1763457.pdf

1763457.pdf

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  1 Electric Drive TechnologyTrends, Challenges, and Opportunities for Future Electric Vehicles Iqbal Husain, Fellow, IEEE, Burak Ozpineci, Fellow, IEEE, Md Sariful Islam, Student Member, IEEE, Emre Gurpinar, Senior Member, IEEE, Gui-Jia Su, Senior Member, IEEE, Wensong Yu, Member, IEEE, Shajjad Chowdhury, Member, IEEE, Lincoln Xue, Senior Member, IEEE, Dhrubo Rahman, Student Member, IEEE and Raj Sahu, Member, IEEE Abstract—The transition to electric road transport technolo- gies requires electric traction drive systems to offer improved performances and capabilities such as fuel efficiency (in terms of MPGe i.e., miles per gallon of gasoline-equivalent), extended range, and fast charging options. The enhanced electrification and transformed mobility are translating to a demand for higher power and more efficient electric traction drive systems that lead to better fuel economy for a given battery charge. To accelerate the mass-market adoption of electrified transportation, the US Department of Energy (DOE), in collaboration with the automotive industry, has announced the technical targets for light-duty electric vehicles for 2025. This paper discusses the electric drive technology trends for passenger electric and hybrid electric vehicles with commercially available solutions in terms of materials, electric machine and inverter designs, maximum speed, component cooling, power density, and performance. The emerging materials and technologies for power electronics and electric motors are presented, identifying the challenges and opportunities for even more aggressive designs to meet the need for next generation electric vehicles. Some innovative drive and motor designs with the potential to meet the DOE 2025 targets are also discussed. Index Terms—EV, HEV, Traction inverter, Electric machines, Drive technology, Heavy rare-earth free machines, WBG inverter I. INTRODUCTION THE global number of electric vehicles (EVs) sold that include both battery EVs (BEVs) and plug-in hybrid EVs (PHEVs) exceeded the five million mark in 2019 with the ratio between BEVs and PHEVs tilting toward the former. The adoption rate has been steadily increasing, taking only six months for one million BEVs to be sold as opposed to the five years it took for the first million BEVs to be sold. Environmental concerns and energy challenges prompted the This manuscript has been co-authored by UT-Battelle, LLC, under con- tract DE-AC05-00OR22725 and NC State University under contract DE- EE0008705 with the US Department of Energy (DOE). The publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Iqbal Husain, Md Sariful Islam, Wensong Yu, and Dhrubo Rahman are with the department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC, USA 27606 ([email protected]; [email protected]; [email protected]; [email protected]). Burak Ozpineci, Emre Gurpinar, Gui-Jia Su, Shajjad Chowdhury, Lincoln Xue, and Raj Sahu are with the Oak RidgeNational Laboratory, Knoxville, TN, USA ([email protected]; [email protected]; [email protected]; chowd- [email protected]; [email protected]; [email protected]). societal demand for clean, efficient, and sustainable vehicles for urban transportation. Traditional original equipment man- ufacturers and new generation manufacturers are responding to the demand with many models of varying range and features. In the past few years, this drastic improvement in vehicle electrification has coincided with a radical transfor- mation of society’s understanding of transportation in which autonomous driving and mobility as a service have opened the door to freedom of movement. These changes of im- proved electrification and transformed mobility translate to a demand for higher power and more efficient electric traction drive systems that lead to better fuel economy for a given battery charge. Recent evolution of wide-bandgap (WBG) semiconductor-based drives, with their capabilities of high- frequency and high-temperature operation, is a catalyst to increase the operating speed of traction machines. Winding, lamination, and permanent magnet material developments in recent years are enabling electric motor design to push the boundaries for maximum speed and power/torque density with design innovations. A. Electric Drivetrain Targets The US Department of Energy (DOE), in collaboration with the US Council for Automotive Research, has announced targets for electric passenger vehicles for 2025 in a roadmap developed in 2017 [1]. DOE aims to reach 33 kW/L power density, 300,000 mile/15-year lifetime, and $6/kW cost for a 100 kW electric traction drive to enable a highly efficient, compact, reliable, and affordable building platform for pas- senger vehicles. This is an 88% reduction in volume and 25% reduction in cost while doubling the target power and lifetime mileage compared with 2020 goals. For the inverter, a 100 kW integrated design with 100 kW/L at $2.70/kW is expected. This represents an 18% cost reduction and 87% volume reduction compared with 2020 goals. For the motor, the power density target is 50 kW/L at a cost of $3.30/kW, which represent an 89% reduction in volume and 30% reduction in cost compared with 2020 goals. To achieve these targets by 2025, the inverter components have to be integrated more intimately using power modules with the latest WBG technology. New materials and optimiza- tion technologies will also be required to reduce volume while new thermal management techniques will be applied to take the heat out more efficiently. For the motor, high-power density topologies and high-efficiency materials will help reach the tar- gets. The rest of this paper describes some of the recent power
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  2 electronics and electricmotors technologies being proposed to accelerate adoption of electrified transportation. B. Trends in Inverter Design Traction inverters used in BEVs/HEVs are dominated by the 3-phase voltage source inverter (VSI) topology because of its high efficiency, low cost, and simple control requirements. The inverter along with the filters, gate drive, sensors, and controller are shown in Fig. 1. The two key components of the inverter drive are the power devices and the direct-current (DC)-link capacitors; the former is by far the most expensive component, accounting for around 50% of the drive cost. The power device of choice adopted by the automotive industry since the introduction of the first modern-day EV, EV1 in 1996, is the Si-IGBT (insulated-gate bipolar transistor), which is still favored because of its cost, decent efficiency, and good short circuit capability of around 10 µs. Recent advances in WBG devices, particularly SiC and GaN devices, promise significant power density and efficiency increases in traction inverter drives. A large DC-link capacitor (Cdc) is used to reduce the ripple current and voltage generated due to the pulse-width modulated (PWM) operation of the inverter stage. These bulky capacitors used in automotive traction inverters are large polypropylene film capacitors chosen for the working voltage (1.2 to 1.5 times the DC-link nominal voltage), root mean square (RMS) current, operating temperature, lifetime, and parasitic inductance and resistance. The inverter power devices, shielded alternating-current (AC) cables, and motor all have significant parasitic capacitances to the ground, which is the vehicle chassis. The high dv/dt due to each PWM switching event at the AC inverter terminals induces common mode (CM) pulses to the ground, which cause severe electro- magnetic interference (EMI). CM filters, an example of which is shown in Fig. 1, are used to mitigate the EMI. The amount of CM choke and capacitors allowed on the DC bus is limited by the SAE International Standard J1772 for DC charging. The parasitic capacitance of the traction electric motor between its winding and chassis ground (denoted by Cp2 in Fig. 1) can be significant and is typically between 5 and 25 nF. In systems in which the inverter and the motor are separated by a distance and connected through shielded AC cables, there will be additional parasitic capacitances. These capacitances and the dv/dt from the PWM voltage waveforms of the inverter will result in significant CM currents such as motor inter-turn currents and bearing currents, which essentially have to be supplied by the inverter. Exceedingly high dv/dt can result in insulation damage of the motor windings. Although an output dv/dt filter can be placed, the best solution is a tightly coupled packaging such as mounting the inverter directly to the motor housing as has been done in the Chevy Volt [2]. The key parameters and specifications of select traction inverters are given in Table I. All the above example inverters are built with Si-based IGBTs, except for Tesla Model 3, which uses SiC MOSFETs (metal–oxide–semiconductor field- effect transistors) as the power devices for the inverter. Among the IGBT-based solutions, Tesla Model S 70D, introduced in Fig. 1. Traction inverter along with its interfacing components and controller. 2012, also has a unique feature of being built with 36 TO- 247 discrete IGBTs, whereas all other inverters are built with IGBT modules. Among the vehicles listed in Table I, only the Toyota Prius 2016 model, which is an HEV, has a DC- DC boost converter in front of the VSI. Modern EVs do not have this boost converter for cost savings and to keep the efficiency of the electric drive high. A few takeaways from Table I are the nominal DC-link voltage of 400 V for BEVs and PHEVs, the maximum fundamental frequency of 1,200 Hz, and the best reported power density of 30.1 kW/L for the Tesla Model S 70D inverter. The maximum fundamental frequency is limited by the maximum inverter PWM switching frequency of IGBT-based solutions where a PWM frequency that is around an order of magnitude higher is needed for an adequate motor current controller bandwidth. With the advent of SiC technology, 800 V nominal DC- link voltage is being considered for passenger vehicles to provide system-level benefits to the EV powertrain. A higher DC-link voltage offers mass savings opportunities across the vehicle, which translates to better mileage range for the same battery capacity. Higher voltage levels mean the cables have to carry less current, which would reduce mass. The reliability of DC-link capacitors can also be increased in higher-voltage, lower-current inverters by avoiding the need to parallel the film capacitors. Traction inverters with high power ratings (100–500 kW) are expected in the future as more BEV models are introduced, and as the fast-charging infrastructure expands. The overarching design goal for traction inverters is enhancing high efficiency and reliability while maintaining low cost, volume, and mass. To achieve higher power density and re- duced weight, the trend is toward higher switching frequency, use of WBG semiconductor devices, and tightly integrated packaging with advanced thermal management systems and materials. The appealing features of high-temperature and low-loss operation of SiC devices open up opportunities for integrated motor drives. Manufacturing of traction inverters directly into the machine housing effectively reduces the mass, volume and cost of the system. C. Trends in High-Speed Electric Machine Design The electric motor for propulsion in the EVs/HEVs is desired to have a high starting torque to meet acceleration re- quirements, high power density to reduce volume, and high ef- ficiency to extend the battery range. The candidate motor drive should also have a wide constant power speed range (CPSR)
