energies

Article Investigation of an Inset Micro Permanent Synchronous Motor Using Soft Magnetic Composite Material

Da-Chen Pang 1 , Zhen-Jia Shi 1, Pei-Xuan Xie 1, Hua-Chih Huang 1 and Gia-Thinh Bui 2,* 1 Department of Mechanical Engineering, National Kaohsiung University of Science and Technology, 415 Jian Gong Rd., Sanmin Dist., Kaohsiung 80778, Taiwan; [email protected] (D.-C.P.); [email protected] (Z.-J.S.); [email protected] (P.-X.X.); [email protected] (H.-C.H.) 2 Department of Electrical and Mechanical Engineering, Hai Phong University, 171 Phan Dang Luu Road, Kien An District, Hai Phong City 180000, Vietnam * Correspondence: [email protected]

 Received: 11 August 2020; Accepted: 25 August 2020; Published: 27 August 2020 

Abstract: This paper presents the world’s smallest inset permanent magnet synchronous motor (PMSM) with a soft magnetic composite (SMC) core, providing ease of manufacturing for micromachine applications without silicon steel laminations. The inset motor can offer an additional reluctance and higher torque density with a lower usage amount of permanent magnet. A 15 mm diameter inset motor was developed with the thickness of a tile-type permanent magnet which is limited to 1 mm by the manufacturer. The motor was designed with high torque density and low torque ripple by varying the interpole iron width for the . Two inset motors were made using both SMC and silicon steel materials for comparison. The performance of the SMC motor was inferior to the silicon steel motor, but it still meets the specifications of the commercial market. If the thickness of the tile-type permanent magnet is further reduced, the micro inset motor with a SMC core can be easily mass-manufactured using powder sintering.

Keywords: inset permanent magnet synchronous motor; soft magnetic composite; silicon steel; micromotor

1. Introduction The permanent magnet synchronous motor (PMSM) has inherent advantages of having high efficiency and a high torque density compared to other industrial motors. The inset PMSM is superior in performance to the surface-mounted type as it has a strong rotor structure, low eddy current loss, and wide flux-weakening capability [1–3]. The inset PMSM is currently used in electric vehicles [4], wind power generation [5], and flywheel energy storage systems [6]. However, the inset PMSM is mainly utilized for medium to large size applications rather than small size ones due to the difficulty of manufacturing it. Hwang et al. [7] made the smallest inset motor with an external diameter of 78 mm and a length of 50 mm. The rotor had a particular slot shape design for magnet retention and ease of assembly. Buttgenbach [8] reviewed the latest development of electromagnetic micromotors with breakthroughs in lithography, electroplating, and other micromachining technologies to expand new possibilities in microfluidics, micropositioning, and microrobots. One of the major technological challenges is to improve the magnetic properties of electroplated soft magnetic materials. Pang and Lai [9] developed a 1 mm diameter micromotor and its was made with a nickel–cobalt electroplated shell and flexible print circuit coils. The motor performance was limited by the nickel–cobalt shell with a saturation flux density of 0.5 T and a relative permeability of 600.

Energies 2020, 13, 4445; doi:10.3390/en13174445 www.mdpi.com/journal/energies Energies 2020, 13, 4445 2 of 11

Soft magnetic composite (SMC) material was first used as a motor core in 1995 [10] and has unique three-dimensional (3D) isotropic magnetic properties and low eddy current loss [11]. Höganäs offers commercial SMC Somaloy products for different performance levels and applications [12,13]. For example, the Somaloy 1000 3P provides the best mechanical strength and permeability with a saturation flux density of 1.42 T and a maximum relative permeability of 950. SMCs are widely used in various motor types, such as PMSMs [14], brushless DC motors [15], reluctance motors [16], and induction motors [17]. Kim [18] confirmed the superior performance of an axial-flux PMSM core material using SMC over silicon steel at a high-speed region. Furthermore, the SMC motors made by powder metallurgy are suitable for 3D integrated structures without assembly and meet competitive price in mass production [19–22]. In 2019, Wang et al. [23] made the smallest SMC BLDC PM motor, with an external diameter of 40.6 mm and a length of 32.5 mm, for small aircraft electric motors. The inset PMSM provides excellent performance and is a great candidate for micromotors, but its development is currently limited due to the difficulty of micromachining. The SMC material provides better magnetic properties than the electroplating nickel alloy used in micromotors. The powder sintered SMC material offers a net-shaping stator and rotor for ease of microfabrication. In this paper, a micro inset PMSM design is proposed for higher torque density and lower torque ripple. The inset PMSMs were made using two different core materials: electrical steel and SMC. The performances of both motors were experimentally tested to demonstrate the feasibility of their miniaturization.

2. Inset Permanent Magnet Synchronous Motors Design and Analysis

2.1. Inset Permanent Magnet Synchronous Motors Design The micro inset motor was designed with a six-slot stator and a four-pole rotor. The stator had an external diameter of 15 mm, an internal diameter of 8.2 mm, and stack length of 5 mm. The rotor had an external diameter of 7.4 mm, root diameter 5.4 mm and an internal diameter of 2 mm. The tile-type permanent were made of sintered NdFeB, grade N48H (Teslar Technology Co., Ltd., Taichung, Taiwan) with radial magnetization. The thickness of the permanent magnet was limited to 1 mm by the manufacturer. Each winding coil had a total of 60 turns and resistance of 2.65 Ω. Table1 lists the design specifications of the inset permanent magnet synchronous motor. Figure1 shows the exploded view and assembly drawing of the inset motor.

