777 /: INDUCTION 



275 



A mechanical-electrical method of measuring slip is shown in 

 Fig. 2">:*. Two cylinders of- insulating material are driven, one 

 by the induction motor shaft and the other by a small synchron- 

 ous motor having the same number of poles as the induction 

 motor. Each of these cylinders is fitted with a slip-ring, to which 

 a small contact piece is connected. The synchronous motor 

 always runs at the speed of the rotating field. Therefore, every 

 time the induction motor slips one revolution, the contact pieces 

 touch each other, closing the circuit between the two slip-rings. 

 This is indicated by a flash of the light connected in series with 

 the rings through the brushes 6, Fig. 253. 



In the Electrical Engineering Laboratories at Harvard Uni- 

 versity, the induction motor and the synchronous motor jointly 



-Measurement of slip by means of synchronous motor. 



drive a differential through gears, a method developed in these 

 laboratories. The speed of the differential is the revolutions 

 slip of the induction motor. If desired, the speed of the differ- 

 ential, and hence the slip, may be measured with a speed counter 

 with considerable accuracy. By changing invars the apparatus 

 i< adapted to machines having any number of poles. 



113. The Induction Regulator.- Without auxiliary apparatus. 



it i- practically impossible to maintain the proper voltage at all 



the distribution )> B system. becaUM \\ith a tivd voltage 



at tin- -fation bus-bars, the voltage at the ends of short feeders 



will ordinarily be greater than \\\< 6 al the end> of long 



feeders. <)\\ing to the ohmic and reactive drops in the lino, 



oltagC at the load end of the feeder may vary considerably 



with the ] p. In order to maintain a more oon- 



voltage at the distribution point, without using an e: 



