Synchronous Reluctance Motor Vs. Induction Motor at Low-Power Industrial Applications: Design and Comparison

energies

Article Synchronous Reluctance Motor vs. Induction Motor at Low-Power Industrial Applications: Design and Comparison

Nezih Gokhan Ozcelik, Ugur Emre Dogru, Murat Imeryuz and Lale T. Ergene *

Electrical Engineering Department, Istanbul Technical University, 34469 Istanbul, Turkey; [email protected] (N.G.O.); [email protected] (U.E.D.); [email protected] (M.I.) * Correspondence: [email protected]; Tel.: +90-212-285-3634

Received: 19 March 2019; Accepted: 4 June 2019; Published: 8 June 2019

Abstract: Although three-phase induction motors are the most common motor type in industry, a growing interest has arisen in emerging electric motor technologies like synchronous reluctance motors and permanent magnet motors. Synchronous reluctance motors are a step forward compared to permanent magnet motors when the cost of the system is considered. The main focus of this study is low-power industrial applications, which generally use three-phase induction motors. In this study, the synchronous reluctance motor family is compared at three diﬀerent power levels: 2.2 kW, 4 kW, and 5.5 kW. The aim of this study is to design and compare synchronous reluctance motors, which can be alternative to the reference induction motors. Finite element analysis is performed for the reference induction motors initially. Their stators are kept the same and the rotors are redesigned to satisfy output power requirements of the induction motors. Detailed design, analysis, and optimization processes are applied to the synchronous reluctance motors considering eﬃciency, power density, and manufacturing. The results are evaluated, and the optimized designs are chosen for each power level. They are prototyped and tested to measure their performance.

Keywords: synchronous reluctance motor; induction motor; ﬁnite element method; synchronous motor; reluctance motor

1. Introduction Increasing attention to energy efficiency makes electric motor technology important when considering the percentage of their energy consumption. Different types of electric motors are used in low-power industrial applications dependent on the needs of the system. However, market research shows that a great majority of industrial motors are still induction motors. Permanent magnet motors and synchronous reluctance motors are becoming an alternative solution for low-power industrial applications. Even though permanent magnet motors are more efficient compared to induction motors, their cost is still higher because of the permanent magnet. Demagnetization is another drawback of permanent magnet machines. Synchronous reluctance motors (SynRMs) have become prominent since the rotor does not have any conductor or any permanent magnet. It costs almost the same as the induction motor, and its efficiency is higher at the same power ratings. There are many studies on synchronous reluctance machines in the literature. Kostko published the first paper titled “Polyphase Reaction Synchronous Motors” in 1923 [1]. The theory of synchronous reluctance machine was established the first time in this paper. However, SynRM was not an alternative to industrial induction machines in those days due to the lack of starting capacities and relatively low efficiencies. Cruickshank et al. published a paper titled “Axially Laminated Anisotropic Rotors for Reluctance Motors” and they proposed the axially laminated SynRM structure [2]. This motor was

Energies 2019, 12, 2190; doi:10.3390/en12112190 www.mdpi.com/journal/energies Energies 2019, 12, 2190 2 of 20 more efficient than Kostko’s first design and became popular between the 1960s and 1970s because of its increased capabilities. The axial alignment of laminations required new special production lines, so this type of motor was not industrially viable in terms of manufacturing. Cruickshank et al. formed the mathematical theory of the axially laminated SynRM in another study [3]. In this work, it is shown that axially laminated SynRMs are still a little behind the induction machines even though they have higher saliency ratios than the transverse laminated SynRM. Honsinger formed the general analytical model of SynRM in his paper titled “The Inductances Ld and Lq of Reluctance Machines” in 1971 [4]. A new analytical method was proposed to calculate the axes inductances in this paper. Before this study, obtaining high saliency ratios, and eventually, higher torque, were arbitrary. SynRM became popular because of the developments in power electronics and manufacturing technologies in 1990s. Lipo proposed the synchronous reluctance motor as a viable alternative for AC drive systems [5]. As Lipo and Matsuo’s research focused on rotor design optimization [6], Miller et al. studied driver designs for SynRM [7–9]. Vagati et al. had a paper on the design criteria of synchronous reluctance motors [10]. Vagati et al. had also a study on the experimental comparison of induction and synchronous reluctance motors. They showed that a SynRM can produce 20% to 40% higher torque compared to an IM in the case of the increased rated current for the same power loss or same temperature [11]. Another comparison of these two motors for a traction application was evaluated by Kamper et al. [12]. There are studies carried out only to maximize the saliency ratio [13]. Different optimization methodologies were evaluated by researchers. Moghaddam et al. evaluated the optimization of barrier shapes [14–16], Bianchi et al. focused on torque ripple reduction [17], and Lin et al. applied multi-objective optimization algorithms to a six-phase synchronous reluctance machine to increase the efficiency [18]. A paper was written by Kamper et al. which remarked on the importance of the finite element method (FEM) to analyze the SynRM [19]. Another work focusing on the multi-objective optimization algorithm of synchronous reluctance motors was published by Cupertino et al. [20]. Ergene et al. published a paper on a permanent magnet assisted SynRM (PMaSynRM), which is designed and prototyped for home appliance [21]. In this study, the rotors of the synchronous reluctance motors are designed for three different frames: 2.2 kW, 4 kW, and 5.5 kW. The stators of the SynRMs are kept the same as those of the reference induction motors at the same frames. The designed synchronous reluctance motors are prototyped and compared to the reference induction motors in terms of efficiency, losses, power density, active material, etc.

2. Torque Equations The synchronous reluctance motor does not have a permanent magnet and conductor in its rotor compared to the permanent magnet motor and induction motor. The torque is equal to the reluctance torque, which is produced by saliency between d and q axes inductances. The ratio between d and q axes inductances is deﬁned as the saliency ratio. This ratio directly aﬀects the torque production of the synchronous reluctance motor. The general torque equation of this motor is in Equation (1) as [21]:

3 p T = (λqsi λ iqs) (1) 2 2 ds − ds where p is the pole pair number, λds, λqs are d and q axis stator ﬂux linkages, ids, iqs are d and q axis stator currents. Rotor currents are zero at steady-state, so the ﬂux linkages can be expressed as a function of d-q axis inductances Ld and Lq, respectively, as given in Equations (2) and (3).

λds = Ldsids (2)

λqs = Lqsiqs (3) Energies 2019, 12, 2190 3 of 20

The reluctance torque in d-q inductances are given in Equation (4). Energies 2019, 12, x FOR PEER REVIEW 3 of 20 Energies 2019, 12, x FOR PEER REVIEW 3 p 3 of 20 T = L Lqs i iqs (4) The saliency ratio between the Ld and L2q inductances2 ds − hasds a vital role in obtaining higher torque values.The saliency ratio between the Ld and Lq inductances has a vital role in obtaining higher torque The saliency ratio between the L and L inductances has a vital role in obtaining higher values. d q 3.torque Design values. Methodology and Parameters 3. Design Methodology and Parameters 3. DesignThe design Methodology methodology and consists Parameters of three main stages: Electromagnetic, structural, and thermal designs.The These design design methodology stages are consists given in of Figure three main 1 as blocks,stages: andElectromagnetic, each main block structural, affects andeach thermal other. The design methodology consists of three main stages: Electromagnetic, structural, and thermal Designdesigns. criteria These block design is stagesthe input are givenfor the in electr Figureomagnetic 1 as blocks, design. and Theeach electromagnetic main block affects design each block other. designs. These design stages are given in Figure1 as blocks, and each main block a ﬀects each includesDesign criteriamotor sizing, block optimizing,is the input andfor thefinite electr elementomagnetic analysis design. of the The optimized electromagnetic motor design. design Ergene block other. Design criteria block is the input for the electromagnetic design. The electromagnetic design et.alincludes proposed motor a multi-parametersizing, optimizing, chart, and includingfinite element rotor analysis geometric of th parameterse optimized for motor 2.2 kW design. motor Ergene [22]. block includes motor sizing, optimizing, and ﬁnite element analysis of the optimized motor design. Atet.al the proposed end of thea multi-parameter electromagnetic chart, design including proced rotorure, a geometric structural parameters design procedure for 2.2 kW begins. motor The [22]. Ergene et al proposed a multi-parameter chart, including rotor geometric parameters for 2.2 kW procedureAt the end supplies of the feedbackelectromagnetic to previous design design proced proceduresure, a structural and updates design the procedure design following begins. theThe motor [22]. At the end of the electromagnetic design procedure, a structural design procedure begins. sameprocedure design supplies steps, respectively. feedback to previous design procedures and updates the design following the The procedure supplies feedback to previous design procedures and updates the design following the same design steps, respectively. same design steps, respectively. ELECTROMAGNETIC STRUCTURAL THERMAL DESIGN DESIGN ELECTROMAGNETIC STRUCTURALDESIGN THERMAL DESIGN DESIGN DESIGN Pre-Design Pre-Design Design Structural Thermal Analysis Analysis Final CriteriaDesign Optimization Structural Thermal Final Criteria Optimization Analysis Analysis

FEM FEM

Figure 1. Electric motor design procedure. Figure 1. ElectricElectric motor motor design design procedure. In this study, the reference induction motors and the newly designed synchronous reluctance In this study, the reference induction motors and the newly designed synchronous reluctance motorsIn are this compared study, the for reference three different induction power motors levels and at the 2.2 newly kW, 4 designedkW, and synchronous5.5 kW for low-power reluctance motors are compared for three diﬀerent power levels at 2.2 kW, 4 kW, and 5.5 kW for low-power industrialmotors are applications. compared forThe three stators different and ampere-turns power levels ofat the2.2 kW,induction 4 kW, motorsand 5.5 are kW kept for low-powerthe same industrial applications. The stators and ampere-turns of the induction motors are kept the same duringindustrial synchronous applications. reluctance The stators moto rsand design ampere-turns procedures of tothe make induction a fair comparison.motors are keptThe rotorsthe same of during synchronous reluctance motors design procedures to make a fair comparison. The rotors of theduring induction synchronous motors arereluctance designed moto againrs design consider proceduresing the reluctance to make amotor fair comparison. parameters. The The rotors design of the induction motors are designed again considering the reluctance motor parameters. The design procedurethe induction steps motors are given are indesigned Figure 2 again schematically. considering the reluctance motor parameters. The design procedure steps are given in Figure2 schematically. procedure steps are given in Figure 2 schematically.