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  3 TABLE I REPRESENTATIVE SETOF TRACTION INVERTERS USED IN COMMERCIALLY AVAILABLE EVS/HEVS Model Type DC-link (V) Max. f1 (Hz) Power (kW) Power density (PD) (kW/L) Specific power (kW/kg) Nissan Leaf (2012) BEV 345 693 80 7.1 4.7 Tesla Model S 70D (2015) BEV 375 493 193 30.1 33.3 Chevy Volt (2016) PHEV 430 800 180 17.3 21.7 Cadillac CT6 (2016) PHEV 360-430 667 215 22.6 16 Toyota Prius (2016) HEV 600 1133 162 23.7 13.6 Audi A3 e-Tron (2016) PHEV 396 — 75 9.4 7.4 Tesla Model 3 (2017) BEV 375/400 900 200 — — US DOE (2025) BEV — — 100 100 — f1 : fundamental frequency, Max.: maximum TABLE II REPRESENTATIVE SET OF ELECTRIC MACHINES USED IN COMMERCIALLY AVAILABLE EVS/HEVS Model (year) Power (kW) Torque (Nm) Max. speed (r/min) CPSR Slot/pole Windings Cooling kW/L Toyota Prius (2010) 60 207 13,500 4.87 48/8 Distributed Water-jacket 4.8 Nissan Leaf (2012) 80 280 10,390 3.8 48/8 Distributed Water-jacket 4.2 Tesla Model S 60 (2013) 225 430 14,800 2.96 —/4 Distributed Water-jacket — with shaft cooling Honda Accord (2014) 124 — 14,000 — 12/8 Concentrated — 2.9 BMW i3 (2016) 125 250 11,400 3.0 72/12 Distributed Water-jacket 9.1 Chevy Volt (2016) 125 370 12,000 3.71 72/12 Distributed-Hairpin Oil cooled — Tesla Model 3 (2017) 192 410 18,000 4.02 54/6 Distributed — — Toyota Prius (2017) 53 163 17,000 5.4 48/8 Distributed-Hairpin Water-jacket 5.7 Nissan Leaf (2017) 80 280 10,390 3.8 48/8 Distributed Water-jacket 4.2 Chevy Bolt (2017) 150 360 8,810 2.20 48/8 Distributed-Hairpin Oil cooled — BMW iX3 (2020) 210 400 15,000 3 48/6 Hairpin — — Nissan Leaf (2020) 160 340 — — 48/8 Distributed Water-jacket — US DOE (2025) 100 144 20,000 ≥ 3 — — — 50 to facilitate a single-gear transmission stage which would help improve the transmission power density and simplify its controls. However, the power density of the electric machine has to be sacrificed to achieve a wider CPSR; in addition, the wide CPSR would also lead to an increase in the motor drive power requirements. Flexible drive control, high reliability, fault tolerance, and low acoustic noise are also essential features for the electric motor drive unit for an BEV/HEV application. The recent BEVs and HEVs primarily employ two types of machines: AC induction machines and interior permanent magnet (IPM) machines. The IPM synchronous motor (IPMSM) using NdFeB magnets has became the design choice of traction electric machines primarily because of the unparalleled power/torque density and efficiency that can be achieved to meet the demanding requirements of an electric traction drivetrain. The rotor structure is also robust since the magnets are buried inside the rotor providing inherent magnet retention. Although the cost is high for IPM machines because of the use of rare-earth and heavy rare-earth materials, they are preferred because no other machine can match the power density of IPMSMs. However, there are efforts to design IPMs with non-heavy rare earth magnet materials to match the performance of those that are common in today’s BEVs. From a motor controls perspective, the permanent magnet (PM) machines without the rotor cage have a low inertia that helps the electrical response time, although the induction motor electrical response characteristics will be the fastest because of the smaller electrical time constant determined by the motor’s leakage inductances. With a higher power density, Fig. 2. Copper and magnet mass distributions of IPM motors in production vehicles. the IPMSM is smaller in size than an induction motor with the same power rating. IPMSMs are more efficient and easier to cool because of the absence of rotor copper loss compared with the induction machines. Induction motors have lower cost and zero cogging torque because of the absence of permanent magnets. Induction motors can sustain a higher peak stator current at several times the rated current without the danger of demagnetizing the magnets. The key specifications of electric machines used in produc- tion BEVs/HEVs in recent years are summarized in Table II [3]–[7]. All the machines listed in Table II are IPMSMs except for the Tesla Model S 60, which is an induction machine. The design trend over the years has been to increase the number of poles and the maximum speed to increase the torque and
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  4 (a) (b) (c) Fig.3. IPM rotors: (a) double-V-shape in Chevy Bolt 2016 [3], (b) V-shape in Tesla Model 3 2017 [4], and (c) U-shape in Toyota Prius 2017 [5]. (a) Double-layer winding (b) Hairpin winding (Active length: 60mm) (c) Concentrated winding Fig. 4. Stator windings: (a) BMW i3 2016 [5], (b) Toyota Prius 2017 [5], and (c) Honda Accord 2005 [3]. power density of these machines since this will help with packaging within the vehicle system platform and improving the fuel efficiency. A high pole count design reduces the end turn length and the amount of magnet materials, which helps increase the efficiency and reduces the cost, respectively. The motor designs have also progressed toward reducing the copper-to-magnet mass ratio in the higher–power density ma- chines as shown in Fig. 2. Higher maximum speed operation helps increase the power density and reduce the system mass, although the burden then falls on the transmission gear, which must proportionally increase in gear ratio to match the motor speed with the vehicle wheel speed. The design trend has also been to increase the DC-link voltage to accommodate the higher back-EMFs (electromotive forces) of higher-speed machines. The limitation on maximum speed comes from the high-frequency motor losses and the drive and transmission constraints. The CPSR for the available EV/HEV motors has been in the range of 3 to 4. The high speed electric machines is a viable option ap- proach for increasing the BEV powertrain drive power density although the approach burdens the transmission system and its design. An alternative approach to the high speed electric machine path for BEV applications is the direct drive in-wheel (IW) electric machine which is also being actively pursued by several manufacturers. One of the configurations for IW approach is the axial flux machine that has the potential to offer the highest torque density as has been demonstrated by the YASA and General Motors prototypes [8], [9]. However, the more attractive solution for an IW configuration from the manufacturing point of view is the radial flux configuration [10], [11]. The advantages of the IW machines are: increased space for passenger and battery-pack, individual motor control at each wheel providing better ride and performance, and elimination of the mechanical gears and transmission. There are also several concerns that require attention for the mass adoption of IW direct drive machines. These include manage- ment of unsprung mass, torque disturbance due to faults, and requirement of larger amount of PM material to achieve high torque at low speed which would increase cost. The concerns are quite challenging for IW direct drives which makes the high speed, low torque machines with mechanical gear as the predominant choice in the commercially available BEVs. The maximum operating speed and pole number combi- nation in production BEVs/HEVs translates to a maximum fundamental frequency of 1,200 Hz. The constraint on the motor fundamental frequency also comes from the bandwidth requirement of the current controller supplied by the IGBT- based inverter drives whose maximum PWM frequency is around 10 kHz. The bandwidth constraint will be alleviated once the industry adopts WBG-based drives, which offer the opportunity for a much higher PWM frequency. The desired performances of traction electric machines with high power density, high efficiency, and low torque ripple with negligible cogging torque are achieved through a comprehen- sive rotor and stator design. On the rotor side, three variants of rotor designs are common for IPM machines used in traction applications: V-shape, double-V-shape, and U-shape, which are shown in Fig. 3. Each of the designs has its advantages and disadvantages [12]. Generally, the double V-shape rotor has the highest torque density and efficiency but has higher magnet losses, which burdens the thermal management. The magnet utilization is the best for V-shape rotors but would have relatively lower corner speeds than the other two design types due to higher no-load voltage. The U-shape is a trade-off between the V- and double-V designs that offers some design flexibility. On the stator side, the winding choices are between dis- tributed vs. concentrated and between stranded vs. hairpin. The distributed windings create sinusoidal airgap flux density or have a minimum presence of harmonic contents in their
