JP3336588B2 - PWM pulse generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PWM pulse generator, and more particularly to an asynchronous PWM modulation that does not synchronize the phase of a fundamental wave of an output voltage command of a power converter with the phase of a carrier frequency, and a power conversion method. The present invention relates to a synchronous PWM switching technique for synchronizing a phase of an output voltage of a device and a phase of a carrier frequency or a phase of an output PWM pulse.

[0002]

2. Description of the Related Art Conventionally, as an inverter technology of this kind,
JP-A-4-143947 and JP-A-6-312
No. 924 discloses that an asynchronous P
A method of continuously and smoothly outputting from a zero voltage to a maximum voltage by a WM method and a one-pulse mode that conducts at 180 ° is described. According to these known examples, there is no need to change the number of pulses as in the case of using a synchronization pulse.
Since there is no change in the number of pulses, there are advantages that there is no jump of current due to the change in the number of pulses, there is no need to synchronize the modulated wave with the PWM pulse, and it is possible to easily generate a pulse. Can be easily realized by software,
There is an advantage. In addition, regarding the case where the generation of the PWM pulse is realized by software, in the synchronous PWM method,
It is necessary to calculate and store a pulse pattern in advance for each number of pulses, which requires a large storage capacity.
The WM method also has an advantage that the capacity of a storage device for storing pulse data may be small. By the way, in the above-described known example, the generation of the asynchronous PWM pulse uses a carrier comparison method of comparing a modulated wave serving as an inverter output voltage command with a carrier waveform such as a triangular wave. In order to output the output voltage faithfully in accordance with the inverter output voltage command, the ratio N = fc / of the frequency of the modulated wave, that is, the frequency fc of the carrier to the inverter output frequency finv.
It is necessary to set finv sufficiently high (more than ten times).

[0003]

However, when the carrier frequency fc is increased, the switching frequency fsw of the switching element constituting the inverter is increased.
The problem that the loss accompanying switching becomes large arises. On the other hand, if the frequency fc of the carrier wave is reduced to reduce the switching loss, when the inverter frequency finv increases, the ratio N of these frequencies decreases, and the motor current pulsates. For this reason, in the above-mentioned known example, the motor current does not pulsate using the asynchronous PWM mode, the switching loss is suppressed low, and the output from zero voltage to one pulse cannot be continuously and smoothly output. As means for solving such a problem, when the inverter frequency is low, asynchronous PWM is used, and in an operation state where fc / finv is reduced, synchronous 3-pulse introduced in Japanese Patent Application No. 8-153698 is used. And then switch to one pulse. As a result, even if the carrier frequency, that is, the switching frequency is lowered, the operation of the inverter can be smoothly continued without the occurrence of current pulsation or the like. However, in the example in which the synchronous three-pulse is introduced, switching between the asynchronous PWM and the synchronous three-pulse method is not described.
Since the number of pulses of M and the pattern of the three synchronous pulses are significantly different, relatively large voltage fluctuations occur transiently when switching with asynchronous PWM, and the motor current may jump at the time of switching. Is not taken into account, for example, the switching element that constitutes the device may be destroyed. On the other hand, in order to avoid such destruction, it is conceivable to increase the rating of the switching element or to limit the output current at the time of switching.
When the rating of the switching element is increased, the size of the switching element increases, and the price increases. Further, if the output current is limited at the time of switching, another problem such as a failure to obtain a predetermined output and torque occurs.

SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, it is an object of the present invention to suppress an excessive change in an inverter output voltage which occurs when switching between an asynchronous PWM mode and a synchronous PWM mode, and to smoothly switch between an asynchronous PWM mode and a synchronous PWM mode. To perform the switching.

[0005]

