Transformer Lab Experiment

• Transformer Experiment Lab Report
• Single Phase Transformer Lab

1. EECE 211L Lab #10: Transformer Characteristics and Impedance Matching Introduction: The objective of this experiment is to observe how the parameters of voltage, current, and impedance reflection are related to a transformer's turns ratio. In addition, the concept of impedance matching will be explored.
2. Using the lab equipments shown in Figure 3.2, do the following: 1. Connect the high voltage side of the transformer to a dc power source as shown in Figure 3.4. Adjust the voltage source such that you measure 0.2A and 0.4A in the windings.

Wrap several hundred turns of magnet wire around these two bars. You may make these windings with an equal or unequal number of turns, depending on whether or not you want the transformer to be able to 'step' voltage up or down. I recommend equal turns, to begin with, then experiment later with coils of unequal turn count. List of experiments sl.no name of the experiment page no 1 open circuit & short circuit test on a single phase transformer 3 2 sumpners test 10 3 scott connection of transformers 15 4 no load and blocked rotor test on a 3- ɸ induction motor 18 5 regulation of alternator using synchronous impedance method 23.

Definition

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled wires.

Basics

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 V_S is scaled from the primary V_P 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 N_S more than N_P — or stepped down, by making it less.

A key application of transformers is to reduce the current beforetransmitting electrical energy over long distances through wires. Mostwires have resistanceand so dissipate electrical energy at a rate proportional to the squareof the current through the wire. By transforming electrical power to ahigh-voltage, and therefore low-current form for transmission and backagain 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 designsexist to perform specialized roles throughout home and industry.

Basic principles

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 strengthof its magnetic field; since the secondary coil is wrapped around thesame 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 theprimary current are within the iron and pass through the secondary coilas well as the primary coil.

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that

where V_S is the instantaneous voltage, N_S is the number of turns in the secondary coil and Φ equals the total magnetic fluxthrough one turn of the coil. If the turns of the coil are orientedperpendicular to the magnetic field lines, the flux is the product ofthe magnetic field strength B and the area Athrough which it cuts. The area is constant, being equal to thecross-sectional area of the transformer core, whereas the magneticfield 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 V_S and V_P gives the basic equation for stepping up or stepping down the voltage

Ideal power equation

If the secondary coil is attached to a load that allows current toflow, electrical power is transmitted from the primary circuit to thesecondary circuit. Ideally, the transformer is perfectly efficient; allthe 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

P_incoming = I_PV_P = P_outgoing = I_SV_S

giving the ideal transformer equation

Thus, if the voltage is stepped up (V_S > V_P), then the current is stepped down (I_S < I_P)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 Z_S 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 Z_P of the primary circuit appears to the secondary to be .

Technical discussion

The simplified description above avoids several complicatingfactors, in particular the primary current required to establish amagnetic field in the core, and the contribution to the field due tocurrent 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, themagnetising current is negligible, although a presence is stillrequired 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 V_P and V_Smeasured at the terminals of the transformer, are equal to thecorresponding EMFs. The primary EMF, acting as it does in opposition tothe primary voltage, is sometimes termed the 'back EMF'. This is due to Lenz's lawwhich states that the induction of EMF would always be such that itwill oppose development of any such change in magnetic field.

Practical considerations

Flux leakage

The ideal transformer model assumes that all flux generated by theprimary winding links all the turns of every winding, including itself.In practice, some flux traverses paths that take it outside thewindings. 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 bedeliberately 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 underdirect-current excitation, with the core flux increasing linearly withtime. In practice, the flux would rise very rapidly to the point where magnetic saturationof the core occurred, causing a huge increase in the magnetisingcurrent and overheating the transformer. All practical transformersmust 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 densityB 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 morecompact because a given core is able to transfer more power withoutreaching saturation, and fewer turns are needed to achieve the sameimpedance. However properties such as core loss and conductor skin effectalso increase with frequency. Aircraft and military equipmenttraditionally employ 400 Hz power supplies which are lessefficient but this is more than offset by the reduction in core andwinding weight.

