<figure><img src="image_1.png" /></figure>

HAL
open science

# Design and Manufacturing of Axial Flux Permanent Magnet Machines for Electric Vehicle Applications

Abdenour Abdelli, Adrien Gilson, Emmanuel Godefroy, Gianluca Zito

<figure><img src="image_2.png" /></figure>

## To cite this version:

Abdenour Abdelli, Adrien Gilson, Emmanuel Godefroy, Gianluca Zito. Design and Manufacturing of Axial Flux Permanent Magnet Machines for Electric Vehicle Applications. EPJ Web of Conferences, 2025, 330, pp.01003. 10.1051/epjconf/202533001003. hal-05215106

HAL Id: hal-05215106

https://ifp.hal.science/hal-05215106v1

Submitted on 19 Aug 2025

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_3.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Design and Manufacturing of Axial Flux Permanent Magnet Machines for Electric Vehicle Applications

Abdenour Abdelli¹*, Adrien Gilson¹, Emmanuel Godefroy¹, and Gianluca Zito¹

¹IFP Energies nouvelles Institut Carnot IFPEN Transports Energie

1 et 4 avenue de Bois-Préau, 92852 Rueil-Malmaison, France

**Abstract.** This paper deals with the design and prototype of an open slot 18-slot/12-pole axial flux permanent magnet (AFPM) machine that meet typical electric vehicle requirements. Multiphysics design is conducted to consider mechanical and thermal constraints in the design stage. The electromagnetic design is based on combination of the 2D and 3D FEA approaches. The rotor losses reduction is performed by magnet segmentation and optimization of the rotor disc. The designed machine is manufactured and tested on the test bench. The experimental results are compared with the simulation results. The impact of the manufacturing process on performance, especially iron losses is investigated.

## 1. Introduction

The high power and torque densities of axial flux permanent magnet (AFPM) machines are well known. Conventional radial flux permanent magnet (PM) machines are put under pressure by their attractive competitors, especially in applications where axial length is limited [1], [2], [3]. There are various configurations of AFPM machines, including: The available options include single stator-single rotor (SSSR), double stator-single rotor (DSSR), single stator-double rotor (SSDR) and yokeless and segmented armature (YASA) [4]. This work focuses on the DSSR configuration with tooth coil windings and open slot. Designing a DSSR for automotive traction is a challenge, particularly in terms of mechanical and thermal aspects. In fact, the tooth coil windings are an attractive solution for these types of machines, as it has a smaller end winding compared to traditional distributed windings and saves on manufacturing costs 5. In principle, such windings are only possible in machines where rotors have a very low conductivity. The drawback of tooth coils is that the stator current linkage generates a variety of sub- and super-space-harmonic components in the air gap flux density [5]. These harmonic components cause hysteresis and eddy-current losses. Also, the large open slot induces space harmonics increasing rotor losses even in no load condition 6. Consequently, machine performance decreases due to high rotor losses, particularly if the rotor is not laminated and the magnets are not segmented.

As a result, the temperature of the permanent magnet rises, which can lead to irreversible demagnetization.

The use of wedge materials, such as glass-fibre-based ones, is common practice in the closing of motor slots after the winding operation. The purpose of this is to prevent either the coils falling or any interference caused by the air gap or the rotor. Glass-fibre wedges have been used for several years and have several advantages, including being mechanically strong enough, having a relative permeability of 1 and not causing heat or other losses. The

use of Magnoval and soft magnetic composite (SMC) materials is one way to reduce the rotor losses [7-8], as demonstrated by some other materials that were tested for use in closing the slot opening. Magnoval's permeability varies between 2 and 3 depending on the magnitude of the flux density. Glass cloth, iron powder and a high-temperature-resistant epoxy resin are the materials used to make Magnoval. The permeability of the SMC materials is up to 850. From a machine design perspective, therefore, these materials are helpful as they can provide a path for the main flux and reduce rotor losses [5].

Therefore, to improve the machine's thermal performance, these factors must be considered at the design stage. Conversely, axial rotor misalignment produces an axial force that deforms the rotor disc [9]. To account for these mechanical and thermal constraints, it is advisable to take a multiphysics design approach that considers electromagnetic, thermal and mechanical aspects during the machine design phase.

The aim of this article is to design and manufacture an AFPM machine with tooth coil windings and open slots. The design procedure is based on a multiphysics approach to take account of thermal and mechanical constraints.

