Performance Analysis of Permanent Magnet Motors for Electric Vehicles (EV) Traction Considering Driving Cycles

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Article Performance Analysis of Permanent Magnet Motors for Electric Vehicles (EV) Traction Considering Driving Cycles

Thanh Anh Huynh 1 ID and Min-Fu Hsieh 2,* ID 1 Department of Systems and Naval Mechatronic Engineering, National Cheng Kung University, Tainan 70101, Taiwan; [email protected] 2 Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan * Correspondence: [email protected]; Tel.: +866-06-275-7575 (ext. 62366)

Received: 1 May 2018; Accepted: 25 May 2018; Published: 29 May 2018 Abstract: This paper evaluates the electromagnetic and thermal performance of several traction motors for electric vehicles (EVs). Two different driving cycles are employed for the evaluation of the motors, one for urban and the other for highway driving. The electromagnetic performance to be assessed includes maximum motor torque output for vehicle acceleration and the ﬂux weakening capability for wide operating range under current and voltage limits. Thermal analysis is performed to evaluate the health status of the magnets and windings for the prescribed driving cycles. Two types of traction motors are investigated: two interior permanent magnet motors and one permanent magnet-assisted synchronous reluctance motor. The analysis results demonstrate the beneﬁts and disadvantages of these motors for EV traction and provide suggestions for traction motor design. Finally, experiments are conducted to validate the analysis.

Keywords: Electric vehicles; IPMSM; SynRM; PMa-SynRM; driving cycle

1. Introduction Electric vehicles (EVs) have become more and more attractive in recent years, as they are considered as the most viable solution to help protect the environment and to achieve high energy efficiency for transportation [1]. In EVs, traction motors are the key component for propulsion, which requires high torque and power density, wide speed range, high efficiency, high reliability, low noise, and reasonable cost. Interior permanent magnet synchronous machines (IPMSMs) have emerged as the brightest candidates, which have been widely used for EV (or hybrid EV) tractions, such as Toyota Prius and Nissan Leaf. These electric motors may be designed with different rotor topologies, where rare-earth magnet is often employed to achieve high performance [2]. However, the material cost of rare-earth magnet is high and the supply is mainly controlled by few countries owning the mineral resources. Some EVs adopt propulsion solutions without rare earth permanent magnet (PM), such as Tesla Model S, which uses copper rotor induction motors (IMs). However, the starting current of IMs can be high and this is disadvantageous for battery duration [3]. In response to the demands for rare-earth-free or less-rare-earth traction motors, PM-assisted synchronous reluctance motors (PMa-SynRMs) can be an excellent candidate [4]. The PMa-SynRM is often designed with ferrite magnets or small amount of rare-earth magnets in the rotor core. The weaker or less PM can be potentially demagnetized during high performance operation (e.g., high armature reaction or flux weakening). Also, the rotor should be well designed in order to gain as much reluctance torque as possible in order to compensate the loss in electromagnetic torque due to less amount of PM used [4,5]. On the other hand, the EVs require high torque for starting at zero speed, acceleration, climbing, and high power for cruising at highway speeds [6]. Therefore, traction motors are usually designed

Energies 2018, 11, 1385; doi:10.3390/en11061385 www.mdpi.com/journal/energies Energies 2018, 11, 1385 2 of 24 to produce peak torque or power that is several times the rated values. However, for frequent urban or suburban driving, traction motors can be mostly operated in low torque, low power, and low efficiency conditions. This would lead to energy waste and the cruise range reduces. To investigate the effect of driving conditions on design of traction motors, some established driving cycles could be employed to test the vehicles. Degano and Carraro [7,8], based on the torque profiles of selected driving cycles, determined some representative operating points of traction motors for PMa-SynRM global optimization. Nguyen and Wang [6,9] proposed a method to calculate the losses and to optimize the design of a surface-mounted PM motors (SPMSM), according to a selected driving cycle. Sarigiannidis [10,11] evaluated the energy consumption, optimized the design, and compared the performance of a fractional-slot concentrated-winding SPMSM and IPMSM based on the New European Drive Cycle (NEDC). Yang [12] determined the target torque and the speed curve based on a driving scenario and the design for an axial-flux permanent magnet (AFPM) was optimized to minimize energy consumption. Most of these studies focused on evaluating and minimizing the losses and energy consumption of traction motors. However, the evaluation of traction motor designs by employing driving cycle test results has not been comprehensively studied. The performance of an IPMSM was evaluated using a driving cycle [13]; however, the thermal issue was not considered for the traction motor to satisfy the driving cycle. Thermal imaging was employed in diagnosis of faults that are related to temperature rise for industrial induction motors [14,15]. The thermal problem can be even more crucial for traction motors with high performance operations, and therefore, the thermal analysis is considered to be critical. The design factors of tractions motors should involve flux linkages, inductances, torque generation, and thermal condition. More study is also required for EV tractions while using PMa-SynRM. This paper compares two IPMSMs and one PMa-SynRM (all using rare-earth magnet) that are based on two standard driving cycles: the Urban Dynamometer Driving Schedule (UDDS) and Highway Fuel Economy Driving Schedule (HWFET) [16]. These driving cycles are employed to comprehensively test the traction motor performance in terms of torque, efficiency, and thermal condition. The torque–speed envelope is derived under the current and voltage limits that the inverter can supply. Thermal analysis is performed to check the temperature of the magnets and windings under various loading conditions. Based on the analysis, the benefits and the disadvantages of the IPMSMs and PMa-SynRM for EV tractions can be fully elaborated. Some suggestions regarding the design of EV traction motors are provided. Finally, experiments are conducted to verify the analysis.

2. Target Traction Motors and EV As previously mentioned, two IPMSMs and one PMa-SynRM with rare-earth magnet are investigated. Among these motors, the PMa-SynRM is a less-magnet option with a dominant reluctance torque. This would make its torque performance at high speed different to that of the IPMSMs, whose electromagnetic torque dominates. For fair comparison, some design constraints for the motors are listed, as follows:

(a) output power: greater or equal to 10 kW (this makes a total power of 20 kW for the EVs. Explained later); (b) torque density: greater or equal to 30 Nm/L; 2 (c) maximum current density: 16–17 Arms/mm ; (d) outer stator diameter: 160 mm; (e) air gap: 0.5 mm; (f) number of slots: 36; (g) winding conﬁguration: distributed winding; and, (h) efﬁciency: greater or equal to 95%.

Figure1a shows the design of the IPMSM with double-layers PM. This rotor design can reduce the torque ripple due to the harmonics that are induced by the ﬂux barriers/magnets with the stator Energies 2018, 11, 1385 3 of 24

Energies 2018, 11, x FOR PEER REVIEW 3 of 24 teeth. The reluctance torque can also be improved although the electromagnetic torque (hereafter called PM torque) may may be be slightly slightly reduced. reduced. The The greater greater number number of of magnet magnet poles poles usually usually generates generates a ahigher higher torque torque for for the the same same current current level; level; however, however, it italso also requires requires more more room room for for e eachach pole. pole.

Figure 1. 1. TractionTraction motors motors using rarerare-earth-earth permanent magnet ( (PM):PM): (a) double-layers double-layers interiorinterior permanent magnet magnet synchronous synchronous machines machines (IPMSM). (IPMSM). Rotor Rotor material: material: 50CS1300. 50CS1300. Stator material: 25CS1500HF;25CS1500HF; ( b)) V V-shaped-shaped IPMSM. IPMSM. Rotor Rotor material: material: 35CS550. 35CS550. Stator Stator material: material: 25CS1500HF 25CS1500HF;; and, and, (c) (PMc) PM-assisted-assisted synchronous synchronous reluctance reluctance motors motors (PMa- (PMa-SynRM).SynRM). Rotor material: Rotor material: 50CS1300. 50CS1300. Stator material: Stator ◦ material:25CS1500HF. 25CS1500HF. Permanent Permanent magnet: magnet:NdFeB-N35H, NdFeB-N35H, Br = 1.2TB @r =20 1.2T °C, @Hc20 = 915C, HkA/mc = 915. All kA/m. of the Allcases of theare casesdistributed are distributed windings windings..