In general, operation of a transformer at its designed voltage butat a higher frequency than intended will lead to reduced magnetisingcurrent. At a frequency lower than the design value, with the ratedvoltage applied, the magnetising current may increase to an excessivelevel. Operation of a transformer at other than its design frequencymay require assessment of voltages, losses, and cooling to establish ifsafe operation is practical. For example, transformers may need to beequipped 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 ofimportance for the determination of the transient response of thewindings to impulse and switching surge voltages.

Energy losses

An ideal transformer would have no energy losses, and wouldtherefore be 100% efficient. Despite the transformer being amongst themost 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 forelectricity distribution usually perform better than 95%.A small transformer, such as a plug-in 'power brick' used for low-powerconsumer electronics, may be no more than 85% efficient; althoughindividual power loss is small, the aggregate losses from the verylarge number of such devices is coming under increased scrutiny.

Transformer losses are attributable to several causes and may bedifferentiated between those originating in the windings, sometimestermed 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 currentslosses contribute to over 99% of the no-load loss. The no-load loss canbe significant, meaning that even an idle transformer constitutes adrain on an electrical supply, and lending impetus to development oflow-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 hysteresiswithin the core. For a given core material, the loss is proportional tothe frequency, and is a function of the peak flux density to which itis subjected.

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 heatingof the core material. The eddy current loss is a complex function ofthe square of supply frequency and inverse square of the materialthickness.

Magnetostriction

Magnetic flux in a ferromagnetic material, such as the core, causesit to physically expand and contract slightly with each cycle of themagnetic 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 fieldcauses fluctuating electromagnetic forces between the primary andsecondary 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 toits magnetic fields is returned to the supply with the next half-cycle.However, any leakage flux that intercepts nearby conductive materialssuch as the transformer's support structure will give rise to eddycurrents and be converted to heat.

Equivalent circuit

The physical limitations of the practical transformer may be broughttogether as an equivalent circuit model (shown below) built around anideal lossless transformer. Power loss in the windings is current-dependent and is easily represented as in-series resistances R_P and R_S.Flux leakage results in a fraction of the applied voltage droppedwithout contributing to the mutual coupling, and thus can be modeled asself-inductancesX_P and X_Sin series with the perfectly-coupled region. Iron losses are causedmostly by hysteresis and eddy current effects in the core, and tend tobe proportional to the square of the core flux for operation at a givenfrequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance R_C in parallel with the ideal transformer.

A core with finite permeability requires a magnetizing current I_Mto maintain the mutual flux in the core. The magnetizing current is inphase with the flux; saturation effects cause the relationship betweenthe two to be non-linear, but for simplicity this effect tends to beignored 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 X_M in parallel with the core loss component. R_C and X_M 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 impedanceR_S and X_S 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 equivalentcircuit', though it retains a number of approximations, such as anassumption of linearity.Analysis may be simplified by moving the magnetising branch to the leftof the primary impedance, an implicit assumption that the magnetisingcurrent is low, and then summing primary and referred secondaryimpedances.

Types

A variety of specialised transformer designs has been created tofulfil certain engineering applications, though they share severalcommonalities. Several of the more important transformer types include:

Autotransformer

An autotransformerhas only a single winding with two end terminals, plus a third at anintermediate tap point. The primary voltage is applied across two ofthe terminals, and the secondary voltage taken from one of these andthe third terminal. The primary and secondary circuits therefore have anumber of windings turns in common.Since the volts-per-turn is the same in both windings, each develops avoltage in proportion to its number of turns. By exposing part of thewinding 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.

Polyphase transformers

For three-phasesupplies, a bank of three individual single-phase transformers can beused, or all three phases can be incorporated as a single three-phasetransformer. In this case, the magnetic circuits are connectedtogether, 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.

Resonant transformers

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, eachpulse of current develops an oscillation in the secondary coil. Due toresonance, a very high voltage develops across the secondary, until itis 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.

