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At much higher frequencies the transformer core size required drops dramatically: a physically small transformer can handle power levels that would require a massive iron core at mains frequency.
The development of switching power semiconductor devices made switch-mode power supplies viable, to generate a high frequency, then change the voltage level with a small transformer.
Large power transformers are vulnerable to insulation failure due to transient voltages with high-frequency components, such as caused in switching or by lightning.
Transformer energy losses are dominated by winding and core losses. Transformers' efficiency tends to improve with increasing transformer capacity.
The efficiency of typical distribution transformers is between about 98 and 99 percent. As transformer losses vary with load, it is often useful to tabulate no-load loss, full-load loss, half-load loss, and so on.
Hysteresis and eddy current losses are constant at all load levels and dominate at no load, while winding loss increases as load increases.
The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply. Designing energy efficient transformers for lower loss requires a larger core, good-quality silicon steel , or even amorphous steel for the core and thicker wire, increasing initial cost.
The choice of construction represents a trade-off between initial cost and operating cost. Closed-core transformers are constructed in 'core form' or 'shell form'.
When windings surround the core, the transformer is core form; when windings are surrounded by the core, the transformer is shell form.
At higher voltage and power ratings, shell form transformers tend to be more prevalent. Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel.
Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation. The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce their magnitude.
Thinner laminations reduce losses,  but are more laborious and expensive to construct. One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of 'E-I transformer'.
The cut-core or C-core type is made by winding a steel strip around a rectangular form and then bonding the layers together.
It is then cut in two, forming two C shapes, and the core assembled by binding the two C halves together with a steel strap. A steel core's remanence means that it retains a static magnetic field when power is removed.
When power is then reapplied, the residual field will cause a high inrush current until the effect of the remaining magnetism is reduced, usually after a few cycles of the applied AC waveform.
On transformers connected to long, overhead power transmission lines, induced currents due to geomagnetic disturbances during solar storms can cause saturation of the core and operation of transformer protection devices.
Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability silicon steel or amorphous non-crystalline metal alloy.
The higher initial cost of the core material is offset over the life of the transformer by its lower losses at light load.
Powdered iron cores are used in circuits such as switch-mode power supplies that operate above mains frequencies and up to a few tens of kilohertz.
These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF band , cores made from non-conductive magnetic ceramic materials called ferrites are common.
Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite.
The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The primary and secondary coils are often wound concentrically to cover the entire surface of the core.
This minimizes the length of wire needed and provides screening to minimize the core's magnetic field from generating electromagnetic interference.
Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size about half , lower weight about half , less mechanical hum making them superior in audio amplifiers , lower exterior magnetic field about one tenth , low off-load losses making them more efficient in standby circuits , single-bolt mounting, and greater choice of shapes.
The main disadvantages are higher cost and limited power capacity see Classification parameters below. Because of the lack of a residual gap in the magnetic path, toroidal transformers also tend to exhibit higher inrush current, compared to laminated E-I types.
Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to hundreds of megahertz, to reduce losses, physical size, and weight of inductive components.
A drawback of toroidal transformer construction is the higher labor cost of winding. This is because it is necessary to pass the entire length of a coil winding through the core aperture each time a single turn is added to the coil.
As a consequence, toroidal transformers rated more than a few kVA are uncommon. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings.
A transformer can be produced by placing the windings near each other, an arrangement termed an "air-core" transformer. An air-core transformer eliminates loss due to hysteresis in the core material.
Air-core transformers are unsuitable for use in power distribution,  but are frequently employed in radio-frequency applications.
The electrical conductor used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn.
For small transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire.
Larger power transformers may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.
High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided Litz wire to minimize the skin-effect and proximity effect losses.
The transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size, aiding manufacture.
The windings of signal transformers minimize leakage inductance and stray capacitance to improve high-frequency response. Coils are split into sections, and those sections interleaved between the sections of the other winding.
Power-frequency transformers may have taps at intermediate points on the winding, usually on the higher voltage winding side, for voltage adjustment.
Taps may be manually reconnected, or a manual or automatic switch may be provided for changing taps. Automatic on-load tap changers are used in electric power transmission or distribution, on equipment such as arc furnace transformers, or for automatic voltage regulators for sensitive loads.
Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker.
A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar.
Small dry-type and liquid-immersed transformers are often self-cooled by natural convection and radiation heat dissipation. As power ratings increase, transformers are often cooled by forced-air cooling, forced-oil cooling, water-cooling, or combinations of these.
The mineral oil and paper insulation system has been extensively studied and used for more than years. Building regulations in many jurisdictions require indoor liquid-filled transformers to either use dielectric fluids that are less flammable than oil, or be installed in fire-resistant rooms.
The tank of liquid filled transformers often has radiators through which the liquid coolant circulates by natural convection or fins. Some large transformers employ electric fans for forced-air cooling, pumps for forced-liquid cooling, or have heat exchangers for water-cooling.
Polychlorinated biphenyls have properties that once favored their use as a dielectric coolant , though concerns over their environmental persistence led to a widespread ban on their use.
Some transformers, instead of being liquid-filled, have their windings enclosed in sealed, pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas.
Insulation must be provided between the individual turns of the windings, between the windings, between windings and core, and at the terminals of the winding.
Inter-turn insulation of small transformers may be a layer of insulating varnish on the wire. Layer of paper or polymer films may be inserted between layers of windings, and between primary and secondary windings.
A transformer may be coated or dipped in a polymer resin to improve the strength of windings and protect them from moisture or corrosion.
