25-cycle motor will in general have greater starting 



FIG. 235. Westinghouse squirrel-cage induction motors as headstock motors 

 for wood-working lathes. 



torque and break-down torque than a 60-cycle motor. On the 

 other hand, the magnetizing current in general is higher, because 

 of the higher flux densities employed in the 25-cycle design. 



Because of its low rotor resistance, the squirrel-cage motor has 

 excellent operating characteristics for constant-speed work. 

 The slip is small and the speed regulation is good. In addition, 

 the motor is simple, rugged, and requires but little attention. 

 Some of its fields of application are in machine shops, in wood- 

 working shops, in cement mills, in textile mills; in fact it is used 

 in most cases where the load requires constant speed with but 

 little starting torque. Figure 235 shows the application of small 



