Electric Kart as a Student Project

STREIT Lubos1, STEPANEK Jan2, ELIS Ludek3, BEDNAR Bedrich4

UNIVERSITY OF WEST BOHEMIA – 1RICE, 2,4KEV, 3KAE
Univerzitni 8
Pilsen, Czech Republic
Tel.: +420 / 377 635 700.
Fax: +420 / 377 631 112.
1
LLOYD@rice.zcu.cz, QUIDO@kev.zcu.cz, 3LUDAELIS@kae.zcu.cz, 4BEAD@kev.zcu.cz
2

URL: http://www.zcu.cz

Acknowledgements
This research has been supported by the European Regional Development Fund and Ministry of
Education, Youth and Sports of the Czech Republic under project No. CZ.1.05/2.1.00/03.0094:
Regional Innovation Centre for Electrical Engineering (RICE) and the Czech Science Foundation
under the project GA CR 102/09/1164.
The great thanks also belong to the MS kart company for cooperation on this project.

Keywords
MOSFET, Parallel operation, DC machine, Battery Management Systems (BMS)

Abstract
Paper deals with the construction of the electric-kart as a student project. The project was solved by
two master thesis and a few semester thesis. The power of this kart is equal to the kart for rent
equipped with the combustion engine. The chassis is equipped with Li-ion batteries and DC-motor.

Introduction
The electric kart is designed to achieve similar power as the commonly used kart for rent. The nominal
power is 5 kW or 15 kW during overload [1]. The topology of the kart is shown in the Fig. 1. It
includes following main parts: LiFeYPO4 batteries [2], MOSFET buck/boost converter, permanent
magnet DC motor, sensors (temperature, RPM, accelerometer) and three CAN units: Motor Control
Unit (µPcan1), Battery Management System (BMS) Unit and Display Unit. Main parts of the electric
kart are labelled in the Fig. 2.

Four LiFeYPO4 batteries connected in series have a nominal voltage 48 V. The rated capacity of each
battery is 80 Ah. The rotational speed of the DC motor ME 0708 is 3 200 RPM. The motor is coupled
with rear axle of the kart by a chain. The gear ratio is 1:3.5. The pedal position sensing is provided by
linear Hall sensor A1302. A permanent magnet is mounted to the pedal and Hall sensor is fixed to the
chassis. The battery pack is connected to the converter by a main contactor. The contactor is switched
on after a pre-charging of converter’s capacitors is done. And it can be switched off by an emergency
switch placed on the steering wheel.

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 Fig. 1: Topology of Electric Kart Fig. 2: Placement of Electric Equipment

Can Units
Three microprocessor units are used as nodes of the distributed control system. These units are
mutually connected via CAN bus. Each unit is equipped by a microprocessor AT90CAN32 and it’s
designed as a board with universal interfaces. This unit board is shown in the next Fig. 3. These units
will be described in following chapters.

Fig. 3: AT90CAN32 with Universal Interface

Motor Control Unit (µPcan1)

The primary function of this unit is to control the DC motor current. Therefore it measures this current
by Hall Effect current sensor LEM HTFS 400-P and it generates switching pulses for the buck/boost
MOSFET converter. Switching pulses for the converter (one for the top switch and the negated for the
bottom switch) are generated in the microprocessor by a counter with compare units. The counter
overflow period is set to 62.5 µs (16 kHz). The counter counts in up/down counting mode. Two
compare registers are set to the required duty cycle of the PWM.
The next functionality implemented in this unit is the speed control of converter’s and motor’s fans.
The speed of fans is controlled to decrease noise when the kart velocity is near zero. The speed of fans
reach maximal speed when high current flows through the motor and the converter or the temperature
of cooled parts is still high. The speed of fans is continuously controlled during the standard operation
(the kart is riding). The motor control unit transmits CAN message which includes these values: kart
velocity, travelled distance, motor current, throttle pedal position, motor temperature, converter
temperature and error flags.

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Display Unit (µPcan2)
The main function of this unit is displaying of the important information to the pilot. All values from
CAN messages can also be displayed for debugging purposes. The EA DOGL128, a 128x64-pixel
graphics display, is used. The unit includes Navimec navigation buttons for better navigating in the
menu. This unit with main screen is shown in the Fig. 4. Display shows actual speed, travelled
distance (Trip), battery state of charge (bar and %) and speed bar. For dynamic displaying of values
the LED bar is used. This bar shows DC motor current. The last visible part is key switch. This switch
turns on/off the whole kart electrical system.

Fig. 4: Display Unit

A digital accelerometer ADXL345 by Analog Devices is used for sensing of acceleration, deceleration
and centrifugal force and/or vibration. The display unit transmits CAN message which includes these
values: ambient temperature, pedal position (if it measures instead of motor control unit), acceleration,
deceleration, centrifugal force, error flags.

Battery Management System Unit (µPcan 3)

The battery management unit (BMS) handles the battery recharging process and batteries energy
balancing. The BMS unit communicates with batteries via RS 485 bus. Each of all four batteries is
equipped by Monitoring Unit with RS 485 bus. This Monitoring Unit measures voltage and
temperature of the battery and it is able to balance the battery voltage between batteries via switching
of a balancing MOSFET. The redundant energy is wasted by MOSFET power dissipation during
balancing. The Monitoring Unit was developed by a bachelor thesis [5]. A PCB of the unit is shown in
Fig. 5.