During the design phase, the reduction of the rotor losses is carried out by the magnet segmentation and the choice of the rotor disc material. The objective of this work is also to show the impact of the manufacturing process on performance, particularly in terms of iron losses.

The paper is divided into four sections. The first section presents the electromagnetic design of the DSSR-AFPM machine, which has 18 slots and 12 poles and meets the requirements of electric vehicles. The design procedure is based on the combination of 2D, and 3D approach. The second part is devoted to minimizing rotor losses. This is done by the magnet's segmentation and the choice of the rotor disc material. The third part deals with the mechanical and thermal design of the machine. The impact of the axial misalignment of the rotor and the thermal behavior are also analyzed. The final part is dedicated to

<footnote>* Corresponding author: abdenour.abdelli@ifpen.fr</footnote>
prototyping and comparison of the experimental measurements with 3D FEA results.

## 2. Design of an AFPM Machine

This section describes the electromagnetic design of an 18-slot, 12-pole DSSR-AFPM that meets the requirements of electric vehicles. The design process combines 2D and 3D FEA approaches.

### 2.1. Specifications

The DSSR-AFPM has been designed to meet the requirements of electric vehicles, and its main specifications are provided in Table 1.

<table><caption>Table 1. Main specifications</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>Maximum speed (rpm)</td><td>8000</td></tr><tr><td>Maximum torque (N.m)</td><td>350</td></tr><tr><td>Maximum power (kW)</td><td>140</td></tr><tr><td>Active outer diameter (mm)</td><td>245</td></tr><tr><td>Active inner diameter (mm)</td><td>140</td></tr></tbody></table>

### 2.2. Electromagnetic design

The combination of the 2D and 3D approaches is used to design the 18-slot 12-pole DSSR-AFPM machine. The design procedure 10. Fig.1 gives 1/6 of 3D parts of the designed machine and its equivalent 2D. The double layer coil tooth windings are selected, and all coils of each phase are in parallel, and the two stators are also connected in parallel. The current density is set to 21 A/mm², and the slot fill factor is fixed at 51%. The line current has been set to 495 Arms and the DC-bus voltage is maintained at 400 V. The main characteristics of this configuration are given in Table 2. The torque and power characteristics as a function of rotational speed are shown in Figure 2. These were calculated using 3D FEA.

<figure><img src="image_4.png" /><figcaption>Fig.1. 1/6 of the geometrical model and tooth coil winding distribution: a) 3D model and b) 2D model</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig.2. Torque and power characteristics as function of speed</figcaption></figure>

<table><caption>Table 2. Main parameters of the designed machine</caption><thead><tr><td>Configuration</td><td>DSSR</td></tr></thead><tbody><tr><td>Active outer diameter (D<sub>out</sub>) [mm]</td><td>245</td></tr><tr><td>λ (D<sub>in</sub>/D<sub>out</sub>)</td><td>0.57</td></tr><tr><td>Slot number (Q<sub>s</sub>)</td><td>18</td></tr><tr><td>Pole number (2p)</td><td>12</td></tr><tr><td>Air gap [mm]</td><td>1</td></tr><tr><td>Magnet thickness [mm]</td><td>12</td></tr><tr><td>Magnet fill factor (%)</td><td>67</td></tr><tr><td>Number of parallel coils per stator</td><td>6</td></tr><tr><td>Number of turns per winding</td><td>54</td></tr><tr><td>Slot fill factor [%]</td><td>51</td></tr><tr><td>Current density [A/mm<sup>2</sup>]</td><td>21</td></tr><tr><td>Maximum rms current line inverter [A]</td><td>495</td></tr></tbody></table>

## 3. Rotor losses reduction

In our previous work, we investigated the impact of the number of PM segments in the radial ($N_{rs}$) and tangential directions on eddy current losses using 3D FEA [10]. The best radial segmentation is 15 magnet elements.