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  5 magneto-motive force (MMF).The distributed windings can be divided into two categories: single-layer winding and double-layer winding (two-phase winding can share the slot). The double-layer distributed winding has fewer harmonic con- tents compared with the single-layer winding. Also, the dis- tributed winding–based IPM machines have higher reluctance torque than the concentrated winding–based IPM machines. However, concentrated winding, which is used in the Honda Accord, has the advantage of short end turn lengths, higher fill factor, and modular structure [13], but it suffers from unwanted MMF harmonics [14], which create high core loss and magnet loss. The third winding option is the hairpin winding, which has a better slot fill factor, power density, overload capability, and thermal performance compared with the stranded conductor–based winding. Hairpin/bar conductors are present in GM designs and the Toyota Prius 2017 model. The downside of hairpin winding is the AC conductor loss, especially during high-speed/high-frequency operation [15], and the output power decreases at a faster rate beyond the base speed of the machine. Considering the pros and cons, the preferred winding choice for the traction motors has been the stranded conductor with distributed windings. Several stators of different EVs/HEVs are presented in Fig. 4. For the machine design, slot/pole/phase q is a unifying parameter that incorporates several constraints of motor de- sign with respect to maximum fundamental frequency, high speed losses, and mode order for NVH (noise, vibration and harshness) performance. In general, the greatest common denominator between slot and pole numbers (GCD (slot, pole)) is the dominant vibration mode order. The higher mode orders are less problematic for NVH performance because the core deformation is inversely proportional to the fourth-order of mode order [16]. In the industry, slot/pole/phase q = 2 is the most common (7 out of 10 motors in Table II have q = 2) because it is a reasonable compromise to limit the stator cost, high speed losses, the maximum fundamental frequency to 1.2 kHz, and the minimum mode order to 8. The power density of an electric machine strongly depends on the effectiveness of the heat extraction method. Forced liq- uid cooling is the popular choice for traction electric machines. Among different forced liquid cooling methods, the housing water jacket cooling method is the most common cooling approach. The typical cooling fluid is a 50% water/50% ethylene-glycol mixture. In the water jacket method, cooling channels are situated in a thermally conductive frame [17] at the outer surface of the stator. The heat generated in the coils, stator core, and rotor core is initially transferred to the housing jacket through conduction and then moved to ambient via convection of the cooling fluid. Even though the housing water jacket is efficient to cool down the coil, it is inadequate to dissipate the heat generated in the end-winding because of the high thermal resistance between the source and the sink. Therefore, a need exists to develop an efficient end-winding cooling technique in conjunction with a housing water jacket to increase the electric machine’s power density. II. TRACTION INVERTER DRIVE Advanced power devices, novel materials, new capacitor variants, and application specific heat sinks provide the oppor- tunity to significantly boost the power density and performance of traction inverter drives, although they bring many design challenges that need to be addressed. In this section, the com- ponent developments and features are presented along with the design challenges and solutions for the next-generation high- power density traction inverters. A. Power Semiconductor Devices Power semiconductor devices based on wide-bandgap (WBG) and ultra wide-bandgap (UWBG) materials are con- sidered to be the disruptive technology for high-performance power electronic systems. The superior material properties of WBG (SiC, GaN), such as high-bandgap Eg, breakdown field Ec, saturation velocity vs, and thermal conductivity λ, enable the development of power semiconductor devices that can have increased power handling capabilities with smaller sizes and reduced losses in comparison with well-established unipolar and bipolar Si-based devices. The specific on-resistance of a unipolar active power semiconductor (e.g., MOSFETs) drift re- gion based on different semiconductor materials is presented in Fig. 5. WBG power devices can enable highly efficient power electronic systems with increased switching frequency and power density. Furthermore, ultra-WBG (diamond, Ga2O3) materials can take the power device performance beyond the limits of WBG-based devices and enable high voltage (≥10 kV) unipolar devices for various applications such as motor drives, transmission and distribution systems. SiC devices have been identified as the key solutions for high switching frequency, high voltage, and high-temperature applications since the beginning of the century [18]. Currently, there are commercially available vertical SiC MOSFET and diode dies from multiple suppliers up to 1.7 kV blocking voltage and 13 mΩ on-state resistance. The performance of higher-voltage SiC devices, at 3.3, 6.5, and 10 kV, are not yet commercially available from multiple suppliers due to market challenges but are presented as promising solutions for high- voltage systems [19]. GaN devices emerged into the low-voltage (<600 V) power device market, with lateral enhancement-mode high-electron- mobility transistors (HEMTs) structure, on a Si substrate. The 2D electron gas formed by a GaN-AlGaN layer in an HEMT provides excellent switching and conduction perfor- mance, and the Si substrate provides a competitive device cost in comparison with Si MOSFETs. As seen in Fig. 5, the vertical GaN outperforms the SiC and Si counterparts in terms of specific on-resistance [21]. However, the lateral structure limits the development of high-voltage HEMTs with low on-state resistance due to lateral blocking, and low thermal conductivity of the Si substrate. It should also be noted that lateral devices with higher blocking voltage will occupy larger area in the wafer, which will lead to lower number of devices per wafer manufactured that increases the cost. On the other hand, high cost of the GaN substrate limits the commercialization of vertical GaN devices despite the fact that
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  6 Fig. 5. Specificon-resistance of a unipolar device drift region based on Baliga’s figure of merit [20]. high performance is validated in high voltage applications. Vertical GaN is expected to be a competitive alternative to SiC above the 900 V blocking class. The system-level benefits of WBG devices accelerated the development of ultra-WBG devices, based on gallium oxide (Ga2O3), which has almost 4.4 times higher energy bandgap and 26.6 times higher breakdown field than Si. These proper- ties make Ga2O3 an excellent candidate for high-temperature, high-voltage power devices, as presented in Fig. 5. However, the thermal conductivity of Ga2O3 is at least 5 times less than Si and 16 times less than SiC, and is foreseen as the main bar- rier for the application of Ga2O3 in high-power applications. Recent development of vertical Ga2O3 diodes show promising results for future power electronic applications. B. Integrated Power Modules Power semiconductor modules used in inverters are re- sponsible for electric power transfer between the source and the load. The efficiency of such systems has improved considerably because of recent advancements in WBG-based power semiconductor devices such as SiC, MOSFETs, and GaN HEMTs. However, despite attaining high efficiencies, a significant amount of power is dissipated in a small area because of increased power demand from the electrical load, increased power density of power modules, and reduced chip size. Therefore, the performance of the materials used for packaging, integration of power modules, and design of thermal management systems have emerged as focal points of the next generation of power electronic systems, especially in application domains related to EVs [22]. The illustration of a conventional power module cross- section is shown in Fig. 6, where various components of the structure are highlighted. The structure is composed of different materials such as aluminum for bond wires, copper for the electrical terminals, and ceramic-based direct-bonded copper substrate (DBC). This multi-layered, multi-material- based structure has limited heat extraction capabilities because of layer count and limited lateral heat spreading. SiC MOSFET-based power modules are offered by major device and module manufacturers such as Infineon, Wolfspeed, ROHM, and Semikron for a variety of circuit topologies. Op- erating temperatures of these modules are limited to 150–175 ◦ C, with a structure based on the illustration in Fig. 6. The commercial SiC dies are rated up to 175◦ C to eliminate reliability issues observed at the gate interface and body diode at elevated temperatures [23]. Although the maximum allowable temperature in WBG-based modules is not higher than Si-based modules, the temperature dependency of the electrical performance of SiC MOSFETs, is less than Si counterparts. Therefore, using SiC MOSFETs in traction drive systems can reduce the cooling requirements by elevating the operating device temperature without compromising system performance. In EV applications, liquid cooled heat sinks are commonly used with a 50%-50% water-ethylene glycol mix as the coolant with 65◦ C inlet temperature. WBG power electron- ics also provide the opportunity for sharing a single cooling loop of 105◦ C for multiple powertrain components, thereby eliminating the need for additional 65◦ C or 80◦ C cooling loops and simplifying the thermal management system. Some commercial SiC modules use direct substrate cooling (by eliminating the base plate) to improve thermal performance; some examples are Infineon Easy 1B and Semikron MiniSKiiP modules [24]. All these packages used to manufacture SiC- based modules were initially designed for Si-IGBT devices. The advantages of these packages are low cost, high matu- rity of the design, and easy adoption by design engineers. However, they do not meet the needs of the high-performance power electronics packaging for WBG devices because of high parasitic inductance for single-layer conductor layout, and high thermal impedance due to multi-material layers in between the device and the heat sink. To overcome the challenges of the commercial power mod- ules, several high power density power electronics packaging architectures have been proposed by academia and industry. GE Global Research has proposed an embedded power module structure approach called “GE Power Overlay” (POL) [25]. Siemens has also proposed an embedded power module struc- ture called “Siemens SiPLIT.” [26], shown in Fig. 7(a). Delphi has developed a bespoke double-sided planar mod- ule for SiC devices based on paralleled SiC MOSFET dies sandwiched between two DBC substrates [27], shown in Fig. 7(b). Unlike the other solutions presented earlier, this structure allows double-sided cooling but accommodates only Fig. 6. Illustration of a conventional power module using a DBC-based substrate (TIM: thermal interface material).
