New trends in electric motors and selection for electric vehicle propulsion systems

Abstract

Abstract The increase in the numbers of electric vehicles (EVs) is seen as an upgrading of the existing vehicles for various reasons. This calls for an in‐depth analysis of the heart of these vehicles—the motor. A motor in an electric vehicle propulsion system is a crucial component that has the ability to affect the efficiency, weight, cost, reliability, power output and performance. Hence a detailed comparative study, that compares the existing types and topologies of various motors, is the need of the hour. The various motors that can be used in electric traction, namely DC, induction, switched reluctance, permanent magnet brushless AC motors and permanent magnet brushless DC motors, are reviewed in view of their capabilities with respect to EV propulsion. A detailed review is presented of existing motors and the application of power electronic techniques to EVs, and recommendations for some new designs of brushless DC motors. These include permanent magnet hybrid motors, permanent magnet spoke motors and permanent magnet inset motors.

Full text

Received: 2 February 2019
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Accepted: 24 February 2020
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Revised: 2 December 2019
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IET Electrical Systems in Transportation
DOI: 10.1049/els2.12018
ORIGINAL RESEARCH PAPER
New trends in electric motors and selection for electric vehicle
propulsion systems
Sreedhar Madichetty
1
|Sukumar Mishra
2
|Malabika Basu
3
1
Mahindra Ecole Centrale, Hyderabad, India
2
IIT Delhi, New Delhi, India
3
Technological University Dublin, Dublin, Ireland
Correspondence
Sreedhar Madichetty, Mahindra University‐Ecole
Centrale School of Engineering, Department of
EEE, Survey No 62/1A, Near Tech Mahindra
Campus, Hyderabad, 500043, India.
Email: sreedhar.803@gmail.com
Funding information
SERB/NPDF/2017/000568
Abstract
The increase in the numbers of electric vehicles (EVs) is seen as an upgrading of the
existing vehicles for various reasons. This calls for an in‐depth analysis of the heart of
these vehicles—the motor. A motor in an electric vehicle propulsion system is a crucial
component that has the ability to affect the efciency, weight, cost, reliability, power
output and performance. Hence a detailed comparative study, that compares the existing
types and topologies of various motors, is the need of the hour. The various motors that
can be used in electric traction, namely DC, induction, switched reluctance, permanent
magnet brushless AC motors and permanent magnet brushless DC motors, are reviewed
in view of their capabilities with respect to EV propulsion. A detailed review is presented
of existing motors and the application of power electronic techniques to EVs, and rec-
ommendations for some new designs of brushless DC motors. These include permanent
magnet hybrid motors, permanent magnet spoke motors and permanent magnet inset
motors.
1
|
INTRODUCTION
The increasing inclination towards electric vehicles has been
due to a large number of factors. The greatest advantage of
electric vehicles over fossil fuel‐powered vehicles is the
increased efciency from the source of energy to the wheel.
Some of the other factors that have caused an increase in
trends towards electric vehicles include better torque–speed
capabilities, ability to operate without any transmission, low
maintenance, zero emissions and noiseless operation. These
vehicles can also be described as green vehicles if the source
of electricity is a renewable source such as solar, hydro, wind,
etc. Hence, electric vehicles are seen as the future of
transportation.
There are three main stages in the history of EVs. Electric
motors during the 1900s had a market penetration equivalent
to that of the steam and internal combustion (IC) engine‐
powered vehicles. At that time, long journeys were infre-
quent and so the short range of electric vehicles was not a huge
limitation [1, 2]. The rst hybrid electric vehicle was created by
Porsche in the 1989. The intention was to improve the ef-
ciency of the IC engine by operating it along with an electric
traction motor. With the advancement in power electronics a
lot was made possible in terms of power conversion and motor
control. This lead to the resurgence of electric vehicles in the
automotive industry [3]. The low power density and the high
cost of batteries caused electric vehicles to lose competition
against the IC engine powered vehicles [4]. In the present
scenario electric vehicles and hybrid electric vehicles are re‐
entering the market with advanced technologies such as
high‐density batteries, high power density motors, and more
efcient power converter technologies [5].
The motors that are used in electric vehicles are quite
different to the motors used in other applications. These
motors require certain specications to be fullled and need to
be in alignment with the traction effort of the vehicle. Also,
these motors cannot afford to be heavy as the range of the
vehicle depends on the weight. High efciency, high power
density, efcient regenerative braking, robustness in harsh
conditions, low maintenance and reliability are the main criteria
sought after in an electric traction motor [5].
The most common types of electric vehicles are battery
electric vehicles and hybrid electric vehicles. The commonly
used motors in these vehicles are DC, induction, permanent
magnet AC motors and permanent magnet DC motors
(brushless DC motors) [6]. There is also mention of switched
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reluctance motors in electric vehicles due to their excellent
torque–speed characteristics that are very much in alignment
with the demands of traction. Permanent‐magnet (PM)
motors, which have the highest efciency, appear to be one of
the best options. However, the market is dominated by asyn-
chronous machines. The explanation for this paradox can be
expressed in terms of the low utilisation factor of the motor in
vehicles and the cost of materials [7].
A study is presented herein of the above‐mentioned
motors and their new trends that have arisen due to signif-
icant improvements are also discussed. With respect to the
direction of ux, brushless DC motors can be categorised as
axial and radial ux motors, whereas with respect to the rotor
position, the motors can be classied as surface permanent
magnet and interior permanent magnet motors. These
different classications of the brushless DC motors are
discussed. Some new trends in the brushless DC motors,
namely the permanent magnet hybrid motor, permanent
magnet spoke motor and the permanent magnet inset motor
are described, including their advantages with regards to their
use in electric traction. Data for electric motors in the most
representative electric cars are also presented. Finally, the
trends of electric vehicles around the world are briey
discussed.
2
|
MOTOR TOPOLOGIES
There are different types of electric motors that can be found
in modern electric vehicles [8]. There are also a great number
of motor topologies possible, with various specications that
can be used to power an EV. This results in market segments
having DC motors, induction motors, brushless AC motors
(BLAC) and brushless DC (BLDC) motors [9].
Motors that do not run at a constant speed do not have
either nominal speed or nominal power [5]. The design of the
motor is done by making trade‐offs between weight and ef-
ciency. Usually, motors are rated at lower values than their peak
power capability. The peak power capability of a motor can
vary between a few kilowatts for electric cycles to 200 kW in
electric cars. Market demands play a key role in deciding the
power of the machine.
The efciency rating for a variable‐speed motor can be
characterised by power–speed or torque–speed efciency
maps. Electric motors have an optimal working condition
and efciency decays when the working points are out of
the optimal region, depending on the type of motor [5]. The
different points that each driving cycle applies to the motor
decide the efciency of the traction motor, as the efciency
depends on the working points. Hence, the performance of
a motor for a wide range of speeds and powers is dened
by the design. The efciency of a motor also depends on
the voltage level at the input. The high‐voltage‐rated ma-
chines are inherently more efcient. When a motor is
operated at voltages below the rated voltage, then the ef-
ciency of the motor is decreased. This mainly happens at
low state of charge [5].
3
|
MOTOR CHARACTERISTICS
The major requirements of an electric vehicle propulsion sys-
tem, as mentioned in the literature, are summarised as [10, 11]:
1. High instant power and high power density.
2. A high torque at low speeds for starting and climbing, as
well as high power at high speed for cruising.
3. A very wide speed range, including constant torque and
constant power regions.
4. A fast torque response.
5. A high efciency over wide speed and torque ranges.
6. A high efciency for regenerative braking.
7. A high reliability and robustness for various vehicle oper-
ating conditions and
8. A reasonable cost.
Apart from these, the electric propulsion system must be
fault tolerant [12, 13]. Market acceptance is the nal criterion
based on which the type of motor to be used for electric
traction is decided. This acceptance is decided based on the
comparative availability and the cost of the power converter
technology [14].
Figure 1a shows the typical characteristics of the electric
motor used in electric vehicles. As can be observed from the
gure, the motor operates at a constant torque for the entire
speed range until the base speed is achieved. After attaining the
base speed, the motor then reduces the torque in a proportion
of the speed and hence maintains a constant power until the
maximum speed. Beyond the maximum speed, the constant
power region eventually degrades at higher speeds. In this re-
gion, the torque decreases in proportion to the square of the
speed [14].
The characteristics of the traction effort with respect to
speed are also similar to those of electric motors. There is a
short constant torque range for low speeds and a wide constant
power region for higher speeds. This can be seen in Figure 1b.
This prole is derived from the basic characteristics of the
power source and transmission [14].
This means that for any source of electric vehicle propul-
sion the prole of traction effort versus speed must always be a
constant torque region followed with a constant power region
with respect to speed.
4
|
COMPARATIVE STUDY
4.1
|
DC motors
The brushed DC motors are known to have high torque at low
speeds and easy control. They also have torque–speed
characteristics suitable for the traction environment [15] but
not for EVs.
The main advantages of these type of motors are their low
cost, well established technology, simple yet robust control and
reliability. These motors were earlier preferred for use as
traction motors but due to the advancement in power
MADICHETTY ET AL.
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187

