Permanent magnet motor driving system with low cost and high power density

Abstract

The invention discloses a permanent magnet motor driving system with low cost and high power density, and belongs to the technical field of distributed electric driving systems of new energy automobiles. The low-cost and high-power-density permanent magnet motor driving system comprises a permanent magnet synchronous motor, a hybrid driving topological structure and a control unit, and the permanent magnet synchronous motor is of a surface-mounted structure. The hybrid drive topological structure is a low-cost hybrid drive topological structure of a SiC semiconductor power device and a conventional Si semiconductor power device. The low-cost and high-power-density permanent magnet motor driving system can solve the problems that after a new energy electric driving system is high in speed, loss of the electric driving system is increased, and cost is increased, effectively reduces stator alternating current loss of a permanent magnet motor at the high speed, improves operation efficiency of the electric driving system, and reduces cost of the electric driving system. The use of copper wire materials and expensive SiC power devices is reduced, and the cost of the electric driver system is reduced.

Description

Technical Field

[0001] The present invention relates to a motor drive system, and more particularly to a permanent magnet motor drive system with low cost and high power density, belonging to the technical field of distributed electric drive systems for new energy vehicles. Background Art

[0002] The battery energy storage technology, motor design technology, and motor drive control technology of the electric drive system for new energy electric vehicles are the core technologies for energy conservation and consumption reduction and improving energy utilization efficiency. New energy vehicles mainly rely on the motor drive control system to output power. Therefore, the cost, power density, and operating efficiency of the motor drive control system have a serious impact on the cost and driving range of new energy vehicles.

[0003] Currently, the drive mode of new energy vehicles is gradually changing from centralized drive to distributed drive. The distributed drive electric drive system has the following advantages for new energy vehicles: easy to control, better response characteristics, and can precisely control the movement of the vehicle; can cooperate with the braking energy recovery system to reduce energy loss; has multiple drive forms, can achieve various vehicle dynamics control functions, and achieve the electronization and automation of vehicle dynamics control. Now new energy vehicles are gradually changing from centralized drive to distributed drive. The distributed drive installs a motor in each wheel, and the motors of each wheel can be controlled separately, which significantly increases the number of drive motors and motor controllers in new energy vehicles. Therefore, the impact of the cost and performance of the electric drive system on the cost and performance of new energy vehicles becomes more prominent.

[0004] In the electric drive system of new energy vehicles, the currently mainly used is a three-phase sine wave permanent magnet synchronous flat wire motor. This flat wire motor has the characteristics of small volume, high efficiency, and low noise. Compared with round wire windings, at the same power, the flat wire winding has a higher slot fill factor, lower material cost, slower temperature rise, lighter weight, and greater power density, and has now become the main actuator of new energy vehicles. The volume size of the motor is directly related to its output torque. Therefore, in order to further improve the power density of the drive motor, the typical speed of the drive motor in the electric drive system of new energy vehicles has been increased from 12000 rpm to 22000 rpm or even higher. High speed can improve the motor power density and working efficiency and reduce the motor volume, but it will increase the motor current frequency. Although compared with round wire windings, the flat wire winding has significant improvements in slot fill factor, material cost, temperature rise, weight, power density, etc. at the same power, it has the problem of relatively large AC resistance. Therefore, when the motor speed is continuously increasing, the disadvantage of large AC losses in the flat wire motor becomes more obvious.

[0005] Currently, the drive topology of sine-wave flat wire motors widely used in new energy vehicles is gradually upgrading from a three-phase half-bridge to a three-phase four-leg drive topology. Compared with the traditional three-phase half-bridge drive topology, the three-phase four-leg drive topology has the following advantages when driving a permanent magnet synchronous motor: Enhanced motor protection ability: The three-phase four-leg topology can achieve better motor protection by adding an extra leg. In the case of single-phase open circuit or phase loss, the three-phase four-leg topology can still maintain partial operation of the system, improving the fault tolerance and reliability of the system. Improved system dynamic performance: The three-phase four-leg topology can achieve faster dynamic response and more precise control through specific control strategies, which is very beneficial for application scenarios that require rapid acceleration and deceleration. Optimized current distribution: In the three-phase four-leg topology, the current can flow through more paths, which helps to optimize the current distribution, reduce the current burden on individual switching elements, and thus extend the life of the driver. Improved thermal management: Due to the more uniform current distribution, the three-phase four-leg topology can help improve the thermal management of the motor and the driver, reduce the formation of hot spots, and improve the overall thermal stability. Suitable for high power density applications: Due to its structural characteristics, the three-phase four-leg topology is suitable for driving permanent magnet synchronous motors with high power density, which is particularly important for applications with limited space. The above numerous advantages have enabled the three-phase four-leg topology to start replacing the three-phase half-bridge topology and become the main topology in the electric drive system of new energy vehicles.

