7$ SYNCHRONOUS ALTERNATORS. [Exp. 



Beginning with the field weakly excited, increase the field cur- 

 rent by steps so that the short-circuit armature current (/s) is 

 increased from, say, normal to i or 2 times* normal full-load 

 current. At each step read field and armature currents and plot 

 as in Curves 2 and 3 of Fig. I. 



In the short-circuit test, we may have either the field or the 

 armature under normal full-load working conditions, but not 

 both at the same time. 



ii. The curve for short-circuit current, will (as in Fig. i) 

 be a straight line through a w r ide working range, and may be 

 extended as a straight linef beyond the observed data. The 

 ultimate bending of the curve depends upon the relative satura- 

 tions of various parts of the magnetic circuit, armature, teeth, 

 poles, etc. 



Fig. i shows that normal excitation, OH = 7.33 amperes, 

 gives a short-circuit current of 116 amperes. (Normal excita- 

 tion is the excitation giving rated voltage, 575, at full load, unity 

 power factor; for this machine see Figs. 6 and 7 the corre- 

 sponding no-load voltage is found to be 627.) 



* ( ice). By taking the run quickly, even higher values of current can 

 be reached. 



Running an alternator on short circuit, as described, affords the best 

 means for drying armature insulation. An alternator in shipment may 

 have been unduly exposed to weather or have been allowed to stand in a 

 damp place. The insulation readily takes up moisture and is much 

 impaired thereby. In such a case, as soon as the alternator is installed 

 it should be run for one day with the armature short-circuited, the field 

 excitation being so low that the normal armature current flows; there is 

 no high voltage to break down the insulation. The armature is thus baked 

 and the insulation restored. This precaution, particularly in the case of 

 high voltage machines, may avoid a break-down of insulation upon 

 starting up. 



t ( na). Extrapolation as a straight line (2) gives (after saturation is 

 reached) a diminishing value for synchronous impedances Z = o-4-/s, 

 as used later. It thus favors the machine by giving a smaller impedance 

 drop; in the electromotive force method this is justifiable because it par- 

 tially offsets the pessimistic tendency of that method. This justification 

 is empirical. 



Curve (3) has been extrapolated by assuming Eo + Is to be constant. 



