THE DYNAMO AND THE ELECTRIC MOTOR
Most of my readers will have seen the small working models of electric
tramcars which can be bought at any electrical supply stores. These
usually require a current of about one ampere at three or four volts.
If we connect such a car to the battery recommended for it, and keep
it running continuously, we find that the battery soon begins to
show signs of exhaustion. Now if we imagine our little car increased
to the size of an electric street car, and further imagine, say, a
hundred such cars carrying heavy loads day after day from morning to
night, we shall realize that a battery of cells capable of supplying
the current necessary to run these cars would be so colossal as to be
utterly impracticable. We therefore must look beyond the voltaic cell
for a source of current for such a purpose, and this source we find in
a machine called the “dynamo,” from the Greek word _dynamis_, meaning
force.
Oersted’s discovery of the production of magnetism by electricity
naturally suggested the possibility of producing electricity from
magnetism. In the year 1831 one of the most brilliant of our British
scientists, Michael Faraday, discovered that a current of electricity
could be induced in a coil of wire either by moving the coil towards
or away from a magnet, or by moving a magnet towards or away from the
coil. This may be shown in a simple way by connecting the ends of a
coil of insulated wire to a galvanometer, and moving a bar magnet in
and out of the coil; when the galvanometer shows that a current is
induced in the coil on the insertion of the magnet, and again on its
withdrawal. We have seen that a magnet is surrounded by a field of
magnetic force, and Faraday found that the current was induced when the
lines of force were cut across.
Utilizing this discovery Faraday constructed the first dynamo, which
consisted of a copper plate or disc rotated between the poles of a
powerful horse-shoe magnet, so as to cut the lines of force. The
current flowed either from the shaft to the rim, or _vice versa_,
according to the direction of rotation; and it was conducted away by
means of two wires with spring contacts, one pressing against the
shaft, and the other against the circumference of the disc. This
machine was miserably inefficient, but it was the very first dynamo,
and from it have been slowly evolved the mighty dynamos used to-day
in electric power stations throughout the world. There is a little
story told of Faraday which is worth repeating even if it is not true.
Speaking of his discovery that a magnet could be made to produce
an electric current, a lady once said to him, “This is all very
interesting, but what is the use of it?” “Madam,” replied Faraday,
“what is the use of a baby?” In Faraday’s “baby” dynamo, as in all
others, some kind of power must be used to produce the necessary
motion, so that all dynamos are really machines for converting
mechanical energy into electrical energy.
The copper disc in this first dynamo did not prove satisfactory, and
Faraday soon substituted for it rotating coils of wire. In 1832 a
dynamo was constructed in which a length of insulated wire was wound
upon two bobbins having soft iron cores, and a powerful horse-shoe
magnet was fixed to a rotating spindle in such a position that its
poles faced the cores of the bobbins. This machine gave a fair
current, but it was found that the magnet gradually lost its magnetism
on account of the vibration caused by its rotation. The next step
was to make the magnet a fixture, and to rotate the bobbins of wire.
This was a great improvement, and the power of machines built on this
principle was much increased by having a number of rotating coils
and several magnets. One such machine had 64 separate coils rotating
between the poles of 40 large magnets. Finally, permanent magnets were
superseded by electro-magnets, which gave a much more powerful field of
force.
[Illustration: FIG. 19.--Diagram showing principle of Dynamo producing
Alternating Current.]
Having seen something of the underlying principle and of the history
of the dynamo, we must turn our attention to its actual working. Fig.
19 is a rough representation of a dynamo in its simplest form. The
two poles of the magnet are shown marked north and south, and between
them revolves the coil of wire A¹ A², mounted on a spindle SS. This
revolving coil is called the armature. To each of the insulated rings
RR is fixed one end of the coil, and BB are two brushes of copper or
carbon, one pressing on each ring. From these brushes the current is
led away into the main circuit, and in this case we may suppose that
the current is used to light a lamp.
In speaking of the induction coil we saw that the currents induced on
making and on breaking the circuit flowed in opposite directions, and
similarly, Faraday found that the currents induced in a coil of wire on
inserting and on withdrawing his magnet flowed in opposite directions.
In the present case the magnet is stationary and the coil moves, but
the effect is just the same. Now if we suppose the armature to be
revolving in a clockwise direction, then A¹ is descending and entering
the magnetic field in front of the north pole, consequently a current
is induced in the coil, and of course in the main circuit also, in one
direction. Continuing its course, A¹ passes away from this portion
of the magnetic field, and thus a current is induced in the opposite
direction. In this way we get a current which reverses its direction
every half-revolution, and such a current is called an alternating
current. If, as in our diagram, there are only two magnetic poles, the
current flows backwards and forwards once every revolution, but by
using a number of magnets, arranged so that the coil passes in turn the
poles of each, it can be made to flow backwards and forwards several
times. One complete flow backwards and forwards is called a period, and
the number of periods per second is called the periodicity or frequency
of the current. A dynamo with one coil or set of coils gives what is
called “single-phase” current, that is, a current having one wave which
keeps flowing backwards and forwards. If there are two distinct sets
of coils we get a two-phase current, in which there are two separate
waves, one rising as the other falls. Similarly, by using more sets of
coils, we may obtain three-phase or polyphase currents.
[Illustration: FIG. 20.--Diagram showing principle of Dynamo producing
Continuous Current.]
Alternating current is unsuitable for certain purposes, such as
electroplating; and by making a small alteration in our dynamo we get
a continuous or direct current, which does not reverse its direction.
