THE INDUCTION MOTOR 



263 



quency, as the slip is slightly greater than 50 per cent, (see page 

 236, equation 65). Therefore, the synchronous speed of No. 2 

 motor is practically half that of No. 1 motor. The rotors each 

 operate at a speed which is slightly less than half the synchron- 

 ous speed of the first motor. The rotors so adjust their speeds 

 that their combined torque is just sufficient to carry the load. 

 It is not necessary that the two motors have the same 

 numl>er of poles. The various speeds for combinations in which 

 the two motors have a different number of poles may be 

 determined as follows: 



Line 



No 2 



Fio. 247. Concatenation of induction motors. 



Let N be the speed of the combination, /i and / 2 the stator 

 frequencies, PI and P 2 the number of poles and Si and s 2 the slips. 



The speed of the first rotor 



* 



N l 



(i _ Kl ) (page 236, from equation 65) 



The speed of the second rotor 



he two rotors are rigidly coupled, the speed N\ equals the 

 speed N*. 



from which 



Pi 



;:dl in conip;iiiM.n with /', + 7 J 2 , when the combina- 

 tion is operating near its synchronous speed, and it may be 

 Then 



P 



Sl l\ H 



