Review and Development of Electric Motor Systems and Electric Powertrains for New Energy Vehicles

Abstract

This paper presents a review on the recent research and technical progress of electric motor systems and electric powertrains for new energy vehicles. Through the analysis and comparison of direct current motor, induction motor, and synchronous motor, it is found that permanent magnet synchronous motor has better overall performance; by comparison with converters with Si-based IGBTs, it is found converters with SiC MOSFETs show significantly higher efficiency and increase driving mileage per charge. In addition, the pros and cons of different control strategies and algorithms are demonstrated. Next, by comparing series, parallel, and power split hybrid powertrains, the series–parallel compound hybrid powertrains are found to provide better fuel economy. Different electric powertrains, hybrid powertrains, and range-extended electric systems are also detailed, and their advantages and disadvantages are described. Finally, the technology roadmap over the next 15 years is proposed regarding traction motor, power electronic converter and electric powertrain as well as the key materials and components at each time frame.

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Automotive Innovation
https://doi.org/10.1007/s42154-021-00139-z
Review andDevelopment ofElectric Motor Systems andElectric
Powertrains forNew Energy Vehicles
WilliamCai1· XiaogangWu1· MinghaoZhou1· YafeiLiang2· YujinWang3
Received: 27 July 2020 / Accepted: 27 January 2021
© The Author(s) 2021
Abstract
This paper presents a review on the recent research and technical progress of electric motor systems and electric powertrains
for new energy vehicles. Through the analysis and comparison of direct current motor, induction motor, and synchronous
motor, it is found that permanent magnet synchronous motor has better overall performance; by comparison with converters
with Si-based IGBTs, it is found converters with SiC MOSFETs show significantly higher efficiency and increase driving
mileage per charge. In addition, the pros and cons of different control strategies and algorithms are demonstrated. Next, by
comparing series, parallel, and power split hybrid powertrains, the series–parallel compound hybrid powertrains are found
to provide better fuel economy. Different electric powertrains, hybrid powertrains, and range-extended electric systems are
also detailed, and their advantages and disadvantages are described. Finally, the technology roadmap over the next 15years
is proposed regarding traction motor, power electronic converter and electric powertrain as well as the key materials and
components at each time frame.
Keywords New energy vehicle· Traction motor· Motor control· Power electronics converter· Control algorithm·
Permanent magnet synchronous motor· Electric motor· Electric powertrain
Abbreviations
ADRC Active disturbance rejection control
AC Alternating current
APU Auxiliary power unit
CM Common mode
CMEMI Common-mode electromagnetic
interference
DC Direct current
DP Dynamic program
DTC Direct torque control
EMI Electromagnetic interference
EV Electric vehicle
FEM Finite element method
FLC Fuzzy logic control
FOC Field orientation control
JJE Jing-Jin Electric
HEV Hybrid electric vehicle
ICE Internal combustion engine
IGBT Insulated-gate bipolar transistor
IM Induction motor
MOSFET Metal–oxide–semiconductor field-effect
transistor
MPC Model predictive control
NEV New energy vehicle
NNC Neural network control
PHEV Plug-in hybrid electric vehicle
PID Proportion integration differentiation
PM Permanent magnet
* William Cai
william_cai88@163.com
Xiaogang Wu
xgwu@hrbust.edu.cn
Minghao Zhou
zhouminghao@aliyun.com
Yafei Liang
lyftcl@yahoo.com.cn
Yujin Wang
wangyujin3099@163.com
1 Institute ofElectrical andPower Electronics Engineering,
Harbin University ofScience andTechnology,
Harbin150080, Heilongjiang, China
2 Technical Center Beijing Electric Vehicle Co., Ltd,
DaxingDistrict,Beijing100176, China
3 Electromechanical Department, Taiyuan University,
Taiyuan030032, Shanxi, China

W.Cai et al.
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PM-BLDCM Permanent magnet brushless direct current
motor
PM-HEM Permanent magnet hybrid excitation motor
PMDCM Permanent magnet direct current motor
PMM Permanent magnet motor
PMSM Permanent magnet synchronous motor
RC Robust control
SMC Sliding mode control
SOH State of health
SRM Switched reluctance motor
SPWM Sinusoidal pulse width modulation
SSV Six-step voltage
SVPWM Space vector pulse width modulation
V/F Voltage per frequency
WBG Wide bandgap
WLTC Worldwide light-duty test cycle
1 Introduction
With the thriving economy, car demand has increased.
However, fuel-powered vehicles emit carbon dioxide and
nitrogen oxide, causing greenhouse effect on the climate
and toxic effect on human health [1, 2]. Moreover, a large
amount of diesel consumption has caused the global energy
crisis. According to statistical data, the increase of two-
thirds of the petroleum consumption comes from trans-
portation industries, which is extremely unfavorable to the
sustainable development of human society [3, 4]. Driven by
the global emission reduction targets in the Paris Climate
Agreement, new energy vehicles (NEVs) have become an
important development direction of the automotive industry
[5, 6]. While European countries and their car manufacturers
have scheduled to limit sales of the fuel vehicle, the NEV
Industry Development Plan (2021–2035) has been prom-
ulgated by the Chinese government, in which the goals are
set for the next 15years. According to the statistics of the
China Association of Automobile Manufacturers (CAAM),
the production and sales of NEVs in China reached 1.242
and 1.206 million units, respectively, in 2019[7], 1.366 and
1.367 million units, respectively, in 2020.
Compared with industry motors, NEV traction motors
should be adapted for harsh operating environments. Their
operation modes are frequently switched between motoring
and generating. Frequent starting and stopping, high rate of
acceleration/deceleration, high torque at low speed and high
power at vehicle high-speed climbing, high power density,
large highly efficient operating area, low vibration and noise,
high reliability and high performance-to-price ratio are
required by the automotive industry [8, 9]. Traction motors
and motor power electronic controllers are the core parts for
converting the electromechanical energy in NEVs [10, 11].
The electric powertrain systems integrated gears, clutch,
and other mechanical components with the traction motors
and motor controllers are also an indispensable system part
of NEVs. The vehicle structure and propulsion are simplified
in the e-powertrain dramatically, whose topologies greatly
influence the NEV performance [12].
Therefore, the requirements for the electric drive sys-
tems in NEVs mainly include the following aspects: (1)
high torque density and good torque control capability for
vehicle dynamic performance; (2) reliability and durability
for the required vehicle safety and life; (3) high efficiency
within operation spectrum [13, 14] and high performance-
to-cost ratio for the energy economy and the users’ capital
investment.
The technologies of traction motors, their power elec-
tronic controllers, and electric powertrains are summarized.
The advantages and disadvantages of existing technologies
and their prospects and development are discussed, provid-
ing reference for the researchers and engineers in NEVs
powertrain system areas.
2 Development ofNEV Traction Motors
2.1 Classification andCharacteristics ofNEV
Traction Motor
The traction motors in NEVs mainly include direct cur-
rent motors (DCMs), induction motors (IMs), permanent
magnet motors (PMMs), and switched reluctance motors
(SRMs). Among them, PMM is divided into PM DC motor
(PMDCM), PM synchronous motor (PMSM), PM brushless
DC motor (PM-BLDCM) and PM hybrid excitation motor
(PM-HEM) [15]. To reduce the dependence on PM materi-
als, excitation synchronous motor is also installed onboard
vehicles, as shown in Fig.1.
2.1.1 Direct Current Motor
DCM is used as the traction motor in electric vehicles (EVs)
from the late nineteenth century because of its simple speed
regulation. However, low efficiency, large mass, and poor
reliability due to brushes and commutators make DCMs no
longer suitable for high-speed NEVs. They are used only
in low-speed EVs, such as carts for logistic cargo moving
inside plants, and shuttle bus in scenic areas.
2.1.2 Switched Reluctance Motor
The SRM stator and rotor are composed of silicon steel lami-
nates, and a salient pole structure is adopted. There are no
windings, slip rings or PMs on the rotor, and only simple
concentrated windings are installed on the stator. The rotor

