A low harmonic optimized synchronous modulation method and device for an open-winding permanent magnet synchronous motor

Abstract

The present invention discloses a low-harmonic optimized synchronous modulation method and device for an open-winding permanent magnet synchronous motor. The method comprises: during the inverter control process, adjusting the phase difference between the reference voltage of the head-end inverter and the reference voltage of the tail-end inverter to 120° to eliminate the influence of the three-integer-multiple-of-three harmonic voltage on the open-winding permanent magnet synchronous motor; and optimizing the switching angles of the head-end inverter and the tail-end inverter using a current harmonic minimum PWM method to eliminate the influence of other subharmonic voltages on the open-winding permanent magnet synchronous motor. The present invention achieves harmonic cancellation by setting the reference voltage of the head-end and tail-end inverters of the open-winding transmission system to a phase shift of 120°, thereby eliminating the influence of the three-integer-multiple-of-three harmonic voltage on the open-winding permanent magnet synchronous motor; and optimizing the switching angle of a single three-level inverter using a current harmonic minimum PWM method to eliminate the influence of other subharmonics.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of permanent magnet synchronous motors, and in particular to a low harmonic optimized synchronous modulation method and device for an open-winding permanent magnet synchronous motor. Background Art

[0002] In recent years, open-winding permanent magnet synchronous motors have been widely used in situations requiring high reliability and high power due to their advantages such as flexible control, good fault tolerance, and high power density. Compared with traditional permanent magnet synchronous motors, the power of open-winding permanent magnet synchronous motors can be distributed to two inverters, which can effectively reduce the capacity of a single inverter. The three-level NPC inverter has many advantages such as high output power, low total distortion rate of the output waveform, low device voltage stress, and low system electromagnetic interference. Therefore, when the dual three-level NPC inverter is used for power supply at both ends, when a fault occurs on one side of the system, it can be switched to a single inverter to power the system, thereby enhancing the fault tolerance of the system. Compared with the isolated DC bus system, the common DC bus type open-winding permanent magnet synchronous motor drive system eliminates a DC power supply, simplifies the system structure and saves costs. The topology diagram of the open-winding permanent magnet synchronous motor of the dual three-level NPC under the common DC bus is shown as follows: Figure 1 shown.

[0003] However, in applications using high-power open-winding permanent magnet synchronous motor (PMSM) transmission systems with a common DC bus, the switching frequency of three-level PWM inverters is typically limited to a few hundred Hz to minimize power device and system energy losses, due to factors such as power device switching losses, inverter heat dissipation requirements, and the presence of zero-sequence currents. This leads to the following disadvantages: the common DC bus introduces a zero-sequence path, which easily generates zero-sequence currents. The ratio of the switching frequency of a three-level NPC inverter to the motor operating frequency, or the carrier ratio, is often less than 10, significantly increasing the low-order harmonics in the inverter's output voltage and current, leading to increased motor losses and severe heat generation. Furthermore, harmonics increase current peaks, increasing current stress on the switching devices.

[0004] To address the above technical issues, existing technologies generally use synchronous modulation to reduce harmonic content. This requires the inverter output voltage to meet quarter-cycle symmetry, half-wave symmetry, and three-phase symmetry, thereby eliminating even-order voltage harmonics at the inverter output. However, other harmonics cannot be completely offset, and the motor harmonic current cannot reach the theoretical minimum. Alternatively, the SHEPWM method can eliminate specific voltage harmonics, but it does not consider the load characteristics of open-winding motors, and its performance cannot reach optimal levels. A detailed analysis is as follows:

[0005] 1. When the DC terminal voltage ratio of the inverter at both ends of an open-winding permanent magnet synchronous motor is constant, the purpose of eliminating 6k±1 (k=1, 3, 5...) harmonics can be achieved by phase shifting the triangular carrier of the SPWM method. However, this method does not filter out the harmonic components sufficiently and the effect is not good.

