JP6148578B2 - Power converter - Google Patents

Description

  Embodiments described herein relate generally to a power converter.

  The cable connecting the inverter and the electric motor is a bundle of three-phase wires, and further a bundle of ground wires. As the cable becomes longer, it acts as a distributed constant line consisting of wiring inductance and stray capacitance. For this reason, vibration occurs when the voltage output from the inverter changes stepwise. In this case, the longer the cable, the larger the wiring inductance and stray capacitance, and the lower the vibration frequency. If the voltage changes stepwise again before the vibration is attenuated, the vibration increases, and a surge voltage having an amplitude exceeding the DC voltage Vdc of the inverter is applied to the motor input terminal.

  The amplitude of the surge voltage becomes the largest when the step change of the line voltage at the inverter output terminal and the phase of the oscillating surge voltage overlap. For example, when the line voltage at the output terminal of the inverter changes to 0 V, Vdc, and 0 V, the maximum surge voltage that appears at the motor input terminal has an amplitude that is almost twice the DC voltage Vdc. Further, when the line voltage at the output terminal of the inverter changes to 0V, Vdc, 0V, -Vdc, 0V or 0V, Vdc, -Vdc, 0V, the surge voltage appearing at the motor input terminal is at most about the DC voltage Vdc. The amplitude is 4 times.

Japanese Patent No. 4032707

  A surge voltage having an amplitude four times as high as the DC voltage Vdc may degrade the insulation of the motor. Therefore, a power converter is provided that can reduce the amplitude of the surge voltage that appears between the lines at the motor input end.

  The power conversion device of the embodiment includes an inverter, a modulation factor calculation unit, a command signal generation unit, and a drive signal generation unit. The inverter includes a switching element connected in a bridge, and the switching element performs a switching operation according to a drive signal, thereby converting the input DC voltage into an AC voltage having a variable voltage value and frequency and outputting the AC voltage to the electric motor.

  The modulation factor calculation means calculates and outputs a modulation factor that is a voltage ratio between the amplitude of the phase voltage output from the inverter and the DC voltage. The command signal generating means outputs a three-phase command signal based on the modulation factor and the phase of the phase voltage. The drive signal generating means outputs a drive signal for PWM driving the switching element based on the comparison between the command signal and the triangular wave signal.

  In accordance with the length of the cable connecting the inverter and the electric motor, a minimum separation time to be secured before the line voltage output from the inverter is inverted from the first polarity to the opposite second polarity is determined. The modulation factor calculating means calculates an upper limit value of the modulation factor necessary for ensuring the minimum separation time based on the minimum separation time and the frequency of the triangular wave signal, and for the command value of the phase voltage output by the inverter The calculated modulation rate is limited to an upper limit value or less and output.

The block diagram of the power converter device which shows 1st Embodiment The figure which shows the command signal of two phase modulation Waveform diagram when the two-phase command signals are equal when the modulation rate is limited to the upper limit or less Waveform diagram when the two-phase command signals are equal when the modulation rate is not limited to the upper limit or less Characteristic diagram showing relationship between cable length Lc and minimum separation time Tmin A characteristic diagram showing the relationship between the modulation rate and the separation time t FIG. 1 equivalent diagram showing the second embodiment FIG. 1 equivalent view showing the third embodiment Waveform diagram when the command signal is clamped at the upper limit Waveform diagram when command signals are different from each other FIG. 9 equivalent diagram showing the fourth embodiment

(First embodiment)
The first embodiment will be described with reference to FIGS. The power conversion device 1 includes an inverter 2, a storage unit 3, a modulation factor calculation unit 4, a command signal generation unit 5, a drive signal generation unit 6, and a drive circuit 7.

  The inverter 2 includes switching elements 8up to 8wn such as IGBTs and FETs connected in a three-phase bridge. When the switching elements 8up to 8wn perform switching operation in accordance with a drive signal given through the drive circuit 7, the inverter 2 converts the DC voltage Vdc input from the DC voltage source 9 into an AC voltage having a variable voltage value and frequency and outputs it. To do. The input terminal of the electric motor 11 is connected to the output terminal of the inverter 2 via the cable 10. The DC voltage source 9 is constituted by a rectifier circuit and a smoothing circuit that receive, for example, a three-phase AC voltage.