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structure enables simple, robust, low cost, and high-speed
operation of SRMs. Moreover, its inverter’s reliable topo-
logical structure prevents it from short-circuit faults [16].
High efficiency and simple control are SRMs’ advantages.
However, torque fluctuation, noise, and vibration are serious
preventing it from applications in NEVs.
2.1.3 Induction Motor
Squirrel-cage IMs are widely used in NEVs. Their stator and
rotor are composed of laminated silicon steel sheets, and
three-phase windings are inserted inside the stator lamina-
tion stack and aluminum or copper bars in the rotor slots
with rings at both ends. IMs are characterized as having a
simple and strong structure, low cost, high reliability, small
torque ripple, low noise, and maintenance-free. IMs can
be easily run at high speed over 15,000rpm with a wide
constant power range. However, IMs control circuit is com-
plex, and their efficiency and power density are relatively
low compared to PMSMs, leading to its increasingly lower
market share globally [17].
2.1.4 Permanent Magnet Motor
(1) Permanent magnet direct current motor
When field windings and magnetic poles of conventional
DCMs are replaced with PMs, a PM-DCM is established.
PM-DCMs show higher power density and efficiency, but
it needs more maintenance and exhibits low life and torque
fluctuation due to the commutator and brush system; these
are still the concerns to be solved for EV applications.
(2) Permanent magnet synchronous motor
In PMSM, its stator with three-phase windings is the same
or similar to IM or synchronous motor stator, and PMs
replace the excitation winding of traditional synchronous
motors. According to the position of PMs on or in the rotor,
PMSMs can be divided into surface-mounted PMSM (SPM)
and interior embedded type (IPM). Well-designed IPMs are
featured by high reluctance torque, high efficiency, high
power factor, low heat, simple structure, small package and
low noise. With the development of power electronics con-
trol strategy, IPMs have become dominant in traction motor
applications. In addition, owing to the fully enclosed struc-
ture, IPMs, being maintenance-free, show low wind friction
losses and low windy noise.
(3) Permanent magnet brushless DC motor
PM-BLDCM is a special PMSM structurally and theoreti-
cally, but its windings are concentrated normally and the
stator current waveshape is trapezoidal, instead of sinusoidal
DC motor
FieldDC motor
PM DC motor
Switched
reluctance motor
Unilateral excitaonswitched reluctance motor
PM aided switched reluctance motor
AC motor
Inducon motor
Synchronous motor
PM synchronous machine
Reluctance synch ronous motor
Excitaon synchronous
machine
Hybrid excitaon
synchronous machine
Trapezoidal wave PM synchronous motor
Salient machine
Non-salient synchronous machine
Claw pole synchronous motor
PM clawpol emotor
Field and PM hybrid excitaon
synchronous motor
Wound rotor inducon motor
Commutator
reliability
Torque fluctuaon
vibraon noise
Torque fluctuaon
vibraon noise
demagnezaon
heang
Cage rotor inducon motor
Sinusoidal AC PM reluctance
synchronous motor
Fig. 1 Traction motors of NEVs

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in SPM. The commutator-brush system is not required.
However, the torque ripple and noise appear during electri-
cal commutation, and it is difficult to achieve the maximum
speed beyond twice the base speed.
(4) Permanent magnet hybrid excitation motor
By adding excitation windings to PMSM, the motor has
both PMs and excitation windings and becomes a hybrid
excited motor, which is PM-HEM. This motor has the mini-
mum flux leakage, high flux density in the air gap, high
power density, and good torque-speed characteristics. How-
ever, its topology and control are relatively complex owing
to two separate excitations.
The performance comparison of the aforementioned
motors is shown in Table1.
In Table1, ●, ●●, ●●● represents the low (poor),
medium, and high (good) indices, respectively. Thus,
PMSM, especially IPM, is the best choice for NEV traction
motors [18].
2.2 Research ofNEVs PMSM
A new type of DC saturated hybrid excitation motors was
proposed in Ref. [19]. By introducing additional DC field
excitation with step-down DC saturation capability, the mag-
neto resistive effect was constructed in the rear pole Ver-
nier PMM (CP-VPMM). In this topology, the bidirectional
flux control of the stator DC excitation reluctance motor
and the good torque density in CP-VPMM are combined.
An airgap-harmonic-oriented design method was proposed
[20]. The magnetic flux enhancement was adopted, and its
characteristics was Ld > Lq, which could be used in sensor-
less motor controls. By integrating a special design of sta-
tor teeth, the air gap length on the q-axis was increased to
obtain high reluctance torque, good fault tolerance, and high
reliability. Next, a hybrid circuit method to improve the effi-
ciency of wound synchronous traction motors was proposed
[21]. By changing the connection between windings U, V, W
and windings X, Y, Z, the efficiency at high speed could be
improved. The torque ripples and the coupling between the
internal/external magnetic fields of a compound excitation
PMM are reduced through finite element analysis in Ref.
[22].
For the PMSM with a complex structure of double sali-
ency, dual stators, and dual air gaps [23], a multiple sensitive
objective optimization method was used to select the key
dimensional parameters. Furthermore, the optimized geo-
metrical dimensions were obtained by the response surface
optimization method. Next, a six-phase fractional slot con-
centrated winding PMSM with fault tolerance capability was
discussed [24]. Based on the analysis of magnet electromo-
tive force harmonics, a new pole-slot matching scheme was
proposed to reduce the eddy current loss of PMs caused by
the concentrated windings and to reduce the number of rotor
poles, and consequently the stator core losses.
2.3 Development ofthePMSM Technology
The future technologies of traction motors for NEVs focus
on the key factors of high efficiency, high speed, high power
density, low vibration and noise, better electromagnetic
compatibility (EMC) and low cost. In the EV development
2025 roadmap proposed by the US Department of Energy,
the EV motors are aimed at achieving high efficiency (97%),
high power density (50kW/L), and low cost (3.3 $/kW). In
the “Energy-saving and New Energy Vehicle Technology
Roadmap 2.0”, the goals for 2025 are set as a specific power
(power-to-mass ratio) of 5.0kW/kg, power density (power-
to-volume ratio) of 35kW/L, and the peak efficiency of 97%
for traction motors. To achieve these goals, global NEV
traction motor suppliers and research institutions are col-
laborating to improve innovation chain and supplier chain,
including components and materials.
2.3.1 High Slot Filled Ratio Winding Technologies
By adopting high slot filled ratio windings with flat/rectan-
gular wires or hairpin windings [25] , the winding heating
can be greatly reduced, and the utilization rate of the wind-
ing copper materials can be increased by 15%–20%, which
is the main method to improve torque density, power density,
and efficiency. For example, the power density of 4.6kW/kg
is achieved in GM VOLT motor through hairpin wingdings.
2.3.2 High‑Speed Motor Technology
The motor size is proportional to its torque. For a motor
with given power requirement, its power equals torque mul-
tiplied by speed. By increasing the operation speed, the
torque requirement for the motor can be reduced, thereby
reducing the motor volume and weight, and its power density
increased with the speed. For example, the traction motor
speed of 17,900 r/min is used in Tesla Model 3, and the
Table 1 Comparison of NEV traction motors
Index DCM IM IPM SRM
Efficiency ● ●● ●●● ●
Speed ● ●●● ●● ●●●
Size ● ●● ●●● ●●
Reliability ● ●● ●●● ●●●
Control simplicity ●●● ●● ● ●●
Performance ● ●● ●●● ●●

