ALTERNATORS. 



221 



In order to illustrate what has been said, the curves of 

 impedance and watts absorbed in Figs. 101 and 102 have 

 been drawn. They were taken on a 1 kw. 4-pole alternator, 

 with closed ring armature winding and solid poles. The 

 armature while stationary was supplied with an alternating 

 current of 10 amperes, the voltage at the terminals and the 

 watts absorbed were observed for a series of positions of 

 the armature. In Fig. 101 is shown the impedance of the 

 armature (1) with the field circuit open, (2) with the field 

 terminals short-circuited, (3) with the normal excitation of 

 1 ampere. 



Fig. 102 shows the readings taken on the wattmeter in 

 the same experiment, the numbers on the curve indicating 

 the conditions as before. It will be noticed that the losses 



250 



20 40 



80 100 120 140 160 180 



Degrees. 



FIG. 102. POWER TAKEN BY ARMATURE IN DIFFERENT POSITIONS WHEN SUPPLIED 

 WITH CONSTANT CURRENT OF 10 AMPERES 



Curve I. Field winding open. 

 II. 1 ampere, excitation. 

 III. Field winding short-circuited. 



vary considerably with the position of the armature, reaching 

 a maximum of 330 watts, when the coils are exactly opposite 

 to the magnet poles. Further, this loss is very much less 

 when the field windings are short-circuited. In the position 

 where the armature coils lie between the poles the losses 

 are the same, whether the field windings are short-circuited 

 or not. This tends to show that the effect of the armature 

 currents on the magnets is very small in this position. We 



