A Study on the Rotor Design of Line Start Synchronous Reluctance Motor for IE4 Eﬃciency and Improving Power Factor

energies

Article A Study on the Rotor Design of Line Start Synchronous Reluctance Motor for IE4 Eﬃciency and Improving Power Factor

Hyunwoo Kim 1 , Yeji Park 1, Seung-Taek Oh 1, Hyungkwan Jang 1, Sung-Hong Won 2, Yon-Do Chun 3 and Ju Lee 1,* 1 Department of Electrical Engineering, Hanyang University, Seoul 04763, Korea; [email protected] (H.K.); [email protected] (Y.P.); [email protected] (S.-T.O.); [email protected] (H.J.) 2 Department of Electrical Engineering, Dongyang Mirae University, Seoul 08221, Korea; [email protected] 3 Electric Machines and Drives Research Center, Korea Electrotechnology Research Institute, Changwon 51543, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2220-0342

Received: 15 October 2020; Accepted: 3 November 2020; Published: 4 November 2020

Abstract: As international regulations of motor efficiency are strengthened, the line-start synchronous reluctance motor (LS-SynRM) is being studied to improve the efficiency of the electrical motor in industrial applications. However, in industrial applications, the power factor is also an important performance index, but the LS-SynRM has poor power factor due to the saliency characteristic. In this paper, the rotor design of LS-SynRM is performed to improve the efficiency and power factor. First, the barrier design is performed to improve the efficiency and power factor using the response surface method (RSM). Second, the rotor slot design is performed according to the length of bar for synchronization. Lastly, the rib design is performed to satisfy the power factor and the mechanical reliability. The final model through the design process is analyzed using finite element analysis (FEA), and the objective performance is satisfied. To verify the FEA result, the final model is manufactured, and experiment is performed.

Keywords: ﬁnite element analysis; international electrotechnical committee; line start synchronous reluctance motor; power factor; super premium eﬃciency

1. Introduction In industrial applications, electrical energy consumption of motors account for 35% to 40% of electrical energy generated in the world. If this electrical energy consumption can be reduced, several environmental impacts, such as the emissions of CO2 or global warming, can be reduced [1,2]. For this reason, international regulations regarding motor efficiency were enacted by the International Electrotechnical Committee (IEC) 60034-30. According to IEC 60034-30, motor efficiency was classified as IE1 to IE5. Recently, the efficiency standards of industrial applications have been strengthened, and industrial motors may need to meet IE4 or IE5 efficiency class [3]. With various types of motors, the three-phase squirrel-cage induction motor (SCIM) is the most used, because of its simple structure, line-start ability, robustness, and low manufacturing cost. However, it is evident that the induction motor (IM) does not meet high efficiency due to rotor copper loss [4–6]. Due to a global trend on improving the efficiency of electric motors, there is ongoing research that focuses on this area. In References [7,8], the aluminum in the rotor slot is replaced by copper to reduce the rotor copper loss. Copper die-casting induction motors can improve efficiency by

Energies 2020, 13, 5774; doi:10.3390/en13215774 www.mdpi.com/journal/energies Energies 2020, 13, 5774 2 of 15 about 2–3% compared to the aluminum die-casting induction motors. However, copper has a higher melting point than aluminum, so a die-casting process is difficult, and the manufacturing cost increases. In industrial variable-speed applications, a variable-speed drive (VSD) is used to improve the efficiency of motors [9–11]. When using the VSD system, a synchronous motor can be controlled to improve the efficiency. However, the synchronous motor along with the VSD system is more expensive than SCIMs. Line-start synchronous machines (LSSMs) are actively studied because they can meet the efficiency of IE4. In addition, a LSSM is direct-on-line (DOL) motor, such as IM, so it does not require an inverter, in which the cost of electric system can be reduced [12–17]. LSSM can be classified into two types; line-start permanent magnet synchronous motor (LSPM) and line-start synchronous reluctance motor (LS-SynRM). LSPM has the high efficiency and power factor because of the permanent magnet. However, the permanent magnet of LSPM must use a rare-earth magnet, such as neodymium, because of the high starting current. Furthermore, the resources of rare-earth magnets are limited, and lead to high costs [18,19]. Therefore, the LS-SynRM has received attention for its high efficiency and for the fact that it does not use a rare-earth magnet. A reactive power of an electrical motor depends on the power factor. If the reactive power is increased by the low power factor, the copper loss in stator winding increases under the same output power and input voltage condition because of the current increases [20]. This increased copper loss decreases the efficiency of the electrical motor. For this reason, there are the power factor standards of industrial motors, according to IEC 60034. The electrical motor must meet the power factor standards to use industrial applications [21]. However, LS-SynRM has a lower power factor than IM and LSPM because of a saliency characteristic and may not meet the power factor standards [22–24]. Therefore, the design method considering the power factor is required. This paper is a design process of LS-SynRM, for super premium efficiency, and for improving the power factor. The efficiency and the power factor of LS-SynRM is determined by dq-axis inductance, which is determined by the barrier design. Therefore, the design parameters are selected for IE4 efficiency and the power factor. Based on the selected design parameters, the barrier optimal design is performed using the response surface method (RSM) and finite element analysis (FEA). In addition, the rotor slot design is performed for synchronization and the rib design is performed to improve the power factor and satisfy the mechanical reliability through the safety factor. To verify the efficiency and the power factor of the design result, the final model is manufactured, and the experiment is performed to verify the IE4 efficiency and power factor. This paper is organized as follows. In Section2, the characteristics of LS-SynRM are discussed in respect to the structure and the operation in asynchronous and synchronous speed. In Section3, the characteristic of the reference machine is analyzed. In Section4, the design of LS-SynRM is performed using RSM and FEA and the final model is analyzed using FEA. In Section5, to verify FEA results, the experiment is performed and the results of LS-SynRM are compared with FEA results. Finally, Section6 contains the conclusion.

