31-10-2012, 06:15 PM
Selection of Electric Motor Drives for Electric Vehicles
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Abstract
In this study, six kinds of the drivetrain systems of
electric motor drives for EVs are discussed. Furthermore, the
requirements of EVs on electric motor drives are presented. The
comparative investigation on the efficiency, weight, cost, cooling,
maximum speed, and fault-tolerance, safety, and reliability is
carried out for switched reluctance motor, induction motor,
permanent magnet blushless dc motor, and brushed dc motor
drives, in order to find most appropriate electric motor drives for
electric vehicle applications. The study shows that switched
reluctance motor drives are the prior choice for electric vehicles.
INTRODUCTION
Conventional vehicles are driven by internal combustion
engine (ICE) and thus they are also named internal
combustion engine vehicles (ICEVs). The vehicle is named
the electric vehicle (EV) if an electric motor or a few electric
motors are used to drive wheels of a vehicle. In addition, the
vehicle is named the hybrid electric vehicle (HEV) if both an
electric motor and an ICE impel wheels of a vehicle. Electric
vehicles are only discussed in this paper.
A system schematic of EVs is illustrated in Fig. 1 [1]. In
EVs, the battery is the original energy source and provides
electric power to electric motor drives and other equipments,
such as lighting devices.
DRIVETRAIN SYSTEMS OF EVS
For EVs, output characteristics of electric motors differ
from those of ICEs. Typically, the electric motor eliminates
the necessity for a motor to be idle while at a stop, it is
allowed to produce large torque at low speed, and it offers a
wide range of speed variations. It may be possible to develop
lighter, more compact, more efficient systems by taking
advantages of the characteristics of electric motors. The
choices of drivetrain systems in an EV include mainly: (a)
propulsion mode, such as front-wheel drive, rear-wheel drive,
or four-wheel drive; (b) number of electric motors in a vehicle;
© drive approach, for instance, indirect or direct drive; and (d)
number of transmission gear levels. Therefore, the possible
drivetrain systems in EVs have the following six
configurations.
Conventional Type
For the conventional type of the drivetrain system in EVs,
the conventional ICE is replaced by an electric motor, as
shown in Fig. 4. This configuration does not change the
typical structure of drivetrain system in ICEVs and hence is
implemented easily.
Requirements of EVs on Electric Motor Drives
Selection of electric motor drives for EVs is a very
important step that requires special attention. In fact, the
automotive industry is still seeking for the most appropriate
motor drive for EVs or HEVs. Previous literatures [2] [3] [6]
discussed the major requirements of EVs on electric motor
drives. Therefore, selecting the most appropriate motor drives
for an EV is a challenging issue.
In this paper, the basic requests are summarized as follows:
1) a high instant power and a high power density; 2) a high
torque at low speed for starting and climbing, as well as a high
power at high speed for cruising; 3) a very wide speed range
with constant-power region; 4) a fast torque response; 5) a
high efficiency over the wide speed range with constant torque
and constant power regions; 6) a high efficiency for
regenerative braking; 7) downsizing, weight reduction
Switched Reluctance Motor Drives
SRM drives are gaining much interest and are recognized to
have a potential for EV applications. These motor drives have
definite advantages such as simple and rugged construction,
fault-tolerant operation, simple control, and outstanding
torque–speed characteristics. SRM drives can inherently
operate with an extremely long constant-power range.
The torque-speed characteristics of SRM drives match very
well with the EV load characteristics. The SRM drive has high
speed operation capability with a wide constant power region.
The motor has high starting torque and high torque-inertia
ratio. The rotor structure is extremely simple without any
windings, magnets, commutators or brushes. The faulttolerance
of the motor is also extremely good. Because of its
simple construction and low rotor inertia, SRM has very rapid
acceleration and extremely high speed operation.
CONCLUSIONS
In this study, six types of the drivetrain systems of electric
motor drives for EVs have been discussed. The drivetrain
schemes with the single-level reduction gear are well suitable
for electric motor drives with wide speed range and highly
maximum speed in EVs. Furthermore, the main requests of
EVs on electric motor drives and expected output
characteristics of electric motor drives are presented in this
paper