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motor speed of 25,000rpm is aimed to be achieved by 2035
in NEV Technology Roadmap 2.0 of China.
2.3.3 Efficient Thermal Management Technologies
Soil cooling, oil and water combined cooling, and new cool-
ing topologies are used to improve cooling technologies and
heat transferring of traction motors, and the power density
of the motors is raised consequently.
PMSMs for NEVs have made continuous progress glob-
ally in power density, system integration, efficiency, maxi-
mum working speed, winding manufacturing process, and
cooling technologies. The technical indicators of typical
motor products are shown in Table2.
3 Research ofNEV Motor Control
Nowadays, PMSM requires its control strategies to have fast
system dynamic response, high dynamic/static precision,
and strong anti-interference ability. However, the PMSM
models are nonlinear; with strong coupling, time-varying
parameters, multiple variables, and large disturbance, its
control algorithms are complex. Therefore, the performance
of the motors is affected directly by the control strategies.
Typical control strategies include constant voltage/frequency
ratio, classical proportion integration differentiation (PID),
field-oriented approach, direct torque, sliding mode variable
structure, adaptive and intelligent controls.
3.1 Motor Control Technologies
Among the PMSM control technologies, the variable-
voltage-variable-frequency (VVVF) control method has
absolute advantages in performance through the following
three methods: the constant voltages-per-frequency (i.e.,
V/F = const) control which is open-loop type and is based on
the steady-state motor models, the field-orientation control
(FOC) and direct torque control (DTC). The latter two are
close-loop types and are based on dynamic motor models.
The comparison of the three motor control methods is pre-
sented in Table3.
3.1.1 Constant V/F Ratio Control
Constant V/F ratio control, also known as constant flux con-
trol, can obtain the constant flux by guaranteeing that the
stator voltage per frequency maintains constant. The state
feedback control was adopted in an N-T coordinate system,
and a new sensorless V/F control method for PMSMs was
proposed [26]. When the motor was running at low speed,
the T-axis current is used to keep the system at high stability.
To operate the motor stable at medium and high frequen-
cies, a velocity stability loop is added, and an active power
disturbance component is extracted for compensation [27].
V/F control is a relatively common method for IMs’ speed
control with the advantages of simplicity, effectiveness, and
high robustness to parameter variation. However, since
it is an open-loop control, the control accuracy, dynamic
response, and load capacity of the systems are reduced due
to drifts of speed and flux in the V/F open-loop control,
Table 2 Indicators of typical
traction motors for passenger
EVs
Technical indicators A brand in China GM
Bolt Germany Bosch Tesla
Model 3
Peak power (kW) 130 130 150 165
Max. speed (r/min) 13,200 8810 16,000 17,900
Peak torque (N·m) 315 360 310 416
Peak efficiency (%) 97 97 97 97
Power density (kW/kg) 4.56 4.60 4.40 4.50
Cooling method Oil Water Water Water
Table 3 Performance
comparison of the three motor
control technologies
Control technology Structural
complex-
ity
Robustness to
parameters pertur-
bation
Starting performance Torque ripple Speed range
V/F control Simple Low Rough High Narrow
FOC Complex High Smoothly High Wide
DTC Complex High Smoothly Relatively high Wide

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which leads to poor startup capability, high torque ripple,
and narrow speed range. Therefore, V/F controls are seldom
used in vehicular traction motor control.
3.1.2 FOC
FOC was proposed by Blaschke in the 1970s. The stator
current was decoupled into the torque component and mag-
netized under the constant rotor flux in the special dq0 coor-
dinate system, and the control of alternating current (AC)
motors can be equivalent to that of an unexcited DC motor.
FOC can achieve smooth starting, low torque ripple and
wide speed range, suitable for high dynamic response of
machinery under tough working conditions.
A vector control strategy is proposed based on the motor
speed-torque-current diagram [28]. The power demand
and the energy consumption were effectively reduced, and
the vehicle driving range was extended. A flux-weakening
control strategy was proposed through an estimator with
improved uncertainty and disturbance. A flux-weakening
adjusting factor to smooth the torque ripple at motor corner
speed is introduced [29]. The robustness at the flux-weak-
ening area is enhanced consequently.
Setting up a motor dynamic model in the two-phase rotat-
ing coordinate is the key to a successful FOC, laying basis
for high dynamic response under harsh working conditions.
3.1.3 DTC
The DTC was proposed by Depenbrock, with the current
loop in the FOC system being removed and no complex
coordinate transformation required. The two-bit bangbang
control is used to generate PWM modulation signals in a
two-phase static coordinate. DTC has the advantages of
simple structure, fast dynamic response, low sensitivity to
parameter perturbation, and strong robustness, therefore
suitable for applications requiring rapid dynamic response
and wide speed regulation. However, it also has the disad-
vantages of the current and torque ripples at low speeds and
the requirement for high sample frequency. Many scholars
combine the space vector pulse width modulation (SVPWM)
and DTC to reduce these ripples.
An improved control strategy using the quadratic esti-
mation method (QEM) and the harmonic voltage elimina-
tion (HVEM) methods was proposed [30]. The final volt-
age vector suppressing harmonic current of the stator was
obtained; thus, the fast dynamic response and good steady
performance were kept unchanged. A novel multi-machine
robust DTC scheme based on the nonlinear model prediction
(NMP) method was proposed [31]. It achieved the accel-
eration slip regulation (ASR) and anti-lock braking system
(ABS) functions of four wheels PMSMs and better driving
performance and vehicle stability. A fuzzy model predictive
DTC (FMP-DTC) strategy for an IPM of EV is proposed
[32]. The weighting factor adjustment was no more required
for optimal switch state selection. The instantaneous torque
response, small torque ripple, and accurate speed tracking
were achieved. A voltage vector allocation strategy based
on a dual-space vector PWM control scheme was proposed
[33]. By selecting the most appropriate mode, the switch-
ing frequencies of the two inverters could be balanced and
reduced, and the power-sharing in the maximum range could
be obtained.
DTC control, even though simple, has an excellent
dynamic and static performance. However, it has a limita-
tion on the increase of inverter switching frequency. There
is no current loop and the current protection should be done
directly, so additional measurements to limit currents are
needed. The “dead-time effect” is also obvious at low speed,
and the change of stator resistance will distort stator current
and flux linkage.
3.2 Current Control Strategies (CCS)
The strategies of PM control include id = 0, maximum torque
per ampere (MTPA), maximum torque per volt (MTPV),
flux weakening (FWC), unit power factor (cosΦ = 1) con-
trols. The performance comparison of these current control
strategies is shown in Table4.
3.2.1 id = 0 Controls
The advantages of this control strategy are algorithm sim-
plicity, small computations, and no demagnetization effect,
Table 4 Performance
comparison of current control
strategies
Current con-
trol strategy Complexity of
the algorithm Maximum
torque capac-
ity
Speed range Robustness Efficiency Power factor
id = 0 Simple Small General wide High Low Low
MTPA Complex Large Wide Low High General high
MTPV Complex Large Wide Low High General high
FWC Complex Small Wide Low High General high
cosΦ = 1 Simple Small Wide High High High