The V-shaped IPMSM that is shown in Figure 1b is known to possess the advantages of flux The V-shaped IPMSM that is shown in Figure1b is known to possess the advantages of flux weakening capability [17]. The PM position and dimension for the V-shaped design are also arranged weakening capability [17]. The PM position and dimension for the V-shaped design are also arranged to satisfy the constraints given previously. The iron loss of the V-shaped design is expected to be to satisfy the constraints given previously. The iron loss of the V-shaped design is expected to be greater than that of the double-layers one because the harmonics in the air gap flux density greater than that of the double-layers one because the harmonics in the air gap flux density distribution distribution may increase the stator core loss. may increase the stator core loss. Figure 1c shows the design of the PMa-SynRM using rare earth PM with each piece having the Figure1c shows the design of the PMa-SynRM using rare earth PM with each piece having the same dimension. This PMa-SynRM adopts five flux barriers per pole to reduce the torque ripple and same dimension. This PMa-SynRM adopts five flux barriers per pole to reduce the torque ripple and to increase the reluctance torque. Only four of the barriers are filled with PMs (except the outermost to increase the reluctance torque. Only four of the barriers are filled with PMs (except the outermost one) for each pole, as seen in Figure 1c. one) for each pole, as seen in Figure1c. Using thin electrical steel laminations for motor core would reduce iron losses and enhance Using thin electrical steel laminations for motor core would reduce iron losses and enhance traction motor efficiency [18,19]. However, thin laminations are usually expensive [18]. Therefore, as traction motor efficiency [18,19]. However, thin laminations are usually expensive [18]. Therefore, a compromise between loss reduction and material cost, thin laminations can be used for the stator as a compromise between loss reduction and material cost, thin laminations can be used for the stator (with alternating flux) and common laminations (e.g., thicker with a possible higher saturation flux (with alternating flux) and common laminations (e.g., thicker with a possible higher saturation flux density) are for the rotor. This may improve the efficiency by 2% to 2.5%, as demonstrated in [19]. density) are for the rotor. This may improve the efficiency by 2% to 2.5%, as demonstrated in [19]. This idea is adopted here. In Figure 1, thin laminations (20CS1500HF) are employed for the stator due This idea is adopted here. In Figure1, thin laminations (20CS1500HF) are employed for the stator due to the alternating flux that generates significant iron losses and other materials (50CS1300 or 35CS550) to the alternating flux that generates significant iron losses and other materials (50CS1300 or 35CS550) are utilized in the rotors. Note that all of the laminations are produced by China Steel Corporation in are utilized in the rotors. Note that all of the laminations are produced by China Steel Corporation Taiwan. The Neodymium Iron Boron (NdFeB) magnet used for all the three motors is N35H with a in Taiwan. The Neodymium Iron Boron (NdFeB) magnet used for all the three motors is N35H with remanence (Br) and coercive force (Hc) of 1.2 T and 915 kA/m, respectively. a remanence (Br) and coercive force (Hc) of 1.2 T and 915 kA/m, respectively. The main specifications and parameters of the motors are given in Table 1. As can be seen, the The main specifications and parameters of the motors are given in Table1. As can be seen, PMa-SynRM is axially longer than the other two. All of the motors are designed to meet the the PMa-SynRM is axially longer than the other two. All of the motors are designed to meet the prescribed minimum requirements. Note that the design processes are not the focus in this paper, prescribed minimum requirements. Note that the design processes are not the focus in this paper, and thus will not be detailed here. and thus will not be detailed here. As shown in Figure 2, two traction motors, one in the front, and the other the rear drivetrains, As shown in Figure2, two traction motors, one in the front, and the other the rear drivetrains, are employed to a light-duty vehicle and each is connected to the front or rear axles via a differential. are employed to a light-duty vehicle and each is connected to the front or rear axles via a differential. With the driving independence of the front and rear wheels, this offers advantages, such as fault With the driving independence of the front and rear wheels, this offers advantages, such as fault tolerance, excellent safety, and enhanced steering ability with an appropriate distribution of driving tolerance, excellent safety, and enhanced steering ability with an appropriate distribution of driving torque to the front and rear wheels according to operating conditions. This also helps to improve the torque to the front and rear wheels according to operating conditions. This also helps to improve stability of the vehicle by precisely distributing the braking torque to the front and rear wheels upon the detection of a wheel slip or wheel lock [20]. Table 2 lists the parameters of the target EV. The

Energies 2018, 11, 1385 4 of 24 Energies 2018, 11, x FOR PEER REVIEW 4 of 24 analysisthe stability process of the for vehicle the traction by precisely motors distributingwhile considering the braking driving torque cycles to is the summarized front and rearin the wheels flow chartupon shown the detection in Figure of 3. a wheel slip or wheel lock [20]. Table2 lists the parameters of the target EV. The analysis process for the traction motors while considering driving cycles is summarized in the ﬂow chart shown in Figure3. Table 1. Motor specifications. Parameter (Unit) Double Layers V-Shaped PMa-SynRM Table 1. Motor speciﬁcations. Max. power (kW) 10 10 10 Max.Parameter torque (Nm) (Unit) Double56 Layers V-Shaped56 PMa-SynRM65 Max. speed (rpm) 7000 7000 7000 Max. power (kW) 10 10 10 RatedMax. power torque (kW) (Nm) 6.6 56 566.6 65 6.6 RatedMax. torque speed (Nm) (rpm) 700030 700030 700030 Base speed @Rated maximum power (kW)torque (rpm) ≥1800 6.6 ≥ 6.61800 6.6≥1500 Base speedRated @ rated torque torque (Nm) (rpm) ≥2100 30 ≥ 302100 30≥2100 Base speed @ maximum torque (rpm) ≥1800 ≥1800 ≥1500 Base speedNo. @of rated slots torque (rpm) ≥210036 ≥210036 ≥210036 No.No. of poles of slots 368 368 36 4 No.No. of ofturns poles 84 85 4 6 Max.No. current of turns (A) 110 4 5100 6 80 Max. current (A) 110 100 80 Outer rotor diameter (mm) 94 109 94 Outer rotor diameter (mm) 94 109 94 StackStack length length (mm) (mm) 86 9090 120120 CoilCoil pitch pitch 4 33 9 9 3 PMPM amount amount per per pole pole (mm (mm3) ) 7740 81008100 57605760 2 CurrentCurrent density density @ @rated rated torque torque (A/mm (A/mm2)) 8.3 8.38.3 8.38.3

Figure 2. SchematicSchematic of a front and rear drives of light-dutylight-duty vehicle.

Table 2. Vehicle specifications. Table 2. Vehicle specifications. Parameter (Unit) Value RadiusParameter of wheels (Unit) (m) Value0.265 RadiusVehicle of wheelsmass (kg) (m) 0.265950 MassVehicle correction mass coefficient (kg) 1.04 950 Mass correction coefficient 1.04 2 Gravitational acceleration acceleration (m/s (m/s2) ) 9.8 RollingRolling resistanceresistance coefficient coefficient 0.011 0.011 Air massmass density density (kg/m (kg/m3) 3) 1.2251.225 AerodynamicAerodynamic drag drag coefficient coefficient 0.4 2 Vehicle frontal frontal area area (m (m) 2) 2.14 Differential gear ratio 7 BatteryDifferential pack nominal gear voltage ratio (V) 3107 Battery pack nominal voltage (V) 310

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Figure 3. The analysis analysis process for traction motors with driving cycle.