2. Characteristic of LS-SynRM

2.1. Structure of LS-SynRM Figure1 shows the rotor structure of a general LS-SynRM. LS-SynRM has a structure in which the squirrel cage slots are placed on the outer side of the rotor, and the barrier is arranged on the inner side of the rotor. Because of this structure, LS-SynRM operates as IMs at the asynchronous speed and as SynRM at the synchronous speed. Moreover, the rotor design of LS-SynRM is classified as a rotor slot and barrier. The rotor slot design determines a starting characteristic of LS-SynRM and the barrier design determines the performance of LS-SynRM, such as the efficiency and power factor. Therefore, the rotor slot design is important to reach the synchronous speed, and the barrier design is important to satisfy the super-premium efficiency (IE4). Energies 2020, 13, 5774 3 of 15 Energies 2020, 13, x FOR PEER REVIEW 3 of 14 Energies 2020, 13, x FOR PEER REVIEW 3 of 14

Figure 1. Rotor structure of the line-start synchronous reluctance motor (LS-SynRM). Figure 1. Rotor structure of the line-start synchronous reluctance motor (LS-SynRM). 2.2. Asynchronous Speed 2.2. Asynchronous Speed In asynchronousasynchronous speed, the LS-SynRM has the slip, which generates an induced voltage on a conductorIn asynchronous in a a rotor rotor slot. slot. speed, An An inducedthe induced LS-SynRM current current has that that the is generated isslip, generated which by generates bythe the induced induced an inducedvoltage voltage providesvoltage provides on the a themagneticconductor magnetic torque in torquea rotor that thatslot.LS-SynRM LS-SynRMAn induced can can becurrent synchronization. be synchronization. that is generated Moreover, Moreover, by the in induced case in caseof voltage the of LS-SynRM, the provides LS-SynRM, thethe thereluctancemagnetic reluctance torquetorque torque thatis generated is LS-SynRM generated due duecan to toabe saliency a synchronization. saliency difference, diﬀerence, Moreover, which which is is the thein differencecase diﬀerence of the between betweenLS-SynRM, dq-axis the inductance.reluctance torque Therefore, is generated the torque due of to LS-SynRMLS-Syn a saliencyRM is difference, expressed asaswhich EquationEquation is the (1)(1) difference [[25,26].25,26]. between dq-axis inductance. Therefore, the torque of LS-SynTT=+RM Tis expressedsin(2 stωα + as ) Equation (1) [25,26]. =ecagerel+ ( + ) (1) Te T=+cage Trel sin 2sωαωt +α (1) TTecagerel Tsin(2 st ) (1) where Te is the torque of LS-SynRM at asynchronous speed, Tcage is the magnetic torque generated by where Te is the torque of LS-SynRM at asynchronous speed, Tcage is the magnetic torque generated by thewhere induced Te is thecurrent, torque T relof is LS-Syn the amplitudeRM at asynchronous of reluctance speed, torque T producedcage is the magnetic by the saliency, torque generateds is slip, ω byis the induced current, T is the amplitude of reluctance torque produced by the saliency, s is slip, ω is thethe electricalinduced current,angular Tfrequency,relrel is the amplitude and α is theof reluctance phase angle torque of pulsating produced torque. by the saliency, s is slip, ω is thethe electricalelectricalThe magnetic angularangular torque frequency,frequency, is constant and andα α isand is the the depends phase phase angle angle on the of of pulsating slippulsating as in torque. thetorque. induction motor, and the averageTheThe of magneticmagnetic reluctance torquetorque torque isis is constantconstant zero, but andand this dependsdepends torque pulsates onon thethe sliptwiceslip asas the inin slip thethe frequency.inductioninduction motor,Figuremotor, 2andand shows thethe averageaaverage speed-torque ofof reluctancereluctance curve torqueof torque LS-SynRM is is zero, zero, at but but an this asynchronothis torque torque pulsates pulsatesus speed. twice twice The the amplitudethe slip slip frequency. frequency. of the Figurepulsating Figure2 shows 2 torque shows a speed-torqueanda speed-torque the average curve curve torque of of LS-SynRM dependLS-SynRM on at theat an an asynchronousslip. asynchrono In a synchronousus speed. speed. The speed,The amplitude amplitude the maxi of ofmum the the pulsating pulsatingtorque is torquetorquecalled andtheand pull-out thethe average average torque, torque torque which depend depend is the on maximumon the the slip. slip. In load In a synchronousa torque synchronous that speed,can speed, be thesynchronized the maximum maximum [26]. torque torque is called is called the pull-outthe pull-out torque, torque, which which is the is maximum the maximum load load torque torque that canthat becan synchronized be synchronized [26]. [26].

Figure 2. Speed-torque curve of LS-SynRM. Figure 2. Speed-torque curve of LS-SynRM. Figure 2. Speed-torque curve of LS-SynRM. 2.3. Synchronous Speed 2.3. Synchronous Speed At a synchronous speed, there are no eddy current currentss in the rotor slot because the the slip slip is is zero. zero. Therefore,At a synchronous the LS-SynRM speed, is operated there are as no the eddy synchronoussync currenthronouss in reluctance the rotor motorslot because and the the eefficiency slipﬃciency is zero. is improvedTherefore, comparedthe LS-SynRM to SCIM is operated due to theas thesecondar synchronousy copper reluctance loss. The motorefficiency and ofthe LS-SynRM efficiency isis expressedimproved ascompared follows. to SCIM due to the secondary copper loss. The efficiency of LS-SynRM is expressed as follows.

