JPH07322636A - Pulse width modulation method of three-phase inverter - Google Patents

Abstract

PURPOSE:To realize a smooth changeover of the pulse mode by reducing the discontinuous variation width of output voltage at the time when the pulse mode is changed over from three pulse mode to one pulse mode. CONSTITUTION:Inverter output-voltage command values are given as voltage- vector command values VP1-VP6 based on the concept of spatial voltage vectors U, V, W, and two kinds of voltage vectors in the direction + or -60 deg. relative to the voltage-vector command values VP1-VP6 and voltage vectors in the same direction as the voltage-vector command values VP1-VP6 or zero voltage vectors are used as three kinds of voltage vectors for combining the voltage-vector command values VP1-VP6. The variable range of output voltage is made larger than three pulse modes by a sine-wave triangular-wave comparison system under a switching state corresponding to a minimum on-off time determined by a switching element, and the discontinuous variation width of output voltage at a time when pulse modes are changed over to one pulse mode can be reduced, thus realizing the smooth changeover of the pulse modes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse width modulation method for a three-phase inverter that changes the output voltage of a three-phase inverter that converts DC power into AC power, for example.

[0002]

2. Description of the Related Art FIG. 11 shows, for example, Hitachi Review VOL.68 No.8.
(1986-8) FIGS. 23 to 28 are diagrams for explaining a conventional pulse width modulation method described in “Inverter train control system”. FIG. 11A shows the relationship between the sine wave which is the U-phase modulation wave and the triangular wave which is the carrier wave in the 3-pulse mode (final mode of PWM control) of the sine wave triangle wave comparison method. The magnitudes of the modulated wave and the triangular wave are compared, and when the modulated wave is larger than the carrier wave, FIG.
The ON signal is supplied to the upper arm switch SU1 of the U phase of the three-phase inverter circuit shown in principle in the above, and the OFF signal is supplied to the lower arm switch SU2 thereof. When the magnitude relationship between the modulated wave and the carrier wave is other than the above, the upper arm switch SU
An OFF signal is supplied to 1 and an ON signal is supplied to the lower arm switch SU2. As a result, the U-phase voltage between the U-phase output point U and the ground G has the waveform shown in FIG.

Although not shown, similarly, by delaying the modulated wave by 120 ° with respect to the U phase and comparing it with the carrier wave,
The V-phase voltage shown in FIG. 11C is obtained. FIG. 11 (d)
Is the voltage between the output points of the U-phase and V-phase, that is, the U-phase V-phase line voltage. The magnitude of this line voltage can be made variable by changing the amplitude of the modulated wave to change the phase width θ between zero phase voltages. On the other hand, FIG. 11 (e)
Is a U-phase V-phase line voltage waveform corresponding to a state in which the phase width θ is zero, that is, a square wave with a full width of 120 °. This state is called a one-pulse mode in which all voltages are output, and the maximum value of the line voltage that can be output by the three-phase inverter is obtained.

[0004]

In the conventional pulse width modulation method, the inverter output voltage can be made variable as described above, but the switch of each arm shown in FIG. 13 is actually as shown in FIG. 14, for example. A diode 10 and a GTO (gate turn-off thyristor) 11 are connected in antiparallel, and a switching state is controlled by supplying an ON / OFF signal to the gate of the GTO 11.

The minimum on / off time is defined for the safe use of the GTO 11, and the switching state should not be changed in a time interval shorter than this. Due to this restriction, that is, in the 3-pulse mode, there is a minimum value of the phase width θ in order to secure the switching margin time of the switches of the upper and lower arms of each phase of the inverter. Therefore, in the conventional pulse width modulation method, as shown in FIG. 12, when the inverter output voltage is output in proportion to the inverter frequency and up to 1 pulse mode is used, the output voltage corresponding to the minimum value of the phase width θ is 3 pulses. It is necessary to switch from the mode to the 1-pulse mode. At that time, the output voltage in the 1-pulse mode is increased by the slit of the phase width θ, and as shown by the broken line in FIG. There is a problem in that a voltage jump of about% occurs, the output voltage changes discontinuously at that point, a sudden change in voltage or torque occurs, and voltage oscillation of the filter easily occurs.

