energies

Article The Design of a Permanent In-Wheel Motor with Dual- and Dual-Field-Excitation Used in Electric Vehicles

Peng Gao *, Yuxi Gu and Xiaoyuan Wang

School of Electrical and Information Engineering, Tianjin University, No. 92 Weijin Road, Tianjin 300072, China; [email protected] (Y.G.); [email protected] (X.W.) * Correspondence: [email protected]; Tel.: +86-022-2740-6705

Received: 15 January 2018; Accepted: 8 February 2018; Published: 12 February 2018

Abstract: The in-wheel motor has received more attention owing to its simple structure, high transmission efficiency, flexible control, and easy integration design. It is difficult to achieve high performance with conventional motors due to their dimensions and structure. This paper presents a new dual-stator and dual-field-excitation permanent-magnet in-wheel motor (DDPMIM) that is based on the structure of the conventional in-wheel motor and the structure of both the radial and axial magnetic field motor. The finite element analysis (FEA) model of the DDPMIM is established and compared with that of the conventional in-wheel motor. The results show that the DDPMIM achieves a higher output torque at low speeds and that the flux-weakening control strategy is not needed in the full speed range.

Keywords: dual-stator and dual-field-excitation permanent-magnet in-wheel motors (DDPMIM); back electromotive force (EMF); electric vehicles (EVs); in-wheel motors; axial flux motor; radial flux motor

1. Introduction Electric vehicles (EVs) will soon become a common mode of transport. Electric motors used in EVs have to satisfy many requirements: (1) a high power density and a high torque density; (2) a high output torque at low speeds and a high power output at high speeds; (3) an ability to operate within a wide range of speed in a highly efficient manner; (4) high reliability; and (5) low cost [1]. The in-wheel motor used in an EV has gained more attention owing to its structure, its high transmission efficiency, its flexible control, and its easy integration design [2–4]. However, many problems have yet to be solved: Since the motor is mounted inside the wheel, both the riding comfort and the handling safety are affected as the unsprung mass is increased. The structure and size of the motor are limited by the wheel and the structural characteristics of the EV’s suspension. The power density and torque density of EVs also need to be improved. In order to improve the force indexes of in-wheel motors, many advanced technologies have been employed. External construction has been used to increase the utilization of the inner space of the wheel. Fractional slots have been used to effectively reduce the ends of the windings and the stator tooth harmonic components, improving the utilization of the windings [5]. The axial length of the motor has also been shortened [6–8]. High-quality silicon steel plates and permanent (PMs) have been used to decrease loss and improve efficiency. However, dramatic increases in performance have not gained using a conventional in-wheel motor structure. Both the method of excitation and the low inductance make it difficult to output a constant torque under the flux-weakening control strategy at high speed. One alternative that has been considered is the use of switched stator windings [9], whereby two or more different windings are included in one machine. They are connected in series at

Energies 2018, 11, 424; doi:10.3390/en11020424 www.mdpi.com/journal/energies Energies 2018, 11, 424 2 of 13

Energies 2018, 11, x 2 of 13 low speeds and connected in parallel at high speeds [10–13]. This has improved output torque capacity switched stator windings [9], whereby two or more different windings are included in one machine. at low speeds and extended the wide range of high speeds. They are connected in series at low speeds and connected in parallel at high speeds [10–13]. This has Aimed at the existing problems of the conventional in-wheel motor, the dual-winding and improved output torque capacity at low speeds and extended the wide range of high speeds. dual-field-excitation permanent-magnet in-wheel motor (DDPMIM) is proposed in this paper based on Aimed at the existing problems of the conventional in-wheel motor, the dual-winding and dual- bothfield-excitation the structure permanent-magnet of the conventional in-wheel in-wheel motor motor (DDPMIM) and the traits is ofproposed the axial-flux in this permanent-magnet paper based on machineboth the (AFPM). structure Firstly, of the thisconventional paper describes in-wheel the motor structure and the of thetraits DDPMIM. of the axial-flux Secondly, permanent- the design ofmagnet a DDPMIM machine is described (AFPM). Firstly, based onthis the paper structural describes dimensions the structure of anof in-wheelthe DDPMIM. motor. Secondly, The winding the switchingdesign of circuit a DDPMIM and process is described are detailed. based Aon finitethe stru elementctural analysisdimensions (FEA) of an model in-wheel of the motor. DDPMIM The is established,winding switching and the back circuit electromotive and process force are (EMF)detailed. waveform, A finite electromagneticelement analysis torque,(FEA) andmodel mechanical of the characteristicsDDPMIM is established, of the motor and are the analyzed. back electromotive Results show force that (EMF) the waveform, DDPMIM electromagnetic motor has higher torque, torque atand low mechanical speeds, higher characteristics power at of high the speed,motor are and analyzed. a lower Results torque ripple.show that The the flux-weakening DDPMIM motor control has strategyhigher fortorque the DDPMIMat low speeds, is not higher needed power at the at full high speed speed, range. and a lower torque ripple. The flux- weakening control strategy for the DDPMIM is not needed at the full speed range. 2. Work Principle of the DDPMIM 2. Work Principle of the DDPMIM A conventional outer rotor in-wheel motor with a fractional slot winding and a surface-mount HalbachA permanent-magnetconventional outer rotor array in-wheel from a previousmotor with work a fractional was studied slot winding [14]. Table and1 providesa surface-mount the main parametersHalbach permanent-magnet of the conventional array in-wheel from a motor.previous The wo ratedrk was power studied of [14]. the prototypeTable 1 provides is 8.5 kW the andmain the ratedparameters speed is of 650 the r/min conventional (Figure in-wheel1). According motor. to Th electricale rated power machines of the theory, prototype the diameteris 8.5 kW and of the the air gaprated should speed be is designed 650 r/min such (Figure that 1). there According is a substantial to electrical amount machines of unusedtheory, the space diameter inside of the the motor. air Thus,gap itshould is possible be designed that a conventional such that there in-wheel is a subs motortantial can amount be redesigned of unused as aspace DDPMIM inside due the tomotor. the free spaceThus, inside it is possible the motor. that a conventional in-wheel motor can be redesigned as a DDPMIM due to the free space inside the motor. Table 1. Main parameters of the conventional in-wheel motor prototype. Table 1. Main parameters of the conventional in-wheel motor prototype.

