2-B] 



EFFICIENCY. 



49 



the field; and the (almost constant) rotation loss W , obtained 

 from the curve in Fig. 2 for the particular speed and excitation. 



The output is found by subtracting these losses from the in- 

 put; the efficiency is output divided by input. 



15. Curves should be plotted with power output (or more 

 simply with armature current) as abscissae, showing separate ant 1 

 total losses, input, output, efficiency, total current and speed ; also 

 useful torque (watts output -=- speed) ; see Fig. 3. Compare the 

 curves of Fig. 3 with the curves for a transformer, Fig. 4, Exp. 

 5-A, and Fig. 8, Exp. 5-8. 



Field R I? Loss 



POWER OUTPUT 

 FIG. 3. Losses and efficiency of a shunt motor. 



Maximum efficiency occurs when the variable loss (armature 

 RI 2 ) equals the constant losses; see 28. 



It is seen that efficiency at light loads is low; this is true of 

 both generators and motors. For this reason several generators 

 are commonly run in parallel in a central station ; as the load on 

 the station decreases, the generators are cut out one at a time, 

 so that the remaining generators will be more or less fully loaded 

 and will run nearer the point of maximum efficiency. 



