HOW ELECTRICITY IS GENERATED BY DYNAMOS.
=126. The Dynamo=, _Dynamo-Electric Machine_ or _Generator_, is a
machine for converting mechanical energy into an electric current,
through electromagnetic induction. The dynamo is a machine that will
convert steam power, for example, into an electric current. Strictly
speaking, a dynamo creates electrical pressure, or electromotive force,
and not electricity, just as a force-pump creates water-pressure, and
not water. They are generally run by steam or water power.
[Illustration: Fig. 137.]
=127. Induced Currents.= We have already spoken about currents being
induced by moving a coil of wire in a magnetic field. We shall now
see how this principle is used in the dynamo which is a generator of
induced currents.
[Illustration: Fig. 138.]
Fig. 137 shows how a current can be generated by a bar magnet and
a coil of wire. Fig. 138 shows how a current can be generated by a
horseshoe magnet and a coil of wire having an iron core. The ends of
the coil are to be connected to an astatic galvanoscope; this forms a
closed circuit. The coil may be moved past the magnet, or the magnet
past the coil.
[Illustration: Fig. 139.]
[Illustration: Fig. 140.]
[Illustration: Fig. 141.]
[Illustration: Fig. 142.]
Fig. 139 shows how a current can be generated by two coils, H being
connected to an astatic galvanoscope and E to a battery. By suddenly
bringing E toward H or the core of E past that of H, a current is
produced. We have in this arrangement the main features of a dynamo.
We can reverse the operation, holding E in one position and moving H
rapidly toward it. In this case H would represent the armature and E
the field-magnet. When H is moved toward E, the induced current in H
flows in one direction, and when H is suddenly withdrawn from E the
current is reversed in H. (See "Study," Chapter XXV., for experiments.)
[Illustration: Fig. 143.]
=128. Induced Currents by Rotary Motion.= The motions of the coils in
straight lines are not suitable for producing currents strong enough
for commercial purposes. In order to generate currents of considerable
strength and pressure, the coils of wire have to be pushed past
magnets, or electromagnets, with great speed. In the dynamo the coils
are so wound that they can be given a rapid rotary motion as they fly
past strong electromagnets. In this way the coil can keep on passing
the same magnets, in the same direction, as long as force is applied to
the shaft that carries them.
[Illustration: Fig. 144.]
=129. Field-Magnets; Armature; Commutator.= What we need then, to
produce an induced current by a rotary motion, is a strong magnetic
field, a rotating coil of wire properly placed in the field, and some
means of leading the current from the machine.
[Illustration: Fig. 145.]
[Illustration: Fig. 146.]
If a loop of wire, Fig. 140, be so arranged on bearings at its ends
that it can be made to revolve, a current will flow through it in
one direction during one-half of the revolution, and in the opposite
direction during the other half, it being insulated from all external
conductors. This agrees with the experiments suggested in § 127, when
the current generated in a coil passed in one direction during its
motion _toward_ the strongest part of the field, and in the opposite
direction when the coil passed _out_ of it. A coil must be cut by
lines of force to generate a current. A current inside of the machine,
as in Fig. 140, would be of no value; it must be led out to external
conductors where it can do work. Some sort of sliding contact is
necessary to connect a revolving conductor with outside stationary
ones. The magnet, called the _field-magnet_, is merely to furnish lines
of magnetic force. The one turn of wire represents the simplest form of
_armature_.
Fig. 141 shows the ends of a coil joined to two rings, X, Y, insulated
from each other, and rotating with the coil. The two stationary pieces
of carbon, A, B, called _brushes_, press against the rings, and to
these are joined wires, which complete the circuit, and which lead out
where the current can do work. The arrows show the direction of the
current during one-half of a revolution. The rings form a _collector_,
and this arrangement gives an _alternating current_.
[Illustration: Fig. 147.]
In Fig. 142 the ends of the coil are joined to the two halves of a
cylinder. These halves, X and Y, are insulated from each other, and
from the axis. The current flows from X onto the brush A, through some
external circuit, to do the work, and thence back through brush B onto
Y. By the time that Y gets around to A, the direction of the current in
the loop has reversed, so that it passes toward Y, but it still enters
the outside circuit through A, because Y is then in contact with A.
This device is called a _commutator_, and it allows a constant or
_direct current_ to leave the machine.
[Illustration: Fig. 148.]
In regular machines, the field-magnets are electromagnets, the whole
or a part of the current from the dynamo passing around them on its
way out, to excite them and make a powerful field between the poles.
To lessen the resistance to the lines of force on their way from the
N to the S pole of the field-magnets, the armature coils are wound on
an iron core; this greatly increases the strength of the field, as
the lines of force have to jump across but two small air-gaps. There
are many loops of wire on regular armatures, and many segments to the
commutator, carefully insulated from each other, each getting its
current from the coil attached to it.
=130. Types of Dynamos.= While there is an almost endless number of
different makes and shapes of dynamos, they may be divided into two
great types; the _continuous_ or _direct current_, and the _alternating
current_ dynamo. Direct current machines give out a current which
constantly flows in one direction, and this is because a commutator is
used. Alternating currents come from collectors or rings, as shown in
Fig. 141; and as an alternating current cannot be used to excite the
fields, an outside current from a small direct current machine must be
used. These are called exciters.
[Illustration: Fig. 149.]
In direct current machines enough residual magnetism is left in the
field to induce a slight current in the armature when the machine is
started. This immediately adds strength to the field-magnets, which, in
turn, induce a stronger current in the armature.
=131. Winding of Dynamos.= There are several ways of winding dynamos,
depending upon the special uses to be made of the current.
The _series wound_ dynamo, Fig. 143, is so arranged that the entire
current passes around the field-magnet cores on its way from the
machine. In the _shunt wound_ dynamo, Fig. 144, a part, only, of the
current from the machine is carried around the field-magnet cores
through many turns of fine wire. The _compound wound_ dynamo is really
a combination of the two methods just given. In _separately-excited_
dynamos, the current from a separate machine is used to excite the
field-magnets.
=132. Various Machines.= Fig. 145 shows a hand power dynamo
which produces a current for experimental work. Fig. 146 shows a
magneto-electrical generator which produces a current for medical use.
Figs. 147, 148 show forms of dynamos, and Fig. 149 shows how arc lamps
are connected in series to dynamos.
[Illustration]