transformer - Overview, Basic principles, Practical considerations, Construction, Transformer types, Uses of transformers

A device, with no moving parts, which transfers an alternating current (AC) from one circuit (called the primary winding) to one or more other circuits (secondary winding) by electromagnetic induction, usually with a change in voltage. There is no electrical connection between the two circuits. It is often used for converting the high voltage from AC power supplies to the normal domestic supply voltage.

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:This article is about transformers as used in electrical and electronics applications.

A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. A changing current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Nikola Tesla in 1891 invented the Tesla coil, which is a high-voltage, air-core, dual-tuned resonant transformer for generating very high voltages at high frequency.

Many others have patents on transformers.

Overview

The transformer is one of the simplest of electrical devices, yet transformer designs and materials continue to be improved. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.

Audio frequency transformers (at the time called repeating coils) were used by the earliest experimenters in the development of the telephone. While some electronics applications of the transformer have been made obsolete by new technologies, transformers are still found in many electronic devices.

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids.

Transformers alone cannot do the following:

Convert DC to AC or vice versa Change the voltage or current of DC Change the AC supply frequency.

However, transformers are components of the systems that perform all these functions.

An analogy

The transformer may be considered as a simple two-wheel 'gearbox' for electrical voltage and current. In a gearbox, mechanical power (torque multiplied by speed) is constant (neglecting losses) and is equivalent to electrical power (voltage multiplied by current) which is also constant.

The gear ratio is equivalent to the transformer step-up or step-down ratio. A step-up transformer acts analogously to a reduction gear (in which mechanical power is transferred from a small, rapidly rotating gear to a large, slowly rotating gear): it trades current (speed) for voltage (torque), by transferring power from a primary coil to a secondary coil having more turns. A step-down transformer acts analogously to a multiplier gear (in which mechanical power is transferred from a large gear to a small gear): it trades voltage (torque) for current (speed), by transferring power from a primary coil to a secondary coil having fewer turns.

Basic principles

Coupling by mutual induction

A simple transformer consists of two electrical conductors called the primary winding and the secondary winding.

Simplified analysis

If a time-varying voltage is applied to the primary winding of turns, a current will flow in it producing a magnetomotive force (MMF). The primary MMF produces a varying magnetic flux in the core, and, with an open circuit secondary winding, induces a back electromotive force (EMF) in opposition to .

where

vP and vS are the voltages across the primary winding and secondary winding, NP and NS are the numbers of turns in the primary winding and secondary winding, dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and secondary windings.

where

vp and vs are voltages across primary and secondary, Np and Ns are the numbers of turns in the primary and secondary , respectively.

Hence in an ideal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings. This leads to the most common use of the transformer: to convert electrical energy at one voltage to energy at a different voltage by means of windings with different numbers of turns.

The EMF in the secondary winding, if connected to an electrical circuit, will cause current to flow in the secondary circuit. Since the reduced flux reduces the EMF induced in the primary winding, increased current flows in the primary circuit.

For example, suppose a power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.

Analysis of the ideal transformer

This treats the windings as a pair of mutually coupled coils with both primary and secondary windings passing currents and with each coil linked with the same magnetic flux.

In the ideal transformer at no load, i.e. with the secondary load removed, the voltage applied to the primary winding is opposed by an induced EMF in the winding equal to the applied voltage in accordance with Faraday's law of induction.

Further on, the balance of the primary and secondary MMF:s i.e.

That is: The ratio between the primary and secondary currents is the inverse of the ratio between the corresponding voltages.

DC voltages and currents

A DC voltage applied to a winding of an ideal transformer will cause a DC voltage to be induced in the other winding. However, using a transformer with DC voltages would require the magnetic flux in the core (and current supplied by the DC voltage source) to increase without bound.

It is possible to draw DC current from a transformer, as a DC current merely represents a constant offset to the flux in the core. Most transformers are designed to be driven to near saturation without any DC current components, so having a DC current will make the transformer saturate more easily.