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which is generally applicable in low-power servo systems.
However, its power factor is low. For the interior PMSM,
this method does not utilize the reluctance torque of the
motor, which reduces the motor torque performance. Thus,
it is only used in surface-mounted PM motors (SPM).
3.2.2 MTPA Controls
MTPA strategy makes full use of the reluctance torque of
motor, so the maximum torque output is greatly improved.
With the same output torque, the stator current of this
method is minimum, which reduces copper losses and
improves efficiency. However, this control strategy is com-
plex and the parameter robustness is not very high.
3.2.3 MTPV Controls
MTPVs make full use of the voltage limit ellipse and DC
bus voltage. The high inverter capacity, the maximum out-
put torque at flux weakening range and the quick system
response can be achieved by this method. However, this con-
trol algorithm is relatively complex.
3.2.4 FW Controls (FWC)
In this method, the flux of the PM motor is reduced by
increasing the d-axis demagnetization current, which guar-
antees the voltage balance and improves the speed adjusting
range. However, this control strategy is sensitive to motor
parameter perturbations, which leads to low robustness.
3.2.5 cosΦ = 1 controls
The unit power factor control based on cosΦ = 1 makes the
power factor equal to 1 by controlling the d-axis and q-axis
currents of PMSM simultaneously without reactive power
output. This control strategy makes full use of the motor
inverter capacity, but its maximum motor torque capacity
is reduced.
3.3 Control Algorithms
Besides PID control, many other advanced control algo-
rithms are introduced. The comparison of several control
methods is shown in Table5.
3.3.1 PID Control
The classic PID control method is stable, reliable, con-
veniently adjustable, and simply structured, making a good
method for linear and stationary objects. However, PMSM
is a strong-coupling and nonlinear object, where the param-
eters change and interact complexly. To improve the motor
speed regulation performance, PID control is combined with
other control methods, such as adaptive PI, neural network
PI, and fuzzy PI controls. However, in terms of motor torque
tracking accuracy, response speed, torque ripple suppres-
sion, and parameter robustness, the algorithms above are not
efficient for achieving excellent dynamic and static perfor-
mance. Therefore, several other advanced control algorithms
are proposed.
3.3.2 Adaptive Control
The adaptive control algorithm handles system uncertainties
by adjusting the controller parameters online, thus having
strong robustness. Among them, model reference adap-
tive control is the most common. Its system is composed
of a reference model, an adjustable system, and an adaptive
mechanism. However, the design of the reference model and
adjustable system relies on the precise motor model, which
is seriously influenced by the motor parameter perturbations.
3.3.3 H∞ Control
As a typical robust control (RC) method, the H∞ control
algorithm aims at minimizing the sensitivity of the controller
uncertainties to maintain the system control performance. Its
Table 5 Comparison of control methods
Control method Advantages Disadvantages
PID Simplicity, easy to speed, strong applicability Long response time, poor steady-state performance
Adaptive control Parameter self-correction, strong robustness Strong dependency on model accuracy, weak dynamic
performance
H∞ control Strong robustness, disturbance resistance Complicated solution process
ADRC Smooth response, strong disturbance resistance Delay in the approximation process, multiple parameters
MPC Simple design, fast dynamic response Complexity, strong dependency of model accuracy
NNC Convenient parameter setting, strong self-learning ability Slow convergence, oscillations
FLC Simple design, model independence, high fault tolerance Low systematism of fuzzy rule design
SMC Simple structure, easy to design, high robustness Singularity, chattering phenomena

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robustness and disturbance rejection are both strong, but the
solution process is complex.
3.3.4 Active Disturbance Rejection Control (ADRC)
ADRC uses a disturbance observer to estimate the system
uncertainties and then introduces the disturbance rejection
into the control signals to compensate the uncertainties.
ADRC provides a strong disturbance rejection. However,
its observer’s design parameters are numerous, the approxi-
mation process is delayed, and a certain steady-state error
exists, which affects the motor control accuracy.
3.3.5 Model Predictive Control (MPC)
MPC is simple in design and has a fast dynamic response.
Its action is based on solving an optimal control problem
of open loop in the finite-time domain at every sampling
moment. However, this control algorithm is complicated and
depends on the motor model parameters.
3.3.6 Neural Network Control (NNC)
The NNC method can achieve a smooth start, small torque
ripple, wide speed range, and high robustness with a sim-
ple parameter setting, strong self-learning ability, and low
motor parameter sensitivity. However, the NNC structure is
relatively complex, and online iterative computation leads
to poor real-time performance. Thus, it is more suitable for
off-line parameter identification.
3.3.7 Fuzzy Logic Control (FLC)
FLC has a simple structure, good robustness, and a small
impact on the motor startup. It is well applied in the design
of the AC servo motor control system. However, in practi-
cal applications, its design relies on experience and expert
knowledge.
3.3.8 Sliding Mode Control (SMC)
The SMC algorithm, being invariable to external distur-
bances, has a simple structure, low sensitivity to the internal
parameter perturbations, and high control accuracy. It is suit-
able for the control of nonlinear uncertain systems, but has a
high torque ripple. Chattering, singularity, and mismatched
uncertainty limit its applications. Advanced SMC algorithms
were proposed to suppress the chattering and even elimi-
nate it by reducing the switching gain and frequency and by
smoothing the control signals.
Collaborative optimization for axial flux PMSM con-
trol system was proposed [34]. Fuzzy control improved
the torque ripple, and SMC improved the motor dynamic
performance, effectively enhancing the range and accelera-
tion performance of EVs. A variable SMC controller based
on a speed loop was proposed [35]. It combined MTPA to
control the IPM and obtained significant control reliability
and flux-weakening performance.
3.4 PWM Control
Among numerous PWM methods, space vector pulse width
modulation (SVPWM), sinusoidal pulse width modulation
(SPWM), and six-step voltage (SSV) are the most common.
Their performance comparison is shown in Table6. For
the given DC bus input voltage and the phase output cur-
rent capability, high DC bus voltage utilization can help the
motor output more power at and after the corner speed (i.e.,
to the flux-weakening range).
3.4.1 SVPWM
SVPWM enables the motor to obtain a circular magnetic
field with constant amplitude. Compared with SPWM,
15.47% higher DC bus voltage utilization can be achieved,
which allows more power to be outputted at high-speed oper-
ation. Low current waveform distortion or a small account
of current harmonic components can also be achieved. Fur-
thermore, the rotating magnetic field is closer to the cir-
cle, which greatly improves the motor performance. Thus,
SVPWM is the dominant modulation in motor control.
3.4.2 SPWM
SPWM focuses on solving the problem of three-phase sym-
metrical sinusoidal voltage frequency and voltage regulation
from the standpoint of the motor power supply. However,
its total harmonic distortion is larger than that in SVPWM,
which impacts negatively the control performance. More
severely, the amplitude of its fundamental phase voltage can
only be 1/2 of the DC bus voltage, which may only be used
in the low-speed range before the corner speed.
Table 6 Performance comparison of modulation methods
Modulation
method Structural
complex-
ity
Harmonic
compo-
nent
Torque ripple Robustness
SVPWM Complex Low Low High
SPWM Simple High High Low
SSV Simple High High Low

