Journal of the Society of Motion Picture Engineers (1930-1949)

to the coil of SI is rectified to avoid chatter of the relay contacts at break. The coil of relay S2 is in parallel with that of SI through a resistance which prevents operation of S2 unless or until the voltage rises to 300 v. This provides protection for the running condensers in case the load is disconnected while the operating switch Dl is closed. Opening the output circuit of phase 1 allows the voltage of the resonant circuit in the primary of T2 to rise well above the condenser rating unless this protection is provided. S2 in turn operates the relay S3 which opens the input circuit and locks up in this position until the input potential is removed. Tap changing on T2 was considered as an alternative method of resonating the primary with a single condenser, but this was abandoned since tap changing in such a resonant circuit causes excessive arcing, and because adjustment equal to that obtained by six condensers and switches would require a sixty-point tap switch. Voltage adjustment for T2 may be obtained without phase shift by means of series resistance in the resonant circuit or by resistance shunted across the primary of T2, but either method results in serious PR losses. The Variac T3 when connected across the input line, as shown, functions as an efficient voltage divider and does not contribute a reactive component to the resonant circuit since the exciting current is supplied by the line. Also, the range of adjustment is wide and very smooth. The three small voltmeters Ml, 2 and 3 are the indicators used to determine and maintain phase balance. M2 is a 150-v meter connected across the incoming line which serves as a pilot meter on the single-phase supply, and also effectively indicates the voltage across phase leads 2 and 3 during operation since this voltage is twice the line voltage except where a heavy load may introduce appreciable PR loss in Tl. Ml and M3 are 300-v meters across the other two output phases. With this arrangement, both threephase output and single-phase input are shown in addition to the basic function of balance indication. Adjustment and Operation The selection of the proper capacitance in the primary circuit of T2 need only be determined once for any given motor and line frequency. Therefore, this information may be obtained in the shop before the camera goes on the set or location. Once obtained, the condenser values for different motors, or combinations of motors, may be tabulated and attached to the converter for ready reference. To determine capacitor value, the motor is connected for operation, preferably driving a camera or other normal load. The condenser values of 1, 2, 4, 8, 15 and 30 /-if are marked on the plate adjacent to the switches which connect them in parallel. Thus, the values shown are additive as the switch handles are toward the marked plate. About 25 /uf should be connected as a preliminary value of capacitance for 60-cycle operation (approximately 35 juf for 50-cycle). The Variac is positioned about center and the motor started by closing the "line" switch Dl. The two outside meters Ml and 3 are then observed and the Variac adjusted until they read alike. If this balanced reading is higher than the pointer position of the middle meter M2 (twice the indicated voltage) then the capacitance should be reduced, or vice versa, and Ml and 2 again balanced by Variac adjustment. This is continued until all three meters show the same pointer position. With very little practice this adjustment can be accomplished in less than a minute. Having established and noted the capacitor value the unit is ready for operation; further adjustment for load variation being made by changing the Variac to make the pointer of Ml read the same as A. L. Holcomb: Three-Phase From Single-Phase 35