International projectionist (July-Dec 1934)

December 1934 INTERNATIONAL PROJECTIONIST hour, or 1% inches in ten minutes; at 60 amperes, lO1/^ inches per hour, or 1% inches in ten minutes; and at 65 amperes, 13V2 inches per hour, or 21/4 inches in ten minutes. (2) The burning ratio between the positive and negative carbons of the 6and 7-mm. trim at 40 amperes is 1.8 to 1; at 45 amperes, 2.7 to 1; and at 50 amperes, 3.6 to 1. With the 6.5 and 8-mm. trim the burning ratio at 50 amperes is 1.6 to 1; at 55 amperes, 2.2 to 1; at 60 amperes, 2.6 to 1; and at 65 amperes, 3 to 1. The tests definitely established that the burning time and the ratio of consumption of the positive and negative carbons do not change whether the arc current be supplied by a single-phase rectifier, a polyphase rectifier, or a motorgenerator set, and that the design of the lamp also has no effect upon the aforementioned characteristics. The rate of consumption of the positive carbon is greatly affected by the current density; whereas the burning rate of the 6-mm. negative carbon is affected little if any by the current density between the limits tested. However, the burning rate of the 6.5-mm. negative carbon varied slightly with the current density, particularly for currents greater than 55 amperes. Although the operating limits of these carbon trims are generally understood to be 40 to 50 amperes for the 6 and 7-mm. trim, and 50 to 65 amperes for the 6.5and 8-mm. trim, the tests conducted by the Committee established definitely that best results are achieved when the trims are operated within the upper limits of their rated capacities. Correct Arc Gap (3) Tests were conducted to determine the effect of the arc-gap upon the burning time and the burning ratio of the carbons. If the arc gap be increased the current automatically decreases and the positive carbon then burns at a slightly lower rate. Likewise, if the arc gap be decreased, the arc current increases and the positive carbon burns faster. It is apparent, then, that since a change of current changes materially the burning time of the positive carbon, and only slightly affects the burning rate of the negative, any change of arc gap will change the current and thus the ratio of burning of the positive and negative. This change of ratio tends to move the arc out of focus with the mirror. It is of utmost importance, therefore, that the arc control mechanism be sensitive enough to hold the arc gap constant (5/16 to 11/32 inch), and that the current also be held constant if frequent focusing of the arc is to be avoided. Lamps having individual feed adjustments for positive and negative carbons, thus allowing the burning ratio to be changed, permit adjustment to any desired current density within the limits of the rating of the carbons. However, lamps having single-feed screws are necessarily limited in opera TABLE I Arc Voltages and Currents for 1-Mm. Pos. and 6-Mm. Neg. Suprex Carbons Source of D.C. Amperes Volts Three-Phase Rectifier 40 30 Single-Phase Rectifier 40 27 M-G Set 40 30.S Three-Phase Rectifier 45 32.5 Single-Phase Rectifier 45 28 M-G Set 45 33 Three-Phase Rectifier 50 34.5 Single-Phase Rectifier 50 29 M-G Set 50 35 tion to the current density, or burning ratio, for which the screw is designed. Deviation from the given ratio will entail constant attention and frequent manual adjustment. (4) In testing the 6 and 7-mm. carbon combination to determine what differences if any occurred in the voltage TABLE II z £ Bi '/f/VI/VG /?/ ]rs ofSu, ?/? EX CAh BOAS ^ * ^ pn.<;/T/i T CA/ffin V ft MM 1 ^ * ^--* 1 as cca r/ ISC C/l/fBi ?/V 6 S/vm £ * O ^ 0 Amper es SO 5 ? 5 4 Si i 5, 9 6 o 6 2 6 4 66 FIGURE 2. Burning rate of Suprex carbons; 6.5-mm. and 8mm. combination Arc Voltages and Currents for 8-Mm. Pos. and 6.5-Mm. Neg. Suprex Carbons Source of D.C. Amperes Volts Three-Phase Rectifier 50 30.5 Single-Phase Rectifier 50 27 M-G Set 50 31 Three-Phase Rectifier 55 32.5 Single-Phase Rectifier 55 28 M-G Set 55 33 Three-Phase Rectifier 60 34.5 Single-Phase Rectifier 60 29 M-G Set 60 35 Three-Phase Rectifier 65 38.5 Single-Phase Rectifier 65 M-G Set 65 39 across the arc when various sources of power were used, single and three-phase rectifiers and motor-generators were used. In each test the same lamp and the same carbons were used and the same arc gap was maintained under identical conditions. The results obtained are shown in Table I. It will be noted that there is a difference of 3% to 6 volts across the arc in the case of the single-phase rectifier, and a difference of % volt in the case of the three-phase rectifier, as compared with the motor-generator. This difference is due to the a-c. component of the rectified current. The d-c. voltmeter records only the d-c. value, and since the a-c. component is not registered, it is apparent that the greater the a-c. component, the greater will be the difference of voltage as measured with a d-c. voltmeter. Similar tests were made with the 6.5 and 8-mm. carbon trim, with results as shown in Table II. The tests indicate that the d-c. arc voltage for a given arc gap depends upon the source of the current; but with the 6 and 7-mm. carbons, the voltage will range between 30 volts at 40 amperes and 35 volts at 50 amperes; and in the case of the 6.5 and 8-mm. combination, between 30 volts at 50 amperes and 39 volts at 65 amperes — the figures for both trims being based upon a current supply of acceptable smoothness. Single-Phase Rectifiers (5) Single-phase rectifiers do not deliver current of the same smoothness as do three-phase rectifiers. The three-phase full-wave rectifier fills in with overlapping waves the gaps that exist when a singlephase rectifier is used (Fig. 3). Tests were made to determine the visual effect of an alternating component upon the projected light under normal operating conditions and with the shutter running at the standard speed of 90 feet a minute. With single-phase rectifiers the flicker was easily noticeable; whereas with both three-phase rectifiers and motor-generators there was no discernible flicker. These tests indicated that good screen re