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3.4.3 SSV
SSV can adjust the power by controlling the voltage ampli-
tude, flux, and torque, which can provide the highest DC bus
voltage utilization ratio, thus beneficial to a greater power
output at speeds beyond the motor corner speed. However,
its harmonics are rich in the phase current and in the airgap
magnetic field, leading to the fifth- and seventh-order har-
monics with higher amplitude.
3.5 Power Electronic Devices inControl
The new generation of insulated-gate bipolar transistor
(IGBT) chips for NEVs were launched by international com-
ponent suppliers like Infineon, Fuji, Mitsubishi, and Rene-
sas. For example, Infineon IGBTs are based on an 8-in. or a
12-in. technology platform, while IGBTs are manufactured
on a 6-in. or an 8-in. wafer in China. Nevertheless, there are
still some gaps in the device performance indicators, key
process technology, production quality control, and cost. The
IGBT module packaging with vehicle standard (equivalent
to imported modules like HP1, HP2, and HP Drive) is close
to the international average level in performance and reli-
ability, and their large-scale application in automotive just
begin in China.
Compared with traditional silicon devices, wide bandgap
(WBG) power devices represented by SiC and GaN show
strong advantages in voltage, operating temperature, switch-
ing frequency, and switching losses, which makes them more
suitable for NEV inverter requiring tolerance to high temper-
ature, high voltage, high frequency, and high power density
[35]. The material property comparison among Si, SiC, and
GaN is shown in Fig.2 [36].
WBG device applications are explored in electrical drive
systems. When the motor controller’s power density exceeds
25kW/L, the WBG semiconductor can be used for heating
reduction due to low conduction loss at a high-frequency
switching operation [37]. A buck converter prototype with a
SiC power module was established for continuous operation
at high-temperature operation [38]. The SiC module’s high
performance could still be found at its junction tempera-
ture of 225°C. An integrated low-voltage rating of the GaN
module was used to set up a three-phase full bridge inverter,
resulting in the reduction in not only the inverter weight
and volume but also the device resistance and conduction
loss [39]. However, if the WBG device switching frequency
was increased to 50–100kHz, the EMC influence caused
by its high dv/dt on the efficiency would be more prominent
[40]. Electromagnetic interference (EMI) levels of SiC and
Si devices with similar topology were compared under the
same working conditions [41]. The results showed that the
miller effect caused by the parasitic parameters in SiC JFET
devices was the main reason of high EMI. An insulated
metal substrate was installed on the division of the motor
drive inverter with a SiC JFET for restraining the common
mode (CM) and EMI [42]. It was shown that the third-order
LCL filter had better performance than the fourth-order of
LCL. Stray inductance between power electronics and the
converter output was utilized as a filter, combined with an
additional RC link, for a high-frequency 100–1MW inverter
with SiC [43]. The test showed that even for a measured
value of 47kV/μs, the inverter output dv/dt could be limited
to 7.5kV/μs.
To handle the ground-drain current, CM electromagnetic
interference (CMEMI) and bearing current in the motor
inverter with WBG devices, a new concept of CM voltage
cancelation through a balancing inverter topology and a
double-winding stator structure was proposed [44]. In a test
based on GaN, although the parasitic capacitance in the case
of asymmetric windings limited the CM cancelation, the
ground current amplitude could be reduced by 90%, and the
conduction CMEMI emission could be reduced by an aver-
age of 20dB without using any filter. In Ref. [45], a standard
driver circuit was adopted by adding a simple coupling cir-
cuit, to drive two series-connected SiC metal–oxide–semi-
conductor field-effect transistors (SiC MOSFETs), and a
limiting buffer circuit was used for voltage balancing. It
has the advantages of low cost, simple structure, and high
reliability.
3.6 Motor Controller Development
High efficiency, high density, and good EMC performance
are the development directions of motor controllers. By
adopting power electronics integration technology, the
weight and volume of the whole controller can be reduced
effectively, power density can be increased, and manufac-
turing cost can be reduced. Power electronics integration
Fig. 2 Property comparison of SiC, Si, and GaN

W.Cai et al.
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technology is mainly divided into three levels: monolithic
integration, hybrid integration, and system integration.
Hybrid integration schemes are mostly adopted in motor
controllers such as Toyota Prius and GM Volt. Module
packaging, interconnection, and efficient cooling are the
core of power electronics hybrid integration. The compari-
son among global advanced products is shown in Table7.
For SiC motor controllers, full use of the high-temper-
ature tolerance, high efficiency, and high frequency of SiC
MOSFET devices is the key to improving the power density
and efficiency further. SiC MOSFET inverter was applied
in Tesla Model 3, shown in Fig.3b. Its SiC motor controller
is composed of 24 SiC MOSFET chips grouped in parallel
and mounted on a pin–fin radiator to achieve high current
output (800Arms). Through the laser welding process, each
SiC MOSFET is connected to a copper busbar, which greatly
improves the connection reliability. Full SiC inverters are
launched for vehicle applications by other companies. It
was found by Toyota that under load conditions, the loss
of SiC power control unit (PCU) of the prototype vehicle
was reduced by 30% compared with IGBT PCU in Fig.3f.
Double-sided welding and double-sided cooling technology
were adopted by Denso to achieve small size and high effi-
ciency of its SiC controller in Fig.3a, which was used in
Toyota fuel cell vehicles.
Typical full SiC controllers are shown in Fig.3.
SiC MOSFET controller was developed by Jing-Jin Elec-
tric (JJE) for VW commercial vehicles, whose power den-
sity is over 40kW/L. The hairpin winding motor and SiC
controller prototypes were developed for EU passenger car
OEM by JJE at the end of 2019. In 2020, 300–600kW series
of SiC MOSFET inverters, shown in Fig.3e, were developed
by JJE for TRATON group, a VW commercial vehicle divi-
sion. The SiC inverter, shown in Fig.3c, is also embarked
onboard of BYD EV-HAN in July 2020.
Table 7 Comparison of power
density of motor controllers Items Toyota Prius Gen4 Bosch Gen3 Conti. Gen3
Power density (kW/L) 25.0 25.0 23.0
Power ratio weight (kW/kg) 23.2 22.8 21.0
Peak power (kW) 105 125 135
DC voltage class (V) 200–600 300–450 300–450
Device current (A) 350Arms 400Arms 450Arms
Device package form Custom Custom Custom
(a) Denso SiC controller (b) Tesla SiC controller (c) BYD SiC controller
(d) Hitachi all-SiC controller (e) JJE SiC controller (f) Toyota full SiC PCU
Fig. 3 Typical SiC motor controllers

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4 NEV Powertrain Development
4.1 BEV Powertrain Topology
Battery EV (BEV) powertrain generally includes the motor,
power electronics control system, and reducer or transmis-
sion. Its configuration depends mainly on the layout of the
electric drive system inside the vehicle. Electric drive sys-
tems can be categorized into single-motor drive (or lump
e-drive), distributed-motor drive, and range-extended drive
systems [46].
4.1.1 Single‑Motor Drive System (or Lump e‑Drive System)
A single-motor drive system is similar to the scheme of tra-
ditional internal combustion engine (ICE) vehicles, but the
electrical motor replaces the ICE, and other configurations
are modified accordingly. However, this configuration has a
large demand for chassis space.
A single-motor drive system can be mounted on the front
or rear drives, and its reducer/transmission structures can be
divided into three categories, as shown in Fig.4.
An EV motor and a high-accuracy vehicle analysis model
were proposed in Ref. [47]. The design space of the motor
could be accurately described and the entire electric drive
system was optimized. The influence of a non-deterministic
modulation scheme on the transmission system was con-
ducted based on the single IPM. The transmission mechani-
cal part was modeled as a dual oscillator, and different
inverter modulation schemes (hysteresis controller, PWM
method) were applied. The inverter-generated harmonics
and switching frequency were taken as the optimization tar-
gets. The simulation results showed that the pulse frequency
could be reduced in the hysteresis controller. In addition,
the inverter-generated partial harmonic energy could com-
pensate for the filter absence. The IPM drive system with
five-phase windings was comprehensively compared with
IPM systems with three phases [48]. The proposed five-
phase motor has the advantages of low cost, small torque
ripple, high power density, good capability of flux weaken-
ing, strong fault tolerance, high reliability, and more design
freedom.
4.1.2 Distributed Electric Drive System
Multiple motors are distributed to the corresponding vehicle
wheels. According to the motor location, it can be catego-
rized into three types: wheel rim, hub type, and combined,
as shown in Fig.5.
Connecting wheels directly to the motors can realize the
precise measurement of the wheel torque and rapid response
to the driving requirement. The vehicle chassis configuration
is simplified, the user available space is expanded, the vehi-
cle weight and the energy consumption are reduced, and the
driving range per charge is consequently increased. There-
fore, the distributed motor drive system has been recognized
as one of the most promising electrified propulsion systems.
However, there is no mass production of vehicles with the
(a) Electric drive system with a clutch
(b) Electric drive system without the clutch
(c) Integrated electric drive system
Fig. 4 Single-motor electric drive systems

