DYNAMOS AND ELECTRIC MOTORS.
A simple dynamo--Continuous-current dynamos--Multipolar
dynamos--Exciting the field magnets--Alternating current
dynamos--The transmission of power--The electric motor--Electric
lighting--The incandescent lamp--Arc lamps--"Series" and "parallel"
arrangement of lamps--Current for electric lamps--Electroplating.
In previous chapters we have incidentally referred to the conversion of
mechanical work into electrical energy. In this we shall examine how it
is done--how the silently spinning dynamo develops power, and why the
motor spins when current is passed through it.
We must begin by returning to our first electrical diagram (Fig. 50),
and calling to mind the invisible "lines of force" which permeate the
ether in the immediate neighbourhood of a magnet's poles, called the
_magnetic field_ of the magnet.
Many years ago (1831) the great Michael Faraday discovered that if a
loop of wire were moved up and down between the poles of an
electro-magnet (Fig. 66) a current was induced in the loop, its
direction depending upon that in which the loop was moved. The energy
required to cut the lines of force passed in some mysterious way into
the wire. Why this is so we cannot say, but, taking advantage of the
fact, electricians have gradually developed the enormous machines which
now send vehicles spinning over metal tracks, light our streets and
houses, and supply energy to innumerable factories.
[Illustration: FIG. 66.]
The strength of the current induced in a circuit cutting the lines of
force of a magnet is called its pressure, voltage, or electro-motive
force (expressed shortly E.M.F.). It may be compared with the
pounds-to-the-square-inch of steam. In order to produce an E.M.F. of one
volt it is calculated that 100,000,000 lines of force must be cut every
second.
The voltage depends on three things:--(1.) The _strength_ of the magnet:
the stronger it is, the greater the number of lines of force coming from
it. (2.) The _length_ of the conductor cutting the lines of force: the
longer it is, the more lines it will cut. (3.) The _speed_ at which the
conductor moves: the faster it travels, the more lines it will cut in a
given time. It follows that a powerful dynamo, or mechanical producer of
current, must have strong magnets and a long conductor; and the latter
must be moved at a high speed across the lines of force.
A SIMPLE DYNAMO.
In Fig. 67 we have the simplest possible form of dynamo--a single turn
of wire, _w x y z_, mounted on a spindle, and having one end attached to
an insulated ring C, the other to an insulated ring C^1. Two small
brushes, B B^1, of wire gauze or carbon, rubbing continuously against
these collecting rings, connect them with a wire which completes the
circuit. The armature, as the revolving coil is called, is mounted
between the poles of a magnet, where the lines of force are thickest.
These lines are _supposed_ to stream from the N. to the S. pole.
In Fig. 67 the armature has reached a position in which _y z_ and _w x_
are cutting no, or very few, lines of force, as they move practically
parallel to the lines. This is called the _zero_ position.
[Illustration: FIG. 67.]
[Illustration: FIG. 68.]
In Fig. 68 the armature, moving at right angles to the lines of force,
cuts a maximum number in a given time, and the current induced in the
coil is therefore now most intense. Here we must stop a moment to
consider how to decide in which direction the current flows. The
armature is revolving in a clockwise direction, and _y z_, therefore, is
moving downwards. Now, suppose that you rest your _left_ hand on the N.
pole of the magnet so that the arm lies in a line with the magnet. Point
your forefinger towards the S. pole. It will indicate the _direction of
the lines of force_. Bend your other three fingers downwards over the
edge of the N. pole. They will indicate the _direction in which the
conductor is moving_ across the magnetic field. Stick out the thumb at
right angles to the forefinger. It points in the direction in which the
_induced_ current is moving through the nearer half of the coil.
Therefore lines of force, conductor, and induced current travel in
planes which, like the top and two adjacent sides of a box, are at right
angles to one another.
While current travels from _z_ to _y_--that is, _from_ the ring C^1 to
_y_--it also travels from _x_ to _w_, because _w x_ rises while _y z_
descends. So that a current circulates through the coil and the exterior
part of the circuit, including the lamp. After _z y_ has passed the
lowest possible point of the circle it begins to ascend, _w x_ to
descend. The direction of the current is therefore reversed; and as the
change is repeated every half-revolution this form of dynamo is called
an _alternator_ or creator of alternating currents. A well-known type of
alternator is the magneto machine which sends shocks through any one who
completes the external circuit by holding the brass handles connected by
wires to the brushes. The faster the handle of the machine is turned the
more frequent is the alternation, and the stronger the current.
