Transformadores

Transformer

Un transformador es un dispositivo eléctrico que transfiere energía eléctrica entre dos o más circuitos a través de la inducción electromagnética. La inducción electromagnética produce una fuerza electromotriz dentro de un conductor que está expuesta al tiempo campos magnéticos variables. Los transformadores se utilizan para aumentar o disminuir las tensiones alternas en aplicaciones de energía eléctrica.

Una corriente variable en el arrollamiento primario crea un flujo magnético variable el núcleo del transformador del transformador y un campo variable que incide sobre devanado secundario del transformador. Este campo magnético variable en el devanado secundario induce una fuerza variable electromotriz (EMF) o la tensión en el devanado secundario debido a la inducción electromagnética. Haciendo uso de la ley de Faraday (descubierto en 1831), en relación con las propiedades básicas de permeabilidad magnética, los transformadores pueden ser diseñados para cambiar de manera eficiente voltajes de CA de un nivel de tensión a otro dentro de las redes de energía.

Desde la invención del primer transformador de potencial constante en 1885, los transformadores se han convertido en esencial para la transmisión, distribución y utilización de energía eléctrica alterna actual. Una amplia gama de diseños de transformador se encuentra en aplicaciones de potencia electrónicos y eléctricos. Transformadores varían en tamaño desde los transformadores de RF de menos de un centímetro cúbico de volumen de unidades de interconexión de la red eléctrica que pesa cientos de toneladas.

Principios básicos

Transformador ideal

Para fines de simplificación o de aproximación, es muy común para analizar el transformador como un modelo de transformador ideal como se presenta en las dos imágenes. Un transformador ideal es un transformador teórico y lineal que es sin pérdidas y perfectamente acoplado; es decir, no hay pérdidas de energía y el flujo es totalmente limitado por el núcleo magnético. perfecto acoplamiento implica infinitamente alta permeabilidad magnética del núcleo y inductancias sinuosas y la fuerza magnetomotriz neta cero.

Montado en un poste el transformador de distribución con toma central devanado secundario se utiliza para proporcionar energía 'split-fase "para el servicio residencial y comercial, que en América del Norte se clasifican normalmente 120/240 V.

transformador de medida, con el punto de polaridad y X1 marcas en el terminal del lado de BT

La identidad transformador ideal se muestra en la ecuación. (5) es una aproximación razonable para el transformador comercial típico, con una relación de voltaje y de bobinado relación de vueltas tanto inversamente proporcional a la relación de corriente correspondiente.

Por la ley de Ohm y la identidad transformador ideal:

la impedancia de carga del circuito secundario se puede expresar como eq. (6)

la impedancia de carga aparente a que se refiere el circuito primario se deriva de la ecuación. (7) para ser igual a la relación de vueltas al cuadrado veces la impedancia de carga del circuito secundario.

transformador ideal conectado con la fuente VP sobre el primario y la carga ZL de impedancia en el secundario, donde 0 <ZL <∞.

Una corriente variable en el devanado primario del transformador se crea un flujo magnético variable en el núcleo y un campo magnético variable que incide sobre el devanado secundario. Este campo magnético variable en el secundario induce una fuerza variable electromotriz (EMF) o la tensión en el devanado secundario. Los devanados primario y secundario están envueltos alrededor de un núcleo de infinitamente alta permeabilidad magnética de modo que todo el flujo magnético pasa a través tanto de los devanados primario y secundario. Con una fuente de tensión conectada a la impedancia de devanado primario y carga conectada al arrollamiento secundario, las corrientes del transformador fluyen en las direcciones indicadas.

(Véase también la polaridad.)

De acuerdo con la ley de inducción de Faraday, ya que el mismo flujo magnético pasa a través tanto de los devanados primario y secundario en un transformador ideal, se induce un voltaje en cada devanado, de acuerdo con eq. (1) en el caso arrollamiento secundario, de acuerdo con eq. (2) en el caso devanado primario. La FEM primaria se denomina a veces fuerza contraelectromotriz. Esto está de acuerdo con la ley de Lenz, que establece que la inducción de la EMF siempre se opone el desarrollo de cualquier cambio en el campo magnético.

