432 , ELECTRICAL MEASUREMENTS 



60 complete vibrations a second. When the alternator gives 

 exactly 60 cycles, the contacts KI and K 2 will be made at a definite 

 point in the voltage wave and no effect will be produced. If, 

 however, the direct-current motor tends to speed up and drive 

 the alternator a little above synchronism, the contacts KI and 

 Kz are made when the electromotive force has a higher value 

 and a larger current will flow from the secondary of the trans- 

 former through the lamp. This throws an additional load on the 

 motor, the magnitude of which is dependent upon the departure 

 from synchronism. 



As there is a resistance in the armature circuit of the motor, 

 this load slows the machine down and brings it back to synchron- 

 ism. Thus by a series of small and rapidly applied loads (120 

 a second) the motor is held in check. If the motor tends to slow 

 down, the contact is made when the voltage is smaller, the load 

 on the motor is decreased and the speed maintained constant. 

 Motors up to 1 kw. may be controlled in this manner. As with 

 all adjustable tuning forks, the weights W should be set exactly 

 opposite each other. The adjustment of the contacts is impor- 

 tant. When the fork is at rest, the contact springs should be 

 equally distant from KI and KZ, about ^2 m - To reduce the 

 sparking, the condensers C\ and C 2 are shunted around the breaks 

 at KI, KZ, and K 3 . The binding posts at C\ allow the insertion 

 of additional capacity around the breaks K\ and Kz, if this be 

 found necessary. The transformer is used in order to separate 

 entirely the load from the synchronizing current, this being 

 necessary in order to avoid leakage, etc. Usually a 1 : 1 ratio is 

 employed, but when the larger sized motors are used the sparking 

 at the contacts may be reduced by transforming to a higher vol- 

 tage, 1:2, and using a high-voltage lamp for the load. 



The Wenner Speed Controller. This device enables motors 

 of 5 kw. and under to be controlled by an electrically driven 

 tuning fork of the desired frequency. The connections are 

 indicated in Pig. 254. 



When the extra field resistance a is in circuit, the field current 

 is cut down and the motor increases in speed, while if a is short- 

 circuited, the field is increased and the speed decreased. The 

 fundamental idea is to arrange a special rotating switch and a 

 contact controlled by the tuning fork, so that when the motor 



