JP2008099508A - Power converter and air conditioner using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power converter capable of improving efficiency in motor driving during low-speed operation and reducing noise, by allowing the applied voltage between terminals of a motor to be a DC voltage of an inverter or lower. <P>SOLUTION: The power converter comprises two DC power sources equipped with a neutral-point output terminal connected between DC terminals, three pairs of upper/lower arms connected in series among DC terminals, with the same polarity, three pairs of intermediate arms consisting of a bidirectional switch, connected between the neutral point output terminal and three connection points between the upper arm and the lower arm of each pair; a main circuit device consisting of a three-phase motor connected to three connection points; and a main circuit control means which performs on/off control by alternately fixing upper and lower arms of any one phase to be on-state, and executing pulse width modulation to generate each drive signal of the upper, lower, and intermediate arms of remaining two phases. The main circuit control means turns on/off the intermediate arm at twice the duty ratio, when the duty ratio acquired by the pulse width modulation is 50% or lower. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a power converter such as an inverter that performs variable speed of a motor, and an air conditioner using the same.

  Conventionally, in air conditioners, technological development for improving energy saving performance has been continuously performed.

  In recent years, the system efficiency has come to be regarded as important especially at light loads that occur frequently during the total operation time, that is, during low-speed operation of the compressor.

  In general, it is well known that the efficiency of a motor drive system using an inverter is higher when the voltage is lower than the DC voltage required for driving the motor, and the efficiency is higher as the voltage is lower, and the motor can be driven with lower noise.

  Therefore, even in the air conditioner power converter, the output voltage supplied to the inverter during low-speed operation of the compressor is lower than that during high-speed operation, thereby improving efficiency and reducing noise during light loads. can do.

  Conventionally, as one of inverter power supply devices that realize such a power conversion device, short-circuit means for short-circuiting an AC power supply via a reactor and short-circuit control means for controlling the short-circuit means (power factor improvement means), A system has been devised that includes boosting means by switching operation by the short-circuit control means and switches the rectifier circuit to a full-wave rectifier circuit or a double voltage rectifier circuit according to the load by the switch control means (see, for example, Patent Document 1). ).

  FIG. 12 shows the configuration of the power supply device described in Patent Document 1. As shown in FIG. 12, the conventional power supply device includes a rectifier circuit 12 that rectifies an AC voltage from an AC power supply 11 to generate a DC voltage, a reactor 13 connected in series to the AC power supply 11, and the reactor 13 Power factor correction control means 106 for controlling the short-circuit elements (104 and 105) for short-circuiting the AC power supply 11 via the AC and the voltage doubler / total for controlling the rectifier circuit 12 to be switched to a full wave rectifier circuit or a double voltage rectifier circuit Wave rectification control means 107, and the voltage doubler / full wave rectification control means 107 is based on the output signal of the power factor correction control means 106 and the input current to the power supply apparatus or the output current from the power supply apparatus. By controlling to switch 12 to full-wave rectifier circuit or voltage doubler rectifier circuit, high output voltage is supplied to the load during heavy load from a wide range of output voltage variable range, and light load It can be supplied to the load low output voltage is.

Although not specified in Patent Document 1, the load (inverter load unit 15) in the air conditioner is generally a three-phase voltage source inverter as shown in FIG.
JP-A-11-206130

  However, the conventional power conversion device (power supply device) includes a switching means between a full-wave rectifier circuit and a voltage doubler rectifier circuit, and a boosting means by a short circuit of an AC power supply via a reactor. As a drive voltage, it is impossible to obtain a DC voltage lower than a DC voltage (approximately AC power supply voltage × √2) normally obtained from an AC power supply by a full-wave rectifier circuit. It was one constraint condition.

  The present invention solves the above-described conventional problems, and performs switching operation using three sets of intermediate arms connected between the intermediate voltage terminal provided in the DC power supply unit and each output phase at light load. By reducing the instantaneous voltage applied between the motor terminals to about ½ of the conventional voltage, the switching loss in the inverter unit and the high-frequency component of the current flowing in the motor are reduced. An object of the present invention is to provide a power converter capable of greatly improving the system efficiency and reducing noise.

