EGEE 401
Energy in a Changing World

Motion to Electrical

PrintPrint

Electrical Generation

electricity generator driven by steam
Figure 2.5: A large electricity generator driven by steam at CalEnergy's Leathers geothermal power plant in Imperial County, California (from Explain That Stuff!).
Credit: Photo by Warren Gretz, courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Electrical generation is the transformation of motion energy into electrical energy.

An electrical generator is a machine that converts mechanical energy into electricity. It works on the phenomenon of electromagnetic induction, discovered by Michael Faraday nearly 200 years ago. When an electrical current runs through a wire, a magnetic field is created around it. Likewise, if a changing magnetic field is created around a wire, electricity will move through the wire. This is accomplished through the relative motion of a magnet and wire.

"Electric generators are essentially very large quantities of copper wire spinning around inside very large magnets, at very high speeds. A commercial utility electric generator—for example, a 180–megawatt generator at the Hawaiian Electric Company's Kahe power plant on Oahu—can be quite large. It is 20 feet in diameter, 50 feet long, and weighs over 50 tons. The copper coils (called the "armature") spin at 3600 revolutions per minute.” (Source: The Electricity Forum)

I also like this explanation from Explain That Stuff!, after making the point that a generator is essentially an electric motor working in reverse, the article continues:

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 [flashlight] 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.) "

This should give you a good idea of how the pieces fit together: whenever we can get something to turn, we can make electricity. Steam, falling water, and wind are examples of ways to turn a turbine, spinning a wire inside a magnetic, generating electricity!