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Benchmarking of electric and hybrid vehicle electric machines, power

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Conference Paper · September 2015

DOI: 10.1109/OPTIM.2015.7426993

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 Benchmarking of Electric and Hybrid Vehicle Electric
Machines, Power Electronics, and Batteries
Bulent Sarlioglu, Senior Member, IEEE, Casey T. Morris, Di Han, Silong Li
Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC)
University of Wisconsin-Madison
Madison, WI 53706 USA

Abstract—Electric and hybrid vehicles continue to demand a The objective of this paper is to review the state-of-the-art
share of the automotive market. This movement has been mainly technologies in electric motor, power electronics, and energy
enabled by recent technological improvements in the realm of storage for automotive applications. The current trends in the
electric motors, power electronics, and energy storage. While technology are presented, as well as any other performance
many electric vehicles are on the market, they are not as cost
competitive as the conventional vehicles, and this is needed for
requirements set for the future. The metrics and benchmarking
large-scale market penetration. In order for this to become a of the technologies are used to describe the current and
reality, continued research is needed to develop new technologies. potential future states of the technologies.
This paper presents the metrics and benchmarks used to gauge The paper is organized as follows: Section II details the
the performance of various systems. In addition, the current current and future motor trend technology, Section III covers
state-of-the-art technologies are presented in terms of these power electronics, and Section IV details energy storage.
metrics as well as the future goals and trends in the industry. Final conclusions are drawn in Section V.
With continued development, the market for electric and hybrid
vehicles will only continue to increase. II. MOTOR TECHNOLOGY
Keywords—Electric machines, electric vehicle, energy storage, Electric motors and generators are widely used in the
future goal, hybrid vehicle, power electronics, trend. vehicles, even long time before the fast development of
electric vehicles (EV) and hybrid electric vehicles (HEV). In
I. INTRODUCTION vehicles, the electric motors/generators are commonly used in
The electric and hybrid vehicle market has been growing many different ways from small rating to very large rating,
over the years and continues to grow today. Influenced by such as traction motor, hybrid coolant water pump, alternator,
both government regulations and consumer demand, auto starter, engine cooling fan, electric power steering, sun roof,
manufacturers have continued to pursue technologies to power window, parking brake engagement, power window,
improve efficiency and fuel economy. As the market power seat, A/C compressor, blower for cabin conditioning,
progresses, continued research and development is needed to windshield wiper, and many linear actuators. Typically, more
enable large-scale market penetration of electric and hybrid than 10 electric motors/generators can be found in any of the
vehicles in the future. In particular, there are three main areas modern vehicles. This paper will only focus on the traction
in which current research aims to improve the vehicle: electric motors for EV/HEV.
motors, power electronics, and energy storage. This paper Among all passenger EVs/HEVs, a very small number of
describes the evolution of these technologies and the road map models use induction motor, including the electric cars made
for future development and implementation in electric and by TESLA. General Motors Company has an induction motor
hybrid vehicles [1], [2]. design for Chevy Spark. The Hyundai Sonata uses surface
The electric vehicle can trace its inception as far back as mount permanent magnet (SPM) machine. Almost all the
the early 20th century; however, was quickly overmatched by other major car companies use interior permanent magnet
the influx of the internal combustion engine (ICE). The (IPM) machines for EVs and HEVs. Other machine types may
advantage that the ICE had was its energy storage capacity, have been studied in research, but have not been used in
i.e. it was capable of providing longer ranges at a lower fuel production. In some other applications, such as electric bike or
cost. However, by the end of the 20th century, technology off-highway vehicle, machine topologies such as switched
improvements in electric machines, power electronics, and reluctance machine are also used.
energy storage sparked increased efforts in electric vehicle Induction machine is one of the oldest motor
development. Today, led by major car companies around the technologies. Fig. 1 shows the induction motor used in
world, electric and hybrid vehicles have a share of the TESLA Roadster Sport electric motor [5]. The comparison of
marketplace. New technologies will continue to revolutionize the torque speed curve of TESLA induction motor and
the industry and lead to large-scale adaptation of these cars internal combustion engine is shown in Fig. 2. As can be seen
[3], [4]. from the figure, the electric motor has higher performance at
low speed even compared to high performance internal low
speed even compared to high performance internal The rotors of different types of PM machines in the
combustion engine. Similarly, the induction machine design EV/HEV applications are shown in the Fig. 4 [7]-[10].
from General Motors is shown in Fig. 3 where bar wound Typically, one layer or two layer V-shaped interior PM are
stator windings are used. used. The rating and performance of the electric motors in
some passenger EVs/HEVs are shown in Table I [11]. It can
be seen that there is a trend of using higher speed electric
motors, since the motor weight and volume will reduce
rapidly as the rotational speed becomes higher.