Fig. 20 shows the new arrangement. Instead of the two rings in Fig.
19, we have now a single ring divided into two parts, each half being
connected to one end of the revolving coil. Each brush, therefore,
remains on one portion of the ring for half a revolution, and then
passes over on to the other portion. During one half-revolution we
will suppose the current to be flowing from brush B¹ in the direction
of the lamp. Then during the next half-revolution the current flows in
the opposite direction; but brush B¹ has passed on to the other half of
the ring, and so the current is still leaving by it. In this way the
current must always flow in the same direction in the main circuit,
leaving by brush B¹ and returning by brush B². This arrangement for
making the alternating current into a continuous current is called a
_commutator_.
[Illustration: PLATE IV.
_By permission of_ _Lancashire Dynamo & Motor Co. Ltd._
A TYPICAL DYNAMO AND ITS PARTS.]
In actual practice a dynamo has a set of electro-magnets, and the
armature consists of many coils of wire mounted on a core of iron,
which has the effect of concentrating the lines of force. The armature
generally revolves in small dynamos, but in large ones it is usually a
fixture, while the electro-magnets revolve. Plate IV. shows a typical
dynamo and its parts.
As we saw in an earlier chapter, an electro-magnet has magnetic powers
only while a current is being passed through its winding, and so some
means of supplying current to the electro-magnets in a dynamo must
be provided. It is a remarkable fact that it is almost impossible to
obtain a piece of iron which has not some traces of magnetism, and
so when a dynamo is first set up there is often sufficient magnetism
in the iron of the electro-magnets to produce a very weak field. The
rapid cutting of the feeble lines of force of this field sets up a
weak current, which, acting upon the electro-magnets, gradually brings
them up to full strength. Once the dynamo is generating current it
keeps on feeding its magnets by sending either the whole or a part of
its current through them. After it has once been set going the dynamo
is always able to start again, because the magnet cores retain enough
magnetism to set up a weak field. If there is not enough magnetism in
the cores to start a dynamo for the first time, a current from some
outside source is sent round the magnets.
The foregoing remarks apply to continuous current dynamos only.
Alternating current can be used for exciting electro-magnets, but in
this case the magnetic field produced is alternating also, so that each
pole of the magnet has north and south magnetism alternately. This will
not do for dynamo field magnets, and therefore an alternating current
dynamo cannot feed its own magnets. The electro-magnets in such dynamos
are supplied with current from a separate continuous current dynamo,
which may be of quite small size.
It is a very interesting fact that electric current can be generated
by a dynamo in which the earth itself is used to provide the magnetic
field, no permanent or electro-magnets being used at all. A simple form
of dynamo of this kind consists of a rectangular loop of copper wire
rotating about an axis pointing east and west, so that the loop cuts
the lines of force of the Earth’s magnetic field.
The dynamo provides us with a constant supply of electric current, but
this current is no use unless we can make it do work for us. If we
reverse the usual order of things in regard to a dynamo, and supply
the machine with current instead of mechanical power, we find that
the armature begins to revolve rapidly, and the machine is no longer
a dynamo, but has become an electric motor. This shows us that an
electric motor is simply a dynamo reversed. Let us suppose that we wish
to use the dynamo in Fig. 20 as a motor. In order to supply the current
we will take away the lamp and substitute a second continuous-current
dynamo. We know from Chapter VII. that when a current is sent through
a coil of wire the coil becomes a magnet with a north and a south
pole. The coil in our dynamo becomes a magnet as soon as the current
is switched on, and the attraction between its poles and the opposite
poles of the magnet causes it to make half a revolution. At this point
the commutator reverses the current, and consequently the polarity of
the coil, so that there is now repulsion where previously there was
attraction, and the coil makes another half-revolution. So the process
goes on until the armature attains a very high speed. In general
construction there is practically no difference between a dynamo and a
motor, but there are differences in detail which adapt each to its own
particular work. By making certain alterations in their construction
electric motors can be run with alternating current.
The fact that a dynamo could be reversed and run as a motor was known
probably as early as 1838, but the great value of this reversibility
does not seem to have been realized until 1873. At an industrial
exhibition held at Vienna in that year, it so happened that a workman
or machinery attendant connected two cables to a dynamo which was
standing idle, and he was much surprised to find that it at once began
to revolve at a great speed. It was then seen that the cables led to
another dynamo which was running, and that the current from this source
had made the first dynamo into a motor. There are many versions of
this story, but the important point in all is that this was the first
occasion on which general attention was drawn to the possibilities of
the electric motor.
The practical advantages afforded by the electric motor are many and
great. Once we have installed a sufficiently powerful dynamo and a
steam or other engine to drive it, we can place motors just where
they are required, either close to the dynamo or miles away, driving
them simply by means of a connecting cable. In factories, motors can
be placed close to the machines they are required to drive, anywhere
in the building, thus doing away with all complicated and dangerous
systems of shafting and belts. In many cases where it would be either
utterly impossible or at least extremely inconvenient to use any form
of steam, gas, or oil engine, electric motors can be employed without
the slightest difficulty. In order to realize this, one only has to
think of the positions in which electrically-driven ventilating fans
are placed, or of the unpleasantly familiar electric drill of the
dentist. An electric motor is small and compact, gives off no fumes and
practically no heat, makes very little noise, is capable of running for
very long periods at high speed and with the utmost steadiness, and
requires extremely little attention.