3. Theoretical Background 3. Theoretical Background To satisfy the required performance of the EV shown in Table 2 and Figure 2, the designed To satisfy the required performance of the EV shown in Table2 and Figure2, the designed motors motorsshould possess should possessexcellent excellent capabilities capabilities in terms inof ﬂux-weakeningterms of flux-weakening and peak and torque peak output torque under output the underinverter the current inverter and current voltage and limits. voltage Thus, limits. the ﬁrst Thus, constraint the first is theconstraint maximum is the current maximum and the current maximum and the maximum voltage for the IPMSMs and PMa-SynRM, which can be given by [21]: voltage for the IPMSMs and PMa-SynRM, which can be given by [21]: IIIIq 2  2  (1) s d2 q2 s_ max Is = Id + Iq ≤ Is_max (1)

q 2 2 V VLILI p  2  2 DCVDC V s_ max= p (L I d d+ λ m) + qL qI ≤ √ (2)(2) s_max d d m q q  3 s3ωs where Is and Is_max areare armature armature current current and and the the maximum maximum current current provided provided by by the the inverter, inverter, respectively; Id and IIq areare dd-- andand qq--axisaxis currents, currents, respectively; Ld andand LLqq areare dd-- andand qq--axisaxis inductances, inductances, respectively; p is numbernumber ofof polepole pair;pair; λλmm isis PMPM fluxflux linkage;linkage;ω ωss isis rotorrotor speed;speed;V Vs_maxs_max is maximum phase voltage; and, VDC isis battery battery voltage voltage applied applied to to the the inverter. inverter. The induced back electromotive force atat the maximummaximum speed isis determineddetermined by:by: VV E = pλ ω ≤ DC√DC (3) E pmm s s__max max  (3) 3 3 where E isis the inducedinduced back electromotiveelectromotive forceforce (BEMF)(BEMF) andand ωωs_maxs_max isis maximum maximum rotor rotor speed. speed. To maximize the fluxflux weakening capability and constant constant-power-power speed range that is required by the traction motor, the followingfollowing idealideal conditioncondition shouldshould bebe satisfiedsatisfied [[2121]]::

  LI λmm= L dd I ss (4)(4) for IPMSM and for IPMSM and

λmm= LIL qq I ss (5)(5) for PMa-SynRM [22]. However, this is only an ideal condition. Practically, the stator flux linkage should be carefully designed with a limit, so that PM would not be demagnetized. If the design cannot

Energies 2018, 11, 1385 6 of 24 for PMa-SynRM [22]. However, this is only an ideal condition. Practically, the stator flux linkage should be carefully designed with a limit, so that PM would not be demagnetized. If the design cannot fully meet the conditions in Equations (4) or (5), a reasonable high constant power speed range can still be achieved if the values of both sides are as close as possible. Note that the conventional d-q-axis definitions with respect to PM flux linkage are different for IPMSMs and PMa-SynRMs [21,22], since the IPMSM is essentially a PM motor, while the PMa-SynRM is basically a SynRM with enhanced performance using PMs. Note that the definitions of d- and q-axis for the two types of motors are indicated in Figure1, and these are used for all of the analysis and results that are presented. Traction motors should be maintained at high efficiency for a speed range that is as wide as possible. This may be achieved by employing the maximum torque per ampere (MTPA) control at low speed for peak torque and the flux weakening control at high speed. For a wider speed range, a lower back EMF is needed (i.e., lower λm), and this indicates an increase of d-axis inductance and reluctance torque referring to Equation (4) for IPMSMs [22]. For the two types of motors, the output torque can be calculated by [6]:

3 T = p λd Id, Iq Iq − λq Id, Iq Id 2 3 λd(Id,Iq)−λd(Id=0,Iq) λq(Id,Iq) = p λ I = 0, Iq Iq + I Iq − I Iq (6) 2 d d Id d Iq d 3 = 2 p λm Iq Iq + Ld Id, Iq − Lq Id, Iq Id Iq where λd and λq are, respectively, d- and q-axis flux linkage, p is number of pole pair, Id and Iq are, respectively, d- and q-axis currents excitations. The flux linkages of the motors can be defined by: λm Iq = λd Id = 0, Iq (7) λd Id, Iq = Ld Id, Iq Id + λm Iq (8) λq Id, Iq = Lq Id, Iq Iq (9)

It can be seen from Equations (1)–(6) that a small value of pλm is preferred for low induced BEMF at high speed. In contrast, a large value is desirable for high torque and high efﬁciency. Therefore, the PMa-SynRMs may require higher total electric loading than that for IPMSMs to achieve the same torque that is required.

4. Performance Evaluation of Traction Motors The design characteristics of the two types of motors under no-load and load conditions are evaluated using finite element analysis (FEA). The PM flux linkage at no-load is given in Figure4. As can be seen in Figure4a, the PM flux linkage of the V-shaped IPMSM is higher than that of the double-layer one. This is due to a higher amount of PM that is used in the V-shaped design. However, the total torque of the two IPMSMs is designed to be equivalent at the same current density by using full coil pitch windings on the double-layer IPMSM and the short-pitch windings for the V-shaped one. Figure4b shows the PM flux linkage at no-load of the PMa-SynRM, which is apparently smaller than that of the IPMSM. The major torque of IPMSMs is contributed by PM flux. This is different for PMa-SynRM, whose torque mainly comes from the flux linkage that is produced by current excitation. Thus, using large amount of PM does not help improve the output torque of the PMa-SynRM, but increase the material cost [5]. Therefore, the PM volume in PMa-SynRM should be kept small, but the risk of demagnetization should be avoided by maintaining the required performance [5]. Inductance variation plays a critical role in the speed range of the IPMSM and PMa-SynRM, as indicated in Equations (4) and (5). Therefore, accurate calculation of the inductance variation in the entire operating range is necessary, especially when saturation occurs. Figures5–7 show the inductance Energies 2018, 11, 1385 7 of 24 Energies 2018, 11, x FOR PEER REVIEW 7 of 24 Energies 2018, 11, x FOR PEER REVIEW 7 of 24 variationinductance for thefor entirethe double operating-layerrange. IPMSM As is can smaller be observed, than that the of variation the V-shaped of the d-axis one. T inductancehis might be for inductance for the double-layer IPMSM is smaller than that of the V-shaped one. This might be thepartially double-layer caused IPMSMby the iron is smaller bridges than in thatthe V of-shaped the V-shaped rotor that one. creates This might additional be partially flux paths. caused This by partially caused by the iron bridges in the V-shaped rotor that creates additional flux paths. This theleads iron to bridges a higher in saliency the V-shaped ratio and rotor a wider that creates speed additional range for the flux double paths.-layer This leadscase, where to a higher the saliency saliency leads to a higher saliency ratio and a wider speed range for the double-layer case, where the saliency ratioratio and S is adefined wider speedas: range for the double-layer case, where the saliency ratio S is defined as: ratio S is defined as: SLL = q d (10) SSLL Lqq/Ldd (10)(10) for IPMSM, or forfor IPMSM, IPMSM, or or  SSLL= Ld/L qq (11)(11) SLL d q (11) forfor PMa-SynRM. PMa-SynRM. The The inductance inductance variationvariation inin Figure7 7 shows shows aa high saliency ratio of of the the PMa PMa-SynRM.-SynRM. for PMa-SynRM. The inductance variation in Figure 7 shows a high saliency ratio of the PMa-SynRM. ThisThis indicates indicates that that the the high highreluctance reluctance torquetorque cancan helphelp to maintain the operati operationon at at high high speed speed for for the the This indicates that the high reluctance torque can help to maintain the operation at high speed for the PMa-SynRM.PMa-SynRM. PMa-SynRM.

