The Technology: How it works
DRIVE SYSTEM
An electric vehicle drive system is the part of the EV that transmits
mechanical energy to the wheels causing the vehicle to move. As
previously mentioned, EVs are much different from standard vehicles
and do not require a transmission. All that is needed to get an
EV moving is an electric motor. The power is supplied to the electric
motor by the batteries which in turn is
regulated by the control system. There
are several different drive system designs in use today. One, for
example, uses a single large motor that is coupled to the rear wheels,
while another design uses two smaller motors to power each wheel
separately. The most efficient design today uses motors that are
attached directly to the wheel, a.k.a. "wheel motors".
source
By eliminating drive shafts and differentials, mechanical losses
between the motor and wheels are kept to a minimum.
Electric motors convert electrical energy into mechanical
energy. There are two types of electric motors in use today: direct
current (DC) motors and alternating current (AC) motors. DC motors
are comprised of three main components:
- A set of field coils around the perimeter of the motor that
creates the magnetic forces which provide torque.
- A rotor or armature mounted on bearings that turn inside the
magnetic field created by the field coils.
- A commutating device that reverses the magnetic forces and
makes the rotor turn which produces the mechanical force used
to turn the wheels.
AC motors are similar to DC motors and have the first two components
listed, but since there is a continuous current reversal, a commutating
device is not needed. Neither motor is actually considered better
then the other. Each has its advantages and disadvantages as listed
below.
Electric Motor Comparison
|
AC Motor
|
DC Motor
|
|
Single-speed transmission
|
Multi-speed transmission
|
|
Light weight
|
Heavier at equivalent power
|
|
Less expensive
|
More expensive
|
|
95% Efficiency at full load
|
85-95% Efficiency at full load
|
|
More expensive controller
|
Simple controller
|
|
Motor/controller/inverter more expensive
|
Motor/controller less expensive
|
Table courtesy of http://www.etvi.org/High_Tech/New_Electric_HiTech_right.html
While an AC motor is less expensive, an AC system is more expensive
due to the cost of complex electronics dealing with the AC inverter
and motor controller. AC motors are the most commonly used motors
in home appliances and machine tools. They are very reliable and
because they are so simple, they should easily last the life of
the vehicle with little or no maintenance. Some typical motor characteristics
are listed in the table below.
Motor Characteristics
| |
DC
Brush Type |
Brushless
DC Permanent Magnet |
AC
Induction |
| Peak efficiency(%) |
85-89 |
95-97 |
94-95 |
| Efficiency at 10% Load |
80-87 |
73-82 |
93-94 |
| Max. RPM |
4,000-6,000 |
4,000-10,000 |
9,000-15,000 |
| Cost per shaft Hp |
$100-150 |
$100-130 |
$50-75 |
| Relative Cost of Controller
to DC Brush Type |
1 |
3-5 |
6-8 |
| 1 Hp = 746 Watts |
|
|
|
Table courtesy of http://www.etvi.org/High_Tech/New_Electric_HiTech_right.html
The controller reverses the field coils of the motor when the vehicle
is in braking mode and therefore the motor becomes a generator returning
energy back to the batteries. This regenerative breaking can return
as high as 10% or more of the energy consumed by the drive system
to the batteries over the course of a single charge. See source
below for a more complex explanation of regenerative breaking.
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Source: http://www.etvi.org/High_Tech/New_Electric_HiTech_right.html
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