[0006] 2. The SHEPWM method can eliminate selected voltage harmonics regardless of load characteristics. This is because it can completely eliminate low-order harmonics, the switching angle is relatively simple to calculate, and the switching angle distribution is continuous with the modulation ratio, making SHEPWM easier to implement than other optimized PWM methods. Furthermore, SHEPWM requires simultaneous elimination of adjacent harmonics. For example, eliminating the 5th and 7th harmonics simultaneously can eliminate the 6th-order torque ripple generated by them. If only the 5th harmonic is eliminated, the increase in the 7th harmonic will cause the 6th-order torque ripple amplitude to even exceed that of non-optimized PWM.

[0007] 3. Adopt the ISCBPWM strategy. Under the carrier ratio of odd multiples of 3, a carrier-based switching sequence with a P-type small vector is used at the first sampling point of each 60° sector. Under the carrier ratio of even multiples of 3, a carrier-based switching sequence with a P-type and N-type small vector is used at the first sampling point of adjacent sectors. In this way, the output voltage waveform satisfies the three-phase symmetry and half-wave symmetry of the pulse generation law under any integer multiple of 3, thereby eliminating the triple frequency harmonics and even harmonics. However, the ISCBPWM strategy is a relatively complex method and requires online calculation, which has high requirements on the performance of the microprocessor. Its robustness is not as good as the CHMPWM modulation strategy.

[0008] In summary, under low-carrier ratio conditions, the increase in low-order harmonics in open-winding permanent magnet synchronous motors can lead to current distortion and harmonic torque, impacting system stability. Conventional synchronous modulation methods cannot optimize the system's harmonics, and changing the inverter topology and increasing the number of switching devices incurs high costs and losses. Therefore, to address the shortcomings and deficiencies of existing technologies, optimizing synchronous modulation to maintain low harmonic content in the output voltage and current at a lower switching frequency is a key issue that must be addressed when designing pulse-width modulation algorithms for open-winding three-level NPC inverters. Summary of the Invention

[0009] The technical problem to be solved by the present invention is: in response to the technical problems existing in the prior art, the present invention provides a low-harmonic optimized synchronous modulation method and device for an open-winding permanent magnet synchronous motor that eliminates various harmonics.

[0010] In order to solve the above technical problems, the technical solution proposed by the present invention is:

[0011] A low harmonic optimized synchronous modulation method for an open-winding permanent magnet synchronous motor, comprising:

[0012] During the inverter control process, the phase difference between the reference voltage of the head-end inverter and the reference voltage of the tail-end inverter is adjusted to 120 degrees to eliminate the influence of the integer multiple harmonic voltage of three on the open-winding permanent magnet synchronous motor;

[0013] The current harmonic minimum PWM method is adopted to optimize the switching angles of the head-end inverter and the tail-end inverter to eliminate the influence of other subharmonic voltages on the open-winding permanent magnet synchronous motor.

[0014] Preferably, the process of optimizing the switching angles of the head-end inverter and the tail-end inverter by adopting the current harmonic minimum PWM method is as follows: converting the problem of solving the optimal harmonic current of the common DC bus type open-winding permanent magnet synchronous motor into a conditional extreme value optimization problem.

[0015] Preferably, the specific process of conditional extreme value optimization is:

[0016] The constraints and objective function of the optimization problem of minimum current harmonic PWM are as follows:

[0017] The objective function is as follows

[0018]

[0019] in is the harmonic distortion value of the current, I n is the nth harmonic current of the open-winding permanent magnet synchronous motor, and I1 is the fundamental current;

[0020] The constraint function is as follows:

[0021]

[0022] where m is the modulation index; α i is the i-th switching angle; U d is the DC bus voltage; N is the number of switching angles within 0° to 90° corresponding to the fundamental phase; n is the harmonic order;

[0023] To solve the objective function The minimum value of is taken as the optimization target, and the transcendental equation is solved through offline calculation to obtain the switching angle of the three-level NPC inverter.

[0024] Preferably, after the switching angle of the three-level NPC inverter is obtained, a digital signal processor DSP or a programmable logic device FPGA is used to generate a trigger pulse signal of the three-level NPC inverter.

