EFFICIENCY TEST 



213 



generator by belt; and (2) when both machines are running light 

 with the belt off. The difference in the power drawn from the mains 

 in the two cases gives w b . Thus w w b gives us the power actually 

 wasted in the machines. Part of this waste will occur in the motor, 

 the remainder in the generator. Now, since the motor is necessarily 

 more heavily loaded than the generator, it will waste a larger amount 



w 



M 



G, G t C, 



f MOTOR J 



KErURATORJ 



FIG. 138. Efficiency Test of Two Similar Induction Motors. 



of power; the power wasted in each machine may be taken to be 

 proportional to the slip. Thus, if s m , s g stand for the slips of motor 

 and generator respectively, the power lost is 



and 



w w b 



# T s g 



W W b 



. , 

 s, n m the motor 



s in the enerator 



Now, if w g = power developed by generator (algebraical sum of 

 readings of W 8 and W 4 in Fig. 138) and transmitted to motor, the 

 total power received by motor is w g + w w b . On subtracting 

 from this the power wasted in the motor, we obtain the useful 

 power transmitted to the generator, and on dividing this latter by 

 Wg + w w b , we obtain the motor efficiency. Similarly, the gene- 

 rator efficiency is obtained by dividing its useful output w g by the 

 power transmitted to it by the motor.* 



* Having subtracted to 6 from the power drawn from the mains, we deal with the 

 machines as if they were connected by means of a perfectly pliable belt which 

 encounters no resistance in its motion through the air. 



