FIELD STRENGTH DUE TO STATOR WINDINGS. 151 



the negative parts of the curves of Fig. 61, and then add all the 



FIG. 61. 



curves together. The result of the adding these curves together is 

 to produce the dotted curve in Fig. 62, which represents the value 

 of the induction at any instant. 



FIG. 62. 



From Fig. 62 we see that the maximum value of the induction 

 is proportional to 



i + 2i sin 30 



that is, to 2i ; and the minimum value of the induction is pro- 

 portional to 



2i sin 60, or i\/3 



Thus the induction varies between limits which are propor- 

 tional to 2 and 1*732, and the variation of the induction from its 

 mean value is about 7 per cent. 



Comparing the result with that which was obtained for a di- 

 phase induction motor, it is seen that increasing the number of 

 currents round the stator diminishes the percentage variation of 

 the induction from its mean value. 



The steady running of an induction motor depends upon the 

 constancy of the torque (or moment of turning couple) of the 

 rotor, and consequently upon the constancy of the strength of 

 the magnetic field in which the rotor revolves. It might be sup- 

 posed, therefore, that the greater the number of currents, in 

 different phases, exciting the stator, the steadier would be the 

 action of the motor. Practical tests, however, indicate that there 

 is no perceptible difference as regards steady running between a 



