Thermal Mapping of a High-Speed Electric Motor Used for Traction Applications and Analysis of Various Cooling Methods—A Review

energies

Review Thermal Mapping of a High-Speed Electric Motor Used for Traction Applications and Analysis of Various Cooling Methods—A Review

Edison Gundabattini 1,* , Arkadiusz Mystkowski 2 , Adam Idzkowski 2 , Raja Singh R. 3 and Darius Gnanaraj Solomon 4

1 Department of Thermal and Energy Engineering, Vellore Institute of Technology (VIT), School of Mechanical Engineering, Vellore 632 014, Tamilnadu, India 2 Faculty of Electrical Engineering, Bialystok University of Technology, Wiejska 45D, 15 351 Bialystok, Poland; [email protected] (A.M.); [email protected] (A.I.) 3 Department of Energy and Power Electronics, Vellore Institute of Technology (VIT), School of Electrical Engineering, Vellore 632 014, Tamilnadu, India; [email protected] 4 Department of Design and Automation, Vellore Institute of Technology (VIT), School of Mechanical Engineering, Vellore 632 014, Tamilnadu, India; [email protected] * Correspondence: [email protected]

Abstract: This paper gives a comprehensive review of advanced cooling schemes and their applications to the permanent magnet synchronous motors (PMSMs), as well as investigating the electrical motor’s topologies its thermal design issues, materials and performances. Particularly, the electromagnetic and electric performances, machine sizing, together with the structural design, are given. In addition, the work addresses the motor’s material design and properties along with its insulation Citation: Gundabattini, E.; performance, which is the main goal of optimization. Mainly, thermal mapping with analysis is Mystkowski, A.; Idzkowski, A.; R., provided according to the different cooling methods, including air-cooling, water-cooling, oil-cooling, R.S.; Solomon, D.G. Thermal heat-pipe-cooling, potting silicon gelatin cooling, and as well as cooling strategies for tubes and Mapping of a High-Speed Electric microchannels. The most common special features and demands of the PMSMs are described in Motor Used for Traction Applications and Analysis of Various Cooling the appearance of the motor’s failures caused by uncontrolled temperature rise. In addition, heat Methods—A Review. Energies 2021, sources and energy losses, including copper loss, core loss versus motor speed, and output power, are 14, 1472. https://doi.org/10.3390/ analyzed. The review of the proposed cooling methods that will achieve the required heat transfer of en14051472 the PMSM is presented with numerical simulations and measurements data. A review of numerical methods and results, including the finite element methods (FEM), such as the Ansys CFD software, to Academic Editor: Won-Ho Kim obtain a high-accuracy thermal mapping model of the PMSM system is given. The revived methods and design requirements due to PMSM temperature profile and cooling flow at different rotor speeds Received: 10 February 2021 and torque loads are investigated. Finally, the motor design recommendations, including the newly Accepted: 2 March 2021 developed cooling solutions, which enable it to effectively redistribute the temperature and heat Published: 8 March 2021 transfer, increasing the efficiency of the PMSM machine, are laid out.

Publisher’s Note: MDPI stays neutral Keywords: thermal mapping; traction motor; PMSM; thermal management; temperature analysis; with regard to jurisdictional claims in thermal design; thermally conductive materials; cooling scheme; temperature sensors published maps and institutional afﬁl- iations.

1. Introduction Electric vehicles are replacing conventional internal combustion engine vehicles, which Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. are an important part of meeting global goals on climate change. The propulsive force This article is an open access article created in the electric vehicle is due to the friction between the road and tire. Traction distributed under the terms and control in electric vehicles has gained tremendous attention because, relative to traditional conditions of the Creative Commons internal combustion engines, traction control can generate very rapid and reliable torques, Attribution (CC BY) license (https:// improving drive performance, protection and stability [1]. The traction motor is the core creativecommons.org/licenses/by/ of electric vehicles, converting electricity into mechanical energy or vice versa, thereby 4.0/). interfacing the battery source with the wheels of the vehicle. Traction motors are to be

Energies 2021, 14, 1472. https://doi.org/10.3390/en14051472 https://www.mdpi.com/journal/energies Energies 2021, 14, 1472 2 of 32

designed to realize up to five times the base speed, with a great torque-power density and good efficiency map for highway cruising and hill climbing. For multi-quadrant operation and multiple-motor synchronization, traction motors require good controllability, greater steady-state precision, and the best dynamic efficiency [2]. In addition, the traction motor should have low acoustic noise, and greater consistency and strength for the vehicular setting. The high-speed electric motor achieves the mentioned traction requirement. The typical advantages of high-speed traction motors include higher efficiencies, more compact- ness, and low cost due to reduced mechanical transmission. However, there are various electrical, thermal and mechanical limitations in high-speed traction motors [3]. The maximum speed of a system is constrained by the centrifugal force and critical speed-related elastic instability. On the other side, the stator and rotor temperature is restricted, and thus the peak power, maximum power, and constant power can be maintained [4]. In general, high speeds also imply high voltages and fluxes, which are to be taken into account in electromagnetic as well as electrical design [5]. The mechanical limitations in high-speed electrical motors mainly include bearing friction, ventilation losses due to non-linear friction, and vibrations caused by hysteresis [6]. Similar to the mechanical stress on high-speed motor drives, the thermal stress influences the performance and life span of the system because of the higher loss of densities arising from a smaller surface area and volume [7]. The conventional method of optimization, considering only electromagnetic and mechanical factors for a given particular electrical loading, can no longer be taken into consideration in order to design a stable and optimal system, and the thermal design must be part of the machine design process [8]. Importantly, the thermal augmentation of an electric motor affects the core, insulation and permanent magnets, which deteriorates the machine’s performance and life span. Iron losses are the common term for loss occurrences associated with the presence of non-constant magnetic fields in laminated ferromagnetic materials [9]. The iron losses are caused by the magnetic hysteresis loop, termed hysteresis loss, and spatial eddy currents termed eddy current losses. The current redistribution generates internal eddy currents at higher frequencies due to higher speeds, which plays a critical role in iron losses. This happens because of ferromagnetic compounds that conduct electricity as well. It is easy to heat up the core material made of stacked iron, which implies the thin insulating coatings on the plate are destroyed. Due to high temperatures, the insulation across the conductors has its lifespan decreased; a constant over the temperature of 10 ◦C halves the existing lifetime [10,11]. Similarly, at a higher temperature, permanent magnets can be irreversibly demagnetized, and thus a temperature profile of the motor (thermal mapping) has to be estimated. Thermal mapping is estimating the temperatures of the components of an electric traction motor in particular. This is essential because the components are made of different materials and their individual thermal expansion may lead to thermo-mechanical stresses. Heating in the motor through copper, magnet, lamination and bearing frictional losses is a cause of concern. Thermal gradients set due to these heat sources could lead to stresses in the radial and axial directions, causing uneven deformations. As such, thermal mapping is a very important parameter to assess the performance of any electric motor [12]. Electric motor temperature variations in rotors, stators and windings are analyzed under constant motor torque and speed (80 Nm and 2500 rpm). This in general represents a situation wherein an EV has to move on an inclined surface. Researchers observed that the double layer interior permanent magnet motor works effectively over a wide range of speed and load. The motor temperature was observed to be under the safe limits, in spite of a temporary overload condition, as it is thermally stable, as given in the illustration in Figure1[13]. Energies 2021, 14, 1472 3 of 32 Energies 2021, 14, 1472 3 of 33

FigureFigure 1.1. RiseRise inin temperaturetemperature underunder instantaneousinstantaneous loadload scenarioscenario [13[13].].

TheThe accurateaccurate coolingcooling ofof thethe electricalelectrical motormotor isis notnot easy.easy.Most Mostof of thethe standardstandardelectrical electrical machinesmachines are are air-cooled, air-cooled, often often with with a ventilator a ventila mountedtor mounted on the on shaft, the withshaft, external with external cooling fins.cooling At higherfins. At speeds, higher thespeeds, motor the can motor be very can noisybe very and noisy take and up atake reasonable up a reasonable amount ofamount torque, of sotorque, it needs so it independent needs independent cooling. cooling. It is stillIt is verystill very tough tough and and complicated complicated to quantifyto quantify the the cooling cooling of of the the spinning spinning parts, parts, as as this this must must be be done done by by the the airflow airflow in in the theair air gapgap andand in the end-winding area. area. For For this this purpose, purpose, the the cooling cooling fins fins are are often often placed placed at the at theends ends of the of therotor rotor [14]. [14 The]. The cooling cooling system’s system’s challenge challenge is to is effectively to effectively eliminate eliminate this thisheat heatand retain and retain the temperature the temperature of the of motor the motor within within the specified the specified range, range,while reducing while reducing energy energyconsumption. consumption. In addition, In addition, in military in military ground ground vehicles, vehicles, it is itimportant is important to consider to consider the thedesign, design, the the requisite requisite reliability, reliability, and and stringent stringent operating operating conditions. Therefore,Therefore, smartsmart thermalthermal managementmanagement control control with with compact compact design design and and dynamic dynamic heat heat transfer transfer capability capability is essentialis essential for for traction traction motors motors [15 ].[15]. Based Based on theon powerthe power level, level, the operating the operating conditions conditions and theand system the system environment, environment, the cooling the cooling method me shouldthod beshould adapted. be Usually,adapted. the Usually, industrial the motorsindustrial are motors air-cooled, are whichair-cooled, cools thewhich machine cools throughthe machine natural through or forced natural convection or forced by employingconvection aby fan employing mounted ona fan its mounted shaft. However, on its shaft. this type However, of cooling this willtype not of cooling be suitable will fornot traction be suitable motors for due traction to the inadequatemotors due space to the for radialinadequate fins, thespace waterproof for radial design, fins, and the thewaterproof uncertain design, ambient and temperature the uncertain and ambient humidity. temperature Moreover, and for traction,humidity. the Moreover, high-power for densitytraction, motors the high-power are preferred, density which motors generate are preferred, much heat; which hence generate the selection much heat; of cooling hence mustthe selection be done of properly. cooling must be done properly. LiquidLiquid coolingcooling isis oneone ofof thethe mostmost efficientefficient coolingcooling approachesapproaches forfor tractiontraction motormotor drives which contain a pump, radiator, and assorted hoses. The heat is transferred by drives which contain a pump, radiator, and assorted hoses. The heat is transferred by the the coolant through heat pipes inside the motor and passed through the radiator. The coolant through heat pipes inside the motor and passed through the radiator. The effectiveness of liquid cooling has been expansively analyzed in [16]. Soparat et al. built effectiveness of liquid cooling has been expansively analyzed in [16]. Soparat et al. built a a liquid cooling system based on an electric motor to substitute a traditional fan-cooled liquid cooling system based on an electric motor to substitute a traditional fan-cooled configuration with enhanced cooling ability. Farsane et al. checked the efficiency of a configuration with enhanced cooling ability. Farsane et al. checked the efficiency of a liquid cooling system experimentally for an enclosed electric motor [17]. In addition to liquid cooling system experimentally for an enclosed electric motor [17]. In addition to device architecture, control theory has been applied to minimize the energy consumption device architecture, control theory has been applied to minimize the energy consumption of the thermal management system. In order to manage the electro-mechanical actuators of the thermal management system. In order to manage the electro-mechanical actuators (e.g., fans and pumps), Tao et al. introduced optimum control theory to preserve the (e.g., fans and pumps), Tao et al. introduced optimum control theory to preserve the temperature of the e-motor for a hybrid electric vehicle (HEV), as well as to reduce power temperature of the e-motor for a hybrid electric vehicle (HEV), as well as to reduce power consumption [18]. Moreno et al., by ultra-capacitors and neural networks, established andconsumption confirmed [18]. an energyMoreno storage et al., by system ultra-capacitors for the HEV and [19 neural]. The networks, real-time achievability established and of thermalconfirmed control an energy optimization storage algorithms system for in wiredthe HEV and autonomous[19]. The real-time hybrid electricachievability vehicles of hasthermal been investigatedcontrol optimization by Zhu et al.algorithms [20]. in wired and autonomous hybrid electric vehiclesAdvanced has been materials investigated with highby Zhu thermal et al. conductivity[20]. and cutting-edge electric motor coolingAdvanced structures materials were explored with high with thermal the development conductivity of and revolutionary cutting-edge manufacturing electric motor technologiescooling structures [21]. A were motor explored cooling systemwith the by development means of heat of pipes revolutionary was realized manufacturing in [22]. To ab- sorbtechnologies heat, the heat[21]. pipeA motor evaporator cooling was system implanted by me intoans of the heat motor pipes housing; was realized however, in the[22]. con- To denserabsorb washeat, mounted the heat in pipe a heat evaporator rejection cooling was implanted chamber. into A heat the pipe-based motor housing; thermal however, bus for motorthe condenser cooling was was developed mounted by in Shoai-Naini a heat reject et al.,ion with cooling system chamber. efficiency Astudied heat pipe-based for differ- entthermal drive bus cycles for [motor23]. A cooling motor refrigeration was developed system by Shoai-Naini with L-shaped et al., heat with pipes system was proposedefficiency bystudied for different drive cycles [23]. A motor refrigeration system with L-shaped heat

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pipes was proposed by Putra et al. [24]. Furthermore, new internal mechanisms of the Putramotor et that al. [benefit24]. Furthermore, from a device new cooling internal have mechanisms been studied. of the A motorpermanent that benefitmagnet from motor a deviceconcept cooling with a have hollow been shaft studied. that Aallows permanent coolan magnett movement motor to concept help facilitate with a hollow cooling shaft was thatproposed allows by coolant Lee et movemental. [21]. Air-cooling to help facilitate is simple cooling and provides was proposed a simple by foundation, Lee et al. [21 but]. Air-coolingthe cooling isefficiency simple and might provides not be a simplesatisfactory. foundation, In addition, but the a coolingcooling efficiency fan is normally might notattached be satisfactory. to the motor In addition, shaft, awhich cooling absorbs fan is normallyenergy and attached could to be the operated motor shaft, indirectly. which absorbsLiquid energycooling andis efficient, could be but operated operating indirectly. the coolant Liquid pump cooling and is efficient,radiator butfan operatingconsumes theelectricity. coolant On pump the andother radiator hand, owing fan consumes to the cooling electricity. sides, Onliquid the cooling other hand,adds additional owing to theweight cooling and sides, complexity. liquid coolingHeat pipes adds can additional work passively weight and with complexity. the temperature Heat pipes gradient; can worknevertheless, passively there with theare temperature heat transmission gradient; nevertheless,restrictions because there are heatof the transmission operating restrictionstemperatures, because fluid of properties, the operating capillary temperatures, limit, etc. fluid [25]. properties, capillary limit, etc. [25]. TheThe proposed proposed review review design design is is shown shown in in Figure Figure2; this 2; this article article predominantly predominantly focuses focuses on thermalon thermal design design approaches approaches that were that appliedwere appl in differentied in different kinds of kinds ultra-high-speed of ultra-high-speed traction motors.tractionIt motors. also discusses It also discusses electromagnetic electromagne failure analysistic failure due analysis to heat due generation to heat generation at various hotat spotsvarious of thehot motor spots and of coolingthe motor flow fluidand dynamics.cooling flow We presentfluid dynamics. a comprehensive We present review a ofcomprehensive electromagnetic review functional of electromagnetic failure processes functional and the failure directions processes ofheat and transmissionthe directions insideof heat the transmission system. This inside article the outlines system. and This evaluates article theoutlines strengths and andevaluates limitations the strengths of these thermaland limitations design methodologies of these thermal through design loss methodologies calculation and through life estimation loss calculation that would and affect life theestimation performance that would of the motor.affect the Finally, performanc the improvede of the and motor. successful Finally, ways the ofimproved conducting and traditionalsuccessful coolingways of methodsconducting were traditional taken into cooling account. methods were taken into account.

FigureFigure 2.2.Proposed Proposed reviewreview perception.perception.