La relación de tensión de devanado del transformador este modo se ve que ser directamente proporcional a la relación de vueltas de bobinado de acuerdo con eq. (3).

De acuerdo con la ley de conservación de la energía, cualquier impedancia de carga conectada a resultados devanado secundario del transformador ideal en la conservación de la potencia aparente, activa y reactiva consistente con eq. (4).

transformador ideal y la ley de inducción

. (5)

. . . (2)

. . . (1)^[a]

By Ohm's Law and ideal transformer identity

By law of Conservation of Energy, apparent,real and reactive power are each conserved in the input and output

. . . (4)

Combining (3) & (4) with this endnote^[b] yields the ideal transformer identity

Turns ratio . . . (3) where

Combining ratio of (1) & (2)

Ideal transformer equations (eq.)

By Faraday's law of induction:

for step-up transformers, a < 1

for step-down transformers, a > 1

Polaridad

. . . (6)

Una convención de puntos se utiliza a menudo en los esquemas eléctricos del transformador, placa de características o marcas del terminal para definir la polaridad relativa de los devanados del transformador. Positivamente el aumento de la corriente instantánea de entrar en el bobinado primario del punto final induce tensión de polaridad positiva a los arrollamientos secundarios del punto final.

Apparent load impedance Z'_L (Z_L referred to the primary)

Transformador real

Las desviaciones del ideal

. (7)

El modelo transformador ideal descuida los siguientes aspectos básicos lineales en los transformadores reales.

pérdidas en el núcleo, llamadas colectivamente las pérdidas por corrientes de magnetización, consisten en

pérdidas de histéresis debido a la aplicación no lineal de la tensión aplicada en el núcleo del transformador, y

Pérdidas por corrientes parásitas debidas al calentamiento por efecto Joule en el núcleo que son proporcionales al cuadrado de los transformadores de tensión aplicadas.

Mientras que los bobinados en el modelo ideal no tienen resistencias e inductancias infinitos, los bobinados de un transformador real tienen resistencias e inductancias finitos no nulo asociados con:

pérdidas por efecto Joule debido a la resistencia en los devanados primarios y secundarios

flujo de fuga que escapa del núcleo y pasa a través de un devanado solamente resulta en impedancia reactiva primaria y secundaria.

Fuga de inductancia

El modelo transformador ideal supone que todo el flujo generado por los enlaces devanado primario todas las espiras de cada bobinado, incluido él mismo. En la práctica, algunos flujo atraviesa caminos que lo llevan fuera de los arrollamientos. Tal flujo se denomina flujo de fuga, y los resultados en la inductancia de fuga en serie con los devanados del transformador mutuamente acopladas. resultados flujo de dispersión en la energía que se almacenan alternativamente en y dados de alta de los campos magnéticos con cada ciclo de la fuente de alimentación. No es directamente una pérdida de potencia, pero da lugar a la regulación de voltaje inferior, haciendo que el voltaje secundario no ser directamente proporcional a la tensión primaria, en particular bajo carga pesada. Los transformadores son, por tanto, normalmente diseñado para tener muy baja inductancia de fuga.

En algunas aplicaciones se desea una mayor fuga y largos caminos magnéticos, cámaras de aire, o derivaciones de derivación magnética deliberadamente puede ser introducido en un diseño del transformador para limitar la corriente de cortocircuito se suministrará. transformadores con fugas se pueden utilizar para alimentar cargas que exhiben resistencia negativa, como arcos eléctricos, lámparas de vapor de mercurio, y las señales de neón o para el manejo de cargas de manera segura que se convierten periódicamente en cortocircuito, tales como soldadura por arco eléctrico.

Los espacios de aire también se utilizan para mantener un transformador se sature, especialmente transformadores de audio-frecuencia en circuitos que tienen una componente de corriente continua que fluye en los devanados.