The present invention has been made in order to solve such a problem, and the width of discontinuous change of the output voltage when switching from the 3-pulse mode to the 1-pulse mode can be reduced, and the pulse mode can be smoothly switched. An object is to provide a pulse width modulation method for a three-phase inverter that can be realized.

[0007]

A pulse width modulation method for a three-phase inverter according to a first aspect of the present invention uses an inverter output voltage command value as a space voltage vector command value every 60 ° with respect to the inverter frequency. As three types of spatial voltage vectors for synthesizing the spatial voltage vector command value, two types of spatial voltage vectors in the directions of ± 60 ° with respect to the spatial voltage vector command value and the spatial voltage vector in the same direction as the spatial voltage vector command value or The zero voltage vector is used.

In the pulse width modulation method for a three-phase inverter according to the second aspect of the present invention, the zero voltage vector which has a smaller change in switching state among the zero voltage vectors which can be output by the three-phase inverter is used as the zero voltage vector. It is a thing.

A pulse width modulation method for a three-phase inverter according to the third aspect of the invention is such that when the inverter output voltage corresponding to the inverter output voltage command value is smaller than a predetermined value,
This is to switch to the 3-pulse mode of the sine wave triangular wave comparison method.

A pulse width modulation method for a three-phase inverter according to a fourth aspect of the present invention switches to a three-pulse mode of a sine wave triangular wave comparison method when the inverter frequency is lower than a predetermined value.

A pulse width modulation method for a three-phase inverter according to a fifth aspect of the invention is a one-pulse mode when the output time of two types of space voltage vectors in ± 60 ° directions is shorter than a predetermined set time. To switch to.

A pulse width modulation method for a three-phase inverter according to a sixth aspect of the invention is characterized in that when the output time of two types of space voltage vectors in the ± 60 ° direction becomes shorter than a predetermined set time, the space voltage vector The spatial voltage vector in the direction of the command value is output only for the time corresponding to 60 ° with the inverter frequency as the reference.

A pulse width modulation method for a three-phase inverter according to a seventh aspect of the invention is characterized in that when the inverter output voltage corresponding to the inverter output voltage command value becomes a predetermined value or more,
It switches to the 1-pulse mode.

In the pulse width modulation method for a three-phase inverter according to the eighth aspect of the present invention, the one-pulse mode is switched to when the inverter frequency is equal to or higher than a predetermined value.

[0015]

According to the first aspect of the present invention, the inverter output voltage command value is given as a space voltage vector command value at every 60 ° with respect to the inverter frequency, and three types of space for combining the space voltage vector command values are provided. As the voltage vector, two types of space voltage vectors in the direction of ± 60 ° with respect to the space voltage vector command value and the space voltage vector or the zero voltage vector in the same direction as the space voltage vector command value are used. In the switching state corresponding to, the output voltage can be increased compared to the conventional 3-pulse mode of the sinusoidal triangular wave comparison method, the discontinuity of the output voltage when switching to the 1-pulse mode can be reduced, and smooth pulse mode switching is possible. Becomes

According to the second aspect of the invention, since the zero voltage vector that can be output by the three-phase inverter is the one having the smaller change in the switching state, it is used as the zero voltage vector. It becomes possible to reduce the power loss.

According to the third aspect of the present invention, in the region where the inverter output voltage is small, the three-pulse mode of the conventional sinusoidal triangular wave comparison method has a smaller harmonic component of the inverter output voltage, so that the inverter output voltage is reduced. Is smaller than a predetermined value, the sine wave triangular wave comparison system is switched to the 3-pulse mode, and the harmonic component of the inverter output voltage can be reduced in a region where the inverter output voltage is small.

