GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 181 



If the supply E.M.F., E, is varied, which amounts to the same 

 thing as changing the radius OC of the circle, the particular load for 

 which the reactive current vanishes may be varied at will. For 

 example, Fig. 5 (in which four circles of different radii are drawn), 

 shows that it is possible to have the current in phase (power-factor 

 equal to unity) with a certain useful load dC<2 or with zero load, instead 

 of the load dG, and that it is possible even to have negative lag (or 

 a leading current) at all loads, by suitable variation of the supply 

 E.M.F. E. 



It is seen that in any case it is impossible to prevent the reactive 

 current from varying when the load changes. Since this current 

 produces a magnetizing or a demagnetizing action equal to that of 



K Id \/ 2 inductive ampere-turns, 



FIG. 5. 



it is not possible to maintain constant potential by means of constant 

 excitation if the impedance of the circuit is not negligible, but that 

 it will be necessary to increase or decrease the excitation ampere- 

 turns by an amount equal and contrary to that of the ampere-turns 

 of the reactive current in order that the total inducing flux may 

 remain constant. (The variation of the magnetic leakage may intro- 

 duce a slight complication, as will be seen later.) 



With a given value of /-, if the impedance Z is varied, we see that, 

 the greater its value, the larger the scale of amperes will be, and, con- 

 sequently, the smaller will be the variation of load which corresponds 

 to a given angular displacement of the vector OC. The variations 

 of the active current with the load will therefore increase in propor- 

 tion with the impedance. 