ParameterParameter Value Value Unit Unit Rated powerRated power 8.5 8.5 kW kW Rated speedRated speed 650 650 r/min r/min Max speedMax speed 1300 1300 r/min r/min Slot numberSlot number 27 27 - - Pole numberPole number 24 24 - - Stator diameterStator diameter 296.4 296.4 mm mm Stator innerStator diameter inner diameter 195 195 mm mm Rotor diameterRotor diameter 328 328 mm mm Rotor innerRotor diameter inner diameter 298 298 mm mm Number ofNumber turns of of winding turns of winding 13 13 - - Thickness of permanent magnets 5 mm Thickness of permanent magnets 5 mm

FigureFigure 1.1. Prototype of of in-wheel in-wheel motor. motor.

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The DDPMIM designed in this study is shown in Figure 2. It consists of two units: the radial flux The DDPMIM designed in this study is shown in Figure2. It consists of two units: the radial flux motor unit and the AFPM unit. The former unit includes Stator Core 1, a copper wire winding, a rotor motor unit and the AFPM unit. The former unit includes Stator Core 1, a copper wire winding, a rotor yoke, and PMs that are magnetized in the radial direction. Stator Core 1 is made up of a laminated yoke, and PMs that are magnetized in the radial direction. Stator Core 1 is made up of a laminated silicon steel sheet. This unit requires the diameter of the air gap in the traditional structure to be as silicon steel sheet. This unit requires the diameter of the air gap in the traditional structure to be as large as possible. The AFPM unit includes Halbach array PMs, Printed Circuit Board (PCB) Winding large as possible. The AFPM unit includes Halbach array PMs, Printed Circuit Board (PCB) Winding A, A, PCB Winding B, a motor cover, and Stator Core 2, which is made of soft magnetic composites PCB Winding B, a motor cover, and Stator Core 2, which is made of soft magnetic composites (SMCs). (SMCs). The AFPM unit is divided into two parts, the left part and the right part. The AFPM unit is divided into two parts, the left part and the right part.

FigureFigure 2. 2.The The dual-stator dual-stator and and dual-field-excitation dual-field-excitation permanent-magnet permanent-magnet in-wheelin-wheel motormotor (DDPMIM)(DDPMIM) structure.structure. PM: PM: permanent permanent magnet; magnet; PCB: PCB: Printed Printed Circuit Circuit Board. Board.

TheThe ideal ideal torque-speed torque-speed curve curve of of the the DDPMIM DDPMIM is shownis shown in Figurein Figure3. T 3.1 T>1T >2 T>2 T>3 T,3 and, andn1 n<1

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Figure 3. The DDPMIM ideal torque-speed curve. FigureFigure 3.3. TheThe DDPMIMDDPMIM idealideal torque-speedtorque-speed curve. curve. 3. Design of DDPMIM 3.3. DesignDesign ofof DDPMIMDDPMIM 3.1. Analysis of Back EMF 3.1.3.1. AnalysisAnalysis ofof BackBack EMFEMF Based on the theory of the permanent-magnet (PMSM), the induced voltage Based on the theory of the permanent-magnet synchronous motor (PMSM), the induced voltage of theBased copper on conductorthe theory isof proportionalthe permanent-magnet to the back synchronous EMF. The structure motor (PMSM), of the PCB the winding induced is voltage shown of the copper conductor is proportional to the back EMF. The structure of the PCB winding is shown in ofin theFigure copper 4. If conductor the air-gap is flux proportional density in tothe the PMSM back EMF.is a constant The structure B, the induced of the PCB voltage winding for the is showncopper Figure4. If the air-gap flux density in the PMSM is a constant B, the induced voltage for the copper inconductor Figure 4. canIf the be air-gap deduced flux as density in the PMSM is a constant B, the induced voltage for the copper conductorconductor cancan bebe deduceddeduced asas BRω ()22− R Rb 22b2 a 2 eBlvB==ωRb ldl =BRBωω()R −− RR (1) Rb bb a a e = eBlvBBlv=== Bωwω Raldl ldl= = 2 (1)(1) Ra Ra 22 where l is the length of the copper conductor, ω is the angular velocity, Ra is the inner radius of the wherewherel lis is the the length length of of the the copper copper conductor, conductor,ω ωis is the the angular angular velocity,velocity,R Raa isis thethe innerinner radiusradius ofof thethe conductor, and Rb is the outside radius. conductor,conductor, and andR Rbb isis thethe outsideoutside radius.radius.