W.Cai et al.
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distributed e-drive systems, except for the wheel side motor
drive system used in BYD K9 buses. Now, the research
mainly focuses on the control strategies such as torque dis-
tribution optimization [49, 50].
An algorithm for four-wheel distributed drive control
was proposed for a fast, energy-saving, and easy-to-realize
torque allocation [51]. The experimental results showed that
the proposed algorithm was reasonable, and the system effi-
ciency was improved significantly in the lateral acceleration
range of the entire ramp. The particle swarm optimization
algorithm was used for searching the optimal global size of
four-wheel-driven EV [52]. The simulation results showed
that the particle swarm optimization method combined with
the real-time torque allocation strategy could effectively
reduce the size of the main components of the powertrain
and reduce energy consumption. Next, a hierarchical control
strategy for a four-wheel distributed drive system was pro-
posed to meet the driver’s operating instructions and main-
tain the lateral stability of the vehicle [53]. The control strat-
egy was divided into two layers. The upper layer realized
nonlinear MPC, and the lower layer controlled the wheels
through a PID controller. The experimental results showed
that the driver’s longitudinal and lateral motion commands
were performed with a good real-time performance.
Hub motor for passenger vehicles has no global mass pro-
duction owing to the constraints of cost, reliability, braking
safety, and control. The mechanical, electrical, and ther-
mal issues are not well solved. Only the samples or small
demonstration prototypes could be seen in the market. As
for commercial engineering vehicles like buses and heavy-
duty trucks, hub motors are already used owing to relatively
unstrung layout space in wheels, low vehicle speed, rela-
tively low sensitivity to unstrung mass increment, which pro-
vides low floor, and large space to bus passengers.
4.1.3 Extended Range EV (EREV orREEV) System
An augmented electric drive system differs from single-
motor and distributed motor drive systems because it con-
tains an auxiliary power unit (APU). The system configura-
tion is shown in Fig.6, which is also classified as plug-in
HEV (PHEV). A low-power engine is usually used. Com-
pared with the battery drive system, the battery capacity here
can be reduced appropriately, which has a good application
prospect in mid-sized vehicles. However, this drive system
has a high cost and needs support from ground chargers [54,
55].
The configuration and performance details of traction
motors A and B in the electric drive system of Chevrolet
EREV were introduced [56]. The simulation results showed
that the bar-wound winding (hairpin winding) motor had
better performance than that of the motors with the strand
round wire windings. In addition, the rotor with cavitation
at the magnet top was specially designed to improve the spin
loss by reducing the flux density harmonics in the motor
airgap. Owing to the advantages of the Volt electric drive
system and control algorithm, its noise reduction was also
significant. A multi-objective optimal energy management
method was proposed for APU fuel consumption and battery
state of health (SOH) in extended-range electric buses [57].
The APU fuel consumption and battery SOH were used as
optimization targets, and the dynamic program (DP) algo-
rithm was used to solve the multi-objective problem. The
simulation results showed that the optimal economic effect
could be obtained if the battery pack parameters and the
control strategy were set to the minimum without battery
replacement. Furthermore, a novel energy management
(b)
(c)
(a) Wheel side motor drive system
Wheel rim–hub motor drive system
Wheel hub motor drive system
Fig. 5 Distributed electric drive systems

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optimization strategy aiming at the APU configuration and
control method in EREVs was proposed to solve the power
distribution issue of APU and battery [58]. The model of the
APU control system was established, and two strategies for
APU power tracking were proposed according to different
power change rates under the condition of changing APU
dynamic response characteristics and control parameters.
The experimental results showed that the APU power change
rate under different conditions could significantly affect fuel
consumption.
4.2 Hybrid Powertrain
An engine, electric motor(s), and power batteries are com-
bined, and two power sources are matched and optimized for
greatly reducing vehicle emissions and fossil energy con-
sumption [59, 60]. According to the combination of power
sources, HEV powertrains can be classified into three types:
serial, parallel, and series–parallel compound.
4.2.1 Series Hybrid Powertrain
The series hybrid power system is shown in Fig.7. The
engine and the motor are connected in series. The engine
does not propel wheels mechanically, and it only drives the
generator to generate electricity through burning fuel. The
generated electric power is sent to the traction motor, which
creates the traction torque to drive the whole vehicle. Simul-
taneously, the power battery can also supply the electric
power to the traction motor to operate the vehicle.
As only the traction motor drives the wheels, the engine is
not affected by the driving conditions, which could be set for
continuous operation at the most efficient point. However,
the energy to drive the vehicle undergoes two conversions:
mechanical to electrical and then to mechanical. Thus, the
fuel energy utilization rate is relatively low.
In the study of serial hybrid powertrains, the combined
optimization of an energy management strategy and driv-
ing speed was investigated for minimizing fuel consumption
[61]. An energy management strategy based on the road con-
dition and the driving time was proposed to find the optimal
driving speed and energy power allocation for specified driv-
ing tasks. A power follower control strategy was combined
with the DC side voltage control strategy, and a novel idea
for energy management of hybrid EVs was proposed [62].
Experimental results showed that this series hybrid vehi-
cle had better fuel economy than the single control scheme.
Furthermore, a multi-function framework for hybrid pow-
ertrains considering the driving conditions was proposed
[63]. Using a diesel-powered traditional vehicle as a hybrid
target, a hybrid topology combining the series-connected
hybrid and wheel–motor systems was presented. Compared
with the traditional vehicles, the acceleration performance
and the climbing gradient were improved by 18% and 10%,
respectively. For the PM generator in the series hybrid con-
figuration, the hybrid excitation topology was proposed [64],
together with the integrated passive rectifier, replacing the
PMSM and the active power electronic converter, which
facilitated the constant control of the PM flux linkage. This
design was confirmed to provide higher output voltage and
power density.
4.2.2 Parallel Hybrid Powertrain
The parallel hybrid power system is shown in Fig.8. The
engine and the motor shafts are connected in parallel. The
vehicle can be driven by the engine and the motor together
or by one of them alone. There is no dedicated generator
in this configuration, and the power battery pack can only
be charged by the traction motor operating in its generat-
ing mode. However, the engine working condition is often
affected by the vehicle driving cycle, and it cannot always
run at the optimal working point. Compared with the series
hybrid power system, a more complicated transmission is
required.
An energy management strategy model based on a deep
recursive neural network was proposed for the optimal
Fig. 6 EREV system
Fig. 7 Series hybrid power system

