Design of a Novel Modular Axial-Flux Double Rotor Switched Reluctance Drive

energies

Article Design of a Novel Modular Axial-Flux Double Rotor Switched Reluctance Drive

Pere Andrada * , Balduí Blanqué , Eusebi Martínez, José Ignacio Perat, José Antonio Sánchez and Marcel Torrent

GAECE, Department of Electrical Engineering (DEE), Universitat Politècnica de Catalunya UPC BARCELONATECH. EPSEVG, Víctor Balaguer 1, 08800 Vilanova i la Geltrú, Spain; [email protected] (B.B.); [email protected] (E.M.); [email protected] (J.I.P.); [email protected] (J.A.S.); [email protected] (M.T.) * Correspondence: [email protected]

Received: 11 February 2020; Accepted: 28 February 2020; Published: 4 March 2020

Abstract: Nowadays, there is a renewed interest in switched reluctance machines and especially in axial-flux switched reluctance machines (AFSRM). This paper presents a comprehensive design procedure for modular AFSRM with an inner stator and two exterior rotors that have a new distribution of the stator and rotor poles, resulting in short magnetic paths with no flux reversal. After a description of the proposed machine, the output torque equation is derived from a simplified non-linear energy conversion loop and guidelines for its design are given. Once the preliminary sizing has been carried out the different modules of the AFSRM, the magnetically active parts made with SMC, are reshaped or refined using 3D printing and 3D electromagnetic finite element analysis until they reach their definitive shape and dimensions. Finally, an AFSRM has been built following the proposed design procedure and has been validated by experimental measurements.

Keywords: switched reluctance machines; axial-ﬂux machines; modular construction; soft magnetic composites; design of electric drives

1. Introduction Nowadays, many applications such as fans, pumps, machine tools, and electric vehicles demand the use of axial-flux electric machines. Most usual axial-flux electric machines are DC commutator motors, brushless DC motors, or synchronous motors, all of them excited with permanent magnets. These machines can be designed in many diverse ways, such as single stage or multistage, with or without armature slots, with or without armature core, or with internal or external rotors to better adapt to the requirements of application in which they have to be used [1]. The development of these machines has gone in parallel with that of the permanent magnets. However, the abrupt increase of the raw material cost in rare earth permanent magnets happened in 2011, shifted research in the field of electrical machines towards topologies with a lower volume of permanent magnets and even without permanent magnets. This circumstance has renewed the interest for switched reluctance machines, especially in the field of electric traction [2]. Some studies carried out in axial-flux switched reluctance motors (AFSRM) demonstrate that they have higher torque density than conventional radial flux switched reluctance machines [3]. The design of AFSRM is not something new; in 1990 Krishnan et al. [4] were the first to give the basics of the design of AFSRM. Later, Arihara and Akatsu proposed the basic design methodology of a single side AFSRM [5], Labak and Kar [6] presented the design procedure and a prototype of a novel five-phase pancake-shaped AFSRM. Madhavan and Fernandes [7,8] developed a new segmented AFSRM with higher number of rotor segments and gave the basis of its design. Ebrahimi and Feyzi made some contributions to the design of AFSRM

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Energiesmodular2020, stator13, 1161 [9]. Recently, Lin proposed a variation of the output equation for this type of machine2 of 17 [10] and Torkaman et al. [11] provided a review of the design of AFSRM including a comparison, considering the assessment of some performance indices among different topologies. This paper withproposes modular a new stator procedure, [9]. Recently, taking Lin into proposed account saturation, a variation for of the the sizing output of equation a particular for thistype type of an ofAFSRM machine with [10 ]an and inner Torkaman stator and et al.two [11 exterior] provided rotors a reviewthat have of thea new design distribution of AFSRM of the including stator and a comparison,rotor poles consideringresulting in theshort assessment magnetic ofpaths some with performance no flux reversal indices that among have di beenﬀerent presented topologies. in [12–16]. This paperThe machine proposes has a new been procedure, built modularly taking intowith account stator and saturation, rotor poles for themade sizing with of SMC a particular parts. The type paper of anis AFSRM organized with as an follows. inner stator In Section and two 2, a exterior description rotors of that the haveproposed a new AFSRM distribution is presented. of the stator Section and 3 rotordevelops poles resultingthe output in torque short magnetic equation pathsof the with AFSRM. no ﬂux In Section reversal 4 that guidelines have been for presentedsizing the AFSRM in [12–16 are]. Thegiven. machine Section has 5 beenexposes built a methodology modularly with of design stator andof an rotor AFSRM, poles intended made with for SMCthe propulsion parts. The of paper a light iselectric organized scooter, as follows. following In Sectionthe aforementioned2, a description sizing of the guidelines. proposed The AFSRM experimental is presented. assessment Section of3 the developsdesigned the AFSRM output is torque presented equation in Section of the AFSRM. 6. Finally, In Section in Section4 guidelines 7, conclusions for sizing from the this AFSRM study are are given.drawn. Section 5 exposes a methodology of design of an AFSRM, intended for the propulsion of a light electric scooter, following the aforementioned sizing guidelines. The experimental assessment of the designed2. Description AFSRM of is the presented Proposed in Section Axial‐6Flux. Finally, Switched in Section Reluctance7, conclusions Machines from (AFSRM) this study Machine are drawn.

2. DescriptionIn this paper, of the a Proposed three phase Axial-Flux AFSRM Switchedwith an inner Reluctance stator of Machines twelve poles (AFSRM) and two Machine outer rotors with ten poles is designed. It is a particular case of a new type of AFSRM with a new arrangement of In this paper, a three phase AFSRM with an inner stator of twelve poles and two outer rotors with the stator and the rotor poles and with short flux paths without flux reversal, which has been ten poles is designed. It is a particular case of a new type of AFSRM with a new arrangement of the previously reported [13]. The ten ferromagnetic poles of each rotor are joined on the opposite side of stator and the rotor poles and with short ﬂux paths without ﬂux reversal, which has been previously the air‐gap, by means of an annular disk also of ferromagnetic material. The stator is formed by reported [13]. The ten ferromagnetic poles of each rotor are joined on the opposite side of the air-gap, twelve poles of triangular shape of ferromagnetic material, sticking out with the same length at both by means of an annular disk also of ferromagnetic material. The stator is formed by twelve poles ends of an aluminum structural disk attached to a hollow shaft. An exploded view of such a motor is of triangular shape of ferromagnetic material, sticking out with the same length at both ends of an shown in Figure 1. aluminum structural disk attached to a hollow shaft. An exploded view of such a motor is shown in Figure1.