electronics there has been a shift towards the AC motors,
namely induction and synchronous motors. This is due to the
fact that the inversion of current that is done using commu-
tators in the DC motors is now being done by highly efcient
and controllable inverters.
The main disadvantages of these motors have been their
low power density, requirement of maintenance of the coal
brush (about every 3000 h), and low efciency [5]. It is also
difcult to reduce the size of DC motors, which makes them
heavy and expensive. Along with this, the maximum motor
speed is also restricted by the friction between brushes and
commutator [15].
4.2
|
Induction motors
Induction motors are the most mature technology amongst the
various commutator‐less motor drives. The squirrel cage type
induction motors are the most acceptable motors for use in
electric propulsion. They have advantages such as reliability,
low maintenance, low cost and robustness [14]. The typical
characteristics of an induction motor are shown in Figure 2a.
The critical speed of an induction motor is around twice
the synchronous speed and can be written as [14]:
Nc¼2* Nsð1Þ
where N
c
is the critical speed in rpm and N
s
is the synchronous
speed of the machine in rpm.
It is this speed at which the motor reaches breakdown
torque limit. If the motor is attempted to be operated at speeds
beyond the critical speed and at maximum current, it will begin
to stall due to the breakdown torque [14]. Hence, the break-
down torque of the motor is a limitation to the constant power
operation. The larger the value of breakdown torque, the wider
is the constant power region. From Figure 2a it can be seen
that, on increasing the speed beyond the critical speed, the
rotor stalls and this makes the slip unity.
Apart from this, there are other disadvantages of induction
motors. These include high losses, low efciency, low power
factor, and low inverter usage factor which is dominant at high
speeds [14]. The efciency of induction motors is inherently
lower than that of permanent magnet motors due to the
presence of rotor bars. This means higher rotor copper losses,
due to which the efciency of the motor at high‐speed ranges
is low [14]. Some of the newly proposed techniques have
attempted to overcome these disadvantages. These are vector
control, multiphase pole‐changing IM drive, and use of dual
inverters. There is also mention of a new type of design of
induction motors known as axial ux induction motors. The
above‐mentioned techniques and axial ux induction motors
are discussed subsequently.
4.2.1
|
Vector control of induction motors
To improve the dynamic performance of the induction motor
drive vector control technique is preferred. Vector control of
IMs is a technique which allows the decoupling of the torque
control from eld control. This requires coordinate trans-
formations on the line to provide fast torque control of the
induction motor. A method called direct torque control is
employed. [15] Direct torque control consists of three parts:
hysteresis control for controlling torque and ux, an optimal
switching vector look‐up table and a motor model. The motor
model estimates the developed torque, stator ux and shaft
speed based on the measurements of two stator phase currents
and battery voltage. Torque and ux reference signals are
produced by using a torque and ux hysteresis control method.
A switching vector look‐up table gives the optimum selection
of the switching vectors for all the possible stator ux‐linkage
space‐vector positions. Speed is controlled using a PI speed
controller [14]. The advantages of using the direct control
method are that the system gives a quick response and is a
simple conguration for control of induction motors. This is
capable of working in all four quadrants and includes regen-
erative braking. This thereby improves the overall efciency of
the system [14].
Vector control of induction motors is a variable‐frequency
drive control method, also known as the eld‐oriented control.
In this method, the stator currents of the three‐phase induc-
tion motors are split into two orthogonal components. These
components are then mathematically manipulated as two vec-
tors. One of the vectors identies the magnetic ux of the
motor and the other identies the torque of the motor [16].
FIGURE 1 (a) Typical characteristics of
an electric motor. (b) Characteristics of the
traction effort [14]
188
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MADICHETTY ET AL.

