A Wireless-Driven, Micro, Axial-Flux, Single-Phase Switched Reluctance Motor

energies

Article A Wireless-Driven, Micro, Axial-Flux, Single-Phase Switched Reluctance Motor

Da-Chen Pang * and Chih-Ting Wang Department of Mechanical Engineering, National Kaohsiung University of Science and Technology, 415 Jian Gong Rd., Sanmin Dist., Kaohsiung 80778, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-909-129-294

Received: 31 July 2018; Accepted: 15 October 2018; Published: 16 October 2018

Abstract: This study proposes a novel, axial-ﬂux, single-phase switched reluctance motor for micro machines with wireless-driven capability. The rotor and stator each have two poles, and the stator utilizes two permanent magnets to provide the required parking position and rotational torque. By reducing the number of magnetic poles and coils in the stator, and by utilizing a cylindrical design for its stator components, the micro motor is able to be easily manufactured and assembled. Safety and convenience are also achieved through the use of a wireless drive, which negates the need for power connections or batteries. This study utilizes the topology method in rotor design to reduce excessive torque ripple. For this study, an actual micro, axial-ﬂux, single-phase switched reluctance motor with a diameter of 5.5 mm and length of 4.4 mm was built in combination with a wireless charging module and motor circuitry found on the market. With an induced current of 0.7 A, the motor achieved a maximum of 900 rpm, indicating possible applications with respect to toys, micro-pumps, dosing pumps, and vessels for gases, liquids, or vacuum that do not require feedthrough.

Keywords: single-phase switched reluctance motor; micromotor; wireless driven; topological method

1. Introduction In 1984, Compter [1] developed the first single-phase switched reluctance motor with a two-pole rotor, two-pole stator, and two permanent magnets in the stator to solve the starting problem. Motor operation was improved by utilizing one positional sensor for control. In 1987, Chan [2] presented two single-phase switched reluctance motor designs, one for an axial-flux-type motor and the other for a radial-flux-type motor, with six poles in the rotors and stators. Horst [3,4] patented two single-phase switched reluctance motors with different rotor designs utilizing salient teeth and shifted poles. Both motors utilized two poles in the rotor and stator and one permanent magnet to provide a suitable starting position. In 1993, Torok and Loreth [5] fabricated a single-phase switched reluctance motor with four poles in the rotor and stator, two permanent magnets, and only two coils. In 1988, Lim et al. [6] proposed a single-phase switched reluctance motor with six poles in the rotor and stator with both axial and radial air gaps to increase torque output. In 2000, Stephenson and Jenkinson [7] produced a different design for a single-phase switched reluctance motor with four poles in the rotor and stator for high-speed fan applications. In 2001, Stanley [8] presented a single-phase switched reluctance motor with four poles in the rotor and stator without permanent magnets. The stator contained four sets of auxiliary coils to provide starting capability in both directions. In 2003, Higuchi et al. [9] proposed a single-phase switched reluctance motor with a tooth-slotted rotor design that used the magnetic saturation effect to produce starting torque in any position instead of using permanent magnets. In 2010, Jakobsen and Lu [10] presented a single-phase switched reluctance motor with four poles in the stator and rotor with a new stator pole design and an arrangement of permanent magnets to improve the starting torque. In 2011, Lu et al. [11] proposed an improved design, especially with regards to the

Energies 2018, 11, 2772; doi:10.3390/en11102772 www.mdpi.com/journal/energies Energies 2018, 11, 2772 2 of 17 arrangement of the permanent magnets, for low-cost pump applications. In 2012, Asgar et al. [12] developed a single-phase switched reluctance motor with a four-pole stator and two-pole rotor that utilized a skewed rotor design without the use of permanent magnets. In 2013, Yang et al. [13] fabricateda single-phase switched reluctance motor with six poles in the stator and rotor that employed the same skewed angle, reducing the acoustic noise and vibration. In 2015, Xu et al. [14] proposed a single-phase hybrid switched reluctance motor with a two-pole stator and four-pole rotor for hammer-breaker applications. In 2016, Infanuti et al. [15] demonstrated a single-phase switched reluctance motor with a four-pole rotor and two-pole stator with Ferrite permanent magnets for better efficiency. The rotor poles had a variable air-gap to provide starting torque in any position. In 2017, Jeong et al. [16] produced a single-phase hybrid switched reluctance motor, with four poles in both the stator and rotor, which applied permanent magnets and a non-uniform air-gap to enable self-starting and reduce torque ripple. Single-phase switched reluctance motors are the lowest-cost motor with the minimum number of components [17] and are especially suited for application in micro machines. However, past studies have not proposed any micro designs, as the smallest motor still has a diameter of 32 mm [12]. Axial-flux motors have a flat structure, produce a higher torque, and possess a higher torque density, making them suitable for application in places where space is limited. With the exception of one of the motors presented by Chan [2], the single-phase switched reluctance motors described above all utilize a radial-flux design; furthermore, while Chan proposed an axial-flux design, his design did not use any permanent magnets to assist starting. This paper proposes a micro, axial-flux, single-phase switched reluctance motor with two poles for both the stator and rotor in combination with two permanent magnets to solve the issue of starting. The magnetic poles of the coils and permanent magnets utilize a cylindrical design for ease of manufacturing and assembly. The rotor structure was designed using the topology method to reduce torque ripple and avoid any dead zones. This research demonstrates the first wireless-driven, single-phase switched reluctance motor especially suited for micro machines, with possible future applications in toys, micro pumps, and gas, liquid, and vacuum containers that do not require feedthrough.

2. Micro Single-Phase Reluctance Switched Motor Design and Analysis

2.1. Single-Phase Switched Reluctance Motor Design This motor was designed with a two-pole stator and two-pole rotor; two permanent magnets were placed on the stator. The diameter of the stator was 5.5 mm, its length was 4.4 mm, and the stator magnetic poles and permanent magnet utilized a cylindrical structure that facilitates the assembly of micro motors. As the magnetic rods utilized in the prototype design of this study are difﬁcult to purchase, each stator pole was assembled using three magnetic rods made of nickel wire with a diameter of 0.75 mm. The permanent magnets were FLN8 AlNiCo magnets (Mingyen Electronics Industry Co., Ltd., Taichung, Taiwan). with axial magnetization. The rotor structure and pole plate utilized a 35CS300 silicon steel plate with a thickness of 0.35 mm. The design speciﬁcations of the single-phase switched reluctance motor are shown in Table1; the exploded view and assembly drawings of the single-phase switched reluctance motor are shown in Figure1.

Table 1. Speciﬁcations of the micro, axial-ﬂux, single-phase switched reluctance motor.

Mechanical Speciﬁcations Poles 2 Poles 2 Number of permanent magnets 2 External diameter 4.5 mm External diameter 5.5 mm Root diameter 2 mm Stator Diameter of magnetic rods 0.75 mm × 3Rotor Internal diameter 1 mm Diameter of permanent magnets 1.2 mm Length 0.35 mm Length of magnetic rods 4 mm Air gap between rotor and stator pole 0.06 mm Length of permanent magnets 2.65 mm Air gap between rotor and permanent magnet 0.2 mm Electrical Speciﬁcations Phase 1 Step angle 90◦ Number of turns of coil 120 Diameter of coil 0.07 mm Maximum current 1 A Resistance of coil 3.4 Ω Energies 2018, 11, x 3 of 17

Energies 2018, 11, 2772 3 of 17 Energies 2018, 11, x 3 of 17 Rotor

MagneticRotor rods & coils

Magnetic rods Permanent & coils magnet

Permanent magnet Pole plate for PM Pole plate for PM Base

Base

Figure 1. Exploded view and assembly drawings of the micro , axial-flux, single-phase switched reluctance motor. FigureFigure 1. 1.Exploded Exploded view view and and assembly assembly drawings drawings of the ofmicro the, axial micro,-flux axial-ﬂux,, single-phase single-phase switched reluctance switched reluctancemotor. motor. 2.2. Rotor Design of the Single-Phase Switched Reluctance Motor 2.2. Rotor Design of the Single-Phase Switched Reluctance Motor 2.2T. heRotor design Design of ofthe the single Single-phase-Phase Switchedswitched Reluctance reluctance Motor motor was critical because improper design oftenThe produceThe design designs ofzero theof the single-phase or single negative-phase switched torque switched and reluctance reluctance excessive motor motor torque was wa critical s ripple critical because, limitingbecause improper improperperformance design design often and producesapplicationsoften produce zero. This ors negative zero study or first torque negative determine and torque excessived the and stator torqueexcessive structure ripple, torque limiting including ripple performance, limiting magnetic performance and rods, applications. coil s and, and Thispermanentapplications study firstmagnets,. determined This studyand then thefirst stator consider determine structureedd rotor the including statorstructure structure magnetic. The design including rods, of coils, this magnetic and motor permanent rods, was coildetermined magnets,s, and usingandpermanent then JMAG considered finitemagnets, element rotor and structure. analysisthen consider The software designed rotor developed of thisstructure motor by. T wasJSOLhe design determined Corporation of this using motor, Tokyo, JMAG wa Japans determined finite. Figure element 2 showsanalysisusing the softwareJMAG definitions finite developed element of the byanalysis coordinate JSOL software Corporation, and developed rotating Tokyo, angleby Japan. JSOL of Corporation Figure the motor2 shows , soTokyo, the that definitions Japan the axis. Figure of the2 the ◦ permanentcoordinateshows the and magnet definitions rotatings wa angle s ofat the0 of°. thecoordinate motor so and that rotating the axis of angle the permanentof the motor magnets so that was the at axis0 . of the permanent magnets was at 0°.