Number of Rotor Barriers Shape of Rotor Barriers Air gap Insulation Ratio Number of Rotor Barriers Shape of Rotor Barriers Air gap Insulation Ratio

1 Barriers g=0.25 kq=0.6 Angular 1 Barriers g=0.25 kq=0.6 Angular g=0.34 kq=0.7 2 Barriers Round g=0.34 kq=0.7 Design 2 Barriers Round kq=0.8 Design 3 Barriers Angular Cut-off kq=0.8 3 Barriers Cut-off -RoundAngular g=0.34 kq=0.9 4 Barriers -Round g=0.34 kq=0.9 4 Barriers

FigureFigure 2. 2. SynchronousSynchronous reluctance reluctance motor motor (SynRM) (SynRM) electromagnetic electromagnetic design design parameters. parameters. Figure 2. Synchronous reluctance motor (SynRM) electromagnetic design parameters. In the design step, the rotor barrier effect is analyzed by considering a different number of rotor barriersIn andthe designshapes. step, The theair gaprotor is barrier also a parameter,effect is anal althoughyzed by theconsidering stator dimensions a different and number the ampere- of rotor turnsbarriers are andkept shapes. the same. The Uniform air gap isand also salient a parameter, air gap althoughstructures the with stator different dimensions radial andair gap the lengthsampere- areturns considered are kept forthe the same. design. Uniform Lastly, and the salient insulation air gap ratios structures of the synchronouwith differents reluctance radial air motorsgap lengths are are considered for the design. Lastly, the insulation ratios of the synchronous reluctance motors are

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In the design step, the rotor barrier eﬀect is analyzed by considering a diﬀerent number of rotor barriers and shapes. The air gap is also a parameter, although the stator dimensions and the

Energiesampere-turns 2019, 12, x are FOR kept PEER the REVIEW same. Uniform and salient air gap structures with diﬀerent radial air4 of gap 20 lengths are considered for the design. Lastly, the insulation ratios of the synchronous reluctance motors evaluated.are evaluated. Efficiency, Eﬃciency, cost, cost, and and power power density density of ofea eachch design design are are compared compared to to those those of of the the reference reference induction motor to obtain the optimum design for low-power industrial applications, such as pumps and fans.

4. Multi Multi Parameter Design Steps of SynRMs The parameters, such as barrier number, end bridge thickness, air gap dimension, and insulation ratio, are used on the motor model in in FEA FEA to to obtain obtain an an optimized optimized motor motor in in terms terms of of cost, cost, efficiency, eﬃciency, and serial production limits [[22].22]. The rotor design parameters are shown in Figure3 3..

Figure 3. SynRM design parameters.

Some constraints are imposed on each step. Th Thee outer and inner stator diameter, the winding distribution, thethe statorstator slot slot number, number, the the stator, stator, and and rotor rotor material material are defined are defined as constant as constant in the analysis. in the analysis.Specific constraints Specific constraints are only used are ononly certain used steps. on certain The barrier steps. number The barrier parameter number is examined parameter first. is Theexamined thickness first. e ffTheect thickness of end bridges effect isof analyzedend bridges for is two analyzed flux barriers. for two The flux L barriers.d/Lq ratio The varies Ld/L dueq ratio to variesboth the due flux to barriers’both the numbersflux barriers’ and numbers their shape. and Another their shape. parameter Another analyzed parameter is the analyzed flux barrier is the width flux variation.barrier width It is variation. gradually It decreased is gradually in thedecreased outward in radialthe outward direction. radial The direction. air gap e Theffect air is evaluatedgap effect inis evaluatedthree different in three models. different Two ofmodels. them are Two the of uniform them are air the gap uniform in the radial air gap direction, in the radial and the direction, third model and theis the third cut-o modelff. Each is the air gapcut-off. model Each is employedair gap model on two is employed different numbers on two differe of barriersnt numbers and three of dibarriersfferent andflux barrierthree different structure. flux The insulationbarrier structure. ratio is the The final insulation parameter ratio for the is simulations.the final parameter This ratio directlyfor the simulations.influences the This saliency ratio directly ratio (Ld influences/Lq). Four the diff erentsaliency insulation ratio (L ratiosd/Lq). Four are examined different oninsulation four diff ratioserent areflux examined barrier motor on four models different with variableflux barrier and motor constant models width with angular variable flux and barriers. constant width angular flux barriers. 5. Finite Element Analysis Results 5. Finite Element Analysis Results 5.1. Analysis of the Reference Induction Motors

5.1. AnalysisThe reference of the Reference induction Induction motors Motors have 2.2 kW, 4 kW and 5.5 kW rated power values as listed in Table1. These induction motors are the mass production commercial type squirrel cage induction The reference induction motors have 2.2 kW 4 kW and 5.5 kW rated power values as listed in motors. The rated voltage is 400 V (line-to-line) for 2.2 kW motor, while it is 690 V for the 4 kW and Table 1. These induction motors are the mass production commercial type squirrel cage induction motors. The rated voltage is 400 V (line-to-line) for 2.2 kW motor, while it is 690 V for the 4 kW and 5.5 kW motors. The current must be increased to provide the same amount of power if the voltage is 400 V for 4.4 and 5.5 kW. The motor faces danger if the current exceeds the motor's nameplate ratings.

Energies 2019, 12, 2190 5 of 20

5.5 kW motors. The current must be increased to provide the same amount of power if the voltage is EnergiesEnergies 2019 2019, 12, 12, x, FORx FOR PEER PEER REVIEW REVIEW 5 5of of 20 20 400 V for 4.4 and 5.5 kW. The motor faces danger if the current exceeds the motor’s nameplate ratings. TableTable 1. 1. Reference Reference Induction Induction Motors’ Motors’ Nameplate. Nameplate. Table 1. Reference Induction Motors’ Nameplate. PowerPower (kW) (kW) 5.5 5.5 4 4 2.2 2.2 VoltageVoltagePower (V) (V) (kW) 5.5 400/690 400/690 4 400/690 400/690 2.2 230/400 230/400 CurrentCurrentVoltage (A) (A) (V) 400/ 11.1/6.4690 11.1/6.4 400/690 8.3/4.8 8.3/4.8 230/400 8.5/4.9 8.5/4.9 FrequencyCurrent (Hz) (A) 11.1/6.4 50 8.3/4.8 50 8.5/4.9 50 FrequencyFrequency (Hz) (Hz) 50 50 50 50 50 50 NumberNumberNumber of of Poles Poles of Poles 4 4 4 4 4 4 4 4 4 −1 1 SpeedSpeedSpeed (min (min (min)−1 ) − ) 146514651465 145514551455 1450 14501450 Slip Slip 0.02330.0233 0.030.03 0.033 0.033 ESlipﬃciency (%) 89.60.0233 88.60.03 86.7 0.033 EfficiencyEfficiency (%) (%) 89.6 89.6 88.6 88.6 86.7 86.7

First, the numerical models of the motors are built using a two dimensional (2D) finite First,First, the the numerical numerical models models of of the the motors motors are are built built using using a atwo two dimensional dimensional (2D) (2D) finite finite element element element method (FEM) based commercial software (FLUX2D) with 2D time-stepping vector potential methodmethod (FEM) (FEM) based based commercial commercial software software (FLUX2D) (FLUX2D) with with 2D 2D time-stepping time-stepping vector vector potential potential formulation. The 3D effects like the end turns of stator winding are included using lumped circuit formulation.formulation. The The 3D 3D effects effects like like the the end end turns turns of of stator stator winding winding are are included included using using lumped lumped circuit circuit parameters in the software. The lumped circuit parameters of the motors are calculated using parametersparameters in in the the software. software. The The lumped lumped circuit circuit parameters parameters of of the the moto motorsrs are are calculated calculated using using SIEMENS-SPEED software (V11.04.010, SPEED Laboratory, University of Glasgow, Glasgow, Scotland), SIEMENS-SPEEDSIEMENS-SPEED softwaresoftware (V11.04.010,(V11.04.010, SPEEDSPEED LaboLaboratory,ratory, UniversityUniversity ofof Glasgow,Glasgow, Glasgow,Glasgow, which is an analytical motor modelling tool. The outputs are injected to FEM. Whole motor geometries Scotland),Scotland), which which is is an an analytical analytical motor motor modelling modelling tool. tool. The The outputs outputs are are injected injected to to FEM. FEM. Whole Whole motor motor are considered for numerical accuracy. geometriesgeometries are are considered considered for for numerical numerical accuracy. accuracy. Both the rotor and stator laminations’ magnetic material are Cogent SURA M700 35A silicon steel BothBoth the the rotor rotor and and stator stator laminations’ laminations’ magnetic magnetic material material are are Cogent Cogent SURA SURA M700 M700 35A 35A silicon silicon for these three motors, and its magnetic flux density and magnetic field intensity values are shown in steelsteel for for these these three three motors, motors, and and its its magnetic magnetic flux flux density density and and magnetic magnetic fi fieldeld intensity intensity values values are are Figure4a. Only the 5.5 kW induction motor has a double cage rotor, as the other two motors have deep shownshown in in Figure Figure 4a. 4a. Only Only the the 5.5 5.5 kW kW induction induction motor motor has has a adouble double cage cage rotor, rotor, as as the the other other two two motors motors rotor bars. The mesh of the 4 kW reference induction motor is given in Figure4b. havehave deep deep rotor rotor bars. bars. The The mesh mesh of of the the 4 4kW kW reference reference induction induction motor motor is is given given in in Figure Figure 4b. 4b.

