A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled wires.
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled wires. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
The secondary induced voltage VS is scaled from the primary VP by a factor ideally equal to the ratio of the number of turns of wire in their respective windings:
By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up — by making NS more than NP — or stepped down, by making it less.
A key application of transformers is to reduce the current before
transmitting electrical energy over long distances through wires. Most
wires have resistance
and so dissipate electrical energy at a rate proportional to the square
of the current through the wire. By transforming electrical power to a
high-voltage, and therefore low-current form for transmission and back
again afterwards, transformers enable the economic transmission of power over long distances. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.
Transformers are some of the most efficient electrical 'machines', with some large units able to transfer 99.75% of their input power to their output. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tonnes used to interconnect portions of national power grids.
All operate with the same basic principles, though a variety of designs
exist to perform specialized roles throughout home and industry.
The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism) and, second, that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction).
By changing the current in the primary coil, one changes the strength
of its magnetic field; since the secondary coil is wrapped around the
same magnetic field, a voltage is induced across the secondary.
A simplified transformer design is shown to the right. A current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron;
this ensures that most of the magnetic field lines produced by the
primary current are within the iron and pass through the secondary coil
as well as the primary coil.
The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that
where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ equals the total magnetic flux
through one turn of the coil. If the turns of the coil are oriented
perpendicular to the magnetic field lines, the flux is the product of
the magnetic field strength B and the area A
through which it cuts. The area is constant, being equal to the
cross-sectional area of the transformer core, whereas the magnetic
field varies with time according to the excitation of the primary.
Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals
Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage
Ideal power equation
The ideal transformer as a circuit element
If the secondary coil is attached to a load that allows current to
flow, electrical power is transmitted from the primary circuit to the
secondary circuit. Ideally, the transformer is perfectly efficient; all
the incoming energy is transformed from the primary circuit to the magnetic field and thence to the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power
- Pincoming = IPVP = Poutgoing = ISVS
giving the ideal transformer equation
Thus, if the voltage is stepped up (VS > VP), then the current is stepped down (IS < IP)
by the same factor. In practice, most transformers are very efficient
(see below), so that this formula is a good approximation.
The impedance in one circuit is transformed by the square of the turns ratio. For example, if an impedance ZS is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of . This relationship is reciprocal, so that the impedance ZP of the primary circuit appears to the secondary to be .
The simplified description above avoids several complicating
factors, in particular the primary current required to establish a
magnetic field in the core, and the contribution to the field due to
current in the secondary circuit.
Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance. When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core.. The current required to create the flux is termed the magnetising current;
since the ideal core has been assumed to have near-zero reluctance, the
magnetising current is negligible, although a presence is still
required to create the magnetic field.
The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS
measured at the terminals of the transformer, are equal to the
corresponding EMFs. The primary EMF, acting as it does in opposition to
the primary voltage, is sometimes termed the "back EMF". This is due to Lenz's law
which states that the induction of EMF would always be such that it
will oppose development of any such change in magnetic field.
The ideal transformer model assumes that all flux generated by the
primary winding links all the turns of every winding, including itself.
In practice, some flux traverses paths that take it outside the
windings. Such flux is termed leakage flux, and manifests itself as self-inductance in series with the mutually coupled transformer windings. Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not itself directly a source of power loss, but results in poorer voltage regulation, causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load. Distribution transformers are therefore normally designed to have very low leakage inductance.
However, in some applications, leakage can be a desirable property,
and long magnetic paths, air gaps, or magnetic bypass shunts may be
deliberately introduced to a transformer's design to limit the short-circuit current it will supply. Leaky transformers may be used to supply loads that exhibit negative resistance, such as electric arcs, mercury vapor lamps, and neon signs; or for safely handling loads that become periodically short-circuited such as electric arc welders.
Air gaps are also used to keep a transformer from saturating,
especially audio-frequency transformers that have a DC component added.
Effect of frequency
The time-derivative term in Faraday's Law shows that the flux in the core is the integral of the applied voltage.