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2. Topologies of Electrical Motors 2. TopologiesThere are ofseveral Electrical categories Motors and groupings of electric motors, such as DC and AC motors,There linear are and several rotating categories motors, and synchr groupingsonous ofand electric asynchronous motors, such(induction) as DC andmotors AC [26].motors, The linearmost andcommon rotating motor motors, topologies synchronous used andin electric asynchronous vehicles (induction) (EVs) are motorsinduction [26]. machinesThe most common(IM), permanent motor topologies magnet used synchr in electriconous vehiclesmachines (EVs) (PMSM) are induction and switched machines reluctance(IM), permanent machines magnet (SRM). synchronous Comparing different machines topologies, (PMSM) andinterior switched PM (IPM) reluctance motors ma-for passengerchines (SRM). car applications Comparing differenthave higher topologies, overload interior capability PM (IPM) and motorsefficiency for than passenger IM and car surface-mountedapplications have PM higher (SPM) overload machines capability [27]. and efficiency than IM and surface-mounted PMLastly, (SPM) machinesthe traditional [27]. surface inset PM synchronous machines (SIPMSM) and the novel Lastly,SIPMSM the have traditional been trial-manufactured surface inset PM and synchronous compared machines[28]. The permanent (SIPMSM) magnet and the innovel the novel SIPMSM SIPMSM have been is a trial-manufacturedmagnetic pole of unequal and compared thickness [28 with]. The different permanent inner magnet and outerin the radians, novel SIPMSM which results is a magnetic in the poleuneven of unequal distribution thickness of the with radial different air-gap inner flux and density outer andradians, a remarkable which results magnetic in the congregate uneven distribution effect. of the radial air-gap flux density and a remarkableIn addition, magnetic different congregate approaches effect. have been discussed for an AC armature employing conventionalIn addition, copper different coils on approaches the stator. have Supe beenrconducting discussed AC for homopolar an AC armature motors employing for high- speedconventional applications, copper which coils on employ the stator. a Superconductinghigh-temperature AC superc homopolaronductor motors (HTS) for high-DC excitationspeed applications, coil, are analyz whiched employ and constructed a high-temperature [29]. superconductor (HTS) DC excitation coil,In are recent analyzed years, and the constructed consideration [29]. of the lack of rare-earth PMs compared to the PMSMIn has recent attracted years, theresearchers consideration to study of the synchronous lack of rare-earth reluctance PMs compared motors (SynRM) to the PMSM for tractionhas attracted applications researchers [30]. toAnother study synchronousmotor type is reluctance the permanent motors magnet (SynRM) synchronous for traction reluctanceapplications motor [30]. (PMSynRM), Another motor which type benefits is the permanentfrom a ferrite-magnet. magnet synchronous The performance reluctance of PMSynRMmotor (PMSynRM), is competitive which benefitswith PMSM, from a ferrite-magnet.and is attracting The performancethe attention of PMSynRMof motor manufacturers.is competitive with PMSM, and is attracting the attention of motor manufacturers. DCDC motors, motors, suchsuch asas brush brush (BDC) (BDC) motors motors or brushless or brushless (BLDC) (BLDC) motors, motors, are more are suitable more suitablefor low-power for low-power and low-speed and low-speed applications, applications, such as electric such as carts. electric A general carts. overviewA general of overviewelectrical of motor electrical topologies motor istopologies presented is in presented Figure3. in Figure 3.

FigureFigure 3. 3. ElectricalElectrical motors motors and and the the selected selected topologies topologies used used in in electric electric vehicles vehicles (EVs). (EVs).

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Advanced electric machine topologies, such as stator permanent magnet (stator-PM) motors [31], hybrid-excitation motors [32,33], ﬂux memory motors and redundant motor structures [34], are currently considered in electric motor design and manufacturing.

3. Design Analysis of Electrical Motors The design principles represent the accumulated wisdom of researchers, designers and related fields. Many common aspects, which include the physical properties, cost and availability of materials, and the ease of manufacturing, are involved. The design goals are also numerous, i.e., efficiency, torque, back-electromotive force (EMF) waveform, cost, minimum weight, reliability regarding high temperature and mechanical stress. Therefore, some broad design approaches could be mentioned in terms of diverse fields of knowledge. For making a computer-aided design (CAD) and an analysis of electric motors, some commercial and high-cost electromagnetic tools, such as the Ansys Motor-CAD, COMSOL Multiphysics, as well as free non-commercial software, are available. The majority of software is only for 2D problems. If 3D electromagnetic simulation is needed, then the choice is among COMSOL or Ansys software. The comparison of the main features is presented in Table1. Ansys Motor-CAD packages contain permanent magnet-based electric machines, asynchronous electric machines with an induced rotor field, and synchronous electric machines without permanent magnets. COMSOL Multiphysics can be used for designing permanent magnet motors, brushless DC motors, and induction motors. MotorAnalysis currently supports induction motors and generators, permanent magnet synchronous motors (PMSM) and generators, and brushless DC (BLDC) machines. SyR-e (synchronous reluctance–evolution) is an open source tool for synchronous reluctance SynRM, PMSynRM as well as permanent magnet SPM and IPM machine designs. It requires Matlab/Octave and Finite Element Method Magnetics (FEMM) software installed. As an example, in publi- cation [35], a design methodology and a design flowchart using a free Motor Analysis-PM software are explored and analyzed for a 50 kW permanent magnet (PM) motor, employed in an industrial electrical vehicle (Toyota Prius).

3.1. Electromagnetic and Electrical Performance The working principle of electrical motors is grounded in electromagnetic theory. The electromagnetic design relies on computing several electromagnetic and electric parameters using numerical and analytical methods. In our study, the calculation of magnetic flux density distribution and current density in the electrical machine is executed (see Figure4). For this purpose, the analytical methods [36,37], as well as the 2D or 3D finite element (FE) numerical methods, are used [38–40]. Numerical methods, i.e., a magneto-static FE analysis and a transient FE analysis, are performed. The analytical methods are based on models in D-Q domain (direct axis and quadrature axis). They are symbolized by a steady-state D-Q analysis and a dynamic D-Q analysis. The time plots, air gap distribution plots or cross-section distribution plots (maps) of the selected quantities can be obtained by performing a magnetostatic FE analysis. A combination of analytical and numerical methods is used in the software to obtain steady-state performance characteristics (torque– rotation speed) and efficiency maps, including field-weakening operations [35]. The numerical data are calculated from the FE analysis solution, including voltages, inductances, torque ripple percentage, motor constants, short circuit current, and power factor. Thus, power losses and efficiency can be analyzed. The power losses mostly consist of mechanical loss, core loss, stray loss and copper loss [40]. Energies 2021, 14, 1472 7 of 32

Table 1. The comparison of motor design software.

Ansys Comsol Motor Wizard Motor Altair Features Motor-CAD Multiphysics Solidworks Solve Flux Motor Geometry templates Yes Yes Yes Yes Yes dxf, dwg, Import CAD files dxf dxf, dwg dxf dxf sldprt Automatic winding Yes Yes Yes Yes Yes patterns generation Customize and select materials Yes Yes Yes Yes Yes Magnetostatic fields analysis Yes Yes Yes Yes Yes Transient electromagnetic Yes Yes Yes Yes Yes field analysis 3D electromagnetic simulations Yes Yes No No No Analytical analysis Yes Yes Yes Yes Yes Parametric simulation Yes Yes Yes Yes Yes Motor characteristic curves Yes Yes Yes Yes Yes Efficiency maps Yes Yes Yes Yes Yes Free software No No No No No Motor JMAG-Express Features FEMM SyR-e Emetor Analysis Online Geometry templates No Yes Yes Yes Yes Import CAD files dxf dxf dxf No No Automatic winding No Yes Yes Yes Yes patterns generation Customize and select materials Yes Yes Yes Yes Yes With XFEMM Magnetostatic fields analysis Yes Yes No Yes support Transient electromagnetic With With XFEMM Yes No No field analysis scripting support 3D electromagnetic simulations No No No No No Analytical analysis No Yes Yes Yes No Parametric simulation No No Yes Yes No With With XFEMM Motor characteristic curves Yes Yes No scripting support With scripting Efficiency maps No Yes Yes No out of GUI Free licence software Yes Yes Yes Yes Yes

3.2. Thermal Design and Machine Sizing The purpose of thermal design is to obtain the thermal distribution in a motor. To this aim, all heat transfer paths must be considered in the network, and appropriate algorithms should be applied to estimate the thermal resistances for such paths. An example is a commercial software package Motor-CAD, which uses the lumped parameter thermal network scheme [41]. It is based on motor geometry and cooling types selected by the user. The operating temperatures can vary in different locations. This can also have an inﬂuence on the electromagnetic performance of the electric motor. This investigation is central in a motor design procedure to precisely envisage demagnetization, insulation failure and cooling performance. An example is the IPM motor, wherein the permanent magnets may fail if the estimated temperature is greater than 150 ◦C due to the demagnetization effect. Therefore, the complete analysis requires a coupling of electromagnetic and thermal calculations. This helps to ﬁnd the structural design for removing heat generated inside the motor to the outside, with a minimum heat transfer and radiation area [42]. Energies 2021, 14, 1472 7 of 33

JMAG- Motor Features FEMM SyR-e Express Emetor Analysis Online Geometry templates No Yes Yes Yes Yes Import CAD files dxf dxf dxf No No Automatic winding patterns No Yes Yes Yes Yes generation Customize and select Yes Yes Yes Yes Yes materials With Magnetostatic fields analysis Yes Yes XFEMM No Yes support With Transient electromagnetic With Yes XFEMM No No field analysis scripting support 3D electromagnetic No No No No No simulations Analytical analysis No Yes Yes Yes No Parametric simulation No No Yes Yes No With With Motor characteristic curves Yes XFEMM Yes No scripting support With Efficiency maps No Yes scripting out Yes No of GUI Free licence software Yes Yes Yes Yes Yes

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FigureFigure 4. 4. MagneticMagnetic fluxflux densitydensity (T) (T) and and stator stator current current density density (A/m (A/m2) distributions2) distributions for the for surface-mounted the surface- mountedpermanent permanent magnet (SPM) magnet motor (SPM) (power motor 10 kW, (power speed 10 2500 kW, rpm, speed RMS 2500 supply rpm, current RMS supply 208 A). current 208 A). The heat network method and the finite element method are two common methods thatFor this can purpose, be used the for analytical thermal analysis. methods In [36,37], this way, as well the as thermal the 2D mapsor 3D finite of magnet element (or (FE)winding) numerical temperature methods, can are be used created. [38–40]. This Numerical is important meth whenods, considering i.e., a magneto-static the process FE of analysismachine and sizing, a transient as given FE in analysis, Figure5. Theare performed. machine stack The length analytical can bemethods analyzed are under based the on modelsmaximum in D-Q temperature domain (direct limit. axis For and each quadrature cooling type, axis). the They winding are symbolized (or magnet) by a tempera- steady- stateture canD-Q be analysis simulated and fora dynamic each defined D-Q analysis. point of powerThe time (copper plots, andair gap core) distribution losses and plots stack orlength cross-section combination. distribution plots (maps) of the selected quantities can be obtained by performing a magnetostatic FE analysis. A combination of analytical and numerical methods3.3. Structural is used (Mechanical) in the software Design to obtain steady-state performance characteristics (torque–rotationThe structural speed) design and considers efficiency machine maps, including stresses and field-weakening strains. The analysis operations methods [35]. Thefor structuralnumerical designdata are are calculated spin analysis, from static the FE stress analysis analysis, solution, fatigue including analysis andvoltages, rotor inductances,dynamics analysis torque [ripple43]. The percentage, commonly moto usedr constants, material short properties circuit in current, stress andand fatiguepower factor.analyses Thus, are power density, losses yield and strength, efficiency ultimate can be tensile analyzed. strength, The power Young’s losses module, mostly Poisson’s consist ofratio, mechanical and stress–life loss, core (S-N) loss, curve. stray A loss precise and stresscopper analysis loss [40]. identifies the locations where the stresses are maximum and where consequent fatigue analysis could be explored. Fatigue 3.2.performance Thermal Design depends and Machine on raw Sizing material form, material orientation, machining process, surfaceThe finish, purpose surface of thermal hardness design and final is to coatings. obtain the The thermal result of distribution the simulation in a is motor. the life To in this aim, all heat transfer paths must be considered in the network, and appropriate algorithms should be applied to estimate the thermal resistances for such paths. An example is a commercial software package Motor-CAD, which uses the lumped parameter thermal network scheme [41]. It is based on motor geometry and cooling types selected by the user. The operating temperatures can vary in different locations. This can also have an influence on the electromagnetic performance of the electric motor. This investigation is central in a motor design procedure to precisely envisage demagnetization, insulation failure and cooling performance. An example is the IPM motor, wherein the permanent magnets may fail if the estimated temperature is greater than 150 °C due to the demagnetization effect. Therefore, the complete analysis requires a coupling of electromagnetic and thermal calculations. This helps to find the structural design for removing heat generated inside the motor to the outside, with a minimum heat transfer and radiation area [42]. The heat network method and the finite element method are two common methods that can be used for thermal analysis. In this way, the thermal maps of magnet (or winding) temperature can be created. This is important when considering the process of machine sizing, as given in Figure 5. The machine stack length can be analyzed under the

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maximum temperature limit. For each cooling type, the winding (or magnet) temperature thecan number be simulated of cycles for each to failure, defined which point is of estimated power (copper under and the givencore) losses load torque and stack variation length andcombination. material properties.

FigureFigure 5.5. FlowFlow diagramdiagram ofof classicclassic sizingsizing methodologymethodology usedused inin thethe designingdesigning ofof electricelectric motors.motors.

3.3. StructuralDynamics (Mechanical) and vibration Design analyses are other issues in motor design. The electromagnetic analysisThe is structural executed design on the motorconsiders and machine the vibration stresses loads and from strains. this analysisThe analysis are used methods in a structuralfor structural analysis design [44 ].are This spin approach analysis, is usedstatic for stress the optimizationanalysis, fatigue of magnetic analysis torque and rotor and fordynamics reducing analysis the noise [43]. radiated The commonly by the motor. used Thematerial mechanical properties causes in ofstress the unbalancedand fatigue magneticanalyses are forces density, (UMF) yield are mainlystrength, induced ultimate by tensile the air strength, gap eccentricity Young’s between module, the Poisson’s stator andratio, the and rotor. stress–life The electromagnetic (S-N) curve. A causes precise can stress be condensed analysis identifies into four categories:the locations winding where topologythe stresses asymmetry, are maximum magnetization and where unevenness, conseque shortnt fatigue circuit analysis and open could circuit. be Normally,explored. theFatigue air gap performance flux and the depends electromagnetic on raw force material dissemination form, material are even orientation, and proportioned. machining A short circuit in the rotor or stator slot can change the magnetic flux of the air-gap. Thereby, process, surface finish, surface hardness and final coatings. The result of the simulation is the UMF is generated, leading to radial vibration. the life in the number of cycles to failure, which is estimated under the given load torque 3.4.variation Material and Design material properties. Dynamics and vibration analyses are other issues in motor design. The Selecting a proper material is an important issue while creating the electromagnetic, electromagnetic analysis is executed on the motor and the vibration loads from this thermal and structural designs of electrical machines. Motors are typically made from analysis are used in a structural analysis [44]. This approach is used for the optimization silicon steels. Another group of materials is the powder metal magnetic materials, which of magnetic torque and for reducing the noise radiated by the motor. The mechanical can be classified as either sintered (for DC applications) or soft magnetic composite (SMC) causes of the unbalanced magnetic forces (UMF) are mainly induced by the air gap types (for AC applications). The SMC, amorphous and nanocrystalline materials show eccentricity between the stator and the rotor. The electromagnetic causes can be better characteristics, such as high saturation flux density and low losses [45]. Besides condensed into four categories: winding topology asymmetry, magnetization this, the newly developed metal amorphous nanocomposite materials (MANCs) are a class unevenness, short circuit and open circuit. Normally, the air gap flux and the of soft magnetic materials that are resourceful at converting energy at high frequencies, allowingelectromagnetic smaller force motors dissemination to provide similar are even power and[ 46proportioned.]. They can beA usedshort for circuit designing in the motorsrotor or with stator new slot topologies, can change greater the efficacymagnetic and flux lower of the manufacturing air-gap. Thereby, cost. the UMF is generated, leading to radial vibration. 3.5. Energy Management System Design The powertrain transfers the mechanical energy of the motor to the wheels of the electric vehicle. The powertrain constituents of an EV are the battery pack, on-board charger, DC–AC converters, controller, and motor. The power transfer is managed by the electronic control unit (ECU) that regulates the frequency and magnitude of the voltage

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delivered to the electric motor in order to manage the speed and acceleration, which are communicated with the use of acceleration/brake pedals. The ECU can reverse the direction of rotation of the motor for the reverse movement of the vehicle. In addition, the ECU can control the conversion of kinetic energy into electricity and recharge the battery during braking. The communication between different ECUs in a vehicle is usually carried out via CAN protocol. The aim of energy management systems (EMS) in electric vehicles is to raise battery life by enhancing thermal stability, and to distribute the optimum power among the powertrain constituents. Some reviews on EMS in EV can be found in [47]. More advanced systems, e.g., a multi-stage energy management system (MS-EMS) for a hybrid electric vehicle (HEV), are designed and simulated. This research aims to minimize the operating costs of a multi-source HEV. The properly optimized MS-EMS deﬁnes the most appropriate power split among the remaining energy sources due to their availability and operating constraints [48].

4. Insulation Materials and Performance Next-generation electric machines are in pursuit of identifying electrically insulating polymer materials. In order to achieve a better cooling performance and thereby increase the power density and torque density, those insulating materials have to have a good “k”, or thermal conductivity. Secondly, to compensate for the stresses developed by the high temperatures caused by the hot spots of the motor, those insulating materials must also have a good “Y”, elastic modulus. In fact, the next-generation electronic devices are also seeking a similar kind of material with a good “k” and “Y”. Extremely high thermal conductivity and elastic modulus are shown by carbon nanotubes (CNTs). For better performance, multiwalled CNT (MWCNT)/polyamide-6 (PA6)/poly(p-phenylene sulﬁde) (PPS) composites, having great “k”, electrical insulation and “Y”, were designed and fabricated. Nano-sized PPS domains (MWCNT-PPS nano domain-linked structure) were used to cap the MWCNT ends to thwart electrically conductive tracks from developing. MWCNT-PPS nano domain-linked conﬁgurations were evenly disseminated in the PA6 matrix, for boosted elastic modulus and heat resistance, thermal conductivity (k) and Energies 2021, 14, 1472 electrically insulating properties. Composites of this kind are highly capable of electrically11 of 33 insulating materials for the great output of electric motors [49]. Figure6 shows the materials used for insulation in electric motors.

