3-A] 



CHARACTERISTICS. 



69 



HI 

 OL 



6 



UR Amp. 



TERMINAL! VOLTAGE, 575 



U 



u. . 



4000 



3000 



2000 



1000 



cent., the same as already obtained from the external character- 

 istic, Fig. 4. 



For constant terminal voltage of 575, the excitation must be 

 increased from 6.66 amperes at no load to 7.33 amperes, at full 

 load. This will be found to check approximately with the arma- 

 ture characteristic, Fig. 5 ; an exact check can not be expected. 



Fig. 2. shows that, as we 

 go above saturation, there is 10 

 less difference between the 

 no-load and full-load voltages, 

 t. e., the regulation is better 



(7). 



11. Armature Character- 

 istic or Field Compounding 

 Curve. This curve is taken 

 for an alternator* in the same 

 way as for a direct current 

 generator (26, Exp. i-B). 

 The curve in Fig. 5, taken 

 for a constant terminal volt- 

 age of 575 at unity power factor, shows that in going from no 

 load to full load (43.4 amperes) the excitation is increased from 

 6.6 to 7.25 amperes. This checks with the increase 6.66 to 7.33 

 amperes in Fig. 2. 



Armature characteristics for lower power factors than unity 

 will rise more rapidly ( 13). 



* ( na). Composite Winding. Although an alternator can not be com- 

 pounded by a series winding carrying the line or armature current, as in 

 the case of a direct current generator (since the field winding requires a 

 direct current and the line or armature current is alternating), the result 

 can be accomplished by rectifying part of the alternating current and 

 passing it through what is called an auxiliary field winding. Such an 

 alternator is said to be composite wound. The alternating current to be 

 rectified is commonly derived from the secondary of a transformer, 

 through the primary of which flows the line or armature current; for 

 the core of this transformer the armature frame or spider is used. The 



20 40 60 80 

 ^ARMATURE AMPERES 



FIG. 5. Armature characteristic, or 

 field compounding curve ; unity power 

 factor ; speed constant. 