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Energies 2018, 11, 2772 4 of 17 푇푇푚푎푥푚푎푥 −−푇푇푚푖푛푚푖푛 푇푇푟푖푝푝푙푒푟푖푝푝푙푒 == ××100100%% ((11)) 푇푇푚푎푥푚푎푥 ++푇푇푚푖푛푚푖푛 The objective function is the maximum average torque and minimum relative torque ripple, and theTThehe set objectiveobjective constraints functionfunction are isis that thethe the maximummaximum average averageaverage torque torque musttorque be andand greater minimumminimum than relativerelative 14 µN·m torquetorque and the rippleripple relative,, andand · thetorquethe setset constraints rippleconstraints must areare be that lowerthat thethe than averageaverage 70%. torquetorque These twomustmust values bebe greatergreater were thanthan selected 1414 μμN afterN·mm multipleandand thethe relativerelative analyses torquetorque were rippleperformed.ripple mustmust bebe Even lowerlower with thanthan a variety 70%.70%. TheseThese of different twotwo valuesvalues designs, werewere selectedselected the maximum afterafter multiplemultiple average analysesanalyses torque we waswerere 15.47 performedperformedµN·m.. · EvenandEven the with with minimum a a variety variety relative of of differentdifferent torque designs, designs,ripple was the the 63.33%. maximum maximum The objectiveaverage average torque torque function wawa wasss 115.5. set4747 as μ μNN·mm andand the the minimumminimum relativerelative torquetorque rippleripple wawass 63.33%.63.33%. TheThe objectiveobjective functionfunction wawass setset asas T 푇−−푇TC Objective function = 푇−푇퐶퐶 × 100% + T − T (2) ObjectiveObjective functionfunction == ××100100%%++((푇푇푟푖푝푝푙푒퐶푟푖푝푝푙푒퐶rippleC−−푇푇푟푖푝푝푙푒푟푖푝푝푙푒ripple)) ((22)) T푇푇C퐶퐶 wherewhere TT isis thethe averageaverage torquetorque andand TTTCCC iisiss thethe constraintconstraint ofof thethe averageaverage torquetorque atatat 141414 µ μμNNN···m.mm..T TTripplerippleripple iisiss the the relativerelative torque torque rippleripple andand TTTrippleCrippleCrippleC iisiss thethe the constraintconstraint constraint ofof of thethe the relativerelative relative torquetorque ripple ripple at 70%. 70%. The minimum minimum valuevalue of of the the objective objective functionfunction waswas selectedselected asas thethe geometricgeometric shapeshape forfor rotorrotor optimization.optimization.optimization.

2.2.1.2.2.1. Step Step 1: 1: DesignDesign ofof RotorRotor MagneticMagnetic Pole Pole Width Width and and Root Root Diameter Diameter TTheThehe first ﬁrstfirst step step of of this this study study wa waswass to to change change the the rotor rotor magnetic magnetic pole pole width width from from 1.8 1.851.855 m mmmmm to to 0. 0.60.66 m mmmmm inin dede decrementscrementscrements ofof of 0.00.0 0.0555 mmm mmm toto achieve toachieve achieve thethe optimal theoptimal optimal magneticmagnetic magnetic polepole widthwidth pole width ofof 1.1.66 mm ofm.m. 1.6 NextNext mm.,, thethe Next, rootroot diameterdiameter the root changeddiameterchanged fromfrom changed 2.2.55 mm frommm toto 2.5 1.71.75 mm5 mmmm to inin 1.75 dedecrementscrements mm in decrements ofof 0.00.055 mmmm of forfor 0.05 anan mm optimaloptimal for an rootroot optimal diameterdiameter root ofof diameter 2.2.00 mmm.m. TheofThe 2.0 designdesign mm. process Theprocess design isis shownshown process inin Figure isFigure shown 3.3. InIn in stepstep Figure 11--1,1,3. thethe In steprotorrotor 1-1, magneticmagnetic the rotor polepole magnetic widthwidth wawa poless 1.1.8 width855 mmmm was andand the1.85the rootroot mm diameterdiameter and the root wawass diameter 2.2.55 mmm;m; wasstepstep 2.5 11--22 mm; showsshows step aa 1-2rotorrotor shows magneticmagnetic a rotor polepole magnetic widthwidth of poleof 1.1.66 width mmmm andand of 1.6 aa root mmroot diameteranddiameter a root ofof diameter 2.2.55 mmmm,, achievingachieving of 2.5 mm, thethe achieving optimaloptimal magneticmagnetic the optimal polepole magnetic widthwidth;; stepstep pole 11-- width;33 showsshows step aa rotorrotor 1-3 magnetic showsmagnetic a rotor polepole widthmagneticwidth ofof 1.1. pole66 mmm widthm andand of aa rootroot 1.6 mm diameterdiameter and a rootofof 1.71.7 diameter55 mmmm resultingresulting of 1.75 mm inin thethe resulting worstworst averageaverage in the worst torquetorque average ofof 1133..1717 torque μN·mμN·m of;; step13.17step 11µ--44N showsshows·m; step thatthat 1-4 thethe shows rotorrotor that magneticmagnetic the rotor polepole magnetic widthwidth wawa poless 1.1. width66 mmmm was andand 1.6 thethemm rootroot and diameterdiameter the root wawa diameterss 2.2.00 mmmm was forfor the2.0the mmbestbest for performanceperformance the best performance ofof stepstep 11,, withwith of step anan average 1,average with an torquetorque average ofof 11 torque4.784.78 μμN·m ofN·m 14.78 andandµ relativeNrelative·m and torquetorque relative rippleripple torque ofof 1ripple102.502.5%.%. of TheThe 102.5%. torquetorque The--angleangle torque-angle curvecurve ofof curveeacheach processprocess of each isis process shownshown is inin shown FigureFigure in 44 Figure,, andand aa4 ,comparisoncomparison and a comparison ofof thethe torquetorque of the characteristictorquecharacteristic characteristicsss isis shownshown is showninin TableTable in 22 Table.. 2.

11--11 11--22 11--33 11--44

TTavgavg== 14.6214.62μμNmNm TrippleTripple== 161.56%161.56% TTavgavg== 14.5214.52μμNmNm TrippleTripple== 101.32%101.32% TTavgavg== 13.1713.17μμNmNm TrippleTripple== 107.41%107.41% TTavgavg== 14.7814.78μμNmNm TrippleTripple=102.50%=102.50%

FigureFigure 3. 3. StepStep 11 1::: DD Designesignesign processprocess ofof thethe rr rotorotorotor magneticmagnetic pole pole width width and and root root diameter diameter.diameter..

5050

4040

3030

11--11

Nm)

Nm) μ μ 1-2 2020 1-2 11--33

Torque( 10 Torque( 10 11--44

--1010 00 3030 6060 9090 120120 150150 180180 AngleAngle (degree)(degree)

FigureFigure 4. 4. StepStep 11 1::: TT Torque-angleorqueorque--angleangle curve curve for for each each step step.step..