  The storage means 3, the modulation factor calculation means 4, the command signal generation means 5 and the drive signal generation means 6 are constituted by a microcomputer. The storage means 3 is an electrically rewritable nonvolatile memory. The storage means 3 continues until the length Lc of the cable 10 and the line voltage output from the inverter 2 are inverted from Vdc having a positive polarity (first polarity) to −Vdc having a negative polarity (second polarity). The data indicating the relationship with the minimum separation time Tmin to be secured (see FIG. 5) is written in advance.

The modulation factor calculation means 4 calculates a modulation factor α which is a voltage ratio between the amplitude command value V (peak value) of the phase voltage output from the inverter 2 and the DC voltage Vdc. The modulation factor α is defined by U ^* / Un (U ^* : effective line voltage of the output voltage of the power converter 1, Un: effective line voltage of the three-phase AC input voltage to the power converter 1). Is. When this relationship is expressed as a voltage ratio between the amplitude command value V and the DC voltage Vdc, Vdc = 2 ^1/2 Un, V = 2 ^1/2 V ^* = 2 ^1/2 (U ^* / 3 ^1/2 ). From the relationship, the modulation rate α is expressed by the equation (1).
α = 3 ^1/2 (V / Vdc) (1)

  Further, the modulation factor calculation means 4 calculates the upper limit value αmax of the modulation factor α necessary for securing the minimum separation time Tmin based on the minimum separation time Tmin and the frequency (carrier frequency fc) of a triangular wave signal Vc described later. Then, the modulation factor α is limited to the upper limit value αmax or less and output.

  The command signal generation means 5 outputs three-phase command signals Vur, Vvr, Vwr based on the modulation factor α output from the modulation factor calculation unit 4 and the phase θ of the phase voltage. The drive signal generator 6 compares the command signals Vur, Vvr, and Vwr with the triangular wave signal Vc that is a carrier, and outputs a drive signal for PWM driving the switching elements 8up to 8wn. The drive signal generating means 6 is realized by, for example, a three-phase PWM timer provided in the microcomputer, and the command signals Vur, Vvr, Vwr are updated for each peak position of the triangular wave signal Vc.

  Next, the operation of this embodiment will be described with reference to FIGS. The command signal generation means 5 outputs command signals Vur, Vvr, Vwr for two-phase modulation. As shown in FIG. 2, in the two-phase modulation, the phase voltage command signal having the maximum amplitude is fixed to the peak value of the triangular wave signal Vc, and the phase voltage command signal having the minimum amplitude is set to the bottom value of the triangular wave signal Vc. This is a modulation method fixed to. At this time, the two-phase modulation command signals Vur, Vvr, and Vwr are generated by adding an offset voltage to the other two phases so that the line voltage maintains a sine wave. The two-phase modulation has a higher voltage utilization rate than the three-phase modulation, and can reduce the number of switching times, thereby reducing loss.

  3 and 4 show the command signals Vur, Vvr, Vwr, the triangular wave signal Vc, the phase voltages Vu, Vv, Vw at the output terminal of the inverter 2 and the inverter when the U-phase command signal Vur and the V-phase command signal Vvr become equal. 2 shows the waveforms of the line voltages Vuv, Vvw, Vwu at the output terminal 2 and the line voltages Vuv, Vvw, Vwu at the input terminal of the motor 11.

  Among the line voltages Vuv, Vvw, and Vwu, the waveform of non-vibration (actually some vibration exists) is the waveform of the output end of the inverter 2, and the vibrational waveform is the waveform of the input end of the electric motor 11. . FIG. 3 is a waveform of the present embodiment that limits the modulation factor α to a predetermined upper limit value αmax or less, and FIG. 4 is a waveform of a conventional configuration in which no upper limit is provided for the modulation factor α. Hereinafter, the surge voltage generated mainly near the peak value of the triangular wave signal Vc will be described, but the surge voltage generated near the bottom value is the same.

  When two phases, for example, the U-phase command signal Vur and the V-phase command signal Vvr are equal, the line voltage Vuv changes to 0V, Vdc, 0V, -Vdc, and 0V. When the cable 10 becomes long, it acts as a distributed constant line. For this reason, when the line voltage Vuv changes stepwise, reflection occurs between the power converter 1 and the electric motor 11, and a surge voltage that attenuates while repeating vibration is generated. If the line voltage Vuv changes again stepwise before the vibration is attenuated, the amplitude of the surge voltage may increase.