Rb Rb

l θ l θ

copper coil copper coil

Ra R PCB-board a PCB-board

Figure 4. DDPMIM winding PCB. Figure 4. DDPMIM winding PCB. Figure 4. DDPMIM winding PCB. A high Rb and a low Ra yields a high induced voltage according to Equation (1). If Ra equals zero, A high Rb and a low Ra yields a highR induced voltage according to Equation (1). If Ra equals zero, the inducedA high R voltageb and a islow proportional Ra yields a high to b inducedsquared. voltage In order according to increase to Equation the back (1). EMF, If R thea equals key iszero, to the induced voltage is proportional to Rb squared. In order to increase the back EMF, the key is to the induced voltage is proportional to Rb squared. In order to increase the back EMF, the key is to

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Energies 2018, 11, x 5 of 13 increase the outside diameter of the PCB and make full use of the space near the outside edge and increaseincrease the the magnetic outside flux diameter density of as the much PCB as and possible. make full use of the space near the outside edge and increase the magnetic flux density as much as possible. 3.2. Selection of Slots and Poles 3.2. Selection of Slots and Poles To reduce the back EMF harmonics, the ends of the windings, and the cogging torque in the PMSM,To the reduce fractional-slot the back concentrated EMF harmonics, windings the areends considered. of the windings, After the and selection the cogging of the slots/polestorque in the combination,PMSM, the thefractional-slot electrical angle concentratedα between windings slots equals are considered. to After the selection of the slots/poles combination, the electrical angle α between slots equals to 2pπ α = 2 pπ (2) α =Z (2) Z where Z is the number of slots, and p is the number of pole pairs. where Z is the number of slots, and p is the number of pole pairs. Results of [5] show that it is useful to improve the sinusoidal property of the back EMF waveform Results of [5] show that it is useful to improve the sinusoidal property of the back EMF waveform and increase the utilization of windings, as Z and p satisfy the relationship as follows: and increase the utilization of windings, as Z and p satisfy the relationship as follows: 2p 2≈pZZ≈ (3)(3)

TheThe number number of slotsof slotsZ can Z can be be deduced deduced as as

( ZpiZ=±2 2 () is an even number Z = 2p ± 2i (Z is an even number)  =±− (4)(4) Z=Zpi2p2± (2()( 2i − 1 )( Z Zis an is odd an odd number number) ) wherewherei has i has as lowas low a value a value as possible.as possible. ForFor a PMSM a PMSM with with fractional-slot fractional-slot windings, windings, an an unbalanced unbalanced magnetic magnetic pull pull will will be be produced produced when when thethe denominator denominator of theof the slots slots per per pole pole per per phase phaseq is q anis an even even number number [15 [15].].

3.3.3.3. Structure Structure of AFPMof AFPM TheThe copper copper wire wire winding winding and and the the PCB PCB winding winding are are connected connected in seriesin series when when DDPMIM DDPMIM operates operates withinwithin a low- a low- or or middle-speed middle-speed range. range. To To keep keep the the current current density density in thein the two two windings windings equal equal to eachto each other,other, the the area area of theirof their current-carrying current-carrying conductors conductors should should be be equivalent equivalent or close.or close. The The AFPM AFPM without without teethteeth is consideredis considered first first because because it has it no has cogging no cogging torque. Figuretorque.5 showsFigure a5 cross shows section a cross of the section magnetic of the circuitmagnetic of the circuit AFPM of with the aAFPM PCB statorwith a and PCB no stator teeth. and no teeth.

Figure 5. Analytical model of the generalized magnetic circuit.

The flux density Bm of PM at the working point and the air-gap flex density Bδ can be deduced respectively as [16]

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σKB The flux density Bm of PM at the working= point andFr the air-gap flex density Bδ can be deduced Bm respectively as [16] + δ (5) σKFrμ σKFBr h = M Bm δ (5) σKF + µr B hM B = r δ B δ = σK +r μ (6) Bδ Frδ (6) σK + µr hM F hM wherewhereσ σis is the the fluxflux leakageleakage coefficient,coefficient, δ isis the the air-gap air-gap length, length, KKF Fis isthe the direction direction coefficient coefficient of ofthe the air- air-gapgap flux flux density, density, μµr risis the the relative relative magnetic magnetic permeability, permeability, hM isis the length ofof thethe PMPMin in the the direction direction ofof magnetization, magnetization, and andB rBisr is the the remanent remanent flux flux density density of of PM. PM. EquationsEquations (5) (5) and and (6) (6) indicate indicate that thatB mBmand andB Bδ δare are related related to toδ δand andh MhM.. The The relationships relationships between between BBδ,δ,δ ,δ and, andh MhMis is shown shown in in Figure Figure6 .6.