In order to solve the above-mentioned problems, the PW is asynchronously output with a modulated wave which is an output command of an inverter.
An asynchronous PWM mode that generates M pulses; and a synchronous three-pulse mode that generates three pulses in a half cycle of the modulated wave in synchronization with the modulated wave, and switches between the two pulse modes during operation. A PWM pulse generator for controlling the inverter by a pulse from the controller, comprising: a synchronous 4-pulse mode for generating four pulses in a half cycle of the modulated wave in synchronization with the modulated wave; When switching between the synchronous 4-pulse mode and the synchronous 3-pulse mode, the rising phase θ41 and the falling phase of the pulse closest to 0 ° of the inverter output voltage phase of the synchronous 4-pulse mode are switched through the synchronous 4-pulse mode. The phase θ42, the rising phase θ43 of the next rising pulse, the falling phase θ44, and the three-pulse inverter output voltage phase The rising phase θ31, falling phase θ32, and rising phase θ33 of the pulse closest to the phase 0 ° within the range of 0 ° to 90 ° are defined as θ31 ≧ θ41, θ32 ≦ θ42, θ33 ≧
θ43, the phase θ44 is the phase 90 °
Approach.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 illustrates a P according to one embodiment of the present invention.
3 shows a WM pulse generator. Here, three-level PWM
Description will be made using a PWM pulse generator for an inverter. In FIG. 1, an operation command generating means 1 includes an inverter output voltage command E and an inverter frequency command fin for driving a three-level PWM inverter (not shown).
Generate v. The integrator 24 outputs the inverter frequency command f
Inv is input, integrated, and the phase command θ is output. PW
The M pulse generating means 2 receives the inverter output voltage command E and the phase command θ to generate a sine wave which becomes a modulation wave of the inverter, and a triangular wave generating means 212 which generates a carrier waveform having a phase θc. And a comparator 213 for comparing a carrier waveform with a sine wave which is a modulation wave of the inverter.
Asynchronous pulse generating means 21 for generating M pulses, four synchronous pulses, three synchronous pulses for driving the inverter based on the rising and falling phases of the pulse specified by the inverter output voltage command E and the phase command θ. Synchronous pulse generating means 221 and 22 for generating one synchronous pulse
2 and 223, a synchronous PWM pulse generating means 22 for generating a synchronous PWM pulse for driving the inverter based on an inverter operation command for outputting an inverter output voltage command E and an inverter frequency command finv, and an inverter output voltage command E , An output pulse of the asynchronous pulse means 21 for generating an asynchronous PWM pulse based on the inverter frequency command finv and the phase θc of the triangular wave and the synchronous PWM signal.
Any PW from the output pulse of the M pulse generation means 22
PWM pulse selecting means 23 for selecting the M pulse, and generates a PWM pulse for driving the three-level PWM inverter.

First, asynchronous PWM pulse generating means 2
1 and frequency command fi
A method of generating a PWM pulse based on nv will be described with reference to FIG. In the asynchronous PWM pulse generation means 21, the integrator 24 uses the inverter frequency command finv
And a sine wave having a constant amplitude is created based on the phase θ of the inverter output voltage obtained by multiplying the constant by the sine wave generation means 211, and the sine wave is multiplied by the inverter voltage command E to modulate the inverter. A waveform is created, and an asynchronous PWM pulse is obtained by a triangular wave comparison method in which the comparator 213 compares the generated triangular wave with the triangular wave generated by the triangular wave generating means 212. Next, a method of determining the rising and falling phases of the synchronous pulse based on the inverter output voltage command E in the synchronous PWM pulse generating means 22 will be described with reference to FIG. In FIG. 2, θ31 and θ32 are the rising phase and falling phase of the first pulse, respectively, θ33 is the rising phase of the second pulse, the falling phase of the second pulse, and the third phase.
Since the rising phase and the rising phase of the pulse are synchronous pulses, they are respectively π (180 ⁰ ) −θ33,
π (180 ⁰ ) −θ32 and π (180 ⁰ ) −θ31. Here, θ31 <θ32 <θ33 (1) and, for the output voltage command E, if E = k · (cos θ31−cos θ32 + cos θ33) (2), a fundamental wave according to the output voltage command is obtained. Here, k is a coefficient having a constant value. The rising and falling phases are compared with the phase command θ obtained by integrating the inverter frequency command, and the rising and falling of the pulse are performed.

In the asynchronous PWM described above, the frequency of the triangular wave is set to be more than ten times the operating frequency of the inverter in order to obtain a stable inverter output voltage. At an inverter frequency at which the inverter frequency becomes higher and the ratio of the triangular wave frequency to the inverter frequency becomes ten and several times or less, it is necessary to switch to a synchronous pulse in order to stabilize the inverter output voltage. As described above, Japanese Patent Application No. 8-153698 does not mention the switching method between asynchronous PWM and synchronous three pulses. In particular, the number of pulses is large as in asynchronous PWM.
When switching to the pulse mode in which the position of the pulse is unspecified, there is a concern that the output voltage of the inverter fluctuates transiently and the current jumps significantly. Further, since the synchronous 3 pulse has a lower switching frequency than the asynchronous pulse, even if the current does not jump in the switching to the asynchronous PWM, a large current ripple constantly occurs and torque pulsation occurs. It is necessary to increase the current capacity of the switching element.