[0025] Preferably, a table lookup method is used to generate a pulse signal. After the DSP or FPGA issues the modulation level and frequency instructions, it looks up the table accordingly and controls the frequency division number to generate the corresponding frequency, trigger angle extension, trigger angle interlocking and delay, and protection logic processing.

[0026] The present invention also discloses a low-harmonic optimized synchronous modulation device for an open-winding permanent magnet synchronous motor, comprising a motor torque controller, a voltage observer, a coordinate conversion unit, a modulation amplitude angle calculation module, a first angle generator, a second angle generator, a first switching angle selection optimizer, and a second switching angle selection optimizer; the output of the motor torque controller is used as a given of the voltage observer to obtain the dq axis voltage of the stator, which is then converted by the coordinate conversion unit and calculated by the modulation amplitude and angle of the modulation amplitude angle calculation module, and then sent to the first angle generator and the second angle generator to output preliminary first switching angle and second switching angle respectively; the first switching angle is optimized by the current harmonic minimum PWM method of the first switching angle selection optimizer to obtain the optimized first switching angle, and then the head-end inverter is controlled by the corresponding pulse generator; the second switching angle is optimized by the current harmonic minimum PWM method of the second switching angle selection optimizer to obtain the optimized second switching angle, and then the tail-end inverter is controlled by the corresponding pulse generator.

[0027] Preferably, a position sensor is further included for detecting position information of the open-winding permanent magnet synchronous motor and sending the position information to the coordinate conversion unit for position closed-loop control.

[0028] Preferably, a speed sensor is further included to detect the speed information of the open-winding permanent magnet synchronous motor and send the speed information to the input end of the motor torque controller for speed closed-loop control.

[0029] Compared with the prior art, the advantages of the present invention are:

[0030] The present invention achieves harmonic cancellation by setting the reference voltage of the inverters at the head and tail ends of the open-winding transmission system to a phase shift of 120 degrees, eliminating the impact of three-integer subharmonic voltages on the open-winding permanent magnet synchronous motor. The current harmonic minimum PWM (CHMPWM) method is adopted to optimize the switching angle of a single three-level inverter to eliminate the impact of other subharmonics. Compared with the five-level modulation method, this method has simpler calculations, reduces the performance requirements of the control part processor, and improves robustness.

[0031] The present invention systematically considers the load characteristics of the open-winding permanent magnet synchronous motor, greatly reduces the current harmonics, and minimizes the THD of the load current, thereby indirectly significantly optimizing various performance indicators such as losses caused by harmonics, current peaks, and torque ripple. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The topology diagram of the open-winding permanent magnet synchronous motor of the dual three-level NPC under the common DC bus in the prior art is shown.

[0033] Figure 2 Schematic diagram of the topology of the open-winding permanent magnet synchronous motor with dual three-level NPC under a common DC bus in the present invention

[0034] Figure 3 This is a flow chart of the modulation method based on phase shifting and harmonic elimination optimization of CHMPWM in the present invention.

[0035] Figure 4 This is a control block diagram of a dual three-level NPC open-winding permanent magnet synchronous motor system under a common DC bus according to the present invention.

[0036] Figure 5 This is a three-phase current waveform diagram of the motor under the CHMPWM modulation strategy of the present invention.

[0037] Figure 6 This is the phase voltage waveform of the motor under the CHMPWM modulation strategy of the present invention.

[0038] Figure 7 This is the current spectrum diagram after FFT analysis of the three-phase current of the motor under the harmonic minimum modulation strategy of the present invention.

[0039] Figure 8 This is a voltage spectrum diagram after FFT analysis of the three-phase voltage of the motor under the harmonic minimum modulation strategy of the present invention.