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Table 2. Step 1: Comparison of the torque characteristics of each step. Table 2. Step 1: Comparison of the torque characteristics of each step. Step 1-1 1-2 1-3 1-4 Maximum tSteporque (μN·m) 34.52 1-1 47.07 1-2 29.37 1-3 38.11 1-4 MMaximuminimum torque torque (µμNN··m)m) 34.52−8.12 47.07−0.31 29.37−1.05 38.11−0.47 AMinimumverage torque torque ( (μµNN·m)m) −14.628.12 −14.520.31 −13.171.05 −14.780.47 RelativeAverage ripple torque torque (µN·m) (%) 14.62161.56 14.52101.32 13.17107.41 14.78102.50 Relative ripple torque (%) 161.56 101.32 107.41 102.50 2.2.2. Step 2: Design of Rotor Magnetic Pole Quadrants 1 and 3 2.2.2. Step 2: Design of Rotor Magnetic Pole Quadrants 1 and 3 Step 2 was the axial symmetrical topology design of the rotor magnetic poles for quadrants 1 and 3. The meshStep 2 dimensions was the axial we symmetricalre 0.25 mm topology × 0.25 mdesignm. We of theb eg rotoran by magnetic establishing poles five for quadrants-layer meshes 1 and by 3. Theadding mesh two dimensions layers of weremeshes 0.25 to mm the× original0.25 mm. magnetic We began pole by width; establishing the outer five-layer layer meshes was defined by adding as layer two 1layers and layer of meshes 5 wa tos the the closest original to magnetic the X-axis. pole The width; design the outerprocess layer wa wass shown defined in asFigure layer 5. 1 andIn step layer 2-1, 5 wasthe firstthe closest design to decrease the X-axis.d the The average design processtorque wasand shownrelative in torque Figure ripple5. In step but 2-1, still the generate first designd a dead decreased zone ◦ betweenthe average the torque angle ands of relative160°and torque 180°; ripple step 2 but-2 stillremove generatedd the meshes a dead zone of the between first and the anglessecond of layer 160 sand to ◦ 180solve; stepthe dead 2-2 removed zone issuethe; meshesstep 2-3 of remove the firstd the and meshes second from layers the to third solve layer the dead for an zone evident issue; decrease step 2-3 inremoved relative the torque meshes ripple from; thestep third 2-4 remove layer ford an more evident meshes decrease from in the relative first four torque layers ripple; to stepachieve 2-4 removed the best performancemore meshes in from step the 2,with first four an average layers to torque achieve of the 13.3 best8 μ performanceN·m and a relative in step torque 2,with anripple average of 83 torque.75%. Theof 13.38 torqueµN·-mangle and curve a relative of eachtorque process ripple of is 83.75%. shown The in Figure torque-angle 6, and curve a comparison of each process of the is torque shown characteristicin Figure6, ands is a shown comparison in Table of the 3. It torque should characteristics be noted that is removing shown in Tablethe meshes3. It should from bethe noted fifth layer that dremovingid not improve the meshes output from performance the fifth layer. did not improve output performance.

2-1 2-2 2-3 2-4

Tavg = 13.01μNm Tripple = 100.92% Tavg = 13.61μNm Tripple = 98.82% Tavg = 13.62μNm Tripple = 93.80% Tavg = 13.38μNm Tripple =83.75%

Figure 5. Step 2 2:: D Designesign process process of the r rotorotor magnetic pole of quadrant quadrantss 1 and 3.3.

40 35 30 25

2-1 Nm)

Nm) 20

μ μ 15 2-2

10 2-3 Torque( Torque( 5 2-4 0 -5 -10 0 30 60 90 120 150 180 Angle (degree)

Figure 6. StepStep 2 2:: T Torque-angleorque-angle curve for each step.step.

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TableTable 33.. StepStep 22:: CComparisonomparison tabletable ofof thethe torquetorque characteristiccharacteristic ofof eacheach stepstep.. Table 3. Step 2: Comparison table of the torque characteristic of each step. StepStep 22--11 22--22 22--33 22--44 MMaximumaximum ttorqueorqueStep ((μμNN··mm)) 28.0728.07 2-1 29.50 2-229.50 2-328.1428.14 2-434.2134.21 · MMinimuminimumMaximum ttorqueorque torque ((μμ (NµNN·m·mm))) −−28.070.130.13 29.500.180.18 28.140.900.90 34.213.033.03 AAverageverageMinimum ttorqueorque torque ((μμN (Nµ·N·mm·m))) 13.0113.01−0.13 0.1813.6113.61 0.9013.6213.62 3.0313.3813.38 RelativeRelativeAverage rrippleipple torque ttorqueorque (µN ·(%)(%)m) 100.92100.9213.01 13.6198.8298.82 13.6293.8093.80 13.3883.7583.75 Relative ripple torque (%) 100.92 98.82 93.80 83.75 22..22..33.. StepStep 3:3: DesignDesign ofof RotorRotor MagneticMagnetic PolePole QuadrantQuadrantss 22 andand 44 2.2.3. Step 3: Design of Rotor Magnetic Pole Quadrants 2 and 4 StepStep 33 wawass thethe axialaxial symmetricalsymmetrical topologytopology designdesign ofof thethe rotorrotor magneticmagnetic polespoles forfor quadrantsquadrants 22 andand 4.4. TheTheStep meshmesh 3 was dimensionsdimensions the axial symmetricalwewerere 0.20.255 mmm topologym ×× 0.20.255 m designmm.m. WeWe of bb theegegaa rotornn byby magneticestablishingestablishing poles threethree for-- quadrantslayerlayer meshes;meshes; 2 and thethe 4. outerTheouter mesh llayerayer dimensions wawass defineddefined were asas layerlayer 0.25 mm 11 andand× layerlayer0.25 mm. 33 wawa Wess closestclosest began toto by thethe establishing XX--axis.axis. TheThe three-layer designdesign processprocess meshes; isis shownshown the outer inin FigurelayerFigure was 77.. InIn defined stepstep 33- as-1,1, layerththee designdesign 1 and layerincreaseincrease 3 wasdd thethe closest averageaverage to the torquetorqueX-axis. andand The decreasedecrease designd processd thethe relativerelative is shown torquetorque in Figure rippleripple7;.; stepInstep step 33--2 3-1,2 removeremove the designdd thethe increased meshesmeshes from thefrom average thethe firstfirst torque andand and secondsecond decreased layerslayers the forfor relative anan evidentevident torque decreasedecrease ripple; step inin the 3-2the removedaverageaverage torquethetorque meshes and and from relativerelative the firsttorquetorque and ripple second ripple;; layersstep step 3 3 for--33 remove anremove evidentdd moremore decrease meshes meshes in the f from averagerom thethe torque first first and and and second secondrelative layertorque layerss withripple;withoutout step anyany 3-3 improvementimprovement removed more;; stepstep meshes 33--44 remove fromremove thedd first oneone and meshmesh second fromfrom layers thethe thirdthird without layerlayer any ofof improvement; thethe stepstep 33--22 stepdesigndesign 3-4 withremovedwithoutout anyany one improvement meshimprovement from the; third; stepstep layer 33--22 waswas of the chosenchosen step 3-2 asas design thethe bestbest without designdesign any forfor improvement; stepstep 3,3, withwith an stepan averageaverage 3-2 was torquetorque chosen ofasof 13.10 the13.10 best μμN·mN·m design andand for aa relativerelative step 3, with torquetorque an rippleaverageripple ofof torque 79.0979.09%.%. of TheThe 13.10 torquetorqueµN·m--angleangle and a curvecurve relative ofof torque eacheach processprocess ripple of isis shown79.09%.shown inThein FigureFigure torque-angle 88,, andand aa curve comparisoncomparison of each process ofof thethe torquetorque is shown characteristiccharacteristic in Figure8, andss isis ashownshown comparison inin TableTable of the 4.4. ItIt torque shouldshould characteristics bebe notednoted thatthat is theshownthe designdesign in Table ofof thethe4. rotor Itrotor should magneticmagnetic be noted polespoles that inin the quadrantsquadrants design of 22 the andand rotor 44 haha magneticdd aa limitedlimited poles effecteffect in in quadrantsin termsterms ofof 2 improvingimproving and 4 had outputaoutput limited performanceperformance effect in terms.. of improving output performance.

33--11 33--22 33--33 33--44

TTavgavg== 13.52 13.52μμNmNm TrippleTripple== 82.44% 82.44% TTavgavg== 13.10 13.10μμNmNm TrippleTripple== 79.09% 79.09% TTavgavg== 12.20 12.20 μ μNmNm TrippleTripple== 80.87% 80.87% TTavgavg== 12.73 12.73μμNmNm TrippleTripple== 85.37% 85.37%

FigureFigure 7. 7. StepStep 3 3:3:: DD Designesignesign processprocess of of the the rotor rotor magnetic magnetic pole polespoless for for quadrant 2 2 and and 4 4.4..

4040 3535 3030 2525

33--11 Nm)

Nm) 2020

μ μ 1515 33--22

1010 33--33 Torque( Torque( 55 33--44 00 --55 --1010 00 3030 6060 9090 120120 150150 180180 AngleAngle (degree)(degree)

FigureFigure 8. 8. StepStep 33 3::: TT Torque-angleorqueorque--angleangle curve curve for for each each step step.step..

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TableTable 4 4.. Step 3 3:: C Comparisonomparison of of the the torquetorque characteristiccharacteristic characteristicsss ofof eacheach stepstep step...

