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Electromagnetic noise analysis of DC permanent magnet motor

                                    Electromagnetic noise analysis of DC permanent magnet motor

    There are many specific forms of motor noise, such as electromagnetic noise, aerodynamic noise and mechanical noise. The time-varying electromagnetic force acting on the stator and rotor of the motor can cause electromagnetic vibration, which makes the motor radiate electromagnetic noise. Therefore, the electromagnetic vibration of the motor is the main source of vibration and noise of the motor. It is necessary to study it. And control. Noise problems caused by motors involve the coupling of electromagnetic, vibration and acoustic fields. Maxwell module of ANSYS software has strong electromagnetic field analysis ability, Mechanical module has structural vibration analysis ability and strong acoustic analysis module Acoustics ACT, which has been verified by engineering practice, and can be completed. Seamless transmission of data between them can realize coupled field analysis, which provides a good solution to the problem of electromagnetic vibration and noise of motors. This paper completes the whole process of electromagnetic, harmonic response, acoustic and coupling analysis through a noise analysis case of a DC permanent magnet motor, and obtains the corresponding sound pressure, sound pressure level and sound speed at different frequencies.

1 Preface 

    Noise is a disorderly combination of different frequencies and sound intensity. It can interfere with people's conversation, reduce people's thinking and cause fatigue, thus affecting rest, work and sleep. Long-term exposure to noise can also damage people's hearing. Therefore, we need to study and control it. Electromagnetic noise is mainly caused by electromagnetic force generated by electromagnetic field in air gap, which causes vibration of yoke of iron core, and finally radiates noise through motor shell. This process involves electromagnetic field analysis, vibration analysis and acoustic analysis of transmission medium. It is a complex problem of multi-field and multi-learning coupling. This paper provides a perfect solution to motor noise problem by using the powerful multi-disciplinary coupling analysis ability of ANSYS.

2.Electromagnetic force analysis of magnetic steel

    The air gap magnetic field of the motor exists in the small air gap of the stator and rotor of the motor. Air gap magnetic field is the medium of electromechanical energy conversion, and also generates radial and tangential electromagnetic force on the stator and rotor of the motor. These electromagnetic forces not only convert electrical energy into mechanical energy, but also act on the structure of stator and rotor, causing electromagnetic vibration. Rotor vibration is mainly transmitted to the shell through the bearing. Because the bearing has elasticity and a certain damping effect, the high-frequency vibration transmitted to the shell will obviously attenuate. The stator is usually connected directly to the motor housing. Compared with the rotor vibration, the stator vibration is easier to cause the decoupling vibration and noise of the housing.

The rotor teeth are ampere force, forming electromagnetic torque. Vibration and noise are generated on the permanent magnet and stator core by Maxwell force. For permanent magnet direct current motors, the main force causing motor vibration is the permanent magnet, including radial and tangential electromagnetic force.

2.1 simplification of motor model

    The electromagnetic force analysis of motor only takes into account the parts of the motor, such as stator and rotor, winding, magnetic steel and so on. Maxwell 2D is used to calculate the radial and tangential magnetic pull force on the inner surface of the magnet steel by using the 1/4 model of the motor. The model is simplified by NX software (simplifying the stator case, such as removing the fine surface, lines, small rounded corners, etc.)



       Figure 1: simplified DC motor model        Figure 2: simplified 2D model

2.2 Maxwell electromagnetic force analysis

   The specific steps are as follows:

   1) meshing, in which the grid is encrypted individually.

   2) coupling analysis options to activate transient electromagnetic fields and harmonic response analysis.

   3) Maxwell automatically calculates the transient electromagnetic force of the last complete cycle.

  4) software automatically completes Fourier transform, and time-domain electromagnetic force is converted into electromagnetic force in frequency domain.

  5) analyze settings and submit solutions.