(a()a ) (b()b )

FigureFigureFigure 4. 4. 4. ((a a()a) B-H) B-H B-H curve curve curve of of of the the the M700 M700 M700 Si-steel; Si-steel; Si-steel; ( (b b()b) The) The The mesh mesh mesh discretization discretization discretization of of of 4 4 4kW kW kW motor. motor. motor.

t TheThe equi-flux equi-ﬂux equi-flux lines lines showingshowing showing thethe the four-polefour-pole four-pole distributiondistri distributionbution and and and color color color shade shade shade of of ﬂuxof flux flux densities densities densities at at at =t =t0.3 =0.3 0.3 s sfor sfor for the the the three three three reference reference reference induction induction induction motors motors motors are are are given given given in in Figurein Figure Figure5. 5. 5.

(a()a ) (b()b ) (c()c )

FigureFigure 5. 5. The The equi-ﬂuxequi-flux equi-flux lines lines lines and and and ﬂux flux flux densities densities densities of (ofa )of 2.2( a()a kW,)2.2 2.2 kW, ( bkW,) 4 ( kW, b()b )4 and 4kW, kW, (c )and 5.5and kW( c()c )5.5 Induction 5.5 kW kW Induction Induction motors. motors.motors.

TheThe normal normal components components of of the the air air gap gap fl fluxux densities densities for for a ageometrical geometrical cycle cycle at at t t= =0.5 0.5 s sand and the the torquetorque profiles profiles of of the the motors motors are are presented presented in in Figu Figurere 6 6for for the the rated rated load load at at the the rated rated voltage voltage and and

Energies 2019, 12, 2190 6 of 20

The normal components of the air gap ﬂux densities for a geometrical cycle at t = 0.5 s and the torqueEnergies proﬁles 2019, 12, ofx FOR the PEER motors REVIEW arepresented in Figure6 for the rated load at the rated voltage and6 of speed 20 given in Table1. Four pole distributions with the slotting from Figure6a,c,e can be easily seen for speed given in Table 1. Four pole distributions with the slotting from Figure 6a,c,e can be easily seen eachfor motor.each motor.

1.5 45.0 1.0 25.0 0.5

0.0 5.0 0.0 0.0 0.1 0.1 0.2 0.2 0.3 0.3 Time [s] 0.02 0.04 0.06 0.08 0.10 0.12 0.14 -0.5 -15.0 Torque [Nm] Flux Density [T] Density Flux

-1.0 -35.0 -1.5 Airgap Path -55.0

(a) (b) 150.0

2.0 100.0 1.5 1.0 50.0 0.5 0.0 0.0 -0.02 0.03 0.08 0.13 0.18 0.23 0.28 0.33 -0.5 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Flux Density [T] Density Flux Torque [Nm] -1.0 -50.0 Time [s] -1.5 -2.0 -100.0 Airgap Path -150.0 (c) (d) 200.0 1.5 150.0 1.0 100.0 0.5

0.0 50.0 0.00 0.10 0.20 0.30 0.40 -0.5 0.0 Flux Density [T] Density Flux Torque [Nm] 0.04 0.06 0.08 0.10 0.12 0.14 -1.0 -50.0 Time [s] -1.5 Airgap Path -100.0 -150.0 (e) (f)

FigureFigure 6. 6.The The normal normal componentcomponent of the flux ﬂux density density in in the the air air gap gap (a ()a 2.2) 2.2 kW, kW, (c) ( c4) kW, 4 kW, and and (e) (5.5e) 5.5kW kW motorsmotors and and torque torque variation variation ofof ((bb)) 2.22.2 kW,kW, ((d) 4 kW, and ( (ff)) 5.5 5.5 kW kW motors. motors.

TheThe FEM FEM analysis analysis results results of of the the motors motors are are compared compared to to the the test test results results to to validate validate the the motor motor FEM models.FEM models. The test The bed test for bed the for motor the testsmotor is tests given is in given Figure in7 Figu. There test 7. The bed test has bed a reference has a reference induction motor,induction hysteresis motor, eddy hysteresis current eddy brake, current and brake, measurement and measurement system. system. The no-load and rated load tests are performed due to the IEC standard 60034-2-1. The stator resistances are obtained as given in Table2. Eight di ﬀerent voltage (1.1, 1, 0.95, 0.9, 0.6, 0.5, 0.4, 0.3 times of the rated voltage) are applied and measured for the no-load test. No-load stator current and no-load input power are measured for each voltage. The ﬁrst four voltages (1.1, 1.0, 0.95; 0.90 times of the rated voltage) are used to calculate the iron losses (Pfe). The last four (0.60, 0.50, 0.40, 0.30 times of the rated voltage) are used to calculate the friction and windage losses (Pfw). The no-load test measurements and calculated loss terms are given in Tables3–5 for 2.2 kW, 4 kW, and 5.5 kW reference motors, respectively.

Energies 2019, 12, 2190 7 of 20 Energies 2019, 12, x FOR PEER REVIEW 7 of 20

FigureFigure 7. 7. TestTest bench. bench. Table 2. Stator resistances per phase for the reference induction motors. The no-load and rated load tests are performed due to the IEC standard 60034-2-1. The stator resistances are obtained as givenInitial in Table Condition 2. Eight different voltage (1.1, Thermal 1, 0.95, Regime 0.9, 0.6, 0.5, 0.4, 0.3 Power times of the rated voltage)Temperature are applied (◦C) and Resistance measured (Ohms) for the no-load Temperature test. (No-load◦C) Resistance stator current (Ohms) and no-load2.2 input kW power are measured 22 for each voltage. 1.91 The first four 64.1voltages (1.1, 1.0, 0.95; 2.196 0.90 times of the rated4 kW voltage) are used 22 to calculate the iron 2.35 losses (Pfe). The last 72.8 four (0.60, 0.50, 0.40, 3.159 0.30 times of the rated5.5 kW voltage) are used 22 to calculate the 1.32 friction and windage 67 losses (Pfw). The 1.603 no-load test measurements and calculated loss terms are given in Tables 3–5 for 2.2 kW, 4 kW, and 5.5 kW reference motors, respectively.Table 3. No-Load Test results of 2.2 kW induction motor.

2 V (V)Table 2. StatorV resistancesI (A) per phase P0 for(W) the reference Pcu0 (W)induction motors. Pfe + Pfw0 (W) 120 14,400 0.86 43 4.9 38.1 Initial Condition Thermal Regime Power 160 25,600 1.13 55 8.5 46.5 Temperature200 40,000 (°C) Resistance 1.43 (Ohms) 70 Temperature 13.5 (°C) Resistance 56.5 (Ohms) 2.2 kW 240 22 57,600 1.73 1.91 88 19.6 64.1 68.4 2.196 4 kW 305 22 93,025 2.28 2.35 124 34.1 72.8 89.9 3.159 370 136,900 3.02 178 60.2 117.8 5.5 kW 22 1.32 67 1.603 400 160,000 3.49 210 80.1 129.9 435 189,225 4.22 271 117.5 153.5 Table 3. No-Load Test results of 2.2 kW induction motor.

V (V)Table 4. VNo-Load2 I (A) Test resultsP0 (W) of 4Pcu0 kW (W) induction Pfe + motor. Pfw0 (W)

120 2 14,400 0.86 43 4.9 38.1 V (V) V I (A) P0 (W) Pcu0 (W) Pfe + Pfw0 (W) 160 25,600 1.13 55 8.5 46.5 120 14,400 0.95 36 8.6 27.4 200 40,000 1.43 70 13.5 56.5 160 25,600 1.49 48 21 27 200240 40,000 57,600 1.811.73 88 68 19.6 31 68.4 36.9 240305 57,600 93,025 2.242.28 124 93 34.1 47.5 89.9 45.4 305370 93,025 136,900 3.013.02 178 140 60.2 85.8 117.8 54.2 370400 136,900 160,000 4.13.49 210 210 80.1 159.3 129.9 50.69 400 160,000 4.7 263 209.3 53.7 435 189,225 4.22 271 117.5 153.5 435 189,225 5.4 366 276.3 89.7

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Table 5. No-Load Test results of 5.5 kW induction motor.