For this reason, vector control is said to decouple torque
control from ux control [17]. This can be seen in the
equations below [18]:
λr¼LmIsϕð2Þ
Ir¼−Lm
LrIsTð3Þ
where λris the rotor ux vector, Iris the rotor current vector,
L
m
and L
r
are the mutual and rotor inductances, respectively,
Isϕand IsTare the projections of stator current Isonto λrand
jλr.Isϕis the component of the stator current Isthat identies
magnetic ux and IsTidenties the toque of the motor.
The need for vector control arises due to the limiting
nature of the breakdown torque which causes stalling of the
motor, when it is operated at speeds higher than the critical
speed [17]. The vector control of the induction motor makes
the control of the motor similar to that of the separately
excited DC motor. In the separately excited DC motor the eld
ux and armature ux linkages are independent of each
other [19].
This means that the control of torque is possible without
changing the eld ux. This helps the motor increase the
breakdown torque limit and hence increases the constant
power region of the motor. In this method, the ux and torque
reference values are sent by the drive's speed control to a
control system. This control system then calculates the
required values of current components and the speed is varied
by changing the ux and the torque. The proportional integral
controller is usually used to control the measured signals
according to the reference values [16].
4.2.2
|
Multiphase pole‐changing IM drive
There are mainly two techniques to change the number of
poles of an induction machine. The rst technique requires a
specially designed induction motor. The coils of windings are
rearranged to change the number of poles of the stator. The
second technique also requires a specially designed machine.
This machine has two stator windings. For low‐speed opera-
tions, one winding is used and other winding is used for
high‐speed operations [20]. The main ux of pole changing
induction motors can be visualised as in Figure 3d.
The synchronous speed of the induction motor is inversely
proportional to the number of poles of the motor.
Ns¼120 * f
P⇒Ns∝1
Pð4Þ
where N
s
is the speed of the induction motor in rpm, fis the
frequency of supply in Hz and Pis the number of poles of the
machine.
FIGURE 2 (a) Typical characteristics of an induction motor [14]. (b) Torque–speed characteristics of a brushless AC (BLAC) motor [6]. (c) Torque–speed
characteristics of a switched reluctance motor [14]. (d) Torque–speed characteristics of a brushless DC (BLDC) motor [14]
MADICHETTY ET AL.
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189

Increasing the synchronous speed will lead to an increase in
the critical speed, which is around twice the synchronous
speed. The breakdown torque, which is dependent on the
critical speed, also increases now and hence the constant power
region of the motor is increased. This means that reducing the
number of poles using multiphase pole‐changing drives, at high
speeds, will widen the constant power region. A six‐phase
pole‐changing squirrel cage induction motor is presented in
[16]. The research proposes a new sinusoidal pulse width
modulation (PWM) strategy in a way that the two carriers of
the six‐phase inverter are out of phase during four‐pole
operation and are in phase during two‐pole operation. With
the use of double Fourier series, it is possible to eliminate the
dc‐link harmonics centred on the odd multiples of the carrier
frequency. The use of this new PWM strategy can signicantly
reduce the DC‐link harmonic currents, which can improve the
battery lifetime also. Figure 3shows the main ux of a pole‐
changing IM drive. The multiphase pole‐changing IM drive is
able to extend the constant power region without oversizing
the motor to solve the problem of breakdown torque [13].
4.2.3
|
Dual inverters
The reactive voltage and the back electromotive force (emf) of
an induction motor increase with an increase in electric angular
speed. This is shown by the following equations [18]:
Vs¼RsIsþjωeσLsIsþjωeLm
Lrλrð5Þ
Vr¼jωeσLsIsð6Þ
e¼jωeLm
Lrλrð7Þ
where Vsis the source voltage, V
r
is the reactive voltage, e
is the back emf, R
s
is the stator resistance, ω
e
is the
electric angular frequency, σis the total leakage coefcient,
L
s
,L
m
and L
r
are the stator, mutual and rotor inductances,
respectively, Isis the stator current, and λris the rotor
ux.
FIGURE 3 (a) Flux orientation in axial and radial ux motors [21]. (b) Basic idea showing the dual‐inverter control scheme: Inverter 2 only takes care of the
reactive voltage component [22]. (c) Designs of single‐and double‐sided axial‐ux induction motors [21]. (d) Main ux of a pole‐changing IM drive [20]: (i) four‐
pole conguration; (ii) twelve‐pole conguration
190
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MADICHETTY ET AL.

