Transformer

TRANSFORMER, a device used for changing 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 around 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 ratio and indicates the voltage increase of the transformer. If U₂ > U₁, then the transformer increases the voltage; if n₂ < n₁, it decreases it. One and the same transformer can be a step-up or step-down transformer depending on which winding the current from the circuit is fed into.

The work of the current in the first winding is I₁V₁ = W₁, the work of the current in the second winding is I₂V₂ = 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, then according to the law of conservation of energy we have I₁V₁ = I₂V₂, from which I₁/I₂ = V₂/V₁ = n₂/n₁. It is evident from the formulas that the voltage increases by the same factor that the current strength decreases. The product I₁n₁ = I₂n₂ is called "ampere-turns" in technology and is used for calculation. Thus, knowing the maximum current strength

in the primary winding (or the maximum current strength in the secondary winding and the voltage in it). These formulas are only approximately correct, since part of the energy of the primary current goes into heating the windings and the core (so-called Joule heat) and into 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 steel, covered with varnish for insulation or separated by paper. The energy used for heating and Foucault currents is called energy loss in the transformer. For good transformers, the 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, separate for each phase. A direct current transformer consists of two generators of high and low voltage, of which one drives the second (see Umformer). Since losses due to heating of the conductor are proportional to I², it is advantageous to use small current strengths when transmitting energy over a long distance; in this case, to maintain power, it is necessary to increase the voltage accordingly, for which step-up transformers are used. In medicine, transformers are used both in X-ray installations and for d'Arsonvalization; the latter is based on 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 high-voltage alternating currents are often obtained. With a number of alternations of about 100,000 per hour, during each half-period, those ionization and chemical processes that are harmful to the organism do not have time to develop, while the electrical and light effects of this current are great.