JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, SEPTEMBER 2017 1 Comparison of main magnetic force computation methods for noise and vibration assessment in electrical machines Raphael¨ PILE13, Emile DEVILLERS23, Jean LE BESNERAIS3 1 Universite´ de Toulouse, UPS, INSA, INP, ISAE, UT1, UTM, LAAS, ITAV, F-31077 Toulouse Cedex 4, France 2 L2EP, Ecole Centrale de Lille, Villeneuve d’Ascq 59651, France 3 EOMYS ENGINEERING, Lille-Hellemmes 59260, France (www.eomys.com) In the vibro-acoustic analysis of electrical machines, the Maxwell Tensor in the air-gap is widely used to compute the magnetic forces applying on the stator. In this paper, the Maxwell magnetic forces experienced by each tooth are compared with different calculation methods such as the Virtual Work Principle based nodal forces (VWP) or the Maxwell Tensor magnetic pressure (MT) following the stator surface. Moreover, the paper focuses on a Surface Permanent Magnet Synchronous Machine (SPMSM). Firstly, the magnetic saturation in iron cores is neglected (linear B-H curve). The saturation effect will be considered in a second part. Homogeneous media are considered and all simulations are performed in 2D. The technique of equivalent force per tooth is justified by finding similar resultant force harmonics between VWP and MT in the linear case for the particular topology of this paper. The link between slot’s magnetic flux and tangential force harmonics is also highlighted. The results of the saturated case are provided at the end of the paper. Index Terms—Electromagnetic forces, Maxwell Tensor, Virtual Work Principle, Electrical Machines. I. INTRODUCTION TABLE I MAGNETO-MECHANICAL COUPLING AMONG SOFTWARE FOR N electrical machines, the study of noise and vibrations due VIBRO-ACOUSTIC I to magnetic forces first requires the accurate calculation of EM Soft Struct. Soft Projection tool Force calc. Ref. Maxwell stress distribution which depends on the time and LMS LMS VWP [6] [7] space distribution of the magnetic flux density. Indeed a very NASTRAN JMAG MT [8] JMAG ALTAIR JMAG MT [8] small magnetic force harmonic can induce large acoustic noise ABAQUS JMAG MT [8] and vibrations due to a resonance with a structural mode of JMAG JMAG VWP [8] MT the stator. The magnetic flux can be determined everywhere ANSYS ANSYS [9] VWP in the machine with a Finite Element Analysis (FEA), only in LMS LMS VWP [10] FLUX the air-gap and windings with semi-analytical methods such ALTAIR FLUX MT [11] as Sub-Domain Model (SDM) [1], or only in the middle of the NASTRAN FLUX MT [11] MATLAB MATLAB VWP [11] [10] air-gap using the permeance magneto-motive force (PMMF). COMSOL COMSOL MT [12] COMSOL MT In order to compute magnetic forces, a various range of MATLAB MATLAB [13] [14] methods can be found in the literature including: fictive VWP MT ANSYS ANSYS [15] [16] magnetic currents and magnetic masses methods [2], energy VWP ANSYS methods [3], Maxwell Tensor (MT) methods [4], and Virtual LMS LMS VWP [17] Work Principle adapted to Finite Element (VWP) [5]. Then a NASTRAN NASTRAN MT [15] MT OPERA OPERA OPERA [18] compatible force computation method for the vibro-acoustic VWP objectives must be chosen. As shown in the Table I, the FEMAG FEMAG FEMAG MT [19] two flagship methods are the VWP and the MT but the INFO NASTRAN FSI Mapper MT [20] LYTICA ACTRAN FSI Mapper MT [20] preference for one method or another is not clearly justified MT GETDP GETDP GETDP [21] [22] in the documentation and articles linked to these software (see VWP MT column ”Ref.” of Table I). MANATEE ALTAIR MATLAB [23] VWP The discussion of local magnetic pressure is still on going [24] [25] [26] and should be the aim of further work. The VWP is built to account for local magnetic behavior, while the Maxwell tensor gives the momentum flux across any acoustic studies to obtain the local force distribution in the air surface, closed or not [24]. Historically the Maxwell Tensor gap which applies on the stator structure [8] [11] [15] [20]. has been used by electrical machine designers to accurately However such a method can have strong limitations depending compute global quantities such as electromagnetic torque, on the geometry as shown by [17]. which is the moment of the global magnetic force applied Consequently, a common method to compute the magneto- on a cylinder surrounding the rotor on its axis of rotation. mechanical excitations is to apply one integrated force per Under the common form the MT cannot be rigorously related stator’s tooth. It corresponds to the ”Lumped Force Mapping” to a local magnetic pressure but it is often used