A Review of Motors for Electric Vehicles

Abstract

the need for clean energy and the need to cut carbon dioxide emission from internal combustion engines have led researchers and engineers into exploring and developing new drive systems. The development of hybrid cars has greatly reduced the emission level of vehicles. However, this is not enough. The purely electrical vehicles are 100% clean and as such their deployment is of great importance. Therefore, these vehicles replace the internal combustion engine in the conventional cars and automobiles with electric motors. Hence, the need for highly improved motors that can perform optimally is of concern for researchers in the field. In this paper, a review of different electric motors with respect to their design simplicity, cost, ruggedness and efficiency is presented. Finally, the brushless DC motor is proven to be an efficient and most suitable candidate for propulsion drive in electric vehicles and hybrid electric vehicles. However, its control is insufficient. A conceptual method to improve its control is also presented.

Full text

111Equation Chapter 1 Section 1 A Review of Motors
for Electric Vehicles
1Tahir Aja Zarma, 2Ahmadu Adamu Galadima, and 3Maruf A.Aminu
123Department of Electrical and Electronics Engineering
123Nile University of Nigeria, Abuja
1tahiraja@nileuniversity.edu.ng, 1engrtahiraja@gmail.com
Abstract— the need for clean energy and the need to cut carbon
dioxide emission from internal combustion engines have led
researchers and engineers into exploring and developing new
drive systems. The development of hybrid cars has greatly
reduced the emission level of vehicles. However, this is not
enough. The purely electrical vehicles are 100% clean and as
such their deployment is of great importance. Therefore, these
vehicles replace the internal combustion engine in the
conventional cars and automobiles with electric motors. Hence,
the need for highly improved motors that can perform optimally
is of concern for researchers in the field. In this paper, a review of
different electric motors with respect to their design simplicity,
cost, ruggedness and efficiency is presented. Finally, the brushless
DC motor is proven to be an efficient and most suitable candidate
for propulsion drive in electric vehicles and hybrid electric
vehicles. However, its control is insufficient. A conceptual method
to improve its control is also presented.
Keywords- Electric motors, EVs, Internal Combustion Engines.
I. INTRODUCTION
Following growing outcry from environmental activist and
government policies, it has become imperative that gas
emission needs to be cut significantly because of its effect on
the ozone layer [1]. Conventional mobility systems (vehicles)
are driven by internal combustion engines (ICE) and thus they
burn gas, petrol into carbon dioxide thereby affecting the
environment. While research has been directed towards clean
energies, it gave rise to advancement and the evolution of
hybrid electric vehicles which use both ICE and Electric
motors to propel their wheels [2-4]. Furthermore, purely
electric vehicles have since been in the market and running
[5]. These vehicles use one or more electric motors for their
propulsion [6]. Researchers are now exploring renewable
forms of energies, from solar, wind, wave tides, to renewable
energy powered vehicular systems for sustainable mobility [7-
10].
An electric vehicle consists of three major subsystems; the
energy source subsystem, the auxiliary subsystem and the
electric propulsion subsystem [3, 11, 12]. The electronic
propulsion subsystem comprises of electronic controller, the
power converter, the mechanical transmission and the electric
motor. In this work, a review of different motors available as
propulsion for electric vehicle is presented. The block diagram
of electric vehicle is as shown in Figure 1.
Figure 1 Block of diagram of electric vehicle
The development of electrical drives dates back to the 18th
century when Faraday demonstrated the principle of the
electromagnetic induction [13]. Following the invention of
faraday’s law, electric motors were invented and that bred the
two major classes of motors; Alternating Current and Direct
Current motors.
Typically, an electric motor consists of a rotor, stator,
windings, air gap and commutators/converters. Depending on
different arrangement of these components different types of
electric motors are constructed [14]. Those electric motors that
do not require brushes for commutation or energy conversion
are called brushless permanent magnet motors [15].
Furthermore, motors can be categorized according to the shape
of their back-EMF. Their shape can either be sinusoidal or
trapezoidal. Based on these shapes, they can be Permanent
Magnet AC Synchronous Motors (PMSM) or Brushless DC
motors (BLDC) respectively [16].
For an Electric motor to be successfully deployed as the drive
for EVs, it should be highly efficient, it should have great
power density and should be cost-effective [11]. However, the
specification of the motors depends on its application purpose.
This application could range from home, regular vehicular and
heavy duty vehicles. Furthermore, the performance of motors
depends mainly on vehicle duty cycle, thermal characteristics
and the cooling mechanism implemented [11]. The
classification of various motors used in traction is shown in
Figure 1. A brief literature review on the motors used for
traction in EV/HEVs is presented below. In this work,
literature review of both AC motors and DC motors is
presented looking at the features mentioned earlier.

