Design of a Small Personal Electric Vehicle
as an Educational Project
Tiene Nobels, Wim Deprez, Ief Pardon, Stijn Stevens, Ondrej Viktorin,
Johan Driesen, Jeroen Van Den Keybus, Ronnie Belmans
Kaholieke Universiteit Leuven, Dept. Elektrotechniek, Div. Electa
Kasteelpark Arenberg 10, B-3001 Leuven, Belgium
URL: http://www.esat.kuleuven.ac.be/electa
Phone +32 16 321026
Fax +32 16 321985
Tiene.Nobels@esat.kuleuven.ac.be
Ief.Pardon@esat.kuleuven.ac.be
Johan.Driesen@esat.kuleuven.ac.be
ondrej.viktorin@esat.kuleuven.ac.be
Abstract – Engineering education should comprise more than
theoretical knowledge. A design project with close relationship
to present technology and the student’s environment provides
hands-on engineering experience and training of general
engineering skills.
Electric vehicles are an example of
technology. Building an electric go-kart is an economical and
real-life educational design project.
I. INTRODUCTION
The electrical energy-engineering curriculum at the
K.U.Leuven comprises 2 years of general science and
engineering education, 1 year of general electrical
engineering and a 2-year electrical energy specialisation.
The design project to be developed is intended for 4th year
students.
The objectives of the design project are mainly to provide
the students with hands-on experience, especially in the
domain of electrical installations and drives. Also general
engineering skills such as organizing meetings, gathering
information and working in team are targeted.
Wim.Deprez@esat.kuleuven.ac.be
Stijn.Stevens@esat.kuleuven.ac.be
Jeroen.Vandenkeybus@esat.kuleuven.ac.be
Ronnie.Belmans@esat.kuleuven.ac.be
the performance of conventional go-karts for rental
purposes.
The final goal is an optimal power management of the gokart, such that its velocity and acceleration is similar with
that of conventional go-karts.
Both the choice of
components and the control of these components are part of
the students’ assignment.
The main components of an electric go-kart are the motor
and the batteries, but peripheral equipment such as brakes
and sensors are essential for a safe vehicle too. Before the
motor and batteries can be selected, the necessary go-kart
power, torque and energy have to be determined.
However some boundary conditions, mostly of practical
nature, will impose limits on the design.
• An empty go-kart chassis as depicted in Fig. 1 is
available to the students. Although the mechanical
design is not the target, space management is
important.
II. DESIGN SUBJECT
Motivation is an important aspect of the learning trajectory.
Therefore, the topic has to be relevant, both to the
curriculum as to present technology problems. Seeking the
design topic in the student’s environment stimulates
motivation too.
Electric vehicles (EV) represent an emerging and high-tech
market [1] in which electrical installation and drive
technology are main topics. Go-karts are an example of
vehicles students are familiar with. These days most gokarts are still gasoline powered, mostly due to the
extremely high energy density of fossil fuel. However, the
exhaust gases of such go-karts are intolerable for indoor
karting. Electric go-karts offer a solution to this problem.
III. ASSIGNMENT
The design task is to build an electric go-kart. The
performance of this electric go-kart has to be comparable to
Fig. 1: Picture of the bare chassis.
•
A power electronic converter is inevitably necessary
for the speed control of any electric motor. At our
department a rapid prototyping platform has been
developed [2]. This platform consists of a freely
programmable processing core and an extendable
number of modules, e.g. a precision measurement
module, A/D converter module or an inverter
comprising 4 half bridges (see Fig. 2). The IGBTs in
the half bridges limit the effective current to 23 A,
imposing severe limitations. The processing core of the
platform is composed of a DSP and an FPGA-board.
The DSP program can be written directly in C-code or
the user-friendlier MATLAB/Simulink-environment can
be used.
•
encourage the students to bring out the best. Presently
however, hardware limitations prohibit a real go-kart
race, as only one go-kart chassis is available.
open-ended: another important aspect of the design
project is the ‘open end’: nor the students, nor the
lecturer can predict the (details of the) final result.
Students contribute largely in all parts of the project,
which incorporates a certain failure risk. On the other
hand, satisfaction in case of success will be all the
larger.
B. Organization
Fig. 2: Rapid prototyping platform with A/D converter,
DSP and FPGA-board
•
•
Cost is in real life one of the most important
limitations. In this project cost will be a factor in
making design decisions as well.