<table><caption>Table 3: Losses as function of the rotor disc materials</caption><thead><tr><td>Rotor material /Losses</td><td>Laminated steel</td><td>Aluminium</td><td>Stainless steel</td><td>Titanium</td></tr></thead><tbody><tr><td>Rotor losses [kW]</td><td>0.48</td><td>2.60</td><td><strong>0.96</strong></td><td>1.228</td></tr><tr><td>Stator iron losses [kW]</td><td>0.80</td><td>0.74</td><td><strong>0.71</strong></td><td>0.71</td></tr><tr><td>Copper [kW]</td><td>0.66</td><td>0.66</td><td><strong>0.66</strong></td><td>0.66</td></tr><tr><td>Torque [Nm]</td><td>139</td><td>137.84</td><td><strong>138.77</strong></td><td>139.37</td></tr><tr><td>Power [kW]</td><td>94.61</td><td>93.82</td><td><strong>94.46</strong></td><td>94.86</td></tr><tr><td>Efficiency</td><td>98.01%</td><td>95.92%</td><td><strong>97.60 %</strong></td><td>97.34%</td></tr></tbody></table>

The eddy-current distribution in the PMs for a radial segmentation of $N_{rs} = 15$ is shown in Figure 3a), while Figure 3b) shows the distribution in the magnet rotor holder.

As it can be seen, the highest current densities are found on the arms of the magnet rotor holder, especially in front of magnets. The shape of the arms can be optimized in order to reduce eddy current losses, even if the rotor disc material is an electrical conductor.
<figure><img src="image_6.png" /><figcaption>Fig. 3. Current density distribution a) in the magnets b) rotor disc</figcaption></figure>

Table 3 shows the rotor losses and the efficiency calculated by 3D FEA for the rated operating point corresponding to rotational speed 6500 rpm and current phase of 250 A. With $N_{rs} = 15$ set to 15, the use of magnet segmentation reduces the electromagnetic torque by around 5%.

Note that these losses have been calculated for different types of materials used for magnet rotor holder. As can be seen, the eddy current losses in the rotor increase with increasing electrical conductivity of the used material, and these losses have a significant impact on the efficiency. So, to minimize the rotor losses, each magnet is segmented to 15 segments, and the magnet rotor holder is made with a stainless steel.

# 4. Mechanical and thermal design

## 4.1. Analysis of Axial Force and axial deformation

At the operating point of 700 A of current phase and 4500 rpm (Fig.2), the axial force due to rotor misalignment is evaluated by 3D FEA at each rotor position for different air gap values of (E1: airgap of the stator1, E2 airgap of the stator2). Fig. 4 shows the z-component of the axial unbalanced force. As can be seen, the axial misalignment of the rotor has a significant effect on axial unbalanced force. Note that, in the case where the stator air gap (E1) is equal to that of stator 2 (E2), the axial force is almost zero.

A 3D mechanical analysis is carried out applying an axial force of 150 N to the rotor (equivalent force for a 20% axial misalignment of the rotor). The study is carried out considering the rotor disc is made of stainless steel. The maximum axial deformation of the rotor in the axial direction is about 0.035 mm. In this application, the magnets are glued to the contour of each rotor slot. A 3D mechanical analysis due to centrifugal force is carried out, taking into account the impact of the adhesive on shear forces. However, this study is not reported here. In summary, the design of the rotor is feasible and reliable.

<figure><img src="image_7.png" /><figcaption>Fig. 4. Axial force as a function of axial misalignment of the rotor</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. Axial deformation of the rotor</figcaption></figure>

## 4.2. Thermal analysis

The temperature is very critical for the functionality and performance of electric machine because the property of both isolation and permanent magnet are strongly depending on it. In this application, the cooling system is based on water jacket cooling. The heat transfer coefficient (HTC) of the water jacket was calculated using a CFD simulation. For a water flow rate of 6 L/min and a water temperature of 30°C, the HTC of the water jacket is 1700 W/m²/K. As the stator yoke is not shrink-fitted, a thermal contact resistance between it and the cooling jacket is taken into consideration and its value is set to 450 W/m²/K. The same value is used to account for the thermal resistance between the magnets and their holder. Finally, the thermal resistance between the winding and the stator iron is set to 250 W/m²/K. The 3D Ansys
mechanical software is used to carry out the thermal analysis. Temperature distribution is estimated using losses that are calculated from electromagnetic analysis.

The temperature distribution for the rated operating point corresponding to rotational speed 4500 and torque 140 Nm is shown in Fig.6, which is about 40% of the maximal torque. The highest temperature of 132°C is reached by the magnets. based on this analysis, we can conclude that continuous power is about 40% of the maximum power.