  In order to solve the above-described conventional problems, a power converter according to the present invention includes a DC power source connected between DC terminals and provided with a neutral point output terminal, and 3 connected in series with the same polarity between the DC terminals. Three sets of intermediate arms each consisting of a pair of upper and lower arms and bidirectional switches respectively connected between connection points U, V, W between the upper and lower arms of each set and a neutral point output terminal The main circuit device composed of the arm and the three-phase motor connected to the connection points U, V, and W and the upper arm or lower arm of any one phase are alternately fixed to the ON state, and pulse width modulation is performed. In the power conversion device including main circuit control means for generating drive signals for the remaining two-phase upper arm, lower arm, and intermediate arm and performing on / off control, the main circuit control means is any one of the main circuit control means. In the period when the upper arm of the phase is fixed on When the duty ratio obtained by pulse width modulation is lower than the first duty ratio having a preset value of 50% or more, the upper arm and the lower arm are complementarily turned on / off at the duty ratio. When the duty ratio is equal to or higher than the first duty ratio, the upper arm and the intermediate arm are complementarily turned on / off at a duty ratio that is 1/2 of the duty ratio, When the duty ratio is higher than a second duty ratio having a preset value of 50% or less during the period in which the arm is fixed on, the upper arm and the lower arm are complementarily turned on and off. When the duty ratio is less than or equal to the second duty ratio, the intermediate arm and the lower arm are complementarily turned on / off at a duty ratio twice the duty ratio.

  Since the power converter of the present invention can make the instantaneous voltage applied between the terminals of the motor smaller than the DC voltage supplied to the inverter, the motor can be driven with high efficiency, especially at light loads. .

The first invention includes a DC power source having a neutral point output terminal connected between DC terminals, three upper and lower arms connected in series with the same polarity between the DC terminals, Connected to the connection points U, V, W, and three sets of intermediate arms each consisting of a bidirectional switch connected between the connection points U, V, W of the upper arm and the lower arm and the neutral point output terminal. The main circuit device consisting of the three-phase motor and the upper arm or lower arm of any one phase are alternately fixed to the ON state, and the remaining two phases of the upper arm, lower arm, and middle are subjected to pulse width modulation. In a power conversion device including main circuit control means for generating drive signals for each arm and performing on / off control, the main circuit control means is a period in which any one phase of the upper arm is fixed to on The duty ratio obtained by pulse width modulation When the duty ratio is lower than the first duty ratio having a value of 50% or more set in advance, the upper arm and the lower arm are complementarily turned on and off at the duty ratio, and the duty ratio is equal to the first duty ratio. When the duty ratio is greater than or equal to the duty ratio, the upper arm and the intermediate arm are complementarily turned on / off at a duty ratio of ½ of the duty ratio, and the lower arm of any one phase is fixed on. When the duty ratio is higher than a preset second duty ratio having a value of 50% or less, the upper arm and the lower arm are complementarily turned on / off, and the duty ratio is equal to the second duty ratio. When the duty ratio is less than or equal to, the intermediate arm and the lower arm are complementarily turned on and off at a duty ratio that is twice the duty ratio. With this configuration, the instantaneous voltage applied across the motor terminals can be reduced to a maximum of half of the DC voltage supplied to the inverter. Can be driven.

  In the second invention, MOSFETs having a breakdown voltage lower than the breakdown voltages of the upper arm and the lower arm are connected in series in the reverse direction to form an intermediate arm, and the MOSFETs connected in series are simultaneously turned on / off. As a result, it is possible to further reduce the on-time loss of the elements constituting the intermediate arm as compared with the elements of the upper arm and the lower arm, so that it is possible to further increase the system efficiency at a light load.

