Why do cars use DC current?
AC vs DC
There are two types of electrical current, AC and DC. One flows back and forth alternating directions: alternating current. The other flows consistently in the same direction direct current. This page discusses the differences between the two, along with the advantages each carries along with it.
Brief summary of AC and DC
- Alternating current is produced within most types of power plants by spinning generators. The direction of current reverses, or alternates, 50-60 times per second depending on a country’s standards. [1] Alternating current is the current that travels through power lines and comes through the power outlets found throughout a home or building. There are various reasons why AC was the current of choice to perform this task, which will be discussed below.
- Direct current is produced by power sources like batteries, fuel cells, and solar panels. Such power sources have two terminals that are positive and negative respectively, which creates a relatively constant voltage for electrons to flow through. Current always flows in the same direction between these two terminals. [1]
Figure 1. An animation from a PhET simulation of alternating current which has been slowed down considerably. [2]
Figure 2. An animation from a PhET simulation of direct current which has been slowed down considerably. [3]
Advantages of each
Alternating current uses of varying voltage and flow of electrons within a conductor. Direct current flows in one direction and with a relatively constant current (charge flowing by a point per unit time). The way each can be manipulated, however, is what is important, and provides clear advantages for certain applications among the two.
Advantages of AC
AC is the current of choice for power plants and the electrical grid as a whole. When a plug is connected to an electrical outlet, alternating current comes out, providing power to countless devices like light bulbs and refrigerators. AC is preferred for this application because:
There are cheap and reliable ways of increasing or decreasing the voltage using transformers, which minimizes power loss in electrical transmission.
Resistance reduces the energy transmitted in a wire. By increasing the voltage on the wires to very high voltages for long distance transmission, this loss can be reduced. The loss of power ( [math]P_[/math] ) is given by the equation: [4]
- [math]I[/math] is the current in amperes
- [math]R[/math] is the resistance in ohms
Increasing the voltages the grid transmits electricity reduces this lost power. As the voltage gets higher, the current decreases proportionally because the transmitted electrical power (energy per unit time) remains the same. For example, if the voltage is increased by a factor of 100, the current must decrease by a factor of 100 and the resulting power lost will be decreased by 100 2 = 10000. However there is a limit, being that at extremely high voltages (2000 kV) the electricity begins to discharge resulting in high losses. [4]
Efficient transmission saves power companies and consumers a lot of money, which helps reduce pollution since power plants do not need to make up for lost electricity by using more fuel.
Other advantages of AC include: [5]
- Low maintenance costs of high speed AC motors.
- Easy to interrupt the current (ie. with a circuit breaker) due to the current going to zero naturally every 1/2 cycle. For example, a circuit breaker can interrupt about 1/20th as much DC as AC current.
Advantages of DC
A big advantage of direct current is that it is easier to change the speed of a DC electric motor than it is for an AC one. This is useful in many applications, such as electric and hybrid cars. [5]
Direct current is used in essentially all consumer electronics, since transistors (the building blocks of modern electronics) rely on it to operate. Devices that use DC current include cell phones, laptops, TVs and much more.
Direct current may also be used to transmit electricity with even greater efficiency than alternating current over extremely large distances by use of HVDC transmission (high-voltage direct current). Along with higher efficiency, HVDC also allows for different AC systems (ie. 50 Hz and 60 Hz) to be connected. [6]
For Further Reading
For further information please see the related pages below:
- Electrical transmission
- Electrical grid
- Electrical generation
- Distribution grid
- Or explore a random page!
References
- ↑ 1.01.1 How Stuff Works. (Accessed December 30, 2015). Direct Current Versus Alternating Current [Online], Available: http://science.howstuffworks.com/electricity8.htm
- ↑http://phet.colorado.edu/sims/circuit-construction-kit/circuit-construction-kit-ac_en.jnlp
- ↑http://phet.colorado.edu/sims/circuit-construction-kit/circuit-construction-kit-ac_en.jnlp
- ↑ 4.04.1 R. Paynter and B.J. Boydell, «Transmission Lines and Substations» in Introduction to Electricity, 1st ed., Upper Saddle River, NJ: Pearson, 2011, ch.25, sec.3, pp.1102-1104
- ↑ 5.05.1 Private communication with M. Pigman power engineer for Tacoma Power, Sept. 17th, 2015.
- ↑ Spark Fun. (Accessed December 30, 2015). Alternating Current vs. Direct Current [Online], Available: https://learn.sparkfun.com/tutorials/alternating-current-ac-vs-direct-current-dc
How Electric Vehicles Work
Take a tour through an electric vehicle with us. You’ll be surprised at how straightforward they are.
How electric vehicles move
EV’s are like an automatic car. They have a forward and reverse mode. When you place the vehicle in gear and press on the accelerator pedal these things happen:
- Power is converted from the DC battery to AC for the electric motor
- The accelerator pedal sends a signal to the controller which adjusts the vehicle’s speed by changing the frequency of the AC power from the inverter to the motor
- The motor connects and turns the wheels through a cog
- When the brakes are pressed or the car is decelerating, the motor becomes an alternator and produces power, which is sent back to the battery
AC/DC and electric cars
AC stands for Alternating Current. In AC, the current changes direction at a determined frequency, like the pendulum on a clock.
DC stands for Direct Current. In DC, the current flows in one direction only, from positive to negative.