(a) (b)

FigureFigure 1. 1.(a ()a Exploded) Exploded view view of of the the proposed proposed axial-ﬂux axial‐flux switched switched reluctance reluctance machines machines (AFSRM) (AFSRM) (left (left),), (b()b representation) representation of of the the ﬂux flux path path when when there there is is alignment alignment between between stator stator and and rotor rotor poles poles (right (right)[)15 [15].].

AsAs it it is is depicted depicted in in Figure Figure2, two2, two coils coils are are wound wound in in both both opposite opposite ends ends of of the the stator stator poles poles and and connectedconnected in in series. series. The The coils coils of two of two adjacent adjacent stator stator poles poles are also are connected also connected in series in forming series aforming double a electromagnet.double electromagnet. Having the Having described the machinedescribed six machine double six electromagnets, double electromagnets, each phase iseach formed phase by is connectingformed by in connecting parallel the in two parallel double the electromagnets two double diametrically electromagnets opposed. diametrically The terminals opposed. of theThe phaseterminals windings of the leave phase the windings machine leave through the themachine hollow through shaft. the hollow shaft.

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(a) (b)

Figure 2. (a) schematic drawing of the coil’s arrangement of the two double electromagnets diametrically opposed (stator poles 1–2 and 7–8) [15], connected in parallel to form phase A. (b) upper view of the layout of the different phases (red: Phase A; blue: Phase B and black: Phase C).

3. Output Torque Equation for AFSRM The starting point of the( adesign) of electric rotating machines is the output( torqueb) equation, which relates the torque with the main dimensions (bore diameter and axial length in radial flux machines), FigureFigure 2. (2.a ) schematic(a) schematic drawing drawing of the coil’sof the arrangement coil’s arrangement of the two doubleof the electromagnetstwo double electromagnets diametrically magneticopposeddiametrically loading, (stator andopposed poles electric 1–2 (stator and loading 7–8) poles [15 1–2[17].], connected and In 7–8) this [15], paper, in parallel connected the to output form in parallel phase torque to A. form equation (b) upper phase view ofA. the(b) of upperAFSRM the is derivedlayoutview not of the of dilayouttheﬀerent expression of phases the different (red: of the Phase phases average A; (red: blue: torque Phase Phase A;disregarding B blue: and black:Phase PhaseB the and saturation C).black: Phase [4,7,9] C). but of the energy converted into mechanical work in each stroke, energy conversion loop, represented by the 3. Output Torque Equation for AFSRM area3. Output W (OACEO) Torque shown Equation in Figure for AFSRM 3 [18], multiplied by the number of strokes per revolution, s, given by: The starting point of the design of electric rotating machines is the output torque equation, which The starting point of the design of electricm N rotating machines is the output torque equation, which relates the torque with the main dimensions (bore diameter and axial length in radial ﬂux machines), relates the torque with the main dimensionss (bore diameter and axial length in radial flux machines),(1) magnetic loading, and electric loading [17]. In this2π paper, the output torque equation of the AFSRM is magnetic loading, and electric loading [17]. In this paper, the output torque equation of the AFSRM wherederived m not is the of thenumber expression of phases of the and average NR the torque number disregarding of rotor poles. the saturation [4,7,9] but of the energy is derived not of the expression of the average torque disregarding the saturation [4,7,9] but of the convertedIf the phase into mechanical resistance is work neglected in each and stroke, the aligned energy magnetization conversion loop, curve represented is approximated by the by area two W energy converted into mechanical work in each stroke, energy conversion loop, represented by the straight(OACEO) lines, shown OD in with Figure a slope3[18], Lau, multiplied and DB by parallel the number to the of unaligned strokes per magnetization revolution, s, givencurve by: also a area W (OACEO) shown in Figure 3 [18], multiplied by the number of strokes per revolution, s, given straight line, OA, with a slope Lu [15]. The current between points A and C is flat‐topped and has a by: m NR value of IB, being C the point where solid states switches=m N turn‐off in which flux linkage, ψs, is equal (1)to 2π kd Las IB, being Las the saturated inductance, sslope of the straight line OB, and kd the magnetic flux 2π (1) linkagewhere m ratio. is the number of phases and NR the number of rotor poles. where m is the number of phases and NR the number of rotor poles. If the phase resistance is neglected and the aligned magnetization curve is approximated by two straight lines, OD with a slope Lau, and DB parallel to the unaligned magnetization curve also a straight line, OA, with a slope Lu [15]. The current between points A and C is flat‐topped and has a value of IB, being C the point where solid state switches turn‐off in which flux linkage, ψs, is equal to kd Las IB, being Las the saturated inductance, slope of the straight line OB, and kd the magnetic flux linkage ratio.

FigureFigure 3. 3. EnergyEnergy conversion conversion loop. loop.

If the phase resistance is neglected and the aligned magnetization curve is approximated by two straight lines, OD with a slope Lau, and DB parallel to the unaligned magnetization curve also a straight line, OA, with a slope Lu [15]. The current between points A and C is flat-topped and has a value of IB, being C the point where solid state switches turn-off in which flux linkage, ψs, is equal to

Figure 3. Energy conversion loop.

