Three-phase pole-mounted step-down transformer.

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled electrical conductors. 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. Look up transformer in Wiktionary, the free dictionary. ... Image File history File links Download high resolution version (1578x1536, 1482 KB) Summary Taken by User:Light current summer 2005 near Seaton, Devon using Olympus C745 on auto Licensing I, the creator of this work, hereby release it into the public domain. ... Image File history File links Download high resolution version (1578x1536, 1482 KB) Summary Taken by User:Light current summer 2005 near Seaton, Devon using Olympus C745 on auto Licensing I, the creator of this work, hereby release it into the public domain. ... Electrical energy can refer to several closely related things. ... A simple electric circuit made up of a voltage source and a resistor. ... In electronics, inductive coupling refers to the transfer of energy from one circuit component to another through a shared magnetic field. ... In science and engineering, conductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons. ... This box:      Electric current is the flow (movement) of electric charge. ... For magnetic induction, see Magnetic field. ... If an electric circuit has a well-defined output terminal, the circuit connected to this terminal (or its input impedance) is the load. ...

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. City lights viewed in a motion blurred exposure. ...

Transformers are some of the most efficient electrical 'machines',^[1] with some large units able to transfer 99.75% of their input power to their output.^[2] Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons 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 efficiency of an entity (a device, component, or system) in electronics and electrical engineering is defined as useful power output divided by the total electrical power consumed (a fractional expression). ... Microphones redirects here. ... Transmission towers Transmission lines in Lund, Sweden Electric power transmission, or more accurately Electrical energy transmission, is the second process in the delivery of electricity to consumers. ...

Applications

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 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.^[3] All but a fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.^[4] Transformers are used extensively in consumer electronic products to step down the supply voltage to a level suitable for the low voltage circuits they contain. In these kind of applications the transformer may also act as a key safety component that electrically isolates the end user from direct contact with the potentially lethal supply voltage. Power line redirects here. ... A wire is a single, usually cylindrical, elongated strand of drawn metal. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... In physics, power (symbol: P) is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time. ... Power line redirects here. ... Transmission lines in Lund, Sweden Electric company redirects here. ... This article is about machines that produce electricity. ... If an electric circuit has a well-defined output terminal, the circuit connected to this terminal (or its input impedance) is the load. ... For delivered electrical power, see Electrical power industry. ...

Basic principles

The transformer is based on two principles: firstly that an electric current can produce a magnetic field (electromagnetism) and secondly 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, it changes the strength of its magnetic field; since the changing magnetic field extends into the secondary coil, a voltage is induced across the secondary. This box:      Electric current is the flow (movement) of electric charge. ... For the indie-pop band, see The Magnetic Fields. ... Electromagnetism is the physics of the electromagnetic field: a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles. ... For magnetic induction, see Magnetic field. ...

An ideal step-down transformer showing magnetic flux in the core

A simplified transformer design is shown to the left. 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. Image File history File links Transformer3d_col3. ... Image File history File links Transformer3d_col3. ... For the indie-pop band, see The Magnetic Fields. ... In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. ... General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ...

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that: Faradays law of induction (more generally, the law of electromagnetic induction) states that the induced emf (electromotive force) in a closed loop equals the negative of the time rate of change of magnetic flux through the loop. ...

where V_S is the instantaneous voltage, N_S is the number of turns in the secondary coil and Φ equals the 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,^[1] the instantaneous voltage across the primary winding equals International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... Magnetic flux, represented by the Greek letter Φ (phi), is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. ... For the indie-pop band, see The Magnetic Fields. ...

Taking the ratio of the two equations for V_S and V_P gives the basic equation^[5] 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 into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... For the indie-pop band, see The Magnetic Fields. ... For delivered electrical power, see Electrical power industry. ...

P_incoming = I_PV_P = P_outgoing = I_SV_S

giving the ideal transformer equation

If the voltage is increased (stepped up) (V_S > V_P), then the current is decreased (stepped down) (I_S < I_P) by the same factor. Transformers are efficient so this formula is a reasonable approximation.

