JP2000350476A - Dc-ac power conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC-AC power conversion circuit which is capable of highly efficient operation over its entire revolution range, when it is used for motor drive. SOLUTION: For example, the series circuit of for example two capacitors 1 and 2 is connected in parallel to a DC power source, and the parallel circuit of a mechanical switch 4 and a semiconductor switch element 5, capable of bidirectional switching, is connected between one of the three-phase AC outputs of an inverter, where the semiconductor switch elements and six pairs of diodes connected in back-to-back with them are wired in bridge, and the intermediate potential point between the capacitors 1 and 2, and both switches 4 and 5 of this parallel circuit are turned off, so that the three-phase system (for high speed) of an inverter and both switches 4 and 5 of the parallel circuit are both turned off, and besides the semiconductor switch elements Ta and Tb are turned off at all times, thus this circuit is operated as a two-phase system (for low speed) of inverter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-AC power conversion circuit suitable for variable-speed driving of a motor and the like.

[0002]

2. Description of the Related Art FIG. 7 shows a first conventional example. The figure shows that a DC intermediate capacitor 15 and six sets of semiconductor switch elements are connected to three.
In this example, a three-phase motor 17 serving as a load is operated at a variable speed by switching a semiconductor switch element with an inverter circuit 16 connected in a phase bridge. FIG. 8 shows the second
The following shows a conventional example. DC intermediate capacitor 1 connected in series
8 and 19, and an inverter circuit 20 in which four sets of semiconductor switch elements are connected in a two-phase bridge, and an intermediate point potential between 18 and 19 and an AC output part of the inverter circuit 20 in a two-phase bridge connection are connected to a three-phase motor 17. This is an example in which the variable speed operation is performed by switching of the semiconductor switch element.

[0003]

In the conventional circuit example shown in FIG. 7, when the motor is operated at a low speed, the voltage applied to the motor must be reduced. It needs to be modulated (PWM). At this time, the voltage is applied to the motor by turning on the switches of the upper arm and the lower arm, so that the DC voltage of the DC intermediate capacitor 15 is turned on and off to the motor. As a result, a high-frequency current flows through the motor, so that the eddy current loss increases and the motor efficiency decreases. In order to reduce the eddy current loss, the DC power supply 21 may be changed to perform pulse amplitude modulation (PAM) operation. However, since the DC power supply is usually made by rectifying a commercial AC power supply, the DC power supply has a lower limit. Is limited, and the PAM operation is performed from a certain high rotational speed.

On the other hand, in the method shown in FIG. 8, a voltage can be applied to the motor by turning on one semiconductor switch element, and the voltage value is の of the sum of the DC voltages of the capacitors 18 and 19. Therefore, the eddy current loss is smaller than that in the system of FIG. 7, and the voltage applied to the motor is low on average, so that the PAM operation can be performed from a low rotation speed. Further, in the method of FIG. 7, the number of semiconductor switch elements is larger than that of the example of FIG.

In the high-speed operation region in which the PAM operation is performed in the system of FIG. 7, the DC voltage of the capacitors 18 and 19 in the system of FIG. 8 is lower than that of the system of FIG. It needs to be higher (approximately twice) than the method. Therefore, the method shown in FIG. 8 requires a semiconductor switch element having a high withstand voltage, resulting in an increase in cost and an increase in loss in a DC boosting circuit. Thus, it can be said that the two-phase system of FIG. 8 is suitable for the low-speed operation of the motor, and the three-phase system of FIG. 7 is suitable for the high-speed operation.

As a bidirectional switch, there are a mechanical switch such as a contactor and a semiconductor switch element using a transistor and a diode. The former is such that when it is completely turned on, its conduction loss is negligible. Although it has the advantage of being small, it takes several tens of milliseconds to turn from on to off or from off to on.
During that period, since a phenomenon such as chattering occurs, there is a disadvantage that switching cannot be performed in synchronization with a semiconductor switching element used in the inverter unit. In addition, since the latter is the same semiconductor switch element, it can be switched in synchronization with the semiconductor switch element used in the inverter section. Efficiency is adversely affected. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a DC-AC power conversion circuit that enables high-efficiency operation in the entire rotational speed range of a motor.

[0007]

In order to solve such a problem, according to the present invention, a series circuit of a plurality of capacitors is connected to a DC power supply in parallel, and a semiconductor switch element and an anti-parallel circuit are connected to the semiconductor switch element. A machine including a contactor between one of three-phase AC outputs of an inverter for bridging six sets of connected diodes and converting DC to three-phase AC, and an intermediate potential point of the series-connected capacitor. A parallel circuit of a dynamic switch and a bidirectional switch including a semiconductor switch element and capable of bidirectional switching is connected, the mechanical switch and the bidirectional switch are turned off, and the mechanical switch is used as a three-phase inverter. A two-phase bridge by turning on a bidirectional switch and constantly turning off a semiconductor switch element for a predetermined phase of the inverter. It characterized by using as a line type 3-phase inverter.

In the first aspect of the present invention, when turning off the mechanical switch and the bidirectional switch, the bidirectional switch can be turned off a predetermined time after the mechanical switch is turned off (claim). Item 2
Invention). According to the second aspect of the invention, it is possible to synchronize the timing of turning off the bidirectional switch with the timing of switching the semiconductor switch element of each phase arm of the inverter (the third aspect of the invention). Further, according to the first aspect of the present invention, the on-timing of the mechanical switch and the bidirectional switch can be synchronized with the off-timing of the upper and lower arm semiconductor switch elements of a predetermined phase of the inverter. Item 4)).