0.9

0.8

0.7

0.6

Flux density/T Flux 0.5

0.4

0.3 25 20 7 6 15 5 4 3 10 2 1 0 5 Air gap/mm Magnet width/mm Figure 6. Air gap flux density. Figure 6. Air gap flux density.

Figure 6 shows that, as hM increases, Bδ first increases and then decreases. The magnetomotive Figure6 shows that, as hM increases, Bδ first increases and then decreases. The magnetomotive force in the magnetic circuit increased linearly with hM. Reluctance also increased as the permeability of force in the magnetic circuit increased linearly with hM. Reluctance also increased as the permeability ofthe the PM PM approached approached the the air. air. UnderUnder thethe limitationlimitation of the space, an an AFPM AFPM with with a aPCB PCB stator stator and and no noteeth teeth was was designed. designed. The Themodel model was was established established according according to the to thecalculation calculation resu resultlt based based on Figure on Figure 6. The6. The harmonic harmonic spectra spectra were werecalculated calculated via viafast fastFourier Fourier transform transform (FFT). (FFT). Th Thee fundamental fundamental amplitude amplitude of of the the back EMFEMF ofof the the DDPMIMDDPMIM was was 153.5 153.5 V V at at 650 650 r/min. r/min. ComparedCompared with with the the conventional conventional in-wheel in-wheel motor, motor, the the DDPMIM DDPMIM hadhad a a 10.9% 10.9% higher higher amplitude. amplitude. The The output output torque torque of the ofDDPMIM the DDPMIM was143 was N ·143m, whichN·m, which is only is a 15.3%only a improvement15.3% improvement at the same at the rated same current. rated Thecurrent. air-gap The flux air-gap density flux was density relatively was relatively lower due lower to the due large to equivalentthe large equivalent air-gap length. air-gap In addition, length. In the addition, back EMF the and back the EMF output and torque the output decreased torque due decreased to the lower due outsideto the lower radius outside and the radius length and of thethe PCBlength winding. of the PCB Therefore, winding. the Therefore, structure the of the structure AFPM of with the a AFPM PCB statorwith anda PCB no stator teeth isand not no suitable teeth is for not DDPMIM. suitable for DDPMIM. TheThe structure structure of of an an AFPM AFPM was was designed designed with with teeth teeth and and a a PCB PCB stator stator and and is is shown shown in in Figure Figure7 .7. TheThe PM PM disc disc is is arranged arranged in in a a 45 45°◦ Halbach Halbach array. array. There There are are four four magnets magnets in in one one pole. pole.

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FigureFigure 7. 7.3D3D view view of ofthe the axial-flux axial-flux permanent-magnet permanent-magnet machine machine (AFPM) (AFPM) with with a PCB a PCB stator stator and and teeth. teeth.

4.4. Finite Finite Element Element Analysis Analysis (FEA) (FEA) of of Conventional Conventional In-Wheel In-Wheel Motor Motor and and DDPMIM DDPMIM

4.1.4.1. FEA FEA Model Model TheThe formulations formulations of of 3D 3D FEA FEA are are  11 ∂A  ∇×1 ∇×AJ = −σϕ∂A +∇ +1 ∇× M ∇ × ∇μμ × A = Ja −a σ ∂ + ∇ϕ + ∇ × M (7)(7) µ  ∂tt 0µ0

 ∂A∂A  ∇ ∇+∇=σ σϕ+ ∇ϕ =00 (8) ∂t ∂t where A and ϕ are the magnetic vector and electric scalar potentials, respectively, J is the where A and φ are the magnetic vector and electric scalar potentials, respectively,a Ja is the armature currentcurrent density, density, and andM Mis is the the magnetization magnetization of of the the permanent permanent magnet. magnet. TheThe DDPMIM DDPMIM in in this this paper paper is is based based on on the the dimensions dimensions shown shown in in Table Table1. 1. Table Table2 provides 2 provides the the mainmain parameters parameters of of the the DDPMIM. DDPMIM.

Table 2. Main parameters of the DDPMIM. Table 2. Main parameters of the DDPMIM.

ParameterParameter DDPMIM-ValueDDPMIM-Value Unit Unit RatedRated power power 12 12 kW kW SlotSlot number number 36 36 - - PolePole number number 32 32 - - Stator core 1 diameter 296.4 mm Stator core 1 diameter 296.4 mm Stator core 1 inner diameter 210 mm StatorRotor core diameter 1 inner (radial diameter flux unit) 328 210 mm mm RotorRotor diameter inner diameter (radial (radial flux fluxunit) unit) 298 328 mm mm Rotor innerStator diameter 2 core (radial diameter flux unit) 210 298 mm mm StatorStator 2 core innerdiameter diameter 45 210 mm mm PCB winding diameter 195 mm StatorPCB 2 windingcore inner inner diameter diameter 60 45 mm mm RotorPCB winding (Halbach diameter PMs) diameter 190 195 mm mm PCBRotor winding (Halbach inner PMs) innerdiameter diameter 70 60 mm mm NumberRotor (Halbach of turns of PMs) copper diameter wire winding 9 190 - mm Number of turns of PCB winding (A or B) 8 - Rotor (Halbach PMs) inner diameter 70 mm Thickness of PMs 4.5 mm Number ofThickness turns of copper of Halbach wire PMs winding 5 9 mm - Number of turns of PCB winding (A or B) 8 - Thickness of PMs 4.5 mm It is difficult to calculate the coupling characteristics of the magnetic field between the axial flux Thickness of Halbach PMs 5 mm motor unit and the radial flux motor unit. The FEA model of the DDPMIM is built, as shown in