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kd Las IB, being Las the saturated inductance, slope of the straight line OB, and kd the magnetic ﬂux linkage ratio. With these assumptions from Figure3, the torque [19] can be evaluated as:

mN T = sW = R k k L I2 (2) 2π d L as B

Being, kL, inductance ratio given by: Lu 1 Las Lu kL = 1 1 kd − (3) −Lau −2 Lau Lu − The flux linkage is the product of the number of turns of the phase, Nf, by the magnetic flux density in the stator pole (magnetic loading), Bp, and by the area of the stator pole. Due to the particular geometry of the AFSRM, with Do and Di which are outer and inner stator diameters, respectively, and considering that the stator pole angle in the average stator diameter is approximately equal to a stroke, the flux linkage is given by: π ψ 2 2 S= Las IB= Bp Do Di Nf (4) 2NRm − Considering the linear current density (electric loading) as:

2m N I 2m N I 4m N I A = f B = f B = f B (5) ( + ) Do Di πDAV π(D0 + Di) π 2

The torque, substituting (3)–(5) in (2) is:

π 2 2 T = k k BpA D D (Do + D ) (6) 16m d L o − i i Introducing the relationship between inner and outer diameter, diameter ratio ξ, as:

D ξ = i (7) Do

Then the torque equation can be expressed as:

π 3 2 T = k k BpAD (1 + ξ) (1 ξ) (8) 16m d L o − An alternative expression is: 2 T = To(1 + ξ) (1 ξ) (9) − With: π T = k k B AD3 (10) o 16m d L p o Therefore, the torque per unit is given by:

T t = = (1 + ξ)2(1 ξ) (11) To −

The torque per unit with respect to the diameter ratio is depicted in Figure4, where it can be seen that the maximum torque per unit is achieved when the diameter ratio is one third, nevertheless good values of ξ can be obtained in the margin between one ﬁfth and one half. Energies 2020, 13, 1161 5 of 17 Energies 2020, 13, x FOR PEER REVIEW 5 of 17

Figure 4.4. Torque p.u.,p.u., t,t, versusversus diameterdiameter ratioratio ξξ..

InIn regardsregards toto thethe otherother termsterms ofof the the Formula formula (10)(10) thethe followingfollowing considerationsconsiderations shouldshould bebe takentaken intointo account:account:

• BBpp, magnetic loading, maximum stator pole magnetic flux flux density, itit shouldshould bebe lowerlower thanthan • magneticmagnetic fluxflux densitydensity ofof saturationsaturation ofof thethe magneticmagnetic material material used used • A,A, electricelectric loadingloading oror linearlinear currentcurrent density,density, dependsdepends onon coolingcooling conditionsconditions ofof thethe machine,machine, itit • shouldshould bebe comprisedcomprised betweenbetween 75,00075,000 andand 200,000200,000 A A/m./m. • kkd, fluxflux linkage duty cycle, usually isis betweenbetween 0.50.5 andand 0.8.0.8. • d • kkL, inductance ratio, depends on thethe levellevel of saturationsaturation ofof thethe machinemachine • L 4.4. Design Guidelines for the Sizing of the Proposed AFSRMAFSRM TheThe AFSRMAFSRM designdesign procedureprocedure startsstarts withwith thethe completecomplete definitiondefinition ofof allall thethe specificationsspecifications andand restrictionsrestrictions thatthat thethe applicationapplication imposesimposes onon thethe machinemachine toto bebe designed.designed. After thethe selectionselection ofof thethe materialsmaterials toto bebe used,used, thethe mainmain dimensionsdimensions areare obtainedobtained usingusing thethe outputoutput torquetorque equation.equation. Then, by meansmeans ofof simplesimple formulas,formulas, thethe geometrical dimensionsdimensions ofof thethe stator, thethe numbernumber ofof turnsturns ofof thethe phase windings,windings, andand thethe geometricalgeometrical dimensionsdimensions ofof thethe twotwo rotorsrotors are are determined. determined.

4.1.4.1. Design of Stator Polar Pieces and Determination ofof thethe NumberNumber ofof TurnsTurns PerPer Pole Pole TheThe statorstator polepole piecespieces areare arrangedarranged accordingaccording to FigureFigure5 5,, where where γ γ thethe angleangle betweenbetween axesaxes ofof consecutiveconsecutive doubledouble electromagnets,electromagnets, withwith ZZ numbernumber ofof doubledouble electromagnets,electromagnets, is is given given by: by: 360o γ 360 (12) γ = Z (12) Z And δ is the angle between axes of stator poles of two consecutive double electromagnets: And δ is the angle between axes of stator poles of two consecutive double electromagnets: 360 N Z δ o (13) 360 [ZN NR Z] δ = − (13) ZNR

The polar pieces of the stator have a triangular shape and considering that in the stator pieces whose symmetry axis form an angle δ the inner sides of these pieces are parallel, that is, the distance ws is constant between Do and Di along the two stator poles, the angle ϕs is approximately:

ϕs = 2δ(1 ξ) (14) −

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Figure 5. Stator section showing the disposition and dimensions of stator poles.

InThe practice, polar pieces it could of bethe advisable, stator have in a order triangular to reduce shape stator and mass, considering that: that in the stator pieces whose symmetry axis form an angle δ the inner sides of these pieces are parallel, that is, the distance ws is constant between Do and Di along the ϕtwos < stator2δ(1 poles,ξ) the angle φs is approximately: (15) − Being: φ 2δ1ξ (14) δ In practice, it could be advisable, in orderws =to Dreducesin stator mass, that: (16) i 2

Therefore, the area of a stator pole, Asp,φ is: 2𝛿1ξ (15)

Being: 1 2 ϕs Asp= (D0 Di) tg (17) 4 − δ 2 w Dsin (16) Otherwise, the voltage equation of the AFSRM can2 be approximated, being ω the speed of the machineTherefore, and neglecting the area of phase a stator resistance, pole, A by:sp, is: 1 φ dψ ∆ψ ∆ψ θ ∆ψ s Asp= Ds D tgs ∆ s (17) U = RfI 4 2 = ω (18) dt − ≈ ∆t ≈ ∆θ ∆t ∆θ Otherwise, the voltage equation of the AFSRM can be approximated, being ω the speed of the From Figure3, the increment of ﬂux linkage between points A and C can be given by: machine and neglecting phase resistance, by:

d∆ψψ = k LasΔψI = kΔψN ΔθBpAspΔψ (19) U Rs Id B d f ω (18) dt Δt Δθ Δt Δθ And the variation of position between points A and C can be approximate by: From Figure 3, the increment of flux linkage between points A and C can be given by: Δψ = k L I = 2kπ N B A ∆ θas sp (20)(19) ≈ mNR And the variation of position between points A and C can be approximate by: Then the number of turns per phase, Nf, is given by: 2π Δθ (20) 2mNπU Nf= (21) m NRkdBpAspω Then the number of turns per phase, Nf, is given by:

The number of turns per coil, Np, depends on the2π numberU of double electromagnets per phase, two N= (21) in the AFRM considered, each consisting of fourm N coils,kB andAsp ofω their connection (x = 1 series connection and x = 1/2 for parallel connection), thus: The number of turns per coil, Np, depends on the number of double electromagnets per phase, two in the AFRM considered, each consisting of four coils, and of their connection (x = 1 series connection and x = 1/2 for parallel connection), thus:

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N N =N (22) N =p 8xf (22) p 8x The wire section, sc, considering the connection of the double electromagnets per phase, is determinedThe wire by section, the ratio sc ,between considering RMS the (Root connection Mean Square of the value) double phase electromagnets current I and per the phase, current is determineddensity, Δ, as: by the ratio between RMS (Root Mean Square value) phase current I and the current density, ∆, as: xx I I s s== (23)(23) c ∆Δ

TheThe copper copper area, area, A Acucu,, intointo aa slotslot isis approximatelyapproximately equalequal to: to:

Acu 2Ns= khw (24) Acu = 2Npsc= kvhews (24)

Being kv the slot fill factor, and he the height of the protruding stator pole at one side of the structuralBeing disk: kv the slot ﬁll factor, and he the height of the protruding stator pole at one side of the structural disk: 2N2Npssc he h== (25)(25) kkvwws

TheThe total total height height of of a a stator stator pole, pole, h hetet,, is:is: h 2h h het et= 2he + hcece (26)(26)

With hce thickness of stator structural disk, Figure 6. With hce thickness of stator structural disk, Figure6.

Figure 6. Arrangement of stator poles into the structural disk. Figure 6. Arrangement of stator poles into the structural disk.

4.2.4.2. Design Design of of Rotor Rotor Pole Pole and and Yoke Yoke TheThe rotor rotor pole pole pieces pieces are are equally equally displaced, displaced, Figure Figure7 ,7, by by the the angle angleα α equalequal to: to: 360 α 360o (27) α = N (27) NR The rotor poles have the same length, half the difference between outer an inner diameter than The rotor poles have the same length, half the diﬀerence between outer an inner diameter than stator poles. The angle φr should verify: stator poles. The angle ϕr should verify: φ φ ϕr > ϕs (28)(28) TheThe height height of of the the rotor rotor pole, pole, Figure Figure8 ,8, can can be be estimated estimated approximately approximately by: by: 1 h 1 h (29) hr 3he (29) ≈ 3

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Figure 7. Rotor section showing disposition and dimensions of rotor poles.

The thickness of the rotor yoke, hcr, is determined considering that the yoke area is approximately half than the rotor pole area in order to reduce mass and despite this increase the flux density in the rotor yoke, therefore the yoke area can be expressed by:

D D 1 D D D D φ h 2 tg (30) 2 cr 4 2 2 2

D D φ h tg (31) cr 4 2 The air‐gap, g, should be as small as possible compatible with mechanical tolerances but it is advisable not to be less than 0.5 mm. Active axial length of the AFSRM, LAX, excluding the thickness of the two covers is equal to:

LAX 2g2h hce+ 2h 2 hcr (32) FigureFigure 7. Rotor 7. Rotor section section showing showing disposition disposition and and dimensions dimensions of of rotor rotor poles.poles.

D D 1 D D D D φ h 2 tg (30) 2 cr 4 2 2 2

LAX 2g2h hce+ 2h 2 hcr (32) FigureFigure 8. 8.Arrangement Arrangement of of rotor rotor poles poles andand rotorrotor yoke.

5. Methodology of Design The thickness of the rotor yoke, hcr, is determined considering that the yoke area is approximately half thanIn the order rotor to design pole area an AFSRM in order intended to reduce for mass the propulsion and despite of a this light increase electric thescooter, ﬂux the density following in the rotormethodology yoke, therefore is proposed. the yoke Once area the can materials be expressed are selected, by: the geometry is defined, and the number of turns per phase is determined according the sizing guidelines are exposed in Section 4, some adjustments will be made( dueDo toD thei) modular1 ( Dconstructiono Di) (D ofo theD machinei) ϕr using 3D printing and 3D − hcr − 2 − tg (30) 2 ≈ 4 2 2 2

(Do Di) ϕr hcr − tg (31) ≈ 4 2 The air-gap, g, should be as small as possible compatible with mechanical tolerances but it is advisable not to be less than 0.5 mm. Active axial length of the AFSRM, LAX, excluding the thickness of the two covers is equal to: Figure 8. Arrangement of rotor poles and rotor yoke. LAX = 2g + 2he + hce+ 2hr + 2 hcr (32) 5. Methodology of Design 5. Methodology of Design In order to design an AFSRM intended for the propulsion of a light electric scooter, the following methodologyIn order to design is proposed. an AFSRM Once intended the materials for the are propulsion selected, the of geometry a light electric is defined, scooter, and the the following number methodologyof turns per is proposed.phase is determined Once the materials according are the selected, sizing guidelines the geometry are exposed is deﬁned, in andSection the 4, number some of turnsadjustments per phase will is be determined made due to according the modular the construction sizing guidelines of the machine are exposed using 3D in Sectionprinting4 and, some 3D

