JPH11206186A - Control method of neutral point clamp inverter - Google Patents

Abstract

(57) [Abstract] (With correction) [PROBLEMS] In an AC motor controlled by a neutral point clamp type inverter, it is possible to re-run or regenerate immediately after an overvoltage suppression circuit operates. A switching element (8U) is provided on the DC side of an inverter (11) when filter capacitors (7p, 7n) become overvoltage.
In a control method of a neutral point clamp type inverter for controlling an AC motor 4 at a variable speed by an inverter including an overvoltage suppression circuit for firing and discharging 1 to 8W4,
When the switching element of the overvoltage suppression circuit is fired while the AC motor is rotating and the voltage of the filter capacitor reaches a predetermined value, the clamp diode 10U1 of the inverter is driven.
The switching elements of the inverter interposed between the AC motor and the AC motor are all ignited over the respective phases.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method of a neutral point clamp type inverter for driving an AC motor at a variable speed, and more particularly to a control method when an overvoltage suppression circuit provided in the neutral point clamp type inverter operates. About.

[0002]

2. Description of the Related Art Conventionally, a DC power supply and a filter capacitor are connected between DC terminals of an inverter for converting DC to AC, and a series connection of a resistor and a switching element is provided at both ends of the filter capacitor for overvoltage protection. Is connected. An example in which such a configuration is applied to a neutral point clamp type inverter is disclosed in
-244702.

Here, an example in which the overvoltage suppression circuit operates will be described. In the configuration in which the AC motor is controlled by the inverter, when the mode is switched from the power running operation mode to the regenerative braking operation mode, the AC motor becomes a generator to generate AC power, and the power is converted into DC power by the inverter. Returned to DC power supply. However, when there is no other load on the DC power supply side, the energy is not absorbed, but is absorbed by the filter capacitor, and the DC voltage rises. In this case, the switching element of the overvoltage suppression circuit is fired to absorb energy in the overvoltage suppression resistor, thereby preventing the overvoltage. When the switching element of the overvoltage suppression circuit is fired, the inverter is stopped and the contactor for supplying power from the DC power supply to the filter capacitor is opened.

[0004]

In particular, when a thyristor element having no self-extinguishing function is used as a switching element of the overvoltage suppression circuit, the thyristor element is still operated by the residual induced voltage of the AC motor even after the filter capacitor is discharged. Current continues to flow through For this reason, the thyristor element is not turned off for several seconds to several tens of seconds until the current becomes equal to or less than the holding current of the thyristor element, so that the contactor for starting the inverter cannot be turned on again, and powering or There is a problem that it is not ready for regeneration.

SUMMARY OF THE INVENTION It is an object of the present invention to control an AC motor by a neutral point clamp type inverter. In the present invention, a neutral point clamp type inverter capable of re-powering or regenerating immediately after the overvoltage suppression circuit is operated. It is to provide a control method.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a neutral point clamp type inverter for converting DC power stored in a filter capacitor into AC power having a variable frequency and a variable voltage, or vice versa. A control method of a neutral point clamp type inverter for controlling an AC motor at a variable speed by the inverter, comprising an overvoltage suppressing circuit that ignites and discharges a switching element when the filter capacitor becomes overvoltage on the DC side of the inverter. While the AC motor is rotating, the switching element of the overvoltage suppression circuit is fired, and when the voltage of the filter capacitor reaches a predetermined value, the inverter interposed between the clamp diode of the inverter and the AC motor. Are fired over all the phases.

