07-12-2012, 01:15 PM
An Electric Gearshift With Ultracapacitors for the Power Train of an Electric Vehicle With a Directly Driven Wheel Motor
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Abstract
A novel electric gearshift with ultracapacitors is designed
for the power train of a directly driven electric vehicle. The
power train consists of two major subsystems: electric propulsion
and energy source. The electric propulsion subsystem is composed
of a dedicated wheel motor and its drive; the energy source subsystem
is equipped with a stack of lead–acid batteries and ultracapacitor
cells. This electric gearshift combines different parallel and
serial connections of batteries, motor windings, and ultracapacitors
to accommodate various driving patterns in the permissible
range of speed and torque. A control core, which is realized by a
field-programmable gate array, is employed to manage the energy
source and direct optimal propulsion to extend the speed range
of constant power, and thereby improve vehicle performance in
terms of efficiency, acceleration, and driving range.
INTRODUCTION
IN THE PAST decade, air pollution and shortages of petroleum
have increased the awareness for critical issues of
environment preservation and the depletion of existing energy
resources. Statistics show that the most severe air pollution
originates from imperfect combustion of petroleum by internal
combustion engine (ICE) vehicles. Electric vehicles (EVs)
are, therefore, important for environmental reasons and, thus,
have become more attractive and promising. Some commercial
hybrid cars are currently in use, although pure EVs with zero
emissions are still under research and development. In the research
on EVs, an energy source that converts into electricity is
a key technology; for propulsion system, it is important to find
more efficient ways of consuming limited onboard power while
the energy-management system coordinates these subsystems
to achieve the best overall performance of the EV.
EV CONFIGURATION AND SPECIFICATIONS
The basic configuration of EV with a directly driven wheel
motor is composed of two subsystems: the electric propulsion
subsystem and the energy source subsystem, as shown in Fig. 1.
The propulsion subsystem consists of a directly driven wheel
motor, a power converter, and an optimal current controller,
while the energy source subsystem has a stack of lead–acid
batteries and an ultracapacitor bank. The electric gearshift
connects the stator windings, batteries, and ultracapacitor cells
in series and parallel by latching relays, which are controlled by
the energy-management unit.
ELECTRIC GEARSHIFT IN DRIVING MODE
Driving Gearshift Without Ultracapacitors
For an energy source with a given power rating, the ideal
profile for traction torque versus vehicle speed has a constant
torque for speed below the base speed and has a constant power
above the base speed. For ICE-powered vehicles, a multigear
transmission was designed to achieve the constant torque and
constant power profile. For EVs, the wheel motor has a similar
speed–torque characteristic. However, its constant power range
is usually short and is limited by the capability of field weakening,
which is made possible only by exciting the stator windings
with an opposite magnetic field against the permanent magnets
on the rotor [15], or by weakening the rotational electromotive
force by employing the advanced conduction angle [16].
To extend the constant power range of EVs, the electric
gearshift is first scheduled with serial and parallel connections
of the stator windings and batteries without ultracapacitor cells.
From electric circuitry laws, when the wheel motor operates at
a stage of low speed and high torque, the batteries are connected
in parallel to provide lower voltage, and the stator windings are
connected in series to comply with the low-speed requirement.
As the stage of high speed and low torque is demanded, the
batteries are connected in series to achieve higher voltage, and
the stator windings operate in parallel accordingly.
Driving Gearshift With Ultracapacitors
Lead–acid batteries do not allow large current discharges
within a short period, which becomes shorter within their lifetime
and recharge capacity. The reason why the battery does not
promptly respond to the discharge is that the chemical reaction
usually takes place with an inevitable time delay. In contrast to
the battery, the discharge of electrons from the ultracapacitor
is a physical reaction; its high power density makes an instant
and huge current output easy and possible. This characteristic
is more suitable for the vehicle starting and accelerating when
the wheel motor requires an instant and huge current to build up
enough torque.
IMPLEMENTATION AND EXPERIMENTS
The electric gearshifts of driving and regenerative braking
have been designed as above. Their control sequences are coded
in the FPGA based on the schedule in Table V. The mechanical
latching relays are used as switching components, which allow
very large currents to flow through. However, the induced
voltage from inductance components could send a spike to
penetrate MOSFETs, and simultaneous conduction of many
relays with an instant and impulsive current will cause short
circuits to damage the system. In this regard, MOSFETs must
be disabled for 100 ms before the subsequent gear is engaged,
during which time, the remaining energy stored in capacitances
is dissipated and all the relays are open. The electric gearshift
has been implemented on an electric motorcycle with a directly
driven wheel motor, as shown in Fig. 17. The scooter data used
in this paper are shown in Table VI.
SUMMARY AND CONCLUSION
The proposed electric gearshift has been successfully
designed, analyzed, and implemented on an EV with a dedicated
wheel motor. The wheel motor is a disc-type axialflux
permanent-magnet brushless dc motor designed by the
Propulsion Control Laboratory at National Taiwan University.
The electric gearshift employs natural and physical properties
of motor windings, conventional lead–acid batteries, and
ultracapacitors, among which various connections are created
in serial and parallel modes. The driving gearshift has four
gears, while the regenerative braking gearshift has five. The
objective of the driving gearshift is to relieve heavy load from
batteries and to allow ultracapacitors to deliver large currents
within a short period, such as starting from rest and accelerating
from low speed.