Table 1. Design specifications of the inset permanent magnet synchronous motor.

Mechanical Specifications Slots 6 Poles 4 External diameter 15 mm External diameter 7.4 mm Stator Internal diameter 8.2 mmRotor Internal diameter 2 mm Stack length 5 mm PM thickness 1 mm Air gap length 0.4 mm Interpole iron width 1.6 mm Electrical Specifications

Phase 3 Step angle 60◦ Number of turns of coil 60 turns Diameter of coil 0.162 mm Maximum current 1 A Resistance of coil 2.65 Ω Energies 2020, 13, x FOR PEER REVIEW 3 of 11 Energies 2020,, 13,, 4445x FOR PEER REVIEW 3 of 11 Energies 2020, 13, x FOR PEER REVIEW 3 of 11

Figure 1. Exploded view and assembly drawing of the inset permanent magnet synchronous motor. Figure 1. ExplodedExploded view view and and assembly assembly drawing of the inset permanent magnet synchronous motor. Figure 1. Exploded view and assembly drawing of the inset permanent magnet synchronous motor. To demonstrate the feasibility of powder sintering the SMC material, the micro inset motors To demonstratedemonstrate the feasibility of powder sintering the SMC material, the micro inset motors wereTo developed demonstrate using the two feasibility different of soft powder magnetic sintering materials: the SMC a silicon material, steel thesheet micro (35CS300) inset motors (China were developed developed using using two two different different soft soft magnetic magnetic materials: materials: a silicon a silicon steel sheet steel (35CS300) sheet (35CS300) (China wereSteel developedCorporation, using Kaohsiung, two different Taiwan) soft and magnetic Somaloy materials: 700 1P (Högnäs a silicon AB, steel Bruksgatan, sheet (35CS300) Sweden). (China The (ChinaSteel Corporation, Steel Corporation, Kaohsiung, Kaohsiung, Taiwan) Taiwan) and Somaloy and Somaloy 700 1P 700 (Högnäs 1P (Högnäs AB, Bruksgatan, AB, Bruksgatan, Sweden). Sweden). The Steelmagnetization Corporation, curves Kaohsiung, of both materialsTaiwan) andare shownSomaloy in 700Figure 1P (Högnäs2. AB, Bruksgatan, Sweden). The Themagnetization magnetization curves curves of both of both materials materials are shown are shown in Figure in Figure 2. 2. magnetization curves of both materials are shown in Figure 2.

Figure 2. Magnetization curves of silicon steel (35CS300) and Somaloy 700 1P. Figure 2. MagnetizationMagnetization curves curves of of silicon silicon steel (35CS300) and Somaloy 700 1P 1P.. Figure 2. Magnetization curves of silicon steel (35CS300) and Somaloy 700 1P. 2.2.2.2. Rotor Rotor Design Design of of the the Inset Inset Permanent Permanent Magnet Magnet Synchronous Synchronous Motor Motor 2.2. Rotor Design of the Inset Permanent Magnet Synchronous Motor 2.2. Rotor Design of the Inset Permanent Magnet Synchronous Motor TheThe design design parameters ofof thethe insetinset motormotor were were fixed fixed for for this this study, study, allowing allowing variation variation only only in thein The design parameters of the inset motor were fixed for this study, allowing variation only in theinterpole interpoleThe design iron iron width parameters width of of the the rotor.of rotor. the Figureinset Figure motor3 shows3 shows were an an illustrationfixed illustration for this of of study, the the inset inset allowing magnet magnet variation rotor.rotor. ThereThere only was in the interpole iron width of the rotor. Figure 3 shows an illustration of the inset magnet rotor. There was thenono interpoleair air gap gap between between iron width the the ofinterpole interpole the rotor. iron iron Figure and and 3the the shows pe permanentrmanent an illustration magnet. of The Thethe surfinset surface-mountedace-mounted magnet rotor. motor motor There was was was a a no air gap between the interpole iron and the permanent magnet. The surface-mounted motor was a nospecialspecial air gap case case between where where thethe interpoleinterpole ironiron widthand width the was was pe setrmanent set to to zero. zero. magnet. The surface-mounted motor was a special case where the interpole iron width was set to zero. special case where the interpole iron width was set to zero.

FigureFigure 3. 3. IllustrationIllustration of of the the inset inset magnet magnet rotor. rotor. Figure 3. Illustration of the inset magnet rotor. Figure 3. Illustration of the inset magnet rotor. The 2D electromagnetic analysis of inset motors was determined using JMAG finite element The 2D electromagnetic analysis of inset motors was determined using JMAG finite element analysisThe software2D electromagnetic developed byanalysis the JSOL of inset Corporation, motors was Tokyo, determined Japan. Various using JMAG motor finite designs element were analysisThe software2D electromagnetic developed analysisby the JSOL of inset Corpor motoation,rs was Tokyo, determined Japan. Varioususing JMAG motor finite designs element were evaluatedanalysis software with the developed interpole iron by the width JSOL ranging Corpor fromation, 0 mm Tokyo, to 2 mm.Japan. The Various analytical motor models designs assumed were analysisevaluated software with the developed interpole ironby the width JSOL ranging Corpor fromation, 0 mmTokyo, to 2 Japan.mm. The Various analytical motor models designs assumed were thatevaluated the motors with the were interpole made ofiron silicon width steel ranging (35CS300) from 0 andmm drivento 2 mm. by The the analytical single-phase models square assumed wave evaluatedthat the motors with the were interpole made ironof silicon width steel ranging (35CS3 from00) 0 andmm todriven 2 mm. by The the analytical single-phase models square assumed wave that the motors were made of silicon steel (35CS300) and driven by the single-phase square wave that the motors were made of silicon steel (35CS300) and driven by the single-phase square wave