Fig. 1 Induction motor used in TESLA Roadster Sport Electric Motor [5].

350

300
250
Torque (LB∙ FT)

200 Fig. 3 Induction machine for EV from General Motors [6].

150
The specific power and power density of each IPM
100 machines used in different EHs/HEVs are shown in Table II
50 [12]. It can be seen that the specific power is typically below 2
0 kW/kg, except the 2008 Lexus LS600h. With the increasing
1.5 3.0 4.5 6.0 7.5 9.0 10.5 12.0 13.5 speed of electric motor, the power density and specific power
Speed (RPM X1000) can increase to a higher level. As a future trend, the
Department of Energy makes targets power density and
specific power at 2020 as 5.7 kW/L and 1.6 kW/kg [10].
Fig. 2 Torque speed curve of the induction motor used in TESLA Roadster The efficiency maps of several IPM motors in passenger
Sport Electric Motor [5]. EV/HEV are shown in Fig. 5 [7], [10], [11].

Fig. 4 Rotor laminations of different types of PM machines in EV/HEV [7], [8], [9], [10].
 TABLE. I
ELECTRIC MOTOR RATING AND PERFORMANCE [11]

Parameter 2004 Prius 2006 2007 Camry 2008 LS 2010 Prius 2011 Sonata 2012 Leaf 2014 Chevy
Accord 600h Accord Volt
Peak Power 50 12.4 70 110 60 30 80 124 111
(kW)
Peak Torque 400 136 270 300 207 205 280 N/A 368
(Nm)
Rotational 6,000 6,000 14,000 10,230 13,500 6,000 10,400 N/A N/A
speed (rpm)
Cooling Heat sink Air- Heat sink Double- Direct cooled, Heat sink Heat sink N/A N/A
with cooled with sided, single side with with
water/glycol heat sink water/glycol water/glycol water/glycol water/glycol water/glycol
loop loop loop loop loop loop

TABLE. II
POWER DENSITY AND SPECIFIC POWER OF ELECTRIC MOTORS [12]

Parameter 2004 Prius 2006 Accord 2007 Camry 2008 LS 600h 2010 Prius 2011 Sonata 2012 Leaf
Peak power density (kW/L) 3.3 1.5 5.9 6.6 4.8 3.0 4.2
Peak specific power 1.1 0.5 1.7 2.5 1.6 1.1 1.4
(kW/kg)
Magnet mass (kg) 1.232 N/A 0.928 1.349 0.768 N/A 1.895
Magnet mass per rating 24.64 N/A 8.84 8.43 12.8 N/A 23.69
(g/kW)

6 and Fig. 7 that for the concentrated windings, the end

A. Concentrated Windings and Distributed Windings
winding length can be greatly reduced. As a consequence, the
Among all the production EVs/HEVs, most motors use length of the motor can be reduced. In addition, the slot fill
distributed winding configuration, as shown in Fig. 6 [7]. The factor of concentrated windings can be made higher compared
2011 Hyundai Sonata uses concentrated winding with 24 to distributed windings in general. Therefore, the copper loss
stator teeth, as shown in Fig. 7 [11]. of concentrated windings is typically smaller. For
From the advantage point of view, it can be seen from Fig. concentrated windings, it is possible to utilize segmented

(a) (b)

(c) (d)
Fig. 5 Efficiency maps of several IPM motor in EVs/HEVs [7], [10], [11], (a) 2011 Sonata, (b) 2010 Prius, (c) 2012 Nissan Leaf, (d) 2008 Lexus LS600h.
stator, so the machine manufacturability will be improved. magnet for EVs/HEVs. For example, the General Motors
The PM machine using concentrated windings will have announced the ferrite IPM generator for the next generation of
improved constant power speed range compared to the PM Chevy Volt.
machine using distributed windings. Finally, the smaller The disadvantage of ferrite magnet is that its energy
mutual inductances in concentrated windings will also product is much lower compared to rare earth magnets. As a
improve the machine fault tolerance capability. consequence, the ferrite PM motor with same power and
speed rating tends to have larger size and higher weight.
Another difference between ferrite and rare earth magnets is
that the ferrite magnet has positive temperature coefficient for
coercivity, Hc. Therefore, the ferrite magnet becomes harder
to be demagnetized at higher temperature, vice versa for rare
earth magnets. Therefore, corner point designs with respect to
temperature should be carefully analyzed for performance.
Compared to NdFeB and ferrite magnets, samarium cobalt
(SmCo) permanent magnet can operate at much higher
temperature. SmCo magnets have lower remnant flux density
and coercivity, and are less sensitive to temperature variation.
However, SmCo magnet is more brittle and manufacturing
should incorporate this issue, but are still considered an
Fig. 6 Distributed winding configuration of Prius 2004 motor [7].
alternative to NeFeB magnets.
III. POWER ELECTRONICS
Power electronics is an enabling technology for the
realization of HEVs and EVs. Specific requirements of power
electronics are low cost, high reliability, high specific and
power density. Depending upon the cars configuration many
different converters can be needed. For example, in Toyota
Prius, there exists a dc/dc converter between the battery and
inverter dc bus inputs, two inverters to convert the dc to ac
power and vice versa during regeneration, a high battery
voltage to low voltage (12 V) dc/dc converter. Additional
power electronics converters are needed for on-board charging
which converts the AC plug power to the battery voltage
levels and an inverter for compressor for A/C.
Fig. 7 Concentrated winding configuration of Hyundai Sonata 2011 motor
[11].