As described above, when the switching element of the overvoltage suppression circuit is ignited and the voltage of the filter capacitor reaches a predetermined value, the switching element of the inverter interposed between the clamp diode of the inverter and the AC motor is connected to each phase. , A closed circuit including the AC motor, the switching element of the inverter, and the clamp diode is formed, and the current generated by the electromotive force remaining in the AC motor is circulated and attenuated by the closed circuit. As a result, the current from the AC motor does not flow through the switching element of the overvoltage suppression circuit, so that the switching element of the overvoltage suppression circuit can be quickly extinguished after the discharge of the filter capacitor is completed.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the present invention,
The configuration of a neutral point-clamped inverter driven by an AC motor to which the present invention is applied will be described with reference to FIG. In the figure, reference numeral 1 denotes a DC power supply, which may be a DC power supply converted from an AC power supply by a converter. The DC power supply 1 is connected between the DC terminals of the neutral point clamp type inverter 11 via the contactor 2 and the filter reactor 3. The two terminals of the filter capacitors 7p and 7n divided in series are connected between the DC terminals, and an overvoltage suppression circuit composed of a resistor 5 and a thyristor element 6 in series is connected to both ends.

[0011] Eleven neutral point clamp type inverters are:
The circuit configuration for one phase (one arm) in a three-phase (U, V, W) configuration is as follows. The U-phase arm is a series body of at least four switching elements (8U1, 8U2, 8U3, 8U4) such as IGBTs connected in series.
(Upper arms 8U1, 8U2, lower arms 8U3, 8U4), both ends of which are connected to both ends where filter capacitors 7p and 7n are connected in series. Freewheel diodes (9U1, 9U2, 9U3, 9U4) connected in parallel with opposite polarities are connected to the respective switching elements. Switching elements 8U1 and 8U constituting upper arm
Switching element 8 forming the connection point 2 and the lower arm
The connection point between U3 and 8U4 is connected to the filter capacitors 7p and 7p via the clamp diodes 10U1 and 10U2, respectively.
7n (also referred to as a neutral potential point).
An armature winding of a corresponding phase of the AC motor 4 such as an induction motor is connected to a connection point (AC output terminal) between the upper arm and the lower arm.

Next, the operation of the above configuration will be described. When the contactor 2 is turned on, the series circuit of the filter capacitors 7p and 7n is charged to the voltage of the DC power supply 1. When an ignition pulse signal from a control device (not shown) is supplied to the switching element of the neutral point clamping type inverter 11 so as to control the AC motor 4 at a variable speed, the potential of the neutral point is applied to the AC output terminal. The resulting voltage waveform is obtained. For this reason, the inverter having this configuration is generally called a neutral point clamp type inverter.

The present invention resides in a control method when an overvoltage is applied to the filter capacitor in the regenerative braking mode in such a configuration, which will be described below. In the embodiment, since control is performed by software by a microcomputer in a control device for controlling the inverter, the control steps will be described with reference to a flowchart in FIG.

(Step 1) The DC terminal voltage value Vd of the filter capacitor is read.

(Step 2) Compare Vd with a predetermined overvoltage value Vro, and if Vd <Vro, continue operation;
If d ≧ Vro, go to step 3.

(Step 3) The contactor 2 is opened, the inverter is stopped, and the thyristor element 6 of the overvoltage suppression circuit is
Is fired.

(Step 4) Compare Vd with a predetermined value Vr1, and if Vd> Vr1, return to step 3; if Vd ≦ Vr1, proceed to step 5.

(Step 5) The switching elements 8U2, 8U3, 8V2, 8V3, 8W2, 8 of the inverter 11
Only W3 is fired.

Note that the predetermined value Vr1 may be several seconds to ten seconds.
In a few seconds, the voltage of the filter capacitor is set to a voltage value until the electric charge of the filter capacitor is discharged by the resistor and becomes the holding current of the thyristor element.

According to the above control steps, when the filter capacitor becomes overvoltage, the thyristor element 6 of the overvoltage suppression circuit is fired, so that energy is absorbed by the resistor 5 and the voltage of the filter capacitor is attenuated. Go.
At that time, the neutral point clamp type inverter 11
Is stopped, and the main circuit is opened from the DC power supply 1 by opening the contactor 2.

