Hey Guys,
Last time we looked at the fall and rise of electric
vehicles. I realized that there is a decent interest amongst the student
community to learn more about electric vehicles. I am also very glad that there
are groups in BITS-Pilani Goa campus working on electric car prototypes and I
am sure other campuses would also be making such contribution. I strongly believe
that such experience in college will definitely benefit students who are
interested in pursuing career/ higher studies in this domain or even in fields
like power electronics/ motor controls and firmware for automotive powertrain
systems.
For the people out there who are not really up to speed
but are super interested and would like to get into this, today I would like to
give a brief architectural overview of electric vehicles. The major difference
between gasoline and electric vehicles is that there is no internal combustion
engine in electric vehicles. This is replaced by an electric motor (AC or DC)
and this motor is charged by a battery on the car. So, as gasoline engine cars
require petrol or diesel as a fuel, these cars need charge or current. People who know the complexity of an engine would instantly
realize the simplicity of electric car.
As you can see in the figure below the electric car
consists of 4 major components.
A Battery: A battery is the heart of the car. This is the
energy storage device (analogous to fuel tank in normal car). This stores DC
voltage when it’s charged. The voltage levels are generally in 100s of volts
(~400V). This battery has certain number of cells in series and parallel. The
number of cells in series and parallel is decided on how much voltage (~range)
and how much current (~power) you want in your car respectively.
A Charger: (not shown in the figure) It is used to charge
the battery up to the desired voltage. This takes in AC voltage from the wall,
rectifies it and then charges the battery. The charger is rated at certain KW
which determines how much current it can consume which in turn dictates how
much time you will need to charge the battery. You will study the concepts used
in charging in ES1.
A Motor: Usually a 3ph AC induction motor is used in electric
cars (due to efficiency/ reliability reasons).It takes in 3ph AC current and
produces torque due to magnetic induction across the stator and rotor. The
shaft of the motor is connected to the halfshaft of the car through a reduction
gear box. You guys generally study these in ES2.
An Inverter: Inverter is a power electronics device that
is needed in between a battery and the motor to convert the DC current that the
battery produces to AC current that the motor consumes. It has IGBTs as switches
which switch at high frequencies like 10KHz while converting DC to AC.
Having said this there are a couple peculiarities of
electric vehicles:
Regenerative Braking: When you apply brakes you are essentially
applying negative torque to the motor to speed it down. That means the motor
consumes negative current which you can visualize as a current going from motor
to battery through the inverter. This results in charging the battery. This
phenomenon makes electric vehicles very attractive in the sense that you can recharge
the battery using the energy spent in braking.
Max torque at zero speed: If you see the motor torque
speed characteristic as shown below you will realize that we can extract
maximum torque from the motor even at zero speeds. This means that we will not
need a gear box to shift to the right gear to attain right amount of torque
from the motor at various speeds (as we require in an engine). Hence you can
achieve high amounts of accelerations/ power from the vehicle from dead stop
positions.
Below is a very interesting link of a drag race between
BMW M5 (known for its acceleration) and Tesla ModelS (electric sedan company
that I work for :)). See
how Tesla gets a head start due to full torque at zero speed!!
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