[0006] When a sine-wave permanent magnet synchronous motor runs at high speed, the back electromotive force of the motor will increase. According to the working principle of the sine-wave permanent magnet motor, the stator current frequency will increase accordingly. To meet the current control performance of the motor and suppress the harmonic losses of the motor, the switching frequency of the power elements of the motor driver usually has to be more than 30 times the current frequency, that is, the carrier frequency is required to be greater than 30. Therefore, as the motor speed increases, the switching frequency of the power devices also increases. This will cause an increase in the switching losses of the power elements in the motor driver. Currently, in order to reduce the driver losses, electric drive products often have to use relatively expensive SiC power elements, which in turn leads to an increase in the cost of the motor driver. And when the drive topology of the permanent magnet motor is upgraded from a three-phase half-bridge circuit to a three-phase four-leg topology, the increase in the number of power devices will make the cost of the driver increase more severely after upgrading the power devices from ordinary Si devices to SiC devices. Summary of the Invention

[0007] The present invention aims to solve the problem of increased losses and rising costs of the electric drive system after the high-speed operation of the new energy electric drive system, and thus proposes a permanent magnet motor drive system with low cost and high power density. It effectively reduces the stator AC losses of the permanent magnet motor at high speed, improves the operating efficiency of the electric drive system, reduces the use of copper wire materials and expensive SiC power devices, and lowers the cost of the electric drive system.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A permanent magnet motor drive system with low cost and high power density, comprising a permanent magnet synchronous motor, a hybrid drive topology, and a control unit.

[0010] The permanent magnet synchronous motor is of a surface-mounted three-phase four-wire structure. Three-phase lines and a neutral point line are led out from the stator side, and a Halbach permanent magnet array is adopted on the rotor side. The permanent magnet array includes a radially magnetized permanent magnet in the middle and two flux-concentrating permanent magnets on both sides, forming a 180° electrical angle square-wave back electromotive force.

[0011] The hybrid drive topology includes a front-stage DC-DC conversion circuit based on SiC MOSFETs and a rear-stage three-phase four-leg DC-AC conversion circuit based on SiIGBTs, where the fourth leg is connected to the motor neutral point.

[0012] The control unit adjusts the duty ratio of the front-stage DC-DC and the slow-switching logic of the rear-stage three-phase four-leg, and combines the leading angle control technology to make the motor phase current in phase or out of phase with the back electromotive force, realizing unity power factor control and efficient energy conversion.

[0013] Further, the Halbach permanent magnet array includes five permanent magnets, with one in the middle being radially magnetized, and two on each of the left and right sides being of flux-concentrating structure. Among them, the inner permanent magnet close to the middle permanent magnet is radially magnetized, with a remanence density higher than that of the middle permanent magnet, and the outer permanent magnet is parallel magnetized, with the magnetization direction pointing to the adjacent magnetic pole and having a flux-concentrating effect.

[0014] Further, the Halbach permanent magnet array forms a 180° electrical angle square-wave back electromotive force.

[0015] Further, the permanent magnet synchronous motor is of an 8-pole 48-slot tooth-slot matching structure.

[0016] Further, the front-stage DC-DC conversion circuit of the hybrid drive topology includes SiC MOSFET power devices, SiC fast-recovery diodes, filter inductors, and voltage-regulating capacitors for adjusting the input bus voltage.

[0017] Further, the duty ratio control formula of the front-stage DC-DC conversion circuit is:

[0018]

[0019] where I s is the phase current of the permanent magnet synchronous motor, R s is the phase resistance of the permanent magnet synchronous motor, n p is the number of pole pairs of the permanent magnet synchronous motor, ω ris the angular velocity of the permanent magnet synchronous motor rotation, ψ f is the rotor permanent magnet flux linkage of the permanent magnet synchronous motor, V dc is the input bus voltage, and the input bus voltage satisfies V dc ≥I s R s +n p ω r_max ψ f , ω r_max is the angular velocity corresponding to the maximum speed of the permanent magnet synchronous motor.

