THEORY OF SYNCHRONOUS MOTOR. 



133 



To find the point P' on the hyperbola corresponding to mini- 

 mum current, we have to bring the points e and e' into coincidence. 

 The point P is obviously got by taking OE\ equal to E, and 

 through E\ drawing E\P parallel to OT. 



The resulting parallelogram OP'TiEi- shows that the generator 

 E.M.F. is in phase with the current. 



Suppose, now, that the generator field is kept constant, while 

 that of the motor is varied. 



When the motor field is small, as e.y. Oe (Fig. 44), we see 



FIG. 44. 



that the current leads before the motor E.M.F., and lags behind 

 the generator E.M.F. 



When e exactly opposes i, the latter lags behind E. 



When E and i are in phase (minimum current), i lags behind e. 



When e is still further increased, as e.g. Oe', i leads before E, 

 and lags behind e. 



We see, therefore, that by properly adjusting the excitation of 

 the field of the motor, the current may be in phase with, or may 

 lead before, or lag behind, either generator or motor E.M.F. s. 

 This is a point which should be borne in mind, as it has an 

 important bearing on the regulation of a power-transmission plant. 



