194 



ALTERNATING CURRENTS 



may now investigate assuming LI, 12, and M to remain constant 

 what effect is produced by a change in the primary (or stator) 

 resistance. 



HI. Case of Negligible Primary Resistance. 

 Hey land's Circle Diagram 



We shall in the first place consider the ideal case of a motor 

 whose stator winding is of absolutely negligible resistance. The point 

 O (Fig. 129) in this case moves up to A, K moves up to B, the 

 diameters BF and BO of the two circles coincide, and the diagram 

 assumes the simple form shown in Fig. 130, U and X being the 



FIG. 130. Heyland's Circle Diagram. 



centres of the two circles. We can easily express the maximum 

 value of the power factor in this ideal case in terms of the dispersion 

 coefficient o-. The maximum power factor cos OQ is obtained when 

 the primary current vector OP becomes tangential to the smaller 

 circle. We have in this case 



