The influences of magnetic characteristics of permanent magnet materials, rotor structure, armature winding mode and control strategy on the performance of permanent magnet synchronous drive motor are analyzed.The optimal design method to improve the performance of permanent magnet synchronous drive motor for new energy vehicle is presented：Ndfeb rare earth permanent magnet material with high remanence , high intrinsic coercivity and high maximum magnetic energy is selected; a permanent magnetic steel rotor with good steady state performance and high power density is used; fractional slot concentrated winding with high slot filling rate, low copper material consumption and small tooth slot torque; and the control strategy of direct torque magnetic filed weaking and velocity expansion.
The introduction
Permanent magnet synchronous drive motor has high power density, high efficiency, small ripple torque and wide speed range, which is the best choice for driving motor of energy-saving and environment-friendly electric vehicle. In order to give full play to the value of permanent magnet synchronous drive motor, this paper, on the basis of continuing to break through the bottleneck of permanent magnet material research, optimizes the motor structure design, improves the performance of permanent magnet synchronous drive motor, and promotes the better development of new energy vehicles.
The influence of permanent magnet materials on the performance of permanent magnet synchronous drive motor
In recent years, permanent magnet materials develop fast, and build a wide variety. The most commonly used main categories are: ferrite permanent magnet materials, aluminum nickel cobalt permanent magnet materials and ndfeb rare earth permanent magnet materials.
The prominent advantages of ferrite permanent magnet material are that it does not contain rare earth elements, cobalt, nickel and other precious metals, with low price, simple manufacturing process, strong coercivity, strong anti-demagnetization ability, low density and light quality. However, ferrite permanent magnet material is hard and brittle, which can not be processed electronically. The motor with ferrite permanent magnet material has low power and efficiency.
The characteristics of al - ni - co permanent magnet are low temperature coefficient, high remanence induction and low coercivity.It is easily magnetized and demagnetized, but it contains cobalt, a precious metal, which makes it expensive.
ndfeb rare earth permanent magnet materials with its excellent magnetic properties become the main portion of permanent magnet materials, its magnetic properties are far more than ferrite and aluminum nickel cobalt and other magnetic materials.
The new generation of ndfeb permanent magnet material has been developed until now, and its residual magnetic induction intensity at room temperature has reached 147 T.The maximum of the intrinsic coercivity force can exceed 1000 kA/m, and the maximum magnetic energy product (BH) can be up to 398 kj/m, which is 5 ~ 12 times of ferrite permanent magnet material and 3 ~ 10 times of alnico permanent magnet material.The shortage of ndfeb permanent magnet material is that the Curie temperature is low, when using at high temperature, the magnetic loss is large. The thermal stability, corrosion resistance and oxidation resistance is poor, so it should be coated on its surface based on the use of the environment of the magnet , to meet the requirements of the vehicle environment.
The magnetic properties and mechanical properties of ndfeb rare earth permanent magnet materials are significantly higher than ferrite and al-ni-co permanent magnet materials, with good processing performance. So ndfeb rare earth permanent magnet material is more suitable for new energy vehicles permanent magnet synchronous drive motor.
The influence of rotor structure on the performance of permanent magnet synchronous drive motor
Permanent magnet synchronous drive motor can be divided into two types of rotor structure: surface type and Plug-in type, and surface type rotor structure can be divided into surface type and embedded type.According to the excitation direction of permanent magnetic steel, the Plug-in permanent magnetic rotor structure can be divided into radial rotor structure, tangential rotor structure and mixed magnetic circuit with radial and tangential.
In the surface mounted rotor structure, the inductance of d-axis and q-axis is equal, and the rotor has no salient pole effect, so no reluctance torque is generated. Since permanent magnetic steel is directly exposed to air gap magnetic field, permanent magnetic steel is prone to demagnetisation, and its magnetic weaking capacity is limited.With the embedded rotor structure, the q-axis inductance is greater than the d-axis inductance, and the rotor has the salient pole effect, so the reluctance torque is generated.
The power density of the motor can be improved effectively by using reluctance torque.The dynamic performance of the embedded structure is better than that of the surface mount, but the magnetic flux leakage coefficient and manufacturing cost are higher than that of the surface mount.The permanent magnetic steel of the plug-in rotor structure is located in the inner part of the rotor, and there are pole shoes made of ferromagnetic material between the outer surface of the permanent magnetic steel and the inner circle of stator core.To protect the permanent magnetic steel in the core of the plug - in rotor.The magnetoresistive torque is produced by the asymmetry of the rotor magnetic circuit.It is helpful to improve overload capacity and power density of permanent magnet synchronous drive motor.And easy for " magnetic weaking" and speed improing.
To sum up, the plug-in rotor structure has good dynamic and steady performance, can provide large torque and high power, and high power density.In addition, different structure types of the plug- in rotor have great influence on PMSM.Therefore, the research on the design of the Plug-in rotor structure is strengthened.It is important to explore the rotor structure with good economy and excellent performance to improve the performance of permanent magnet synchronous drive motor.
The effect of armature winding on the performance of permanent magnet synchronous drive motor
The armature winding of permanent magnet synchronous motor can be divided into distributed winding and centralized winding according to the shape of winding and the way of wire embedding.According to the number of slots per pole and phase of the motor, it can be divided into integer slot winding group and fractional slot winding group.