2 V (V) V I (A) P0 (W) Pcu0 (W) Pfe + Pfw0 (W) 120 14,400 1.6 68 12.3 55.7 160 25,600 2.1 88 21 67 200 40,000 2.63 120 33.3 86.7 240 57,600 3.22 151 49.8 101.2 305 93,025 4.26 213 87 126 370 136,900 5.62 298 151.8 146.2 400 160,000 6.47 349 201.3 147.7 435 189,225 8.07 434 312.9 121.1

The load curve test is performed after the thermal regime of winding temperatures is achieved. There are six operating points due to the standard: 1.25, 1.15, 1, 0.75, 0.5, 0.25 times of the rated load are performed. The input power, Pin; output torque, Tout; stator line current, I1; phase-to-phase voltage, V; and speed, n; measurements should be taken for each loading test. The load test results are given for the reference motors in Tables6–8, respectively. The output powers for the motors are calculated using the measured output torque and speed.

Table 6. Load Test results of 2.2 kW IM.

Parameters 25% 50% 75% 100% 115% 125% Voltage V (V) 402 401 401 401 400 401 Phase Current I1 (A) 3.5 3.82 4.37 4.92 5.83 6.72 Input Power Pin (W) 771 1336 1943 2560 3187 3822 Torque T (Nm) 3.6 7.3 10.9 14.6 18.2 21.9 1 Speed n (min− ) 1487 1473 1458 1445 1427 1410 Output Power Pout (W) 560 1126 1663 2208 2718 3232 Eﬃciency 72.7 84.2 85.6 86.3 85.3 84.6

Table 7. Load Test results of 4 kW IM.

Parameters 25% 50% 75% 100% 115% 125% Voltage V (V) 402 400 401 401 400 401 Phase Current I1 (A) 5.19 5.88 6.91 8.22 9.73 11.41 Input Power Pin (W) 1264 2327 3415 4500 5655 6819 Torque T (Nm) 6.6 13.2 19.8 26.3 32.9 39.5 1 Speed n (min− ) 1491 1480 1466 1457 1439 1426 Output Power Pout (W) 1030 2045 3038 4011 4955 5896 Eﬃciency 81.5 88 89 89.1 87.6 86.5

Table 8. Load Test results of 5.5 kW IM.

Parameters 25% 50% 75% 100% 115% 125% Voltage V (V) 402 400 399 400 401 400 Phase Current I1 (A) 6.74 7.77 9.27 11.1 13.11 15.24 Input Power Pin (W) 1701 3161 4632 6122 7652 9188 Torque T (Nm) 9.1 18.1 27.2 36.2 45.3 54.3 1 Speed n (min− ) 1491 1484 1476 1467 1455 1445 Output Power Pout (W) 1420 2814 4202 5558 6899 8213 Eﬃciency 83.5 89 90.7 90.8 90.2 89.4

The FEM and test results of the reference induction motors are given in Table9 in terms of the torque, current, power, and eﬃciency for the rated values. The diﬀerence between FEM and test results are in a 10% range and are acceptable. Energies 2019, 12, x FOR PEER REVIEW 9 of 20

Table 8. Load Test results of 5.5 kW IM.

Parameters 25% 50% 75% 100% 115% 125% Voltage V (V) 402 400 399 400 401 400 Phase Current I1 (A) 6.74 7.77 9.27 11.1 13.11 15.24 Input Power Pin (W) 1701 3161 4632 6122 7652 9188 Torque T (Nm) 9.1 18.1 27.2 36.2 45.3 54.3 Speed n (min−1) 1491 1484 1476 1467 1455 1445 Output Power Pout (W) 1420 2814 4202 5558 6899 8213 Efficiency 83.5 89 90.7 90.8 90.2 89.4

The FEM and test results of the reference induction motors are given in Table 9 in terms of the torque, current, power, and efficiency for the rated values. The difference between FEM and test results are in a 10% range and are acceptable.

Energies 2019, 12, 2190 Table 9. Induction Motors’ FEM and TEST results. 9 of 20 2.2 kW 4 kW 4 kW 5.5 kW FEM 5.5 kW Test Parameter 2.2 kW FEM Table 9. InductionTest Motors’ FEM andTest TEST results. Torque (Nm) 14.6 14.5 26.26 26.3 36.1 36.2 Parameter 2.2 kW FEM 2.2 kW Test 4 kW FEM 4 kW Test 5.5 kW FEM 5.5 kW Test Current (Arms) 4.8 4.95 8 8.22 11.1 11.7 TorqueOutput (Nm) Power (W) 14.6 2215 14.5 2203 26.26 3957 3984 26.3 5535 36.1 5482 36.2 Current (Arms) 4.8 4.95 8 8.22 11.1 11.7 OutputEfficiency Power (W) (%) 2215 86 2203 86.3 3957 89.53 89.1 3984 89.8 5535 90.8 5482 Efficiency (%) 86 86.3 89.53 89.1 89.8 90.8 5.2. Analysis of Synchronous Reluctance Motors 5.2. AnalysisAfter the of analysis Synchronous of the Reluctance reference Motors induction motors, synchronous reluctance motor models are builtAfter in three the analysispower ratings of the for reference the multi-parameter induction motors, analyses synchronous and their reluctance cross-effect motor analyses models at arethe builtsame intime. three Complete powerratings motor models for the are multi-parameter used to obtain analyseshigh precision and their during cross-e pre-modellingffect analyses and at post- the sameprocessing time. steps Complete as in induction motor models motor are models. used to obtain high precision during pre-modelling and post-processingGraphical stepsillustrations as in induction of the rotor motor barrier models. numbers and barrier shapes considered for the numericalGraphical analysis illustrations are given of the in rotorFigure barrier 8. The numbers number and of barrier flux barriers shapes consideredis analyzed for to the evaluate numerical the analysisinfluence are on given the performance in Figure8. The of the number motor. of The flux finit barrierse element is analyzed analysis to evaluateis performed the influence up to four on theflux performancebarriers since of flux the barriers motor. The saturate finite after element six, analysisand Ld-L isq performedinductances up show to four no fluxdifference. barriers Three since fluxflux barrier shapes are evaluated. Rotor barrier number illustrations are given a to d for barrier number 1 barriers saturate after six, and Ld-Lq inductances show no difference. Three flux barrier shapes are evaluated.to 4, accordingly, Rotor barrier at section number (A) and illustrations barrier shape are given illustrations a to d for are barrier given number e to h for 1 to angular, 4, accordingly, round, atangular-round, section (A) and and barrier cut-off, shape accordin illustrationsgly, in areFigure given 8. eThe to hiron for angular,guides form round, an angular-round,angle with the andflux cut-obarriersff, accordingly, at the angular in Figure (A) barrier8. The shape. iron guides Off-centere form and round angle withpieces the create flux barriersiron flux at guides the angular at the (A)round barrier (R) flux shape. barriers. Off-centered Angular-ro roundund pieces(A-R) is create the last iron design, flux guideswhich is at a the combination round (R) of flux the barriers. angular Angular-roundand round barriers. (A-R) is the last design, which is a combination of the angular and round barriers.

(A) (B)

Figure 8. SynRM design parameters (A The number of rotor barriers per pole: (a) one barrier, (b) two barriers, (c) three barriers, (d) four barriers, B Barrier Shape: (e) angular, (f) round, (g) angular-round, (h) cut-oﬀ).

The torque characteristic results at rated load are given in Figure9. The torque ripple due to the number of ﬂux barriers are listed in Table 10.

Table 10. Output Torques and their ripples due to the rotor barriers’ number for 2.2 kW SynRM motor.

Parameter 1 Barrier 2 Barriers 3 Barriers 4 Barriers Torque (Nm) 15.1 17.9 19.1 24.4 Torque Ripple (%) 77 46 41 27 Energies 2019, 12, x FOR PEER REVIEW 10 of 20

Figure 8. SynRM design parameters (A: The number of rotor barriers per pole: (a) one barrier, (b) two barriers, (c) three barriers, (d) four barriers, B: Barrier Shape: (e) angular, (f) round, (g) angular-round, (h) cut-off).

The torque characteristic results at rated load are given in Figure 9. The torque ripple due to the Energiesnumber 2019 of, flux12, x FORbarriers PEER are REVIEW listed in Table 10. 10 of 20

Figure 8. SynRM design parameters (A: The number of rotor barriers per pole: (a) one barrier, (b) two 80 barriers, (c) three barriers,1 Barrier (d) four barriers, B: 2Barrier Barriers Shape: (e) angular, (f) round, (g) angular-round, (h) cut-off). 3 Barriers 4 Barriers 60

EnergiesThe2019 torque, 12, 2190 characteristic results at rated load are given in Figure 9. The torque ripple due 10to ofthe 20 number of40 flux barriers are listed in Table 10.

8020 1 Barrier 2 Barriers 3 Barriers 4 Barriers

Torque [Nm] 600 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 Time [s] -2040

-4020

FigureTorque [Nm] 0 9. Torque profiles of the 2.2 kW SynRM due to the number of rotor barriers (Data from [22]). 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 Table 10. Output Torques and their ripples due to the rotor barriers’ number for Time2.2 kW [s] SynRM -20 motor.

-40 Parameter 1 Barrier 2 Barriers 3 Barriers 4 Barriers Torque (Nm) 15.1 17.9 19.1 24.4 Torque Ripple (%) 77 46 41 27 FigureFigure 9. 9. TorqueTorque profiles proﬁles of of the the 2.2 2.2 kW kW SynRM SynRM due due to to the the number of rotor barrier barrierss (Data from [[22]).22]).

