Application Limits of the Airgap Maxwell Tensor Raphaël Pile, Guillaume Parent, Emile Devillers, Thomas Henneron, Yvonnick Le Menach, Jean Le Besnerais, Jean-Philippe Lecointe

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Raphaël Pile, Guillaume Parent, Emile Devillers, Thomas Henneron, Yvonnick Le Menach, et al.. Application Limits of the Airgap Maxwell Tensor. 2019. ￿hal-02169268￿

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Raphael¨ Pile1,2, Guillaume Parent3, Emile Devillers1,2, Thomas Henneron2, Yvonnick Le Menach2, Jean Le Besnerais1, and Jean-Philippe Lecointe3 1EOMYS ENGINEERING, Lille-Hellemmes 59260, France 2Univ. Lille, Arts et Metiers ParisTech, Centrale Lille, HEI, EA 2697 - L2EP - Laboratoire d’Electrotechnique et d’Electronique de Puissance, F-59000 Lille, France 3Univ. Artois, EA 4025, Laboratoire Systemes` Electrotechniques´ et Environnement (LSEE), F-62400 Bethune,´ France

Airgap Maxwell tensor is widely used in numerical simulations to accurately compute global magnetic forces and torque but also to estimate magnetic surface force waves, for instance when evaluating magnetic stress harmonics responsible for electromagnetic vibrations and acoustic noise in electrical machines. This article shows that airgap surface forces based on Maxwell tensor with a circular contour depend on the radius of application. The dependence of surface forces on radius is analytically explained using an academic case of a slotless electric machine. The corrective coefficients that should be applied to transfer the airgap surface forces to stator (or rotor) bore radius are presented. The coefficients depend on the airgap geometry and surface forces wavenumber. The use of transfer coefficients is recommended to quantify the deviation due to the radius of application in the airgap. In addition, the transfer coefficients could be applied to correct the radial surface force component on the tip of the teeth assuming infinite permeability.

Index Terms—Maxwell tensor, Magnetic stress, Electrical machines, Magneto-mechanical, Vibroacoustic.