[0020] Furthermore, the switching frequency of the front-stage DC-DC conversion circuit satisfies:

[0021]

[0022] Furthermore, the latter-stage three-phase four-leg DC-AC conversion circuit of the hybrid drive topology includes eight SiIGBT power devices and eight fast-recovery diodes. Each Si IGBT power device is connected in parallel with a fast-recovery diode. The power elements are connected in series in pairs to form a half-bridge drive circuit. Among them, three half-bridges are connected to the three-phase windings of the motor, and the fourth half-bridge is connected to the neutral point to form a three-phase four-leg structure.

[0023] Furthermore, the slow switching logic divides six sectors based on the back electromotive force phase angle of the motor. The switching state of the latter-stage Si IGBT only switches once or three times in each sector. The advance commutation angle compensation inductance delay is adopted to synchronize the phase current with the back electromotive force phase.

[0024] Furthermore, the calculation formula of the advance commutation angle is as follows:

[0025]

[0026] where, L s is the stator inductance of the permanent magnet motor.

[0027] The beneficial effects of the present invention are:

[0028] 1. The back electromotive force and driving current of the permanent magnet synchronous motor of the present invention are square waves with the same phase. Compared with the traditional sine-wave permanent magnet synchronous motor, under the same back electromotive force peak value and current peak value, the electromagnetic output power of the permanent magnet motor of the electric drive system proposed by the present invention can be increased to 2 times the original. Therefore, under the same power, the use of ferromagnetic materials and winding copper materials of the permanent magnet motor can be significantly reduced, achieving cost reduction and efficiency improvement.

[0029] 2. In the present invention, the current in each phase winding is direct current during the duration of each sector, thereby basically eliminating the increase in the AC copper loss of the motor stator winding caused by the high-frequency current during the high-speed rotation of the motor. Compared with the traditional permanent magnet synchronous motor, the stator loss of the motor is reduced, and the operating efficiency of the motor is improved.

[0030] 3. The present invention significantly reduces the number of expensive SiC power devices used, and can reduce the number of SiC devices used by up to 87.5%, reducing the cost of the driver.

[0031] 4. In the present invention, a triple-frequency current is injected into the original fundamental frequency current. The combined action of the two currents can improve the utilization rate of the power supply voltage and further increase the output torque.

[0032] 5. The phase current amplitude of the electric drive system of the present invention is only 50% of the phase current amplitude of the traditional sinusoidal permanent magnet synchronous motor. Therefore, without changing the use of the ferromagnetic material and copper winding material of the motor, the phase current of the motor is reduced, the heating of the copper winding of the motor is reduced, and the operating efficiency of the motor is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a schematic structural diagram of an embodiment of the permanent magnet synchronous motor of the present invention;

[0034] Figure 2 is a schematic structural diagram of an embodiment of the Halbach permanent magnet array of the present invention;

[0035] Figure 3 is a schematic diagram of the air-gap magnetic flux density waveform of the permanent magnet synchronous motor of the present invention;

[0036] Figure 4 is a schematic diagram of the back electromotive force waveform of the permanent magnet synchronous motor of the present invention;

[0037] Figure 5 is a schematic structural diagram of an embodiment of the hybrid drive topology of the present invention;

[0038] Figure 6 is a schematic circuit topology diagram of the hybrid drive topology of the present invention;

[0039] Figure 7 is a schematic diagram of the simulation model of the low-cost and high-power-density permanent magnet motor drive system of the present invention;

[0040] Figure 8 is a schematic diagram of the simulation result of the speed loop of the present invention;

[0041] Figure 9 is a schematic diagram of the simulation result of the speed loop with sudden load addition of the present invention;

[0042] Figure 10 It is a schematic diagram of the three-phase back electromotive force and current waveforms of the present invention.

[0043] In the figure: 1. Stator core; 2. Stator winding; 3. Permanent magnet array; 4. Rotor core; 5. Rotating shaft; 6. First permanent magnet; 7. Second permanent magnet. Detailed implementation manners

[0044] Detailed implementation manner one: In combination with Figures 1-10 To illustrate this implementation manner, the low-cost high-power-density permanent magnet motor drive system described in this implementation manner includes a permanent magnet synchronous motor, a hybrid drive topology, and a control unit.

[0045] The permanent magnet synchronous motor is a surface-mounted three-phase four-wire structure. Three-phase lines and a neutral point line are led out from the stator side, and a Halbach permanent magnet array is used on the rotor side. The permanent magnet array includes radially magnetized permanent magnets in the middle and flux concentrating permanent magnets on both sides, forming a 180° electrical angle square-wave back electromotive force.

