Study on Influence of Stator Electromagnetic Force
The electromagnetic noise of the stator in the motor is mainly affected by two factors, the electromagnetic excitation force and the structural response and acoustic radiation caused by the corresponding excitation force. A review of the research.
Professor ZQZhu from the University of Sheffield, UK, etc. used the analytical method to study the electromagnetic force and noise of the permanent magnet motor stator, the theoretical study of the electromagnetic force of the permanent magnet brushless motor, and the vibration of the permanent magnet brushless DC motor with 10 poles and 9 slots. The noise is studied, the relationship between the electromagnetic force and the stator tooth width is theoretically studied, and the relationship between the torque ripple and the optimization results of vibration and noise is analyzed.Professor Tang Renyuan and Song Zhihuan from Shenyang University of Technology provided a complete analytical method to study the electromagnetic force and its harmonics in the permanent magnet motor, which provided theoretical support for further research on the noise theory of the permanent magnet motor. The electromagnetic vibration noise source is analyzed around the permanent magnet synchronous motor powered by the sine wave and the frequency converter, the characteristic frequency of the air gap magnetic field, the normal electromagnetic force and the vibration noise is studied, and the reason for the torque ripple is analyzed. The torque pulsation was simulated and verified experimentally using the Element, and the torque pulsation under different slot-pole fit conditions, as well as the effects of air gap length, pole arc coefficient, chamfered angle, and slot width on the torque pulsation were analyzed.The electromagnetic radial force and tangential force model, and the corresponding modal simulation is carried out, the electromagnetic force and vibration noise response are analyzed in the frequency domain and the acoustic radiation model is analyzed, and the corresponding simulation and experimental research are carried out. It is pointed out that the main modes of the permanent magnet motor stator are shown in the figure.
The main mode of permanent magnet motor
Motor body structure optimization technologyThe main magnetic flux in the motor enters the air gap substantially radially, and generates radial forces on the stator and rotor, causing electromagnetic vibration and noise. At the same time, it generates tangential moment and axial force, causing tangential vibration and axial vibration. In many occasions, such as asymmetric motors or single-phase motors, the generated tangential vibration is very large, and it is easy to cause resonance of components connected to the motor, resulting in radiated noise. In order to calculate electromagnetic noise, and to analyze and control these noises, it is necessary to know their source, which is the force wave that generates vibration and noise. For this reason, the analysis of electromagnetic force waves is carried out through the analysis of the air-gap magnetic field.Assuming that the magnetic flux density wave produced by the stator is , and the magnetic flux density wave produced by the rotor is , then their composite magnetic flux density wave in the air gap can be expressed as follows:
Factors such as stator and rotor slotting, winding distribution, input current waveform distortion, air-gap permeance fluctuation, rotor eccentricity, and the same unbalance can all lead to mechanical deformation and then vibration. The space harmonics, time harmonics, slot harmonics, eccentricity harmonics and magnetic saturation of magnetomotive force all generate higher harmonics of force and torque. Especially the radial force wave in the AC motor, it will act on the stator and rotor of the motor at the same time and produce magnetic circuit distortion.The stator-frame and rotor-casing structure is the main radiation source of motor noise. If the radial force is close to or equal to the natural frequency of the stator-base system, resonance will occur, which will cause deformation of the motor stator system and generate vibration and acoustic noise.In most cases, the magnetostrictive noise caused by the low-frequency 2f, high-order radial force is negligible (f is the fundamental frequency of the motor, p is the number of motor pole pairs). However, the radial force induced by magnetostriction can reach about 50% of the radial force induced by the air-gap magnetic field.For a motor driven by an inverter, due to the existence of high-order time harmonics in the current of its stator windings, the time harmonics will generate additional pulsating torque, which is usually larger than the pulsating torque generated by the space harmonics. big. In addition, the voltage ripple generated by the rectifier unit is also transmitted to the inverter through the intermediate circuit, resulting in another kind of pulsating torque.As far as the electromagnetic noise of permanent magnet synchronous motor is concerned, Maxwell force and magnetostrictive force are the main factors causing motor vibration and noise.
