International projectionist (Jan-Dec 1957)

^— — ^^— ■— MHH— «WU111«11 OOMCKSBBBBTi NTERNATIONAL PROJECTIONIST Volume 32 JANUARY 1957 Number 1 Transmitting Power to Arc Lamps By ROBERT A. MITCHELL This practical guide to evaluating an arc-lamp power supply and maintaining it at peak efficiency examines every aspect of current transmission from the power plant to the carbon. THE BEST projection lamp in the world is almost useless if not properly powered. The high intensity carbon arc refuses to tolerate improper voltage characteristics or variations in the current supplied to it. In lamps not having automatic control of the crater position and feeding of the carbons, a fluctuating power supply may also seriously interfere with proper functioning of the feed mechanism. In certain sections of the country, projection arcs in theatres unfortunately labor under the disadvantage of an unsatisfactory power supply and faulty transmission of the power from the source to the lamps. Aside from the fire hazard created by overheated cables, DC arc-circuit wiring of inadequate size, while not detrimental to arc-lamp operation when motor-generators are used, waste electrical energy and fail to deliver the maximum available current to the arcs when rectifiers supply the DC. (When generators supply the power, voltage drop in the transmission line can be compensated by reducing the amount of voltage drop in the ballast resistors. Ballast rheostats are not used with rectifiers.) Erratic arc power due to corroded and loose contacts is not uncommon; and when a faulty connection is located inside the lamphouse, the heat of the lamp conceals the identity of the "burning" contact. Uneven Current Supply Even perfect rectifiers cannot cope with the unfavorable characteristics of AC input current supplied by some commercial power companies. To prevent flickering of the screen illumination, only motor-generator sets should be used in localities where the AC input current fluctuates in voltage or exhibits different and varying degrees of power factor between the several phases of polyphase current. AC VOLTMETERS (200-300 V.) Normal range, 220-240 volts \\ i « 1 ' 3-phase To generator input line or rectifier — *i FIG. 1. How AC voltmeters are connected to a 3-phase power line. The power factor of current from AC mains is extremely important to the users of rectifiers operated on 3-phase current. Differences in power factor will make the projected light flicker, and a low power factor will seriously reduce arc current and give a dim light on the screen. And yet the AC input ammeters and voltmeters may indicate maximum power consumption. The power factor of alternating current is 1 only when the current (amperes) and pressure (volts) are in step with one another as the current undergoes its rapid alterations. If the current leads or lags behind the voltage, the power factor decreases ; that is, the number of watts consumed in a circuit (measured by a wattmeter) is less than the mathematical product of volts times amperes. This is why the terms "volt-amperes" and "kilovoltamperes" are often used in place of "watts" and "kilowatts" in discussions of power consumption in AC circuits. When the power factor is 0 (current and voltage 90 degreees out of phase), the current is called "wattless," and is incapable of operating electrical devices. Power factors of 0.8 to 0.9 are usual in 3-phase lines; and a power factor less than 0.7 in one or more of the three phases will occasional}7 occur. The disparity usually arises through INTERNATIONAL PROJECTIONIST JANUARY 1957