Instrument transformers

The physical limitations of the practical transformer may be broughttogether as an equivalent circuit model (shown below) built around anideal lossless transformer. Power loss in the windings is current-dependent and is easily represented as in-series resistances R_P and R_S.Flux leakage results in a fraction of the applied voltage droppedwithout contributing to the mutual coupling, and thus can be modeled asself-inductancesX_P and X_Sin series with the perfectly-coupled region. Iron losses are causedmostly by hysteresis and eddy current effects in the core, and tend tobe proportional to the square of the core flux for operation at a givenfrequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance R_C in parallel with the ideal transformer.

A core with finite permeability requires a magnetizing current I_Mto maintain the mutual flux in the core. The magnetizing current is inphase with the flux; saturation effects cause the relationship betweenthe two to be non-linear, but for simplicity this effect tends to beignored 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 X_M in parallel with the core loss component. R_C and X_M 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 impedanceR_S and X_S 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 equivalentcircuit', though it retains a number of approximations, such as anassumption of linearity.Analysis may be simplified by moving the magnetising branch to the leftof the primary impedance, an implicit assumption that the magnetisingcurrent is low, and then summing primary and referred secondaryimpedances.

Types

A variety of specialised transformer designs has been created tofulfil certain engineering applications, though they share severalcommonalities. Several of the more important transformer types include:

Autotransformer

An autotransformerhas only a single winding with two end terminals, plus a third at anintermediate tap point. The primary voltage is applied across two ofthe terminals, and the secondary voltage taken from one of these andthe third terminal. The primary and secondary circuits therefore have anumber of windings turns in common.Since the volts-per-turn is the same in both windings, each develops avoltage in proportion to its number of turns. By exposing part of thewinding 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.

Polyphase transformers

For three-phasesupplies, a bank of three individual single-phase transformers can beused, or all three phases can be incorporated as a single three-phasetransformer. In this case, the magnetic circuits are connectedtogether, 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.

Resonant transformers

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, eachpulse of current develops an oscillation in the secondary coil. Due toresonance, a very high voltage develops across the secondary, until itis 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.

Instrument transformers

A current transformeris a measurement device designed to provide a current in its secondarycoil proportional to the current flowing in its primary. Currenttransformers are commonly used in metering and protective relaying,where they facilitate the safe measurement of large currents. Thecurrent transformer isolates measurement and control circuitry from thehigh voltages typically present on the circuit being measured.

Voltage transformers (VTs) are used for metering and protection inhigh-voltage circuits. They are designed to present negligible load tothe supply being measured and to have a precise voltage ratio toaccurately step down high voltages so that metering and protectiverelay equipment can be operated at a lower potential.

Classification

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.

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 of free space,and the core thus serves to greatly reduce the magnetising current, andconfine the flux to a path which closely couples the windings.Early transformer developers soon realised that cores constructed fromsolid iron resulted in prohibitive eddy-current losses, and theirdesigns mitigated this effect with cores consisting of bundles ofinsulated iron wires.Later designs constructed the core by stacking layers of thin steellaminations, a principle that has remained in use. Each lamination isinsulated from its neighbors by a thin non-conducting layer ofinsulation. 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 highlyelliptical paths that enclose little flux, and so reduce theirmagnitude. 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 to10 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 tomanufacture. The cut-core or C-core type is made by winding a steelstrip around a rectangular form and then bonding the layers together.It is then cut in two, forming two C shapes, and the core assembled bybinding 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 remanencemeans 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 fusesmust be selected to allow this harmless inrush to pass. On transformersconnected to long, overhead power transmission lines, induced currentsdue 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.

Solid cores

Powdered ironcores are used in circuits (such as switch-mode power supplies) thatoperate above main frequencies and up to a few tens of kilohertz. Thesematerials 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 (sometimescalled 'slugs') which allow adjustment of the coupling coefficient (andbandwidth) 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, butmore expensive cores with circular cross-sections are also available.The primary and secondary coils are often wound concentrically to coverthe entire surface of the core. This minimises the length of wireneeded, and also provides screening to minimize the core's magneticfield from generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminatedE-I types for a similar power level. Crazybump free. Other advantages compared to E-Itypes, include smaller size (about half), lower weight (about half),less mechanical hum (making them superior in audio amplifiers), lowerexterior magnetic field (about one tenth), low off-load losses (makingthem more efficient in standby circuits), single-bolt mounting, andgreater choice of shapes. The main disadvantages are higher cost andlimited rating.