[0046] The hybrid drive topology includes a front-stage DC-DC conversion circuit based on SiC MOSFETs and a rear-stage three-phase four-leg DC-AC conversion circuit based on Si IGBTs, where the fourth leg is connected to the motor neutral point.

[0047] The control unit adjusts the duty ratio of the front-stage DC-DC and the slow-switching logic of the rear-stage three-phase four-leg, and combines the leading angle control technology to make the motor phase current and the back electromotive force in the same phase or opposite phase, realizing unity power factor control and efficient energy conversion.

[0048] The present invention includes a permanent magnet synchronous motor with a special surface-mounted structure, a low-cost hybrid drive topology of a new type of SiC semiconductor power device and a conventional Si semiconductor power device, and a control unit with high-performance electric drive system control technology. It can effectively reduce the amount of copper material used in the motor. Under the same output power, the use of copper winding materials can be reduced by 50%. At the same time, it can reduce the use of expensive SiC power devices, and the number of SiC devices can be reduced by up to 87.5% at most, effectively reducing the manufacturing cost of the electric drive system. And it reduces the impact of the increase in AC resistance after high-speed operation on the energy consumption of the electric drive system, improving the system efficiency. At the same time, it can achieve cost reduction and efficiency improvement of high-speed new energy electric drive systems, especially suitable for new energy vehicle distributed electric drive systems using multiple sets of motors and multiple sets of drivers.

[0049] Such as Figures 1-4As shown, the permanent magnet synchronous motor is a surface-mounted three-phase four-wire permanent magnet synchronous motor with a square-wave back electromotive force. The stator side leads out the three-phase lines and the neutral point line of the stator, and the rotor side mounts the permanent magnets on the surface of the rotor of the motor. By adjusting the position and structure of the permanent magnets in the rotor of the motor, the magnetic field of the rotor of the permanent magnet motor is realized to have a magnetic field with a square-wave back electromotive force of nearly 180° electrical angle. The motor adopts a Halbach permanent magnet array to achieve a square-wave air-gap magnetic density distribution, and the air-gap magnetic density is close to a square wave. Preferably, the permanent magnet synchronous motor has an 8-pole 48-slot tooth-slot combination structure, and the 8-pole 48-slot structure is often used in the new energy vehicle hub motor drive system. The 1 / 8 model of the permanent magnet synchronous motor is as shown in Figure 1 shown.

[0050] The Halbach permanent magnet array under one pole is composed of 5 independent permanent magnets. The Halbach permanent magnet array includes five permanent magnets, with the middle one being radially magnetized, and two on each of the left and right sides being magnetic concentration structures, and the remanence density of the edge permanent magnets is higher than that of the middle permanent magnet to widen the flat-top region of the square-wave air-gap magnetic density. Among them, the inner permanent magnet close to the middle permanent magnet is radially magnetized, and its remanence density is higher than that of the middle permanent magnet. The outer permanent magnet is parallelly magnetized, and the magnetization direction points to the adjacent magnetic pole and has a magnetic concentration effect. The middle permanent magnet is radially magnetized to ensure that the middle part of the air-gap magnetic density under this pole has a square-wave waveform; in addition, in order to widen the angular ratio of the square wave under one pole, two permanent magnets are arranged on each of the left and right edges. The first permanent magnet functions to concentrate magnetic flux, and the remanence density of the second permanent magnet is slightly higher than that of the middle permanent magnet to suppress the decrease of the air-gap magnetic density at the flat-top edge, as shown in Figure 2 shown.

[0051] Specifically, a simulation experiment is carried out on the permanent magnet synchronous motor through simulation software, and the air-gap magnetic density waveform of the motor is as shown in Figure 3 shown. The flat-top part of the square-wave air-gap magnetic density accounts for more than 94.5% of the electrical angle of a single pole, and the square-wave ratio of the back electromotive force waveform exceeds 170° within 180° electrical angle. Under one pole, the flat-top ratio of the square-wave magnetic density > 94.5% (42.5°

[0052] / 45°), that is, the back electromotive force of the motor presents a square-wave waveform > 170° in half an electrical cycle (180°), as shown in Figure 4 shown.

[0053] As shown in Figures 5-6 shown, the hybrid drive topology adopts a three-phase four-leg structure, connects the fourth leg to the neutral point of the three-phase winding, and through controlling the switching logic of the four legs, makes the triple-frequency current and the fundamental-frequency current act together to generate a square-wave phase current of nearly 180° electrical angle, improving the utilization rate of the power supply voltage.