FigureFigure 6. AA survey survey of of different different materials used for insulation in electric machines.

Nano-fillersNano-ﬁllers are mixedmixed withwith epoxy epoxy resin resin for usingfor using the nanocompositethe nanocomposite as a dielec-as a tric/insulator in power apparatus. SiO and TiO are mixed in appropriate proportions dielectric/insulator in power apparatus. SiO2 2 and TiO2 2 are mixed in appropriate proportions with epoxy epoxy resin, resin, and and divergent divergent electric electric fields ﬁelds are areused used in experimentation. in experimentation. Classical Classical tests, suchtests, as such switching as switching impulse impulse tests, lightning tests, lightning impulse impulse tests, and tests, DC and voltage DC voltage and AC and power AC frequencypower frequency tests, are tests, carried are carried out on out epoxy on epoxy dielectrics. dielectrics. A Anon-classi non-classicalcal tests, tests, the the highhigh- frequencyfrequency high high voltage voltage breakdown breakdown test, test, is is carri carrieded out to assess the possibility of using the nanocomposite in high-speed switching devices. The nanocomposites are found to have nanocomposite in high-speed switching devices. The nanocomposites are found to have better voltage endurance characteristics and may function as a credible shielding substance for power apparatus [50]. Epoxy resin was doped with SiO2 particles to fabricate the insulation used in high-voltage motors. At different temperatures, AC breakdown strength was measured. Conductivity and surface morphology tests were conducted. It was shown that the doped nano-SiO2 particle in dielectric composite improved electrical breakdown strengths, and this composite is suitable for insulating high-voltage motors [51]. A carbon fiber magnesium oxide (CF–MgO) hybrid was developed, since it is thermally conductive but electrically insulating, as the filler for the polymer mix. When this filler was introduced into Nylon6, the highest thermal conductivity was obtained. The insulation was found to be higher with a 10 wt. % addition of the hybrid fiber [52]. The mica-epoxy-based composite has compatible thermal characteristics and insulation properties. The thermal characterization of mica composite; its components, zinc naphthenate, epoxy resin, anhydride methylhexahydrophtalic and mica tape, are provided. The intrinsic properties of this composite give a great thermal and electrical performance [53].

5. Thermal Mapping and Analysis Thermal mapping is the scientifically proven technique for creating accurate temperature profiles of the machine. Thermal mapping helps designers to measure the temperature of the motor components under steady-state and intermittent operating conditions for the precise modeling of thermal behavior. Through mapping, it is possible to detect the components that are likely to heat up and deploy the tools to sustain the system under secure operating conditions. Understanding the key paths of heat transfer offers possibilities for motor manufacturers to dramatically increase the power performance of motor efficiency and make design decisions. As the automotive sector begins to migrate toward more electrically dominated propulsion systems, thermal management is crucial for electric motors. The complexities associated with thermal management for electric motors are increasing, aiming to minimize the size of the component, and reduce weight and costs without compromising reliability and efficiency. An optimized thermal architecture will help to significantly increase the rated capacity of the unit, almost without any increase in its production costs. The thermal control of

Energies 2021, 14, 1472 11 of 32

better voltage endurance characteristics and may function as a credible shielding substance for power apparatus [50]. Epoxy resin was doped with SiO2 particles to fabricate the insulation used in high-voltage motors. At different temperatures, AC breakdown strength was measured. Conductivity and surface morphology tests were conducted. It was shown that the doped nano-SiO2 particle in dielectric composite improved electrical breakdown strengths, and this composite is suitable for insulating high-voltage motors [51]. A carbon fiber magnesium oxide (CF–MgO) hybrid was developed, since it is thermally conductive but electrically insulating, as the filler for the polymer mix. When this filler was introduced into Nylon6, the highest thermal conductivity was obtained. The insulation was found to be higher with a 10 wt. % addition of the hybrid fiber [52]. The mica-epoxy-based composite has compatible thermal characteristics and insulation properties. The thermal characterization of mica composite; its components, zinc naphthenate, epoxy resin, anhydride methylhexahydrophtalic and mica tape, are provided. The intrinsic properties of this composite give a great thermal and electrical performance [53].

5. Thermal Mapping and Analysis Thermal mapping is the scientifically proven technique for creating accurate temperature profiles of the machine. Thermal mapping helps designers to measure the temperature of the motor components under steady-state and intermittent operating conditions for the precise modeling of thermal behavior. Through mapping, it is possible to detect the components that are likely to heat up and deploy the tools to sustain the system under secure operating conditions. Understanding the key paths of heat transfer offers possibilities for motor manufacturers to dramatically increase the power performance of motor efficiency and make design decisions. As the automotive sector begins to migrate toward more electrically dominated propulsion systems, thermal management is crucial for electric motors. The complexities associated with thermal management for electric motors are increasing, aiming to minimize the size of the component, and reduce weight and costs without compromising reliability and efficiency. An optimized thermal architecture will help to significantly increase the rated capacity of the unit, almost without any increase in its production costs. The thermal control of electric motors is a dynamic task, because of the multiple thermal transfer pathways within the motor, i.e., multiple materials and thermal interfaces, from which the heat must travel to be expelled. There are various methods for obtaining the thermal information from the machines, such as the infrared thermography (contactless) method and thermistor (contact) method [54–56].

5.1. Infrared Thermography The identification of infrared radiation emitted by the machine with a temperature above absolute zero is the basis of infrared thermography. This radiation is transformed by thermography into visible light, which results in a thermal image. The surface temperature field of the machine is mapped, which makes it possible to assess the strength of radiation. Thermal vision cameras allow the digital measuring of the temperature distribution of the test machine. This temperature map is graphically represented. Since all temperatures are allocated a different color, the thermal picture of the machine is seen in the viewport. In reality, when the data are stored as a temperature chart, based on the adopted color scale and the relationship to the temperature scale, the same entity can appear different. Thermo-vision is an efficient and non-invasive diagnostic way of obtaining the surface temperature field of the tested objects in the form of photographs using thermographic cameras.

5.2. Thermistor Thermistor is a conjunction of the words “thermally sensitive resistor”. A thermistor is a speciﬁc category of resistor that changes its physical resistance during temperature change. Thermistors are made of semiconductor material of a ceramic form using metal oxide processing, such as nickel, manganese, cobalt, etc. The thermistor is normally pressed Energies 2021, 14, 1472 12 of 32

into small discs or balls that are sealed up to provide a rapid reaction to temperature changes. It is a passive resistive device, meaning that to achieve a measurable voltage output, we need to transfer a current through it. Thermistors are then wired in series via an appropriate biasing resistor to form a voltage divider circuit, with the resistor chosen to achieve a voltage output at a pre-determined temperature value. Their key advantage over forms of snap-action is their reaction speed to any temperature, their accuracy, and their repeatability adjustments. Major thermistor types have a negative temperature coefﬁcient of resistance, that is, their resistance value decreases with a temperature rise.

5.3. Resistive Temperature Detectors Precision temperature sensors manufactured from high-purity conducting metals such as titanium, copper, or nickel, wrapped into a coil and whose electrical resistance varies as a function of temperature, analogous to a thermistor, are known as resistive temperature detectors. Resistive temperature detectors have positive temperature coefﬁcients, but their performance is highly linear in contrast to the thermistor, providing very precise temperature readings. However, they have very low thermal sensitivity, which means that a temperature rise only causes a very slight change in performance. Resistive temperature detectors are passive resistive devices that create an output voltage that increases linearly with temperature by passing a constant current through the temperature sensor. As the resistive temperature detector is a resistive device, a current should pass through it for observing voltage. Even then, as the current passes through it, some differences in resistance due to the self-heating of the resistive wires, the I2R law, produces an error in the readings. To prevent this, the resistive temperature detectors are normally wired into a Wheatstone Bridge network with additional wires for lead compensation and/or access to a continuous current source.

5.4. Fiber Optic Temperature Measurements Even in dangerous environments, ﬁber optic temperature sensors without external effects (radio signals, magnetic ﬁelds, microwaves) can operate rapidly and easily. They can be used conveniently to calculate an engine’s stator winding temperature or the temperature of the bearing. The sensors help to easily sense variations in temperature to record the real conditions and trigger preventive steps [54–56]. Table2 shows the comparison of different measuring techniques against operating parameters.

5.5. Permanent Magnet Synchronous Motor Electric motors with greater power density and a proven record of extended running period are the best resources for meeting the demands. They are the prime movers in the automotive, rail, marine, and aerospace conveyance sectors [57]. The synchronous motor and induction motor are the two types of traction that motors mostly utilize in EVs [58]. In hybrid drives, basic recommendations have been made for the design of a traction motor, such as that of the spatially constricted space and the necessity for a high power density and high torque density. This allows the motor to support greater power inside a smaller volume, resulting in an increase in the generation of heat within the engine and a decrease in the productive cooling space. With a high power density, high performance, high reliability and compact size (Figure7), the PMSM is a popular option for the growing electric vehicle industry, as well as for vehicle energy and power applications [59–61]. PMSMs are also known for their slight vibration noise, great torque density, decent torque stability and intense control accuracy [62]. Energies 2021, 14, 1472 13 of 32

Table 2. Comparison of different thermal mapping techniques against operating parameters.

Resistive Temperature Infrared Parameters Thermistor Detectors Thermography Measurement Type Contact type Contact type Non-contact type Cost High Low Costlier Accuracy Low Less Higher Range High, up to 660 ◦C Low, up to 130 ◦C Very high, up to 2000 ◦C Mapping Need calculation Need calculations Easy Reuse Not possible Not possible Possible Response Slow, 1 to 7 s Fast, less than 1 s Faster, 1 to 250 ms Both internal and Both internal and Only surface Measurement area external temperature external temperature temperature Materials Metals Semiconductor materials Glass and plastic Loss Less Medium Very much less Positive Negative Temperature coefficient Not applicable temperature coefficient temperature coefficient Temperature Linear Nonlinear Accurate Characteristics Sensitivity Less sensitive High sensitivity Accurate Size Large Small Immaterial Hysteresis effect Less More No Very high No risk of ceramic materials Reliability Highly reliable contamination and Energies 2021, 14, 1472 easily damaged 14 of 33 mechanical effect, no wear and tear.

FigureFigure 7. Special 7. Special features features of permanent of permanent ma magnetgnet synchronous synchronous motor motor (PMSM). (PMSM).

Currently,Currently, the the enclosed enclosed PMSMs PMSMs have have been been sw swiftlyiftly industrialized industrialized across across the the globe. globe. RotorRotor cooling cooling is a is challenging a challenging assignment assignment in inmotor motor design, design, as asthe the heat heat dissipation dissipation is is unidirectionalunidirectional [63]. [63 On]. On the the thermal thermal design design side, side, the the PMSM PMSM has has many many limitations, limitations, such as its its vulnerabilityvulnerability to to coil coil insulation insulation failures failur andes and magnet magnet demagnetization demagnetization under under extreme extreme thermal thermalenvironments environments [59,60]. [59,60]. The normal The motornormal has motor deprived has conditionsdeprived ofconditions heat dissipation of heat that dissipationare constrained that are by constrained the spatial by and the environmental spatial and environmental challenges, and challenges, a great rise and in operatinga great risetemperature in operating boosts temperature torque andboosts power torque densities and power [64]. densities [64]. An investigation has shown that the stator winding is the most heated unit of the motor, the rotor winding is the next most heated, and the frame is the least heated unit. It is also known that the highest temperature value of both stator and rotor windings is reduced by about 70% of its length from the supply of cooling air [65]. It is important to avoid such harm in order to envisage a precise temperature distribution in various elements of the electric motor. Therefore, it is important to have effective cooling for greater heat dissipation in PMSMs [59]. Despite their high performance, electric motors are thermally restricted by many forms of losses in some operating points. The essential components of temperature overheating decrease efficiency (derating), and suitable cooling must be implemented to decrease the derating. The design of effective electric motor cooling models in traction drives with improved torque and power densities is difficult because the heat release in various parts of the motor differs significantly from the operative conditions [66,67].

5.6. PMSM Thermal Analysis Electric motor effectiveness is estimated by its electromagnetic configuration and thermal design. A detailed thermal analysis is still in operation, with emphasis on the temperature captured by the sensors kept in the vicinity of the copper windings, to recommend potential methods to lessen the motor working temperatures. Although the electromagnetic losses of the windings depend on temperature, improving overall cooling often means achieving enhanced motor output in terms of torque supplied, the power developed and the efficiency achieved, as that is the real objective. In view of the electrical power densities, an effective thermal simulation of the engine and the combined cooling system is crucial for the efficient thermal control of the engine. In fact, high temperatures are unfavorable in terms of the efficiency and life of many materials, so good cooling systems are necessary. Excessive heat lowers the battery life, weakens the motor magnets’ rare earth, and facilitates the dissipation of the copper windings by Joule heating. It is mainly important to determine the heat sources in a thermal model and also to predict the

Energies 2021, 14, 1472 14 of 32

An investigation has shown that the stator winding is the most heated unit of the motor, the rotor winding is the next most heated, and the frame is the least heated unit. It is also known that the highest temperature value of both stator and rotor windings is reduced by about 70% of its length from the supply of cooling air [65]. It is important to avoid such harm in order to envisage a precise temperature distribution in various elements of the electric motor. Therefore, it is important to have effective cooling for greater heat dissipation in PMSMs [59]. Despite their high performance, electric motors are thermally restricted by many forms of losses in some operating points. The essential components of temperature overheating decrease efficiency (derating), and suitable cooling must be implemented to decrease the derating. The design of effective electric motor cooling models in traction drives with improved torque and power densities is difficult because the heat release in various parts of the motor differs significantly from the operative conditions [66,67].

5.6. PMSM Thermal Analysis Electric motor effectiveness is estimated by its electromagnetic configuration and thermal design. A detailed thermal analysis is still in operation, with emphasis on the temperature captured by the sensors kept in the vicinity of the copper windings, to recommend potential methods to lessen the motor working temperatures. Although the electromagnetic losses of the windings depend on temperature, improving overall cooling often means achieving enhanced motor output in terms of torque supplied, the power developed and the efficiency achieved, as that is the real objective. In view of the electrical power densities, an effective thermal simulation of the engine and the combined cooling system is crucial for the efficient thermal control of the engine. In fact, high temperatures are unfavorable in terms of the efficiency and life of many materials, so good cooling systems are necessary. Excessive heat lowers the battery life, weakens the motor magnets’ rare earth, and facilitates the dissipation of the copper windings by Joule heating. It is mainly important to determine the heat sources in a thermal model and also to predict the heat transfer mechanisms across the entire motor. The mechanism of heat transfer Energies 2021, 14, 1472 15 of 33 could be conductive, convective, or radiative, identified by the type of cooling system. The electromagnetic eddy current loss, hysteresis loss in magnets and laminations, and Joule heating are the primary sources of thermal energy. The objective was to lessen the running heat transfer mechanisms across the entire motor. The mechanism of heat transfer could betemperatures conductive, convective, of the motor’s or radiative, hot iden spotstified (to by be the precise, type of incooling the coppersystem. The windings) through the electromagneticoptimizations eddy of thecurrent most loss, critical hysteresis factors loss in formagnets a safe and operation laminations, and and longJoule lifetime [14]. heatingThe are the problem primary ofsources overheating of thermal ener is mostgy. The likely objective to arisewas to with lessen the the running deficiency of the radiation temperatures of the motor’s hot spots (to be precise, in the copper windings) through the optimizationsflow path, eventuallyof the most crit causingical factors operation for a safe operation issuesor and even long loss.lifetime Overheating [14]. would demagne- tizeThe the problem PM and of overheating would reduce is most likely the PMSM’s to arise with dynamic the deficiency efficiency. of the radiation The cooling system can flowbe disabled path, eventually by an overlycausing highoperation coil temperature.issues or even Inloss. addition, Overheating the would lubricating property and demagnetizeresilience ofthe the PM bearings and would are reduce reduced the PMSM’s by overheating, dynamic efficiency. curtailing The cooling the durability of a PMSM. system can be disabled by an overly high coil temperature. In addition, the lubricating propertyOverheated and resilience hot spots of the in thebearings parts are of reduced the electric by overheating, motor often curtailing threaten the to deteriorate the durabilitycapability of ofa PMSM. the rotors, Overheated which hot are spots vital. in the Investigations parts of the electric identified motor often that longitudinal rotor threatenextension to deteriorate due to uncontrolled the capability temperaturesof the rotors, which resulted are vital. insignificant Investigations safety problems and identifiedadded expenses that longitudinal in the rotor making extension of PMSMsdue to uncontrolled [62] (Figure temperatures8). resulted in significant safety problems and added expenses in the making of PMSMs [62] (Figure 8).

