nil-: INDUCTION MOTOR 2i:> 



current, when combined \vitli / ( >, gives the total current /i at 

 this load, and the resulting power-factor is cos 6\. As the load 

 increases, an energy current /: is required. The total current 

 then becomes I .. and the corresponding power-factor becomes 

 cos 2 . It will be observed that the power-factor angle decreases 

 and therefore the power-factor increases as the load on the motor 

 increases. The increased reactance drops in the stator and in 

 the rotor with increase of load tend to oppose this increase of 

 power-factor and when the load exceeds a certain value may even 

 bring about a decrease of power-factor. 



As the power-factor increases, a smaller increase of current 

 is required for a given increase of load than would be necessary 

 if the power-factor were constant. Therefore, the current in- 

 creases more slowly than the load as shown in Fig. 232. At first 

 the efficiency increases rapidly and reaches a maximum value 

 for the same reason that it does in other electrical apparatus. 

 At all loads there are certain fixed losses, such as core loss, fric- 

 tion and windage. In addition there are the load losses (PR) 

 which increase nearly as the square of the load. Therefore, at 

 light loads the efficiency is low because the fixed losses are large 1 

 as compared with the input. As the load increases, the efficiency 

 increases to a maximum, the fixed and variable losses being equal 

 at this point. Beyond this point the PR losses become rela- 

 tively large, causing the efficiency to decrease. 



One disadvantage of the squirrel-cage motor lies in the fact 

 that it takes a very large current at low power-factor on starting, 

 and in spite of this large current it develops but little torque. 

 When the motor is at standstill, the squirrel cage acts as the 

 short-circuited secondary of a transformer, causing the motor to 

 take an excessive current on starting, if full voltage is applied. 



I'liriin- 'Jill shows the variation of torque with slip for two 



rit values of line voltage. It will be noted that for small 



value- of -lip up to and beyond full load, which is the ordinary 



peration. the torque is substantially proportional to 



the slip. At higher values of slip, however, the torque curve 



over and finally reaches a maximum. This maximum is 



-alled tin- break-down ton/n,. Beyond this maximum point the 



tonpi (> as the -lip increases. For most typos of load 



Ntabilitu, as an increase in load is accom panic* 1 