El conocimiento de la inductancia de fuga también es útil cuando los transformadores son operados en paralelo. Se puede demostrar que si la impedancia ciento y la relación asociada bobinado reactancia de fuga a la resistencia (X / R) de dos transformadores fueron hipotéticamente exactamente lo mismo, los transformadores se comparten el poder en proporción a sus respectivas clasificaciones de tensión-corriente (por ejemplo, 500 unidad de kVA en paralelo con 1.000 kVA unidad, la unidad más grande sería llevar el doble de la actual). Sin embargo, las tolerancias de impedancia de transformadores comerciales son significativos. Además, la impedancia Z y X relación / R de diferentes transformadores de capacidad tiende a variar, lo que corresponde 1,000 kVA y 500 kVA unidades Los valores son, para ilustrar, respectivamente, Z ≈ 5.75%,X/R ≈ 3.75 and Z ≈ 5%, X/R ≈ 4.75.

circuito Equivalente

Con referencia al diagrama, un comportamiento físico del transformador práctico puede ser representado por un

modelo de circuito equivalente, que puede incorporar un transformador ideal.

Winding pérdidas por efecto Joule y las reactancias de fuga están representados por las siguientes impedancias de bucle en serie del modelo:

El devanado primario: RP, XP

Devanado secundario: RS, XS.

En el curso normal de transformación circuito equivalente del transformador real equivalente del circuito, RS y XS son en la práctica

por lo general a que se refiere el lado primario multiplicando estas impedancias por las vueltas relación al cuadrado, (NP / NS) 2 = a2.

pérdida en el núcleo y la reactancia está representado por las siguientes impedancias de la pierna de derivación del modelo:

Núcleo de hierro o pérdidas: RC

reactancia de magnetización: XM.

RC y XM se denominan colectivamente de desmagnetización rama del modelo.

Pérdidas en el núcleo son causadas principalmente por efectos de histéresis y por corrientes parásitas en el núcleo y son proporcionales al cuadrado del flujo de núcleo para un funcionamiento a una frecuencia dada. El núcleo permeabilidad finita requiere un IM corriente de magnetización para mantener flujo mutuo en el núcleo. corriente de magnetización está en fase con el flujo, la relación entre los dos es no lineal debido a los efectos de saturación. Sin embargo, todas las impedancias del circuito equivalente mostrado son por tales efectos no linealidad no se reflejan normalmente en los circuitos equivalentes del transformador lineal definición y. Con el suministro sinusoidal, flujo del núcleo se retrasa la FEM inducida en 90 °. Con circuito abierto devanado secundario, la magnetización de la rama I0 es igual transformador de corriente sin carga.

El modelo resultante, aunque a veces denominado circuito equivalente 'exacta' basado en supuestos de linealidad, conserva una serie de aproximaciones. El análisis puede simplificarse suponiendo que la impedancia de magnetización rama es relativamente alta y la reubicación de la rama a la izquierda de las impedancias primarias. Esto introduce error, pero permite la combinación de primaria y se hace referencia resistencias secundarias y las reactancias por simple suma como dos impedancias en serie.

Transformador equivalentes parámetros relación de impedancia de circuitos y transformadores se pueden derivar de las siguientes pruebas: prueba de circuito abierto, la prueba de cortocircuito, sinuosas prueba de resistencia, y la relación de transformación de la prueba.

Basic transformer parameters and construction

Flujo de dispersión de un transformador

Flujo de dispersión

By Faraday's Law of induction shown in eq. (1) and (2), transformer EMFs vary according to the derivative of flux with respect to time. The ideal transformer's core behaves linearly with time for any non-zero frequency. Flux in a real transformer's core behaves non-linearly in relation to magnetization current as the instantaneous flux increases beyond a finite linear range resulting in magnetic saturation associated with increasingly large magnetizing current, which eventually leads to transformer overheating.

The EMF of a transformer at a given flux density increases with frequency. By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Conversely, frequencies used for some railway electrification systems were much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50–60 Hz) for historical reasons concerned mainly with the limitations of early electric traction motors. Consequently, the transformers used to step-down the high overhead line voltages (e.g. 15 kV) were much larger and heavier for the same power rating than those required for the higher frequencies.

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with 'volts per hertz' over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

One example is in traction transformers used for electric multiple unit and high-speed train service operating across regions with different electrical standards. The converter equipment and traction transformers have to accommodate different input frequencies and voltage (ranging from as high as 50 Hz down to 16.7 Hz and rated up to 25 kV) while being suitable for multiple AC asynchronous motor and DC converters and motors with varying harmonics mitigation filtering requirements.