As described above, the filter capacitors 7p and 7n
Continue to discharge, and when the voltage becomes equal to or less than the predetermined value Vr1, each switching element 8U2, 8U3, connected to the output point of each phase of the neutral point clamp type inverter 11
When all 8V2, 8V3, 8W2, 8W3 are fired, the AC motor 4, the switching elements 8U2, 8U3, 8V2, 8V3, 8W2, 8W3 and the clamp diodes 10U1, 10U2, 10V1, 10V are thereby obtained.
A closed circuit composed of 2, 10W1 and 10W2 is formed, and the current flowing due to the induced voltage of the AC motor flows along the path indicated by the arrow shown in FIG. FIG. 2 shows a case where an induced voltage is generated so that a current flows out of the U-phase terminal of the AC motor. In a case where a current flows out of the V-phase or W-phase, the current similarly flows through the closed circuit. Become.

Therefore, after the discharge of the filter capacitor, the current flowing through the thyristor element 6 immediately becomes equal to or lower than the holding current, and the thyristor element is quickly turned off. Therefore, it is not necessary to make the thyristor element of the overvoltage suppression circuit have a breaking ability, and the thyristor element can be quickly turned off only by a very simple change of software. For this reason, after the discharge of the filter capacitor due to the overvoltage, the contactor 2 can be turned on again, and preparation for repowering or regeneration can be completed within a short time. In the above embodiment, the switching element of the overvoltage suppression circuit is a thyristor element, but GTO, ITO is used instead of the switching element.
When a self-extinguishing element such as a GBT is used, zero current interruption can be performed according to the present invention, so that loss due to interruption does not occur. And cost reduction.

[0021]

According to the present invention, during the regenerative braking control of the AC motor by the neutral point clamp type inverter,
After the operation of the overvoltage suppression circuit is started, control is performed so that the regenerative current from the AC motor does not flow to the DC side of the inverter from the time when the filter capacitor voltage becomes equal to or less than a predetermined value. And the power running control or the regenerative control by the inverter can be promptly performed.

[Brief description of the drawings]

FIG. 1 is a control flowchart for explaining an embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating a configuration targeted by the present invention and an operation of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DC power supply, 2 ... Contactor, 4 ... AC motor, 5 ... Resistor, 6 ... Thyristor element, 7p, 7n ... Filter capacitor, 8U1-8W4 ... Switching element, 9U1-9
W4: Freewheel diode, 10U1-10W2
... Clamp diode, 11 ... Neutral point clamp type inverter.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hiroyuki Ozawa 1070 Ma, Hitachinaka City, Ibaraki Prefecture Inside the Mito Plant of Hitachi, Ltd. Mito factory

Claims (2)

[Claims] 1. A neutral point clamp type inverter for converting DC power stored in a filter capacitor into AC power having a variable frequency and a variable voltage, or vice versa, wherein the filter capacitor is provided on the DC side of the inverter. A control method of a neutral point clamp type inverter for controlling an AC motor at a variable speed by the inverter, comprising an overvoltage suppression circuit for firing and discharging a switching element when an overvoltage occurs, wherein the AC motor is rotating. When the switching element of the overvoltage suppression circuit is fired and the voltage of the filter capacitor reaches a predetermined value, the switching element of the inverter, which is interposed between the clamp diode of the inverter and the AC motor, extends over each phase. Neutral point clamp type inverter characterized in that all are ignited Control method. 2. A neutral point clamp type inverter for converting DC power stored in a filter capacitor into AC power having a variable frequency and a variable voltage, or vice versa, wherein the filter capacitor is provided on the DC side of the inverter. A control method of a neutral point clamp type inverter for driving an AC motor by the inverter, wherein the inverter is provided with an overvoltage suppression circuit for igniting and discharging a switching element when an overvoltage occurs, wherein the overvoltage suppression is performed while the AC motor is rotating. When the switching element of the circuit is ignited and the voltage of the filter capacitor reaches a predetermined value, the current of the AC motor is returned between the AC motor and the inverter without flowing to the DC side of the inverter. The inverter is controlled by a neutral point clamp type inverter. Your way.