TableThe rotor 10. Output structure Torques is not and very their strong ripples mechanicallymechanical due to thlye rotor due tobarriers’ the fluxflux number barriers. for 2.2The kW tangential SynRM end bridgesmotor. are designed to reinforce the rotor as shown in Figure 3 3.. MakingMaking endend bridgesbridges veryvery thinthin resultsresults in problems problems for for manufacturing manufacturing processes. processes. Increasing Increasing the the end end bridge bridge thickness thickness of the of therotor rotor causes causes the Parameter 1 Barrier 2 Barriers 3 Barriers 4 Barriers torquethe torque decrease. decrease. The output The output torque torque graph graph for the for si thex different six different structures structures is given is given in Figure in Figure 10. The 10. Torque (Nm) 15.1 17.9 19.1 24.4 normalThe normal components components of flux of flux densities densities for forend end brid bridgege thickness thickness are are given given in in Figure Figure 11.11. Also, Also, the Torque Ripple (%) 77 46 41 27 comparative torque and fluxflux density results are givengiven inin TableTable 1111..

The rotor structure is not very strong mechanically due to the flux barriers. The tangential end 110 3 Barriers 0.5 mm 3 Barriers 1 mm 3 Barriers 1.5 mm bridges are designed to reinforce the4 Barriers rotor 0.5 as mm shown in Figure4 Barriers 3. Making1 mm end bridges4 Barriers very 1.5 mm thin results in problems90 for manufacturing processes. Increasing the end bridge thickness of the rotor causes the torque decrease.70 The output torque graph for the six different structures is given in Figure 10. The normal components of flux densities for end bridge thickness are given in Figure 11. Also, the comparative50 torque and flux density results are given in Table 11.

30 110

Torque [Nm] 3 Barriers 0.5 mm 3 Barriers 1 mm 3 Barriers 1.5 mm 10 4 Barriers 0.5 mm 4 Barriers 1 mm 4 Barriers 1.5 mm 90 -10 0.04 0.06 0.08 0.1 0.12 0.14 70 Time [s] -30 50 -50 30 Figure 10.

Torque [Nm] Torque outputs of 2.2 kW SynRM due to diﬀerent end thicknesses for three and four barriers 10 (Data from [22]).

-10 0.04 0.06 0.08 0.1 0.12 0.14 Table 11. Air Gap Flux Densities and average output torque of 2.2 kW SynRM dueTime to di [s]ﬀerent end thicknesses-30 for three and four barriers.

-50 Parameter 3 Barriers 4 Barriers Thickness (mm) 0.5 1 1.5 0.5 1 1.5 Flux Density (T) 0.78 0.78 0.78 0.79 0.78 0.78 Torque (Nm) 28.9 23.9 19 29.9 24.7 19.8 Energies 2019, 12, x FOR PEER REVIEW 11 of 20

Figure 10. Torque outputs of 2.2 kW SynRM due to different end thicknesses for three and four barriers (Data from [22]).

3 Barriers 0.5 mm 3 Barriers 1 mm 3 Barriers 1.5 mm 4 Barriers 0.5 mm 4 Barriers 1 mm 4 Barriers 1.5 mm 1.2

Energies 2019, 12, x FOR PEER REVIEW 11 of 20 0.7

Figure 0.210. Torque outputs of 2.2 kW SynRM due to different end thicknesses for three and four Energiesbarriers2019, 12 (Data, 2190 from [22]). 11 of 20 -0.3 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Flux Density [T]

-0.8 3 Barriers 0.5 mm 3 Barriers 1 mm 3 Barriers 1.5 mm 4 Barriers 0.5 mm 4 Barriers 1 mm 4 Barriers 1.5 mm 1.2 -1.3 0.7

0.2 Figure 11. Air gap flux density of 2.2 kW SynRM due to different end thicknesses (Data from [22]). -0.3 0.00 0.05 0.10 0.15 0.20 0.25 0.30 TableFlux Density [T] 11. Air Gap Flux Densities and average output torque of 2.2 kW SynRM due to different end thicknesses-0.8 for three and four barriers.

-1.3 Parameter 3 Barriers 4 Barriers Thickness (mm) 0.5 1 1.5 0.5 1 1.5 Flux Density (T) 0.78 0.78 0.78 0.79 0.78 0.78 Figure 11. AirTorque gap flux (Nm) density of 2.2 28.9 kW SynRM23.9 due to di 19fferent end 29.9 thicknesses 24.7 (Data19.8 from [22]). Figure 11. Air gap flux density of 2.2 kW SynRM due to different end thicknesses (Data from [22]). The radial air gap width should be proposed as small as possible by considering the mechanical TableThe radial 11. Air air Gap gap Flux width Densities should and be averageproposed output as sm torqueall as of possible 2.2 kW SynRMby considering due to different the mechanical end limitations. The air gap can be uniform or cut-off in the radial direction for the synchronous reluctance limitations.thicknesses The for air three gap and can four be barriers. uniform or cut-off in the radial direction for the synchronous reluctancemotor designs. motor The designs. flux density, The flux average density, output average torque, output and torque, torque and ripple torque for di ripplefferent for air different gap values air gapare evaluatedvalues are in evaluated detailParameter in Figuresin detail 12 in and Figures 13, respectively. 312 Barriers and 13, respectively. 4 Barriers Thickness (mm) 0.5 1 1.5 0.5 1 1.5 1 Flux Density (T) 0.78 0.78 0.78 0.79 0.78 0.78 Torque (Nm) 28.9 23.9 19 29.9 24.7 19.8 0.95 4 BARRIERS

The radial0.9 airg=0.34 gap mm width g=0.25 should mm be proposed cut-off as sm g=0.34all mmas possibleg=0.25 by mm considering cut-off the mechanical limitations. The air gap can be uniform or cut-off in the radial direction for the synchronous reluctance0.85 motor designs. The flux density, average output torque, and torque ripple for different air gap values are evaluated in detail in Figures 12 and 13, respectively. 0.8

Flux Density [T] 0.751

0.950.7 4 BARRIERS [Nm] Torque Output Average Flux Density 0.650.9 g=0.34 mm g=0.25 mm cut-off g=0.34 mmg=0.25 mm cut-off ara-rara-rara-rara-rara-rara-r 0.85 Barrier Shape a: angular; r: round; a-r: angular-round

0.8 Figure 12. FluxFlux density density and and average average torque torque of of 2.2 2.2 kW kW Sy SynRMnRM for for different diﬀerent number of barriers and air Flux Density [T] gap values. 0.75

0.7 [Nm] Torque Output Average The ratio of the sum of flux barrier widths to the sum of iron guide widths in the q-axis direction Flux Density is defined0.65 as ‘insulation ratio’, kq. If the rotor is completely made of iron without any saliency, the kq would be equalara-rara-rara-rara-rara-rara-r to 0. If the total flux barrier width is equal to that of iron segment width, then kq is equal to 1. Output torque characteristicsBarrier Shape a: angular; of the r: synchronousround; a-r: angular-round reluctance motors are presented in Figure 14. They are operated at the rated load with rated voltage and speed values given in Table 1.

The averageFigure 12. values Flux density at steady and stateaverage and torque torque of ripples2.2 kW Sy asnRM percentage for different are summarizednumber of barriers in Table and 12air. gap values.

Energies 2019, 12, x FOR PEER REVIEW 12 of 20

Energies 2019, 12, 2190 12 of 20 Energies 2019, 12, x FOR PEER REVIEW 12 of 20

Figure 13. Torque ripple of 2.2 kW SynRM for different number of barriers and air gap values.

The ratio of the sum of flux barrier widths to the sum of iron guide widths in the q-axis direction is defined as ‘insulation ratio’, kq. If the rotor is completely made of iron without any saliency, the kq would be equal to 0. If the total flux barrier width is equal to that of iron segment width, then kq is equal to 1. Output torque characteristics of the synchronous reluctance motors are presented in Figure 14. They are operated at the rated load with rated voltage and speed values given in Table 1. The average Figurevalues 13. at Torquesteady ripplestate and of 2.2 torque kW SynRM ripples for as di ﬀpercentageerent number are of summarized barriers and air in gap Table values. 12. Figure 13. Torque ripple of 2.2 kW SynRM for different number of barriers and air gap values. 200 The ratio of the sum of flux barrier widths to the sum of iron guide widths in the q-axis direction is defined as ‘insulation ratio’, kq. If the rotor is completely made of iron without any saliency, the kq would150 be equal to 0. If the total flux barrier 2.2widt kWh SynRM is equal to 4that kW SynRMof iron segment5.5 kW width,SynRM then kq is equal to 1. Output torque characteristics of the synchronous reluctance motors are presented in Figure 14. They are operated at the rated load with rated voltage and speed values given in Table 1. The 100 average values at steady state and torque ripples as percentage are summarized in Table 12.

