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Three-Phase Power From Single-Phase Source
By A. L. HOLCOMB
Described is the development of a nonrotating device for the conversion of single-phase 115-v power to a three-phase 230-v form for the synchronous operation of cameras, sound recorders and other film pulling mechanisms associated with production of motion pictures.
JL HREE-PHASE MOTORS provide several desirable characteristics which are not supplied by single-phase units of equivalent power, and these characteristics, which include smaller size, lighter weight and quieter performance, are of particular value for the operation of cameras in motion picture production. Unfortunately, three-phase power lines are seldom available for location work outside of the studio lot, whereas the single-phase, 115-v source has become readily available in a large percentage of locations. Since sound recorders can conveniently be driven by single -phase motors, due to higher permissible noise, and to less rigorous weight-space requirements than cameras, it has become apparent that a synchronous converter from single-phase, 1 1 5-v, to three-phase, 230-v, for camera operation would be a desirable device. This would permit the operation of all channel equipment
Presented on April 25, 1952, at the Society's Convention at Chicago, 111., by John G. Frayne for the author, A. L. Holcomb, Westrex Corp., 6601 Romaine St., Hollywood 38, Calif.
from a single-phase source without degrading camera performance.
A nonrotating device is preferable for reasons of noise and maintenance and while several such units are commercially available for this duty, they are not well fitted for camera drive since they require factory adjustment for the particular motor and load which the unit is to supply. Operation with other motors or different loads causes phase unbalance which can create operating noise and/or limit the maximum power obtainable from the motor, thus destroying the very features for which a three-phase motor is desirable. Since most cameras used in motion picture production vary widely in power demand with temperature change, it. is only by chance that the factory adjusted phase balance is optimum for any given location condition. If the maximum power condition of the motor is selected for phase balance, an unbalance and noisy operation exist for all lesser loads while adjustment for balance at light load reduces the maximum power obtainable.
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July 1952 Journal of the SMPTE Vol. 59