StepStep 3 3-1--1 3-233--22 3-333--33 3-433--44 · MaximumMaximum to torquerque (μ (µNN··mm)) 33.8733.87 30.68 26.7726.7726.77 27.4027.4027.40 MinimumMinimum torque torque (μ (µNN··mm)) 3.26 3.26 3.583.58 2.832.832.83 2.162.162.16 AverageAverage torque torque (μ (µNN··mm)) 13.5213.52 13.10 12.2012.2012.20 12.7312.7312.73 Relative ripple torque (%) 82.44 79.09 80.87 85.37 Relative ripple torque (%) 82.44 79.09 80.8780.87 85.3785.37

22.2.4.2..22..44.. Step Step 4:4: DetailedDetailed Design Design of of the the Rotor Rotor Magnetic Magnetic Pole Pole SStepSteptep 4 4 wa waswass the the detailed design of the rotor magnetic pole pole byby shrinkingshrinking thethe meshmesh dimensionsdimensions to to 0.120.1250.1255 m mmmmm ×× 0.120.120.1255 m mm.m. Since Since the the degree degree to to which which adjustments adjustments toto to thethe the rotorrotor rotor magneticmagnetic magnetic polespoles poles in in in quadrants quadrants quadrants 22 andand 444 improved improvedimproved the the performance performance was was limited, limited, this this step stepstep only onlyonly presents presentspresents the the analysisthe analysisanalysis results resultsresults forthe forforrotor thethe rotormagneticrotor magneticmagnetic poles inpolespoles quadrants in quadrants 1 and 3.1 Weand began 3. We by beg establishingan by establishingestablishing nine-layer ninenine meshes;--layerlayer themeshes;meshes; outer the layerthe outerouter was layerdefinedlayer wawa asss defineddefined layer 1 as and layer layer 1 and 9 was layer the 9 closest was the to closest the X-axis. to thethe The XX--axis.axis. design TheThe process designdesign is processprocess shown isis in shownshown Figure in in9 . FigureInFigure step 9.9. 4-1, InIn thestepstep design 4--1, thethe added design meshes addedto meshes the first to andthe firstfirst second andand layer secondsecond without layerlayer withoutwithout any improvement; any any improvement; improvement; step 4-2 stepremovedstep 44--22 removeremove meshesd frommeshes the from first the layer first for layer an evident for an evident increase increasincreas in averageee inin averageaverage torque torque andtorque relative andand relativerelative torque torqueripple;torque step ripple ripple 4-3;; stepstep removed 4-3 remove meshesd from meshes the third,from fourth,the third, third, and fourth, fourth,fifth layers and and without fifth fifth layers layers any improvement;withoutwithout any any improvementstepimprovement 4-4 used;; astepstep diagonal 4-4 use designd a diagonal for the design salient for pole the forsalient an evident polepole forfor increase anan evidentevident in averageincreaseincrease torqueinin averageaverage and torquerelativetorque andand torque relativerelative ripple; torque step ripple 4-4 was; step selected 4-4 was as selected the best as design the bestbest for designdesign step 4, forfor with stepstep an 44 average,, withwith anan torque averageaverage of torque13.75torqueµ Nofof· m1133. and.7575 μ aN·m relative and torquea relative ripple torque of 82.85%.The ripple of 82.85 torque-angle%.%.TheThe torquetorque curve--angleangle of each curvecurve process ofof eacheach is shown processprocess in isFigureis shownshown 10 ,inin and FigureFigure a comparison 1010,, and a comparison of the torque of characteristics the torque characteristiccharacteristic is shown inss is Tableis shownshown5. inin TableTable 5.5.

44--11 4-2 4--3 44--44

TTavgavg== 12.6912.69μμNmNm TrippleTripple == 80.08%80.08% Tavg = 13.32μNm Tripple = 80.86% TTavgavg== 13.2413.24μμNmNm TrippleTripple== 82.16% 82.16% TTavgavg== 13.75 13.75μμNmNm TrippleTripple== 82.85% 82.85%

Figure 9. StepStep 4 4:: D Detailedetailed d designesign process process of of thethe the rr rotorotorotor magneticmagnetic polepole pole...

40 35 30 25 44--11 Nm) 20

Nm) 20 μ μ 4-2 15 4-2 4-3

10 4-3 Torque(

Torque( 4-4 5 4-4 0 --5 --10 0 30 60 90 120120 150150 180180 Angle (degree)

Figure 10. Step 4: Torque-angle curve for each step. FigureFigure 10. 10. StepStep 4 4:: T Torque-angleorque-angle curve for each step step..

Energies 2018, 11, 2772 8 of 17 EnergiesEnergies 20182018,, 1111,, xx 88 ofof 1717

TableTable 5 5.5.. StepStep 44 4::: CC Comparisonomparisonomparison ofof thethe torquetorque characteristiccharacteristic characteristicsss of of each each step step.step..

StepStepStep 44 4-1--11 4-244--22 4-344--33 4-444--44 MMaximumaximumMaximum ttorqueorque torque ((μμ (µNNN···mm)m)) 33.2633.2633.26 31.53 31.53 33.9033.9033.90 29.0429.0429.04 MMinimuminimumMinimum ttorqueorque torque ((μμ (µNNN···mm)m)) 3.683.68 3.343.34 3.323.323.32 2.722.722.72 AAverageverageAverage ttorqueorque torque ((μμ (µNNN···mm)m)) 12.6712.6712.67 13.32 13.32 13.2413.2413.24 13.7513.7513.75 Relative ripple torque (%) 80.08 80.86 82.16 82.85 RelativeRelative rrippleipple ttorqueorque (%)(%) 80.0880.08 80.8680.86 82.1682.16 82.8582.85

TTThehehe electromagneticelectromagnetic analysisanalysis resultsresults forfor thethe singlesingle single-phase--phasephase switchedswitched reluctancereluctance motormotor designdesign usingusing thethe topotopo topologylogylogy methodmethod method areare are presentedpresented presented inin in toptop top viewview view andand and 3D3D 3D viewview view diagramdiagram diagramsss inin FiguresFigures in Figures 1111––14. 14.11 –TheThe14. motormotor The motor wawass drivendrivenwas driven byby anan by inputinput an input currentcurrent current ofof 11 AA of atat1 angleangle A atss angles betweenbetween between 00°° andand0 90°90°◦ and,, andand 90 by◦by, and aa permanentpermanent by a permanent magnetmagnet magnet atat anglesangles at betweenbetweenangles between 90°90° andand 90 180°180°◦ and.. TheThe 180 magneticmagnetic◦. The magnetic fluxflux densitydensity flux density distributiondistribution distribution forfor whenwhen for the whenthe coilscoils the begbeg coilsaann began toto exciteexcite to exciteatat anan angleangleat an of angleof 0°0° isis of shownshown 0◦ is shown inin FigureFigure in Figure11.11. TThehe 11 maximummaximum. The maximum fluxflux densitydensity flux densityofof 1.81.866 TT of occuroccur 1.86redred T occurred inin thethe rotorrotor in the,, whilewhile rotor, aa densitydensitywhile a of densityof 0.90.911 TT of occuroccur 0.91redred T occurred inin thethe airair in gap thegap..air TheThe gap. magneticmagnetic The magnetic fluxflux densitydensity flux density distributiondistribution distribution whenwhen thethe when motormotor the produceproducemotor produceddd thethe maximummaximum the maximum torquetorque atat torque anan angleangle at an ofof angle 74°74° isis of shownshown 74◦ is inin shown FigureFigure in 12.12. Figure TheThe maximum12maximum. The maximum fluxflux densitydensity flux ofofdensity 1.1.9944 TT of occuroccur 1.94redred T occurred inin thethe rotorrotor in the,, whilewhile rotor, aa while densitydensity a density ofof 0.0.6633 of TT 0.63occuroccur Tredred occurred inin thethe in airair the gap.gap. air The gap.The magneticmagnetic The magnetic fluxflux densitydensityflux density ddistributionistribution distribution whenwhen when thethe currentcurrent the current wawass switchedswitched was switched offoff atat off anan at angleangle an angle ofof 91°91° of isis 91 shownshown◦ is shown inin FigureFigure in Figure 13.13. TheThe 13 . maximummaximumThe maximum fluxflux fluxdensitydensity density ofof 0.0. of9999 0.99 TT occuroccur T occurredredred inin the inthe the polepole pole plateplate plate,,, whilewhile while aa density adensity density ofof of 0.20.2 0.2999 TTT occuroccur occurredredred inin in thethe airair gap.gap. TheThe The magneticmagnetic magnetic fluxflux flux densitydensity density distributiondistribution distribution whenwhen when thethe the permanentpermanent permanent magnetmagnet magnet produceproduce produceddd thethe maximummaximum torquetorque atat anan angleangle ofof 135°135° 135◦ isisis shownshown shown inin in FigureFigure Figure 14.14.14. TheThe The maximummaximum maximum fluxflux flux densitydensity density of of 1.01.0 1.0555 T T occuroccur occurredredred inin the the polepole plateplate plate,,, while while a a density density of of 0.2 0.270.277 T T occur occurredoccurredred in in the the air air gap. gap.

FigureFigureFigure 11.11. 11. TopTopTop viewview andand 3D3D viewview ofof magneticmagnetic fluxflux ﬂux density density plots plots of of the the motor motor at at a a rotating rotating angle angle of of 00°.◦°..

FigureFigureFigure 12.12. 12. TopTopTop viewview view andand and 3D3D 3D viewview view ofof of magneticmagnetic magnetic fluxflux ﬂux densitydensity plots plots of of the the motormotor atat aa rotatingrotating angleangleangle of ofof 74 7474◦°.°..

Energies 2018, 11, 2772 9 of 17 Energies 20182018,, 1111,, xx 99 ofof 1717

FigureFigure 13. 13. TopTop view view andand and 3D3D 3D view view of of magnetic magnetic flux ﬂux density density plots of the motormotor atat aa rotatingrotatingrotating angle angleangle of ofof 91 9191◦°.°..

