264 ALTERNATING CURRENTS 



If the stator of the second motor is so connected that its rotor 

 tends to turn in a direction opposite to that of the rotor of the 

 first motor, equation (68) becomes 



/iX!20 si/i X 120 



p (i - so = p --- (i - s 2 ; 



Again neglecting the term PiS 2 , the slip becomes 



.. - p^ (70) 



The set will not start of itself if connected in concatenation 

 with the rotors tending to turn in opposite directions. It 

 must first be brought up to speed either by an auxiliary motor 

 or by one motor alone, before the second one is connected. 



As an example of the speeds obtainable with two motors hav- 

 ing a different number of poles, consider two 60-cycle motors, 

 one having 4 poles and the other 20 poles. The following syn- 

 chronous speeds are obtainable: 



Four-pole motor alone: 1,800 r.p.m. 



Twenty-pole motor alone : 360 r.p.m. 



When the 4-pole and 20-pole motors are in concatenation, 

 aiding, the slip of the first motor, from equation (69), 



20 20 

 = 20 + 4 ~ 24 



The synchronous speed of the set is 



N = (1 - sO 1,800 = 1,800 = 300 r.p.m. 



When the 4-pole and the 20-pole motors are in concatenation, 

 opposing, the slip of the first motor, from equation (70) , 



20 = 20 

 = 20 - 4 16 



The synchronous speed of the set is 



N == (1 - si) 1,800 = - | (1,800) = - 450 r.p.m., or the set 



now rotates in the opposite direction. 



Four different synchronous speeds are obtainable with these 

 two motors, 1,800 r.p.m., 360 r.p.m., 300 r.p.m., 450 r.p.m. 



It is to be noted that the synchronous speed resulting from 



