The motion picture projectionist (Nov 1931-Jan 1933)

28 Motion Picture Projectionist December, 1931 Motor Generator Sets for Arc Circuits By John H. Hertner In the following article Mr. Hertner gives us an account of the purpose, development and advantages of rotating conversion equipment for arc circuits. As the president of the Hertner Electric Company, the author possesses a thorough knowledge of his subject and his words are, therefore, well deserving of the attention of our readers. — The Editor. THE various changes in the motion picture theatre and in motion picture projection have been closely followed by changes in the auxiliary apparatus used therewith. A review of the industry from its beginning shows an almost continuous growth and change. Going back to the time when the carbon arc was first used as a source of light it was generally customary to supply it with whatever form of electrical energy that was available, whether A.C. or D.C. If the voltage was too high sufficient ballast was used to bring the arc voltage to about normal and the current to the amount required, if the power was D.C. If A.C. power was supplied a choke coil was generally introduced, cutting the power cost over that required were resistance used. Advantages of D.C. Recognized The advantages of direct current were soon recognized and various types of rectifying apparatus were employed where A.C. power was furnished. Thus we are still finding mercury arc rectifiers in service. A considerable number of rotary converters were installed. For small units the vacuum tube rectifier found considerable application. The largest field, however, is that of the motor generator set. Here again there has been considerable adaptation of the design to fit the conditions of service. The earliest projection rooms, very aptly called "booths," were notoriously small, and since space elsewhere was often not to be had, the designer found a demand for a machine occupying a minimum of floor space. This led to quite a large number of vertical sets being sold while the horizontal units were generally of the close coupled, twobearing type. There is still a considerable sale for vertical sets. The most obvious design electrically in a motor generator is to have the output high enough so that the ballast resistance will stabilize the arc in order that it will not require attention too often. The regulation of the unit should be such that it is "flat compound," or that regardless of current load up to a reasonable overload, Standard the voltage is maintained practically constant so that a number of arcs can be run at one time without the one interfering with the other. That ballast resistance is necessary when operating off constant voltage is readily apparent when it is remembered that the resistance coefficient of a carbon arc is negative, that is, the greater the current the less the resistance. Were an arc established across a constant voltage and the ballast then shorted out the current would quickly climb to a value which would blow the fuses, destroy the carbons or the leads, or create some other damage, to the point where it would kill itself, not by going out on account of its diminishing value, but by the destruction caused by its excessive value. In other words, without ballast the arc once started would take an amount of current limited only by what the line and the generator could furnish and would endure only as long as the carbons could stand the overload. Years ago unit rates for electricity were higher than they are today. The theatre owner was keen to save what Table 1-A MULTIPLE ARC TYPE A.C. Motors -110 -220—440 or 550 Volts -2 or 3 Phase -60 Cycle D.C . Amps D.C. Amps D.C. Type Ballast Type Continuous 5 Min. Int. Volts Rheostats MA-20-40 20 40 80 AAA MA-25-50 25 50 80 AAA MA-30-60 30 60 80 AAA MA-40-80 40 80 80 G MA-60-120 60 120 85 H MA-80-160 80 160 85 J MA-100-200 100 200 90 K MA-125-250 125 250 90 L MA-150-300 150 300 90 L MA-200-400 200 400 100 L ever he could on his power bill. Usually he had two projectors and nothing else to feed off his generator. The two projectors naturally took the same number of amperes. This led, about fifteen years ago, to the introduction of the series type of generator which delivers constant current at a predetermined value regardless of arc length and therefore needs no ballast. It thus operates one or two arcs in series, saving the power usually wasted in ballast. Where the arc voltage was, for instance, 55 volts and the generator 85 volts or a ballast drop of 30 in a multiple generator, the energy consumption of the series machine is but two-thirds of that of the multiple type. The more recent introduction of a multitude of spot lights, stereopticons, dissolvers, etc., requires the production of currents of various amounts simultaneously, besides the horizontal arcs of the present reflector lamps are more sensitive than those of the old "vertical" arcs, hence there has been a decided swing back to the less efficient multiple type of generator. As the theatre grew the arc amperes in direct current went up successively from 35 to 50 and to 75 amperes on the old type of vertical arcs and when the high intensity lamp came out it raised this to about double, bringing about the need for larger and larger generators. Then came the so-called "low intensity" reflector lamps with an efficiency so high that the demand for current at the arc went down in some houses as low as 15 amperes. This brought about the building of very small generators, of about 2% K.W. capacity. High Current in Demand The talking picture with its special screen has again reversed the trend and higher current values are again demanded in spite of better light efficiency. We have then, today, call at the arc of from about 15 amperes to upward of 200 amperes, the range embracing from the low intensity reflector at the lower end to high intensity condenser at the other. The arc voltage of these various sizes and kinds of arcs varies considerably. In a general way this arc voltage varies with the amperes, being greater with higher ampere values. This condition is modified, however, by the type of lamp or rather the kind of carbon. Thus a reflector high intensity burning 65 amperes will have a lower arc voltage than a reflector low intensity with half the amperes. If the manufacturer of the motor generator were to hold all his product to a standard voltage it might be too (Continued on page 39)