i-B] 



COMPOUND GENERATOR. 



RHEOSTAT 



FIG. i. Connections for no- 

 load saturation curve. 



generator, Fig. I, and is driven without load at constant speed. 

 Readings are taken of field current, terminal voltage and speed. 

 The field current is varied by adjust- 

 ing the field rheostat by steps from its 

 position of maximum to minimum re- 

 sistance. This gives the ascending 

 curve ; the resistance is then increased 

 again to its maximum for the descend- 

 ing curve. If the rheostat, with re- 

 sistance all in, does not sufficiently 

 reduce the field current, a second 

 rheostat may be placed in series with it. The machine " builds 

 up " from its residual magnetism as does the series generator ; 

 if the field winding is connected to the armature in the wrong 



direction, the machine will not 

 pick up but will tend to be- 

 come demagnetized. Should 

 the direction of rotation be 

 reversed, the field connection 

 should be reversed. 



8. Curves. Voltage read- 

 ings are corrected by propor- 

 tion for any variation in speed 

 (9, Exp. i-A), and the 

 curves plotted as in Fig. 2. 



9. Interpretation of 

 Curves. The curves in Fig. 2 

 show the saturation of the 

 iron and are much the same 

 as the characteristic of a 



series dynamo. The current through the armature is small, 

 being only a few per cent, of full-load current ; the resistance drop 

 through the armature may accordingly be neglected and the meas- 

 ured terminal voltage be taken as (practically) equal to the total 



2 3 



FIELD AMPERES 



FIG. 2. No-load saturation curve. 



