Summary:
This article introduces the development status of the new energy drive motor industry, and discusses in detail the current mainstream motors in the industry:
1. Current status of drive motor
The drive motor is the core power source of new energy vehicles and hybrid vehicles. Based on the electromagnetic induction effect, the drive motor converts the electrical energy provided by the vehicle into mechanical energy to drive the vehicle. According to the division of input current, the driving motor can be divided into two types: DC motor and AC motor. The current mainstream driving motor is an AC motor, which transmits AC power to the stator (usually a silicon steel sheet with a copper coil, which is fixed), and generates a rotating magnetic field. The rotor (made of electromagnets, permanent magnets or silicon steel sheets) is affected by the rotating magnetic field. , resulting in a rotational torque. According to the speed consistency of the rotor and the stator, AC motors can be divided into synchronous motors and asynchronous motors. The current mainstream drive motors in the new energy market are mainly permanent magnet synchronous motors and squirrel-cage asynchronous induction motors, among which permanent magnet synchronous motors account for the absolute mainstream share of motors in domestic car manufacturers. The main components of the drive motor generally include: rotor, stator, wire harness, housing, end cover, etc., among which the stator, winding, bearing, and rotor are in order of value content, and the cost accounts for 19%, 17%, and 12% respectively. , 11%. The driving motor assembly is divided into the downstream, the stator core, rotor core, stator winding, bearing, end cover and other components are the midstream, and the raw materials such as copper wire, aluminum alloy, silicon steel sheet, permanent magnet and so on are the upstream.
Drive motors for new energy vehicles are a type of industrial motors. In view of the special environment of vehicle working conditions, the special differences in performance between drive motors for new energy vehicles and traditional industrial motors are mainly reflected in large starting torque, high power density, and speed range. wide, strong heat dissipation requirements, wide high-efficiency range, and excellent NVH performance.
1) Large starting torque: New energy vehicles put more emphasis on performance indicators such as starting response and acceleration from 100 kilometers. The driving motor of new energy vehicles requires higher torque when starting or at low speed, so as to increase the speed of the vehicle to the expected value in the shortest time.
2) High power density: Vehicle drive motors have strict weight requirements, volume requirements and power requirements. The weight and size of the motor directly affect the power performance and layout space of the car. The direction and difficulty of motor design is to increase the power weight density and power volume density as much as possible.
3) Wide speed regulation range: The wide speed regulation range of the drive motor can save the traditional multi-speed gearbox for new energy vehicles, and only use the fixed gear set to achieve a wide speed regulation range and effectively reduce costs.
4) Strong heat dissipation requirements: Due to the high power density of new energy vehicle drive motors, heat dissipation problems follow. The volume of a 150KW traditional powertrain assembly is about 409L, while the volume of an electric vehicle powertrain assembly with the same peak power can reach 82L, which is only 20% of the traditional powertrain. The small size and high power design can bring about problems such as heat dissipation, mechanical vibration, electromagnetic compatibility, and NVH howling. The energy conversion efficiency of the motor is above 90%, the peak efficiency is about 95%, and the average energy loss accounts for about 10%, and this 10% energy loss is mostly reflected in the form of heat generation, so the heat dissipation requirements of the drive motor are strong.
5) Wide range of high efficiency: New energy vehicles, especially pure electric vehicles, are powered by on-board battery packs, and the efficiency of the motor directly affects the cruising range, so the requirements for the efficiency of the motor are very high. The driving motor of new energy vehicles needs to have as wide a high-efficiency operating range as possible. Under normal urban road conditions, the car will not start frequently, nor will it continue to run at ultra-high speed. It is more about accelerating and overtaking or decelerating and braking while driving at a constant speed. action, so the operating efficiency of the middle section is especially important.
6) Excellent NVH performance: Compared with traditional industrial motors, drive motors for new energy vehicles need to have better NVH performance due to customers' pickiness. Various manufacturers and OEMs have spent huge amounts of manpower and financial resources on optimizing motor noise. At present, the NVH development of drive motors has matured.