However, the PM machine with concentrated windings

usually require high rotor pole numbers (for example, 2011
Sonata motor has 16 rotor poles). As a consequence, the
excitation fundamental frequency will become very high at
high speed operation. Due to this reason, the proximity and
skin effect loss could be an issue for concentrated windings.
In addition, the magnet eddy current loss in PM motors
utilizing concentrated windings is higher compared to those Fig. 8. Schematic of powertrain in HEV [14].
using distributed windings. Therefore, high rotor loss and
difficulty of rotor cooling are challenging in concentrated
A. System Configurations of the Powertrain
winding PM machines. In addition, the vibration risk of
concentrated windings PM machines is high. A detailed trade- Typically, hybrid/electric vehicles have two inverters in the
off study is needed to choose either concentrated or electric power train as shown in Fig. 8. Inverter 1 is used to
distributed winding machine for vehicle traction application. drive the “motor”, which assists the ICE in providing
mechanical drive power for the vehicle and also act as a
B. Rare Earth Magnets and Ferrite Magnets generator to recharge the battery during regenerative breaking.
For most of the EVs/HEVs, neodymium iron boron Inverter 2 connects to the “generator”, which transfers power
(NdFeB) is typically used as the magnet material for PM from the ICE to recharge the battery and also to utilize the
machines. The NdFeB magnet has very high remanence and ICE as a power source to supply motor [7].
energy product, which is preferable for designing high torque Depending on system consideration, there may be a
density and high power density PM machines. However, due bidirectional DC-DC converter between battery pack and DC
to the high price of rare earth materials, some car bus, which is shared by the two inverters. The DC-DC
manufacturers started to design PM machines using ferrite
 Fig. 9. Power conversion system of 2010 Prius PCU between battery and electric machines [7].

TABLE. III
POWER DENSITY AND SPECIFIC POWER OF CONVERTERS [7]

Converter Type Power (kW) DC Bus Voltage (V) Volume (L) Power density (kW/L) Mass (kg) Specific Power (kW/kg)
PCU for 2010 Prius 60 200 16.2 3.70 13 4.62
PCU for 2007 Camry 70 250 11.7 5.98 17.4 4.02
PCU for 2008 LS 600h 110 288 13.7 8.03 17.9 6.15
Motor inverter for 2010 Prius 60 200 5.4 11.11 3.6 16.67
Buck/Boost for 2010 Prius 27 200 4.8 5.63 5.1 5.29
Motor inverter for 2004 Prius 50 200 8.7 5.75 8.8 5.68
Buck/Boost for 2004 Prius 20 200 5.1 3.92 4.8 4.17
Motor inverter for 2007 Camry 70 250 6 11.67 7.5 9.33
Buck/Boost for 2007 Camry 30 250 3.5 8.57 6.6 4.55
Motor inverter for 2008 LS 600h 110 288 6.4 17.19 7.4 14.86
Buck/Boost for 2008 LS 600h 36.5 288 4 9.13 6.9 5.29

Fig. 10. Comparison of PCU module from four HEV model [7].

