A three-phase bridge series type frequency conversion drive system and layout method

Abstract

The present invention relates to the technology of motor variable-frequency drive, and particularly relates to a three-phase bridge series-type variable-frequency drive system and a layout method. The system includes a rectifier, m three-phase units, and an AC motor; each three-phase unit includes a three-phase bridge inverter, a capacitor connected in parallel on the DC side, and a DC bypass switch; the positive and negative DC input terminals of the m three-phase units are connected in series in sequence and connected to the DC output terminal of the rectifier; the stator winding of the AC motor includes m mutually isolated sub-windings; the AC output terminals of the m three-phase units are respectively connected to the three-phase terminals of the m sub-windings of the AC motor; m is a positive integer. This system independently supplies power to each sub-winding of the motor stator with each three-phase unit, and by using the isolation between the sub-windings of the motor stator, the phase-shifting transformer installed to achieve input-side power isolation for each unit and the three-phase uncontrolled rectifier circuit of each unit in a conventional cascaded inverter are omitted, improving the power density of the variable-frequency drive system.

Description

Technical Field

[0001] The present invention belongs to the technical field of motor variable frequency drive, and particularly relates to a three-phase bridge series type variable frequency drive system and a layout method thereof. Background Art

[0002] An AC motor variable frequency drive system consists of a power electronic converter (also known as a frequency converter) capable of outputting a variable frequency voltage and a motor. In the field of high-voltage motor drive, in order to reduce the du / dt of the motor voltage and extend the service life of the motor, a high-voltage frequency converter with a multi-level output voltage waveform is usually adopted. There are three common topologies of high-voltage frequency converters: flying capacitor type three-level (multi-level) converter, diode clamped type three-level (multi-level) converter, and H-bridge cascaded type multi-level converter. The latter two are more mature in current market applications. In high-voltage and high-power applications, the three-level converter directly uses high-voltage power electronic switching devices, which has a simple structure and few components. However, it is limited by the withstand voltage and current parameters of the switching devices and can only be used in motor drive systems below 6 kV. Moreover, the cost of high-voltage switching devices is high, which is not conducive to maintenance.

[0003] The H-bridge cascaded type multi-level converter applied to a high-voltage frequency converter consists of 3m units (m units for each phase) and a phase-shifting transformer with a complex structure. The phase-shifting transformer provides an isolated three-phase power supply for all units. Each unit consists of a low-voltage (800 - 1200 V) three-phase bridge uncontrolled rectifier, a DC capacitor, and a single-phase H-bridge inverter. Then, the outputs of the H-bridge inverters of m units are connected in series as one phase, so as to synthesize a high voltage of 3 - 10 kV. This structure can obtain a higher number of output levels, greatly reduce the output harmonics, and the unit structure makes it convenient to replace and is conducive to maintenance. However, the phase-shifting transformer greatly increases the volume, weight, and cost of the equipment. Its volume may account for about one-third of the total volume of the equipment, and its weight may account for about one-half of the total weight of the equipment. At the same time, since there is an H-bridge inverter on the output side of all units of this frequency converter, the total number of switching devices of all H-bridge inverters is m × 4 × 3 = 12m. If the three-phase bridge uncontrolled rectifier on the input side of each unit is further considered, the number of switching devices is even more. In addition, since the H-bridge inverter is a single-phase circuit, the unit requires a large-capacity DC capacitor to alleviate the double-frequency fluctuation of the DC voltage. Summary of the Invention

