Electricity is the flow of electrons, but electrons can flow in several different ways, so there are several different kinds of electricity. If all the electrons flow in one direction --- like the cars on a one-way street --- it is direct current (DC), and if the electrons flow smoothly back and forth --- like the water in waves --- it is alternating current (AC). Generators are devices that convert mechanical energy into electricity, and generators can produce either DC or AC.
In 1820, Hans Christian Oersted discovered that when electricity flows through a wire magnetism is produced around the wire and when a wire moves through a magnetic field electricity starts to flow in the wire (the principle that generators work on). In generators --- both AC and DC --- the part that moves is really a shaft upon which there are thousands of loops of wire. Two things are needed to generate electricity: something has to turn the shaft and some means must be used to get the electricity out of the rotating loops and then out of the generator. The force that turns the shaft often is driven by steam, falling water or wind power.
DC Generator Components
All generators have the same three components: the stator, the rotor and some way (usually brushes riding on rings) to get the induced electricity from the rotor to the world outside the generator. The only difference between an AC generator and a DC generator is how the electricity is taken out of the generator. The stator is the fixed outer part of the generator that provides the magnetism. In small DC motors this is a permanent magnet. In larger generators this can be an electromagnet powered by the electricity that is being produced (it is necessary to turn these a few times before they start working). The rotor is the rotating part that is turned by mechanical energy. The rotor contains a single wire that is looped many times.
Rings and Brushes
Both AC and DC generators produce electricity in the same way --- it is how the electricity is taken out of the generator that makes a generator AC or DC. AC generators have two rings --- one connected to each end or the rotor winding. As the generator rotates, the induced current first flows in one direction and then switches and flows in the other direction as the rotor approaches the north and south poles of the magnet. The DC generator has one ring with two gaps. Each half-ring is connected to one end of the rotor winding. The two brushes are touching opposite sides of the split ring. No matter which pole the rotor is approaching, the same brush is always positive and other brush is always negative because each brush takes the current off the half of the ring that has the correct polarity. This means that the output of a DC generator is DC current.
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