Thus, the reactive component of voltage for an induction
motor becomes high during high‐speed operation. This leads
to a poor power factor of the motor. If two inverters are used
instead of one to supply power to the motor, the decrease in
power factor can be compensated. This can be done using a
oating DC‐link capacitor bank to supply the required reactive
power. The advantages of this system are that it provides
reactive voltage support for the main DC‐link that is con-
nected to the battery and also allows voltage boosting to
enhance the motor terminal voltage [18]. At high speeds when
the requirement for reactive power increases, the secondary
inverter connected to the capacitor bank compensates for the
reactive power requirement. This helps to maintain the unity
power factor of the main DC‐link connected to the source. As
the oating DC‐link capacitor bank only supplies reactive
power to the induction motor, no power source is required to
be connected to it. When the power factor is maintained at
unity, it is possible to extend the constant power region of the
motor. This is because the oating DC‐link is actually sup-
plying voltage boost to the motor [18]. Figure 3b shows the
basic idea of such a dual converter scheme.
4.2.4
|
Axial ux induction motors
Axial ux machines are more common in the brushless per-
manent magnet types of motors. Axial ux induction motors
could play a role in the electric propulsion as the rare Earth
magnets are limited. The work in [23, 24] proposed that light
construction, excellent mechanical and dynamical performance
are properties that make the axial ux induction machine well
adaptable to medium‐speed operations in the range of
3000–15,000 rpm.
The axial ux induction motor can have better cooling due
to a greater diameter‐to‐length ratio. These motors also allow
inner diameter to be much larger than the shaft diameter. This
is a great advantage in cases such as the in‐wheel motors where
the diameter‐to‐length ratio is high, as the axial ux IM can
signicantly increase the torque density where the length of the
machine is a limiting design constraint [24]. The different to-
pologies in which axial ux induction motors can exist are:
(i) Single‐sided machines
(ii) Double‐sided machines.
Single‐sided rotor axial flux induction motor
This type of machine contains one stator and one rotor, and
the stator can be slotted or slot‐less. The ux in these machines
ows in the circumferential direction. This means that the ux
enters and leaves the stator and rotor on the same side.
Double‐sided rotor axial flux induction motor
Double‐sided axial ux motors can have two congurations. In
one conguration it can have one rotor and two stators on
each side. In the other conguration, it can have one stator in
the centre and two rotors on two of its sides. This can be seen
in Figure 3c.
The radial length from the stator inner radius to the outer
radius is the active part of the motor which produces torque in
the axial motors. The axial length is dependent on the proper
yoke design of the stator and rotor. As the number of poles
increases, the active radial part of the machine remains un-
changed and the axial length depends on the ux density in the
stator yokes [25]. The ux path in a radial ux machine is
relatively long compared to the axial ux equivalent. The larger
the ux path the greater is the requirement for magnetising
current and hence the power factor drops. A large fraction of
the length of a radial ux machine is in the end turn region of
the windings. This makes the axial ux motors have a shorter
prole in terms of length of the rotor [21].
The axial ux machines are not used very commonly
because of the large attraction forces that exist between the
stator and the rotor. This causes difculties in manufacturing.
The difculty in maintaining the air gap uniform along with the
high cost of the manufacturing for the laminated stator core is
another hurdle in their adoption [21].
4.3
|
Permanent magnet synchronous
(PMS) motors (or brushless AC)
The brushless AC motor is another type of AC motor used in
electric vehicle propulsion. There have been many car manu-
facturers using this technology to power their vehicles.
These motors are fed sinusoidal AC current which pro-
duces a sinusoidal eld. The interaction between the sinusoidal
eld and sinusoidal current produces a constant and a smooth
torque. This is the inherent property of BLAC motors that
differentiates them from BLDC motors. In contrast, brushless
DC motors are fed rectangular currents, which produces a
rectangular eld. Though the torque produced by the inter-
action of rectangular eld and rectangular current is higher
than that produced by the sinusoidal eld and current, the
torque is not smooth and has ripples [26].
BLAC motors have advantages such as better efciency,
higher power density and better heat dissipation to the envi-
ronment [6]. Better heat dissipation make these motors suitable
for operation in harsh environments. The disadvantage of
these motors is that they have a narrow constant power region.
The constant power region must be wide enough to accom-
modate higher speeds. Thus, this calls for a need of power
converters that can widen the speed range above the base
speed. These converters control the conduction angle at higher
speeds and help in improving the efciency of these motors.
Figure 2b shows the torque–speed characteristics of the
brushless AC motor [6].
4.4
|
Switched reluctance motor
The switched reluctance motors have emerged as a new
competitor to the existing types of motors used in electric
traction. This has mainly been due to the rising concern about
the magnetic material [27]. The use of rotor salient poles is the
MADICHETTY ET AL.
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191

