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CHAPTER VI 



EFFICIENCY 



LET af in Fig. 30 represent the maximum current in a 

 motor of R ohms resistance, and let al> represent the 

 tension of the line to which it is connected. Complete 

 the rectangle afdb and join ad. The vertical intercept 

 between af and ad represents the watts taken from the 

 line ; this is nothing at a, and a maximum at /. Con- 

 struct the curve of mechanical watts by taking the 

 difference between the line watts and the heat watts, as 

 explained in Chapter II. 



The ratio of the mechanical watts to the line watts is the 

 efficiency of conversion, and expresses the proportion 

 of the electrical energy put into the motor that is converted 

 into work. It is a maximum when the current is nothing, 

 and is nothing when the current is a maximum. This is 

 true for every form of continuous-current motor, whether 

 shunt or series wound. 



The mechanical watts is equal to cMn, where n is the 

 revolutions per second, and M the induction factor. Since 

 M is given by 



