DETAILED STUDY OF OPERATION WITH NORMAL LOAD 49 



minimum current corresponding to the same power would be OD = 

 2.8 amperes, approximately. 



II. OPERATION OF A MOTOR WITH CONSTANT EXCITATION, 

 SUPPLIED AT CONSTANT E.M.F. 



The two diagrams lend themselves equally well to the discussion 

 of this case, which is usually the most frequent one in practice. In 

 the first diagram (Fig. 29) the E.M-F. vector of the motor will remain 

 stationary and that of the generator will turn round the pole O, when 



FIG. 29. 



the load varies. The extremity A\ describes a circle having O for its 

 center and E^ for its radius. If the lines of equal load have been drawn, 

 their points of intersection with the circle will define completely the 

 conditions (currents and lags) for each load. The useful active cur- 



rents will be obtained by subtracting the loss-current -~ from the 



4u 



active currents themselves. 



It will be noted that the power is zero when A\ is on the power 

 line passing through d. If, in consequence of overload, the motor- 



