I. Introduction Yokeless and segmented armature axial flux machine (YASA) is being considered for an advanced electric traction and propulsion in electric vehicles applications, since it has compact construction and high torque density. Compared with other axial flux machine, YASA could have higher winding fill factor and lower stator iron losses because of the absence of the stator yoke [1]. However, it is very difficult to design the stator cooling structure of YASA especially large power motor for electric vehicles as its stator is in the middle of two rotors. [2] gives a stator water-cooling structure for a 30KW YASA machine, in which fins are insert the space between stator modules with U-shape water-cooling pipe. But the manufacturing process is very complicated. Recently, heat pipe, as a small size but highly efficient passive cooling element, is attracting more and more attention [3]. Therefore, in this paper, a novel stator cooling structure combined flat heat pipe with housing water-cooling is presented. II. Stator Cooling Structure of YASA For an electric drive motor, the sealing is the primary consideration. As a result, open fan cooling can not be applied and the entire machine must be assembled in a housing. In addition, the weight of the motor should be as light as possible ensuring the mechanical strength. [4] introduces a stator mechanical construction of YASA with a excellent plastic material named polyetheretherketon (PEEK), which verifies the feasibility of stator support made by PEEK. Considering the low density, high thermostability, workability and nonelectroconductibility of PEEK, it is chosen as the material of stator carrier. In consideration of the fixation improvement of stator modules and the accuracy enhancement of rotor assembly, the ultimate cooling structure is shown in Fig. 1. It is obvious that the stator irons and concentrated windings are imbedded in two plates made of PEEK for full fixation. And the PEEK plates are bolted to the protruding middle part of the inner surface of the housing. A aluminum alloy ring is placed inside to secure the bearing to avoid unbalance air gap. What's more, several heat pipe is inserted in the gap in parallel between stator modules to enhance the heat transfer efficiency of stator to housing water-cooling. Considering the space limitation, the flat instead of round heat pipe is selected. And The thickness, height and length of flat heat pipe selected are 2mm (one third of the total gap width), 4mm and 85mm, respectively. For the reducing thermal contact resistance and improving mechanical strength, the remainder of stator support is impregnated with epoxy resin. The spiral type of housing cooling structure is adopted, because of its simple processing. III. CFD simulation of cooling structure Before simulation, the properties of epoxy resin and heat pipe should be identified. The selection principle of epoxy is high thermal conductivity, low thermal expansivity, low viscosity and high mechanical stability [5]. Finally, EP2000 from Lord is chosen after comparison. And the thermal conductivity of EP2000 is 1.9 W/mK. The heat pipe is the heat-transfer device which combines the principles of both thermal conductivity and phase transition to transfer heat between two solid interfaces effectively [6]. Its equivalent heat conductivity can reach hundreds of times more than copper, which is the metal with the best thermal properties. In simulation, a heat pipe is usually equivalent to a copper bar of high thermal conductivity [7]. But its performance should be determined in detail. Different size of heat pipes have been measured in laboratory under different conditions. And it is found that the performance of them depends on their size and the input power of heat source. With the increase of input power, the equivalent heat conductivity will get higher, and finally plateau. But the relationship between performance with size is still under study. In the most extreme case, the input power is 5W, the average temperature of a 100mm long, 2.5mm high and 0.8mm thick heat pipe is 308K. And the equivalent thermal conductivity is calculated as 14801W/mK. Therefore, the thermal conductivity of heat pipe in CFD is set as 10000W/mK tentatively. The stator temperature distribution of CFD simulation with heat pipes is shown in Fig. 2 (a), whereas the result without heat pipes is given in Fig. 2 (b). As is shown in Fig. 2, it is evident that the heat pipes accelerate the release of heat from the stator, and reduce the temperature of the stator significantly. The average temperature of stator reduces from 480K to 352K, which is 128K lower. IV. Conclusion In this paper, a novel stator cooling system for yokeless and segmented armature axial flux machine with flat heat pipe is developed. The proposed structure can increase the heat transfer efficiency of stator and improve the temperature of stator sufficiently. And the validity of this cooling structure is verified by CFD simulation. However, there are also many works to be done in the future. The thermal properties of heat pipes, especially the relationship between size parameters with thermal conductivity, should be identified. And the mechanical stability of above construction needs to be confirmed.

**References:**

[1] A. Gerlando, G. Foglia and M. Iacchetti, IEEE Transactions on Energy Conversion, Vol. 31, no. 2, pp. 750-758, 2016. [2] J. Chang, Y. Fan and J. Wu, IEEE Transactions on Industrial Electronics, early access. [3] J. Huang, S. Naini and R. Miller, IEEE Transactions on Vehicular Technology, vol. 68, no. 5, pp. 4467-4478, 2019. [4] B. Zhang, T. Seidler and R. Dierken, IEEE Transactions on Industrial Electronics, vol. 63, no. 4, pp. 2062-2071, 2016. [5] S. Nategh, A. Boglietti and D. Barber, IEEE Transactions on Energy Conversion, vol. 35, no. 2, pp. 1026-1035, 2020. [6] A. Faghri, Heat Pipe Science and Technology, Global Digital Press, 2016. [7] G. Fang, Y. Wei and Z. Yan, Applied Tthermal Engineering, vol. 152, pp. 594-604, 2019.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3a896721b5bca598cb368406564fbfc1.jpg)
Fig. 1 Stator cooling structure of YASA with flat heat pipe

