Converter system combining a two-quadrant voltage-source rectifier and a four-quadrant voltage-source inverter, and a motor drive embodying the same

In an AC motor drive system including:

• an AC/DC converter having AC input lines, opposite poles and main thyristors forming a bridge across respective said AC input lines and across said poles;
• a constant voltage-source inverter having AC output lines and DC terminals;
• a DC-link capacitor across said inverter DC terminals;
• and an AC motor energized by said AC output lines;
• the combination of:
• first and second diode means operatively connected each between a corresponding pole of said thyristor bridge and an associated end of said DC-link capacitor for providing a current path between conducting main thyristor switches when the bridge is operating in the forward mode;
• first and second GTO switches cross-coupled between a corresponding end of said DC-link capacitor and a corresponding pole of said thyristor bridge;
• first means responsive to motor braking for initiating cyclical and alternate control of said first and second GTO switches and for providing a corresponding firing angle for said main thyristor bridge to provide a regenerative mode for bypassing said first and second diode means and for passing said current path through said DC-link capacitor in the opposite direction, thereby to allow energy feedback from said DC terminals through said converter onto said AC input lines;
• and
• second means for inhibiting conduction of said first and second GTO switches during a time interval for each GTO switch so as to define a duty cycle in relation with an unbalance between the voltage input of said inverter and the voltage output of said AC/DC converter, thereby to match said inverter voltage input and said converter voltage output substantially.