The motion picture projectionist (Nov 1931-Jan 1933)

December, 1931 Motion Picture Projectionist 23 The Use of Resistance in Arc Circuits In this month's issue the author concludes his dissertation on the use of resistance in arc circuits with a consideration of the application of booster circuits, rheostat installation, points to bear in mind when ordering resistance equipment and the desirability of simplicity of control. Mr. Boyden is an electrical engineer and a member of the Engineering Staff of the Ward Leonard Electric Company. — The Editor. Part II IN arc circuits having a maximum current exceeding 110 amperes, it is advisable to reduce the starting current to some value lower than that used for normal operation in order to avoid fracturing the carbon due to inrush current, when the arc is struck and to obtain a satisfactory crater in the positive carbon. To provide the reduced current, usually from V2 to % of normal current, an extra terminal is placed on the rheostat. The fixed section of the rheostat is generally connected to this terminal thereby providing a circuit in which the current is limited to the minimum current as specified on the rheostat name plate. Two wires from the rheostat are carried to the lamphouse and connected to knife switches as shown in Fig. 4. The lead from the fixed section of the resistance is connected to one side of the double pole switch and the lead from the variable section to the booster switch. Then the current in the circuit when striking the arc after closing the double pole switch is a minimum. After the carbons are warmed and the crater formed, the normal current is supplied by closing the booster switch. Purpose of Booster Circuit The use of the booster circuit increases the importance of the minimum current specified for the rheostat. For example, a rheostat having a rating of 60-180 amperes normally operating on 150 amperes would have a warming-up current of 60 amperes. This current is too low to be practical with carbons rated at 150 amperes, but by fixing the minimum current at 90 amperes, the desired heating and burning would be obtained. However, the boosting current need not limit the specification of the minimum current because it is always possible to obtain any desired current below this value. A rheostat rated at 150-210 amperes may be connected so as to furnish a warming-up current of 60 or 90 amperes by supplying an extra terminal. With the increased use of higher amperages in arc circuits the use of booster circuits will become By Wilson G. Boyden more general. Lamp house manufacturers are furnishing equipment with the necessary switches as standard for all Hi-Intensity lamps. Standard rheostats are now supplied with the extra terminal for the connection to the booster switch on all capacities above 90 amperes. Installation of Rheostats The use of increased current on arc circuits in order to provide suitable light on porous screens results in higher wattages in the ballast rheostats than was the practice before the use of sound in the theatre. The present trend in theatre design is to provide separate rooms for the rheostat equipment fitted with suitable racks for mounting the rheostats above the floor level and equipped with fans and ventilators for providing continuous air circulation. Under any circumstances, it is important that sufficient ventilation be provided to carry off the heat given off by the rheostats, and that the rheostats be so placed as to insure free circulation of air through the resistance elements. The fact that a rheostat becomes heated, when carrying current, is to be expected as its function is to dissipate energy in the form of heat, but it must not be placed in such a position or covered over so that the heat cannot be freely radiated. Rheostats should not be located near inflammable material or in any location providing a fire hazard. If possible, it is well to provide at least one foot of free air space on the sides and back of the rheostats. Local Underwriters' requirements or codes will govern all installations, but it is up to the projectionist to keep the rheostat room free from inflammable material, fans operating, and ventilation through and around the rheostat unimpeded. How to Order Rheostats In writing specifications for projection arc rheostats information as to the supply voltage, the maximum and minimum arc current and the arc Connection Diagram for Ar.c Cir-co :-^^i tijH ■ ifilililimH Fig. 4 Typical Arc Rheostat voltage should always be included. The supply voltage may be obtained from the motor generator or converter nameplate or if the current is taken from the distributing lines of a power company a voltmeter may be used to measure the voltage. The lamp house or carbon manufacturers are in the best position to recommend the correct values of arc voltage and current. A Moot Question Ask a dozen projectionists what the correct arc voltage is for a Hi intensity lamp using 13.6 MM H. I. positive and 9/16" cored negative carbons operating at 125 amperes. The answers will be the same only in the case of those who answer "I do not know." Ask lamp house manufacturers and the carbon manufacturers the same question, and you will be surprised that such a lack of agreement exists. Go out with a voltmeter and measure the actual arc voltages under operating conditions and you will have some interesting results. At a theatre in A, we find the voltage to be 63, while an identical lamp house operating under apparently similar conditions, using the identical carbons and same value of current, at a theatre in B shows a voltage of 83. The projectionist at A is entirely satisfied with the results while the one in B complains that his rheostat, which is exactly the same as that used in the theatre at A, is not delivering the current. Since it was designed for an arc voltage of 65, how could it? Still everybody expects the rheostat manufacturer to make a rheostat that will fit both applications.