Journal of the Society of Motion Picture Engineers (1930-1949)

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TRANSFORMER T2 SINGLE PHASE 1 15V INPUT THREE PHASE LOAD TRANSFORMER Tl Fig. 1. Simplified schematic of the Converter. Thus, it appears that a satisfactory single-phase to three-phase converter for camera operation must provide either a circuit that does not unbalance when supplying different motors and varying loads, or one which provides some clear indication of unbalance and a ready means of correction which is not too complicated for field adjustment. Consideration of the problem indicates that the required phase shift from two conductors electrically 180° apart to three conductors displaced 120° cannot be obtained on a nonrotating basis without introducing reactance in various forms in the conversion circuit. Since the electrical load is also reactive, it will combine with the circuit reactance to determine the phase shift obtained unless isolation can be introduced between the conversion circuit and motor. Adequate isolation does not appear to be practical because of space-weight and efficiency factors and thus the motor reactance which varies from motor to motor and with load on any given motor must be considered as a component part of the conversion circuit. Therefore, a conversion circuit which will inherently maintain phase balance in the presence of the prescribed conditions is not considered practical, and as a result development has been focused on the alternative method, which would provide convenient indication and adjustment. Basic Circuit The Scott transformer connection, which is relatively old in the art and described in most electrical engineering handbooks, is the basis of the conversion circuit shown in Fig. 1. This transformer connection was originally used for the conversion of two-phase to three-phase or vice versa. By the addition of a capacitor of the right value in series with the primary of transformer T2, as shown in Fig. 1, the current in this primary can be shifted 90° with respect to the primary of Tl and thus the equivalent of a two-phase primary circuit is provided from a single-phase source. In the secondary, a three-phase electrical displacement exists only when connected to a balanced three-phase inductive load, which is the load condition presented by a threephase motor. The circuit functions to produce 120° phase displacement between the three output leads by means of the vector addition of both a proper voltage and a 90° phase shift with respect to the mid-tap of Tl. A portion of the inductive load is reflected through T2 where it is effectively resonated at line frequency by the series capacitor. This provides the 90° phase shift mentioned, since at resonance there will be no reactive component in the primary of T2 and the current will be in phase A. L. Holcomb: Three-Phase From Single-Phase 33