  If the line voltage changes only to 0V, Vdc, and 0V, the amplitude of the surge voltage generated at the input terminal of the electric motor 11 will be almost twice the DC voltage Vdc at maximum. However, when the two-phase command signal values are equal and switched, the line voltage changes from 0V, Vdc (first polarity), 0V, -Vdc (second polarity), 0V and the first polarity to the opposite second polarity. Change. Moreover, since the width of the period during which the line voltage is Vdc and the width of the period during which −Vdc is reduced, the amplitude of the surge voltage generated at the input end of the motor 11 is further increased.

  In this case, as the two-phase command signal value approaches the peak value of the triangular wave signal Vc, the time from the time when the line voltage changes from Vdc to 0 V to the time when it changes to −Vdc (hereinafter referred to as the separation time t). ) And the surge voltage amplitude increases. When the step change of the line voltage and the phase of the surge voltage overlap, the amplitude of the surge voltage is at most about four times the DC voltage Vdc. In order to prevent insulation deterioration of the electric motor 11, it is necessary to suppress such an excessive (approximately four times) surge voltage.

  In order to suppress the amplitude of the surge voltage, it is effective to secure the separation time t to a predetermined value or more to attenuate the vibration once generated. In the case of the conventional configuration shown in FIG. 4, as the modulation factor α increases, the command signal value when the two-phase command signals become equal approaches the peak value of the triangular wave signal Vc as much as possible. It could be as short as zero.

  Therefore, the modulation factor calculation means 4 provided in the power conversion device 1 of the present embodiment sets an upper limit on the modulation factor α as described below. FIG. 5 shows a minimum value (hereinafter referred to as a minimum separation time) of the separation time t required to suppress the amplitude of the surge voltage to a predetermined value lower than four times the DC voltage Vdc with respect to the cable length Lc for a certain carrier frequency fc. Time Tmin). Since the vibration frequency of the surge voltage decreases as the cable 10 becomes longer, the minimum separation time Tmin also becomes longer. The data shown in FIG. 5 is stored in the storage means 3 in advance, and when the cable length Lc is input, the minimum separation time Tmin is read and given to the modulation factor calculation means 4.

FIG. 6 shows the relationship between the modulation rate α and the separation time t when the two-phase command signals are equal in the two-phase modulation described above. The relationship among the modulation factor α, the carrier frequency fc, and the separation time t is expressed by equation (2).
Separation time t = (1-3 ^1/2 / 2 · α) / fc (2)

  As the modulation factor α decreases, the command signal value moves away from the peak value or bottom value of the triangular wave signal Vc. Since the triangular wave signal Vc exceeds the maximum command signal value of the three phases and then reaches the maximum command signal value again after passing through the peak, no phase voltage changes, and thus the line voltage does not change. Accordingly, the separation time t becomes longer as the modulation rate α becomes lower. Further, the separation time t becomes shorter in inverse proportion to the carrier frequency fc.

  The modulation factor calculating means 4 has the data shown in the equation (2) or FIG. 6, and when the minimum separation time Tmin and the carrier frequency fc are input, the upper limit of the modulation factor necessary for ensuring the minimum separation time Tmin. The value αmax is calculated. For example, when the carrier frequency fc is 16 kHz, the modulation factor upper limit value αmax that satisfies the minimum separation time Tmin of 10 μs or more is 0.97. The modulation factor calculation means 4 calculates a modulation factor α, which is a voltage ratio between the phase voltage amplitude command value V output by the inverter 2 and the DC voltage Vdc, and limits the modulation factor α to an upper limit value αmax or less. To do. Vur (before limitation) and Vvr (before limitation) indicated by broken lines in FIG. 3 indicate the U-phase command signal Vur and the V-phase command signal Vvr before the modulation rate α is limited.

  As a result, as shown in FIG. 3, the separation time t is secured more than the minimum separation time Tmin. For example, when the U-phase command signal Vur and the V-phase command signal Vvr become equal, the line voltage Vuv changes 0 V, −Vdc, 0 V after the vibration caused by the changes 0 V, Vdc, 0 V is sufficiently reduced. . Therefore, compared with FIG. 4, the amplitude of the surge voltage generated at the input end of the electric motor 11 is suppressed.

  According to the first embodiment described above, the power conversion device 1 is configured such that the line voltage has a reverse polarity when the two-phase command signals are equal according to the vibration frequency and the attenuation characteristic determined by the length of the cable 10 and the like. The minimum separation time Tmin when changing to is determined. Then, the modulation rate α is limited to an upper limit value αmax or less obtained based on the minimum separation time Tmin and the carrier frequency fc. Thereby, the amplitude of the surge voltage generated when the two-phase command signals are equal can be suppressed to a desired value (a predetermined value lower than four times the DC voltage Vdc in this embodiment). Further, since the power conversion device 1 generates the three-phase command signals Vur, Vvr, and Vwr based on the limited modulation factor α, the line voltage is not distorted.