According to the fourth aspect of the invention, when the inverter output voltage and the inverter frequency are in a proportional relationship, the inverter output voltage is small in a region where the inverter frequency is small, and the three-pulse mode of the conventional sinusoidal triangular wave comparison system is used. Since the harmonic component of the inverter output voltage becomes smaller, the inverter output voltage is switched to the 3-pulse mode of the sine wave triangular wave comparison method when the inverter frequency is lower than a predetermined value. It is possible to reduce the harmonic component of.

In the invention of claim 5, ± 60 °
When the output time of the two types of space voltage vectors in the direction becomes shorter than the predetermined set time, the mode is switched to the 1-pulse mode, which prevents the switching state from being changed at a time interval shorter than the minimum on / off time of the switching element. It becomes possible.

In the invention of claim 6, ± 60 °
When the output time of the two types of space voltage vector in the direction becomes shorter than the predetermined set time, the space voltage vector in the direction of the space voltage vector command value is output for a time corresponding to 60 ° based on the inverter frequency, and the automatic Since it is purposely switched to the 1-pulse mode, it is possible to prevent the switching state from being changed at a time interval shorter than the minimum on / off time of the switching element.

In the invention of claim 7, the output time of the two types of space voltage vectors in the directions of ± 60 ° becomes shorter as the inverter output voltage becomes larger.
When the inverter output voltage is equal to or higher than a predetermined value, the one-pulse mode is switched to, and it is possible to prevent the switching state from being changed at a time interval shorter than the minimum on / off time of the switching element.

According to the eighth aspect of the present invention, when the inverter output voltage and the inverter frequency are in a proportional relationship, the inverter output voltage increases as the inverter frequency increases, and two types of spatial voltage in the ± 60 ° direction. Since the output time of the vector becomes shorter, it is switched to the 1-pulse mode when the inverter frequency becomes equal to or higher than a predetermined value, and it is prevented that the switching state is changed at a time interval shorter than the minimum on / off time of the switching element. It becomes possible.

[0023]

【Example】

Example 1. An embodiment of a pulse width modulation method according to the present invention will be described below with reference to the drawings. 1 and 2 show the relationship between the space voltage vector command value and the space voltage vector that synthesizes the space voltage vector command value according to the present invention. 1 corresponds to the case where the phase sequence is U, V, W and the rotation direction is clockwise, and FIG. 2 corresponds to the case where the phase sequence is U, W, V and the rotation direction is counterclockwise.

Here, the space voltage vector will be described. The space voltage vector V is defined by the equation (1). Here, Vu, Vv, and Vw are phase voltages, and α = −2π / 3. In other words, it is a form of three-phase / two-phase conversion, in which two phases are divided into a real part and an imaginary part and treated as a vector.

[0025]

[Equation 1]

Table 1 is obtained by calculating the space voltage vector by the equation (1) corresponding to the switching state of the three-phase inverter shown in FIG. Re is a real part and Im is an imaginary part.

[0027]

[Table 1]

In Table 1, the switching state 0 of each phase is shown in FIG.
The switching elements of SU2, SV2, and SW2 of the lower arm of 3 are on, that is, the phase voltage is zero, while switching state 1 is SU1, SV1, and SW1 of the upper arm of FIG.
The switching element is ON, that is, the phase voltage corresponds to the DC voltage (Ed). Table 1 shows that there are eight types of voltage vectors that can be output by the three-phase inverter of FIG. 13 from v ₀ to v ₇ corresponding to the switching state of each phase, and v ₀ and v ₇ have zero magnitude (“zero Voltage vector ”), and the others have a size of (2/3) · Ed and are 6
It shows that the voltage vectors have different directions by 0 °. FIG. 3 is a diagram in which the spatial voltage vector of Table 1 is displayed on the complex plane.