Figure 4. Comparison of PM flux linkage of IPMSM and PMa-SynRM: (a) IPMSM and (b) PMa- FigureFigure 4. 4Comparison. Comparison of of PM PM ﬂux flux linkage linkage of IPMSM of IPMSM and andPMa-SynRM: PMa-SynRM: (a) IPMSM (a) IPMSM and ( b and) PMa-SynRM. (b) PMa- SynRM. Reference angle starts from q-axis. ReferenceSynRM. Reference angle starts angle from startsq-axis. from q-axis.

Figure 5. d-q-axis inductance of double-layer PM IPMSM: (a) Ld, (b) Lq. Figure 5. d-q-axis inductance of double-layer PM IPMSM: (a) Ld, (b) Lq. Figure 5. d-q-axis inductance of double-layer PM IPMSM: (a) Ld,(b) Lq.

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Figure 6. d-q-axis inductance of V-shaped PM IPMSM: (a) Ld, (b) Lq. Figure 6. d-q-axis inductance of V-shaped PM IPMSM: (a) Ld,(b) Lq. Figure 6. d-q-axis inductance of V-shaped PM IPMSM: (a) Ld, (b) Lq.

FigureFigure 7 .7 d. d-q--qaxis-axis inductance inductance of PMa--SynRM:SynRM: ( a()a )L dL, d(,b ()b L) qL. q. Figure 7. d-q-axis inductance of PMa-SynRM: (a) Ld,(b) Lq. FigureFigure 8 shows8 shows the the flux flux linkage linkage characteristics characteristics of of the the IPMSM IPMSM (double (double-layer)-layer) with with different different current excitations and phase angles that were calculated using Equations (7)–(9) based on the PM Figurecurrent8 shows excitations the flux and linkagephase angles characteristics that were calculated of the IPMSM using Equations (double-layer) (7)–(9) withbased differenton the PM current fluxflux linkage linkage and and inductance inductance previously previously obtained.obtained. It isis observedobserved that that the the IPMSM IPMSM can can be be potentially potentially excitations and phase angles that were calculated using Equations (7)–(9) based on the PM flux linkage demagnetizeddemagnetized with with high high current current excitation excitation and deep flux flux weakening. weakening. Therefore, Therefore, the the armature armature and inductancecurrent,current, curre previously current ntangle, angle, obtained.torque torque for for MTPA ItMTPA is observed control control,, and that flux the weakeningweakening IPMSM operations operations can bepotentially should should be be carefully carefully demagnetized with highanalyzed currentanalyzed in excitationinorder order to to achieve achieve and deepa asufficient sufficient flux weakening. andand safesafe speed Therefore, range.range. Figures Figures the 9 9 armatureand and 10 10 show show current, the the calculated calculated current angle, armature current, current angle, and torque characteristics of the IPMSM for wide speed range. At torque forarmature MTPA current, control, current and angle, flux weakeningand torque characteristics operations of should the IPM beSM carefully for wide analyzedspeed range. in At order to the armature current below 54.8 A, the IPMSM cannot achieve the maximum speed (7000 rpm), as the armature current below 54.8 A, the IPMSM cannot achieve the maximum speed (7000 rpm), as achieve a sufficientshown in Figure and 10. safe This speed is because range. the low Figures armature9 andcurrent 10 cannot show sufficiently the calculated weaken the armature PM flux current, shown in Figure 10. This is because the low armature current cannot sufficiently weaken the PM flux current angle,to extend and the torque operating characteristics speed within ofthe the voltage IPMSM limit. forThe widePM flux speed can be range. effectively At weakened the armature for current to extend the operating speed within the voltage limit. The PM flux can be effectively weakened for the armature current greater than 54.8 A (current density 9.7 Arms/mm2), for the cases beyond which below 54.8 A, the IPMSM cannot achieve the maximum speed (7000 rpm),2 as shown in Figure 10. This is thea armature wide speed current range greater can be than reached 54.8 withA (current the voltage density limit. 9.7 BeyondArms/mm the), basefor the speed, cases the beyond armatu whichre because the low armature current cannot sufficiently weaken the PM flux to extend the operating a widecurrent speed is maintained range can and be the reached current withphase the angle voltage is increased limit. for Beyond flux weakening the base until speed, the the maximum armatu re speed withincurrentspeed. the is Notemaintained voltage that the limit.and base the speed The current depends PM phase flux on angle can the voltagebeis increased effectively or the for power flux weakened weakeninglimit. High for armatureuntil the the armature maximumcurrent current 2 greater thanspeed.would 54.8 Note also A that (currentlead the to abase low density speedbase speed. depends 9.7 AFigurerms on/mm 10 the shows voltage), for that theor the the casesIPMSM power beyond hits limit. the voltageHigh which armature limit a wide at a currentbase speed range can be reachedwouldspeed also of with 2500lead the torpm a voltage low(achievable base limit.speed. base Figure Beyondspeed), 10an showsthearmature base that current speed,the IPMSM of the54.8 hits armatureA, andthe voltagea current current limit density at is a maintained ofbase speed9.7 Aofrms 2500/mm rpm2 without (achievable force cooling. base speed),Nevertheless, an armature the speed current range ofof 54.8the IPMSM A, and cana current be enhanced density if of and the current phase angle is increased for flux weakening until the maximum speed. Note that the 9.7 betterArms/mm cool2ing without is used force so that cooling. more currentNevertheless, can be input the speed to improve range theof thetorque IPMSM at high can speed. be enhanced if base speedbetter depends coolContrarily,ing is on used the the so PMa voltagethat-SynRM more or currentcan the achieve power can bethe input limit.maximum to Highimprove speed armature the (7000 torque rpm) current at with high low speed. would armature also lead to a low basecurrent speed.Contrarily, (lower Figure thethan 10 PMa 40 shows A)-SynRM for the that MTPAcan the achieve and IPMSM flux the weakening maximum hits the operations, voltage speed (7000 limit as shown rpm) at a within base Figure low speeds armature11 and of 2500 rpm 12. This is due to the low PM flux linkage, which can be easily weakened with a small armature 2 (achievablecurrent base (lower speed), than an 40 armatureA) for the MTPA current and of flux 54.8 weakening A, and a currentoperations, density as shown of 9.7 in Figure Arms/mms 11 andwithout 12. current. This is Therefore,due to the the low torque PM flux-speed linkage, curve canwhich be can extended be easily under weakened voltage limit with to a a small high armature speed. force cooling.However, Nevertheless, the base speed the speedreduces rangeat maximum of the torque IPMSM where can the be voltage enhanced saturates if better due to cooling the high is used so that morecurrent. current Therefore, can be inputthe torque to improve-speed curve the can torque be extended at high speed. under voltage limit to a high speed. However, the base speed reduces at maximum torque where the voltage saturates due to the high Contrarily, the PMa-SynRM can achieve the maximum speed (7000 rpm) with low armature current (lower than 40 A) for the MTPA and flux weakening operations, as shown in Figures 11 and 12. This is due to the low PM flux linkage, which can be easily weakened with a small armature current. Therefore, the torque-speed curve can be extended under voltage limit to a high speed. However, the base speed reduces at maximum torque where the voltage saturates due to the high current and winding resistance. The high current weakens the PM flux and it increases the reluctance torque, leading to speed extension under the voltage limit. Energies 2018, 11, x FOR PEER REVIEW 9 of 24 Energies 2018, 11, x FOR PEER REVIEW 9 of 24 current and winding resistance. The high current weakens the PM flux and it increases the reluctance Energiescurrent2018 and, 11 winding, 1385 resistance. The high current weakens the PM flux and it increases the reluctance9 of 24 torque, leading to speed extension under the voltage limit. torque, leading to speed extension under the voltage limit.