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  7 (a) (b) (c) Fig. 7. Highpower density modules, (a) Siemens SiPLIT POL structure, (b) Delphi Viper structure, and (c) Oak Ridge National Laboratory planar-bond-all structure. one switch (five dies in parallel per switch) per module. Oak Ridge National Laboratory (ORNL) has also developed a double-sided power module architecture targeted for WBG devices [28]. The planar-bond-all structure of the proposed architecture is shown in Fig. 7(c). The package features sandwiching of power semiconductor switches between two DBC substrates and using copper shims to eliminate wire bonds for the power loop. Two cold plates (coolers) are directly bonded to the outside of these substrates, allowing double-sided, integrated cooling. The enclosed area of the main current loop in this new interconnection configuration is reduced dramatically with the replacement of wire bonds with copper shims. The elimination of wire bonds leads to a significant reduction in electrically parasitic inductance and resistance, allowing for full use of WBG switches. As an alternative to DBC for improved thermal performance in SiC based power modules, ORNL, in collaboration with Momentive and Henkel, has developed and experimentally validated an advanced graphite-core insulated metal substrate (IMS) substrate. This substrate is based on IMS technol- ogy, where the thermally annealed pyrolytic graphite (TPG)- encapsulated copper core replaces the solid copper core. The graphite material contains millions of stacked graphene layers, showing excellent thermal conductivity and very low mass density, thus increasing overall thermal performance and decreasing the overall weight of the substrate. The structure of the graphite-core IMS is shown in Fig. 8. Graphite-embedded substrates can provide increased current density for SiC MOS- FETs regardless of the thermal management strategy employed to cool the substrate. Using this technology improves the current density of the power module by 10% (Fig. 8(c)). SiC MOSFETs demand more stringent requirements than those for Si-IGBTs for short-circuit protection. Compared with Si-IGBTs, SiC MOSFETs behave differently under short- circuit and overcurrent faults and have higher short-circuit current densities and smaller thermal capacitances due to smaller die sizes than similarly rated Si-IGBTs. These lead to a large difference in short-circuit withstand time, 2-4 µs for SiC MOSFETs vs. 10 µs for Si-IGBTs. Therefore SiC MOSFETs require not only a much faster response time but also higher noise immunities for protection circuits. These stringent requirements present design challenges for applying the desaturation-based protection circuits widely used for Si- IGBTs. One alternative is to use some of the MOSFET cells for drain current-sensing and thus avoids many of the challenges. In addition, soft turn-off is more important for SiC MOSFETs to prevent destructively high spike voltages due to their faster switching speeds. In the area of reliability assessment, understanding the failure modes and mechanisms in WBG devices is a critical ongoing effort particularly since these are likely to be used at elevated temperatures and higher fields than their Si- IGBT counterparts. JEDEC, the global standards body for the microelectronics industry, has a dedicated committee JC-70: Wide Bandgap Power Electronic Conversion Semiconductors to pursue WBG standards activities. JEDEC JESD-22 standard for reliability assessment of packaged devices and AEC-Q101 (AEC stands for Automotive Electronics Council) for stress assessment in discrete devices are currently being used by suppliers to qualify WBG parts for automotive applications. In Europe, the power module qualification is standardized under AQG 324: Automotive Power Module Qualification maintained by European Center for Power Electronics (ECPE). C. DC-Link Capacitors Capacitors are one of the essential passive components used in a traction inverter to keep the DC-link voltage constant, suppress high-frequency current components, and regulate current flow. The vast majority of these capacitors are used in a VSI to decouple the load from the supply; thus, a capacitor absorbs a large ripple current due to the inverter switching action. The DC-link capacitor for a three-phase VSI can take up to 60% of the RMS load current [29]. Since the capacitor bank must store a certain amount of energy to maintain a stable DC voltage level, the DC-link capacitor takes up a substantial amount of space, limiting the inverter’s power density. Several types of capacitors can be used in EV traction applications, which can be divided into two primary groups: electrostatic and electrolytic capacitors. Although the elec- trolytic capacitors are the most popular choice for conventional motor drive applications, their short lifespan, limited current conduction capability, and low-frequency operation make them untenable for use as DC-link capacitors in EV inverters. In contrast, electrostatic capacitors have nonpolar construc- tion in which plastic films and ceramic are used as dielectrics,
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  8 (a) (b) (c) Fig. 8. (a)TPG-embedded copper core, (b) TPG-embedded IMS, and (c) current density comparison of SiC MOSFET with different substrates [28]. while a variety of materials are used as electrodes. Among all the electrostatic capacitors, the polymer film is used as the DC-link capacitor for electric drive application because of its reliability, high-current conduction capability, high-frequency operation, and lower losses compared with aluminum elec- trolytic capacitors. Film capacitors, which use plastic/polymers as the dielectric, have very low-temperature dependency; thus, the change in the dielectric characteristic is minimal. The rela- tive permittivity of these dielectrics is low (e.g., 2–3), and film capacitors are therefore bulkier than electrolytic capacitors for the given capacitance. Furthermore, the operating temperature of commercially available film capacitors is low (i.e., 105◦ C) and has a limit on self-temperature rise (10–20◦ C) [30], thus necessitating an active cooling strategy. Capacitors based on glass-based dielectric show promise to overcome the challenges associated with achieving a high breakdown voltage using alkali-free glass material [31]. How- ever, the material itself is rigid and can crack from mechanical and thermal stress. Moreover, the dielectric constant of the material is not high enough to compete with some of the ceramic-based capacitors. Ceramic capacitors, which use ceramic dielectrics with high Fig. 9. Impedance variation of film, MLCC, and PLZT-based capacitors. dielectric constants, are promising candidates to improve the power density of EV traction inverters. These capacitors have a much higher RMS current rating and can withstand higher temperatures than film capacitors. Depending on dielectric materials, ceramic capacitors can be classified into three categories: class 1, class 2, and lead-lanthanum-zirconate- titanate (PLZT)-based capacitor. The class 1 dielectric has a low temperature, and a DC bias dependency; thus they can be used in an application in which constant capacitance is required (e.g., resonant tank, filter applications). The class 2 ceramic capacitors have much higher energy density than the class 1 ceramic capacitors; thus, class 2 ceramic capacitors can be a suitable choice for DC-link applications. The most common class 2 dielectric is barium titanate, which is a ferroelectric dielectric material, and its parameters are highly temperature dependent. Moreover, the capacitance of class 2 ceramic capacitors decreases rapidly with the DC bias voltage. Reliability issues are also associated with ceramic capacitors; the ceramic dielectric material is rigid and can crack from mechanical and thermal stress, thus creating a short-circuit between DC terminals. Therefore, class 2 multilayer ceramic capacitors (MLCCs) have not gained popularity for safety- critical applications, such as EV traction inverters. The PLZT-based ceramic capacitors show characteristics slightly different from the class 2 ceramic capacitors; unlike ferroelectric materials, capacitance increases with DC bias [32]. The PLZT capacitor uses an antiferroelectric dielectric material that can withstand higher currents and temperatures. PLZT capacitors have better reliability since they use series connection of two MLCC geometries in one component, meaning the capacitor will be operational in the event of a crack in the dielectric. In [33], the results of highly accelerated life testing are presented comparing class 2 and PLZT-based capacitors, where the PLZT-based capacitors show a much lower failure rate. The PLZT-based capacitor also shows a decrease in capacitance after a certain temperature, thus al- lowing natural current balancing among the parallel branches. Additionally, the self-resonant point of these capacitors is also at a higher frequency, thus allowing these capacitors to be used for high-frequency applications (Fig. 9). The use of ceramic-based capacitors increases the inverter’s power density but also imposes a challenge on electrical per- formance. Unlike the film and aluminum electrolytic capaci- tors, the ceramic capacitor cannot be produced as a large block,
• 9.
  9 Fig. 10. Segmentedtraction drive system. mainly because of the brittle nature of ceramic materials; thus, several hundreds of them will be required for DC-link application. Therefore, proper packaging of this large number of capacitors is needed; otherwise, it may introduce current asymmetry among the parallel capacitor branches that can lead to thermal runaway. Moreover, additional layout inductance will lead to significant voltage overshoot across semiconductor devices. Special care is needed when packaging these large number of capacitors for EV traction applications. D. Segmented Inverter for Capacitor Volume Reduction A design approach for reducing the capacitor volume is by using different inverter topologies such as the segmented inverter [34], [35] that can significantly reduce the DC-link ripple current and capacitance requirement. Fig. 10 illustrates the modifications of a standard VSI based drive (Fig. 1) to the segmented drive system. The inverter switches and motor stator windings are respectively separated into two sets of switches (indicated in orange and blue in the figure) and two sets of windings (a1, b1, c1) and (a2, b2, c2). Each group of switches (orange or blue) is connected as a three-phase inverter bridge and connects to one set of the stator windings, forming an independent drive unit. Because switches in high power inverter modules are comprised of multiple switch and diode dies connected in parallel, only minor modifications to the switch connections are needed to form the segmented arrangement. Fig. 11 plots a comparison of simulated capacitor ripple cur- rents at various levels of power factors vs. inverter modulation index for the standard and segmented inverters. The capacitor ripple currents are normalized against the rms value of the phase current. The plots show a more than 50% reduction in peak capacitor ripple currents with the segmented inverter which results in needs for smaller capacitors. Between using newer smaller volume capacitor technologies and new inverter topologies that do not require large DC-link capacitances, the overall capacitor size can be reduced to improve the inverter power density. E. Inverter Design Optimization WBG devices are generally desired to be operated at high frequencies to realize high power density of the traction drive by reducing the size of passive components. Under fast switching conditions, the impact of high di/dt on the parasitic inductances cannot be ignored, especially in multichip high- power modules. The challenges are to be overcome through Fig. 11. Comparison of simulated capacitor ripple currents at various power factors vs inverter modulation index for the standard and segmented drives. design optimization while evaluating the adverse effects using advanced analytical tools. 1) Inverter Loop Inductance Minimization: In inverter ap- plications, the parasitic commutation loop inductance deter- mines the magnitude of the voltage spike across the switches at turn-off. High commutation loop inductance leads to large voltage spikes, which can cause device breakdown. Tradi- tionally, copper-based laminated busbars have been the go-to solution to ensure high efficiency and safe operation in trac- tion applications, and the operating frequency was generally limited to 10-20 kHz. With the increased operating frequencies used in the WBG systems and the attendant increase in signal edge rates, the busbar design plays a critical role in traction applications for system performance, safety, efficiency, and electromagnetic emissions. The commutation loop in the inverter is established by the the laminated DC bus and the DC-link capacitor bank. More than one commutation loop may exist in the inverter as shown in Fig. 12 [36]. The inverter’s DC bus voltage is constrained by the voltage overshoot resulting from the energy stored in the parasitic inductance (Lp) seen by the power module’s drain and source power terminals during turn-off. This parasitic inductance, combined with higher di/dt, negatively affects the power module voltage and current use in two ways: First, voltage overshoot, ∆Vovershoot = −Lp · di/dt (1) adds to the DC bus voltage which constrains it to an artificially low value because of the safety margin. Second, module current must also be limited or slowed down during turn- off to limit the di/dt which will result in additional switching losses penalizing the device thermally and the overall system efficiency. 2) Inverter Busbars: The DC bussing is a multi-physical design problem in the electrical, thermal, and mechanical domains. Electrically, the DC bus needs to be a low equivalent series resistance (ESR) (i.e., high-conductivity material, large cross-sectional conduction area) and low equivalent series
• 10.