(Second Embodiment)
The second embodiment will be described with reference to FIGS. 7 and 6 (explained above). The power conversion device 21 shown in FIG. 7 replaces the power conversion device 1 shown in FIG. 1 with the modulation rate calculation means 4 and the drive signal generation means 6 replaced with the modulation rate calculation means 22 and the drive signal generation means 23, respectively. It has a configuration. The modulation factor calculating means 22 calculates a modulation factor α which is a voltage ratio between the amplitude command value V and the DC voltage Vdc.

  FIG. 6 shows the relationship between the modulation rate α and the separation time t in the two-phase modulation when the carrier frequencies fc (8 kHz, 12 kHz, 16 kHz) are different. The lower the carrier frequency fc is, the smaller the slope of the triangular wave signal Vc is, so that the separation time t becomes longer even with the same modulation rate α.

  Therefore, the drive signal generator 23 generates the carrier of the triangular wave signal Vc necessary for securing the minimum separation time Tmin based on the minimum separation time Tmin given from the storage unit 3 and the maximum modulation rate α during operation. The upper limit value fcmax of the frequency fc is calculated. For example, when the minimum separation time Tmin is 10 μs and the maximum modulation rate α during operation is 1, the upper limit value fcmax is 13.397 kHz. The drive signal generation means 23 sets a carrier frequency fc equal to or lower than the upper limit value fcmax in the three-phase PWM timer, and outputs a drive signal based on a comparison between the command signals Vur, Vvr, Vwr and the triangular wave signal Vc. Other operations such as a method for determining the minimum separation time Tmin are the same as those in the first embodiment.

  According to the second embodiment described above, the power conversion device 21 determines the minimum separation time Tmin, and sets the carrier frequency fc based on the minimum separation time Tmin and the maximum value of the modulation factor α. Limit to fcmax or less. As a result, even when the two-phase command signals are equal, the amplitude of the surge voltage can be suppressed to a desired value (a predetermined value lower than four times the DC voltage Vdc in this embodiment). Moreover, since the power converter device 21 can output a voltage equal to the command value V of the phase voltage, the current and torque of the electric motor 11 can be controlled with high accuracy according to the command value.

(Third embodiment)
A third embodiment will be described with reference to FIGS. The power conversion device 31 illustrated in FIG. 8 has a configuration in which the drive signal generation unit 6 is replaced with the drive signal generation unit 32 with respect to the power conversion device 1 illustrated in FIG. In the drive signal generation means 32, any of the three-phase command signals Vur, Vvr, Vwr is within a signal width range corresponding to 1/2 of the minimum separation time Tmin from the peak value or bottom value of the triangular wave signal Vc. The command signal is shifted out of the signal range.

  Specifically, when the slope of the triangular wave signal Vc is 1, (peak value−Tmin / 2) is the command signal upper limit value and (bottom value + Tmin / 2) is the command signal lower limit value. The drive signal generating means 32 clamps at the command signal upper limit value when the command signal is larger than the command signal upper limit value (see FIG. 9), and clamps at the command signal lower limit value when the command signal is smaller than the command signal lower limit value. To do. The drive signal generating means 32 outputs a drive signal based on the comparison between the clamped command signals Vur, Vvr, Vwr and the triangular wave signal Vc.

  FIG. 10 is a waveform diagram corresponding to FIGS. 3 and 4 when the command signals Vur, Vvr, and Vwr are different from each other. Since the command signals Vur and Vvr are not interchanged, the line voltage Vuv changes to the same polarity as 0 V, Vdc (first polarity), 0 V, Vdc (first polarity), and 0 V. Since the command signals Vur and Vvr are separated from each other, the width of the period during which the line voltage Vuv is Vdc is longer and the vibration is likely to be attenuated than when the command signals Vur and Vvr are equal. However, when the time from the time when the line voltage changes from Vdc to 0 V to the time when it changes to Vdc again (also referred to as the separation time t) is shortened, a surge voltage having an amplitude about three times the DC voltage Vdc is generated. May occur.