Energies 2018, 11, x 8 of 13 Energies 2018, 11, 424 8 of 13 It is difficult to calculate the coupling characteristics of the magnetic field between the axial flux motor unit and the radial flux motor unit. The FEA model of the DDPMIM is built, as shown in Figure Figure8a. The initial 3D mesh is also shown in Figure8b. In the 3D FEA, the model is divided into a 8a. The initial 3D mesh is also shown in Figure 8b. In the 3D FEA, the model is divided into a mesh mesh of tetrahedrally shaped elements. The accuracy of the solution highly depends on the size of the of tetrahedrally shaped elements. The accuracy of the solution highly depends on the size of the mesh mesh elements. The adaptive mesh method is used in the FEA software, but it is essential to apply elements. The adaptive mesh method is used in the FEA software, but it is essential to apply a a maximum element size to different components. The calculation accuracy and time are taken into maximum element size to different components. The calculation accuracy and time are taken into consideration. The maximum element sizes of each component of the DDPMIM are shown in Table3. consideration. The maximum element sizes of each component of the DDPMIM are shown in Table 3.

(a) (b)

Figure 8. 3D model of the DDPMIM. (a) 3D FEA model; (b) 3D mesh. Figure 8. 3D model of the DDPMIM. (a) 3D FEA model; (b) 3D mesh. Table 3. Maximum element size of the DDPMIM. Table 3. Maximum element size of the DDPMIM. Components Maximum Element Size-Value Unit AirboxComponents of the whole model Maximum Element Size-Value 5 Unit mm Air gap of the radial flux unit 0.5 mm Airbox of the whole model 5 mm Air gapAir of the gap radial of the flux AFPM unit unit 0.5 0.5 mm mm Air gap of theStator AFPM core unit 1 0.5 1.5 mm mm StatorStator core 1 core 2 1.5 1.5 mm mm Stator coreRotor 2 yoke 1.5 1.5 mm mm Rotor yokePMs 1.52 mmmm PMs 2 mm 4.2. Analysis of Magnetic Field 4.2. Analysis of Magnetic Field The magnetic field of the DDPMIM was calculated. Figure 9 shows the flux density distribution in theThe radial magnetic flux motor. field of Figure the DDPMIM 9a shows was the calculated.flux density Figure distribution9 shows in the the flux calculation density distribution of the static inmagnetic the radial field flux at motor. no lo Figuread. Figure9a shows 9b shows the flux the density flux density distribution distribution in the calculation at some point of the in static the magneticcalculation field of atthe no transient load. Figure magnetic9b shows field. the The flux wi densitynding distributioncurrent is twice at some the pointrated in current the calculation (29.5 A). ofThe the rotor transient speed magnetic is 650 r/min. field. In The both winding cases, the current flux isdensity twice thein the rated area current near the (29.5 inner A). Thediameter rotor speedof the isStator 650 r/min.Core 1 Inis zero. both cases,In addition, the flux the density diameter in theof the area Halbach near the PMs inner of diameterAFPM is smaller of the Stator than Corethe inner 1 is zero.diameter In addition, of the Stator the diameter Core 1, and of the the Halbach magnetic PMs field of generated AFPM is smallerby Halbach than PMs the inneralong diameter the axis. ofBased the Statoron the Coreabove 1, analysis, and the magneticthere is no field magnetic generated couplin by Halbachg between PMs the along two motor the axis. units. Based The on difficulty the above in analysis,control will there not is be no increased. magnetic couplingFigure 10between shows an the arro twow motorplot of units.the magnetic The difficulty flux of in one-fourth control will of notthe beAFPM increased. unit. Figure 10 shows an arrow plot of the magnetic flux of one-fourth of the AFPM unit.

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(a) (b) (a) (b) Figure 9. Flux density distribution. (a) No-load; (b) Load. (a) (b) FigureFigure 9. Flux9. Flux density density distribution. distribution. ( a(a)) No-load; No-load; ((bb)) Load.Load. Figure 9. Flux density distribution. (a) No-load; (b) Load. Stator Rotor Stator Rotor Stator Rotor

Figure 10. Axial magnetic flux arrow plot.