EnergiesEnergies 20202020,, 1313,, 4445x FOR PEER REVIEW 4 of 11 current of 1 A at a rotational speed of 1000 RPM under a no-load condition. The final motor design currentwas chosen of 1 based A at a on rotational its high speedtorque of density 1000 RPM and underlow torque a no-load ripple. condition. The final motor design was chosen based on its high torque density and low torque ripple. Torque ripple (Tripple) was defined as Torque ripple (Tripple) was defined as 𝑇 𝑇 𝑇 100% (1) Tmax𝑇Tmin Tripple = − 100% (1) Tavg × where Tmax, Tmin, and Tavg are the maximum, minimum, and average , respectively. Torque density (TV) was defined as where Tmax, Tmin, and Tavg are the maximum, minimum, and average torques, respectively. Torque density (T ) was defined as 𝑇 V 𝑇 (2) T𝑉𝑜𝑙avg Tv = (2) where Vol is the volume of the magnet. Vol whereTheVol preliminaryis the volume analysis of the of magnet. the motor designs was assessed with interpole iron widths of 0, 0.5, 1, 1.5,The and preliminary 2 mm, as shown analysis in of Figure the motor 4. The designs torque was characteristics assessed with and interpole volume ironof the widths magnet of 0,were 0.5, 1,obtained 1.5, and as 2 shown mm, as in shown Table in2. FigureThe best4. Thedesign torque can be characteristics found in the and interpole volume iron of thewidths magnet between were 1 obtainedmm and as2 mm. shown in Table2. The best design can be found in the interpole iron widths between 1 mm and 2 mm.

Figure 4. Motor designs with various interpole iron widths. Figure 4. Motor designs with various interpole iron widths. Table 2. Motor characteristics with interpole iron widths of 0, 0.5, 1, 1.5, and 2 mm.

Table 2. Motor characteristics with interpole iron widths of 0, 30.5, 1, 1.5, and 2 mm. 3 d (mm) Tavg (mN-m) Tripple (%) Vol (mm ) Tv (mN-m/mm ) d (mm)0 Tavg 3.608(mN-m) T 90.33ripple (%) 25.53Vol (mm3) Tv 0.141(mN-m/mm3) 00.5 3.608 3.503 82.7390.33 23.0225.53 0.1520.141 0.51 3.503 3.316 68.0082.73 20.5123.02 0.1620.152 1.5 3.065 70.43 17.96 0.171 1 23.316 2.763 98.3068.00 15.3620.51 0.1800.162 1.5 3.065 70.43 17.96 0.171 2 2.763 98.30 15.36 0.180 To find the final motor design, further analysis was conducted with interpole iron widths changing from 1.1 mm to 1.9 mm in increments of 0.1 mm. The motor characteristics were calculated as shown To find the final motor design, further analysis was conducted with interpole iron widths changing in Table3. The final design was selected with an interpole iron width of 1.6 mm. The motor had an from 1.1 mm to 1.9 mm in increments of 0.1 mm. The motor characteristics were calculated as shown in average torque of 3.008 mN-m, a torque ripple of 74.77%, and a torque density of 0.172 mN-m/mm3. Table 3. The final design was selected with an interpole iron width of 1.6 mm. The motor had an average torque of 3.008 mN-m, a torque ripple of 74.77%, and a torque density of 0.172 mN-m/mm3. Table 3. Motor characteristics with interpole iron widths between 1.1 mm and 1.9 mm.

3 3 Tabled (mm) 3. Motor characteristicsTavg (mN-m) with interpoleTripple (%) iron widthsVol between(mm ) 1.1 mmTv and(mN-m 1.9 mm/mm. ) 1.1 3.270 62.96 20.00 0.164 d (mm) Tavg (mN-m) Tripple (%) Vol (mm3) Tv (mN-m/mm3) 1.2 3.222 62.70 19.49 0.165 1.11.3 3.270 3.172 64.6862.96 18.9820.00 0.1670.164 1.21.4 3.222 3.119 67.2562.70 18.4719.49 0.1690.165 1.31.5 3.172 3.065 70.4364.68 17.9618.98 0.1710.167 1.6 3.008 74.77 17.44 0.172 1.41.7 3.119 2.950 80.4067.25 16.9218.47 0.1740.169 1.51.8 3.065 2.889 87.7370.43 16.4017.96 0.1760.171 1.61.9 3.008 2.827 93.8774.77 15.8817.44 0.1780.172 1.7 2.950 80.40 16.92 0.174 1.8 2.889 87.73 16.40 0.176 1.9 2.827 93.87 15.88 0.178

Energies 2020, 13, 4445 5 of 11 EnergiesEnergies 2020 2020, ,13 13, ,x x FOR FOR PEER PEER REVIEW REVIEW 5 5of of 11 11