![](https://assets.underline.io/uploads/markdown_image/1/image/15c0ece6138def21a245c72bc21caa06.jpg)
Fig. 2 Temperature distribution of stator (a) cooling without heat pipe (b) cooling with heat pipe

 A novel stator cooling construction for yokeless and segmented armature axial flux machine with heat pipe

I. INTRODUCTION Axial flux permanent magnet (AFPM) machines with tooth-wound concentrated windings (TCWs) and segmented stator were founded a growing interest for high-performance drive applications such as electric vehicles, aircrafts, ships, etc. [1] Four dual-rotor single-stator (DRSS) axial flux permanent magnet vernier (AFPMV) machines with segmented stator and consequent-pole (CP) embedded rotors are proposed and compared in this paper. The configuration and operating principle of AFPMV machines are introduced first. Then, the critical design parameters and electromagnetic performances of proposed AFPMV machines are analyzed and compared both in no-load and load conditions. Finite element analysis (FEA) results demonstrate that the machine with Halbach arrays employed on the stator split teeth features higher torque capability and lower rotor eddy current loss compared to its counterparts with conventional PMs or stator magnet less machines. In addition, for better flux-weakening capability, the AFPMV machine with hybrid field excitation from both rotor PM and stator DC coils is also discussed. II. MACHINE CONFIGURATIONS AND OPERATION PRICIPLE The proposed AFPMV machine A is shown in Fig.1(a) as the benchmark. The machine has a single stator sandwiched between two-disc rotors which embedded CPs. The stator consists of 12 segments, each with three split teeth on the major tooth. TCWs are employed to shorten the end length and suppress copper loss. With the absent of the stator yoke, such machines take the advantageous performance of lower stator core loss. Furthermore, as shown in the Fig.1(b) and Fig.1(c), the radially magnetized PMs and the Halbach arrays are equipped in the split slots to form machine B and C, respectively. Finally, each stator segment of machine D is wounded with a DC field winding on the middle split tooth, which is illustrated in the Fig.1(d). The flux modulation effect has enhanced the torque producing capability according to the pole ratio of rotor and AC windings [2]. In the vernier machine with modulation poles fixed on the stator, the angular speed of the working flux harmonic component is ωp = -NPMωPM /(Nm-NPM) where NPM and Nm are the pole-pair number of the magnetic fields excited with PM and the number of modulation teeth, respectively, ωp and ωPM are the speed of the working rotation magnetic field produced by the stator windings and the speed of the PMs, respectively. III. PERFORMANCE ANALYSES AND COMPARISONS A. No-Load Flux Linkage and No-Load Back-EMFs The flux distributions of four investigated machines were conducted by the FEA, the simulation results show that the machine C tend has lower flux leakage in the stator split teeth and higher magnetic density compared with the machine B because of the PM with circumferentially magnetization. Lower magnetic density is distributed in machine D when the DC winding produces the reverse flux, that is, the flux adjustment can be realized through the DC winding in the split teeth. The waveforms of no-load back-EMFs of the models are illustrated in Fig.2, in which the DC winding of the machine D operates in the flux-weakening condition. It can be seen that the amplitude of the no-load back-EMFs increase with the introduction of the PM in the split teeth. For the machine C, with the increase of circumferential PM volume, the amplitude of the back-EMF first increases and then decreases, that is, it is necessary to balance the flux leakage and the main flux. B. Output Torque and Losses Fig. 3 shows the output torque of the machines, the average output torque of the machines with PMs in the stators generate an average output torque at least 37% higher than those of the machines without PMs in the stators. The results of comprehensive comparison are shown in table I, which verify that the machine with Halbach arrays employed in the split slots tend to have higher output torque and lower stator core loss and rotor PM loss. And the parametric sensitivity studies and design optimizations will be presented in the final paper. III. CONCLUSION This paper extends the theory of magnetic field modulation into AFPM machines with DRSS and TCWs, making it particularly suitable for low-speed, high-torque, and axial length limited applications. The comparative analyses of four topologies of AFPVM machines are conducted by FEA. The results verify that it is a feasible method to modulate the magnetic field in the air gap with the split teeth in the segmented stator modules of the AFPM machine. By comparing the relative merits and disadvantages of the performances of the proposed machines, it is concluded that the machine with Halbach PMs embedded in the stator split slots shows the characteristics of higher torque density and lower loss compared with other topologies, and machine with DC field winding in the stator split slots shows the adequate flux-weakening performance.

**References:**

 [1] F. Giulii Capponi, G. De Donato and F. Caricchi, "Recent Advances in Axial-Flux Permanent-Magnet Machine Technology," IEEE Trans. Ind. Appl., vol. 48, no. 6, pp. 2190-2205, (2012). [2] R. Qu, D. Li and J. Wang, "Relationship between magnetic gears and vernier machines," 2011 International Conference on Electrical Machines and Systems, Beijing, pp. 1-6, (2011).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a0475924f8f2f552126552b15ab27ef7.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/fd7314d2948f6f81a1feaf16a2df87d4.jpg)

 Electromagnetic Performance Analysis and Comparison of Fractional-Slot Non-Overlapping Winding Dual-rotor Axial Flux Permanent Magnet Vernier Machines with Segmented Stators

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A novel stator cooling construction for yokeless and segmented armature axial flux machine with heat pipe

Electromagnetic Performance Analysis and Comparison of Fractional-Slot Non-Overlapping Winding Dual-rotor Axial Flux Permanent Magnet Vernier Machines with Segmented Stators

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