in vibro- method proposed in [15]. A comparable integrated force per JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, SEPTEMBER 2017 2 tooth can be obtained using the VWP or the MT. For the VWP, the integrated force per tooth is directly the resultant Stator of the nodal forces computed inside the tooth. For the MT, it is obtained by integrating the magnetic pressure in the air Windings gap over a path ”embracing/surrounding” the tooth. Thus, this paper proposes a comparison of the most common methods Magnet under local and integrated form with and without saturation effect. In order to perform the computation, MANATEE [23] is used because it is a privileged simulation framework for the vibro-acoustics study of electrical machines which can compute magnetic field with all these methods. Moreover it Rotor includes vibro-acoustic models showing good agreement with experiments. Tooth n°2 The numerical values of the targeted Surface Permanent Magnet Synchronous Machine (SPMSM) machine with con- Air gap centrated windings can be found in Table II. This validation case parameter is used all along this paper. This geometry Fig. 1. Electrical motor flux map generated with FEMM [29] in loaded case which can be found in the article [27] is privileged because the long and thin stator’s teeth are concentrating considerable radial and tangential efforts on teeth’s tips such that the lumped equivalence between the magnetic co-energy variation and the force method is adapted. The geometry is presented on Fig. force applied on a solid determined by the domain Ω, such 4 and the flux lines in load case (no saturation) is illustrated that the force amplitude in the direction s 2 fx; y; zg is: on Fig. 1. A linear B-H curve and homogeneous media are H considered for the whole machine. The geometry used in this @ Z Z Fs = B · dH dΩ (1) paper has been successfully used with lumped force mapping @s Ω 0 by [28] with a MT applied in the air-gap. Applying this equation to a mesh element e: TABLE II Z Z H ! INPUT PARAMETERS OF THE SPMSM 12S-10P MACHINE T −1 @J −1 @jJj Fs = −B · J · · H + B · dH jJ j e @s 0 @s Parameter Value & Unit Numerical (2) Rotation speed N 400 [rpm] with J the Jacobian matrix of the element e. The derivatives of Total time steps 160 J can be determined knowing the type of element (triangular, Number of element (min/max) [9142 ; 10798] rectangular, quadrilateral, tetrahedron ...). With a linear case, Number of nodes (min/max) [4888 ; 5564] Geometrical the integrand of B can be simplified as follow: Rry 11.9 [mm] Z H Z H Rrbo 23.9 [mm] µ 2 Rm 26.8 [mm] B : dH = µH : dH = jHj (3) 0 0 2 Rsbo 27.9 [mm] Rslot 46.8 [mm] with H constant in each element. Thus the natural way to Rsy 50.0 [mm] Slot angular width 18 [deg] implement the VWP algorithm is to loop on each element, Tooth angular width 12 [deg] compute the previous formulas and add the element contribu- Depth 50 [mm] tion to its nodes. For a given node i: Magnetic −1 µ0 4π1e-7 [H:m ] Z @ µ @j j µ 1e5 i X T −1 J 2 −1 J stator Fs = −B · J · · H + jHj jJ j µmagnet 1.05 @s 2 @s 8eji2e e µair 1 (4) Magnets Brm 1.2 [T] Windings Phase current RMS 10 [A] Number of 1 B. Discussion parallel circuits per phase Number of turns per coil 34 An overview of the magnetic force distribution according to Eq. (4) is presented on Fig. 2. A first observation is the concentration of nodal forces at the iron-air interface. II. NODAL FORCES WITH VIRTUAL WORK PRINCIPLE This result is expected because of the linear homogeneous A. Formulas & implementation hypothesis media for the stator. In a linear electromagnetic Let H be the magnetic field, B the magnetic flux density, media, the electromagnetic co-energy density is: Hi and Bi their respective components in cartesian frame. jBj2 The nodal force expression proposed in [5] is based on an (x; B(x)) = (1/µ) (5) 2 JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, SEPTEMBER 2017 3 Fig. 3. Barycenter of forces trajectory during one revolution according to Fig. 2. Example of nodal magnetic force repartition using the VWP on the VWP nodal forces stator’s mesh average, the center of force is close to the center of the tooth tip Then the variation of energy density due to a virtual (or real) and under 2% of the total tooth height. The asymmetrical path displacement p : x 2 2 ! p(x) 2 2 in case of weak- R R and the difference between the average center and the tooth’s coupling ( B(p(x)) ≈ B(x)) is: tip center are explained by the asymmetrical average tangential (p(x);B(x)) − (x; B(x)) ≈ forces between both edges, which produce the electromagnetic jB(x)j2 torque.