CH. VI EFFICIENCY 135 



will be 91 per cent., and the corresponding current 100 

 amperes. Compare also Fig. 35. 



If the torque loss is constant the current c f required 

 to make up the loss will vary inversely as the induction 

 factor. Hence the higher the value of M the greater will 

 be the efficiency and the smaller the current at which the 

 efficiency is a maximum. 



The larger we make M the slower the motor will run 

 on a line of given tension, and the greater will be the 

 torque for a given current. Since w = ce the power at 

 maximum efficiency will be nearly proportional to the 

 current at maximum efficiency ; hence, by increasing the 

 induction factor we can increase the maximum efficiency, 

 but in so doing we decrease the power at that efficiency. 



Example 36. If in Example 34 the induction 

 factor is doubled, the torque loss remaining unaltered, 

 the maximum efficiency will be increased to 92 per 

 cent., the current for this efficiency will be 50 amperes, 

 and the power 6'0 kilowatts. 



The preceding equations assume that the induction 

 factor remains constant. If M varies, as in series- wound 

 motors, we may still assume that the torque losses remain 

 the same at all loads, but we can no longer assume that 

 c f is always a measure of those losses. In a series-wound 

 motor running without external load, the value of M is 

 small, and hence the current c f is large. As the load 

 increases, the proportion of the total current required to 

 make up the torque loss becomes smaller. Our equa- 

 tions assume that for all currents the torque loss is 

 represented by c f amperes. If then M is greater for 

 any current than it was when the motor was running 

 without external load, we must reduce c f in our equa- 