In this example, using these voltage vectors,
At every time Ts = 1 / (6 · finv) corresponding to 60 ° with reference to the inverter frequency finv, the space voltage vector command value v _p (v _p1 ~
v _p6 ), and the space voltage vector command value v _p is shown in Table 2.
And the voltage vectors V ₁ , V ₂ and V ₃ shown in Table 3 are combined.

[0030]

[Table 2]

[0031]

[Table 3]

Table 2 corresponds to FIG. 1 and Table 3 corresponds to FIG. K in the table determines the magnitude of the inverter output voltage, and the line voltage is expressed by equation (2) using K, and K = π / 3 corresponds to the one-pulse mode. The line voltage here is the magnitude of the voltage vector command value,
When K = π / 3, v _p = √ (2/3) · Ed, and the fundamental wave component is √6 / π · Ed.

Line voltage = √6 · K · Ed / π, 0 ≦ K ≦ π / 3 (2)

A method of calculating the output times T ₁ , T ₂ , T ₃ of the three types of voltage vectors V ₁ , V ₂ , V ₃ for synthesizing the voltage vector command value v _p will be described below. Since the following equations (3) and (4) are established between the voltage vector command value vp and the voltage vectors V ₁ , V ₂ , and V ₃ , the voltage vector command value v _p and the time Ts = 1 / ( 6 · finv), the voltage vector command value v _p is given in Table 2 or Table 3.
Since it is determined which one of v _p1 to v _p6 of the above is corresponded, the output times T _{1 to} T ₃ can be obtained. Using this result, the voltage vector command value v _p can be synthesized by outputting the voltage vectors V ₁ , V ₂ , and V ₃ in order as shown in FIG. 4 for the times T ₁ , T ₂ , and T _3. .

V _p · Ts = V ₁ · T ₁ + V ₂ · T ₂ + V ₃ · T ₃ (3)

Ts = T ₁ + T ₂ + T ₃ (4)

Specifically, in Table 2, v _p = v _p1 , V ₁ = v ₂ ,
In the case of V ₂ = v ₁ and V ₃ = v ₆ , T ₁ , T ₂ and T ₃ will be calculated. The magnitude of the voltage vector command value has a one-to-one correspondence with the magnitude of the line voltage, and is therefore set as in equation (5).

V _p1 = √6 · K · Ed / π (5)

By substituting the equation (5) into the equation (3) and rearranging by using the fact that the imaginary part is zero, the equation (6) is obtained.

√6 · K · Ts / π = √ (2/3) · T ₂ + √ (2/3) · T ·· (6)

Further, the equation (4) becomes the equation (7).

Ts = 2T + T ₂ (7)

As a result, T ₁ , T ₂ , and T ₃ are obtained as in the following equations (8) and (9).

T = T ₁ = T ₃ = (1-3K / π) Ts (8)

T ₂ = (6K / π−1) Ts (9)

[0046] Here, (7) from the equation, when determining the conditions to be T ₂ = 0, with K = π / 6, it can be seen that the T ₂ = 0. That is, when the line voltage corresponding to K = π / 6 is output, the output time of V ₂ becomes zero. Therefore,
When K <π / 6, it is understood that the voltage vector according to the command cannot be synthesized unless the zero voltage vector is used as V ₂ as shown in Table 2 or Table 3. FIG. 5 shows K in Table 2.
The synthesis of v _p1 corresponding to <π / 6 is shown in FIG.
It is the figure which showed the synthesis _{| combination} of _vp1 corresponding to / 6. As described above, when K <π / 6 and K ₂ is used by switching between the voltage vector command value and the voltage vector (for example, v ₁ ) and the zero voltage vector (for example, v ₇ ) in the same direction as V ₂ , K is π /
It can be applied to a region smaller than 6.