20050 Moment [Nm] 0 150 2.2 kW SynRM 4 kW SynRM 5.5 kW SynRM 0.04 0.06 0.08 0.10 0.12 0.14

100-50

-10050 Time [s}

Moment [Nm] Figure 14. TheThe output output torque variation for different diﬀerent power levels. 0 TableTable0.04 12. AverageAverage Output 0.06 Torques and 0.08 their torque ripplesrippl 0.10es forfor SynRMSynRM 0.12 atat didifferentﬀerent powerpower 0.14 ratings.ratings. 4 Barriers -50 4 Barriers Parameter Parameter 2.2 kW2.2 kW 4 4kW kW 5.5 kW 5.5 kW Torque (Nm)Torque (Nm) 14.1 14.1 25.2 25.2 35.03 35.03 -100 Torque RippleTorque (%) Ripple (%) 18Time 18 [s} 17 17 22 22 Structural analysisFigure of the14. Thenewly output designed torque variationmotors isfor another different important power levels. issue to be considered. The stressStructural analysis analysis of the of prototyped the newly designedmotors is motorssimulated is another in ANSYS important in order issue to satisfy to be the considered. motor’s The stressTable analysis 12. Average of the Output prototyped Torques motorsand their is torque simulated ripples in for ANSYS SynRM inat orderdifferent to power satisfy ratings. the motor’s mechanical integrity. The simulation is performed at 1500 min 1 and the mechanic characteristics of 4 Barriers− the rotor are given in Table 13.Parameter 2.2 kW 4 kW 5.5 kW Torque (Nm) 14.1 25.2 35.03 Torque Ripple (%) 18 17 22 Structural analysis of the newly designed motors is another important issue to be considered. The stress analysis of the prototyped motors is simulated in ANSYS in order to satisfy the motor’s

Energies 2019, 12, x FOR PEER REVIEW 13 of 20

Energies 2019, 12, 2190 13 of 20 mechanical integrity. The simulation is performed at 1500 min−1 and the mechanic characteristics of the rotor are given in Table 13. Table 13. Mechanical Characteristics of Rotor Si-Steel. Table 13. Mechanical Characteristics of Rotor Si-Steel. Parameters M700 Parameters M700 3 7800 Mass DensityMass (kg Density/m ) (kg/m3) 7800 Young Modulus (GPa) 210 PoissonYoung Coeﬃcient Modulus (GPa) 210 0.31 Tensile Yield StrengthPoisson (MPa) Coefficient 0.31 300 Tensile UltimateTensile Strength Yield Strength (MPa) (MPa) 300 410 Tensile Ultimate Strength (MPa) 410 The mechanical analysis is performed to see the total deformation and the equivalent stress on The mechanical analysis is performed to see the total deformation and the equivalent stress on the rotor under the rated operation condition. The simulations and results for total deformation of the rotor under the rated operation condition. The simulations and results for total deformation of 2.22.2 kW kW motor motor are are given given inin FigureFigure 15 and Table 14,14, respectively. respectively. The The minimum minimum total total deformation deformation and and maximummaximum elastic elastic strainstrain valuesvalues areare listedlisted for one and four four barriers. barriers. A A four-barrier four-barrier structure structure is ismore more robustrobust than than a a single single barrier barrier withwith thethe safetysafety factor of of 12.217. 12.217.

(a) (b) (c) (d)

(e) (f) (g) (h)

Figure 15. Structural analysis of 2.2 kW motor with four-rotor barrier per pole (a) total deformation Figure 15. Structural analysis of 2.2 kW motor with four-rotor barrier per pole (a) total deformation (axial view), (b) total deformation (radial view), (c) Equivalent Elastic Strain (axial view), (d) (axial view), (b) total deformation (radial view), (c) Equivalent Elastic Strain (axial view), (d) Equivalent Equivalent Elastic Strain (radial view), (e) Equivalent Stress (axial view), (f) Equivalent Stress (radial Elastic Strain (radial view), (e) Equivalent Stress (axial view), (f) Equivalent Stress (radial view), view), (g) Safety Factor (axial view), (h) Safety Factor (radial view). (g) Safety Factor (axial view), (h) Safety Factor (radial view).

TableTable 14.14. Structural analysis analysis results results of of 2.2 2.2 kW kW motor. motor.

ParameterParameter 1 Barrier 1 Barrier 4 Barriers 4 Barriers Maximum Total Deformation (µm) 29.296 16.755 Maximum Total Deformation (µm) 29.296 16.755 Maximum Elastic Strain (mm/mm) 1.942 × 10−4 1.1719 × 10−4 Maximum Elastic Strain (mm/mm) 1.942 10 4 1.1719 10 4 × − × − Maximum StressMaximum (MPa) Stress (MPa) 25,424 25,424 24,555 24,555 Minimum SafetyMinimum Factor Safety Factor 64.525 64.525 12.217 12.217 Maximum SafetyMaximum Factor Safety Factor 15 15 15 15

6.6. Prototype Prototype Synchronous Synchronous ReluctanceReluctance Motors TheThe overall overall results results areare evaluatedevaluated afterafter thethe multi-parametermulti-parameter design design proc process.ess. The The synchronous synchronous reluctancereluctance models models for for prototypingprototyping are chosen considering considering technical technical parameters parameters like like efficiency, eﬃciency, torque torque ripple, production process, etc. The same stator, fan, and housing frames of the reference induction ripple, production process, etc. The same stator, fan, and housing frames of the reference induction motors are used for newly designed synchronous reluctance motors. Rotor laminations are produced motors are used for newly designed synchronous reluctance motors. Rotor laminations are produced by using wire erosion cut. The lamination structure and motor views for three different power levels by using wire erosion cut. The lamination structure and motor views for three diﬀerent power levels are given in Figure 16 and Figure 17a, accordingly. are given in Figures 16 and 17a, accordingly.

Energies 2019,, 12,, x 2190 FOR PEER REVIEW 1414 of of 20 20 Energies 2019, 12, x FOR PEER REVIEW 14 of 20

Figure 16. Prototype motor sample lamination and the rotor [22]. Figure 16. Prototype motor sample lamination and the rotor [[22].22].

(a) (b) (a) (b) FigureFigure 17. (a) PrototypePrototype SynRMSynRM Motors Motors at at 5.5 5.5 kW, kW, 4 kW, 4 kW, and and 2.2 kW;2.2 (kW;b) Prototype (b) Prototype motor motor and its and driver. its driver.Figure 17. (a) Prototype SynRM Motors at 5.5 kW, 4 kW, and 2.2 kW; (b) Prototype motor and its Thedriver. ideal motor model achieved from multi-parametric FEM analyses is the angular four variable-widthThe ideal motor barriers model rotor thatachieved has kqfrom= 1 insulationmulti-parametric ratio. TheFEM 4-kW analyses prototyped is the motorangular and four its variable-widthtest setThe are ideal shown barriersmotor in Figure modelrotor 17 that achievedb. Thehas testkq =from system1 insulation multi-para has a ratio. genericmetric The variable 4-kWFEM prototypedanalyses frequency is motorthe angular driver.and its four Thetest setinvertervariable-width are shown is supplied inbarriers Figure by a rotor variable17b. thatThe AC has test source. k qsystem = 1 insulation However, has a generic theratio. input The variable voltage 4-kW frequencyprototyped of the inverter motor motor is fixeddriver. and duringits The test invertertheset are test. shown is In supplied the in inverter Figure by operatinga 17b.variable The menu,ACtest source.system the output However,has a powergeneric the and variableinput speed voltage frequency level of thethe motor motorsinverter driver. are is setfixed The to duringtheinverter required the is test.supplied rated In the output. by inverter a variable The operating inverter AC source. menu, sets the However,th speede output and the power output input and voltage power speed of oflevel thethe of device inverter the motors under is fixed are the settest.during to Two the the required power test. In analyzers therated inverter output. are usedoperating The for inverter the menu, measurements se tsth ethe output speed in power theand test output and systems. speed power level The of first theof the onedevice motors is located under are thebetweenset totest. the Two the required variable power rated analyzers AC output. source are andThe used the inverter inverter,for the se tsmeas and theurements thespeed second and in oneoutput the is test located power systems. between of the The device thefirst inverter oneunder is locatedandthe test. the between motor.Two power Thus, the variable analyzers the motor AC aree sourceffi ciency,used and for inverter thethe inverter,meas effiurementsciency, and andthe in second systemthe test one e ffisystems. ciencyis located canThe between be first obtained one the is inverterseparately.located betweenand As the a motor.the result, variable Thus, SynRM AC the e sourceffimotorciency andefficiency, is the calculated inverter, inverter by and direct efficiency, the measurementsecond and one system is located of converterefficiency between outputcan thebe obtainedpower,inverter corresponding andseparately. the motor. As to aThus, motor result, the input SynRM motor power efficiencyefficiency, (Pin) and is inverter calculated the mechanical efficiency, by direct power and measurement outputsystem atefficiency the of motor converter can shaft be output(Pobtainedmech). power, The separately. test corresponding system As hasa result, atorque to SynRMmotor sensor input efficiency to measurepower is calculated(P thein) and output the by torque mechanicaldirect of measurement motor. power The output measurement of converter at the motorisoutput performed shaft power, (P duemech corresponding). to The the test IEC system standards to motor has a 60034-2-3input torque power sensor loading (P toin) measureand test. the Six mechanicalthe operating output points torquepower are ofoutput motor. performed at The the measurementduemotor to shaft the standard: (P ismech performed). The 1.25, test 1.15,due system to 1, the 0.75, has IEC a 0.5, torque standards and sensor 25 times 60034-2-3 to ofmeasure the loading rated the load test.output Six at the torqueoperating rated of speed.motor. points Theare performedinputmeasurement power, due Pis into performed; the output standard: torque, due 1.25, to T outthe 1.15,; statorIEC 1, standards 0.75, line 0.5, current; and 60034-2-3 25 I1 ;times phase-to-phase loading of the ratedtest. voltage,Six load operating at the V; andrated points speed, speed. are n; Themeasurementsperformed input power, due areto Pthein taken; outputstandard: for torque, each 1.25, loading. T1.15,out; stator 1, The0.75, line load 0.5, current; testand results25 timesI1; phase-to-phase are of giventhe rated in Tables load voltage, at 15 the– V;17 rated andfor 2.2speed,speed. kW, n;4The kW,measurements input and power, 5.5 kW areP SynRMs,in; outputtaken respectively.for torque, each Tloading.out; stator The line load current; test resultsI1; phase-to-phase are given in voltage, Tables V;15–17 and forspeed, 2.2 kW,n; measurements 4 kW, and 5.5 arekW taken SynRMs, for eachrespectively. loading. The load test results are given in Tables 15–17 for 2.2 kW, 4 kW, and 5.5 kW SynRMs, respectively.

