ALTERNATING CURRENTS, CHOKING COILS, TRANSFORMERS, CONVERTERS AND
RECTIFIERS
DIRECT CURRENT.--When a current of electricity is generated by a cell,
it is assumed to move along the wire in one direction, in a steady,
continuous flow, and is called a _direct_ current. This direct current
is a natural one if generated by a cell.
ALTERNATING CURRENT.--On the other hand, the natural current generated
by a dynamo is alternating in its character--that is, it is not a
direct, steady flow in one direction, but, instead, it flows for an
instant in one direction, then in the other direction, and so on.
A direct-current dynamo such as we have shown in Chapter IV, is much
easier to explain, hence it is illustrated to show the third method used
in generating an electric current.
It is a difficult matter to explain the principle and operation of
alternating current machines, without becoming, in a measure, too
technical for the purposes of this book, but it is important to know the
fundamentals involved, so that the operation and uses of certain
apparatus, like the choking coil, transformers, rectifiers and
converters, may be explained.
THE MAGNETIC FIELD.--It has been stated that when a wire passes through
the magnetic field of a magnet, so as to cut the lines of force flowing
out from the end of a magnet, the wire will receive a charge of
electricity.
[Illustration: _Fig. 102._ CUTTING A MAGNETIC FIELD]
To explain this, study Fig. 102, in which is a bar magnet (A). If we
take a metal wire (B) and bend it in the form of a loop, as shown, and
mount the ends on journal-bearing blocks, the wire may be rotated so
that the loop will pass through the magnetic field. When this takes
place, the wire receives a charge of electricity, which moves, say, in
the direction of the darts, and will make a complete circuit if the ends
of the looped wire are joined, as shown by the conductor (D).
ACTION OF THE MAGNETIZED WIRE.--You will remember, also that we have
pointed out how, when a current passes over a wire, it has a magnetic
field extending out around it at all points, so that while it is passing
through the magnetic field of the magnet (A), it becomes, in a measure,
a magnet of its own and tries to set up in business for itself as a
generator of electricity. But when the loop leaves the magnetic field,
the magnetic or electrical impulse in the wire also leaves it.
THE MOVEMENT OF A CURRENT IN A CHARGED WIRE.--Your attention is
directed, also, to another statement, heretofore made, namely, that when
a current from a charged wire passes by induction to a wire across
space, so as to charge it with an electric current, it moves along the
charged wire in a direction opposite to that of the current in the
charging wire.
Now, the darts show the direction in which the current moves while it is
approaching and passing through the magnetic field. But the moment the
loop is about to pass out of the magnetic field, the current in the loop
surges back in the opposite direction, and when the loop has made a
revolution and is again entering the magnetic field, it must again
change the direction of flow in the current, and thus produce
alternations in the flow thereof.
Let us illustrate this by showing the four positions of the revolving
loop. In Fig. 103 the loop (B) is in the middle of the magnetic field,
moving upwardly in the direction of the curved dart (A), and while in
that position the voltage, or the electrical impulse, is the most
intense. The current used flows in the direction of the darts (C) or to
the left.
In Fig. 104, the loop (A) has gone beyond the influence of the magnetic
field, and now the current in the loop tries to return, or reverse
itself, as shown by the dart (D). It is a reaction that causes the
current to die out, so that when the loop has reached the point farthest
from the magnet, as shown in Fig. 105, there is no current in the loop,
or, if there is any, it moves faintly in the direction of the dart (E).
[Illustration: _Figs. 103-106._ ILLUSTRATING ALTERNATIONS]
CURRENT REVERSING ITSELF.--When the loop reaches its lowest point (Fig.
106) it again comes within the magnetic field and the current commences
to flow back to its original direction, as shown by darts (C).
SELF-INDUCTION.--This tendency of a current to reverse itself, under the
conditions cited, is called self-induction, or inductance, and it would
be well to keep this in mind in pursuing the study of alternating
currents.
You will see from the foregoing, that the alternations, or the change of
direction of the current, depends upon the speed of rotation of the loop
past the end of the magnet.
[Illustration: _Figs. 107-108._ FORM FOR INCREASING ALTERNATIONS]
Instead, therefore, of using a single loop, we may make four loops (Fig.
107), which at the same speed as we had in the case of the single loop,
will give four alternations, instead of one, and still further, to
increase the periods of alternation, we may use the four loops and two
magnets, as in Fig. 108. By having a sufficient number of loops and of
magnets, there may be 40, 50, 60, 80, 100 or 120 such alternating
periods in each second. Time, therefore, is an element in the operation
of alternating currents.