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PP Tω η ==out out = e e P PP+++ Tω P P Energies 2020, 13, 5774 in out loss e e copper core (2)4 of 15 3 P TLLI=−()sin22 β edqa22 improved compared to SCIM due to the secondary copper loss. The efficiency of LS-SynRM is where η is the efficiency, Pout is mechanical power, Pin is electrical input power, Ploss is total loss of expressed as follows. e e copper core motor, T is torque of motor, ω is synchronousPout Pout speed, P Tiseω coppere loss, and P is core loss, P is η = = + = + + the number of pole, Ld and Lq are dP-,in q-axisPout inductance,Ploss Teω respectively,e Pcopper Pcore Ia is the current, and β is the (2) 3 P 2 Te = (L Lq)I sin 2β current phase angle. 2 2 d − a The efficiency is determined by the reluctance torque, the core loss, and the copper loss. The core where η is the efficiency, Pout is mechanical power, Pin is electrical input power, Ploss is total loss of loss depends on the magnetic flux density and input frequency, which is determined when designing motor, Te is torque of motor, ωe is synchronous speed, Pcopper is copper loss, and Pcore is core loss, P is electric machines. Therefore, the copper loss must be reduced to improve the efficiency. This should the number of pole, Ld and Lq are d-, q-axis inductance, respectively, Ia is the current, and β is the increase the torque per current, which should increase the rotor saliency difference. current phase angle. The power factor of the LS-SynRM is determined by the phase difference between the voltage The efficiency is determined by the reluctance torque, the core loss, and the copper loss. The core and current. Figure 3a shows a vector diagram of the LS-SynRM, and the power factor of LS-SynRM loss depends on the magnetic flux density and input frequency, which is determined when designing can be expressed as follows. electric machines. Therefore, the copper loss must be reduced to improve the efficiency. This should ρ −1 L increase the torque per current, whichcosϕρ should== increase the , rotor saliencyd difference. ρ 2 L The power factor of the LS-SynRM is determined+ 1 by the phaseq difference between the voltage(3) and current. Figure3a shows a vector diagram ofcos the22ββ LS-SynRM, sin and the power factor of LS-SynRM can be expressed as follows. where cosφ is the power factor, and ρ is the saliency ratio. ρ 1 Ld The power factor is determinedcos ϕ by= ther saliency− ratio, andρ = the current phase angle. Figure 3b (3) 2 Lq shows the power factor according to the saliencyρ ra+tio and1 the current phase angle. The power factor cos2 β sin2 β increases as the saliency ratio increases. Therefore, the saliency difference and ratio are important wheredesign cos parametersϕ is the power for IE4 factor, efficiency and ρ andis the improving saliency the ratio. power factor.

(a) (b)

FigureFigure 3. 3.Characteristic Characteristic of LS-SynRM ( (aa)) vector vector diagram, diagram, and and (b ()b power) power factor. factor.

3. ReferenceThe power Machine factor is determined by the saliency ratio and the current phase angle. Figure3b showsIn the order power to compare factor according the efficiency to the and saliency power ratiofactor and of LS-SynRM, the current 2.2 phase kW four angle. pole The IM poweris selected factor increasesas the reference as the saliency motor. Figure ratio 4 increases. show the FEA Therefore, model and the Table saliency 1 shows diﬀerence the specification and ratio areof reference important designmachine. parameters ANSYS forMaxwell IE4 e ﬃwasciency used and to analyze improving the performance the power factor. of the reference machine.

3. Reference Machine In order to compare the eﬃciency and power factor of LS-SynRM, 2.2 kW four pole IM is selected as the reference motor. Figure4 show the FEA model and Table1 shows the speciﬁcation of reference machine. ANSYS Maxwell was used to analyze the performance of the reference machine.

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FigureFigure 4. 4. FiniteFinite element element analysis analysis (FEA (FEA)) model model of of reference reference machine machine..

TableTable 1. 1. SpecificationSpeciﬁcation of of reference reference machine. machine.

ItemItem ValueValue Unit Unit OutputOutput power power 2.2 2.2 kW kW Line voltageLine voltage 380 380 Vrms V rms Input frequencyInput frequency 60 60 Hz Hz Outer diameterOuter diameter (stator/rotor) (stator/rotor) 180 /109.4 180/109.4 mm mm Inner diameter (stator/rotor) 110/32 mm NumberInner of slot diameter (stator/rotor) (stator/rotor) 36/28 110/32 mm – NumberStack length of slot (stator/rotor) 120 36/28 – mm Air gap lengthStack length 0.3 120 mm mm Air gap length 0.3 mm Table2 shows the FEA and experiment results of the reference machine. The e fficiency of the referenceTable machine 2 shows is the 90.7% FEA that and is experiment IE3 efficiency results according of the to reference IEC 60034. machine. Because The IE4 efficiency efficiency isof 91%,the referencethe reference machine machine is 90.7% does that not is satisfy IE3 efficiency IE4 effi ciency.according In general,to IEC 60034. the induction Because IE4 motor efficiency has the is 91%, rotor thecopper reference loss due machine to the slip,does and not thissatisfy loss IE4 accounts efficien forcy. 23% In general, of total lossesthe induction in Table2 motor. If the has rotor the copper rotor copperloss is reduced,loss due to the the effi slip,ciency and of this the loss electrical accounts machine for 23% can of satisfy total losses IE4 e ffiinciency. Table 2. LS-SynRM If the rotor does copper not losshave is the reduced, rotor copper the efficiency loss because of the thiselectrical motor ma is operatedchine can atsatisfy a synchronous IE4 efficiency. speed, LS-SynRM where slip does is zero. not haveTherefore, the rotor LS-SynRM copper loss can because satisfy IE4 this e ffimotorciency, is oper andated the design at a synchronous process of LS-SynRMspeed, where has slip studied is zero. to Therefore,improve the LS-SynRM efficiency. can satisfy IE4 efficiency, and the design process of LS-SynRM has studied to improve the efficiency. Table 2. FEA and experiment result of reference model. Table 2. FEA and experiment result of reference model. Value Item Value Unit Item FEA Experiment Unit FEA Experiment Power 2.2 2.2 kW Power 2.2 2.2 kW Speed 1764.96 1762 rpm TorqueSpeed 121764.96 1762 11.9 rpm Nm Core lossTorque 58.2712 11.9 63 Nm W Stator copperCore loss loss 72.6 58.27 63 78 W W Rotor copperStator loss copper loss 40.53 72.6 78 51 W W Total loss 214.3 220 W EfficiencyRotor copper loss 91.2 40.53 51 90.7 W % Power factorTotal loss 83.8 214.3 220 80 W % Efficiency 91.2 90.7 % Power factor 83.8 80 %

