274 



ALTERNATING CURRENTS 



motor mains. When the current in the arc is passing through 

 its zero values, the arc emits but little light. Therefore, during 

 these periods the sectors on the disc are but dimly illuminated. 

 In one-half cycle the armature of the motor would advance one 

 pole if there were no slip. During this time each black sector 

 would advance to the position just occupied by the adjacent 

 black sector which preceded it. The same is true of the white 

 sectors. During the period of advancement the sectors are but 

 faintly visible because the current in the arc is passing through 

 zero. Each black sector and each white sector is not, therefore, 

 clearly visible until it has reached the position just occupied by 

 the sector of the same color just preceding it. As the disc is 



\v 



Line 



Fio. 252. Stroboscopic method for measuring slip. 



intensely illuminated twice every cycle, while the arc current is 

 passing through its maximum values, all the sectors are clearly 

 visible twice every cycle. Therefore, if the disc rotated at 

 synchronous speed it would appear stationary. Due to the fact 

 that each conductor on the rotor does not advance one pole each 

 half cycle, the sectors will not reach the position of the next 

 adjacent sector of the same color, but will fall short of this 

 distance, due to the slip. The sectors on the disc will then 

 appear not stationary, but will seem to be rotating slowly back- 

 ward. The number of revolutions per minute that they appear 

 to rotate is the revolutions slip of the rotor. Figure 252 shows 

 a stroboscope for a four-pole machine. Occasionally, black and 

 white stripes are painted on the face of the pulley, Fig. 252, to 

 serve the same purpose. 



