International projectionist (Jan-Dec 1936)

I. P. has received a communication from Mr. J. K. Elderkin, of Forest Manufacturing Corp., in which is set forth some pointed critical comment of an article1 by Mr. C. C. Dash, of Hertner Electric Co., contributed originally to the S.M.P.E., which discusses various units of power supply for projection arcs. In line with its well-established policy of providing a forum for the expression of opinion by those who have something interesting to say on technical matters, I. P. presents here Mr. Dash's paper and Mr. Elderkin's comments thereon. Needless to say, further comment by either of the aforementioned gentlemen, or anybody else, on the matter under discussion is invited. — Editor. Sources of D. C. Supply for the Suprex H. I. Arc Two Comments on Relative Efficiencies of Various Power Supply Units By C. C. DASH HERTNER ELECTRIC COMPANY THE introduction of the non-rotating, high-intensity reflecting arc has presented a problem to the electrical manufacturers in the production of a satisfactory source of d. c. for use with this arc, the operating characteristics of which are quite different from those of the previous types used in motion picture projection. The new arc is much more susceptible to changes of voltage in the d. c. source than the old arc, hence must be handled more carefully, and more precautions must be taken in selecting current supply equipment. (A) D. c. power service from central station. (B) A. c. to d. c. converting equipment. (1) Non-rotating equipment: (a) Hot cathode tube rectifiers. (b) Copper-oxide rectifiers. (2) Rotating equipment: (a) Synchronous converters. (b) Motor -generator sets. ( 1 ) Generator for each lamp, drooping characteristic, no ballast. (2) Generator, flat-compounded using ballast. A.— D. C. LINE SERVICE When d. c. service is obtained from a central station, the auxiliary equipment used with the arc need be only properly designed ballast rheostats having a sufficient voltage-drop to limit the current to the proper value at the correct arc voltage. In the average theatre installation the voltage delivered by the central station usually increases at about nine o'clock p. m. and it is necessary to have some easily operated regulating device upon the ballast rheostat so that the resistance can be changed in order to compensate for this increase in supply voltage. When considering the overall efficiency of using d. c. from a central station, it must be borne in mind that the current cost per kilowatt hour from such a source is usually much lighter than that paid per unit when the power is obtained from a. c. mains. For an arc voltage of 35 volts and a current of 50 amperes, the total power drawn from the d. c. line at 115 volts is 5750 watts, of which 1750 would be utilized in the lamp and 4000 consumed in the ballast resistance in the form of heat. The overall efficiency, therefore, is 30.4 per cent. The inrush of current when the arc is struck, using the form of supply, is but very little greater than 60 the normal operating value, and does not exceed 10 to 15 per cent more than the normal operating current. The power equipment in the central station or in a central system usually consists of a multiplicity of types and sizes of generators. In some instances storage batteries are floated across the d. c. line, so that there is practically no ripple and the current is practically true d. c. The noise in an arc is a function of the ripple in the d. c. supply, and inasmuch as there is no ripple with such an arrangement, there is practically no hum in the arc. The current through the arc is very steady, due to the large ballast drop, but the results attained with a meter for measuring the steadiness of the light indicate considerable variation of the luminous intensity upon the screen due to the fact that the arc voltage can vary considerably without a compensating change in the current, and thereby affect the light output. The operating characteristic when working on the d. c. power FIGURE 1 50 *40 Volt-ampere charac ^ teristic of the hot cathode tube rectifier 20 10 1 "An Investigation of Sources of D. C. for the Non-Rotating, High-Intensity Reflecting Arc," S.M.P.E. Journal, XXVI (Jan., 1936) No. 1, p. 79. 5. 10 20 30 40 50 AMPERES 60 70 [14]