[Illustration: FIG. 69.]
CONTINUOUS-CURRENT DYNAMOS.
An alternating current is not so convenient for some purposes as a
continuous current. It is therefore sometimes desirable (even necessary)
to convert the alternating into a uni-directional or continuous current.
How this is done is shown in Figs. 69 and 70. In place of the two
collecting rings C C^1, we now have a single ring split longitudinally
into two portions, one of which is connected to each end of the coil _w
x y z_. In Fig. 69 brush B has just passed the gap on to segment C,
brush B^1 on to segment C^1. For half a revolution these remain
respectively in contact; then, just as _y z_ begins to rise and _w x_ to
descend, the brushes cross the gaps again and exchange segments, so that
the current is perpetually flowing one way through the circuit. The
effect of the commutator[17] is, in fact, equivalent to transposing the
brushes of the collecting rings of the alternator every time the coil
reaches a zero position.
Figs. 71 and 72 give end views in section of the coil and the
commutator, with the coil in the position of minimum and maximum
efficiency. The arrow denotes the direction of movement; the double
dotted lines the commutator end of the revolving coil.
[Illustration: FIG. 70.]
PRACTICAL CONTINUOUS-CURRENT DYNAMOS.
The electrical output of our simple dynamo would be increased if,
instead of a single turn of wire, we used a coil of many turns. A
further improvement would result from mounting on the shaft, inside the
coil, a core or drum of iron, to entice the lines of force within reach
of the revolving coil. It is evident that any lines which pass through
the air outside the circle described by the coil cannot be cut, and are
wasted.
[Illustration: FIG. 71.]
[Illustration: FIG. 72.]
The core is not a solid mass of iron, but built up of a number of very
thin iron discs threaded on the shaft and insulated from one another to
prevent electric eddies, which would interfere with the induced current
in the conductor.[18] Sometimes there are openings through the core from
end to end to ventilate and cool it.
[Illustration: FIG. 73.]
We have already noticed that in the case of a single coil the current
rises and falls in a series of pulsations. Such a form of armature would
be unsuitable for large dynamos, which accordingly have a number of
coils wound over their drums, at equal distances round the
circumference, and a commutator divided into an equal number of
segments. The subject of drum winding is too complicated for brief
treatment, and we must therefore be content with noticing that the coils
are so connected to their respective commutator segments and to one
another that they mutually assist one another. A glance at Fig. 73 will
help to explain this. Here we have in section a number of conductors on
the right of the drum (marked with a cross to show that current is
moving, as it were, into the page), connected with conductors on the
left (marked with a dot to signify current coming out of the page). If
the "crossed" and "dotted" conductors were respectively the "up" and
"down" turns of a single coil terminating in a simple split commutator
(Fig. 69), when the coil had been revolved through an angle of 90° some
of the up turns would be ascending and some descending, so that
conflicting currents would arise. Yet we want to utilize the whole
surface of the drum; and by winding a number of coils in the manner
hinted at, each coil, as it passes the zero point, top or bottom, at
once generates a current in the desired direction and reinforces that in
all the other turns of its own and of other coils on the same side of a
line drawn vertically through the centre. There is thus practically no
fluctuation in the pressure of the current generated.
The action of single and multiple coil windings may be compared to that
of single and multiple pumps. Water is ejected by a single pump in
gulps; whereas the flow from a pipe fed by several pumps arranged to
deliver consecutively is much more constant.
MULTIPOLAR DYNAMOS.
Hitherto we have considered the magnetic field produced by one bi-polar
magnet only. Large dynamos have four, six, eight, or more field magnets
set inside a casing, from which their cores project towards the armature
so as almost to touch it (Fig. 74). The magnet coils are wound to give
N. and S. poles alternately at their armature ends round the field; and
the lines of force from each N. pole stream each way to the two adjacent
S. poles across the path of the armature coils. In dynamos of this kind
several pairs of collecting brushes pick current off the commutator at
equidistant points on its circumference.
[Illustration: FIG. 74.--A Holmes continuous current dynamo: A,
armature; C, commutator; M, field magnets.]
EXCITING THE FIELD MAGNETS.