  In a third aspect of the present invention, in the air conditioner including the load detection means and the power conversion device according to claim 1 or 2, when the load is heavier than a predetermined load, the intermediate arm is always turned off, The upper arm and the lower arm are complementarily turned on / off at a duty ratio obtained by pulse width modulation based on the two-arm modulation method. As a result, the current rating of the elements required for the intermediate arm can be reduced, so that it is possible to obtain a power conversion device with high system efficiency at light loads with a lower cost configuration. Furthermore, when the neutral point voltage is generated by dividing the capacitor, it can be operated in a range that does not affect the DC voltage balance of the capacitor. It will be easy.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1 shows a configuration of a power conversion device according to a first embodiment of the present invention. As shown in FIG. 1, the power conversion device according to the present invention has the same polarity between the DC power sources V1 and V2 having a neutral point output terminal O connected between the DC terminals PN and the DC terminal PN. Three sets of upper arms (S11, S21, S31) and lower arms (S12, S22, S32) connected in series at the center, and connection points (U, V, W) of the upper and lower arms of each set and the middle It consists of three sets of intermediate arms (S13, S23, S33) each consisting of a bidirectional switch connected to the sex point output terminal O, and a three-phase motor 1 connected to the connection points U, V, W. Main circuit device 2, voltage command means 3 for providing a voltage command value to be supplied to the three-phase motor 1, pulse width modulation means 4 for performing pulse width modulation based on the voltage command value, and pulse width modulation means 4 Based on the modulation results of the upper arm, lower arm, And a main circuit control unit 6 consisting arm driving signal output means 5 for outputting a respective driving signal beam.

  The pulse width modulation method in the present invention will be described below. The pulse width modulation method in the present invention is a two-arm modulation in which the upper arm or the lower arm is alternately turned on every phase angle of 60 °. In the following description, it is described assuming a sinusoidal current drive. However, the present invention is not limited to sinusoidal current drive.

  FIG. 2 shows voltage command waveforms (Su, Sv, Sw) at the terminal voltage of each phase during high-speed rotation, and FIG. 3 shows voltage command waveforms (Su, Sv, Sw) at the terminal voltage of each phase during low-speed rotation. Indicates. FIG. 4 is a time chart showing a state of pulse width modulation of the V phase during a period in which the lower arm S12 of the U phase is fixed on.

The pulse width modulation means 4 performs pulse width modulation using two types of triangular waves Vc1 and Vc2 having the same phase as the carrier wave and having a sufficiently high frequency compared to the rotation speed (frequency) of the three-phase motor 1. Su is a voltage command value at the U-phase terminal voltage, Sv is a voltage command value at the V-phase terminal voltage, Vc1 is a triangular wave carrier having the same amplitude as the DC terminal voltage Vdc, and Vc2 is 1/2 of Vc1. It is a triangular wave carrier wave having an amplitude of. The pulse width modulation means 4 compares the threshold voltage Vth1 and Sv, which are set in advance to ½ or less of the DC voltage Vdc, at every peak timing of the triangular wave carrier wave Vc1.

  FIG. 5 is a time chart showing a state of V-phase pulse width modulation when Sv is larger than Vth1. As shown in FIG. 5, when the voltage command value Sv is larger than Vth1, the pulse width modulation means 4 performs pulse width modulation using Vc1 as a carrier wave. That is, the upper arm is turned on when Sv is larger than the carrier wave Vc1, and the lower arm is turned on when Sv is smaller than the carrier wave Vc1 (dead time is necessary, but not shown in the figure). The intermediate arm is always off.

  The arm drive pulse output means 5 outputs the gate drive signals of the upper arm S21, the intermediate arm S23, and the lower arm S22 based on the modulation result obtained by the pulse width modulation means 4, and constitutes the corresponding arm. Turn the element on and off. Since the voltage command value Sw at the W-phase terminal voltage is the same as Sv, the description is omitted. The voltage command value Su at the terminal voltage of the U phase is always zero during the period because the lower arm S12 is fixed on, and is always turned off during the period by the arm drive pulse output means 5.

  FIG. 6 is a time chart showing the state of V-phase pulse width modulation when Sv is smaller than Vth1. As shown in FIG. 6, when the voltage command value Sv is smaller than Vth1, the pulse width modulation means 4 performs pulse width modulation using Vc2 as a carrier wave. That is, when the Sv is larger than the carrier wave Vc2, the intermediate arm is turned on, and when the Sv is smaller than the carrier wave Vc2, the lower arm is turned on (dead time is necessary but omitted in the figure). The upper arm is always off.

