Design of a new type yokeless and segmented armature axial flux machine

Abstract

This paper takes the demand of high power density motor for new energy vehicles as the application background. In order to solve the problem of low power density of radial flux motor, a novel direct cooling yokeless and segmented armature axial flux machine (YASA) is proposed. The thermal-fluid multi-physical coupling field of YASA motor has been studied by combining numerical analysis and computational fluid dynamics (CFD). On this basis, the internal relationship among the output power, flow resistance and structural parameters of the cooling system has been studied, and then the parameters of cooling system has been multi-objectives optimized. On the premise of meeting the pressure loss of the cooling system, the heat transfer efficiency has been improved, and then the output power of the motor has been increased.

Keywords

YASA cooling system power density CFD pressure loss

1. Introduction

Permanent magnet synchronous motor (PMSM) is widely applied to electric vehicle (EV) due to its high power density and high efficiency. According to the direction of flux, permanent magnet synchronous motor can be divided into radial flux motor and axial flux motor. At present, radial flux permanent magnet synchronous motor (RFPMSM) is mainly used on EV, the power density of which can reach 2.0–3.6 kW/kg [1,2]. From the current research situation, improving the speed and reluctance torque are two main ways to increase the power density of the RFPMSM [3]. However, with the increase of motor speed, the problems of bearing life, motor noise and controller switching loss become more and more prominent [4]. In comparison with the RFPMSM, the axial flux permanent magnet synchronous motor (AFPMSM) especially the yokeless and segmented armature axial flux machine (YASA) exhibits many distinguished advantages such as high torque density, short axial length, and high efficiency [5]. Firstly, the absence of the stator yoke makes it be more efficient and high power density. Secondly, the centralized short-distance windings can effectively reduce the copper consumption and improve the efficiency [6]. Owing to processing difficulties, the stator and rotors of the in-wheel motor are fabricated using soft magnetic composites (SMCs) [7].

In addition to the electromagnetic performance, the output power of YASA is mainly limited by the thermal performance [8]. The greater output power of the motor, the greater heat generated. The cooling system can take the heat away in time to prevent the internal temperature of the motor from exceeding the allowable working temperature. In addition, with the increase of the internal temperature of the motor, the winding resistance increases, the magnetic performance of the permanent magnet decreases, and the efficiency continue to deteriorate [9]. Therefore, the cooling system is particularly important to improve the performance of the motor. Due to the absence of the stator yoke, it is difficult to integrate cooling system, which limits the output power of the motor. Providing an efficient cooling system and improving the heat dissipation efficiency has become a key issue for this kind of motor. B. Zhang et al. [10] arranged multiple copper tubes in parallel on the back of stator as the cooling system. The disadvantage of the cooling system is that the contact area between the cooling system and the winding is small and the motor maintained low cooling efficiency.

Fig. 1.

Geometry of the direct cooling YASA.

Table 1

Parameters of the YASA prototype

Parameters	Symbol	Value
Rated power	P _rated	30 kW
Peak power	P _peak	65 kW
Peak torque	T _peak	140 Nm
Maximum current	I _peak	250 A
Max speed	N _max	6000 rpm
Max efficiency	𝜂_max	95%
Rated voltage	V _rated	350 VDC
Number of poles	P	20
Number of slots	S _n	18
Dimension		Φ230 mm × 171 mm
Weight		12 kg

2. YASA prototype

A new type of direct cooling YASA is introduced in this paper. The main characteristics of this kind of YASA are that the fins are extended on the water-cooling casing, the water-cooling pipes are integrated inside the fins, and the windings are in close to the fins. The geometry of the YASA machine under study is shown in Fig. 1, while the parameters of the YASA are given in Table 1.

For the traditional water-cooling motor, the heat generated by the current is firstly conducted from winding to stator, and then from the stator to the water-cooling casing. The large thermal resistance will cause a large temperature difference between the winding and the coolant, which will severely limit the output power of the motor. For this kind of motor, the heat generated from the windings is directly conducted to the fins, and then taken away by the liquid in the water-cooling pipe inside the fins to achieve the purpose of heat evacuation.

The cooling system can reduce the thermal resistance between the stator winding and the water-cooling pipeline effectively, and improve the output power of the motor.

The fins reduce the space of copper windings and hence increase the loss, but at the same time they improve heat evacuation. As the diameter of the water-cooling pipes increases, the pressure loss of the liquid will decrease, the power of the cooling system’s pump on vehicle will decrease. But at the same time the width of the inward heat extraction fins will increase, the slot area available for the windings will decrease, causing a lower winding factor and hence higher winding resistance and losses. Similarly, as the parallel number of the water-cooling pipe increase, the pressure loss of the liquid will decrease, the power of the cooling system’s pump on vehicle will decrease. But on the premise of constant cooling liquid flow, the velocity of liquid will decrease, this leads to a worse heat evacuation. Thus the diameter and the parallel number of the water-cooling pipes and should be selected carefully.

