Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1988-08-10
1990-01-02
Wong, Peter S.
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363129, H02J 336
Patent
active
048917419
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention generally concerns improvements in or relating to AC/DC converters for interchanging electrical energy between AC and DC circuits. The invention will be described hereinafter with particular reference to polyphase rectifier drives for DC motors which employ naturally-commutated phase-controlled thyristor circuits to determine the armature current of the motor, but the invention is susceptible of wider application (for example in power-sharing electrical power distribution systems) and has application also to circuits employing controlled rectifier devices other than thyristors and to circuits wherein forced commutation is utilized rather than natural commutation. As employed herein the term AC/DC converter is intended to embrace DC to AC converters as well as AC to DC converters.
BACKGROUND OF THE INVENTION
DC motor drives are utilized in industry to serve a wide variety of purposes and commonly employ an AC to DC phase-controlled thyristor converter for controlling the motor. The converter, through the controlled switching of thyristors, changes the AC supply voltage to a controllable DC output voltage which is applied to the armature windings of the motor. By use of thyristors, commutation (that is the transfer of current from one thyristor to another and hence from one phase of the supply to the next phase) is achieved naturally in that the polyphase AC supply waveforms are responsible for turning off a conducting thyristor in much the same way as a similar circuit of diodes would behave and no special or additional circuitry is required for the commutation process; such commutation is known in the art as natural or line commutation. Thyristor AC/DC converters are therefore relatively simple in principle, though complex circuitry is commonly employed for controlling thyristor ignition, and are widely utilised in many industrial fields.
FIG. 1 of the accompanying drawings is a schematic illustration of a conventional three-phase, phase-controlled, naturally-commutated thyristor bridge, shown typically with a DC motor load. As is well known, control circuitry (which has been omitted from FIG. 1) is employed for providing triggering signals to the thyristors in the bridge at defined time instances in the waveforms of the three phases of the AC supply to determine the signed magnitude of the DC output provided to the motor M. Such control circuitry is commonly sophisticated and might, for example, employ algorithms running in microprocessors, phase locked loops, etc., as is disclosed, for example, in the book "Thyristor DC Drives" by Paresh C. San (published by John Wiley & Sons in 1981).
The circuit of FIG. 1 is known as a full converter in that the motor terminal voltage can be reversed, in dependence upon the triggering of the thyristors, so that the thyristor converter operates in a so-called inversion mode in which power can be transferred from the motor back through the thyristor bridge and into the AC supply. By virtue of this facility, regenerative braking can be accomplished in a controlled manner by means of the illustrated circuit, with the kinetic energy of a drive system coupled to the DC motor being converted into electrical energy by the motor and returned through the thyristor bridge to the AC supply. The circuit of FIG. 1 thus is useful as a controller for a DC motor which enables the motor speed and torque to be precisely determined in forward and in reverse operation of the motor, enabling the motor to be effectively and controllably braked by regeneration.
The bridge arrangement shown in FIG. 1 provides for control only in two quadrants in the armature current/back emf domain. FIG. 2 of the accompanying drawings shows this domain, and it will be readily appreciated by those skilled in the art that control is possible only in the first two quadrants with the circuit of FIG. 1. To achieve full four quadrant control, dual converters in which a similar but oppositely connected thyristor bridge is additionally connected across the motor terminals are
REFERENCES:
patent: 4315305 (1982-02-01), Siemon
patent: 4393442 (1983-07-01), Kahkipuro
patent: 4409648 (1983-10-01), Kobari et al.
patent: 4468724 (1984-08-01), Omae et al.
patent: 4490780 (1984-12-01), Nondahl
Slattery Daniel T.
Watson Jeremy D.
SSD Limited
Wong Peter S.
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