DIRECT CURRENT MOTORS. 



[Exp. 



g 



FIG. 5. 



The following is suggested as a method for conducting the test. 

 30. Varying Excitation, Shunt Machine. First let us consider 

 the case of a shunt machine, in which the excitation varied during 

 the load run. Make three no-load runs at three excitations covering 

 the range of excitations used in the load run. From these no-load 

 runs, after plotting W Q --S, plot three curves (A, B, C in Fig. 5) 

 showing W for different speeds as before. 



To get W^ for a particular speed, erect a perpendicular in Fig. 5, 



corresponding to that speed. 

 This perpendicular intersects 

 the three curves A, B, C, giv- 

 ing (for a particular speed) 

 the values of W for different 

 field currents. For each 

 speed a derived curve may 

 now be plotted giving W 

 for different field currents. 

 31. Compound Generator. 

 In testing a compound gen- 

 erator, first make a load run 

 to ascertain the equivalent shunt excitation and then make no-load 

 runs as a shunt motor. 



Load Run. Make a load run as a compound generator, and note 

 the values of terminal voltage and speed at three (or more) differ- 

 ent loads ; in each case ascertain the equivalent shunt excitation, i. e., 

 the field current which would give the same terminal voltage (and 

 hence the same flux density) with the machine run as a shunt* gen- 

 erator at the same speed. 



No-load Runs. Knowing this equivalent shunt excitation, make 

 the three corresponding no-load runs as a shunt motor at constant 

 excitation, in each run using one of the three equivalent shunt field 

 currents just determined. 



*(3ia). This equivalent shunt excitation may be determined after 

 each reading: without stopping the machine, the series winding should 

 be first short circuited and then opened; or, the machine may be stopped 

 and started again. Instead of this the equivalent excitation can be found 

 from a separate shunt run (like an armature characteristic 26, Exp. i-B) 

 in which is determined the field current which will give for each load 

 the same terminal voltage as in the compound run. 



SPEED 



Rotation losses W , for different 

 excitations. 