  10 inductance (ESL) (i.e.,thin and wide planes) structure. The main DC bussing design parameters affecting ESR and ESL include copper thickness and the width and length of the DC+ and DC- planes, and separation distance between them. Thermally, a low temperature rise (e.g., less than 80◦ C from room temperature) due to the maximum expected RMS current passing through the structure is required. Mechanically, a high level of robustness against normal shock and vibration during use is necessary. The laminated busbar is widely used in high-power and integrated converters for its low parasitic inductance advantage to restrain the voltage overshoot, EMI, and switching losses [37], [38]. In the design approach, effort must be made to ensure that magnetic field cancellation is maximized, thereby minimizing the parasitic inductance of the bussing system seen by the power module. The following design guidelines are applied to achieve a low and balanced inductance structure: 1) The length of the commutation loop is designed to be as short as possible through compact placement of the components while ensuring that the thermal and temperature rise requirements of the busbars are met. 2) The busbar layers are placed as close as possible to reduce the distance between the commutation loops, while ensuring appropriate isolation requirements. Opti- mization can be done by reducing the number of layers, or changing the order of stacking the laminated layers. 3) Components are to be distributed symmetrically for a balanced design of the commutation loops. With an increased demand for high-power density power converters, busbar form-factor and interconnection with power modules play significant roles in the overall system-level de- sign. A higher level of power module and passive component packaging and integration can be achieved by using a PCB- based busbar architecture compared to a laminated busbar. The high density packaging is more convenient in a stacked-layer design approach to increase the power density of the overall inverter [39]. Routing of high current conductors through a multi-layer PCB reduces the overall volume while adding flexibility. The multi-layer busbar with thick copper traces can be designed to minimize the overall loop inductance, and this also enables the mounting of the DC-bus and local snubber capacitors in a symmetric layout using minimum amount of space. The power modules are positioned such that the copper traces connect directly onto the module pins. Because of the high bus voltage, the PCB design should adhere to Fig. 12. DC-power loop current paths for a half-bridge power module [36]. Fig. 13. PCB busbar for 135 kW SiC-based traction inverter. various standards (such as UL-796, IPC-2221a specifications) to withstand voltages in the range of several kilo-Volts between the conductive elements on the board. Furthermore, thermal stress needs to be considered so that the temperature rise during operation does not exceed the capability of the PCB material. In the PCB busbar design shown in Fig. 13, minimization of the loop inductance is addressed by selecting a vertical stacking architecture with the power planes distributed in a specific order [36]. For the DC side, the positive and negative power planes are stacked in pairs with maximum overlapping between the positive and negative layers. Designing the busbar in this way fully uses the magnetic coupling existing between the PCB conducting layers, thus enabling effective flux can- cellation. In the case of AC planes, the phases do not have any vertical overlap with each other. The overlap between the AC and DC planes is minimized to ensure that the parasitic capac- itance between the DC and AC power planes is low, thereby reducing the busbar’s contribution to the switching losses. Also, having a pair of DC+ and DC- layers close to the module facilitated in lowering the commutation loop inductance. The shielding layer, which is simply a layer of copper, provides a path of the induced currents and minimizes the loop inductance as well. Taking advantage of the planar architecture of the inverter, DC-link capacitors (typically film-type) have been mounted very close to the power devices. Furthermore, the PCB allows mounting of high-frequency snubber capacitors (ceramic-type) significantly closer to the commutation loop to provide low-impedance decoupling during turn-off to reduce the device voltage spike. 3) WBG Impact on EMI and its Mitigation: The high switching frequency and fast switching times (dv/dt) of WBG embedded PWM inverters pose challenges to EMI issues to the vehicle system and harm the traction motor operation and lifetime. Slowing down the WBG devices to the same level of traditional switching devices can alleviate the issues but also loses the system benefit of efficiency and power-density from upgrading to WBG devices. EMI relies on three necessary aspects: noise source, noise propagation path, and noise victim (or receiver), as depicted in Fig. 14. For EV systems, the switching actions of power inverter is the major noise source. It causes the strongest in-
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  11 Fig. 14. EMIoccurs between noise sources and victims through various propagation paths and mechanisms. terference with the converter control circuitry itself, including gate drivers, isolators, and sensing, leading to false triggering and malfunction. Because of the broadband nature, the noise source prominently affects the performance of AM and FM radio [40]. With the growing trend of equipping advanced driver-assistance system and infotainment, which incorporates more sensors, electronics, actuators, and complex wire harness, EMI becomes even more critical. The International Special Committee on Radio Interference (CISPR) has CISPR 12 [41] and CISPR 25 [42] standards containing limits and procedures for the measurement of radio disturbances. Although CISPR 12 targets protecting off-board receivers from interference from the entire vehicle, CISPR 25 protects the receivers on the vehicle and it contains both whole-vehicle and component-level tests. Notably, although CISPR 12 is required for conformity assessment, CISPR 25 mainly serves as the basis for internal production specifications defined by automotive manufacturer. A typical switching waveform from a power inverter is shown as a trapezoidal waveform (Wav1) in Fig. 15. The speed of transition (dv/dt) is characterized by rise time (tr), and the switching frequency (fsw) can be represented by the pulse width (ton) if assuming 50% duty cycle in this example (ton ≈ 1/fsw/2). The frequency spectrum, plotted on the left as the green curve, clearly shows an envelope with two slopes divided by a frequency governed by the rise time tr. Wav2 is a waveform where both tr and ton are 1/10 of those of Wav1, correlating the characteristics of traditional Si devices and WBG devices. The amplitude of frequency components for Wav2 can be 20 dB higher at high frequency range. More detailed analytical expressions can be found in [43]. Moreover, WBG devices tend to suffer from more severe voltage oscillation due to smaller parasitic capacitance, leading to additional noise components at around the oscillation frequency, as shown in Wav3 (Fig. 15). EMI noise can be suppressed from the source such as through power-loop inductance minimization to reduce the oscillation shown in Wav3 (Fig. 15). The oscillation can also be suppressed by advanced gate driver design through fine adjustment of the switching transitions without increasing switching loss. By varying the switching frequency within a certain range instead of a fixed switching frequency, the “spiky” spectrum can spread across a wider band thereby easing the filtering requirement. It has been reported that a 20 dB reduction on conducted EMI can be achieved with this frequency dithering approach. Using alternative inverter circuit topologies with increased number of switches, it is possible Fig. 15. Typical 50% duty-cycle switching waveforms in power converter (right) and their frequency spectrums (left). While Wav1 depicts Si device switching waveforms, Wav2 and Wav3 can represent SiC/GaN devices switch- ing at higher frequency, faster slew rate, and more oscillation to create more switching states which can theoretically cancel out the EMI noise, especially the CM portion [44]. Filtering and shielding can block the noise from propagation to the victim. As majority of CM noise propagates across parasitic capacitance of the system, it can be expected that the noise at higher frequency for WBG-based system will see an even lower impedance which is easier to leak. Therefore, for a WBG system, it will be more effective to contain the CM noise as close as possible to the noise source. Some research has proposed power module designs with integrated filter and shielding structures [45]. As the general design principle for EMI filter stays similar compared to Si-based inverters, without direct dealing with the noise source and optimizing power module design, the filter size will be likely bigger and heavier if switching at higher frequency with WBG devices [46]. 4) Power Electronics Thermal Management: Thermal man- agement is a necessity to be able to increase the power conversion density of the power electronics. Traditionally, fin- based heat sinks such as straight-fin and pin-fin have been used to cool the power electronic system with air- and liquid cooling-based systems. Liquid-cooling helps to achieve higher power density due to higher heat transfer capabilities of the coolant and are widely employed in automotive and defense applications. Variety of coolants and fin designs have been theoretically and experimentally studied in literature to achieve the best possible cooling of the system. Advanced cooling concepts such as micro-channel cooling, jet impingement, two- phase cold plates, double-sided packaging and direct substrate cooling are also being implemented for such high-power density power electronic applications. With advancements in AI, another area of interest is to use such techniques to develop complex thermal management structures. In this area, ORNL has developed complex heat sink structures using AI, as shown in Fig. 16, which were 3D printed using Additive Manufacturing Technology and have been demonstrated to outperform their conventional counter- parts [47], [48]. These structures were optimized for minimum junction temperatures for high power density applications. In similar direction, new cooling structures are being devel- oped which alleviate intrinsic thermal management issues in power electronics. For example, a thermal imbalance mitiga- tion scheme proposed by Sahu et. al. mitigates the intrinsic