Figure 8. Flux linkage variations on (a) d- and (b) q-axis of IPMSM with respect to current and current FigureFigure 8. FluxFlux linkage linkage variations variations on on(a) (da-) andd- and (b) q (-baxis) q- axisof IPMSM of IPMSM with respect with respect to current to current and current and angle. currentangle. angle.

Figure 9. (a) Current and (b) current angle variation of IPMSM with speed. Figure 9.9. (a) Current and ((b) currentcurrent angleangle variationvariation ofof IPMSMIPMSM withwith speed.speed.

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Figure 10. (a) Torque-current angle and (b) torque-speed curves of IPMSM. FigureFigure 10. 10.( (aa)) Torque-currentTorque-current angleangle and ( b)) torque torque-speed-speed curves curves of of IPMSM. IPMSM.

Figure 11. (a) Current and (b) current angle variations of PMa-SynRM with speed. Figure 11. (a) Current and (b) current angle variations of PMa-SynRM with speed. Figure 11. (a) Current and (b) current angle variations of PMa-SynRM with speed.

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Figure 12. The torque-speed curves of PMa-SynRM. Figure 12. The torque-speed curves of PMa-SynRM.

5. Driving Cycle Applications 5. Driving Cycle Applications The combination of UDDS and HWFET driving cycles is used to evaluate the traction motor The combination of UDDS and HWFET driving cycles is used to evaluate the traction motor performance based on the vehicle specifications, including the battery and the drivetrain properties, performance based on the vehicle specifications, including the battery and the drivetrain properties, as shown in Table 2. The combined UDDS and HWFET is converted into traction motor speed, as as shown in Table2. The combined UDDS and HWFET is converted into traction motor speed, shown in Figure 13a. The UDDS is suitable for urban lightweight vehicles with 23 start-stop cycles as shown in Figure 13a. The UDDS is suitable for urban lightweight vehicles with 23 start-stop cycles and a maximum speed of 91.2 km/h. There are two phases of speed: a “cold start” phase of 505 s for and a maximum speed of 91.2 km/h. There are two phases of speed: a “cold start” phase of 505 s a distance of 5.78 km with 41.2 km/h average speed, and a “transient phase” of 864 s in the total for a distance of 5.78 km with 41.2 km/h average speed, and a “transient phase” of 864 s in the total duration of 1369 s. The HWFET is used to test the thermal condition of the traction motors without duration of 1369 s. The HWFET is used to test the thermal condition of the traction motors without stop cycles. It is suitable for highway driving with an average speed of 77 km/h and a peak speed of stop cycles. It is suitable for highway driving with an average speed of 77 km/h and a peak speed of 97 km/h in a total duration of 765 s and 16.45 km route [16]. 97 km/h in a total duration of 765 s and 16.45 km route [16]. The torque of the traction motor should be generated and delivered to satisfy the vehicle The torque of the traction motor should be generated and delivered to satisfy the vehicle dynamics dynamics during the operation of the prescribed driving cycles, i.e., short-term overload (peak) during the operation of the prescribed driving cycles, i.e., short-term overload (peak) torque output torque output for acceleration/climbing, and the effective energy utilization in the medium- and high- for acceleration/climbing, and the effective energy utilization in the medium- and high-speed ranges. speed ranges. Therefore, the root-mean-square (RMS) torque is defined to relate the operating Therefore, the root-mean-square (RMS) torque is defined to relate the operating condition to the condition to the thermal limits of the traction motors in the driving cycle, as given by [8]: thermal limits of the traction motors in the driving cycle, as given by [8]: 1 t2 Tv T2 () t dt rms u  t2 (12) u tcycle Z u 1 t1 2 Trms = t T (t)dt (12) tcycle where tcycle is the time period of the entire driving cyclet1 and T(t) is the instantaneous torque. Figure 13 shows the motor speed, load torque, and power in the motor mode, and the t T t whereregenerativecycle is the braking time period mode of over the entirethe combined driving cycle UDDS and and( ) is HYFET the instantaneous driving cycles, torque. where the maximumFigure speed13 shows is 97 the km/h. motor It speed, can be load seen torque, that for and the power target in thevehicle, motor the mode, driving and thecycle regenerative requires a brakingmaximum mode torque over and the peak combined power UDDS of approximately and HYFET 60 driving Nm and cycles, 20 kW, where respecti the maximumvely. However, speed the is 97RMS km/h. torque It can is about be seen 29.5 that Nm for and the the target average vehicle, power the driving is only cycle5.4 kW requires for the a whole maximum cycle. torque Note andthat peakthe torque power and of power approximately that is required 60 Nm by and the 20 vehicle kW, respectively. calculated above However, is assumed the RMS to be torque evenly is shared about 29.5by the Nm front and and the rear average motors power, as seen is only in Figure 5.4 kW 2. for Therefore, the whole the cycle. motor Note specifications that the torque in Table and 1 powersatisfy thatthis isvehicle required if the by prescribed the vehicle driving calculated cycles above are used. is assumed The performance to be evenly of shared the IPMSMs by the andfront PMa and- rearSynRM motors, will be as seenevaluated in Figure based2. Therefore,on the torque the and motor power specifications that is required in Table for1 thesatisfy prescribed this vehicle drivin ifg thecycles. prescribed Note that driving, practically, cycles are this used. lightweight The performance vehicle may of the not IPMSMs be suitable and for PMa-SynRM highway driving. will be evaluatedHowever, basedit would on make the torque a useful and reference power that if the is required highway for driving the prescribed can be included driving in cycles. the analysis Note that, for practically,design considerations this lightweight of traction vehicle motors. may not be suitable for highway driving. However, it would make a useful reference if the highway driving can be included in the analysis for design considerations of traction motors.

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Figure 13. Motor (a) speed; (b) torque; and, (c) power over the driving cycles. Figure 13. Motor (a) speed; (b) torque; and, (c) power over the driving cycles. 6. Comparison of Motor Performance in Driving Cycles 6. Comparison of Motor Performance in Driving Cycles Again, the FEA (JMAG software) is used for simulation of the motor electromagnetic performance.Again, the FEA Figure (JMAG 14 showssoftware) the is torque used- forspeed simulation curves of the motor IPMSM electromagnetic (double-layer) performance. and PMa- FigureSynRM 14 shows under the the torque-speed peak and continuous curves of conditions, the IPMSM where (double-layer) the maximum and PMa-SynRMtorque of the underIPMSM the and peak andPMa continuous-SynRM conditions, is 57 Nm andwhere 68 the Nm, maximum respectively. torque The of rated the IPMSM conditions and of PMa-SynRM the motors is should 57 Nm be and 68 Nm,determined respectively. using Thethermal rated analysis, conditions which of will the be motors discussed should later. be Nevertheless, determined at using this stage, thermal the analysis, rated whichtorque will of be both discussed types of later. motors Nevertheless, is preliminarily at this determined stage, the byrated the root torque-mean of-square both typestorque of that motors was is preliminarilycalculated determinedfrom the driving by the cycles root-mean-square using Equation torque (12), which that was is 29.5 calculated Nm. Then from, beyond the driving the rated cycles usingtorque Equation in Figure (12), 14, which the speed is 29.5 gradually Nm. Then, increases beyond by the increasing rated torque the current in Figure angles 14 ,for the flux speed weakeni graduallyng with the same current amplitude. Thus, the two IPMSMs have similar characteristics. increases by increasing the current angles for ﬂux weakening with the same current amplitude. Thus, the two IPMSMs have similar characteristics. Energies 2018, 11, 1385 13 of 24 Energies 2018, 11, x FOR PEER REVIEW 13 of 24

Figure 14. Torque distribution profiles for (a) IPMSMs and (b) PMa-SynRMs over the driving cycles. Figure 14. Torque distribution proﬁles for (a) IPMSMs and (b) PMa-SynRMs over the driving cycles.