main characteristic feature of this motor. The difference be-
tween the direct axis and quadrature axis synchronous reluc-
tance is the cause for the torque production in these motors, as
there is no excitation eld in the rotor [5]. The rotor is cheap,
robust and insensitive to temperature [28, 29]. The peak ef-
ciency of the reluctance motor is comparable to the induction
motor but the efciency remains high for a large range of
speed. The efciency of the hybrid switched reluctance motors
can go as high as 95% [30].
The advantages of this type of motor are its simple yet
rugged construction, ability to tolerate faults, simple control,
and outstanding torque–speed characteristics. The switched
reluctance motor's greatest advantage lies in the large constant
power region. This is usually inherently small in the other
motors and power electronics or other means are used to
extend this region [14]. Apart from these advantages, the high
rotor inductance ratio makes the sensorless control easier to
implement [30, 31]. Switched reluctance motors are also easy to
cool when compared to the other types of motors. This is
again advantageous, as it can be used in harsh ambient con-
ditions [15].
However, even after these advantages the switched reluc-
tance motor is not extensively used in electric vehicles due to
certain disadvantages [31, 32]. These disadvantages include
high acoustic noise that is due to the high torque ripple, need
of special converter topology, excessive bus current ripple, and
electromagnetic interference noise generation. However, all
these disadvantages and advantages are crucial for EV appli-
cations. Figure 2c shows the typical torque–speed characteris-
tics of the switched reluctance motor [14].
4.5
|
Brushless DC motor
The brushless DC motors are one of the largest competitors to
all the existing types of motors used in electric vehicle pro-
pulsion. They can be imagined as a reversal of the stator and
rotor in the permanent magnet DC motors. Also, the inversion
from DC to AC is done with the use of power electronic
converters instead of a mechanical commutator. In contrast to
brushless AC motors, these are not fed sinusoidal current
waves. Instead, they are fed rectangular waves due to the
sequential commutation of winding currents. This is one
reason why they are called brushless DC motors [33–35].
There are quite a few advantages to these motors, these
include high power density, which means that the weight and
volume are reduced for the same amount of output power;
higher efciency intrinsically, which is due to the use of per-
manent magnets that eliminate the rotor excitation currents
which account for half of the losses in the form of Joule losses
for non‐self‐excited synchronous motors; efcient and reduced
dissipation of heat to the surroundings, which means that the
temperature increase does not handicap the performance of
the motor [14]; reduced maintenance, as there is no need for
replacement of brushes; and high torque due to the interaction
between the rectangular ux and the rectangular current. This
torque is higher than the torque due to the interaction between
the sinusoidal ux and sinusoidal current motors in BLAC
motors. The recent trends in electric vehicles have shown the
use of direct‐drive motors and in‐wheel motors. The perma-
nent magnet brushless DC motors are well suited for these
type of applications also [5, 14].
The main disadvantage of these motors is that the constant
power region is narrow due to the limited eld weakening
capabilities which come from the presence of permanent
magnets [10]. This is because, at high speeds, there is a risk of
permanent demagnetisation of the PMs [11]. Figure 2d shows
the typical torque–speed characteristics of a brushless DC
motor [14].
There exist various topologies in which PM brushless
motors can be congured. If the position of the magnets is
considered, they can be classied as surface permanent magnet
(SPM) and interior permanent magnet (IPM) brushless DC
motors. If the direction of ux is considered, they can be
classied as axial ux and radial ux motors. Figure 4shows
one of the many possible arrangements of magnets in the
surface and interior permanent magnet motor designs. A type
of hybrid permanent magnet brushless motors are known to
exist in which the air gap eld that is provided by the per-
manent magnets is weakened by the DC eld excitation during
high‐speed operation. The direction and magnitude of this DC
eld excitation determine the strengthening or weakening of
the PM magnetic eld. Another type of hybrid motor uses
SPM and IPM magnet arrangements in its rotor sections. The
PM inset motor is one type of BLDC motor that has charac-
teristics of both BLDC motors and multi‐phase reluctance
motors. A novel design of BLDC motors, known as the spoke
motor, is also present in the recent trends due to its high
torque and power density. These types of BLDC motors are
discussed subsequently.
4.5.1
|
Axial ux and radial ux motors
When compared to radial ux motors, axial ux motors are
well suited for applications where the axial length is limited and
a very at prole is required. Figure 3a shows the direction of
FIGURE 4 Rotors of radial ux motors with surface‐mounted and
interior permanent magnet designs [5]
192
-
MADICHETTY ET AL.

ux in the axial as well as radial ux motors. Also, in appli-
cations where rapid acceleration or deceleration is required,
axial ux motors are preferred over the radial ux motors [36].
There are various congurations in which the axial ux
motors can exist. Based on the type of the stator these motors
can be classied as slotted or slotless (Figure 5). Based on the
number of air gaps they can be classied as single gap or
double gap. According to these criteria, the axial ux motors
can be classied as:
1. Single‐gap slotted axial machine
2. Dual‐gap slotted axial machine
3. Single‐gap slotless axial machine
4. Dual‐gap slotless axial machine
The radial ux motors occupy a volume larger than any of
the axial ux motors. The diameter of slotless motors is
greater when compared to the slotted type of motors, but the
length in the axial direction is the smallest for the slotless
motors. This leads to the attest prole amongst all the types.
The radial eld motor has the maximum moment of inertia
when compared to any axial motor at all power levels. All the
axial ux motors have almost the same moment of inertia,
with the dual‐gap slotless motor having the least. This is
because the dual‐gap slotless motor does not contain any steel
in the rotor and the density of the magnetic material is slightly
lower than that of steel. Both axial ux slotless machines
require the maximum magnet volume. This increases the
weight of these motors. Slotted axial ux motors require less
magnetic material than radial ux motors, for all power
ratings [36].
In terms of copper losses in these machines, the radial
motors and the slotted axial ux motors require a lower
number of turns per coil for the same generated back emf,
when compared to the slotless motors. Hence the copper
losses are lower in radial and slotted axial motors. All the
machines, axial or radial, slotted or slotless, have similar iron
losses. At high power levels the axial single‐gap slotless has the
least amount of iron losses due to the least weight of steel used.
If sufcient cooling is available then the single‐gap slotless
axial motor can be used when minimum weight is required for
a given power rating. Both the iron and copper losses can be
reduced by increasing the amount of iron and copper, but this
will increase the package size and the weight of the motor.
When the sums of copper and iron losses are compared, the
radial ux and the dual‐gap axial motors have similar values.
Also, the friction and the preload losses will be lower in the
dual‐gap and the radial ux machines due to the high attractive
forces between the rotor and the stator. A low speed of
operation can lead to windage losses [36].
An important indicator of the acceleration of the rotor is
the torque per unit moment of inertia. The radial ux motor
has the lowest torque for a given moment of inertia. This is due
to the longest rotor for any power rating. The dual‐gap slotless
axial motor has the largest value for the torque per unit
moment of inertia. In this case, the slotted motors come be-
tween radial and slotless motors [36].
Active weight in the motor is the weight of the copper and
the iron that is required by the magnetic circuit. Active volume
is the volume of the active weights. The power per unit active
weight of axial and radial motors are similar except at higher
power levels, when it is less for the radial ux motors. The
power per unit active volume of the radial ux motors is
signicantly low for all power levels [36].
4.5.2
|
Surface permanent magnet (SPM) and
interior permanent magnet (IPM) motors
When performances of the SPM and IPM motors are
compared, it is observed that the IPM can have a higher
loading capacity, both at low and high speeds (Figure 6). This is
not true in the case of SPM motors. This overload capacity of
IPM motors is much higher in the case of higher saliency
motors. The saliency ratio (ɛ) dened as below [38]:
ε¼Lq
Ldð8Þ
is the ratio between the quadrature axis inductance (L
q
) and the
direct axis inductance (L
d
). A high saliency ratio value means
high anisotropy of the machine.
The SPM motor is not able to go beyond the continuous
power rating, independent of the current load that is applied.
The important fact to be considered is that the IPM motor has
a superior overloading capacity when compared to the SPM,
only for the designs where rotor anisotropy is maximum [38].
For enhancing the overload capacity of the IPM motor at high
speeds a higher value of PM ux can be set according to the
following equation [38]:
λm¼Ld:1
2:ði1þi0Þ ð9Þ
FIGURE 5 a) A slotted machine rotor and (b) a
slotless machine rotor [37]
MADICHETTY ET AL.
-
193

