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DC Motor

From http://www.howstuffworks.com/motor.htm

Parts of an Electric Motor

 

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 at the right:

  • An armature or rotor
  • A commutator
  • Brushes
  • An axle
  • A field magnet
  • A DC power supply of some sort

 

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 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. You can learn more about electromagnets by reading How an Electromagnet Works.

 

The basic idea behind an electromagnet is extremely simple: by running electric current through a wire you can create a magnetic field. By using this simple principle you can create all sorts of things, including motors, solenoids, read/write heads for disk and tape drives, speakers, and so on.

How does an Electromagnet Work?

An electromagnet starts with a battery (or some other source of power) and a wire. What a battery produces is electrons. If you look at a battery, say at a normal D cell from a flashlight, you can see there are two ends marked plus (+) and minus (-). Electrons collect at the negative end of the battery, and, if you let them, they will gladly flow to the positive end. The way you "let them" flow is with a wire. If you attach a wire directly between the positive and negative terminals of a D cell, 3 things will happen:

    1. Electrons will flow from the negative side of the battery to the positive side as fast as they can.
    2. The battery will drain fairly quickly (in a matter of several minutes). For that reason it is generally not a good idea to connect the two terminals of a battery to one another directly. Normally you connect some kind of load in the middle of the wire so the electrons can do useful work. The load might be a motor, a light bulb, a radio or whatever.
    3. A small magnetic field is generated in the wire. It is this small magnetic field that is the basis of an electromagnet.

You probably knew about items 1 and 2 in this list, but item 3 might be a surprise to you. Yet this definitely happens in all wires carrying electricity. You can prove it to yourself with the following experiment. For the experiment you will need:

    1. An AA, C or D cell (a normal flashlight battery)
    2. A piece of wire (if you have no wire around the house, go buy a spool of insulated thin copper wire down at the local Radio Shack or hardware store. 4-strand telephone wire is perfect - cut the outer plastic sheet and you will find four perfect wires within).
    3. A compass

Put the compass on the table and, with the wire near the compass, connect the wire between the positive and negative ends of the battery for a few seconds. What you will notice is that the compass needle swings. Initially the compass will be pointing toward the earth's North Pole (whatever direction is for you), as shown in the figure on the right. When you connect the wire to the battery, the compass needle swings because the needle is itself a small magnet with a north and south end. Being small it is sensitive to small magnetic fields. Therefore the compass is affected by the magnetic field created in the wire by the flow of electrons.

The figure below shows the shape of the magnetic field around the wire. In this figure, imagine that you have cut the wire and are looking at it end-on. The green circle in the figure is the cross-section of wire itself. A circular magnetic field develops around the wire, as shown by the circular lines. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire). You can see that the field is perpendicular to the wire and the field has a direction that depends on which direction of the current flowing in the wire. The compass needle aligns itself with this field (perpendicular to the wire). If you flip the battery around and repeat the experiment, you will see that the compass needle aligns itself in the opposite direction.

Because the magnetic field around a wire is circular and perpendicular to the wire, an easy way to amplify the wire's magnetic field is to coil the wire, as shown below:


For example, if you wrap your wire around a nail 10 times, connect the wire to the battery and bring one end of the nail near the compass, you will find that it has a much larger effect on the compass. In fact, the nail behaves just like a bar magnet. However, the magnet exists only when the current is flowing from the battery. What you have created is an electromagnet. You will find that this magnet is able to pick up small steel things like paper clips, staples and thumb tacks.

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 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 to the left 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 "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 have the chance to take apart a small 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:

  1. It causes the motor to have better dynamics. In a two-pole motor, if the electromagnet is at the balance point, perfectly horizontal between the two poles of the field magnet when the motor starts, you can imagine the armature getting "stuck" there. That never happens in a three-pole motor.
  2. Each time the commutator hits the point where it flips the field in a two-pole motor, the commutator shorts out the battery (directly connects the positive and negative terminals) for a moment. This shorting wastes energy and drains the battery needlessly. A three-pole motor solves this problem as well.

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.