How Electric Cars Work?
or
What Makes the EV Tick?
by Marshall Brain
A Short Explanation
Electric cars are something that show up in the news all the time.
So - what is inside these unique creatures?
The heart of an electric car is the combination of:
- The electric motor
- The motor's controller
- A Link from the throttle to the controller
- The batteries
Electric cars can use AC or DC motors:
A simple DC controller connected to the batteries and the DC motor.
If the driver floors the accelerator pedal, the controller delivers the
full 96 volts from the batteries to the motor. If the driver takes his/her
foot off the accelerator, the controller delivers zero volts to the motor.
For any setting in between, the controller "chops" the 96 volts thousands of
times per second to create an average voltage somewhere between 0 and 96 volts.
A majority of the electric cars on the road today are "home brew" conversion vehicles.
A typical conversion uses a DC controller and a DC motor.
An AC controller hooks to an AC motor. Using six sets of power transistors, the
controller takes in 300 volts DC and produces 240 volts AC, 3-phase. The controller
additionally provides a charging system for the batteries, and a DC-to-DC converter
to recharge the 12-volt accessory battery.
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 or higher, 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 can overdrive them (up to a factor of 10-to-1) for short
periods of time.
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 regen feature. During braking, the motor turns into a generator and delivers power
back to the batteries.
The DC-to-DC converter is normally a separate box under the hood, but sometimes this
box is built into the controller.
Any electric car that uses batteries needs a charging system to recharge the batteries.
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.
Usually, the person doing the conversion has a "donor vehicle" that will act as the
platform for the conversion. Almost always, the donor vehicle is a normal gasoline-powered
car that gets converted to electric. Most donor vehicles have a manual transmission.
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Charging Current
When lead-acid batteries are at a low state of charge, nearly all the
charging current is absorbed by the chemical reaction. Once the state of charge reaches
a certain point, at about 80 percent of capacity, more and more energy goes into heat
and electrolysis of the water. The resulting bubbling of electrolyte is informally
called "boiling." For the charging system to minimize the boiling, the charging
current must cut back for the last 20 percent of the charging process.
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The person doing the conversion has a lot of choices when it comes to battery technology.
Lead Acid, Nickel-Metal Hydride, Lithium Ion, & Lithium Polymer. The vast majority of home
conversions use lead-acid batteries, and there are several different options: Marine
deep-cycle lead-acid batteries, Golf-cart batteries, and High-performance sealed batteries.
The batteries can have a flooded, gelled or AGM (absorbed glass mat) electrolyte.
Flooded batteries tend to have the lowest cost but also the lowest peak power.
The EV Challenge (www.evchallenge.org) is an innovative
educational program for middle and high school students that centers around building
electric-powered cars.
Great Links
For More information, Contact your local EV Organization:
The Electric Vehicle Society of Canada is a non-profit group. We are concerned with clean, safe,
and innovative electric transportation. We Hold monthly meetings at Centennial College, Toronto,
on the third Thirsday, except July & August.
The Electric Vehicle Society of Canada
21 Burritt Road, Toronto, Ontario M1R 3S5
Howard Hutt, President
416-755-4324 or
hwhutt@rogers.com
We offer Yearly Membership to Corporations & Schools for $100.00 Individuals at $30.00,
Seniors for $20.00, and Students are Free.
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