Last but not least is it important to come to a realistic
design: the electric go-kart should not only be designed
(on paper/PC) but also built. The understanding of the
difference between theory (paper design) and practice
(prototype) represents one of the main didactical goals
of this project.
As this project is a long-term project, many ideas for
expansion are at hand. These topics are not essential for
the go-kart, but they can improve the efficiency and the
novelty of the design. Examples are data logging,
efficiency improvement by the implementation of a
separate two-wheel drive and the use of renewable energy.
For above reasons, the economic value of electric go-karts
is probably considerable. Consequently the research may
be of interest to the karting industry.
IV. DIDACTICAL CONSIDERATIONS
A. Design properties
Properties of an educationally good design project are e.g.
[6]:
• a group enables development of teamwork skills. This
is of major relevance for later professional life.
• motivation plays a major role in the efficiency of
education. It can be stimulated for example by the
design subject choice, by introducing a competitive
aspect or by a substantial student contribution to
decision making.
• a common project to all groups: different groups with
the same task enable learning not only from team
members, but also from competitors. Additionally the
evaluation will be more consistent. Moreover
competition improves motivation, thus a race would
The design project is spread over two semesters. Students
are arbitrarily split up in two competitive groups. During
the first semester students and teaching assistants are
embedded in a role-play where both groups of students act
as an engineering agency. Teaching assistants act as a
company consulting outside engineering agencies. The
design evolves as the result of an interaction between
students and teaching assistants: the students have
meetings, perform basic research and propose solutions,
which are subsequently evaluated by the teaching
assistants. At the end of the first semester both engineering
agencies present their paper design of a go-kart driven by
an electric motor.
The practical implementation and the final test-drive follow
in the second semester. The role-play stops but the
students stay in their competitive group. The tools
available to both groups include help/support of teaching
assistants.
C. Evaluation
Evaluation is an important aspect of education as it has a
large influence on the motivation and the goals of the
students. Because a number of general engineering skills
are an important part of the objectives, permanent
evaluation represents a large amount of the points to be
earned. However, to an engineer also the goal is important.
For this reason, the final result will be considered as well.
V. TECHNICAL DESIGN
A. Design specifications
The goal is to build an electric go-kart comparable to
conventional fuel go-karts for rental purposes. Therefore
the design specifications can be based on the characteristics
of such karts. Accordingly, we aim at a maximum speed of
50 km/h and heats of 15 minutes.
An initial selection of the motor torque is made based on
available data from combustion engine driven go-karts,
given in Fig. 3. Conventional fuel go-karts for rental
purposes have typically a reduction gear ratio of 5 [5].
Consequently a maximum torque of 70 Nm at 550 rpm on
the wheels is needed. A 4 kW electro motor, running at
3000 rpm and delivering a torque of approximately 13 Nm,
meets the requirement, assuming a reduction ratio of 5 to 6
on the electrical go-kart.
disadvantages such as cost and temperature deteriorating.
The dc motor can be relatively light, but its main advantage
is the easy control compared to alternating current
machines.
Nevertheless the teaching staff has a preference for an
induction motor (with cage rotor) since:
• the induction motor is an extremely robust and durable
machine that requires little or no maintenance, one of
the main reasons why the induction motor is so often
used in industry.
• the low cost of an induction motor is very attractive.
• also didactical reasons play a role: it is the most
common type of motor in industry and consequently
deserves a place in the engineering curriculum.
Moreover a dc motor has already been covered in an
earlier design project [4].
Fig. 3: Torque-speed characteristic of a
conventional go-kart engine [5].
Taking into account the above mentioned reduction ratio
and the wheel circumference, the electro motor has to be
able to run at 5000 rpm to satisfy the 50 km/h condition.
B. Voltage choice
Not only the necessary power and energy, but also the
voltage has implications on the motor and battery choice.
The batteries always deliver a dc voltage (‘Voltage level 1’
in Fig. 4), but the motor needs an other voltage (‘Voltage
level 2’).
Voltage
level 1
Voltage
level 2
An interesting fact is that the students engaged in the pilotproject chose the induction type independently of the
teaching staff.