Motor stator vibration characteristicsThe electromagnetic noise of the motor is not only related to the frequency, order and amplitude of the electromagnetic force wave generated by the air gap magnetic field, but also related to the natural mode of the motor structure. Electromagnetic noise is mainly generated by the vibration of the motor stator and casing. Therefore, predicting the natural frequency of the stator through theoretical formulas or simulations in advance, and staggering the electromagnetic force frequency and the natural frequency of the stator, is an effective means to reduce electromagnetic noise.When the frequency of the radial force wave of the motor is equal to or close to the natural frequency of a certain order of the stator, resonance will be caused. At this time, even if the amplitude of the radial force wave is not large, it will cause a large vibration of the stator, thereby generating a large electromagnetic noise. For motor noise, the most important thing is to study the natural modes with radial vibration as the main, the axial order is zero, and the spatial mode shape is below the sixth order, as shown in the figure.
Stator vibration form
When analyzing the vibration characteristics of the motor, due to the limited influence of damping on the mode shape and frequency of the motor stator, it can be ignored. Structural damping is the reduction of vibration levels near the resonant frequency by applying a high energy dissipation mechanism, as shown, and is only considered at or near the resonant frequency.
damping effect
After adding windings to the stator, the surface of the windings in the iron core slot is treated with varnish, the insulating paper, varnish and copper wire are attached to each other, and the insulating paper in the slot is also closely attached to the teeth of the iron core. Therefore, the in-slot winding has a certain stiffness contribution to the iron core and cannot be treated as an additional mass. When the finite element method is used for analysis, it is necessary to obtain parameters that characterize various mechanical properties according to the material of the windings in the cogging. During the implementation of the process, try to ensure the quality of the dipping paint, increase the tension of the coil winding, improve the tightness of the winding and the iron core, increase the rigidity of the motor structure, increase the natural frequency to avoid resonance, reduce the vibration amplitude, and reduce electromagnetic waves. noise.The natural frequency of the stator after being pressed into the casing is different from that of the single stator core. The casing can significantly improve the solid frequency of the stator structure, especially the low-order solid frequency. The increase of rotational speed operating points increases the difficulty of avoiding resonance in motor design. When designing the motor, the complexity of the shell structure should be minimized, and the natural frequency of the motor structure can be increased by appropriately increasing the thickness of the shell to avoid the occurrence of resonance. In addition, it is very important to reasonably set the contact relationship between the stator core and the casing when using finite element estimation.
Electromagnetic Analysis of MotorsAs an important indicator of the electromagnetic design of the motor, the magnetic density can usually reflect the working state of the motor. Therefore, we first extract and check the magnetic density value, the first is to verify the accuracy of the simulation, and the second is to provide a basis for the subsequent extraction of electromagnetic force. The extracted motor magnetic density cloud diagram is shown in the following figure.
It can be seen from the cloud map that the magnetic density at the position of the magnetic isolation bridge is much higher than the inflection point of the BH curve of the stator and rotor core, which can play a better magnetic isolation effect.
Air gap flux density curveExtract the magnetic densities of the motor air gap and tooth position, draw a curve, and you can see the specific values of the motor air gap magnetic density and tooth magnetic density. The magnetic density of the tooth is a certain distance from the inflection point of the material, which is presumed to be caused by the high iron loss when the motor is designed at high speed.
Motor Modal AnalysisBased on the motor structure model and grid, define the material, define the stator core as structural steel, and define the casing as aluminum material, and conduct modal analysis on the motor as a whole. The overall mode of the motor is obtained as shown in the figure below.
first-order mode shape
second-order mode shape
third-order mode shape
Motor vibration analysisThe harmonic response of the motor is analyzed, and the results of vibration acceleration at various speeds are shown in the figure below.