2. Noise mechanism of permanent magnet synchronous motor
The three most common types of AC motors are permanent magnet synchronous motors, induction asynchronous motors, and synchronous reluctance motors. The stator of the AC motor is basically the same, the main difference is the rotor. The stator is mainly composed of iron core and coil. The stator core is made of laminated silicon steel sheets; the enameled wire is wound into a coil, embedded in the core slot, and then insulated; the insulated core is inserted into the casing to obtain the stator. The stator winding is connected to the AC power supply (usually three-phase AC, the combination of the three-phase AC and the coil with a space angle of 120° forms a synthetic magnetic field like a rotating magnetic field with uniform strength), forming a rotating magnetic field, which pulls the rotor rotate.
The rotor of the permanent magnet synchronous motor is composed of iron core, magnetic steel and shaft press-fitted. The magnetic steel provides the magnetic flux of the motor and is made of rare earth neodymium iron boron by powder metallurgy, which has the greatest impact on the performance of the motor.
The rotor speed of the permanent magnet synchronous motor is synchronized with the speed of the stator magnetic field, that is, the frequency of the alternating current. The stator winding is connected with alternating current to generate a rotating magnetic field, and the magnetic field of the rotor permanent magnet is induced by the rotating magnetic field of the stator to rotate accordingly, and then output power. The advantages of permanent magnet synchronous motors are high power density, high operating efficiency, large and smooth torque, simple and compact structure, and good speed regulation performance.
The advantages of permanent magnet synchronous motors are high power density, high operating efficiency, large and smooth torque, simple and compact structure, and good speed regulation performance.
1) High power density: The NdFeB magnetic material of the permanent magnet synchronous motor has excellent magnetic properties. After magnetization, it is not necessary to continue to increase external energy to build a strong permanent magnetic field without additional circuits for excitation (that is, to give conductors Electricity generates a magnetic field). Under the rated power, the power density of the permanent magnet synchronous motor with the same insulation material and heat dissipation conditions is more than 2 times that of the induction asynchronous motor.
2) High operating efficiency: Since the rotor uses permanent magnets, the rotor of the permanent magnet synchronous motor usually does not need to be energized and excited, which can reduce related energy loss and have high efficiency.
3) Large and smooth torque: within the rated current range, increasing the current can quickly increase the motor torque. In addition, the rotating magnetic field formed by the three-phase alternating current in the stator is also relatively stable, which makes the output torque ripple of the motor smaller. Asynchronous induction motors offer significant advantages.
4) Good speed regulation performance: The relationship between electricity, magnetism and force of permanent magnet synchronous motor is simpler than that of induction asynchronous motor, and it is easier to adjust and control speed. The state equation of the asynchronous motor is fourth-order, and the equation of the rotor and the stator is coupled (the current in the rotor is generated by the rotation induction of the stator magnetic field); the state equation of the permanent magnet synchronous motor is second-order, and the magnetic field of the permanent magnet exists independently , and its speed regulation performance is less difficult to control than induction asynchronous motors.
5) The structure is simple and compact: the permanent magnet synchronous drive motor does not need to set the excitation power structure and the excitation winding structure, which reduces the structural complexity, and the motor structure is relatively compact, which ensures that the motor operation is more reliable.
Permanent magnet synchronous motor noise includes motor controller noise, motor electromagnetic noise, mechanical noise, battery cooling noise, etc., and is mainly composed of motor electromagnetic noise and mechanical noise. Among them, mechanical noise includes: bearing noise, reducer whine, rotor eccentricity, etc., which are mainly generated by the operation of mechanical parts. The electromagnetic noise of the motor is mainly composed of torque fluctuations (cogging torque, ripple torque, time harmonics, etc.) , time harmonics, etc.) act on the motor shell, which excites the modal response of the motor shell and causes resonance. Figure 1 is a diagram of the mechanism of electromagnetic noise.
