CHAPTER VI. TRANSFORMERS.
Where alternating current is available in commercial wireless telegraph stations, the induction coil has been gradually superseded by the more modern transformer for charging the oscillation condenser. Since the transformer is less expensive to construct than an induction coil capable of transmitting the same distance, it is favored by many amateur experimenters. A one quarter kilowatt transformer has a sending range of over 50 miles when used with a properly constructed aerial about 80 feet high. This is probably the size best suited to the average private installation.
The secondary of a wireless transformer seldom develops potentials exceeding 15,000 to 20,000 volts, while those of an induction coil range from 1 5,000 to 300,000 volts. However, the strength of the secondary current of a transformer is so much greater than that of an induction coil, that more powerful and penetrating waves are developed. For these reasons a transformer is always rated by its output in watts or kilowatts rather than by the spark length produced at the secondary terminals. The spark of a one quarter kilowatt transformer is only 0.25 to 0.50 of an inch, while the spark of a one half kilowatt transformer may be the same length but still represent more energy.
There are two distinct types of transformers in use, known as the "open" and "closed," accordingly whether the core is straight like the core of an induction coil or in the form of a hollow rectangle. The open core type is used in the government stations and by the United Wireless Telegraph Co. It is the simpler and more easily constructed of the two, but is also less efficient and requires that more material be expended to bring it up to a definite rating. In principle it is simply an induction coil operated on alternating current minus the interrupter and condenser. In view of the greater currents employed, the windings must be larger and heavier than those of the induction coil to prevent heating.
Before commencing the construction of a transformer one should read the chapter on induction coils and use the same care emphasized there in regard to building coils.
Open Core Transformer.—The transformer described below will transmit from 20 to 75 miles and consume about 300 watts on the no volt 60 cycle alternating current.
The core is 16 inches long and 2 inches in diameter. It is built up of soft iron wires in the same manner as if it were the core of an induction coil.
The primary is composed of two layers of No. 14 double cotton covered B. S. gauge magnet wire. The primary is 14 inches long and is wrapped with a layer of micanite cloth 3/8 inch thick to separate it from the secondary winding.
It is never advisable to use shellacked cotton cloth as insulation. When cotton is dried and shellacked, it is at first a good insulator, but soon becomes capable of absorbing moisture. Shellac carbonizes at a low temperature, and if a transformer or coil having any of this sort of insulation entering into its construction is overheated, the insulation is liable to become a conductor. Micanite cloth is the best insulation for transformers. The dielectric strengths of the different forms of micanite are shown by the following table.
The secondary is wound in ten sections over the micanite insulation. Each section is 6 inches in diameter and 1 1/4 inches thick and is wound with No. 30 B. S. single silk covered wire. The sections are separated by disks of blotting paper 1/8 inch thick and 7 inches in diameter, treated as described in Chapter IV. The completed transformer should be placed in a box and covered with oil.
A One Quarter Kilowatt Closed Core Transformer. The simplest form of a closed core transformer consists of two independent coils of insulated wire wound upon an iron ring. When an alternating current is passed through one of the coils, known as the primary, it generates a magnetic flux in the iron core. The lines of force passing through the core induce in the secondary coil an electromotive force the magnitude of which is in nearly the same ratio to the primary inducing electromotive force as the number of turns of wire in the secondary is to the number of turns in the primary. For example, if it is desired to raise the potential of the no volt alternating current to 22,000 volts, the number of turns in the secondary of the transformer must be at least 200 times the number in the primary.
A circular ring of iron wire presents several theoretical advantages as a transformer core but it would be difficult to form and afterwards place the windings in position. The core is therefore usually in the form of a hollow rectangle, built up of very thin sheets or laminations of soft iron carefully insulated from one another by a coat of varnish. If the core were solid or the separate laminations not insulated from one another, heavy currents, known as eddy currents, would be set up in the iron and cause heating. A considerable loss in the efficiency of the transformer would also result.
One half of both the secondary and the primary windings of a properly designed transformer are placed on opposite sides or "legs" of the core in order to reduce the magnetic leakage and increase the efficiency. The only difficulty involved in such construction is the proper insulation of the primary from the secondary, but if careful attention is given to this point no difficulty will be experienced.
Core.—The dimensions and method of assembling the core laminations are indicated by Fig. 44. Long strips are cut from soft Russian or Swedish stovepipe iron. The strips, which are 1 3/4 inches wide are then cut up into short lengths, one half of which are 7 3/4 inches long and the other half 4 3/4 inches. Enough are cut to form two piles of each size 1 3/4 inches high when compressed. The completed core will then form a hollow rectangle 9 1/2 x 6 1/2 x 1 3/4 inches.
Fig. 44. Assembly and Dimensions of Core.
The strips must be dipped in some good insulating varnish such as P. & B. compound and thoroughly dried before they are assembled. Both "legs" (the longest sides) are laid on a table with the alternate ends overlapping as shown by A and B in Fig. 44. After the short pieces C and D have been slipped between the overlapping ends the whole core is squared up. The strips D are then carefully removed and one end of the core thus left open until all the windings are in place. Three or four layers of well varnished linen cloth are wound tightly over the "legs" preparatory to winding on the primary.
Primary.—Four fiber heads, H, 4 3/4 inches square, 1/2 inch thick and having a square hole 1 7/8 x 1 7/8 inches cut in the center are required. One of the fiber heads is placed on each end of the assembled "leg" as shown in Fig. 48.
Fig. 45. Fiber Head and Separator.