[0040] Figure 9 This is a diagram of the three-phase sinusoidal fundamental voltage of two groups of converters after phase shifting and the three-phase voltage after superimposing the zero-sequence component. DETAILED DESCRIPTION

[0041] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0042] like Figure 3 As shown, the embodiment of the present invention provides a low harmonic optimization synchronous modulation method for an open-winding permanent magnet synchronous motor, comprising:

[0043] During the inverter control process, the phase difference between the reference voltage of the head-end inverter and the reference voltage of the tail-end inverter is adjusted to 120 degrees to eliminate the influence of the integer multiple harmonic voltage of three on the open-winding permanent magnet synchronous motor;

[0044] The current harmonic minimum PWM method is adopted to optimize the switching angles of the head-end inverter and the tail-end inverter to eliminate the influence of other subharmonic voltages on the open-winding permanent magnet synchronous motor.

[0045] Specifically, before eliminating the integer multiple harmonic voltage of three, the current harmonics of the open-winding permanent magnet synchronous motor at a low carrier ratio are analyzed as follows:

[0046] Determine the number of switching angles N required within 1 / 4 fundamental wave period based on the inverter switching frequency;

[0047] The optimized PWM strategy generally requires that the generated voltage waveform have half-wave symmetry and 1 / 4 cycle symmetry. In a three-phase system, symmetry between the three phases is also generally required. Through the harmonic analysis of the output voltage of the three-level inverter, the expression of the voltage harmonics of the three-level inverter at the beginning and end of the winding at N switching angles within 1 / 4 fundamental wave period is obtained:

[0048]

[0049]

[0050] In formula (1), U A1 、U B1 、U C1 is the three-phase output voltage U of the first-end inverter A2 、U B2 、U C2 The three-phase output voltage of the tail inverter, ω e is the fundamental angular frequency.

[0051] In formula (2), n is the harmonic order; U mn is the amplitude of the nth harmonic, n=1 is the fundamental wave; α i is the i-th switching angle; U d is the DC bus voltage; N is the number of switching angles within 0° to 90° corresponding to the fundamental phase.

[0052] For a common DC bus open-winding permanent magnet synchronous motor drive system, the above voltage is input to the first and last inverters of the open-winding motor. Therefore, the phase voltage acting on the open-winding motor winding is the difference between the two inverter voltages.

[0053] First, define the switching function S ij , represents the switching function of the converter phase bridge arm (3):

[0054]

[0055] In formula (3), i represents the symbol of the phase bridge arm, j represents the converter to which it belongs, and each group of bridge arms outputs three states: P, 0, and N according to different switching results. The output phase voltages of the two groups of converters at the head end and the tail end are respectively:

[0056]

[0057] Since the phase voltages of the two sets of converters are both referenced to point O, the dual three-level output phase voltages Ua, Ub, and Uc can be obtained according to Kirchhoff's voltage law as shown in formula (5):

[0058]

[0059] Equations (1)-(5) show that the inverter output voltage includes the fundamental wave and various harmonics, thus generating harmonic currents in the open-winding permanent magnet synchronous motor. For open-winding permanent magnet synchronous motors with a common DC bus, since the zero-sequence impedance of the winding is very small, a small integer multiple of 3 zero-sequence voltage will generate a large zero-sequence current, increasing the loss of the open-winding permanent magnet synchronous motor and potentially causing torque ripple.

[0060] In order to solve the above technical problems, the phase shifting and harmonic elimination method is adopted. By setting the reference voltages of the head-end inverter and the tail-end inverter of the open-winding transmission system to a phase difference of 120°, the influence of the integer multiple harmonic voltage of 3 on the open-winding permanent magnet synchronous motor is eliminated.

[0061]

[0062] In formula (6), is the integer multiple harmonic voltage of three-phase 3 output by the head-end inverter, is the three-phase integer multiple harmonic voltage of 3 output by the tail inverter. It can be seen from formula (6) that when n is an integer multiple of 3, the subtraction value of the integer multiple harmonic voltage output by the head-end inverter and the tail-end inverter is 0, indicating that the integer multiple harmonics of 3 can be offset. After phase shifting, the three-phase sinusoidal fundamental voltage of the two groups of converters ( Figure 9 solid line) and the three-phase voltage after superimposing the zero-sequence component ( Figure 9 (middle dashed line) Figure 9 As shown, the converter 1 vector lags the converter 2 vector by 120°. Figure 9 It can be seen that the zero-sequence components of converters 1 and 2 can cancel each other out, which can suppress low-frequency harmonics. However, whether they can suppress high-frequency components still needs to be analyzed from the unit switching cycle.