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torque distribution of single-axle parallel hybrid vehicles
[65]. Better performance in terms of fuel economy and
accuracy was provided. Further, an adaptive neuro-fuzzy
reasoning system was combined with the equivalent power
consumption minimization strategy, and a practical adaptive
energy management strategy for parallel hybrid buses was
proposed [66]. The results showed that the fuel economy was
improved by this control strategy. A torque balance thresh-
old change strategy was proposed for the energy manage-
ment of parallel hybrid vehicles [67]. When the engine was
active, it runs at constant torque, to ensure its operation at
highly efficient points. Further, an electro-hydraulic parallel
hybrid powertrain for urban vehicles was introduced [68].
The power consumption and battery discharge stress of the
electric powertrain were reduced by hydraulic system. The
vehicle driving range per charge was increased, and the
battery life was improved. Aiming at the mode conversion
of parallel hybrid vehicles, a control method based on an
adaptive double-loop control framework was proposed [69].
The experimental results showed that the proposed control
method could effectively improve the performance of HEVs.
In addition, the time of the mode conversion process was
shortened and the vehicle turbulence within an acceptable
range could be controlled.
4.2.3 Series andParallel Compound Powertrain
In compound powertrains, the relationship between the
generator and the motor can be either series or parallel, as
shown in Fig.9. The engine can directly output propulsion
power through nodes 1, 2, 4 to drive the vehicle together
with the motor or to generate electric power through nodes 1,
3 when only the motor drives the vehicle mechanically. Usu-
ally, when the vehicle is running at low speed, the driving
system works mainly in series. When the vehicle is running
stably at high speed, the main operation mode is parallel.
The optimal matching of all components to the greatest
extent can be achieved and the contradiction between the
fuel energy utilization rate and the engine’s best working
condition can be balanced with the compound hybrid sys-
tem. Compared with the parallel configuration, this com-
pound is more complex and has higher requirements on the
power combination components.
The system efficiency was improved by optimizing the
configuration, e.g., by adding gears to components or gear-
boxes with several transferring ratios [70]. To solve the
working area mismatch between the engine and the motor,
a new multi-mode coupling drive system was designed by
coupling a distributed drive system with a centralized drive
system and adding a clutch [71]. The results showed that
the system efficiency was improved because the engine and
motor operating points fall within their effective ranges.
The optimal transferring ratio and motor size were deter-
mined based on the requirements for fuel economy, accelera-
tion, and maximum speed performance [72]. The proposed
method had the advantage of obtaining a more accurate
compact EV powertrain. To solve the power allocation and
management in the hybrid configuration, an energy man-
agement strategy based on nonlinear MPC was proposed
[73]. Compared with the charge depletion strategy, the fuel
economy was improved by 18.86% and 10.36%, through
the nonlinear MPC and equivalent power consumption
minimization strategies, respectively. Aiming at the PMM
performance optimization, the brushless dual-rotor motor
with axial magnetic field modulation was investigated [74],
which could be connected to the traditional PMSM, forming
a dynamic composite device. The axial tilting moment was
analyzed, and the necessary conditions to avoid the axial
tilting moment were given.
Fig. 8 Parallel hybrid power system
Fig. 9 Series and parallel compound powertrain

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4.3 Comparison ofElectric Drive Systems
4.3.1 Comparison ofBEV Drive Systems
The advantages and disadvantages of BEV drive systems
are listed in Table8.
It can be seen from Table8 that the three drive sys-
tems have their advantages and disadvantages. The main
advantages of the centralized motor system are simple to
control and relatively low research and development (R&D)
cost. However, the low efficiency is its obvious disadvan-
tage. The distributed system has the least mechanical trans-
mission mechanisms and the highest transferring efficiency.
However, the latter contains multiple motors and involves
a more complex electronic control technology, which has
no mass production thus far. For the EREV system, owing
Table 8 Advantages/
disadvantages of BEV drive
systems
Performance indicators Centralized system Distributed system Additive system
Structural complexity Complex Simple Complex
R&D costs Low Middle High
System efficiency Low High Middle
Mileage per charge Short Middle Long
Control difficulty Low High Middle
Table 9 Advantages/
disadvantages of hybrid drive
systems
Performance indicators Series hybrid system Parallel hybrid system Compound hybrid system
Structural complexity Simple Simple Complex
Fuel economy Poor Good Preferable
Suitable working condition Heavy traffic Intercity/highway Suitable for inter city
Engine operating point Always the best Difficult to be the best Adjustable to the best
Control difficulty Low Middle High
Pure Electric Powertrain, Electromechanical Coupling Assembly, Hub/Wheel Assembly
Tracon MotorReducer/ TransmissionPower Electronics Controller
Hardware support: advanced tesng and test equipment, intelligent manufacturing equipment,
basic parts and manufacturing technology, advanced material s and devices, high-performance
mul-core IC chips, wide bandgap power electronics chips and devices
Soware support: algorithm and sowareoperang system /soware architecture, advanced
manufacturing process, volume pr od ucon quality assurance system, development verificaon
standards and specificaons, operang system /soware architecture, safety standards,
diagnosc methods, mul-physics simulaon and opmizaon design, integraon and
packaging technology, reliability and life evaluaon technology
Silicon steel sheets,
electromagnec wires,
insulang materials,
permanent magnets,
bearing, posion sensors,
temperature sensors
Power electronics devices,
integrated circuits,
film capacitors,
voltage and current sensors,
applicaon soware,
control strategies
Gears and shaing,
clutches,
planetary gears,
shaing,
sealing and lubricaon,
actuators
Powertrain
Parts
Core materials and
components
Foundaon support
Fig. 10 Technical architecture of the NEV electric drive system

W.Cai et al.
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to the APU’s existence, the driving distance per charging is
increased. However, a lot of space in the system is occupied
by the transmission structures, and the working efficiency
is not very high.
4.3.2 Hybrid Powertrain Comparison
The advantages and disadvantages of different hybrid pow-
ertrains are shown in Table9.
The advantages of the series configuration are mainly
reflected in the simple structure, relatively easy control, and
optimal engine working point. The main parallel configura-
tion advantages are good fuel economy and relatively simple
structure. The series–parallel compound configuration com-
bines the characteristics of both series and parallel configu-
rations, so it can adapt to a variety of driving conditions.
However, the system is complex and the control is difficult.
5 Technical Innovation andDevelopment
Forecast oftheNEV Electric Drive Systems
The technical architecture of the NEV electric drive system
is shown in Fig.10. It mainly includes powertrains, core sub-
assemblies, key materials, components, and basic support.
The innovation of the NEV electric drive system tech-
nology can be summarized as the overall improvement of
the entire supplier chain technology from materials, parts/
components, and the motor systems to the powertrains.
Fig. 11 Grain boundary diffusion and grain boundary modulation of
the PM
Table 10 Objective parameters on the technical roadmap of electric drive systems
Performance of powertrains, components, and materials 2020 2025 2030 2035
Specific power of e-motors (kW/kg) 4 5 6 7
Area percentage at the e-motor efficiency ≥ 80% 88 90% 93% 95%
E-motor cost (¥/kW) 35 30 25 20
Power density of power electronics converters (kW/L) 25 40 50 70
Area ratio of the converter efficiency ≥ 90% 88.5% 90% 92% 94%
Power electronics converter cost (¥/kW) 40 30 25 20
Specific power of EV e-powertrains (kW/kg) 1.8–1.9 2.0 2.4 3.0
Efficiency of EV e-powertrains under CLTC 86% 87% 88.5% 90%
EV e-powertrain cost (¥/kW) 75–80 72 60 50
Mass reduction percentage of HEV powertrains Base 20% 35% 50%
Efficiency of HEV powertrains under WLTC 81% 83% 84% 86%
Torque density of direct drive hub motors (Nm/kg) N/A 20 24 30
Loss of silicon steel sheets (kW @P1.0/400, @P1.0/800 14, 37 13, 36 12, 33 11, 30
Magnetic polarization of silicon steel sheets (T) 1.66 1.67 1.68
Remanence of PM martials (T) 1.35 1.40 1.45 1.50
Coercive force of PM martials (kA/m) 2300 2300 2400 2400
Percentage of heavy rare earth materials (Dy + Te) 4% 3% 2% 1%
Corona guard life of flat enameled wires (h) 100@12kHz 100@20kHz 100@30kHz 100@50kHz
Thermal tolerance temperature of insulation (°C) 180–200 200 220 240
Thermal conductivity of insulation systems (W/mk) > 0.25 0.3 0.4 0.5
Limiting speed of bearings with d ≤ 35mm(rpm) 16,000 20,000 25,000 28,000
Fatigue life of bearings 5L10 6L10 8L10
Current density of IGBT chips at 750VDC (A/cm2) 300 350 400 450
Current density of SiC chips at 1200VDC (A/cm2) 350 450 600 800
Thermal tolerance temperature of capacitors (°C) 120 140 150 175
Master frequency of multi-core locked step MCU N/A 300MHz 600MHz 1000MHz