FigureFigure 14. 14. TopTop view view andand and 3D3D 3D view view of of magnetic magnetic flux ﬂux density density plots ofof thethe motormotor at atat a aa rotating rotatingrotating angle angleangle of ofof 135 135135◦.°.°.

SinceSince the the maximum maximum flux ﬂux density density of of the the rotor wa wasss larger larger than than the the 1.631.63 TT fromfromfrom thethethe manufacturer’smanufacturer’smanufacturer’s catalog,catalog, there there there wa wasss some some concern concern regarding regarding magnetic magnetic saturation, saturation, saturation, and and and thisthis this wa wasss confirmed confirmedconﬁrmed later later later in in the the experimentexperimentexperiment... ItIt It shouldshould should bebe be notednoted noted thatthat that therethere there wa wasss magnetic magnetic leakage leakage at at the the bottom bottom of of the the three three magnetic rods,rods, with with thisthis leakageleakageleakage being beingbeing especially especiallyespecially evident evidentevident in inin Figures FiguresFigures 13 1313 and andand 14 14,,14, because becausebecause the thethe distance distance between between the thethemagnetic magnetic magnetic rods rods rods and theand andpermanent the the permanent permanent magnets magnets magnets was too wa small.ss too too Ifsmall. small. three If If small three three magnetic small small magnetic magnetic rods are rods rodsreplaced are are replacedreplacedby one large by by rodone one and large large the rod rod distance and and the the is increased, distance distance isthe is increased, increased, problem will the the be problem problem solved; will willthis was be be solved; solved; demonstrated this this was was in demonstrateda simulation that inin aa is simulationsimulation not shown thatthat here. isis not shown here.

3.3. Fabrication Fabrication and and Testing Testing of the the Single Single-Phase--Phase Switched ReluctanceReluctance MotorMotor 3.1. Motor Fabrication 3..1.. Motor Fabrication The parts of the single-phase switched reluctance motor were fabricated by mechanical machining The parts of the single--phase switched reluctance motor werere fabricated fabricated by by mechanical processes and a prototype was hand-assembled. As shown in Figure 15, the components of the machining processes and a prototype wass hand--assembleded.. As shown in Figure 15, the components motor (from left to right) were the rotor, pole plate, permanent magnet, magnetic rod with coil, base, of the motor (from left to right) werere thethe rotor,rotor, polepole plate,plate, permanentpermanent magnet,magnet, magneticmagnetic rodrod with coil, and fixture with fixed shaft. A top view of the motor prototype is shown in Figure 16. The rotor, base, and fixture with fixedfixed shaft.shaft. A toptop viewview ofof thethe motormotor prototypeprototype isis shownshown inin FigureFigure 16.16. The rotor, pole plate, base, and fixture were made by wire electrical discharge machining. The magnetic rod pole plate,, base,, andand fixturefixture werere made by wire electrical discharge machining. The magnetic rod used Goodfellow Cambridge Ltd. (Huntingdon, UK) NI005159 nickel wires that were cut and grinded used Goodfellow Cambridge Ltd. ((Huntingdon, UK)) NI005159 nickel wires that werere cutcut and and before being wrapped around the coils. The permanent FLN8 AlNiCo magnet was produced by grinded before being wrapped around the coilss.. TheThe permanentpermanent FLN8 AlNiCo magnet wass produced Mingyen Electronics Industry Co., Ltd. (Taichung, Taiwan). The assembly of the motor was carried out by Mingyen Electronics Industry Co., Ltd. ((Taichung,, Taiwan). Taiwan). The assemblyssembly of of the the motor motor wass by first inserting the magnetic rod with coil into the base, placing the pole plate and permanent magnet carriedcarried outout by first inserting the magnetic rod with coil intointo thethe basebase,, placingplacing thethe pole plate and on the base, and installing the fixture and fixed shaft. A copper wire with a diameter of 0.06 mm was permanent magnet on the base,, and installinginstalling thethe fi fixture and fixed shaft.shaft. A A copper copper wire wire with with a a wrapped around the fixed shaft to secure the axial air gap of the motor. The rotor was placed on the diameter of 0.06 mm wass wrapped around the fixed shaft toto securesecure thethe axial air gap of the motor. The shaft to complete the assembly process. rotorrotor wass placedplaced onon thethe shaftshaft toto completecomplete thethe assembly process..

Energies 2018, 11, 2772 10 of 17 Energies 2018,, 11,, xx 10 of 17

FFigureigure 15. 15. MeMechanicalchanical parts parts of of the the axial axial-ﬂux,-flux, single single-phase-phase switched switched reluctance reluctance motor. motor.

Figure 16. Prototype of the axial-ﬂux, single-phase switched reluctance motor. Figure 16. Prototype of the axial-flux, single-phase switched reluctance motor. 3.2. Torque-Angle Curve Testing 3.2. Torque-Angle Curve Testing The experimental testing of the torque-angle curve is very important for a single-phase switched reluctanceThe experimental motor because testing the output of the torquetorque- isangle varied curve at different is very important angle positions. for a single Torque-phase is measured switched by reluctancehanging different motor because small weights, the output metal torque balls is and varied adhesive at different tapes on angle the outerposition diameters. Torque of theis measured rotor [18]. byTypically, hanging the different mass of small the metal weight balls, metal is 18 mg balls and and the adhesive adhesive tapes tape on 1 mg. the outer The total diameter weight of was the rotor measured [18]. Typically,using the Mettler-Toledothe mass of the AB54 metal Analytical ball is 18 Balance mg and (Mettler-Toledo the adhesive AG,tape Greifensee,1 mg. The Switzerland)total weight withwas measureda repeatability using of the0.1 mg Mettler and- aToledo linear accuracy AB54 Analytical of 0.2 mg. Balance The ﬁrst (Mettler test consisted-Toledo of AG, measuring Greifensee, the Switzerland)passive torque-angle with a repeatability curve at every of 0.1 3◦ intervalmg and froma linear 0◦ toaccuracy 180◦ without of 0.2 mg. any The input first current. test consisted The motor of measurwas designeding the topassive operate torque at a minimum-angle curve activation at every current 3° interval of 0.5 from A and 0° ato maximum 180° without current any ofinput 1 A. current.The torque-angle The motor curves was designed were measured to operate at currentsat a minimum from 0.5activation A to 1 A current in increments of 0.5 A and of 0.1 a maximum A at every current3◦ interval of 1 from A. The 0◦ totorque 90◦. -angle curves were measured at currents from 0.5 A to 1 A in increments of 0.1Table A at 6every shows 3° the interval theoretical from and0° to experimental 90°. torque characteristics of the single-phase switched reluctanceTable 6 motor, shows includingthe theoretical the minimumand experimental torque, torque maximum characteristics torque, average of the single torque,-phase and switched relative reluctancetorque ripple, motor at various, including current the levels.minimum Figure torque, 17 shows maximum the theoretical torque, and average experimental torque, torque-angleand relative torquecurves ripple of the, at single-phase various current switched levels. reluctance Figure 17 shows motor. the The theoretical test results and showed experimental that the torque maximum-angle curvesdetent torqueof the single was 12-phaseµN·m switched at a 30◦ angle.reluctance At a minimummotor. The activation test results current showed of 0.5 that A, the the maximum minimum detenttorque torque was 1.54 waµsN 12·m, μN·m also atat aa 3030°◦ angle.angle. WhenAt a m theinimum current activation ranges from current 0.6 Aof to0.5 1 A A,, the the minimum torquetorque wa wass 1.54 always μN·m 1.92, alsoµN ·atm a at 30° a 91 angle.◦ angle, When meaning the current that the ranges coil wasfrom no 0. longer6 A to 1 excited. A, the minimum When the torquerated current was always was1 1.92 A, theμN·m motor at a produced 91° angle,an meaning average that torque the coil of 10.34 was noµN longer·m with excited. a relative When torque the ratedripple current of 80.66%. was It 1 should A, the bemotor noted produce that thed theoreticalan average and torque experimental of 10.34 μ torque-angleN·m with a relative curves didtorque not ripplematch of well 80.66%. at larger It should currents, be noted and thus that thethe motortheoretical friction and was experimental tested to clarify torque this-angle issue. curves did not match well at larger currents, and thus the motor friction was tested to clarify this issue. Table 6. Comparisons of theoretical and experimental torque characteristics of the single-phase Tableswitched 6. C reluctanceomparisons motor. of theoretical and experimental torque characteristics of the single-phase switched reluctance motor. Max. Torque (µN·m) Min. Torque (µN·m) Average Torque (µN·m) Relative Ripple Torque (%) Current (A) Max. Torque (μN·m) Min. Torque (μN·m) Average Torque (μN·m) Relative Ripple Torque (%) Current (A) Max.Theory Torque Experiment (μN·m) Min. Theory Torque Experiment (μN·m) Average Theory Torque Experiment (μN·m) TheoryRelative Ripple Experiment Torque (%) Current (A) Theory Experiment Theory Experiment Theory Experiment Theory Experiment 0.5Theory 12.03 Experiment 9.26 Theory 1.98 Experiment 1.54 Theory 5.65 Experiment 4.29 71.77Theory Experiment 71.43 0.5 0.612.03 15.37 9.26 11.69 1.98 2.72 1.54 1.92 5.65 7.33 5.564.29 69.8971.77 67.2371.43 0.6 0.715.37 18.75 11.69 14.33 2.72 2.72 1.92 1.92 7.33 8.99 6.775.56 74.6469.89 72.4567.23 0.7 0.818.75 22.20 14.33 16.98 2.72 2.72 1.92 1.92 10.628.99 8.016.77 78.1574.64 76.2372.45 0.8 0.922.20 25.61 16.98 19.40 2.72 2.72 1.92 1.92 10.62 12.21 9.268.01 80.7878.15 78.8976.23 0.8 122.20 29.04 16.98 21.39 2.72 2.72 1.92 1.92 10.62 13.75 10.348.01 82.8578.15 80.6676.23 0.9 25.61 19.40 2.72 1.92 12.21 9.26 80.78 78.89 1 29.04 21.39 2.72 1.92 13.75 10.34 82.85 80.66