An embodiment of the present invention will be specifically described below with reference to FIGS. 3, 4, 5, 6, and 7. In the present embodiment, a synchronous pulse for covering the inverter frequency between the asynchronous PWM and the synchronous three pulses is introduced.
Here, introduction of four synchronous pulses will be described as an example of the synchronous pulse. FIG. 3 is an example of a synchronous four pulse. In FIG. 3, θ41 and θ42 are the rising phase and falling phase of the first pulse, and θ43 and θ44.
Are the rising phase and the falling phase of the second pulse, and the rising phase and the falling phase for the third pulse and the fourth pulse are synchronous PWM, so that π (180 °) −θ44, π ( 180 °) -θ4
3, π (180 °) -θ42, π (180 °) -θ41
It is. Here, if θ41 <θ42 <θ43 <θ44 (3) and E = k · (cosθ41−cosθ42 + cosθ43−cosθ44) with respect to the output voltage command E, the output voltage command is satisfied. Is obtained. It can easily be imagined that the introduction of the synchronous 4 pulses will result in the steady current ripple being smaller than the synchronous 3 pulses.
The generation of the current ripple which occurs more steadily than the case where three synchronous pulses are introduced, and the generation of the torque pulsation caused by the generation of the current ripple are suppressed, and the current capacity of the switching element does not increase.

Next, switching between synchronous four-pulse asynchronous PWM and synchronous three-pulse switching will be described. First, FIG. 4 shows a method of generating a pulse pattern of four synchronous pulses according to the present embodiment. As shown in FIG. 4, the rising and falling phases that determine the pulse pattern of the synchronous four pulses, that is, θ41 to θ44, are comparisons between a sine wave that is a modulated wave in advance and a triangular wave synchronized with the sine wave, that is, This is a value obtained by the triangular wave comparison method. In the example of the synchronous 4-pulse shown in FIG. 4, 10 °, 50 °, 90 °, 130 °
°, 170 ° are the peaks of the triangular wave, and 30 °, 70 °,
110 ° and 150 ° are triangular wave valleys. Although FIG. 4 shows 0 ° to π (180 °), π (180
°) to 2π (360 °).
Further, the contents described below can be applied to the period from π (180 °) to 2π (360 °). On the other hand, asynchronous PW
At M, the relationship between the phase of the sine wave and the peaks and valleys of the triangular wave is undefined. Therefore, for example, if switching is performed at a specific phase of a sine wave, continuity between the triangular wave in the synchronous four pulses and the triangular wave in the asynchronous PWM cannot be maintained.
As a result, depending on the state of the pulse in the asynchronous PWM mode at the time of switching, the period during which the pulse is on may continue for a long time, and conversely, the period during which the pulse may be off may continue for a long time. This causes the current to jump.

Referring to FIG. 5, an asynchronous P according to the present embodiment will be described.
Switching between the WM and the synchronous 4-pulse will be described. In the present embodiment, as shown in FIG. 5, in the PWM pulse selecting means 23, the phase with respect to the sine wave of the peak or the valley of the triangular wave in the asynchronous PWM becomes the modulated wave referred to when determining the pulse pattern of the synchronous PWM pulse. Switch when the phase of the peak or valley of the triangular wave synchronized with the sine wave matches. FIG. 5A shows a case where the switching is performed at the valley of the synchronization carrier, and FIG. 5B shows a case where the switching is performed at the top of the synchronization carrier. In FIG. 5A, the left side of the point p is the asynchronous PWM mode, and the right side of the p point is the synchronous four-pulse mode, and the asynchronous PWM is at the point p where the valley of the asynchronous pulse carrier and the valley of the synchronous four pulse carrier coincide. To synchronous 4 pulses. Similarly, in FIG. 5B, the asynchronous PWM is switched to the synchronous four pulses at a point q where the peak of the carrier of the asynchronous pulse and the carrier peak of the synchronous four pulses coincide. Here, in the asynchronous PWM pulse, since the relationship between the peak and the valley of the triangular wave and the phase of the modulation wave is random, the switching is determined by the operating condition of the inverter to be switched (determined by the inverter frequency and the modulation rate). Although it is not performed immediately after it has been reached, it always matches and switches within a few cycles. In this case, the switching timing is slightly delayed (several cycles), but this does not cause a problem in the operation of the inverter. If it is desired to perform the switching immediately after the operating condition of the inverter to be switched is satisfied, the phase of the peak or valley of the triangular wave referred to when creating the pulse pattern of the synchronous four pulses is set to θ.
In the case of x, a certain width such as θx ± 5 ° may be provided. As a result, in the present embodiment, in the triangular wave comparison method, the frequency of the triangular wave is switched at the top or the valley of the triangular wave with continuity, so that the current can be switched smoothly without jumping up.