I.INTRODUCTION application of airgap surface force based on MT and the surface forces at stator bore radius. Due to numerous numerical AXWELL tensor (MT) is used to accurately compute artifacts in practical electrical machines, only a slotless stator M overall forces on a given body. In particular, electrical and rotor with constant permeability is studied in this article. It machines designers are used to apply MT technique in the allows to obtain the analytic expression of the airgap potential middle of the airgap to calculate the electromagnetic torque and resulting surface forces at any radius. Thus the surface experienced by the rotor. This technique has been naturally force transfer deviation is quantified by analytically-derived extended to the study of magnetic force waves experienced by transfer coefficients (20) and (24) which depend on airgap the outer structure - generally the stator - for vibroacoustic geometry and magnetic surface force wavenumber. analysis. The method is generally based on a circular path in the airgap to ease physical interpretations with the Fourier II.PROBLEM DEFINITION transform [1,2]. In that case, one may look for a circular path as close as possible from the stator bore radius to accurately A. Maxwell Tensor estimate forces experienced by the stator. Indeed, for magnetic According to [3,7], for an incompressible linearly linear media [3], the theoretical magnetic surface force density magnetizable media the magnetic flux density B is related to at the air-ferromagnetic interface is exactly given by MT the magnetic field H ∀x ∈ Ω by B(x) = µ(x,B)H(x) such expression. that the magnetic stress tensor reduces to the following form: However, the radius of the circular path has a strong H · H T = −µ I + µHH (1) influence on calculated stress harmonics [4]–[6]. In Finite m 2 Element Analysis (FEA) simulations, the mesh is generally I the finest in the airgap so that it is recommended to evaluate with the identity tensor. In particular, the Vacuum Maxwell µ = µ forces in the middle of the airgap. Besides, 2D FEA based stress tensor is obtained for 0. The theoretic application on scalar potential cannot ensure high accuracy of both radial of this magnetic stress tensor for the computation of magnetic P and tangential flux density at the air-ferromagnetic interface, surface force density on a contour which exactly corresponds which are both involved in MT expression. Then the numerical to the air-ferromagnetic interface (defined by its local normal n application of the MT at the interface is source of numerical vector ) leads to [3,5]:     errors [5]. 1 1 1 2 µ0 − µ 2 P = − Bn − Ht n (2) Therefore, it is interesting to know how to transfer surface 2 µ0 µ 2 forces based on MT calculated in the airgap to another radius, where Bn and Ht are the magnetic flux density locally normal in particular at tooth tip radius. Such a transformation would (resp. magnetic field locally tangential) to the interface. also be useful when using analytical electromagnetic models such as permeance magneto-motive force models, which focus on the accurate calculation of the flux density distribution at B. Airgap Surface Force the airgap radius. One of the most used method to compute magnetic surface Therefore, this article compares the deviation between the forces based on MT is to use Stoke’s theorem [8] along a Stator the magnetic potential z-component Az is imposed at radius Γ Rag such that ∀θ ∈ [0, 2π]: ∗ Rag Az(Rag, θ) = βn sin(nθ + ψn), n ∈ N (5) The continuity of tangential magnetic field can be expressed R s with the magnetic potential at the interface between the air Rotor (permeability µ0) and ferromagnetic media (permeability µ) ∀θ ∈ [0, 2π]: Fig. 1. Slotless electrical machine used to compare magnetic surface force on the stator and in the airgap 1 ∂Az − 1 ∂Az + (Rs , θ) = (Rs , θ) (6) µ0 ∂r µ ∂r Thus the boundary condition at r = R with µ → ∞ becomes: circular closed boundary Γ in the airgap at a given radius Rag s r as illustrated in Fig 1. The result gives the total magnetic force ∂Az (Rs, θ) = 0 (7) Fm acting on either the stator or rotor [1,5]: ∂r I For these boundary conditions, Poisson’s equation is solved µ0 2 Fm = − H n + µ0HnH dΓ (3) for the 2D magnetic vector potential in polar coordinates ∀θ ∈ Γ 2 [0, 2π] and ∀r ∈ [Rag, Rs]: Then the airgap MT approximation is to assume the term 1 ∂ 1 ∂A  ∂2A under the integral to be the magnetic surface force density in z + z = 0 (8) the polar referential: r ∂r r ∂r ∂θ2 This partial derivative problem can be solved 1 2 µ0 2 Pr(Rag) ≈ Br(Rag) − Hθ(Rag) analytically [9]. Thus the magnetic potential, flux density and 2µ0 2 (4) field can be analytically computed in the studied domain. The P (R ) ≈ B (R )H (R ) θ ag r ag θ ag next section provides the calculation steps which lead to the with Pr the radial magnetic surface force density, Pθ the analytical expression of flux density (15) in the slotless case. tangential magnetic surface force density, Br the radial magnetic flux density, and Hθ the tangential magnetic field. D. Analytic Magnetic Solving in Slotless Case Neglecting the ferromagnetic permeability µ contribution A solution exists and is unique for the previous system into (2) gives an expression very similar to (4) justifying the composed of (8), (7) and (5) according to [8]. Then a method use of airgap surface force approximation. consists in stating a function Az and to check if it fulfills The application of (2) at the air-ferromagnetic interface the boundary conditions: a solution similar to [9] is searched remains source of numerical errors [5]: in the case of ∀θ ∈ [0, 2π] and ∀r ∈ [Rag, Rs]: numerical model based on FEA, the continuity of both normal γ E (r, R ) + α E (R , r) magnetic flux Bn density and tangential magnetic field Ht n n s n n ag Az(r, θ) = sin(nθ + ψn) (9) cannot be ensured on the interface between materials. As a En(Rag, Rs) consequence, publications often use airgap approximation (4) with En a polynomial function defined by: for the vibroacoustic design of electrical machines because 2 n n the middle of the airgap is better meshed, it is less En :(x, y) ∈ R → (x/y) − (y/x) (10) sensitive to variational formulation, and it allows the physical Under this form, the vector Az is satisfying Poisson’s understanding of each magnetic force wave (for instance with equation (8). Then the coefficient γn, αn, have to be analytical models) [1,2]. determined in order to satisfy the boundary conditions. If a correct set of these coefficients is found, then Az would be the C. Studied Slotless Case unique solution of the problem. First, satisfying the boundary condition (5) in the airgap leads to: Understanding the sources of airgap surface force (4) variations is a difficult task because of numerous artifacts such γn = βn (11) as slotting effect, sharp geometries, interference between the The second boundary condition (7) leads to: wavenumbers, etc. To avoid these main artifacts, a specific case is defined: αn = 2βn/Fn(Rag, Rs) (12)