Energies 2018, 11, 2772 11 of 17 Energies 2018, 11, x 11 of 17

0A Theory 40 0.5A Theory

0.6A Theory 30 0.7A Theory

20 0.8A Theory m)

- 0.9A Theory

N μ 10 1A Theory

0A Exp

Torque ( Torque 0 0.5A Exp

0.6A Exp -10 0.7A Exp

-20 0.8A Exp 0 15 30 45 60 75 90 105 120 135 150 165 180 0.9A Exp Angle (degree) 1A Exp

FigureFigure 17. Theoretical 17. Theoretical and and experimental experimental torque torque-angle-angle curves curves of the single-phasesingle-phase switched switched reluctance reluctance motor. motor.

33.3..3. F Frictionriction Testing Testing AAss friction friction greatly greatly affects affects the the performance performance of of a a micro micro motor, motor, this test measures measures the the friction friction torque torque byby hanging hanging tiny tiny weight weightss on on the the outer outer diameter diameter of the of therotor. rotor. For Forthis thistest, test,the permanent the permanent magnets magnets were ◦ ◦ ◦ removedwere removed and measurements and measurements were were taken taken at every at every 3° interval 3 interval between between 0° and 0 and360° 360to calculateto calculate the averagethe average friction friction torque, torque, which which was found was found to be 1.19 to be μN·m 1.19.µ InN order·m. In to order find the to findimpact the of impact the friction of the, itfriction, was assume it wasd assumed that the friction that the wa frictions entirely was against entirely the against output the torque. output The torque. study The add studyed the added friction the torfrictionque to torque the output to the output torque torque,, and the and output the output characteristics characteristics were were recalculated. recalculated. Table Table 77 showsshows the calculatedcalculated values of the maximum torque and average torque of the motor with and without friction correctioncorrection.. Figure Figure 18 18 shows shows t thehe theoretical theoretical and and experimental experimental tor torque–angleque–angle curves curves of of the the motor motor with frictionfriction correction. correction. At At a a rated rated current current of of 1 1 A A,, the deviation deviationss between the theoretical and measured maximummaximum torque torque and and average average torque torque we werere 26% 26% and 16% 16%,, respectively respectively.. It It should should be be noted that that the deviationdeviation increase increasedd as the input current increase increased.d. IItt wa wass found found that that the the deviation between the the theoretical and measured torque torque-angle-angle curves wa wass duedue to magnetic saturation in the rotor structure. After careful examin examinationation of the electromagnetic analysisanalysis,, it it wa wass found found that that the the maximum maximum flux flux densities densities of the of therotor rotor reach reacheded 1.86 1.86T and T 1.9 and4 T 1.94 at a T74 at° ◦ anglea 74 anglewith 0. with5 A 0.5 and A and1 A 1currents A currents,, respectively respectively.. The The maximum maximum flux flux densit densityy of of the the rotor rotor made made of of siliconsilicon steel steel 35C300 35C300 can can only only handle handle 1 1.63.63 T T.. It It is is suggested suggested that that the the motor motor be be redesigned redesigned by by increasing increasing thethe rotor rotor thickness to avoid magnetic saturation in the future.

Table 7.7. TorqueTorque characteristics of of the singlesingle-phase-phase switched reluctance reluctance motor motor with and without frictionfriction correction correction..

MaximumMaximum Torque Torque (μ (µN·mN·m)) AverageAverage Torque Torque (µ (NμN·m·m) ) CurrentCurrent (A) (A) ExperimentExperiment without ExperimentExperiment with ExperimentExperiment without ExperimentExperiment with TheoryTheory Theory withoutCorrection Correction withCorrection Correction withoutCorrection Correction withCorrection Correction 0.5 0.512.03 12.03 9.26 9.26 10.45 10.45 5.65 4.29 5.485.48 0.6 0.615.37 15.37 11.69 11.69 12.88 12.88 7.33 5.56 6.756.75 0.7 0.718.75 18.75 14.33 14.33 15.52 15.52 8.99 6.77 7.967.96 0.8 0.822.20 22.20 16.98 16.98 18.17 18.17 10.62 8.01 9.209.20 0.9 0.925.61 25.61 19.40 19.40 20.59 20.59 12.21 9.26 10.4510.45 1 129.04 29.04 21.39 21.39 21.58 21.58 13.75 10.34 11.5311.53

Energies 2018, 11, 2772 12 of 17 EnergiesEnergies 2018 2018, 11, 11, x, x 12 12of of17 17

0A Theory 40 0A Theory 40 0.5A Theory 0.5A Theory 0.6A Theory 30 0.6A Theory 30 0.7A Theory 0.7A Theory 20 0.8A Theory

20 0.8A Theory m)

- 0.9A Theory

N - μ 0.9A Theory

N 10 μ 10 1A Theory 1A Theory 0A Exp+Friction Torque ( Torque 0 0A Exp+Friction Torque ( Torque 0 0.5A Exp+Friction 0.5A Exp+Friction -10 0.6A Exp+Friction 0.6A Exp+Friction -10 0.7A Exp+Friction 0.7A Exp+Friction -20 0.8A Exp+Friction 0 15 30 45 60 75 90 105 120 135 150 165 180 -20 0.8A Exp+Friction 0 15 30 45 60 75 90 105 120 135 150 165 180 0.9A Exp+Friction Angle (degree) 1A0.9A Exp+Friction Exp+Friction Angle (degree) 1A Exp+Friction

Figure 18. Theoretical and experimental torque-angle curves of the motor with friction correction. FigureFigure 18 18.. TheoreticalTheoretical and and experimental experimental torque torque-angle-angle curves of the motor with friction correction correction.. 3.4. Motor Speed Testing 3.4. Motor Speed Testing 3.4. Motor Speed Testing The speed of this motor was tested by a direct connection to the motor driver with a square wave The speed of this motor was tested by a direct connection to the motor driver with a square wave currentThe speedinput ,of and this the motor voltage wa sand tested current by a directsignals connection of the coil to we there motormeasured driver at variouswith a square excitation wave current input, and the voltage and current signals of the coil were measured at various excitation frequencies. currentfrequencies. input, Atand a ratedthe voltage current and of 1 current A, the motorsignals achieve of thed coil a maximum were measured rotation atspeed various of 150 excitation0 rpm At a rated current of 1 A, the motor achieved a maximum rotation speed of 1500 rpm with an excitation frequencies.with an excitation At a rated frequency current of of 50 1 H Az., Thethe voltagemotor achieve and currentd a maximum signals we rotationre 4.4 V and speed 1 A of, as 150 shown0 rpm frequency of 50 Hz. The voltage and current signals were 4.4 V and 1 A, as shown in Figure 19. within Figure an excitation 19. frequency of 50 Hz. The voltage and current signals were 4.4 V and 1 A, as shown in Figure 19.