The impedance in one circuit is transformed by the square of the turns ratio.^[1] 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 .

Detailed operation

The simplified description above neglects 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.^[6] When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core.^[6]. 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 still required to create the magnetic field. Magnetic reluctance can be thought of as having an analogous function to resistance in an electrical circuit except that it cannot consume energy. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Magnetic flux, represented by the Greek letter Φ (phi), is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. ... A magnetic circuit is a closed path containing a magnetic flux. ...

The changing magnetic field induces an electromotive force (EMF) across each winding.^[7] Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages V_P and V_S 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".^[8] 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. Electromotive force (emf) is the amount of energy gained per unit charge that passes through a device in the opposite direction to the electric field existing across that device. ... A force that runs against the current which induces it, it is caused by a changing electromagnetic field. ... Lenzs law (pronounced (IPA) ) gives the direction of the induced electromotive force (emf) and current resulting from electromagnetic induction. ...

Practical considerations

Flux leakage in a two-winding transformer

Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ...

Flux leakage

Main article: Leakage inductance

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.^[9] Such flux is termed leakage flux, and results in self-inductance in series with the mutually coupled transformer windings.^[8] Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not directly a 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.^[9] Distribution transformers are therefore normally designed to have very low leakage inductance. Leakage inductance is that property of an electrical transformer that causes a winding to appear to have some pure inductance in series with the mutually-coupled transformer windings. ... Inductance is a physical characteristic of an inductor, which is an electrical device that produces at any time a voltage proportional to the instantaneous rate of change in current flowing through it. ... Electrical circuit components can be connected together in one of two ways: series or parallel. ... For the indie-pop band, see The Magnetic Fields. ... In electrical engineering, particularly power engineering, voltage regulation is the ability of a system to provide near constant voltage over a wide range of load conditions. ... Leakage inductance is that property of an electrical transformer that causes a winding to appear to have some pure inductance in series with the mutually-coupled transformer windings. ...

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.^[8] 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.^[10] Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have a direct current flowing through the windings. This article is about the 1986 movie. ... A VI curve with a negative differential resistance region Negative resistance or negative differential resistance (NDR) is a property of electrical circuit elements composed of certain materials in which, over certain voltage ranges, current is a decreasing function of voltage. ... A 3000 volt electricity arc between two nails Electricity arcs between the power rail and electrical pickup shoe on a London Underground train An electric arc can melt calcium oxide An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current... A Mercury-vapor lamp is a gas discharge lamp which uses mercury in an excited state to produce light. ... Neon signs are often used to advertise for hotels, bars and entertainment venues. ... Manual Metal Arc welding, also known as stick or MMA welding is one of the most common forms of welding. ...

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.^[11] Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time.^[12] 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.^[12] Faradays law of induction (more generally, the law of electromagnetic induction) states that the induced emf (electromotive force) in a closed loop equals the negative of the time rate of change of magnetic flux through the loop. ... This article is about the concept of integrals in calculus. ... For magnetic materials, saturation is the state when the material can not absorb a stronger magnetic field, such that an increase of magnetization produces no significant change in magnetic flux density. ...

The EMF of a transformer at a given flux density increases with frequency.^[6] By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation, and fewer turns are needed to achieve the same impedance. However properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight.^[13] The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. ...

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 will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect the transformer from overvoltage at higher than rated frequency. Automotive style miniature relay A relay is an electrical switch that opens and closes under the control of another electrical circuit. ...

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.

Energy losses

An ideal transformer would have no energy losses, and would therefore be 100% efficient. In practical transformers 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 98%.^[14]

Experimental transformers using superconducting windings achieving efficiencies of 99.85%,^[15] While the increase in efficiency is small, when applied to large heavily-loaded transformers the annual savings in energy losses is significant. A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. ...

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.^[16]

The losses vary with load current, and may be expressed as "no-load" or "full-load" loss. 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, which encourages development of low-loss transformers (also see energy efficient transformer).^[17] Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... A system with hysteresis can be summarised as a system that may be in any number of states, independent of the inputs to the system. ... As the circular plate moves down through a small region of constant magnetic field directed into the page, eddy currents are induced in the plate. ... In a typical grid, electric transformer loss typically contributes about 40-50% of the total transmission & distribution loss. ...