Ferrite toroidal cores are used at higher frequencies, typicallybetween 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 ofwindings. As a consequence, toroidal transformers are uncommon aboveratings of a few kVA. Small distribution transformers may achieve someof 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 functioningtransformer can be produced simply by placing the windings in closeproximity to each other, an arrangement termed an 'air-core'transformer. The air which comprises the magnetic circuit isessentially lossless, and so an air-core transformer eliminates lossdue to hysteresis in the core material.The leakage inductance is inevitably high, resulting in very poorregulation, and so such designs are unsuitable for use in powerdistribution. 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.

Windings

The conducting materialused for the windings depends upon the application, but in all casesthe individual turns must be electrically insulated from each other toensure that the current travels throughout every turn.For small power and signal transformers, in which currents are low andthe potential difference between adjacent turns is small, the coils areoften wound from enamelled magnet wire,such as Formvar wire. Larger power transformers operating at highvoltages may be wound with copper rectangular strip conductorsinsulated 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 currentwould otherwise exist in high-current windings.Each strand is individually insulated, and the strands are arranged sothat at certain points in the winding, or throughout the whole winding,each portion occupies different relative positions in the completeconductor. The transposition equalizes the current flowing in eachstrand of the conductor, and reduces eddy current losses in the windingitself. The stranded conductor is also more flexible than a solidconductor of similar size, aiding manufacture.

For signal transformers, the windings may be arranged in a way tominimise leakage inductance and stray capacitance to improvehigh-frequency response. This can be done by splitting up each coilinto sections, and those sections placed in layers between the sectionsof the other winding. This is known as a stacked type or interleavedwinding.

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 voltageratio. The taps may be connected to an automatic on-load tap changerfor voltage regulation of distribution circuits. Audio-frequencytransformers, used for the distribution of audio to public addressloudspeakers, have taps to allow adjustment of impedance to eachspeaker. A center-tapped transformer is often used in the output stageof 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 sealingthe windings and helping to prevent the possible formation of coronaand absorption of dirt or water. This produces transformers more suitedto damp or dirty environments, but at increased manufacturing cost.

Coolant

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 heatfrom 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 oilcirculates by natural convection; large transformers employ forcedcirculation of the oil by electric pumps, aided by external fans orwater-cooled heat exchangers. Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vaporbefore the cooling oil is introduced. This helps prevent electricalbreakdown 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 withmineral oils commonly also contained polychlorinated biphenyls ascontaminants 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 capabilitydeteriorate, the transformer casing is completely sealed. Experimentalpower 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.

Terminals

Very small transformers will have wire leads connected directly tothe ends of the coils, and brought out to the base of the unit forcircuit connections. Larger transformers may have heavy boltedterminals, 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.

History

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 severallighting fixtures with different luminous intensities from a singlesource of electric power'. Evidently, the induction coil in this systemoperated as a transformer.

Lucien Gaulard and John Dixon Gibbs,who first exhibited a device with an open iron core called a 'secondarygenerator' in London in 1882 and then sold the idea to American companyWestinghouse. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system.

William Stanley,an engineer for Westinghouse, built the first commercial device in 1885after George Westinghouse had bought Gaulard and Gibbs' patents. Thecore was made from interlocking E-shaped iron plates. This design wasfirst 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 electronicsapplications obsolete, transformers are still found in many electronicdevices. 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.)

Definition

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled wires.

Basics

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 V_S is scaled from the primary V_P 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 N_S more than N_P — or stepped down, by making it less.

A key application of transformers is to reduce the current beforetransmitting electrical energy over long distances through wires. Mostwires have resistanceand so dissipate electrical energy at a rate proportional to the squareof the current through the wire. By transforming electrical power to ahigh-voltage, and therefore low-current form for transmission and backagain 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 designsexist to perform specialized roles throughout home and industry.