3. Structure optimization

The section view of the stator is shown in Fig. 2, there are two water-cooling pipes in every heat extraction fins, and coolant flows in from one port to the other. The diameter of the water-cooling pipe is denoted with d _l, the width of the fin is denoted with l _f, the width of the coil is denoted with l _c, and the parallel number of the water-cooling pipes is denoted with N. For this kind of YASA, there are 18 fins on the water-cooling casing, the number of pipes in parallel N can be 1, 2, 3, 6, 9, 18, different parallel number of the water-cooling pipes can be shown in Fig. 3.

Fig. 2.

Section view of the stator.

Fig. 3.

Different parallel number of water-cooling pipes.

The output power of the motor is $\begin{eqnarray}\displaystyle P_{\text{out}}=T{\omega}. & & \displaystyle\end{eqnarray}$ (1) The output torque of the motor is $\begin{eqnarray}\displaystyle T=K_{T}I & & \displaystyle\end{eqnarray}$ (2) where K _T is the torque constant of the motor.

Considering the stator, whereby the heat generation in the copper windings is defined as $\begin{eqnarray}\displaystyle {\Phi}=3I^{2}R & & \displaystyle\end{eqnarray}$ (3) Where I is the phase current, and R is the phase resistance. The heat transferred from the motor stator to the coolant is $\begin{eqnarray}\displaystyle {\Phi}=\frac{T_{w}-T_{f}}{R_{th}}=hA(T_{w}-T_{f}) & & \displaystyle\end{eqnarray}$ (4) Where R _th is the thermal resistance between the water-cooling casing and the fluid, T _w is the temperature of the water-cooling casing, T _f is the temperature of the coolant. h is thermal-convection coefficients, A is the heat transfer area. $\begin{eqnarray}\displaystyle A=l_{p}{\pi}d_{l} & & \displaystyle\end{eqnarray}$ (5) Where l _p is the length of the cooling pipe. d _l is the diameter of the cooling pipe.

Most time, the output power of the motor is limited by the maximum temperature of the stator, the heat generated by the stator is proportional to the product of h and A. The water-cooling system can remove most of the generated heat. This paper mainly improves the factor h × A by optimizing the structure of the water-cooling system, and then improves the output power of the motor.

To abbreviate the expressions of the convection coefficients, four important parameters, Nusselt number Nu, Reynolds number Re, Prandtl number Pr are introduced, and they are dimensionless numbers. $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle Re=\frac{Vl_{f}}{{\nu}}\end{eqnarray}$ (6) $\begin{eqnarray} \displaystyle \text{} & \text{} & \displaystyle Pr =\frac{{\nu}}{{\alpha}}=\frac{{\mu}C_{p}}{{\lambda}} \end{eqnarray}$ (7) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle Nu=\frac{hl_{f}}{{\lambda}}\end{eqnarray}$ (8) Where V is the fluid velocity, 𝜐 is the kinematic viscosity, l _f is the feature size, μ is the dynamic viscosity, 𝛼 is the volume expansion coefficient, C _p is the specific heat capacity at constant pressure, 𝜆 is the material’s thermal conductivity.

For this kind of motor, multiple pipes can be connected in parallel, the feature size l _f can be express as $\begin{eqnarray} \displaystyle l_{f}=\sqrt{N}d_{l}. & & \displaystyle \end{eqnarray}$ (9) And the fluid velocity can be expressed as $\begin{eqnarray}\displaystyle V=\frac{Q}{N{\pi}\left({\displaystyle \frac{d_{l}}{2}}\right)^{2}} & & \displaystyle\end{eqnarray}$ (10) Where Q is the water flow rate, which is kept constant as 8 L/min in this paper.

The Reynolds number Re can exceed 10000 under the given water flow rates (8 L/min). Therefore, the water flow type is turbulent. The forced water-cooled convection coefficient can be calculated from [11] $\begin{eqnarray}\displaystyle Nu=0.023Re^{0.8}Pr^{0.4} & & \displaystyle\end{eqnarray}$ (11)

From formula (6)–(11), we can get the thermal-convection coefficients h. $\begin{eqnarray}\displaystyle h=0.023\left(\frac{4Q}{{\nu}{\pi}d_{l}\sqrt{N}}\right)^{0.8}\left(\frac{{\mu}C_{p}}{{\lambda}}\right)^{0.4}\frac{{\lambda}}{d_{l}\sqrt{N}} & & \displaystyle\end{eqnarray}$ (12) And the product of h and A can be expressed as $\begin{eqnarray}\displaystyle hA=0.023\left(\frac{4Q}{{\nu}{\pi}d_{l}\sqrt{N}}\right)^{0.8}\left(\frac{{\mu}C_{p}}{{\lambda}}\right)^{0.4}\frac{{\pi}{\lambda}l_{p}}{\sqrt{N}}. & & \displaystyle\end{eqnarray}$ (13)

It is important to notice from Eq. (13) that the factor h × A is the function of the diameter of the cooling pipe d _l and the number of pipes in parallel N, while other parameters are all constant. This makes the study of the variation of these parameters on the machine temperature very interesting and useful in the selection of these parameters. The relationship between the factor h × A and N, d _l is shown in Fig. 4.

