244 



ALTERNATING CURRENTS 



Figure 232 'shows the ordinary characteristic curves of a 10- 

 hp. squirrel-cage motor. It will be noted that the torque, speed 



4 5 6 78 9 10 11 12 



Horse Power Output 

 FIG. 232. Operating characteristics of a squirrel-cage induction motor. 



and efficiency curves are very similar to those of a shunt motor. 



The power-factor increases with the load for the following reason : 

 At no load the motor takes a current 7 , (Fig. 233). 7 



is mostly magnetizing current, although there is a small 



energy component necessary to 

 supply the no-load losses. The 

 power-factor at no load is cos 6 , 

 the value of which may be as low 

 as 0.10 to 0.15. The back electro- 

 motive force of the motor remains 

 nearly constant from no load to full 

 load. Therefore, the flux must re- 

 main substantially constant, just as 

 it does in the transformer, so that 

 the magnetizing current changes 



but slightly from no load to full load. As load is applied to the 



motor, an energy current 7} is required to carry the load. This 



. / M 



FIG. 233. Increase of power- 

 factor with increase of load. 