converter can be used to regulate the DC link voltage during i.e. power density and specific power, are used in this section
transient operations [15]. to illustrate the state-of-the-art power converter techniques.
The schematics for the two inverters and DC-DC Typically, the size and weight of power converters are
converter are shown in Fig. 9. As can be seen, the two related to their power rating. Hence, power density is defined
inverters are basic two-level voltage source inverter as ratio of converter volume to converter power, and specific
topologies, and the DC-DC converter is a bidirectional power is the ratio of weight over power.
buck/boost topology. The specific power and power density of each type of
converters and power control unit (PCU) modules are listed in
B. Power Density and Specific Power
Table III for four HEV models, i.e. 2010 Prius, 2004 Prius,
One critical requirement for power electronic converters 2007 Camry, and 2008 LS 600h. The PCU modules from four
in automotive applications is to minimize their size and HEV models are also compared side by side in Fig. 10.
weight. Carrying too large or too heavy power converters on As can be seen, the 2008 Lexus LS 600h has the most
board will result in space reduction of passenger compartment compact design among the models compared, with an 8.03
and decrease in fuel economy. Hence, two important metrics, kW/L power density and 6.15 kW/kg specific power. Its sub-
modules such as DC-DC converters and motor inverters also
have the highest power density and specific power. This can
be partially due to the relative large power rating of LS 600h

Fig. 12. Power loss on converters of various switching frequencies under

full load condition [14].
Fig. 11. Compartments of 2010 Prius PCU [7].
Overall Efficiency Comparison at Full Load
as a Function of Switching Frequency
TABLE IV 100
COMPARISON OF KEY PARAMETERS FOR DIFFERENT DEVICES [14] IGBT
99.5 Hybrid
Si IGBT SiC Si IGBT SiC FET
99

Overall Efficiency [%]

Device Parameters w/Si MOSFET w/SiC
Diode w/SiC SBD SBD 98.5
Maximum junction
175 135 150 98
temperature, Tj-max (C)
Thermal resistance junction 97.5
0.7 0.44 0.34
to case, Rjc (C/W)
97
Turn on delay time, tdon (s) 2.00e-8 1.30e-8 4.00e-8
Voltage fall time, tfv (s) 1.11e-7 2.40e-8 2.30e-7 96.5
Turn-off delay time, tdoff (s) 1.02e-7 4.00e-8 2.32e-7
Voltage rise time, trv (s) 4.00e-8 3.80e-8 5.30e-8 96
0 50 100 150 200
Voltage drop across drain to Switching Frequency [kHz]
4.0 2.2 2.5
source, Vds (V) Fig. 13. Efficiencies of converters at various switching frequency under
Turn on energy, Eon (J) 1.95e-3 3.15e-4 9.15e-4 full load condition [14].
Turn-off energy, Eoff (J) 1.35e-3 2.85e-4 2.01e-3
reduction can be achieved by replacing Si devices with WBG
PCU (110 kW), which is also the largest among all. devices. As a result, improvements on fuel economy and
reduction in cooling systems will be achieved. On the other
As an example, Fig. 11 shows the compartments of the 2010 hand, by increasing the switching frequency of WBG
Prius PCU. As can be seen, in order to achieve high power converters significantly, the considerable reduction in filter
density and specific power, an optimized packaging layout size will be resulted, while the efficiency is not penalized
solution is the key point on top of others. compared to the Si counterparts.
Fig. 12 and Fig.13 shows the DC/DC converter loss and
C. Application of SiC and GaN Devices efficiency as a function of switching frequencies using three
Wide bandgap (WBG) semiconductor material (SiC and different device combinations. Corresponding converter size
GaN) based power switching devices will potentially become and weight as a function of switching frequency is also shown
a game changer in automobile power electronic converters is Fig. 14. Fig. 15 shows the whole PCU size comparison
with their superior properties compared to the conventional Si between Si technology and SiC technology. It is also claimed
material based technology. that SiC PCU can achieve 90% loss reduction compared to Si
The major advantages of WBG devices are lower on-state PCU, thus leading to a 10% MPG improvement in HEVs [13].
resistance, faster switching transition, and higher temperature
capability, compared to Si devices. Table IV compares the Si D. Future Trends in Power Electronics
IGBT (IRG7PH30K10PbF) and Si diode (DSEP 29-12A) Future trends in power converters in hybrid/electric
combination, the SiC MOSFET (CMF20120D) and SiC diode vehicles are high power rating, high efficiency, low weight,
(C4D40120D) combination, and a hybrid Si IGBT and SiC and low volume. Continuing improvements in automotive
diode module (GA35XCP12-247). converter depend on improvement in semiconductor material,
As can be seen from Table IV, although the thermal innovation on packaging technology, new design on cooling
characteristics of SiC devices are not maximized because of system, and revolution in converter topologies.
packaging limitations, it is obvious that SiC MOSFET, with
IV. ENERGY STORAGE
SiC SBD combination, has only 1/7 switching loss and half
conduction loss, compared to Si IGBT/diode combination. A. Battery Metrics
The benefits of using WBG devices in automobile Perhaps the most important, and currently most
applications can be explored in two aspects. By keeping the expensive, component relative to the feasibility of hybrid
switching frequencies of converter the same, significant loss electric and especially electric-only vehicles is the energy
 TABLE V
COMPARISON OF BATTERY CELL TYPES

Fig. 15. Size comparison of Si PCU (left) and SiC PCU

Weight and Volume of Passive Components in Converters
9 9
(right) [16].