The universal EMF equation

If the flux in the core is sinusoidal, the relationship for either winding between its number of turns, voltage, magnetic flux density and core cross-sectional area is given by the universal emf equation (from Faraday's law):

where

E is the sinusoidal rms or root mean square voltage of the winding, f is the frequency in hertz, N is the number of turns of wire on the winding, a is the cross-sectional area of the core in square metres B is the peak magnetic flux density in teslas P is the power in volt amperes or watts,

Other consistent systems of units can be used with the appropriate conversions in the equation.

Practical considerations

Classifications

Transformers are adapted to numerous engineering applications and may be classified in many ways:

By power level (from fraction of a volt-ampere(VA) to over a thousand MVA), By application (power supply, impedance matching, circuit isolation), By frequency range (power, audio, radio frequency(RF)) By voltage class (a few volts to about 750 kilovolts) By cooling type (air cooled, oil filled, fan cooled, water cooled, etc.) By purpose (distribution, rectifier, arc furnace, amplifier output, etc.). Isolating Intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses (otherwise the output voltage would be a little less than, rather than the same as, the input voltage).

Circuit symbols

Standard symbols

Transformer with two windings and iron core.

Losses

An ideal transformer would have no losses, and would therefore be 100% efficient. Large power transformers (over 50 MVA) may attain an efficiency as high as 99.75%. Small transformers, such as a plug-in "power brick" used to power small consumer electronics, may be less than 85% efficient.

Transformer losses arise from:

Winding resistance

Current flowing through the windings causes resistive heating of the conductors (I2 R loss).

Eddy currents

Induced eddy currents circulate within the core, causing resistive heating.

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis within the magnetic core.

Magnetostriction

Magnetic flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction.

Mechanical losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings.

Cooling system

Large power transformers may be equipped with cooling fans, oil pumps or water-cooled heat exchangers designed to remove the heat caused by copper and iron losses. The power used to operate the cooling system is typically considered part of the losses of the transformer.

Operation at different frequencies

The equation shows that the EMF of a transformer at a given flux density increases with frequency. By operating at higher frequencies, transformers can be physically more compact without reaching saturation, and a given core is able to transfer more power. Generally, operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetising (no load primary) current.

Steel cores develop a larger hysteresis loss due to eddy currents as the operating frequency is increased.

Flyback transformers are built using ferrite cores.

Switching power supply transformers usually operate between 30-1000 kHz.

Operation of a power 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 at hydroelectric generating stations may be equipped with over-excitation protection, so-called "volts per hertz" protection relays, to protect the transformer from overvoltage at higher-than-rated frequency which may occur if a generator loses its connected load.

Construction

Cores

Steel cores

Transformers for use at power or audio frequencies have cores made of many thin laminations of silicon steel. Very thin laminations are generally used on high frequency transformers.

The cut core or C-core is made by winding a silicon steel strip around a rectangular form.

A steel core's remanence means that it retains a static magnetic field when power is removed. 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 false operation of transformer protection devices.

Distribution transformers can achieve low off-load losses by using cores made with low loss high permeability silicon steel and amorphous (non-crystalline) steel, so-called "metal glasses" — the high cost of the core material is offset by the lower losses incurred at light load, over the life of the transformer. In order to maintain good voltage regulation, distribution transformers are designed to have very low leakage inductance.

Certain special purpose transformers use long magnetic paths, insert air gaps, or add magnetic shunts (which bypass a portion of magnetic flux that would otherwise link the primary and secondary windings) in order to intentionally add leakage inductance. Gaps are also used to keep a transformer from saturating, especially audio transformers which have a DC component added.

Solid cores

Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above mains frequencies and up to a few tens of kilohertz.

At even higher, radio-frequencies (RF), other types of cores made from non-conductive magnetic ceramic materials, called ferrites, are common. Some RF transformers also have moveable cores (sometimes called slugs) which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

Air cores

High-frequency transformers may also use air cores. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.

Toroidal cores

Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel or permalloy wound into a coil, from powdered iron, or ferrite, depending on operating frequency.

