MEASUREMENT OF SLIP 209 



comparable with the slip, and may even exceed it considerably at 

 light loads. Special methods have, therefore, been devised for 

 measuring small values of the slip. 



In one of these, the shaft of the motor carries a cardboard disc 

 divided into a number of sectors, alternately black and white, the 

 total number of sectors being equal to twice the number of poles. 

 The disc is illuminated by means of an arc lamp, which derives 

 its current from the same source as the motor. Now, an alternating- 

 current arc is extinguished twice during each complete wave of 

 current, so that the light coming from it will in reality consist of a 

 succession of flashes. If we suppose that the motor is being driven 

 at synchronous speed, then since the time taken by a white sector to 

 move into the position of the white sector next in advance of it is 

 equal to a half-period, it follows that as each flash reaches its 

 maximum brightness the white and black sectors are in the same 

 relative positions, and thus the disc appears to be stationary. If slip 

 is allowed to take place, the positions of the sectors will be retarded 

 relatively to the flashes, and the disc will appear to rotate.* By 

 counting the number of apparent revolutions of the disc during any 

 convenient interval of time, we obtain the number of slip revolutions. 

 If we at the same time determine the number of revolutions of the 

 motor (during the same interval of time, while counting the slip 

 revolutions), then the slip is given by 



slip revolutions 



motor revolutions -f- slip revolutions 



Instead of a cardboard disc divided into white and black sectors, 

 a simple radial chalk line drawn across the web of the pulley is 

 sufficient, provided the pulley is illuminated mainly by the light of 

 the arc. The experiment may be conducted in daylight. 



A disadvantage connected with the use of the arc is that as it 

 draws a fairly heavy current from the mains, it is liable to unbalance 

 the three-phase system, so that the phase p.d.'s across the stator 

 windings are not alike, and the motor is not running under entirely 

 normal conditions. 



Samojloff f has, however, found that if the rotating disc be in 

 darkness, an incandescent lamp (whose periodic partial extinctions, 

 due to alternate heating and cooling of the filament, are ordinarily 

 quite imperceptible to the eye) may be successfully employed for the 

 same purpose. With an incandescent lamp, which only takes a 

 small current, no serious unbalancing can, of course, take place. 



Although by the above means small values of the slip are 

 easily determined with a high degree of accuracy, the method 



* In a direction opposed to the direction of rotation of the motor, 

 t Annalen der Physik, 3. 2. p. 353 (1900). 



P 