[0063] Furthermore, considering the characteristics of the open-winding permanent magnet synchronous motor, the harmonic current model of the open-winding permanent magnet synchronous motor is analyzed:

[0064] The 5th and 7th harmonic components of the system in the three-phase stationary coordinate system appear as the 6th harmonic in the fundamental wave dq synchronous rotating coordinate system, and the integer multiples of 3 have been offset by the phase shifting method to eliminate the harmonics, so the dq axis synchronous rotating coordinate system U dn , U qn As shown in formula (7):

[0065]

[0066] When only harmonics are considered, the relationship between the stator harmonic current and stator harmonic voltage of the open-winding permanent magnet synchronous motor is shown in formula (8):

[0067]

[0068] In formula (8), ψ f is the rotor fundamental flux amplitude, L d is the d-axis component of the motor inductance, L q is the q-axis component of the motor inductance, w e is the rotor electrical angular velocity, i dn is the nth harmonic current component of the d-axis, i qn is the nth harmonic current component of the q-axis; for the planar open-winding permanent magnet synchronous motor L q =L d =L.

[0069] When a high-power open-winding permanent magnet synchronous motor drive system with a common DC bus operates normally, the zero-sequence current harmonics are very small and can even be ignored. Secondly, at high speeds, the stator and rotor resistance voltage drop of the motor can be ignored. Also, considering that the motor's self-inductance is usually much larger than its leakage inductance, the relationship between the harmonic current amplitude and the harmonic voltage amplitude, based on the simplified harmonic equivalent circuit in the dq-axis coordinate system, is expressed as Equation (9):

[0070]

[0071] In order to minimize the THD of all harmonic currents, the optimal harmonic current problem of the open-winding permanent magnet synchronous motor with a common DC bus is transformed into a conditional extreme value optimization problem. The constraints and objective function of the optimization problem for minimum current harmonic PWM are as follows:

[0072] The objective function is formula (10), and the constraint function is formula (11)

[0073]

[0074] in is the harmonic distortion value of the current, I n is the nth harmonic current of the open-winding permanent magnet synchronous motor, and I1 is the fundamental current.

[0075] Combining the characteristics of open-winding permanent magnet synchronous motor and the influence of load angle on harmonic current, a new current harmonic evaluation index σ is defined here: i :

[0076]

[0077] where m is the modulation index,

[0078] Specifically, to solve The minimum value of is taken as the optimization target, and the transcendental equation is solved through offline calculation to obtain the switching angle of the three-level NPC inverter. The digital sine function table formed is used to control the three-level NPC inverter using the lookup table method.

[0079] After determining the switching angle using the CHMPWM modulation strategy, a digital signal processor (DSP) or programmable logic device (FPGA) is typically used. Pulse generation, which involves extensive timing and logic control, is more effectively handled by the FPGA. Specifically, a table lookup method is used to generate the pulse signal. After the DSP or FPGA issues the modulation level and frequency command, it then uses the table lookup to control the frequency division factor to generate the corresponding frequency, trigger angle extension, trigger angle interlocking and delay, and protection logic.

[0080] The present invention achieves harmonic cancellation by setting the reference voltage of the inverters at the head and tail ends of the open-winding transmission system to a phase shift of 120 degrees, eliminating the impact of three-integer subharmonic voltages on the open-winding permanent magnet synchronous motor. The current harmonic minimum PWM (CHMPWM) method is adopted to optimize the switching angle of a single three-level inverter to eliminate the impact of other subharmonics. Compared with the five-level modulation method, this method has simpler calculations, reduces the performance requirements of the control part processor, and improves robustness.