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  12 (a) (b) Fig. 16. 3Dprinted heat sinks developed by ORNL [47], [48]. Fig. 17. AI-optimized heat sink for thermal imbalance mitigation in power module [28]. heat spreading issues in insulated metal-based substrates by developing suitable liquid-cooled heat sinks using AI and Multiphysics finite element simulations (Fig. 17) [28]. III. TRACTION ELECTRIC MOTOR A. Advanced Materials for Electric Machines A comprehensive effort through materials development, cooling techniques, and designs are required to push the state- of-the-art power density of 9.1 kW/L (BMW i3 2016) to 50 kW/L (DOE 2025 goal) for traction electric machines. Cost and availability of raw materials is also an important consideration for the traction electric machines which is driving the materials research and development efforts. Few of the emerging materials are discussed next which include ultra-conductive copper (UCC) conductor for windings, grain boundary diffusion (GBD) processed magnets, and low loss lamination materials. Copper (Cu) is the widely used conductor for windings, but recently, the carbon nanotube (CNT) incorporated Cu windings are also gaining interest for traction electric machines [49]. A coating process on both sides of Cu using CNT layer showed an increase of 14% in conductivity than conventional Cu. The same method can be applied to round Cu to develop a multi-strand Cu-CNT conductor. Another process that has been developed in [50] showed that the Cu-CNT composite could achieve a 28% higher conductivity compared to conven- tional Cu. Although these composites are still commercially unavailable, the UCC conductor’s availability will significantly increase the power density of the electric machines. Sintered neodymium-iron-boron (NdFeB) magnets are the magnet choice for IPM machines because of their high energy density and magnet knee-point well into the third quadrant of the B-H characteristics. However, the cost of these NdFeB magnets, due to the use of heavy rare earth (HRE) materials, is also the primary disadvantage of PM type traction electric machines. The resistance to demagnetization of NdFeB mag- nets, and by extension, its high temperature performance is improved by adding dysprosium (Dy) or terbium (Tb) which are HRE elements. The price instability, supply uncertainty, and the cost of Dy and Tb is a major concern for BEV/HEV manufacturers. Therefore, the reduction or complete removal of HRE contents from magnets without degrading performance will lower the cost of IPM machine in BEVs/HEVs. Two types of magnets, the GBD magnets and the HRE-free magnets are available in the market having less Dy or zero HRE content. In the GBD process, the HRE rich compound is applied at the surface of the magnet which diffuses into the grain boundaries right below the magnet surface, but not so much in the grain boundaries in the interior of the magnet. The result of the diffusion process is that the corners and edges of the magnets are much more rich with HRE rather than in the interior [51]. The corners and the edges are more susceptible to demagnetization in IPM machines. The B-H properties of HRE-free (NEOREC45mhf) and HRE (G48UH) PMs are compared in Fig. 18. It can be seen that both magnets have similar remnant flux density at room temperature. However, the knee point flux density of HRE-free PM is at higher flux density compared to that of HRE PM at any operating temperature. The maximum operating temperature of HRE- free PM is within 150–160◦ C, whereas HRE PM can operate up to 220◦ C, which makes the former more vulnerable to demagnetization. Promising new materials are also emerging for the sta- tor/rotor laminations to improve magnetic properties, and re- duce cores losses which will be useful for meeting the desired goals for the next-generation high-speed electric machines. One of the low loss electrical lamination material is the 6.5% Si steel [52] which is gaining interest for high-speed electric machines. The material has a lower specific core- loss than traditional steel while maintaining similar magnetic properties and has been used for high-frequency transformers and inductors. However, the brittleness of this material during stamping limits its application for mass production. A promis- ing material introduced by GE is the dual-phase material that can control the permeability in selective regions [53]. The dual-phase materials can exhibit non-magnetic behavior in the bridges of the machines while having strong magnetic properties in the other parts of the rotor lamination. These materials can then reduce the flux leakage while maintaining the structural integrity since the bridges are one of the major sources for flux leakage, which adversely affects the power density and power factor of the machine. The main limitation of the dual phase material is a lower saturation flux density
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  13 TABLE III PROPERTIES OFADVANCED MATERIALS Properties 6.5% Conventional Dual M19 HyperCo steel steel phase steel 50 Bsat (T) 1.8 2 1.56 (0.25) 2 2.4 Yield 275 350 275 (565) 350 435 (MPa) Core loss 18 27 — — 20 1 T, 1 kHz µ — — 1100 (1) 2,100 — than M19 steel. The respective properties of the dual phase material and 6% Si steel with respect to conventional materials are presented in [54]. Another lamination material, Hyperco 50, is a 49% cobalt, 2% vanadium, and balanced iron alloy that has the maximum saturation flux density while maintaining excellent mechanical strength. The saturation flux density is 2.4 T, which will help to increase the power density of the machines. The yield strength is 435 MPa for 0.15 mm lamination thickness, which will allow to make thinner bridges. The specific core-loss is 22 W/kg at 800 Hz, 1 T. Therefore, the power density, efficiency, and power factor of the electric machines can be improved significantly using this material. The main impediment for the use of Hyperco in automotive traction applications is its high cost. The advanced lamination materials are summarized in Table III. B. Heavy Rare-Earth Free Machine Designs A couple of promising HRE-free machine designs to meet the DOE 2025 goals, including relative advantages and disad- vantages are discussed in this section. 1) Outer Rotor Halbach PM machine with Slotted Stator: This configuration uses outer rotor topology to maximize the torque density (as opposed to inner rotor topology) as shown in Fig. 19; outer rotor also naturally provides natural retention against centrifugal force. The Halbach configuration maximizes the airgap flux density due to flux concentration. Additionally, the stator is based on fractional slot concentrated winding. The concentrated winding has a shorter end-turn length, high-fill factor, and can have a segmented stator. Fig. 18. B-H properties of HRE and HRE-free PMs. Three different HRE-free configurations with a maximum speed of 20,000 r/min have been compared and reported in [55]. This configuration can have the maximum power density compared to other HRE-free topologies (IPM machine). The challenges of magnet loss during high-speed operation and magnet demagnetization during three-phase short circuit fault are being worked on for further improvement. The outer rotor configurations also leave a hollow space in the middle where the drive can be housed to increase the system-level power density. 2) Slotless-Halbach PM Machine with Embedded Cooling: This outer rotor configuration with Halbach magnets uses a slotless topology with winding embedded liquid cooling (WELC) to reduce the frequency-dependent core-loss and magnet-loss during high-speed operation [56]. This electro- magnetic configuration is illustrated in Fig. 20(a) while the WELC concept is shown in Fig. 20(b). The outer rotor Hal- bach PM maximizes torque density and reduces the harmonic contents of airgap flux density. The stator adopts fractional slot concentrated winding to minimize the end turn extension. The winding support uses commercially available non-magnetic thermally conductive plastic with conductivity varying be- tween 3 to 14 W/m-K [57]. With the winding embedded cool- ing channels passing through the winding support structure, the heat source (winding) comes into the direct contact of the heat sink (low temperature coolant). Additionally, an axial water jacket cooling is added at the stator frame to further improve the thermal performance, as shown in Fig. 20(b). CFD analysis has shown that a maximum current density (for 18 s) of 33 A(rms)/mm2 and a continuous current density of 23 A(rms)/mm2 are feasible [58]. This translates to an improve- ment of 50% over conventional housing water jacket cooling. The main disadvantage of this Slotless-Halbach topology is very high three-phase short circuit fault which increases the risk of demagnetization. 3) Segmented Magnet IPM Machine: The segmented mag- net IPM configuration is very similar to the state-of-the-art configurations using inner rotor topology where each pole has two magnets arranged in V-shape. However, instead of using a single bar in each pocket, each magnet is circum- ferentially segmented into three or five small magnets. The corner magnets are thicker than the middle magnets to provide higher demagnetization withstand capability. The magnets are Fig. 19. Outer rotor Halbach configuration (18-slot/12-pole).
