International projectionist (Jan-Dec 1946)

A 6-Phase, Full-Wave Rectifier THE greatly extended use of metallic disc-type rectifiers during the war has resulted in important developments in this field. Particular reference is made to the outstanding progress made with magnesium-copper sulphide [M-CS] rectifiers within the heavy current field. In its present state the M-CS rectifier stack is a component of great dependability. One of the reasons for its increasing use is its ability to operate in high ambient temperatures and to withstand heavy intermittent overloads. Although ordinarily operated at a temperature well below 200° F.. the M-CS stacks can safely stand an operating temperature of 265° F. This compares with a maximum temperature of 180° F. for the nearest of other types of metallic disc rectifiers. Because the M-CS stacks do not need to be operated close to their maximum safe temperature limit these stacks apply themselves well to thermostatic protection by a slowly operating thermostat which does not cause nuisance interruptions from temporary overloads. Various Types of Rectifiers Two objections have in the past been raised against rectifiers for motion picture projection : 1. The pulsation or "ripple" has a noticeable effect on the light from the Front view of new 6-phase, full-wave projection rectifier unit. By MERLE H. CHAMBERLIN Member, I. A. Local 165 Chief Projectionist, Metro-Goldwyn-Mayer Studio, Culver City, California arc when projected on the screen through the intervals of the shutter. To analyze this let us consider first a rectifier of the half-wave type, fed from an ordinary single phase light circuit. Such a rectifier or '"electric valve" only lets through one-half of each cycle of the A. C. voltage and blocks out the other half. The result is one impulse per cycle, or 60 impulses per second with 60-cycle A. C. supply. This pulsation or ripple would show up as objectionable flicker on the screen. For this reason single-phase rectifiers for arc lamps are of the full-wave type, i.e., using both halves of the A. C. cycle. As there is one impulse for each half cycle this type gives 120 impulses per second on 60-cycle supply. Rectifiers which employ bulbs usually have a stabilizing choke in the D. C. circuit which tends to smooth out the ripple. However, with single-phase supply a pronounced ripple is practically unavoidable and for this reason single-phase rectifiers are ordinarily only used where 3-phase supply is not available. The next better step is the use of a 3-phase. half-wave rectifier fed from a 3-phase power line. Such a rectifier gives 180 impulses per second. We now come to the commonly used tvpe of rectifier for larger motion picture projectors, the 3-phase, full-wave [bridge type] rectifier. This give 360 impulses per second when operated on 60-cycle current. 720 Impulses Per Second Although this type has been generally accepted in the industry and furnished by the leading manufacturers up to the present, even the 3-phase, full-wave rectifier does not deliver a perfectly flicker-free light. To overcome this a new 6-phase, full-wave rectifier has recently been developed for motion picture projection.* This is equivalent to 12-phase, half-wave rectification, such as is used in large central power stations supplying direct current to electric railroads and street cars. With 3-phase, 60-cycle A. C. supply this rectifier delivers 720 impulses per second, and with the close overlapping of the phases thus obtained the ripple has prac * By the McColpin-Christie Corp., Ltd., of Los Angeles, Calif. tically disappeared and the improvement in the light is very gratifying. This represents a major step in the development of rectifiers for projection. 2. A second objection to rectifiers has been brought forth in the past. Unlike the more common electrical loads on a power system, all single-phase and 3-phase rectifiers produce in the power line certain '"harmonics" which in unfavorable cases may be picked up by sound systems. With the 6-phase, full-wave rectifier of proper design the current drawn from the power line is so nearly a pure sine Switching panel for new 6-phase projection rectifier unit. wave that harmonics of a disturbing magnitude are not produced. 6-Phase Unit Principle A few words to explain how 6-phase rectification is possible from a 3-phase supply line may be of interest. Without going into too many technicalities, the system consists of splitting each leg or phase of the rectifier transformer into two. separate parts, then cross-connecting a part of one legto a part of another leg in such a way that each part gives an impulse which follows by 30 electrical degrees (1/12 of a cycle of 360°) after the preceding impulse. This is analogous to ,& 12-cylinder double acting combustion motor, as compared to a 6-cylinder motor, using only a very light flywheel. Like the 12-cylinder motor, the rectifier delivers a perfectly smooth flow of power. Several of these 6-phase rectifiers were thoroughly tested by us at the M-G-M {Continued on page 24) 15 INTERNATIONAL PROJECTIONIST « December 1946