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The 3-phase transistor-core power inverter described in this paper follows the trend away from mechanical devices toward lighter, more efficient, electronic equipment which, in most cases, are more reliable than the mechanical devices they replace. A single-phase transistor-core inverter has already replaced the mechanical multivibrator in one of our missile systems; it is only natural, then, for an electronic 3-phase inverter to follow as a power source for the 3-phase gyroscope motors which are part of the instrumentation in the same missile. This paper presents: 1. The need for a 3-phase inverter and discusses the reasons for the use of 3-phase gyroscope motors rather than d-c singlephase or 2-phase motors. 2. The waveshape characteristics of a 3-phase square-wave system and points out some of the system's limitations. 3. The operation of the 3-phase inverter which is an extended version of the one described by A. G. Milnes.1 4. The reasons for adding additional circuitry to Milnes' inverter to make it more stable over wide load ranges.