ALTERNATORS. 195 



Many of the experiments which follow are almost identical 

 with similar tests upon direct-current machines, described 

 in the author's " Dynamo and Motor Testing." In such cases 

 the experiments are only discussed briefly here, special 

 attention being given 'to points of difference in the results or 

 conclusions from those previously given. 



Relation Between Speed and Voltage of an Alternator. 



The formula for the voltage of an alternator 



nNZ 

 = 10" x 60* 



shows that the voltage will vary in the same proportion 

 as the speed if the remaining factors remain constant. 



n = revolutions per minute. 



N = number of armature conductors acting in series. 

 Z = number of magnetic lines entering the armature. 

 k = constant, depending upon wave form of electro- 

 motive force. 



Hence, as in the case of a -direct-current generator, if an 

 alternator be constantly excited and driven at various speeds, 

 the relation of speed to voltage will be constant. If plotted 

 on squared paper with speed horizontal and voltage plotted 

 vertically the relation would be shown as a straight line, 

 exactly as in the case of a direct-current dynamo. 



The Magnetic Circuit. The magnetic circuit of the alter- 

 nator is designed^ exactly as in the case of a direct-current 

 dynamo. The magnetisation curve, showing the relation 

 between excitation and magnetic flux, is also of the same 

 importance in the case of alternating generators. This 

 relation is obtained experimentally by running the alternator 

 at a constant speed and varying the excitation. The armature 

 voltage corresponding to each value of the excitation is then 

 observed. The curve is either plotted to show the relation 

 between " ampere-turns " on the magnets and magnetic 

 flux through the armature, or, more usually, the curve shows 

 the relation between the actual quantities observed, viz., 

 ( xciting current and armature voltage. 



The ampere turns can be calculated by multiplying the 

 exciting . current by the number of magnet windings, while 

 the useful magnetic flux can be calculated from the observed 



