International projectionist (Jan 1943-Dec 1944)

FIGURE 1. FOUR COMMON CAUSES OF ARC REFLECTOR BREAKAGE (A) Poorly designed carbon savers (B) Arc Current Excessive (C) Reflector holder clamps too tight (D) Excess draft through draft pipe on opening door of lamphouse too soon after shutdown. suprex type arc lamp will not function within 25 per cent of its normal efficiency. Great care should be exercised to prevent burning these parts. The most common cause of arc lamp inefficiency can be traced to improper arc supply source. Whether motor generators or rectifiers are used for this purpose they should conform to the lamp manufacturer's exact specifications as well as that of the carbon manufacturer. A projection arc lamp which is specified for one application should never be operated above that particular current rating. If an attempt is made to convert or adapt such lamp for use with a higher arc amperage, poor operation can be expected. For example, a low intensity type lamp designed to operate properly at 30 amperes will not dissipate the heat created at 40 or 45 amperes. Results will be warped doors, broken reflectors, broken sight glass and unsteady arc control operation which combines to make the final efficiency much less than if the change had never been attempted. In addition to these faults there is the excessive waste of carbons and current without the sought-after increase in screen light. Arc lamps should always be operated within their specified hmits for top efficiency. Proper Handling of Reflectors The reflector is the most delicate and important single item used in the modern projection arc lamp. Special attention must be given the reflector in order to keep it operating efficiently for the longest period of time. The reflector should not be handled when hot. The chances of accidental breakage are far too great. Besides, if the reflective surface is touched with the fingers when hot, the fingerprints will be practically etched into the glass to the extent that they cannot be removed. An oily cloth will produce cloudiness no matter how small the amount of oil on the cloth. New cheesecloth should be kept on hand for cleaning the reflector. Castile soap and clean water is the best cleaning agent. If the reflector is not systematically cleaned, it will rapidly lose its efficiency. A habit should be formed of cleaning the reflector every operating day. If a wooden salad bowl can be obtained which is a bit larger in diameter than the reflector, it will be found that this vessel will be ideal for positioning the reflector while cleaning. Cleaning will lessen the effect of "pitting" which is inherent in suprex type carbons. After cleaning the reflector it should be carefully dried before replacing in lamphouse. Otherwise a crack may result. Care must be exercised in the amount of exhaust draft used to carry off the carbon gas and fumes. Excessive draft will break reflectors. Also, unsteady arc burning will result from this cause. The lamphouse doors should always be kept closed for a short time after each changeover. The reflector must have ample room in the reflector holder when installing. After reflector is properly mounted there should be about 3/32" play from side to side, otherwise the reflector will crack under the stress of heat expansion. Carbon-savers of certain designs are very dangerous to reflectors because the metal clamps on these savers do not always have sufficient clearance through the center hole in the reflector. If the slight est negative carbon adjustment is made during operation, the metal clamp may be fed up on the glass, breaking the reflector. If the metal in the saver only touches the inside of the center hole without stress, breakage can result from the high heat which is conducted by this metal. It is false economy to break reflectors with poorly designed carbonsavers. Excessive arc current will surely break or destroy the efficiency of reflectors. Arc lamps should not be operated at the extreme limits if it is wished to conserve on parts and have trouble-free operation. An arc lamp reflector can be ruined in an attempt to obtain more light than the design will allow. See Figure 1. Higher amperage by no means assures more screen light; the reverse is more often the case as excessive "tailflame" shadows a large portion of the reflector surface as well as distorting the crater or gas ball which produces the useful light. It is good, sound practice to operate the arc lamp at an amperage below the specified limit. Are Control Unit Consistent operation of the modern reflector arc lamp depends largely upon the proper functioning of the arc control unit. Arc control units generally consist of a motor, contactor relay, or potentiometer combined with mechanical adjustments to position and maintain the arc gap in proper relation to the reflector and projector film aperture. These units operate on the change of voltage across the arc as the carbons are consumed. This voltage being low, it is necessary to have very accurate electrical as well as mechanical adjustments in order to feed the carbons uniformly. The following suggestions should keep the arc control operating at its highest efficiency : The motor used on the average control is a d.c. 1/30 or 1/50 h.p. wound to operate at the prescribed arc voltage of the particular application. The motor should be inspected monthly. The brushes should be removed and examined. The commutator should be checked for roughness. 2/0 sandpaper can be used to keep the commutator surface smooth and bright. Arc control motor bearings must not be over lubricated. One drop of oil twice a week at each bearing is sufficient. Packed ball bearings need attention only about once every two years. Manufacturers' specified brushes should be used for replacement, otherwise poor motor operation may result from "sticky" brushes or brushes of the wTong carbon stock. The motor should be kept clean. A few minutes a day spent in cleaning this item will repay the effort by faultless 14 INTERNATIONAL PROJECTIONIST