Large power transformers are vulnerable to insulation failure due to transient voltages with high-frequency components, such as caused in switching or by lightning.

At much higher frequencies the transformer core size required drops dramatically: a physically small and cheap transformer can handle power levels that would require a massive iron core at mains frequency. The development of switching power semiconductor devices and complex integrated circuits made switch-mode power supplies viable, to generate a high frequency from a much lower one (or DC), change the voltage level with a small transformer, and, if necessary, rectify the changed voltage.

Energy losses

Real transformer energy losses are dominated by winding resistance joule 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 express these losses in terms of 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 overwhelmingly without load, while variable winding joule losses dominating increasingly 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.

Transformer losses arise from:

Winding joule losses

Current flowing through a winding's conductor causes joule heating. As frequency increases, skin effect and proximity effect causes the winding's resistance and, hence, losses to increase.

Core losses

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. According to Steinmetz's formula, the heat energy due to hysteresis is given by

Power transformer over-excitation condition caused by decreased frequency; flux (green), iron core's magnetic characteristics (red) and magnetizing current (blue).

Effect of frequency

If the flux does not contain even harmonics the following equation can be used for half-cycle average voltage E_avg of any waveshape:

Transformer universal EMF equation

If the flux in the core is purely sinusoidal, the relationship for either winding between its rmsvoltage E_rms of the winding, and the supply frequency f, number of turns N, core cross-sectional area a in m² and peak magnetic flux density B_peak in Wb/m² or T (tesla) is given by the universal EMF equation:

, and,

hysteresis loss is thus given by

where, f is the frequency, η is the hysteresis coefficient and β_max is the maximum flux density, the empirical exponent of which varies from about 1.4 to 1.8 but is often given as 1.6 for iron.

Eddy current losses

Ferromagnetic materials are also good conductors and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness. Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.

Magnetostriction related transformer hum

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction, the frictional energy of which produces an audible noise known as mains hum or transformer hum. This transformer hum is especially objectionable in transformers supplied at power frequencies and in high-frequency flyback transformers associated with PAL system CRTs.

Stray losses

Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat. There are also radiative losses due to the oscillating magnetic field but these are usually small.

Mechanical vibration and audible noise transmission

In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings. This energy incites vibration transmission in interconnected metalwork, thus amplifying audibletransformer hum.

Core form and shell form transformers

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. Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils. Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at the lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 kV or 75 MVA). At higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design tends to be preferred for extra-high voltage and higher MVA applications because, though more labor-intensive to manufacture, shell form transformers are characterized as having inherently better kVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage.

Construction

Cores

Laminated steel cores

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. The steel has a permeability many times that offree space and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation. The universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation.

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.^[55] Thin laminations are generally used on high-frequency transformers, with some of very thin steel laminations able to operate up to 10 kHz.

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'.^[ Such a design tends to exhibit more losses, but is very economical to manufacture. 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. They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance.

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. Overcurrent protection devices such asfuses must be selected to allow this harmless inrush to pass. 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.

Core form = core type; shell form = shell type

Laminated core transformer showing edge of laminations at top of photo

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 ferritesare common. Some radio-frequency transformers also have movable cores (sometimes called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

Toroidal cores

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. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. 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.

Air cores

A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the windings near each other, an arrangement termed an 'air-core' transformer. The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material. The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution. They have however very high bandwidth, and are frequently employed in radio-frequency applications, for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings. They're also used for resonant transformers such as Tesla coils where they can achieve reasonably low loss in spite of the high leakage inductance.

Windings

The conducting material 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 power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound fromenamelled magnet wire, such as Formvar wire. Larger power transformers operating at high voltages 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. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. 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. Acenter-tapped transformer is often used in the output stage of an audio poweramplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar.

