Pulse Width Modulation Method for Synthesizing Multiphase Full-Parameter Controllable AC Voltage from Three-Phase AC

Abstract

The present invention discloses a pulse width modulation method for synthesizing a multi-phase fully parameter controllable AC voltage from three-phase AC. By pulse width modulation and chopping synthesis of a three-phase AC input voltage, an AC output voltage with controllable frequency, amplitude and phase can be obtained. In addition, harmonics can be added to the AC output voltage, and each parameter of the harmonics can also be controlled. The present invention is easy to output multiple AC voltages, and by designing the parameters of the output voltage, multiple output voltages can also be formed into a multi-phase system. The current harmonics generated by the present invention can offset each other to avoid current harmonic pollution to the input node. At the same time, the present invention can generate a harmonic AC voltage, which can increase the utilization rate of the voltage waveform and improve the energy transmission density.

Description

Technical Field

[0001] The present invention belongs to the technical field of electric energy conversion, and in particular relates to a pulse width modulation method for synthesizing a multi-phase fully-parameter controllable AC voltage from three-phase AC. Background Art

[0002] Historically, the earliest transmission method used was direct current (DC), which emerged in Russia in 1874. At the time, the transmission voltage was only 100V. With advancements in DC generator manufacturing technology, the DC transmission voltage had risen to 6000V by 1885. However, further increasing the rated voltage of high-power DC generators presented a series of technical difficulties, such as insulation. The inability to directly boost DC voltage significantly limited transmission distances, making it impossible to meet the demands for increased transmission capacity and distance. In the late 1880s, three-phase AC generators and transformers were invented. In 1891, the world's first three-phase AC power station was completed in Germany. Since then, AC transmission has become the dominant transmission system.

[0003] DC and AC transmission each have their own unique advantages. DC transmission offers lower line construction and operating costs, is more suitable for interconnecting two power systems, and is easier to implement via urban underground and submarine cables. AC transmission, on the other hand, offers advantages such as lower substation costs, voltage step-up and step-down transformers, easier fault clearance, and reduced reactive power compensation requirements.

[0004] With the advancement of power electronics technology, power transmission methods are no longer limited to DC and sinusoidal AC. Power electronics devices can be used to inject designed harmonics into the power waveform to achieve a desired voltage waveform. Currently, research on such devices is limited, leaving much room for further research. Summary of the Invention

[0005] The purpose of the present invention is to provide a pulse width modulation method for synthesizing a three-phase AC into a multi-phase fully parameter-controllable AC voltage, which can obtain an AC output voltage with controllable frequency, amplitude, and phase, and is easy to form multiple phases. In addition, harmonics can be added to the AC output voltage, and each parameter of the harmonics is also controllable.

[0006] The technical solution adopted by the present invention is a pulse width modulation method for synthesizing a multi-phase fully parameter controllable AC voltage from three-phase AC, designing a modulation wave containing a fundamental wave control term and a harmonic wave control term, and using the modulation wave, a direct AC chopper performs pulse width modulation chopping on the three-phase AC input voltage, and then performs series synthesis and low-pass filtering on the voltage obtained after chopping to obtain an AC output voltage containing a controllable fundamental wave and harmonics.

[0007] The present invention is also characterized in that:

[0008] The pulse width modulation method for synthesizing a three-phase AC into a multi-phase fully parameter controllable AC voltage is specifically implemented in the following steps:

[0009] Step 1: The three-phase AC input voltage is expressed as U-phase voltage, V-phase voltage, and W-phase voltage respectively;

[0010] Step 2: Design the calculation formula and modulation wave that can realize the control of the fundamental and harmonic waves of the output voltage. Use the direct cross-cross chopper and modulation wave to perform pulse width modulation chopping on the U-phase voltage, V-phase voltage, and W-phase voltage respectively to obtain the three-phase modulation voltage, which is expressed as: the modulation voltage V of the U phase DU , V-phase modulation voltage V DV , W phase modulation voltage V DW ;

[0011] Step 3: Add the three-phase modulated voltages in series and perform low-pass filtering to obtain the AC output voltage V O .