The primary winding is wound in six layers, 4 1/2 inches long, three layers on each "leg." About three pounds of No. 16 B. S. gauge double cotton covered magnet wire are required for the winding. The terminals of the two halves of the primary are led through the fiber heads at the same end of the transformer. The windings are not to be carried close up to the fiber heads but begin and end about 1/4 inch from them, so that the remaining space may be filled by winding in a strip of micanite cloth 1/4 inch wide. The primary and secondary windings are separated by a strip of micanite cloth 5 inches wide, wound over both of the primary windings close up to the heads until a layer 1/2 inch thick is formed.
Secondary.—The form on which the secondary sections are wound is illustrated in Fig. 46. All the parts are cut out of wood except the shaft and are made of the dimensions indicated. If the center of the form is slightly tapered it will greatly facilitate the removal of the completed sections. Sixteen sections are required. When removed from the winder they will be in the form of hollow squares 4 1/2 x 4 1/2 x 7/16 inches.
Fig. 46. Section Form.
About ten pounds of No. 34 B. S. gauge silk covered wire are required to wind the sections. Cotton covered wire must not be used to avoid expense, because with it a sufficient number of secondary turns cannot be secured to bring the secondary current up to the proper voltage. By observing explicitly the instructions and precautions given below no trouble will be experienced in handling enameled wire and forming the sections. The form should be placed in a lathe chuck or some other machine which is convenient and whereby the form may be rapidly revolved under the control of the operator.
Saw slots are cut in the wooden flanges and the center of the form as shown in the illustration so that silk threads may be passed under and around the completed section and tied so that a possible "cave in" of the wire is prevented. After tying up the section should be removed from the form by unscrewing the nut and taking off the flange.
Fig. 47. Methods of Connecting Sections.
When winding the wire it must be very carefully watched for loops or kinks and only be laid on in even layers. It must also be borne in mind that enameled wire cannot be as tightly wound as fiber covered wire for reasons heretofore explained. In case the wire becomes broken, it must be smoothly spliced and soldered. Do not under any consideration use acid as a flux or heat the wire with a flame. Acid will corrode the fine wire, and the flame will badly oxidize or melt it. Use a short piece of No. 8 B. S. gauge tinned copper wire set in a small file handle as a soldering iron, and rosin as a flux. Paraffin some silk binding tape such as dressmakers use and wrap the joint with a small piece. The sections as they are removed from the winder must be taped and then carefully marked with an arrow which points in the direction of the winding.
Fig. 47 illustrates the two methods of connecting up the sections. It will be noticed in the second method that the arrow denoting the direction of winding points down on every alternate section. This does not indicate necessarily that every alternate section is wound in an opposite direction from the other, but that they have merely been turned around so that the arrows come on a reverse side of the core and point in an opposite direction. This precaution must be taken in order that the current will flow through all the sections, and is made necessary because the inside terminal of one section is connected to the inside terminal of the adjacent section and the outside terminal of that section is connected to the outside terminal of the next adjacent section. The first method, A, illustrated in Fig. 44, is less complicated and does not require this reversal, but for various reasons is not to be recommended in place of B.
Eight of the completed and taped sections are placed on each "leg" of the transformer, with one of the fiber separators between each pair as in Figs. 48 and 49. When each "leg" has been completely assembled, solder all the secondary terminals together so as to connect as in Fig. 47.
Fig. 48. Assembly of Leg.
Then place the remaining fiber head, S, on each of the "legs" and finish assembling the core by slipping in the end strips D.
Square the core up perfectly true and fasten by four fiber strips M, Fig. 49, 9 3/4 inches long, 1 3/4 inches wide and 1/2-inch thick. The strips are placed in the position shown in Fig. 46 and a hole P bored in the end of each. Four 1/4-inch bolts, two of which are 3 inches long and two 3 1/2 inches, pass through the holes in the strips, so that when the nuts are screwed on the core is clamped firmly. The two longer bolts are placed at the same end of the transformer.
Fig. 49. Transformer with One Secondary removed.
The terminals of the primary lead out to four binding posts mounted on the fiber strips. The pillars which support the secondary binding posts are fiber rods, 1 inch in diameter and 2 inches long. The lower end of each is bored and tapped to fit the upper ends of the longer bolts which clamp the fiber strips together. An insulating shield must be placed between the two secondary windings to prevent sparks from jumping across. A piece of fiber 5 x 5 x 1/8 inches will serve nicely for this purpose. If the primary windings are placed in series the transformer will consume about 300 watts. When the transformer is placed in a box and the box filled with some boiled amber petroleum, the windings may be connected in parallel and the transformer will consume about 500 watts. It will then transmit over 100 miles providing the aerial is at least 100 feet high.
The wiring connections are diagramed in Fig. 50. A variable inductance or reactance coil is connected in series with the primary circuit to steady the current, as explained in the paragraph under reactance. A reactance suitable for this transformer may be constructed by winding two layers of No. 12 B. S. gauge cotton covered wire, six inches long, around a hollow wooden tube made of cigar box wood. The core is built up of soft sheet iron to form a rectangle 8 x 1 3/4 x 1 3/4 inches which will just slide in and out of the tube. The windings should be about six inches long.
Fig. 50. Wiring Diagram.
Five half-gallon Leyden jars form about the right capacity for the secondary of the transformer when the windings are in series. Twice that number must be used when the windings are in parallel.
The secondary sections must always be kept in series, as otherwise the voltage would not be high enough to properly charge the condenser.