Magneto-Vibro-Acoustic Analysis Linking Flux® to ANSYS® Mechanical Hamza ENNASSIRI - GREAH & Farid ZIDAT - CEDRAT.

Nowadays electrical devices have invaded practically every sector of daily life. Many studies are made to optimize machines and make them more energy efficient. The focus is

also placed on aspects related to comfort. Indeed, manufacturers in the automotive, aerospace and household appliance sectors, among others, are looking to master the sound and vibrations of these electrical devices. To master these vibro-acoustic aspects, designers are addressing the issue from the early design stages. There are multiple origins of noise and vibration: magnetic, mechanical or aerodynamic. This means having to decouple and / or combine these physical phenomena.To assist designers in their work, Flux® can be coupled to several 3D mechanical vibrations codes of the market.

To study the influence of magnetic forces on noise and vibration in an electromagnetic device, coupling must be done between magnetic and mechanical phenomena. CEDRAT has developed a coupling between Flux to LMS Virtual.Lab and Flux to NASTRAN in version 11.2.2. In the latest version, Flux 12.1, it is now possible to export magnetic forces in a format that can be easily read by ANSYS Mechanical. This coupling was done with the help of the GREAH laboratory. In this article we present the coupling methodology between Flux and ANSYS Mechanical. A wound rotor synchronous motor is studied as an example.

EM Model of the synchronous motor

Fig. 1: 3-phase 4-pole synchronous motor geometry and windings.

Figure 1 and Table 1 give the geometry and characteristics of the electric motor studied. It is a 3-phase 4-pole synchronous machine running at 7500 RPM with a generated power of 55 kW. From Flux 12 onwards, forces acting on the stator of the machine can be exported for use in MSC Nastran, LMS Virtual.Lab or ANSYS Mechanical. This functionality is supported for Flux 2D, Skew and 3D.

The example explained here uses Flux 2D. As seen in Figure 2, only 1/4th of the 4-pole machine needs to be modeled and only a one-quarter mechanical turn should be computed to obtain the results for a full round. After the meshing of stator and rotor, an extra mesh is created to complete a 360° stator teeth support mesh. This support replicates the discretization of the stator teeth near the air gap of the normal 2D EM mesh and this discretization is copied to the remaining 3 quarters to make up a full round. Figure 2 further explains how the magnetic forces on the EM nodes can be computed using the magnetic pressure.

Fig. 2: Process in Flux 2D for computing forces based on the magnetic pressure and export to text file.

Export .DAT file of:

- EM stator teeth surface mesh extruded for X layers.- Magnetic forces (Fx, Fy, Fz) on each node for each time step

Mechanical model of the synchronous motor» 1- Geometry creation

A CAD file is exported from Flux and imported into ANSYS Mechanical to build the geometry of the synchronous motor.

The geometry used in a 2D or 3D Flux project can be used in a mechanical structural project but not without some elementary modifications as volumes conversion to a binary structure and the indication of faces contact characteristics.

Fig. 3: Geometry creation in ANSYS Mechanical.

Table 1: Synchronous machine electrical and mechanical characteristics.

Machine type 3 Phase Synchronous Machine

Mechanical Power 55 KW

Phase current (Peak) 70 A

Torque 70 Nm

Speed in case study 7500 RPM = 125 Hz

# poles 4

# stator poles 48

Stator Outer Radius 130 mm

Stator Inner Radius 85 mm

Stator Stack Length 170 mm

E M m o d e l m e s h i n g including completion to 360° stator teeth surface.

2D -3D geometryexport form Flux

Importing the geometry into Ansys

Volumes conversion to a binary structure using the Ansys design modeler

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» 4- Results

Parts of simulation results are illustrated in Figure 6. The circumferential modes (2) and (3) associated with the longitudinal mode zero (0) are exited respectively at 463 and 1189 [Hz] frequencies. A step by step transient structural analysis using time varying loads is done to compute the device displacements, which will be used to compute the sound pressure level in the air surrounding it.

Fig. 6: ANSYS Mechanical results.

ConclusionSimulation tools help designers to make smart design decisions before making a prototype and the different tests required to validate it. As this article illustrates, it is now possible to study the electromagnetic and vibro-acoustic performance of electric motors and other electromagnetic devices, using a direct coupled approach between magnetic and mechanical phenomena. First, an electromagnetic analysis is performed under Flux, after which the magnetic forces on the stator surface are computed, displayed and later exported to a text file to be imported via a small macro. The force export is available for Flux 2D, Skew and 3D.

In ANSYS Mechanical, the forces are imported and mapped onto the structural model of the electric motor and converted to the frequency domain. Using a modal approach, the vibration and acoustic response can be computed, giving access to mode participations, total acoustic power, and sound directivity.

» 2- Magnetic force importation

The next step after building the model is to prepare the constraints that should be applied on the stator teeth for each time step. The headers of the DAT file contain the necessary parameters that should be scheduled in the coupling macro interface as illustrated in Figure 4, where the macro execution allows users to import the data file containing the forces exported from Flux to ANSYS Mechanical in an easy way.

Fig. 4: Magnetic forces import.

» 3- Display of the imported magnetic forces

After the local forces computation and import to the mechanical project comes then the materials definition and affectation to the different structure volumes. The determination of the material properties is a difficult step in a vibro-acoustic analysis, especially in order to take into account the stator core lamination, the winding and end-winding characteristics. In this case, orthotropic mechanical properties are used. Those properties were derived from literature in studies based on experimental methods. The machine’s geometry is exported from Flux and a mechanical mesh is applied to it, in order to investigate its vibrations. In addition, a layer of air is added on top of the structure in order to compute the emitted sound power level (Noise). It is important to note that the mesh elements of the air layer should be selected for the application of fluid structure interaction conditions, while the elements on the external radius of the spherical air layer are selected for the application of absorbing boundary conditions.

After material affectation and mesh it is possible to display the imported magnetic forces, as shown in Figure 5

Transfer loads to the elements of the mechanical mesh (case of multiple slice).

Data file from Flux force calculation

Macro execution

Graphical interface of the coupling macro

Simulation tools help designers to make smart design decisions before making a prototype and the different tests required to validate it. It is now possible to study the electromagnetic and vibro-acoustic performance of electric motors, using a direct coupled approach between magnetic and mechanical phenomena.

Fig. 5: Displaying of the imported magnetic forces.

Circumferential mode (2,0 at f= 463.2646 Hz) Circumferential mode (3,0 at f= 1189.0431 Hz)