i-B] COMPOUND GENERATOR. 19" 



For the total shunt characteristic (Curve B), plot total arma- 

 ture current* (line current plus field current) as abscissae, and 

 total generated voltage (terminal voltage plus armature RI drop) 

 as ordinates. 



16. Interpretation (Armature Reactions and Regulation). 

 An ideal characteristic would be the straight horizontal line, Curve 

 C, indicating a constant voltage at all loads. As a matter of fact 

 the terminal voltage (Curve A) decreases with load. There are, 

 at constant speed,f three causes for this: (i) armature resistance 

 drop, (2) armature reactions which reduce the magnetic flux 

 and (3) decreased field excitation as the voltage decreases. 



The difference between Curves A and B shows the effect of 

 (i) resistance drop; the difference between B and C shows the 

 effect of (2) armature reaction and (3) decreased excitation, 

 and of (4) if speed varies. 



The difference between B and C will show the effect of arma- 

 ture reactions (2) alone J if a run is made at constant excitation 

 and constant speed, thus eliminating (3) and (4). This is the 

 practical method for determining armature reactions. The ma- 

 chine may be self excited or (preferably) separately excited. 



17. The regulation of the generator is shown by the drop 

 in Curve A. To express regulation numerically as a per cent., 

 the rated voltage at full load is taken as 100 per cent. In a 

 commercial test, therefore, the curve is taken by beginning at 

 full load at rated voltage (100 per cent.) and proceeding to open 

 circuit. The regulation || is the per cent, variation from normal 



* The difference between line and armature currents is so small that for 

 many practical purposes the distinction between them can be neglected. 



t (i6a). Should the generator slow down under load, as when driven 

 by an induction motor, this would constitute a fourth cause (4). 



$ ( i6b). Included, as a part of armature reaction, is the effect of local 

 self-induction of the armature conductors, when traversed by the arma- 

 ture current which (in any one conductor) is rapidly reversing in direc- 

 tion. 



II A. I. E. E. Standardization Rules 187, et seq. 



