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Ac motor drive with improved voltage-source inverter

  • US 4,959,602 A
  • Filed: 06/02/1989
  • Issued: 09/25/1990
  • Est. Priority Date: 06/02/1989
  • Status: Expired due to Term
First Claim
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1. In an AC motor drive system including:

  • an AC/DC converter having three-phase AC input lines, opposite poles, main thyristors forming a bridge across said AC input lines and said poles and means for sequentially firing said main thyristors;

    a precharged DC-link capacitor across said poles;

    an inverter having AC output lines, DC terminals connecting said DC-link capacitor to the inverter, and an AC motor supplied by said AC output lines;

    with 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 thyristors when the motor drive system is operating in one of the motoring and regenerating modes;

    with additional first and second static switches cross-coupled between respective ends of said DC-link capacitor and respective poles of said thyristor bridge;

    with means for controlling said first and second additional static switches (a) OFF for passing said current path through said DC-link capacitor in one direction when the motor drive is in the motoring mode and (b) ON for passing current in the opposite direction when the motor drive is in the regenerating mode, thereby to allow feedback from said DC terminals through said converter onto said AC input lines;

    means for sensing the current of said current path to derive current representative signal;

    one forced-commutated switch being used as one of said first and second additional static switches and an auxiliary thyristor being used as the other of said first and second additional static switches, said DC-link capacitor being bypassed (1) in the motoring mode by said forced-commutated switch when turned ON and (2) in the regenerating mode by said auxiliary thyristor being ON whereas said forced-commutated switch is turned OFF;

    the combination of;

    current control loop means operative upon said controlling means for applying to said forced-commutated switch a duty-cycle OFF/ON in accordance with, respectively, a given excess and a given want of said current relative to a reference current related to a boost voltage for said DC-link capacitor;

    voltage control loop means being provided to generate said reference current and responsive to a present voltage of said DC-link capacitor and to a predetermined desired voltage for voltage boosting thereof to derive a voltage error, the sign of said voltage error being changed to the opposite when in the regenerating mode, said current reference being established in relation to said voltage error;

    whereby said current control loop is effective in establishing said forced-commutated switch duty-cycle.

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