where λ
m
is the PM ux linkage, L
d
is the direct axis induc-
tance, i
1
is the rated current and i
0
is the overload current.
When the motors are supplied by the same current and
maximum voltage, the two motors have very similar power
outputs. Saturation, in both the motors, has the ability to affect
the rated torque. Overload torques, however, are different
because of saturation and cross‐saturation effects. These ef-
fects are more pronounced in the case of the SPM motor. Due
to this, the SPM motor has a lower overload torque at low
speed [38].
When compared with SPM motors, the IPM motors tend
to have higher copper losses and lower iron losses. At base
speed, the SPM motor tends to have lower overall losses, but
at higher speed there exist losses due to permanent magnets.
The PM losses in the SPM motor can be reduced by axial
segmentation. Increasing the number of poles is advanta-
geous to the SPM motor as it helps to improve continuous
power density. However, even after the addition of poles, the
IPM motor tends to have a greater overload capacity than the
SPM motors. The IPM motors, on the other hand, are also
more susceptible to the losses due to harmonics. Hence, it is
required to have a calculated number of stator slots and rotor
segments in this motor. The SPM motor is more subject to
PM losses [38].
The SPM and IPM motors both give the same continuous
power for the same size of inverter and active parts. When
manufacturing is considered, the SPM motor is easier to
manufacture than the IPM motor [38].
4.5.3
|
Spoke motor
Recent trends in electric motor design have shown a drift to-
wards the spoke brushless DC motor. These motors are known
for their high power density. The design of a spoke motor is of
an interior permanent magnet motor. The superiority of this
motor is due to the higher saliency ratio, which concentrates
the magnetic ux generated. This concentration of ux is the
reason for higher torque density per unit volume [39]. The
spoke BLDC motor is inherently an IPM motor, where the
magnets are arranged at both sides of the rotor pole, as shown
in Figure 6a. This is the reason for ux concentration [39].
The spoke BLDC motor has an average back emf that is
about 34% higher than the SPM motor [39]. This back emf has
severe harmonics in its spatial distribution, when compared to
SPM motors. The inductances in an SPM motor are constant
on positions of rotor and otherwise. However, in the case of
the spoke motor, the inductances are highly variable with
respect to rotor position. The variation of inductances in
spatial distribution leads to the generation of the reluctance
torque. These inductances have a value that is larger than that
in the SPM motors. The reduction in the reluctance in the case
of the spoke motor is the reason for the larger value of
inductances [39].
In a brushless DC motor there is always an incoming phase
and an outgoing phase. Due to the phase inductance there is a
commutation torque ripple that is generated in the motor. The
spatial distribution of harmonics in the back emf and varying
inductances in the spoke motor have a direct consequence on
the phase current and the instantaneous torque. The phase
current ripple is the cause for commutation torque. The
reluctance torque hardly contributes in the effective torque.
When the phase current has an advanced phase angle, then the
phase current increases, due to constructive addition of two
consecutive peaks. The increase in current then leads to an
increase in the electromagnetic torque and also the reluctance
torque. This reluctance torque is now able to contribute to the
output torque [39].
The leakage ux is generated between the magnets because
the magnets are inside the rotor. This leakage ux leads to a
FIGURE 6 (a) The rotor design of
SPM and spoke motor [39]. (b) Air hole and
ux barrier design of the spoke motor [39]
194
-
MADICHETTY ET AL.