An ideal transformer would, at least hypothetically, work under
direct-current excitation, with the core flux increasing linearly with
time. In practice, the flux would rise very rapidly to the point where magnetic saturation
of the core occurred, causing a huge increase in the magnetising
current and overheating the transformer. All practical transformers
must therefore operate under alternating (or pulsed) current conditions.
Transformer universal EMF equation
If the flux in the core is sinusoidal, the relationship for either winding between its rms EMF E, and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation:
The EMF of a transformer at a given flux density increases with frequency, an effect predicted by the universal transformer EMF equation.
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
traditionally employ 400 Hz power supplies which are less
efficient but this is more than offset by the reduction in core and
In general, operation of a transformer at its designed voltage but
at a higher frequency than intended will lead to reduced magnetising
current. At a frequency lower than the design value, with the rated
voltage applied, the magnetising current may increase to an excessive
level. 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.
Knowledge of natural frequencies of transformer windings is of
importance for the determination of the transient response of the
windings to impulse and switching surge voltages.
An ideal transformer would have no energy losses, and would
therefore be 100% efficient. Despite the transformer being amongst the
most efficient of electrical machines, with experimental models using superconducting windings achieving efficiencies of 99.85%,
energy is dissipated in the windings, core, and surrounding structures.
Larger transformers are generally more efficient, and those rated for
electricity distribution usually perform better than 95%.
A small transformer, such as a plug-in "power brick" used for low-power
consumer electronics, may be no more than 85% efficient; although
individual power loss is small, the aggregate losses from the very
large number of such devices is coming under increased scrutiny.
Transformer losses are attributable to several causes and may be
differentiated between those originating in the windings, sometimes
termed copper loss, and those arising from the magnetic circuit, sometimes termed iron loss. The losses vary with load current, and may furthermore be expressed as "no-load" or "full-load" loss, respectively. Winding resistance dominates load losses, whereas hysteresis and eddy currents
losses contribute to over 99% of the no-load loss. The no-load loss can
be significant, meaning that even an idle transformer constitutes a
drain on an electrical supply, and lending impetus to development of
low-loss transformers (also see energy efficient transformer).
Losses in the transformer arise from:
- Winding resistance
- Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.
- Hysteresis losses
- Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis
within the core. For a given core material, the loss is proportional to
the frequency, and is a function of the peak flux density to which it
- Eddy currents
- Ferromagnetic materials are also good conductors, and a solid 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
- 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. This produces the buzzing sound commonly associated with transformers, and in turn causes losses due to frictional heating in susceptible cores.
- Mechanical losses
- In addition to magnetostriction, the alternating magnetic field
causes fluctuating electromagnetic forces between the primary and
secondary windings. These incite vibrations within nearby metalwork,
adding to the buzzing noise, and consuming a small amount of power.
- Stray losses
- Leakage inductance is by itself 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.
The physical limitations of the practical transformer may be brought
together as an equivalent circuit model (shown below) built around an
ideal lossless transformer. Power loss in the windings is current-dependent and is easily represented as in-series resistances RP and RS.
Flux leakage results in a fraction of the applied voltage dropped
without contributing to the mutual coupling, and thus can be modeled as
self-inductances XP and XS
in series with the perfectly-coupled region. Iron losses are caused
mostly by hysteresis and eddy current effects in the core, and tend to
be proportional to the square of the core flux for operation at a given
frequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance RC in parallel with the ideal transformer.
A core with finite permeability requires a magnetizing current IM
to maintain the mutual flux in the core. The magnetizing current is in
phase with the flux; saturation effects cause the relationship between
the two to be non-linear, but for simplicity this effect tends to be
ignored in most circuit equivalents. With a sinusoidal supply, the core flux lags the induced EMF by 90° and this effect can be modeled as a magnetising reactance XM in parallel with the core loss component. RC and XM are sometimes together termed the magnetising branch of the model. If the secondary winding is made open-circuit, the current I0 taken by the magnetising branch represents the transformer's no-load current.
The secondary impedance RS and XS is frequently moved (or "referred") to the primary side after multiplying the components by the impedance scaling factor .
The resulting model is sometimes termed the "exact equivalent
circuit", though it retains a number of approximations, such as an
assumption of linearity.