Energies 2020, 13, 1161 9 of 17 Energies 2020, 13, x FOR PEER REVIEW 9 of 17 Energies 2020, 13, x FOR PEER REVIEW 9 of 17 adjustmentsanalysis of electromagnetic will be made due fields to the with modular finite construction elements of(3D the FEA). machine Finally, using once 3D printing the definitive and 3D analysis of electromagnetic fields with finite elements (3D FEA). Finally, once the definitive analysisdimensions of electromagnetic and parameters fields of the with AFSRM finite have elements been (3D specified, FEA). Finally, an electromagnetic once the definitive evaluation dimensions of the dimensions and parameters of the AFSRM have been specified, an electromagnetic evaluation of the andAFSRM parameters is will be of thecarried AFSRM out haveby means been specified, of 3D FEA. an electromagneticIt is important to evaluation add that of the the design AFSRM process is will AFSRM is will be carried out by means of 3D FEA. It is important to add that the design process beshould carried considerer out by means a thermal of 3D analysis FEA. It in is operational important to conditions, add that the but design this will process be an objective should considerer of a future a should considerer a thermal analysis in operational conditions, but this will be an objective of a future thermalpaper. analysis in operational conditions, but this will be an objective of a future paper. paper. 5.1. Specifications and Restrictions 5.1. Specifications and Restrictions 5.1. Specifications and Restrictions The AFSRM must develop a torque of 80 Nm at 335 rpm and provide a constant power of 2.8 kW The AFSRM must develop a torque of 80 Nm at 335 rpm and provide a constant power of 2.8 betweenThe 335 AFSRM rpm and must 1215 develop rpm. Due a torque to the of machine 80 Nm at has 335 to rpm be placed and provide inside a constant wheel of power 13 the of 2.8 kW between 335 rpm and 1215 rpm. Due to the machine has to be placed inside a wheel of 13’’00 the maximumkW between external 335 dimensions rpm and 1215 were rpm. limited Due to the a diameter machine of has the to motor be placed frame inside of 308 a wheel mm and of 13’’ an the maximum external dimensions were limited to a diameter of the motor frame of 308 mm and an axial axialmaximum length of 116external mm. dimensions The battery were voltage limited was 96to a V diameter and the AFSRM of the motor was controlledframe of 308 using mm the and SRM an axial length of 116 mm. The battery voltage was 96 V and the AFSRM was controlled using the SRM controllerlength shown of 116 in mm. Figure The9 andbattery described voltage in was [20]. 96 V and the AFSRM was controlled using the SRM controller shown in Figure 9 and described in [20]. controller shown in Figure 9 and described in [20].

Figure 9. View of the power electronic controller. FigureFigure 9. View 9. View of the of power the power electronic electronic controller. controller. The construction of the magnetic circuit of the proposed AFSRM, due to the difficulties of built TheThe construction construction of the of magnetic the magnetic circuit circuit of the of proposed the proposed AFSRM, AFSRM, due to due the to di ﬃtheculties difficulties of built of it built it with electrical steel, was made using SMC (Soft Magnetic Composites). First, commercially withit electrical with electrical steel, was steel, made was using made SMC using (Soft SMC Magnetic (Soft Composites). Magnetic Composites). First, commercially First, commercially available available Somaloy 700 3P was the selected material but finally a SMC material, with a density of3 7.33 Somaloyavailable 700 3P Somaloy was the 700 selected 3P was material the selected but ﬁnally material a SMC but material,finally a SMC with amaterial, density ofwith 7.33 a density g/cm and of 7.33 g/cm3 and with the B‐H curve shown in Figure 10, was used. withg/cm the B-H3 and curve with shownthe B‐H in curve Figure shown 10, was in used.Figure 10, was used.

Figure 10. B‐H curve of the SMC used. FigureFigure 10. B-H 10. B curve‐H curve of the of SMC the SMC used. used.

Energies 2020, 13, x FOR PEER REVIEW 10 of 17 Energies 2020, 13, 1161 10 of 17 5.2. First Approach to the Dimensions of the AFSRM

5.2. FirstThe ApproachAFSRM to to be the designed Dimensions has of to the be AFSRM able to provide a torque of 80 Nm at 335 rpm. Taking into account the characteristics of the SMC used, the electric loading considered, Bp, is of 1.6 T. Since the plannedThe cooling AFSRM is to natural be designed the electric has to loading, be able toA, provide is of 121,000 a torque A/m. of With 80 Nm these at 335 values rpm. and Taking taking into a account the characteristics of the SMC used, the electric loading considered, Bp, is of 1.6 T. Since the diameter ratio, ξ, of 0.5, a magnetic flux linkage duty cycle, kd, of 0.8 and estimating inductance ratio, planned cooling is natural the electric loading, A, is of 121,000 A/m. With these values and taking a kL, about 0.4 an outer diameter of 260 mm is obtained from output torque equation (8). Value that, of ξ course,diameter should ratio, be, oflower 0.5, athan magnetic the external ﬂux linkage diameter duty of cycle, the frame. kd, of Then 0.8 and following estimating the inductanceproposed sizing ratio, guidelines,kL, about 0.4 the an main outer dimensions diameter of of 260 the mm stator is obtainedand rotor from as well output as the torque main Equationwinding data (8). Value are obtained that, of ascourse, it is shown should in be the lower second than column the external of Table diameter A1 in ofthe the Appendix frame. Then A. It followingis important the to proposed point out sizing that theguidelines, number the of turns main dimensionsper pole is calculated of the stator according and rotor to as formula well as (21) the mainconsidering winding that data AFSRM are obtained has to giveas it is2.8 shown kW at in 1215 the rpm second with column an RMS of Tablecurrent A1 of in 60 the A Appendix and a currentA. It isdensity important of 5 A/mm to point2. out that the number of turns per pole is calculated according to Formula (21) considering that AFSRM has to give 2 5.3.2.8 kWDefinitive at 1215 Design rpm with of the an SMC RMS Parts current of 60 A and a current density of 5 A/mm .