Dry-type transformer winding insulation systems can be either of standard open-wound 'dip-and-bake' construction or of higher quality designs that include vacuum pressure impregnation (VPI), vacuum pressure encapsulation (VPE), and cast coil encapsulation processes. In the VPI process, a combination of heat, vacuum and pressure is used to thoroughly seal, bind, and eliminate entrained air voids in the winding polyester resin insulation coat layer, thus increasing resistance to corona. VPE windings are similar to VPI windings but provide more protection against environmental effects, such as from water, dirt or corrosive ambients, by multiple dips including typically in terms of final epoxy coat.

Laminating the core greatly reduces eddy-current losses

Cut view through transformer windings. White: insulator. Green spiral: Grain oriented silicon steel. Black: Primary winding made ofoxygen-free copper. Red: Secondary winding. Top left: Toroidal transformer. Right: C-core, but E-core would be similar. The black windings are made of film. Top: Equally low capacitance between all ends of both windings. Since most cores are at least moderately conductive they also need insulation. Bottom: Lowest capacitance for one end of the secondary winding needed for low-power high-voltage transformers. Bottom left: Reduction of leakage inductance would lead to increase of capacitance.

Solid cores

Windings are usually arranged concentrically to minimize flux leakage.

Small toroidal core transformer

Power transformer inrush current caused by residual flux at switching instant; flux (green), iron core's magnetic characteristics (red) and magnetizing current (blue).

Cooling

See also: Arrhenius equation

To place the cooling problem in perspective, the accepted rule of thumb is that the life expectancy of insulation in all electric s including all transformers is halved for about every 7 °C to 10 °C increase in operating temperature, this life expectancy halving rule holding more narrowly when the increase is between about 7 °C to 8 °C in the case of transformer winding cellulose insulation.

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. Large transformers are filled with transformer oil that both cools and insulates the windings. Transformer oil is a highly refined mineral oil that cools the windings and insulation by circulating within the transformer tank. The mineral oil and paper insulation system has been extensively studied and used for more than 100 years. It is estimated that 50% of power transformers will survive 50 years of use, that the average age of failure of power transformers is about 10 to 15 years, and that about 30% of power transformer failures are due to insulation and overloading failures. Prolonged operation at elevated temperature degrades insulating properties of winding insulation and dielectric coolant, which not only shortens transformer life but can ultimately lead to catastrophic transformer failure.With a great body of empirical study as a guide,transformer oil testing including dissolved gas analysis provides valuable maintenance information. This underlines the need to monitor, model, forecast and manage oil and winding conductor insulation temperature conditions under varying, possibly difficult, power loading conditions.

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. Air-cooled dry transformers can be more economical where they eliminate the cost of a fire-resistant transformer room.

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. An oil-immersed transformer may be equipped with a Buchholz relay, which, depending on severity of gas accumulation due to internal arcing, is used to either alarm or de-energize the transformer. Oil-immersed transformer installations usually include fire protection measures such as walls, oil containment, and fire-suppression sprinkler systems.

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. Today, non-toxic, stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. PCBs for new equipment were banned in 1981 and in 2000 for use in existing equipment in United Kingdom Legislation enacted in Canada between 1977 and 1985 essentially bans PCB use in transformers manufactured in or imported into the country after 1980, the maximum allowable level of PCB contamination in existing mineral oil transformers being 50 ppm.

Some transformers, instead of being liquid-filled, have their windings enclosed in sealed, pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas.

Experimental power transformers in the 500‐to‐1,000 kVA range have been built with liquid nitrogen or helium cooled superconducting windings, which eliminates winding losses without affecting core losses.

Insulation drying

Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly dried of residual moisture before the oil is introduced. Drying is carried out at the factory, and may also be required as a field service. Drying may be done by circulating hot air around the core, 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. The heating can be controlled very well, and it is energy efficient. The method is called low-frequency heating (LFH) since the current used is at a much lower frequency than that of the power grid, which is normally 50 or 60 Hz. A lower frequency reduces the effect of inductance, so the voltage required can be reduced. The LFH drying method is also used for service of older transformers.

Bushings

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.