[0012] Step 2: Use direct AC chopper and modulation wave to perform pulse width modulation chopping on U phase voltage, V phase voltage and W phase voltage respectively. The specific process is: use modulation wave D to chop the U phase AC input voltage. U , and get the voltage V DU ; V-phase AC input voltage adopts modulation wave D V , and get the voltage V DV ;W phase AC input voltage adopts modulation wave D W , and get the voltage V DW ;

[0013] Assume that the U-phase AC input voltage U u , V-phase AC transmission voltage U v , W-phase AC input voltage U w The expression is:

[0014]

[0015] Where V in represents the input voltage amplitude, ω1 represents the angular frequency of the input three-phase voltage;

[0016] Modulation voltage V DU 、V DV 、V DW The expression is:

[0017]

[0018] Among them, D U Indicates the U-phase modulation wave, D V Indicates V-phase modulation wave, D W Indicates the W-phase modulated wave.

[0019] Step 2: Design the calculation formula and modulation wave that can realize the control of output voltage fundamental wave and harmonic wave. The specific process is as follows:

[0020] Determine the calculation formulas D1 and D2 based on the desired output voltage frequency, amplitude, and phase:

[0021]

[0022] In formula (3), ω1 is the angular frequency of the input three-phase voltage, and ω2 is the angular frequency of the output fundamental voltage. D and V DK is the amplitude of each AC quantity in the calculation formula, is the initial phase difference of the output voltage fundamental wave relative to the input voltage, is the initial phase difference of the Kth harmonic in the output voltage relative to the input voltage;

[0023] The process of performing Clarke inverse transform on equations D1 and D2 is shown in equation (4):

[0024]

[0025] Step 2: When the direct-type AC / AC chopper is a bipolar direct-type AC / AC chopper, the modulation wave is expressed as follows according to formula (4):

[0026]

[0027] In formula (5), the sine wave with a frequency of ω1-ω2 is the fundamental control term in the modulation wave, which is used to control the fundamental parameters in the output voltage. The sine wave with a frequency of ω1-Kω2 is the harmonic control term in the modulation wave, which is used to control the harmonic parameters in the output voltage.

[0028] In step 3, the AC output voltage V O The expression is:

[0029] V O =V DU +V DV +V DW (6)

[0030] From equations (1)-(6), the AC output voltage V O Expressed as:

[0031]

[0032] If the output voltage is required to contain only the fundamental wave, the harmonic control term in the modulation wave is 0, and the modulation wave is expressed as:

[0033]

[0034] AC output voltage V O Expressed as:

[0035]

[0036] Step 2: When the direct AC / AC chopper is a unipolar direct AC / AC chopper, set D U 、D V 、D W After unipolarization, the instantaneous values ​​of the three modulation waves will be greater than zero, and the output voltage coefficient will become half of the original value. According to formula (4), the modulation wave expression is:

[0037]

[0038]

[0039]

[0040] In step 3, the AC output voltage V O The expression is:

[0041] V O =V DU +V DV +V DW (6)

[0042] From equations (1)-(4), (6), (10)-(12), we can get the AC output voltage V O Expressed as:

[0043]

[0044] If the output voltage is required to contain only the fundamental wave, the harmonic control term in the modulation wave is 0, and the modulation wave is expressed as:

[0045]

[0046] AC output voltage V O Expressed as:

[0047]

[0048] The beneficial effects of the present invention are:

[0049] (1) The present invention is a direct AC-AC conversion method and does not contain a DC link or DC capacitors. However, DC capacitors can lead to high failure rates, short life cycles, and expensive maintenance costs.

[0050] (2) The output voltage fundamental wave and Kth harmonic of the present invention are all fully parameter-controllable. Their frequency, amplitude and phase can be independently controlled.

[0051] (3) The harmonic AC voltage generated by the present invention can increase the utilization rate of the voltage waveform and improve the energy transmission density.

[0052] (4) The present invention can easily output multiple AC voltages. By designing the parameters of the output voltages, multiple output voltages can also be used to form a multi-phase system.

[0053] (5) The current harmonics generated by the present invention can cancel each other out, thus avoiding current harmonic pollution to the input node. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 This is a flow chart of a pulse width modulation method for synthesizing a multi-phase fully parameter controllable AC voltage from three-phase AC according to the present invention;

[0055] Figure 2 It is a topological structure used in implementing the pulse width modulation method of synthesizing a multi-phase fully parameter controllable AC voltage from a three-phase AC according to the present invention;

[0056] Figure 3 The figure shows the modulation wave D when the input voltage is 50Hz and the output voltage is 15Hz and the third harmonic is added. U , D V and D W waveform;

[0057] Figure 4 The figure shows the three-phase output voltage V when the input voltage is 50Hz, the output voltage is 15Hz, and the third harmonic is added. OA , V OB and V OC Waveform diagram;