5.3. DefinitiveThe modular Design construction of the SMC of Parts the AFSRM, the use of SMC, and the fact that its different parts, in addition to channeling the magnetic flux, must give mechanical consistency to the AFSRM, so that The modular construction of the AFSRM, the use of SMC, and the fact that its different parts, in stator poles must fit in the structural disk and rotor poles must be glued on the covers, makes the addition to channeling the magnetic flux, must give mechanical consistency to the AFSRM, so that dimensions determined by the above‐mentioned procedure should be slightly modified. At this stator poles must fit in the structural disk and rotor poles must be glued on the covers, makes the point, it is important to keep in mind that in prototyping, the pieces of SMC are made by machining dimensions determined by the above-mentioned procedure should be slightly modified. At this point, blocks of SMC with dimensional limitations that condition the size of the pieces to be performed, in it is important to keep in mind that in prototyping, the pieces of SMC are made by machining blocks of addition some machining operations should be avoided, circumstances that restricts the assembly SMC with dimensional limitations that condition the size of the pieces to be performed, in addition possibilities of the SMC parts in the machine. In order to deal with them, two tools have been of great some machining operations should be avoided, circumstances that restricts the assembly possibilities importance to refine and reshape the parts of SMC of the machine. The first is 3D printing that enables of the SMC parts in the machine. In order to deal with them, two tools have been of great importance to easily assess the constructive modifications of the parts introduced to ease the assembly of the to refine and reshape the parts of SMC of the machine. The first is 3D printing that enables to easily different parts into the machine. The second is 3D‐FEA [21] that, in addition to enabling the assess the constructive modifications of the parts introduced to ease the assembly of the different parts determination of the magnetization and static torque curves that provide an assessment of the into the machine. The second is 3D-FEA [21] that, in addition to enabling the determination of the AFSRM performance, allows observing the variations in the magnetic flux density due to the magnetization and static torque curves that provide an assessment of the AFSRM performance, allows modifications made in the stator or rotor poles, which facilitates the evaluation of the convenience of observing the variations in the magnetic flux density due to the modifications made in the stator or carrying out these modifications. rotor poles, which facilitates the evaluation of the convenience of carrying out these modifications. Figure 11 depicts the disposition of stator poles into the structural disk (a) made by means of 3D Figure 11 depicts the disposition of stator poles into the structural disk (a) made by means of 3D printing and the 3D‐FEA (b) showing the map of flux magnetic density when there is full alignment printing and the 3D-FEA (b) showing the map of flux magnetic density when there is full alignment with the rotor pole for a current of 100 A. with the rotor pole for a current of 100 A.

(a) (b)

Figure 11. (a) Stator pole arrangement into into the the structural disk with parts made by 3 D printing and (b) map of magnetic flux ﬂux density in the stator poles obtained by 3D 3D-FEA‐FEA when there is full alignment with the rotor poles for a current of 100100 A.A.

Figure 12 shows the evolution of the stator polar pieces from the first’s pieces obtained by 3D printing to the final of SMC. The evolution of the rotor pole pieces is also depicted in Figure 13. The final pieces of SMC both of stator and rotor are those that best respond, in shape and dimensions, to

Energies 2020, 13, 1161 11 of 17

Figure 12 shows the evolution of the stator polar pieces from the first’s pieces obtained by 3D printing to the final of SMC. The evolution of the rotor pole pieces is also depicted in Figure 13. The Energies 2020, 13, x FOR PEER REVIEW 11 of 17 finalEnergies pieces 2020, 13 of, x SMC FOR PEER both REVIEW of stator and rotor are those that best respond, in shape and dimensions,11 of to17 the conditioning factors of the materials, to the structural requirements, and to a minimization of its the conditioning factors of the materials, to the structural requirements, and to a minimization of its mass. Circumstances that modify some of the dimensions or parameters of the machine are indicated mass. Circumstances that modify some of the dimensions or parameters of the machine are indicated in the third column (final value) of Table A1 of the AppendixA. in the third column (final value) of Table A1 of the Appendix A.

Figure 12. Evolution of stator pole pieces.

Figure 13. Evolution of rotor pole pieces. Figure 13. Evolution of rotor pole pieces.

5.4. 3D Finite Element Analysis Once all the dimensions and parameters of the AFSRM prototype are completely defined, defined, as it was mentionedmentioned in in Section Section 5.3 5.3,, the the magnetization magnetization curves curves and and the the static static torque torque curves curves of the of the machine machine can becan obtained be obtained by 3D by FEA. 3D FEA. In Figure In Figure 14, magnetization 14, magnetization curves curves are represented, are represented, while while in Figure in Figure 15, static 15, torquestatic torque curves curves for diff erentfor different values of values current of are current shown. are The shown. information The information given from these given curves from can these be enoughcurves can for abe first enough assessment for a offirst the assessment AFSRM or can of bethe used AFSRM in a moreor can complete be used simulation in a more considering, complete besidessimulation AFSRM, considering, the power besides converter AFSRM, and the control power converter strategies and performed control strategies using Matlab-Simulink performed using as describedMatlab‐Simulink in [12]. as Anyway, described once in the[12]. simulation Anyway, hasonce shown the simulation that the proposed has shown solution that the matches proposed the previoussolution matches expectations the previous of the drive expectations it is time toof build the drive a prototype it is time with to the build corrected a prototype values with derived the ofcorrected the exposed values design derived procedure. of the exposed In Figure design 16a, procedure. a view of the In completeFigure 16a, stator, a view including of the complete winding, andstator, the including two covers winding, with the and rotor the poletwo covers pieces gluedwith the to themrotor canpole be pieces seen, glued while to Figure them 16canb showsbe seen, a photographywhile Figure 16b of the shows complete a photography AFSRM prototype. of the complete AFSRM prototype.

Energies 2020, 13, x FOR PEER REVIEW 12 of 17

Energies 2020, 13, 1161 12 of 17 Energies 2020, 13, x FOR PEER REVIEW 12 of 17 Energies 2020, 13, x FOR PEER REVIEW 12 of 17

Figure 14. Aligned and unaligned magnetization curves for AFSRM prototype.

Figure 14. Aligned and unaligned magnetization curves for AFSRM prototype. FigureFigure 14. 14.Aligned Aligned and and unaligned unaligned magnetization magnetization curves curves for for AFSRM AFSRM prototype. prototype.

