Mathematical Modelling of Engineering Problems
Vol. 5, No. 2, June, 2018, pp. 51-57
Journal homepage: http://iieta.org/Journals/MMEP
Operations of electric vehicle traction system
Ahmed M. Youssef
Aircraft Electric Equipment Dep., MTC, Cairo 11865, Egypt
Corresponding Author Email: ammyk.khater@mtc.edu.eg
https://doi.org/10.18280/mmep.050201
ABSTRACT
Received: 4 April 2018
Accepted: 4 May 2018
After revealing that around 1.3 million deaths every year is due to greenhouse gas
emissions and fossil-fuel pollutants being pumped into the atmosphere by transportation
industry and vehicles, a global revolt may be needed to accelerate the transition to more
clean energy and transport systems. Electric vehicles are considered one of the pillars of
eco-friendly solutions since they produce no exhaust gases. Somehow, the limitation of
driving mileage still represents an obstacle for developing these vehicles. In general, an
electric vehicle propulsion system consists of an electric energy supply system and a
traction system; interconnected to each other through a high voltage DC-link. This paper
is concerned with the traction system; which constitutes a Brushless Direct Current
(BLDC) motor, its voltage source inverter and drive system, in order to tackle this
obstacle. It deals with the four quadrants of the speed-torque profile of the motor, with
special emphasis on regenerative braking.
Keywords:
electric vehicle, four quadrant operation,
BLDC motor, drive system, rechargeable
energy storage system, and regenerative
braking
reverse directions, as well as acceleration and deceleration
occur frequently, therefore in electric vehicles the inputs of the
driver are: a key switch, a forward/reverse direction switch, an
accelerator pedal, and a braking pedal. Consequently, dealing
with the electric vehicle traction system involves modeling the
traction BLDC motor and controlling its drive for its four
quadrants of the speed-torque profile.
Traditional vehicle has either hydraulic or pneumatic
braking system, which creates friction torque between brakeshoes and wheels to decelerate the vehicle. This braking
technique dissipates kinetic energy as heat energy; thereby
causes a loss of energy. In urban driving, studies show that
about one third to one half of the energy required for operation
of a vehicle is consumed during braking [2]. This leads the
researches to be focused on energy-saving braking methods.
Electric braking to BLDC motor drive can be implemented
by three methods namely; plugging, dynamic braking, and
regenerative braking. Although both plugging and dynamic
braking methods provide fast braking responses, they are
highly dissipative processes. In the plugging method, all the
kinetic energy ends up as heat in the motor, while in the
dynamic braking method the kinetic energy of the motor and
load is dissipated in the form of heat energy through some
external resistance. In the regenerative braking method, the
kinetic energy of the motor is converted into an electrical one,
and is returned back to the supply, thus it is an energy-saving
method. By applying the regenerative braking method, the
driving range of electric vehicles can be extended by 8-25%
depending on the driving condition, and consequently the
obstacle for developing electric vehicles due to their
limitations of driving mileage is tackled.
The paper is organized in six sections: section (2) describes
briefly the components of the propulsion system of the fuel
cell/battery powered electric vehicle. Section (3) explains the
principle of operation of the traction system components and
details the four quadrant operation of BLDC motor. Methods
1. INTRODUCTION
Because transportation is considered crucial to our economy
and lives, transportation industry is growing continually. The
majority of transportation industries and vehicles rely on
natural resources; mainly fossil fuels, for their operations.
Since these resources are finite and being consumed at a fast
rate, their prices increase dramatically. Besides, burning of
these fossil fuels results in emission of greenhouse gases, air
pollution, and global warming. Rapid consumption of natural
resources and environmental impact of combustion of fossil
fuels have focused the researchers on sustainable and clean
energy sources transportation.
Electric vehicles are considered one of the pillars of ecofriendly solutions since they produce no exhaust gases.
Somehow, the limitation of driving mileage still represents an
obstacle for developing these vehicles. An electric vehicle
propulsion system consists of supply system and traction
system; interconnected to each other through an interfacing
and control system. This paper is concerned with the electric
vehicle traction system, which consists of traction electric
motor together with its voltage source inverter and drive
system, in order to tackle this obstacle.
Since the electric motor is the main component of any
electric vehicle traction system, selecting a proper type of
motor with suitable rating is very important. Possible motor
candidates for powering electric vehicles are: the induction
motor, the switch reluctance motor, and the BLDC motor.
BLDC motors are characterized by their capability of offering
higher torque and power density together with higher
reliability and efficiency, compactness, longer operating life,
higher dynamic response, better speed versus torque
characteristics, and noiseless operation compared to the
motors of the same size and other types. Therefore BLDC
motors are preferred for electric vehicle applications [1].
