2-B] EFFICIENCY. 59 



Results. Results are worked up as in the preceding paragraph. 

 Curves are plotted as in Fig. 5 and derived curves found showing 

 the variation of W with field current for any speed. Such a derived 

 curve is plotted for each speed observed in the load run. 



32. Compound or Differential Motor. A load run is first made 

 to find the equivalent shunt excitation; no-load runs are then made 

 as a shunt motor. 



Load Run. Make a load run as a compound or differential motor, 

 and note the speed at three (or more) different loads so chosen as 

 to cover the speed variation of the run. In each case ascertain the 

 equivalent shunt excitation, i. e., the field current which would give 

 the same speed* (and hence the same flux density) with the machine 

 run as a shunt motor, the load and the line voltage being the same 

 as before. 



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

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

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

 currents just determined. 



Results. The results are worked up as in the preceding para- 

 graphs. From the three no-load runs three curves are plotted, as 

 in Fig. 5, showing W 9 for varying speed at different excitations. 

 From these curves a derived curve may be plotted showing the varia- 

 tion of W with field excitation for any speed. Such a derived curve 

 is plotted for each speed observed in the load run, and from it the 

 value of W obtained for the corresponding excitation. 



*(32a). The equivalent shunt excitation may be determined after 

 each reading by cutting out the series coil as in 3ia. 



The adjustment to a definite speed is, however, difficult without some 

 particularly sensitive tachometer. X avoid this adjustment, proceed as 

 follows : 



Determine say five shunt speed characteristics, that is make five runs 

 at different constant shunt excitations, determining speed for different 

 loads. For each excitation plot speed as ordinates and armature current 

 as abscissae. By interpolating between these curves, we can find the shunt 

 excitation that gives a particular speed for a particular armature current. 

 This will give the equivalent shunt excitation corresponding to any speed 

 and armature current found in the load run as a differential or com- 

 pound motor. Knowing the equivalent shunt excitation, the correspond- 

 ing no-load runs are made. 