The motor manufacturer ELNOR Motors, has in its product
range a 4 kW, 400 V, 3000-rpm three-phase induction
motor. The motor is operated above its nominal speed.
Consequently, flux weakening is used to limit the inverter
output voltage. This essentially means that torque reduces
above 3000 rpm, similar to conventional fuel go-karts
engines (Fig. 3).
The available rapid prototyping platform limits the
effective output current to 23 A. Consequently, the
effective voltage for an induction motor of 4 kW is at least
100 V and accordingly, the dc bus voltage should be at
least 142 V.
The downsize of a reduced motor voltage is an increased
ohmic loss and reduced main inductance. In conjunction
with ELNOR Motors, 130 V was selected. They rewound
a 4 kW, 400 V, 3000-rpm three-phase induction motor to a
4 kW, 130 V, 3000-rpm three-phase induction motor.
When space vector modulation (SVM) is used, the
minimally required dc bus voltage Udc is 184 V [7].
U dc = 3 ⋅ Uˆ phase = 2 ⋅ U line
Batteries
Power
Converter
(1)
Motor
Fig. 4: Energy flow.
The voltage levels have consequences for the losses in the
wiring and the power electronic switches as well. Indeed,
as the voltage level increases, the currents are reduced for
the same power. Both low wiring ohmic losses and the
possibility to use low-cost industry standard IGBTs require
the use of a fairly high voltage. However, a high voltage
requires many batteries connected in series. Moreover
Attention should be paid to safety requirements in case of
higher voltages.
C. Motor choice
Current electrical go-karts employ mostly dc motors [3].
Permanent magnet-machines are interesting due to their
extremely high power density, but they also have
D. Battery choice
In electric vehicles standard car batteries do not serve the
purpose: they can deliver large currents, but only for a
short time. EVs need deep cycle batteries, which have a
certain power capacity but more important a considerable
energy content.
Standard lead acid batteries can be drained in 15 minutes,
but the large weight is an important disadvantage [9].
Nevertheless, they are used in this project for cost reasons.
Allowing the batteries to discharge down to 80 %, which is
acceptable for standard batteries, means that an initial dc
bus voltage of 230 V is needed [8]. This is achieved by
connecting 20 12 V batteries in series.
The battery capacity was selected based on the observation
that the mass of a battery is proportional to its energy
content [8]. Complying with the objective of heats of 15
minutes, 20 batteries of 12 V and 12 Ah, weighing nearly
80 kg, are selected. Compared to conventional fuel gokarts the inertia drastically increases, resulting for the
electric go-kart in lower acceleration rates or higher torque
requirements. Cost issues force us to settle for lower
acceleration ratios.
For safety reasons and in order not to upset the go-kart’s
weight balance, placement of the batteries merits particular
attention.
mechanical
brake device
brake
brake cylinder
pedal
E. Control
To control the induction motor, Field Oriented Control
(FOC) is used. It enables control over both the excitation
flux-linkage and the torque-producing current in a
decoupled way [7] and is extensively dealt with by the
students in earlier courses and laboratory sessions.
Implementing FOC implies acquisition of the motor shaft
position, using an incremental encoder.
Sensorless
operation of FOC is avoided due to the added complexity
of the control model.
In order to control the go-kart I/O is indispensable. Sensors
detect the wanted direction, acceleration/deceleration and
actual motor position (see Fig. 5).
Acceleration
Brake
Sensors
Direction
T*
slot
Fig. 7: Close-up of the brake pedal mechanism
Safety precautions enforce the use of a mechanical brake,
but energy considerations demand regenerative braking.
Implementing a delay in the mechanical brake, during
which regenerative braking is active, combines both.
Using a slot in the mechanical brake device effects the
delay (see Fig. 7).
To indicate the wanted direction (for- or backwards) to the
control system a binary switch is used.
VI. EDUCATIONAL IMPLEMENTATION
PE
FOC
M
Rotor position
Fig. 5: Control scheme
A motor vehicle is typically torque controlled. FOC
enables to control the torque-producing current. The
accelerator and brake pedal positions are converted into
electrical signals with a potentiometer with an A/D
converter (see Fig. 6).