FigureFigure 8. An 8. Anoverview overview of failures of failurescaused by causeduncontrolled by uncontrolledtemperature. temperature. The most overriding stator/rotor core losses were estimated by 2D FEA for all the components of an electric motor. The analysis of the stator and rotor core losses was investigated for an uninterrupted power situation, Figure 9. Core losses for the elements of the motor increased with the speed of the rotor, as predicted, but the losses of the stator yoke were the least responsive compared to others. The copper losses under continuous power functioning settings were another main loss factor (Figure 10). These losses were calculated at 180° C, and the performance, output power and torque of the electric motor under continuous speed power were estimated (Figure 11) [68].

Figure 9. Core component losses with speed [68].

Energies 2021, 14, 1472 15 of 33

heat transfer mechanisms across the entire motor. The mechanism of heat transfer could be conductive, convective, or radiative, identified by the type of cooling system. The electromagnetic eddy current loss, hysteresis loss in magnets and laminations, and Joule heating are the primary sources of thermal energy. The objective was to lessen the running temperatures of the motor’s hot spots (to be precise, in the copper windings) through the optimizations of the most critical factors for a safe operation and long lifetime [14]. The problem of overheating is most likely to arise with the deficiency of the radiation flow path, eventually causing operation issues or even loss. Overheating would demagnetize the PM and would reduce the PMSM’s dynamic efficiency. The cooling system can be disabled by an overly high coil temperature. In addition, the lubricating property and resilience of the bearings are reduced by overheating, curtailing the durability of a PMSM. Overheated hot spots in the parts of the electric motor often threaten to deteriorate the capability of the rotors, which are vital. Investigations identified that longitudinal rotor extension due to uncontrolled temperatures resulted in significant safety problems and added expenses in the making of PMSMs [62] (Figure 8).

Energies 2021, 14, 1472 15 of 32 Figure 8. An overview of failures caused by uncontrolled temperature.

The most overriding stator/rotor core losses were estimated by 2D FEA for all the The most overriding stator/rotor core losses were estimated by 2D FEA for all the components of an electric motor. The analysis of the stator and rotor core losses was components of an electric motor. The analysis of the stator and rotor core losses was investigated for an uninterrupted power situation, Figure 9. Core losses for the elements investigated for an uninterrupted power situation, Figure9. Core losses for the elements of of the motor increased with the speed of the rotor, as predicted, but the losses of the stator the motor increased with the speed of the rotor, as predicted, but the losses of the stator yoke were the least responsive compared to others. The copper losses under continuous yoke were the least responsive compared to others. The copper losses under continuous power functioning settings were another main loss factor (Figure 10). These losses were power functioning settings were another main loss factor (Figure 10). These losses were calculated at 180° C, and the performance, output power and torque of the electric motor calculated at 180 ◦C, and the performance, output power and torque of the electric motor under continuous speed power were estimated (Figure 1111))[ [68].68].

Energies 2021,, 1414,, 14721472 16 of 33

Figure 9. Core component losses with speed [[68].68].

FigureFigure 10. CopperCopper loss loss under under continuous continuous power with speed [68]. [68].

FigureFigure 11. 11. ElectricElectric motor motor efficiency, efﬁciency, output output power power and torque and under torque continuous under continuous power with power speed [68]. with speed [68]. 6. Cooling Methods 6. Cooling Methods The operating stability and durability of the PMSM are constrained by greater temperaturestemperaturesThe operating atat thethestability hothot spotsspots and andand durability thethe inefficieninefficien of thett heatheat PMSM dissipation,dissipation, are constrained asas discusseddiscussed by greater earlier.earlier. temper- TheThe properatures atquantification the hot spots of and electric the inefﬁcient motor heat heat generation dissipation, and as sophisticated discussed earlier. coupled The propermotor cooling practices are topics of considerable importance [69]. Generally, air-cooling, water- cooling, oil-cooling, water–glycol-cooling and phase change evaporative-cooling are the active cooling strategies for the thermally stable operationoperation ofof anan electricelectric motor.motor. TheThe activeactive cooling system structures, such as a housing with fins, housing with water jacket, oil spray end winding cooling, etc., were integrated into the thermal scheme to envisage the precise temperature.temperature. TheThe simplesimple andand economicaleconomical air-cair-cooled motor (ACM) with a fan at the end of thethe shaftshaft influencesinfluences thethe airair insideinside andand promotpromotes heat exchange with the environment. The air-cooling system, however, does not completelytely meetmeet thethe increasiincreasing demand for heat dissipation [14,58,62]. Investigations were done thoroughly on the temperature control approaches, such as an oil spray end winding cooling, liquid cooling by automatic transmissiontransmission fluidfluid (ATF),(ATF), forcedforced air-cooling,air-cooling, coolingcooling throughthrough phasephase changechange materials,materials, andand thethe pottingpotting ofof statorstator windingswindings withwith highhigh ththermal conductivity materials [57]. Researchers across the globe have analyzed various PMSM structures,structures, suchsuch asas thethe rotorrotor andand statorstator stacks, and the rotor and stator cooling ducts, of a PMSM [70].

6.1. Air-Cooling IfIf thethe airflowairflow inin thethe end-spaceend-space couldcould bebe imimproved by wafters, in a self-ventilating cooling system, the cooling efficiency of the rotor would be improved. A modern inclined flowflow deflectordeflector waswas intendedintended toto promotepromote thethe effectivenesseffectiveness ofof thethe rotor’srotor’s cooling.cooling. TheThe difference in air pressure generated across thethe rotatingrotating rotorrotor facilitatedfacilitated thethe airflowairflow throughthrough thethe ventvent holesholes [63].[63]. ResearchersResearchers acrossacross thethe globeglobe designeddesigned andand analyzedanalyzed aa popularpopular air-cooling system wherein the fan is fitted externally at the end of the shaft. Normal rotor, winding and PM temperatures were reported to be substantially reduced by air

Energies 2021, 14, 1472 16 of 32

quantification of electric motor heat generation and sophisticated coupled motor cooling practices are topics of considerable importance [69]. Generally, air-cooling, water-cooling, oil-cooling, water–glycol-cooling and phase change evaporative-cooling are the active cooling strategies for the thermally stable operation of an electric motor. The active cooling system structures, such as a housing with fins, housing with water jacket, oil spray end winding cooling, etc., were integrated into the thermal scheme to envisage the precise temperature. The simple and economical air-cooled motor (ACM) with a fan at the end of the shaft influences the air inside and promotes heat exchange with the environment. The air-cooling system, however, does not completely meet the increasing demand for heat dissipation [14,58,62]. Investigations were done thoroughly on the temperature control approaches, such as an oil spray end winding cooling, liquid cooling by automatic transmission fluid (ATF), forced air-cooling, cooling through phase change materials, and the potting of stator windings with high thermal conductivity materials [57]. Researchers across the globe have analyzed various PMSM structures, such as the rotor and stator stacks, and the rotor and stator cooling ducts, of a PMSM [70].

6.1. Air-Cooling If the airflow in the end-space could be improved by wafters, in a self-ventilating cooling system, the cooling efficiency of the rotor would be improved. A modern inclined flow deflector was intended to promote the effectiveness of the rotor’s cooling. The difference in air pressure generated across the rotating rotor facilitated the airflow through the vent holes [63]. Researchers across the globe designed and analyzed a popular air- cooling system wherein the fan is fitted externally at the end of the shaft. Normal rotor, winding and PM temperatures were reported to be substantially reduced by air convection and the effect of the airflow path on the motor temperatures [71–74]. The external fan can, however, raise the electric motor volume and decrease the power density, and this was not acceptable to fully sealed motors [63]. The rotor is the essential factor when it comes to temperature. Temperature sensors detected spikes with the increase in high-frequency current-borne iron losses, and the thermally insulating air gap at greater speeds of the traction drive. The integrated rotor internal air-cooling system was examined, and the potential load was calculated to maximize the thermal use of the electric motor [75]. In the recent past, with just a resin–varnish layer, a fully sealed fan-cooled induction motor was tested with two separate impregnating ingredients, such as epoxy and silicone potting. The products covered and over-molded the total area between the end winding and the metal housing’s inner surface. The novel cooling method was based on a vortex tube that generated a cold air stream to cool the unit. This strategy was evaluated and compared with a typical water-cooling technique. A simulation was conducted to offer insight into the possibilities of the modern technique of cooling. A finite volume CFD was used to evaluate the cooling path in terms of fluid velocity, pressure drops and flow volume in order to make the simulation quite precise and comprehensive in the traction motor [59].

6.2. Water Jacket Cooling Thermal analysis was explored for an efficient brushless synchronous electric traction motor with PMs and with cooling of the water jacket. The effect of the water mass flow rate on the temperature of different electric motor components has previously been studied. As the mass flow rate of the water increased, it was observed that the temperature of all the system’s heat sources decreased with the rise in the pressure drop across the water jacket (Figure 12)[14]. Energies 2021, 14, 1472 17 of 33

convection and the effect of the airflow path on the motor temperatures [71–74]. The external fan can, however, raise the electric motor volume and decrease the power density, and this was not acceptable to fully sealed motors [63]. The rotor is the essential factor when it comes to temperature. Temperature sensors detected spikes with the increase in high-frequency current-borne iron losses, and the thermally insulating air gap at greater speeds of the traction drive. The integrated rotor internal air-cooling system was examined, and the potential load was calculated to maximize the thermal use of the electric motor [75]. In the recent past, with just a resin–varnish layer, a fully sealed fan-cooled induction motor was tested with two separate impregnating ingredients, such as epoxy and silicone potting. The products covered and over-molded the total area between the end winding and the metal housing’s inner surface. The novel cooling method was based on a vortex tube that generated a cold air stream to cool the unit. This strategy was evaluated and compared with a typical water-cooling technique. A simulation was conducted to offer insight into the possibilities of the modern technique of cooling. A finite volume CFD was used to evaluate the cooling path in terms of fluid velocity, pressure drops and flow volume in order to make the simulation quite precise and comprehensive in the traction motor [59].

6.2. Water Jacket Cooling Thermal analysis was explored for an efficient brushless synchronous electric traction motor with PMs and with cooling of the water jacket. The effect of the water mass flow rate on the temperature of different electric motor components has previously been studied. As the mass flow rate of the water increased, it was observed that the temperature Energies 2021, 14, 1472 of all the system’s heat sources decreased with the rise in the pressure drop across17 of the 32 water jacket (Figure 12) [14].

FigureFigure 12.12.Brushless Brushless PMSMPMSM andand waterwater jacketjacket coolingcooling [14[14].].

TheThe coolingcooling systemsystem withwith fluxflux barriersbarriers waswas investigatedinvestigated toto improveimprove thethe coolingcooling effect effect withwith properproper heatheat dissipationdissipation asas aa partpart ofof the the thermal thermal optimization optimizationscheme scheme ininthe the PMSM PMSM stator.stator. TheThe resultsresults werewere compared compared to to those those of of thethestandard standard cooling cooling jacket jacket by by incorporating incorporating temperature-measuringtemperature-measuring sensors.sensors. ThisThis isis presentedpresented inin FiguresFigures 1313 and and 14 14.. ThisThis methodmethod ofof coolingcooling waswas similarsimilar toto thethe traditionaltraditional designdesign ofof aa water-coolingwater-cooling jacketjacket aroundaround thethe stator.stator. AsAs therethere waswas nono sheetsheet betweenbetween thethe waterwater jacketjacket andand thethe flux flux barriers, barriers,the the chilling chilling liquid liquid circulatingcirculating withinwithin thethe fluxflux barriersbarriers substantiallysubstantially increasedincreased thethe chillingchilling field. field. TheThe fluxflux Energies 2021, 14, 1472 barriers were isolated from the air-gap by a thin aluminum layer to avoid cooling liquid18 of 33 barriers were isolated from the air-gap by a thin aluminum layer to avoid cooling liquid flowingflowing throughthrough thethe air-gapair-gap andand creatingcreating aashort shortcircuit. circuit. AA moremore turbulentturbulent flowflow waswas developed,developed, leadingleading toto greatergreater heatheat dissipation.dissipation. ThisThis coolingcooling techniquetechnique maymay bebe aadecent decent substitutesubstitute forfor thethe traditionaltraditional jacketjacket inin aa limitedlimitedand andharsh harshsetting setting because because of of the the turbulent turbulent flowflow andand thethe increasedincreased cooling cooling area area [ 60[60].].

FigureFigure 13.13.Temperature Temperature proﬁle profile in in ﬂux flux barrier barrier cooling cooling of of PMSM PMSM [ 60[60].].

Figure 14. Temperature profile for a standard cooling jacket [60].

Using different techniques, such as flat wire winding, oil spray end winding cooling and aluminum winding, performance enhancement or cost savings could be accomplished. Based on cooling systems with integrated spiral stator water jackets, a spiral shaft groove, and oil spray components, the performance of the electric motor was predicted. At a 50 °C inlet temperature, with a fluid flow rate of 6.5 L/min and 50–50% water and ethylene glycol mixture, forced convection heat extraction was enacted on both elements. In addition, using an epoxy-type material, the stator end winding region was potted. The entire cooling model was implemented and evaluated for its viability [76]. The external surface of the driving motor stator was cooled by cooling water flowing through the water jacket in an innovative cooling method. In addition, airflow caused by an axial cooling fan mounted at the end of the rotor was used to cool the inner stator and rotor. With an innovative helical flow channel, the water jacket enveloped the stator. The

Energies 2021, 14, 1472 18 of 33

substitute for the traditional jacket in a limited and harsh setting because of the turbulent flow and the increased cooling area [60].

Energies 2021, 14, 1472 18 of 32

Figure 13. Temperature profile in flux barrier cooling of PMSM [60].

FigureFigure 14.14. TemperatureTemperature proﬁleprofile for for a a standard standard cooling cooling jacket jacket [ 60[60].].

UsingUsing differentdifferent techniques,techniques, suchsuch asas flatflat wirewire winding,winding, oiloil sprayspray endend windingwinding coolingcooling andand aluminumaluminum winding, winding, performance performance enhancement enhancement or cost savingsor cost could savings be accomplished. could be Basedaccomplished. on cooling Based systems on withcooling integrated systems spiral with statorintegrated water spiral jackets, stator a spiral water shaft jackets, groove, a andspiral oil shaft spray groove, components, and oil the spray performance components of the, the electric performance motor was of the predicted. electric Atmotor a 50 was◦C inletpredicted. temperature, At a 50 with °C inlet a fluid temperature, flow rate of 6.5with L/min a fluid and flow 50–50% rate waterof 6.5 andL/min ethylene and 50–50% glycol mixture,water and forced ethylene convection glycol mixture, heat extraction forced conv wasection enacted heat on extraction both elements. was enacted In addition, on both usingelements. an epoxy-type In addition, material, using an the epoxy-type stator end winding material, region the stator was potted. end winding The entire region cooling was modelpotted. was The implemented entire cooling and model evaluated was implem for itsented viability and [76 evaluated]. for its viability [76]. TheThe externalexternal surfacesurface ofof thethe drivingdriving motormotor statorstator waswas cooledcooled byby coolingcooling waterwater flowingflowing Energies 2021, 14, 1472 19 of 33 throughthrough thethe waterwater jacketjacket inin anan innovativeinnovative coolingcooling method.method. InIn addition,addition, airflowairflow causedcaused byby anan axialaxial coolingcooling fanfan mountedmounted atat thethe endend ofof thethe rotorrotor waswas usedused toto coolcool thethe innerinner statorstator andand rotor.rotor. WithWith anan innovativeinnovative helicalhelical flowflow channel,channel, thethe waterwater jacketjacket envelopedenveloped thethe stator.stator. TheThe arrangementarrangement was was such such that that the the cooling cooling air air reached reached the the engine engine through through the radial holes acrossacross the the left left end end coil, coil, and and the the other other through through the the tiny tiny stator–rotor stator–rotor ga gap.p. Therefore, Therefore, via the statorstator and and rotor rotor of of the the motor, motor, an an effort effort was was made made to to boost boost the the water water jacket’s jacket’s cooling cooling system system throughthrough the the forced forced cooling cooling air, air, as presented in Figure 15[ [77].77].

FigureFigure 15. 15. CoolingCooling flow ﬂow velocity velocity distribution distribution in in PMSM PMSM at at a a rotor rotor speed speed of 35 krpm [77]. [77].

