Motion Picture News (Oct-Dec 1930)

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D e c e m b er 6 . 19 30 Motion Picture News 47 ARCS FROM GENERATOR SETS How Inefficiency Makes an Operator A Switchboard Operator as Well As a Projectionist 1 1 the positive carbon starts to bum off eccentrically, a serious pulsation of current will result with each revolution of the carbon. This effect is also cumulative as the current is increased at the time when the arc is longest. It can be seen, therefore, that a series arc generator cannot be expected to give satisfactory results on high intensity lamps with a continuous arc feed even though an attempt is made to use ballast resistance in series with the lamp. There have been attempts made to operate the low current reflector lamps with continuous arc feeds, and the same conditions obtain as with the high intensity lamp except that the difficulties caused by the rotating carbon are not encountered. Voltage Drop for Ballast Resistance An electric arc is unstable when connected to a source of constant voltage without a ballast resistance in series with it. The generator voltage or supply voltage for stable arc operation will be in excess of the arc voltage by the amount of voltage drop in the ballast resistor. For satisfactory operation this ballast resistance voltage drop should not be less than SO per cent, of the arc voltage and for economical reasons should not be much over this value. The combination of the ballast rheostat and an electric arc in series connected to a constant voltage direct current source is a simple electrical engineering problem, but as the conditions vary it becomes rather obscure to one not familiar with the problem. An electric arc supplied with direct current cannot be figured 60 JO 40 30 ZO "^ ^S ^ IMVM.T 1° i 5 ARC AMPE RES 10 zo •40 fo 60 70 Figure 3: Current, Voltage Variation with 85 and 1 00-Volt Generators Feeding a High Intensity Arc Set for a 60-Volt Arc at 70 Amperes Figure 5: Installation Diagram of Two Multiple Arc Units Using Panel "D" as a rheostat on account of the fact that the resistance of the arc is a variable quantity. Carbon has a negative temperature coefficient of resistance, and the arc stream has apparently the same characteristic. This characteristic of the arc is shown in the low current reflector arc which, when adjusted for a constant arc length, has an arc voltage which is practically constant although the current may vary within the limits for which the carbon is designed. Resistance at Definite Arc Lengths If we assume a certain size carbon which is rated to operate from 25 to 30 amperes with a definite arc length corresponding to an arc voltage of 50 volts, at 25 amperes the apparent resistance is 2 ohms, and at 30 amperes, 1.67 ohms. If we take a piece of ordinary resistance material and change the current from 25 to 30 amperes, the voltage will vary directly as the current changes. On account of this constant voltage condition we find that the variation of current in an arc with change of arc voltage caused by a change in arc length is much greater with a small difference between arc voltage and supply voltage than with a (Continued on page 65)