INDUCTION MOTOR 



345 



running torque of the motor. R% + r 2 = x 2 , where r 2 and Xz are 

 the resistance and the reactance of the rotor. os , osi, os 2 and os 3 

 are the corresponding stator currents and cos 0o, cos 61, etc., are the 

 power factors at start. 



FIG. 323. Starting torque with various rotor resistances. 



The curves in Fig. 324 are the " speed-torque " characteristics 

 for the motor operating with the various resistances in the rotor. 

 The maximum torque is the same in all cases but it is reached at 

 different speeds. One current curve holds in all cases. 



If a resistance R>Rs is in- 

 serted in the rctor windings 

 the starting current is further 

 reduced and the power factor 

 is improved but the starting 

 torque is decreased. R^ may 

 be made of such value that 

 the starting torque $4^4 is equal 

 to full-load torque and the 

 starting current os 4 is equal to 

 full-load current. Curve (4) 

 is the speed-torque character- 

 istic for this case. 



Thus by inserting resistance 

 in the rotor any starting torque 

 up to the maximum running 

 torque or "pull out" torque may be obtained. The starting 

 current is reduced and the power factor is improved. 



In starting a heavy load resistance Rz is used and the motor gives 

 its maximum torque at start. The resistance is then cut out 

 gradually as the speed increases and the motor operates with 

 short-circuited rotor with characteristics as shown in curve (0). 



25 50 75 100 125 150 175 200 225 250 

 Torque in Percent of Full Load 



FIG. 324. Speed-torque characteristics 

 of an induction motor with various 

 rotor resistances. 



