Claims
- 1. 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.
BACKGROUND OF THE INVENTION
This application is a continuation-in-part of application Ser. No. 10,879 filed Feb. 4, 1987, which was a continuation-in-part of application Ser. No. 807,568 filed Dec. 11, 1985, (now abandoned) by the present inventor and owned by the present assignee.
The present patent application is related to U.S. Pat. No. 4,697,131, issued Sept. 29, 1987, for "Improved Voltage Source Inverter and Variable Frequency, Constant Voltage AC Motor Drive Embodying the Same" incorporated by reference.
A voltage-source inverter includes an AC/DC converter supplying a DC voltage to the inverter through a DC-link including a capacitor and an inductance, as generally known. The AC/DC converter and the inverter are both generally in the form of a bridge including static power switches connected across the DC terminals and the AC lines.
Voltage-source inverters with fixed voltage DC links are increasingly being used in high performance motor drive applications. Typically, such motor drives operate through the four quadrants in terms of positive or negative current and motoring or regenerating mode. However, the DC link is commonly supplied from a single-quadrant rectifier bridge, i.e. not accepting energy returned from the motor when braking, thus not returned to the AC power supply.
In a modern voltage-source inverter controlled motor drive, the drive is inherently capable of operating in four quadrants of the speed-torque plane. The first quadrant is where both speed and torque are positive. The third quadrant is where both speed and torque are negative. Thus, in those two instances the product torque speed is positive. This is when power flow goes into the mechanical load of the motor. In contrast, the second and fourth quadrants are those where the product torque times speed is negative, i.e. when the motor is acting as a generator and power is flowing from the mechanical load through the motor and back to the inverter side of the drive.
The problem is that in a voltage-source inverter, which can accept return power flow, the power can flow back through the inverter to the DC-link capacitor, but not back through the AC/DC converter. In other words, instead of a single-quadrant rectifier bridge, a two-quadrant power converter is required.
The prior art shows power circuits and control schemes which can be used to obtain two-quadrant power conversion, in a voltage-source inverter motor drive. See for instance U.S. Pat. Nos. 4,353,023 and 4,434,393 using an antiparallel regenerative thyristor bridge circuit connected between the inverter and the rectifier through switching transistors fired in synchronism with the thyristors. This approach requires at least six additional thyristors and associated control, which is a costly solution. A more simple solution has been proposed to deal with the negative flow of power into the DC-link from the inverter, consisting in using a resistor-chopper. This solution, however, because it employs a resistor, is cost effective, but has an adverse impact on efficiency and it causes enclosure temperatures to rise.
The prior art also shows a voltage-source inverter in which the rectifier bridge has its DC output connected to the respective poles of the DC link capacitor via respective diodes, and in which two thyristors are connected for cross-coupling between the two poles of the DC link capacitor. See U.S. Pat. No. 4,375,612 of Wirth.
In the cross-referenced patent application, instead of two cross-coupled thyristors, one GTO device is combined with one thyristor, whereby in the regenerative mode upon turning off the GTO device the freewheeling current through one diode and the added thyristor is rapidly forced to zero by the negative supply voltage returning the inductive energy to the AC side of the rectifier. Accordingly, a totally no-current situation is created involving the added thyristor, thereby allowing recovery time before proceeding with the next thyristor commutation in the bridge.
The invention is applicable to a voltage-source inverter (VSI) motor drive operating through the four voltage-current quadrants, i.e. involving regenerative mode operation by controlling the VSI on the AC input side so as to reverse the DC-link polarity thereof, while switching across the DC-link terminal connections to the inverter side thereof so as to match the polarities.
The invention resides in the implementation of such matching of polarities with cross-coupled GTO devices associated with the two diodes coupled with the respective poles of the DC-link capacitor between the output of the rectifier thyristor bridge and the DC-link terminals of the voltage-source inverter motor drive. To this effect, the GTO devices are interrupted with a duty cycle selected in response to the voltage difference between the inverter DC input voltage and a constant reference voltage.
US Referenced Citations (6)
Non-Patent Literature Citations (1)
| Entry |
| Peter Wood-Switching Power Converters, Van Nostrand, pp. 41-44. |
Continuation in Parts (2)
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Number |
Date |
Country |
| Parent |
10879 |
Feb 1987 |
|
| Parent |
807568 |
Dec 1985 |
|
Patent Grant
4788635