There are two types of zero voltage vector, v ₀ and v ₇ , and the output voltage does not change regardless of which voltage vector is used. Focusing attention, since it is preferable to reduce the change in switching state in terms of power loss and the like, it is preferable to use v ₀ and v ₇ separately as shown in Tables 2 and 3, for example, if the voltage vector command value is v _p1 , v
_p3, v using v ₇ when the _p5, may be used to v ₀ when the voltage vector command values _{v p2, v p4, v p6} . Further, although the GTO is assumed as the switching element, it may be a transistor, an IGBT, an FET, or the like as long as the switching state can be controlled by a gate signal or a base signal.

Although the details of the relationship between the fundamental wave component and the magnitude of the voltage vector command value obtained in this embodiment will be omitted, if the above equation (2) is used from the waveform harmonic analysis result, The wave component is expressed by the following equation.

Fundamental wave component = √6 / π · Ed · sin (3/2 · K) (10)

On the other hand, in the case of the conventional three-pulse mode of the sine wave triangular wave comparison method, the fundamental wave component is expressed by the following equation according to the similar harmonic analysis.

Fundamental wave component = 3√6 / π ² · K · Ed (11)

Here, the inverter frequency finv is 100
Considering the case of shifting from the 3-pulse mode to the 1-pulse mode by setting H _Z , T ₁ , and T ₂ to 200 μs, the value of K at this time is K = 0.92 from the above equation (8). Therefore,
Substituting this K into the above equations (10) and (11) respectively and calculating the fundamental wave component, in this embodiment, it is approximately 0.98 · √6 / π · Ed, the conventional sinusoidal triangular wave comparison system. Is about 0.88 · √6 / π · Ed. Therefore, as described above, since K = π / 3, that is, the fundamental wave component in the 1-pulse mode is √6 / π · Ed, the voltage change at the transition from the 3-pulse mode to the 1-pulse mode is the same as that of the conventional sinusoidal triangular wave comparison. Although it was 12% in the system, it becomes 2% in this embodiment, and the transition from the 3-pulse mode to the 1-pulse mode can be realized almost continuously, and the output voltage error at the transition from the 3-pulse mode to the 1-pulse mode can be realized. It can be seen that the pulse mode is smoothly switched by reducing the width of continuous change.

Example 2. As described in the first embodiment, it is possible to apply the 3-pulse mode of the pulse width modulation method based on the spatial voltage vector even in the region where the inverter output voltage is small. In the region where the output voltage is small, the 3-pulse mode based on the conventional sinusoidal triangular wave comparison method is preferable because it has less harmonic components. Therefore, as shown in FIG. 7, the inverter output voltage E ₀ is set, the 3-pulse mode based on the spatial voltage vector is applied in the inverter output voltage range higher than this, and the conventional three-pulse mode is applied in the inverter output voltage range smaller than the voltage E ₀ . The 3-pulse mode of the sine wave triangular wave comparison method is applied.

When the load of the three-phase inverter is an induction motor, constant V / f (voltage / frequency) control is often used. At this time, instead of the inverter output voltage E ₀ as shown in FIG. Set f0, f0
In the above inverter frequency region, the 3-pulse mode based on the spatial voltage vector can be applied, and in the inverter frequency region smaller than f ₀ , the conventional 3-pulse mode of the sinusoidal triangular wave comparison system can be applied.

Example 3. When the inverter output voltage increases, K in the equations (8) and (9) also increases, and the output times T ₁ and T ₃ of the voltage vectors V ₁ and V ₃ become shorter and V ₂ becomes V ₂
The output time T _{2 of} is longer. That is, the influence of the above-mentioned minimum on / off time of the switching element appears
It can be seen that the outputs are V ₁ and V ₃ . Assuming that this minimum on / off time is Tmin, T = T ₁ = T ₃ obtained by the equation (8).
When T <Tmin, the outputs of V ₁ and V ₃ are eliminated,
If V ₂ is output only for Ts time, 1 is automatically output.
Can shift to pulse mode.