Transformers are among the most efficient of machines, but all exhibit losses

Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, sometimes termed iron loss. Losses in the transformer arise from: Download high resolution version (1024x1545, 96 KB)Closeup of singlephase polemount transformer This is a picture I took of the singlephase polemount transformer supplying my parents cottage on Old Lakeshore Road, about 12 miles from Dunville Ontario. ... Download high resolution version (1024x1545, 96 KB)Closeup of singlephase polemount transformer This is a picture I took of the singlephase polemount transformer supplying my parents cottage on Old Lakeshore Road, about 12 miles from Dunville Ontario. ... This article needs to be cleaned up to conform to a higher standard of quality. ... Core loss (or iron loss) is a form of energy loss that occurs in electrical transformers and other inductors. ...

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 is subjected.^[17]

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 thickness.^[17]

Magnetostriction

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,^[5] 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.^[18]

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.^[4]

Joule heating is the process by which the passage of an electric current through a conductor releases heat. ... The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. ... A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor. ... A system with hysteresis can be summarised as a system that may be in any number of states, independent of the inputs to the system. ... Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. ... In science and engineering, conductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons. ... As the circular plate moves down through a small region of constant magnetic field directed into the page, eddy currents are induced in the plate. ... Joule heating is the process by which the passage of an electric current through a conductor releases heat. ... Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. ...

Equivalent circuit

Refer to the diagram below

The physical limitations of the practical transformer may be brought together as an equivalent circuit model (shown below) built around an ideal lossless transformer.^[19] 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 dropped without contributing to the mutual coupling, and thus can be modeled as self-inductances X_P and X_S 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.^[20] 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. Inductance is a physical characteristic of an inductor, which is an electrical device that produces at any time a voltage proportional to the instantaneous rate of change in current flowing through it. ...

A core with finite permeability requires a magnetizing current I_M 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.^[20] 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.^[19] In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. ... In mathematics, the trigonometric functions are functions of an angle, important when studying triangles and modeling periodic phenomena. ... Electrical circuit components can be connected together in one of two ways: series or parallel. ...

The secondary impedance R_S and X_S is frequently moved (or "referred") to the primary side after multiplying the components by the impedance scaling factor . Electrical impedance, or simply impedance, is a measure of opposition to a sinusoidal alternating electric current. ...

Transformer equivalent circuit, with secondary impedances referred to the primary side

The resulting model is sometimes termed the "exact equivalent circuit", though it retains a number of approximations, such as an assumption of linearity.^[19] 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 impedances, resulting in so-called equivalent impedance. Image File history File links This is a lossless scalable vector image. ... The word linear comes from the Latin word linearis, which means created by lines. ...

The parameters of equivalent circuit of a transformer can be calculated from the results of two transformer tests: open-circuit test and short-circuit test.

Types

For more details on this topic, see Transformer types.

A variety of specialised transformer designs has been created to fulfill certain engineering applications, though they share several commonalities. Several of the more important transformer types include: A variety of specialised transformer types have been created for the electrical transformer to fulfil certain niche roles. ...

Autotransformer

Main article: Autotransformer

An autotransformer with a sliding brush contact

An autotransformer 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.^[21] 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.^[22] An autotransformer is an electrical transformer with only one winding. ... Image File history File links Variable_Transformer_01. ... Image File history File links Variable_Transformer_01. ... An autotransformer is an electrical transformer with only one winding. ... An autotransformer is an electrical transformer with only one winding. ... A pair of carbon brushes In electrical engineering, brushes conduct current between stationary wires and moving parts, most commonly in a rotating shaft. ...

Polyphase transformers

For more details on this topic, see Three-phase electric power.

