344 



ELECTRICAL ENGINEERING 



Two methods of reducing the starting current are in use. (1) 

 The voltage impressed on the stator is reduced by multi-voltage 

 taps on the supply transformers or by using a potential starter. 

 (2) Resistance is inserted in series with the rotor windings. 



(1) When the impressed voltage is reduced, the starting current 

 is reduced in proportion to it, but the starting torque is reduced 

 as the square of the voltage. 



In Fig. 322 Ei.Sih represents the starting torque of the motor 



O 



FIG. 322. Starting at reduced voltage. 



at full voltage and osi represents the starting current, neglecting 



I 

 the exciting current, and -~ 



represents the starting torque at 

 half voltage and os 2 represents the starting current. 



Since o$ 2 = - r the starting current is reduced to one half its 



2 



value at full voltage, but the starting torque is reduced to one 

 quarter. The power factor is not changed. 



Thus starting with reduced voltage gives very small starting 

 torque and low power factor. 



A squirrel-cage rotor may be used. 



(2) When resistance is inserted in the rotor windings the start- 

 ing current is reduced and is brought more nearly in phase and the 

 starting torque is increased. 



In Fig. 323 s t represents the starting torque when the rotor 

 circuits are closed without any starting resistance. 



Siti is the starting torque when resistance Ri is inserted. 



82^2 is the starting torque when resistance R^>R\ is inserted. 



s 3 Z 3 is the maximum possible starting torque and is obtained by 

 inserting a resistance R s >Rz', it is the same as the maximum 