It can be seen in Figure 14, that, under the UDDS and HWFET driving cycles, some operating pointsIt are can higher be seen than in Figurethe rated 14 ,conditions that, under of theboth UDDS the IPMSM and HWFET and PMa driving-SynRM. cycles, For the some PMa operating-SynRM, pointssome operating are higher points than the are rated even conditions out of the ofmaximum both the IPMSMcapability. and This PMa-SynRM. appears that For the PMa-SynRM,10 kW PMa- SynRMsome operating is insufficient points for are high even speed out operation of the maximum of the vehicles capability. due to Thisthe small appears torque. that Even the 10for kWthe IPMSM,PMa-SynRM the rated is insufficient power is forslightly high too speed low operation for high speed of the driving. vehicles It due should to the be small noted torque. that operation Even for inthe highway IPMSM, themode rated usually power lasts is slightly for a long too low period for high so that speed the driving. rated condition It should beof notedthe traction that operation motors shouldin highway cover mode this driving usually cycle. lasts From for a longthe previous period so results, that the it is rated found condition that the ofPMa the-SynRM traction seems motors to beshould suitable cover for this low driving speed cycle.operation From, such the previousas urban results,bus, since it is its found output that power the PMa-SynRM drops at high seems speed. to Inbe contrast, suitable forthe lowIPMSM speed can operation, be easily suchimproved as urban for bus,high sincespeed its operation output power by improving drops at highthe cooling speed. capabilityIn contrast, or the modifying IPMSM canthe bedesign easily (no improved forced cooling for high so speed far). This operation accounts by improvingfor the importance the cooling of thermalcapability design or modifying and analysis, the designwhich (nowill forcedbe discussed cooling later. so far). Note This that accounts the data forsampling the importance time is one of secondthermal for design all ofand the operating analysis, which points will of both be discussed driving cycles later. (i.e., Note all that of the the dots data in sampling Figure 14). time is one secondFigure for all 15a,b of the shows operating the efficiency points of both map driving of the double cycles (i.e.,-layers all andof the the dots V-shaped in Figure IPMSMs. 14). The highestFigure efficiency 15a,b shows (dark thered efficiencyregions) for map the of two the double-layerscases is beyond and 96% the and V-shaped 95%, respectively, IPMSMs. The and highest this isefficiency within a (dark speed red range regions) of 2000 for rpm the two to 4000 cases rpm is beyond and a torque 96% and lower 95%, than respectively, 30 Nm. The and double this is- withinlayers IPMSMa speed rangehas a slightly of 2000 higher rpm to effic 4000iency rpm andfor a a wider torque operating lower than range 30 Nm. than The the double-layersV-shaped one. IPMSM This could has bea slightly due to highera higher efficiency total Ampere for a- widerturns for operating the V-shaped range thanIPMSM. the V-shapedFor the two one. IPMSMs, This could the operating be due to pointsa higher for total the Ampere-turnsUDDS driving for cycle the (blue V-shaped dots) IPMSM. mostly Forlocate the around two IPMSMs, 2000 rpm the operatingto 4000 rpm, points whe forre the highUDDS efficiency driving cycleregion (blue is. Meanwhile, dots) mostly the locate operating around points 2000 rpmfor the to 4000HWFET rpm, driving where thecycle high (green efficiency dots) fallregion within is. Meanwhile, the lower efficiency the operating regions points, ranging for thefrom HWFET 5000 rpm driving to 7000 cycle rpm (green for the dots) two fallIPMSMs. within This the maylower be efficiency caused by regions, a large ranging part of fromthe armature 5000 rpm cu torrent 7000 that rpm is for used the twoto weaken IPMSMs. the This flux may at high be caused speed whereby a large deep part flux of the weakening armature is current required that (Figure is used 8). to weaken Contrarily, the flux the atPMa high-SynRM speed whereachieves deep higher flux weakeningefficiency at is high required speed (Figure due to8 ).the Contrarily, reduced armature the PMa-SynRM current, achieves as shown higher in Figure efficiency 15c. Therefore, at high speed it is suitabledue to the for reduced the HWFET armature and current,the higher as shownspeed operating in Figure 15pointsc. Therefore, of the UDDS it is suitable in terms for of the efficiency. HWFET Theand themaximum higher speedefficiency operating is 95% pointsbeyond of 2000 the UDDS rpm and in terms it is maintained of efficiency. un Thetil the maximum maximum efficiency speed. However,is 95% beyond this PMa 2000- rpmSynRM and requires it is maintained a redesign until or thedecent maximum forced speed.cooling However, in order to this fully PMa-SynRM cover the operating points of the driving cycles. Again, the data sampling time is also one second here.

Energies 2018, 11, 1385 14 of 24 requires a redesign or decent forced cooling in order to fully cover the operating points of the driving cycles.Energies Again, 2018, the 11, x data FOR PEER sampling REVIEW time is also one second here. 14 of 24

Figure 15. Efficiency map of IPMSMs and PMa-SynRM over the driving cycles: (a) double layers Figure 15. Efﬁciency map of IPMSMs and PMa-SynRM over the driving cycles: (a) double layers magnet IPMSM, (b) v-shaped magnets IPMSM, and (c) PMa-SynRM. magnet IPMSM, (b) v-shaped magnets IPMSM, and (c) PMa-SynRM. 7. Thermal Analysis with Driving Cycle 7. Thermal Analysis with Driving Cycle Thermal analysis (using MotorCAD) for the IPMSMs and PMa-SynRM is conducted for the Thermalprescribed analysis driving (using cycles. MotorCAD)This would be for critical the IPMSMs for the anddesign PMa-SynRM of traction motorsis conducted since the for the prescribedperformance driving can cycles. be limi Thisted wouldby thermal be critical conditions. for the First, design the operating of traction range motors of the since traction the performance motors can befor limited the driving by thermal cycles under conditions. prescribed First, temperature the operating limits range (PM: of 130 the °C traction; winding: motors 140 °C for) without the driving cyclesforced under cooling prescribed is investigated. temperature As shown limits in Figure (PM:130 13b,◦ theC; winding:output torque 140 of◦ C)the without traction motors forced varies cooling is significantly within a driving cycle. Therefore, to represent the equivalent effect, the root-mean- investigated. As shown in Figure 13b, the output torque of the traction motors varies signiﬁcantly square torque within a cycle is taken, as calculated by Equation (12). (29.5 Nm, as previously within a driving cycle. Therefore, to represent the equivalent effect, the root-mean-square torque discussed). within a cycle is taken, as calculated by Equation (12). (29.5 Nm, as previously discussed).