The air gap flux density waveformsFigure 10. Axial of the magnetic axial fl fluxux motor arrow unit plot. and the radial flux motor unit Figure 10. Axial magnetic flux arrow plot. were obtained, as shown in FigureFigure 11. 10. The Axial latter magnetic is a trapezoidal flux arrow plot.waveform, which leads to torque ThedeclineThe air withair gap gap ripple flux flux density under density the waveforms waveformssinusoidal ofline of the currentthe axial axial control flux flux motor motor method. unitunit The andand former the radial has better flux flux featuresmotor motor unit unit suchThe as highair gap magnitude flux density and a waveformsfine sinusoidal of theproperty axial flaccordingux motor to unit the Halbachand the array.radial flux motor unit werewere obtained, obtained, as shownas shown in in Figure Figure 11 11.. The The latter latter is is a a trapezoidal trapezoidal waveform,waveform, which which leads leads to to torque torque declinewere obtained, with ripple as shown under inthe Figure sinusoidal 11. The line latter current is a control trapezoidal method. waveform, The former which has leads better to features torque declinedecline with with ripple ripple under under the the sinusoidal sinusoidal line line current current control control method. method. TheThe former has has better better features features such as high magnitude and1.5 a fine sinusoidal property according to the Halbach array. suchsuch as high as high magnitude magnitude and and a fine a fine sinusoidal sinusoidal property property according according to to the the HalbachHalbach array.array. 1 1.5 0.51.5 1 01 0.5 -0.50.5

Air gap flux density/T flux gap Air 0 -1 0 Axial flux unit -0.5 -1.5 Radial flux unit

Air gap flux density/T flux gap -0.5Air -1 Air gap flux density/T flux gap Air 0 90 180 270 360 Axial flux unit -1 Electric degree/deg -1.5 RadialAxial fluxflux unitunit -1.5 Figure 11.Radial Flux fluxdensity unit distribution. 0 90 180 270 360 0 90 Electric180 degree/deg 270 360

Electric degree/deg

Figure 11. Flux density distribution. Figure 11. Flux density distribution. Figure 11. Flux density distribution.

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Energies 2018, 11, x 10 of 13 Figure 12 presents no-load back EMF waveforms at the rated speed of the two in-wheel motors. Figure 12 presents no-load back EMF waveforms at the rated speed of the two in-wheel motors. Connect-1 represents the copper wire winding, PCB Winding A, and PCB Winding B connected Connect-1 represents the copper wire winding, PCB Winding A, and PCB Winding B connected in in series. Connect-2 represents the copper wire winding and PCB Winding A connected in series. series. Connect-2 represents the copper wire winding and PCB Winding A connected in series. Connect-3 represents the copper wire winding alone. Connect-3 represents the copper wire winding alone.

400

300

200

100

0 EMF/V -100 Conventional -200 Connect-1 Connect-2 -300 Connect-3 -400 0 45 90 135 180 225 270 315 360 Electric degree/deg

Figure 12. Back electromotive force (EMF) waveforms. Figure 12. Back electromotive force (EMF) waveforms.

TheThe harmonic harmonic spectra spectra were were calculated calculated via via FFT, FFT, and and results results are are given given in in Table Table4. Comparison4. Comparison of of thethe fundamental fundamental amplitude amplitude (F (F amp) amp) and and the the total total harmonics harmonics distortion distortion (THD) (THD) shows shows that that the the back back EMFEMF waveform waveform is improvedis improved by by connecting connecting the the axial axial flux flux motor motor unit. unit.

TableTable 4. Main4. Main parameters parameters of of the the in-wheel in-wheel motor. motor. Item Conventional Connect-1 Connect-2 Connect-3 Item Conventional Connect-1 Connect-2 Connect-3 F_amp 251.0 207.9 282.2 356.4 F_ampTHD 251.0 5.6% 207.9 5.2% 282.22% 1.7% 356.4 THD 5.6% 5.2% 2% 1.7% 4.3. Analysis of Load Characteristic 4.3. Analysis of Load Characteristic The circuit of winding switch is shown in Figure 13. There are four switches in one phase used to Thechange circuit the ofconnecting winding switchforms of is shownthe copper in Figure wire winding,13. There PCB are four Winding switches A, and in one PCB phase Winding used B. toThe change DDPMIM the connecting operating forms state, of the the winding copper wireconnectio winding,n state, PCB and Winding the switch A, andes state PCB are Winding shown B. in TheTable DDPMIM 5. Take operatingPhase A for state, example. the winding The copper connection wire winding, state, and PCB the Winding switches A stateand PCB are shown Winding in B Tableare 5connected. Take Phase in series A for example.with KA1 The and copper KA3 turned wire winding, on and with PCB KA2 Winding and KA4 A and turned PCB Windingoff when B the areDDPMIM connected operates in series within with KA1a low-speed and KA3 range. turned The on andterminal with voltage KA2 and increases KA4 turned with offan whenincrease the in DDPMIMspeed. When operates this within voltage a low-speedapproaches range. the inverter The terminal limited voltage voltage, increases the copper with wire an increase winding in and speed. PCB WhenWinding this voltage A are connected approaches in the series inverter with limitedKA1 and voltage, KA4 turned the copper on and wire with winding KA2 and and KA3 PCB Windingturned off. AThe are connected copper wire in series winding with works KA1 and independently KA4 turned onwith and KA2 with and KA2 KA4 and turned KA3 turned on and off. KA1 The and copper KA3 wireturned winding off when works the independently DDPMIM operates with KA2 within and a KA4 high-speed turned onrange. and KA1 and KA3 turned off when the DDPMIM operates within a high-speed range.