  The arm drive pulse output means 5 outputs the gate drive signals of the upper arm S21, the intermediate arm S23, and the lower arm S22 based on the modulation result obtained by the pulse width modulation means 4, and constitutes the corresponding arm. Turn the element on and off. Here, the reason why the threshold voltage Vth1 is set to ½ or less of the DC voltage Vdc is to secure a margin for the ripple voltage of the DC voltage Vdc and other variations. Therefore, in the case where fluctuations in DC voltage Vdc and other variations can be ignored, threshold voltage Vth1 may be set to ½ of Vdc.

  By these controls, the pulse width modulation means 4 is a voltage command value from the voltage command means 3 (more precisely, the terminal voltage generated by adjusting the voltage equal to each phase for every 60 degrees of phase angle for modulation). When the duty ratio is higher than 50% (or a value of 50% or less) when the voltage command value is pulse-width modulated with the triangular wave carrier wave Vc1, the upper arm and the lower arm are the same as in the conventional two-arm modulation method. The arm is turned on and off in a complementary manner. Conversely, when the duty ratio when pulse width modulation is performed with the triangular wave carrier wave Vc1 is lower than 50% (or a value of 50% or less), the intermediate arm and the lower arm have a duty ratio that is twice the duty ratio. Are complementarily turned on and off.

  FIG. 7 is a time chart showing a state of pulse width modulation of the V phase during a period when the lower arm S12 of the U phase is fixed on at a light load. Normally, the voltage required to drive the three-phase motor 1 becomes lower during low-speed operation. Therefore, in the power conversion device according to the present embodiment, as the load becomes lighter, not the upper arm S21 but the middle. The rate at which the arm S23 is turned on increases. Therefore, when the load is lighter than a certain load, the ON period of the upper arm disappears as shown in FIG.

Next, the state of pulse width modulation of the V phase during the period in which the upper arm S11 of the U phase is fixed on will be described. FIG. 8 is a time chart showing a state of V-phase pulse width modulation when Sv is smaller than Vth2. As shown in FIG. 8, when the voltage command value Sv is smaller than Vth2, the pulse width modulation means 4 performs pulse width modulation using Vc1 as a carrier wave. That is, the upper arm is turned on when Sv is larger than the carrier wave Vc1, the lower arm is turned on when Sv is smaller than the carrier wave Vc1 (dead time is necessary, but omitted in the figure), The intermediate arm is always off.

  FIG. 9 is a time chart showing a state of V-phase pulse width modulation when Sv is Vth2 or more. As shown in FIG. 9, when the voltage command value Sv is larger than Vth2, the pulse width modulation means 4 has an amplitude half that of Vc1, and the height of the peak of the triangular wave is Vc3 equal to Vc1. Pulse width modulation is performed as a carrier wave. That is, the upper arm is turned on when Sv is larger than the carrier wave Vc3, the intermediate arm is turned on when Sv is smaller than the carrier wave Vc3 (dead time is necessary, but omitted in the figure), The lower arm is always off.

  Through these controls, the pulse width modulation means 4 is supplied with the voltage command value from the voltage command means 3 (more precisely, the voltage command value of the terminal voltage generated by adjusting the voltage equal to every phase angle 60 degrees for modulation. ) Is lower than 50% (or a value of 50% or more) when the pulse width modulation is performed with the triangular wave carrier wave Vc1, the upper arm and the lower arm are connected as in the conventional two-arm modulation method. Complementary on / off. Conversely, when the duty ratio when pulse width modulation is performed with the triangular wave carrier wave Vc1 is higher than 50% (or a value of 50% or more), the intermediate arm and the lower arm have a duty ratio that is ½ of the duty ratio. Are complementarily turned on and off.

  By the above control, the power conversion device of the present invention performs switching operation of only the upper arm and the lower arm by a general two-arm modulation at the time of heavy load. The proportion of increases.

