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Let's start by looking at the overall plan of a simple 2-pole DC electric motor. A simple motor has 6 parts, as shown in the diagram below.
An electric motor is all about magnets and magnetism: a motor uses magnets to create motion. If you have ever played with magnets you know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have 2 bar magnets with their ends marked North and South, then the North end of one magnet will attract the South end of the other. On the other hand, the North end of one magnet will repel the North end of the other (and similarly South will repel South). Inside an electric motor these attracting and repelling forces create rotational motion.
In the diagram above and below you can see two magnets in the motor, the armature (or rotor) is an electromagnet, while the field magnet is a permanent magnet (the field magnet could be an electromagnet as well, but in most small motors it is not to save power).
Electromagnets and Motors
To understand how an electric motor works, the key is to understand how the electromagnet
works. An electromagnet is the basis of an electric motor. You can understand how things
work in the motor by imagining the following scenario. Say that you created a simple
electromagnet by wrapping 100 loops of wire around a nail and connecting it to a battery.
The nail would become a magnet and have a North and South pole while the battery is
connected. Now say that you take your nail electromagnet, run an axle through the middle
of it, and you suspended it in the middle of a horseshoe magnet as shown in the figure
below. If you were to attach a battery to the electromagnet so that the North end of the
nail appeared as shown, the basic law of magnetism tells you what would happen: The North
end of the electromagnet would be repelled from the north end of the horseshoe magnet and
attracted to the south end of the horseshoe magnet. The South end of the electromagnet
would be repelled in a similar way. The nail would move about half a turn and then stop in
the position shown.
You can see that this half-turn of motion is simple and obvious because of the way magnets naturally attract and repel one another. The key to an electric motor is to then go one step further so that, at the moment that this half-turn of motion completes, the field of the electromagnet flips. The flip causes the electromagnet to complete another half-turn of motion. You flip the magnetic field simply by changing the direction of the electrons flowing in the wire (you do that by flipping the battery over). If the field of the electromagnet flipped at just the right moment at the end of each half-turn of motion, the electric motor would spin freely.
The Armature
The armature takes the place of the nail in an electric motor. The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core. The armature has an axle, and the commutator is attached to the axle. In the diagram above you can see three different views of the same armature: front, side and end-on. In the end-on view the winding is eliminated to make the commutator more obvious. You can see that the commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet.
The Commutator and brushes
The "flipping the electric field" part of an electric motor is accomplished by two parts: the commutator and the brushes. The diagram at the right shows how the commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator.
Putting It All Together
When you put all of these parts together, what you have is a complete electric motor:
In this figure, the armature winding has been left out so that it is easier to see the commutator in action. The key thing to notice is that as the armature passes through the horizontal position, the poles of the electromagnet flip. Because of the flip, the North pole of the electromagnet is always above the axle so it can repel the field magnet's North pole and attract the field magnet's South pole.
If you ever take apart an electric motor you will find that it contains the same pieces described above: two small permanent magnets, a commutator, two brushes and an electromagnet made by winding wire around a piece of metal. Almost always, however, the rotor will have three poles rather than the two poles as shown in this article. There are two good reasons for a motor to have three poles:
It is possible to have any number of poles, depending on the size of the motor and the specific application it is being used in.
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