      Figure 3: Maxwell 2D model            Figure 4: activation cylinder harmonic

force calculation options

Figure 5: magnetic density distribution cloud chart of magnetic

Figure 6: force density distribution of rotor tip.

    Here, the radial and tangential magnetic pull density distributions of the stator inner surface calculated in Maxwell will be used as excitation sources and coupled into Mechanical for harmonic response analysis in the frequency domain.

3. harmonic response analysis

    Select the harmonic response in the analysis system window of Workbench, and drag the harmonic response to the right side of Maxwell in the project schematic window. Connect D4 and B5 to achieve seamless transmission of electromagnetic force results to harmonic response analysis.


Figure 7: Maxwell, Mechanical Coupling Analysis Process

 3.1 Model simplification

    Harmonic response analysis only considers the stator housing of the motor, magnetic steel, ignoring the rest of the structure, simplifying the model with NX software (including removing small faces, lines, small rounded corners, etc.)


Figure 8: Simplified harmonic response analysis 3D model

3.2 Mechanical Harmonic Response Analysis

   Specific steps are as follows:

   1) Meshing

   2) Import Maxwell electromagnetic force

   3) Define the noise source transfer file

   4) Harmonic response analysis solves the solution and submits the solution

3.2.1 Harmonic response analysis boundary conditions


3.2.2 Harmonic response analysis material parameters


Figure 10: Cloud maximum deformation distribution cloud

Figure 11: Stator equivalent stress distribution cloud


Figure 12: Amplitude-frequency and phase-frequency curves of the Y-direction deformation of the outer surface of the stator

    Here, the mechanical load response calculation of the outer surface of the stator casing displacement, velocity, acceleration and other response loads will be used as excitation sources, coupled to the Acoustics ACT acoustic calculation module for acoustic calculation.

4 Acoustic analysis

    Select Harmonic response in the Workbench's Analysis System window and drag Harmonicresponse to the Project Schematic window as shown below.


Figure 13: Maxwell, Mechanical, Acoustics ACT Coupling Analysis Process


4.1 Creating an acoustic model

    The Acoustics ACT acoustic analysis only considers the noise propagation medium (air) outside the stator casing, regardless of the internal structure of the motor. With the NX processing model, the acoustic area outside the stator housing is created and several key viewing areas are cut out as shown below.


Figure 14: Acoustic Analysis 3D Model

Figure 15: Acoustic Analysis 3D Model - Side Structure4.2 Acoustics ACT Acoustic Analysis

Specific steps are as follows:

1) Meshing

2) Define a coupled noise source

3) Noise calculation boundary condition application

4) Noise analysis setup and submit solution

4.2.1 Acoustic analysis boundary conditions


     Figure 16: Acoustic analysis boundary conditions



4.3Acoustic analysis results


Figure 17: Sound pressure distribution of the stator shell area

Figure 18: Sound pressure distribution of the stator shell area

Figure 19: Motor stator housing area sound velocity distribution vector

    Through Acoustics ACT acoustic calculation, the sound pressure, sound pressure level and sound velocity of the air outside the stator of the motor at various frequencies are obtained.

5 Conclusion

   1) The transient electromagnetic force on the stator magnet of the motor is obtained by 2D electromagnetic field simulation calculation;

   2) Through electromagnetic and structural coupling analysis, the transient electromagnetic force on the magnetic steel calculated by 2D electromagnetic field is automatically introduced into Mechanical for harmonic response analysis, and the harmonic response results of displacement, velocity and acceleration of the stator shell of the motor are obtained.

   3) Through the structural and acoustic coupling analysis, the harmonic response analysis result of the stator shell part of the motor is automatically imported into the AcousticsACT acoustic calculation module, and the acoustic analysis is performed as the excitation of the noise analysis.

Finally, through the powerful electromagnetic, vibration and noise coupling field analysis function of ANSYS, the vibration and noise analysis of DC permanent magnet motor is completed, and the sound pressure, sound pressure level and sound velocity of the air outside the stator of the motor are obtained.