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4.4. DesignDesign ofof LS-SynRMLS-SynRM 4.1. Design Process 4.1. Design Process Figure5 shows the design process of LS-SynRM for IE4 e fficiency and improving the power factor. Figure 5 shows the design process of LS-SynRM for IE4 efficiency and improving the power The stator, rotor, and number of rotor slot are constrained under the same conditions as the reference factor. The stator, rotor, and number of rotor slot are constrained under the same conditions as the machine. The main design process is three steps. First, the design of the barrier is performed to reference machine. The main design process is three steps. First, the design of the barrier is performed improve the efficiency and power factor using RSM. The efficiency and power factor of LS-SynRM to improve the efficiency and power factor using RSM. The efficiency and power factor of LS-SynRM are determined by the saliency difference and ratio. Therefore, there are two steps in the design of are determined by the saliency difference and ratio. Therefore, there are two steps in the design of the barriers: the number of barriers and thickness of barriers and segments. Second, the design of the barriers: the number of barriers and thickness of barriers and segments. Second, the design of the the rotor slot is performed to reach LS-SynRM into the synchronous speed. Because synchronization rotor slot is performed to reach LS-SynRM into the synchronous speed. Because synchronization is is determined by the resistance and leakage inductance of the rotor bar, the design is performed determined by the resistance and leakage inductance of the rotor bar, the design is performed according to the depth of the rotor slot. Lastly, the design of each rib is performed considering the according to the depth of the rotor slot. Lastly, the design of each rib is performed considering the power factor and mechanical reliability. The thickness of the ribs affects the leakage flux, which affect power factor and mechanical reliability. The thickness of the ribs affects the leakage flux, which affect the performance of LS-SynRM, and is determined by the thickness of each rib. Therefore, the thickness the performance of LS-SynRM, and is determined by the thickness of each rib. Therefore, the of each rib must be designed considering the performance and safety factor. If the efficiency and thickness of each rib must be designed considering the performance and safety factor. If the efficiency power factor does not satisfy, the design parameter and range are reselected, and RSM is performed to and power factor does not satisfy, the design parameter and range are reselected, and RSM is satisfy the performance. Table3 shows IEC 60034 standard and the design objective of the e fficiency performed to satisfy the performance. Table 3 shows IEC 60034 standard and the design objective of and power factor. Considering the margin based on Table2, FEM performance is selected as 91.5% the efficiency and power factor. Considering the margin based on Table 2, FEM performance is efficiency and 81% power factor. selected as 91.5% efficiency and 81% power factor.

FigureFigure 5.5. DesignDesign processprocess ofof LS-SynRM.LS-SynRM.

Table 3. International Electrotechnical Committee (IEC) 60034 standard and design objective of the efficiency and power factor.

Items IEC 60034 Design Objective Unit Efficiency 91 ≥91.5 % Performance Power factor 77 ≥81 %

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Table 3. International Electrotechnical Committee (IEC) 60034 standard and design objective of the eﬃciency and power factor.

Items IEC 60034 Design Objective Unit Eﬃciency 91 91.5 % Performance ≥ Energies 2020, 13, x FOR PEER REVIEWPower factor 77 81 % 7 of 14 ≥

4.2. Design of Barrier From EquationEquation (3), thethe powerpower factorfactor of LS-SynRMLS-SynRM is determineddetermined by the saliencysaliency ratio, whichwhich is ratioratio dd-axis-axis inductanceinductance to toq q-axis-axis inductance. inductance. The Thedq dq-axis-axis inductances inductances are are determined determined by by the the number number of barriersof barriers and and thickness thickness of of ﬂux flux barrier barrier and and segments segments [27 [27].]. Therefore, Therefore, the the barrier barrier designdesign isis performedperformed according toto thethe numbernumber ofof barriersbarriers andand thicknessthickness ofof ﬂuxflux barrierbarrier andand segments.segments.

4.2.1. Number of Barrier Figure6 6 shows shows the the FEA FEA model model according according to to the the number number of barriers.of barriers. The The number number of rotor of rotor slots slots is the is samethe same as the as reference the reference machine, machine, and the and total the length total of length barrier of and barrier segment and is segment constant, is for constant, comparison for undercomparison the same under conditions. the same Furthermore, conditions. Furthermore, for each layer for of each each model, layer of the each thickness model, of the the thickness barrier and of segmentthe barrier are and compared segment equally. are compared Table4 shows equally. the comparison Table 4 shows of the dqthe-axis comparison inductances, of thethe saliencydq-axis ratio,inductances, the power the factor,saliency and ratio, efficiency the power under factor, the rated and current efficiency condition. under the From rated Table current4, the largercondition. the saliencyFrom Table ratio, 4, thethe largerlarger the the power saliency factor. ratio, Moreover, the larger the the effi powerciency fa ofctor. each Moreover, model satisfies the efficiency more than of IE4each e ffimodelciency. satisfies Therefore, more the than design IE4 efficiency. of the barrier, Ther accordingefore, the todesign the thickness of the barrier, of the according flux barrier to and the segments,thickness of is the performed flux barrier based and on segments, model 3. is performed based on model 3.