For three-phase 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.^[23] A number of winding configurations are possible, giving rise to different attributes and phase shifts.^[24] One particular polyphase configuration is the zigzag transformer, used for grounding and in the suppression of harmonic currents.^[25] Three-phase power transformer which is the sole transfer point for electricity to a suburban shopping mall in Canada. ... Three phase systems have 3 waveforms (usually carrying power) that are 2/3π radians (120 degrees,1/3 of a cycle) offset in time. ... This article is about a portion of a periodic process. ... A zigzag transformer is a special purpose transformer. ... It has been suggested that Ground conductor be merged into this article or section. ... This article is about the components of sound. ...

Resonant transformers

A resonant transformer uses the inductance of its windings in combination with external capacitors connected in series or parallel with the windings, and/or the capacitance of the windings themselves, to create one or more resonant circuits. For example, it may use the inductance of the primary winding in series with a capacitor. Resonance can aid in achieving a very high voltage across the secondary. Resonant transformers such as the Tesla coil can generate very high voltages, and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator.^[26] 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.^[27] In an electrical circuit, resonance occurs at a particular frequency when the inductive reactance and the capacitive reactance are of equal magnitude, causing electrical energy to oscillate between the magnetic field of the inductor and the electric field of the capacitor. ... Categories: Technology stubs ... Tesla Coil at Questacon, the Australian National Science Centre museum A Tesla coil (also teslacoil) is a type of resonant transformer, named after its inventor, Nikola Tesla. ... Van de Graf generator. ... In electronics, the superheterodyne receiver (also known by its full name, the supersonic heterodyne receiver, or by the abbreviated form superhet) is a technique for selectively recovering the information from radio waves of a particular frequency. ...

Leakage transformers

A leakage transformer, also called a stray-field transformer, has a significantly higher leakage inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core between primary and secondary, which is sometimes adjustable with a set screw. This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings. The output and input currents are low enough to prevent thermal overload under all load conditions – even if the secondary is shorted. Leakage inductance is that property of an electrical transformer that causes a winding to appear to have some pure inductance in series with the mutually-coupled transformer windings. ...

Leakage transformers are used for arc welding and high voltage discharge lamps (cold cathode fluorescent lamps, which are series-connected up to 7.5 kV AC). It acts then both as a voltage transformer and as a magnetic ballast. Manual Metal Arc welding, also known as stick or MMA welding is one of the most common forms of welding. ... An automotive (ignition system) ballast resistor An electrical ballast (sometimes called control gear) is a device intended to limit the amount of current flowing in an electric circuit. ...

Other applications are short-circuit-proof extra-low voltage transformers for toys or doorbell installations. The use of extra low voltage (ELV) in an electrical circuit is one of several means to provide protection against electrical shock. ... Ancient shop doorbell A doorbell is a signaling device commonly found near a door. ...

Instrument transformers

Current transformers, designed to be looped around conductors

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.^[28] Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... A CT for operation on a 110 kV grid A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the current flowing in its primary. ... Power meter redirects here. ... It is the part of electrical power engineering that deals with protecting the electrical power system from faults by isolating the faulty part from the rest of the network. ...

Voltage transformers (VTs)--also referred to as Potential transformers (PTs)--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.^[29]

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.

In the United States the term (Volt-amps) in Electrical terms, means watts. ... The waveform of 230 volt, 50 Hz compared with 110 V, 60 Hz. ... An audio frequency (abbreviation: AF) is any frequency from about 20 hertz to about 20 kilohertz, which is the approximate range of sound frequencies that is audible to humans. ... It has been suggested that this article or section be merged with Radio waves. ... 11kV/400V-230V transformer in an older suburb of Wellington, New Zealand Electricity distribution is the penultimate stage in the delivery (before retail) of electricity to end users. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ... Steel mill with two arc furnaces An electric arc furnace is a system that heats charged material by means of an electric arc. ...

Construction

Cores

Laminated core transformer showing edge of laminations at top of unit.

Transformer, 115 VAC to 6. ... Transformer, 115 VAC to 6. ...