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  14 displaced from thestarting of the cavity which reduces the effect of the demagnetization field on the edges as illustrated in Fig. 21. We consider the design of a segmented magnet IPM ma- chine for demonstrating the potentials and challenges to be overcome for high-speed HRE free IPM machines. In this design, a double layer distributed winding with stranded wire is considered to minimize the torque pulsations and magnet loss. The maximum operating speed and peak power have been considered to be 20,000 r/min and 100 kW, respec- tively, following the DOE 2025 research goal [1]. Following the specifications of the commercially available powertrain system, a CPSR of 3 has been selected that translates to a base speed of 6,667 r/min. The slot/pole combination has been chosen to keep the maximum fundamental frequency less than 1,500 Hz to restrict the frequency-dependent losses within a manageable range. The stator winding consists of a double layer lap winding with a turn/coil ratio of 12. There are four parallel paths in the three-phase winding system so that it is possible to feed from a one-inverter (three-phase) or two parallel inverters (six-phase) or a four-leg inverter source. The detailed design and control methodologies for the IPM machines are presented in [15], [60]. Multi-objective and multi-physics optimizations are re- quired to achieve the target specifications ensuring efficient use of the materials. For the presented design, the objective (a) (b) Fig. 20. (a) Slotless-Halbach machine with integrated drive and (b) WELC concept [58]. Fig. 21. Segmented magnet IPM machine: 2D-FEA model (1/4th model) [59]. TABLE IV DESIGN SPECIFICATIONS AND PERFORMANCES OF SEGMENTED MAGNET IPM MACHINE Parameters Values Outer diameter (mm) 190 Active length (mm) 60 End-turn extension (mm) 40 Volume including end-turn (Liter) 2.83 Peak torque (Nm) ≥ 143 Base speed (r/min) 6,667 Peak power (kW) 100 Power density (kW/Liter) 35 Peak current (A) 230 DC-link voltage (V) 670 Magnet (kg) 0.78 Dy free Magnet (grade) NEOREC45mhf Lamination (Hyperco 50) (mm) 0.15 Slot/pole 48/8 Maximum speed (r/min) 20,000 Winding material Copper-pure functions and constraints are as follows: max(T), min(TR) = f(MW, ML, γ, TW, MA, Ror, BW) VLL ≤ Vdc, MV ≤ 1p.u, J ≤ 30A(rms)/mm2 (2) where T, TR, MW, ML, γ, TW, MA, Ror, BW, VLL, MV, J are torque, torque ripple, magnet width, magnet length, phase advancement angle, magnet angle, rotor outer radius, bridge width, line-line voltage, magnet volume, and current density, respectively. The output of the finite element based optimiza- tion process is the detailed specifications which is provided in Table IV. The subsequent step in the design process is to evaluate the performance data such as torque-speed, power-speed, current-speed, efficiency map, and different loss components. The design process includes a loss analysis to obtain the conductor loss, core loss, magnet loss, and mechanical loss. The analysis helps generate the efficiency map, shown in Fig. 22, for performance evaluation as well as to provide the
• 15.
  15 heat source inputsfor the thermal modeling of the machine. The mechanical losses are not considered for the presented analysis. The conductor loss has been extracted at 100◦ C to take care of the temperature effect on loss [60]. The core loss is obtained using 2D-FEA for a lamination thickness of 0.15 mm. Punching, stamping, and pressing of the core materials during the manufacturing stage changes the material properties, which makes the FEA predicted core loss estimation less reliable. Machine designers typically multiply the FEA predicted core losses by a factor of 1.5 to 2. One of the challenging losses for HRE-free high-speed IPM machines is the magnet loss. Magnet losses increase the magnets’ temperature, and sub- sequently, push toward the demagnetization zone. It is also challenging to extract heat from a rotary environment. For the designed machine, the magnet loss is less than 1 W in the entire operating range, even at 20,000 r/min. Once the design has been finalized, different motor parame- ters need to be extracted, such as Ld, Lq, Ke, to develop an ini- tial control algorithm. Motor parameters vary under different loading conditions due to non-linear machine characteristics and saturation. Ld has less variation with loading, whereas Lq has higher variation due to the saturation effect. The back- EMF constant (phase-peak/speed) also varies with loading and will drop from the no-load value as shown in Fig. 23. The thermal performance must be evaluated in the entire operating range to determine the peak and continuous power envelop for the designed machine. Typically, the motor needs to provide the peak torque for a short time (10 s, 15 s, or 18 s) depending on the requirement. In contrast, the continuous output power is the power that can be extracted from the motor without violating the thermal limit. A spiral housing water jacket using forced convection liquid cooling with water- ethylene-glycol and end-winding potting using CoolThermSC [61] has been adopted for the design (Fig. 24). The fluid flow rate is 6.5 liter/minute, and the inlet temperature is 65◦ C. The thermal analysis has been carried out using MotorCAD based on a lumped parameter thermal network (LPTN). The maximum stator temperature is limited to 180◦ C, and the magnet temperature is limited to 140◦ C for safe operation. Thermal analysis shows that the continuous output power is 80% of the peak power with end-winding potting which means the peak power can be further increased. Although, the thermal performance can be enhanced significantly with end-winding potting, the manufacturing issues of potting for large traction machines need to overcome before it can be a viable option [61]. The demagnetization of magnets in IPM machines must be evaluated during the design process since the traction power will get limited or even reduce to zero for the partial or complete irreversible demagnetization. The maximum oper- ating temperature for the HRE-free PM’s safe operation is selected as 140◦ C. One of the worst-case conditions is the three-phase short-circuit fault at the maximum operating speed (≥20,000 r/min). For demagnetization evaluation, the PMs’ flux density is compared with the knee point flux density (0.34 T at 140◦ C) at transient peak current condition for the designed IPM machine. The results, given in Fig. 25, show that the PMs are safe from irreversible demagnetization Fig. 22. Efficiency map of the segmented magnet IPM machine. Fig. 23. Back-EMF constant variation as a function of loading. validating that the segmented magnets help to improve the demagnetization performance. Another worst-case operating condition is when the entire available phase currents are pushed through the negative d-axis. In this situation, the demagnetization ratio (demagnetized area/total area) should be less than one percent for the safe operation of PMs. To check demagnetization under this condition, a line is drawn just inside the PM (0.1 mm inside), and the normal flux density (in the magnetization direction) at this line is compared with knee point flux density to check demagnetization. The results for the designed machine under this scenario are shown in Fig. 26. Although the segmented PM design can enhance the demagnetization performance as presented, further work is required in cavity/pole design to improve the demagnetization or to increase the rotor operating temperature beyond 150◦ C. With regards to the structural integrity for next-generation high-speed IPM machines operating with maximum speeds over 20,000 r/min, the centrifugal forces will have a tremen- dous effect on the rotor bridges, especially at the center bridges. The center bridges need to be as large as possible from the mechanical point of view. In contrast, the electromagnetic design requires bridges having a thickness as minimum as possible. The increase in the thickness of the bridges reduces the PM flux linkage, PM use, and subsequently, the power density. The effect of operating speed and bridge thickness
• 16.
  16 Fig. 24. End-windingpotting and housing water jacket cooling. Fig. 25. PMs flux density distribution at transient peak condition with three- phase short circuit current at 20,000 r/min. on mechanical stress is presented in [62]. The worst-case structural performance of the presented design is evaluated at 20,000 r/min using Ansys Mechanical and the maximum stress as a function of operating speed is presented in Fig. 27. The maximum stress is at the center bridges. The HyperCo material has a yield strength of 435 MPa. The results show that a proper selection of bridge thickness, rotor outer diameter, and maximum operating speed results in mechanically stable design, even at 20,000 r/min. Rounded edges also help to reduce the mechanical stress [62]. The bearing and shaft will also require special attention besides structural stress. C. WBG Impact on Motor Insulation High dv/dt voltage is detrimental to motor winding insula- tion in two ways. On one hand, due to impedance mismatch between the inverter output cable and the motor, the motor terminal may see a voltage higher than two times of the nominal values. This over-voltage increases with longer cable and faster rise time. On the other hand, switching voltage with high dv/dt causes an uneven distribution of applied voltage across the turns and coils of a motor winding. Some insulation layer between winding turns may need to sustain higher voltage than others, leading to partial discharge and gradual insulation breakdown [63]. The first mechanism is mainly mitigated by placing the traction inverter as close as possible to the motor. Tesla Model 3 was the first commercial EV adopting SiC MOSFETs, and its teardown conducted by Electrek [64] shows that the motor and inverter unit are attached to each other (Fig. 28). Extensive ongoing research on the integration of motor and inverter will further shrink this connection [35]. The second mechanism can be addressed by strengthening the insulation but can also be handled by adding a dv/dt or sine-wave filter between the inverter and motor. Both Fig. 26. Normal flux density inside PM when all available currents in the negative d-axis. Fig. 27. Stress as a function of speed. filters can be RLC filters but with different corner frequency because dv/dt filter targets to reduce the dv/dt of the inverter switching edges, while sine-wave filter eventually produces only the fundamental component of the inverter PWM output. Therefore, the corner frequency of dv/dt filter is higher than that of sine-wave filter, thus has a smaller size. In [65], the inverter interconnect parasitic inductance was used to reduce the filter size and [66] takes a step further to integrate all RLC into one structure, which achieved 80% filter weight reduction. The usage of WBG devices at high switching frequency makes the sine-wave filtering approach attractive. In [67], GaN-based 100 kHz inverter achieves pure sine-wave output with very low filter volume and 2% higher efficiency than a Si-IGBT–based 3 kW inverter. With the needs of high-power, high-density, traction inverter for heavy duty applications, higher battery voltage such as 800 to 1500 V may be needed. In this case, motor insulation design will be more challenging for higher nominal voltage, so dv/dt issue would be more suitable to be addressed outside of the motor, for which related topic is expected to be of increasing importance in the years to come. In summary, for WBG drive systems, the rules for insulation solution should be a combination of adopting highly integrated motor drive with minimized cable length and controlling switching dv/dt of WBG by various active/passive techniques. IV. CONCLUSION There are a lot of challenges yet to be addressed in inverters and motors to make future electric vehicles more efficient and affordable. High power density targets are important for
• 17.