Similar to conventional vehicles, driving in forward and
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for electric braking of BLDC motor are discussed in section
(4). Simulation results of the proposed traction system, with
special emphasis on regenerative braking, are detailed in
section (5). Finally, the paper concludes with a brief summary
in section (6).
[1], whereas the battery pack is connected to the DC-bus via a
bidirectional DC/DC converter. Original 12 V accessories,
such as: lights, horn, and so on are powered by tapping the full
battery pack voltage and cuts it down to a regulated 12 V
output using a buck DC/DC converter.
It is obvious that high performance electric motor drive
systems are central to modern electric vehicle propulsion
systems. The benefits accruing from the application of such
drives are precision control of torque, speed and position
which promote superior electric vehicle dynamical
performance [3]. This paper is one of a series of papers by the
author on the modeling and simulation of the overall fuel
cell/battery powered electric vehicle configuration
components. It carries out the four quadrants of the speedtorque profile of the traction BLDC motor, with special
emphasis on regenerative braking as an energy-saving method
for extending the driving range of the electric vehicle.
2. SYSTEM DESCRIPTION
The basic configuration of the fuel cell/battery powered
electric vehicle propulsion system is shown in Figure 1. Its
traction system constitutes a BLDC motor, and its associated
voltage source inverter and drive system.
The power supply system is composed of a fuel cell stack
as a main source, and a battery pack as an auxiliary source to
power the propulsion motor during starting the vehicle and to
assist the propulsion of the vehicle during transients. The fuel
cell stack is connected to the DC-bus through a boost converter
Figure 1. Fuel cell/battery powered electric vehicle configuration [1]
3. TRACTION
SYSTEM
OPERATIONS
COMPONENTS
inverter/switching power supply, which produces an AC
electric signal to drive the motor, i.e. the motor accomplishes
commutation electronically. The commutation instants are
determined by the rotor position. Detecting the rotor position
in BLDC motors is performed either by position sensors like
hall sensor, position encoder and resolver etc. or by sensorless
techniques.
AND
Complete electric vehicle traction system is composed of
BLDC motor, inverter bridge, rotor position sensor, controller
and driver circuit. A BLDC motor is a synchronous motor with
permanent magnets on the rotor and armature windings on the
stator. It is powered by a DC electric source via an integrated
Figure 2. Overall structure of the proposed traction drive system
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rotation. Therefore, it takes six steps to complete an electrical
cycle. The hall sensor signals are fed to the control circuit
which controls the direction and speed of the motor by
producing pulse-width modulation (PWM) signals for
triggering the electronic switches; MOSFET or IGBT, of the
six-step inverter bridge via an interface driver.
Figure 2 shows the overall structure of the proposed traction
drive system. The switching sequence for clockwise and
counter-clockwise rotations, the current direction and the
position sensors’ signals are shown in Table 1. The inverter
bridge structure is shown in Figure 3.
A three-phase BLDC motor has three stator windings,
which are oriented 120o apart. When the motor rotates, each
winding generates a voltage called back-EMF, which has an
opposite polarity to the energized voltage. There are two types
of stator windings: trapezoidal and sinusoidal, which refers to
the shape of the back-EMF signal. Trapezoidal motor is a more
attractive alternative for most applications due to its
simplicity, lower price and higher efficiency [4].
A three-phase trapezoidal stator windings BLDC motor
requires three hall sensors to detect its rotor position, each hall
sensor is typically mounted 120o apart and produces “1”
whenever it faces the North pole of the rotor, i.e. every 60 o
Table 1. Switching sequence of BLDC motor
Figure 3. Inverter bridge structure
According to the speed-torque profile of the BLDC motor
shown in Figure 4, there are four quadrants of operation;
forward motoring, forward braking, reverse motoring and
reverse braking, respectively.
During motoring modes; first and third quadrants, the
magnitude of the supply voltage is greater than the back-EMF,
NC (Normally Closed) contacts in Figure 2 are closed and NO
(Normally Open) contacts are opened, and thus the energy
flows from the supply to the motor. In these modes, both the
speed and the torque have the same sign; positive for forward
motoring or negative for reverse motoring. Whereas in
generating (braking) modes; second and fourth quadrants, the
magnitude of the supply voltage is less than the back-EMF,
and hence the energy flows from the motor to the supply, i.e.
the motor acts as a generator. During these modes, NC
contacts are opened and NO contacts are closed to connect the
motor to the chargeable battery via the rectifier, to store the
generated energy. In such modes, the torque has an opposite
sign to the speed, which means a “brake” is being applied to
decelerate the motor.