• Slotless stator and rotor to avoid sharp geometries and with Fn a polynomial function defined by: slotting effect. 2 n n • Single-wave excitation to avoid interference between Fn :(x, y) ∈ R → (x/y) + (y/x) (13) different wavenumbers. Thus, the unique solution of the system is entirely defined • Analytical solving to avoid meshing and numerical errors. with the geometrical parameters and excitation’s wavenumber. The case presented in Fig. 1 proposes to fulfill all these The flux density is derived from the magnetic potential: constraints. The stator bore radius is Rs and the middle airgap 1 ∂Az ∂Az radius is R . In order to have only one magnetic wavenumber, B = curl(A) = er − eθ = Brer + Bθeθ (14) ag r ∂θ ∂r With (14) and (9) both radial and tangential magnetic flux can 1 be analytically expressed for this slotless machine: n = 1 n = 2

2En(Rag,r) n = 4 ) nβ En(r, Rs) + 0.9 r F (R ,R ) ( n n ag s n = 8 Br(r, θ) = cos(nθ + ψn) n r En(Rag, Rs) 2

(15) R 2Fn(Rag,r) Fn(r, Rs) + 0.8 nβn Fn(Rag,Rs) Bθ(r, θ) = − sin(nθ + ψn) r En(Rag, Rs) 0.9 0.92 0.94 0.96 0.98 1

