212 ALTERNATING CURRENTS 



F. Blanc * finds that for well-constructed motors of ^ to 300 h.p. 

 the friction loss may be represented by the formula 



friction loss = C\/h.p. x D 



D being the rotor diameter in cms., and C a constant whose value 

 ranges from 20 to 30. 



125. Efficiency Tests. Methods of Meunier, 

 Sumpner, and Weekes 



A very accurate method of determining the efficiency of two 

 motors of about the same output is the following modification of the 

 well-known Hopkinson test for dynamos, which appears to have 

 been first suggested by M. P. Meunier,f and independently by Dr. 

 Sumpner and Mr. E. W. Weekes,| who have tested the method 

 practically with very satisfactory results. The arrangement of con- 

 nections is shown in Fig. 138. The two motors are belted together, 

 the sizes of the pulleys being different, so that while one of the 

 machines which has the larger pulley is running as a motor, the 

 other which has the smaller pulley is being driven mechanically 

 above the speed of synchronism. It will be shown ( 129) that 

 when the rotor of an induction motor whose stator is connected 

 across the supply mains is driven above the speed of synchronism, 

 the motor will act as a generator, supplying power to the mains. 

 Thus one of the machines shown in Fig. 138 will act as a motor, 

 receiving power partly from the mains, partly (the greater amount) 

 from the second machine, which acts as a generator. The stator 

 terminals of the generator are marked GI, Gg, G 3 , those of the motor, 

 MI, M 2 , M 3 . The difference in the diameters of the pulleys must 

 be sufficient to give slips corresponding to the maximum load 

 when the rotors are short-circuited. Smaller loads may be obtained 

 by introducing suitable resistances into the rotor circuits. Let 

 w = power supplied from mains (algebraical sum of readings of 

 wattmeters Wi and W 2 in Fig. 138). This power is required to 

 make up for the total losses. Now, one source of loss is that 

 represented by the power w b required to bend the belt and drive it 

 against air friction. In order to find w^, the wattmeters Wi and W 2 

 are read (1) when both stators are across the mains, and the rotor of 

 the motor is short-circuited, driving the open-circuited rotor of the 



* Elektrotechnische Zeitschrift, vol. xxi. p. 131 (1900). 

 t Eelairage Elecirique, vol. xxxiv. p. 228 (1903). 



% Electrical Engineer, vol. xxxiii. p. 918 ; also vol. xxxiv. p. 310 (1904). 

 An allowance can easily be made for the power wasted in the regulating 

 resistances. 