0.5 Current Current (A) 1 0

0.5 Current Current (A)

0 Current 4 Voltage 3 Current 4 Voltage 2

3 Voltage (V) Voltage 1 2

0 Voltage (V) Voltage 1

0 0 1 2 3 4 5 6 7 8 9 10 Time (ms)

0 1 2 3 4 5 6 7 8 9 10 FigureFigure 19. 19Voltage. Voltage and and current current signalssignals ofof thethe single-phaseTimesingle (ms)-phase switched switched reluctance reluctance motor motor at ata amaximum maximum

speedspeed of of 1500 1500 rpm. rpm. Figure 19. Voltage and current signals of the single-phase switched reluctance motor at a maximum 4.4 Wireless-Driven. Wirelessspeed of -150Driven0 rpm Single-Phase Single. -Phase SwitchedSwitched Reluctance Motor Motor TheThe single-phase single-phase switched switched reluctancereluctance motor wa wass easier easier to to apply apply to to a awireless wireless-driven-driven circuit circuit than than 4typical. typicalWireless three-phase three-Driven-phase Single motors. motors.-Phase The The motor motorSwitched driver Reluctance wa wass combined combined Motor with with a a commercial commercial wireless wireless charging charging modulemoduleThe to single to produce produce-phase square square switched wave wave reluctance signalssignals transmittedtransmitted motor wa wirelessly wirelesslys easier to to toapply the the single single-phaseto a -wirelessphase switched switched-driven reluctancecircuit reluctance than typicalmotor.motor. The three The experiment- phase experiment motors. used use theThed XKT-412the motor XKT driver- wireless412 wireless wa transmissions combined transmission modulewith amodule commercial and XKT335 and XKT335 wireless wireless wireless charging charging moduleICcharging produced to produceIC byproduced Shenzhen square by Shenzhenwave Core Ketaisignals Core Electronics transmitted Ketai Electronics Co., wirelessly Ltd. Co (Shenzhen,., toLtd. the (Shenzhen, single China).-phase China). The switched operating The operating reluctance voltage motor.ofvoltage the wireless The of the experiment chargingwireless charging usemoduled the wasmodule XKT 5- V412 wa to s wireless12 5 VV withto 1 transmission2 aV frequency with a frequency module of 0 to 2of and MHz. 0 to XKT335 2 ItM canHz. producewirelessIt can charginga maximumproduce IC a producedmaximum current of bycurrent 1 AShenzhen with of a1 transmissionA Core with Ketai a transmission Electronics distance distance of Co 0., mm Ltd. of to (Shenzhen, 0 10 m mm.m to 1 The0 China). m schematicm. The The schematic operating diagram voltageofdiagram the wireless-driven, of ofthe the wireless wireless charging- axial-ﬂux,driven, axial module single-phase-flux, wasingles 5- phase switchedV to 1 switched2 Vreluctance with reluctance a frequency motor motor isof shown is0 shownto 2 inM inHz. Figure Figure It can 20 , produceand20,a and photograph aa maximumphotograph of thecurrent of actualthe actualof set-up1 A set with- isup shown ais transmissionshown in Figure in Figure distance21. 21. of 0 mm to 10 mm. The schematic diagram of the wireless-driven, axial-flux, single-phase switched reluctance motor is shown in Figure 20, and a photograph of the actual set-up is shown in Figure 21.

Energies 2018, 11, x 13 of 17 Energies 2018, 11, x 13 of 17 Energies 2018, 11, 2772 13 of 17 Wireless Charging Energies 2018, 11, x 13 of 17 Wireless Charging Wireless Charging

DC Power Motor Driver Transmitter DCsupplyPower Motor Driver Transmitter supplyDC Power Motor Driver Distance : 0~6mmTransmitter supply Distance : 0~6mm Oscilloscope Motor Distance : 0~6mmReceiver Oscilloscope Motor Receiver Oscilloscope Motor Receiver

Figure 20. Schematic diagram of the wireless-driven, single-phase switched reluctance motor system. FigureFigureFigure 20 20.. Schematic Schematic20. Schematic diagram diagram diagram of of of the the the wireless wireless-driven, wireless--drivendriven,, singlesingle single-phase-phase-phase switched switched switched reluctance reluctance reluctance motor motor motor system system system.. .

Figure 21. Photograph of the actual wireless-driven, single-phase switched reluctance motor test Figure 21. Photograph of the actual wireless-driven, single-phase switched reluctance motor test system. FiguresystemFigure .21 21.. Photograph ofof the the actual actual wireless-driven, wireless-driven single-phase, single-phase switched switched reluctance reluctance motor test motor system. test system4.1. Current. -Distance Curve Testing 44.1..1. Current Current-Distance-Distance Curve Testing 4.1. CurrentAs- Distancethe output Curve characteristics Testing of the wireless-driven circuit are greatly affected by changes in AoutputAss the loadoutput and characteristics distance, this study of the first wireless wireless-driven tested -thedriven relationship circuit circuit betweenare are greatly the affected output current by changes and in outputAdistance.s loadthe output and In order distance, characteristics to avoid this the study motor of the firstﬁrst inductance wireless testtesteded -theeffect, driven relationship the circuit experiment are between greatly compare the affectedd output the results by currentchanges with and in outputdistance.two load test In orderloadsorderand :distance, to3.4to avoidΩavoid for th the thisthee motor motor studymotor and inductance firstinductance a pure test resistanceed effect, theeffect, relationship of the 3.6the experiment Ω. experiment In the between experiment, compared compare the the output the dpower the results currentresults supply with with and two distance.twotest test loads:was loads setIn 3.4 toorder: aΩ3.4 voltagefor Ωto for theavoid ofth motor e5 motorVthe and andmotor currentand a pureainductance pure of resistance1 Aresistance, and effect, the of measurementsof 3.6the 3.6Ω experimentΩ.. In In the the of experiment,experiment, the compare wirelessd- thethereceiving the power results end supply with twowawass testset we re to loads taken a voltage: 3.4 at Ωa distance forof 5th Ve motorandof every current and 1 ma mpure of from 1 A,Aresistance, 0 and mm the to of 6measurements m3.6m. Ω. The In currentsthe experiment, of theof the wireless-receivingwireless mot theor - powerreceivingand pure supply end resistance loads reached their maximum values of 0.9 A and 0.84 A, respectively, and fell quickly waweweresre set taken to a at voltage a distance of 5 Vof and every current 1 mmmm of from 1 A, 0and mmmm the to measurements 6 mm.mm. The currents of the ofwireless the mot motor-receivingor and pure end over a 2 mm distance. Figure 22 shows the current-distance curves of the single-phase switched weresistancere taken loadsloads at a reacheddistancereachedtheir theirof every maximum maximum 1 mm values fromvalues 0 of mof 0.9m 0. Ato9 andA6 andm 0.84m. 0 The.8 A,4 respectively,Acurrents, respectively of the and, motand fellor quicklyfe lland quickly pure over reluctance motor and pure resistance loads. resistanceovera 2 mm a 2 distance m loadsm distance. .reach Figureed Figure22 their shows maximum 22 the shows current-distance values the current of 0.-9distance curves A and of 0 curves.8 the4 A single-phase, respectively of the single switched, -andphase fe reluctancell switched quickly overreluctancemotor a and2 m motor purem distance. resistance and pure Figure loads.resistance 22 shows loads. the current-distance curves of the single-phase switched 0.95 reluctance motor and pure resistance0.9 loads. 0.85 0.95 0.8 0.9 Motor Resistance 0.950.75 3.4Ω 0.850.9 Pure Resistance 0.7 3.6Ω

0.850.80.65 Motor Resistance Current (A) Current 3.4Ω 0.750.8 0.6 MotorPure Resistance Resistance 0.750.70.55 3.4Ω3.6Ω 0.65 0.5 Pure Resistance Current (A) Current 0.7 0 1 2 3 4 5 6 3.6Ω 0.650.6 Current (A) Current Distance (mm)

0.550.6 Figure 22. Current-distance0.550.5 curves of the single-phase switched reluctance motor and pure resistance loads. 0 1 2 3 4 5 6 0.5 Distance (mm) 0 1 2 3 4 5 6 Distance (mm)

FigureFigure 22. Current 22. Current-distance-distance curves curves of ofthe the single single-phase-phase switched switched reluctance motor motor and and pure pure resistance resistance loads. loads. Figure 22. Current-distance curves of the single-phase switched reluctance motor and pure resistance loads.

Energies 2018, 11, x 14 of 17

4.2. Current-Frequency Curve Testing

TheEnergies purpose2018, 11 ,of 2772 this experiment was to examine the excitation frequency effect on 14the of 17output currentEnergies of the 2018 ,wireless 11, x charging module. To avoid the influence of the motor inductance14 of 17 , the experiment was conducted with a pure resistance load of 3.6 Ω at a 2 mm distance. The output 44.2..2. Current Current-Frequency-Frequency Curve Testing currents were set at 0.5 A, 0.6 A, and 0.7 A at 1.25 Hz, and the measurements were taken with an excitation frequencyThe purpose at of every this experiment 10 Hz between wa wass to examine1.25 Hz theand excitation 100 Hz. frequency The test effect results on showed the output that an current of of the the wireless wireless charging charging module. module. To avoid To avoid the inﬂuence the influence of the motor of the inductance, motor inductance the experiment, the increase in frequency led to a gradual decrease of current for increases in frequency up to 70 Hz, after wasexperiment conducted was with conducted a pure resistance with a pure load resistance of 3.6 Ω at load a 2 mm of 3.6 distance. Ω at a The2 m outputm distance. currents The were output set which the current gradually increased with further increases in the frequency. At 90 Hz, the current currentsat 0.5 A, 0.6were A, set and at 0.7 0.5 A A at, 0. 1.256 A Hz,, and and 0.7 the A measurementsat 1.25 Hz, and were the measurements taken with an excitationwere taken frequency with an was atexcitationat its every maximum. 10 frequency Hz between Figure at every 1.2523 shows Hz10 andHz the between 100 current Hz. The1.2-5frequency testHz resultsand 10 curves0 showed Hz. The of that testthe an wirelessresults increase showed charging in frequency that modulean . increaseled to a gradualin frequency decrease led to of a currentgradual for decrease increases of current in frequency for increases up to 70in frequency Hz, after which up to 7 the0 H currentz, after whichgradually the increasedcurrent gradually with further increase increasesd with in further the frequency. increases At in 90 the Hz, frequency the current. At was 90 atHz, its the maximum. current 0.8 waFigures at its23 maximum.shows the current-frequency Figure 23 shows the curves current of the-frequency wireless curves charging of the module. wireless charging module. 0.7 0.6 0.8 0.5 I=0.7A 0.7 0.4 I=0.6A 0.6 I=0.5A 0.3

