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.
[Illustration: Fig. 224.]
[Illustration: Fig. 225.]
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.)
[Illustration: Fig. 226.]
=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.
[Illustration: Fig. 227.]
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.
[Illustration: Fig. 228.]
[Illustration: Figs. 229 to 231.]
[Illustration: Fig. 232.]
[Illustration: Fig. 233.]
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.
[Illustration: Fig. 234.]
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.
[Illustration: Fig. 235.]
[Illustration: Fig. 236.]
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.