(a) (b) (c)

Figure 6. FEA model according to the number of barriers (a) model 1, (b) model 2, and (c) model 3. Figure 6. FEA model according to the number of barriers (a) model 1, (b) model 2, and (c) model 3.

Table 4.4. FEA result of LS-SynRM accordingaccording toto thethe numbernumber ofof barriers.barriers.

L Ld LLq SaliencySaliency Power FactoPowerr Factor Model Model d q (mH)(mH) (mH)(mH) Ratio Ratio (%) (%) Model 1Model 33.6 1 33.6 301.6 301.6 8.97 8.97 78.7 78.7 Model 2Model 31.7 2 31.7 297.7 297.7 9.38 9.38 79.3 79.3 Model 3Model 31.2 3 31.2 309.5 309.5 9.92 9.92 80.5 80.5

4.2.2. Thickness of Flux Barriers and Segments 4.2.2. Thickness of Flux Barriers and Segments To maximize the power factor, the optimal design is performed using RSM and FEA. Figure 7 To maximize the power factor, the optimal design is performed using RSM and FEA. Figure7 shows the design parameters for the optimal design [28]. The range of design parameters is selected shows the design parameters for the optimal design [28]. The range of design parameters is selected within the rotor constraints, as shown in Table 5. Furthermore, the objective function is maximizing within the rotor constraints, as shown in Table5. Furthermore, the objective function is maximizing the power factor and the efficiency. Figure 8a shows the RSM result and Figure 8b shows the optimal the power factor and the eﬃciency. Figure8a shows the RSM result and Figure8b shows the optimal design result. Table 6 shows the optimal design result. In Table 6, RSM and FEA results are similar, so the optimal result is valid.

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design result. Table6 shows the optimal design result. In Table6, RSM and FEA results are similar,

soEnergies the optimal 2020, 13, x result FOR PEER is valid. REVIEW 8 of 14

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FigureFigure 7.7. Design parameters for the optimaloptimal designdesign usingusing thethe responseresponse surfacesurface methodmethod (RSM).(RSM).

Table 5. Range of design parameters.

Items αr (degM) Kw Wshaft (mm) Figure 7. Design parametersRange for the 4.5–9optimal design 0.4–0.6 using the response 3–6 surface method (RSM).

(a) (b)

Figure 8. Optimization using RSM under rated current condition. (a) RSM result, (b) optimal design result.

(a)Table 5. Range of design parameters. (b)

Items αr (degM) Kw Wshaft (mm) FigureFigure 8. 8. OptimizationOptimization using using RSM RSM under under rated rated current current condition. condition. (a) RSM (a) result, RSM result,(b) optimal (b) optimal design Range 4.5–9 0.4–0.6 3–6 result.design result.

TableTableTable 5. Range 6.6. OptimalOptimal of design designdesign parameters. result.result.

Items αr (degM) PowerKw FactorFactoWshaftr (mm)Efficiency Eﬃ ciency ParameterParameter Range 4.5–9 0.4–0.6(%) 3–6 (%) (%) αr Wshaft αr KWw shaft RSM FEA RSM FEA Kw RSM FEA RSM FEA (degM) (degM) Table(mm) 6.(mm) Optimal design result. 7.236 0.56097.236 0.5609 4 4 81.19 81.19 81.02 81.02 91.59 91.5991.75 91.75 Power Factor Efficiency Parameter (%) (%) 4.3.4.3. Design of Rotor Slot αr Wshaft Kw RSM FEA RSM FEA InIn general,general, the the starting starting(degM) characteristic characteristic is(mm) determined is determined by the rotorby the resistance rotor resistance and leakage and inductance. leakage Thisinductance. rotor resistance This rotor and resistance leakage7.236 is 0.5609and determined leakage 4 is by de 81.19 thetermined depth81.02 and by91.59 the thickness depth 91.75 ofand the thickness rotor slot. ofHowever, the rotor theslot. barriers However, are constraintsthe barriers on are the constraints thickness on and the depth thickness of the and rotor depth slot. Therefore,of the rotor for slot. synchronization, Therefore, for 4.3.synchronization, Design of Rotor the Slot design parameters of the rotor slot are determined, as shown in Figure 9a. In order to analyze synchronization, the analysis, according the selected design parameters, is In general, the starting characteristic is determined by the rotor resistance and leakage performed by the time-step FEA, which is the mechanical and electromagnetic transient analysis inductance. This rotor resistance and leakage is determined by the depth and thickness of the rotor [29,30], considering inertia of LS-SynRM. Figure 9b shows the time step FEA result. The slot. However, the barriers are constraints on the thickness and depth of the rotor slot. Therefore, for synchronization region is determined by the depth of the rotor slot. Figure 10 shows the time step synchronization, the design parameters of the rotor slot are determined, as shown in Figure 9a. In order to analyze synchronization, the analysis, according the selected design parameters, is performed by the time-step FEA, which is the mechanical and electromagnetic transient analysis [29,30], considering inertia of LS-SynRM. Figure 9b shows the time step FEA result. The synchronization region is determined by the depth of the rotor slot. Figure 10 shows the time step

