Three-phase inverter

A polyphase inverter for providing six-step output waveforms to a first phase, a second phase, and a third phase, said second phase having a phase angle of 120.degree. relative to first phase, said third phase having a phase angle of 240.degree. relative to said first phase, comprising:

• a direct current source of electrical energy;
• a plurality of switch means connected to said direct current source;
• gating means for selectively actuating said switching means for selected periods of time to produce six-step output waveforms having a predetermined fundamental frequency in each of said phases;
• clock means for generating clock signals having a clock frequency which is a multiple of the fundamental frequency;
• and
• interval timer means connected between said clock means and said gating means for generating timing signals for controlling said selective periods of time to produce a plurality of notches in said six-step output waveforms to modulate said six-step output waveforms to provide a constant volts/Hertz value and to control the percentages of selected harmonics therein, said first phase having notches symmetrically located about the 75.degree., 105.degree., 255.degree., and 285.degree. points therein with the notches in the output waveforms of said second and third phases being out of phase with the notches in said first phase by 120.degree. and 240.degree., respectively.

A method for converting direct current into polyphase alternating current, comprising the steps of:

• connecting a plurality of switching means to a direct current source of electrical energy;
• selectively actuating said switching means for selected periods of time with a gating means output to produce output waveforms having a fundamental frequency in each of a plurality of phases;
• varying said selected periods of time to control the percentage of selected harmonics in said output waveform in each phase;
• and
• maintaining the amplitude of the fundamental frequency in the output waveform at a substantially constant volts/Hertz value in each phase by varying the width of pulses generated upon actuation of said switching means.

A method for converting direct current into three-phase alternating current, comprising the steps of:

• generating six-step waveforms having a predetermined fundamental frequency in a first phase, a second phase and a third phase, the waveform of said second phase having a phase angle of 120.degree. relative to the waveform of said first phase, the waveform of said third phase having a phase angle of 240.degree. relative to the waveform of said first phase;
• modulating the six-step waveforms to produce notches therein, the notches in the first phase being symmetrically located about the 75.degree., 105.degree., 255.degree. and 285.degree. in the output waveform thereof, the notches in the second and third phases having phase angles of 120.degree. and 240.degree., respectively, with the notches in the first phase;
• and
• controlling the notch angles to vary the fundamental frequency while maintaining a substantially constant ratio of voltage to fundamental frequency in the output of each phase.