Energies 2019, 12, 2190 15 of 20

Table 15. Load Test results of 2.2 kW SynRM.

Parameters 25% 50% 75% 100% 115% 125% Voltage V (V) 396.6 436.5 455.5 452.1 453.8 450.3 Phase Current I1 (A) 2.34 3.25 4.03 4.91 5.95 7.16 Input Power Pin (W) 651 1275 1887 2490 3159 3843 Torque T (Nm) 3.52 7.04 10.56 14.1 17.6 21.1 1 Speed n (min− ) 1500 1500 1500 1500 1500 1500 Output Power Pout (W) 553.1 1106 1659 2212 2765 3318 Eﬃciency 85 86.8 87.9 88.9 87.6 86.4

Table 16. Load Test results of 4 kW SynRM.

Parameters 25% 50% 75% 100% 115% 125% Voltage V (V) 461 469 485 478 457 473 Phase Current I1 (A) 3.84 5.59 7.14 8.8 10.9 13.38 Input Power Pin (W) 1147 2240 3326 4413 5540 6745 Torque T (Nm) 6.38 12.75 19.13 25.5 31.88 38.26 1 Speed n (min− ) 1500 1500 1500 1500 1500 1500 Output Power Pout (W) 1001.5 2003 3004.6 4006.1 5007.7 6009.2 Eﬃciency 87.3 89.4 90.3 90.8 90.4 89.1

Table 17. Load Test results of 5.5 kW SynRM.

Parameters 25% 50% 75% 100% 115% Voltage V (V) 485 457 469 469.4 462 Phase Current I1 (A) 5.34 7.61 9.59 11.8 13.79 Input Power Pin (W) 1580 3052 4571 6013 7067 Torque T (Nm) 8.83 17.64 26.46 35.3 42.56 1 Speed n (min− ) 1500 1500 1500 1500 1459 Output Power Pout (W) 1387 2770 4156 5550 6501 Eﬃciency 87.8 90.8 90.9 92.3 92

Since the rated output power levels have been desired to be constant, the input voltage of SynRMs are obtained slightly higher than the rated values of IMs as given in Tables 15–17.

7. Comparative Results In this section, the comparative results are given for the reference induction motors and newly designed synchronous reluctance motors for three diﬀerent power ranges.

7.1. Performance Table 18 shows the general motor parameters for both induction motors and synchronous reluctance motors at the rated speed and torque. The torque is slightly diﬀerent since the output power is the same and the speeds are naturally diﬀerent.

Table 18. Comparative Measured Performance Results (Frame-Size-Power Range).

2.2 kW (4 Pole) 4 kW (4 Pole) 5.5 kW (4 Pole) Parameter 100 Frame 112 Frame 132 Frame IM SynRM IM SynRM IM SynRM Voltage (V) 400 452.1 400 478 400 469.4 Current (A) 4.95 4.91 8.3 8.8 11.1 11.8 1 Speed (min− ) 1450 1500 1455 1500 1465 1500 Torque (Nm) 14.48 14 26.25 25.46 35.85 35.01 cos θ 0.78 0.647 0.79 0.6 0.79 0.6 EnergiesEnergies 2019 2019, ,12 12, ,x x FOR FOR PEER PEER REVIEW REVIEW 1616 ofof 2020

TableTable 18. 18. Comparative Comparative Measured Measured Performance Performance Results Results (Frame-Size-Power (Frame-Size-Power Range). Range).

2.22.2 kWkW (4(4 Pole)Pole) 44 kWkW (4(4 Pole)Pole) 5.55.5 kWkW (4(4 Pole)Pole) ParameterParameter 100100 FrameFrame 112112 FrameFrame 132132 FrameFrame IMIM SynRMSynRM IMIM SynRMSynRM IMIM SynRMSynRM VoltageVoltage (V)(V) 400400 452.1452.1 400400 478 478 400 400 469.4469.4 CurrentCurrent (A)(A) 4.954.95 4.914.91 8.38.3 8.8 8.8 11.111.1 11.811.8 SpeedSpeed (min(min−−11)) 14501450 15001500 14551455 15001500 14651465 15001500 Torque (Nm) 14.48 14 26.25 25.46 35.85 35.01 Energies 2019, 12Torque, 2190 (Nm) 14.48 14 26.25 25.46 35.85 35.01 16 of 20 coscos θθ 0.78 0.78 0.6470.647 0.790.79 0.60.6 0.790.79 0.60.6

TheThe referencereference IMsIMs andand newlynewly designeddesigned SynRMsSynRMs areare forcedforced toto startstart withwith fullfull loadload conditioncondition toto determinedetermine thethe startingstarting capabilitycapability ofof of thethe the motors. motors. AsAs a a result, result, thethe motorsmotors cancan startstart atat fullfull loadload conditioncondition forfor each eacheach power powerpower rating ratingrating with withwith no problems. nono problems.problems. The controller TheThe cocontrollerntroller is not designed isis notnot designeddesigned by the authors. byby thethe The authors.authors. prototyped TheThe SynRMprototypedprototyped motors SynRMSynRM are drivenmotorsmotors by areare a drivendriven generic byby commercial aa genericgeneric commercialcommercial controller. controller. Therecontroller. were ThereThere no problems werewere nono eitherproblemsproblems for startingeithereither forfor or startingstarting operating. oror operating.operating. TorqueTorque vsvs vs currentcurrent current graphsgraphs graphs ofof of IMsIMs IMs andand and SynRMsSynRMs SynRMs areare are givengiven given FigureFigure Figure 1818. 18.. TheseThese datadata setssets areare evaluatedevaluated −11 −1 1 underunder 15001500 minmin−−1 forfor SynRMs SynRMs and and near near 1500 1500 min min−1for forfor IMs.IMs. IMs.

45 2525 45 5050 40 40 4545 20 20 3535 4040 35 3030 35 15 30 15 2525 30 2525 2020 1010 2020 Torque [Nm] Torque [Nm]

Torque [Nm] 15 Torque [Nm] Torque [Nm] Torque [Nm] 15 15 10 15 5 2,2 kW IM 10 4 kW IM 10 5 2,2 kW IM 4 kW IM 10 5,5 kW IM 5 5,5 kW IM 2,2 kW SynRM 5 4 kW SynRM 5 5,5 kW SynRM 2,2 kW SynRM 4 kW SynRM 5 5,5 kW SynRM 0 0 0 00 0 3813 38134914 24682468 4914 Current [A] Current [A] Current [A] Current [A] Current [A] Current [A] ((aa)) ((bb)) ((cc))

FigureFigureFigure 18. 18. Comparison Comparison of of Torque Torque vs. vs. Current CurrentCurrent for for ( (aa))) 2.2 2.22.2 kW, kW,kW, ((bb)) 44 kW,kW, andand and (( c(cc))) 5.55.5 5.5 kW.kW. kW.

IMsIMs perform perform higher higher torque torque valuesvalues values atat at the the same same currentcu currentrrent values values as as givengiven inin FigureFigure 18 18 since since SynRMsSynRMs havehave lowerlower powerpower factors.factors. InputInput powerpower vs.vs. ToTorqueTorquerque graphsgraphs areare presentedpresented in inin Figure FigureFigure 19 19.19..

4000 7500 4000 7500 80008000 2.2 kW IM 2.2 kW IM 4 kW IM 5.5 kW IM 4 kW IM 5.5 kW IM 3500 6500 3500 2.2 kW SynRM 6500 70007000 2.2 kW SynRM 4 kW 5.5 kW SynRM 4 kW 5.5 kW SynRM SynRM 3000 5500 SynRM 3000 5500 60006000

2500 4500 2500 4500 50005000 [W] [W] [W] [W] [W] [W] in in in in in P

P 3500 2000 in P P P 2000 3500 4000 P 4000 1500 2500 1500 2500 30003000

1000 15001500 1000 20002000 500 500 500 500 1000 11121 1000 11121 5254552545 5254552545 TorqueTorque [Nm] [Nm] TorqueTorque [Nm] [Nm] TorqueTorque [Nm] [Nm]

((aa)) ((bb)) ((cc))

FigureFigure 19. 19. Comparison Comparison of ofof Input InputInput Power PowerPower vs. vs.vs. Torque Torque for for ( (aa))) 2.2 2.22.2 kW, kW,kW, ((b)b) 4 4 kW,kW, andand and (( c(cc))) 5.55.5 5.5 kW.kW. kW.