The resin may be impregnated into the winding insulation using combinations of vacuum and pressure during the coating process, eliminating all air voids in the winding.
In the limit, the entire coil may be placed in a mold, and resin cast around it as a solid block, encapsulating the windings.
Large oil-filled power transformers use windings wrapped with insulating paper, which is impregnated with oil during assembly of the transformer.
Oil-filled transformers use highly refined mineral oil to insulate and cool the windings and core. Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly dried of residual moisture before the oil is introduced.
Drying may be done by circulating hot air around the core, by circulating externally heated transformer oil, or by vapor-phase drying VPD where an evaporated solvent transfers heat by condensation on the coil and core.
For small transformers, resistance heating by injection of current into the windings is used. Larger transformers are provided with high-voltage insulated bushings made of polymers or porcelain.
A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.
Various specific electrical application designs require a variety of transformer types. Although they all share the basic characteristic transformer principles, they are customized in construction or electrical properties for certain installation requirements or circuit conditions.
In electric power transmission , transformers allow transmission of electric power at high voltages, which reduces the loss due to heating of the wires.
This allows generating plants to be located economically at a distance from electrical consumers. In many electronic devices, a transformer is used to convert voltage from the distribution wiring to convenient values for the circuit requirements, either directly at the power line frequency or through a switch mode power supply.
Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers.
Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires. A balun transformer converts a signal that is referenced to ground to a signal that has balanced voltages to ground , such as between external cables and internal circuits.
Isolation transformers prevent leakage of current into the secondary circuit and are used in medical equipment and at construction sites.
Resonant transformers are used for coupling between stages of radio receivers, or in high-voltage Tesla coils.
Electromagnetic induction , the principle of the operation of the transformer, was discovered independently by Michael Faraday in and Joseph Henry in Faraday performed early experiments on induction between coils of wire, including winding a pair of coils around an iron ring, thus creating the first toroidal closed-core transformer.
The first type of transformer to see wide use was the induction coil , invented by Rev. Nicholas Callan of Maynooth College , Ireland in Induction coils evolved from scientists' and inventors' efforts to get higher voltages from batteries.
Since batteries produce direct current DC rather than AC, induction coils relied upon vibrating electrical contacts that regularly interrupted the current in the primary to create the flux changes necessary for induction.
Between the s and the s, efforts to build better induction coils, mostly by trial and error, slowly revealed the basic principles of transformers.
By the s, efficient generators producing alternating current AC were available, and it was found AC could power an induction coil directly, without an interrupter.
In , Russian engineer Pavel Yablochkov invented a lighting system based on a set of induction coils where the primary windings were connected to a source of AC.
The secondary windings could be connected to several 'electric candles' arc lamps of his own design.
The coils Yablochkov employed functioned essentially as transformers. In , the Ganz factory , Budapest, Hungary, began producing equipment for electric lighting and, by , had installed over fifty systems in Austria-Hungary.
Their AC systems used arc and incandescent lamps, generators, and other equipment. Lucien Gaulard and John Dixon Gibbs first exhibited a device with an open iron core called a 'secondary generator' in London in , then sold the idea to the Westinghouse company in the United States.
Induction coils with open magnetic circuits are inefficient at transferring power to loads. Until about , the paradigm for AC power transmission from a high voltage supply to a low voltage load was a series circuit.
Open-core transformers with a ratio near were connected with their primaries in series to allow use of a high voltage for transmission while presenting a low voltage to the lamps.
The inherent flaw in this method was that turning off a single lamp or other electric device affected the voltage supplied to all others on the same circuit.
Many adjustable transformer designs were introduced to compensate for this problematic characteristic of the series circuit, including those employing methods of adjusting the core or bypassing the magnetic flux around part of a coil.
In both designs, the magnetic flux linking the primary and secondary windings traveled almost entirely within the confines of the iron core, with no intentional path through air see Toroidal cores below.
The new transformers were 3. Transformers today are designed on the principles discovered by the three engineers.
They also popularized the word 'transformer' to describe a device for altering the EMF of an electric current  although the term had already been in use by He assigned to William Stanley the task of developing a device for commercial use in United States.
This design  was first used commercially in the US in  but Westinghouse was intent on improving the Stanley design to make it unlike the ZBD type easy and cheap to produce.
Pre-wound copper coils could then be slid into place, and straight iron plates laid in to create a closed magnetic circuit. Westinghouse otained a patent for the new low-cost design in In , Nikola Tesla invented the Tesla coil , an air-cored, dual-tuned resonant transformer for producing very high voltages at high frequency.
Audio frequency transformers " repeating coils " were used by early experimenters in the development of the telephone.
From Wikipedia, the free encyclopedia. Electrical device that transfers energy through electromagnetic induction from one circuit to another circuit.
It may be used to step up or step down the voltage. This article is about the electrical device.
For the media and toy franchise, see Transformers. For other uses, see Transformer disambiguation. Combining the ratio of eq.
Main article: Leakage inductance. See also: Steinmetz equivalent circuit. Main article: Transformer types. Main article: Induction coil.
High-voltage transformer fire barriers Inductive coupling Load profile Magnetization Paraformer Polyphase system Power inverter Rectiformer Voltage converter.
The negative sign in eq. The Lineman's and Cableman's Handbook 11th ed. New York: McGraw-Hill. Archived from the original PDF on Retrieved Transactions of the American Institute of Electrical Engineers.
Tipler, Physics , Worth Publishers, Inc. Alternating Current Machines 5th ed. London: Pitman. Dalessandro, F.