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5.1 Core material Technology Innovation
oftheElectric Drive System
In the electric drive system, the development of rare earth
PM materials contributes to the PMSM development. The
magnet performance characteristics have been significantly
improved, such as magnetic energy product, magnetic decli-
nation, sinusoidal magnetization, magnet block splitting and
bonding, light rare earth element usage, surface coating, and
PM measurement. The grade over N50UH Nd-Fe-B PM is
mass-produced, and its residual magnetism is close to 90%
of the theoretical limit of Nd2Fe14B compound. However, the
coercivity is still lower than 30% of the theoretical limit, so
there is much room for improvement.
Heavy rare earth grain boundary diffusion and grain
boundary modulation technologies can be adopted for future
development, as shown in Fig.11. These technologies can
greatly reduce the usage of heavy rare earth materials like
dysprosium and/or terbium and improve the performance
and quality of magnets. Moreover, some mixed PM materials
(including ferrite) are used for partially replacing Nd-Fe-B
materials by some OEMs like Toyota and GM.
Therefore, besides PM materials, corona guard insulation
materials, electromagnetic wires, high-performance mag-
netic materials, and manufacturing processing have drawn
much attention in the electric motor industries. Insulating
materials and their insulation structure design, providing
high performance, high reliability, high electric and ther-
mal life, high thermal conductivity, corona guard, and oil
compatibility, are also paid considerable attention since the
high-speed motor and the high-frequency inverter are the
development trend.
5.2 Innovation ofPower Electronics
In the electric drive system, power electronics plays an
important role in the traction motor system and electric pow-
ertrain performance. In the future, the NEV industry will
focus on the following power electronics research.
(1) The trench technology has become the mainstream in
IGBT chips for vehicle applications. It can enhance the
ability of electron injection and reduce the switching-
on, turning-off, and conduction losses. With the devel-
opment of groove technology, the refinement of groove
plays a key role in improving the overall performance
of IGBT chips for EVs.
(2) Rapid thick epitaxial growth and the material inspec-
tion technology is drawing much attention for SiC
MOSFET chips. Wafer preparation and inspection as
well as low-sensitivity and high-density SiC or GaN
module packaging are directly related to successful
NEV application on the third-generation WBG power
semiconductors.
(3) Single-chip functional integration improvement, sys-
tem complexity reduction, power density increase, and
reliability improvement of power electronic chips and
packages are focused. IGBTs or SiC MOSFETs and
SiC diodes are integrated on the same chip to improve
the current density of the power module. Current and
temperature sensors are integrated on the electronic
power switch chip to detect transient output current
and transient junction temperature fluctuations, thus
improving the reliability and power density of package
modules.
(4) The technologies of copper binding wire, copper termi-
nal direct bonding, belt bonding and flexible connec-
tion are adopted to replace the traditional aluminum
binding wire technology. The contact area of the silicon
wafer is increased for evenly distributing the thermo-
electric stress in the power contact part, reducing the
peak temperature of the silicon wafer and improving
the power cycle life of the module.
(5) Double-side welding, single-side/double-side cool-
ing, and integrating heat dissipation sink can reduce
the thermal resistance of the chip, improve the heat
dissipation capacity, and improve the power cycle reli-
ability, which has become a new trend of the packag-
ing technologies of the next-generation IGBT and SiC
MOSFET module.
5.3 Innovation oftheMotor andPowertrain System
In the domestic market of passenger BEV powertrains, the
integration products of 3-in-1 and multi-in-1 electric drive
assembly are at the same level as those of the global suppli-
ers. Tesla’s integrated structure design is relatively advanced
and is based on the new electric chassis design and forward
research. The specific power of motors exceeds 4–4.6kW/kg
globally. However, there is no first-mover advantage at the
time of product launch. To improve the driving efficiency,
the SiC MOSFET inverters with discrete devices were com-
mercialized earlier than others, and their highest efficiency
of the 3-in-1 electric drive system is approximately 94%.
Compared with the Si-based motor drive system, power-
trains based SiC provided higher peak and operating effi-
ciencies under the vehicle driving cycles.
In the future, the systematic integration and supplier
chain innovation of NEV electric powertrains and their key
components should be focused, which include:
(1) Configuration technology of electromechanical cou-
pling assembly;

W.Cai et al.
1 3
(2) Sealing (condensation), heat dissipation, and lubrica-
tion of highly integrated electromechanical coupling
systems;
(3) Key parts and components such as shaft with gears,
clutch, planetary gears, and actuators;
(4) Direct drive hub motor, new-type wheel hub electric
drive system, and innovative design technology of the
brake system;
(5) NVH design, noise suppression, detection, and evalu-
ation technology;
(6) Validation standards and specifications for electrome-
chanical coupling devices.
5.4 2021–2035 Technology Roadmap ofElectric
Powertrains, Key Components, andMaterials
The technology roadmap of electric powertrains, electric
motors, power electronics converters, and key components,
as well as materials, is described in Ref. [75]. The future
performance parameters of the electric drives, their sub-
assembly, parts/components, and materials are predicted
and shown in Table10 in terms of time frame.
6 Conclusions
The current states of the NEV motor systems and powertrain
technologies are systematically reviewed; the technologi-
cal innovations and applications in materials, devices, and
powertrains are summarized in details; and different con-
trol algorithms are compared. Although the performances
of traction motor and powertrain products is dramatically
improved, more R&D is required for more innovative tech-
nologies, such as motor design optimizations and control
algorithms, multi-physical simulation analysis, robustness
design, system integration with jointly considered motors,
controllers and reducers/transmissions, a next-generation
motor system based on SiC devices, PM motors with less
or without heavy rare earth PM materials, efficient motor
cooling methods, and new material development and
applications.
Vehicular dynamic performances, energy saving features,
safety, and comfortabilities are mainly summarized from the
electric drive systems. The technical roadmaps of electric
powertrains, traction motor systems, key components, and
materials are summarized in terms of the time frame of
2025, 2030, and 2035, which can be used as a reference by
researchers and engineers in the OEMs and NEV industry
supplier chains, the government officers, or investors for
investing strategies.
Acknowledgements We are grateful to Maotong Yang and Guanning
Guo from the Institute of Electrical and Power Electronics Engineering
at Harbin University of Science and Technology for their help in trans-
lating figures and tables as well as partial content of this paper.
Compliance with Ethical Standards
Conflict of interest On behalf of all authors, the corresponding author
states that there is no conflict of interest.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.
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