<figure><img src="image_9.png" /><figcaption>**Fig.6:** Temperature distribution at 140 N.m and 4500 rpm a) rotor b) stator</figcaption></figure>

# 5. Comparison between simulation and experimental results

## 5.1. Prototype and construction

A DSSR-AFPM machine with 18 slots and 12 poles has been designed to provide hands-on experience of manufacturing the tooth coil machine and verify the amount of computed losses. The pre-assembled prototype machine is shown in Figure 7. A 0.56 mm diameter round wire is used to wind the stator. The slot fill factor is about 42% compared to 51% used during the design phase. The total number of turns per coil is therefore 48 turns compared to the 54 turns initially planned. To have meaningful comparison between the measurements and the 3D FEA calculations, the same turns per coil of 48 is used.

<figure><img src="image_10.png" /><figcaption>**Fig.7.** Prototype a) Stator with tooth coils b) Rotor disc filled with the magnets, (c) Complete prototype machine.</figcaption></figure>

## 5.2. Experimental setup

Experimental evaluation of the designed motor's performance is conducted. The photograph of the Figure 8 illustrates the designed prototype mounted on the test rig.

<figure><img src="image_11.png" /><figcaption>**Fig.8.** Test bench</figcaption></figure>

## 5.3. No-Load Condition

Figure 9 compares the back-EMF at 1000 rpm measured and modelled using 3D FEA. The two back EMFs are very similar. Figure 10 also presents the harmonic content of the measured and calculated back EMF. The same harmonic content is obtained through both simulation and experimental measurement. In this machine, the back EMF spectrum contains both even and odd harmonics. However, it is the odd harmonics that dominate.

To highlight the impact of the manufacturing process on iron losses, two stators is manufactured by the same supplier (i.e., the same sheet steel, only the process is different). Stator.1 is manufactured by stamping and welding, and Stator.2 by machining and welding. With the same rotor disc, both stator configurations are used to evaluate no-load losses. Fig.11 shows the no-load losses measured with the two stators and those calculated by 3D FEA. The no-load loss consists of the stator iron loss, eddy-current in the rotor, and mechanical loss, with the majority being iron loss.

As we can see, the manufacturing process has a major impact on iron losses. From one process to another, the
iron losses can be increased by 35%. The difference between losses calculated by 3D FEA and measured losses varies from 30 to 60% [9. Thus, it is important to quantify the impact of the manufacturing process on iron losses to better estimate the machine's performances during the design phase, particularly in terms of thermal performance. To achieve this, a loss separation procedure needs to be carried out, which will be the subject of future work.

<figure><img src="image_12.png" /><figcaption>Fig. 9. Back-EMF waveforms at 1000 rpm</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. Harmonic content at 1000 rpm</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 11. No load losses</figcaption></figure>

## 5.4. Load Condition

To perform the loading test, the prototype works as a motor. The torque is measured at 1000 rpm for different phase current values ranging from 20 to 400 A. The cooling water temperature is set to 25 °C.

<figure><img src="image_15.png" /><figcaption>Fig. 12. Measured torque at 1000 rpm</figcaption></figure>

The variation in torque as a function of current is calculated by 3D FEA for a magnet temperature of 25°C. The torque calculated is an electromagnetic torque. The mechanical torque on the shaft can be approximated by subtracting the no-load torque at 1000 rpm from the electromagnetic torque. At 1000 rpm, the no-load torque varies from 1.44 to 2.13 N.m. As we can see on the Fig.12,the maximum deviation is around 3.5%.

## 6. Conclusion

In this paper, a design and prototyping of an 18 slot - 12 pole DSSR-AFPM machine that meet a set of automotive specifications is presented. Tooth coil windings with open slot induces space harmonics increasing the rotor losses even at no load conditions. So, at design stage, the rotor losses are reduced by magnet segmentation and the material selection.

* The magnet is segmented in 15 segments.

* For process reasons, a disc rotor with stainless steel is chosen, knowing that a fibre-glass disc rotor is better than a stainless-steel disc rotor.

* Rotor losses are further reduced by using a glass fibre wedge with iron powder, the results of this study is not reported in this article.

In the design procedure, mechanical and thermal stresses are taken into account to ensure:

* The mechanical integrity of the rotor structure subjected to both axial force due to the axial offset of the rotor and centrifugal force at high speed.