Figure 1 Mechanism diagram of electromagnetic noise
Electromagnetic noise is the unique and most important noise of the drive motor. The magnetic field in the air gap makes the magnetic density change, resulting in alternating electromagnetic force, which acts on the stator slots to generate electromagnetic force (radial force and tangential force), and the electromagnetic force is a time-related alternating force. Variable force, depending on air gap shape, cogging shape, etc. The radial magnetic induction intensity is greater than the tangential magnetic induction intensity, the radial electromagnetic force does not generate electromagnetic torque, and the radial force is greater than the tangential force. The tangential electromagnetic force generates torque to drive the rotor to rotate, the main function is to maintain the establishment of the alternating magnetic field, the radial force wave excites the stator structure, and the stator structure vibrates to generate radiation noise, especially the frequency of the radial force electromagnetic force wave and the stator structure mode When they are consistent, the radiation noise is very obvious, and the noise generated by the electromagnetic force is a high-frequency howling sound, which is easier for the human ear to recognize.
Cogging torque is one of the unique problems of permanent magnet motors. When the stator winding of gear permanent magnet motors is not energized, the torque generated by the interaction between the magnetic field of the permanent magnets and the reluctance of the slotted stator is the mutual interaction between the permanent magnets and the stator. caused by tangential force. The rotor and stator of an unpowered permanent magnet motor have a tendency to self-adjust to the position of minimum reluctance, resulting in a periodic torque. The cogging torque has nothing to do with the armature current, it is a function of the relative position of the stator and rotor, and it is related to the design characteristics of the motor body (such as the structure size of the pole slot and the cogging and the magnetic pole, etc.). The cogging torque will cause the torque pulsation of the permanent magnet motor, and then cause the speed fluctuation, causing the motor to vibrate and make noise, especially when the frequency of the pulsating torque is consistent with the resonance frequency of the armature current, resonance will occur, which will inevitably enlarge the cogging Torque vibration and noise.
Ripple torque is the torque generated by the interaction between the magnetomotive force of the stator and the electromagnetic characteristics of the rotor when the permanent magnet motor is loaded. When the stator and rotor have the same harmonic induction intensity, the ripple torque is proportional to the harmonic order, so the higher harmonic has a larger ripple torque. Using the rotor chute, the electromagnetic torque and induced electromotive force formed are similar to the average value of the same rotor bar evenly distributed in a circle, which can effectively weaken the harmonic electromotive force generated by the tooth harmonic magnetic field, thereby weakening the harmonic electromotive force generated by these The additional torque caused by the harmonic magnetic field reduces electromagnetic vibration and noise. Although the rotor chute will also reduce the fundamental electromotive force induced by the rotor, the degree of chute generally selected is much smaller than the pole pitch, so it has little effect on the basic performance of the motor. Therefore, small and medium-sized cast aluminum rotor asynchronous motors Commonly used rotor chute.
3. NVH advantage of flat wire motor
The special requirements of new energy vehicle drive motors in terms of performance are mainly reflected in high power density, wide range of high efficiency, wide range of speed regulation, large starting torque, and strong heat dissipation requirements. Therefore, the development trend of drive motors for new energy vehicles will also revolve around these performances. The current mainstream directions are flat wire motors, oil-cooled motors, and all-in-one electric drive assemblies. Flat wire motors have higher power density, larger gaps between flat copper wires, and easy penetration of cooling oil. The development of flat wire motors has promoted the application of direct oil cooling technology. At the same time, the cooling oil has good insulation and can be used as lubricating oil for reducers and gear bearings. It can also collect waste heat from the motor and use it to keep the battery pack warm. The direct oil cooling technology accelerates the integration of the vehicle's thermal management system. It also plays a role in promoting the development of the all-in-one electric drive system assembly.
The flat wire motor refers to replacing the traditional cylindrical enamelled copper wire in the stator winding with enamelled copper flat wire processed into a hairpin shape. In the cross section of the stator winding of the round wire motor, there are a lot of gaps between the round copper wires, while in the cross section of the stator winding of the flat wire motor, the rectangular copper wire can better fill the space and improve the slot fullness rate, which is flat The essential difference between wire motors and round wire motors.
Compared with round wire motors, the primary advantages of flat wire motors are reduced losses and increased efficiency. Among the losses of permanent magnet synchronous motors, copper loss (mainly the loss in the stator winding) accounts for about 65%, iron loss (the loss in the stator core and rotor core) accounts for about 20%, and the remaining losses account for relatively low. Flat wire motors and round wire motors have the same level of iron loss, the main difference lies in copper loss.