ElementElement heat heat losses losses and and the the subsequent subsequent temp temperatureerature distribution distribution of of the the water water jacketjacket- cooledcooled motor motor were were calculated calculated against against electrom electromagneticagnetic output, output, i.e., i.e., efficiency. efficiency. The The winding winding temperaturetemperature was was calculated calculated using using compatible compatible sensors sensors at at a a higher higher current current density. density. Higher Higher coolantcoolant flowflow raterate tests tests were were performed performed with with higher higher forced forced convection convection heat transferheat transfer coeffi- 2 coefficientscients of up of to 5000up to W/(m 5000 W/(m·K). The2∙K). results The results indicated indicated a substantial a substantial reduction reduction in the winding in the winding temperature [69]. The water-cooled model was examined considering both the radial heat flow in the active portion of the system and the axial heat flow through the end-winding area and the shaft [78]. The PMSM’s water jacket is not only essential for heat dissipation, but also for mechanical safety. In the PM drive motors of EVs, the water jacket is easily damaged while starting, accelerating and climbing in a harsh environment. To ensure steady EV function, the optimal configuration of the PMSM’s water jacket was investigated. In order to determine the optimum volume and scale of the water channel in the water jacket optimization system, a convection heat transfer factor was examined. Secondly, through mechanical optimization design, the mechanical stress on the water jacket was investigated via calculating the optimal thickness of the inner layer of the water jacket. A thorough thermal analysis of the PMSM was done to achieve the best water-cooling jacket [79]. Numerical results for thermal simulations were obtained using CFD software ANSYS Fluent for a BMW i3 PM synchronous motor. Outputs such as torque, winding and stator losses, and peak winding temperatures were analyzed for direct stator cooling (rectangular and circular channel) and a direct winding heat exchanger with the conventional jacket-cooling technique. The direct winding heat exchanger was proven to be more effective in lowering the winding temperature, because it offers double the surface area and much less thermal resistance compared to the other cooling methods at the same heat transfer coefficient (Figure 16) [80].

Figure 16. Comparing direct winding heat exchanger cooling with direct stator cooling [80].

Energies 2021, 14, 1472 19 of 33

arrangement was such that the cooling air reached the engine through the radial holes across the left end coil, and the other through the tiny stator–rotor gap. Therefore, via the stator and rotor of the motor, an effort was made to boost the water jacket’s cooling system through the forced cooling air, as presented in Figure 15 [77].

Figure 15. Cooling flow velocity distribution in PMSM at a rotor speed of 35 krpm [77].

Element heat losses and the subsequent temperature distribution of the water jacket- cooled motor were calculated against electromagnetic output, i.e., efficiency. The winding Energies 2021, 14, 1472 temperature was calculated using compatible sensors at a higher current density. Higher19 of 32 coolant flow rate tests were performed with higher forced convection heat transfer coefficients of up to 5000 W/(m2∙K). The results indicated a substantial reduction in the temperaturewinding temperature [69]. The water-cooled[69]. The water-cooled model was model examined was consideringexamined considering both the radial both heat the flowradial in heat the activeflow in portion the active of the portion system of and the the system axial and heat the flow axial through heat flow the end-winding through the areaend-winding and the shaft area [and78]. the shaft [78]. TheThe PMSM’sPMSM’s waterwater jacketjacket is is not not only only essential essential for for heat heat dissipation, dissipation, but but also also for me-for chanicalmechanical safety. safety. In theIn the PM PM drive drive motors motors of of EVs, EVs, the the water water jacket jacket is is easily easily damaged damaged whilewhile starting,starting, accelerating accelerating and and climbing climbing in in a aharsh harsh environment. environment. To Toensure ensure steady steady EV EVfunction, function,the optimal the optimal configuration configuration of the of thePMSM’s PMSM’s water water jacket jacket was was investigated. investigated. In Inorder order to todetermine determine the the optimum optimum volume volume and and scale scale of of the the water water channel channel in in the waterwater jacketjacket optimizationoptimization system, a convectionconvection heatheat transfertransfer factorfactor waswas examined.examined. Secondly,Secondly, throughthrough mechanicalmechanical optimizationoptimization design,design, thethe mechanicalmechanical stressstress onon thethe waterwater jacketjacket waswas investigatedinvestigated viavia calculatingcalculating thethe optimaloptimal thicknessthickness of the the innerinner layerlayer ofof thethe waterwater jacket.jacket. AA thoroughthorough thermalthermal analysis of the the PMSM PMSM was was done done to to achieve achieve the the best best water-cooling water-cooling jacket jacket [79]. [79 ]. NumericalNumerical resultsresults forfor thermalthermal simulationssimulations werewere obtainedobtained usingusing CFDCFD softwaresoftware ANSYSANSYS FluentFluent for a BMW i3 PM synchronous motor.motor. OutputsOutputs suchsuch asas torque,torque, windingwinding andand statorstator losses,losses, andand peak peak winding winding temperatures temperatures were analyzedwere analyzed for direct for stator direct cooling stator (rectangular cooling and(rectangular circular channel)and circular anda channel) direct winding and a heat direct exchanger winding with heat the exchanger conventional with jacket- the coolingconventional technique. jacket-cooling The direct technique. winding The heat di exchangerrect winding was heat proven exchanger to be morewas proven effective to inbe loweringmore effective the winding in lowering temperature, the winding because temperature, it offers double because the surface it offers area double and much the lesssurface thermal area resistanceand much comparedless thermal to resistance the other coolingcompared methods to the atother the cooling same heat methods transfer at coefficientthe same heat (Figure transfer 16)[ 80coefficient]. (Figure 16) [80].

Energies 2021, 14, 1472 20 of 33

FigureFigure 16.16. Comparing direct windingwinding heatheat exchangerexchanger coolingcooling withwith directdirect statorstator coolingcooling [[80].80].

ResearchersResearchers insertedinserted waterwater coldcold platesplates (WCP)(WCP) radiallyradially intointo thethe corecore laminations laminations such such thatthat hothot spotsspots inin laminationslaminations andand windingswindings werewere closeclose toto the the coolant. coolant. TheThe heatheat transfer transfer pathpath inin thethe WCPWCP isis inin anan axialaxial directiondirection withwith betterbetter thermalthermal conductivity,conductivity, andand itit isis inin the the radialradial directiondirection inin WJWJ withwith lowerlowerthermal thermal conductivity.conductivity. TheThe performanceperformance ofof thethe PMSMPMSM withwith WCPWCP andand withwith WJWJ waswas analyzed analyzed usingusing CFD,CFD, andand the the results results were were compared. compared. TheThe windingwinding and and PM PM temperatures temperatures with with the WCPthe WCP were lesswere than less those than with those the with WJ. Inthe addition, WJ. In ataddition, higher torques, at higher the torques, PMSM withthe PMSM WCP displayed with WCP a betterdisplayed thermal a better advantage thermal compared advantage to WJcompared (Figure 17to ).WJ It was(Figure observed 17). It that was the observ windinged that temperature the winding could temperature be reduced could by over be ◦ 20reducedC with by WCP over compared20 °C with to WCP WJat compar the rateded to condition WJ at the [81 rated]. condition [81].

FigureFigure 17.17.Thermal Thermal advantageadvantage withwith thethe waterwater coldcold platesplatescompared compared to to water water jacket-cooling jacket-cooling [ 81[81].].

6.3. Liquid/Oil-Based Cooling In motors, liquid-cooling is designed to transfer the heat produced by stators and windings through the water jackets to the cooling fluid. Using encapsulating varnish or Epoxylite, the heat dissipation of the end winding could be dissipated through the casing [16,17,82]. A 30,000 rpm speed automotive traction motor oil-based shaft cooling system was investigated. Thermal analysis was done analytically and experimentally. The results in Figure 18 indicate that the shaft temperature decreased despite the rise in the speed of the shaft for different iron losses with oil-based cooling. The results in Figure 19 also show that the shaft temperature decreased as the oil flow rates increased for different iron losses with oil-based cooling [83].

Figure 18. Variation in temperature with different speeds as regards iron loss [83].

Energies 2021, 14, 1472 20 of 33

Researchers inserted water cold plates (WCP) radially into the core laminations such that hot spots in laminations and windings were close to the coolant. The heat transfer path in the WCP is in an axial direction with better thermal conductivity, and it is in the radial direction in WJ with lower thermal conductivity. The performance of the PMSM with WCP and with WJ was analyzed using CFD, and the results were compared. The winding and PM temperatures with the WCP were less than those with the WJ. In addition, at higher torques, the PMSM with WCP displayed a better thermal advantage compared to WJ (Figure 17). It was observed that the winding temperature could be reduced by over 20 °C with WCP compared to WJ at the rated condition [81].

Energies 2021, 14, 1472 20 of 32 Figure 17. Thermal advantage with the water cold plates compared to water jacket-cooling [81].

6.3.6.3. Liquid/Oil-Based Liquid/Oil-Based Cooling Cooling InIn motors, motors, liquid-cooling liquid-cooling is is designed designed to to transfer transfer the the heat heat produced produced by by stators and windingswindings through through the the water water jackets jackets to to the the cooling cooling fluid. ﬂuid. Using Using encapsulating encapsulating varnish varnish or Epoxylite,or Epoxylite, the heat the heat dissipation dissipation of the of end the wi endnding winding could couldbe dissipated be dissipated through through the casing the [16,17,82].casing [16 ,A17 30,000,82]. A rpm 30,000 speed rpm automotive speed automotive traction tractionmotor oil-based motor oil-based shaft cooling shaft coolingsystem wassystem investigated. was investigated. Thermal Thermal analysis analysis was done was analytically done analytically and experimentally. and experimentally. The results The inresults Figure in 18 Figure indicate 18 indicatethat the thatshaft the temperat shaft temperatureure decreased decreased despite the despite rise in the the rise speed in the of thespeed shaft of for the different shaft for iron different losses iron with losses oil-based with cooling. oil-based The cooling. results The in Figure results 19 in also Figure show 19 thatalso the show shaft that temperature the shaft temperature decreased as decreased the oil flow as therates oil increased ﬂow rates for increased different for iron different losses withiron lossesoil-based with cooling oil-based [83]. cooling [83].

Energies 2021, 14, 1472 21 of 33

FigureFigure 18. 18. VariationVariation in in temperature temperature with differ differentent speeds as regards iron loss [[83].83].

FigureFigure 19.19. Variation inin shaftshaft temperaturetemperature withwith differentdifferent ﬂowflowrates ratesas asregards regards iron iron loss loss [ 83[83].].

The oil-coolingoil-cooling conﬁgurationconfiguration for for the the end end winding winding and and stator stator core core was was investigated investigated as theas the inner inner rotor rotor motor motor was was considered considered to to be be the the research research focus. focus. The The results results revealed revealed thatthat thethe heatheat insideinside thethe motormotor waswas carriedcarried awayaway byby thethe lubricatinglubricating ﬂuidfluid viavia heatheat conductionconduction andand convection.convection. The analysis comparingcomparing this withwith thethe water-cooledwater-cooled motormotor revealedrevealed thatthat thethe raterate ofof temperature increaseincrease ofof thethe oil-cooled motormotor was slower than that of the water- cooledcooled motor. It It was was also also found found that that the the temp temperatureerature difference difference across across the themotor motor front front and ◦ andback back decreased decreased by 18 by °C 18 inC inhalf half an an hour hour (Figure (Figure 20). 20). In In general, general, because of complexcomplex pipelinepipeline connections,connections, waterwater jacket-coolingjacket-cooling waswas challenging.challenging. It waswas alsoalso foundfound thatthat withwith thethe water-cooled circuit, circuit, the the sealing sealing structure structure had had insulation insulation concerns concerns and and issues. issues. The The oiloil-cooledcooled system system was was tested tested for for its its cooling cooling effect effect on on the the core core of of the stator.stator. InvestigationsInvestigations foundfound thatthat notnot onlyonly waswas thethe coolingcooling effecteffect viable,viable, butbut itit alsoalso workedworked forfor extendedextended periodsperiods under rated conditions. For the end winding or stator core cooling, the cooling structure and cooling methods were configured as the motor losses were primarily concentrated there. The cooling structure was designed in such a way that the cooling oil and the heat source were kept closer for a better cooling effect. Therefore, coolant oil was transmitted through the holes of the end oil cover and the stator core. The oil-immersed end winding and the axial oil passage cooling of the yoke of the stator and stator core were investigated for their effectiveness as regards cooling, and so too were the enhanced hub motor power density and torque density [64].

Figure 20. Contrast between water-cooled motor and oil-cooled motor based on the test results [64].

To increase the performance of the EV motor for greater power density, the optimization of the liquid cooling system is vital. Following the design assessment of parts such as lubricating oil and the motive seal of electric cars, revolutionary cooling topologies of electrical machines were investigated. Different winding insulation schemes were

Energies 2021, 14, 1472 21 of 33

Figure 19. Variation in shaft temperature with different flow rates as regards iron loss [83].

The oil-cooling configuration for the end winding and stator core was investigated as the inner rotor motor was considered to be the research focus. The results revealed that the heat inside the motor was carried away by the lubricating fluid via heat conduction and convection. The analysis comparing this with the water-cooled motor revealed that the rate of temperature increase of the oil-cooled motor was slower than that of the water- cooled motor. It was also found that the temperature difference across the motor front and back decreased by 18 °C in half an hour (Figure 20). In general, because of complex pipeline connections, water jacket-cooling was challenging. It was also found that with Energies 2021, 14, 1472 21 of 32 the water-cooled circuit, the sealing structure had insulation concerns and issues. The oil- cooled system was tested for its cooling effect on the core of the stator. Investigations found that not only was the cooling effect viable, but it also worked for extended periods underunder rated conditions. For For the the end end winding or stator core cooling, the cooling structure andand cooling methods were configured conﬁgured as the motor losses were primarilyprimarily concentrated there.there. The The cooling cooling structure structure was was designed designed in in su suchch a a way that the cooling oil and the heat sourcesource were were kept closer for a better cooling effect. Therefore, coolant oil was transmitted throughthrough the the holes holes of the end oil cover and th thee stator core. The oil-immersed end winding andand the the axial axial oil oil passage passage cooling cooling of the the yoke yoke of of the the stator stator and and stator stator core core were were investigated investigated forfor their their effectiveness effectiveness as as regards regards cooling, and so too too were were the enhanced enhanced hub motor power densitydensity and torque density [64]. [64].

FigureFigure 20. 20. ContrastContrast between between water-cooled water-cooled motor motor and and oil-c oil-cooledooled motor motor based based on on the the test test results results [64]. [64 ].

ToTo increaseincrease the the performance performance of theof EVthe motorEV motor for greater for greater power density,power thedensity, optimiza- the optimizationtion of the liquid of the cooling liquid cooling system system is vital. is Following vital. Following the design the design assessment assessment of parts of parts such suchas lubricating as lubricating oil and oil and the motivethe motive seal seal of electric of electric cars, cars, revolutionary revolutionary cooling cooling topologies topologies of ofelectrical electrical machines machines were were investigated. investigated. Different Different winding winding insulation insulation schemes schemes were studied, were such as impregnation using standard varnish only, as well as potting with a standard epoxy and a highly thermally conductive potting silicon gelatin (PSG) material. A stator winding fixed cooler structure was also investigated for its effectiveness on cooling and service life. There were jacket openings on the fixed coolers for the stator winding. The jacket was supplied with oil through gravity, whereas the stator was supplied through pressure-based injection. Stator winding rotary coolers would enhance the end winding heat transfer. Under the operation of centrifugal force, the liquid would enter the stator winding rotary cooler. For the customized design of the electric motor cooling system, the stator winding rotary cooler and the air-gap cooling cavity were found to be convenient. This is one of the innovative designs of the cooling topologies for EV electric motors that could uphold efficacy at high speeds and substantial loads [84].

6.4. Oil Spray Cooling The best way to lessen the temperatures of an electric motor is to keep the heat sinks in the vicinity of the heat sources. In modern PMSMs, three types of parallel strategies were implemented and tested (Figure 21): the injection of oil into the revolving shaft through holes; oil injection into the housing through nozzles; external cooling jacket. Direct spray cooling was undertaken with rotor shaft nozzles equipped for the cooling of end windings. The cooling oil (dielectric fluid) was injected into the electric motor’s (mostly air-filled) interior with a jet at variable volume flow speeds through the revolving shaft’s holes [66]. The end winding is in general considered to be one of the prime heat sources in the electric motor. Spray nozzles were used to spray oil to cool the stator end windings (Figure 22). Investigations were done and various cooling schemes were analyzed for their effectiveness in bringing down the temperature of the heat sources. End winding spray cooling with water jacket cooling is considered to be the best, as the temperature rise permitted through this scheme is slower compared to the other cooling schemes as the rotor speeds increase (Figure 23). In addition, in comparison to the other cooling schemes, Energies 2021, 14, 1472 22 of 33

studied, such as impregnation using standard varnish only, as well as potting with a standard epoxy and a highly thermally conductive potting silicon gelatin (PSG) material. A stator winding fixed cooler structure was also investigated for its effectiveness on cooling and service life. There were jacket openings on the fixed coolers for the stator winding. The jacket was supplied with oil through gravity, whereas the stator was supplied through pressure-based injection. Stator winding rotary coolers would enhance the end winding heat transfer. Under the operation of centrifugal force, the liquid would enter the stator winding rotary cooler. For the customized design of the electric motor cooling system, the stator winding rotary cooler and the air-gap cooling cavity were found to be convenient. This is one of the innovative designs of the cooling topologies for EV electric motors that could uphold efficacy at high speeds and substantial loads [84].

