Analysis of single-phase-to-three-phase converters

"Reprinted from Transactions of the American Institute of Electrical Engineers, Vol. 75, Page 403, January, 1956.""December 1, 1956.""A static phase converter is a device which supplies polyphase power to a load from a single-phase power source using only passive circuit elements. Ideally such a device should produce balanced polyphase voltages at the load. Several recent articles 1, 2, 3 have been published concerning the operation of 3-phase induction motors connected through static phase converters to singlephase lines. These articles discuss the simple form of phase converter using only capacitors. This capacitor-only type of phase converter can be adjusted to give balanced 3-phase power only for loads possessing a lagging power factor of 0.5. For any other load it is impossible to obtain balanced conditions. In contrast, the Add-A-Phase phase converter, consisting of an autotransformer in addition to the capacitors, may be set to supply balanced 3-phase power to any load possessing a lagging power factor regardless of its value. This more general type of phase converter circuit will be referred to in this article as the autotransformer type of phase converter. At the point at which balanced operation occurs with an induction motor load, the speed, torque, horsepower, efficiency, and other motor quantities will be identical to those obtained when the motor is connected to a balanced 3-phase power source of the same voltage. It is the purpose to present an analysis of the autotransformer phase converter, treating the capacitor-only phase converter as a special case of the more general analysis. The analysis indicates that many of the limitations placed on the application of the capacitor-only phase converter may be relaxed when the autotransformer phase converter is used. "--Page 1