International projectionist (Jan-Dec 1957)

the operation of streetlights from one phase and the operation of inductive devices, such as motors, from one of the remaining phases. Voltage disparities in 3-phase lines are equally troublesome to the users of arc rectifiers. These may be checked by three identical AC voltmeters of suitable voltage range, each one connected across one phase (Fig. 1). An enclosed 3-pole switch should be used in lieu of a relay for turning the voltmeters on and off. Voltages as high as 220 are extremely dangerous! Voltmeters, Ammeters More useful to the projectionist, however, are the meters in the DC arc circuit. Every arc circuit should have a DC voltmeter to indicate the output voltage at the generator or rectifier terminals, and also a DC ammeter to indicate the amount of current flowing through the arc and all other components of the circuit. It is preferable to have two ammeters, one for each lamp, although it is possible to get by with only one ammeter if the shunt be inserted into the circuit between one of the generator output terminals and the ballast resistors. See Figs. 2A and 2B. (Use of one ammeter eliminates the possibility of any differences in meter accuracy.) An extra DC voltmeter connected across the lamphouse carbon-holder terminals, while not absolutely required, is useful for keeping the projectionist posted on the prevailing arc drop (voltage difference between the positive and negative carbons). This voltage is always less than the DCsupply output voltage when the arc is burning, and varies according to the length of the arc gap. The closer the carbons are brought together, the lower the voltage reading. A few theatres are entirely without meters of any kind in the arc-lamp circuits. It's rather surprising that the projectionists are willing to get along without them. With no meters to keep them informed as to the output of the DC conversion apparatus and the current consumed in the lamps, they can only guess that everything is working efficiently. They often guess wrong. All projection-room circuits should be adequately metered, for that matter. An AC voltmeter reading from 100 to 150 volts should be installed in every projection room to indicate the prevailing line voltage. And the absence of a generator or rectifier DC output voltmeter is unthinkable. In many cases the original arc-lamp ammeters are never replaced when they break down. There is no good reason why an electric meter should ever break down, of course; but many are injured by excessive heat and strong magnetic fields in the lamphouse. A grossly inaccurate meter is little better than none at all. The arc ammeters should be placed either in the control cabinet or on the projection-room wall where they can be seen by the projectionist as he adjusts the arc gap or the positioning of the carbons of each lamp. If two ammeters are used, one for each lamp, they should be conspicuously labelled "Projector 1" and "Projector 2," with corresponding numerals neatly and legibly painted on the lamphouses. Before discussing the possible weak points in a projection-arc circuit — points represented by electrical appara tus and by wiring, switches, fuse blocks, and connections — let's remind ourselves of a simple fact of physics, namely, that all wasted electrical power is converted into heat at the piont where it is wasted. An inefficient AC-to-DC converting apparatus or a loose connection offers resistance to the flow of current; and as long as the current flows, the resistance develops heat. The heat may do a great deal of harm by damaging costly units or by starting fires. Spotting Inefficiency Because all forms of energy — light, sound, electricity, and kinetic energy — all eventually end up as heat, the quantity of energy originally present may be ascertained by measuring the amount of heat it produces. The heat produced anywhere in an arc-lamp circuit thus represents electrical energy which is totally lost so far as screen illumination is concerned. As a matter of fact the power-transmitting efficiency of a motor-generator set or a rectifier can also be found by measuring the amount of heat produced by its operation. Accurate measurement of heat is so difficult, however, that the efficiency of electrical apparatus is usually determined by dividing the output power by the input power in watts. Nevertheless, the heat-measurement method is scientifically valid. There is not much difference between the efficiencies of rectifiers and motorgenerator sets. Stack (selenium) and tube-type rectifiers including their transformers, reactances, and other integral arc-current components have power-handling efficiencies of 70-90%, while motor-generator sets have overall No. 1 Ballast ) Ammeter for both lamps FIG. 2. Diagram showing the placement of meters in an arc-lamp circuit. (A) illustrates circuit having two ammeters, one for each lamp; (B) shows the alternative hookup employing one ammeter for both lamps. INTERNATIONAL PROJECTIONIST • JANUARY 1957