Basic principles

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 strengthof its magnetic field; since the secondary coil is wrapped around thesame 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 theprimary current are within the iron and pass through the secondary coilas well as the primary coil.

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that

where V_S is the instantaneous voltage, N_S is the number of turns in the secondary coil and Φ equals the total magnetic fluxthrough one turn of the coil. If the turns of the coil are orientedperpendicular to the magnetic field lines, the flux is the product ofthe magnetic field strength B and the area Athrough which it cuts. The area is constant, being equal to thecross-sectional area of the transformer core, whereas the magneticfield 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 V_S and V_P gives the basic equation for stepping up or stepping down the voltage

Ideal power equation

If the secondary coil is attached to a load that allows current toflow, electrical power is transmitted from the primary circuit to thesecondary circuit. Ideally, the transformer is perfectly efficient; allthe 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

P_incoming = I_PV_P = P_outgoing = I_SV_S

giving the ideal transformer equation

Thus, if the voltage is stepped up (V_S > V_P), then the current is stepped down (I_S < I_P)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 Z_S 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 Z_P of the primary circuit appears to the secondary to be .

Technical discussion

The simplified description above avoids several complicatingfactors, in particular the primary current required to establish amagnetic field in the core, and the contribution to the field due tocurrent 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, themagnetising current is negligible, although a presence is stillrequired 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 V_P and V_Smeasured at the terminals of the transformer, are equal to thecorresponding EMFs. The primary EMF, acting as it does in opposition tothe primary voltage, is sometimes termed the 'back EMF'. This is due to Lenz's lawwhich states that the induction of EMF would always be such that itwill oppose development of any such change in magnetic field.

Practical considerations

Flux leakage

The ideal transformer model assumes that all flux generated by theprimary winding links all the turns of every winding, including itself.In practice, some flux traverses paths that take it outside thewindings. 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 bedeliberately 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 underdirect-current excitation, with the core flux increasing linearly withtime. In practice, the flux would rise very rapidly to the point where magnetic saturationof the core occurred, causing a huge increase in the magnetisingcurrent and overheating the transformer. All practical transformersmust 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 densityB 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 morecompact because a given core is able to transfer more power withoutreaching saturation, and fewer turns are needed to achieve the sameimpedance. However properties such as core loss and conductor skin effectalso increase with frequency. Aircraft and military equipmenttraditionally employ 400 Hz power supplies which are lessefficient but this is more than offset by the reduction in core andwinding weight.

In general, operation of a transformer at its designed voltage butat a higher frequency than intended will lead to reduced magnetisingcurrent. At a frequency lower than the design value, with the ratedvoltage applied, the magnetising current may increase to an excessivelevel. Operation of a transformer at other than its design frequencymay require assessment of voltages, losses, and cooling to establish ifsafe operation is practical. For example, transformers may need to beequipped 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 ofimportance for the determination of the transient response of thewindings to impulse and switching surge voltages.

Energy losses

An ideal transformer would have no energy losses, and wouldtherefore be 100% efficient. Despite the transformer being amongst themost 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 forelectricity distribution usually perform better than 95%.A small transformer, such as a plug-in 'power brick' used for low-powerconsumer electronics, may be no more than 85% efficient; althoughindividual power loss is small, the aggregate losses from the verylarge number of such devices is coming under increased scrutiny.

Transformer losses are attributable to several causes and may bedifferentiated between those originating in the windings, sometimestermed 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 currentslosses contribute to over 99% of the no-load loss. The no-load loss canbe significant, meaning that even an idle transformer constitutes adrain on an electrical supply, and lending impetus to development oflow-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 hysteresiswithin the core. For a given core material, the loss is proportional tothe frequency, and is a function of the peak flux density to which itis subjected.

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 heatingof the core material. The eddy current loss is a complex function ofthe square of supply frequency and inverse square of the materialthickness.