Next, a method of creating a pulse pattern for smoothly switching between the synchronous 4 pulse and the synchronous 3 pulse will be described. FIG. 8 shows respective pulse patterns at the time of switching between synchronous 4 pulses and synchronous 3 pulses. FIG.
In this case, since the pulse patterns of the synchronous 4 pulse and the synchronous 3 pulse are greatly different, if the switching is performed as it is, the inverter output voltage fluctuates transiently, and as a result, the current may jump. FIG. 6 shows respective pulse patterns of four synchronous pulses and three synchronous pulses at the time of switching according to the present embodiment. The operating conditions for switching to asynchronous PWM for synchronous 4 pulses, that is, when the modulation rate is relatively low even for synchronous 4 pulses, the pulse needs to be a pulse obtained by comparison with a triangular wave for smooth switching. However, except for the switching point, it is only necessary to satisfy Expressions (3) and (4), and θ41 to θ44 can be freely determined. Similarly, in the three synchronous pulses, θ31 to θ33 can be freely determined as long as the expressions (1) and (2) are satisfied. Therefore, at the modulation degree at which the switching between the synchronous 4 pulse and the synchronous 3 pulse is performed, θ41 to θ44 are set so that the relationship between θ41 to θ44 and θ31 to θ33 satisfies the equations (5) to (7).
And θ31 to θ33 are set. The PWM waveform at this time is the waveform shown in FIG. FIG. 6 shows an example in which three synchronous pulses are created in synchronization with four synchronous pulses. As is apparent from comparison with FIG. 8, the first and third pulses of the three synchronous pulses are made narrow and the second pulse is made wide. I do. Note that four synchronous pulses may be created in synchronization with the three synchronous pulses. As described above, θ31 to θ3 are set so as to satisfy Expressions (1) to (7).
By setting 3, θ41 to θ44, the on-period and off-period in the synchronous four pulses substantially coincide with the on-period and off-period in the synchronous three pulses. For this reason, the pulse shapes are similar to each other, and the synchronization 4
Switching between the pulse and the synchronous three pulses can be performed smoothly regardless of the phase. As a result, a large fluctuation does not occur in the inverter output voltage, and the current does not jump. θ31 ≧ θ41 (5) θ32 ≦ θ42 (6) θ33 ≧ θ43 (7)

In this embodiment, the switching from asynchronous PWM to synchronous 3 pulses is performed via synchronous 4 pulses. However, when asynchronous PWM is directly switched to synchronous 3 pulses without introducing synchronous 4 pulses. In the operating condition for switching from asynchronous PWM to synchronous three pulses, the pulse pattern of synchronous three pulses is determined by a comparison method using a sine wave serving as a modulation wave and a triangular wave synchronized with the sine wave.
As in the case of switching from asynchronous PWM to four synchronous pulses, the phase of the triangular wave with respect to the sine wave of the triangular wave in the asynchronous PWM coincides with the phase of the triangular wave with the peak or valley which was referred to when determining the pulse pattern of the synchronous PWM pulse. Asynchronous PWM by switching to the case
To synchronous 3 pulses can be performed smoothly.
Also, if this is applied, the pulse pattern obtained as a result of comparing the pulse pattern with the triangular wave synchronized with the modulating wave even in other synchronous pulse modes other than the synchronous 4-pulse, such as the synchronous 5-pulse mode, from the asynchronous PWM. The switching to the asynchronous PWM is performed when the phase with respect to the modulation wave of the peak or the valley of the triangular wave in the asynchronous PWM coincides with the phase of the peak or the valley referred to in determining the synchronous pulse pattern. It is clear that the switching to the synchronous three pulses can be performed smoothly. That is, the present invention can easily be applied to switching between asynchronous and other synchronous pulse modes. Further, in the present embodiment, the synchronous 4 pulse is taken as an example of the PWM mode inserted between the asynchronous PWM mode and the synchronous 3 pulse. However, if it is desired to further reduce the current ripple or if there is a margin in the switching frequency, If the pulse pattern obtained by comparing a sine wave whose modulation pattern and the inverter frequency is a modulated wave and a triangular wave synchronized with the sine wave at the modulation degree at which the switching to the asynchronous PWM is performed is limited to four synchronous pulses There is no.

