THEORY OF THE MONOPHASE INDUCTION MOTOR. 161 



possibly attain synchronism, since that would make KI or K 2 zero, 

 and 



would then be negative. There is, therefore, a limiting speed 

 below synchronism beyond which a monophase motor cannot go. 



HO. Starting Devices. The difficulty encountered with 

 monophase motors is that they will not start without some auxiliary 

 starting device. The usual starting device is an additional stator 

 winding, round which flows a current out of phase with the main, 

 or running current. It is not necessary to have an additional 

 source of power with which to supply these additional windings 

 with current; they may be arranged in parallel with the main 

 windings on the stator, so that the motor current divides, part going 

 round the main coils, and part round the additional coils. 



If the ratio of the reactance to the resistance of the two stator 

 windings is different, the currents in the two circuits will differ in 

 phase, and, by making the difference of phase sufficiently large, a 

 considerable starting-torque will result. 



Some makers produce the phase difference in the stator wind- 

 ings by making the self-induction in one circuit large, and intro- 

 ducing capacity into the other circuit by means of condensers 

 made of iron plates in a solution of carbonate of soda. 



The splitting of the current causes the magnetic field to 

 rotate, and the motor starts as a di-phase motor. As soon as the 

 rotor is run up to a speed which may be considerably below syn- 

 chronism, the additional stator circuit is broken ; and as there is 

 now a considerable torque exerted on the rotor, the machine will 

 continue to run, and will be able to give out power with a high 

 degree of efficiency. 



On referring to equation (23), we find by differentiation with 

 respect to s 2 , and remembering that KI + *c 2 = 2, that T\ 1\ is a 

 maximum when 



+ 2 



' 



and that the maximum torque is given by 



m ^ = 2 I IC1K 2 KB, - ICii) ^ ^ ^ ^ (25) 



That is, for a given slip, the maximum torque is inversely propor- 



M 



