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An Electric Motor is a device for transforming electricity into mechanical power. A generator, or dynamo, is constructed in almost the same identical manner as a motor but its purpose is just the opposite. A dynamo transforms mechanical power into electricity. A dynamo produces electric current, but a motor consumes it. Some machines can be used either as a motor or dynamo—not all however.

Of course most experimenters have in all probability seen many electric motors, but it is more than likely that the exact operation is not thoroughly understood. Here is your chance to learn.

The little motors described in this book can each be made in two or three hours out of a few scraps of sheet iron, magnet wire and screws. The cost of the necessary materials is practically negligible.

One of the main advantages of these little motors is that they illustrate the actual principles that are used in the large motors, such as are employed everywhere for practicable power purposes.

The iron parts may be made out of sheet iron or the ordinary so-called "tin" used in cocoa cans, etc. Thin tin can be cut with an ordinary pair of shears. Sheet iron such as is used in making stovepipes, etc., is an excellent material to use in making these little motors. Sheet iron is usually heavier than tin and will have to be cut with a pair of "snips." Greater skill will also then be required in bending the parts. It is worth while noting however, that the extra difficulty involved in using the heavier material is worth the trouble because it makes possible a more powerful and efficient motor.

The first and easiest type of motor to make is the "Simplex."

The Principle on which an Electric Motor Operates is really very simple. If a current of electricity is passed through a copper wire, the wire will attract to itself iron filings, etc., as long as the current continues to flow. As soon as the current is shut off, the filings drop away because the magnetism immediately disappears with the cessation of the current.

FIG. 1.—If a current of electricity is passed through a wire, the wire will attract to itself iron filings.

If a wire, carrying a current of electricity is formed into a loop, the entire space enclosed by the loop will possess the properties of a magnet.

By forming the wire into several loops or a spiral the combined effect of all the individual turns is concentrated in a small space and a much more powerful field is produced. If the coil is provided with an iron core, the magnetism is much more concentrated and will exercise a very powerful attractive effect upon any neighboring masses of iron or steel. Such a coil is called an electromagnet.

FIG. 2.—If a wire carrying a current of electricity is formed into a loop, the space enclosed by the loop will become magnetic. The arrows represent the paths of the lines of magnetic force.

Electromagnets play a very important part in the construction of electric motors. The strength of an electro magnetic coil is proportional to its ampere turns. The number of ampere turns in a coil is obtained by multiplying the number of amperes flowing through the coil by the number of turns of wire composing it.

FIG. 3.—By forming the wire into several loops or a spiral so that the effect of the individual turns is concentrated in a small space, an Electromagnet is made.

You can easily see the effect of turns of wire on an electromagnet by winding two or three turns of wire around a nail and connecting it to a battery. These two or three turns will probably create enough magnetism to enable the nail to lift up one or two ordinary carpet tacks.

FIG 4—The strength of an electromagnet is proportional to the ampere turns. The magnet illustrated above does not possess sufficient turns to be very strong.

Then increase the number of turns to forty or fifty and note that the magnetism of the nail has increased greatly and that it now possesses power to pick up a larger number of tacks at a time.

From this one may be led to believe that the more turns of wire an electromagnet possesses, the stronger it will be, and while to a certain extent this is true, it should be remembered that it is not simply turns that count but ampere turns and if the number of turns of wire is increased beyond a certain point the resistance of the coil to the electric current will become so great that the current in amperes flowing through the coil is greatly reduced and consequently also the magnetism is decreased.

FIG. 5.—An increase in the number of turns of wire has resulted in considerable increase in the magnetism and this magnet is able to lift a much greater weight than that shown in Figure 4.

It will be found that the magnetism of an electromagnet is strongest at the ends. These places are called the poles.

If you bring one pole of a small electromagnet, formed by winding a nail with a few turns of wire, near a compass needle, you will find that it will attract one end of the compass needle and repel the other. The end of the compass needle which points North is called a North pole. The ends of the electromagnet which attracts the North pole of the compass needle is a South pole.

One of the most important laws of magnetism is that like poles repel each other and unlike poles attract each other. A North and a South pole therefore tend to pull toward each other, whereas two North poles or two South poles repel one another.

Figure 6 illustrates the principle of an electric motor.

It consists of a bar of iron marked "A" called the Armature and wound with a coil of wire called the armature winding. The armature is the part of the motor which revolves.

FIG. 6.—The Principle of the Electric Motor.

Each end of the armature winding is connected to one half of a brass ring called the commutator and marked "C, C," in the illustration. The two halves of the commutator are insulated from each other and are mounted on the armature shaft so that they revolve together with the armature.

The armature revolves between the ends of a horseshoe shaped piece of iron called the field. The field is also wound with a coil of wire called the field winding or sometimes the field coil.

The armature and the field are both electromagnets.

Two strips of copper, "B, B," bear against the commutator. These are the brushes, and their purpose is to lead the current to the armature coil.

One brush is connected to one end of the field coil. The other end of the field coil and the other brush are connected to a source of electric current.

As soon as the current is turned on, the armature and the field both become magnets. The North pole of the field attracts the South pole of the armature and vice-versa. The armature starts to move so that the poles will come opposite but as the commutator moves around and is turned over, the current flows through the armature coil in the opposite direction. This reverses the magnetism of the armature and that which was the South pole become the North pole and vice-versa.

FIG. 7.—Diagrams showing the difference between a Shunt and a Series Motor.

The armature poles will therefore have to move 180 degrees in order that the South pole may come opposite the North pole of the field. Before it gets there, however, the commutator will have turned over again, reversing the current in the armature and making it necessary to continue its journey again. This process keeps up and so the armature revolves always trying to seek a new position which it is prevented from remaining at by the action of the commutator.

Motors are said to be series or shunt wound depending on whether all the current flowing through the armature also passes through the field or whether it divides between the two as shown in Figure 7.