2.3. Electromagnetic Analysis of Inset Permanent Magnet Synchronous Motor 2.3.2.3. Electromagnetic Electromagnetic Analysis Analysis ofof InsetInset PermanentPermanent Magnet Synchronous MotorMotor TheThe performances performances ofof thethe insetinset motors motors were analyzedanalyzed using bothboth siliconsilicon steelsteel 35CS30035CS300 and and Somaloy Somaloy 700700 1P 1P1P materials materials for for for comparison. comparison. comparison. TheThe The outputoutput output torque torque was calculated was calculated atat aa single-phasingle-pha at a single-phasesese square square wave squarewave current current wave inputcurrentinput of of input1 1 A A at at ofrotational rotational 1 A at rotational speedspeed ofof 10001000 speed RPMRPM of 1000under RPM a no-loa underd condition. a no-load FigureFigure condition. 55 shows shows Figure the the torque-angle 5torque-angle shows the curvestorque-anglecurves of of the the curvessilicon silicon ofsteel steel the and and silicon SMCSMC steel motors.motors. and SMC The silic motors.on steel The motor silicon hadhad steel anan motor averageaverage had torque torque an average of of 3.01 3.01 torque mN- mN- mofm and 3.01 and torque mN-mtorque ripple andripple torque of of 74.8%. 74.8%. ripple TheThe of SMCSMC 74.8%. motor The had SMC an motor average had torquetorque an average ofof 2.732.73 torquemN-m mN-m ofand and 2.73 torque torque mN-m ripple ripple and oftorqueof 64.4%. 64.4%. ripple The The ofBEMF BEMF 64.4%. was was The computedcomputed BEMF was atat computedaa rotational at speed a rotational of 9000 speed RPM.RPM. of FigureFigure 9000 RPM. 6a,b6a,b shows shows Figure the6 thea,b BEMF- BEMF- shows angletheangle BEMF-angle curves curves for for the curvesthe two two for motors.motors. the two TheThe motors. BEMFBEMF of The the BEMF silicon of steelsteel the silicon andand SMCSMC steel motorsmotors and SMC is is 3.77 3.77 motors V V and andis 3.34 3.34 3.77 V, V, V respectively.andrespectively. 3.34 V, respectively. The The motor motor characteristics Thecharacteristics motor characteristics areare summarized are summarized in Table 4.4. in Table4.

FigureFigureFigure 5. 5.5. Torque-angle Torque-angleTorque-angle curves curves of of 35CS300 35CS300 and Somaloy 70 7000 1P1P motorsmotors atat anan inputinput current current of of 1 1 A A.A. .

FigureFigure6. 6.BEMF-angle BEMF-angle curvescurves of (a) 35CS30035CS300 and ( b)) Somaloy Somaloy 700 700 1P 1P motors motors at at a a rotational rotational speed speed of of Figure 6. BEMF-angle curves of (a) 35CS300 and (b) Somaloy 700 1P motors at a rotational speed of 90009000 RPM. RPM. 9000 RPM. Table 4. Theoretical properties of 35CS300 and Somaloy 700 1P motors. Table 4. Theoretical properties of 35CS300 and Somaloy 700 1P motors. Table 4. Theoretical properties of 35CS300 and Somaloy 700 1P motors. Motor Type and Model Tavg (mN-m) Tripple (%) BEMF (V) Motor Type and Model Tavg (mN-m) Tripple (%) BEMF (V) 35CS300 3.008avg 74.77ripple 3.766 Motor Type35CS300 and Model T 3.008(mN-m) T 74.77 (%) BEMF3.766 (V) Somaloy 700 1P 2.731 64.39 3.335 Somaloy35CS300 700 1P 3.008 2.731 64.3974.77 3.3353.766 DiSomaloyfferenceDifference 700 1P 9.2% 2.7319.2% 13.9%13.9% 64.39 11.4% 3.335 11.4% Difference 9.2% 13.9% 11.4% TheThe performanceperformance of the SMC SMC motor motor was was poorer poorer than than that that of ofthe the silicon silicon steel steel motor motor because because of its of itslower lowerThe saturation performance saturation magnetic magnetic of the flux SMC flux density motor density andwas and permeability.poorer permeability. than that The of maximum The the silicon maximum torque steel torque motor of the because ofsilicon the silicon steel of its lower saturation magnetic flux density and permeability. The maximum torque of the silicon steel steelmotor motor occurred occurred at a at94° a 94angle◦ angle and and the themagnetic magnetic flux flux density density distribution distribution is isshown shown inin Figure Figure 7a.7a. motorMagneticMagnetic occurred saturationsaturation at a occurredoccurred94° angle at and the tooththe magnetic and yoke flux of thedensity stator distribution since since the the maximum maximum is shown flux flux in densityFigure density of7a. of 1.72Magnetic1.72 T T was was saturation higher higher than than occurred the the saturation saturation at the magnetictooth magnetic and flux yoke flux density of density the of stator 1.63 of T1.63since for T 35CS300. thefor maximum35CS300. The maximum The flux maximum density torque of 1.72 T was higher than the saturation magnetic flux density of 1.63 T for 35CS300. The maximum oftorque the SMC of the motor SMC motor occurred occurred at an at 86 an◦ angle 86° angle and and the the magnetic magnetic flux flux density density distribution distribution is shown shown in in torqueFigureFigure7 ofb.7b. the There There SMC waswas motor alsoalso occurred magnetic magnetic at saturation saturation an 86° angle in in this andthis case thecase magnetic because because the fluxthe maximum madensityximum distribution flux flux density density is shown of of 1.75 1.75 in T Figure 7b. There was also magnetic saturation in this case because the maximum flux density of 1.75

Energies 2020, 13, 4445 6 of 11 Energies 2020, 13, x FOR PEER REVIEW 6 of 11 Energies 2020, 13, x FOR PEER REVIEW 6 of 11 wasT was higher higher than than the the saturation saturation magnetic magnetic flux flux density density of of 1.31 1.31 T forT for Somaloy Somaloy 700 700 1P. 1P. If theIf the geometries geometries of T was higher than the saturation magnetic flux density of 1.31 T for Somaloy 700 1P. If the geometries theof the stator stator are are redesigned, redesigned, the the magnetic magnetic saturation saturation problem problem in thein the motors motors will will be resolved.be resolved. of the stator are redesigned, the magnetic saturation problem in the motors will be resolved.

