Generators

Last updated: February 27, 2009.

Oil may be the world's favorite fuel, but not for much longer. Modern homes are powered mostly by electricity and it won't be much longer before most of us are driving electric cars as well. Electricity is superbly convenient. You can produce it in all kinds of different ways using everything from coal and oil to wind and waves. You can make it in one place and use it on the other side of the world if you want to. And, once you've produced it, you can store it in batteries and use it days, weeks, months, or even years later. What makes electric power possible—and indeed practical—is a superb electromagnetic device called an electricity generator: a kind of electric motor working in reverse that converts ordinary energy into electricity. Let's take a closer look at generators and find out how they work!

Photo: A typical electricity generator. This one can make up to 225kW of electric power and is used for testing prototype wind turbines. Photo by Lee Fingersh courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Where does electricity come from?

Photo: Most of our electricity comes from power plants flowing through huge electrical equipment like this, but virtually all of it starts off in a generator. Photo by courtesy of US Department of Energy.

The best way to understand electricity is to start by giving it its proper name: electrical energy. If you want to run anything electrical, from a toaster or a toothbrush to an MP3 player or a television, you need to feed it a steady supply of electrical energy. Where are you going to get that from? There's a basic law of physics called the conservation of energy that explains how you can get energy—and how you can't. According to this law, there's a fixed amount of energy in the universe and some good news and some bad news about what we can do with it. The bad news is that we can't create more energy than we have already; the good news is that we can't destroy any energy either. All we can ever do with energy is convert it from one form into another.

If you want to find some electricity to power your television, you won't be making energy out of thin air: the conservation of energy tells us that's impossible. What you'll be doing is using energy converted from some other form into the electrical energy you need. Generally, that happens in a power plant some distance from your home. Plug in your TV and electrical energy flows into it through a cable. The cable is much longer than you might think: it actually runs all the way from your TV—underground or through the air—to the power plant where electrical energy is being prepared for you from an energy-rich fuel such as coal, oil, gas, or atomic fuel. In these eco-friendly times, some of your electricity will also be coming from wind turbines, hydroelectric power plants (which make power using the energy in dammed rivers), or geothermal energy (Earth's internal heat). Wherever your energy comes from, it'll almost certainly be turned into electricity with the help of a generator. Only solar cells make electricity without using generators.

Photo: A large electricity generator driven by steam at CalEnergy's Leathers geothermal power plant in Imperial County, California. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

How generators work

Photo: A motor from an electric lawn mower turns electrical energy into motion. An electricity generator has exactly the same components but works in the opposite way, turning motion into electrical energy.

If you've read our detailed article about electric motors, you'll already know pretty much how generators work: a generator is essentially just an electric motor working in reverse. If you've not read that article, you might like to take a quick look before reading on— but here's a quick summary either way. An electric motor is essentially just a tight coil of copper wire wrapped around an iron core that's free to rotate at high speed inside a powerful, permanent magnet. When you feed electricity into the copper coil, it becomes a temporary, electrically powered magnet—in other words, an electromagnet—and generates a magnetic field all around it. This temporary magnetic field pushes against the magnetic field that the permanent magnet creates and forces the coil to rotate. By a bit of clever design, the coil can be made to rotate continuously in the same direction, spinning round and round and powering anything from an electric toothbrush to an electric train.

So how is a generator different? Suppose you have an electric toothbrush with a rechargeable battery inside. Instead of letting the battery power the motor that pushes the brush, what if you did the opposite? What if you turned the brush back and forth repeatedly? What you'd be doing would be manually turning the electric motor's axle around. That would make the copper coil inside the motor turn around repeatedly inside its permanent magnet. If you move an electric wire inside a magnetic field, you make electricity flow through the wire—in effect, you generate electricity. So keep turning the toothbrush long enough and, in theory, you would generate enough electricity to recharge its battery. That, in effect, is how a generator works. (Actually, it's a little bit more tricky than this and you can't actually recharge your toothbrush this way, though you're welcome to try!)

In practice, you need to put in a huge amount of physical effort to generate even small amounts of electricity. You'll know this if you have a bicycle with dynamo lights powered from the wheels: you have to pedal somewhat harder to make the lights glow—and that's just to produce the tiny amount of electricity you need to power a couple of torch bulbs. A dynamo is simply a very small electricity generator. At the opposite extreme, in real power plants, gigantic electricity generators are powered by steam turbines. These are a bit like spinning propellers or windmills driven using steam. The steam is made by boiling water using energy released from burning coal, oil, or some other fuel. (Note how the conservation of energy applies here too. The energy that powers the generator comes from the turbine. The energy that powers the turbine comes from the fuel. And the fuel—if it's coal or oil—originally came from plants powered by the Sun's energy. The point is simple: energy always has to come from somewhere.)

Portable generators

Most of the time we take electricity for granted. We switch on lights, TVs, or clothes washers without stopping to think that the electrical energy we're using has to come from somewhere. But what if you're working outdoors, in the middle of nowhere, and there's no electricity supply you can use to power your chainsaw or your electric drill?

One possibility is to use cordless tools with rechargeable batteries. Another option is go for pneumatic tools, such as jackhammers. These are entirely mechanical and powered by compressed air instead of electricity. A third option is to use a portable electricity generator. It's simply a small gasoline engine (petrol engine), similar to the compact engine you get on a motorcycle, with an electricity generator attached. As the engine chugs away, burning up gasoline, it pushes a piston back and forth, turning a generator and producing a steady electric current as its output. With the help of a transformer, you can use a generator like this to produce pretty much any voltage you need, anywhere you need it. As long as you have enough gasoline, you can make your own electricity supply indefinitely. But remember the conservation of energy: run out of gas and you run out of electricity as well!

Photo: A portable electricity generator powered by a diesel engine. Photo by JoAnn S. Makinano courtesy of US Air Force and Defense Imagery.