[0058] Figure 5 The figure shows the modulation method and Figure 2 For the topology shown, when the input voltage is 50Hz and the output voltage is 15Hz, the three-phase input voltage U is added when the third harmonic is U , U V and U W And three-phase input current I U , I V and I W Waveform diagram;

[0059] Figure 6 The figure shows that when the input voltage is 50Hz, the output voltage is 15Hz, and it only contains the fundamental wave, the modulation wave D U , D V and D W waveform;

[0060] Figure 7 The figure shows the modulation method and Figure 2 The topology shown, the output voltage is 15Hz, and only contains the fundamental wave, the three-phase output voltage V OA , V OB and V OCWaveform diagram. DETAILED DESCRIPTION

[0061] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0062] The pulse width modulation method of the present invention for synthesizing a multi-phase fully parameter controllable AC voltage from a three-phase AC is as follows: Figure 1 As shown in the figure, a modulation wave containing fundamental and harmonic control terms is designed. The direct cross-chopter uses the modulation wave to perform pulse width modulation chopping on the three-phase AC input voltage, and the voltage obtained after chopping is synthesized in series and low-pass filtered to obtain an AC output voltage containing both fundamental and harmonic controllable. When the harmonic part is 0, the modulation wave is only controlled by the fundamental wave. A topology structure is used as shown in the figure. Figure 2 As shown, please follow the steps below:

[0063] The three-phase AC input voltage is expressed as U-phase voltage, V-phase voltage, and W-phase voltage respectively; the calculation formula and modulation wave that can realize the fundamental wave and harmonic control of the output voltage are designed, and the modulation wave D is used for the U-phase AC input voltage. U , and get the voltage V DU ; V-phase AC input voltage adopts modulation wave D V , and get the voltage V DV ;W phase AC input voltage adopts modulation wave D W , and get the voltage V DW ; Assume that the U-phase AC input voltage U u , V-phase AC transmission voltage U v , W-phase AC input voltage U w The expression is:

[0064]

[0065] Where V in represents the input voltage amplitude, ω1 represents the angular frequency of the input three-phase voltage;

[0066] Modulation voltage V DU 、V DV 、V DW The expression is:

[0067]

[0068] Among them, D U Indicates the U-phase modulation wave, D V Indicates V-phase modulation wave, D W Indicates the W-phase modulated wave.

[0069] The calculation formula and modulation wave for controlling the fundamental and harmonic waves of the output voltage are designed as follows:

[0070] Determine the calculation formulas D1 and D2 based on the desired output voltage frequency, amplitude, and phase:

[0071]

[0072] In formula (3), ω1 is the angular frequency of the input three-phase voltage, and ω2 is the angular frequency of the output fundamental voltage. D and V DK is the amplitude of each AC quantity in the calculation formula, is the initial phase difference of the output voltage fundamental wave relative to the input voltage, is the initial phase difference of the Kth harmonic in the output voltage relative to the input voltage;

[0073] The process of performing Clarke inverse transform on equations D1 and D2 is shown in equation (4):

[0074]

[0075] The direct-type cross-intersection chopper can adopt a bipolar direct-type AC / AC chopper or a unipolar direct-type AC / AC chopper. When a bipolar direct-type AC / AC chopper is adopted, the modulation wave is expressed as follows according to formula (4):

[0076]

[0077] In formula (5), the sine wave with a frequency of ω1-ω2 is the fundamental control term in the modulation wave, which is used to control the fundamental parameters in the output voltage. The sine wave with a frequency of ω1-Kω2 is the harmonic control term in the modulation wave, which is used to control the harmonic parameters in the output voltage.

[0078] The three-phase modulated voltage is synthesized by series addition and low-pass filtering to obtain the AC output voltage V O , the process is:

[0079] AC output voltage V O The expression is:

[0080] V O =V DU +V DV +V DW (6)

[0081] From equations (1)-(6), the AC output voltage V O Expressed as:

[0082]

[0083] If the output voltage is required to contain only the fundamental wave, the harmonic control term in the modulation wave is 0, and the modulation wave is expressed as:

[0084]

[0085] AC output voltage V O Expressed as:

[0086]

[0087] From formula (7), it can be seen that the frequency, amplitude and phase of the output voltage fundamental wave and each K-th harmonic are controllable by adjusting the calculation formula D1 and D2, or the modulation wave D U 、D V 、D W The corresponding parameters in can realize full parameter control of the output voltage fundamental and harmonics.