Analysis may be simplified by moving the magnetising branch to the left
of the primary impedance, an implicit assumption that the magnetising
current is low, and then summing primary and referred secondary
A variety of specialised transformer designs has been created to
fulfil certain engineering applications, though they share several
commonalities. Several of the more important transformer types include:
has only a single winding with two end terminals, plus a third at an
intermediate tap point. The primary voltage is applied across two of
the terminals, and the secondary voltage taken from one of these and
the third terminal. The primary and secondary circuits therefore have a
number of windings turns in common.
Since the volts-per-turn is the same in both windings, each develops a
voltage in proportion to its number of turns. By exposing part of the
winding coils and making the secondary connection through a sliding brush, an autotransformer with a near-continuously variable turns ratio is obtained, allowing for very fine control of voltage.
supplies, a bank of three individual single-phase transformers can be
used, or all three phases can be incorporated as a single three-phase
transformer. In this case, the magnetic circuits are connected
together, the core thus containing a three-phase flow of flux. A number of winding configurations are possible, giving rise to different attributes and phase shifts. One particular polyphase configuration is the zigzag transformer, used for grounding and in the suppression of harmonic currents.
A resonant transformer uses the inductance of its primary winding in series with a capacitor to form a tuned resonant circuit.
When the primary winding is driven at its resonant frequency, each
pulse of current develops an oscillation in the secondary coil. Due to
resonance, a very high voltage develops across the secondary, until it
is limited by some process such as electrical breakdown. Resonant transformers such as the Tesla coil can generate very high voltages, able to provide much higher current than electrostatic machines such as the Van de Graaff generator. Another application of the resonant transformer is to couple between stages of a superheterodyne receiver, where the selectivity of the receiver is provided by tuned transformers in the intermediate-frequency amplifiers.
A current transformer
is a measurement device designed to provide a current in its secondary
coil proportional to the current flowing in its primary. Current
transformers are commonly used in metering and protective relaying,
where they facilitate the safe measurement of large currents. The
current transformer isolates measurement and control circuitry from the
high voltages typically present on the circuit being measured.
Voltage transformers (VTs) are used for metering and protection in
high-voltage circuits. They are designed to present negligible load to
the supply being measured and to have a precise voltage ratio to
accurately step down high voltages so that metering and protective
relay equipment can be operated at a lower potential.
The many uses to which transformers are put leads them to be classified in a number of different ways:
- By power level: from a fraction of a volt-ampere (VA) to over a thousand MVA;
- By frequency range: power-, audio-, or radio frequency;
- By voltage class: from a few volts to hundreds of kilovolts;
- By cooling type: air cooled, oil filled, fan cooled, or water cooled;
- By application function: such as power supply, impedance matching, output voltage and current stabilizer, or circuit isolation;
- By end purpose: distribution, rectifier, arc furnace, amplifier output;
- By winding turns ratio: step-up, step-down, isolating (near equal ratio), variable.
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 of free space,
and the core thus serves to greatly reduce the magnetising current, and
confine the flux to a path which closely couples the windings.
Early transformer developers soon realised 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.
Thin laminations are generally used on high frequency transformers,
with some types of very thin steel laminations able to operate up to
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 remanent magnetism is reduced, usually after a few cycles of the applied alternating current. Overcurrent protection devices such as fuses
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.
cores are used in circuits (such as switch-mode power supplies) that
operate above main 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.
Some radio-frequency transformers also have moveable cores (sometimes
called 'slugs') which allow adjustment of the coupling coefficient (and
bandwidth) of tuned radio-frequency circuits.
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 minimises the length of wire
needed, and also 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
Ferrite toroidal cores are used at higher frequencies, typically
between a few tens of kilohertz to a megahertz, to reduce losses,
physical size, and weight of switch-mode power supplies.
A drawback of toroidal transformer construction is the higher cost of
windings. As a consequence, toroidal transformers are uncommon above
ratings of a few kVA. 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 physical core is not an absolute requisite and a functioning
transformer can be produced simply by placing the windings in close
proximity to 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.