  The drive signal generation means 32 provided in the power conversion device 31 of the present embodiment limits the command signals Vur, Vvr, Vwr of each phase to be equal to or less than the command signal upper limit value and equal to or greater than the command signal lower limit value. Thereby, the separation time t of the line voltage can be secured for the minimum separation time Tmin or more, so that the surge voltage amplitude when the line voltage changes to the same polarity across 0 V can also be suppressed. In addition, operations and effects similar to those of the first embodiment can be obtained.

(Fourth embodiment)
A fourth embodiment will be described with reference to FIG. In addition to the function of the drive signal generation unit 32 described in the third embodiment, the drive signal generation unit of the present embodiment has the command signal input from the command signal generation unit 5 equal to the peak value or the bottom value of the triangular wave signal Vc. During the period (period Ta shown in FIG. 11), the clamp process (shift process) is stopped.

  When the command signal becomes equal to the peak value or the bottom value of the triangular wave signal Vc, the switching element of that phase stops the switching operation. On the other hand, when the clamping process is executed, the switching operation continues even in the period Ta. According to this embodiment, the same effects as those of the third embodiment can be obtained, switching loss can be reduced, and the phase voltage output from the inverter 2 can be brought close to the phase voltage command value V.

(Other embodiments)
In addition to the plurality of embodiments described above, the following configuration may be adopted.
Not only the two-phase modulation of the 60 ° switching method, but two-phase modulation or three-phase modulation of the 120 ° switching method may be used.

According to the embodiment described above, the amplitude of the surge voltage appearing between the lines at the motor input end can be reduced.
Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the invention described in the claims and equivalents thereof in the same manner as included in the scope and gist of the invention.

  In the drawings, 1, 21 and 31 are power converters, 2 are inverters, 4 and 22 are modulation rate calculation means, 5 is command signal generation means, 6, 23 and 32 are drive signal generation means, and 8up to 8wn are switching elements. Reference numeral 10 denotes a cable, and 11 denotes an electric motor.

Claims (4)

An inverter that includes a bridge-connected switching element, and the switching element performs a switching operation according to a drive signal, thereby converting the input DC voltage into an AC voltage having a variable voltage value and frequency and outputting the AC voltage to the motor;
A modulation factor calculating means for calculating and outputting a modulation factor which is a voltage ratio between the amplitude of the phase voltage output by the inverter and the DC voltage;
Command signal generating means for outputting a three-phase command signal based on the modulation factor and the phase of the phase voltage;
Drive signal generating means for outputting the drive signal for PWM driving the switching element based on a comparison between the command signal and a triangular wave signal;
According to the length of the cable connecting the inverter and the electric motor, a minimum separation time to be secured before the line voltage output from the inverter is reversed from the first polarity to the opposite second polarity is determined. And
The modulation factor calculating means calculates an upper limit value of the modulation factor necessary for ensuring the minimum separation time based on the minimum separation time and the frequency of the triangular wave signal, and outputs a phase voltage output from the inverter The power conversion device, wherein the modulation factor calculated for the command value is limited to the upper limit value or less and is output. An inverter that includes a bridge-connected switching element, and the switching element performs a switching operation according to a drive signal, thereby converting the input DC voltage into an AC voltage having a variable voltage value and frequency and outputting the AC voltage to the motor;
A modulation factor calculating means for calculating and outputting a modulation factor which is a voltage ratio between the amplitude of the phase voltage output by the inverter and the DC voltage;
Command signal generating means for outputting a three-phase command signal based on the modulation factor and the phase of the phase voltage;
Drive signal generating means for outputting the drive signal for PWM driving the switching element based on a comparison between the command signal and a triangular wave signal;
According to the length of the cable connecting the inverter and the electric motor, a minimum separation time to be secured before the line voltage output from the inverter is reversed from the first polarity to the opposite second polarity is determined. And
The drive signal generating means calculates an upper limit value of the frequency of the triangular wave signal necessary for securing the minimum separation time based on the minimum separation time and the modulation factor, and the command signal and the upper limit value. A power converter that outputs the drive signal based on a comparison with a triangular wave signal having the following frequency.   The drive signal generating means is configured such that the command signal output from the command signal generating means is within a signal width range of the triangular wave signal corresponding to ½ of the minimum separation time from the peak value or bottom value of the triangular wave signal. The command signal is shifted outside the range of the signal width, and the drive signal is output based on a comparison between the command signal after the shift and the triangular wave signal. 2. The power conversion device according to 2.   4. The power conversion according to claim 3, wherein the drive signal generation unit stops the shift process for a period in which a command signal output from the command signal generation unit is equal to a peak value or a bottom value of the triangular wave signal. apparatus.

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