[0004] Aiming at the problems existing in the background art, the present invention provides a variable frequency drive system composed of a frequency converter composed of m input series-connected three-phase bridge inverters and a motor with m sets of independent stator windings. The outputs of each unit of the frequency converter are coordinated with the winding structure of the motor, and the electrical isolation of the output side of each unit is realized by the electrical isolation of each winding of the motor, thereby eliminating the complex and heavy phase-shifting transformer, and the three-phase bridge inverters of each unit also reduce the capacity requirement of the DC-side capacitor.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions: A three-phase bridge series type variable frequency drive system for an AC power grid, comprising an AC power source, a rectifier, m three-phase units, and an AC motor; each three-phase unit includes a three-phase bridge inverter, a capacitor C connected in parallel on the DC side, and a DC bypass switch S p ; the positive and negative DC input terminals of the m three-phase units are connected in series in sequence and connected to the DC output terminal of the rectifier; the stator winding of the AC motor includes m mutually isolated sub-windings, and each sub-winding is connected in star or delta; the AC output terminals of the m three-phase units are respectively connected to the three-phase terminals of the m sub-windings of the AC motor; the magnetic flux of each phase of the stator of the motor is jointly generated by the corresponding windings of each sub-winding, and each sub-winding adopts a distributed and uniform distribution or a centralized layout in space; m is a positive integer.

[0006] A three-phase bridge series type variable frequency drive system for a DC power grid, comprising a DC power source, m three-phase units, and an AC motor; each three-phase unit includes a three-phase bridge inverter, a capacitor C connected in parallel on the DC side, and a DC bypass switch S p ; the positive and negative DC input terminals of the m three-phase units are connected in series in sequence and connected to the DC output terminal of the rectifier; the stator winding of the AC motor includes m mutually isolated sub-windings, and each sub-winding is connected in star or delta; the AC output terminals of the m three-phase units are respectively connected to the three-phase terminals of the m sub-windings of the AC motor; the magnetic flux of each phase of the stator of the motor is jointly generated by the corresponding windings of each sub-winding, and each sub-winding adopts a uniform distribution or a centralized layout in space; m is a positive integer.

[0007] In the above three-phase bridge series type variable frequency drive system, the switching tubes of the three-phase bridge inverter are selected from metal-oxide semiconductor field effect transistors MOSFET, insulated gate bipolar transistors IGBT, or other types of fully controlled switching devices; the DC bypass switch S p is a mechanical switch or a fully controlled power electronic switch device.

[0008] In the above three-phase bridge series type variable frequency drive system, the rectifier is selected from a six-pulse rectifier circuit, a twelve-pulse rectifier circuit, or an eighteen-pulse rectifier circuit.

[0009] In the above three-phase bridge series type variable frequency drive system, each sub-winding adopts a centralized layout in space to form a three-phase motor drive system.

[0010] In the above three-phase bridge series type variable frequency drive system, each sub-winding adopts a distributed and uniform distribution in space to form a variable frequency drive system for a multi-phase motor or a multi-pole pair motor.

[0011] A layout method for a three-phase bridge series-type variable frequency drive system, including the following methods:

[0012] Method 1: Each sub-winding of the motor stator adopts a concentrated layout, and the output voltage waveforms of m three-phase units are in the same phase. Then, the magnetic fields generated by all sub-windings are superposed in the same phase to form a three-phase motor drive system;

[0013] Method 2: If m = k × j, where both k and j are positive integers and k ≥ 1, divide the m pairs of sub-windings of the motor stator into j groups, with every k pairs of sub-windings in one group. The a-phase, b-phase, and c-phase of the same pair of sub-windings are 120° apart in space. The sub-windings in the same group are arranged at the same position, and the output voltages of the connected three-phase units are in the same phase. The a-phase windings of adjacent groups among all j groups are 120° / j apart in space in sequence, and the b-phase and c-phase of adjacent groups are also 120° / j apart in space in sequence. The output voltage phases of the adjacent three-phase units are 120° / j apart in sequence. Then, the frequency converter of m three-phase units and the motor of m pairs of sub-windings form a 3j-phase motor drive system with k pairs of sub-windings per phase.

