The Vibroacoustic Aspects of Axial Flux PM Motor for E-mobility
In collaboration with University of l’Aquila, we evaluated the NVH, electromagnetic, and thermal aspects on electric motors. In particular, an innovative axial-flux permanent magnet motor has been optimized and simulated to increase its electromagnetic performance and to decrease its vibrations, noise and working temperatures. This multiphysics approach allows to design an electric motor solution that meets the requirements and the NVH specifics imposed by an initial project
Here you can find the discussed points within this presentation.
In the last few years, the electric car market and the e-mobility world in general , have considerably growing and, besides the need to improve the battery packs and the power electronic technologies, there is a great interest in finding innovative and more efficient solutions for one of the main components of the electric powertrain: the electric motor.
Nowadays, the types of electric motors for automotive applications that are most frequently adopted are the radial flux machines, either induction squirrel cage ones and permanent magnet ones.
The aim of this case study is to propose an innovative and different solution to power a battery electric vehicle: the idea is to place an axial flux machine directly into the wheels’ rim of the vehicle, without any gear reducer or mechanical differential.
This type of motor is particularly suitable for electrical vehicles and it has been thought of as a direct-drive application for the automotive sector because it is ideal for low speed applications due to the large number of poles that can accommodate.
The axial flux motor is characterized by a flux moving axially between its rotor and stator. It is also called “disc-type motor” and it has a high power-weight ratio.
Moreover, the axial flux PM motor cooling is better than the one of other types of motors.
The torque is proportional to the cube of its medium radius and, depending on the required power or torque, a multiple-stadium configuration can be realized.
One of the reason why until now the axial flux motors haven’t had a big diffusion in the electric motor world is because they are difficult to study.
Fortunately, the capability of modern software tools allows for a fast and accurate design process even in case of complex 3D structures, as we will see in the following slides.
In the designing process of an electrical machine, many analyses must be conducted in order to study and evaluate every single possible aspect of its behavior and performances before constructing a prototype and then testing it.
Here, an overview of our typical workflow. In the next slides will be shown in detail.
After having preliminarily designed an electrical machine and before starting to realize the prototype, in order to save time, money and materials, it is good practice to conduct some FEM electromagnetic analyses to check its performances and electrical parameters, such as the torque, the phase voltage, the magnetic flux density, the losses and, as a consequence, the machine efficiency at each operating point and in this particular case important is to compute the electromagnetic forces. These forces will be used as input for the NVH analysis.
The computational analyses of automotive NVH (Noise, Vibration, Harshness) performances are extremely important. They can be carried out with finite element analysis software in order to evaluate if any powertrain component encounters resonance phenomena and the overall sound quality.
In an electrical machine, the noise that can be produced depends on the machine type, on its size, design, topology, materials, manufacturing, power, speed, support, bearings, load…
It is possible to distinguish three main categories of the vibration and noise produced by an electrical machine:
- The electromagnetic vibration and noise: they consist of vibrations and noise due to higher harmonics of time and space, to eccentricity, to phase unbalance, to magnetic saturation, to slot opening and to magnetostriction, which all produce parasitic effects in the electrical machine.
- The mechanical vibration and noise: they are associated with the mechanical assembly of the electrical machine and they are due to bearings, rotor unbalance, shaft misalignment, sliding contacts, bearing defects
- The aerodynamic vibration and noise: these are produced by the cooling air flow that passes through or over the electrical machine due to the fan.
In order to evaluate the vibrations and the noise of the first AFPM electrical machine, first the modal analysis and then the frequency response analysis can be carried out.
The modal analysis is used to determine the natural frequencies and the mode shapes of the AFPM motor’s structure. For each structure’s vibrational mode, there is an associated natural frequency by which the mode is oscillating.
The results of the modal analysis are the maximum displacements of the mechanical structure of the AFPM machine and they can be seen, in ascending order of frequency.
The frequency response analysis is conducted through the Frequency Response Function (FRF) between an input signal (the excitation forces at the air gap) and an output signal (the machine structure’s response).
This analysis has been conducted on the mechanical machine’s model in a free-clamped condition, so the surface on one side of the frame is free to move and the one on the other side is clamped to the vehicle’s structure.
First of all, it is necessary to export the magnetic forces (from Flux) and import them into HyperWorks to define them as loads.
The analyzed points that have been considered are 5: one on the free side of the frame (in particular, its velocity’s deformation is shown because of the relation to the sound radiation), one on the extreme part of the shaft (in particular, its acceleration’s deformation is shown), one on the stator tooth’s face that is facing the air gap (in particular, its velocity’s deformation is shown), one on the most radially outer point of the rotor disc (in particular, its displacements are shown) and one on the most radially outer point on the permanent magnet (in particular, its displacements are shown in order to check if there could be any contacts between the PMs and the stator teeth).
In conclusion
The initial requirements have been fully satisfied by the designed motor’s electromagnetic performance, showing good power and torque densities for traction applications;
The NVH analyses performed on the AFPM motor have shown positive results, making the electrical machine suitable, in terms of comfort and environmental impact, to be used in the automotive field.
Credits
Matteo Betti Technology Manager