[0088] In addition, through the voltage synthesis and modulation method of the present invention, multiple output voltages can be obtained, and the phase angle Set to 0, N-phase AC voltage can be constructed. At this time, the expression of N-phase AC voltage is:

[0089]

[0090] When a bipolar direct AC / AC chopper is used, the instantaneous value of the modulation wave is between -1 and 1.

[0091] When the direct AC / AC chopper is a unipolar direct AC / AC chopper, D U 、D V 、D W After unipolarization, the instantaneous values ​​of the three modulation waves are between 0 and 1, and the output voltage coefficient becomes half of the original value. According to formula (4), the modulation wave expression is:

[0092]

[0093]

[0094]

[0095] AC output voltage V O The expression is:

[0096] V O =V DU +V DV +V DW (6)

[0097] From equations (1)-(4), (6), (10)-(12), we can get the AC output voltage V O Expressed as:

[0098]

[0099] If the output voltage is required to contain only the fundamental wave, the harmonic control term in the modulation wave is 0, and the modulation wave is expressed as:

[0100]

[0101] AC output voltage V O Expressed as:

[0102]

[0103] Example

[0104] like Figure 2 The topology shown is a topology for running the present invention. The present invention can be used but is not limited to Figure 2 Run on the topology shown.

[0105] like Figure 2 As shown, the three-phase core-type multi-winding transformer has four windings on each core leg, namely T a1 、T a2 、T a3 、T a4 , T b1 、T b2 、T b3 、T b4 , T c1 、T c2 、T c3 、T c4 The winding ratio of all windings is the same; each secondary winding has two wires leading out, the one with a black solid dot is the positive polarity wire, and the other is the negative polarity wire. Winding T a1 , T b1 , T c1 The winding is the primary winding, connected to a three-phase voltage. The remaining windings are the secondary windings, connected to each unipolar direct AC / AC chopper. Winding T a2 Give unipolar direct AC / AC chopper U a Power supply; winding T a3 Give unipolar direct AC / AC chopper U b Power supply; winding T a4 Give unipolar direct AC / AC chopper U c Power supply; winding T b2 Give the unipolar direct AC / AC chopper V a Power supply; winding T b3 Give the unipolar direct AC / AC chopper V b Power supply; winding T b4 Give the unipolar direct AC / AC chopper V c Power supply; winding T c2 For unipolar direct AC / AC chopper Wa Power supply; winding T c3 For unipolar direct AC / AC chopper W b Power supply; winding T c4 For unipolar direct AC / AC chopper W c Power supply; The positive input port of each unipolar direct AC / AC chopper is connected to the positive polarity line of its corresponding winding, and the negative input port is connected to the negative polarity line of its corresponding winding. a The negative terminal of the output port and the chopper V a The positive connection of the output port, chopper V a The negative terminal of the output port and the chopper W a The positive connection of the output port, chopper U a The positive terminal of the output port and the chopper W a The negative pole of the output port is connected to the input port 2 of the A-phase LC filter, and the output port 2 of the A-phase LC filter constitutes the output voltage of the A-phase. b The negative terminal of the chopper V b Positive connection of output port, chopper V b The negative pole of the output port and the chopper W b Positive connection of output port, chopper U b The positive terminal of the output port and the chopper W b The negative pole of the output port is connected to the second input port of the B-phase LC filter, and the second output port of the B-phase LC filter constitutes the B-phase output voltage. c The negative terminal of the output port and the chopper V c The positive connection of the output port, chopper V c The negative terminal of the output port and the chopper W c The positive connection of the output port, chopper U c The positive terminal of the output port and the chopper W c The negative pole of the output port is connected to the input port 2 of the C-phase LC filter, and the output port 2 of the C-phase LC filter constitutes the C-phase output voltage. The negative poles of the output ports 2 of the three LC filters are connected to a point N, and the positive poles of the output ports 2 of the three LC filters constitute the output three-phase voltage V OA 、V OB 、V OC .

[0106] Assume that the input voltage expression of the primary winding of the transformer is (14)

[0107]

[0108] Since the winding ratios of the windings are the same, the input voltages of the unipolar AC / AC choppers are the same as the input voltages of the primary windings. The output voltage of phase A is used as an example for deduction, and the other phases are symmetrical and the same.

