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BELL SYSTEM TECHNICAL JOURNAL 



1700 r.p.m. a belt drive is used to reduce the motor speed of 1800 r.p.m, 

 to the proper value; small adjustments in speed being provided by- 

 means of a F pulley having an adjustable diameter. 



In order to insure continuity of operation in case of failure of the 

 power supply the generator is also arranged to be driven by a d-c. 

 motor equipped with a mechanical governor. This motor is supplied 

 from the central office battery and is automatically switched thereto 

 in case the voltage of the commercial power supply falls below a 

 predetermined value. 



In the second method the VF carrier-supply unit is governed by 

 means of a vacuum-tube circuit whose output is controlled by fre- 

 quency variations in the highest frequency channel, and the resulting 

 d-c. current is applied to the motor field. The operation of this device, 

 which has been described elsewhere,^^ may be briefly explained as 



Fig. 19 — Principle of tuned-circuit speed-regulator. 



follows: It depends for accomplishing its purpose on producing varia- 

 tions in the strength of the current through an auxiliary regulating 

 field associated with the motor which drives the multi-frequency 

 generator. The essential features are shown in Fig. 19. The action 

 is as follows: The voltage of channel 12 (2295 cycles) of the generator 

 whose speed is to be controlled is applied simultaneously through a 

 divided circuit to the input and output of tube V^ which may be called 

 the phase-detector tube. The plate voltage is applied directly through 

 transformer 7*3, there being no B battery in the ordinary sense. The 

 grid voltage on the other hand is applied through the bridge W, one 

 of the arms of which is an anti-resonant circuit tuned to 2295 cycles. 

 The result is that the magnitude of the grid-filament voltage and its 

 phase relative to the plate voltage are dependent on frequency. When 

 the latter has its correct value the anti-resonant arm exactly balances 

 the bridge, and the a-c. voltage across T^ is nil. At higher frequencies 