Ferrite toroid 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.

Toroidal transformers are more efficient than the cheaper laminated EI types of similar power level.

A drawback of toroidal transformer construction is the higher cost of windings. 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.

When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core.

Windings

The wire of the adjacent turns in a coil, and in the different windings, must be electrically insulated from each other. Transformers for years have used Formvar wire which is a varnished type of magnet wire.

The conducting material used for the winding depends upon the application. Small power and signal transformers are wound with solid copper wire, insulated usually with enamel, and sometimes additional insulation. Larger power transformers may be wound with wire, copper, or aluminum rectangular conductors. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made of Litz wire to minimize the skin effect losses in the conductors. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. (see reference (1) below)

For signal transformers, the windings may be arranged in a way to minimise leakage inductance and stray capacitance to improve high-frequency response.

Windings on both the primary and secondary of power transformers may have external connections (called taps) to intermediate points on the winding to allow adjustment of the voltage ratio. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull type circuit. Tapped transformers are also used as components of amplifiers, oscillators, and for feedback linearization of amplifier circuits.

Insulation

The turns of the windings must be insulated from each other to ensure that the current travels through the entire winding. The potential difference between adjacent turns is usually small, so that enamel insulation is usually sufficient for small power transformers. Supplemental sheet or tape insulation is usually employed between winding layers in larger transformers.

The transformer may also be immersed in transformer oil that provides further insulation. Although the oil is primarily used to cool the transformer, it also helps to reduce the formation of corona discharge within high voltage transformers. To ensure that the insulating capability of the transformer oil does not deteriorate, the transformer casing is completely sealed against moisture ingress.

Certain power transformers have the windings protected by epoxy resin.

Shielding

Where transformers are intended for minimum electrostatic coupling between primary and secondary circuits, an electrostatic shield can be placed between windings to reduce the capacitance between primary and secondary windings.

Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer, or to prevent the transformer from affecting the operation of nearby devices that may be sensitive to stray fields such as CRTs.

Coolant

Small signal transformers do not generate significant amounts of heat. Power transformers rated up to a few kilowatts rely on natural convective air cooling. Transformers handling higher power, or having a high duty cycle can be fan-cooled.

Some dry transformers are enclosed in pressurized tanks and are cooled by nitrogen or sulfur hexafluoride gas.

The windings of high-power or high-voltage transformers are immersed in transformer oil — a highly-refined mineral oil, that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable.

The oil cools the transformer, and provides part of the electrical insulation between internal live parts. Very large or high-power transformers (with capacities of millions of watts) may have cooling fans, oil pumps and even oil to water heat exchangers. Oil-filled transformers undergo prolonged drying processes, using vapor-phase heat transfer, electrical self-heating, the application of a vacuum, or combinations of these, to ensure that the transformer is completely free of water vapor before the cooling oil is introduced.

Oil-filled power transformers may be equipped with Buchholz relays which are safety devices that sense gas build-up inside the transformer (a side effect of an electric arc inside the windings), and thus switches off the transformer.

Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss.

Terminals

Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections.

Enclosure

Small transformers often have no enclosure.

Transformer types

Autotransformers

An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. While theoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 volts across the entire winding, and 115 volts between one end and the tap. As the same winding is used for input and output, the flux in the core is partially cancelled, and a smaller core can be used. For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than a true (two-winding) transformer of the same rating.

In practice, transformer losses mean that autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if used to step up.

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 can be obtained, allowing for very small increments of voltage.

Polyphase transformers

For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer.

Resonant transformers

A resonant transformer operates at the resonant frequency of one or more of its coils and (usually) an external capacitor. When the primary coil is driven by a periodic source of alternating current, such as a square or Sawtooth wave at the resonant frequency, each pulse of current helps to build up an oscillation in the secondary coil. These devices are used to generate high alternating voltages, and the current available from this device can be much larger than that from electrostatic machines such as the Van de Graaff generator or Wimshurst machine.

Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.