Figure 7.7. MagneticMagnetic flux flux density density distributions distributions of of ( (aa)) 35CS300 35CS300 and and ( b) Somaloy 700 1P motors atat Figure 7. Magnetic flux density distributions of (a) 35CS300 and (b) Somaloy 700 1P motors at maximum torques.torques. maximum torques. 2.4. Improved Designs of Inset Permanent Magnet Synchronous Motors 2.4. Improved Designs of Inset Permanent Magnet Synchronous Motors 2.4. Improved Designs of Inset Permanent Magnet Synchronous Motors The magnetic saturation problem of two motors can be solved by increasing the cross-sectional The magnetic saturation problem of two motors can be solved by increasing the cross-sectional areasThe and magnetic chamfers saturation in the yoke problem and tooth. of two The motors performance can be of solved the SMC by increasing motor was the further cross-sectional enhanced areas and chamfers in the yoke and tooth. The performance of the SMC motor was further enhanced by byareas changing and chamfers the soft in magnetthe yoke material and tooth. to The Somaloy performance 1000 3P. of The the statorSMC motor of the was silicon further steel enhanced motor was by changing the soft magnet material to Somaloy 1000 3P. The stator of the silicon steel motor was redesignedchanging the with soft an magnet external material diameter to of 15.5Somaloy mm, a100 yoke0 3P. width The of stator 1.3 mm, of andthe asilicon tooth widthsteel motor of 2.5 mm.was redesigned with an external diameter of 15.5 mm, a yoke width of 1.3 mm, and a tooth width of 2.5 mm. Theredesigned maximum with torque an external of the diameter silicon steel of 15.5 motor mm, occurred a yoke width at a 97 of 1.3angle mm, and and the a tooth magnetic width flux of 2.5 density mm. The maximum torque of the silicon steel motor occurred at a 97°◦ angle and the magnetic flux density distributionThe maximum is showntorque inof the Figure silicon8a. Thesteel statormotor ofoccurr the SMCed at motora 97° angle was redesignedand the magnetic with anflux external density distribution is shown in Figure 8a. The stator of the SMC motor was redesigned with an external diameterdistribution of 15.5is shown mm, a in yoke Figure width 8a. of The 1.4 mm,stator and of the a tooth SMC width motor of 2.7was mm. redesigned The maximum with an torque external of diameter of 15.5 mm, a yoke width of 1.4 mm, and a tooth width of 2.7 mm. The maximum torque of thediameter SMC motor of 15.5 occurred mm, a yoke at a 97widthangle of and1.4 mm, the magnetic and a tooth flux width density of distribution2.7 mm. The is maximum shown in Figuretorque8 b.of the SMC motor occurred at a 97°◦ angle and the magnetic flux density distribution is shown in Figure Thethe SMC motor motor characteristics occurred at of a the 97° improved angle and designsthe magnet areic summarized flux density in distribution Table5. The is averageshown in torque, Figure 8b. The motor characteristics of the improved designs are summarized in Table 5. The average torque, torque8b. The ripple,motor andcharacteristics BEMF all of increased the improved without design magnetics are summarized saturation. However,in Table 5. theThe finalaverage designs torque, of torque ripple, and BEMF all increased without magnetic saturation. However, the final designs of the thetorque two ripple, motors and are BEMF unchanged all increased and the without same asmagnetic shown insaturation. Table1, to However, demonstrate the final the performancedesigns of the two motors are unchanged and the same as shown in Table 1, to demonstrate the performance ditwofference motors of theare twounchanged soft magnet and materials.the same as shown in Table 1, to demonstrate the performance difference of the two soft magnet materials. difference of the two soft magnet materials.

Figure 8. Magnetic flux density distributions of (a) 35CS300 and (b) Somaloy 1000 3P in improved Figure 8. Magnetic flux density distributions of (a) 35CS300 and (b) Somaloy 1000 3P in improved motorFigure designs 8. Magnetic at maximum flux density torques. distributions of (a) 35CS300 and (b) Somaloy 1000 3P in improved motor designs at maximum torques. . motor designsTable at maximum 5. Theoretical torques properties of 35CS300 and Somaloy 1000 3P motors. Table 5. Theoretical properties of 35CS300 and Somaloy 1000 3P motors. Motor TypeTable and 5. Model Theoretical Tpropertiesavg (mN-m) of 35CS300 andTripple Somaloy(%) 1000 3P motorsBEMF. (V)

Motor35CS300 Type and Model 3.072Tavg (mN-m) 97.33Tripple (%) BEMF 3.896 (V) Motor Type and Model Tavg (mN-m) Tripple (%) BEMF (V) Somaloy 100035CS300 3P 2.9773.072 95.7797.33 3.896 3.770 35CS300 3.072 97.33 3.896 Somaloy 1000 3P 2.977 95.77 3.770 Somaloy 1000 3P 2.977 95.77 3.770