Figure 15. Static torque curves for AFSRM prototype. . . Figure 15. Static torque curves for AFSRM prototype. Figure 15. Static torque curves for AFSRM prototype. Figure 15. Static torque curves for AFSRM prototype.

(a) (b)

Figure 16. (a) Stator and the two motor covers, showing in the central part the SMC rotor pieces glued Figure 16. (a) Stator and the two(a) motor covers, showing in the central part the SMC rotor(b) pieces glued on the cover and (b) view of the(a ) fully assembled AFSRM prototype.prototype (b) Figure 16. (a) Stator and the two motor covers, showing in the central part the SMC rotor pieces glued Figure 16. (a) Stator and the two motor covers, showing in the central part the SMC rotor pieces glued 6. Experimental on the cover Assessment and (b) view of thethe fully Designed assembled AFSRM AFSRM prototype on the cover and (b) view of the fully assembled AFSRM prototype Once the AFSRM and the controller were built, the drive was tested in a test rig in order to assess 6. Experimental Assessment of the Designed AFSRM its6. performance.Experimental Assessment of the Designed AFSRM Once the AFSRM and the controller were built, the drive was tested in a test rig in order to assess Once the AFSRM and the controller were built, the drive was tested in a test rig in order to assess its performance. its performance.

Energies 2020, 13, 1161 13 of 17 Energies 2020, 13, x FOR PEER REVIEW 13 of 17

6.1. Test RigRig DescriptionDescription The AFSRM prototype was tested in thethe testtest rigrig shownshown inin FigureFigure 1717.. TheThe AFSRMAFSRM waswas coupledcoupled throughthrough a torque-meter,torque‐meter, KistlerKistler 4550A500S10N1KA04550A500S10N1KA0 500500 Nm,Nm, toto aa DCDC machinemachine actingacting asas a brake and connected to a DC source variable in voltage and current, APS DPS 100–450, through the mentioned electronic powerpower controllercontroller [[20],20], controlledcontrolled byby a HIL platform, dSPACE ACEACE 1006.1006. The measurements of torque,torque, speedspeed andand powerpower werewere monitoredmonitored byby meansmeans ofof testingtesting powerpower analyzeranalyzer system,system, eDriveeDrive packagepackage GEN2tA-6ch-2MsGEN2tA‐6ch‐2Ms/s/s ofof HBM.HBM.

FigureFigure 17.17. AFSRM drive test rig.

6.2. AFSRM Drive Control InIn order toto meetmeet thethe requirementsrequirements ofof thethe drive,drive, thethe controlcontrol strategystrategy usedused waswas hysteresishysteresis controlcontrol withwith variablevariable turn-onturn‐on and turn-oturn‐offff anglesangles inin lowlow andand mediummedium speedsspeeds andand singlesingle pulsepulse withwith variablevariable turn–onturn–on andand turn-oturn‐offff angles angles for for high high values values of of speed. speed. The The values values of of reference reference current current and and of of the the turn-on turn‐ andon and turn-o turnff‐offangles angles for for each each point point of of operation operation were were determined determined after after many many simulations simulations performed performed withwith Matlab-Simulink,Matlab‐Simulink, using the the results results of of the the finite finite element element analysis analysis of of the the AFSRM AFSRM [12–14]. [12–14 ]. 6.3. Experimental Results of the AFSRM Prototype 6.3. Experimental Results of the AFSRM Prototype The test rig enabled the experimental determination of torque-speed curves of the AFSRM drive, The test rig enabled the experimental determination of torque‐speed curves of the AFSRM drive, keeping the speed constant while the load was increased acting on the DC machine. Figure 18 shows keeping the speed constant while the load was increased acting on the DC machine. Figure 18 shows these curves, including power isolines for better comparison, with the drive expected requirements these curves, including power isolines for better comparison, with the drive expected requirements that are depicted by a dashed red line. The behavior of the drive at different points of operation was that are depicted by a dashed red line. The behavior of the drive at different points of operation was analyzed considering the aforementioned control strategy. Although turn-on and turn-off angles were analyzed considering the aforementioned control strategy. Although turn‐on and turn‐off angles predetermined by simulation, some adjustments had to be performed to adapt to the actual behavior were predetermined by simulation, some adjustments had to be performed to adapt to the actual of the drive. These adjustments were made to determine the angles that provided a lower absorbed behavior of the drive. These adjustments were made to determine the angles that provided a lower absorbed current and a higher output torque, ensuring that the motor temperature was the same for all operating points. From these adjustments a new table of turn‐on and turn‐off angles was obtained

Energies 2020, 13, 1161 14 of 17

Energies 2020, 13, x FOR PEER REVIEW 14 of 17 currentEnergies and 2020 a, 13 higher, x FOR output PEER REVIEW torque, ensuring that the motor temperature was the same for all operating14 of 17 points. From these adjustments a new table of turn-on and turn-oﬀ angles was obtained that was better that was better adapted to the different operating conditions of the AFSRM. Figure 19 shows the adaptedthat was to thebetter diﬀ adaptederent operating to the conditionsdifferent operating of the AFSRM. conditions Figure of 19 the shows AFSRM. the waveforms Figure 19 shows of phase the waveforms of phase current, phase voltage, and bus current for 500 rpm, 60 Nm with hysteresis current,waveforms phase of voltage, phase andcurrent, bus current phase forvoltage, 500 rpm, and 60 bus Nm current with hysteresis for 500 rpm, control 60 (NmθON =with4 ◦hysteresis, θOFF = control (θON = −4°, θOFF = 12°). Figure 20 depicts the waveforms of phase current, phase voltage− and 12control◦). Figure (θON 20 = depicts −4°, θOFF the = waveforms12°). Figure of20 phase depicts current, the waveforms phase voltage of phase and current, bus current phase for voltage 900 rpm, and bus current for 900 rpm, 30 Nm with hysteresis control (θON = −7°, θOFF = 8°) and Figure 21 shows the 30bus Nm current with hysteresis for 900 rpm, control 30 Nm (θON with= hysteresis7◦, θOFF = control8◦) and (θ FigureON = −7°, 21 θ showsOFF = 8°) the and waveforms Figure 21 ofshows phase the waveforms of phase current, phase voltage,− and bus current for 1400 rpm, 20 Nm with single pulse current,waveforms phase of voltage, phase current, and bus phase current voltage, for 1400 and rpm, bus 20current Nm with for 1400 single rpm, pulse 20 Nm control with (θ singleON = pulse7◦, control (θON = −7°, θOFF = 8°). − θOFFcontrol= 8◦ ).(θON = −7°, θOFF = 8°).

