142 ELEMENTS OF ELECTRICAL ENGINEERING. 



shunt or compound motor supplied with current from constant 

 voltage mains, are nearly constant. On the other hand, the 

 armature loss plus the series field loss is nearly proportional to 

 the square of the current intake of the motor. The efficiency of 

 the motor is therefore very small when the load is small, inas- 

 much as the constant losses, namely, field loss and stray power 

 loss, are then a large part of the motor intake. The efficiency 

 of the motor increases with increase of load, passes through a 

 maximum value for a certain definite load, and then, with further 

 increase of load, the efficiency decreases because of the rapid in- 

 crease of armature loss. 



The efficiencies of a shunt motor or a compound motor may 

 be calculated for a series of assigned values of current or power 

 intake, and from these data a curve may be derived showing the 

 relation between power output and efficiency. Fig. 96 shows a 



40 



Power output in % of full rated load 



25 50 75 too MS 



Fig. 96. Efficiency Curve of a Shunt Motor. 



typical efficiency curve of a shunt motor. The efficiency curve 

 of a compound motor is very similar to the efficiency curve of 

 a shunt motor shown in Fig. 96. 



The upper curves in Fig. 94 show the efficiency of a street 

 railway motor at various loads (current intakes). A street rail- 

 way motor is run for a great portion of the time at about J full 



