International projectionist (Jan-Dec 1936)

TECHNICAL AND ECONOMIC ASPECTS OF COPPEROXIDE RECTIFIERS By C. E. HAMANN COMMERCIAL ENGINEER, GENERAL ELECTRIC COMPANY First to acquaint the field with the application of copper-oxide rectifiers as a source of power supply for projection*, I. P. now presents additional and more explicit data thereon. The appended paper, originally presented before the S.M.P.E., presents data on the design, construction and operating characteristics of c.-o. rectifiers which should make for maximum efficiency in operating these units. — Editor. THE application of the copperoxide rectifier as a d. c. power supply for projection is by no means new. A fan-cooled type of copper-oxide rectifier was developed by General Electric in 1930, and applied successfully to the low-intensity type of lamp as well as the hi-low lamp.1 With the advent of the Suprex type of arc an entirely new field has been opened up. The characteristics of the copperoxide rectifier have been found to be admirably well adapted to the special voltage and current requirements of the Suprex arc. In construction, the transformers and control panel are assembled as a unit and located in the upper part of the casing. The copper-oxide stacks, together with the air baffles and the blower system, are also assembled as a single unit and installed in the lower part of the casing. Aside from these two unit assemblies, the only other parts are the control relays and protective switch. It has been pointed out that singlephase rectification is not suitable for the Suprex arc due to the pronounced ripple in the d. c. voltage output.2 Hence, copper-oxide rectifiers for Suprex sup is not of sufficient magnitude to cause any discernible effect in the light upon the screen. Therefore, the additional cost of a filter reactance does not appear warranted, and is omitted in the commercial design. A typical circuit diagram of a 3-phase unit is illustrated in Fig. 1. The transformer connections are arranged deltadelta. Units for 2-phase service differ only in transformer design. A Scottconnected transformer changes 2-phase to 3-phase, so that the rectifier circuit and the output characteristics are identical to those of the 3-phase unit and therefore need not be discussed separaMy. Circuit Design Data An examination of the wiring diagram will readily disclose the general scheme of the rectifier circuit, but a brief explanation of the control circuit may be in order. It will be noted that the blower motor and the relay holding coils are energized from a separate 110-volt, single-phase circuit. The purpose is two-fold: (a) it provides a simple remote-control arrangement for starting and stopping the rectifiers, and (b) it A-C. S-Phase Supply 12 3 FIGURE 2 (a) Arrangement of single phase "unit bridge" cir + + J + J+ -L cuit; (b) arrange J+ i J+ -L ment of threephase unit-bridge circuit + H (a ■ + ) j , i . — r ■f + — ' ■ (&) I. P., 8 (March, 1935) No. 3., p. 15. 9 (Dec. 1935) No. 6, p. 18. Transformers ply have been developed for polyphase service only. The rectifier circuit is designed for full-wave rectification of all three phases of the a. c. supply, and the resulting d. c. output has a ripple of relatively low magnitude and high frequency (360 peaks per second for 60cycle, 3-phase). / The only noticeable effect of the d. c. ripple is a slight "sing" to the arc, which, it has been determined experimentally, can be eliminated by a small reactance filter in the d. c. circuit. However, visual inspection supplemented by photo-metric tests indicate that the ripple Relays FIGURE 1 Standard arrangement of internal wiring of three-phase D-c circuit copper-oxide rectifier for motion picture projection [17] l}Arc :}110v.a-c }Threa-phase. permits operating the rectifier on 2 phases in the event of failure of any one of the phases of the 3-phase a. c. supply line. In the design of a copper-oxide rectifier it is customary to base all calculations upon established data for the "unit bridge."1 For a single-phase circuit a "unit bridge" consists of four disks or elements arranged in a full-wave "bridge" circuit as shown in Fig. 2a. For a 3-phase circuit, with which this paper is chiefly concerned, the "unit bridge" consists of 6 disks arranged in a 3-phase full wave "bridge" as shown in Fig. 2b. Experience over many years has resulted in establishing certain standard limits of voltage, current, and temperature for the unit bridge. Thus, dividing the safe voltage limit of the unit bridge into the desired voltage rating indicates the number of disks that must be connected in series in each leg of the bridge circuit. Similarly, the current limit of the unit bridge divided into the desired