Classification parameters

Transformers can be classified in many ways, such as the following:

Power capacity: From a fraction of a volt-ampere (VA) to over a thousand MVA.
Duty of a transformer: Continuous, short-time, intermittent, periodic, varying.
Frequency range: Power-frequency, audio-frequency, or radio-frequency.
Voltage class: From a few volts to hundreds of kilovolts.
Cooling type: Dry and liquid-immersed – self-cooled, forced air-cooled; liquid-immersed – forced oil-cooled, water-cooled.
Circuit application: Such as power supply, impedance matching, output voltage and current stabilizer or circuit isolation.
Utilization: Pulse, power, distribution, rectifier, arc furnace, amplifier output, etc..
Basic magnetic form: Core form, shell form.
Constant-potential transformer descriptor: Step-up, step-down, isolation.
General winding configuration: By EIC vector group – various possible two-winding combinations of the phase designations delta, wye or star, and zigzag or interconnected star; other – autotransformer, Scott-T, zigzag grounding transformer winding.
Rectifier phase-shift winding configuration: 2-winding, 6-pulse; 3-winding, 12-pulse; . . . n-winding, [n-1]*6-pulse; polygon; etc..

Types

Various specific electrical application designs require a variety of transformer types. Although they all share the basic characteristic transformer principles, they are customize in construction or electrical properties for certain installation requirements or circuit conditions.

Autotransformer: Transformer in which part of the winding is common to both primary and secondary circuits.
Capacitor voltage transformer: Transformer in which capacitor divider is used to reduce high voltage before application to the primary winding.
Distribution transformer, power transformer: International standards make a distinction in terms of distribution transformers being used to distribute energy from transmission lines and networks for local consumption and power transformers being used to transfer electric energy between the generator and distribution primary circuits.
Phase angle regulating transformer: A specialised transformer used to control the flow of real power on three-phase electricity transmission networks.
Scott-T transformer: Transformer used for phase transformation from three-phase to two-phase and vice versa.
Polyphase transformer: Any transformer with more than one phase.
Grounding transformer: Transformer used for grounding three-phase circuits to create a neutral in a three wire system, using a wye-delta transformer, or more commonly, a zigzag grounding winding.
Leakage transformer: Transformer that has loosely coupled windings.
Resonant transformer: Transformer that uses resonance to generate a high secondary voltage.
Audio transformer: Transformer used in audio equipment.
Output transformer: Transformer used to match the output of a valve amplifier to its load.
Instrument transformer: Potential or current transformer used to accurately and safely represent voltage, current or phase position of high voltage or high power circuits.
Pulse transformer: Specialized small-signal transformer used to transmit digital signaling while providing electrical isolation, commonly used in Ethernet computer networks as 10BASE-T, 100BASE-T and 1000BASE-T.

Cutaway view of liquid-immersed construction transformer. The conservator (reservoir) at top provides liquid-to-atmosphere isolation as coolant level and temperature changes. The walls and fins provide required heat dissipation balance.

Transformers are used to increase (or step-up) voltage before transmitting electrical energy over long distances through wires. Wires have resistance which loses energy through joule heating at a rate corresponding to square of the current. By transforming power to a higher voltage transformers enable economical transmission of power and distribution. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a tiny fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.

Transformers are also used extensively in electronic products to decrease (or step-down) the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage.

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.

History

Discovery of induction

Electromagnetic induction, the principle of the operation of the transformer, was discovered independently by Michael Faraday in 1831, Joseph Henry in 1832, and others. The relationship between EMF and magnetic flux is an equation now known as Faraday's law of induction:

An electrical substation in Melbourne, Australiashowing three of five 220 kV – 66 kV transformers, each with a capacity of 150 MVA

Applications

Transformer at the Limestone Generating Station in Manitoba,Canada

.where

is the magnitude of the EMF in Volts and Φ_B is the magnetic flux through the circuit in webers.

Preceded by Francesco Zantedeschi in 1830, 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. However he only applied individual pulses of current to his transformer, and never discovered the relation between the turns ratio and EMF in the windings.

The first type of transformer to see wide use was the induction coil, invented by Rev.Nicholas Callan of Maynooth College, Ireland in 1836. He was one of the first researchers to realize the more turns the secondary winding has in relation to the primary winding, the larger the induced secondary EMF will be. 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 1830s and the 1870s, efforts to build better induction coils, mostly by trial and error, slowly revealed the basic principles of transformers.