III.TRANSFEROFSURFACEFORCESINSINGLEMAGNETIC Adimensional radius r/Rs WAVE CASE Fig. 2. Comparing stator and airgap Maxwell Tensor magnetic surface force This section aims to compare the airgap surface forces (4) for the harmonic 2n calculated at radius Rag with the exact value calculated at Rs. Then an airgap surface radial force function is defined: and R2n should be very close to 1 for every wavenumbers and for any radius. Fig. 2 shows that these coefficients [R , R ] × [0, 2π] → P : ag s R are not always negligible and they can strongly affect r (r, θ) → 1 B2(r, θ) − B2(r, θ)
2µ0 r θ (16) high magnetic surface force wavenumbers. Nevertheless, the In order to better understand the airgap transfer of the forces, vibro-acoustic of electrical machine generally considers only a Fourier decomposition is performed: low wavenumbers such that the airgap surface force stays relatively accurate in the radial direction with narrow airgaps. Pr(r, θ) = Pr,0(r) + Pr,2n(r) cos(2nθ + 2ψn) (17) A similar method is applied to estimate the impact of the airgap transfer on the tangential surface force, with the The previous result (15) allows to express the magnetic flux following function: under the form:  [Rag, Rs] × [0, 2π] → R Br(r, θ) = Br,n(r) cos(nθ + ψn) Pθ : 1 (23) (18) (r, θ) → B (r, θ)Bθ(r, θ) µ0 r Bθ(r, θ) = Bθ,n(r) sin(nθ + ψn) Then the next result can be easily demonstrated following the Then the airgap radial surface force can be decomposed under same methodology: the form: 2 2 2
R E2n(r, Rs) Pr,0(r) = Br,n(r) − Bθ,n(r) /2 s Pθ(r, θ) = − 2 Pr,2n(Rs) sin(2nθ + 2ψn) (24) 2 2
(19) r 2 Pr,2n(r) = B (r) + B (r) /2 r,n θ,n In order to estimate the relative error ∆, the airgap width This is a classical result: a magnetic wavenumber n in the is defined as g with g = 2 (Rag − Rs)  Rag. Then a Taylor airgap is recomposed into magnetic force wavenumbers 0 and approximation on (24) leads to: 2n. For the studied slotless case ∀θ ∈ [0, 2π], Bθ(Rs, θ) = 0. In order to avoid θ dependency, the amplitude of each ∆ ∝ 2ng/Rag (25) harmonics are compared independently. To this purpose, a However on the studied case, the tangential surface force function is defined ∀r ∈ [Rag, Rs], for k ∈ N: density should be null. A new criterion can be defined: ∆  1 R (r) = P (r)/P (R ) (20) means that airgap surface forces are accurate. Note that the k r,k r,k s R 2π resultant torque is constant for any radius - 0 Pθ(r, θ)rdθ The next step is to introduce the analytic solution of the = 0 - but it is not true for the total radial force because it is magnetic flux (15) into (20) in order to get the analytic R varying linearly with the radius of application: Pr(r, θ)dθ = expression of each function R and R . After some Rs R 0 2n Pr(Rs, θ)dθ. Nevertheless, the transfer coefficient does calculations, the expression of these two functions simplify r not change the unbalanced magnetic forces Fx and Fy in the remarkably well as follows for k = 0 or k = 2n: Cartesian referential. 2 Rk(r) = (Rs/r) Fk(r, Rs)/2 (21) IV. GENERALIZATION TO MULTI-HARMONIC CASE Note that ∀r, F (r, R ) = 1. Then combining (17) with (21), 0 s The previous section demonstrates a new transfer coefficient the relation between the airgap radial surface force and the for single magnetic excitation wave. In this section, exact radial surface force at position r = R can be established: s the coefficient (21) is validated to any combination of r2  2P (r)  P ( , θ) = P (r) + r,2n cos(2nθ + 2ψ ) wavenumbers. r Rs 2 r,0 n ˆ ˆ th Rs F2n(r, Rs) Pr is the Fourier transform of Pr and if P (r, k) is the k (22) harmonic of the airgap surface force (4) : The differences between exact radial surface force density X ˆ and airgap radial surface force depends on the airgap width, Pr(r, θ) = P (r, k) cos(kθ + φk) (26) the radius of application and the wavenumber, but not on k the magnetic excitation βn. If the airgap surface forces are From this airgap surface force, a transfer model is proposed equivalent to the exact surface forces at Rs, then both R0 by assuming that - despite the recombination of several flux TABLE I of an electrical machine stator’s tooth tip - with infinite INJECTEDMAGNETICPOTENTIALSPECTRUMFORTHEVALIDATIONCASE ferromagnetic permeability - then the transfer method (27) WITH R = 42.5 mm AND R = 45.0 mm. AG S can be applied but only for this restricted angular opening. Wavenumber n 5 7 15 17 19 Nevertheless, the transfer of the airgap surface force based on Amplitude βn [T.m] 5.2615 0.6861 0.2119 0.2725 0.1926 MT in front of the slotted area might not be properly predicted Phase ψn [rad] 3.8893 0.7100 0.4700 0.6849 0.5410 with these coefficients. ·105 V. CONCLUSION 0 This paper objective was to understand to what extent ]

2 the surface force density computed at the airgap radius with Maxwell Tensor differs from the exact magnetic force density. For this purpose, a simplified slotless academic machine −1 was defined in order to find analytical surface force transfer coefficients. The study of a single flux density excitation A Pr

Surface force [N/m demonstrates the existence of a a new transfer coefficient. The P Pr final part of the paper shows that these transfer coefficients −2 P S r remain unchanged with a multi-harmonic magnetic excitation. 0 0.5 1 1.5 2 2.5 3 The importance of these transfer coefficients depends on Angular position [rad] the airgap size and force wavenumbers to be taken into ·104 account for vibroacoustic analysis. Only these two factors