[0109] The calculation formulas D1 and D2 are shown in formula (17)

[0110]

[0111] In formula (17), ω1 is the angular frequency of the input three-phase voltage, and ω2 is the angular frequency of the output fundamental voltage. D and V DK is the amplitude of each AC quantity in the calculation formula. is the initial phase difference of the output voltage fundamental wave relative to the input voltage, is the initial phase difference of the Kth harmonic in the output voltage relative to the input voltage. The modulated wave is obtained by performing Clarke inverse transformation on the calculation formula D1 and D2. After the modulated wave is converted to a single polarity, the results are shown in formulas (18)-(20)

[0112]

[0113]

[0114]

[0115] Chopper Aa uses modulation wave D A , chopper Ba adopts modulation wave D B , chopper Ca adopts modulation wave D C Since the output ports of choppers Aa, Ba, and Ca are connected in series, the output voltage V OA The expression of is (21):

[0116]

[0117] V OB and V OC The modulation and synthesis method is similar to V OA The final result of the three-phase output voltage is shown in (22):

[0118]

[0119] Taking the transformer phase A core leg as an example, the harmonic current is analyzed and the output current is set as Equation (23). The input sides of the unipolar direct AC / AC chopper Ua, Ub, and Uc are coupled through the magnetic circuit of the phase A core leg. The coupled current can be expressed as Equation (24).

[0120]

[0121]

[0122] Among them I m is the amplitude coefficient of the output three-phase current. A is the output current expression of phase A. It can be seen that the frequency of the coupled current is ω1, which is the same as the input voltage and current. Therefore, the current harmonics generated by the present invention can cancel each other out, avoiding current harmonic pollution to the input node.

[0123] Figure 3 The figure shows the modulation wave D when the input voltage is 50Hz and the output voltage is 15Hz and the third harmonic is added. A , D B and D C waveform.

[0124] Figure 4 The figure shows the three-phase output voltage V when the input voltage is 50Hz, the output voltage is 15Hz, and the third harmonic is added. OA , V OB and V OC According to the waveform diagram. Figure 4 As can be seen, the frequency, amplitude, and phase of the fundamental and harmonics in the output three-phase voltage all conform to the set values. Therefore, the present invention can produce an AC voltage with fully controllable fundamental and harmonic parameters, and easily constructs a multi-phase system. Furthermore, it can be seen that this waveform has higher space-time utilization and improves energy transmission density.

[0125] Figure 5 The figure shows the modulation method and Figure 2 For the topology shown, when the input voltage is 50Hz and the output voltage is 15Hz, the three-phase input voltage U is added when the third harmonic is A , U B and U C And three-phase input current I A , I B and I C As can be seen from the waveform, the output current is a 50 Hz sine wave. Therefore, the current harmonics generated by the present invention can cancel each other out, and the input current has the same frequency as the input voltage, thus avoiding current harmonic pollution to the input node.

[0126] Figure 6 The figure shows that when the input voltage is 50Hz and the output voltage is 15Hz, and only the fundamental wave is included, the modulation wave D A , D B and D C waveform.

[0127] Figure 7 The figure shows the modulation method and Figure 2 The topology shown, when the output voltage is 15Hz and contains only the fundamental wave, the three-phase output voltage VOA , V OB and V OC The waveform diagram. At this time, the input voltage is 50Hz and the amplitude is 311V. Set V D is 0.5, ψ is 0, and ω2 is 30π. Figure 7 It can be seen that the amplitude of the output three-phase voltage is 116V, the initial phase angle is 0°, and the frequency is 15Hz. The frequency, amplitude, and phase all meet the set values. Therefore, the present invention can obtain an AC voltage fundamental wave with controllable frequency, amplitude, and phase, and no harmonics.

[0128] Through the above-mentioned method, the pulse width modulation method of synthesizing a multi-phase fully parameter-controllable AC voltage from three-phase AC of the present invention can obtain an AC output voltage with controllable frequency, amplitude, and phase by pulse width modulation and chopping synthesis of the three-phase AC input voltage. In addition, harmonics can be added to the AC output voltage, and each parameter of the harmonics is also controllable. The present invention is easy to output multiple AC voltages. By designing the parameters of the output voltage, multiple output voltages can also be formed into a multi-phase system. The current harmonics generated by the present invention can offset each other, avoiding current harmonic pollution to the input node. At the same time, the present invention can generate a harmonic AC voltage, which can increase the utilization rate of the voltage waveform and improve the energy transmission density.

