This paper analyzes the influence of permanent magnet material magnetic properties, rotor structure, armature winding method, and control strategy on the performance of permanent magnet synchronous drive motors . Neodymium iron boron rare-earth permanent magnet materials with high remanence, high intrinsic coercivity, and high maximum energy product are selected. An embedded permanent magnet steel rotor with good steady-state performance and high power density is adopted. A fractional-slot concentrated winding with high slot fill factor, low copper consumption, and low cogging torque, along with a direct torque field weakening speed-enhancing control strategy, are employed. The optimal design method for improving the performance of permanent magnet synchronous drive motors for new energy vehicles is presented.
introduction
Currently, the world faces a severe energy shortage and a rapidly deteriorating ecological environment. Environmental protection issues are becoming increasingly prominent, making the development of a low-carbon economy an urgent priority. New energy vehicles have become the most popular emerging industry in the global energy conservation and environmental protection field. Improvements in automotive electrification technology are attracting increasing attention. As the "engine" of hybrid and pure electric vehicles, the drive motor has become a core component directly related to the performance and energy conservation and emission reduction of new energy vehicles. Permanent magnet synchronous drive motors (PMSMs) possess high power density, high efficiency, low pulsating torque, and a wide field-weakening speed regulation range, making them the best choice for energy-saving and environmentally friendly new energy vehicle drive motors. To better leverage the value of PMSMs, this paper, while continuing to overcome the bottlenecks in permanent magnet material research, optimizes the motor structure design, improves the performance of PMSMs, and promotes the better development of new energy vehicles.
The Influence of Permanent Magnet Materials on the Performance of Permanent Magnet Synchronous Drive Motors
In recent years, permanent magnet materials have developed rapidly and become increasingly diverse. Currently, the most commonly used types include ferrite permanent magnet materials, AlNiCo permanent magnet materials, and NdFeB rare earth permanent magnet materials. The development history of permanent magnet materials is shown in Figure 1.
The outstanding advantages of ferrite permanent magnets are that they do not contain rare earth elements or precious metals such as cobalt and nickel, making them inexpensive, easy to manufacture, highly coercive, resistant to demagnetization, low in density, and lightweight. However, ferrite permanent magnets are hard and brittle, making them unsuitable for electrical machining, resulting in motors with low power and efficiency. AlNiCo permanent magnets are characterized by a low temperature coefficient, high remanence, low coercivity, and ease of magnetization and demagnetization, but they contain cobalt, a precious metal, making them very expensive. Neodymium iron boron rare earth permanent magnets, with their superior magnetic properties, have become the mainstay of permanent magnet materials, far exceeding those of ferrites and AlNiCo.
The latest generation of neodymium iron boron (NdFeB) permanent magnet materials has achieved a remanent magnetic induction of 147 T at room temperature. Its intrinsic coercivity can exceed 1000 kA/m, and its maximum energy product (BH) reaches 398 kJ/m, which is 5-12 times that of ferrite permanent magnets and 3-10 times that of AlNiCo permanent magnets. The drawbacks of NdFeB permanent magnets include a relatively low Curie temperature, resulting in significant magnetic loss at high temperatures, and poor thermal stability, corrosion resistance, and oxidation resistance. Therefore, surface coatings must be applied according to the magnet's intended use environment to meet automotive environmental requirements.
Neodymium iron boron (NdFeB) rare earth permanent magnet materials exhibit significantly higher magnetic and mechanical properties than ferrite and AlNiCo permanent magnet materials, and also offer better processing performance. China's rare earth production accounts for over 80% of the world's total. With abundant rare earth resources, NdFeB rare earth permanent magnet materials are particularly suitable for permanent magnet synchronous drive motors in new energy vehicles.
The Influence of Rotor Structure on the Performance of Permanent Magnet Synchronous Drive Motor
Permanent magnet synchronous drive motors can be classified into two types of rotor structures based on the installation method of permanent magnets on the rotor: surface type and embedded type. Surface type rotor structures can be further divided into surface-mounted type and embedded type. Embedded type can be classified into radial rotor structure, tangential rotor structure and permanent magnet rotor structure with a hybrid magnetic circuit that integrates radial and tangential directions according to the excitation direction of permanent magnets.