Further, the synchronization 4 described in the present embodiment
The pulse pattern generation method used for switching between the pulse and the synchronous three pulse can be applied to other combinations such as synchronous five pulse and synchronous three pulse. FIG. 7 shows an example of switching between synchronous 5 pulses and synchronous 3 pulses. At this time, the phase of the five synchronous pulses only needs to satisfy Expressions (10) and (11). Further, when switching between the synchronous 5 pulse and the synchronous 3 pulse, it is necessary to satisfy the expressions (12) to (14). This is an application of switching between synchronous 4-pulse and synchronous 3-pulse, and can easily be applied to other combinations of pulse numbers. θ51 <θ52 <θ53 <θ54 <θ55 (10) E = k · (cos θ51−cosθ52 + cosθ53−cosθ54 + cosθ55) (11) θ31 ≧ θ51 (12) θ32 ≦ θ52 (13) θ33 ≧ θ53 …… (14)

In the above, the case where the inverter frequency is increased has been described as an example. However, according to the present invention, when the inverter frequency is decreased, if the pulse pattern is determined by the above-described means, the same as the case where the inverter frequency is increased can be obtained. In addition, the switching between the synchronization pulses can be performed smoothly.
On the other hand, when switching from synchronous PWM to asynchronous PWM, at the peak or valley of the carrier of the synchronous pulse,
By starting a triangular wave, which is a carrier used for generating an asynchronous pulse, from a peak or a valley, the switching to the asynchronous PWM can also be performed smoothly.

[0016]

As described above, according to the present invention,
Since switching between the asynchronous PWM mode and the synchronous PWM mode can be performed smoothly, the current does not jump at the time of switching, and the switching element can be prevented from being broken. In addition, by reducing the number of pulses at the time of switching, for example, to four synchronous pulses, it is not necessary to set the switching frequency higher than necessary, and it is possible to suppress an increase in inverter loss. As a result, the capacity of the cooling device of the apparatus can be reduced.

[Brief description of the drawings]

FIG. 1 shows a PWM pulse generator according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a pulse pattern of three synchronous pulses.

FIG. 3 is a diagram showing an example of a pulse pattern of four synchronous pulses.

FIG. 4 is a diagram showing a method for generating a pulse pattern of four synchronous pulses according to the present invention;

FIG. 5 is a diagram showing switching between an asynchronous PWM mode and a synchronous four-pulse mode according to the present invention.

FIG. 6 is a diagram showing an example of respective pulse patterns when switching between synchronous 4 pulses and synchronous 3 pulses according to the present invention.

FIG. 7 is a diagram showing an example of each pulse pattern when switching between synchronous 5 pulses and synchronous 3 pulses according to the present invention.

FIG. 8 is a diagram showing an example of a pulse pattern when switching between synchronous 4 pulses and synchronous 3 pulses;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Inverter operation command generation means, 2 ... PWM pulse generation means, 21 ... Asynchronous pulse generation means, 211 ... Sine wave generation means, 212 ... Triangle wave (carrier) generation means, 21
3 comparator, 22 sync pulse generating means, 221 sync 4 pulse generating means, 222 sync 3 pulse generating means, 2
23: Synchronous one pulse generating means, 23: PWM pulse selecting means

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Wataru Miyake 1070 Ma, Hitachinaka City, Ibaraki Prefecture Inside the Mito Plant, Hitachi, Ltd. (72) Koji Kishimoto 1070 Ma, Hitachinaka City, Ibaraki Prefecture Mito Plant, Hitachi, Ltd. (56) References JP-A-8-331856 (JP, A) JP-A-3-11979 (JP, A) JP-A-53-86432 (JP, A) JP-A-62-58880 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H02M ^7/ 48-7/98

Claims (1)

(57) [Claims] 1. An asynchronous PWM mode for generating a PWM pulse asynchronously with a modulated wave as an output command of an inverter;
A PWM which has a synchronous three-pulse mode for generating three pulses in a half cycle of the modulated wave in synchronization with the modulated wave, and switches the two pulse modes during operation to control the inverter with a pulse from each mode. A pulse generator, comprising: a synchronous four-pulse mode for generating four pulses in a half cycle of the modulated wave in synchronization with the modulated wave;
The synchronous 4-pulse mode is interposed in the switching between the WM mode and the synchronous 3-pulse mode, and when switching between the synchronous 4-pulse and the synchronous 3-pulse is performed, the rising edge of the pulse closest to 0 ° of the inverter output voltage phase of the synchronous 4-pulse is performed. The rising phase θ41, the falling phase θ42, and the rising phase θ43, falling phase θ44 of the next rising pulse, and the phase closest to the phase 0 ° within the range of 0 ° to 90 ° of the inverter output voltage phase of the three synchronous pulses. The rising phase θ31 of the pulse, the falling phase θ32, and the rising phase θ33 of the next rising pulse are represented by θ31 ≧
A PWM pulse generator, wherein the phase θ44 is made closer to the phase 90 ° while maintaining the relationship of θ41, θ32 ≦ θ42, θ33 ≧ θ43.