[0054] The hybrid drive topology is an inverter topology that combines SiC power devices and Si power devices, including a two-stage structure. Its topology is as shown in Figure 5 . The first stage is a DC-DC conversion circuit based on SiC MOSFET power devices, and the second stage is a three-phase four-leg DC-AC conversion circuit composed of Si IGBT power devices. It is a typical circuit topology. The schematic diagram of the circuit topology is as shown in Figure 6 .

[0055] The front-stage DC-DC conversion circuit of the hybrid drive topology includes a SiC MOSFET power device S1, a SiC fast-recovery diode D1, a filter inductor L, and a voltage-regulating capacitor C, which are used to adjust the input bus voltage. The rear-stage three-phase four-leg DC-AC conversion circuit of the hybrid drive topology includes eight Si IGBT power devices Q1-Q8 and eight fast-recovery diodes D1-D8. Each Si IGBT power device is connected in parallel with a fast-recovery diode. The power elements are connected in series in pairs to form a half-bridge drive circuit, and are connected in parallel in pairs to form a three-phase four-leg drive circuit. Among them, three half-bridge circuits are connected to one phase of the permanent magnet motor, and the fourth half-bridge circuit is connected to the neutral point of the winding of the permanent magnet synchronous motor. Thus, by controlling the duty cycle of the high-speed switch S1 and the switching states of the slow switches Q1-Q8, the speed control of the permanent magnet synchronous motor is realized. The front-stage DC-DC conversion circuit adopts a topology that can achieve step-down or buck-boost control of the DC voltage. As long as the required voltage regulation function can be achieved, the control function of the electric drive system can be completed under the control method of the present invention.

[0056] Specifically, the rear-stage DC-AC circuit only functions as a commutator and can use relatively inexpensive Si-based power devices with low switching speeds, including fully controlled devices such as JFET, GTO, or IGBT. Only expensive SiC power devices need to be used in the front-stage DC-DC converter. At least only 1 SiC MOSFET fully controlled device and 1 uncontrollable SiC diode need to be used. Thus, compared with the traditional sine-wave permanent magnet synchronous motor drive controller that requires 8 SiC MOSFET power devices and 8 SiC diodes, this patent significantly reduces the number of expensive SiC power devices used, and can reduce the use of SiC devices by up to 87.5%, reducing the cost of the driver.

[0057] To achieve the stable operation of the motor, the duty cycle D of the high-speed switch S1 S1 needs to be controlled. The duty cycle control formula of the front-stage DC-DC conversion circuit is:

[0058]

[0059] Among them, I s is the phase current of the permanent magnet synchronous motor, and R s is the phase resistance of the permanent magnet synchronous motor, and n p is the number of pole pairs of the permanent magnet synchronous motor, and ω r is the angular velocity of the rotation of the permanent magnet synchronous motor, and ψ f is the rotor permanent magnet flux linkage of the permanent magnet synchronous motor, and V dc is the input bus voltage, and the input bus voltage satisfies V dc ≥I s R s +n p ω r_max ψ f , and ω r_max is the angular velocity corresponding to the highest speed of the permanent magnet synchronous motor.

[0060] The switching frequency of the pre-stage DC-DC conversion circuit satisfies (the switching frequency of S1 needs to satisfy):

[0061]

[0062] For a traditional high-speed permanent magnet synchronous motor drive system, it is often required that Q1 to Q8 adopt the 3D-SVPWM modulation method, and a relatively high switching frequency is used to generate the sinusoidal voltage signal required to drive the motor by means of high-speed chopping of the switch, so as to drive the motor to operate. Since Q1 to Q8 adopt the high-speed PWM drive method, their switching frequency often reaches 10 kHz or even higher, thus generating relatively high switching losses. And since the sinusoidal voltage of the drive motor is formed by high-speed PWM switching chopping, there must be other high-order harmonic voltages in addition to the fundamental sinusoidal voltage in the output voltage of Q1 to Q8. This part of the high-order harmonics will generate harmonic currents and harmonic losses in the motor, further affecting the stability and efficiency of the operation of the electric drive system. In the hybrid drive topology structure of the present invention, Q1 to Q8 no longer adopt the form of high-speed PWM chopping, but adopt the form of slow-switching control, and its switching logic is directly related to the phase angle (electrical angle) of the back electromotive force on the motor rotor, and its switching logic is shown in Table 1.

