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.

To explain this, study Fi, 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 Fi 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 Fi, 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 Fi, there is no current in the loop, or, if there is any, it moves faintly in the direction of the dart (E).

CURRENT REVERSING ITSELF.-When the loop reaches its lowest point (Fi 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.

Instead, therefore, of using a single loop, we may make four loops (Fi, 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 Fi. 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 Fi, 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).

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 Fig 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 Fi, 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.

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, Fi. 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).

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 Fi, 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 Fi. 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, Fi, 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.

HOW AN ALTERNATING DYNAMO IS MADE.-It is now necessary to apply these principles in the construction of an alternating-current machine. Fi is a diagram representing the various elements, and the circuiting.

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.

In Fi, 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.

In Fi 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 x 50 = 10,000 watts, and the secondary coil has 50 turns, we shall have 100 volts and 100 amperes: 100 (V.) x 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) x 25 (amperes) also gives 10,000 watts.

Necessarily, there will be some loss, but the foregoing is offered as the theoretical basis of calculation.