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the design parameters of the rotor slot are determined, as shown in Figure9a. In order to analyze synchronization, the analysis, according the selected design parameters, is performed by the time-step EnergiesFEA, which2020, 13, is x FOR the mechanicalPEER REVIEWand electromagnetic transient analysis [29,30], considering inertia9 of 14 of LS-SynRM.Energies 2020, Figure13, x FOR9b PEER shows REVIEW thetime step FEA result. The synchronization region is determined9 of by 14 FEAthe depthresult offor the the rotorthree slot. selected Figure points. 10 shows If synchron the timeization step fails, FEA the result torque for theand three speed selected pulsate. points. As a FEA result for the three selected points. If synchronization fails, the torque and speed pulsate. As a result,If synchronization the design parameter fails, the torqueis determined and speed as T pulsate.bar1 = 20 (mm) As a result, and T thebar2 = design 13 (mm). parameter is determined result, the design parameter is determined as Tbar1 = 20 (mm) and Tbar2 = 13 (mm). as Tbar1 = 20 (mm) and Tbar2 = 13 (mm).

Tbar 2 Tbar 2

Tbar1 Tbar1

(a) (b) (a) (b) Figure 9. Design parameter of rotor slot and FEA result (a) design parameter, (b) FEA result. FigureFigure 9.9. DesignDesign parameterparameter ofof rotorrotor slotslot andand FEAFEA resultresult ((aa)) design design parameter, parameter, ( b(b)) FEA FEA result. result.

(a) (b) (a) (b) FigureFigure 10. 10. SpeedSpeed and and torque torque curve curve of of the the selected selected three three points points (a (a) )speed speed curve curve (b (b) )torque torque curve. curve. Figure 10. Speed and torque curve of the selected three points (a) speed curve (b) torque curve. 4.4.4.4. Design Design of of Ribs Ribs 4.4. Design of Ribs WhenWhen LS-SynRM LS-SynRM rotates, rotates, the the centrifugal centrifugal force force is is fo focusedcused on on the the outer outer rib. rib. If If the the thickness thickness of of the the outerouter When rib isis designed LS-SynRMdesigned to to berotates, small,be small, the the centrifugal LS-SynRMthe LS-SynRM force is mechanically isis fo mechanicallycused damagedon the outerdamaged during rib. If operation. duringthe thickness operation. Therefore, of the Therefore,theouter design rib the ofis ribdesigneddesign must of be ribto performed bemust small, be performed for the use LS-SynRM as the for industrialuse is as mechanically the application. industrial damaged application. However, during the However, thickness operation. the of thicknesstheTherefore, outer ribof thethe also design outer aﬀects ofrib the rib also power must affects factor,be performed the so thepower design fo rfactor, use of as the so the rib the industrial is performeddesign application.of the considering rib is However, performed the power the consideringfactorthickness and mechanical ofthe the power outer reliability.factor rib andalso Themechanicalaffects mechanical the reliability.power reliability factor, The is mechanicalso determined the design reliability by aof safety the isrib factor,determined is performed as shown by ain consideringsafety the following factor, the as equation.powershown factorin Generally,the andfollowing mechanical the equation. mechanical reliability. Generally, reliability The themechanical is ensuredmechanical ifreliability the reliability safety is factor determined is ensured is higher ifby thethana safetysafety 1.5 [factor, 31factor]. asis highershown than in the 1.5 following [31]. equation. Generally, the mechanical reliability is ensured if the safety factor is higher than 1.5 [31]. σyield SF = σ (4) σyield SF = σmax σ yield (4) SF = max σ (4) max where SF is the safety factor, σyield is the tensile yield strength of material, σmax is the stress of material duringwhere rotation. SF is the safety factor, σyield is the tensile yield strength of material, σmax is the stress of material duringBecause rotation. of the structure of LS-SynRM, there are several bridges that reduce the mechanical stress at the Becauserib. Therefore, of the structure the design of LS-SynRM, parameters there are selected,are several as bridges shown thatin Figure reduce 11. the Considering mechanical stress the manufacture,at the rib. Therefore, the design the parameters design parameters are designed are to selected, be 0.3 (mm) as shown or more. in FigureFigure 12 11. shows Considering the power the manufacture, the design parameters are designed to be 0.3 (mm) or more. Figure 12 shows the power

Energies 2020, 13, 5774 10 of 15

where SF is the safety factor, σyield is the tensile yield strength of material, σmax is the stress of material during rotation. Because of the structure of LS-SynRM, there are several bridges that reduce the mechanical stress Energiesat the 2020 rib., 13 Therefore,, x FOR PEER the REVIEW design parameters are selected, as shown in Figure 11. Considering10 of the14 Energies 2020, 13, x FOR PEER REVIEW 10 of 14 manufacture, the design parameters are designed to be 0.3 (mm) or more. Figure 12 shows the power factor and the safety factor, according to the design parameters. As the thickness of each rib increases, factorfactor and and the the safety safety factor, factor, according according to to the the design design parameters. parameters. As As thethe thicknessthickness ofof eacheach ribrib increases,increases, the power factor is reduced because the leakage magnetic flux is increased. Furthermore, the safety thethe power power factor factor is is reduced reduced because because the the leakage leakage magnetic magnetic ﬂux flux is is increased. increased. Furthermore, Furthermore, the the safety safety factor is increased due to reduced mechanical stress. Considering the objective performance in Table factorfactor is is increased increased due due to to reduced reduced mechanical mechanical stress. stress. Considering Considering the the objective objective performance performance in Tablein Table3, 3,the the design design parameters parameters are are designed designed as asT rib1Trib1= = 0.3 (mm) and Trib2rib2 == 0.70.7 (mm). (mm). 3, the design parameters are designed as Trib1 = 0.3 (mm) and Trib2 = 0.7 (mm).