Battery Electric Vehicles
The key components of a Battery Electric Vehicle are:
- Electric motor
- Inverter
- Battery
- Battery charger
- Controller
- Charging cable
Electric motor
You will find electric motors in everything from juicers and toothbrushes, washing machines and dryers, to robots. They are familiar, reliable and very durable. Electric vehicle motors use AC power.
Inverter
An inverter is a device that converts DC power to the AC power used in an electric vehicle motor. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current. It can also increase or decrease the power or torque of the motor by adjusting the amplitude of the signal.
Battery
An electric vehicle uses a battery to store electrical energy that is ready to use. A battery pack is made up of a number of cells that are grouped into modules. Once the battery has sufficient energy stored, the vehicle is ready to use.
Battery technology has improved hugely in recent years. Current EV batteries are lithium based. These have a very low rate of discharge. This means an EV should not lose charge if it isn’t driven for a few days, or even weeks.
Battery charger
The battery charger converts the AC power available on our electricity network to DC power stored in a battery. It controls the voltage level of the battery cells by adjusting the rate of charge. It will also monitor the cell temperatures and control the charge to help keep the battery healthy.
Controller
The controller is like the brain of a vehicle, managing all of its parameters. It controls the rate of charge using information from the battery. It also translates pressure on the accelerator pedal to adjust speed in the motor inverter.
Charging cable
A charging cable for standard charging is supplied with and stored in the vehicle. It’s used for charging at home or at standard public charge points. A fast charge point will have its own cable.
EV versus ICE
The most significant difference between ICE (Internal Combustion Engine), BEVs and PHEVs is found in the powertrain, the components that generate motive (moving) power and deliver it to the wheels to move the car.
- ICE vehicles burn fuel (usually petrol or diesel) that releases heat to move parts of the engine and other components that deliver power to the wheels. The ignition starts this combustion process.
- BEVs use power stored as electricity in rechargeable batteries and deliver it via one or more electric motors to the wheels.
- PHEVs have both ICE and electric components, along with controls that manage the balance of electric and ICE power used while driving.
How it works
Through the ignition and combustion of a 15:1 air-fuel mix, the ICE engine converts thermal energy into mechanical energy and emits waste exhaust gases in the process. Improvements in efficiency and evolving emission standards, ICE technology has not changed much in the last 100 years.
There are hundreds of moving parts with tight tolerances that must work together to keep the combustion engine running. When the combustion process starts you can hear and feel the vibrations in the vehicle caused by the mechanical and hydraulic systems.
ICE engines produce power in a limited speed range and use gears to maintain acceleration. Fuel keeps burning as long as the engine is switched on, even when the car is idling.
Electrical powertrains convert electrical energy (stored in the battery), into mechanical energy which turns the motor, rotating the wheels. EVs have 90% fewer moving parts than ICE vehicles.
How Do Plug-In Hybrid Electric Cars Work?
Plug-in hybrid electric vehicles (PHEVs) use batteries to power an electric motor and another fuel, such as gasoline, to power an internal combustion engine (ICE). PHEV batteries can be charged using a wall outlet or charging equipment, by the ICE, or through regenerative braking. The vehicle typically runs on electric power until the battery is nearly depleted, and then the car automatically switches over to use the ICE. Learn more about plug-in hybrid electric vehicles.
Key Components of a Plug-In Hybrid Electric Car
Battery (auxiliary): In an electric drive vehicle, the low-voltage auxiliary battery provides electricity to start the car before the traction battery is engaged; it also powers vehicle accessories.
Charge port: The charge port allows the vehicle to connect to an external power supply in order to charge the traction battery pack.
DC/DC converter: This device converts higher-voltage DC power from the traction battery pack to the lower-voltage DC power needed to run vehicle accessories and recharge the auxiliary battery.
Electric generator: Generates electricity from the rotating wheels while braking, transferring that energy back to the traction battery pack. Some vehicles use motor generators that perform both the drive and regeneration functions.
Electric traction motor: Using power from the traction battery pack, this motor drives the vehicle’s wheels. Some vehicles use motor generators that perform both the drive and regeneration functions.
Exhaust system: The exhaust system channels the exhaust gases from the engine out through the tailpipe. A three-way catalyst is designed to reduce engine-out emissions within the exhaust system.
Fuel filler: A nozzle from a fuel dispenser attaches to the receptacle on the vehicle to fill the tank.
Fuel tank (gasoline): This tank stores gasoline on board the vehicle until it’s needed by the engine.
Internal combustion engine (spark-ignited): In this configuration, fuel is injected into either the intake manifold or the combustion chamber, where it is combined with air, and the air/fuel mixture is ignited by the spark from a spark plug.
Onboard charger: Takes the incoming AC electricity supplied via the charge port and converts it to DC power for charging the traction battery. It also communicates with the charging equipment and monitors battery characteristics such as voltage, current, temperature, and state of charge while charging the pack.
Power electronics controller: This unit manages the flow of electrical energy delivered by the traction battery, controlling the speed of the electric traction motor and the torque it produces.
Thermal system (cooling): This system maintains a proper operating temperature range of the engine, electric motor, power electronics, and other components.
Traction battery pack: Stores electricity for use by the electric traction motor.
Transmission: The transmission transfers mechanical power from the engine and/or electric traction motor to drive the wheels.