[0014] Method 3: On the basis of the above Method 2, if j = i × h, where both i and h are positive integers and i > 1, keep the operation of the first, i + 1, 2i + 1, … (h - 1)i + 1 groups of sub-windings in the motor stator winding and their connected three-phase units, while the three-phase units connected to the remaining j - h groups of sub-windings stop output and close their DC bypass switches S p , then the 3j-phase motor drive system operates with 3h phases; the frequency converter controls and adjusts the rectifier on the AC input side so that the amplitude of its DC output voltage matches the number of units that continue to operate after phase conversion; if the input side of the frequency converter is an unregulated DC power supply, the remaining operating units evenly divide the total DC voltage;

[0015] Method 4: On the basis of the above Method 2, when k > 1 and j < m, that is, the number of sub-windings in each group is greater than 1, if some units in the frequency converter fail and withdraw from operation, close the DC bypass switch S p of the faulty unit. At the same time, select the same number of units in other groups to stop output and turn on their DC bypass switches. The remaining units and sub-windings continue to form a symmetric 3j-phase output; the frequency converter controls and adjusts the rectifier on the AC input side so that the amplitude of its DC output voltage matches the number of units that continue to operate after phase conversion; if the input side of the frequency converter is an unregulated DC power supply, the remaining operating units evenly divide the total DC voltage;

[0016] Mode 5: m auxiliary sub-windings are evenly distributed. The a-phase, b-phase, and c-phase of the same auxiliary sub-winding are 120° / m apart from each other in space. The a-phase windings of all sub-windings are 360° / m apart from each other in space, and the b-phase and c-phase windings of all sub-windings are also 360° / m apart from each other. The frequency converter controls the m three-phase units to output voltage waveforms with the same phase, forming a variable-frequency drive system with a pole pair number of m; or more than 2 sub-windings are arranged on the same magnetic pole, and one of the windings and the corresponding three-phase unit are used as a backup to cope with faults.

[0017] Compared with the prior art, the present invention combines a multi-channel independent output three-phase bridge series-connected frequency converter with the stator winding structure of a motor to form a new variable-frequency drive system. It changes the input-side power isolation of each unit of a conventional H-bridge cascaded frequency converter relying on a phase-shifting transformer into output-side isolation between each unit by using the stator sub-windings of the motor. At the same time, changing the output of each unit from single-phase power to three-phase power can reduce the DC fluctuation of the unit. The stator sub-windings of the motor can be flexibly arranged in space. Therefore, the beneficial effects of the present invention are:

[0018] (1) There is no longer a need for a phase-shifting transformer with a complex structure. The AC input side of the frequency converter can adopt a relatively simple 6-pulse rectifier, 12-pulse rectifier, or 18-pulse rectifier circuit according to the requirements of the use environment; or the rectifier circuit can be removed from the DC-powered power grid and directly connected to the DC power grid.

[0019] (2) The structure of the frequency converter is simplified, and the number of components used is greatly reduced. Each unit's three-phase bridge inverter has 6 switching devices, and a total of 6m switching devices are required. Also, since 1 three-phase unit replaces 3 single-phase units, the number of DC capacitors and DC voltage sampling circuits is reduced by two-thirds. If the frequency converter draws power from the AC power grid, the complexity of its input-side rectifier circuit is also less than that of the three-phase bridge uncontrolled rectifier circuits on the input sides of all units of the H-bridge cascaded frequency converter and the required phase-shifting transformer.

[0020] (3) The H-bridge inverter is a single-phase inverter, and its inherent double-frequency fluctuation on the DC side will generate a relatively large AC component in its DC bus current, which not only causes the need for a relatively large-capacity DC capacitor for filtering, but also reduces the service life of the DC capacitor and increases the circuit loss; for the three-phase bridge inverter adopted by the present invention, the AC component of its DC bus is greatly reduced due to the mutual cancellation between the three phases, so a DC capacitor with a smaller capacity can be used, thereby reducing the volume and improving the reliability.

[0021] (4) The stator sub-windings of the motor can be flexibly arranged in space to form motors with different pole pair numbers and different speeds; or they can be coordinated with the output control strategies of each unit of the frequency converter to form a drive system that can change the phase number, or expand the speed regulation range of the motor.

