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.