Fractional slot or integer slot is choosen according to the motor performance and production process. Fractional slot winding group has the following advantages over integer slot winding group:
1) on average, the number of slots corresponding to each pair of magnetic poles is greatly reduced. A small number of large slots are replaced by a large number of small slots, and the number of armature punching slots is relatively small.Armature core manufacturing process is relatively simple, and at the same time can reduce the relative occupation space of the insulation , is conducive to improve the slot filling rate, and thus improve the performance of the motor.
2) generally, when fractional slot is used, the motor coil end is shorter, which not only reduces the resistance of the motor winding by saving copper wire, but also reduces the copper consumption of the motor under the same circumstances, improves the motor efficiency and reduces the temperature rise.
3) when chute is not used, the short distance and distribution effect of winding can be passed.The sinusoidal property of inverse electromotive force waveform is improved to reduce torque pulsation and noise.
4) when adopting pitch l=1(fractal-slot concentrated winding), automatic winding can be adopted, which not only improves labor productivity, simplifies wire embedding process and wiring, but also reduces costs. At the same time, each coil is wound on only one tooth, which shortens the coil circumference and the extension length of the winding end.Further reduce the amount of copper used, each coil end does not overlap.Interphase insulation is not necessary.
5) reasonable selection of pole slot.Compared with integer slot winding group, fractional slot winding is more effective to reduce the cogging torque and improve the output power, and its ability of magnetic weaking and speed improving is also improved to some extent.
Compared with integer slot winding group, the main shortcomings of fractional slot winding group are as follows: the number of slots and the number of poles are strictly restricted, the winding coefficient is slightly lower, the winding inductance is larger, and the armature reaction emf has harmonic wave, which leads to rotor eddy current loss and noise.At present, the shortcomings of the fraction slot winding group can be effectively remedied by selecting the pole-slot matching with low magnetomotive harmonic, adopting the laminated rotor yoke to reduce eddy current loss, adopting the permanent magnet material with high resistivity, appropriately increasing the atmospheric gap and adjusting the width of slot.
Based on the above analysis, fractional slot winding can effectively improve the full slot rate, reduce the copper consumption of the motor, and reduce the cogging torque, both in performance index and economic index.It's more suitable for permanent magnet synchronous drive motor.
Influence of control strategy on performance of permanent magnet synchronous drive motor
The two typical control strategies of PMSM are vector control and direct torque control.Both have their own advantages and disadvantages.Vector control is based on the mathematical model of the controlled permanent magnet synchronous motor. The motor torque is realized by controlling the armature winding current.
Under vector control, the low-speed torque of permanent magnet synchronous drive motor is relatively stable and the speed regulation range is wide. Under the control of rotor field-oriented vector, no reactive excitation current is required, so the maximum electromagnetic torque can be generated per unit current.As opposed to vector control, direct torque control eliminates complex spatial coordinate transformation.Only by using stator flux directional control, direct observation and control of the motor flux and torque can be realized in the stator coordinate system.
At present, the advanced control algorithm has been applied to the two control strategies and achieved good results, such as the direct torque control of permanent magnet synchronous drive motor based on sliding mode control structure solved the problems of large current, flux and torque pulsation in the direct torque control of traditional permanent magnet synchronous drive motor.
A new direct torque control method of permanent magnet synchronous drive motor based on duty cycle control is proposed.Adjust the time of effective voltage vector in real time, the torque ripple is effectively reduced.
In addition, based on the research of vector control and direct torque control strategy, high-performance control technology also develops rapidly, which greatly improves the performance of permanent magnet synchronous drive motor.
1) magnet weaking technology.The permanent magnet synchronous drive motor (PMSM) need a wide speed range, especially for motors directly driving electric vehicle.The speed range of the motor is limited by the mechanical structure strength of the motor itself and the range of constant power zone above the rated speed.Magnetic weaking control is needed for this situation.The plug - in rotor structure makes the motor have salient pole effect.And make full use of reluctance torque to broaden the range of magnetic weaking.
2) torque ripple suppression technology.There are two main reasons for torque pulsation of permanent magnet synchronous drive motor: non-ideal magnetic circuit caused by its own structure and error amplification of introduced parameters by control method.So, by optimizing the structure of permanent magnet synchronous drive motor, the distribution of rotor magnetic field can be improved, and control strategy can be optimized from the motor control level to reduce stator cogging torque and finally achieve torque ripple suppression.
Based on the above analysis, the plug-in permanent magnet synchronous drive motor adoptting the technology of magnetic weaking controlled by direct torque, has the remarkable effect to own performance promotion.
Conclusion
This paper analyzes the influence of permanent magnet material's magnetic characteristics, rotor structure, armature winding and control strategy on the performance of permanent magnet synchronous drive motor. Permanent magnet steel adopts ndfeb rare earth permanent magnet material, rotor adopts plug-in structure, armature winding adopts partial slot winding group, and at the same time, coordinates with the technology of magnetic weaking and speed improving of direct torque control. These all can effectively improve the main performance index of permanent magnet synchronous drive motor.
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