Figure 4. Four quadrant operations
4. ELECTRIC BRAKING METHODS
Electric braking to BLDC motor drive can be implemented
in three ways namely; plugging, dynamic braking, and
regenerative braking. Plugging or plug-reversal is a method of
braking obtained by reversing the armature terminals while
running, thus both the supply voltage and the back-EMF act in
the same direction. The armature current is reversed, thereby
producing a braking torque. This current is very high; even
larger than when starting from rest. Frequent plugging will
cause serious overheating, because each reversal involves the
‘dumping’ of four times the stored kinetic energy as heat in the
windings. Although plugging provides fast braking response,
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it is a highly dissipative process in which all the kinetic energy
ends up as heat in the motor [5].
The second type of braking methods is the dynamic or
rheostatic braking, which brings the motor to rest position by
disconnecting the windings from the power supply and short
circuiting them. Short circuiting the windings leads a high
current which can damage the windings. To limit this current
flowing in these windings, an external resistance called a
braking resistance is connected in series with the windings.
Thus, dynamic braking is a method of dissipating the kinetic
energy of the motor and load in the form of heat energy
through some external resistance [6].
The third type of braking methods is the regenerative
braking. During its braking period, the kinetic energy of the
motor is converted into an electrical one, and is returned back
to the supply, i.e. the motor acts as a generator during this
period. Thus regenerative braking is an energy-saving method.
Applying the regenerative braking method in an electric
vehicle increases the efficiency of the vehicle by recapturing
the wasted kinetic energy. This energy is stored in a
rechargeable battery storage system during regenerative
period, and can be fed back to the inverter mains during peak
power demand occasions; such as vehicle acceleration or
driving uphill, or during shortage of energy from source.
Thereby, regenerative braking can be a method for
extending the driving range of electric vehicles by 8-25%
depending on the driving condition, normally it is more
effective in urban driving rather than highways whereas little
braking occurs, and consequently the obstacle for developing
electric vehicles due to their limitations of driving mileage is
tackled [2].
The structure of the proposed traction drive system, shown
in Figure 2, includes a switching (relay) circuit coupled to the
BLDC motor. As discussed before, whenever the motor is
operating in the braking modes, NC contacts are opened and
NO contacts are closed, thereby the generated voltage gets
rectified and energy-regenerative braking operation is
established by allowing the charge to be stored in a chargeable
battery. When the rechargeable battery storage system is fully
charged, in such a rare case, the regenerative braking cannot
occur. Therefore a module is inserted after rectification to
check the State-Of-Charge (SOC) of the chargeable battery. It
allows the energy-regenerative braking operation to be
performed until full SOC occurs, then it directs the energy to
be dissipated in an external resistive load, i.e. performs
dynamic braking operation. It is obvious that the dynamic
braking time can be varied by varying the value of resistor.
Although the benefits of using regenerative braking system
in electric vehicles, the produced braking power at low speeds
is not sufficient and may fail to stop the vehicle in the required
time; especially in emergency cases, due to relatively low
voltage generated by the motor/generator. Hence, the
mechanical braking system is still indispensable for safety
actions and for avoiding failure operation of electric energy
regeneration. In nowadays electric vehicles, the functions of
both mechanical braking and regenerative braking are
combined into a single foot pedal; such that the first part of the
pedal controls the regenerative braking while the second part
controls the mechanical braking.
5. SIMULATION RESULTS
Figure 5 shows the Simulink model of the proposed traction
drive system. A 3 HP three phase BLDC motor is used. Its
parameters are shown in Table 2 [7].
Table 2. BLDC motor technical specifications
Figure 5. Simulink model of proposed traction drive system
In most traction applications, the BLDC motor control
circuit consists of a speed controller and a current controller to
control the motor performances. Our proposed speed control
loop uses a PI regulator to control the speed of the motor by
comparing the referenced speed with the actual speed of the
motor. To avoid armature over-current and destabilization of
the system due to sudden reference changes, acceleration and
deceleration ramps of 1000 rpm/s are put for the speed
reference change rate to follow. The controller gains (Kp = 3.3
and Ki = 300) are determined by considering the rotor inertia,
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the viscous damping, the number of poles, the phase resistance
and inductance. The speed controller produces the referenced
torque signal for the current controller based on the motor
speed and torque relationship.
A hysteresis current controller is used to generate the
required PWM pulses based on the reference torque signal, the
hall sensors feedback and the phase currents feedback.
Hysteresis-band PWM is basically an instantaneous feedback
current control method of PWM where the actual current
continually tracks the command current within hysteresisband. These PWM signals are fed to the six-step inverter
bridge via an interface driver for triggering the electronic
switches.
The simulation is carried out for a time of 3.5 s and a
discrete power GUI mode is adopted with 2 s sampling time.
To check the behavior of the proposed traction system, first
the motor is operated on no-load condition; that is the applied
reference load torque is maintained at zero. The simulation
results are given in Figures 6-9.