reduction in the torque and output power density. Reducing the
leakage ux can increase the torque further [39].
When irreversible demagnetisation analysis is performed
on the SPM and spoke motor, it is observed that the SPM
motors are only partially demagnetised, whereas the spoke
motors are completely demagnetised by external elds. This
shows that the spoke motor has weak resistance to the external
demagnetising eld [39].
In order to reduce the effect of the demagnetisation in
spoke motors, two designs, namely the air hole type and ux
barrier type rotor designs, are proposed [39]. These are shown
in Figure 7(b). If the reference model is considered to have a
rotor without any air hole or ux barrier design, then it can be
said that the air hole and the ux barrier type rotors perform
better than the reference model. Both the air hole and the ux
barrier type designs have better torque characteristics, but the
ux barrier type design has more torque when compared to the
air hole type design. In terms of back emf, it can be seen that
the ux barrier type rotor exhibits better resistance to external
demagnetising elds. The back emf reduction is about 18% in
the case of the air hole design and it is about 5.3% in the ux
barrier type design. This shows that the ux barrier type spoke
BLDC motor is more robust to the external demagnetising
elds [39].
4.5.4
|
Permanent magnet hybrid motor
The permanent magnet hybrid motor is composed of segments
of the surface and interior permanent magnet arrangements on
a stepped skewed rotor shaft. Placement of the rotor segments
is done in such a way that the average output torque is maxi-
mised [40]. This arrangement is shown in Figure 7a.
When compared to the SPM motors of the similar torque
density, the hybrid PM motor has a wider constant power re-
gion. This motor also has 21.9% reduced magnet weight
compared to conventional SPM motors [40].
When compared to the IPM, the SPM has greater
maximum torque, maximum back emf and also maximum
cogging torque. Although it is preferred to have high values
of back emf and torque, a high value of cogging torque is
not preferred. The greatest advantage of the IPM motor
that is added to this hybrid motor is the wide constant
power region [40].
Hence, the hybrid motor that combines the designs of
SPM and IPM motors, will have properties that are almost an
average of the two motors. This helps to reduce disadvantages
of one type and add advantages of another type in properties
such as the back emf, torque, etc. [40].
Figure 7b shows that the torque–speed characteristics of
the hybrid BLDC motor are an average of that of the SPM and
IPM motors, thus extending the constant power region of the
SPM motor [40].
4.5.5
|
Permanent magnet inset motor
The PM inset motor utilises the advantages of the high power
density and efciency of the brushless DC motor and advan-
tages of high starting torque and wide constant power range of
DC series motor. The originality of this motor is that the
generation of the PM excitation is both by PM and by the
specially controlled stator currents (two particular phases), that
are under the same stator pole. This causes torque of the motor
to be generated in two parts. By the interaction between the PM
excitation and the two phase stator currents, one part of torque
is generated. The other part is the electromagnetic torque that is
generated due to the interaction between the magnetic eld
generated by the two stator currents and the stator currents of
the other phases. This second part of the torque is proportional
to the square of the phase currents, which is the characteristic
of the DC series motor [41].
The design of the inset motor draws inspiration from the
multiphase reluctance motor. An inverter‐fed multiphase
reluctance motor operates similarly to the DC motor with the
only difference being that both the eld and the armature
windings are inside the stator. The armature reaction eld (F
a
)
is produced by the stator currents of the phases that are under
the rotor pole faces. These currents act as armature currents
(i
a
). The eld excitation (F
f
) is produced by the stator currents
that ow in the phases that are in the interpole region. These
currents act as eld currents (i
f
). The use of a multiphase
inverter allows control of each phase current of the multiphase
motor. This makes it possible to control the eld and the
armature currents independently. This is the characteristic
feature of the separately DC motor. The rectangular distribu-
tion of eld and current gives high power density to the
multiphase reluctance motor [41].
FIGURE 7 a) Arrangement of
(i) conventional stepped skew of SPM
motor and (ii) unconventional stepped skew
of permanent magnet hybrid motor [40].
(b) Torque–speed characteristics of SPM,
IPM and permanent magnet hybrid motor
[40]
MADICHETTY ET AL.
-
195

There are two types of torques that can be produced by
this motor. One of the torques, T
a
is a positive torque that is
produced due to the interaction of F
f
and i
a
. The other torque
is undesirable and is called the negative torque T
f
. This torque
is generated by the interaction between F
a
and i
f
[41].
The PM inset motor is designed by placing the permanent
magnets in sunken spaces in a specic way such that the rotor
has a PM magnet width less than half the pole pitch. Such an
arrangement produces two positive torques. This means that
the stator eld current is now used to create two positive
torques, instead of a positive and negative torque. One of the
positive torques is the reluctance torque that equals the
difference between T
a
and T
f
. The other positive torque is the
PM torque, T
m
. This torque is produced by the interaction of i
f
and permanent magnet eld F
m
.
The torque of the PM inset motor is given by the equation
below [41]:
T¼ ½iTd
dθ½L½i þ ½eT½i�ωð10Þ
Here, Tis the motor torque, [i] represents the matrix of
phase currents, [L] represents the matrix of phase inductances,
where L
ii
is the self‐inductance of phase i, and L
ij
is the mutual
inductance between phases i and j, θis the angle between rotor
axis and stator winding axis, [e] represents the rotational back
emf due to the PM material and ωrepresents the angular speed.
The rst term in the above equation represents the reluc-
tance torque component and the second term represents the
PM torque component of the inset motor.
Figure 8shows the different torques, stator poles and the
excitations in the permanent magnet inset motor during
clockwise and anticlockwise rotation of the motor [41].
The PM inset motor has two specialties. Firstly, these are
fed by multiphase rectangular waves to give high power density.
Secondly, the rotor has PM magnet width that is less than one‐
half of the pole pitch, giving it the ability to perform eld
regulation for constant power operations at high speeds. Also,
the salient rotor pole surfaces are eccentric, thereby reducing
the armature reaction [41].
Thus the PM inset motor has the powerful torque of the
PM brushless motor and controllable reluctance torque, which
is proportional to the square of the phase current. Hence, this
motor is able to offer high power density, high starting torque
and high efciency [41].
4.5.6
|
Trends of motors used in electric vehicles
Table 1shows the recent trends in the electric motors used in
some of the most representative electric vehicles in the market
[5]. It can be seen that induction motors and permanent
magnet motors are the most commonly used type of motors in
recent times. DC motors were more prevalent during earlier
times when the electric vehicle technology was evolving. There
are numerous electric vehicles coming into the market. The
electric vehicle market includes electric cars, hybrid electric
cars, electric scooters and motorcycles, electric cycles and also
electric pickup trucks and three wheelers. Table 2shows the
different electric vehicles, specifying the type and power of
their motors.
FIGURE 8 (a) Clockwise and (b) anticlockwise rotation of the inset motor [41]
196
-
MADICHETTY ET AL.

