208 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



sab can be drawn which cuts the circle at two intermediate load-points 

 ab between the extreme points F and M, in such a way that this line 

 will not be far distant from the circular arc of any point in its useful 

 portion, PM. The segments dP and dQ, read off from the diagram 

 (by the proper scale) will determine the currents io and I w correspond- 

 ing to zero-load and normal load; and Eqs. (31) and (32) can be used 

 with these values to deduce the number of constant ampere-turns, 

 Ad, which are necessary, and also the number of turns n of the series- 

 windings. 



The slight residual differences in the E.M.F. e will be obtained by 

 drawing circles of radius E from every point on the right line PS and 



M 



taking their intersections with the axis of voltages BO. The value of 

 the reactance X affects the precision of regulation, because, the greater 

 the length of the arc FM, corresponding to a given variation of load, 

 the more difficult it is to make it coincide with a right line such as PS. 

 To improve the regulation, therefore, the segment dD=ZI w , which 

 measures the maximum load I w , ought to be decreased to a minimum; 

 and, to this end, the reactance (which is the more important factor in 

 the value of Z) ought to be decreased to the lowest value possible. 

 However, as already seen, the reactance cannot be decreased below 

 a certain value A' without causing the zero-load reactive current i to 

 increase. It is therefore necessary to take a suitable mean value be- 

 tween Xo and zero, which will depend on the circumstances and the 

 conditions of the case. In general, the more the load of the converter 

 is uniform and the less often it runs without load, the more it will be 

 possible to reduce X. 



