International projectionist (Jan-Dec 1936)

18 INTERNATIONAL PROJECTIONIST September 1936 50 *• 51 tt kS*~ 40 "5: ""*• ^ ' , "" 50 'y/* R ~£c »* Sj "i :i. Am; ?« •» Output 10 60 70 SO 30 40 50 FIGURE 3 Volt-ampere output characteristics of standard 65-ampere copper-oxide rectifier current rating indicates the number of parallel groups of disks required in each leg of the bridge circuit. The problem of design would be simple except that consideration must be given to the so-called "aging" characteristic of the copper-oxide element.1 This can be defined as a gradual increase of resistance of the unit in the "forward" direction which tends to stabilize after 4,000 to 5,000 hours use. Aging is a function of temperature as well as of time, and the higher the operating temperature the greater will be the change of resistance before stabilization takes place. It is at once apparent that in order to maintain the initial output of a unit it will be necessary to increase somewhat the applied a. c. voltage after aging has taken place. Care must be taken in the design to make sure that the final applied a. c. voltage necessary to maintain the rated output after aging will not exceed a safe value for the particular disk combination under consideration. Fortunately, sufficient data have been collected over a period of years to predict with reasonable accuracy the amount of aging that will take place for any given conditions of temperature. In the commercial design (Fig. 2) eight taps are provided on each transformer secondary winding, making it possible to adjust the applied a. c. voltage in steps of approximately 2 volts. This serves the dual purpose of permitting a wide range of output adjustment and a ready means to compensate for aging. In Fig. 4 is shown a family of curves giving the d. c. volt-ampere output regulation for each of the eight secondary taps on a standard 65-ampere unit. These curves illustrate the inherent regulation of this type of rectifier that makes it possible to operate without any form of external ballast in the arc circuit. It will be seen that the entire range of output, from 40 to 65 amperes, 30 to 35 volts, can be covered with the five lower taps, leaving three additional taps to compensate for aging, an amount that experience shows is more than ample. C.-O. Operating Efficiency The efficiency of a copper-oxide rectifier is regarded as the ratio of the d. c. watts' output to the a. c. watts' input. The losses in the rectifier consist of resistance losses in the forward direction through the copper-oxide elements, leakage in the "blocking" direction, and transformer losses. In the case of the fan-cooled motion picture rectifier, the power consumed by the fan motor and the control relays should be added to the input to obtain the true over-all efficiency. Resistance losses in the copper-oxide elements represent the major part of the total losses. It has been previously shown that this resistance tends to increase with age up to a certain stabilizing point. It is at once obvious that aging tends to reduce somewhat the initial efficiency. There is no rule-of-thumb method of stating the new and aged efficiencies of any rectifier, because the difference depends upon the ratio that the rectifier resistance bears to the total impedance of the rectifier-load circuit. If the rectifier resistance is a small part of the total impedance of the circuit, then a considerable change in rectifier resistance will mean only a slight change in the total impedance of the circuit, and consequently only a slight change in efficiency. Aging being a function of temperature, an adequate system of forced ventilation will permit operating at a considerably higher current-density per unit bridge than would be the case with the conventional air-cooled type of unit. Tests on fan-cooled units operating at various current densities continuously since 1929 have given the necessary data for establishing safe limits with respect to current density and temperature. With a fan-cooling system capable of limiting the temperature rise of the copper-oxide elements to a maximum of 2 or 3 degrees C, a maximum current density of 2 amperes per unit bridge (3phase) appears safe. It may be concluded that under these conditions it will not be necessary to apply an a. c. voltage in excess of 8 volts per unit bridge in order to maintain an output of 7 volts d. c. per unit bridge after aging has taken place. Figure 4 shows the over-all operating efficiency of a standard 65-ampere unit (new) and includes the power consumed by the fan motor, relays and protective switch. Fig. 5 shows the upper and lower limits of the predicted efficiency after aging has taken place. It should be kept in mind that several thousands of hours of continuous use are required before the aging begins to approach stabilization, and this, measured in terms of theatre service, is a matter of years. Inspection of Fig. 4 indicates a somewhat higher efficiency at 40 amperes than at the full-load point of 65 amperes. By increasing the number of parallel groups, and in this way reducing the current-density per unit bridge, it would be possible to make the point of maximum efficiency coincide with the full-load point on the curve; but the increase in size and cost of the unit would more than offset any possible advantage from the slight gain in efficiency. On the other hand, any attempt by the designer to economize on materials by reducing the number of parallel groups and increasing the currentdensity per unit bridge will mean increasing the impressed voltage to a point in excess of the maximum safe limit of 8-volts per unit bridge, thus introducing a risk of possible breakdown by puncturing the oxide film. Long Life Forecast It has been frequently stated that a copper-oxide rectifier properly applied will last indefinitely. Factory life-tests now running into the tenth year as well as hundreds of different industrial applications all tend to bear out this claim.t The percentage of troubles in the field has been gratifyingly small, and such troubles as have occurred are usually traceable either to misunderstanding the operation of the unit or to overloading due to inadequate wiring and equipment. There are at the present, time upward of 600 G. E. motion picture type copper* oxide rectifiers in the field, furnishing ample proof of the acceptance of this type of equipment by the industry. Good engineering and strict adherence to the design limits described above should result in a high degree of reliability, exceptionally long life, and freedom from trouble. Aging, if given proper consideration when designing the rectifier, will result in only a small reduction in operating efficiency over a period of years. A careful check of a number of units in t "Now there are operating on test hundreds of units of various types from 1 to 10 years old, all of which have Riven the same answer, namely, that the life of the rectifier when properly applied is indefinite. The performance with time of a Rectox used to operate a printing telegraph will serve as an example. In 3 years of continuous operation at full load, the output has dropped off only 7 per cent. Tests made on a standard 3-phase elevator-control c.-o. rectifier, so far covering 27.600 hours of operation, show similar results. The rectifier was connected directly to the a.-c. line without intervening resistance, and no adiustment of vo'.taee or load is ever made. In a 7^2 -year test, or 65.000 hours, a number of 2-ampere, 6-volt battery charters are still delivering rated output, with no indication of any limit to the life of the rectifier."— Smith, I. R.: "Copper-Oxide Rectifiers for Motion Picture Arc Supply," /. Soc. Mot. Pic. Eng., (Sept., 1936) p. 331.