Energies 2018, 11, 424 11 of 13 EnergiesEnergies 2018 2018, ,11 11, ,x x 1111 of of 13 13

FigureFigure 13. 13. Circuit CircuitCircuit of of the the winding winding switch. switch.

Table 5. Switch state. TableTable 5.5.Switch Switch state.state. DDPMIMDDPMIM Operating Operating DDPMIM Operating StateLowLow Low Speed Speed Speed Range Range Range Middle Middle Speed Speed RangeRange Range High High High Speed Speed RangeRange Range StateState WindingsWindingsWindings Connection Connection Connection coppercoppercopper wire wire wire winding, winding, winding, PCB PCB PCB coppercopper wire wirewire winding, winding, coppercopper wirewire wire (In Series) Winding A, PCB Winding B PCB Winding A winding (In(In Series) Series) WindingWinding A, A, PCB PCB Winding Winding B B PCBPCB Winding Winding A A windingwinding KA1,KA1,KA1, KA3; KA3; KA3; KA1,KA1,KA1, KA4; KA4; KA2,KA2, KA4;KA4; KA4; ON KB1, KB3; KA1, KA4; KA2, KA4; Switches ONON KB1,KB1, KB3; KB3; KA1,KA1, KA4; KA4; KA2,KA2, KA4; KA4; State KC1, KC3 KA1, KA4 KA2, KA4 SwitchesSwitches KC1,KC1, KC3 KC3 KA1,KA1, KA4 KA4 KA2,KA2, KA4 KA4 KA2, KA4; KA2, KA3; KA1, KA3; StateState KA2,KA2, KA4; KA4; KA2,KA2, KA3; KA3; KA1,KA1, KA3; KA3; OFFOFF KB2,KB2, KB4; KB4; KA2,KA2, KA3; KA1, KA3;KA3; OFF KB2,KC2, KB4; KC4 KA2, KA3KA3; KA1,KA1, KA3 KA3; KC2,KC2, KC4 KC4 KA2,KA2, KA3 KA3 KA1,KA1, KA3 KA3

TheThe conventional conventional in-wheel in-wheel in-wheel motor motor motor and and and the the the DDPMIM DDPMIM DDPMIM are are setare set withset with with the samethe the same same rated rated rated current. current. current. Torque-speed Torque- Torque- speedcurvesspeed curves arecurves shown are are shown inshown Figure in in 14Figure Figure. 14. 14.

200200 180180

150150

120120

9090 Copper wire winding Copper wire winding

Torque(N.m) Copper wire winding Copper wire winding Torque(N.m) + + PCB PCB winding winding A A + + PCB PCB winding winding A A inin series series inin series series

DDPMIM DDPMIM CopperCopper wire wire winding ConventionalConventional winding 0 0 00 200200 400400 600600 800800 10001000 1200120013501350 Speed(r/min) Speed(r/min) FigureFigure 14. 14. Torque-speed Torque-speed curves. curves. Figure 14. Torque-speed curves. For the conventional in-wheel motor, the output torque is 122 N·m at the rated current when the ForFor thethe conventional in-wheel in-wheel motor, motor, the the output output torque torque is is122 122 N·m N·m at atthe the rated rated current current when when the speed is below 1000 r/min. At 1000 r/min, the terminal voltage approaches the limit voltage value of thespeed speed is below is below 1000 1000 r/min. r/min. At 1000 At r/min, 1000 r/min, the term theinal terminal voltage voltage approaches approaches the limit the voltage limit value voltage of the inverter. The flux-weakening control strategy must be employed as the speed increases. The valuethe inverter. of the inverter. The flux-weakening The flux-weakening control controlstrategy strategy must be must employed be employed as the as speed the speed increases. increases. The magnetic field of the conventional in-wheel motor is excited by surface-mount PM and the d-axis Themagnetic magnetic field field of the of the conventional conventional in-wheel in-wheel motor motor is isexcited excited by by surface-mount surface-mount PM PM and the dd-axis-axis inductance is small. A large current will be required in the high-speed range. The efficiency of the inductanceinductance isis small.small. AA largelarge currentcurrent willwill bebe requiredrequired inin thethe high-speedhigh-speed range.range. TheThe efficiencyefficiency ofof thethe motormotor will will be be decreased. decreased. At At 1200 1200 r/min, r/min, the the output output torque torque is is only only 50 50 N·m N·m and and the the output output power power is is 6.8 6.8 kW.kW. WhenWhen thethe DDPMIMDDPMIM operatesoperates fromfrom 00 toto 700700 r/r/min,min, thethe coppercopper wirewire winding,winding, PCBPCB WindingWinding A,A,

