192 



BELL SYSTEM TECHNICAL JOURNAL 



at present provided would be desirable for ordinary motion pictures 

 as well as synchronized pictures. 



D.C. Control Circuit 



A circuit very similar to the one just described is employed in the 

 case of the D.C. motor. Fig. 8 shows a photograph of this motor 

 and its control cabinet. The circuit is shown in Fig. 9. It differs 

 from the A.C. circuit in that an auxiliary regulating field winding is 

 employed on the motor instead of a variable reactor. The source of 

 power for the plates of the vacuum tubes is obtained from the auxiliary 

 720-cycle generator instead of from a 60-cycle transformer as in the 



SHUNT FIELD, I oS^5 

 I SERIES I I '-' UZ 



! I L_b^ i 



COMMUTATOR 



I LJ , , — rj^j 1 



I 1- ^r^ij-J ALTERNATOR 



jRESULATING ] FIELD 



O.C. CONTROL CABINET 



Fig. 9 — D. C. control circuit diagram. 



A.C. circuit. Since a strengthening of the field of the D.C. motor is 

 required in order to reduce the speed it is necessary to reverse the 

 phase relationship of the transformer Ti, so that the current in the 

 detector tube decreases at speeds above 1,200 instead of increasing 

 as in the case of the A.C. circuit. 



The operation of the circuit is as follows: When the line switch is 

 first thrown the motor acts as an ordinary D.C. shunt motor and 

 accelerates. At low speeds the output from the 720-cycle generator 

 is low and consequently there is no plate voltage supplied to the tubes 

 and no current through the auxiliary field winding. The field is, 

 therefore, weak and the motor speeds up. This condition is main- 

 tained until the equilibrium speed of 1,200 R.P.M. is approached. 

 The phase angle of the voltage supplied to the grid of the tube F3 is 

 then in phase with the voltage supplied to the plate so that the grid 

 of the tube goes positive at the same time that the plate is positive. 