* The operation at continuous power equal to 40% of the maximum power.

A comparison of the experimental and simulation results shows that the manufacturing process has a substantial impact on iron losses. Depending on the manufacturing process, the difference between simulated and measured losses can be as high as 60%. This difference in iron losses increases the temperature of the machine and decreases its efficiency. Therefore, it is essential to quantify the impact of the manufacturing process on iron losses in order to develop more predictive models, particularly for thermal analysis. This work will be the subject of future publications.
## References

1. Cavagnino, M., Lazzari, F., Profumo, A.: "A comparison between the axial flux and the radial flux structures for PM synchronous motors", *IEEE Trans. Ind. Appl.*, 2002, 34, (6), pp. 1517-1524.

2. J. F. Gieras, R.-J. Wang, and M. J. Kamper, "Axial Flux Permanent Magnet Brushless Machines," 2008, doi: 10.1007/978-1-4020-8227-6.

3. T. Husain, B. Tekgun, Y. Sozer and M. Hamdan, "Comparison of axial flux machine performance with different rotor and stator configurations," 2017 *IEEE International Electric Machines and Drives Conference (IEMDC)*, 2017, pp. 1-8, doi: 10.1109/IEMDC.2017.8002354.

4. A. Parviainen, M. Niemela, and J. Pyrhonen, "Modeling of axial flux permanent-magnet machines," *IEEE Transactions on Industry Applications*, vol. 40, no. 5, pp. 1333-1340, 2004.

5. P. M. Lindh, J. J. Pyrhönen, P. Ponomarev and D. Vinnikov, "Influence of wedge material on losses of a traction motor with tooth-coil windings," *IECON 2013 - 39th Annual Conference of the IEEE Industrial Electronics Society*, Vienna, Austria, 2013, pp. 2941-2946, doi: 10.1109/IECON.2013.6699598.

6. L. Alberti, E. Fornasiero, N. Bianchi and S. Bolognani, "Rotor Losses Measurements in an Axial Flux Permanent Magnet Machine," in *IEEE Transactions on Energy Conversion*, vol. 26, no. 2, pp. 639-645, June 2011, doi: 10.1109/TEC.2010.2096818.

7. www.hoganas.com on 4.5.2012.

8. www.stenbacka.fi on 4.5.2012.

9. J. Lai, J. Li and T. Xiao, "Design of a Compact Axial Flux Permanent Magnet Machine for Hybrid Electric Vehicle," in *IEEE Transactions on Industrial Electronics*, vol. 68, no. 8, pp. 6630-6639, Aug. 2021, doi: 10.1109/TIE.2020.3008358.

10. A. Abdelli, A. Gilson, B. Chareyron and G. Zito, "Combination of 2D and 3D Finite Element Models in the Design of Axial Flux Permanent Magnet Machines for Electric Vehicle Applications," 2023 *IEEE Workshop on Electrical Machines Design, Control and Diagnosis (WEMDCD)*, Newcastle upon Tyne, United Kingdom, 2023, pp. 1-6, doi: 10.1109/WEMDCD55819.2023.10110931.

11. Gulec, Mehmet & Aydin, M. (2017). Implementation of Different 2D Finite Element Modelling Approaches in Axial Flux Permanent Magnet Disc Machines. IET Electric Power Applications. 12. 10.1049/iet-epa.2017.0434.

12. N. Taran and M. Ardebili, "A novel approach for efficiency and power density optimization of an Axial Flux Permanent Magnet generator through genetic algorithm and finite element analysis," 2014 *IEEE 23rd International Symposium on Industrial Electronics (ISIE)*, Istanbul, Turkey, 2014, pp. 709-714, doi: 10.1109/ISIE.2014.6864699.

13. N. Taran, G. Heins, V. Rallabandi, D. Patterson and D. M. Ionel, "Evaluating the Effects of Electric and Magnetic Loading on the Performance of Single- and Double-Rotor Axial-Flux PM Machines," in *IEEE Transactions on Industry Applications*, vol. 56, no. 4, pp. 3488-3497, July-Aug. 2020, doi: 10.1109/TIA.2020

14. Lampérth MU, Malloy AC, Mlot A, Cordner M. Assessment of Axial Flux Motor Technology for Hybrid Powertrain Integration. *World Electric Vehicle Journal*. 2015; 7(2):187-194.