  17 Fig. 28. TeslaModel 3 electric powertrain where motor and inverter are closely connected [64]. integrating the electric drive with the base that holds the battery. These can be achieved with better and more integrated designs. WBG-based inverters cannot just mimic their Si- based versions just like how the current BEV designs are not mimicking the conventional gas powered vehicle designs. The use of emerging materials with novel designs will be needed to meet the aggressive targets set for future battery electric vehicles. WBG-based inverter drives and HRE-free machine designs will make their way toward commercialization with minimum manufacturing complexity to address both cost and reliability. EMI for WBG-based drive system will be more severe than Si-based alternative. Only with careful planning on mitigation measures at the noise source and the effort to contain the noise as local as possible, the full benefit of WBG can be harvested. 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  19 Iqbal Husain (S’89-M’89-SM’99-F’09)is the Di- rector of the FREEDM NSF Engineering Center and the ABB Distinguished Professor in the department of Electrical & Computer Engineering at North Carolina State University, Raleigh, NC. Prior to joining NC State, he was at the University of Akron where he built a successful power electronics and motor drives program. He was a visiting Professor at Oregon State University, Corvallis, OR in 2001. Dr. Husain’s expertise is in the areas of power elec- tronics, electric machines, motor drives, and system controls. His research is also focused on power electronics integration into power and transportation systems. The primary applications of his work are in the transportation, automotive, aerospace, and power industries. Dr. Husain has also developed innovative graduate and undergraduate courses on electric and hybrid vehicles and published the textbook “Electric and Hybrid Vehicles: Design Fundamentals” on this topic. Dr. Husain received his PhD in electrical engineering from Texas A&M University in 1993. He received the 2006 SAE Vincent Bendix Automotive Electronics Engineering Award, the 2004 College of Engineering Outstanding Researcher Award, the 2000 IEEE Third Millennium Medal and the 1998 IEEE-IAS Outstanding Young Member award. He became an IEEE Fellow in 2009. He was the Editor-in-Chief of the IEEE Electrification Magazine from 2016 to 2020. Burak Ozpineci (S’92-M’02-SM’05-F’20) received the B.S. degree in electrical engineering from Orta Dogu Technical University, Ankara, Turkey, in 1994, and the M.S. and Ph.D. degrees in electri- cal engineering from The University of Tennessee, Knoxville, TN, USA, in 1998 and 2002, respectively. In 2001, he joined the Post-Master’s Program with Power Electronics and Electric Machinery Group, Oak Ridge National Laboratory (ORNL), Knoxville, TN, USA. He became a Full Time Research and De- velopment Staff Member in 2002, the Group Leader of the Power and Energy Systems Group in 2008, and Power Electronics and Electric Machinery Group in 2011. Presently, he is serving as the Section Head for the Vehicle and Mobility System Research Section. He also serves as a Joint Faculty with the Bredesen Center, The University of Tennessee. Dr. Ozpineci is a Fellow of IEEE. Md Sariful Islam (S’16) received the B.Sc. and M.Sc. degrees in Electrical and Electronic En- gineering (EEE) from the Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh, in 2012 and 2014, respectively, and the Ph.D. degree in electrical engineering from the North Carolina State University, Raleigh, NC, USA, in 2020. Currently, he is working as an electromagnetic engineer at Halla Mechatronics, Bay City, MI where he is responsible for designing motor and sensors for automotive applications. His research interests include design, modeling, and control of electric machines with noise, and vibration analysis. He focuses on high-performance electric machines with WBG drives. Dr. Islam is a member of IEEE Industrial Applications Society and serving as a Reviewer for several IEEE journals and conferences on electric machines and drives. Emre Gurpinar (S’11-M’17-SM’20) ) received the B.Sc. degree from Istanbul Technical University, Istanbul, Turkey, in 2009 and the M.Sc. degree from the University of Manchester, Manchester, U.K. in 2010, and the Ph.D. degree from the University of Nottingham, U.K. in 2017, all in electrical engi- neering. In May 2017, he joined the Oak Ridge National Laboratory, Knoxville, TN, USA, where he is working as R&D staff in Electric Drives Research Group. He was a visiting Ph.D. student with the Department of Energy Technology, Aalborg University, Denmark, between August 2015 and October 2015. He was an R&D Power Electronics Engineer with General Electric, U.K. He received ”Outstanding Paper Award” in ASME InterPACK Conference in 2019. His research interests include wide-bandgap power devices, high- frequency converters, packaging and integration of power electronic systems, and electrified transportation. Gui-Jia Su (M’94-SM’01) received the B.S., M.S., and Ph.D. degrees in 1985, 1989, and 1992, respec- tively, all in Electrical Engineering. From 1992 to 1995, he was an assistant professor at Nagaoka University of Technology, Japan. From 1995 to 1998, he was with Sanken Electrical Co., Ltd., Japan, where he engaged in research and devel- opment of uninterruptible power supplies, sensorless PM motor drives, and power factor correction for single- and three-phase rectifiers. In 1998, he started working at the Power Electronics and Electric Ma- chinery Research Center at the Oak Ridge National Laboratory as a research scientist and is currently a distinguished member of the R&D staff. His research interests include DC/DC converters, inverters, wired and wireless battery chargers, and traction motor drives for electric vehicle applications. Dr. Su is a Battelle distinguished inventor and a recipient of the U.S. De- partment of Energy Vehicle Technologies Office Distinguished Achievement Award in 2019. Wensong Yu (M’07) received the M.S. degree from the Central China University of Science and Technology, and the Ph.D. degree from the South China University of Technology, China, in 1995 and 2000, respectively, both in mechanical and electrical engineering. From 2006 to 2013, he was a Postdoc- toral Researcher, Research Scientist, and Research Assistant Professor at the Bradley Department of Electrical and Computer Engineering at Virginia Polytechnic Institute and State University, Blacks- burg, VA, USA. Since 2013, he has been with the Department of Electrical and Computer Engineering at North Carolina State University, Raleigh, NC, USA, as a Research Associate Professor. His current research interests are high-frequency solid-state transformer, advanced soft- switching technique, digital control of multi-switch topology, wide bandgap device applications, ultra-high efficiency inverter, high-voltage power conver- sion and protection, WBG electric vehicle traction drive, distributed energy storage devices, and green energy grid infrastructure.
• 20.
  20 Shajjad Chowdhury (S’15-M’18)received the B.Sc. degree in electrical and electronics engi- neering from the American International University Bangladesh, Dhaka, Bangladesh, in 2009, the M.Sc. degree in power and control engineering from Liv- erpool John Moores University, Liverpool, U.K., in 2011, and the Ph.D. degree in electrical and electron- ics engineering from the University of Nottingham, Nottingham, U.K., in 2016. In January 2017, he joined the Power Electronics, Machines and Control Group, the University of Nottingham, as a Research Fellow. In 2018, he joined Electric Drives Research Group, Oak Ridge National Laboratory, Oak Ridge, TN, USA, as a Postdoctoral Research Associate. His research interests include multilevel converters, modulation schemes, and high-performance ac drives. Lingxiao (Lincoln) Xue (S’13-M’15-SM’20) re- ceived the B.S. and M.S. degrees from Zhejiang University, Hangzhou, China, in 2006 and 2008, respectively, and the Ph.D. degree from Virginia Tech, Blacksburg, VA, USA, in 2015, all in electrical and electronics engineering. He has been a Senior Staff Applications Engineer after joined Navitas Semiconductor in 2015. Presently, he is with Oak Ridge National Laboratory as a R&D staff. Dr. Xue served in the IEEE Power Electronics Society as the Young Professionals Chair and the Chapter Chair of IEEE Power Electronic Society in Coastal Los Angeles Section. His research focuses on power electronics design and architecture, specializing in wide bandgap devices, solid-state lighting, high frequency/density power conversion and transportation electrification. Dhrubo Rahman (S’11) received his B.Sc in Elec- trical and Electronic Engineering (EEE) in 2012 from Bangladesh University of Engineering and Technology (BUET), Dhaka, and his MS in Electri- cal Engineering (EE) from Rensselaer Polytechnic Institute (RPI), Troy, NY in 2015. He is currently working toward his Ph.D. degree from the Depart- ment of Electrical and Computer Engineering, North Carolina State University (NCSU), Raleigh, NC, USA, working at the FREEDM Systems Center as a graduate research assistant. His primary research interest is in the system-level design and optimization of traction drives for electric vehicles using wide bandgap semiconductor devices. Raj Sahu (Member, IEEE) received the B.Tech. (Hons.) and the M.Tech. degrees in Electrical Engi- neering from Indian Institute of Technology, Kharag- pur, India, in 2014, and the Ph.D. degree in Electrical and Computer Engineering from Purdue University in 2019. His research interests include design and analy- sis of power magnetic devices, constrained multi- objective optimization, thermal management design, and AI application for high-power density wide-band gap power electronic module development.