Until current passes through the field magnet coils, no magnetic field
can be created. How are the coils supplied with current? A dynamo,
starting for the first time, is excited by a current from an outside
source; but when it has once begun to generate current it feeds its
magnets itself, and ever afterwards will be self-exciting,[19] owing to
the residual magnetism left in the magnet cores.
[Illustration: FIG. 75.--Partly finished commutator.]
Look carefully at Figs. 77 and 78. In the first of these you will
observe that part of the wire forming the external circuit is wound
round the arms of the field magnet. This is called a _series_ winding.
In this case _all_ the current generated helps to excite the dynamo. At
the start the residual magnetism of the magnet cores gives a weak field.
The armature coils cut this and pass a current through the circuit. The
magnets are further excited, and the field becomes stronger; and so on
till the dynamo is developing full power. Series winding is used where
the current in the external circuit is required to be very constant.
[Illustration: FIG. 76.--The brushes of a Holmes dynamo.]
Fig. 78 shows another method of winding--the _shunt_. Most of the
current generated passes through the external circuit 2, 2; but a part
is switched through a separate winding for the magnets, denoted by the
fine wire 1, 1. Here the strength of the magnetism does not vary
directly with the current, as only a small part of the current serves
the magnets. The shunt winding is therefore used where the voltage (or
pressure) must be constant.
[Illustration: FIG. 77.--Sketch showing a "series" winding.]
[Illustration: FIG. 78.--"Shunt" winding.]
A third method is a combination of the two already named. A winding of
fine wire passes from brush to brush round the magnets; and there is
also a series winding as in Fig. 77. This compound method is adapted
more especially for electric traction.
ALTERNATING DYNAMOS.
These have their field magnets excited by a separate continuous current
dynamo of small size. The field magnets usually revolve inside a fixed
armature (the reverse of the arrangement in a direct-current generator);
or there may be a fixed central armature and field magnets revolving
outside it. This latter arrangement is found in the great power stations
at Niagara Falls, where the enormous field-rings are mounted on the top
ends of vertical shafts, driven by water-turbines at the bottom of pits
178 feet deep, down which water is led to the turbines through great
pipes, or penstocks. The weight of each shaft and the field-ring
attached totals about thirty-five tons. This mass revolves 250 times a
minute, and 5,000 horse power is constantly developed by the dynamo.
Similar dynamos of 10,000 horse power each have been installed on the
Canadian side of the Falls.
[Illustration: FIG. 79.]
TRANSMISSION OF POWER.
Alternating current is used where power has to be transmitted for long
distances, because such a current can be intensified, or stepped up, by
a transformer somewhat similar in principle to a Ruhmkorff coil _minus_
a contact-breaker (see p. 122). A typical example of transformation is
seen in Fig. 79. Alternating current of 5,000 volts pressure is produced
in the generating station and sent through conductors to a distant
station, where a transformer, B, reduces the pressure to 500 volts to
drive an alternating motor, C, which in turn operates a direct current
dynamo, D. This dynamo has its + terminal connected with the insulated
or "live" rail of an electric railway, and its - terminal with the wheel
rails, which are metallically united at the joints to act as a
"return." On its way from the live rail to the return the current passes
through the motors. In the case of trams the conductor is either a cable
carried overhead on standards, from which it passes to the motor through
a trolley arm, or a rail laid underground in a conduit between the
rails. In the top of the conduit is a slit through which an arm carrying
a contact shoe on the end projects from the car. The shoe rubs
continuously on the live rail as the car moves.
To return for a moment to the question of transformation of current.
"Why," it may be asked, "should we not send low-pressure _direct_
current to a distant station straight from the dynamo, instead of
altering its nature and pressure? Or, at any rate, why not use
high-pressure direct current, and transform _that_?" The answer is, that
to transmit a large amount of electrical energy at low pressure (or
voltage) would necessitate large volume (or _amperage_) and a big and
expensive copper conductor to carry it. High-pressure direct current is
not easily generated, since the sparking at the collecting brushes as
they pass over the commutator segments gives trouble. So engineers
prefer high-pressure alternating current, which is easily produced, and
can be sent through a small and inexpensive conductor with little loss.
Also its voltage can be transformed by apparatus having no revolving
parts.
THE ELECTRIC MOTOR.
Anybody who understands the dynamo will also be able to understand the
electric motor, which is merely a reversed dynamo.