Energies 2018, 11, 1385 15 of 24

Energies 2018, 11, x FOR PEER REVIEW 15 of 24 Figure 16 shows the torque-speed proﬁles under a series of temperature limits in the winding and Figure 16 shows the torque-speed profiles under a series of temperature limits in the winding magnet until the maximum limits (PM: 130 ◦C; winding: 140 ◦C) for the double-layer IPMSM. It can be and magnet until the maximum limits (PM: 130 °C; winding: 140 °C) for the double-layer IPMSM. It◦ seencan that be seen the effective that the effective torque of torque this motor of this can motor achieve can achieve 30 Nm 30 under Nm under the temperature the temperature limits limits of 140 of C ◦ for140 winding °C for andwinding 130 Cand for 130 PM. °C Note for PM. that Note the PM that used the PM is N35SH, used is whoseN35SH, product whose product maximum maximum operating ◦ temperatureoperating temperature is 150 C. Similarly, is 150 °C as. shownSimilarly, in Figure as shown 17, in the Figure effective 17, torque the effective for the torque V-shaped for the IPMSM V- ◦ canshaped also achieve IPMSM 30 can Nm, also with achieve a slightly 30 Nm less, with peak a magnet slightly temperature less peak magnet of 124 temperatureC. Therefore, of the 124 PM°C. of theTherefore, V-shaped the IPMSM PM of isthe safer V-shaped than that IPMSM of the is safer double-layer than that one,of the despite double a-layer slightly one, lower despite rated a slightly torque. Thelower analysis rated results torque. show The analysis that the results double-layer show that and the V-shaped double-layer IPMSMs and V can-shaped both operateIPMSMs at can the both rated conditionoperate without at the rated forced condition cooling. However, without forced water cooling.cooling should However, still water be employed cooling to should fully satisfy still be the UDDS,employed and particularly to fully satisfy HWFET the UDDS driving, and cycles. particularly HWFET driving cycles. TheThe thermal thermalanalysis analysis for for the the PMa-SynRM PMa-SynRM without without forced forced cooling cooling is is shown shown in in Figure Figure 18 18,, where where the motorthe motor fails to fails reach to reach the torque, the torque as calculated, as calculated by Equation by Equation (12). (12). Water Water cooling cooling may may help help to to reduce reduce the windingthe winding and magnet and magnet temperature temperature and to fit and into to the fit requirementinto the requirement of the driving of the cycles driving for thecycles PMa-SynRM. for the Nevertheless,PMa-SynRM. the magnetNevertheless, of PMa-SynRM the magnet should of be PMa redesigned-SynRM to sho avoiduld demagnetization. be redesigned to avoid demagnetization.Figure 19 shows the temperature rise of the winding, stator, and PM of all the three motor in the peakFigure and continuous19 shows the conditionstemperature for rise a of period. the winding, As can stator be seen,, and thePM windingof all the three temperature motor in ofthe the peak and continuous conditions for a period. As can be seen, the winding temperature of the PMa- PMa-SynRM is higher than that of the IPMSMs although their current densities are almost the same SynRM is higher than2 that of the IPMSMs although their current densities are almost the same (around 16.5 Arms/mm for peak torque). This is because the phase resistance of the PMa-SynRM (around 16.5 Arms/mm2 for peak torque). This is because the phase resistance of the PMa-SynRM is is double that of the IPMSMs. Therefore, the PMa-SynRM can only operate about 4 min at the peak double that of the IPMSMs. Therefore, the PMa-SynRM can only operate about 4 min at the peak condition and about 26 min at the rated condition, while the IPMSM can achieve 8 and 40 min, condition and about 26 min at the rated condition, while the IPMSM can achieve 8 and 40 min, respectively.respectively. Also, Also, the the high high winding winding temperature temperature of of the the PMa PMa-SynRM-SynRM leads leads to to the the high high magnet magnet temperature,temperature, as as shown shown in in Figure Figure 18 18.. The The above above analysisanalysis can also also be be verified veriﬁed by by the the simulation simulation results results withwith the the driving driving cycles, cycles, as as shown shown in in Figure Figure 20 20,, wherewhere the winding temperature temperature of of the the PMa PMa-SynRM-SynRM is is higherhigher than than that that of of IPMSM, IPMSM and, and therefore, therefore, thethe IPMSMsIPMSMs can operate operate more more safely safely than than the the PMa PMa-SynRM-SynRM forfor the the driving driving cycles. cycles.

Figure 16. The temperature distribution profiles for (a) winding and (b) magnet of double-layer Figure 16. The temperature distribution proﬁles for (a) winding and (b) magnet of double-layer IPMSM. IPMSM.

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Figure 17. The temperature distribution profiles for (a) winding and (b) PM of V-shaped IPMSM. FigureFigure 17. 17The. The temperature temperature distribution distribution proﬁlesprofiles for ( a)) winding winding and and (b (b) )PM PM of of V V-shaped-shaped IPMSM. IPMSM.

Figure 18. The temperature distribution profiles for (a) winding and (b) PM of PMa-SynRM. Figure 18. The temperature distribution profiles for (a) winding and (b) PM of PMa-SynRM. Figure 18. The temperature distribution proﬁles for (a) winding and (b) PM of PMa-SynRM.

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Figure 19. The temperature rise for winding and magnet: (a) peak; and, (b) rated/continuous Figure 19. The temperature rise for winding and magnet: (a) peak; and, (b) rated/continuous Figureoperations. 19. The temperature rise for winding and magnet: (a) peak; and, (b) rated/continuous operations. operations.

Figure 20. The temperature rise for winding and magnet: (a) UDDS, and (b) HWFET. FigureFigure 20. 20.The The temperature temperature riserise forfor winding and magnet: magnet: (a (a) )UDDS, UDDS, and and (b ()b HWFET.) HWFET.

Energies 2018, 11, 1385 18 of 24

8. Discussions According to the previous analysis, the beneﬁts and drawbacks of the IPMSMs and PMa-SynRM for EV application are summarized as below:

- The IPMSMs possess the benefit of high PM flux linkage, low armature current, and current angle to reach the maximum torque. Meanwhile, the torque of PMa-SynRM is almost derived from the armature current. Therefore, a higher armature current and current angle are applied in order to generate the maximum torque for PMa-SynRM. - The PMa-SynRM can achieve very high speed at low armature current because the reluctance torque and saliency ratio are higher than that of IPMSMs. In addition, the PMa-SynRM has a higher efficiency at high speed than that of IPMSMs. This is the advantage of PMa-SynRM for high speed application. - To produce the same power as the IPMSMs, the PMa-SynRM should be designed with a higher motor volume than that of the IPMSM. Moreover, higher electrical loading is needed for the PMa-SynRM, and this can lead to temperature rise that limits the operating range. - The IPMSMs are more advantageous in urban driving because of their higher performance and efficiency at lower speed. - The PMa-SynRM is more suitable than the IPMSM in the highway driving cycle in terms of efficiency. However, the design should be improved by slightly increasing the PM amount so that the winding temperature can be brought down and the torque at high speed can be enhanced. This indicates a better design would lie between the two types of motors that are presented in this paper in terms of PM amount that is employed or sharing between the PM and reluctance torques. Moreover, better cooling may be necessary.