Figure 18. Torque‐speed curves experimentally obtained on the test rig. Drive requirements depicted FigureFigure 18. 18.Torque-speed Torque‐speed curves curves experimentally experimentally obtained obtained on on the the test test rig. rig. Drive Drive requirements requirements depicted depicted by the red dashed line. byby the the red red dashed dashed line. line.

Figure 19. Experimental waveforms of phase current, phase voltage and bus current for 500 rpm, 60 FigureFigure 19. 19.Experimental Experimental waveforms waveforms of of phase phase current,current, phasephase voltage and bus current current for for 500 500 rpm, rpm, 60 Nm60 Nm with with hysteresis hysteresis control control θONθ = −4°,= θ4OFF, θ= 12°.= 12 . Nm with hysteresis control θONON = −4°,− θ◦ OFFOFF = 12°. ◦

Energies 2020, 13, 1161 15 of 17 Energies 2020, 13, x FOR PEER REVIEW 15 of 17

FigureFigure 20. 20. ExperimentalExperimental waveforms waveforms of of phase phase current, current, phase phase voltage voltage and and bus bus current current for for900 900rpm, rpm, 30 Nm30 Nm with with hysteresis hysteresis control control θONθ = −7°,= θ7OFF, =θ 8°. = 8 . ON − ◦ OFF ◦

Phase Voltage [V]] 100

-50

-100 Phase Current [A] 120 100

0 Bus Current [A] 100

40 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.021 Time (s) FigureFigure 21. 21. ExperimentalExperimental waveforms waveforms of of phase phase current, current, phase phase voltage voltage and and bus bus current current for for1400 1400 rpm, rpm, 20 Nm20 Nm with with single single pulse pulse control control θONθ = −7°,= θ7OFF, =θ 8°. = 8 . ON − ◦ OFF ◦ 7.7. Conclusions Conclusions ThisThis paper paper presents presents a a comprehensive comprehensive design design procedure procedure for for an an AFSRM AFSRM with with an an inner inner stator stator and and twotwo exterior rotorsrotors that that have have a newa new distribution distribution of the of stator the stator and rotor and poles rotor resulting poles resulting in short magneticin short magneticpaths with paths no flux with reversal. no flux In reversal. the proposed In the procedure, proposed the procedure, output torque the output equation torque is derived equation from is a derivedsimplified from non-linear a simplified energy non conversion‐linear energy loop conversion therefore loop considering therefore saturation. considering Then, saturation. guidelines Then, for guidelinesthe sizing offor the the stator, sizing rotors, of the and stator, winding rotors, are and given. winding The designare given. is refined The design using is 3D refined printing using and 3D 3D printingFEEA. Following and 3D FEEA. the proposed Following design, the proposed an AFSRM design, prototype an AFSRM was built.prototype Finally, was the built. prototype Finally, wasthe prototypetested in a was test rig.tested Experimental in a test rig. results Experimental proved the goodnessresults proved of the proposedthe goodness procedure. of the proposed procedure. Author Contributions: Conceptualization, P.A.; Methodology, P.A.; Simulation, E.M., B.B., M.T.; Mounting, B.B. Authorand J.A.S.; Contributions: Tests, B.B., J.I.P., Conceptualization, and M.T.; Writing—original P.A.; Methodology, draft preparation,P.A.; Simulation, P.A. E.M., and M.T.; B.B., Writing—reviewM.T.; Mounting, B.B. and andediting, J.A.S.; P.A., Tests, B.B., B.B., E.M., J.I.P., J.I.P., and J.A.S.M.T.; andWriting—original M.T. All authors draft have preparation, read and P.A. agreed and toM.T.; the Writing—review published version and of the manuscript. editing, P.A., B.B., E.M., J.I.P., J.A.S. and M.T. Funding: This research was supported by Spanish Ministry of Economy and Competitiveness (DPI 2014-57086-R Funding:and FEDER This Funds. research was supported by Spanish Ministry of Economy and Competitiveness (DPI 2014‐57086‐ R and FEDER Funds. Acknowledgments: Authors would like to thank AMES S.A. for providing the SMC pieces and specially Mark Acknowledgments:Dougan Chief Metallurgist, Authors Dept. would of like R&D to AMES thank S.A. AMES and S.A. José forAntonio providing Calero the R&D SMC Manager pieces and of AMES specially S.A. Dr. for Marktheir support.Dougan Chief Metallurgist, Dept. of R&D AMES S.A. and Dr. José Antonio Calero R&D Manager of AMES S.A.Conflicts for their of Interest:support. Authors declare no conflict of interest.

Conflicts of Interest: Authors declare no conflict of interest.

Energies 2020, 13, 1161 16 of 17

Appendix A. Main dimensions of the AFSRM prototype

Table A1. Main dimensions and parameters of the AFSRM prototype.

Dimension Value Final Value

Do 260 mm 260 mm ξ 0.5 0.45 Di 130 mm 117.9 mm D 75 mm 75 mm α 36◦ 36◦ γ 60◦ 60◦ δ 24◦ 24◦ ϕs 24◦ 20◦ ϕr 26◦ 24.35◦ ws 27.03 mm 20.47 mm g 0.5 mm 0.6 mm Nf 108 turns 108 turns x 1 1/2 Np 16 turns 32 turns he 25.83 mm 28.9 mm 2 2 sc 10.91 mm 4.78 mm hce 12 mm 12 mm kv 0.5 0.57 hr 8.61 mm 10 mm hcr 7.5 mm 8 mm Lax 96.88 mm 108.8 mm

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