The regulation is ordinarily calculated as follows : Given %IR, the per cent alternating current resistance, or, better yet, the per cent impedance watts, of the transformer; %IX, the per cent reactance of the transformer; p, the power factor of the load, and, q, the reactive factor of the load; then, is a vector diagram, showing the various component voltages.
(6) total losses of a transformer in power and distribution service vary from about of i % to about 4%, de pending on size, frequency, voltage, etc., making the efficiency vary from 96% to 99.5%• It is this fact of high efficiency, coupled with the further fact of the absence of any mechanical motion that has made the transformer a most valuable apparatus and has led to the adoption of alternating currents by most systems.
(7) Temperature Rise.—The maximum load that may safely be put on a transformer is limited by its temperature rise. The insulations commonly used in transformers, being largely organic substances, are subject to rapid deterioration under overheat. Temperature limits vary in different countries, ranging from so° to 6° C rise above the ambient (surrounding) temperature.
(8) Insulation Strength.—Transformers not only produce higher and lower voltages, but they also insulate one circuit from the other and from ground. The safety of both the transformer and the circuits which it links is therefore dependent on the strength of its insulation. For rules of various societies on this subject and for methods of making such tests reference may be made to the appropriate literature listed in the bibliography.
Transformer Connections.—For polyphase circuits, a great number of transformer connections have been developed; the standard connections for three-phase circuits being the Y and the delta connections. Various connections have also been developed for accomplishing transformation from a polyphase system of a given number of phases to one of a different number of phases, with or without voltage transformation.
either cylindrical and arranged one inside the other, in which case they are concentric; or they may be flat coils and sandwiched in with each other (to obtain reasonably low leakage reactance), in which case they are interleaved. The windings may be interleaved with either core or shell type of magnetic circuit. The coils may be wound on circular forms, in which case the transformer is said to be of circular-coil construction; or they may be wound on rectangular forms, as in the so-called pancake or shell-type coils.
Transformers may be oil-immersed or dry. The former is very desirable for the higher voltage and larger power units for im proved insulation and cooling, while the latter is more desirable for miniature transformers for greater convenience. Sometimes considerations of fire risk lead to the use of dry transformers in even large sizes, which then are cooled by an air blast. Based on the method used for cooling, transformers may be, (I) water cooled, in which case cool water is circulated in sealed tubes im mersed in the insulating oil filling the transformer case ; air blast, in which air is blown by fans through the air ducts of the windings; (3) air-cooled or natural draft, depending on natural radiation and convection from the exposed surface of trans former or its case; (4) forced oil, in which the hot oil is pumped out into radiators, cooled and returned into the transformer; (5) air-jet cooled, in which jets of air are blown on the tank and its radiating pipes to break up the stagnant film of air adhering to them and thus to accelerate the cooling by convection of air cur rents.