Imagine in Fig. 70 a dynamo taking the place of the lamp and passing
current through the brushes and commutator into the coil _w x y z_. Now,
any coil through which current passes becomes a magnet with N. and S.
poles at either end. (In Fig. 70 we will assume that the N. pole is
below and the S. pole above the coil.) The coil poles therefore try to
seek the contrary poles of the permanent magnet, and the coil revolves
until its S. pole faces the N. of the magnet, and _vice versâ_. The
lines of force of the coil and the magnet are now parallel. But the
momentum of revolution carries the coil on, and suddenly the commutator
reverses its polarity, and a further half-revolution takes place. Then
comes a further reversal, and so on _ad infinitum_. The rotation of the
motor is therefore merely a question of repulsion and attraction of like
and unlike poles. An ordinary compass needle may be converted into a
tiny motor by presenting the N. and S. poles of a magnet to its S. and
N. poles alternately every half-revolution.
In construction and winding a motor is practically the same as a dynamo.
In fact, either machine can perform either function, though perhaps not
equally well adapted for both. Motors may be run with direct or
alternating current, according to their construction.
On electric cars the motor is generally suspended from the wheel truck,
and a small pinion on the armature shaft gears with a large pinion on a
wheel axle. One great advantage of electric traction is that every
vehicle of a train can carry its own motor, so that the whole weight of
the train may be used to get a grip on the rails when starting. Where a
single steam locomotive is used, the adhesion of its driving-wheels only
is available for overcoming the inertia of the load; and the whole
strain of starting is thrown on to the foremost couplings. Other
advantages may be summed up as follows:--(1) Ease of starting and rapid
acceleration; (2) absence of waste of energy (in the shape of burning
fuel) when the vehicles are at rest; (3) absence of smoke and smell.
ELECTRIC LIGHTING.
Dynamos are used to generate current for two main purposes--(1) To
supply power to motors of all kinds; (2) to light our houses, factories,
and streets. In private houses and theatres incandescent lamps are
generally used; in the open air, in shops, and in larger buildings, such
as railway stations, the arc lamp is more often found.
INCANDESCENT LAMP.
If you take a piece of very fine iron wire and lay it across the
terminals of an accumulator, it becomes white hot and melts, owing to
the heat generated by its resistance to the current. A piece of fine
platinum wire would become white hot without melting, and would give out
an intense light. Here we have the principle of the glow or incandescent
lamp--namely, the interposition in an electric circuit of a conductor
which at once offers a high resistance to the current, but is not
destroyed by the resulting heat.
In Fig. 80 is shown a fan propelling liquid constantly through a pipe.
Let us assume that the liquid is one which develops great friction on
the inside of the pipe. At the contraction, where the speed of travel
is much greater than elsewhere in the circuit, most heat will be
produced.
[Illustration: FIG. 80.--Diagram to show circulation of water through a
pipe.]
In quite the early days of the glow-lamp platinum wire was found to be
unreliable as regards melting, and filaments of carbon are now used. To
prevent the wasting away of the carbon by combination with oxygen the
filament is enclosed in a glass bulb from which practically all air has
been sucked by a mercury pump before sealing.
[Illustration: FIG. 81.--The electrical counterpart of Fig. 80. The
filament takes the place of the contraction in the pipe.]
The manufacture of glow-lamps is now an important industry. One brand of
lamp[20] is made as follows:--First, cotton-wool is dissolved in
chloride of zinc, and forms a treacly solution, which is squirted
through a fine nozzle into a settling solution which hardens it and
makes it coil up like a very fine violin string. After being washed and
dried, it is wound on a plumbago rod and baked in a furnace until only
the carbon element remains. This is the filament in the rough. It is
next removed from the rod and tipped with two short pieces of fine
platinum wire. To make the junction electrically perfect the filament is
plunged in benzine and heated to whiteness by the passage of a strong
current, which deposits the carbon of the benzine on the joints. The
filament is now placed under the glass receiver of an air-pump, the air
is exhausted, hydro-carbon vapour is introduced, and the filament has a
current passed through it to make it white hot. Carbon from the vapour
is deposited all over the filament until the required electrical
resistance is attained. The filament is now ready for enclosure in the
bulb. When the bulb has been exhausted and sealed, the lamp is tested,
and, if passed, goes to the finishing department, where the two platinum
wires (projecting through the glass) are soldered to a couple of brass
plates, which make contact with two terminals in a lamp socket. Finally,
brass caps are affixed with a special water-tight and hard cement.