A voltage regulating transformer uses a resonant winding and allows part of the core to go into saturation on each cycle of the alternating current. This effect stabilizes the output of the regulating transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an ac power supply.

Instrument transformers

Current transformers

A current transformer is a type of "instrument transformer" that is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.

Current transformers (CTs) are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.

Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary.

Specially constructed wideband current transformers are also used (usually with an oscilloscope) to measure waveforms of high frequency or pulsed currents within pulsed power systems. One type of specially constructed wideband transformer provides a voltage output that is proportional to the measured current.

Voltage transformers

Voltage transformers (or potential transformers) are another type of instrument transformer, 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. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protection relays.

The transformer winding high-voltage connection points are typically labelled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. This applies to current transformers as well.

Pulse transformers

A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a constant amplitude). Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other high energy pulsed power applications.

To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load.

The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers.

RF transformers (transmission line transformers)

For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or coaxial cable, wound around ferrite or other types of core.

The core material increases the inductance dramatically, thereby raising its Q factor. The cores of such transformers help improve performance at the lower frequency end of the band. RF transformers sometimes used a third coil (called a tickler winding) to inject feedback into an earlier (detector) stage in antique regenerative radio receivers.

Baluns

Baluns are transformers designed specifially to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar or coaxial cable and are similar to transmission line transformers in construction and operation.

Audio transformers

Audio transformers are usually the factor which limit sound quality;

Transformers are also used in DI boxes to convert impedance from high-impedance instruments (for example, bass guitars) to enable them to be connected to a microphone input on the mixing console.

A particularly critical component is the output transformer of an audio power amplifier. Valve circuits for quality reproduction have long been produced with no other (inter-stage) audio transformers, but an output transformer is needed to couple the relatively high impedance (up to a few hundred ohms depending upon configuration) of the output valve(s) to the low impedance of a loudspeaker. the speakers require high current at low voltage.) Solid-state power amplifiers may need no output transformer at all.

For good low-frequency response a relatively large iron core is required;

Early transistor audio power amplifiers often had output transformers, but they were eliminated as designers discovered how to design amplifiers without them.

Speaker transformers

In the same way that transformers are used to create high voltage power transmission circuits that minimize transmission losses, speaker transformers allow many individual loudspeakers to be powered from a single audio circuit operated at higher-than normal speaker voltages.

At the audio amplifier, a large audio transformer may be used to step-up the low impedance, low-voltage output of the amplifier to the designed line voltage of the speaker circuit. Then, a smaller transformer at each speaker returns the voltage and impedance to ordinary speaker levels. The speaker transformers commonly have multiple primary taps, allowing the volume at each speaker to be adjusted in a number of discrete steps.

Use of a constant-voltage speaker circuit means that there is no need to worry about the impedance presented to the amplifier output (which would clearly be too low if all of the speakers were arranged in parallel and would be too complex a design problem if the speakers were arranged in series-parallel). The use of higher transmission voltage and impedance means that power lost in the connecting wire is minimized, even with the use of small-gauge conductors (and leads to the term constant voltage as the line voltage doesn't change much as additional speakers are added to the system).

Small Signal transformers

Moving coil phonograph cartriges produce a very small voltage. In order for this to be amplified with a reasonable signal-noise ratio, a transformer is usually used to convert the voltage to the range of the more common moving-magnet cartridges.

'Interstage' and coupling transformers

A use for interstage transformers is in the case of push-pull amplifiers where an inverted signal is required.

Uses of transformers

For supplying power from an alternating current power grid to equipment which uses a different voltage. Large, specially constructed power transformers are used for electric arc furnaces used in steelmaking. Rotating transformers are designed so that one winding turns while the other remains stationary. Sliding transformers can pass power or signals from a stationary mounting to a moving part such as a machine tool head. Some rotary transformers are used to couple signals between two parts which rotate in relation to each other. Small transformers are often used internally to couple different stages of radio receivers and audio amplifiers. Transformers may be used as external accessories for impedance matching;