Magnetostriction

Magnetic flux in a ferromagnetic material, such as the core, causesit to physically expand and contract slightly with each cycle of themagnetic 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 fieldcauses fluctuating electromagnetic forces between the primary andsecondary 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 toits magnetic fields is returned to the supply with the next half-cycle.However, any leakage flux that intercepts nearby conductive materialssuch as the transformer's support structure will give rise to eddycurrents and be converted to heat.

Equivalent circuit

The physical limitations of the practical transformer may be broughttogether as an equivalent circuit model (shown below) built around anideal lossless transformer. Power loss in the windings is current-dependent and is easily represented as in-series resistances R_P and R_S.Flux leakage results in a fraction of the applied voltage droppedwithout contributing to the mutual coupling, and thus can be modeled asself-inductancesX_P and X_Sin series with the perfectly-coupled region. Iron losses are causedmostly by hysteresis and eddy current effects in the core, and tend tobe proportional to the square of the core flux for operation at a givenfrequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance R_C in parallel with the ideal transformer.

A core with finite permeability requires a magnetizing current I_Mto maintain the mutual flux in the core. The magnetizing current is inphase with the flux; saturation effects cause the relationship betweenthe two to be non-linear, but for simplicity this effect tends to beignored 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 X_M in parallel with the core loss component. R_C and X_M 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 impedanceR_S and X_S 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 equivalentcircuit', though it retains a number of approximations, such as anassumption of linearity.Analysis may be simplified by moving the magnetising branch to the leftof the primary impedance, an implicit assumption that the magnetisingcurrent is low, and then summing primary and referred secondaryimpedances.

Types

A variety of specialised transformer designs has been created tofulfil certain engineering applications, though they share severalcommonalities. Several of the more important transformer types include:

Autotransformer

An autotransformerhas only a single winding with two end terminals, plus a third at anintermediate tap point. The primary voltage is applied across two ofthe terminals, and the secondary voltage taken from one of these andthe third terminal. The primary and secondary circuits therefore have anumber of windings turns in common.Since the volts-per-turn is the same in both windings, each develops avoltage in proportion to its number of turns. By exposing part of thewinding 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.

Polyphase transformers

For three-phasesupplies, a bank of three individual single-phase transformers can beused, or all three phases can be incorporated as a single three-phasetransformer. In this case, the magnetic circuits are connectedtogether, 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.

Resonant transformers

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, eachpulse of current develops an oscillation in the secondary coil. Due toresonance, a very high voltage develops across the secondary, until itis 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.

Instrument transformers

A current transformeris a measurement device designed to provide a current in its secondarycoil proportional to the current flowing in its primary. Currenttransformers are commonly used in metering and protective relaying,where they facilitate the safe measurement of large currents. Thecurrent transformer isolates measurement and control circuitry from thehigh voltages typically present on the circuit being measured.

Voltage transformers (VTs) are used for metering and protection inhigh-voltage circuits. They are designed to present negligible load tothe supply being measured and to have a precise voltage ratio toaccurately step down high voltages so that metering and protectiverelay equipment can be operated at a lower potential.

Classification

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.

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 of free space,and the core thus serves to greatly reduce the magnetising current, andconfine the flux to a path which closely couples the windings.Early transformer developers soon realised that cores constructed fromsolid iron resulted in prohibitive eddy-current losses, and theirdesigns mitigated this effect with cores consisting of bundles ofinsulated iron wires.Later designs constructed the core by stacking layers of thin steellaminations, a principle that has remained in use. Each lamination isinsulated from its neighbors by a thin non-conducting layer ofinsulation. 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 highlyelliptical paths that enclose little flux, and so reduce theirmagnitude. 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 to10 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 tomanufacture. The cut-core or C-core type is made by winding a steelstrip around a rectangular form and then bonding the layers together.It is then cut in two, forming two C shapes, and the core assembled bybinding 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 remanencemeans 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 fusesmust be selected to allow this harmless inrush to pass. On transformersconnected to long, overhead power transmission lines, induced currentsdue 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.

