3-A] 



CHARACTERISTICS. 



67 



next experiment. Compare also 16, Exp. i-B. As in the case 

 of a shunt generator (19, Exp. i-B), when the iron is highly 

 saturated, the demagnetizing effect of armature reaction is 

 the least and the regulation 

 the best. 



8. The external character- t5~| 



2 t 0.2 



istic, Fig. 4, is practically an 



ellipse.* At one end of the 



characteristic (near open cir- FIG. 3. Connections for loading an 



cuit), an alternator tends to 



regulate for constant voltage; at the present day, this is the 

 usual working part of the characteristic. At the other end 

 (near short circuit), an alternator tends to regulate for constant 



current. The earliest alter- 

 nators were constructed for 

 such operation. Constant 

 current alternators are used 

 (less now than formerly) 

 for series arc lighting. For 

 this service an alternator 

 should have high armature 

 reaction so as to limit the 

 current on short circuit to 

 the desired value; a reac- 

 tance external to the arma- 

 ture will serve equally well. 



U500 

 ^400 

 z300 



5 

 200 



100 



x B 



20 40 60 80 100 



ARMATURE AMPERES 



FIG. 4. External characteristic of an 

 alternator at unity power factor. (The 

 dotted parts of these curves were cal- 

 culated according to Exp. 3-6.) 



9. Full-load Saturation 

 Curve. The machine is run 

 at constant speed so as to give its normal full-loadf current at 

 different field excitations. The connections are as in Fig. 3. To 

 obtain the curve for unity power factor, a non-inductive resistance 



* See discussion of Fig. 7, Exp. 3-8. 



t(9a). Curves taken at intermediate loads (one fourth, one half and 

 three fourths full load) would lie between the no-load and full-load 



