2 -B] EFFICIENCY. 47 



field current, the iron losses are independent of load and depend 

 upon speed alone. 



ii. Friction and Windage. The friction and windage losses 

 are also independent of load and depend alone upon speed, being 

 (for all practical purposes) directly proportional* to speed. 

 Friction includes frictions of brushes as well as of bearings. 



12. Rotation Losses W (Combined Iron Losses, Friction 

 and Windage). In the no-load run the power supplied to the 

 armature (product of armature voltage and current) gives the 

 rotation losses plus a small armature copper loss. This copper 

 loss is subtracted (or neglected as small) to get the rotation 

 losses. These losses are sometimes termed stray power, ,f 



The combined rotation losses W , thus determined at no load, 

 will be present at all loads and will have the same value for the 

 same speed and excitation. If the speed of the motor is very 

 nearly constant, the W losses will be correspondingly constant. 

 Rotation losses are commonly classed among the constant losses,:]: 

 inasmuch as they are independent of load and the variation due 

 to any small change of speed is small. 



For determining efficiency there is no necessity for ascertain- 

 ing the separate losses due to hysteresis, eddy currents, friction 

 and windage, their combined value W being sufficient. 



13. A curve should be plotted showing the rotation losses 

 W for constant field current at different speeds. To plot this 

 curve accurately, it is best|| to first plot for various speeds the 



*(na). Windage increases more rapidly than the first power of the 

 speed; but windage loss is comparatively small and does not, at usual 

 speeds, materially affect the law of variation of the combined friction and 

 windage losses. 



t The term stray power applies to any loss except copper loss. 



$(i2a). The no-load losses are the rotation losses plus the copper 

 loss of the field circuit (and the practically negligible copper loss of the 

 armature) ; the no-load losses are therefore termed "constant." 



|| (i3a). This is advantageous because a straight line can be drawn 

 more accurately than a curved one, when the observed data are few 

 or irregular; two accurate points are sufficient, but three are better as a 