TABLE 1Types of motors used in the
most representative models in the market Sl. No. Name Year Power (in kW) Motor type
1 Audi e‐tron Quattro 2019 300 IM
a
2 Mercedes‐Benz EQC (N293) 2019 300 IM
a
3 Hyundai Kona 2018 150 PM
b
4 Mahindra eVerito 2017 31 IM
a
5 MW Motors Luka EV 2016 50 IM
a
6 Tesla Model X 2015 245 IM
a
7 Kia Soul EV 2014 81.4 PM
b
8 Volkswagen e‐Up 2013 60 PM
b
9 Tesla Model S 2012 215 IM
a
10 Hyundai Blue ON 2012 212 PM
b
11 Ford Focus Electric 2011 110 IM
a
12 Volvo C30 DRIVe 2011 89 PM
b
13 Tata Indica Vista EV 2011 55 PM
b
14 Ford Turneo Connect EV 2011 50 IM
a
15 REVA NXR 2011 13 IM
a
16 BYD F3M 2010 125 PM
b
17 Fiat Doblo 2011 43 IM
a
18 Peugeot iOn 2011 35 PM
b
19 Nissan Leaf 2010 80 PM
b
20 Ford Transit Connect EV 2010 50 IM
a
21 BYD e6 2009 115 PM
b
22 Mitsubishi i MiEV 2009 47 PM
b
23 Tesla Roadster 2008 215 IM
a
24 REVAi 2008 13 IM
a
25 AC Propulsion eBox 2007 150 IM
a
26 Ford Ranger EV 1999 67 IM
a
27 Peugeot Partner 1999 28 DC
c
28 Toyota RAV4 EV 1998 50 PM
b
29 GM S‐10 1997 85 IM
a
30 Nissan Altra 1997 62 PM
b
31 Honda EV Plus 1997 49 DC
c
32 GM EV‐1 1996 102 IM
a
33 Ford Ecostar 1992 56 IM
a
34 Bertone Blitz 1992 52 DC
c
35 City EI 1987 4 DC
c
36 Lucas Chloride 1977 40 DC
c
37 Citicar 1974 6 DC
c
38 Eneld 8000 1969 10 DC
c
a
Induction motor.
b
Permanent magnet motor.
c
DC motor.
MADICHETTY ET AL.
-
197

5
|
CONCLUSION
The DC motors appear to be one of the best choices according
to the traction prole of an electric vehicle. They have excellent
torque speed characteristics and are simple and robust in their
construction. However, these motors require maintenance and
have low power density. The induction motors have easy control
due to the ux‐weakening capabilities, are robust and require
low maintenance. These motors are restricted in their constant
power region by their breakdown torque. They have low ef-
ciencies and operate at poor power factors at high speeds. The
vector control of induction motors helps to improve the dy-
namic performance of the motor and also its constant power
region. The pole‐changing induction motors are capable of
operating with high efciency at low and high speeds due to the
different windings that are used in low‐speed and high‐speed
operations. The dual‐converter strategy can be used to
enhance the power factor of the induction motors at high
speeds. Axial ux induction motors have light construction,
excellent mechanical properties and dynamic performance. This
makes them adaptable to medium‐speed operations. These new
advances on the induction motor add advantages to the motor,
but at the same time increase the cost and size of the system.
The other type in AC motors are the brushless AC motors
(or permanent magnet synchronous motors). They have better
efciency, smooth torque, high power density and better heat
dissipation to the environment. They lack a wide constant
power region and hence require power converters. A switched
reluctance motor is also a type of synchronous motor. These
motors, in contrast to the brushless AC motors, have a wide
constant power region and have the most suitable character-
istics for an electric vehicle. However, these motors suffer
from torque ripple that causes acoustic noise. In addition, they
have excessive current ripple.
The brushless DC motors have been in recent trends due
to the high power density and high torque capabilities. These
motors have undergone and are still undergoing a lot of
research. This has led to the development of various types of
brushless DC motors. Although a very suitable choice, this
motor does also lack the wide constant power region due to its
limited ux‐weakening capabilities.
Axial ux BLDC motors can be used in designs where the
length of the rotor is a constraint. The radial ux motors have
the lowest copper losses when compared to the axial ux
motors. With increasing output power, the radial ux motor
has overall losses close to those of the single air gap slotted
type axial motor, which has the least overall losses. The main
advantage of axial ux motors is their signicantly high power
per unit active volume for all power levels.
If the mounting of permanent magnets is considered, the
interior permanent magnet motors have a higher overload
capacity when compared to the surface permanent magnet
motors. Spoke motors have recently come into the picture, in
which the permanent magnets are arranged as spokes of a
wheel. These motors pack a high power per unit active volume
and have greater values of back emf and torque than con-
ventional SPM motors. The ux barrier type rotor design of
these motors is superior in resisting demagnetisation due to
external elds. Another new type of permanent magnet motor
that is a hybrid of SPM and IPM motors has been designed.
This motor exhibits the advantages of both SPM and IPM
motors and has fewer disadvantages when compared to SPM
motors. The permanent magnet inset motor is a new design of
the BLDC motor that has PM magnets sunk into the rotor
surface. This motor has the advantage of using both the PM
torque and the reluctance torque additively.
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a
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How to cite this article: Madichetty, S., Mishra, S.,
Basu, M.: New trends in electric motors and selection
for electric vehicle propulsion systems. IET. Electr. Syst.
Transp. 11(3), 186–199 (2021). https://doi.org/10.
1049/els2.12018
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