8 8 performance of the vehicle. Volume is equally important

7 7 because there is a very finite amount of space within a vehicle
6 6
for the battery, so being able to put more energy in the same
Total weight [kg]

Total volume [L]

amount of space is greatly valued. Currently many battery
5 5
types exist and various types are used in vehicles to optimize
4 4
the customer demands: total cost and range.
3 3
The two main types of batteries that have been
2 2 implemented in EVs and HEVs are of the Nickel-Metal-
1 1 Hydride (NiMH) family or of the Lithium Ion (Li-Ion) family.
0 0 These vary both in chemical and electrical properties, and
0 50 100 150 200
Switching frequency [kHz] some of their most important qualities are presented in Table
Fig. 14. Total weight and volume of the converters as a function of V along with the specifications of lead acid batteries. Fig. 16
switching frequency [14]. and Fig. 17 also compare different energy storage
technologies, showing important variability in their energy
storage system, namely the battery. In EVs, the battery is the and power characteristics. It is important to note that gasoline
fundamental piece that determines most user-interested for IC engines has specific energy and density that is off the
functions of the vehicle, including range, acceleration, and chart of Figure 17, leaving plenty of room for growth of
cost. Battery technology can still be considered in its infancy, energy storage technology.
so it is expected that it will continue to mature and reduce in As can be seen from these tables and figures, Li-Ion
price and size and increase in capacity. All of these effects are batteries are better than NiMH batteries in terms of energy
crucial to the continued growth of the electric and hybrid density, specific energy and specific power. Specific power
vehicle industry. (W/kg) is important in terms of vehicle performance, such as
According to the studies done in [17] in 2010 the cost of acceleration, and also is a factor in the amount of energy that
an EV battery per kilowatt-hour (kWhr) ranged from $600- can be captured from regeneration braking. Optimizing all of
$1,105. Progress in the last five years have brought the these factors, including the cost mentioned previously, is an
estimated price near $500 per kWhr. Continued reduction in ongoing challenge.
the price per unit of energy is necessary for the continued B. Current Vehicles
penetration of EVs in the marketplace.
In addition, the energy density (Whr/L)/specific energy Globally, all car manufacturers are exploring some type
(Whr/kg) of the battery is equally important for the growth of of hybrid or electric vehicle; however, the battery type, size,
EVs. These are critical because the weight and volume in a and other characteristics vary greatly. Table VI shows a
vehicle are invaluable, effecting the range and acceleration summary of the battery specifications of recent production

Fig. 16. Specific energy vs specific power for energy storage technologies. Fig. 17. Energy density vs specific energy for energy storage in HEVs.
vehicles. Other details important to manufacturers include the V.CONCLUSION
battery cooling mechanism, voltage level, and range. The state-of-the-art technologies and metrics and
Manufacturers must optimize their battery to not only meet benchmarks for the various technologies have been presented
the consumer demands (range, lifetime, charging time, cost, and in these terms, the future of the electric and hybrid vehicle
etc.) but also to work most efficiently with the entire vehicle is exciting. Research and development continues throughout
system, such as proper electric machine development and the industry to improve the fuel economy and efficiency of
control of solid state power converters. These advancements these vehicles. As the various technologies improve, electric
have led to a diverse implementation of batteries in EV/HEV and hybrid vehicles will become cost competitive in all
systems. aspects compared to the ICE vehicle. This is of great
C. Future Goals significance globally as it will enable reduction in CO2 and
NOX emissions, reducing the impact of vehicles on the
Both the U.S. Department of Energy (DOE) and the U.S. environment. These improvements will also aid to meet
Advanced Battery Consortium (USABC) have established requirements set by governments and consumers and can only
different goals for the development of battery technology. In be met by continued research and development efforts.
2012, the DOE issued a 10 year battery challenge to enable ACKNOWLEDGMENTS
large market penetration of EVs [18]. In addition, the The authors would like to thank Mr. Oliver Gross of Fiat
USABC has set similar short-term and long-term goals for Chrysler for Figs. 16, 17 and Table V, created for an energy
battery performances, with different goals for batteries for short course at UW-Madison.
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