Fi
gure 2: Classes of Electric Motors
II. THE ALTERNATING CURRENT MOTORS
This section reviews sinusoidally-powered electric motors.
These are further divided into synchronous and asynchronous
motors.
A. Synchronous Motors
Synchronous motors are motors where the shaft of the rotor is
synchronized with the frequency of the supply current. In
these motors the period of the rotor is exactly same as that of
the supply.
i. Permanent Magnet Synchronous Motor (PMSM)
This motor shares some similarities with the BLDC motor, but
is driven by a sinusoidal signal to achieve lower torque ripple
[17]. The sinusoidal distribution of the multi-phase stator
windings generates a sinusoidal flux density in the air gap that
is different from BLDC motor’s trapezoidal flux density. This
motor possesses feature of an induction motor and a brushless
dc motor. The motor has a permanent magnet rotor and
winding on its stator. Furthermore, the stator of this motor is
designed to produce sinusoidal flux density which resembles
that of an induction motor. The power density of this motor is
higher than induction motors with the same ratings since there
is no stator power dedicated to magnetic field production.
Today, these motors are designed to be more powerful while
also having a lower mass and lower moment of inertia. This
motor can generate torque at zero speed, highly efficient and
produces high power density compared to an induction motor.
However, this motor requires a drive to operate.
To achieve the specifications of high torque at low speed, high
density and high efficiency, this motor uses variable frequency
drive. However, the VFD control technique increases the
complexity of the system and hence requires careful attention
to precisely control its speed. Hence the cost of this motor is
on the higher side as compared to the induction motor [18].
ii. The Stepper Motor
The stepper motor and switched reluctance motor have the
same structure. The stator of a stepper motor consists of
concentrated winding coils, while the rotor is made of soft iron
laminates without coils [19]. Torque is produced in these
motors when the current switches from one set of stator coil to
the next coil, the switching currents from stator windings
generates magnetic attraction between rotor and stator to
rotate the rotor to the next stable position, or "step" [20]. The
rotational speed is determined by the frequency of the current
pulse, and the rotational distance is determined by the number
of pulses. Since each step results in a small displacement, a
stepper motor is typically limited to low-speed position-
control applications [21]. The ability to move a specific step
makes these devices commonly used in positioning
mechanisms. Stepper motors are characterized by their
moving and holding torque which if exceeded the motor slips
and hence the motor loses count. This motor produces torque
through magnetic reluctance, magnetic attraction or both. The
motor doesn’t offer dynamic speed control. The motor can
only be accelerated at full toque to full speed and decelerates
at full torque. Hence, the motor offers greater torque for a
given speed. Therefore, this motor is ideal for precision and
position control purposes, making it unsuitable for EV
application [22].
iii. The Switched Reluctance Motor
The rotor in the Switched Reluctance motor (SR) cannot
generate magnetic field around itself because of the absence of
coils in the rotor, therefore no reactive torque is produced in
an SR motor. Torque in these motors is produced when a stator
phase is energized, the stator pole pair attracts the closest rotor
pole pair toward alignment of the poles [23]. This way, high-
torque ripple is generated which contributes to acoustic noise
and vibration. However, due to its simple design, SR motor is
very economical to build, and is perhaps the most robust
motor available [20]. This motor relatively produces lower
torque compared with the stepper motor. Hence, its use is not
popular in EV application.
B. Asynchrounous Motor (Induction Motor)
In this motor, the current in rotor winding is obtained from the
field of the stator winding by electromagnetic induction. The
rotor current is now utilized for torque production. The
popular asynchronous motor available is the induction motor.
In this motor, a sinusoidal AC current is used to excite the
stator to create a rotating magnetic field that induces a current
in the rotor; the induced current in the rotor generate a relative
magnetic field in the rotor [24]. The magnetic fields in the
rotor and the stator run at slightly different frequencies and
hence generate torque [25]. The induction motors are
characterized with cheaper cost, absence of brushes,
commutators and low maintenance. These features make the
induction motor attractive in EVs. However, the need for
converting the power supply from AC to DC demands more
circuitry and hence complex control schemes [26].
III.
DIRECT CURRENT MOTORS
In this section, the different DC motors available is presented.
Motors such as brushed DC and the Brushless DC are
presented in terms of their respective power density, efficiency
and cost.