Energies 2020,, 13,, 4445x FOR PEER REVIEW 7 of 11 Energies 2020, 13, x FOR PEER REVIEW 7 of 11 3. Fabrication and Assembly of Inset Permanent Magnet Synchronous Motors 3. Fabrication and Assembly of Inset Permanent Magnet Synchronous MotorsMotors 3.1. Fabrication of Motor Stator and Rotor 3.1. Fabrication of Motor Stator and Rotor The two micro inset motors were made from different soft magnetic materials—silicon steel The twotwo micromicro insetinset motorsmotors werewere mademade fromfrom didifferentfferent softsoft magneticmagnetic materials—siliconmaterials—silicon steelsteel (35CS300) and Somaloy 700 1P. The silicon steel sheets were cut by wire electrical discharge (35CS300)(35CS300) andand Somaloy Somaloy 700 700 1P. The1P. siliconThe silicon steel sheetssteel sheets were cut were by wire cut electricalby wire dischargeelectrical machiningdischarge machining (WEDM) and stacked together to form the laminated cores of the stator and rotor as shown (WEDM)machining and (WEDM) stacked and together stacked to formtogether the laminatedto form the cores laminated of the statorcores of and the rotor stator as and shown rotor in Figureas shown9a. in Figure 9a. The SMC cores were first sintered and heat-treated, and then machined by WEDM to Thein Figure SMC cores9a. The were SMC first cores sintered were and first heat-treated, sintered and and heat-treated, then machined and then by WEDM machined to obtain by WEDM the final to obtain the final shapes of the stator and rotor as shown in Figure 9b. shapesobtain the of the final stator shapes and of rotor the stator as shown and inrotor Figure as shown9b. in Figure 9b.

Figure 9. Stator and rotor cores made of (a) 35CS300 and (b) Somaloy 700 1P. Figure 9. Stator and rotor cores made of (a) 35CS300 and (b) Somaloy 700 1P.1P. 3.2. Motor Assembly 3.2. Motor Assembly Before assembly, the dimensions of the motor partsparts had to be checked to ensure that they met Before assembly, the dimensions of the motor parts had to be checked to ensure that they met design specifications.specifications. TheThe precisionprecision andand concentricityconcentricity of of the the axial axial shaft, shaft, stator, stator, and and rotor rotor are are critical critical to design specifications. The precision and concentricity of the axial shaft, stator, and rotor are critical reduceto reduce motor motor vibration. vibration. The statorThe stator slot was slot insulated was insulated with tape with before tape thebefore coils the were coils wound were manually. wound to reduce motor vibration. The stator slot was insulated with tape before the coils were wound Themanually. motor wasThe assembledmotor was withassembled an end with frame, an bearing, end frame, stator bearing, core, axial stator shaft, core, and axial rotor, shaft, and and a bearing rotor, manually. The motor was assembled with an end frame, bearing, stator core, axial shaft, and rotor, and framea bearing on theand other frame side. on the Pictures other ofside. the Pictures mechanical of the parts mechanical and assembled parts and motors assembled are shown motors in and a bearing and frame on the other side. Pictures of the mechanical parts and assembled motors Figuresare shown 10 andin Figures 11. 10 and 11. are shown in Figures 10 and 11.

Figure 10. Mechanical parts of ( a) 35CS300 and ( b) Somaloy 700 1P motors.motors. Figure 10. Mechanical parts of (a) 35CS300 and (b) Somaloy 700 1P motors.

Energies 2020, 13, x FOR PEER REVIEW 7 of 11

3. Fabrication and Assembly of Inset Permanent Magnet Synchronous Motors

3.1. Fabrication of Motor Stator and Rotor The two micro inset motors were made from different soft magnetic materials—silicon steel (35CS300) and Somaloy 700 1P. The silicon steel sheets were cut by wire electrical discharge machining (WEDM) and stacked together to form the laminated cores of the stator and rotor as shown in Figure 9a. The SMC cores were first sintered and heat-treated, and then machined by WEDM to obtain the final shapes of the stator and rotor as shown in Figure 9b.

Figure 9. Stator and rotor cores made of (a) 35CS300 and (b) Somaloy 700 1P.

3.2. Motor Assembly Before assembly, the dimensions of the motor parts had to be checked to ensure that they met design specifications. The precision and concentricity of the axial shaft, stator, and rotor are critical to reduce motor vibration. The stator slot was insulated with tape before the coils were wound manually. The motor was assembled with an end frame, bearing, stator core, axial shaft, and rotor, and a bearing and frame on the other side. Pictures of the mechanical parts and assembled motors are shown in Figures 10 and 11.

Energies 2020, 13, 4445 8 of 11 Figure 10. Mechanical parts of (a) 35CS300 and (b) Somaloy 700 1P motors.

Energies 2020, 13, x FOR PEER REVIEW 8 of 11

Figure 11. Assembled motors made of (a) 35CS300 and (b) Somaloy 700 1P. Figure 11. Assembled motors made of (a) 35CS300 and (b) Somaloy 700 1P.

4.4. PropertiesProperties TestingTesting ofof InsetInset PermanentPermanent MagnetMagnet Synchronous Motors

4.1.4.1. MotorMotor SpeedSpeed TestingTesting TheThe speed speed ofof thethe motorsmotors waswas measuredmeasured using the square wave current current drive drive and and the the voltage voltage and and currentcurrent signals signals ofof thethe windingwinding coilcoil werewere measuredmeasured atat various excitation . frequencies. At At a a rated rated current current ofof 1 1 A, A, the the maximum maximum rotational rotational speed speed of of the the motor motor was was 9000 9000 RPM RPM at at an an excitation excitation frequency of 300of 300 Hz. TheHz. voltage The voltage and current and current signals signals of the of motors the motors made ofmade silicon of silicon steel (35CS300) steel (35CS300) and Somaloy and Somaloy 700 1P cores700 are1P showncores are in shown Figure in12 a,b.Figure 12a,b.