Fig. 4.

The relationship between the factor h × A and N, d _l.

The pressure loss of the cooling system mainly contain two parts, one is the frictional head loss, and another is local head loss. $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle p_{f}={\xi}\frac{l_{t}}{l_{f}}\frac{{\rho}V^{2}}{2}\end{eqnarray}$ (14) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle p_{m}={\zeta}\frac{{\rho}V^{2}}{2}\end{eqnarray}$ (15) where p _f is the frictional head loss, p _m is the local head loss, 𝜉 is the friction loss factor, l _t is the total length of the cooling circuit, ζ is the local loss factor. $\begin{eqnarray}\displaystyle l_{t}=\frac{18}{N}l_{p}. & & \displaystyle\end{eqnarray}$ (16)

The total pressure loss is $\begin{eqnarray}\displaystyle {\rm\Delta}p=p_{f}+p_{m}. & & \displaystyle\end{eqnarray}$ (17)

When 4000 ≤ Re ≤ 100000 [11] $\begin{eqnarray}\displaystyle {\xi}=\frac{0.316}{\sqrt[4]{Re}}. & & \displaystyle\end{eqnarray}$ (18)

The relationship between the pressure loss and N, d _l is shown in Fig. 5.

Fig. 5.

The relationship between the pressure loss and N, d _l.

Fig. 6.

The relationship between the evaluation indicator and N.

From the analysis result, we can get the conclusion that as the decrease of the parallel number N and diameter of water pipes d _l, fluid velocity increase, and the turbulence is intensified, the heat dissipation capacity of cooling system enhanced. However, at the same time, the pressure loss will increase and the power of the cooling system will increase under the condition of constant flow rate.

In order to maximize heat transfer efficiency with minimal pressure loss, an evaluation indicator commonly used in engineering is defined as $\begin{eqnarray}\displaystyle {\eta}=\frac{h}{({\rm\Delta}p)^{1/3}}. & & \displaystyle\end{eqnarray}$ (19)

As demonstrated in Fig. 6, it can be noted that, 𝜂 is maximum when the number of pipes in parallel N equal to 5.2. As mentioned above, the number of pipes in parallel N only can be 1, 2, 3, 6, 9, 18, finally N = 6 is selected.

In practical applications, the pressure loss of the EV cooling system must be less than 100 kPa. Figure 7 illustrates that the diameter of the cooling pipe d _l must be more than 2.8 mm under the condition of N = 6, the pressure loss can meet the requirement of application. Considering the actual processing requirements, the d _l is 3 mm.

Fig. 7.

The relationship between the pressure loss and d _l.

A 3D CFD model is developed to verify the results of the analytical models. The material properties of YASA part are given in Table 2.

Table 2

Material properties of the YASA part

Part	Material	Density	Thermal conductivity	Specific heat capacity
		kg/m³	W/m ⋅ K	J/k ⋅ K
Housing	Aluminum alloy (AL6061)	2712	167	896
Water-cooling casing	Aluminum alloy (AL6061)	2712	167	896
Armature	SMC (10003P)	7520	25	584
Magnet	NdFeB (N38SH)	7500	8.949	502
Winding	Copper	8890	385	392
Insulation	Paper	930	0.18	1340
Potting	Epoxy (WH2000)	1540	1.23	600
Gap	Air	1.205	0.0259	1005

It can also be seen that the good correspondence between the CFD results and the analytical method.

Fig. 8.

The tempture of the stator of YASA.

Fig. 9.

The pressure loss of the cooling system.

4. Experimental validation

In order to validate the optimized results, the optimized YASA is manufactured and tested, as shown in Fig. 10. The winding and the core temperatures are measured by an embedded resistance temperature detector (PT100) sensor, while the pressure loss is measured by two liquid pressure sensors (UNIK5000).

Fig. 10.

The optimizized YASA.

Fig. 11.

Winding temperature obtained by CFD and test, 140 A, waterflow of 8 L/min.

As shown in Fig. 11, the winding temperature obtained from CFD and test are compared when the YASA is supplied with a phase current of 140 A (the output power is 42.5 kw) with the water-cooling system. The pressure loss get by the experimental test is 0.12 Mpa, while the loss get by the CFD is 0.117 Mpa, which is shown in Fig. 9. The results show a good correspondence between the CFD and the experimental work.

5. Conclusion

In this paper, a new type of direct cooling YASA is introduced. Some optimization to increase the output power by improving the heat transfer efficiency of the water-cooling system are presented. Through the numerical analysis, the optimal diameter and the parallel number of the water-cooling pipes are revealed.

The CFD result show that, the continuous output power of this YASA can exceed 42.5 kW, which is 41.6% than the prototype, which agrees well with the experimental test. This work can provide a useful method to study thermal performance of water-cooling YASA.

Footnotes

Acknowledgements

The research work is supported by science and technology innovation 2025 major project (2018B10067) and technology innovation of new energy vehicle and intelligent network vehicle project (IMIZX2018001).

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