In a surface-mounted rotor structure, the d-axis and q-axis inductances are equal, and the rotor does not exhibit a salient pole effect, thus no reluctance torque is generated. However, because the permanent magnets are directly exposed to the air gap magnetic field, they are prone to demagnetization, limiting their field-weakening capability. In an embedded rotor structure, the q-axis inductance is greater than the d-axis inductance, and the rotor exhibits a salient pole effect, thus generating reluctance torque.
Reluctance torque can effectively improve the power density of a motor. Embedded structures offer improved dynamic performance compared to surface-mounted structures, but their leakage flux and manufacturing costs are higher. In an embedded rotor structure, the permanent magnet is located inside the rotor, with pole shoes made of ferromagnetic material between the outer surface of the permanent magnet and the inner circle of the stator core. These protect the permanent magnet within the embedded rotor core. The asymmetry in its rotor magnetic circuit structure generates reluctance torque, which helps improve the overload capacity and power density of permanent magnet synchronous drive motors. Furthermore, it facilitates speed expansion through "field weakening."
The selection of a suitable rotor structure has a significant impact on the performance of permanent magnet synchronous drive motors. Toyota's hybrid vehicles, including the Prius (2003, 2004, 2010), 2007 Camry, and 2008 LS600h, and Honda's 2005 Accord, all use permanent magnet synchronous drive motors as their main drive motors, but their rotor structures differ. The 2005 Accord has a surface-mounted structure, while the Prius, 2007 Camry, and 2008 LS600h have an internal structure. The 2003 Prius has a straight rotor structure, the 2004 Prius, 2010 Prius, and 2007 Camry have a V-shaped rotor structure, and the 2008 LS600h has a triangular structure, as shown in Figure 2. Key parameters are listed in Table 1.
As shown in Table 1, the 2004 Prius, 2007 CaII, 2010 Prius, and 2008 LS600h drive motors with built-in rotor structures have significantly higher maximum power, maximum speed, and power density than the 2005 Accord drive motor with a surface rotor structure. Furthermore, different permanent magnet built-in structures also have a significant impact on the parameters of the drive motors.
In summary, the built-in rotor structure exhibits superior dynamic and steady-state performance, providing high torque and power with high power density. Furthermore, different structural types within the built-in rotor significantly impact the performance of permanent magnet synchronous drive motors. Therefore, strengthening research on built-in rotor structure design and exploring economical and high-performance rotor structures is crucial for improving the performance of permanent magnet synchronous drive motors.
The impact of armature windings on the performance of permanent magnet synchronous drive motors
Permanent magnet synchronous drive motor armature windings can be classified into distributed windings and concentrated windings based on the shape and winding method of the coils. Based on the number of slots per pole per phase, q = kJ/m, they can be classified into integer slot windings and fractional slot windings.
The choice between fractional slots and integer slots depends on motor performance and manufacturing processes. Fractional slot windings offer the following advantages over integer slot windings:
1) The number of slots per pair of magnetic poles is greatly reduced, with fewer large slots replacing more small slots, resulting in fewer slots in the armature laminations. The armature core manufacturing process is relatively simple, and the space occupied by slot insulation is reduced, which helps to improve the slot fill factor and thus improve motor performance.
2) When using a fractional slot, the motor coil ends are shorter, which not only reduces the motor winding resistance by saving copper wire, but also reduces the motor copper loss under the same conditions, thereby improving motor efficiency and reducing temperature rise.
3) When skewed slots are not used, the sinusoidal nature of the back EMF waveform can be improved through the short-pitch and distributed effect of the windings, thereby reducing the torque ripple and noise of the motor.
4) When using a pitch l = 1 (fractional slot concentrated winding), automatic winding can be adopted, which not only improves labor productivity and simplifies the winding process and wiring, but also reduces costs. At the same time, each coil is wound on only one tooth, shortening the coil circumference and the extension length of the winding end. This further reduces the amount of copper used, and there is no overlap at the ends of the coils. Phase-to-phase insulation is not required.