Fig. 6: Close-up of the gas and brake potentiometer
The practical implementation of the go-kart is carried out
in the second semester and comprises:
• implementing a torque control scheme of FOC for an
ac motor drive on the rapid prototyping platform
• applying control sensors
• mechanical assembly
The students do not have to do the mechanical assembly
themselves; our competent technicians do it according to
the instructions. Additionally they provide a test bench
upon which the performance of the motor control system
can be fully tested.
Both engineering agencies split up their group in two
divisions. One division attends to the implementing of a
torque control scheme of FOC, the scheme-division, the
other division attends to the applying of control sensors, the
sensor-division.
Possibilities for student guidance range from providing
them with a fully working, previously tested control
scheme, which they should adapt and optimise or having
them build a complete FOC scheme from scratch. An
often-recurring question in designing practical labs is
which compromise between the two possibilities
maximizes the ratio between acquired competence and
invested time. In fact, two compromises were offered to
the students: the first one comprises a full-fledged control
system from a PhD student, which has to be downgraded.
The second one is a basic induction motor control
simulation model. Both engineering agencies chose the first
approach putting a big effort into comprehending the high
performance FOC scheme. Because it is possible to
program the rapid prototyping platform with
MATLAB/Simulink, the scheme can easily be tested off-
line with a MATLAB model representing the motor.
Students determine the motor parameters by measurements
on the test bench.
In a next stage the FOC scheme has to be interfaced with
the sensors and the general control (see Fig. 5).
For use on the test bench a test box with a binary switch
and two potentiometers is available.
A demonstration on the test bench is followed by the final
implementation on the go-kart. In order to allow all-day
testing the kart has been cable powered instead of battery
powered as no multiple battery packs were available.
VIII. CONCLUSIONS
An educational design project to provide students with
hands-on engineering experience and to train general
engineering skills has been presented. The choice of an
electric go-kart as a design target encourages intensive
student enrolment and, at the same time, is based on a
current technological challenge. As this project is meant to
be a long-term project, permanent renewal is necessary to
maintain the earlier stated “open-end” property. Therefore
next year separate two-wheel drive will be implemented
(see Fig. 9).
Fig. 9: Schematic drawing of a separate two-wheel drive.
ACKNOWLEDGMENT
Fig. 8: Final test-drive-day
The authors kindly thank motor designer ELNOR Motors
for providing the authors with the 130 V induction motors.
VII. EDUCATIONAL REVIEW OF THE PROJECT
A survey of appreciation amongst the students was made.
In general students were satisfied with the design project in
spite of the fact that it was a pilot-project accompanied
with the necessary growing pains. Even though much
progress is possible, students found the project to be very
instructive and at the end very rewarding.
Because of the interaction during the role-play between
company and engineering agencies (i.e. teaching assistants
and students) the technical design is quasi-identical for
both engineering agencies. Both engineering agencies
designed an electrical go-kart with a four-quadrant control.
Driving backwards and regenerative braking is possible.
During the implementation phase the general idea was to
let the students sort it out within their group, but we learned
that a minimum of coaching is necessary.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
http://www.toyota.com/prius, last visited 25 May 2004.
Van den Keybus J., “Development of a universal power
measurement and control platform for low-voltage grid-coupled
applications in a deregulated electricity market”, PhD
dissertation, K.U.Leuven, December 2003.
http://www.lynchmotor.com, last visited 25 May 2004.
Daems W., De Smedt B., Vanasssche P., Gielen G., Sansen W.,
De Man H., PeopleMover: An example of interdisciplinary
project-based education in electrical engineering, IEEE Trans.
on Education, vol. 46, no.1, Feb. 2003, pp. 157-167.
http://www.honda-engines.com/gx270.htm, last visited 21 May
2004.
Gregson P.H., Little T.A., Using Contests to Teach Design to EE
Juniors, IEEE Trans. on Education, vol. 42, no.3, August 1999,
pp.229-232.
Terörde G., “High performance motion controle of ac motor
drives with & without shaft sensor”, PhD dissertation,
K.U.Leuven, September 2002.
Vincent C. A. & Scrosati B., “Modern Bateries: An introduction
to electrochemical power sources”, reference book, second
edition, 1997
S.R. Holm, H. Polinder, J.A. Ferreira, P. van Gelder and R. Dill,
“A Comparison of Energy Storage Technologies as Energy
Bu®er in Renewable Energy Sources with respect to Power
Capability”, Young Researchers Symposium, Leuven, February
2002.