Energies 2021, 14, 1472 23 of 33

FigureFigure 21.21. Direct oil injection intointo thethe shaft,shaft, housinghousing andand coolingcooling jacket—schematicjacket—schematic [[66].66]. Energies 2021, 14, 1472 23 of 33

The end winding is in general considered to be one of the prime heat sources in the electric motor. Spray nozzles were used to spray oil to cool the stator end windings (Figure 22). Investigations were done and various cooling schemes were analyzed for their effectiveness in bringing down the temperature of the heat sources. End winding spray cooling with water jacket cooling is considered to be the best, as the temperature rise permitted through this scheme is slower compared to the other cooling schemes as the rotor speeds increase (Figure 23). In addition, in comparison to the other cooling schemes, such as microchannels and cooling tubes, end winding oil spray cooling is proven to be the best (see Figure 24) [68].

Figure 22. Oil spray-cooling at the end winding [68]. FigureFigure 22. 22. Oil Oil spray-cooling at at the the end end winding winding [68]. [68].

FigureFigure 23. 23.Rise Rise in in temperature temperature with with cooling cooling schemes schemes under under 30 30 kW kW rated rated power power [ 68[68].]. Figure 23. Rise in temperature with cooling schemes under 30 kW rated power [68]. The uninterrupted performance of the electric motor was investigated, and the resultsThe indicated uninterrupted a significant performance improvement of theand electricgain was motor achieved was with investigated, the oil spray- and the cooling (SC) in contrast to stator spiral water jacket (WJ)-cooling (Figure 24). The stator results indicated a significant improvement and gain was achieved with the oil spray- spiral WJ system has a strong but minimal heat removal capability, hence it was suggested cooling (SC) in contrast to stator spiral water jacket (WJ)-cooling (Figure 24). The stator to be utilized in combination with SC. The results also indicate that the maximum value spiralof continuous WJ system torque has awas strong greater but than minimal before, he increasingat removal from capability, 185 Nm henceto 315 itNm was when suggested toboth be utilizedcooling insystems combination were involved. with SC. In The addition, results the also maximum indicate value that theof continuousmaximum value ofpower continuous was greater torque than was before, greater increasing than before, from 68 increasing kW to 98 kW from with 185 the Nm combination to 315 Nm of when bothboth coolingthe cooling systems systems were (Figure involved. 25). OilIn addition,spray-cooling the systemsmaximum were value found of tocontinuous be powereconomical was greater to establish, than and before, were increasingrecommended from for 68 utilization kW to 98 as kWa secondary with the mode. combination This of bothstrategy the wascooling suggested systems to remove (Figure the heat25). anOild tospray-cooling enhance the performance systems were of a tractionfound to be economicalmotor considerably. to establish, The andoil spray-cooling were recommen showedded great for utilization potential in as dissipating a secondary the mode.heat This strategyas a single was unit suggested as well [76]. to remove the heat and to enhance the performance of a traction motor considerably. The oil spray-cooling showed great potential in dissipating the heat as a single unit as well [76].

Figure 24. Variation in continuous torque with spray-cooling and without spray-cooling [76].

Energies 2021, 14, 1472 23 of 33

Figure 22. Oil spray-cooling at the end winding [68].

Figure 23. Rise in temperature with cooling schemes under 30 kW rated power [68].

The uninterrupted performance of the electric motor was investigated, and the results indicated a significant improvement and gain was achieved with the oil spray- cooling (SC) in contrast to stator spiral water jacket (WJ)-cooling (Figure 24). The stator spiral WJ system has a strong but minimal heat removal capability, hence it was suggested to be utilized in combination with SC. The results also indicate that the maximum value of continuous torque was greater than before, increasing from 185 Nm to 315 Nm when both cooling systems were involved. In addition, the maximum value of continuous power was greater than before, increasing from 68 kW to 98 kW with the combination of both the cooling systems (Figure 25). Oil spray-cooling systems were found to be economical to establish, and were recommended for utilization as a secondary mode. This strategy was suggested to remove the heat and to enhance the performance of a traction Energies 2021, 14, 1472 23 of 32 motor considerably. The oil spray-cooling showed great potential in dissipating the heat as a single unit as well [76].

Figure 24. VariationVariation in continuous torque with spray-cooling and without spray-cooling [[76].76].

The uninterrupted performance of the electric motor was investigated, and the results indicated a signiﬁcant improvement and gain was achieved with the oil spray-cooling (SC) in contrast to stator spiral water jacket (WJ)-cooling (Figure 24). The stator spiral WJ system has a strong but minimal heat removal capability, hence it was suggested to be utilized in combination with SC. The results also indicate that the maximum value of continuous torque was greater than before, increasing from 185 Nm to 315 Nm when both cooling systems were involved. In addition, the maximum value of continuous power was greater than before, increasing from 68 kW to 98 kW with the combination of both the cooling systems (Figure 25). Oil spray-cooling systems were found to be economical to establish, and were recommended for utilization as a secondary mode. This strategy was suggested Energies 2021, 14, 1472 24 of 33 to remove the heat and to enhance the performance of a traction motor considerably. The oil spray-cooling showed great potential in dissipating the heat as a single unit as well [76].

Figure 25. VariationVariation in in continuous continuous output output power power with with spray-cooling spray-cooling and and withou withoutt spray-cooling spray-cooling [76]. [76].

6.5. Cooling Tubes and Microchannels Researchers across the globe are investigatinginvestigating various cooling schemes in an attempt to bring down the temperatures at the hot spotsspots ofof PMSMs.PMSMs. In order to obtain a betterbetter cooling effect,effect, oneone of of the the cooling cooling schemes schemes investigated investigated was was cooling cooling via cooling via cooling tubes insidetubes insidethe stator the slots,stator along slots, with along a coolingwith a cooling jacket (Figure jacket 26(Figure). This 26). cooling This schemecooling could scheme fulﬁl could the fulfilobjectives, the objectives, but was foundbut was to befound complex. to be complex. Another coolingAnother scheme cooling investigated scheme investigated involved involvedhaving a coolinghaving jacket,a cooling with jacket, circumferential with circumferential cooling passages, cooling shrunk-ﬁt passages, at shrunk-fit the stator at outer the statordiameter outer (Figure diameter 27). (Figure These cooling27). These schemes cooling were schemes compared were compared to microchannels to microchannels with end windingwith end oilwinding spray-cooling oil spray-cooling (Figure 28 (Figure)[68]. 28) [68].

Figure 26. Structure of cooling tubes inside stator slots [68].

Figure 27. Stator cooling with micro channels [68].

Energies 2021, 14, 1472 24 of 33

Figure 25. Variation in continuous output power with spray-cooling and without spray-cooling [76].

Figure6.5. Cooling 25. Variation Tubes in andcontinuous Microchannels output power with spray-cooling and without spray-cooling [76].

6.5. CoolingResearchers Tubes and across Microchannels the globe are investigating various cooling schemes in an attempt to bringResearchers down acrossthe temperatures the globe are investigatinat the hot spotsg various of PMSMs.cooling schemes In order in anto attemptobtain a better tocooling bring downeffect, the one temperatures of the cooling at the schemes hot spots investigated of PMSMs. In was order cooling to obtain via a coolingbetter tubes coolinginside theeffect, stator one slots, of the along cooling with schemes a cooling investigated jacket (Figure was cooling 26). This via coolingcooling schemetubes could insidefulfil thethe statorobjectives, slots, alongbut was with found a cooling to be jacket complex. (Figure Another 26). This coolingcooling schemescheme could investigated fulfilinvolved the objectives, having a butcooling was foundjacket, to with be complex. circumferential Another cooling scheme passages, investigated shrunk-fit at the involvedstator outer having diameter a cooling (Figure jacket, 27). with These circumferential cooling schemes cooling were passages, compared shrunk-fit to microchannels at the Energies 2021, 14, 1472 24 of 32 statorwith endouter winding diameter oil (Figure spray-cooling 27). These cooling(Figure schemes 28) [68]. were compared to microchannels with end winding oil spray-cooling (Figure 28) [68].

FigureFigure 26. 26. Structure Structure of ofcooling cooling tubes tubes inside inside stator stator slots [[68].slots68]. [68].

Energies 2021, 14, 1472 25 of 33 Figure 27. Stator cooling with micro channels [68]. FigureFigure 27. 27.Stator Stator cooling cooling with with micro micro channels channels [68]. [68].

FigureFigure 28. 28.Comparison Comparison of cooling of cooling schemes schemes [68]. [68]. A compact microchannel thermal management cooling jacket was investigated for coolingA the compact stator core. microchannel In this cooling scheme,thermal hollow management conductors cooling were employed jacket in was direct investigated for windingcooling cooling.the stator The core. cooling In jacket’sthis cooling ring-shaped scheme, construction hollow conductors was such that were the inner employed in direct surfacewinding fit closelycooling. to theThe core cooling of the stator, jacket’s and ring-shaped the outer surface construction fit closely to thewas control such that the inner electronics.surface fit Insideclosely the to cooling the core jacket, of athe complicated stator, and fluid the path outer was builtsurface to bring fit closely down to the control the values of pressure drop and pumping strength, and to raise its thermal efficiency. As theelectronics. stator winding Inside was the known cooling to be jacket, a hot spot, a complicated hollow conductors fluid enabled path was the coolantbuilt to to bring down the directlyvalues accessof pressure the end drop windings, and topumping overcome strength the thermal, and resistance to raise between its ther themal stator efficiency. As the andstator the winding winding. Thewas same known coolant to andbe coolinga hot spot, loop couldhollow be shared conductors by the jacketenabled and the coolant to thedirectly winding access cooling the mechanism, end windings, minimizing to overcome the complexity the of thermal the pumping, resistance radiator between and the stator reservoir structures [85]. and the winding. The same coolant and cooling loop could be shared by the jacket and the winding cooling mechanism, minimizing the complexity of the pumping, radiator and reservoir structures [85].

6.6. Heat Pipe-Cooling An innovative phase-change heat pipe-cooling scheme for PMSMs has been investigated. A heat pipe-based straight-embedded module (SEME) (Figure 29), a three- dimensional rounding module (RM) (Figure 30), and thermal management modules for ACMs were analyzed. The experimental results indicate that the operative time required to control the temperature of a PMSM could be extended by 28.6% (under high speed) and 21.4% (under high torque) with this phase-change heat pipe-cooling scheme. In addition, the power density of the PMSM was enhanced while reducing the peak temperature by 22.3% (under rated conditions) (Figure 31) [62].

Figure 29. Heat pipe-based air-cooling with straight-embedded module (SEME) [62].

Energies 2021, 14, 1472 25 of 33

Figure 28. Comparison of cooling schemes [68].

A compact microchannel thermal management cooling jacket was investigated for cooling the stator core. In this cooling scheme, hollow conductors were employed in direct winding cooling. The cooling jacket’s ring-shaped construction was such that the inner surface fit closely to the core of the stator, and the outer surface fit closely to the control electronics. Inside the cooling jacket, a complicated fluid path was built to bring down the values of pressure drop and pumping strength, and to raise its thermal efficiency. As the stator winding was known to be a hot spot, hollow conductors enabled the coolant to directly access the end windings, to overcome the thermal resistance between the stator and the winding. The same coolant and cooling loop could be shared by the jacket and the winding cooling mechanism, minimizing the complexity of the pumping, radiator and Energies 2021, 14, 1472 25 of 32 reservoir structures [85].

6.6. Heat Pipe-Cooling 6.6. Heat Pipe-Cooling An innovative phase-change heat pipe-cooling scheme for PMSMs has been investigated.An innovative A heat phase-change pipe-based straight-e heat pipe-coolingmbedded scheme module for (SEME) PMSMs (Figure has been 29), in- a three- vestigated. A heat pipe-based straight-embedded module (SEME) (Figure 29), a three- dimensional rounding module (RM) (Figure 30), and thermal management modules for dimensional rounding module (RM) (Figure 30), and thermal management modules for ACMsACMs werewere analyzed. analyzed. The The experimental experimental results results indicate indicate that the that operative the operative time required time to required tocontrol control the the temperature temperature of a of PMSM a PMSM could could be extended be extended by 28.6% by 28.6% (under (under high speed) high andspeed) and 21.4%21.4% (under high high torque) torque) with with this this phase-change phase-change heat heat pipe-cooling pipe-cooling scheme. scheme. In addition, In addition, thethe power densitydensity of of the the PMSM PMSM was was enhanced enhanced while while reducing reducing the peak the temperature peak temperature by by 22.3%22.3% (under rated rated conditions) conditions) (Figure (Figure 31)[ 31)62]. [62].

Energies 2021, 14, 1472 26 of 33

FigureFigure 29.29. HeatHeat pipe-based pipe-based air-cooling air-cooling with with straight-embedded straight-embedded module module (SEME) (SEME) [62]. [62].

Figure 30.30.Heat Heat pipe-based pipe-based air-cooling air-cooling with with 3D rounding 3D rounding module module (RM) [ 62(RM)]. [62].

Figure 31. Temperature change with time using heat pipe-based air-cooling under rated conditions [62].

6.7. Potting Silicon Gelatin (PSG) Cooling Investigations into air-cooling for electric motors in EVs are complicated in certain areas with poor heat transfer abilities. Secondly, concerning electric motor topology, the direct liquid-cooling of traction motors could be difficult to introduce. The decay of oil at hot spots inside the windings is a major problem in direct liquid cooling, which would impact the heat exchange. Forced air-cooling and direct liquid-cooling approaches, therefore, face both spatial and inherent issues. Investigators identified and tested liquid- cooled motors with thermally conductive potting materials. The electric motor end winding was permeated with a conventional epoxy-based substance and potted with thermally conductive epoxy. A single material was impregnated with common resins in the global potting motor. The results indicated that the heat transfer was improved considerably by moving from the copper winding to the cooling ducts’ fitted housing. Investigations established that enhancing the heat transfer resulted in a higher power density and enhanced motor life with lower winding temperatures. The end winding potting was designed and applied to a real size PM water-cooled traction motor by researchers. On a potted motor and an unpotted motor, the test findings were examined under steady-state, running and duty cycle conditions. The results indicated a substantial enhancement in the thermal efficiency of the traction motor under both steady-state and duty cycle applications. Furthermore, in the stator and rotor, potting lessened the temperature substantially. The optimal direct heat transfer from the hot spot (end

Energies 2021, 14, 1472 26 of 33

Energies 2021, 14, 1472 26 of 32

Figure 30. Heat pipe-based air-cooling with 3D rounding module (RM) [62].

FigureFigure 31.31. TemperatureTemperature change change with with time time using using heat heat pipe-based pipe-based air-cooling air-cooling under under rated rated conditions [62]. conditions [62]. 6.7. Potting Silicon Gelatin (PSG) Cooling 6.7. PottingInvestigations Silicon Gelatin into air-cooling (PSG) Cooling for electric motors in EVs are complicated in certain areasInvestigations with poor heat into transfer air-cooling abilities. for Secondly, electric concerningmotors in EVs electric are motorcomplicated topology, in certain the di- rectareas liquid-cooling with poor heat of tractiontransfer motors abilities. could Seco bendly, difficult concerning to introduce. electric The motor decay topology, of oil at hotthe spotsdirect inside liquid-cooling the windings of traction is a major motors problem could in be direct difficult liquid to cooling,introduce. which The woulddecay of impact oil at thehot heatspots exchange. inside the Forced windings air-cooling is a major and directproblem liquid-cooling in direct liquid approaches, cooling, therefore, which would face bothimpact spatial the andheat inherent exchange. issues. Forced Investigators air-cooling identified and direct and testedliquid-cooling liquid-cooled approaches, motors withtherefore, thermally face both conductive spatial pottingand inherent materials. issues. The Investigator electric motors identified end winding and tested was perme-liquid- atedcooled with motors a conventional with thermally epoxy-based conductive substance potting and materials. potted with The thermally electric conductivemotor end epoxy.winding A singlewas permeated material was with impregnated a conventional with commonepoxy-based resins substance in the global and potting potted motor. with Thethermally results conductive indicated thatepoxy. the A heat single transfer materi wasal was improved impregnated considerably with common by moving resins from in thethe copperglobal windingpotting motor. to the coolingThe results ducts’ indica fittedted housing. that the Investigations heat transfer established was improved that enhancingconsiderably the by heat moving transfer from resulted the copper in a higher winding power to the density cooling and ducts’ enhanced fitted motor housing. life withInvestigations lower winding established temperatures. that enhancing The end th windinge heat transfer potting resulted was designed in a higher and applied power todensity a real and size PMenhanced water-cooled motor life traction with motorlower bywinding researchers. temperatures. On a potted The motor end winding and an unpotted motor, the test findings were examined under steady-state, running and duty Energies 2021, 14, 1472 potting was designed and applied to a real size PM water-cooled traction motor27 of by 33 cycle conditions. The results indicated a substantial enhancement in the thermal efficiency researchers. On a potted motor and an unpotted motor, the test findings were examined of the traction motor under both steady-state and duty cycle applications. Furthermore, under steady-state, running and duty cycle conditions. The results indicated a substantial in the stator and rotor, potting lessened the temperature substantially. The optimal direct enhancement in the thermal efficiency of the traction motor under both steady-state and heatwinding) transfer to the from coolant-fitted the hot spot frame (end winding)by conduction to the was coolant-fitted facilitated frame by the by stator conduction potting duty cycle applications. Furthermore, in the stator and rotor, potting lessened the was(Figure facilitated 32) [16,21,57]. by the stator potting (Figure 32)[16,21,57]. temperature substantially. The optimal direct heat transfer from the hot spot (end

FigureFigure 32.32. StatorStator pottingpotting effectivenesseffectiveness ofof conductiveconductiveheat heattransfer transferfrom from end end winding winding [ 57[57].].

