EFFICIENCY TEST 215 



no losses whatever are occasioned by the belt drive, and that the 

 total waste of power in the machines is 



w w b 



Although we have explained in detail how to apply the correction 

 due to belt slip, we may state that in most cases this correction is 

 extremely small, and need only be taken account of at heavy loads 

 and where the highest possible degree of accuracy is required. 



126. Alexanderson's Method 



The following approximate method of testing induction motors 

 has been devised by Mr. E. Alexanderson,* and is used by the General 

 Electric Co. of America. The principle of the method is based on 

 the fact that for ordinary loads the slip is practically proportional 

 to the torque. The load consists of a separately excited continuous- 

 current generator belted to the motor. The useful output of the 

 generator is measured by means of a voltmeter and ammeter, and 

 is equal to Vt (V being the p.d. and i the current) ; its total output 

 is Vi + n' 2 , ? being the armature resistance. The slip s of the motor 

 corresponding to this load is measured (by the stroboscopic method 

 of painting a number of wliite sectors on the pulley equal to the 

 number of poles, and illuminating the pulley by an alternating arc 

 supplied from the same circuit as the motor). Let w = total 

 mechanical power of motor, and w\ = power required to balance 

 losses due to rotor bearing friction and windage, belt losses, and 

 frictional and core losses of generator; and let T and TI be the 

 torques corresponding to the total power w and the losses MI 

 respectively. Then obviously 



T 

 The ratio ,-p- is determined by switching off the load from the 



continuous-current generator, but maintaining its excitation, and 

 finding the new value i of the slip which corresponds to the 



T s 

 torque TI. We then have rir = - approximately, and (1) becomes 



ll 6'i 



w s ~ , . . i-i ,w w\s s\ 



= - Subtracting unity from each side, we get - = - - t 



Wi Si Wi 81 



* Electrical World nd Engineer, vol. xliv. p. 212; or The Electrician, vol. liii. 

 p. 869 (1904). 



