POWER LOSSES IN GENERATORS AND- MOTORS. 141 



other words, with the intake of current by the motor, and that 

 the stray power loss varies also with the speed. 



69. Efficiency of conversion and mechanical efficiency of a motor. 



The total electrical power, EJ& delivered to a motor is used : 

 (a) to supply the power lost in heating the field windings, and to 

 supply the armature loss, RJ ; and (&) to force the current 

 through the armature in opposition to the induced electromotive 

 force E a . The part (&) is equal to E<J a and, according to 

 Lenz's law, it is all converted into the actual mechanical power 

 which supplies the stray power loss and the useful power deliv- 

 ered at the pulley. The ratio of the mechanical power, EJ a , 

 developed in the armature to the total electrical power, El, 

 delivered to the motor, is called the efficiency of conversion of the 

 motor. That is : 



Efficiency of conversion = -~j (33) 



A portion S of the total mechanical power developed in the 

 motor armature is used to overcome bearing friction, air friction, 

 and magnetic friction, and the remainder, \EJ a S~\ , is delivered 

 at the motor pulley as useful power. The ratio of the useful 

 power delivered by a motor, to the total mechanical power 

 developed in the motor armature, is called the mechanical efficiency 

 of the motor. That is 



E T S 

 Mechanical efficiency = -^-j|-y-^ (34) 



a a 



The actual or commercial efficiency of a motor is equal to the 

 product of its efficiency of conversion and its mechanical efficiency, 

 inasmuch as the efficiency of conversion is the fractional part of 

 the delivered electrical power which is converted into mechanical 

 power, and the mechanical efficiency is the fractional part of this 

 total mechanical power which is available at the motor pulley. 



70. Variation of the efficiency of shunt and compound motors 

 with load. The shunt field loss and the stray power loss of a 



