2. Differences between gasoline and electric cars:
• The gasoline engine is replaced by an electric motor.
• The electric motor gets its power from a controller.
• The controller gets its power from an array
of rechargeable batteries.
• A gasoline engine, with its fuel lines, exhaust pipes,
coolant hoses and intake manifold, tends to look like a
plumbing project. An electric car is definitely
a wiring project.
• In order to get a feeling for how electric cars work in
general, let's start by looking at a typical electric car to
see how it comes together.
3. Contd….
• The gasoline engine, along with the muffler, catalytic converter,
tailpipe and gas tank, were all removed.
• The clutch assembly was removed. The existing manual
transmission was left in place, and it was pinned in second gear.
• A new AC electric motor was bolted to the transmission with an
adapter plate.
• An electric controller was added to control the AC motor.
• A battery tray was installed in the floor of the car.
• Fifty 12-volt lead-acid batteries were placed in the battery tray (two
sets of 25 to create 300 volts DC).
• Electric motors were added to power things that used to get their
power from the engine: the water pump, power steering pump, air
conditioner.
• A vacuum pump was added for the power brakes (which used
engine vacuum when the car had an engine).
4. Electric vehicles
• The shifter for the manual transmission was replaced
with a switch, disguised as an automatic
transmission shifter, to control forward and reverse.
• A charger was added so that the batteries could be
recharged. This particular car actually has two charging
systems -- one from a normal 120-volt or 240-volt wall
outlet, and the other from a magna-charge inductive
charging paddle.
• The 120/240-volt charging system
• The Magna-Charge inductive paddle charging system
• The gas gauge was replaced with a volt meter.
5.
6. Components
• The heart of an electric car is the combination of:
• The electric motor
• The motor's controller
• The batteries
• The controller takes power from thebatteries and delivers it to
the motor. The accelerator pedal hooks to a pair
of potentiometers (variable resistors), and these potentiometers
provide the signal that tells the controller how much power it is
supposed to deliver. The controller can deliver zero power (when
the car is stopped), full power (when the driver floors the
accelerator pedal), or any power level in between.
• In this car, the controller takes in 300 volts DC from the battery
pack. It converts it into a maximum of 240 volts AC, three-phase, to
send to the motor. It does this using very largetransistors that
rapidly turn the batteries' voltage on and off to create a sine wave.
7. Working:
• When you push on the gas pedal, a cable from
the pedal connects to these two potentiometers:
• The potentiometers hook to the gas pedal and
send a signal to the controller.
• The signal from the potentiometers tells the
controller how much power to deliver to the
electric car's motor. There are two
potentiometers for safety's sake. The controller
reads both potentiometers and makes sure that
their signals are equal. If they are not, then the
controller does not operate. This arrangement
guards against a situation where a potentiometer
fails in the full-on position.
8. Working contd…
• The controller's job in a DC electric car is easy to understand. Let's assume
that the battery pack contains 12 12-volt batteries, wired in series to
create 144 volts. The controller takes in 144 volts DC, and delivers it to the
motor in a controlled way.
• The very simplest DC controller would be a big on/off switch wired to the
accelerator pedal. When you push the pedal, it would turn the switch on,
and when you take your foot off the pedal, it would turn it off. As the
driver, you would have to push and release the accelerator to pulse the
motor on and off to maintain a given speed.
• Obviously, that sort of on/off approach would work but it would be a pain
to drive, so the controller does the pulsing for you. The controller reads
the setting of the accelerator pedal from the potentiometers and
regulates the power accordingly. Let's say that you have the accelerator
pushed halfway down. The controller reads that setting from the
potentiometer and rapidly switches the power to the motor on and off so
that it is on half the time and off half the time. If you have the accelerator
pedal 25 percent of the way down, the controller pulses the power so it is
on 25 percent of the time and off 75 percent of the time.
9. Contd…
• Most controllers pulse the power more than 15,000 times per
second, in order to keep the pulsation outside the range of human
hearing. The pulsed current causes the motor housing to vibrate at
that frequency, so by pulsing at more than 15,000 cycles per
second, the controller and motor are silent to human ears.
• In an AC controller, the job is a little more complicated, but it is the
same idea. The controller creates three pseudo-sine waves. It does
this by taking the DC voltage from the batteries and pulsing it on
and off. In an AC controller, there is the additional need to reverse
the polarity of the voltage 60 times a second. Therefore, you
actually need six sets of transistors in an AC controller, while you
need only one set in a DC controller. In the AC controller, for each
phase you need one set of transistors to pulse the voltage and
another set to reverse the polarity. You replicate that three times
for the three phases -- six total sets of transistors.
10.
11. Electric-car Motors and Batteries
• Electric cars can use AC or DC motors:
• If the motor is a DC motor, then it may run on anything
from 96 to 192 volts. Many of the DC motors used in
electric cars come from the electric forklift industry.
• If it is an AC motor, then it probably is a three-phase AC
motor running at 240 volts AC with a 300 volt battery pack.
• DC installations tend to be simpler and less expensive. A
typical motor will be in the 20,000-watt to 30,000-watt
range. A typical controller will be in the 40,000-watt to
60,000-watt range (for example, a 96-volt controller will
deliver a maximum of 400 or 600 amps). DC motors have
the nice feature that you an overdrive them (up to a factor
of 10-to-1) for short periods of time.
12. Contd…
• That is, a 20,000-watt motor will accept 100,000 watts for a short period
of time and deliver 5 times its rated horsepower. This is great for short
bursts of acceleration. The only limitation is heat build-up in the motor.
Too much overdriving and the motor heats up to the point where it self-
destructs.
• AC installations allow the use of almost any industrial three-phase AC
motor, and that can make finding a motor with a specific size, shape or
power rating easier. AC motors and controllers often have
a regenerative feature. During braking, the motor turns into a generator
and delivers power back to the batteries.
13. Six significant problems with current
lead-acid battery technology:
• They are heavy (a typical lead-acid battery pack weighs 1,000
pounds or more).
• They are bulky (the car we are examining here has 50 lead-acid
batteries, each measuring roughly 6" x 8" by 6").
• They have a limited capacity (a typical lead-acid battery pack might
hold 12 to 15 kilowatt-hours of electricity, giving a car a range of
only 50 miles or so).
• They are slow to charge (typical recharge times for a lead-acid pack
range between four to 10 hours for full charge, depending on the
battery technology and the charger).
• They have a short life (three to four years, perhaps 200 full
charge/discharge cycles).
• They are expensive (perhaps $2,000 for the battery pack shown in
the sample car).
14. Charging an Electric Car
• Any electric car that uses batteries needs a charging system to
recharge the batteries. The charging system has two goals:
• To pump electricity into the batteries as quickly as the batteries will
allow
• To monitor the batteries and avoid damaging them during the
charging process
• The most sophisticated charging systems monitor battery voltage,
current flow and battery temperature to minimize charging time.
The charger sends as much current as it can without raising battery
temperature too much. Less sophisticated chargers might monitor
voltage or amperage only and make certain assumptions about
average battery characteristics. A charger like this might apply
maximum current to the batteries up through 80 percent of their
capacity, and then cut the current back to some preset level for the
final 20 percent to avoid overheating the batteries.