Cut view through transformer windings. White: insulator. Green spiral: Grain oriented silicon steel. Black: Primary winding made of oxygen-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 (bad) conductors 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.
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 from enamelled 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.
For signal transformers, the windings may be arranged in a way to
minimise leakage inductance and stray capacitance to improve
high-frequency response. This can be done by splitting up each coil
into sections, and those sections placed in layers between the sections
of the other winding. This is known as a stacked type or interleaved
Both the primary and secondary windings on power transformers may have external connections, called taps,
to intermediate points on the winding to allow selection of the voltage
ratio. The taps may be connected to an automatic on-load tap changer
for voltage regulation of distribution circuits. 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.
Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum,
one can replace air spaces within the windings with epoxy, thus sealing
the windings and helping to prevent the possible formation of corona
and absorption of dirt or water. This produces transformers more suited
to damp or dirty environments, but at increased manufacturing cost.
Extended operation at high temperatures is particularly damaging to transformer insulation. Small signal transformers do not generate significant heat and need little consideration given to their thermal management. Power transformers rated up to a few kVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans.
Specific provision must be made for cooling high-power transformers,
the larger physical size requiring careful design to transport heat
from the interior. Some power transformers are immersed in specialized transformer oil that acts both as a cooling medium, thereby extending the lifetime of the insulation, and helps to reduce corona discharge. The oil is a highly refined mineral oil that remains stable at high temperatures so that internal arcing will not cause breakdown or fire; transformers to be used indoors must use a non-flammable liquid.
The oil-filled tank often has radiators through which the oil
circulates by natural convection; large transformers employ forced
circulation of the oil by electric pumps, aided by external fans or
water-cooled heat exchangers. Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vapor
before the cooling oil is introduced. This helps prevent electrical
breakdown under load. Oil-filled transformers may be equipped with Buchholz relays, which detect gas evolved during internal arcing and rapidly de-energize the transformer to avert catastrophic failure.
Polychlorinated biphenyls have properties that once favored their use as a coolant, though concerns over their toxicity and 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.
Before 1977, even transformers that were nominally filled only with
mineral oils commonly also contained polychlorinated biphenyls as
contaminants at 10-20 ppm.
Some "dry" transformers are enclosed in pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas.
To ensure that the gas does not leak and its insulating capability
deteriorate, the transformer casing is completely sealed. Experimental
power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.
Very small transformers will have wire leads connected directly to
the ends of the coils, and brought out to the base of the unit for
circuit connections. Larger transformers may have heavy bolted
terminals, bus bars or 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.
The transformer principle was demonstrated in 1831 by Michael Faraday, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee its practical uses. Viable designs would not appear until the 1880s. Within less than a decade, the transformer was instrumental during the "War of Currents" in seeing alternating current systems triumph over their direct current counterparts, a position in which they have remained dominant.
Russian engineer Pavel Yablochkov in 1876 invented a lighting system based on a set of induction coils, where primary windings were connected to a source of alternating current and secondary windings could be connected to several "electric candles".
The patent claimed the system could "provide separate supply to several
lighting fixtures with different luminous intensities from a single
source of electric power". Evidently, the induction coil in this system
operated as a transformer.
Lucien Gaulard and John Dixon Gibbs,
who first exhibited a device with an open iron core called a 'secondary
generator' in London in 1882 and then sold the idea to American company
Westinghouse. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system.
an engineer for Westinghouse, built the first commercial device in 1885
after George Westinghouse had 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 Zipernowsky, Bláthy and Déri from the Ganz company in Budapest created the efficient "ZBD" closed-core model in 1885 based on the design by Gaulard and Gibbs. Their patent application made the first use of the word "transformer". Russian engineer Mikhail Dolivo-Dobrovolsky developed the first three-phase transformer in 1889. In 1891 Nikola Tesla invented the Tesla coil, an air-cored, dual-tuned resonant transformer for generating very high voltages at high frequency. Audio frequency transformers (at the time called repeating coils) were used by the earliest experimenters in the development of the telephone.
While new technologies have made transformers in some electronics
applications obsolete, transformers are still found in many electronic
devices. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)