Figs. 6, 7, 8.—Diagrams illustrating the Principles of Electro-Magnets.
In [Fig. 6] W represents a coil of wire provided with a cotton covering, so that there may be no actual contact between the adjoining convolutions. If the ends p n of this coil are connected with a source of electric energy, an electric current will flow through it, and if a bar, as indicated by N P, of iron or steel is placed within the coil it will become magnetized. If the bar is made of steel and is hardened it will retain the magnetism, and become what is called a permanent magnet; such a magnet, in fact, as we have considered in all the previous figures. If the bar is made of iron it will not retain the magnetism, but will only be a magnet as long as the electric current flows through the coil W. A magnet of the latter type is called an electro-magnet. If the iron is of poor quality—that is, from an electrical standpoint—it will require an appreciable time to lose its magnetism, but if it is soft and high grade, electrically considered, it will lose its magnetism instantly, or nearly so. If we take two bars of soft iron and arrange them side by side, as in [Fig. 7], and wind coils around them as indicated each one will become magnetized when the ends p n of the coils are connected with an electric circuit. If the lower ends of the two bars are joined by a piece, as shown at M, we will have a horseshoe electro-magnet. If we take a round disk of iron, as in [Fig. 8], and wind a coil around it, it will also become a magnet when an electric current traverses the coil. Thus it will be seen that it makes little difference what the shape of the iron may be, providing it is surrounded by a coil of wire and an electric current is passed through the latter. This being the case, it is evident that either of the processes explained in connection with [Figs. 4 and 5] can be made available for the production of a continuous rotation by the aid of electro-magnets. Suppose we make a drum, as shown in [Fig. 9], and wind a wire coil around it in the direction indicated, then when a current passes through the wire the drum will be magnetized, with poles at top and bottom. If the electric current passes through the wire from end p to end n the drum will be magnetized positively at the top and negatively at the bottom, and if the direction of the current through the wire is reversed the polarity of the drum will be reversed. If we construct a horseshoe magnet of the shape shown in [Fig. 10], and place within the circular opening between its ends the drum of [Fig. 9], we will have a device that is capable of developing a continuous rotation, providing we have suitable means for reversing the direction of the electric current through the wire coil; and this machine constitutes an electric motor in its simplest form.
Figs. 9, 10.—Diagrams illustrating the Principles of the Electric Motor.
In an electric motor the horseshoe magnet is called the field magnet, and the rotating part is called the armature, while the device by means of which the direction of the current through the armature coil is reversed is called the commutator. In this last figure it will be noticed that the coils wound upon the field magnet are represented as of wire much finer than that wound upon the armature. In actual practice machines are sometimes wound in this way, and sometimes the field wire is twice as large as that on the armature. When the field wire is very much finer than that of the armature the machine is what is known as shunt wound, which means that only a small portion of the current that passed through the armature passes through the field coils. Although with this type of winding the current that passes through the field coils is very weak, the magnetism developed thereby can be made greater than that of the armature if desired. This result is accomplished by increasing the number of turns of wire in the field coils. Thus if the current through the armature is one hundred times as strong as that through the field coils, the latter can be made to equal the effect of the former by increasing the number of turns in the proportion of one hundred to one, and if the increase is still greater the field coils will develop the strongest magnetism. The reason why a small current passing around a magnet a great many times will develop as strong a magnetization as a large current, can be readily understood when we say that the magnetism is in proportion to the total strength of the electric current that circulates around the magnet. Suppose we have two currents, one of which is one thousand times as strong as the other, then if the weak one is passed through a coil consisting of one thousand turns it will develop just as strong a magnetization as the large current passing through a coil of only one turn. This last explanation enables us to see how it is that the comparatively small current that can pass through the contact between the trolley wire and the trolley wheel can develop in the motor force sufficient to propel a heavy car up a steep grade. When that small current reaches the car motors it passes through a thousand or more turns of wire, and thus its effect is increased a corresponding number of times.
A motor having a single coil of wire upon the armature, as in [Fig. 10], would not give very satisfactory results, owing to the fact that the rotative force developed by it would not be uniform. Such motors are made in very small sizes, but never when a machine of any capacity is required. For large machines it is necessary to wind the armature with a number of coils, so that the rotating force may be uniform, and also so that the current may be reversed by the commutator without producing sparks so large as to destroy the device. When an armature is wound with a number of coils the direction of the current is reversed, by the commutator, in each coil as it reaches the point where its usefulness ends, and where, if it continued to flow in the same direction, it would act to hold the armature back. The effect of this reversal of the current in one coil after another is to maintain the polarity of the armature practically at the same point, so that the strongest pull is exerted between it and the field magnet poles at all times. To explain clearly the way in which the commutator reverses the current in one coil at a time it will be necessary to make use of a diagram illustrating what is called a ring armature. Such a diagram is shown in [Fig. 11]. The ring A is the armature core, and is made of iron; the wire coils are represented as consisting of one turn to each coil, and are marked w w w. The current enters the wire through the spring B, and passes out through C. As can be seen, the current from B can flow through the coils w w in both directions, thus dividing into two currents, each one of which will traverse one half of the wire wound upon the armature. The two half currents will meet at C. If the armature is rotated the springs B and C (which are called commutator brushes) will pass from one turn of the wire coil to another just back of it as the rotation progresses, and each time that contact is made with a new turn the direction of the current in the turn just ahead will be reversed. The current in the wire as a whole, however, will always be in the same direction—that is, in all the turns to the right of the two brushes; the current will flow toward the center of the shaft on the front side of the armature, and away from the shaft in all the turns on the left side. As the direction of the current on opposite sides of the brushes is always the same, the poles of the armature will remain under B and C, therefore the relation between the position of the poles of the armature and the field magnet will be the same substantially as that illustrated in [Fig. 10], and, as a result, the force tending to produce rotation will at all times be the greatest possible for the strength of the current used and the size of the magnets.
Armatures are wound with a number of turns of wire in each coil, unless the machine is very large, and present an appearance more like [Fig. 12]. In this figure the brushes are arranged to make contact with the outer surface of the ring C, which is the commutator. The segments s s are connected with the ends of the armature coils c c c, but are separated from each other by some kind of material that will not conduct electricity—that is, they are electrically insulated. As will be noticed from this, the armature in [Fig. 11] acts as a commutator as well as an armature, its outer surface performing the former office. In the winding the difference between [Figs. 11 and 12] is simply in the number of turns in each coil, there being one turn in [Fig. 11] and several in [Fig. 12].
Figs. 11, 12.—Diagrams illustrating the Method of winding Armatures of Electric Motors and Generators.
The armature shown in [Fig. 10] is of the type called drum armature, but it can be wound so as to produce the same result as the ring, although it is not so easy to explain this style of winding. It will be sufficient for the present explanation to say that whatever type of armature may be used, the winding is always such that the direction of the current through the wire coils is reversed progressively, so that the magnetic polarity is maintained practically at the same point; therefore there is a continuous pull between this point of the armature core and the poles of the field magnet. The commutator is secured to the armature shaft, and the brushes through which the current enters and leaves are held stationary; keeping this fact in mind, it can be seen at once that in [Fig. 12] the current will flow from the brush a through the two sides of the armature wire to brush b, hence all the coils on the right of the vertical line will be traversed by the current in the same direction—that is, either to or from the center of the shaft—and in the coils on the left the direction will be opposite, which is just the same order as was explained in connection with [Fig. 11].