ooling oil and commencing normal operation.
Those credited with the invention of the transformer include:
- Michael Faraday, who invented an 'induction ring' on August 29, 1831. This was the first transformer, although Faraday used it only to demonstrate the principle of electromagnetic induction and did not foresee the use to which it would eventually be put.
- Lucien Gaulard and John Dixon Gibbs, who first exhibited a device called a 'secondary generator' in London in 1881 and then sold the idea to American company Westinghouse. This may have been the first practical power transformer, but was not the first transformer of any kind. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Their early devices used a linear iron core, which was later abandoned in favour of a more efficient circular core.
- William Stanley, an engineer for Westinghouse, who built the first practical device in 1885 after George Westinghouse bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886.
- Hungarian engineers Ottó Bláthy, Miksa Déri and Károly Zipernowsky at the Ganz company in Budapest in 1885, who created the efficient "ZBD" model based on the design by Gaulard and Gibbs.
- Nikola Tesla in 1891 invented the Tesla coil, which is a high-voltage, air-core, dual-tuned resonant transformer for generating very high voltages at high frequency.
Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to gigawatt units used to interconnect large portions of national power grids, all operating with the same basic principles and with many similarities in their parts.
- Up to 1 watt: Signal transformers, interstage coupling
- 1 - 1000 watts: Small power transformers, filament transformers, audio output transformers
- 1 kilowatt - 1 megawatt: Power transformers; larger units in this range may be oil filled
- 1 megawatt and over: Large power transformers, used for substations, large electrical consumers, and for power plants and transmission.
Transformers can be classified into various types according to the ratio of the numbers of turns in the coils, as well as whether or not the primary and secondary are isolated:
- the secondary has more turns than the primary
- the secondary has fewer turns than the primary
- intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses (otherwise the output voltage would be a little less than, rather than the same as, the input voltage).
- the primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer.
In all cases the primary winding, or the secondary winding, or both, may have taps that allow selection of one of several different ratios of primary to secondary turns.
An ideal transformer would have no loss, and would therefore be 100% efficient. However, the coils of a real transformer have resistance. When modeling a real transformer the resistance can be considered as existing in series with the winding of an ideal transformer.
Large power transformers are often more than 98% efficient, in terms of energy supplied to the primary winding of the transformer and coupled to the secondary. The remaining 2% (or less) of the input energy is lost to:
- Winding resistance
- Eddy currents
- Induced currents circulating in the core causing resistive heating of the core.
- Stray losses
- Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects, such as the transformer's support structure, and be converted to heat.
- Hysteresis losses
- Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. Differing core materials will have different levels of hys