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

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Fig. 2. Schematic of the Converter. with the voltage. The current in the primary of Tl will lag approximately 90° since it is predominantly inductive. The secondary voltages of Tl and T2 will thus be 90° out of phase and if the secondary voltage of Tl is made 230 v and that of T2 is 200 v, then the voltage between the lead marked Phase 1 and either Phase 2 or 3 will be: £ri/2 + J'ETZ or 230 v. As combined in the load, the currents in all three phases are approximately 120° apart. From the above it becomes apparent why such circuits require adjustment to a specific load condition since a variation in either phase or voltage of the secondary of T2 will upset the three-phase balance and both factors will vary with any change in impedance or inductance of the load. It also becomes apparent that this correction must provide a separate adjustment of the capacitor to match the load inductance in addition to a voltage correction for T2. Developed Circuit Development of the basic circuit for actual use is shown in Fig. 2. The transformers Tl and T2 appear in the same form as in Fig. 1, but the single capacitor is replaced by six units (C2 to 7) of such sizes that they provide any value from 1 to 60 juf in \-yS. steps and are readily connected as required by means of individual switches. These are oil filled a-c capacitors rated at 330 v. An additional capacitor shown as Cl is an a-c electrolytic unit of 100-/zf capacity normally connected in parallel with the others through the relay SI. This is a necessary feature since the impedance of a synchronous motor is very much lower at the instant of starting than when running; therefore, the capacitance required to approach resonance at line frequency is several times greater at start than is desirable for running phase balance at even maximum load. Unless this initial high capacitance is provided, the output is essentially single-phase and the motor will not start. Gl meets this condition for the short start time and is automatically disconnected by the relay SI when the voltage across the primary of T2 reaches 100 v. The current supply July 1952 Journal of the SMPTE Vol.59