FigureFigure 12. 12.Voltage Voltage andand currentcurrent signalssignals ofof ((aa)) 35CS30035CS300 and ( b) Somaloy 700 700 1P 1P motors motors at at a a rotational rotational speedspeed of of 9000 9000 RPM. RPM.

4.2.4.2. MotorMotor TorqueTorque Testing Testing TheThe experimental experimental torque torque was was measured measured by hangingby hang standarding standard test weights test weights on the externalon the external diameter ofdiameter the rotor of the at every rotor at 10 every◦ interval 10° interval from 0from◦ to 600° ◦to. 60°. The The torque-current torque-current curves curves were were tested tested at currents currents fromfrom 0.50.5 AA toto 11 AA inin incrementsincrements ofof 0.10.1 A.A. TheThe motormotor friction torque was was estimated estimated to to be be 0.08 0.08 mN-m mN-m usingusing a a minimum minimum startingstarting currentcurrent ofof 0.0350.035 A and then used to calculate actual actual output output torques. torques. At At aa ratedrated current current ofof 11 A,A, thethe averageaverage torquestorques forfor the silicon steel and SMC SMC motors motors were were 2.55 2.55 mN-m mN-m and and 2.352.35 mN-m, mN-m, respectively,respectively, after friction friction compensation. compensation. The The torque torque constants constants (KT) (K wereT) were 2.59 mN-m/A 2.59 mN-m and/A and2.37 2.37 mN-m/A, mN-m /respectively.A, respectively. The Thetheoretical theoretical andand experimental experimental KT of K Ttheof thetwo twomotors motors are arecompared compared in inTable Table 6.6 Lastly,. Lastly, the the theoretical theoretical and and experimental experimental torque-current torque-current curves curves are plotted are plotted as shown as shownin Figure in Figure13. The 13 errors. The errorsbetween between the theoretical the theoretical and experimental and experimental KT of the KT siliconof the siliconsteel and steel SMC and motors SMC motors were were−13.1%13.1% and − and12.5%,12.5%, respectively. respectively. The experimental The experimental KT of K theT of SMC the SMC motor motor was was8.5% 8.5% less lessthan than thatthat of − − ofthe the silicon silicon steel steel motor. motor.

Figure 13. Torque-current curves of 35CS300 and Somaloy 700 1P motors.

Energies 2020, 13, x FOR PEER REVIEW 8 of 11

Figure 11. Assembled motors made of (a) 35CS300 and (b) Somaloy 700 1P.

4. Properties Testing of Inset Permanent Magnet Synchronous Motors

4.1. Motor Speed Testing The speed of the motors was measured using the square wave current drive and the voltage and current signals of the winding coil were measured at various excitation frequencies. At a rated current of 1 A, the maximum rotational speed of the motor was 9000 RPM at an excitation frequency of 300 Hz. The voltage and current signals of the motors made of silicon steel (35CS300) and Somaloy 700 1P cores are shown in Figure 12a,b.

Figure 12. Voltage and current signals of (a) 35CS300 and (b) Somaloy 700 1P motors at a rotational speed of 9000 RPM.

4.2. Motor Torque Testing The experimental torque was measured by hanging standard test weights on the external diameter of the rotor at every 10° interval from 0° to 60°. The torque-current curves were tested at currents from 0.5 A to 1 A in increments of 0.1 A. The motor friction torque was estimated to be 0.08 mN-m Energiesusing a2020 minimum, 13, 4445 starting current of 0.035 A and then used to calculate actual output torques.9 At of 11a rated current of 1 A, the average torques for the silicon steel and SMC motors were 2.55 mN-m and 2.35 mN-m, respectively, after friction compensation. The torque constants (KT) were 2.59 mN-m/A and Table 6. Comparison of torque constants (KT) of 35CS300 and Somaloy 700 1P motors. 2.37 mN-m/A, respectively. The theoretical and experimental KT of the two motors are compared in

Table 6. Lastly,KT (mN-m the theoretical/A) and 35CS300 experimental Motor torque-current SMC Motor curves are plotted Diff aserence shown in Figure 13. The errors Theorybetween the theoretical 2.98and experimental KT 2.71of the silicon steel and 9.06% SMC motors were −13.1% and −Experiment12.5%, respectively. The 2.59experimental KT of the 2.37 SMC motor was 8.5% 8.49% less than that of the silicon steelError motor. 13.1% 12.5% N/A − −

Energies 2020, 13, x FOR PEER REVIEW 9 of 11

Table 6. Comparison of torque constants (KT) of 35CS300 and Somaloy 700 1P motors.

