Transformer

TRANSFORMER, a device used to change the voltage of an electric current. A transformer consists of a closed iron frame (see figure) with two windings of insulated wire, a and b, wound upon it with a number of turns n₁ and n₂. When an alternating current is passed through one of them, a current also arises in the second due to induction. The electromotive force induced in the second winding is proportional to the electromotive force in the primary and the ratio of the number of turns: The ratio n₂/n₁ is called the transformation coefficient and indicates the voltage step-up of the transformer. If n₂ > n₁, the transformer steps up the voltage; if n₁ > n₂, it steps it down. One and the same transformer can be a step-up or step-down transformer depending on which winding the current from the circuit is sent into. The work of the current in the first winding I₁E₁ = W₁, the work of the current in the second winding I₂E₂ = W₂, where I is the current strength. Since almost all magnetic lines of force excited by the current of the first winding go into the core, according to the law of conservation of energy we have I₁E₁ = I₂E₂, whence I₁/I₂ = n₂/n₁. From the formulas it is evident that by how many times the voltage increases, by that many times the current strength decreases. The product I₁n₁ = I₂n₂ is called "ampere-turns" in engineering and serves for calculation. Thus, knowing the maximum current strength

in the primary winding (or I; -; |________) the maximum current strength in the secondary winding and the voltage in it. These formulas are correct only approximately, because part of the energy of the primary current goes to heating the windings and the core (so-called Joule heat) and to so-called Foucault currents in the core. To reduce the latter, the core is usually made not solid, but of thin sheets of special transformer sheet iron, coated with varnish for insulation or laid with paper. The energy expended on heating and Foucault currents is called energy loss in the transformer. For good transformers, energy loss does not exceed 5%. Transformers designed for high power are placed in mineral, so-called transformer oil, for better insulation of the secondary winding and cooling of the entire transformer. To increase power, the windings are made of several parts, or sections; the sections of the primary winding are connected in parallel, the sections of the secondary in series. For the transformation of three-phase current, transformers with three primary and secondary windings are used, separately for each phase. A direct-current transformer consists of two high- and low-voltage generators, of which one drives the second (see Rotary converter). Since losses for conductor heating are proportional to I², when transmitting energy over a long distance it is advantageous to use small current strengths, while to preserve power it is necessary to correspondingly increase the voltage, for which step-up transformers are used. In medicine, transformers are used both in X-ray installations and for d'Arsonvalization; the basis of the latter is high-frequency alternating currents, so-called Tesla currents (named after the scientist who proposed them). To obtain Tesla currents, rapid oscillations arising during the spark discharge of a capacitor are passed through the primary coil of a well-insulated transformer without an iron core. If the primary coil contains few turns and the secondary several hundred turns, the voltage increases several tens of times and alternating currents of high voltage are obtained. With a number of alternations of about 100,000 per hour, during each half-period those ionization and chemical processes which are harmful to the organism do not have time to develop, whereas the electrical and light effects of this current are great.