  In the PWM control period using the intermediate arm, the instantaneous voltage applied to the motor is ½ that of the conventional control in which the PWM control is performed using the upper arm and the lower arm. The instantaneous voltage applied across the motor terminals can be reduced to about ½ of the DC voltage supplied to the inverter. Therefore, since the power conversion device of the present invention can reduce the frequency component of the carrier wave of the current flowing through the motor, it is possible to reduce noise mainly including the frequency component of the carrier wave. Further, the switching efficiency of each arm also improves the inverter efficiency because the voltage becomes low.

  As described above, in the power conversion device of the present embodiment, the instantaneous voltage applied between the motor terminals is lighter than the DC voltage Vdc of the inverter when the load is light, that is, when the motor is operating at a low speed. Therefore, compared with the conventional power converter, high efficiency and low noise can be realized particularly in motor driving at light load.

(Embodiment 2)
FIG. 10 shows the configuration of the power conversion device according to the second embodiment of the present invention. As shown in FIG. 10, the power conversion device according to the present embodiment includes an upper arm (S11, S21, S31) and a lower arm (S12, S31) as bidirectional switches that constitute the intermediate arm (S13, S23, S33). MOSFETs having a breakdown voltage lower than the breakdown voltage of the elements constituting S22 and S32) are connected in series in the reverse direction.

In the power conversion device of the present invention, the voltage applied to the intermediate arm when it is off is approximately ½ of the direct-current voltage Vdc because of the circuit configuration, so that the breakdown voltage of the MOSFET constituting the intermediate arm is the upper arm and lower arm A value obtained by adding a margin to approximately ½ of the breakdown voltage of the element configured by Since the on-resistance of the MOSFET is proportional to the power of about 2.5 of the withstand voltage, the power conversion device according to the present embodiment enables conduction in the intermediate arm by simultaneously turning on and off the MOSFETs of each phase connected in series. It is possible to further reduce the loss (loss when turned on).

  As described above, the power conversion device according to the second embodiment can further increase the system efficiency at light loads.

(Embodiment 3)
FIG. 11 shows a configuration of an air conditioner according to Embodiment 3 of the present invention. As shown in FIG. 11, the air conditioner in the present embodiment includes a converter based on a full-wave rectifier circuit including a rectifier circuit 12 and a reactor 13, and load detection means (not shown), and an inverter As described above, the power conversion device described in the first or second embodiment is provided.

  The air conditioner of the present invention performs the operation described in the first or second embodiment when the load detected by the load detecting means is lighter than the predetermined load. Further, when the detected load is heavier than a predetermined load, all the intermediate arms (S13, S23, S33) are always turned off and obtained by pulse width modulation based on a general two-arm modulation system. The upper arm (S11, S21, S31) and the lower arm (S12, S22, S32) are complementarily turned on / off at a duty ratio.

  In order to perform the operation described in the first or second embodiment over the entire load range, the current rating of the switch element constituting the intermediate arm (S13, S23, S33) needs to be greater than the motor current at the maximum load. However, in the invention in the present embodiment, since the maximum load value when using the intermediate arm (S13, S23, S33) is limited, the switches constituting the intermediate arm (S13, S23, S33) The current rating of the element can be reduced. Therefore, the cost required for elements such as MOSFETs and IGBTs constituting the intermediate arm (S13, S23, S33) is reduced, so that the system efficiency at the time of light load with high operation frequency can be increased with an inexpensive system configuration. it can.

  Furthermore, as shown in FIG. 11, when the neutral point voltage is generated by the divided voltage of the capacitor, within the range of the load power or less where the influence on the DC voltage balance of the capacitors C1 and C2 does not become a problem. By turning on / off the intermediate arms (S13, S23, S33), there is also an effect that the control can be easily configured without performing the balance control.

  The load detection means in the present invention may be any means as long as it can relatively determine the magnitude of the load. For example, it detects the number of rotations of the compressor motor, detects the input current from the AC power supply 11, detects the motor current, etc. Is mentioned.

  As described above, the air conditioner in the present embodiment can drive the instantaneous voltage applied between the motor terminals with a DC voltage that is significantly lower than the DC voltage normally obtained by a full-wave rectifier circuit. As a result, high system efficiency can be realized particularly at light loads.