Induction coil, 1900, Bremerhaven, Germany

Induction coils

Faraday's experiment with induction between coils of wire

First alternating current transformers

By the 1870s, efficient generators producing alternating current (AC) were available, and it was found AC could power an induction coil directly, without an interrupter.

In 1876, 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 1878, the Ganz factory, Budapest, Hungary, began equipment for electric lighting and, by 1883, 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 1882, then sold the idea to the Westinghouse company in the United States. They also exhibited the invention in Turin, Italy in 1884, where it was adopted for an electric lighting system. However, the efficiency of their open-core bipolar apparatus remained very low^.

Early series circuit transformer distribution

Induction coils with open magnetic circuits are inefficient at transferring power to loads. Until about 1880, 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 1:1 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. Efficient, practical transformer designs did not appear until the 1880s, but within a decade, the transformer would be instrumental in the War of Currents, and in seeing AC distribution systems triumph over their DC counterparts, a position in which they have remained dominant ever since.

Faraday's ring transformer

In the autumn of 1884, Károly Zipernowsky, Ottó Bláthy and Miksa Déri(ZBD), three engineers associated with the Ganz factory, had determined that open-core devices were impracticable, as they were incapable of reliably regulating voltage. In their joint 1885 patent applications for novel transformers (later called ZBD transformers), they described two designs with closed magnetic circuits where copper windings were either a) wound around iron wire ring core or b) surrounded by iron wire core. The two designs were the first application of the two basic transformer constructions in common use to this day, which can as a class all be termed as either core form or shell form (or alternatively, core type or shell type), as in a) or b), respectively (see images). The Ganz factory had also in the autumn of 1884 made delivery of the world's first five high-efficiency AC transformers, the first of these units having been shipped on September 16, 1884. This first unit had been manufactured to the following specifications: 1,400 W, 40 Hz, 120:72 V, 11.6:19.4 A, ratio 1.67:1, one-phase, shell form.

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 3.4 times more efficient than the open-core bipolar devices of Gaulard and Gibbs. The ZBD patents included two other major interrelated innovations: one concerning the use of parallel connected, instead of series connected, utilization loads, the other concerning the ability to have high turns ratio transformers such that the supply network voltage could be much higher (initially 1,400 to 2,000 V) than the voltage of utilization loads (100 V initially preferred). When employed in parallel connected electric distribution systems, closed-core transformers finally made it technically and economically feasible to provide electric power for lighting in homes, businesses and public spaces. Bláthy had suggested the use of closed cores, Zipernowsky had suggested the use of parallel shunt connections, and Déri had performed the experiments;

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 1882. In 1886, the ZBD engineers designed, and the Ganz factory supplied electrical equipment for, the world's first power station that used AC generators to power a parallel connected common electrical network, the steam-powered Rome-Cerchi power plant.

Although George Westinghouse had bought Gaulard and Gibbs' patents in 1885, the Edison Electric Light Company held an option on the US rights for the ZBD transformers, requiring Westinghouse to pursue alternative designs on the same principles. He assigned to William Stanley the task of developing a device for commercial use in United States. Stanley's first patented design was for induction coils with single cores of soft iron and adjustable gaps to regulate the EMF present in the secondary winding (see image). This design was first used commercially in the US in 1886 but Westinghouse was intent on improving the Stanley design to make it (unlike the ZBD type) easy and cheap to produce.

Westinghouse, Stanley and associates soon developed an easier to manufacture core, consisting of a stack of thin 'E‑shaped' iron plates, insulated by thin sheets of paper or other insulating material. Prewound copper coils could then be slid into place, and straight iron plates laid in to create a closed magnetic circuit. Westinghouse applied for a patent for the new low-cost design in December 1886; it was granted in July 1887

Other early transformers

In 1889, Russian-born engineer Mikhail Dolivo-Dobrovolsky developed the first three-phase transformer at the Allgemeine Elektricitäts-Gesellschaft ('General Electricity Company') in Germany.

In 1891, Nikola Tesla invented the Tesla coil, an air-cored, dual-tuned resonant transformer for generating very high voltages at high frequency.