The torque vs. total losses of the motors are given in Figure 20. The total losses of the SynRMs are less than those of the IMs for the same torque values.

Energies 2019, 12, x FOR PEER REVIEW 17 of 20

The torque vs. total losses of the motors are given in Figure 20. The total losses of the SynRMs Energies 2019, 12, 2190 17 of 20 are less than those of the IMs for the same torque values.

700 1000 800 4 kW IM 5.5 kW IM 2.2 kW IM 900 4 kW SynRM 5.5 kW SynRM 600 2.2 kW SynRM 700 800

500 700 600 600 400 500 500 300 400 400 Total Losses [W] Total Losses Total Losses [W] Total Losses

300 [W] Total Losses 200 300 200 100 100 200 0 0 100 0 10203040 02040 01020 Torque [Nm] Torque [Nm] Torque [Nm] (a) (b) (c)

FigureFigure 20.20. ComparisonComparison ofof TotalTotal LossesLosses vs.vs. Torque for (a) 2.2 kW, (b) 4 kW, and (c) 5.5 kW.

7.2.7.2. Active Active Material Material TheThe synchronoussynchronous reluctancereluctance motorsmotors werewere designeddesigned while keeping the the same same stator stator design, design, ampere-turns,ampere-turns, andand stackstack heights,heights, with the the referenc referencee induction induction motors motors for for all all three three power power ranges ranges in inthis this study study as as indicated indicated above above sections. sections. The The main main di differenceﬀerence is is the the rotor rotor frames frames between between the the reference reference inductioninduction motors motors and and the the newly newly designed designed synchronoussynchronous reluctancereluctance motors. All All other other active active materials, materials, suchsuch as as laminations laminations and and stator stator conductors, conductors, are are kept ke thept the same, same, except except for thefor rotorthe rotor conductor. conductor. The rotorThe ofrotor an induction of an induction motor includes motor includes magnetic magnetic steel and steel aluminum and aluminum alloy as analloy active as an material. active material. The rotor The of a synchronousrotor of a synchronous reluctance reluctance motor includes motor onlyincludes magnetic only magnetic steel. Since steel. the Since stator the laminationstator lamination and stator and windingsstator windings are kept are the kept same, the the same, only the diﬀ onlyerence diff betweenerence between the IM and the SynRM IM and motors’ SynRM materialmotors’ costmaterial is on thecost rotor is on side. the Therotor active side. materialThe active usage material is shown usage for is bothshown of thefor motorsboth of inthe Table motors 19. in This Table will 19. give This an ideawill about give an material idea about costs material of the motors. costs of the motors.

TableTable 19. 19.Rotor Rotor MaterialsMaterials ofof IMIM && SynRMSynRM (in pu).

2.2 kW2.2 kW 4 kW 4 kW 5.5 5.5 kW kW MaterialMaterial IMIM SynRM SynRM IMIM SynRM IM IM SynRM SynRM MagneticMagnetic steel steel 0.79 0.79 0.79 0.79 0.790.79 0.790.79 0.88 0.88 0.88 0.88 AluminumAluminum conductor conductor 0.21 0.21 -- 0.210.21 -- 0.11 0.11 - - Total activeTotal material active material at rotor at rotor1 1 0.790.79 11 0.790.79 1 10.88 0.88

7.3.7.3. Thermal Thermal Performance Performance TheThe thermal thermal regime regime of of the the windings windings should should be be reached reached before before applying applying the the performance performance tests. tests. The thermalThe thermal test is performedtest is performed at the nominal at the nominal load. The load. ambient The andambient winding and temperatureswinding temperatures are measured are beforemeasured starting before the test.starting The the motor test. should The motor be operated should be at theoperated rated loadat the until rated the load winding until the temperature winding temperature does not change at 1 °K or 1 °C in a half hour measurement period. The motor reaches does not change at 1 ◦K or 1 ◦C in a half hour measurement period. The motor reaches the thermal steadythe thermal state regimesteady state if this regime condition if thisis condition satisﬁed. is satisfied. All the electrical, All the electrical, mechanical, mechanical, and thermal and thermal motor motor parameters are recorded, such as input power Pin (W), output torque Tout (Nm), line current I1 parameters are recorded, such as input power Pin (W), output torque Tout (Nm), line current I1 (A), (A), phase-to-phase voltage V (V), speed n (min1 −1), frequency f (Hz), stator phase winding resistance phase-to-phase voltage V (V), speed n (min− ), frequency f (Hz), stator phase winding resistance R1(ohms), and winding temperature (°C). R1(ohms), and winding temperature (◦C). TheThe thermal thermal analysis analysis for for the the SynRMsSynRMs isis performedperformed using MOTORCAD soft softwareware (V10.2.3, (V10.2.3, Motor Motor DesignDesign Limited, Limited, Wrexham, Wrexham, UK). UK). The The geometrygeometry isis appliedapplied withwith coolingcooling types and the fin ﬁn thicknesses thicknesses as input. The analytical thermal calculations are performed, and the results are compared to those at as input. The analytical thermal calculations are performed, and the results are compared to those the rated current values. The initial temperature of the winding for 2.2 kW SynRM is 22 °C and it at the rated current values. The initial temperature of the winding for 2.2 kW SynRM is 22 ◦C and it reaches 55 °C at the end. Similarly, the temperature test is performed for the 2.2 kW induction motor. reaches 55 ◦C at the end. Similarly, the temperature test is performed for the 2.2 kW induction motor. The induction motor reaches higher temperature values at the same ratings. The results are given in Table 20. Energies 2019, 12, x FOR PEER REVIEW 18 of 20

Energies 2019, 12, 2190 18 of 20 The induction motor reaches higher temperature values at the same ratings. The results are given in Table 20. Table 20. Winding Temperatures of 2.2 kW motors.

TableSynRM 20. Winding Temperatures of 2.2 kW motors. IM ParameterSynRM MOTORCAD TestIM Test

TemperatureParameter (◦C) 56.1MOTORCAD 55 Test Test 64.1 Temperature (°C) 56.1 55 64.1 The MOTORCAD results are shown in Figure 21 for the 2.2 kW SynRM. The MOTORCAD results are shown in Figure 21 for the 2.2 kW SynRM.

(a)

(b)

(c)

FigureFigure 21. 21. ThermalThermal analysis analysis of of 2.2 2.2 kW kW SynRM SynRM ( a) radial, (b) axial, ((c)) nodalnodal equivalentequivalent representation.representation.

TheThe lack lack of ofrotor rotor copper copper bars bars affects aﬀects the the thermal resultsresults and and e ﬃefficiencyciency as as well. well.

Energies 2019, 12, 2190 19 of 20

7.4. Vibration and Comfort Performances Since the torque ripple is an important parameter for motor design procedure, after producing prototype SynRM motors, vibration tests are performed. Permitted vibration accelerations are given in IEC 60034-14 standards. The upper limit is deﬁned for the motor producers as 2.5 m/s2 for free suspension between 56 and 132 frame motors [23]. The vibration and torque ripple comparison of SynRM motors is presented for diﬀerent power levels in Table 21. The torque ripple results are output of the FEM analysis. It can be seen that the SynRM motors have higher vibration and torque ripple values than those of reference IMs.

Table 21. Vibration and Torque Ripple Comparison.

Vibration (m/s2) Torque Ripple (%) Power IM SynRM IM SynRM 2.2 kW 0.267 1.43 13.5 18 4 kW 0.273 1.67 15.2 17 5.5 kW 0.316 1.82 20 22

8. Conclusions In low-power industrial applications product groups, efficiency, cost, and power density play an important role when system requirements are considered. In this study, three different power stage induction motors (2.2 kW, 4 kW, and 5.5 kW) are taken as the reference motors. The main aim of this study is to design and make a comparison of synchronous reluctance motors. The main difference between the induction motors and the synchronous reluctance motors is the rotor structure. The stator structure and radial air gap length are kept constant for both IMs and SynRMs. Different types of rotor design parameters are considered to make a complementary design. After the selection of the best models for each power level, the selected models are prototyped and tested for measuring their performance. SynRM has slightly higher efficiency than IM since rotor losses are eliminated. Main drawback of SynRMs are the low-power factors. IMs provides higher torque values at the same current levels and requires lower input power for the same output power. As a result, considering losses, cost, and power density, the newly designed synchronous reluctance motor prototypes give an alternative compared to the reference induction motors. The stator winding can also be designed for a future work to justify the input voltage levels of SynRMs. Increasing the stack length would be another solution to decrease voltage levels. The reliability, torque ripple, vibration improvements, and power factor optimization are worth investigating further.

Author Contributions: All the authors contributed substantially to the paper. Conceptualization, M.I. and L.T.E.; methodology, M.I. and L.T.E.; software, U.E.D. and N.G.O.; validation, U.E.D., N.G.O., M.I. and L.T.E.; writing—original draft preparation, N.G.O., L.T.E. Funding: This research was funded by TUBITAK grant number 1140055 and by ITU-AYP-2014-3 Project financially. Acknowledgments: The authors gratefully thank to Cenap ERTURK and Hakan GEDIK˙ at ARÇELIK˙ Çerkezkoy Plant and CEDRAT, France for supporting the R&D studies at universities. Conflicts of Interest: The authors declare no conflict of interest.

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