214 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



representing the strength of the primary current describes a semi- 

 circle which cuts the axis of x at two points, A and B, such that OA =-- ij v 



A 



represents the zero-load magnetizing current, and O5= , where 



a 



a = -- , in which v\ and v 2 are the leakage coefficients for the two wind- 



ings of the transformer. 



By hypothesis, the voltage of the source of current-supply is con- 

 stant. Therefore, an increase of speed of the converter, by causing an 

 increase in the frequency, will bring about a decrease of the magnetiz - 

 ing current corresponding to zero-load. We will have a new circle, 



Rotary Converter 



Three- Phase Transformer 



Three-Phase 



Inductive 



Resistance 



FIG. 26. 



A 'B', whose diameter will be i'do\ i ) The two circles will inter- 



w / 

 sect each other at a point M, which is on the line drawn, from O, 



tangent to one of the circles. 



It is seen, at once, on the diagram, that the right line ON, which 

 represents the active current in relation to the primary current OM, 

 indicates the critical load already mentioned. In fact, for the value 

 OH , corresponding to a load which is lighter than the critical load, i.e., 

 which makes OK < ON, it is seen that the reactive current has decreased 

 from the value OL to the value OL\, as the result of a speed -increment. 

 On the contrary, for the current OP, corresponding to a load which 

 is heavier than the critical load, i.e., which makes OQ > ON, the reactive 

 current has increased from OR to OR\. It is now easy to analyze the 

 phenomena observed. 



In the first place, when the load was lighter than the critical load 

 of the machine, an increment of speed produced a decrease of the 

 reactive current, and, consequently a decrease in demagnetizing action; 

 and, since the converter was running as a direct current motor, with 

 constant shunt-field-excitation, its total excitation-flux was increased. 

 Therefore, the E.M.F. on the D.C. side being constant, the speed of 