That is, for example, the voltage vector command value v _p1
When the output time of the voltage vectors v ₂ and v _{6 in} the directions of ± 60 ° is shorter than the predetermined time, the output of these voltage vectors is stopped and the voltage vector v _{1 in the} same direction as the voltage vector command value v _p1. Output only. As a result, the 3-pulse mode based on the space voltage vector can be automatically switched to the 1-pulse mode (in this case, the 1-pulse mode according to the present embodiment). Note that Tmin is not necessarily the minimum on / off time, and can be set to any value as long as it is at least the minimum on / off time. Further, the processing in the 1-pulse mode can also be realized by outputting Ed and 0 for each 180 ° of each phase (in this case, the conventional 1-pulse mode).

Example 4. Further, as shown in FIGS. 9 and 10, the inverter output voltage E ₁ or the inverter frequency f ₁ is changed in the same manner as the switching between the 3-pulse mode of the sine-wave triangular wave comparison method and the 3-pulse mode based on the space voltage vector.
It is also possible to set and to switch between the 3-pulse mode and the 1-pulse mode based on the space voltage vector.
Inverter output voltage set values E ₀ and E ₁ and inverter frequencies f ₀ and f ₁ actually have hysteresis characteristics to prevent chattering. Further, although not described above, the processing of the pulse width modulation method is realized as software processing of a microcomputer or the like.

[0058]

According to the first aspect of the present invention, the inverter output voltage command value is given as a space voltage vector command value at every 60 ° with respect to the inverter frequency, and the space voltage vector command value is synthesized. Two types of space voltage vectors in the directions of ± 60 ° with respect to the space voltage vector command value and a space voltage vector or a zero voltage vector in the same direction as the space voltage vector command value are used as the space voltage vector of the type. In the switching state corresponding to the minimum on / off time determined by the above, the variable range of the output voltage can be increased compared to the conventional 3-pulse mode of the sinusoidal triangular wave comparison method, and the discontinuity width of the output voltage when switching to the 1-pulse mode can be reduced. Smooth pulse mode switching becomes possible, and the current and The effect of such lost can cause unwanted phenomena sudden change of click.

According to the second aspect of the present invention, the zero voltage vector that can be output by the three-phase inverter is the one that has the least change in the switching state and is used as the zero voltage vector. The number of times is reduced and the power loss can be reduced.

According to the third aspect of the invention, when the inverter output voltage corresponding to the inverter output voltage command value is smaller than the predetermined value, the sine wave triangular wave comparison system is switched to the 3-pulse mode. This has the effect of reducing the harmonic components of the inverter output voltage in the low voltage region.

According to the fourth aspect of the invention, when the inverter frequency is lower than the predetermined value, the three-pulse mode of the sine wave triangular wave comparison system is switched, and the inverter output voltage and the inverter frequency have a proportional relationship. In some cases, there is an effect such that the harmonic component of the inverter output voltage can be reduced in the region where the inverter output voltage is small.

According to the fifth aspect of the invention, when the output time of the two types of space voltage vectors in the ± 60 ° direction becomes shorter than the predetermined set time, the one pulse mode is switched to. It is possible to prevent the switching state from being changed at a time interval shorter than the minimum on / off time.

According to the sixth aspect of the invention, when the output time of the two types of space voltage vectors in the ± 60 ° direction becomes shorter than the predetermined set time, the space voltage vector in the direction of the space voltage vector command value is set. Is output only for a time corresponding to 60 ° based on the inverter frequency, and is automatically switched to the 1-pulse mode, so that it is possible to prevent the switching state from being changed at a time interval shorter than the minimum on / off time of the switching element. And so on.

According to the seventh aspect of the invention, when the inverter output voltage corresponding to the inverter output voltage command value becomes equal to or higher than the predetermined value, the one pulse mode is switched to. As the inverter output voltage increases, ± 60
Since the output time of the two types of spatial voltage vectors in the ° direction becomes short, there is an effect that it is possible to prevent changing the switching state at a time interval shorter than the minimum on / off time of the switching element.

