Electric power conversion systems – Current conversion – Using semiconductor-type converter
Patent
1997-07-03
1998-11-10
Riley, Shawn
Electric power conversion systems
Current conversion
Using semiconductor-type converter
363 56, 363 74, 363 40, H02M 324
Patent
active
058353712
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to bridge-type power converters in which one end of a series connection of two semiconductor switching elements each having a parallel diode is connected to the positive side of a d.c. bus while the other end is connected to the negative side.
BACKGROUND ART
In bridge-type power converters such as power inverters, the voltage of a d.c. voltage bus is switched by switching elements to provide an a-c output so that the voltage can change sharply. In order to reduce the switching loss, current ripples, and noise, the speed characteristic of elements has been improved, and the voltage change rate has increased. This raises the following problems.
(1) The abrupt voltage change against the ground produces electromagnetic waves, which interfere with other electronic equipment.
(2) Leak current goes to cables and electric motor windings via parasitic capacity.
(3) If a cable is long, there is a distributed constant circuit, forming a stable wave, and a voltage surge is applied to the electric motor.
(4) The high voltage change rate makes uneven a voltage distribution in the motor windings, applying a high voltage to a portion of the windings, causing insulation deterioration.
As a countermeasure, it is possible to provide a gradient to a driving signal for the gate or base of a main semiconductor element to reduce the switching speed. However, this method increases the switching loss.
As another measure an L-C resonance circuit is provided in the d.c. bus to block the d-c voltage so that the inverter bridge operates in the zero voltage period to avoid the sharp voltage change (see PCT patent application U.S. Ser. No. 87/02489). By this method, however, it is impossible to control continuously the time length of an output voltage.
As still another measure, a filter is provided between the inverter and the electric motor. However, if an L-C filter is used, the resistor for damping the continuing oscillation makes a loss. Also, it is necessary to determine a filter constant for individual motors.
FIG. 1 shows a three-phase voltage inverter main circuit. The bridge circuit is composed of six switching elements Q.sub.1 -Q.sub.6 and six free wheel diodes D.sub.1 -D.sub.6. The characteristics of the switching elements Q.sub.1 -Q.sub.6 determine the on/off switching speed. The voltage at the output terminal is switched in very short time periods, raising the aforementioned problem.
DISCLOSURE OF THE INVENTION
It is an object of the invention to provide a bridge-type power converter which is able to reduce the voltage change rate at switching times of the voltage applied to an electric motor which is driven by the bridge-type power converter.
A bridge-type power converter according to one aspect of the invention includes, in each phase, a inductor connected between a connection point of said two switching elements and an output terminal; capacitors connected between the output terminal and a positive or negative side of said d.c. bus for changing an output voltage along a curve determined by a semiconductors switching elements operate, causing abrupt changes of a voltage at the connection point; diodes for changing a current path at a time when said resonance goes approximately one quarter of period to thereby stop said resonance; and constant voltage diodes such as Zener diodes for absorbing energy stored in said inductor during said resonance.
According to another aspect of the invention there is provided a bridge-type power converter, wherein the constant voltage diodes connected to the positive and negative sides of d.c. bus in each phase of a multiple phase bridge-type power converter developed with the circuit according to claim 1 are replaced by diodes forming a high-voltage or low-voltage priority circuit and diodes forming backward current path, respectively.
According to still another aspect of the invention there is provided a bridge-type power converter, wherein the constant voltage diodes are replaced by a constant voltage circuit which comprises tran
REFERENCES:
patent: 4502085 (1985-02-01), Morrison et al.
patent: 4914558 (1990-04-01), Flickinger
Kume Tsuneo
Sawa Toshihiro
Sonoda Sumitoshi
Yamada Kenji
Kabushiki Kaisha Yaskawa Denki
Riley Shawn
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