Let us now illustrate the manner of connecting up and building the
dynamo, so as to derive the current from it. In Fig. 109, the loop (A)
shows, for convenience, a pair of bearings (B). A contact finger (C)
rests on each, and to these the circuit wire (D) is attached. Do not
confuse these contact fingers with the commutator brushes, shown in the
direct-current motor, as they are there merely for the purpose of making
contact between the revolving loop (A) and stationary wire (D).
[Illustration: _Fig. 109._ CONNECTION OF ALTERNATING DYNAMO ARMATURE]
BRUSHES IN A DIRECT-CURRENT DYNAMO.--The object of the brushes in the
direct-current dynamo, in connection with a commutator, is to convert
this _inductance_ of the wire, or this effort to reverse itself into a
current which will go in one direction all the time, and not in both
directions alternately.
To explain this more fully attention is directed to Figs. 110 and 111.
Let A represent the armature, with a pair of grooves (B) for the wires.
The commutator is made of a split tube, the parts so divided being
insulated from each other, and in Fig. 110, the upper one, we shall call
and designate the positive (+) and the lower one the negative (-). The
armature wire (C) has one end attached to the positive commutator
terminal and the other end of this wire is attached to the negative
terminal.
[Illustration: _Fig. 110._ DIRECT CURRENT DYNAMO]
One brush (D) contacts with the positive terminal of the commutator and
the other brush (E) with the negative terminal. Let us assume that the
current impulse imparted to the wire (C) is in the direction of the dart
(F, Fig. 110). The current will then flow through the positive (+)
terminal of the commutator to the brush (D), and from the brush (D)
through the wire (G) to the brush (E), which contacts with the negative
(-) terminal of the commutator. This will continue to be the case, while
the wire (C) is passing the magnetic field, and while the brush (D) is
in contact with the positive (+) terminal. But when the armature makes a
half turn, or when it reaches that point where the brush (D) contacts
with the negative (-) terminal, and the brush (E) contacts with the
positive (+) terminal, a change in the direction of the current through
the wire (G) takes place, unless something has happened to change it
before it has reached the brushes (D, E).
[Illustration: _Fig. 111._ CIRCUIT WIRES IN DIRECT CURRENT DYNAMO]
Now, this change is just exactly what has happened in the wire (C), as
we have explained. The current attempts to reverse itself and start out
on business of its own, so to speak, with the result that when the
brushes (D and E) contact with the negative and positive terminals,
respectively, the surging current in the wire (C) is going in the
direction of the dart (H)--that is, while, in Fig. 110, the current
flows from the wire (C) into the positive terminal, and out of the
negative terminal into the wire (C), the conditions are exactly reversed
in Fig. 111. Here the current in wire C flows _into_ the negative (-)
terminal, and _from_ the positive (+) terminal into the wire C, so that
in either case the current will flow out of the brush D and into the
brush E, through the external circuit (G).
It will be seen, therefore, that in the direct-current motor, advantage
is taken of the surging, or back-and-forth movement, of the current to
pass it along in one direction, whereas in the alternating current no
such change in direction is attempted.
ALTERNATING POSITIVE AND NEGATIVE POLES.--The alternating current,
owing to this surging movement, makes the poles alternately positive and
negative. To express this more clearly, supposing we take a line (A,
Fig. 112), which is called the zero line, or line of no electricity. The
current may be represented by the zigzag line (B). The lines (B) above
zero (A) may be designated as positive, and those below the line as
negative. The polarity reverses at the line A, goes up to D, which is
the maximum intensity or voltage above zero, and, when the current falls
and crosses the line A, it goes in the opposite direction to E, which is
its maximum voltage in the other direction. In point of time, if it
takes one second for the current to go from C to F, on the down line,
then it takes only a half second to go from C to G, so that the line A
represents the time, and the line H the intensity, a complete cycle
being formed from C, D, F, then through F, E, C, and so on.
[Illustration: _Fig. 112._ ALTERNATING POLARITY LINES]
HOW AN ALTERNATING DYNAMO IS MADE.--It is now necessary to apply these
principles in the construction of an alternating-current machine. Fig.
113 is a diagram representing the various elements, and the circuiting.
[Illustration: _Fig. 113._ ALTERNATING CURRENT DYNAMO]
Let A represent the ring or frame containing the inwardly projecting
field magnet cores (B). C is the shaft on which the armature revolves,
and this carries the wheel (D), which has as many radially disposed
magnet cores (E) as there are of the field magnet cores (B).