] 8.6 A were identified. The transfer coefficients have little influence 2 Pr (Airgap) P on low magnetic force wavenumbers but significant influence Pr (New method) S Pr (Reference) on high force wavenumbers. Consequently, these coefficients can be used to estimate the deviation between the surface 3.7 force calculated in the air and the surface force calculated on the stator bore radius. Considering infinite permeability, Surface force [N/m the transfer coefficients could be applied to correct the radial 0 2 4 8 10 12 14 20 22 24 26 30 32 34 36 38 magnetic surface forces from the airgap Maxwell Tensor with Angular wavenumber a real electrical machine, but only in front of a tooth tip. Future research work will address the generalization of Fig. 3. Comparison of several methods for surface force density using Maxwell Tensor these coefficients to slotted areas of electrical machines and to tangential surface forces. The authors expect to density harmonics into one force harmonic - the coefficient use this slotless machine as a reference case in future (21) stays valid: development of magneto-mechanical coupling methods: the simplified geometry allows to solve analytically the magnetic X 2Pˆ(r, k) and mechanical equations such that it could be compared to Pr(Rs, θ) = cos(kθ + φk) (27) Fk(r, Rs) numerical and experimental results in future research work. k

The validity of the assumption is verified for the same slotless REFERENCES case as in Fig. 1 with infinite ferromagnetic permeability. Thus [1] J. F. Gieras, C. Wang, and J. C. Lai, Noise of polyphase electric motors. the flux density is entirely solved analytically using a linear CRC Press, 2005. combination of (15). The numerical inputs of the validation [2] X. Xu, Q. Han, and F. Chu, “Review of electromagnetic vibration in case are presented in Table I. The results are presented in electrical machines,” Energies, vol. 11, no. 7, 2018. A P [3] F. Henrotte and K. Hameyer, “Computation of electromagnetic force Fig. 3: Pr denotes the application of (16) at r = Rag, Pr densities: Maxwell stress tensor vs. virtual work principle,” J. Comput. S denotes the application of (27), and Pr denotes the exact Appl. Math., vol. 168, no. 1-2, pp. 235–243, 2004. [4] J. Mizia, K. Adamiak, A. Eastham, and G. Dawson, “Finite element force value of magnetic surface forces obtained with (16) at r = Rs. 2 calculation: comparison of methods for electric machines,” IEEE Trans. Comparing the L -norm of the two signals with respect to the Magn., vol. 24, no. 1, pp. 447–450, 1988. stator surface forces leads to: [5] Z. Ren, “Comparison of Different Force Calculation Methods in 3D Finite Element Modelling,” IEEE Trans. Magn., vol. 30, no. 5, pp. 3471–3474, P S −5 A S kPr − Pr kL2 ≈ 10 kPr − Pr kL2 (28) 1994. [6] J. Hallal, F. Druesne, and V. Lanfranchi, “Study of electromagnetic P S A perfect match is observed between the Pr and the Pr , which forces computation methods for machine vibration estimation,” in ISEF means that the intuited formula (27) is correct at least for Conference, 2013, pp. 12–14. [7] R. E. Rosensweig, Ferrohydrodynamics. Cambridge University Press, the slotless geometry. A similar result can be found with the 1985. tangential component. [8] L. D. Landau and E. Lifshitz, “The classical theory of fields. vol. 2,” This last result allows the partial use of these coefficients Course of theoretical physics, 1975. [9] T. Lubin, S. Mezani, and A. Rezzoug, “Exact analytical method for for slotted electrical machine: all the previous results hold magnetic field computation in the air gap of cylindrical electrical when the angular domain is restricted. Thus supposing machines considering slotting effects,” IEEE Trans. Magn., vol. 46, no. 4, the previous reasoning is applied on the angular opening pp. 1092–1099, 2010.