JP3759334B2 - DC-AC power conversion circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a DC-AC power conversion circuit suitable for use in variable speed driving of a motor.
[0002]
[Prior art]
FIG. 7 shows a first conventional example. The figure shows an example in which a DC intermediate capacitor 15 and an inverter circuit 16 in which six sets of semiconductor switch elements are connected in a three-phase bridge are provided, and variable speed operation of a three-phase motor 17 as a load is performed by switching the semiconductor switch elements. is there.
FIG. 8 shows a second conventional example. DC intermediate capacitors 18 and 19 connected in series and an inverter circuit 20 in which four sets of semiconductor switch elements are connected in a two-phase bridge are provided, and an AC output unit of the inverter circuit 20 connected in a two-phase bridge with an intermediate point potential between 18 and 19 Is connected to a three-phase motor 17 and variable speed operation is performed by switching a semiconductor switch element.
[0003]
[Problems to be solved by the invention]
In the conventional circuit example of FIG. 7, when the motor is operated at a low speed, it is necessary to lower the voltage applied to the motor. Therefore, the semiconductor switch element of the inverter needs to be pulse width modulated (PWM) for voltage control. At this time, since the voltage is applied to the motor by turning on the switches of the upper arm and the lower arm, the DC voltage of the DC intermediate capacitor 15 is turned on and off in the motor. As a result, high-frequency current flows through the motor, eddy current loss increases, and motor efficiency decreases. In order to reduce this eddy current loss, the DC power supply 21 may be made variable and a pulse amplitude modulation (PAM) operation may be performed. However, since the DC power supply is usually made by rectifying commercial AC power, Is limited, and PAM operation is performed at a certain high rotational speed.
[0004]
On the other hand, in the method of FIG. 8, it is possible to apply a voltage to the motor by turning on one semiconductor switch element, and the voltage value is ½ of the sum of the DC voltages of the capacitors 18 and 19, so The current loss is smaller than that of the method shown in FIG. 7, and the voltage applied to the motor is low on average, so that the PAM operation can be performed at a low rotational speed. Further, since the method of FIG. 7 has a larger number of semiconductor switch elements than the example of FIG. 8, the loss in the semiconductor element portion also increases.
[0005]
In the high-speed operation region in which the PAM operation is performed in the method of FIG. 7, the DC voltage of the capacitors 18 and 19 is smaller in the method of FIG. 8 than in the method of FIG. Need to be high (approximately twice). Therefore, the system shown in FIG. 8 has a problem that a semiconductor switch element having a high breakdown voltage is required, resulting in an increase in cost and a loss in a circuit for boosting direct current. Thus, it can be said that the two-phase method of FIG. 8 is suitable for low-speed operation of the motor, and the three-phase method of FIG. 7 is suitable for high-speed operation.
[0006]
In addition, there are mechanical switches such as contactors and semiconductor switch elements using transistors and diodes as bidirectional switches, and the former has the merit that its conduction loss is negligibly small when it is completely turned on. However, since it takes several tens of ms to turn on from off or from on to off, a phenomenon such as chattering occurs during that period, so it synchronizes with the semiconductor switch element used in the inverter section. There is a demerit that it cannot be switched. In addition, since the latter is the same semiconductor switch element, it can be switched in synchronism with the semiconductor switch element used in the inverter section. However, since voltage drop occurs at the time of conduction, the generated loss in that part increases, This is a negative effect of efficiency.
Accordingly, an object of the present invention is to provide a DC-AC power conversion circuit that enables high-efficiency operation in the entire rotation speed range of a motor.
[0007]
[Means for Solving the Problems]
In order to solve such a problem, according to the first aspect of the present invention, a series circuit of a plurality of capacitors is connected in parallel to a DC power supply, and a semiconductor switch element and six pairs of diodes connected in reverse parallel thereto are bridge-connected. A mechanical switch including a contactor and a semiconductor switch element between one of the three-phase AC outputs of the inverter that converts direct current to three-phase alternating current and an intermediate potential point of the series-connected capacitors. Connect a parallel circuit with a bidirectional switch capable of bidirectional switching,
The mechanical switch and the bidirectional switch are turned off and used as a three-phase inverter, the mechanical switch and the bidirectional switch are turned on, and the phase of the inverter to which the mechanical switch and the bidirectional switch are connected is connected. The semiconductor switch element is always turned off and used as a two-phase bridge-connected three-phase inverter.
In the first aspect of the invention, the motor driven by the inverter can be driven as the two-phase bridge connection type three-phase inverter during low-speed operation, and can be driven as the three-phase inverter during high-speed operation. Item 2).
[0008]
In the first or second aspect of the invention, switching between the two-phase bridge connection type three-phase inverter drive and the three-phase inverter drive can be performed based on a speed command of a motor driven by the inverter. (Invention of Claim 3).
In the invention according to any one of claims 1 to 3, in turning off the mechanical switch and the bidirectional switch, the bidirectional switch can be turned off after a predetermined time after the mechanical switch is turned off. (Invention of claim 4) In the invention of claim 4, the timing of turning off the bidirectional switch and the timing of switching the semiconductor switch element of each phase arm of the inverter can be synchronized (invention). 5 invention).
According to the first aspect of the present invention, the ON timing of the mechanical switch and the bidirectional switch and the OFF state of the upper and lower arm semiconductor switch elements of the phase of the inverter to which the mechanical switch and the bidirectional switch are connected. Timing can be synchronized (invention of claim 6 ).
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing a first embodiment of the present invention.
The direct current intermediate circuit shown in the figure includes capacitors 1 and 2 connected in series, one phase of the three-phase AC output of the three-phase inverter 3, and the intermediate potential point of the capacitor of the direct current section. In this configuration, the target switch 4 and the semiconductor switch element 5 capable of bidirectional switching are connected in a parallel circuit. As a semiconductor switching element capable of bidirectional switching, a semiconductor switching element and a diode combined in series as shown in FIG. 2A or 2B can be used. In general, a plurality of DC intermediate capacitors can be provided.
[0010]
FIG. 3 is a block diagram showing a second embodiment of the present invention, and shows a first control apparatus example in FIG. In the figure, a control block 6 is a block for operating at high speed operation of the motor, and the circuit of FIG. 1 is used as a main circuit in the same manner as FIG. 7 (three-phase method: three-phase inverter). The control block 7 is a block for operating the motor at a low speed, and the circuit of FIG. 1 is used as the main circuit in the same manner as FIG. 8 (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, in order to turn on both the mechanical switch 4 and the semiconductor switch element 5 capable of bidirectional switching, and to make it equivalent to the two-phase system of FIG. This is achieved by always turning it off.
[0011]
First, the comparator 10 compares the motor rotation speed command 8 with a switching value 9 for switching from the two-phase method to the three-phase method. At that time, the OFF command to the semiconductor switch element 5 capable of bidirectional switching is output after a preset time by the timer 11 after the comparator 10 operates, and is controlled in synchronization with this signal. Switch from block 7 to control block 6. At that time, the switching is smoothly performed by synchronously switching off the semiconductor switch element 5 and switching the semiconductor switch of the inverter.
FIG. 4 shows the state at this time. When the command 8 coincides with the switching value 9, the mechanical switch 4 such as a contactor is turned off, and then the semiconductor switch element 5 is turned off after a lapse of the timer time. Recognize.
[0012]
FIG. 5 is a block diagram showing a third embodiment of the present invention, and shows a second control apparatus example in FIG. Reference numeral 13 denotes a comparator.
This is an example when the motor shifts from high speed to low speed, and the motor rotation speed command 8 is compared with a switching value 12 for switching from the three-phase method to the two-phase method in the comparator 13. In response to the output signal 14 indicating that the motor rotational speed command 8 has become the switching value 12 or less, the mechanical switch 4 and the semiconductor switch element 5 capable of bidirectional switching are turned on, and the control blocks 6 to 7 are switched on. Switch to. Thereby, the semiconductor switches Ta and Td are operated so as to be always 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 the contactor and the semiconductor switch element 5 are turned on, and immediately from the three-phase method to the two-phase method. It turns out that it switches.
[0013]
【The invention's effect】
According to the present invention, since the motor is equivalently driven by the two-phase method at low speed operation and equivalently by the three-phase method at high speed operation, it is possible to perform highly efficient operation in the entire rotation speed range. Become.
Further, for example, when shifting from the two-phase method to the three-phase method, a certain set time after the mechanical switch is turned off (until the mechanical switch is actually turned off after an OFF signal is input to the mechanical switch). Since the semiconductor switch element is turned off during the transition period and the semiconductor switch element is turned on during the transitional period, the control of the inverter may be left in the two-phase system, which is affected by the switching operation of the mechanical switch. Not. In addition, the switching is smoothly performed by synchronously switching off the semiconductor switch element and switching the semiconductor switch of the inverter.
On the other hand, when shifting from the three-phase method to the two-phase method, the mechanical switch and the semiconductor switch element are switched on by synchronizing the turn-on of the upper and lower arm semiconductor switch elements with a predetermined one phase of the inverter. Will be smooth.
[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 illustrating an example of a bidirectional switch used in FIG. 1;
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]
DESCRIPTION OF SYMBOLS 1, 2 ... Capacitor, 3 ... Inverter, 4 ... Mechanical switch, 5 ... Semiconductor switch element, 6 ... 3 phase system control block, 7 ... 2 phase system control block, 10, 13 comparator, 11 ... Timer.