A. Brushed DC motor
A brushed DC motor consists of a commutator and brushes
that convert a DC current in an armature coil to an AC current.
As current flows through the armature windings, the
electromagnetic field repels the nearby magnets with the same
polarity, and causes the winging to turn to the attracting
magnets of opposite polarity. As the armature turns, the
commutator reverses the current in the armature coil to repel
the nearby magnets, thus causing the motor to continuously
turn. This motor can be driven by DC power, hence it is very
attractive for low-cost applications. However, some drawbacks
of brushed DC motor are the arcing produced by the armature
coils on the brush-commutator surface generating heat, wear,
and electromagnetic interference (EMI) [27]. These
charactersitics of the brushed motor indicate that it is more
suitable in applications where high efficiency is not a major
concern. This renders use of this type of motor less attractive
in EV applications.
B. Brushless DC motor
The brushless DC (BLDC) motors are the most popular and
widely used in control application [28] and are configured into
single-phase, 2-phase and 3-phase [29]. The simple structure,
ruggedness, and low-cost of a BLDC motor make it a viable
candidate for various general purpose applications. The BLDC
combined with a suitable controlled converter provides several
desired characteristics for an efficient drive system. One major
advantage of BLDC is enhanced speed versus torque
characteristics as compared with other electric motors [30].
Furthermore, the BLDC accomplishes commutation
electronically using rotor position feedback to determine when
to switch the current [31]. This motor is built with a
permanent magnet rotor and wire-wound stator poles. The
rotor is formed from permanent magnet and can alter from
two-pole to eight-pole pairs with alternate North (N) and
South (S) poles [32]. The stator windings work with the
permanent magnets on the rotor to generate a uniform flux
density in the air gap [33]. This permits the stator coils to be
driven by a constant DC voltage (hence the name brushless
DC). The rotor position of a BLDC sensed using hall effect
sensors is very important, this gives the information about
winding that is energized at the moment and the winding that
will be energized in sequence [34]. Whenever the rotor
magnetic poles pass near the hall sensors, they give a high or
low signal, suggesting the N or S pole is passing near the
sensors. The exact order of commutation can be estimated,
depending upon the combination of these three hall sensor
signals.
Furthermore, Sensorless control strategies can be used to
eliminate the position sensors, thus reducing the cost and size
of motor. In fact, control methods, such as back-EMF and
current sensing can provides enough information to estimate
with sufficient precision the rotor position and, therefore, to
operate the motor with synchronous phase currents. Perhaps,
the most popular BEMF methods rely on one technique called
the zero crossing point (ZCP), being the only point to provide
the rotor position information at either 00 or 1800 electrical.
The zero crossing point methods are succeeded by a phase
shift of 300 or 900 to match the commutation instances. Any
detection error of the ZCP results in a sub-optimal phase
current [35].
A conceptual method which uses extended kalman filter for
estimating the exact commutation instance of a winding is
suggested. This method shall be further developed, validated
and reported.
The BLDC motor offers excellent power density as compared
to other motors, higher torque, reduced operational and
mechanical noise, elimination of electromagnetic interference
and offers excellent efficiency. Hence, this motor is the most
popular in EV application [11].
IV.
CONCLUSION
In this work, a review of different motors used as electric
drive trains is presented. Working principles, operational
requirements, excellent features and drawbacks of all motors
available are discussed in detail and presented. The brushless
DC motor has proven to be an efficient candidate for
application in electric drive trains. This motor offers
extraordinary power density, high efficiency and is cheaply
available. The popularity of this motor as used as an electric
drive train is also presented.
V.
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