192 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



pulled out as the load increases, in such a way as to bring back to zero 

 a direct- reading phase-indicator, connected at the terminals of the 

 converters. 



This method is applicable whatever may be the value of E (i.e., 

 the radius of the circle); but the most suitable value is, obviously, 

 the smallest value, which corresponds to E=s'+RI. 



The diagram presupposes that the locus of the point G is a circle; 

 but if the E.M.F. e varies the center O of that circle is displaced and 

 the locus of G is a curve, which can be drawn by points determined 

 from the data. This does not, in any way, necessitate modifications 

 in the method. 



Power-Factor of the Generator. In what precedes, we have seen 

 the necessity of keeping down the value of the E.M.F. , s. It is cus- 

 tomary to express the disadvantages of the useless increase of this 

 E.M.F. by stating the value of the power-factor, i.e., by the value of the 

 cosine of the phase-angle, usually termed the " angle of lag," between 

 the E.M.F. and the current. 



Let ^ denote this phase-angle, and let denote the phase-angle 

 between and E; and let, as before, </> denote the phase-angle between 

 the current and the E.M.F. s. We have 



and this angle of lag, $, is always in the same direction as (see Fig. 

 i), i.e., in the direction of the hands of a watch (in these diagrams). 

 We therefore have 



For the condition of average load, that is to say, in the cases where 

 there is no lag, we have 0=o, and, therefore, reduces to the same 

 value as itself. 



In order to increase the power-factor of the generator as much as 

 possible at average loads, we should, therefore, reduce 0, and, conse- 

 quently, e, as much as possible, as already stated. It is much more 

 important to do this than to reduce to zero the " no-load " lag between 

 the current and the converter E.M.F. Therefore, the reactance used 

 should have a value comprised between zero and the value which 

 renders the no-load lag a minimum. This is all the more desirable 