  As described above, the power conversion device according to the present invention can achieve improved efficiency and low noise during low-speed operation by setting the instantaneous voltage directly applied to the motor to be equal to or less than the DC voltage of the inverter. Therefore, it can be applied not only to air conditioners but also to appliances having motor loads such as heat pump water heaters, refrigerators and washing machines.

The figure which shows the structure of the power converter device in Embodiment 1 of this invention. The figure which shows the voltage command waveform (Su, Sv, Sw) in the terminal voltage of each phase at the time of high speed rotation of the power converter device in the said Embodiment 1. FIG. The figure which shows the voltage command waveform (Su, Sv, Sw) in the terminal voltage of each phase at the time of low speed rotation of the power converter device in the said Embodiment 1. FIG. The time chart which shows the mode of the pulse width modulation of V phase in the period when the lower arm S12 of U phase is fixed to ON of the power converter device in the said Embodiment 1. The time chart which shows the mode of the pulse width modulation of the V phase when Sv is larger than Vth1 of the power converter device of the said Embodiment 1. FIG. The time chart which shows the mode of the pulse width modulation of the V phase in case the Sv is smaller than Vth1 of the power converter device in the said Embodiment 1. FIG. Time chart showing a state of pulse width modulation of the V phase during the period in which the lower arm S12 of the U phase is fixed on at a light load of the power conversion device according to the first embodiment. The time chart which shows the mode of the pulse width modulation of the V phase in case the Sv is smaller than Vth2 of the power converter device in the said Embodiment 1. FIG. The time chart which shows the mode of the pulse width modulation of the V phase when Sv is larger than Vth2 of the power converter device of the said Embodiment 1. FIG. Configuration diagram of power conversion device according to Embodiment 2 of the present invention The block diagram of the air conditioner in Embodiment 3 of this invention Configuration diagram of conventional power converter Configuration diagram of inverter unit in conventional power converter

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Three-phase motor 2 Main circuit device 3 Voltage command means 4 Pulse width modulation means 5 Arm drive signal output means 6 Main circuit control means

Claims (3)

A DC power supply having a neutral point output terminal connected between the DC terminals, three sets of upper and lower arms connected in series with the same polarity between the DC terminals, and each set of upper and lower arms Three intermediate arms each consisting of a bidirectional switch connected between the connection points U, V, W and the neutral point output terminal, and a three-phase motor connected to the connection points U, V, W The main circuit device and any one phase upper arm or lower arm are alternately fixed to the ON state, and pulse width modulation is performed to drive signals of the remaining two phases of the upper arm, lower arm, and intermediate arm. And a power converter comprising a main circuit control means for performing on / off control,
The main circuit control means has a duty ratio obtained by pulse width modulation in a period in which any one phase of the upper arm is fixed on, from a first duty ratio having a preset value of 50% or more. If the duty ratio is lower, the upper arm and the lower arm are complementarily turned on and off at the duty ratio. If the duty ratio is equal to or higher than the first duty ratio, it is ½ of the duty ratio. The upper arm and the intermediate arm are complementarily turned on / off at a duty ratio, and the duty ratio is a preset value of 50% or less during a period when the lower arm of any one phase is fixed on. When the duty ratio is higher than the second duty ratio, the upper arm and the lower arm are complementarily turned on / off, and when the duty ratio is equal to or lower than the second duty ratio, Power converter according to claim complementarily turning on and off the intermediate and lower arms at twice the duty ratio of the ratio. The intermediate arm is configured by serially connecting MOSFETs having a breakdown voltage lower than that of the upper arm and the lower arm in reverse directions, and the MOSFETs connected in series are simultaneously turned on and off. The power converter device described in 1. In the air conditioner provided with the load detection means and the power conversion device according to claim 1 or 2, when the load is heavier than a predetermined load, the intermediate arm is always turned off and the pulse based on the two-arm modulation method is used. An air conditioner characterized in that the upper arm and the lower arm are complementarily turned on and off at a duty ratio obtained by width modulation.

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