Electric power conversion systems – Current conversion – Using semiconductor-type converter
Patent
1986-08-11
1987-10-13
Beha, Jr., William H.
Electric power conversion systems
Current conversion
Using semiconductor-type converter
363 96, H02M 7521, H02M 106
Patent
active
047002883
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to power converters and, in particular, to autonomous inverters.
2. Description of the Prior Art
At present, adjustable electric drives equipped with autonomous inverters are extensively used in many technical fields. They substantially simplify the kinematics of machines and mechanisms, eliminate reduction gears, combine the electric drive and the mechanism into an integrated device, make the technilogical process fully automatic, and maintain desired optimal conditions of the mechanism with high speed and accuracy.
The circuitry of an autonomous inverter, which is one of the basic components of a thyristor frequency changer, largely determines the adjusting characteristics and power performance of the electric drive, as well as its weight, size, cost, and reliability. The latter considerations often dictate the choice of a particular circuit in preference to others, when selecting an autonoumous inverter for a high-power, high-voltage frequency converter. Many autonomous inverters are rejected for purely practical reasons.
In this connection it is a serious problem to provide autonomous inverters which are structurally uncomplicated, functionally reliable to provide improved communtation stability, and have better power performance. They should be smaller in size, lighter in weight, and less costly to manufacture. The problem is especially applicable to autonomous inverters for high-voltage frequency-adjustable electric drives wherein addition of extra gates and capacitors is impractical.
Known in the art is an autonomous inverter (cf., for example, USSR Inventor's Cetrificate No. 505,099 Cl. H02 m 5/42, H02 m 7/472) which is designed for induction motor control. In order to improve commutation stability and ensure faultless starting, this autonomous inverter, comprising thyristor switches, a differential-transformer control transducer whose primary winding is inserted into the circuit composed of a capacitor and one of the thyristor switches, is provided with additional threshold limiters whose inputs are connected to the transducer secondary winding. Outputs of the limiters are connected to inputs of a step phase shifter, the input of the master oscillator is connected to output lines of the autonomous inverter, while the output thereof is connected to the input of the step phase shifter.
The threshold limiters are adjusted to one level of the signal of the differential-transformer transducer in order to maintain the amplitude of the voltage across the switching capacitors constant in all conditions of the autonomous inverter and improve its commutation stability.
This autonomous inverter is deficient in that the switching capacitors are not isolated from the load during the intervals between switchings, and energy-exchange oscillating processes of persistent nature exist between the leakage induction motor of the switching capacitors. This results in distortion of the shape of the output voltage of the autonomous inverter by harmonics. Moreover, a direct component may appear on the switching capacitors of this autonomous inverter, which results in the asymmetry of the restoration angles of the switch thyristors and, consequently, in worse commutation stability. Any control assymetry leads to the voltage direct component appearing on the capacitors due to the presence of the transformer in the control transducer, through which information on the direct component cannot be transmitted.
Also known in the art is an autonomous inverter (cf., for example, W. Echner, D. Kollensperger, "Einrichtung zur Zwangskommutierung fur eine Anordnung zur Steuerung der Drehzahl und Drehrichtung einer Drehstrommaschine", Siemens AG, FRG Patent No. 1638551, Cl. H02P 7/62, filed Feb. 17, 1967, published May 28, 1975) comprising a main bridge thyristor switch, switching capacitors, and an additional thyristor bridge switch.
In order to improve commutation stability, each switching capacitor is provided with a charging device comprising a transforme
REFERENCES:
patent: 3656047 (1972-01-01), Borg
patent: 4183081 (1980-01-01), Cutler et al.
patent: 4255783 (1981-03-01), Messer
patent: 4317166 (1982-09-01), Praha et al.
Baraban Viktor P.
Lazarev Grigory B.
Zabrovsky Stanislav G.
Zvyagin Albert F.
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