3-D Eddy Current Torque Modeling

CMP-843 3-D Eddy Current Torque Modeling Subhra Paul, Walter Bomela and Jonathan Z. Bird Laboratory for Electromechanical Energy Conversion and Control University of North Carolina at Charlotte, NC 28223, USA This paper presents an analytic based eddy current torque analysis procedure. The equations are derived using the second order vector potential and the magnetic rotor is modeled using the magnetic charge sheet concept. The formulation enables the damping and stiffness equations to be derived. The equations are numerically computed and the accuracy is compared against experimental torque and power measurements. Index Terms— eddy currents, torque, Halbach rotor, maglev, second order vector potential. I. INTRODUCTION HEN a magnetic source moves in the vicinity of a Wconductive plate, time varying magnetic fields induce eddy currents in the plate which in turn interacts with the source magnetic field to create velocity dependent drag and lift force. The generated drag force can be utilized in applications such as eddy current braking [1] and rotor vibration damping [2]. While the lift force can be utilized to (a) (b) provide suspension for high-speed maglev trains [3]. Fig. 2 The (a) x-y and (b) z-y view of the problem region. Electrodynamic maglev suspension systems typically rely on a II. GOVERNING EQUATIONS null-flux coil guideway topology in order to maximize the lift- A schematic showing the relevant problem regions is to-drag ratio [4]. Another way to avoid this drag and achieve shown in Fig. 2. The rotor velocity in the x, y and z directions integrated suspension and propulsion for the vehicle at low as well as rotational speed, ω , is shown. The length, l and cost is to rotate the magnetic source rather than translationally m width, w, of the plate are assumed to be large enough so that moving it above a conductive plate guideway. This all fields are zero at the edges. The plate is assumed to have electrodynamic wheel (EDW) concept is illustrated in Fig. constant conductivity be nonmagnetic and simply connected. 1(a) [5]. In order to create large lift force a flux-focusing Halbach rotor, as shown in Fig. 1(b), is used. In this approach A. Conductive Region, ΩII the thrust force is dependent on the slip speed, sl, defined as Utilizing the magnetic vector potential ω sl=ωmro-vx where m, ro and vx are mechanical angular BA= ∇ × (1) velocity, outer radius and translational velocity respectively. the eddy current problems can be formulated as [8] Reitz and Davis [6] and later Langerholc [7] developed dA 2A µ σ force and eddy current torque equations for a coil when there ∇ = 0 (2) dt is only translational motion. This paper presents an analytic where σ = conductivity. Solving (2) leads to a solution based eddy current torque, power, damping and stiffness procedure that is complicated due to the need to solve three analysis procedure for the case when a magnetic rotor is scalar terms and also account for the coupling within the rotated and translationally moved above a conductive plate. magnetic flux density components. This derivation complexity The equations are derived using the second order vector can be avoided by using the SOVP, W, defined as [9, 10] potential[8-10] approach and the magnetic rotor is modeled AW= ∇ × = ∇ ×[W yˆ +y ˆ × ∇ W ] (3) using the magnetic charge sheet concept [11]. a b where Wa and Wb are scalars. Only the transverse electric (TE) potential, Wa, is non-zero when solving eddy current problems in a conductive medium that is infinite in the x-z plane [9]. Therefore, the TE potential exists only normal to the conductive plate as illustrated in Fig. 3. The formulation in terms of TE enables one to think of eddy current problems in terms of reflected and transmitted field components [12]. Substituting (3) into (2) yields (a) (b) dW ∇2W = µ σ a (4) Fig. 1(a) A 2 pole-pair Halbach rotor rotating and translationally moving a 0 dt above an aluminum plate. Isoline plot of the reflected radial flux density and isosurface plot of the eddy current density is shown, (b) An experimental 2 Utilizing the convective derivative pole-pair Halbach rotor with radial and shunt magnets CMP-843 ∞ ∞ dWa∂ W a = +()v ⋅ ∇W (5) III III jξm x jk n z κ mn () y+ h dt∂ t a Wa = ∑ ∑ Cmn e e e (19) m n and assuming a steady state solution such that =−∞ =−∞ where ξ= 2 πm / l (20) −j() Pωm t m Wa(,,,)(,,) x y z t= Wa x y z e (6) allows (4) to be written as kn = 2π n / w (21) ∂WWW ∂ ∂ 2 2 2 2 a a a βmn = λ + γmn (22) ∇Wa = −µ0 σ jP ωm W a+ v x + vy + vz (7) ∂x ∂ y ∂ z λ= −0.5vy µ0 σ (23) where P=pole-pairs. Substituting (3) into (1) yields 2 2 2 2 2 2 γmn= κ mn − µ0 σso (24) ∂WWWWa ∂a ∂ a ∂ a B =xˆ − +yˆ + zˆ , in ΩII (8) 2 2 2 2 2 κ= ξ + k (25) ∂x ∂ y ∂x ∂ z ∂z ∂ y mn m n so= j( Pw m +ξ m v x + k n v z ) (26) B. Nonconductive Regions, ΩI, ΩIII Since the conductivity is zero the governing equation is [13] IV. SOURCE FIELD 2 ∇Wa = 0 , in ΩI, ΩIII (9) In this paper the TE potential due to a Halbach magnetic and the relationship to B simplifies to rotor is calculated directly from the field created by an W equivalent fictitious magnetic charge cylinder as shown in Fig. B ∂ a = ∇ , in ΩI, ΩIII (10) ∂y 4. The surface charge density on the cylinder is defined as [11] s jP()θo− ω m t As the magnetic scalar potential, φ , is defined as ρms(r o , θ o , t )= 2 B r ( r o ) e (27) B where = −µ0 ∇ φ (11) PPP+1 + 1 2 the magnetic scalar potential is related to the SOVP by s 2Brem P (1+µ r )( r i − r o) r o 1 Br () r = (28) P2 r 2P 2r 2PP r + 1 1 ∂Wa (1+ )[(1 −µr ) i − (1 + µr ) o ] φ = − , in ΩI, ΩIII (12) µ0 ∂y is the Halbach rotor radial flux density [15]. Brem= remanent flux density, µr = magnet relative permeability. The origin of C. Boundary Conditions the cylindrical charge sheet is located at (xc,yc,zc)=(0,ro+g,0). The continuity conditions in terms of the TE potential are The scalar potential and magnetic flux density created by this [14] charge cylinder at y=0 can be computed from [11] π w WWWs+ r = II , at y = 0 (13) 2 o /2 a a a so ro ρms(,,)r o θ o t φ (,,,)x y z t = dzo dθ o (29) s r II 4πµ ∫ ∫ r ∂WWW ∂ ∂ 0 0−w /2 MA a+ a = a , at y = 0 (14) o ∂y ∂ y ∂ y 2π w /2 r o ρ(,,)r θ t WWII= III , at y = -h (15) Bso(,,,)x y z t = o ms o o r dz dθ (30) a a 4π ∫ ∫ 3 MA o o 0−w /2 rMA ∂WWII ∂ III o a= a , at y = -h (16) where ∂y ∂ y r MA =(x − ro cos)θ o xˆ + ( y − yc − r o sin)(θ o yˆ + z − zo ) zˆ (31) y Cylindrical magnetic Utilizing (12) the source field in terms of the SOVP is charge sheet s Wa 2π wo /2 so ro W r (xc,yc,zc) A (r ,θ ,z ) W(,,,) x y z t = − ρ(,,)r θ t a . o o o a ms o o z r 4π ∫ ∫ MA 0−w /2 r . o W II W II o M (x,y,z) a a x . ln{rMA + ( y − yc − r o sinθ o )} dz o d θ o (32) III Wa wo The integration with respect to zo is performed analytically Fig. 3 The source, reflected and Fig. 4. Cylindrical magnetic charge transmitted TE potentials. sheet. whereas integration with respect to θo is accomplished numerically. In order to match the modes with the TE III. GENERAL SOLUTION potentials given by (17)-(19), the source field, as given by (32) Applying the separation of variables method to (7) and (9) , is expressed as the following Fourier series within ΩI and noting that the field must decay when moving away from ∞ ∞ S jξ x jk z κ y jP ω t Ws(,,,) x y z t = mn em e n e mn e− m (33) the conductive surface, the solution within each region is a ∑ ∑ 2 m=−∞ n =−∞ κmn ∞ ∞ r I jξm x jk n z− κ mn y where the Fourier coefficients are determined from Wa = ∑ ∑ Cmn e e e (17) m=−∞ n =−∞ 2 w/2 l /2 κ jξ x jk z S mn Wso x z t e−m e − n dxdz ∞ ∞ mn = ∫ ∫ a ( ,0, , ) (34) β y βy j ξ x jk z lw II II mn II − mn λy m n −w/2 − l /2 Wa = ∑ ∑ [Cmn e+ Dmn e] e e e (18) m=−∞ n =−∞ Substituting (17)-(19), (33) into (13)-(16) and solving yields CMP-843 ∞ ∞ ∂Rw (0) jµ σ P r Smn jξ x jk z− jPω t mn 0 W(,,,) x y z t = R() y em e n e m (35) = dmn + µ0 σκmn[ κ mnvy+ s o ]coth (βmn h )/ β mn a ∑ ∑ 2 mn ∂ω d 2 m=−∞ n =−∞ κmn m mn where the reflection function, Rmn(y), is +µ σ[ κ v + s ]{1 − hκcsch2( β h )} (47) 0 mn y o mn mn v s µ0 σ[] κmn y+ o −κ y R() y = e mn (36) ∂Rw (0) µ σκ mn 2 mn 0 mn 2κmn− µ0 σs o + 2 κ mn β mncoth() βmnh = dmn + λ[κmnv y+ s o]()/coth β mnh β mn ∂v d 2 Substituting (35) into (12) and (10), the reflected scalar y mn potential and magnetic flux density are −λh[]() κ v + scsch2 β h (48) mn y o mn ∞ ∞ r Rmn() y jξ x jk z− jPω t 22 φ (,,,)x y z t = S em e n e m (37) and dmn = κmn +γmn + 2κmn β mncoth() β mnh (49) ∑ ∑ mn µ κ m=−∞ n =−∞ o mn Utilizing (39) the angular stiffness constant is obtained from ∞ ∞ ξ m ∂T 1 ∂2U Br jξm x jk n z− jPωm t em m (,,,)x y z t = ∑ ∑ e e e− j xˆ + y ˆ kθ = − = − Re (50) m=−∞ n =−∞ κmn ∂θ ωm ∂t ∂ θ ρms (x ,0, z , t )= constant 2 kn 2 ∞ ∞ |S | − j zˆ S R() y (38) lwP mn mn mn or, kθ = Re[Rmn (0)] (51) κmn ∑ ∑ µ0 m=−∞ n =−∞ κmn Similarly the vertical stiffness can be computed as V.

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