6 DIRECT CURRENT GENERATORS. [Exp. 



a run. This is true of all characteristics or other curves involv- 

 ing the saturation of iron. 



8. Brush Position. During the run the brushes are kept in 

 one position; if for any reason they are changed, the amount 

 should be noted. For a generator the best position is the position 

 of least sparking and of maximum voltage, which locates the 

 brushes on the " diameter " or " line " of commutation. Under 

 load this line is shifted forward from its position at no load, on 

 account of field distortion caused by armature reaction,* and the 

 brushes are accordingly advanced a little to avoid sparking. As 

 it is desirable to keep the brushes in the same position in taking 

 all the curves, with load or without load, it is well to give the 

 brushes at no load a little lead, but not enough to cause much 

 sparking. 



9. Speed Correction. If the speed varied during the run, 

 the values of voltage as read are to be corrected to the values they 

 would be at some assumed constant speed. Since, for any given 

 field current, voltage variesf directly with speed, this correction 

 is simply made by direct proportion; each voltmeter reading is 



*(8a). Armature Reactions. Armature current has a demagnetising 

 effect and a cross-magnetising effect, the two effects together being called 

 armature reaction, as discussed in various text books. The demagnetizing 

 effect due to back ampere-turns weakens the field; the cross-magnetizing 

 effect due to cross ampere-turns distorts the field (weakening it on one 

 side and strengthening it on the other) and shifts forward the line of 

 commutation. In many early machines this made it necessary to shift 

 the brushes forward or back with change of load to avoid sparking; in 

 modern machines the armature reactions are not sufficient to make this 

 necessary and the brushes are kept in one position at all loads. 



If a very accurate determination of the neutral position of the brushes 

 is desired, it can be found by a voltmeter connected to two sliding points 

 which are the exact width of a commutator bar apart. The neutral posi- 

 tion is the position of zero voltage between adjacent commutator bars, and 

 this is shown by the voltmeter. 



t (9a). If the speed can be varied at will, this can be verified for one 

 field excitation. A peripheral speed of 3,000 feet per minute is permissible 

 with the ordinary drum or ring armature. 