9. Experiments The three motors were fabricated, as shown in Figure 21. The experimental results are shown in Figures 22 and 23. In Figure 22, the BEMF (peak value) comparison shows that the simulations and experiments agree well for the three motors. Figure 23 shows the torque and power versus speed curves for the three motors at the prescribed rated conditions. It can be observed that the motors can be achieved the 30 Nm rated torque and 6.6 kW rated power. However, the power for the PMa-SynRM cannot maintain beyond 2100 rpm, while that for the IPMSMs can. Figure 24 shows the efficiency versus speed curves of the three motors at the rated condition. Note that the three motors were tested at different times for different projects so the measured speed ranges varied. The measured maximum efficiency for the double-layers IPMSM, V-shaped IPMSM and PMa-SynRM are 96.7%, 95.6%, and 93%, respectively. The largest error between the simulations and the experiments for the three motors is about 2.5% (based on the simulations). This indicates that the simulations and experiments agree well. The error can be attributed to some factors that are not considered in the simulations (e.g., mechanical losses). Figure 25 shows the peak torque of the IPMSMs and PMa-SynRM at their own base speeds (Table1), where all of the motors can achieve the maximum torque with the maximum current excitation. The comparison in Figure 25 also shows that the FEA and experiments agree well. Unlike the thermal imaging method that is presented in [14,15], this paper employed the thermocouples for temperature measurement of the double-layers IPMSM when considering facility availability, as the test setup shown in Figure 26. To ensure the safety in the experiments, only a light load is applied in simulation and measurement with a 34 Arms current, 1800 rpm speed, and 15 Nm torque for 10 min operation. Figure 27 shows the simulation model and some sample temperatures in different parts of the motor. Figure 28 shows the comparison between the measurement and simulation for the motor housing within 10 min operation. Both cases agree well. Although the above experiments are limited by the laboratory facilities, the accuracy of the simulations is still properly validated. Thus, the simulations that are presented in this paper should be sufficiently reliable. Energies 2018, 11, 1385 19 of 24 EnergiesEnergies 20120188,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 1919 ofof 2424

FigureFigure 21.2211.. PrototypesPrototypes of of IPMSMs IPMSMs and and PMa PMa-SynRM:PMa--SynRM:SynRM: ( (aa))) sta statorstatortor of of double double-layersdouble--layerlayerss IPMSM IPMSM with winding winding,winding,, ((bb)) statorstator ofof V-ShapedVV--ShapedShaped IPMSMIPMSM withoutwithout winding,windingwinding,, ((cc)) statorstator ofof PMa-SynRMPMaPMa--SynRMSynRM withwith winding,winding, ((dd)) rotorrotor ofof doubledouble-layersdouble--layerslayers IPMSM IPMSMIPMSM with with with PM, PM, PM, (e )((e rotore)) rotor rotor of V-shaped of of V V--shapedshaped IPMSM IPMSM IPMSM without without without PM, and PM, PM, ( fandand) rotor ((ff)) of rotorrotor PMa-SynRM of of PMa PMa-- withoutSynRMSynRM withoutwithout PM. PMPM..

Figure 22. Comparison of back electromotive force (BEMF) of IPMSMs and PMa-SynRM. Figure 22.22. Comparison of back electromotive forceforce (BEMF)(BEMF) ofof IPMSMsIPMSMs andand PMa-SynRM.PMa-SynRM.

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Figure 23. Torque-power vs speed curve at rated condition: (a) double layers IPMSM, (b) V-Shaped Figure 23. Torque-power vs speed curve at rated condition: (a) double layers IPMSM, (b) V-Shaped IPMSM, and (c) PMa-SynRM. IPMSM, and (c) PMa-SynRM.

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Figure 24. Efficiency vs speed curve at rated condition: (a) double layers IPMSM, (b) V-Shaped Figure 24. Efﬁciency vs speed curve at rated condition: (a) double layers IPMSM, (b) V-Shaped IPMSM, IPMSM, and (c) PMa-SynRM. and (c) PMa-SynRM.

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Figure 25. Torque comparison of IPMSMs and PMa-SynRM. Figure 25. Torque comparison of IPMSMs and PMa-SynRM. Figure 25. Torque comparison of IPMSMs and PMa-SynRM. Figure 25. Torque comparison of IPMSMs and PMa-SynRM.

Figure 26. Temperature measurement setup for the double layer IPMSM. Figure 26. Temperature measurement setup for the double layer IPMSM. FigureFigure 26. 26.Temperature Temperature measurement measurement setup for for the the double double layer layer IPMSM. IPMSM.

Figure 27. Thermal simulation model and temperature for the double layer IPMSM. Figure 27. Thermal simulation model and temperature for the double layer IPMSM. Figure 27. Thermal simulation model and temperature for the double layer IPMSM. Figure 27. Thermal simulation model and temperature for the double layer IPMSM.

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FigureFigure 28. 28. TemperatureTemperature Measurement Measurement of of double double layer layer IPMSM. IPMSM.

10. Conclusions 10. Conclusions This paper has investigated the characteristics of IPMSMs and PMa-SynRM in terms of This paper has investigated the characteristics of IPMSMs and PMa-SynRM in terms of electromagnetic and thermal performance via driving cycles for EV applications. It is found that the electromagnetic and thermal performance via driving cycles for EV applications. It is found that PMa-SynRM has an advantage in high speed operation for its high efficiency, despite insufficient the PMa-SynRM has an advantage in high speed operation for its high efficiency, despite insufficient torque. Instead, the IPMSMs possess better efficiency at low speed for the urban driving cycle. The torque. Instead, the IPMSMs possess better efficiency at low speed for the urban driving cycle. thermal analysis showed that the IPMSMs possess better performance than the PMa-SynRM subject The thermal analysis showed that the IPMSMs possess better performance than the PMa-SynRM subject to temperature limits. In addition, the analysis results show that the operating range of the motors to temperature limits. In addition, the analysis results show that the operating range of the motors can can be limited if proper cooling is not employed. Therefore, to achieve the same required be limited if proper cooling is not employed. Therefore, to achieve the same required performance, performance, the traction motors should be designed differently, according to the conditions with or the traction motors should be designed differently, according to the conditions with or without cooling without cooling systems and the cooling capacity. The analysis in this paper has demonstrated the systems and the cooling capacity. The analysis in this paper has demonstrated the advantages and advantages and disadvantages of the IPMSMs and PMa-SynRM for EV tractions and the suitability disadvantages of the IPMSMs and PMa-SynRM for EV tractions and the suitability of these motors for of these motors for specific driving cycles. Suggestions have also been provided for the design specific driving cycles. Suggestions have also been provided for the design improvement of traction improvement of traction motors for future study. Three prototypes were fabricated and the motors for future study. Three prototypes were fabricated and the measurement validates the analysis measurement validates the analysis in this paper. in this paper.

AuthorAuthor Contributions: Contributions: ThanhT.A.H. Anh conceived Huynh and conceived conducted and theconducted research the and research wrote theand paper.wrote the M.-F.H. paper suggested. Min-Fu Hsiehthe research suggested topic, the guided research T.A.H. topic, to guided complete Thanh the research Anh Huynh and helpedto complete edit and the ﬁnalizeresearch the and paper. helped M.-F.H. edit and also finalizeprovided the the paper. laboratory Min-Fu space Hsieh and also facilities provided for the this laboratory study. space and facilities for this study.

Acknowledgments:Acknowledgments: ThisThis work work was was in in part part supported supported by by Taiwan’s Taiwan’s Ministry Ministry of of Science Science and and Technology Technology under under contracts MOST 106-2622-8-006-001 and 106-2218-E-006-023. The authors would like to thank Jian-Shin Lai, contracts MOST 106-2622-8-006-001 and 106-2218-E-006-023. The authors would like to thank Jian-Shin Lai, Ping- Ping-Yuan Wang and the lab-mates for the assistance in experiments. Yuan Wang and the lab-mates for the assistance in experiments. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Ruiwu, C.; Chris, M.; Ming, C. Quantitative Comparison of Flux-Switching Permanent-Magnet Motors 1. Ruiwu, C.; Chris, M.; Ming, C. Quantitative Comparison of Flux-Switching Permanent-Magnet Motors with Interior Permanent Magnet Motor for EV, HEV, and PHEV Applications. IEEE Trans. Magn. 2012, 48, with Interior Permanent Magnet Motor for EV, HEV, and PHEV Applications. IEEE Trans. Magn. 2012, 48, 2374–2384. [CrossRef] 2. 2374Won,–2384, H.; Hong, doi:10.1109/TMAG.2012.2190614. Y.K.; Lee, W.; Choi, M. Roles of coercivity and remanent flux density of permanent magnet

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