[0022] (5) Since each series unit has a three-phase output, unlike the H-bridge cascaded inverter, there is no need to add DC voltage balance control between three phases in the control strategy. Instead, only the DC voltage balance control between series units needs to be considered, which simplifies the control algorithm and is beneficial to the stability of the controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is the circuit structure diagram of the three-phase bridge series type variable frequency drive system of the present invention with a rectifier;

[0024] Figure 2 is the circuit structure diagram of the three-phase bridge series type variable frequency drive system of the present invention without a rectifier;

[0025] Figure 3 is the schematic diagram of the concentrated distribution of the stator sub-windings of the motor in an embodiment of the present invention;

[0026] Figure 4 is the schematic diagram of the uniform distribution of the stator sub-windings of the multi-phase motor in an embodiment of the present invention;

[0027] Figure 5 is the output voltage command waveform of the inverter of the six-phase motor drive system in an embodiment of the present invention;

[0028] Figure 6 is the schematic diagram of the uniform distribution of the stator sub-windings of the multi-pole pair motor in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0030] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0031] Next, the present invention will be further described in conjunction with specific embodiments, but it is not a limitation of the present invention.

[0032] This embodiment is applicable to high-voltage and medium-voltage motor drive systems. Each three-phase bridge inverter in the inverter is used to supply power to each stator sub-winding of the motor separately. By using the isolation between the stator sub-windings of the motor, the phase-shifting transformer installed to isolate the input power sources of each unit and the three-phase uncontrolled rectifier of each unit in the conventional cascaded inverter are omitted, and instead, a conventional rectifier with a simple structure is used, which improves the power density of the variable frequency drive system.

[0033] This embodiment is implemented through the following technical solutions. A three-phase bridge series-type variable frequency drive system, a novel variable frequency drive system composed of an inverter that outputs multiple independent three-phase alternating currents and an AC motor with multiple independent stator windings, includes a rectifier, m three-phase units, and an AC motor; each three-phase unit includes a three-phase bridge inverter, a capacitor C connected in parallel on the DC side, and a DC bypass switch S p ; the positive and negative DC inputs of the m three-phase units are connected in series in sequence, and then connected to the DC output terminal of the rectifier; the stator windings of the AC motor include m mutually isolated sub-windings, and each sub-winding can be connected in star or delta; the AC output terminals of the m three-phase units are respectively connected to the three-phase terminals of the m sub-windings of the AC motor; the magnetic flux of each phase of the stator of the motor is jointly generated by the corresponding windings of each sub-winding, and the sub-windings can be distributed evenly and dispersedly or in a centralized layout in space; m is a positive integer.

[0034] Moreover, if the input is direct current, there is no rectifier.

[0035] Moreover, if the input is three-phase alternating current, rectifier circuits such as 6-pulse rectification, 12-pulse rectification, and 18-pulse rectification can be adopted.

[0036] Moreover, the switching devices of the three-phase bridge inverter are all full-controlled power electronic switching devices; the DC bypass switch can be a mechanical switch or a full-controlled power electronic switching device, and is usually in the off state, and only conducts when the three-phase unit stops output due to a fault or control reason during the operation of the inverter.

[0037] Moreover, if the sub-windings of the motor stator are evenly distributed in space and combined with the output voltage control of each three-phase unit, a variable frequency drive system for a multi-phase motor or a multi-pole pair motor is formed.

[0038] During specific implementation, as Figure 1 shown, the three-phase bridge series-type variable frequency drive system includes a conventional rectifier, m three-phase units, and an AC motor; each three-phase unit includes a three-phase bridge inverter, a capacitor C connected in parallel on the DC side, and a DC bypass switch S p . The conventional rectifier can select a six-pulse, twelve-pulse, eighteen-pulse rectifier or other rectifier circuits according to the equipment usage environment. If it is in a DC power grid, as Figure 2 shown, the variable frequency drive system does not require a rectifier and is directly connected to the DC power grid.

[0039] The positive and negative DC input terminals of m three-phase units are connected in series in sequence, and then the positive terminal of the first unit and the negative terminal of the m-th unit are connected to the positive and negative terminals of the total DC bus. For the switching devices of the three-phase bridge inverter, metal-oxide semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), or other types of fully controlled switching devices can be selected according to the magnitude of the output current.