The shaft (C) also carries two pulleys with rings thereon. One of these
rings (F) is for one end of the armature winding, and the other ring
(G) for the other end of the armature wire.
THE WINDINGS.--The winding is as follows: One wire, as at H, is first
coiled around one magnet core, the turnings being to the right. The
outlet terminal of this wire is then carried to the next magnet core and
wound around that, in the opposite direction, and so on, so that the
terminal of the wire is brought out, as at I, all of these wires being
connected to binding posts (J, J'), to which, also, the working circuits
are attached.
THE ARMATURE WIRES.--The armature wires, in like manner, run from the
ring (G) to one armature core, being wound from right to left, then to
the next core, which is wound to the right, afterward to the next core,
which is wound to the left, and so on, the final end of the wire being
connected up with the other ring (F). The north (N) and the south (S)
poles are indicated in the diagram.
CHOKING COIL.--The self-induction in a current of this kind is utilized
in transmitting electricity to great distances. Wires offer resistance,
or they impede the flow of a current, as hereinbefore stated, so that it
is not economical to transmit a direct current over long distances. This
can be done more efficiently by means of the alternating current, which
is subject to far less loss than is the case with the direct current.
It affords a means whereby the flow of a current may be checked or
reduced without depending upon the resistance offered by the wire over
which it is transmitted. This is done by means of what is called a
choking coil. It is merely a coil of wire, wound upon an iron core, and
the current to be choked passes through the coil. To illustrate this,
let us take an arc lamp designed to use a 50-volt current. If a current
is supplied to it carrying 100 volts, it is obvious that there are 50
volts more than are needed. We must take care of this excess of 50 volts
without losing it, as would happen were we to locate a resistance of
some kind in the circuit. This result we accomplish by the introduction
of the choking coil, which has the effect of absorbing the excessive 50
volts, the action being due to its quality of self-induction, referred
to in the foregoing.
[Illustration: _Fig. 114._ CHOKING COIL]
In Fig. 114, A is the choking coil and B an arc lamp, connected up, in
series, with the choking coil.
THE TRANSFORMER.--It is more economical to transmit 10,000 volts a long
distance than 1,000 volts, because the lower the pressure, or the
voltage, the larger must be the conductor to avoid loss. It is for this
reason that 500 volts, or more, are used on electric railways. For
electric light purposes, where the current goes into dwellings, even
this is too high, so a transformer is used to take a high-voltage
current from the main line and transform it into a low voltage. This is
done by means of two distinct coils of wire, wound upon an iron core.
[Illustration: _Fig. 115._ A TRANSFORMER]
In Fig. 115 the core is O-shaped, so that a primary winding (A), from
the electrical source, can be wound upon one limb, and the secondary
winding (B) wound around the other limb. The wires, to supply the
lamps, run from the secondary coil. There is no electrical connection
between the two coils, but the action from the primary to the secondary
coil is solely by induction. When a current passes through the primary
coil, the surging movement, heretofore explained, is transmitted to the
iron core, and the iron core, in turn, transmits this electrical energy
to the secondary coil.
HOW THE VOLTAGE IS DETERMINED.--The voltage produced by the secondary
coil will depend upon several things, namely, the strength of the
magnetism transmitted to it; the rapidity, or periodicity of the
current, and the number of turns of wire around the coil. The voltage is
dependent upon the length of the winding. But the voltage may also be
increased, as well as decreased. If the primary has, we will say, 100
turns of wire, and has 200 volts, and the secondary has 50 turns of
wire, the secondary will give forth only one-half as much as the
primary, or 100 volts.
If, on the other hand, 400 volts would be required, the secondary should
have 200 turns in the winding.
VOLTAGE AND AMPERAGE IN TRANSFORMERS.--It must not be understood that,
by increasing the voltage in this way, we are getting that much more
electricity. If the primary coil, with 100 turns, produces a current of
200 volts and 50 amperes, which would be 200 × 50 = 10,000 watts, and
the secondary coil has 50 turns, we shall have 100 volts and 100
amperes: 100 (V.) × 100 (A.) = 10,000 watts. Or, if, on the other hand,
our secondary winding is composed of 200 turns, we shall have 400 volts
and 25 amperes, 400 (volts) × 25 (amperes) also gives 10,000 watts.
Necessarily, there will be some loss, but the foregoing is offered as
the theoretical basis of calculation.