ARC LAMPS.
In _arc_ lighting, instead of a contraction at a point in the circuit,
there is an actual break of very small extent. Suppose that to the ends
of the wires leading from a dynamo's terminals we attach two carbon
rods, and touch the end of the rods together. The tips become white hot,
and if they are separated slightly, atoms of incandescent carbon leap
from the positive to the negative rod in a continuous and intensely
luminous stream, which is called an _arc_ because the path of the
particles is curved. No arc would be formed unless the carbons were
first touched to start incandescence. If they are separated too far for
the strength of the current to bridge the gap the light will flicker or
go out. The arc lamp is therefore provided with a mechanism which, when
the current is cut off, causes the carbons to fall together, gradually
separates them when it is turned on, and keeps them apart. The principle
employed is the effort of a coil through which a current passes to draw
an iron rod into its centre. Some of the current feeding the lamp is
shunted through a coil, into which projects one end of an iron bar
connected with one carbon point. A spring normally presses the points
together when no current flows. As soon as current circulates through
the coil the bar is drawn upwards against the spring.
SERIES AND PARALLEL ARRANGEMENT OF LAMPS.
When current passes from one lamp to another, as in Fig. 82, the lamps
are said to be in _series_. Should one lamp fail, all in the circuit
would go out. But where arc lamps are thus arranged a special mechanism
on each lamp "short-circuits" it in case of failure, so that current may
pass uninterruptedly to the next.
[Illustration: FIG. 82.--Incandescent lamps connected in "series."]
Fig. 83 shows a number of lamps set _in parallel_. One terminal of each
is attached to the positive conductor, the other to the negative
conductor. Each lamp therefore forms an independent bridge, and does
not affect the efficiency of the rest. _Parallel series_ signifies a
combination of the two systems, and would be illustrated if, in Fig. 83,
two or more lamps were connected in series groups from one conductor to
the other. This arrangement is often used in arc lighting.
[Illustration: FIG. 83.--Incandescent lamps connected in "parallel."]
CURRENT FOR ELECTRIC LAMPS.
This may be either direct or alternating. The former is commonly used
for arc lamps, the latter for incandescent, as it is easily stepped-down
from the high-pressure mains for use in a house. Glow-lamps usually take
current of 110 or 250 volts pressure.
In arc lamps fed with direct current the tip of the positive carbon has
a bowl-shaped depression worn in it, while the negative tip is pointed.
Most of the illumination comes from the inner surface of the bowl, and
the positive carbon is therefore placed uppermost to throw the light
downwards. An alternating current, of course, affects both carbons in
the same manner, and there is no bowl.
The carbons need frequent renewal. A powerful lamp uses about 70 feet of
rod in 1,000 hours if the arc is exposed to the air. Some lamps have
partly enclosed arcs--that is, are surrounded by globes perforated by a
single small hole, which renders combustion very slow, though preventing
a vacuum.
ELECTROPLATING.
Electroplating is the art of coating metals with metals by means of
electricity. Silver, copper, and nickel are the metals most generally
deposited. The article to be coated is suspended in a chemical solution
of the metal to be deposited. Fig. 84 shows a very simple plating
outfit. A is a battery; B a vessel containing, say, an acidulated
solution of sulphate of copper. A spoon, S, hanging in this from a glass
rod, R, is connected with the zinc or negative element, Z, of the
battery, and a plate of copper, P, with the positive element, C. Current
flows in the direction shown by the arrows, from Z to C, C to P, P to
S, S to Z. The copper deposited from the solution on the spoon is
replaced by gradual dissolution of the plate, so that the latter serves
a double purpose.
[Illustration: FIG. 84.--An electroplating outfit.]
In silver plating, P is of silver, and the solution one of cyanide of
potassium and silver salts. Where nickel or silver has to be deposited
on iron, the article is often given a preliminary coating of copper, as
iron does not make a good junction with either of the first two metals,
but has an affinity for copper.
[17] From the Latin _commuto_, "I exchange."
[18] Only the "drum" type of armature is treated here.
[19] This refers to continuous-current dynamos only.
[20] The Robertson.