TheThe temperaturetemperature analysis analysis ﬁndings findings for for the th prototypese prototypes revealed revealed that thethat winding the winding temperaturetemperature was was lessened lessened considerably considerably by both by both potting potting methods. methods. The The same same thermal thermal effect effect as withas with the the end end winding winding potting potting could could be achieved be achieved with with global global potting potting using using resins resins with with con- considerably poorer thermal conductivity. The positions of temperature sensors are denoted by line patterns in Figure 33.

Figure 33. Rise in temperature with and without end winding potting [57].

7. Future Scope The design of an effective electric traction motor is very much the need of the hour, as the demand for EV is gaining momentum all over the globe. Improved and more efficient motor development is possible with a well-designed thermal model and with an effective thermal simulation. The thermal model should also consist of cooling schemes in the pursuit of enhanced output power, torque density and efficiency. An improvised thermal model of an electric traction motor is used to predict and investigate heat release in various parts of the motor, as they differ significantly with the operative conditions of the motor (see Figure 34). Highly thermally conductive and electrically insulating polymer materials, with good elastic modulus, are hugely sought after for the next generation of electric machines and electronic devices.

Energies 2021, 14, 1472 27 of 33

winding) to the coolant-fitted frame by conduction was facilitated by the stator potting (Figure 32) [16,21,57].

Figure 32. Stator potting effectiveness of conductive heat transfer from end winding [57].

Energies 2021, 14, 1472 The temperature analysis findings for the prototypes revealed that the winding27 of 32 temperature was lessened considerably by both potting methods. The same thermal effect as with the end winding potting could be achieved with global potting using resins with siderablyconsiderably poorer poorer thermal thermal conductivity. conductivity. The positions The positions of temperature of temperature sensors aresensors denoted are bydenoted line patterns by line inpatterns Figure in33 .Figure 33.

FigureFigure 33.33.Rise Rise inin temperaturetemperature withwith andand withoutwithoutend endwinding windingpotting potting [ 57[57].].

7.7. FutureFuture ScopeScope TheThe designdesign ofof anan effectiveeffective electricelectric tractiontraction motormotor isis veryvery muchmuch thethe needneed ofof thethe hour,hour, asas thethe demanddemand forfor EVEV isis gaininggaining momentummomentumall all over over the the globe. globe. ImprovedImproved andand moremore efficientefficient motormotor developmentdevelopment isis possiblepossible withwith aa well-designed well-designed thermal thermal model model and and with with an an effectiveeffective thermalthermal simulation.simulation. TheThe thermalthermal modelmodel shouldshould alsoalso consistconsist ofof cooling cooling schemes schemes inin thethe pursuitpursuit ofof enhancedenhanced outputoutput power,power, torquetorque densitydensity andand efficiency. efficiency. An An improvised improvised thermalthermal modelmodel ofof anan electricelectric tractiontraction motormotor isisused usedto topredict predict and and investigate investigate heat heat release release inin variousvarious partsparts ofof thethe motor,motor, as as they they differ differ significantly significantly with with the the operative operative conditions conditions of of the motor (see Figure 34). Highly thermally conductive and electrically insulating polymer Energies 2021, 14, 1472 the motor (see Figure 34). Highly thermally conductive and electrically insulating28 of 33 materials, with good elastic modulus, are hugely sought after for the next generation of polymer materials, with good elastic modulus, are hugely sought after for the next electric machines and electronic devices. generation of electric machines and electronic devices.

Figure 34. 34. FuturisticFuturistic permanent permanent magnet magnet synchronous synchronous moto motorr with with highly highly conductive conductive composites composites [86]. [86]. Future research into this topic may consist of advanced thermo-mechanical and Future research into this topic may consist of advanced thermo-mechanical and thermo-electrical analyses of the PMSM motors in order to verify the optimal solutions thermo-electrical analyses of the PMSM motors in order to verify the optimal solutions of of the cooling schemes and to minimize the hot spots. In addition, future investigations the cooling schemes and to minimize the hot spots. In addition, future investigations would extend proposed and reviewed thermal mapping approaches for the high-speed would extend proposed and reviewed thermal mapping approaches for the high-speed motor spindles and high-power motors, with included uncertainties. Another topic for motor spindles and high-power motors, with included uncertainties. Another topic for future research is to compare the different motor topologies and their performances, which future research is to compare the different motor topologies and their performances, depend on the most suitable cooling system. which depend on the most suitable cooling system. 8. Summary and Conclusions 8. Summary and Conclusions In this paper, a review of the thermal mapping and cooling solutions has been under- takenIn for this the paper, permanent a review magnet of synchronousthe thermal machinesmapping (PMSMs).and cooling The solutions major results has ofbeen the undertakenmotors’ thermal for the analysis, permanent structure magnet design synchr andonous optimization machines (PMSMs). are presented The basedmajor results on the of the motors’ thermal analysis, structure design and optimization are presented based on the review of the recent most important research works and reports. Particularly, this literature review discussed thermal and structural design methodologies and cooling schemes for the PMSM traction machines, with an emphasis on the efficiency improvements over output powers and rotor speeds. The following outcomes are highlighted: • Traction motors for multi-quadrant operation and synchronization need a high controllability which, due to their high power, is a challenging task. In this context, the thermal augmentation of the motor core, insulation and permanent magnets are the main factors that influence the machine’s performance and life span; • Thermal mapping is an essential tool providing thermal motor optimization in order to meet the energy efficiency requirements of modern motors, wherein the PMSMs are mainly developed with novel topologies, namely stator-core solutions, multiphase, and reconfigurable windings; • Nowadays, there are noticeable trends towards higher maximum speeds and motor power density, which cause increased heat losses. These losses cause a number of undesired thermal, thermo-mechanical, thermos-electrical, and tribological effects/uncertainties; • The high-power motor needs an effective cooling system wherein air-cooling is combined with the more effective liquid-cooling. In order to achieve a high motor performance, the effective cooling system must be managed with the smart thermal control system using multiple temperature sensor feedbacks; • Many factors, such as motor topology, geometry, structural design, materials, insulation, etc., have a major influence on motor efficiency and electromagnetic/electrical performance; • Effective numeric simulation tools enable the calculation, modeling, design and simulation of the motor operations in different operating environments. This practice speeds up motor optimization, whereby many motor numeric models can be tested very fast and easily; • The major trend in motor topologies is toward the adoption of permanent magnet synchronous machines with high power density and cost reductions;

Energies 2021, 14, 1472 28 of 32

review of the recent most important research works and reports. Particularly, this literature review discussed thermal and structural design methodologies and cooling schemes for the PMSM traction machines, with an emphasis on the efficiency improvements over output powers and rotor speeds. The following outcomes are highlighted: • Traction motors for multi-quadrant operation and synchronization need a high controllability which, due to their high power, is a challenging task. In this context, the thermal augmentation of the motor core, insulation and permanent magnets are the main factors that influence the machine’s performance and life span; • Thermal mapping is an essential tool providing thermal motor optimization in order to meet the energy efficiency requirements of modern motors, wherein the PMSMs are mainly developed with novel topologies, namely stator-core solutions, multiphase, and reconfigurable windings; • Nowadays, there are noticeable trends towards higher maximum speeds and motor power density, which cause increased heat losses. These losses cause a number of undesired thermal, thermo-mechanical, thermos-electrical, and tribological effects/uncertainties; • The high-power motor needs an effective cooling system wherein air-cooling is combined with the more effective liquid-cooling. In order to achieve a high motor performance, the effective cooling system must be managed with the smart thermal control system using multiple temperature sensor feedbacks; • Many factors, such as motor topology, geometry, structural design, materials, insulation, etc., have a major influence on motor efficiency and electromagnetic/ electrical performance; • Effective numeric simulation tools enable the calculation, modeling, design and simulation of the motor operations in different operating environments. This practice speeds up motor optimization, whereby many motor numeric models can be tested very fast and easily; • The major trend in motor topologies is toward the adoption of permanent magnet synchronous machines with high power density and cost reductions; • Active cooling approaches have become established in PMSM motors due to their fundamentally higher cooling potential than passive cooling solutions; • Fluid-based cooling systems are mainly used for cooling both the stator and the rotor, and are more effective in comparison with air-cooling methods. In particular, the indirect cooling of these components by means of cooling channels in the shaft is widely applied; • Air-based cooling is only applied if the occurring heat losses are low. The main advantage of air-based cooling systems is the considerably lower requirement for space of the peripheral equipment. However, as the air warms up, the cooling effect is small, and in addition, this approach leads to increased acoustic pollution due to turbulent airflow; • Liquid cooling around the stator and through the rotor enables direct operation with a high performance for high-power motors. Using axial channels between the stator windings, the winding temperatures can be reduced up to 60–80%; • The rate of temperature increase of the oil-cooled motor is slower than that of the cooled water-cooled motor. However, these methods have a lower potential in dissipating the heat in comparison with the oil spray-cooling solution; • The cooling solutions using cooling tubes and microchannels enable us to bring down the temperatures at the hot spots and to increase motor thermal efficiency; • In order to increase the heat transfer intensity from the copper winding to the cooling medium, the electric motor’s end winding is potted with thermally conductive epoxy, wherein potting silicon gelatin (PSG) cooling is the most promising solution; • The fast development of advanced and new insulation materials based on carbon fibers (CF–MgO), Nylon6, and mica-epoxy, enable us to achieve better and better motor temperature robustness and efficiency values. These new materials are able to Energies 2021, 14, 1472 29 of 32

offer improved prospects for the design of great performance and/or low-cost motors with innovative manufacturing approaches; • Thermal mapping is an essential tool providing thermal motor optimization in order to meet the energy efﬁciency requirements of modern motors, whereby the PMSMs are mainly developed with novel topologies, namely stator-core solutions, multiphase, and reconﬁgurable winding; • Design optimization achieves both high manufacturing quality and low manufacturing costs, whereby assembling errors can be cleared out, and the low-cost manufacturing technology employed can be used to achieve robust design optimization, incorporating manufacturing tolerances and motor production costs.

Author Contributions: Conceptualization, E.G., R.S.R. and D.G.S.; methodology, E.G.; validation, A.M. and A.I.; formal analysis, E.G. and R.S.R.; investigation, E.G., A.M. and A.I.; writing—original draft preparation, E.G., R.S.R. and D.G.S.; writing—review and editing, A.M. and A.I.; supervision, E.G. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: Authors are thankful to the managements of Vellore Institute of Technology, India and Faculty of Electrical Engineering, Bialystok University of Technology, Poland, for their continued support. This research is supported by Bialystok University of Technology projects (WZ/WE-IA/4/2020 and WZ/WE-IA/2/2020) and financed from a subsidy provided by the Ministry of Science and Higher Education. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Kuntanapreeda, S. Traction control of electric vehicles using sliding-mode controller with tractive force observer. Int. J. Veh. Technol. 2014, 2014, 829097. [CrossRef] 2. Chan, C.C.; Cheng, M. Vehicle Traction Motors. In Encyclopedia of Sustainability Science and Technology; Meyers, R.A., Ed.; Springer: New York, NY, USA, 2012; pp. 11522–11552. 3. Huynh, T.A.; Hsieh, M.F. Performance analysis of permanent magnet motors for electric vehicles (EV) traction considering driving cycles. Energies 2018, 11, 1385. [CrossRef] 4. Singh, K.V.; Bansal, H.O.; Singh, D. A comprehensive review on hybrid electric vehicles: Architectures and components. J. Mod. Transp. 2019, 27, 77–107. [CrossRef] 5. Rahman, M.A.; Chiba, A.; Fukao, T. Super high speed electrical machines—Summary. In Proceedings of the 2004 IEEE Power Engineering Society General Meeting, Denver, CO, USA, 6–10 June 2004. [CrossRef] 6. Tenconi, A.; Vaschetto, S.; Vigliani, A. Electrical machines for high-speed applications: Design considerations and tradeoffs. IEEE Trans. Ind. Electron. 2014, 61, 3022–3029. [CrossRef] 7. Henke, M.; Narjes, G.; Hoffmann, J.; Wohlers, C.; Urbanek, S.; Heister, C.; Steinbrink, J.; Canders, W.-R.; Ponick, B. Challenges and opportunities of very light high-performance electric drives for aviation. Energies 2018, 11, 344. [CrossRef] 8. Cupertino, F.; Leuzzi, R.; Monopoli, V.G.; Cascella, G.L. Design Procedure for High-Speed PM Motors Aided by Optimization Algorithms. Machines 2018, 6, 5. [CrossRef] 9. Rens, J.; Vandenbossche, L.; Dorez, O. Iron Loss Modelling of Electrical Traction Motors for Improved Prediction of Higher Harmonic Losses. World Electr. Veh. J. 2020, 11, 24. [CrossRef] 10. Barré, O.; Napame, B. The Insulation for Machines Having a High Lifespan Expectancy, Design, Tests and Acceptance Criteria Issues. Machines 2017, 5, 7. [CrossRef] 11. Guo, H.; Ding, Q.; Song, Y.; Tang, H.; Wang, L.; Zhao, J. Predicting Temperature of Permanent Magnet Synchronous Motor Based on Deep Neural Network. Energies 2020, 13, 4782. [CrossRef] 12. Frajnkovic, M.; Omerovic, S.; Rozic, U.; Kern, J.; Connes, R.; Rener, K.; Biˇcek,M. Structural Integrity of In-Wheel Motors; SAE Technical Paper; SAE: Warrendale, PA, USA, 2018. [CrossRef] 13. Sarigiannidis, A.; Beniakar, M.; Kakosimos, P.; Kladas, A. Performance evaluation and thermal analysis of interior permanent magnet traction motor over a wide load range. In Proceedings of the 2016 XXII International Conference on Electrical Machines (ICEM), Lausanne, Switzerland, 4–7 September 2016. [CrossRef] 14. Cavazzuti, M.; Gaspari, G.; Pasquale, S.; Stalio, E. Thermal management of a Formula E electric motor: Analysis and optimization. Appl. Therm. Eng. 2019, 157, 113733. [CrossRef] 15. Huang, J.; Shoai Naini, S.; Miller, R.; Rizzo, D.; Sebeck, K.; Shurin, S.; Wagner, J. A Hybrid Electric Vehicle Motor Cooling System- Design, Model, and Control. IEEE Trans. Veh. Technol. 2019, 68, 4467–4478. [CrossRef] 16. Gai, Y.; Kimiabeigi, M.; Chuan Chong, Y.; Widmer, J.D.; Deng, X.; Popescu, M.; Goss, J.; Staton, D.A.; Steven, A. Cooling of automotive traction motors: Schemes, examples, and computation methods. IEEE Trans. Ind. Electron. 2019, 66, 1681–1692. [CrossRef] Energies 2021, 14, 1472 30 of 32