[0009]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention. As the DC intermediate circuit in the figure,
Capacitors 1 and 2 connected in series and three-phase inverter 3
Of one of the three-phase AC outputs and the intermediate potential point of the capacitor in the DC part is connected to a mechanical switch 4 such as a contactor.
And a semiconductor switch element 5 capable of bidirectional switching. As the semiconductor switch element capable of bidirectional switching, a bridge type or a series combination of a semiconductor switch element and a diode as shown in FIG. 2A or 2B can be used. Generally, a plurality of DC intermediate capacitors can be provided.

FIG. 3 is a block diagram showing a second embodiment of the present invention, and shows a first control device example in FIG. In the figure, a control block 6 is a block for operating the motor at a high speed operation, and uses the circuit of FIG. 1 as a main circuit in the same manner as in FIG. 7 (three-phase system: three-phase inverter). The control block 7 is a block for operating the motor at low speed operation.
(Two-phase system: two-phase bridge connection type three-phase inverter). In addition, 10 is a comparator, and 11 is a timer. In the circuit of FIG. 1, the mechanical switch 4 and the semiconductor switch element 5 capable of bidirectional switching are both turned on, so that the semiconductor switches Ta and Td are equivalently equivalent to the two-phase system of FIG. This is achieved by always turning off.

First, the comparator 10 compares the motor speed command 8 with a switching value 9 for switching from the two-phase system to the three-phase system. At this time, an off command to the semiconductor switch element 5 capable of bidirectional switching is output after a time set in advance by the timer 11 after the operation of the comparator 10, and control is performed in synchronization with this signal. Switch from block 7 to control block 6. At this time, the switching is smoothly performed by synchronizing the turning off of the semiconductor switch element 5 and the switching of the semiconductor switch of the inverter. FIG. 4 shows the state at this time. When the command 8 matches the switching value 9, the mechanical switch 4 such as a contactor is turned off, and the semiconductor switch element 5 is turned off after a lapse of a timer time. Understand.

FIG. 5 is a block diagram showing a third embodiment of the present invention, and shows a second control device example in FIG. 13
Is a comparator. This is an example in which the motor shifts from high speed to low speed, and the motor rotation speed command 8 is compared in the comparator 13 with the switching value 12 for switching from the three-phase system to the two-phase system. Motor speed command 8 is the switching value 1
The mechanical switch 4 and the semiconductor switch element 5 capable of bidirectional switching are turned on by an output signal 14 indicating that the number has become 2 or less, and the control block is switched from 6 to 7. Thereby, the semiconductor switch Ta
And Td are always operated to be in the off state. FIG. 6 shows the state at this time. When the command 8 coincides with the switching value 12, both the mechanical switch 4 such as a contactor and the semiconductor switch element 5 are turned on, and immediately from the three-phase system to the two-phase system. It turns out that it changes.

[0013]

According to the present invention, the motor is driven equivalently by the two-phase method at the time of low-speed operation and equivalently by the three-phase method at the time of high-speed operation. Operation becomes possible. Further, for example, when shifting from the two-phase system to the three-phase system, after a certain set time from when the mechanical switch is turned off (from the time the OFF signal is input to the mechanical switch until the mechanical switch is actually turned off) Since the semiconductor switching element is turned off at the time of) and the semiconductor switching element is turned on during the transitional period, the control of the inverter may be kept in the two-phase mode, and the switching operation of the mechanical switch may have an effect. Not done. In addition, the switching is smoothly performed by synchronizing the turning off of the semiconductor switch element and the switching of the semiconductor switch of the inverter. On the other hand, when shifting from the three-phase system to the two-phase system, the switching is performed by synchronizing the ON state of the mechanical switch and the semiconductor switch element with the OFF state of the semiconductor switch elements of the upper and lower arms of a predetermined phase of the inverter. Becomes smoother.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing an example of a bidirectional switch used in FIG.

FIG. 3 is a block diagram showing a first control device in FIG. 1;

4 is an operation explanatory diagram when FIG. 1 is driven by the control device of FIG. 3;

FIG. 5 is a block diagram showing a second control device in FIG. 1;

6 is an operation explanatory diagram when FIG. 1 is driven by the control device of FIG. 5;

FIG. 7 is a schematic diagram showing a first conventional example.

FIG. 8 is a schematic diagram showing a second conventional example.

[Explanation of symbols]

1, 2, capacitor, 3 inverter, 4 mechanical switch, 5 semiconductor switch element, 6 three-phase control block, 7 two-phase control block, 10, 13 comparator,
11 ... Timer.

Claims (4)

[Claims] 1. An inverter for converting a direct current to a three-phase alternating current by connecting a series circuit of a plurality of capacitors in parallel to a direct current power source and bridging a semiconductor switch element and six sets of diodes connected in anti-parallel to the semiconductor switch element. A mechanical switch including a contactor, a bidirectional switch including a semiconductor switch element and capable of bidirectional switching, between one of the three-phase AC outputs and an intermediate potential point of the series-connected capacitor. And turning off the mechanical switch and the bidirectional switch.
Use as a two-phase bridge-connected three-phase inverter by turning on the mechanical switch and the bidirectional switch, and by always turning off the semiconductor switch element for one predetermined phase of the inverter. DC-AC power conversion circuit. 2. The direct current switch according to claim 1, wherein the mechanical switch and the bidirectional switch are turned off, and then the mechanical switch is turned off, and then the bidirectional switch is turned off a predetermined time after the mechanical switch is turned off. -AC power conversion circuit. 3. The DC-AC power conversion circuit according to claim 2, wherein the timing for turning off the bidirectional switch and the timing for switching the semiconductor switch element of each phase arm of the inverter are synchronized. . 4. The semiconductor device according to claim 1, wherein the on timing of the mechanical switch and the bidirectional switch is synchronized with the off timing of a semiconductor switch element of an upper and lower arm of a predetermined phase of the inverter. DC-AC power conversion circuit.