[0063] Table 1 Switching logic table of slow switches Q1 to Q8

[0064]

[0065]

[0066] Among them, the phase angle when the back electromotive force of phase A changes from negative voltage to positive voltage is defined as 0 degrees.

[0067] As can be seen from Table 1, the switching logic of the slow power transistors in the low-cost and high-power-density permanent magnet motor drive system of the present invention is mainly affected by the rotor position of the motor. In one electrical angle cycle, the slow power transistors Q1 to Q6 are only turned on and off once, and Q7 and Q8 are only turned on and off three times. Compared with the switching frequency of 10 kHz in the traditional drive scheme, the switching times of the power transistors are significantly reduced, and the switching losses of the power transistors are reduced.

[0068] Moreover, under the switching logic control shown in Table 1, the motor phase current and the back electromotive force always remain in the same phase (motor state) or the opposite phase (generator state), realizing the unity power factor control of the motor and reducing the energy loss in the electromechanical energy conversion process of the motor.

[0069] Due to the existence of inductance in the motor, when the switching states of the slow switches Q1 to Q8 change according to the logic shown in Table 1, the current in the motor winding changes slowly to the final steady-state current under the limitation of the inductance, which will inevitably affect the output external characteristics of the electric drive system. Conventional control techniques require a bus drive voltage to quickly adjust the current, which is likely to cause oscillations in the electric drive system and bring additional losses. In order to achieve a stable output of the torque of the electric drive system, the motor control method of the present invention proposes an advanced angle control technique for Q1 to Q8 based on the PWM chopper control of the high-speed switch S1 and the phase switching control of the slow and high-speed switches Q1 to Q8. According to the theoretical commutation position shown in Table 1, the method of advanced commutation is adopted to reduce the voltage required for the rapid response of the current. By setting an appropriate advanced commutation angle, when the back electromotive force of the motor commutes, the phase current of the motor has completed commutation, so that the steady-state in-phase of the current and voltage is achieved through advanced commutation control, improving the stability of the operation of the electric drive system. The calculation formula of the advanced commutation angle is as follows:

[0070]

[0071] where L s is the stator inductance of the permanent magnet motor.

[0072] When the system is running, by sampling the motor speed and speed command to the PWM generator, a high-frequency PWM wave is generated to control the on and off of the high-speed MOSFET in the BUCK circuit to adjust the magnitude of the inverter DC bus voltage. Instead of performing high-frequency PWM chopping at the inverter bridge, a slow-switching form is adopted. The on and off of Q1-Q8 are related to the corresponding three-phase back electromotive force, and under the switch logic control shown in Table 1, unity power factor control of the motor can be achieved. However, due to the presence of inductance, the time for the current to enter the steady state is slow. By means of advanced commutation, the voltage required for the rapid response of the current is reduced. By setting an appropriate advanced commutation angle, when the back electromotive force of the motor commutes, the phase current of the motor has completed commutation, so that the steady-state in-phase of the current and voltage is achieved through advanced commutation control, making the motor drive more stable and fast.

[0073] The slow-switching logic divides six sectors based on the phase angle of the motor back electromotive force. The switching state of the subsequent SiIGBT in each sector only switches once or three times. The advanced commutation angle is used to compensate for the inductance delay to synchronize the phase current with the back electromotive force phase. During the operation process, in the present invention, the current in each phase winding is DC within the duration of each sector, thus basically eliminating the increase in the AC copper loss of the motor stator winding caused by the high-frequency of the current during the high-speed rotation of the motor. Compared with the traditional permanent magnet synchronous motor, the stator loss of the motor is reduced and the operation efficiency of the motor is improved.

[0074] Preferably, the present invention does not use a traditional DC-AC converter but adopts a three-phase four-leg topological circuit structure. In the hybrid topology structure, the frequencies of the 8 switches are not the same. The slow switches Q1-Q6 of the first three legs are used, and the slow switches Q7-Q8 of the fourth leg use a three-fold switching frequency, which is equivalent to injecting a triple-frequency current into the original fundamental frequency current. The combined action of the two currents can improve the utilization rate of the power supply voltage and further increase the output torque. Similarly, under the same motor rotor back electromotive force and the same output power, the phase current amplitude of the electric drive system of the present invention is only 50% of the phase current amplitude of the traditional sine-wave permanent magnet synchronous motor. Therefore, without changing the use of the motor ferromagnetic material and copper winding material, the phase current of the motor is reduced, the heating of the motor copper winding is reduced, and the operation efficiency of the motor is improved.