[0040] The stator winding of the AC motor includes m mutually isolated sub-windings, and each sub-winding can be connected into Figure 1 the star or delta as shown, where the external terminals of the i-th sub-winding are respectively defined as a i 、b i 、c i ; The output terminals of m three-phase bridge inverters are respectively connected to the external terminals of m sub-windings.

[0041] The magnetic flux of each phase of the motor stator is jointly generated by the corresponding windings of each sub-winding. The sub-windings can adopt a concentrated layout as shown in Figure 3 or a uniform distribution as shown in Figure 4 、 Figure 6 . Different layouts of the sub-windings can form a multi-phase motor or a three-phase motor with different pole pairs. The layout method is as follows:

[0042] (1) Three-phase motor drive system with concentrated winding layout

[0043] If all m sub-windings adopt the concentrated layout as shown in Figure 3 , and the output voltage waveforms of all m three-phase units are in the same phase, then the magnetic fields generated by all sub-windings are superposed in the same phase to form a three-phase motor drive system.

[0044] (2) Multi-phase and phase-converting motor drive system with dispersed winding layout

[0045] If m = k × j, where both k and j are positive integers and k ≥ 1, divide the m sub-windings of the motor stator into j groups, with every k sub-windings in a group. The a-phase, b-phase, and c-phase of the same sub-winding are 120° apart in space. The sub-windings in the same group are arranged at the same position, and the output voltages of the three-phase units they are connected to are in the same phase. The a-phase windings of adjacent groups among all j groups are 120° / j apart in space in sequence, and the b-phase and c-phase of adjacent groups are also 120° / j apart in space respectively, and the output voltage phases of adjacent groups of three-phase units are 120° / j apart in sequence. Then, the frequency converters of m three-phase units and the motors of m sub-windings form a 3j-phase motor drive system with k sub-windings per phase.

[0046] If k = 1 and m = j, all m sub-windings adopt Figure 4In the shown evenly distributed layout, the a-phase, b-phase, and c-phase of the same sub-winding are 120° apart in space. The a-phase windings of all sub-windings are 120° / m apart in space successively, and the b-phase and c-phase windings of all sub-windings are also 120° / m apart successively. The output voltage phases of m three-phase units are 120° / m apart successively, thus forming a 3m-phase motor drive system. Taking m = 2 and k = 1 as an example, the windings of each phase of the first sub-winding and the corresponding phases of the second sub-winding are 120° / m = 60° apart respectively, that is Figure 4 the δ in Figure 5 is 60°; meanwhile, the output voltage phases of the first and second three-phase units of the frequency converter are 60° apart, and the waveform of the output voltage command is as shown in

[0047] In the same way, when m = 3 and k = 1, δ = 120° / m = 40°, a nine-phase motor drive system can be formed; when m = 4 and k = 1, δ = 30°, a twelve-phase motor drive system can be formed.

[0048] In the above example of m = 2 and k = 1, when the upper three of the six switches of the three-phase bridge of the second three-phase unit of the frequency converter are all turned on and the lower three are all turned off (or the upper three are all turned off and the lower three are all turned on, with the same effect), and the DC bypass switch of this unit is closed, there will be no current in the second sub-winding of the motor, and the motor drive system will change from six-phase operation to three-phase operation. This variable-phase-number frequency conversion drive method can effectively expand the adjustment range of motor torque and speed and improve the performance of the motor drive system.

[0049] Generally speaking, if j = i×h, where both i and h are positive integers and i > 1, the method of this embodiment can be adopted to keep the first, i + 1, 2i + 1, …, (h - 1)i + 1 groups of sub-windings in the motor stator winding and the three-phase units they are connected to running, while the three-phase units connected to the remaining j - h groups of sub-windings stop outputting and close their DC bypass switches S p , then the 3j-phase motor drive system operates in 3h phases; the frequency converter controls and adjusts the rectifier on the AC input side to make the amplitude of its DC output voltage match the number of units that continue to run after the phase change; if the input side of the frequency converter is an unregulated DC power supply, the remaining running units evenly divide the total DC voltage.