Figure 8. Stator back-EMF at no-load
Figure 9. Stator current at no-load
From Figure 6, it can be observed that the rotor speed is
exactly following the reference one. The torque in Figure 7
stays around the reference zero value, except during the speed
climbing and diving periods it jumps to nearly +11 and -11
Nm, respectively. Figure 8 shows the variation in back-EMF
as the speed changes its direction from forward to reverse.
Figure 9 shows that the stator current increases during
acceleration and deceleration periods, and decays during
steady-state speeds.
For checking the behavior of the proposed system in the
four quadrants of the speed-torque profile of the motor, the
reference speed and the applied load torque to the motor are
presented in Table 4.
Figure 6. Rotor speed at no-load
The reference speed presented in Table 3, is sent to the 1000
rpm/s rate limiter before entering the speed controller.
Table 3. Reference speed
Table 4. Quadrant determination
Figures 10-16 show the simulation results for the four
quadrants operation of the motor. The speed set point and the
torque set point are also shown.
From time t = 0.5 s to 1.3 s, the speed set point is 300 rpm
and the applied load torque is positive. That is, the machine is
operating in the first quadrant; forward motoring. As can be
seen in Figure 10, the actual speed is precisely following the
reference with the acceleration ramp.
Figure 7. Electromagnetic torque at no-load
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Figure 10. Rotor speed (four quadrant operation)
machine is operating in the third quadrant; reverse motoring.
NO contacts are opened and NC contacts are closed. As can be
seen in Figure 10, the actual speed tries to follow the speed set
point ramp by decreasing the speed to zero and starting to
speed up with a rotation in reverse direction.
At time t = 2.75 s, the speed set point is commanded to reach
zero value, but due to the 1000 rpm/s rate limiter, it returned
to zero value at 3.02 s. From time t = 2.5 s to 3.02 s, positive
load torque is applied to the motor, while the speed set point
is still negative. This implies that a brake is applied, and that
the machine is operating in the fourth quadrant; reverse
braking.
Figure 11. Electromagnetic torque
From time t = 1.3 s to 2 s, negative load torque is applied to
the motor, while the speed set point is still positive. This
implies that a brake is applied, and that the machine is
operating in the second quadrant; forward braking. Figure 10
shows a deceleration in the rotor speed. At that time, the motor
acts as a generator. NC contacts are opened and NO contacts
are closed, and the generated voltage gets rectified. Because
the rechargeable battery storage system is not fully charged,
the energy-regenerative braking operation is established by
allowing the charge to be stored in the chargeable battery, as
can be seen in the battery parameters Figures 14-16.
Figure 13. Stator back-EMF
Figure 12. Stator current
During the time t = 2 s to 2.5 s, both the speed set point and
the applied load torque to the motor are negative. That is, the
Figure 14. Battery voltage
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As shown in Figure 10, the actual speed is no longer
following the speed set point ramp but a deceleration in the
reverse direction occurs. Again during that time, the motor acts
as a generator. NC contacts are opened and NO contacts are
closed. The motor kinetic energy is converted into an electric
one and is stored in the rechargeable battery storage system, as
shown in the battery parameters Figures 14-16.
near future. Therefore electric vehicles are going to be popular
due to their zero exhaust gases emission. Somehow, the
limitation of driving mileage still represents an obstacle for
developing these vehicles.
The objective of this paper was to study and improve the
efficiency of the electric vehicle traction system in order to
tackle this obstacle. Operational characteristics and
mathematical modeling of the components of the traction
system; BLDC motor, its voltage source inverter and drive
system, are detailed. The four quadrant operation, along with
the different braking methods of BLDC motor drive is
discussed. Discussion proved that the energy management in
the electric vehicle can be well improved through the braking
system used in it.
A simple scheme of an electric vehicle traction drive system
has been presented and successfully simulated. The simulation
results showed that the proposed system is working properly
and the motor reverses the speed smoothly. Energy
improvement is achieved by applying the regenerative braking
method which recaptures the motor wasted kinetic energy,
converts it into an electrical one, and then returns it back to the
supply, thereby extends the driving range of electric vehicle.
Figure 15. Battery current
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[5] Hughes A. (2006). Electric motors and drives:
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Figure 16. Battery state-of-charge
Figure 12 shows the stator current, where the transition from
one quadrant to another is clearly observed in this waveform.
Figure 13 shows the variation of the stator back-EMF relative
to the amplitude and direction of the speed. It is observed that
its amplitude is increased by increasing the speed and is
reduced when braking is applied at time 1.25 s.
6. CONCLUSION
The demand for greener standards of living and production
in today’s world has steered automotive industries towards the
development of an environmentally cleaner transportation in
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