[0075] According to the parameters of the permanent magnet synchronous motor and the hybrid drive topology structure driver of the present invention, a simulation model of a low-cost and high-power-density permanent magnet motor drive system is built, as Figure 7 shown.

[0076] Given that the commanded speed of the motor is 300 r / min, the speed-loop simulation results of the low-cost and high-power-density permanent magnet motor drive system of the present invention are as Figure 8 shown.

[0077] The orange curve is the speed target curve of 300r / min, and the blue curve is the actual speed curve. As shown in the figure above, when the motor starts running, the motor speed has tracked the target speed at 0.01, and the system has a relatively fast speed. The actual speed is not much different from the given speed, and the error fluctuates around 0.02r / min.

[0078] When a load of 8 N.m is suddenly added to the motor at 0.05s, it can be seen that the motor speed stabilizes at 0.052s, indicating that the control system has a strong anti-interference ability. The simulation results are as follows: Figure 9 shown.

[0079] When a load of 8 N.m is suddenly applied to the motor at 0.05 s, the simulation results of the three-phase current and three-phase back electromotive force of the low-cost and high-power density permanent magnet motor drive system of the present invention are as follows: Figure 10 shown.

[0080] from Figure 10 It can be seen from the figure that the back electromotive force of the motor designed by the present invention presents a square wave trend, and its size is directly related to the motor speed, and its direction is directly related to the position of the motor. The main reason for the large fluctuation of the three-phase current before 0.01s is that the motor has a low speed when starting. In order to make the motor speed quickly reach the command value, the control system adjusts the duty cycle of the high-speed switch S1 of the BUCK circuit to increase, so that the three-phase current fluctuates greatly and the output torque increases. Since the motor is started at no load, when the motor speed reaches the command speed, the output torque is basically zero. Therefore, when the motor speed comes up, the current waveform is basically 0 before 0.05s. After 0.05s, the motor is suddenly loaded with a load of 8N.m. Since the speed of the motor has stabilized at this time, the current waveform also enters a steady state and presents a square wave shape. When the back electromotive force of the motor is commutated, according to the control technology proposed by the present invention, the phase current of the motor has completed the commutation in advance, so that the phase current of the motor is always basically kept in the same phase with the back electromotive force of the corresponding phase of the motor, thereby verifying the stability of the operation of the square wave electric drive system of the present invention and the reliability of the speed servo through the simulation results.

[0081] And it can be seen from the simulation results that at the operating speed of 300 rpm and the output torque of 8 Nm, the current amplitude of the motor of the present invention is 8 A, while for the permanent magnet synchronous open winding motor with the same back electromotive force amplitude, output speed and output torque, the current amplitude of its sine wave is 16 A. Therefore, compared with the traditional sine wave permanent magnet synchronous open winding motor, the present invention can significantly reduce the phase current amplitude of the motor, reduce the stator copper loss of the motor, and improve the operating efficiency of the electric drive system. Similarly, when ensuring that the back electromotive force amplitude and phase current amplitude of the motor remain unchanged, the electric drive system of the present invention can increase the power output by 100%. The back electromotive force and drive current of the permanent magnet synchronous motor of the present invention are square waves with the same phase. Compared with the traditional sine wave permanent magnet synchronous motor, at the same back electromotive force peak and current peak, the electromagnetic output power of the permanent magnet motor of the electric drive system proposed by the present invention can be increased to twice the original. Therefore, at the same power, the consumption of ferromagnetic materials and copper winding materials of the electric drive system can be significantly reduced, realizing cost reduction and efficiency improvement.

[0082] The performance stability of the low-cost and high-power-density permanent magnet motor drive system proposed by the present invention is verified through simulation results. It can achieve high-performance servo control through the hybrid drive mode of high-speed switching devices and low-speed switching devices, and significantly reduce the winding current value under the same output torque, thereby reducing the heat generation of the electric drive system, reducing the use of ferromagnetic materials and winding copper materials in the process of motor design and manufacturing, as well as the use of expensive high-speed power switches in the manufacturing process of motor drivers, and reducing the cost of the electric drive system. And because the winding current is a direct current value in each drive sector of the electric drive system, the increase in the AC copper loss of the motor stator winding caused by the high-frequency current during the high-speed rotation of the motor is basically eliminated. Compared with the traditional permanent magnet synchronous motor, the stator loss of the motor is further reduced, and the operating efficiency of the electric drive system is improved.