5) By rationally selecting the pole slot combination, the fractional slot concentrated winding is more effective than the integer slot winding in reducing cogging torque and increasing output power, and its field weakening speed extension capability is also improved to a certain extent.
Compared to integer-slot windings, the main drawbacks of fractional-slot windings are: strict constraints on the selection of the number of slots and poles, slightly lower winding coefficient, larger winding inductance, and harmonics in the armature reaction magnetomotive force leading to rotor eddy current losses and noise. Currently, measures such as selecting pole-slot combinations with lower magnetomotive force harmonics, using laminated rotor yokes to reduce eddy current losses, using high-resistivity permanent magnet materials, appropriately increasing the air gap, and adjusting the slot width can effectively compensate for the shortcomings of fractional-slot windings.
Based on the above analysis, fractional-slot windings can effectively improve slot fill factor, reduce motor copper losses, and decrease cogging torque, resulting in better performance and economic efficiency. This makes them more suitable for permanent magnet synchronous drive motors.
Impact of Control Strategy on the Performance of Permanent Magnet Synchronous Drive Motor
The two typical control strategies for permanent magnet synchronous motors are vector control and direct torque control. Each has its own advantages and disadvantages. Vector control is based on a mathematical model of the controlled permanent magnet synchronous motor, and the motor torque is achieved by controlling the armature winding current.
Permanent magnet synchronous motors exhibit relatively stable low-speed torque and a wide speed range under vector control. With rotor field-oriented vector control, no reactive excitation current is required, thus maximizing electromagnetic torque per unit current. Compared to vector control, direct torque control eliminates complex spatial coordinate transformations. By employing stator flux orientation control, direct observation and control of the motor's flux and torque can be achieved within the stator coordinate system. This offers advantages such as simple control method, fast torque response, and ease of full digitalization.
Currently, advanced control algorithms have been applied to two control strategies with good results, such as direct torque control of permanent magnet synchronous motors based on sliding mode variable structure, which solves the problems of large current, flux linkage and torque ripple in traditional direct torque control of permanent magnet synchronous motors.
A novel direct torque control method for permanent magnet synchronous motors based on duty cycle control calculates the duty cycle of the selected effective voltage vector's action time throughout the entire sampling period using a precise mathematical model and torque error. This allows for real-time adjustment of the effective voltage vector's action time, effectively reducing torque ripple. Furthermore, a cerebellum-based joint controller (CMAC) based on a proportional-integral-differential neural network is introduced into the AC speed control system of the permanent magnet synchronous motor, replacing the traditional speed outer-loop PI controller in a dual-loop control system.
In addition, based on the research of vector control and direct torque control strategies, high-performance control technology has also developed rapidly, greatly improving the performance of permanent magnet synchronous drive motors.
1) Field weakening speed extension technology. Electric vehicles, especially direct-drive electric vehicles, require permanent magnet synchronous drive motors with a wide speed range. However, the speed range of the motor is limited by the mechanical strength of the motor itself and the range of constant power above the base speed. To address this, field weakening control is necessary. An internal rotor structure is used to give the motor a salient pole effect, and reluctance torque is fully utilized to widen the range of the field weakening region.
2) Torque Ripple Suppression Technology. The torque ripple in permanent magnet synchronous drive motors (PMSMs) is mainly caused by two factors: the non-ideal magnetic circuit resulting from the motor's own structure and the amplification of errors in introduced parameters by the control method. Therefore, by optimizing the structure of the PMSM and improving the rotor magnetic field distribution, and also by optimizing the control strategy at the motor control level to reduce stator cogging torque, torque ripple suppression can ultimately be achieved.
Based on the above analysis, the built-in permanent magnet synchronous drive motor utilizes direct torque control field weakening speed extension technology, which significantly improves its performance.
Conclusion
This paper analyzes the influence of permanent magnet material magnetic properties, rotor structure, armature winding, and control strategy on the performance of permanent magnet synchronous drive motors. The permanent magnet steel uses neodymium iron boron rare earth permanent magnet material, the rotor adopts an internal structure, the armature winding uses fractional slot winding, and direct torque field weakening speed-enhancing technology is used simultaneously. This effectively improves the main performance indicators of the permanent magnet synchronous drive motor.