17. Farsane, K.; Desevaux, P.; Panday, P.K. Experimental study of the cooling of a closed type electric motor. Appl. Therm. Eng. 2000, 20, 1321–1334. [CrossRef] 18. Tao, X.; Zhou, K.; Ivanco, A.; Wagner, J.R.; Hofmann, H.; Filipi, Z. A Hybrid Electric Vehicle Thermal Management System Nonlinear Controller Design; SAE Technical Paper; SAE: Warrendale, PA, USA, 2015. [CrossRef] 19. Moreno, J.; Ortúzar, M.E.; Dixon, J.W. Energy-management system for a hybrid electric vehicle, using ultracapacitors and neural networks. IEEE Trans. Ind. Electron. 2006, 53, 614–623. [CrossRef] 20. Zhu, C.; Lu, F.; Zhang, H.; Sun, J.; Mi, C.C. A Real-Time Battery Thermal Management Strategy for Connected and Automated Hybrid Electric Vehicles (CAHEVs) Based on Iterative Dynamic Programming. IEEE Trans. Veh. Technol. 2018, 67, 8077–8084. [CrossRef] 21. Oguma, H.; Tsukimoto, K.; Goya, S.; Okajima, Y.; Ishizaka, K.; Ito, E. Development of Advanced Materials and Manufacturing Technologies for High-efficiency Gas Turbines. Mitsubishi Heavy Ind. Tech. Rev. 2015, 52, 5–14. 22. Bernagozzi, M.; Charmer, S.; Georgoulas, A.; Malavasi, I.; Michè, N.; Marengo, M. Lumped parameter network simulation of a Loop Heat Pipe for energy management systems in full electric vehicles. Appl. Therm. Eng. 2018, 141, 617–629. [CrossRef] 23. Shoai Naini, S.; Huang, J.; Miller, R.; Wagner, J.; Rizzo, D.; Shurin, S.; Sebeck, K. A Thermal Bus for Vehicle Cooling Applications— Design and Analysis. SAE Int. J. Commer. Veh. 2017, 10, 122–131. [CrossRef] 24. Aprianingsih, N.; Winarta, A.; Ariantara, B.; Putra, N. Thermal performance of Pulsating Heat Pipe on Electric Motor as Cooling Application. E3S Web Conf. 2018, 67, 03035. [CrossRef] 25. Jouhara, H.; Chauhan, A.; Nannou, T.; Almahmoud, S.; Delpech, B.; Wrobel, L.C. Heat pipe based systems—Advances and applications. Energy 2017, 128, 729–754. [CrossRef] 26. Lei, G.; Zhu, J.; Guo, Y.; Liu, C.; Ma, B. A Review of Design Optimization Methods for Electrical Machines. Energies 2017, 10, 1962. [CrossRef] 27. Shao, L.; Hartavi Karci, A.E.; Tavernini, D.; Sorniotti, A.; Cheng, M. Design Approaches and Control Strategies for Energy-Efficient Electric Machines for Electric Vehicles—A Review. IEEE Access 2020, 8, 116900–116913. [CrossRef] 28. Qu, B.; Yang, Q.; Li, Y.; Sotelo, M.A.; Ma, S.; Li, Z. A Novel Surface Inset Permanent Magnet Synchronous Motor for Electric Vehicles. Symmetry 2020, 12, 179. [CrossRef] 29. Kalsi, S.; Hamilton, K.; Buckley, R.G.; Badcock, R.A. Superconducting AC Homopolar Machines for High-Speed Applications. Energies 2019, 12, 86. [CrossRef] 30. Heidari, H.; Rassõlkin, A.; Kallaste, A.; Vaimann, T.; Andriushchenko, E.; Belahcen, A.; Lukichev, D.V. A Review of Synchronous Reluctance Motor-Drive Advancements. Sustainability 2021, 13, 729. [CrossRef] 31. Chau, K.T. Stator-Permanent Magnet Motor Drives. In Electric Vehicle Machines and Drives: Design, Analysis and Application; Wiley-IEEE Press: Piscataway, NJ, USA, 2015; pp. 147–194. [CrossRef] 32. Zhang, L.; Fan, Y.; Li, C.; Liu, C. Design and Analysis of a New Six-Phase Fault-Tolerant Hybrid-Excitation Motor for Electric Vehicles. IEEE Trans. Magn. 2015, 51, 8700504. [CrossRef] 33. Asfirane, S.; Hlioui, S.; Amara, Y.; Gabsi, M. Study of a Hybrid Excitation Synchronous Machine: Modeling and Experimental Validation. Math. Comput. Appl. 2019, 24, 34. [CrossRef] 34. Yang, H.; Lin, H.; Zhu, Z.Q. Recent advances in variable flux memory machines for traction applications: A review. CES Trans. Electr. Mach. Syst. 2018, 2, 34–50. [CrossRef] 35. Kuptsov, V.; Fajri, P.; Trzynadlowski, A.; Zhang, G.; Magdaleno-Adame, S. Electromagnetic Analysis and Design Methodology for Permanent Magnet Motors Using MotorAnalysis-PM Software. Machines 2019, 7, 75. [CrossRef] 36. Hsieh, M.; Hsu, Y.; Dorrell, D.G.; Hu, K. Investigation on End Winding Inductance in Motor Stator Windings. IEEE Trans. Magn. 2007, 43, 2513–2515. [CrossRef] 37. Cheng, G.; Guo, X.; Wen, Y.; Wang, Q.; Li, G.; Zhou, R. Electromagnetic Modeling and Analysis of 3-DOF Permanent Magnet Spherical Motor Using Magnetic Equivalent Circuit Method. In Proceedings of the 21st International Conference on Electrical Machines and Systems (ICEMS), Jeju, Korea, 7–10 October 2018; pp. 2643–2648. [CrossRef] 38. Chiver, O.; Micu, E.; Barz, C. Stator Winding Leakage Inductances Determination using Finite Elements Method. In Proceedings of the 11th International Conference on Optimization of Electrical and Electronic Equipment, Brasov, Romania, 22–24 May 2008; pp. 69–74. [CrossRef] 39. Zhao, J.; Quan, X.; Jing, M.; Lin, M.; Li, N. Design, Analysis and Model Predictive Control of an Axial Field Switched-Flux Permanent Magnet Machine for Electric Vehicle/Hybrid Electric Vehicle Applications. Energies 2018, 11, 1859. [CrossRef] 40. Barmpatza, A.C.; Kappatou, J.C. Finite Element Method Investigation and Loss Estimation of a Permanent Magnet Synchronous Generator Feeding a Non-Linear Load. Energies 2018, 11, 3404. [CrossRef] 41. Lee, J.-Y.; Luu, P.T. Electric Motor Design of an Integrated Motor Propulsor for Unmanned Vehicles: The Effect of Waterproofing Can. Energies 2020, 13, 2227. [CrossRef] 42. Tsutsui, A.; Fujisawa, R.; Sekiyama, K.; Yamazaki, Y.; Saeki, S.; Koiwai, K. Loss Analysis of Electric Motors in Hybrid Excavator. Kobelco Technol. Rev. 2019, 37, 15–20. 43. Ramarathnam, S.; Mohammed, A.K.; Bilgin, B.; Sathyan, A.; Dadkhah, H.; Emadi, A. A Review of Structural and Thermal Analysis of Traction Motors. IEEE Trans. Transp. Electrif. 2015, 1, 255–265. [CrossRef] 44. Xu, X.; Han, Q.; Chu, F. Review of Electromagnetic Vibration in Electrical Machines. Energies 2018, 11, 1779. [CrossRef] 45. Krings, A.; Boglietti, A.; Cavagnino, A.; Sprague, S. Soft Magnetic Material Status and Trends in Electric Machines. IEEE Trans. Ind. Electron. 2017, 64, 2405–2414. [CrossRef] Energies 2021, 14, 1472 31 of 32

46. Simizu, S.; Ohodnicki, P.R.; McHenry, M.E. Metal Amorphous Nanocomposite Soft Magnetic Material-Enabled High Power Density, Rare Earth Free Rotational Machines. IEEE Trans. Magn. 2018, 54, 1–5. [CrossRef] 47. Aruna, P.; Vasan, P.V. Review on Energy Management System of Electric Vehicles. In Proceedings of the 2nd International Conference on Power and Embedded Drive Control (ICPEDC), Chennai, India, 21–23 August 2019; pp. 371–374. [CrossRef] 48. Serpi, A.; Porru, M. A Multi-Stage Energy Management System for Multi-Source Hybrid Electric Vehicles. In Proceedings of the IECON 2019—45th Annual Conference of the IEEE Industrial Electronics Society, Lisbon, Portugal, 14–17 October 2019; pp. 5901–5908. [CrossRef] 49. Morishita, T.; Katagiri, Y.; Matsunaga, T.; Muraoka, Y.; Fukumori, K. Design and fabrication of morphologically controlled carbon nanotube/polyamide-6-based composites as electrically insulating materials having enhanced thermal conductivity and elastic modulus. Compos. Sci. Technol. 2017, 142, 41–49. [CrossRef] 50. Ghouse, S.M.; Venkatesh, S.; Rajesh, R.; Natarajan, S. Effects of SiO2 and TiO2 Nano Fillers in Enhancing the Insulation Breakdown Strength of Epoxy Nano Composite Dielectric under Divergent Electric Fields. Int. J. Electr. Eng. Inf. 2013, 5, 501–518. [CrossRef] 51. Lei, Z.; Men, R.; Wang, F.; Li, Y.; Song, J.; Shahsavarian, T.; Li, C.; Fabiani, D. Surface modified nano-SiO2 enhances dielectric properties of stator coil insulation for HV motors. IEEE Trans. Dielectr. Electr. Insul. 2020, 27, 1029–1037. [CrossRef] 52. Zhang, J.; Du, Z.; Zou, W.; Li, H.; Zhang, C. MgO nanoparticles-decorated carbon fibers hybrid for improving thermal conductive and electrical insulating properties of Nylon 6 composite. Compos. Sci. Technol. 2017, 148, 1–8. [CrossRef] 53. Koreeda, T.; Matos, J. Thermal characterization of mica–epoxy composite used as insulation material for high voltage machines. J. Therm. Anal. Calorim. 2011, 106, 619–623. [CrossRef] 54. Glowacz, A. Fault diagnosis of electric impact drills using thermal imaging. Measurement 2021, 171, 108815. [CrossRef] 55. Lipo, T.A. Thermal Design. In Introduction to AC Machine Design; John Wiley & Sons, Ltd.: Hoboken, NJ, USA, 2017; pp. 305–357. [CrossRef] 56. Kim, K.; Feng, S.S. Thermal Mapping Using Infrared Thermography. In Application of Thermo-Fluidic Measurement Techniques; Kim, T., Lu, T.J., Song, S.J., Eds.; Butterworth-Heinemann: Oxford, UK, 2016; pp. 215–250. [CrossRef] 57. Nategh, S.; Boglietti, A.; Barber, D.; Liu, Y.; Brammer, R. Thermal and Manufacturing Aspects of Traction Motors Potting: A Deep Experimental Evaluation. IEEE Trans. Energy Convers. 2020, 35, 1026–1035. [CrossRef] 58. Roy, P.; Towhidi, M.; Ahmed, F.; Bourgault, A.J.; Mukandan, S.; Balamurali, A.; Kar, N.C. A Comprehensive Review of Thermal Design and Analysis of Traction Motors. In Proceedings of the 2019 IEEE 28th International Symposium on Industrial Electronics (ISIE), Vancouver, BC, Canada, 12–14 June 2019; pp. 203–208. [CrossRef] 59. Nollau, A.; Gerling, D. A new cooling approach for traction motors in hybrid drives. In Proceedings of the 2013 International Electric Machines & Drives Conference, Chicago, IL, USA, 12–15 May 2013; pp. 456–461. [CrossRef] 60. Nollau, A.; Gerling, D. A flux barrier cooling for traction motors in hybrid drives. In Proceedings of the 2015 IEEE International Electric Machines & Drives Conference (IEMDC), Coeur d’Alene, ID, USA, 10–13 May 2015; pp. 1103–1108. [CrossRef] 61. Sun, Y.; Zhang, S.; Chen, G.; Tang, Y.; Liang, F. Experimental and numerical investigation on a novel heat pipe based cooling strategy for permanent magnet synchronous motors. Appl. Therm. Eng. 2020, 170, 114970. [CrossRef] 62. Fang, G.; Yuan, W.; Yan, Z.; Sun, Y.; Tang, Y. Thermal management integrated with three-dimensional heat pipes for air-cooled permanent magnet synchronous motor. Appl. Therm. Eng. 2019, 152, 594–604. [CrossRef] 63. Kang, M.; Wang, H.; Guo, L.; Shi, T.; Xia, C. Self-circulation cooling structure design of permanent magnet machines for electric vehicle. Appl. Therm. Eng. 2020, 165, 114593. [CrossRef] 64. Guo, F.; Zhang, C. Oil-cooling method of the permanent magnet synchronous motor for electric vehicle. Energies 2019, 12, 2984. [CrossRef] 65. Pugachev, A.A.; Kosmodamianskiy, A.S. Investigation of induction motor temperature distribution in traction applications. IOP Conf. Ser. Earth Environ. Sci. 2017, 87.[CrossRef] 66. Beck, C.; Schorr, J.; Echtle, H.; Verhagen, J.; Jooss, A.; Krüger, C.; Bargende, M. Numerical and experimental investigation of flow phenomena in rotating step-holes for direct-spray-cooled electric motors. Int. J. Engine Res. 2020.[CrossRef] 67. Gundabattini, E.; Kuppan, R.; Solomon, D.G.; Kalam, A.; Kothari, D.P.; Abu Bakar, R. A review on methods of finding losses and cooling methods to increase efficiency of electric machines. Ain Shams Eng. J. 2020.[CrossRef] 68. EL-Refaie, A.M.; Alexander, J.P.; Galioto, S.; Reddy, P.B.; Huh, K.; de Bock, P.; Shen, X. Advanced high power-density interior permanent magnet motor for traction applications. IEEE Trans. Ind. Appl. 2014, 50, 3235–3248. [CrossRef] 69. Tikadar, A.; Kumar, N.; Joshi, Y.; Kumar, S. Coupled Electro-Thermal Analysis of Permanent Magnet Synchronous Motor for Electric Vehicles. In Proceedings of the 2020 19th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, USA, 21–23 July 2020. [CrossRef] 70. Ruuskanen, V.; Nerg, J.; Pyrhonen, J. Effect of lamination stack ends and radial cooling channels on no-load voltage and in-ductances of permanent-magnet synchronous machines. IEEE Trans. Magn. 2011, 47, 4643–4649. [CrossRef] 71. Kefalas, T.D.; Kladas, A.G. Thermal Investigation of Permanent-Magnet Synchronous Motor for Aerospace Applications. IEEE Trans. Ind. Electron. 2014, 61, 4404–4411. [CrossRef] 72. Asef, P.; Perpina, R.B.; Barzegaran, M.R. An innovative natural air-cooling system technique for temperature-rise suppression on the permanent magnet synchronous machines. Electr. Power Syst. Res. 2018, 154, 174–181. [CrossRef] 73. Fawzal, A.S.; Cirstea, R.M.; Woolmer, T.J.; Dickison, M.; Blundell, M.; Gyftakis, K.N. Air inlet/outlet arrangement for rotor cooling application of axial flux PM machines. Appl. Therm. Eng. 2018, 130, 1520–1529. [CrossRef] Energies 2021, 14, 1472 32 of 32

74. Bornschlegell, A.S.; Pelle, J.; Harmand, S.; Fasquelle, A.; Corriou, J. Thermal Optimization of a High-Power Salient-Pole Electrical Machine. IEEE Trans. Ind. Electron. 2013, 60, 1734–1746. [CrossRef] 75. Jaeger, M.; Ruf, A.; Hameyer, K.; Tongeln, T.G. Thermal Analysis of an Electrical Traction Motor with an Air Cooled Rotor. In Proceedings of the 2018 IEEE Transportation Electrification Conference and Expo (ITEC), Long Beach, CA, USA, 13–15 June 2018; pp. 194–200. [CrossRef] 76. Popescu, M.; Goss, J.; Staton, D.A.; Hawkins, D.; Chong, Y.C.; Boglietti, A. Electrical Vehicles—Practical Solutions for Power Traction Motor Systems. IEEE Trans. Ind. Appl. 2018, 54, 2751–2762. [CrossRef] 77. Sim, K.; Lee, Y.B.; Jang, S.M.; Kim, T.H. Thermal analysis of high-speed permanent magnet motor with cooling flows supported on gas foil bearings: Part I—Coupled thermal and loss modeling. J. Mech. Sci. Technol. 2015, 29, 5469–5476. [CrossRef] 78. Wang, R.J.; Heyns, G.C. Thermal analysis of a water-cooled interior permanent magnet traction machine. In Proceedings of the 2013 IEEE International Conference on Industrial Technology (ICIT), Cape Town, South Africa, 25–28 February 2013; pp. 416–421. [CrossRef] 79. Zhang, B.; Qu, R.; Fan, X.; Wang, J. Thermal and mechanical optimization of water jacket of permanent magnet synchronous machines for EV application. In Proceedings of the 2015 IEEE International Electric Machines & Drives Conference (IEMDC), Coeur d’Alene, ID, USA, 10–13 May 2015; pp. 1329–1335. [CrossRef] 80. Tikadar, A.; Johnston, D.; Kumar, N.; Joshi, Y.; Kumar, S. Comparison of Electro-Thermal Performance of Advanced Cooling Techniques for Electric Vehicle Motors. Appl. Ther. Eng. 2021, 183, 116182. [CrossRef] 81. Fan, X.; Li, D.; Qu, R.; Wang, C.; Fang, H. Water Cold Plates for Efficient Cooling: Verified on a Permanent-Magnet Machine with Concentrated Winding. IEEE Trans. Ind. Electr. 2020, 67, 5325–5336. [CrossRef] 82. Nategh, S.; Huang, Z.; Krings, A.; Wallmark, O.; Leksell, M. Thermal Modeling of Directly Cooled Electric Machines Using Lumped Parameter and Limited CFD Analysis. IEEE Trans. Energy Conver. 2013, 28, 979–990. [CrossRef] 83. Gai, Y.; Chong, Y.C.; Adam, H.; Deng, X.; Popescu, M.; Goss, J. Thermal Analysis of an Oil-Cooled Shaft for a 30 000 r/min Automotive Traction Motor. IEEE Trans. Ind. Appl. 2020, 56, 6053–6061. [CrossRef] 84. Zi-Chao, Z.; Qiang, S.; Ahmed, B. Innovative Design of the Cooling Topologies for Electric Vehicle Motors. IOP Conf. Ser. Mater. Sci. Eng. 2019, 533, 012021. [CrossRef] 85. Yao, Z.; Saadon, Y.; Mandel, R.; McCluskey, F.P. Cooling of Integrated Electric Motors. In Proceedings of the 2020 19th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, USA, 21–23 July 2020; pp. 660–665. [CrossRef] 86. Pang, D.-C.; Shi, Z.-J.; Xie, P.-X.; Huang, H.-C.; Bui, G.-T. Investigation of an Inset Micro Permanent Magnet Synchronous Motor Using Soft Magnetic Composite Material. Energies 2020, 13, 4445. [CrossRef]