INDUCTANCE AND CAPACITY 



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speed rises, the resistance a will be short-circuited for a greater 

 percentage of the time, thus increasing the average strength of 

 the field and, in consequence, bringing the speed back to its 

 original value. 



The contact is controlled by the electrically driven fork and 

 is so arranged that it is closed for half a complete period, that is, 

 for half the time. Contact 2 is a slip ring, which is electrically 

 connected to the half-ring 3; thus its circuit is also made for 

 half the time. As contacts 1 and 3 are in series, the resistance 

 a is short-circuited only when both contacts 1 and 3 are made. 



In the normal running position, switch 3 lags 90 behind switch 

 1 in time phase. Thus the resistance a is short-circuited one- 

 fourth of the time and is in circuit three-fourths of the time. 



FIQ. 254. Diagram for Wenner speed controller. 



If the motor tends to run too fast, it gains on the fork in time 

 phase and the short-circuit around a is closed for a longer time, 

 thus increasing the average strength of the motor field. This 

 slows down the machine. On the other hand, if the motor tends 

 to run too slow, the resistance a is short-circuited for a shorter 

 time, the average strength of the motor field is decreased and the 

 speed comes back to its original value. The resistance b is used 

 in synchronizing, and should be about three-fourths of the re- 

 sistance a. The machine is started with resistance b in circuit. 

 When synchronism is indicated by the lamps, b is replaced by a. 

 The rotating switches 4 and 5, which are in electrical connection 

 with the slip ring 2, are so set that when the motor and the fork 

 are in synchronism the lamps are of equal brilliancy. If the 

 motor tends to hunt, it will be shown by the ammeter. 



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