Solid cores

Powdered ironcores are used in circuits (such as switch-mode power supplies) thatoperate above main frequencies and up to a few tens of kilohertz. Thesematerials 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 (sometimescalled 'slugs') which allow adjustment of the coupling coefficient (andbandwidth) 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, butmore expensive cores with circular cross-sections are also available.The primary and secondary coils are often wound concentrically to coverthe entire surface of the core. This minimises the length of wireneeded, and also provides screening to minimize the core's magneticfield from generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminatedE-I types for a similar power level. Other advantages compared to E-Itypes, include smaller size (about half), lower weight (about half),less mechanical hum (making them superior in audio amplifiers), lowerexterior magnetic field (about one tenth), low off-load losses (makingthem more efficient in standby circuits), single-bolt mounting, andgreater choice of shapes. The main disadvantages are higher cost andlimited rating.

Ferrite toroidal cores are used at higher frequencies, typicallybetween 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 ofwindings. As a consequence, toroidal transformers are uncommon aboveratings of a few kVA. Small distribution transformers may achieve someof 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 functioningtransformer can be produced simply by placing the windings in closeproximity to each other, an arrangement termed an 'air-core'transformer. The air which comprises the magnetic circuit isessentially lossless, and so an air-core transformer eliminates lossdue to hysteresis in the core material.The leakage inductance is inevitably high, resulting in very poorregulation, and so such designs are unsuitable for use in powerdistribution. 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.

Windings

Transformer Experiment Lab Report

The conducting materialused for the windings depends upon the application, but in all casesthe individual turns must be electrically insulated from each other toensure that the current travels throughout every turn.For small power and signal transformers, in which currents are low andthe potential difference between adjacent turns is small, the coils areoften wound from enamelled magnet wire,such as Formvar wire. Larger power transformers operating at highvoltages may be wound with copper rectangular strip conductorsinsulated 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 currentwould otherwise exist in high-current windings.Each strand is individually insulated, and the strands are arranged sothat at certain points in the winding, or throughout the whole winding,each portion occupies different relative positions in the completeconductor. The transposition equalizes the current flowing in eachstrand of the conductor, and reduces eddy current losses in the windingitself. The stranded conductor is also more flexible than a solidconductor of similar size, aiding manufacture.

For signal transformers, the windings may be arranged in a way tominimise leakage inductance and stray capacitance to improvehigh-frequency response. This can be done by splitting up each coilinto sections, and those sections placed in layers between the sectionsof the other winding. This is known as a stacked type or interleavedwinding.

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 voltageratio. The taps may be connected to an automatic on-load tap changerfor voltage regulation of distribution circuits. Audio-frequencytransformers, used for the distribution of audio to public addressloudspeakers, have taps to allow adjustment of impedance to eachspeaker. A center-tapped transformer is often used in the output stageof 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 sealingthe windings and helping to prevent the possible formation of coronaand absorption of dirt or water. This produces transformers more suitedto damp or dirty environments, but at increased manufacturing cost.

Coolant

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 heatfrom 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 oilcirculates by natural convection; large transformers employ forcedcirculation of the oil by electric pumps, aided by external fans orwater-cooled heat exchangers. Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vaporbefore the cooling oil is introduced. This helps prevent electricalbreakdown 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 withmineral oils commonly also contained polychlorinated biphenyls ascontaminants 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 capabilitydeteriorate, the transformer casing is completely sealed. Experimentalpower 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.

Terminals

Very small transformers will have wire leads connected directly tothe ends of the coils, and brought out to the base of the unit forcircuit connections. Larger transformers may have heavy boltedterminals, 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.

History

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 severallighting fixtures with different luminous intensities from a singlesource of electric power'. Evidently, the induction coil in this systemoperated as a transformer.

Lucien Gaulard and John Dixon Gibbs,who first exhibited a device with an open iron core called a 'secondarygenerator' in London in 1882 and then sold the idea to American companyWestinghouse. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system.

William Stanley,an engineer for Westinghouse, built the first commercial device in 1885after George Westinghouse had bought Gaulard and Gibbs' patents. Thecore was made from interlocking E-shaped iron plates. This design wasfirst 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.

Single Phase Transformer Lab

While new technologies have made transformers in some electronicsapplications obsolete, transformers are still found in many electronicdevices. 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.)