0.5 I=0.7A Current (A) Current 0.2 0.4 I=0.6A I=0.5A

0.1 0.3 Current (A) Current 0 0.2 0 10 20 30 40 50 60 70 80 90 100 0.1 Frequency (Hz) 0 0 10 20 30 40 50 60 70 80 90 100 Frequency (Hz) Figure 23. Current-frequency curves of the wireless charging module with pure resistance loads at a 2 mm distance. Figure 23 23.. CurrentCurrent-frequency-frequency curves curves of of the the wireless wireless charging charging module module with with pure pure resistance resistance load loadss at ata 2a m 2 mmm distance distance.. 4.3. Performance Comparison of Wire- and Wireless-Driven Motors 44.3..3. Performanc Performancee Comparison of Wire Wire-- and Wireless-DrivenWireless-Driven Motors This study compared the performances of wire- and wireless-driven, single-phase switched This study compared the performances of wire- and wireless-driven, single-phase switched reluctance Tmotorshis study based compare on voltaged the performancesand current signals. of wire- Theand experiment wireless-driven wa,s singleconducted-phase with switched the motor reluctance motors based on voltage and current signals. The experiment was conducted with the operatedreluctance with a motors0.7 A current based on and voltage a 30 and Hz current excitation signals. frequency The experiment or a 90 wa0 rspm conducted rotation with speed. the motor The motor operatedmotor operated with a 0. with7 A current a 0.7 A and current a 30 andHz excitation a 30 Hz excitation frequency frequency or a 900 rpm or arotation 900 rpm speed. rotation The speed.motor had a wired transmission connected to the power source or a wireless transmission that was 2 mm from haThed a motor wired had transmission a wired transmission connected to connectedthe power source to the poweror a wireless source transmission or a wireless that transmission was 2 mm from that the powerwasthe power 2 source mm from source. The the. The voltage power voltage source. and and current The current voltage signals signals and ofcurrent thethe wire wire signals- and- and of wireless the wireless wire--driven and-driven wireless-driven conditions conditions are are shownshownconditions in the in left the are and left shown andright right in diagrams, the diagrams, left and respectively right respectively diagrams,,, in respectively, FigureFigure 24. 24. in Figure 24.

1 1 1 1

0.5 0.5 Current Current (A)

0.5 Current (A) 0.5 Current Current (A) 0 Current (A) 0 0 0

Current Current 4 4 CurrentVoltage Voltage Current 4 4 3 Voltage 3 Voltage

3 2 23

Voltage (V) Voltage Voltage (V) Voltage

2 1 12 Voltage (V) Voltage 0 (V) Voltage 0 1 1

0 0 1 2 3 4 5 6 7 8 9 10 00 1 2 3 4 5 6 7 8 9 10 Time (ms) Time (ms)

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 Time (ms) Time (ms) Figure 24. Voltage and current signals of the motor at 900 rpm with wire-driven (left) and wireless- driven (right) conditions. FigureFigure 24. Voltage 24. Voltage and andcurrent current signals signals of of the the motor motor at 900900 rpmrpm with with wire-driven wire-driven (left ()left and) wireless-and wireless- drivenThedriven (right test ()right conditionsresults) conditions. show. ed that the wire-driven, single-phase switched reluctance motor exhibited almost no delay, but the wireless-driven motor exhibited an evident time delay. The differences in The test results showed that the wire-driven, single-phase switched reluctance motor exhibited the time response of the wire- and wireless-driven motors are summarized in Table 8. When Thealmost test noresults delay, show but theed wireless-driven that the wire motor-driven exhibited, single an-phase evident switched time delay. reluctance The differences motor in exhibit the ed operating at 0.7 A of current, the wire-driven motor reached a maximum rotation speed of 990 rpm, almosttime no delay, response but of the wire-wireless and- wireless-drivendriven motor motorsexhibit areed summarizedan evident time in Table delay.8. When The operating differences in the timewhile response the wireless of -driven the wire motor- and only wireless reached -90driven0 rpm. The motors time aredelay summarized of the wireless in-driven Table motor 8. When operatingresul atted 0. in7 Aa lower of current, rotation the speed. wire -driven motor reached a maximum rotation speed of 990 rpm, while the wireless-driven motor only reached 900 rpm. The time delay of the wireless-driven motor resulted in a lower rotation speed.

Energies 2018, 11, 2772 15 of 17 at 0.7 A of current, the wire-driven motor reached a maximum rotation speed of 990 rpm, while the wireless-driven motor only reached 900 rpm. The time delay of the wireless-driven motor resulted in a lower rotation speed.

Table 8. Time response comparisons of the wire- and wireless-driven, single-phase switched reluctance motors.

Speed (rpm) Driven Method Stable Time (ms) Rise Time (ms) Lag Time (ms) Wire-driven 0.10 0.08 0.05 900 Wireless-driven 1.26 0.86 0.54

5. Discussion Single-phase switched reluctance motors have the advantages of the lowest cost with the minimum number of components and the simplest controller, but show limited performance in terms of output torque and efﬁciency. This paper proposed a novel axial-ﬂux, micro motor design using the topology method on a rotor structure only. A commercial wireless charge module was used to demonstrate the wireless-driven capability for possible new applications in consumer electronics. There are a few improvements required for its future application.

1. The limitation of the current density of copper wire is due to heat dissipation in the environment. The current density is higher for smaller diameter wire because of a larger surface to mass ratio for heat dissipation. Although the coils were tested with a maximum current of 1 A for over one hour in the laboratory, it is suggested that the diameter of the coils should be increased. The efﬁciency of the motor also improved with lower resistance. It is also noted that the coils are not continuously supplied with current since the duty cycle is only 50%. The diameter of the coils should be increased to 0.16 mm for a current of 0.7 A. 2. The magnetic circuit design of the motor can be improved. The nickel magnetic rods were chosen for use in the laboratory due to their availability. Materials with higher permeability and magnetization levels such as NiFe or FeSiB are recommended. One reason for the selection of AlNiCo magnets was their cheaper price compared to NdFeB magnets. The other reason was that the energy density and size of the permanent magnets are not important because the magnets are on the stator side and the magnetic force of the magnets must be less than the electromagnetic force of the coils. It should be noted that the pole plate for permanent magnets can be included in the base plate. The magnetic path of the permanent magnets and coils can share the same base plate. The motor can be also designed with only one magnet for simplicity. 3. After performance testing, it was found that the commercial wireless charge module applied in this motor was not suitable for serious applications. An operating distance of at least 5 to 10 mm is required for water tanks and vacuum vessels. Since current research on the motor is focused on electric fans, pumps, and hammer breakers for large power outputs, this study tried to evoke possible new applications with wireless technology. The performance of the wireless-driven motor can be greatly enhanced if the wireless driver circuit is designed taking the motor into consideration.

6. Conclusions This paper presented a wireless-driven, axial-ﬂux, single-phase switched reluctance motor that utilizes a two-pole design for both the rotor and stator and two permanent magnets on the stator to provide the starting position and rotational torque when there is no electromagnetic excitation. The study proposed an innovative design with cylindrical stator poles and permanent magnets to facilitate the production and assembly of micro-motors. The topology design utilized in the rotor structure reduced torque ripple. The motor was operated at a minimum activation current of 0.5 A and a maximum current of 1 A and provided an average output torque of 10.34 µN·m or reached Energies 2018, 11, 2772 16 of 17 a maximum rotation speed of 1500 rpm. Restrictions due to the currently available wireless-driven circuit resulted in a maximum current of 0.7 A and a maximum rotating speed of 900 rpm. The current wireless transmission distance was limited to 2 mm, but if the wireless charging module can be improved, the wireless transmission distance can be increased effectively. As the wireless-driven motor does not require a physical power connection or battery, it is suitable for applications such as toys, micro-pumps, water tanks, and pressure vessels that do not require feedthrough.

Author Contributions: D.-C.P. conceived the wireless-driven, micro, axial-flux, single-phase switched reluctance motor; D.-C.P. and C.-T.W. designed the motor; C.-T.W. assembled the motor and performed the experiments; D.-C.P. and C.-T.W. analyzed the data; D.-C.P. and C.-T.W. wrote the draft paper; D.-C.P. finalized the paper. Funding: The financial supports of the MEMS and Precision Machinery Research and Development Center and the Department of Mechanical Engineering at National Kaohsiung University of Science and Technology for the development of the micro motor. Conflicts of Interest: The authors declare no conflict of interest. The founding sources had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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