Transformer

Transformer, electrical device consisting of one coil of wire placed in close proximity to one or more other coils, used to couple two or more alternating-current (AC) circuits together by employing the induction between the coils (see Electricity). The coil connected to the power source is called the primary coil, and the other coils are known as secondaries. A transformer in which the secondary voltage is higher than the primary is called a step-up transformer; if the secondary voltage is less than the primary, the device is known as a step-down transformer. The product of current times voltage is constant in each set of coils, so that in a step-up transformer, the voltage increase in the secondary is accompanied by a corresponding decrease in the current.

Large devices are used in electricity supply, and small units in electronic devices (see Electronics). Industrial and residential power tranformers that operate at the line frequency (50 Hz in the United Kingdom), may be single phase or three-phase, and are designed to handle high voltages and currents. Efficient power transmission requires a step-up transformer at the power-generating station to raise voltages, with a corresponding decrease in current. Line power losses are proportional to the square of the current times the resistance of the power line, so that very high voltages and low currents are used on long-distance transmission lines to reduce losses. At the receiving end, step-down transformers reduce the voltage, and increase the current, to the residential or industrial voltage levels, usually around 240 volts.

Power transformers must be efficient and should dissipate as little power as possible in the form of heat during the transformation process. Efficiencies are normally above 99 per cent and are obtained by using special steel alloys to couple the induced magnetic fields between the primary and secondary windings. The dissipation of even 0.5 per cent of the power transmitted in a large transformer generates large amounts of heat, which requires special cooling provisions. Typical power transformers are installed in sealed containers that have oil or another substance circulating through the coils to transfer the heat to external radiator surfaces, where it can be discharged to the surrounding atmosphere.

In electronic equipment, transformers with capacities in the order of one kilowatt are largely used ahead of a rectifier, which in turn supplies direct current (DC) to the equipment (see Rectification). Such electronic power transformers are usually made of stacks of steel alloy sheets, called laminations, on which copper wire coils are wound. Transformers in the 1 to 100 watt power level are principally used as step-down transformers to couple electronic circuits to loudspeakers in radios, television sets, and high-fidelity equipment (see Sound Recording and Reproduction). Known as audio transformers, these devices use only a small fraction of their power rating to deliver signals in the audible ranges, with minimum distortion. The transformers are judged on their ability to reproduce sound-wave frequencies (from 20 Hz to 25 kHz) with minimal distortion over the full sound power level (see Frequency; Sound).

At power levels of one milliwatt or less, transformers are primarily used to couple ultra-high-frequency (UHF), very-high frequency (VHF), radio-frequency (RF), and intermediate-frequency (IF) signals, and to increase their voltage. These high-frequency transformers usually operate in a tuned or resonant circuit (see Resonance), in which tuning is used to remove unwanted electrical noise at frequencies outside the desired transmission range.