KT (mN-m/A) 35CS300 Motor SMC Motor Difference Theory 2.98 2.71 9.06% Experiment 2.59 2.37 8.49% FigureError 13. Torque-currentTorque-current− curves13.1% of 35CS300 and Somaloy−12.5% 700 1P motors.N/A

4.3. Motor BEMF Testing 4.3. Motor BEMF Testing The experimental BEMF was tested using a micro-dynamometer (HSY. Co., Ltd., Taichung, Taiwan) The experimental BEMF was tested using a micro-dynamometer (HSY. Co., Ltd., Taichung, at every 2000 RPM interval from 1000 to 9000 RPM. At a rotational speed of 9000 RPM, the BEMF Taiwan) at every 2000 RPM interval from 1000 to 9000 RPM. At a rotational speed of 9000 RPM, the for the silicon steel and SMC motors was 3.24 V and 2.96 V, respectively. The BEMF constants (K ) BEMF for the silicon steel and SMC motors was 3.24 V and 2.96 V, respectively. The BEMF constantsV were 3.50 mV/(rad/s) and 3.08 mV/(rad/s), respectively. The theoretical and experimental KV of the (KV) were 3.50 mV/(rad/s) and 3.08 mV/(rad/s), respectively. The theoretical and experimental KV of the two motors are compared in Table7 and voltage-speed curves are plotted as shown in Figure 14. two motors are compared in Table 7 and voltage-speed curves are plotted as shown in Figure 14. The The error between the theoretical and experimental KV of the silicon steel and SMC motors was 12.7% error between the theoretical and experimental KV of the silicon steel and SMC motors was −12.7%− and and 13.7%, respectively. The experimental KV of SMC motor was 12.0% less than that of silicon −13.7%,− respectively. The experimental KV of SMC motor was 12.0% less than that of silicon steel motor. steel motor.

Table 7. Comparison of BEMF constants (KV) of 35CS300 and Somaloy 700 1P motors. Table 7. Comparison of BEMF constants (KV) of 35CS300 and Somaloy 700 1P motors.

KKV V(mV (mV/(rad/s))/(rad/s)) 35CS300 35CS300 Motor Motor SMC Motor SMC Motor Di Differencefference TheoryTheory 4.014.01 3.573.57 11.0%11.0% ExperimentExperiment 3.503.50 3.083.08 12.0%12.0% ErrorError 12.7%−12.7% 13.7%−13.7% N/N/AA − −

Figure 14. Voltage-speedVoltage-speed curves of 35CS300 and Somaloy 700 1P motors.

4.4. Discussion The experimental performance of the micro inset motor using Somaloy 700 1P was worse than that of silicon steel (35CS300). Its torque constant (KT) and BEMF constant (KV) were 8.5% and 12.0% lower, respectively. There were two reasons for this: the saturation magnetic flux density and magnetic permeability of the Somaloy material were inferior, and there was magnetic saturation occurring at the stator yoke and tooth. However, the powder sintered SMC material allows for mass- production and near-net-shape manufacturing without assembly, which saves time and reduces the cost of micromotors. This study demonstrated the feasibility of the inset SMC micromotor, which can achieve the performance of a commercial BLDC motor such as Faulhaber 1509T006B. The inset motor can be further improved by motor design optimization based on Somaloy 1000 3P and SMC magnetic property advancement in the future.

5. Conclusions In this study, a six-slot stator and a four-pole micro inset was developed with an external diameter of 15 mm and a length of 5 mm. The miniaturization of the motor is currently

Energies 2020, 13, 4445 10 of 11

4.4. Discussion The experimental performance of the micro inset motor using Somaloy 700 1P was worse than that of silicon steel (35CS300). Its torque constant (KT) and BEMF constant (KV) were 8.5% and 12.0% lower, respectively. There were two reasons for this: the saturation magnetic flux density and magnetic permeability of the Somaloy material were inferior, and there was magnetic saturation occurring at the stator yoke and tooth. However, the powder sintered SMC material allows for mass-production and near-net-shape manufacturing without assembly, which saves time and reduces the cost of micromotors. This study demonstrated the feasibility of the inset SMC micromotor, which can achieve the performance of a commercial BLDC motor such as Faulhaber 1509T006B. The inset motor can be further improved by motor design optimization based on Somaloy 1000 3P and SMC magnetic property advancement in the future.

5. Conclusions In this study, a six-slot stator and a four-pole micro inset permanent magnet motor was developed with an external diameter of 15 mm and a length of 5 mm. The miniaturization of the motor is currently limited by a 1 mm thick rare earth tile-type permanent magnet. The inset motors were fabricated and tested using two different core materials, silicon steel and soft magnetic composite (SMC). Although the motor performance using silicon steel was better, the SMC core can be easily made without lamination, which is particularly suitable for micromotors. At an input square wave current of 1 A, two motors achieved a stall torque over 2.35 mN-m and a maximum speed of 9000 RPM. Experimental testing found that both motors meet the specifications of commercial motors used for small , portable hand-held power tools, micro pumps, and surveillance camera systems.

Author Contributions: Conceptualization, D.-C.P.; methodology, D.-C.P.; validation, D.-C.P., Z.-J.S., P.-X.X., H.-C.H., and G.-T.B.; formal analysis, D.-C.P., Z.-J.S., P.-X.X. and G.-T.B.; investigation, D.-C.P., Z.-J.S., P.-X.X., H.-C.H., and G.-T.B.; data curation, D.-C.P., Z.-J.S., P.-X.X.; writing—original draft preparation, D.-C.P., Z.-J.S., P.-X.X.; writing—review and editing, D.-C.P., H.-C.H. and G.-T.B.; supervision, D.-C.P. All authors have read and agreed to the published version of the manuscript. Funding: This work was partially funded by the MEMS and Precision Machinery Research and Development Center and the Department of Mechanical Engineering at National Kaohsiung University of Science and Technology. Conflicts of Interest: The authors declare no conflict of interest.

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