Claims (6)

A three-phase AC output of an inverter that connects a DC power supply to a series circuit of a plurality of capacitors in parallel, bridges a semiconductor switch element and six diodes connected in antiparallel thereto, and converts the direct current into a three-phase alternating current A parallel circuit of a mechanical switch including a contactor and a bidirectional switch including a semiconductor switch element capable of bidirectional switching is connected between one of the above and an intermediate potential point of the series-connected capacitors. And
The mechanical switch and the bidirectional switch are turned off and used as a three-phase inverter, the mechanical switch and the bidirectional switch are turned on, and the phase of the inverter to which the mechanical switch and the bidirectional switch are connected is connected. A DC-AC power conversion circuit characterized in that the semiconductor switch element is always turned off and used as a two-phase bridge connection type three-phase inverter. 2. The DC-AC of claim 1, wherein the motor driven by the inverter is driven as the two-phase bridge-connected three-phase inverter during low-speed operation, and is driven as the three-phase inverter during high-speed operation. Power conversion circuit. 3. The direct current according to claim 1, wherein switching between the two-phase bridge connection type three-phase inverter drive and the three-phase inverter drive is performed based on a speed command of a motor driven by the inverter. AC power conversion circuit. 4. The direct current according to claim 1, wherein the mechanical switch and the bidirectional switch are turned off after the mechanical switch is turned off and then the bidirectional switch is turned off after a predetermined time. AC power conversion circuit. 5. The DC-AC power conversion circuit according to claim 4, wherein a timing at which the bidirectional switch is turned off and a timing at which the semiconductor switch element of each phase arm of the inverter is switched are synchronized. The on-timing of the mechanical switch and the bidirectional switch is synchronized with the off-timing of the upper and lower arm semiconductor switch elements of the phase of the inverter to which the mechanical switch and the bidirectional switch are connected. The DC-AC power converter circuit according to claim 1.

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