[0081] The present invention systematically considers the load characteristics of the open-winding permanent magnet synchronous motor, greatly reduces the current harmonics, and minimizes the THD of the load current, thereby indirectly significantly optimizing various performance indicators such as losses caused by harmonics, current peaks, and torque ripple.

[0082] like Figure 4As shown, an embodiment of the present invention provides a low harmonic optimized synchronous modulation device for an open-winding permanent magnet synchronous motor, including a motor torque controller, a voltage observer, a coordinate conversion unit, a modulation amplitude angle calculation module, a first angle generator, a second angle generator, a first switching angle selection optimizer and a second switching angle selection optimizer; the output of the motor torque controller is used as a given of the voltage observer to obtain the dq axis voltage of the stator, which is then converted by the coordinate conversion unit and calculated by the modulation amplitude and angle of the modulation amplitude angle calculation module, and then sent to the first angle generator and the second angle generator to output the preliminary first switching angle and the second switching angle respectively. The first switching angle is optimized by the current harmonic minimum PWM method of the first switching angle selection optimizer to obtain the optimized first switching angle, and then the corresponding pulse generator is used to control the head-end inverter; the second switching angle is optimized by the current harmonic minimum PWM method of the second switching angle selection optimizer to obtain the optimized second switching angle, and then the corresponding pulse generator is used to control the tail-end inverter.

[0083] Furthermore, the system also includes a position sensor and a speed sensor. The position sensor is used to detect the position information of the open-winding permanent magnet synchronous motor and send it to the coordinate conversion unit for position closed-loop control. The speed sensor is used to detect the speed information of the open-winding permanent magnet synchronous motor and send it to the input end of the motor torque controller for speed closed-loop control.

[0084] like Figure 2 As shown in the figure, the open-winding permanent magnet synchronous motor drive system of dual three-level NPC under a common DC bus consists of two sets of inverters, three-level NPC inverter 1# and three-level NPC inverter 2#, including 24 switching tubes and 12 clamping diodes. At the same time, a set of DC units is required, which is implemented by the rectifier unit of the previous stage. In this case, the load is abstracted from the open-winding permanent magnet synchronous motor into a three-phase RL impedance. The six nodes a1, b1, c1 and a2, b2, c2 of the two sets of inverters are connected as series power supplies to the three-phase symmetrical load respectively. The midpoint potential is defined as zero, and the positive bus potential is V dc1 , the negative bus is V dc2 (negative value), each group of converter phase voltage provides V dc1 ,0,V dc2 The above system adopts a common DC bus topology, which only requires one DC bus power supply, greatly reducing the volume, weight and cost of the system, and the power capacity utilization of the common DC bus power supply is high.

[0085] Simulation analysis: Figure 5-8The waveforms are simulated using Matlab / Simulink for the system harmonic minimum modulation method at different switching angles, where the motor DC voltage is 4000V. All control and modulation algorithms are implemented using programming to maximize the approximation to the actual control algorithm. Figure 5 and Figure 6 is the three-phase current and voltage waveform of the motor under the CHMPWM modulation strategy, where Figure 7 and Figure 8 This is the current and voltage spectrum diagram after FFT analysis of the three-phase current and phase voltage of the motor under the system harmonic minimum modulation strategy.

[0086] In summary, the FFT analysis using the CHMPWM modulation strategy demonstrates the superior performance of the system harmonic minimization modulation method in reducing current THD. This method can control the amplitude of harmonics to a very low level, significantly reducing the adverse effects of low-order harmonics. Furthermore, the current peak under this method is significantly lower than that of unoptimized modulation methods with the same modulation ratio and number of switching angles. The CHMPWM modulation method is beneficial for improving the current utilization of the switching devices in the inverter.

[0087] As shown in this disclosure and the claims, unless the context clearly indicates an exception, the words "a", "an", "an" and / or "the" do not specifically refer to the singular, but also include the plural. The words "first", "second" and similar words used in this disclosure do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, words such as "include" or "comprise" mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect.