Figure 11. Design parameter for the power factor and safety factor. Figure 11. Design parameter for the power factor and safety factor. Figure 11. Design parameter for the power factor and safety factor.

(a) (b) (a) (b) Figure 12. FEA result according to the design parameter of rib (a) safety factor, and (b) power factor. FigureFigure 12.12.FEA FEA resultresult accordingaccording toto thethe designdesign parameterparameter of of rib rib ( a(a)) safety safety factor, factor, and and ( b(b)) power power factor. factor.

4.5.4.5. Design Design Result Result 4.5. Design Result FigureFigure 13 13 shows shows the the design design result result through through the the design design process process of of LS-SynRM. LS-SynRM. Table Table 7 shows the Figure 13 shows the design result through the design process of LS-SynRM. Table 7 shows the FEAFEA result result of of LS-SynRM. LS-SynRM. The The efficiency efficiency of of LS-SynRM LS-SynRM is is 91.7% 91.7% and and the the power power factor factor of of LS-SynRM LS-SynRM is is FEA result of LS-SynRM. The efficiency of LS-SynRM is 91.7% and the power factor of LS-SynRM is 81.2%.81.2%. Compared Compared withwith TableTable2, the2, the e ffi ciencyefficiency is improved is improved about about 0.5%, but0.5%, the but power the factorpower is decreasedfactor is 81.2%. Compared with Table 2, the efficiency is improved about 0.5%, but the power factor is decreasedabout 2.6% about than 2.6% the reference than the machine. reference The machine. efficiency The satisfies efficiency the satisfies IE4 efficiency the IE4 and efficiency the power and factor the decreased about 2.6% than the reference machine. The efficiency satisfies the IE4 efficiency and the powersatisfies factor the IECsatisfies 60034 the standard IEC 60034 for standard the industrial for th application.e industrial application. Therefore, the Therefore, LS-SynRM the can LS-SynRM improve power factor satisfies the IEC 60034 standard for the industrial application. Therefore, the LS-SynRM canthe improve efficiency the compared efficiency with compared IM and with be used IM and as an be industrial used as an electric industrial machine. electric machine. can improve the efficiency compared with IM and be used as an industrial electric machine.

Figure 13. Final model of LS-SynRM. Figure 13. Final model of LS-SynRM.

Energies 2020, 13, x FOR PEER REVIEW 10 of 14

factor and the safety factor, according to the design parameters. As the thickness of each rib increases, the power factor is reduced because the leakage magnetic flux is increased. Furthermore, the safety factor is increased due to reduced mechanical stress. Considering the objective performance in Table 3, the design parameters are designed as Trib1 = 0.3 (mm) and Trib2 = 0.7 (mm).

Figure 11. Design parameter for the power factor and safety factor.

(a) (b)

Figure 12. FEA result according to the design parameter of rib (a) safety factor, and (b) power factor.

4.5. Design Result Figure 13 shows the design result through the design process of LS-SynRM. Table 7 shows the FEA result of LS-SynRM. The efficiency of LS-SynRM is 91.7% and the power factor of LS-SynRM is 81.2%. Compared with Table 2, the efficiency is improved about 0.5%, but the power factor is decreased about 2.6% than the reference machine. The efficiency satisfies the IE4 efficiency and the Energiespower2020 factor, 13 ,satisfies 5774 the IEC 60034 standard for the industrial application. Therefore, the LS-SynRM11 of 15 can improve the efficiency compared with IM and be used as an industrial electric machine.

Figure 13. Final model of LS-SynRM. Figure 13. Final model of LS-SynRM.

Table 7. FEA result of the ﬁnal LS-SynRM.

Item Value Unit Power 2.2 kW Speed 1800 rpm Torque 11.7 Nm Core loss 53.3 W Stator copper loss 87.78 W Rotor copper loss 12.3 W Total loss 201.4 W Eﬃciency 91.7 % Power factor 81.2 % Safety factor 1.56 -

5. Experimental Validation

5.1. Manufacture of LS-SynRM To verify the FEA result, the designed LS-SynRM is manufactured. Figure 14a shows the rotor core of the manufactured LS-SynRM. A die-casting process is performed to inject the melted aluminum into the rotor slot. During the die-casting process, a high press is applied to the rotor slot so that the melted aluminum is injected in the rotor slot. Figure 14b shows the endplate, which is required to prevent the melted aluminum from being injected into the barrier. Figure 14c shows the cross-section of the rotor and the aluminum is ﬁlled in the rotor slot. Figure 14d shows the rotor with the end-ring. The width of the end-ring is the same as the reference machine and the height of the end-ring is the diﬀerence between the deepest rotor slot depth and the outer diameter of the rotor.

5.2. Experiment Result The experiment is performed to verify the FEA result. Figure 15 shows the experiment environment. The dynamometer motor is induction motor as 2.2 kW load. Using voltage and frequency control, LS-SynRM runs up and reaches a synchronous speed. After the LS-SynRM reaches a synchronous speed, the load torque is applied through the dynamometer motor to measure the efficiency and power factor of LS-SynRM. In the power analyzer, the efficiency and power factor are calculated using measured voltage, current, torque, and speed. Table8 shows the comparison of FEA and the experiment result of LS-SynRM. When comparing the experiment and FEA results, the experiment current increases by about 5% than the FEA current. This reduces the power factor and increases the copper loss, as shown in Table8. The power factor of FEA and experiment is 81.2% and 77.3%, respectively. However, the efficiencies of FEA and the experiment are similar because the total losses are similar. As a result, it is confirmed that the IE4 class efficiency and the power factor in IEC 60034 is satisfied. Energies 2020, 13, x FOR PEER REVIEW 11 of 14