Fig. 5: PCB of the Monitoring Unit [5]

The PCB of the Monitoring Unit is mounted into an aluminium box. This box has high Ingress
Protection Rating for outdoor use. The balancing MOSFET is screwed on a surface of the box and
connected by wire therefore it is not placed in PCB. A photo of the Monitoring Unit in the aluminium
box and the placement of it in the kart are shown in Fig. 6.

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 Fig. 6: Monitoring Unit – aluminium box and placement in the kart [5]

When the charger is connected the BMS unit controls the charging. The lithium battery charger
POW48V50AT made by GWL Power Company [6] is used for charging. The nominal output current
of this charger is 50 A. It allows achieving the full charge of all batteries in approximately 1.5 hour.
The charger is shown in the Fig. 7.

Fig. 7: Lithium Battery Charger - 48V/50A [6]

The battery state of charge (SoC) is calculated by this BMS unit. It is calculated by the integration of
the measured battery current. The BMS unit also allows logging of selected operational values
(temperatures, currents, voltage, RPM, traveled distance, velocity and battery state of charge) to an SD
card for further processing. It is possible to connect PC by USB through this unit and download the
logged data.

The BMS unit transmits CAN message which includes these values: battery current, 4 battery
voltages, 4 battery temperatures, SoC and error flags.

MOSFET Converter
Power Circuit

The topology of the MOSFET converter is buck/boost – half-bridge. It is composed of four parallel
discrete transistors for buck and four for boost for increasing of the current rating. The buck converter
is used for drive and a boost converter is used for regenerative braking. The main parameters of used
MOSFETs IRFP4568 are: VDSS=150 V, RDS(ON)=4.8 mΩ, ID (TC=25°C)=171 A, ID (TC=100°C)=121 A,
Ciss=10.4 nF. The whole schematic of the Buck/Boost converter is shown in the Fig. 8. The capacity C
is formed from parallel connection of a filtering electrolytic capacitor and a snubber film capacitor.

The converter is designed for nominal continuous current up to 300 A. The switching frequency of the
converter is 16 kHz because the motor inductance is relatively low about 40 µH. Maximal input
voltage is 100 V (battery voltage can be 68 V during charging). Active heat sink with axial fan is used
to cool down the MOSFETs dissipation. The converter mechanical construction is shown in Fig. 9.

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Fig. 8: Buck/Boost Converter – schematics Fig. 9: Buck/Boost Converter – Mechanical Construction

The mechanical construction is composed of two active heat sinks, 4 parallel filtering capacitors, 8
MOSFETs, 2 brass metal plates and 2 copper belts. Two heat sinks are used because current flows
through them. Therefore transistors don’t need insulating thermal pads. Two brass plates are used for
“sandwich” construction of the DC bus to decrease parasitic inductances. The nominal capacitance of
each filtering capacitor is 2 200 µF. It means 8 800 µF in all. SMD transils are used for suppression of
dangerous overvoltage peaks. Copper belts are used for connection between the converter and a
terminal plate.

Driver Circuit
The driver circuit topology is composed of gate drive optocoupler HCPL-316, high current gate driver
IXDN414 and power supplies [3]. Useful features of the HCPL-316 are desaturation detection and
under voltage lock-out. It is possible to use the desaturation detection for short circuit protection or
overloading protection of converter MOSFETs. The desaturation detection measures the voltage drop
across the RDSON resistance to detect the overcurrent. Therefore it’s usable also for MOSFETs.
The next Fig. 10 shows two layer printed circuit board of the driver. Optocouplers HCPL-316 has a
white package. High current drivers IXDN414 are placed close to them.

Fig. 10: Driver Circuit - PCB

The following Fig. 11 shows standard operation of the kart. There are motor current about 300 A –
CH4 [163 A/div] and the converter output voltage CH3 [25 V/div].

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 Fig. 11: Standard Motor Current and Voltage Behaviour

Conclusion
A total weight of the kart is 260 kg (including pilot). The maximal kart acceleration is 4 m/s2 (0.4g).
The maximal measured velocity was 56 km/h. This maximal velocity was measured by the internal
RPM sensor and it was verified by GPS. The traveled distance is about 30 kilometers per one charge
of batteries. It takes approximately 1.5 hour of the riding. The full battery charging takes the same
time. In this paper the right solution of the student project was described. Students demonstrated that
they can work as a team.

References
[1] Vitols, K.; Reinberg, N.; Sokolovs, A.; Galkin, I. Drive selection for electric kart. In Power Electronics
and Motion Control Conference, EPE/PEMC 2010. Ohrid (Macedonia): IEEE, Sept 2010. Page T9-15
to T9-18. ISBN 978-1-4244-7856-9.

[2] Winston Battery Limited. [Online] Winston Battery Limited

[cit.: 2012-04-01] <http://www.thunder-sky.com/>

[3] MOSFET/IGBT Drivers Theory and Applications. [Online] IXYS Corporation, c2001.
[cit.: 2011-04-05] http://www.ixyspower.com/.../IXAN0010.pdf

[4] MS SUPERKART s motorem VM 250 | MS KART. [Online] MS KART, c2010-2012.

[cit.: 2012-04-01]. <http://www.mskart.cz/>

[5] POLACH, Tomáš. Li-ion Battery Measuring Circuits. Pilsen, Czech Republic, 2013. Bachelor thesis.
University of West Bohemia.

[6] EV-Power | Your Complete Power Solutions [Online] Global World Logistic Ltd.
[cit.: 2013-06-01] <http://www.ev-power.eu/>

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