Motor vibration noise assessment
One: The calculation of electromagnetic force generally has two methods to obtain electromagnetic force, one of which is concentrated electromagnetic force method and one is node electromagnetic force method, each has advantages and disadvantages.
The process of calculating the concentrated electromagnetic force is as follows: first calculate the air gap magnetic density, then calculate the electromagnetic force in the air gap by the formula, accumulate the calculation of multiple times, and obtain the complete electromagnetic force spatiotemporal distribution. Then map the electromagnetic force to the structure we need to study, either the stator teeth or the rotor magnet, the surface of each pole of the rotor. Concentrated electromagnetic force calculations are generally performed manually, so the objects selected are generally coarse. (For example, a tooth is an object.)
In this example, the research object selected the rotor surface of each pole and calculated the electromagnetic force time distribution of each pole surface. Then, for each pole, electromagnetic spectrum analysis can be performed to obtain the frequency distribution characteristics of the electromagnetic force. It is also possible to analyze the magnitude of the electromagnetic force at a certain moment. We see that the electromagnetic force of each pole is different. The electromagnetic distribution of the electromagnetic waves is obtained by analyzing the electromagnetic forces of all poles in a unified manner. With the electromagnetic force time distribution and spatial distribution of an object, this electromagnetic force is completely described, and can be accurately applied in subsequent analysis. This is also the advantage of centralized distribution force calculation. The disadvantage is that it requires more manual operation.
The calculation of distributed electromagnetic force is calculated by finite element software. The software can directly calculate the electromagnetic force in each grid and store the electromagnetic force data under all steps. This makes it easy to obtain the electromagnetic force time distribution of each node. Can be directly used as an excitation source for subsequent vibration analysis software. The advantage is that no manual calculation is required, which is very convenient. But this requires strong coupling between structural analysis software and electromagnetic analysis software. On the other hand, it is inconvenient to determine the spatial distribution of electromagnetic force waves, which is not conducive to engineers to judge the nature and source of electromagnetic force.
Two: modal calculation and correction
In the modal calculation, it is necessary to pay attention to the axial elastic modulus and Poisson's coefficient of the stator core is an uncertain quantity, which needs to be obtained through experiments. A predecessor gave his set of test conclusions in the upper right corner of the above figure. If the frequency and mode shape (number of times) of the modality are found to be highly close to the frequency and spatial distribution of the concentrated electromagnetic force calculation in the first step, then this electromagnetic force is very dangerous.
The order that the modal test can obtain depends on the arrangement of the points of the sensor, and the more points, the more orders. Through the modal test, not only the natural frequency but also the damping coefficient can be obtained.
Three: vibration analysis
After applying the electromagnetic force, the predecessor obtained the vibration data of the casing, the thin end cover and the thick end cover through vibration analysis, and found that the thin and thick end caps have large amplitude near the first natural frequency. It is judged that this is the main vibration body of abnormal noise under 400 rpm conditions.
Four: noise prediction analysis
By comparing the noise simulation calculation with the noise spectrum analysis of the black box test, it is found that the common feature of both is that the 580HZ and 1440HZ accessories have strong vibrational spectra, which are just the resonant frequency of the end cap, which is further verified. The front and rear end cap resonances are the cause of abnormal noise.
Finally, according to the waterfall chart drawn in the white box and black box test stages, it is found that the 3rd and 8th order force waves coincide with the natural frequencies of the two end caps at 400 rpm. The third-order force wave can be judged to be caused by the third-order tooth harmonic, which gives the root of the problem.
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