Table 7. FEA result of the final LS-SynRM. Item Value Unit Power 2.2 kW Speed 1800 rpm Torque 11.7 Nm Core loss 53.3 W Stator copper loss 87.78 W Rotor copper loss 12.3 W Total loss 201.4 W Efficiency 91.7 % Power factor 81.2 % Safety factor 1.56 -

5. Experimental Validation

5.1. Manufacture of LS-SynRM To verify the FEA result, the designed LS-SynRM is manufactured. Figure 14a shows the rotor core of the manufactured LS-SynRM. A die-casting process is performed to inject the melted aluminum into the rotor slot. During the die-casting process, a high press is applied to the rotor slot so that the melted aluminum is injected in the rotor slot. Figure 14b shows the endplate, which is required to prevent the melted aluminum from being injected into the barrier. Figure 14c shows the cross-section of the rotor and the aluminum is filled in the rotor slot. Figure 14d shows the rotor with the end-ring. The width of the end-ring is the same as the reference machine and the height of the Energies 2020, 13, 5774 12 of 15 end-ring is the difference between the deepest rotor slot depth and the outer diameter of the rotor.

(a) (b)

Energies 2020, 13, x FOR PEER REVIEW 12 of 14

5.2. Experiment Result The experiment is performed to verify the FEA result. Figure 15 shows the experiment environment. The dynamometer motor is induction motor as 2.2 kW load. Using voltage and frequency control, LS-SynRM runs up and reaches a synchronous speed. After the LS-SynRM reaches a synchronous speed, the load torque is applied through the dynamometer motor to measure the efficiency and power factor of LS-SynRM. In the power analyzer, the efficiency and power factor are calculated using measured voltage, current, torque, and speed. Table 8 shows the comparison of FEA and the experiment result of LS-SynRM. When comparing the experiment and FEA results, the experiment current increases by about 5% than the FEA current. This reduces the power factor and increases the copper loss,( cas) shown in Table 8. The power factor of FEA and(d) experiment is 81.2% and 77.3%, respectively. However, the efficiencies of FEA and the experiment are similar because the total lossesFigure are 14. similar. Manufactured As a result, LS-SynRM it is confirmed ( (aa)) rotor rotor that core, the ( bIE4) endplate class efficiency for die-casting, and the ( cpower) cross-section factor in ofIEC 60034rotor, is ((satisfied.d)) rotorrotor withwith thethe end-ring.end-ring.

Figure 15. The experiment environment and test dynamometer. Figure 15. The experiment environment and test dynamometer.

Table 8. Comparison of FEA and experiment result. Value Item Unit FEA Experiment Power 2.2 2.2 kW Torque 11.7 11.6 Nm Current 4.56 4.8 Arms Core loss 53.3 45.9 W Stator copper loss 87.78 100 W Rotor copper loss 12.3 0 W Total loss 201.4 201.7 W Efficiency 91.7 91.6 % Power factor 81.2 77.3 %

6. Conclusions In this paper, the design method of LS-SynRM is studied to satisfy the efficiency and power factor of IEC 60034 standard. In addition, LS-SynRM is designed considering the mechanical reliability for use in the industrial application. The performance of LS-SynRM is analyzed according to several design parameters, such as the number of barriers, the thickness of the barriers, and the segments, the depth of the rotor slot, and thickness of each rib. The design of the barrier is performed to satisfy the performance of LS-SynRM using RSM. The design of the rotor slot is performed, and the synchronization region is determined by the depth of the rotor slot. The design of each rib is

Energies 2020, 13, 5774 13 of 15

Table 8. Comparison of FEA and experiment result.

Value Item Unit FEA Experiment Power 2.2 2.2 kW Torque 11.7 11.6 Nm Current 4.56 4.8 Arms Core loss 53.3 45.9 W Stator copper loss 87.78 100 W Rotor copper loss 12.3 0 W Total loss 201.4 201.7 W Eﬃciency 91.7 91.6 % Power factor 81.2 77.3 %

6. Conclusions In this paper, the design method of LS-SynRM is studied to satisfy the efficiency and power factor of IEC 60034 standard. In addition, LS-SynRM is designed considering the mechanical reliability for use in the industrial application. The performance of LS-SynRM is analyzed according to several design parameters, such as the number of barriers, the thickness of the barriers, and the segments, the depth of the rotor slot, and thickness of each rib. The design of the barrier is performed to satisfy the performance of LS-SynRM using RSM. The design of the rotor slot is performed, and the synchronization region is determined by the depth of the rotor slot. The design of each rib is performed, and the thickness of each rib is designed considering the power factor and the safety factor. Considering the efficiency, the power factor, and the safety factor, the optimal LS-SynRM is designed. The final LS-SynRM satisfies the efficiency of 91.7%, the power factor of 81.2%, and the safety factor 1.56. The final LS-SynRM is manufactured and the experiment is performed to verify the FEA result. As a result, the efficiency and power factor satisfy the IEC 60034 standard.

Author Contributions: Conceptualization, H.K.; methodology, H.J.; validation, S.-T.O.; formal analysis, H.K. and S.-T.O.; investigation, H.K. and Y.P.; resources, S.-H.W.; data curation, Y.P.; writing—original draft preparation, H.K.; writing—review and editing, S.-T.O. and H.J.; supervision, S.-H.W. and J.L.; project administration, J.L.; funding acquisition, Y.-D.C. and S.-H.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Korea government Ministry of Ministry of Land, Infrastructure and Transport (20CTAP-C157784-01) and in part by a Korea Institute of Energy Technology Evaluation and Planning grant funded by the Korea government (20192010106780, A Construction and Operation of Open Platform for Next-Generation Super Premium Efficiency (IE4) Motors). Conflicts of Interest: The authors declare no conflict of interest.

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