Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, April, 1930 Page 25 interlocking' action between the motors. For any given load on one of the motors, its rotor will assume a definite phase angle with the rotor of the distributor, the magnitude of this angle depending upon the load. There is therefore an e.m.f. acting on the circuit of these two rotors and a synchronizing current will flow, causing a synchronizing torque which will hold the two rotors in this same relative position. The action is similar to the displacement between a synchronous motor and a generator. The angular displacement, for the conditions under which the motors are used, is at the maximum thirty electrical degrees, or fifteen mechanical degrees. For all practical purposes this displacement is negligible as it corresponds to only about two-thousandths of a second. As we will see later the speed of the d-c. driving motor is held very constant and with this system the speed of the camera and recorder motors is independent of line frequency, voltage variations and of load. Signal for Starting When a "take" is to be made, the signal "interlock" is received in the recording room, where the motor controls, the distributor and the d-c. driving motor are located. One phase of the supply line is closed, locking the motors in a stationary position. A synchronizing start mark is made on the picture films, on the sound films and on the wax disc. The signal "start motors" is received in the recording room when all cameras are ready and the motors are started by closing the third line of the 3-phase supply and starting the d-c. motor. The speed of the d-c. motor is brought up to 1200 r.p.m. by means of its automatic control and when stability is reached a switch is closed, lighting a red light in the monitor room and on the stage, signaling that action is to begin and that quiet must be maintained on the stage. The control system for constant speed of the d-c. motor is complicated but consists essentially of a shunt regulating field winding in which the current is governed by a vacuum tube control circuit. In Fig. 2, is shown the diagram of the d-c. motor circuit and of the field control circuit. This is somewhat simplified as there is omitted the special series field used for starting and automatically cuttingout before the motor reaches full speed. The motor is called upon to start under a heavy inertia load, as it must accelerate the rotors of all the induction motors. In addition to this inertia load, it must start under full mechanical load as the induction motors in this system have no torque at standstill and will not develop any torque until the distributor is set in motion and an angular displacement exists between its rotor and the rotors of the motors. For these reasons, a special starting field is used, in addition to the main series winding. As the system accelerates, the torque required of the d-c. motor is greatly reduced and its starting series winding is cut out. The motor is coupled to a small alternator of 720 cycles frequency at 1200 r.p.m. The d-c. armature is tapped at two opposite points through slip rings, giving an a-c. supply of 720 cycles at 1200 r.p.m. for the filament and plate circuit of the vacuum tubes. The control circuit is governed by two low-pass filters, Fi and F2, which have a 720 cycle cutoff. As the motor comes up to speed, the alternator frequency will be below 720 cycles, that is, below the cutoff frequency and a voltage is applied to the plate of tube Ti which produces a comparatively high negative potential on the grids of tubes To and Ta by reason of the voltage drop across Ri. Therefore the space current of T» and T.i through the regulating field is small and the motor continues to speed up. When the speed reaches 1200 r.p.m., the 720 cycle current is regulated by the frequency cutoff characteristic of Fi and F» and the space current of Ti is decreased, reducing the negative grid potential of T,. and T3. This allows a larger flow of current from T2 and T3 through the regulating field and the rise in speed is checked. A feedback circuit for tube Ti is provided. This circuit is made up of R5, R8 and RT and C3 bridged across the regulating field terminals. This circuit gives a compensating action for the static fluctuations. If a governor is too sensitive it will constantly "hunt" about the speed for which it is set. In this case, the governing action is not made too sensitive and the small fluctuations are compensated for by means of the feedback circuit, the speed of the motor being held constant to within 0.1%. Recordings Combined on Film The picture film and the sound film are developed and the two films are used together to obtain the single sound picture film. The sound track is in the form of a band a tenth of an inch wide along the inside edge of the perforations. At the present time disc records are not as much used as sound films for reproduction purposes. Their main use is for "playbacks," that is, as soon as the wax record is made it is reproduced directly without further process. This enables the directors and actors to judge immediately the dramatic effects and quality of a recorded scene without having to wait for the film or record to be processed. This has proved to be a valuable aid in the production of sound pictures. Let us now follow the reverse process, that is, the reproduction of sound from the film. The sound film is run through a standard projector modified by the addition of the sound reproducing attachment which is located between the head mechanism and the lower take-up magazine. A light beam of high intensity is concentrated, by an optical system containing a slit, and focused on the sound track of the film. The spacing of the light bands along this track determines the pitch of the sound while the varying density of these bands determines the quality and loudness. In back of the film is a photo-electric cell which generates a small current whose variations correspond to the variations of the light intensity through the sound track, and therefore to the variations of the original sound. This current is amplified and carried through a volume control, or fader, from which it is fed to the loudspeaker horns. In the disc method the process is similar to a phonograph with an electric pickup. The disc, however, is much larger and revolves at the slow 720 ~ ALTERNATOR D.C. R4 MOTOR FIG. 2 Control circuit arrangement for insuring constant film speed.