Copper loss can be specifically divided into DC loss and AC loss. Both round wire motors and flat wire motors have DC loss. When the number of current phases and current effective value are given, the DC loss is proportional to the DC resistance of the winding. Because round wire is thinner and has higher resistance, round wire motors generally have higher DC losses than flat wire motors under the same conditions. Due to the small cross-sectional size of a single winding of the round wire winding, the AC loss can be ignored, while the flat wire winding is relatively large due to the large cross-sectional size, affected by the skin effect and the proximity effect.
In addition to improving efficiency and reducing losses, the use of flat wire motors has a larger area of high-efficiency range, higher power density, stronger heat dissipation capacity and better NVH performance than round wire motors. From the source analysis, the same power, lower copper loss and iron loss are greatly reduced, and higher efficiency also means lower radial force amplitude of each order, which indirectly brings about a comprehensive reduction of the main radial force of the overall NVH; The flat wire motor needs to insert the wire from the slot, and plug it directly through the end of the iron core, which means a smaller slot design, that is, a smaller cogging torque; higher stator stiffness: flat wire motor winding stiffness Higher, that is to say, to improve the overall rigidity of the stator especially, the same radial force wave, the smaller the vibration amplitude of the shell, and the smaller the radiation noise.
New energy vehicles have higher and higher requirements for cruising range, power density, and energy utilization efficiency, and electric drive systems are gradually developing in the direction of integration, miniaturization, and light weight. The all-in-one electric drive systems that have been released so far include three-in-one, four-in-one, six-in-one, seven-in-one and even eight-in-one electric drive systems, the most common of which is the three-in-one electric drive system. The "all-in-one" integration solution of the electric drive system can share circuits and power semiconductors, reduce the amount of cables, effectively reduce the volume and weight of the electric drive system, increase power density, and achieve lightweight. Play a more effective role in the NVH control of the motor assembly.
4. Drive motor NVH development process
The general motor NVH development process includes eight parts, as shown in Figure 2. They are: design requirements, topology design, electromagnetic design, simulation analysis, A prototype, rectification optimization, B prototype, vehicle matching.
Figure 2 Flowchart of drive motor NVH development
1) In the design requirements stage, according to the performance requirements, compare the competing products to determine the motor performance parameters and NVH performance indicators.
2) In the topology design stage, confirm the motor type, rotor structure, length-to-diameter ratio, number of slots and other structures and their impact on NVH.
3) In the electromagnetic design stage, design the geometric dimensions of the motor, pole-slot fit, winding, material selection and its influence on NVH, cogging torque analysis, etc.
4) The simulation analysis stage includes electromagnetic simulation, structural simulation, multi-physics simulation, and balancing other performance indicators, as shown in Figure 3.
Fig.3 Simulation flow chart of driving motor acoustic response
5) In the prototype stage, the NVH test of the prototype machine, the verification of the simulation results, the structural mode test, and the noise source identification test are carried out.
6) In the stage of rectification and optimization, electromagnetic scheme optimization, structural scheme optimization, performance balance, and optimal scheme selection are carried out.
7) In the B prototype stage, the NVH acceptance of the optimized prototype is carried out on the bench, the NVH verification of the vehicle is carried out, and the single target is achieved.
8) In the vehicle matching stage, the vehicle test verification, suspension design, acoustic package development, goal achievement confirmation, etc. are carried out.
5 Conclusion
This paper introduces the development status of the new energy drive motor industry, and discusses in detail the current mainstream motors in the industry: the structure, working principle and noise mechanism of permanent magnet synchronous motors, and the torque fluctuation and radial force wave that cause electromagnetic noise of permanent magnet synchronous motors. The generation mechanism is explained in detail, the NVH performance advantages of the flat wire motor are introduced, the NVH development process of the permanent magnet synchronous motor and the noise simulation process of the permanent magnet synchronous motor are established, and the direction for the NVH development of the drive motor is provided.
Source | EDC electric drive future