Claims

1. Pulse width modulation method for synthesizing multi-phase fully parameter controllable AC voltage from three-phase AC, It is characterized in that Design a modulation wave containing fundamental wave control items and harmonic wave control items. The direct AC chopper uses the modulation wave to perform pulse width modulation chopping on the three-phase AC input voltage, and then performs series synthesis and low-pass filtering on the voltage obtained after chopping to obtain an AC output voltage containing both fundamental wave and harmonic wave controllable. Follow the steps below to implement it: Step 1: The three-phase AC input voltage is expressed as U Phase voltage, V Phase voltage, W Phase voltage; Step 2: Design a calculation formula and modulation wave that can realize the control of the output voltage fundamental wave and harmonic wave, and use the direct cross-cross chopper and modulation wave to respectively U Phase voltage, V Phase voltage, W The phase voltage is pulse-width modulated and chopped to obtain a three-phase modulated voltage, which is expressed as: U Phase modulation voltage V DU , V Phase modulation voltage V DV , W Phase modulation voltage V DW ; The direct cross-chop and the modulation wave are respectively used to U Phase voltage, V Phase voltage, W The specific process of pulse width modulation chopping of phase voltage is as follows: U Phase AC input voltage adopts modulation wave D U , and get the voltage V DU ; V Phase AC input voltage adopts modulation wave D V , and get the voltage V DV ; W Phase AC input voltage adopts modulation wave D W , and get the voltage V DW ; set up U Phase AC input voltage U u , V Phase AC transmission voltage U v , W Phase AC input voltage U w The expression is: in V in Indicates the input voltage amplitude, Indicates the angular frequency of the input three-phase voltage; Modulation voltage V DU , V DV , V DW The expression is: in, D U express U Phase modulated wave, D V express V Phase modulated wave, D W express W Phase modulated wave; The design can realize the calculation formula and modulation wave specific process of output voltage fundamental wave and harmonic wave control as follows: Determine the calculation formula based on the desired output voltage frequency, amplitude, and phase D 1 , D 2 : In formula (3) ω 1 is the angular frequency of the input three-phase voltage, ω 2 is the angular frequency of the output fundamental voltage, V D and V DK is the amplitude of each AC quantity in the calculation formula, 1 is the initial phase difference of the output voltage fundamental wave relative to the input voltage, K Output voltage K The initial phase difference of the subharmonic relative to the input voltage; The calculation formula D 1 and D 2 The process of performing the Clarke inverse transform is shown in formula (4): （4）； Step 3: Add the three-phase modulated voltages in series and perform low-pass filtering to obtain an AC output voltage. V O .

2. According to the pulse width modulation method of synthesizing a multi-phase fully parameter controllable AC voltage from a three-phase AC according to claim 1, It is characterized in that When the direct-type AC / AC chopper in step 2 is a bipolar direct-type AC / AC chopper, the modulation wave expression obtained according to formula (4) is: The frequency in formula (5) is The sine wave is the fundamental wave control item in the modulation wave, which is used to control the fundamental wave parameters in the output voltage. The frequency is The sine wave is the harmonic control item in the modulation wave, which is used to control the harmonic parameters in the output voltage.

3. According to the pulse width modulation method of synthesizing a multi-phase fully parameter controllable AC voltage from a three-phase AC according to claim 2, It is characterized in that The AC output voltage described in step 3 V O The expression is: (6) From equations (1)-(6), we can get the AC output voltage VO It is expressed as:

4. According to the pulse width modulation method of synthesizing a multi-phase fully parameter controllable AC voltage from three-phase AC as claimed in claim 3, It is characterized in that If the output voltage is required to contain only the fundamental wave, the harmonic control term in the modulation wave is 0, and the modulation wave is expressed as: （8） AC output voltage VO It is expressed as: （9）。 5. The pulse width modulation method for synthesizing a multi-phase fully parameter controllable AC voltage from a three-phase AC according to claim 1, It is characterized in that When the direct-type AC / AC chopper described in step 2 is a unipolar direct-type AC / AC chopper, D U , D V , D W After unipolarization, the instantaneous values ​​of the three modulation waves will be greater than zero, and the output voltage coefficient becomes half of the original value. According to formula (4), the modulation wave expression is:

6. The pulse width modulation method for synthesizing a multi-phase fully parameter controllable AC voltage from a three-phase AC according to claim 5, It is characterized in that The AC output voltage described in step 3 V O The expression is: (6) From equations (1)-(4), (6), (10)-(12), we can get the AC output voltage V O It is expressed as:

7. The pulse width modulation method for synthesizing a multi-phase fully parameter controllable AC voltage from a three-phase AC according to claim 6, It is characterized in that If the output voltage is required to contain only the fundamental wave, the harmonic control term in the modulation wave is 0, and the modulation wave is expressed as: AC output voltage VO It is expressed as:

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