[0050] In addition, if m = 4 and k = 2, the output voltages of the first and second three-phase units can be made 60° apart. At the same time, the output voltages of the third and first, and the fourth and second three-phase units are in the same phase respectively. And the first and third sub-windings of the motor are arranged in the same position, and the second and fourth sub-windings are arranged in the same position, then a six-phase motor drive system with a higher voltage and larger power than the above example of m = 2 and k = 1 can be formed.

[0051] When k > 1 and j < m, that is, the number of sub-windings in each group is greater than 1. If some units in the frequency converter fail and need to be taken out of operation, the DC bypass switch S of the faulty unit can be p closed. At the same time, the same number of units in other groups are also selected to stop output and their DC bypass switches are turned on. The remaining units continue to form a symmetric 3j-phase output, enabling the motor to operate at a lower power. At this time, the frequency converter can control and adjust the rectifier on the AC input side so that the amplitude of its DC output voltage matches the number of units that continue to operate after phase conversion. If the input side of the frequency converter is an unregulated DC power supply, the remaining operating units evenly divide the total DC voltage.

[0052] For example, when m = 6, k = 2, and j = 3, that is, a motor with 6 sub-windings is divided into groups of 2 sub-windings each and arranged in the same position, and the 3 groups of sub-windings are evenly distributed in space. The a-phase, b-phase, and c-phase between the sub-windings of each group differ by δ = 120° / j = 40° in sequence. The output voltages of the 6 three-phase units are also divided into 3 groups and the phases of each group differ by 40°, thus forming a nine-phase motor drive system. If the first sub-winding or the three-phase unit connected to it fails and is taken out of operation, the frequency converter can cause the units connected to the third sub-winding of the second group and the fifth sub-winding of the third group to stop output and close their DC bypass switches, and reduce the output voltage of the rectifier, or keep the total DC voltage unchanged and let the remaining units evenly divide the total DC voltage. In this way, the drive system can still operate in nine phases and maintain a certain power output. If the third and fifth sub-windings or the three-phase units connected to them fail again and are taken out of operation, the frequency converter can use the fourth and sixth sub-windings and the three-phase units connected to them to replace the faulty sub-windings and three-phase units respectively, and continue to maintain the operation of the drive system without stopping.

[0053] In the aforementioned example where m = 6 and k = 2, if all the sub-windings in the same group or the three-phase units connected to them fail and are taken out of operation, and it is impossible to maintain nine-phase operation, the frequency converter can select a complete group of sub-windings and the units connected to them from the remaining two groups to operate in three phases, and still be able to maintain a certain power output without stopping.

[0054] The variable-frequency drive system of this embodiment can form a multi-phase motor drive system, which can improve power, increase torque, broaden the speed regulation range, and adapt to some special application scenarios. In the case of partial unit failures in the frequency converter, methods such as reducing the number of phases or reducing power can be adopted to maintain the operation of the motor and meet the requirements of some application occasions that limit shutdown.

[0055] (3) Multi-pole pair motor drive system with dispersed winding layout

[0056] If the stator sub-windings of the motor are arranged as Figure 6They are evenly distributed in the manner shown, and m sub-windings are evenly and dispersedly distributed. The a-phase, b-phase, and c-phase of the same sub-winding are 120° / m apart from each other in space. The a-phase windings of all sub-windings are 360° / m apart from each other in space, and the b-phase and c-phase windings of all sub-windings are also 360° / m apart from each other in space. The frequency converter controls the m three-phase units to output voltage waveforms with the same phase, forming a variable-frequency drive system with a pole pair number of m. It is also possible to arrange more than 2 sub-windings at the position of the same magnetic pole to increase the motor voltage and power, or use one of the windings and the corresponding three-phase unit as a backup to deal with possible fault situations and improve the reliability of the drive system.