[0088] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions based on the principles of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A low harmonic optimization synchronous modulation method for an open-winding permanent magnet synchronous motor, characterized in that: include: During the inverter control process, the phase difference between the reference voltage of the head-end inverter and the reference voltage of the tail-end inverter is adjusted to 120 degrees to eliminate the influence of the integer multiple harmonic voltage of three on the open-winding permanent magnet synchronous motor; The switching angles of the head-end inverter and tail-end inverter are optimized by adopting the current harmonic minimum PWM method to eliminate the influence of other subharmonic voltages on the open-winding permanent magnet synchronous motor. The process of optimizing the switching angles of the head-end inverter and the tail-end inverter by adopting the current harmonic minimum PWM method is as follows: the problem of solving the optimal harmonic current of the open-winding permanent magnet synchronous motor with a common DC bus is transformed into a conditional extreme value optimization problem; The specific process of conditional extreme value optimization is: The constraints and objective function of the optimization problem of minimum current harmonic PWM are as follows: The objective function is as follows in is the harmonic distortion value of the current, is the nth harmonic current of the open-winding permanent magnet synchronous motor, is the fundamental current; The constraint function is as follows: in m is the modulation index; is the i-th switching angle; U d is the DC bus voltage; N is the number of switching angles within 0°-90° corresponding to the fundamental phase; n is the harmonic order; To solve the objective function The minimum value of is taken as the optimization target, and the transcendental equation is solved through offline calculation to obtain the switching angle of the three-level NPC inverter.

2. The low harmonic optimization synchronous modulation method for an open-winding permanent magnet synchronous motor according to claim 1, characterized in that: After the switching angle of the three-level NPC inverter is obtained, a digital signal processor DSP or a programmable logic device FPGA is used to generate a trigger pulse signal of the three-level NPC inverter.

3. The low harmonic optimization synchronous modulation method for an open-winding permanent magnet synchronous motor according to claim 2, characterized in that: The pulse signal is generated by the table lookup method. After the DSP or FPGA issues the modulation level and frequency instructions, it looks up the table accordingly and controls the frequency division number to generate the corresponding frequency, trigger angle extension, trigger angle interlocking and delay, and protection logic processing.

4. A low harmonic optimization synchronous modulation device for an open-winding permanent magnet synchronous motor, used to execute the steps of the low harmonic optimization synchronous modulation method for an open-winding permanent magnet synchronous motor as claimed in claim 1, 2 or 3, characterized in that: The invention comprises a motor torque controller, a voltage observer, a coordinate conversion unit, a modulation amplitude angle calculation module, a first angle generator, a second angle generator, a first switching angle selection optimizer and a second switching angle selection optimizer; the output of the motor torque controller is used as a given of the voltage observer to obtain the dq axis voltage of the stator, which is then converted by the coordinate conversion unit and calculated by the modulation amplitude and angle of the modulation amplitude angle calculation module before being sent to the first angle generator and the second angle generator to output preliminary first switching angle and second switching angle respectively; the first switching angle is optimized by the current harmonic minimum PWM method of the first switching angle selection optimizer to obtain the optimized first switching angle, which is then controlled by the corresponding pulse generator to control the head-end inverter; the second switching angle is optimized by the current harmonic minimum PWM method of the second switching angle selection optimizer to obtain the optimized second switching angle, which is then controlled by the corresponding pulse generator to control the tail-end inverter.

5. The low harmonic optimization synchronous modulation device for an open-winding permanent magnet synchronous motor according to claim 4, characterized in that: It also includes a position sensor for detecting position information of the open-winding permanent magnet synchronous motor and sending the position information to the coordinate conversion unit for position closed-loop control.

6. The low harmonic optimization synchronous modulation device for an open-winding permanent magnet synchronous motor according to claim 5, characterized in that: The system also includes a speed sensor for detecting speed information of the open-winding permanent magnet synchronous motor and sending the speed information to the input end of the motor torque controller for speed closed-loop control.

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