Energies 2018, 11, 424 12 of 13 motor will be decreased. At 1200 r/min, the output torque is only 50 N·m and the output power is Energies 2018, 11, x 12 of 13 6.8 kW. When the DDPMIM operates from 0 to 700 r/min, the copper wire winding, PCB Winding andA, and PCB PCB Winding Winding B are B connected are connected in series, in series, and the and outp theut output torque torqueis 175 N·m. is 175 This N· ism. a 43.4% This is increase. a 43.4% Fromincrease. 700 Fromto 950 700 r/min, to 950 the r/min, copper the wire copper winding wire an windingd PCB andWinding PCB WindingA are connected A are connected in series, inand series, the outputand the torque output is torque 140 N·m. is 140 From N·m. 950 From to 1300 950 r/ tomin, 1300 the r/min, copper the wire copper winding wire windingworks alone, works and alone, the outputand the torque output is torque 95 N·m. is 95At N 1200·m. Atr/min, 1200 the r/min, output the power output is power 11.9 kW. is 11.9 This kW. is Thisan 75% is an increase. 75% increase. From 950From to 9501100 to r/min, 1100 r/min,the output the outputtorque of torque the DDPMIM of the DDPMIM is lower. is A lower. higher A current higher is current needed is to needed increase to theincrease output the torque. output torque. For comparison purposes, the two motors are loaded with the same ratedrated current.current. At 650 r/minr/min,, the output output power power densities densities ofof the the conventional conventional in wheel in wheel motor motor and andDDPMIM DDPMIM are 213 are W/kg 213 W/kg(0.915 3 3 W/cm(0.9153 W/cm) and 254) and W/kg 254 (0.915 W/kg W/cm (0.9153), W/cmrespectively.), respectively. At the rated At thecondition, rated condition, the output the power output densities power 3 3 aredensities 170 W/kg are 170(0.732 W/kg W/cm (0.7323) and W/cm 252 W/kg) and (1.28 252 W/kgW/cm3 (1.28), respectively. W/cm ), respectively. The PMSM suffers from torque pulsation, which is the main contributing factor of noise and vibration in EVs. As As in-wheel motors are directly installed in the wheels of the vehicles, unsprung weight will be increased. This This will will worsen worsen the the noise noise and and vibration vibration of of EVs, EVs, especially especially at at low low speeds. speeds. The torque at the rated speed (650(650 r/min)r/min) and and the the rated current of the conventional in-wheel motor and DDPMIM DDPMIM were were calculated, calculated, as as shown shown in in Figure Figure 15. 15 The. The torque torque pulsation pulsation of the of theconventional conventional in- wheelin-wheel motor motor is 23.2%, is 23.2%, while while that thatof the of DDPMIM the DDPMIM is 10.6%. is 10.6%. Torque Torque pulsation pulsation is thus isreduced thus reduced by 54.3%, by improving54.3%, improving overall overallride comfort. ride comfort.

200

150

100 Torque/N.m

50 Conventional DDPMIN 0 0 45 90 135 180 225 270 315 360 Source phase angle/deg Figure 15. Torque curves. Figure 15. Torque curves.

5. Conclusions 5. Conclusions The DDPMIM is able to operate within a full speed range without adding flux weakening The DDPMIM is able to operate within a full speed range without adding flux weakening control. control. Within the limited operating voltage of the inverter, different connection forms of windings Within the limited operating voltage of the inverter, different connection forms of windings are used. are used. The DDPMIM is shown here to have a greater output torque and less torque ripple than The DDPMIM is shown here to have a greater output torque and less torque ripple than those of the those of the conventional in-wheel motor, especially at low speeds. conventional in-wheel motor, especially at low speeds. The connection status of the windings, except for the speed and the limited operating voltage of The connection status of the windings, except for the speed and the limited operating voltage of the inverter, mainly depend on the back EMF. The back EMF of each unit and the ratios between the inverter, mainly depend on the back EMF. The back EMF of each unit and the ratios between them them must be carefully designed. However, the space for the axial flux motor unit is limited. It is must be carefully designed. However, the space for the axial flux motor unit is limited. It is important important to increase the back EMF produced by the PCB winding.The arrangement of the PCB to increase the back EMF produced by the PCB winding.The arrangement of the PCB winding and the winding and the PM array must take this into account. PM array must take this into account. While the DDPMIM designed in this study may not be optimal, based on the FEM results, it While the DDPMIM designed in this study may not be optimal, based on the FEM results, it outperforms the conventional structure in-wheel motor and therefore shows promise for future outperforms the conventional structure in-wheel motor and therefore shows promise for future applications. applications.

Acknowledgments: This work is funded by National Natural Science Foundation of China, No. 51577125.

Author Contributions: Peng Gao and Xiaoyuan Wang proposed the new structure of the in-wheel motor. Penag Gao and Yuxi Gu conducted the main work of design and simulation.

Energies 2018, 11, 424 13 of 13

Acknowledgments: This work is funded by National Natural Science Foundation of China, No. 51577125. Author Contributions: Peng Gao and Xiaoyuan Wang proposed the new structure of the in-wheel motor. Penag Gao and Yuxi Gu conducted the main work of design and simulation. Conflicts of Interest: The authors declare no conflict of interest.

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