[0057] The above are only the preferred embodiments of the present invention, and do not limit the implementation manners and protection scope of the present invention. For those skilled in the art, it should be realized that all the solutions obtained by equivalent substitution and obvious changes made by using the content of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A layout method for a three-phase bridge series-type variable frequency drive system for an AC power grid, characterized in that: The system includes a DC power supply or an AC power supply, m a three-phase unit, and an AC motor; each three-phase unit includes a three-phase bridge inverter, a capacitor C connected in parallel on the DC side, and a DC bypass switch S p ; m The positive and negative DC input terminals of the three-phase units are connected in series in sequence and connected to the DC output terminal of the rectifier; the stator windings of the AC motor include m sub-windings that are mutually isolated, and each sub-winding is connected in star or delta; m The AC output terminals of the three-phase units are respectively connected to the m three-phase terminals of the sub-windings of the AC motor; the magnetic fluxes of each phase of the stator of the motor are jointly generated by the corresponding windings of each sub-winding, and each sub-winding is uniformly distributed or centrally arranged in space; m is a positive integer; each sub-winding is uniformly distributed dispersedly in space to form a variable-frequency drive system for a multi-phase motor or a multi-pole pair motor; the method includes the following methods: Method 1: Each sub-winding of the motor stator adopts a concentrated layout, and m the output voltage waveforms of the three-phase units are in the same phase, so the magnetic fields generated by all sub-windings are superposed in the same phase to form a three-phase motor drive system; Method 2: If m = k × j ,in k and j are all positive integers, and k ≥1, the motor stator m The secondary winding is divided into j Group, each k The auxiliary sub-windings are grouped together. The phases a, b and c of the same auxiliary sub-winding are spaced 120 degrees apart. The sub-windings of the same group are arranged at the same position. The output voltages of the connected three-phase units are in the same phase. j The phase a windings of adjacent groups in the group are spaced 120° apart. j The phases b and c of adjacent groups also differ by 120° / j The output voltage phases of adjacent groups of three-phase units differ by 120° / j ,but m The inverter of the three-phase unit and m The motor consists of a secondary winding with each phase k 3 of the auxiliary winding j Phase motor drive system; Method 3: On the basis of Method 2, if j = i × h , where i and h are both positive integers, and i > 1, so that the first, i +1, 2 i +1, …( h -1) i+ group of sub-windings and their connected three-phase units in the motor stator winding remain in operation, while the remaining j - h groups of sub-windings-connected three-phase units stop outputting and close their DC bypass switches S p , then the three- j phase motor drive system operates in three h phases; the frequency converter controls and adjusts the rectifier on the AC input side so that the amplitude of its DC output voltage matches the number of units that continue to operate after phase conversion; if the input side of the frequency converter is an unregulated DC power supply, the remaining operating units evenly divide the total DC voltage; Method 4: On the basis of Method 2, when k > 1 and j < m that is, the number of sub-windings in each group is greater than 1. If some units in the frequency converter fail and stop operating, close the DC bypass switch S of the faulty unit p , and at the same time, select the same number of units in other groups to stop output and turn on their DC bypass switches. The remaining units and the auxiliary winding continue to form a symmetric three j -phase output; the frequency converter controls and adjusts the rectifier on the AC input side so that the amplitude of its DC output voltage matches the number of units that continue to operate after phase conversion; if the input side of the frequency converter is an unregulated DC power supply, the remaining operating units evenly divide the total DC voltage; Method 5 m The secondary sub-windings are evenly and dispersedly distributed, and the a-phase, b-phase, and c-phase of the same secondary sub-winding are 120˚ apart from each other in space / m , and the a-phase windings of all sub-windings are 360˚ apart from each other in space / m , and the b-phase and c-phase windings of all sub-windings are also 360˚ apart from each other / m . The frequency converter controls m a three-phase unit to output voltage waveforms with the same phase, constituting a variable-frequency drive system with a pole pair number of m ; or arrange more than 2 sets of sub-windings on the same magnetic pole, and use one set of windings and the corresponding three-phase unit as a backup to cope with faults.

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