Laminated steel cores

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel.^[30] 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.^[31] 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.^[32] 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.^[23] The universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation. In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. ... Silicon steel is a soft magnetic material, a steel containing silicon, usually with virtually no other alloying elements. ... In physics, free space is a concept of electromagnetic theory, corresponding roughly to the vacuum, the baseline state of the electromagnetic field, or the replacement for the electromagnetic aether. ... For other uses, see Transformer (disambiguation). ...

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,^[30] but are more laborious and expensive to construct.^[33] Thin laminations are generally used on high frequency transformers, with some types of very thin steel laminations able to operate up to 10 kHz.

Laminating the core greatly reduces eddy-current losses

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".^[33] 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.^[33] They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... It has been suggested that this article or section be merged into Shape. ... It has been suggested that this article or section be merged into Shape. ...

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.^[34] 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.^[35] Remanence is the magnetization left behind in a medium after an external magnetic field is removed. ... Heres an example of inrush current transients during capacitor bank energization. ... Automotive style miniature relay A relay is an electrical switch that opens and closes under the control of another electrical circuit. ... 200 A Industrial fuse. ... Geomagnetically induced currents (GIC), affecting the normal operation of long technological conductor systems, are a manifestation at ground level of space weather. ... A geomagnetic storm is a storm in the magnetosphere. ...

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.^[36] Silicon steel is a soft magnetic material, a steel containing silicon, usually with virtually no other alloying elements. ... An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. ...

Solid cores

Powdered iron 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.^[33] 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. General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. ... Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of electric current. ... Very high frequency (VHF) is the radio frequency range from 30 MHz (wavelength 10 m) to 300 MHz (wavelength 1 m). ... This article is about ceramic materials. ... A stack of ferrite magnets Ferrites are electrically non-conductive ferrimagnetic ceramic compound materials, consisting of various mixtures of iron oxides such as Hematite (Fe2O3) or Magnetite (Fe3O4) and the oxides of other metals. ...

Toroidal cores

Small transformer with toroidal core

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.^[37] 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.^[38] 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. Image File history File links Size of this preview: 689 × 600 pixelsFull resolution (710 × 618 pixel, file size: 131 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Size of this preview: 689 × 600 pixelsFull resolution (710 × 618 pixel, file size: 131 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... For other uses, see Steel (disambiguation). ... Permalloy is a nickel iron alloy with about 20% iron and 80% nickel content. ... A stack of ferrite magnets Ferrites are electrically non-conductive ferrimagnetic ceramic compound materials, consisting of various mixtures of iron oxides such as Hematite (Fe2O3) or Magnetite (Fe3O4) and the oxides of other metals. ... Galvanized surface with visible crystallites (grains) of zinc. ... Magnetic reluctance is the resistance of a material to a magnetic field. ... Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a (usually undesirable) disturbance caused in a radio receiver or other electrical circuit by electromagnetic radiation emitted from an external source. ...

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

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 switching-mode power supply for laboratory use. ...

Air cores

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.^[8] The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution.^[8] They have however very high bandwidth, and are frequently employed in radio-frequency applications,^[39] for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings. A system with hysteresis can be summarised as a system that may be in any number of states, independent of the inputs to the system. ... Bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a filter, a communication channel, or a signal spectrum, and is typically measured in hertz. ...

Windings

Windings are usually arranged concentrically to minimise flux leakage

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.^[11] 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.^[40] Image File history File links Size of this preview: 662 × 599 pixelsFull resolution (1099 × 995 pixel, file size: 104 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Size of this preview: 662 × 599 pixelsFull resolution (1099 × 995 pixel, file size: 104 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Transformer_min_stray_field_geometry. ... Image File history File links Transformer_min_stray_field_geometry. ... It has been suggested that Silicon steel be merged into this article or section. ... This article or section does not cite any references or sources. ... Leakage inductance is that property of an electrical transformer that causes a winding to appear to have some pure inductance in series with the mutually-coupled transformer windings. ... In science and engineering, conductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons. ... The enameled wire is a thin wire with a plastic coverage used for made the electric motor coils. ... There are very few or no other articles that link to this one. ...

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.^[11] Large power transformer