[0083] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent variations within the scope of the technical solution of the present invention by using the above-disclosed technical content. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent replacement and improvement made to the above embodiments based on the technical essence of the present invention within the spirit and principle of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A permanent magnet motor drive system with low cost and high power density, characterized in that: It includes a permanent magnet synchronous motor, a hybrid drive topology, and a control unit. The permanent magnet synchronous motor has a surface-mounted three-phase four-wire structure. Three-phase lines and a neutral point line are led out from the stator side, and a Halbach permanent magnet array is used on the rotor side. The permanent magnet array includes radially magnetized permanent magnets in the middle and flux concentrating permanent magnets on both sides, forming a 180° electrical angle square-wave back electromotive force. The hybrid drive topology includes a front-stage DC-DC conversion circuit based on SiC MOSFETs and a rear-stage three-phase four-leg DC-AC conversion circuit based on Si IGBTs, where the fourth leg is connected to the motor neutral point. The control unit adjusts the duty ratio of the front-stage DC-DC and the slow-switching logic of the rear-stage three-phase four-leg, and combines the advanced angle control technology to make the motor phase current in phase or out of phase with the back electromotive force, achieving unity power factor control and efficient energy conversion.

2. The low-cost and high-power-density permanent magnet motor drive system according to claim 1, wherein: The Halbach permanent magnet array includes five permanent magnets. One in the middle is radially magnetized, and two on each of the left and right sides are flux concentrating structures. The inner permanent magnet close to the middle permanent magnet is radially magnetized with a remanence density higher than that of the middle permanent magnet, and the outer permanent magnet is parallel magnetized with the magnetization direction pointing to the adjacent magnetic pole and having a flux concentrating effect.

3. The low-cost high-power-density permanent magnet motor drive system according to claim 2, characterized in that: The Halbach permanent magnet array forms a 180° electrical angle square-wave back electromotive force.

4. The permanent magnet motor drive system with low cost and high power density according to claim 2, characterized in that: The permanent magnet synchronous motor has an 8-pole 48-slot tooth-slot combination structure.

5. The low-cost high-power-density permanent magnet motor drive system according to claim 1, characterized in that: The front-stage DC-DC conversion circuit of the hybrid drive topology includes SiC MOSFET power devices, SiC fast recovery diodes, filter inductors, and voltage stabilizing capacitors for adjusting the input bus voltage.

6. The low-cost high-power-density permanent magnet motor drive system according to claim 5, characterized in that: The duty ratio control formula of the front-stage DC-DC conversion circuit is: Among them, I s is the phase current of the permanent magnet synchronous motor, R s is the phase resistance of the permanent magnet synchronous motor, n p is the number of pole pairs of the permanent magnet synchronous motor, ω r is the angular velocity of rotation of the permanent magnet synchronous motor, ψ f is the rotor permanent magnet flux linkage of the permanent magnet synchronous motor, V dc is the input bus voltage, and the input bus voltage satisfies V dc ≥I s R s +n p ω r_max ψ f , ω r_max is the angular velocity corresponding to the maximum speed of the permanent magnet synchronous motor.

7. The low-cost high-power density permanent magnet motor drive system according to claim 6, characterized in that: The switching frequency of the front-stage DC-DC conversion circuit satisfies:

8. The low-cost high-power density permanent magnet motor drive system according to claim 7, characterized in that: The rear-stage three-phase four-leg DC-AC conversion circuit of the hybrid drive topology includes eight Si IGBT power devices and eight fast recovery diodes. Each Si IGBT power device is paralleled with a fast recovery diode, and the power components are connected in series in pairs to form a half-bridge drive circuit. Among them, three half-bridges are connected to the three-phase windings of the motor, and the fourth half-bridge is connected to the neutral point, forming a three-phase four-leg structure.

9. The low-cost and high-power-density permanent magnet motor drive system according to claim 8, characterized in that: The slow-switching logic divides six sectors based on the phase angle of the motor back electromotive force. The switching state of the rear-stage Si IGBTs in each sector only switches once or three times, and an advanced commutation angle is used to compensate for the inductance delay to make the phase current in phase with the back electromotive force.

10. The low-cost high-power-density permanent magnet motor drive system according to claim 9, characterized in that: The calculation formula of the advanced commutation angle is as follows: Among them, L s is the stator inductance of the permanent magnet motor.

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