According to the eighth aspect of the present invention, when the inverter frequency is equal to or higher than a predetermined value, the mode is switched to the 1-pulse mode, and when the inverter output voltage is proportional to the inverter frequency, the inverter frequency is changed. Since the output time of the two types of space voltage vectors in the directions of ± 60 ° becomes shorter as becomes larger, it is possible to prevent the switching state from being changed at a time interval shorter than the minimum on / off time of the switching element.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a space voltage vector command value and a space voltage vector combining the space voltage vector command value according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a space voltage vector command value and a space voltage vector combining the space voltage vector command value according to an embodiment of the present invention.

FIG. 3 is a diagram showing voltage vectors that can be output by a three-phase inverter.

FIG. 4 is a diagram showing three types of voltage vectors and respective output times.

FIG. 5 is a diagram showing an example of combining space voltage vector command values.

FIG. 6 is a diagram showing an example of combining space voltage vector command values.

FIG. 7 is a diagram for explaining switching of pulse modes based on an inverter output voltage.

FIG. 8 is a diagram for explaining switching of pulse modes based on an inverter frequency.

FIG. 9 is a diagram for explaining switching of pulse modes based on an inverter output voltage.

FIG. 10 is a diagram for explaining switching of pulse modes based on an inverter frequency.

FIG. 11 is a diagram for explaining a conventional pulse width modulation method.

FIG. 12 is a diagram for explaining a change in inverter output voltage when switching from the 3-pulse mode to the 1-pulse mode.

FIG. 13 is a diagram for explaining the principle of a three-phase inverter.

FIG. 14 is a diagram showing a specific configuration example of a switch.

[Explanation of symbols]

V space voltage vector, finv inverter frequency, v _p
Space voltage vector command value.

Claims (8)

[Claims] 1. An inverter output voltage command value is given as a space voltage vector command value at every 60 ° with respect to the inverter frequency, and the space voltage vector command is given as three types of space voltage vectors for synthesizing the space voltage vector command value. A pulse width modulation method for a three-phase inverter, which uses two types of space voltage vectors in ± 60 ° directions with respect to a value and a space voltage vector or a zero voltage vector in the same direction as the space voltage vector command value. . 2. The pulse width of the three-phase inverter according to claim 1, wherein a zero voltage vector having a smaller change in switching state is used as the zero voltage vector among the zero voltage vectors that can be output by the three-phase inverter. Modulation method. 3. When the inverter output voltage corresponding to the inverter output voltage command value is smaller than a predetermined value, the sine wave triangular wave comparison method is switched to the 3-pulse mode. Pulse width modulation method for three-phase inverter. 4. The pulse width modulation method for a three-phase inverter according to claim 1, wherein when the inverter frequency is lower than a predetermined value, the three-pulse mode of the sinusoidal triangular wave comparison method is switched to. 5. The one-pulse mode is switched to when the output time of the two types of space voltage vectors in the ± 60 ° directions becomes shorter than a predetermined set time. Pulse width modulation method for three-phase inverter. 6. When the output time of the two types of space voltage vectors in the ± 60 ° directions is shorter than a predetermined set time, the space voltage vector in the direction of the space voltage vector command value is based on the inverter frequency. 3. The pulse width modulation method for a three-phase inverter according to claim 1, wherein the pulse width modulation is performed for a time corresponding to 60 degrees. 7. The three-phase inverter according to claim 1, wherein when the inverter output voltage corresponding to the inverter output voltage command value exceeds a predetermined value, the one-pulse mode is switched to. Pulse width modulation method. 8. The pulse width modulation method for a three-phase inverter according to claim 1, wherein when the inverter frequency is equal to or higher than a predetermined value, the one pulse mode is switched to.