Chapter XXIV

THE ELECTRIC MOTOR, AND HOW IT DOES WORK.

=160. Currents and Motion.= We have seen, Chapter XII., that when coils of wire are rapidly moved across a strong magnetic field, a current of electricity is generated. We have now to deal with the opposite of this; that is, we are to study how _motion_ can be produced by allowing a current of electricity to pass through the armature of a machine.

Fig. 224 shows, by diagram, a coil H, suspended so that it can move easily, its ends being joined to a current reverser, and this, in turn, to a dry cell D C. A magnet, H M, will attract the core of H when no current passes. When the current is allowed to pass first in one direction and then in the opposite direction, by using the reverser, the core of H will jump back and forth from one pole of H M to the other. There are many ways by which motion can be produced by the current, but to have it practical, the motion must be a rotary one. (See "Study," Chapter XXVI., for numerous experiments.)

=161. The Electric Motor= is a machine for transforming electric energy into mechanical power. The construction of motors is very similar to that of dynamos. They have field-magnets, armature coils, commutator, etc.; in fact, the armature of an ordinary direct current dynamo will revolve if a current be passed through it, entering by one brush and leaving by the other. There are many little differences of construction, for mechanical and electrical reasons, but we may say that the general construction of dynamos and motors is the same.

Fig. 225 shows a coil of wire, the ends of which are connected to copper and zinc plates. These plates are floated in dilute sulphuric acid, and form a simple cell which sends a current through the wire, as shown by the arrows.

We have seen that a current-carrying wire has a magnetic field and acts like a magnet; so it will be easily seen that if a magnet be held near the wire it will be either attracted or repelled, the motion depending upon the poles that come near each other. As shown in the figure, the N pole of the magnet repels the field of the wire, causing it to revolve. We see that this action is just the reverse to that in galvanometers, where the coil is fixed, and the magnet, or magnetic needle, is allowed to move. As soon as the part of the wire, marked A in Fig. 225, gets a little distance from the pole, the opposite side of the wire, B, begins to be attracted by it, the attraction getting stronger and stronger, until it gets opposite the N pole. If the N pole were still held in place, B would vibrate back and forth a few times, and finally come to rest near the pole. If, however, as soon as B gets opposite N the S pole of the magnet be quickly turned toward B, the coil will be repelled and the rotary motion will continue.

Let us now see how this helps to explain electric motors. We may consider the wire of Fig. 225 as one coil of an armature, and the plates, C and Z, as the halves of a commutator. In this arrangement, it must be noted, the current always flows through the armature coil in the same direction, the rotation being kept up by reversing the poles of the field-magnet. In ordinary simple motors the current is reversed in the armature coils, the field-magnets remaining in one position without changing the poles. This produces the same effect as the above. The current is reversed automatically as the brushes allow the current to enter first one commutator bar and then the opposite one as the armature revolves. The regular armatures have many coils and many commutator bars, as will be seen by examining the illustrations shown.

The ordinary galvanometer may be considered a form of motor. By properly opening and closing the circuit, the rotary motion of the needle can be kept up as long as current is supplied. Even an electric bell or telegraph sounder may be considered a motor, giving motion straight forward and back.

=162. The Uses of Motors= are many. It would be impossible to mention all the things that are done with the power from motors. A few illustrations will give an idea of the way motors are attached to machines.

Fig. 226 shows one form of motor, the parts being shown in Fig. 227.

Fig. 228 shows a fan motor run by a battery. They are generally run by the current from the street. Figs. 229-231 show other forms of fan motors. Fig. 232 shows an electric hat polisher. A church organ bellows is shown in Fig. 233, so arranged that it can be pumped by an electric motor. Fig. 234 shows a motor direct connected to a drill press.

=163. Starting Boxes.= If too much current were suddenly allowed to pass into the armature of a motor, the coils would be over-heated, and perhaps destroyed, before it attained its full speed. A rapidly revolving armature will take more current, without being overheated, than one not in motion. A motor at full speed acts like a dynamo, and generates a current which tends to flow from the machine in a direction opposite to that which produces the motion. It is evident, then, that when the armature is at rest, all the current turned on passes through it without meeting with this opposing current.

Fig. 235 shows a starting, stopping, and regulating box, inside of which are a number of German-silver resistance coils properly connected to contact-points at the top. By turning the knob, the field of the motor is immediately charged first through resistance, then direct, and then the current is put on the armature gradually through a series of coils, the amount of current depending upon the distance the switch is turned. Fig. 236 shows a cross section of the same.