220 



ALTERNATORS. 



we can measure the power spent in sending the current 

 through the armature. The power thus measured is the 

 sum of the losses in the armature and in the fields, the 

 latter being due chiefly to eddy currents, as the hysteresis 

 losses will be small at the low induction produced. 



Now, the losses in the fields wiU vary with the position 

 of the armature, being greatest when the flux in the fields 

 is greatest, and being small when the armature is so situated 

 as to produce a minimum of flux through the poles. The 

 hysteresis and eddy-current losses in the armature will, 

 however, not vary very much with the position of the armature. 

 By taking a series of readings of the power developed when 

 supplying a constant current to the armature in a succession 

 of different positions, it is thus possible to form an idea of the 

 approximate amount of the losses which occur in the fields 

 owing to eddy currents. 



If the poles are surrounded by closed circuits having a high 

 conductivity, the magnetic flux will produce currents in 

 these round the pole face. The magnetic effect of these 

 currents will be to oppose the varying flux, and to greatly 



5-6 



300 



250 



5-4 



5-2 



200 2 5-0 



150 m - 4-8 



100 -o 4-6 



- 4-4 



20 40 60 



120 140 160 



80 100 

 Degrees. 

 FIG. 101. CURVES OF ARMATURE IMPEDANCE. 



Curve I. Field winding open. 

 II. 1 ampere excitation. 

 III. Field winding short-circuited. 



CURVE OF " VOLTS" AT TERMINALS WITH 10 AMPS. 



diminish its amount. This is the effect of the amortisseurs 

 often used on the poles of alternating-current machines. 

 The ordinary magnet winding, if short-circuited, will have a 

 somewhat similar effect in redacing the eddy currents in the 

 poles as is illustrated in the curves to be described. 



