i-B] 



COMPOUND GENERATOR. 



opposition to the shunt winding. The effect of the series wind- 

 ing is now to decrease (instead of increase) the magnetization 

 of the iron, as the armature current increases, causing the volt- 

 age to fall off with load more rapidly than with the shunt field 

 alone. As there is no demand for this, generators with differ- 

 ential winding are not used. (In a motor, Exp. 2-A, a differ- 

 ential winding is useful in giving constant speed). 



25. (e) Series Characteristic. This characteristic shows 

 the effect of the series winding alone, with the shunt winding not 

 connected. The procedure is the same as in testing a series 

 generator, the connections being as in Fig. 2, Exp. i-A. 



10000 



8000 



6000 



4000 



CONSTANT SPEED 



CONSTANT TERMINAL VOLTAGE 



1 1 



80 120 160 



ARMATURE AMPERES 



-7,900 



200 



FIG. 7. Armature characteristic or field compounding curve, showing that 

 at full load 2,200 more ampere-turns are needed than at no load for constant 

 terminal voltage. 



26. (/) Armature Characteristic. This curve is used in de- 

 termining the proper number of series turns for compounding a 

 generator; and is therefore frequently called a field compounding 

 curve* It shows, Fig. 7, the variation in field excitation 



* This has also been called an " excitation characteristic," a name which 

 is ambiguous since it may be taken to mean the saturation curve, 6. 



