20 



DIRECT CURRENT GENERATORS. 



lExp. 



full-load voltage (in this case the per cent, increase) in going 

 from full load to no load. 



1 8. Characteristics taken with Low Field Excitation. On 

 short circuit a shunt generator has no field excitation and the 

 short-circuit current (depending on residual magnetism) is com- 

 monly less than normal full-load current. The current, however, 

 is much greater before short circuit is reached. On account of 

 this excessive current, the complete characteristic curve cannot 

 be obtained with the field rheostat in its normal setting. To 

 show the form of the complete shunt characteristic, set the field 

 rheostat for a no-load voltage much below normal, and take 

 Curve E (Fig. 4) from open circuit to short circuit, and Curve F 

 returning from short circuit to open circuit. The form of these 

 curves should be interpreted. 



19. With a weak field, armature reactions cause the terminal 

 voltage to fall off with load more rapidly than with a strong 



field. This is seen by com- 

 paring Curves E and F 

 with Curve A. The effect 

 of armature reactions is 

 least .when the iron is 

 highly saturated, for then 

 any decrease in magneto- 

 motive force (due to arma- 

 ture ampere-turns) does 

 not cause a corresponding 

 decrease in magnetic flux. 



(Compare Fig. 2.) It follows, therefore, that a shunt generator 

 gives the best regulation when worked above the knee of the 

 saturation curve. It will be found (22) that this is not so for 

 a compound generator. 



20. (c) Compound Characteristic. The connections for 

 taking the compound characteristic, Fig. 5, are the same as for 

 the shunt characteristic, Fig. 3, with the addition of the series 



6ERIE8 

 FIELD 



Connections for compound char- 

 acteristic. 



