Electric power conversion systems – Frequency conversion without intermediate conversion to d.c. – By semiconductor converter
Patent
1998-03-09
1999-10-19
Wong, Peter S.
Electric power conversion systems
Frequency conversion without intermediate conversion to d.c.
By semiconductor converter
363161, 363 8, 363 10, H02M 1302, H02M 5257, H02M 5275
Patent
active
059699663
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a power converting apparatus and a power converting method for driving a medium to high voltage AC motor at a variable speed, and particularly to a power converting apparatus and a power converting method of a pulse width modulation (PWM) controlling system.
2. Discussion of the Background
Conventionally, for variable speed driving of a high voltage AC motor, a system which employs a high voltage invertor or another system wherein a step-down transformer and a step-up transformer are connected to the input side and the output side of a low voltage invertor to drive the high voltage AC motor is employed.
FIG. 6 is a circuit diagram of a driving circuit which employs a high voltage invertor of a conventional example, and FIG. 7 is a concept diagram illustrating four quadrature operation based on the relationship between the torque and the speed of a motor. In FIG. 6, reference symbol 10 denotes a high voltage AC motor of an object of driving, 101 an invertor unit, 102 a smoothing capacitor unit, 103 a regenerative converter unit, 104A and 104B each denotes an AC reactor, and 105 denotes a three-phase transformer.
The invertor unit 101 includes three-level invertors of the neutral clamping type and employs, for power elements, a GTO (Gate Turn Off Thyristor, hereinafter referred to simply as GTO) to assure a high withstanding voltage for the elements. The power elements are connected in series to achieve divisional sharing of a voltage, and variable voltage variable frequency (VVVF) power is supplied from a high voltage DC power supply formed from the smoothing capacitor unit 102 to the invertor unit 101. In order to keep divisional sharing of a voltage of the GTOS, well known snubber circuits must be installed individually. In the converter unit which supplies a DC voltage to the smoothing capacitor unit 102, the capacity of the high voltage invertors is generally as high as several hundreds kW or more, and the construction of the regenerative converter unit 103 is used for damping energy processing upon deceleration or for four quadrature operation (forward driving, reverse driving, forward regeneration and reverse regeneration) illustrated in FIG. 7. In FIG. 6, two circuits each composed of a combination of thyristors and GTOs are used in series connection, and control between driving and regeneration is performed depending upon the direction of DC power. The regenerative converter unit 103 is connected to secondary windings of the three-phase transformer 105 through the AC reactors 104A and 104B while primary windings of the three-phase transformer 105 are connected to a high voltage commercial power supply so as to receive supply of power.
FIG. 8 is a circuit diagram showing a driving circuit which employs a low voltage invertor of a conventional example. In FIG. 8, reference numeral 10 denotes a high voltage AC motor of an object of driving, 106 an invertor unit, 107 a smoothing capacitor unit, 108 a regenerative converter unit, 109 an AC reactor, 110 a step-down transformer, and 111 a step-up transformer.
The invertor unit 106 includes IGBTs (Insulated Gate Bipolar Transistors, hereinafter referred to simply as IGBTs) and diodes connected in a three-phase bridge circuit and is pulse width modulation (hereinafter referred to simply as PWM) controlled so that it may output a voltage and a frequency necessary to drive the motor 10 through the step-up transformer 111. Since the invertor unit 106 is a low voltage invertor, it is connected to the high voltage AC motor 10 through the step-up transformer 111. Also the regenerative converter unit 108 is composed of IGBTs and diodes connected in a three-phase bridge circuit similarly as in the invertor unit 106, and is connected to secondary windings of the step-down transformer 110 through the AC reactor 109 while primary windings of the step-down transformer 110 are connected to a high voltage commercial power supply so as to receive supply of power. Meanwhile, also DC buses of th
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Hirano Koichi
Kume Tsuneo
Sawa Toshihiro
Kabushiki Kaisha Yaskawa Denki
Vu Bao Q.
Wong Peter S.
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