EFFICIENCY OF A DYNAMO-ELECTRIC MACHINE 191 



(if there is any) PR loss which varies with the square of 

 the load. 



These various losses are shown plotted as curves in Fig. 

 114. The results are taken from a test of a 5 h.p. shunt 

 motor and are about right for any continuous-current 

 machine of this capacity. The curve marked " total loss " 



1 -2. 3 4 5 6 



H.P. Output 



FIG. 114. Loss Curves of a Motor. 



is plotted by adding the ordinates of the other four curves, the 

 significance of which is indicated in the figure. The shunt 

 field loss is 97 watts at all loads, and the stray power is 

 320 watts at no load and practically the same at full load; 

 the brush contact resistance loss is 3 watts at no load and 

 80 watts at full load, the no-load armature PR loss is less 

 than one watt and at full load it is 242 watts. 



42. Calculation of Efficiency from Loss Curves. If such 

 a set of curves is given for any machine its efficiency 

 curve may be at once obtained. The input to any electric 

 machine must evidently be equal to the output plus all 



