66 



SYNCHRONOUS ALTERNATORS. 



[Exp. 



900 



800 



700 



uj 600 



mal speed and excitation, readings being taken of speed and field 



current to see that they are constant. The connections are shown 



in Fig. 3. 



The characteristic is to be obtained for unity* power factor, a 



non-inductive variable resistance being used for a load. Read- 

 ings are taken of termi- 

 nal voltage and external 

 current from o to 25 per 

 cent, overload. 



In commercial testing, 

 the excitation is adjusted 

 for normal voltage at 

 full load. Fig. 4 shows 

 the characteristic of a 

 25 K.W. alternator in 

 which the voltage in- 

 creases from 575 at full 

 load to 627 at no load 

 a regulation of 9 per 

 8 9 10 n 12 13 u is 16 cent. (See 14, 17, 

 Exp. i-B.) It is de- 

 sirable to have the reg- 

 ulation as " close " as 

 possible, i. e., with the 



500 



400 



300 



200 



100 



01234 



567 



FIELD AMPERES 



FIG. 2. Saturation curve at no load, and at 

 full load (43.4 amperes at unity power factor). 

 Field ampere-turns equal field amperes multi- 

 plied by number of field turns, 464. 



All curves in Exps. 3-A and 3-B relate to 

 the same .s-kilowatt alternator. mO** possible 



tion in the voltage from 



no load to full load. Since with non-inductive load the power 

 factor is unity, the power output is found by taking the product 

 of terminal voltage and external current. 



7. The causes for the decrease in terminal voltage with load 

 are impedance drop in the armature (due to its resistance and 

 inductance) and armature reactions, discussed more fully in the 



* For other power factors, see 13 ; take data as in 14. Also see Fig. 

 7, Exp. 3-B. 



