iS 



DIRECT CURRENT GENERATORS. 



[Exp. 



14. The setting of the rheostat for commercial testing ( 21 a) 

 is made for normal voltage at full load. For the purposes of this 

 experiment, it is usually preferable to set the rheostat for normal 

 voltage (or for any selected value of voltage) at no load; in 

 this case the shunt, compound and differential curves, Fig. 6, all 

 start from the same no load voltage. The load current is then 

 increased from no load up to about 25 per cent, overload and 

 then decreased, if so desired, back to no load. The return curve 

 will fall a little below, on account of hysteresis. 



Data are also to be taken for a characteristic starting at no load 

 with a voltage below normal ( 18). 



Armature resistance is measured by the fall-of -potential method, 

 ( 17, Exp. i-A). 



100 120 

 AMPERES 



FIG. 4. Shunt characteristics. 



15. Curves. The armature RI drop is plotted as Curve D in 

 Fig. 4. 



For the external shunt characteristic (Curve A, Fig. 4), plot 

 observed line current as abscissae and observed terminal voltage 

 as ordinates. 



