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Large units able to transfer 99.75 of their input power to their output.All operate with the same basic principles, although the range of designs is wide.
The first widely used transformer was the induction coil, invented by Irish clergyman Nicholas Call in 1836.He was one of the first to understand the transformer principle was demonstrated in 1831 by Michael Faraday, although he principle that the more turns a transformer winding has, the larger EMF it
Russian engineer Pavel Yablochkov in 1876 invented a lighting system based on a set of induction coils, where primary windings were connected to a source of alternating current and secondary windings could be connected to several "electric candles" The patent claimed the system could "provide separate supply to several lighting fixtures with different luminous intensities from a single source of electric power". Evidently, the induction coil in this system operated as a transformer.
Lucien Gaulard and John Dixon Gibbs, who first exhibited a device with an open iron core called a 'secondary generator' in London in 1882 and then sold the idea to American company Westinghouse also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system.
William Stanley, an engineer from Westinghouse, built the first commercial device in 1885 after George Westinghouse had bought Gaulard and Gibbs' patents. The core was made
from interlocking E-shaped iron plates. This design was first used commercially in 1886. 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.
A key application of transformers is to increase voltage before transmitting electrical energy over long distances through wires.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 form for transmission and back again afterwards, transformers enable economic transmission of power over long distances. Transformers are used extensively in electronic products to step down the supply voltage to a level suitable for the low voltage circuits they contain..
Transformers are also used to couple stages of amplifiers and to match devices such as microphones and record player cartridges to the input impedance of amplifiers. Transformers are also used when it is necessary to couple a differential-mode signal to a ground-referenced signal, and for isolation between external cables and internal circuits.
The transformer is based on two principles: firstly, that an electric current can produce a magnetic field and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil 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.
The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:
Vs = Ns.dF/dt
where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and F 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, the instantaneous voltage across the primary winding equals
Vp = Np.dF/dt
Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage
Vs/Vp = Ns/Np
Ideal power equation
States that 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. The incoming electric power must equal the outgoing power.
Pincoming = IPVP = Poutgoing = ISVS
giving the ideal transformer equation
Vs/Vp = Ns/Np = Ip/Is
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 magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.
The changing magnetic field induces an electromotive force across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs.
Leakage flux of a transformer
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 traces paths that take it outside the windings.Such flux is termed leakage flux, and results in leakage 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. Transformers are therefore normally designed to have very low leakage inductance. 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 in circuits that have a direct current flowing through the windings.
Effect of frequency
Transformer would work with direct-current excitation, with the core flux increasing linearly with time.Also the flux would rise to the point where magnetic saturation of the core occurred, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating current.
An ideal transformer do not have energy losses, and would 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.
Experimental transformers using superconducting windings achieve efficiencies of 99.85, while the increase in efficiency is small, when applied to large heavily-loaded transformers the annual savings in energy losses is significant.
A small transformer, such as a plug-in "wall-wart" or power adapter type used for low-power consumer electronics, may be no more than 85 efficient, with considerable loss even when not supplying any load.
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. 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, termed iron loss. Losses in the transformer arise from:
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.
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.
Eddy currents 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 inversesquare of the material thickness.
Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and in turn causes losses due to frictional heating in susceptible cores.
Inaddition 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.
A wide variety of transformer designs are used for different applications, though they share several common features. Important common transformer types includes
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. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. An adjustable autotransformer is made by exposing part of the winding coils and making the secondary connection through a sliding brush, giving a variable turns ratio.
b) Leakage transformer
Fig1.5 Leakage transformer
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.Leakage transformers are used for arc welding and high voltage discharge lamps. It acts then both as a voltage transformer and as a magnetic ballast.Other applications are short-circuit-proof extra-low voltage transformers for toys or doorbell installations.
c) Resonant transformers
These are a type of the leakage transformer. It uses the leakage inductance of its secondary windings in combination with external capacitors, to create one or more resonant circuits. 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.
d) current transformer
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.
Transformers can be classified in different ways:
1. by frequency range: power, or radio frequency;
2. by power range: a fraction of a volt-ampere to over a thousand MVA;
3. by cooling type: air cooled, oil filled, fan cooled, or water cooled;
4. by application function:such as power supply, impedance matching, output voltage and current stabilizer, or circuit isolation;
5. by endpurpose:distribution, rectifier, arc furnace, amplifier output;
6. By winding turns ratio: step-up, step-down, isolating, variable.
A laminated core transformer showing edge of laminations at top of unit
One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of "E-I transformer". Such a design tends to exhibit more losses, but is very economical to manufacture.
b) Solid cores
Powdered iron cores are used in circuits that operate above main frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic ceramic materials called ferrites are common.Some radio-frequency transformers also have movable cores which allow adjustment of the coupling coefficient of tuned radio-frequency circuits.
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 wound into a coil, powdered iron, or ferrite.The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available.The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference.
Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size, lower weight, less mechanic, lower exterior magnetic field, low off-load losse, single-bolt mounting, and greater choice of shapes.
The main disadvantages are higher cost and limited rating.
d) Air cores
They have 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 are usually arranged concentrically to minimize flux leakage
The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire, such as Formvar wire.
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. For signal transformers, the windings may be arranged in a way to minimize leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interleaved winding.
Both the primary and secondary windings on power transformers may have external connections, called taps, to intermediate points on the winding to allow selection of the voltage ratio. The taps may be connected to an automatic on-load tap changer for voltage regulation of distribution circuits.
Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, one can replace air spaces within the windings with epoxy, thus sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers more suited to damp or dirty environments, but at increased manufacturing cost.
Some power transformers are immersed in transformer oil that both cools and insulates the windings. The oil is a highly refined mineral oil that remains stable at high temperatures. Liquid-filled transformers to be used indoors must use a non-flammable liquid, or must be located in fire and/or explosion resistant rooms.
Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.
8. Power transformers
This is the most common type of transformer, widely used in appliances to convert mains voltage to low voltage to power electronics
a) Widely available in power ratings ranging from mW to MW
b) Insulated laminations minimize eddy current losses
c) Small appliance and electronic transformers may use a split bobbin, giving a high level of insulation between the windings
d) Rectangular core
e) Core laminate stampings are usually in EI shape pairs. Other shape pairs are sometimes used.
f) A screen winding is occasionally used between the 2 power windings
g) Small appliance and electronics transformers may have a thermal cut out built in
h) Occasionally seen in low profile format for use in restricted spaces
i) laminated core made with silicon steel with high permeability
Doughnut shaped toroidal transformers are used to save space compared to EI cores, and sometimes to reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring, and tape for insulation.
Toroidals compared to EI core transformers:
a) Lower external magnetic field
b) Smaller for a given power rating
c) Higher cost in most cases, as winding requires more complex & slower equipment
d) Less robust
e) Central fixing is either
a. bolt, large metal washers & rubber pads
b. bolt & potting resin
f) Overtightening the central fixing bolt may short the windings.
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 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.Since the current in the windings is lower, the transformer is smaller, lighter cheaper and more efficient. For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than an isolating transformer of the same rating.
Constan voltage transformer.
By arranging particular magnetic properties of a transformer core, and installing a resonant tank circuit, a transformer can be arranged to automatically keep the secondary winding voltage constant regardless of any variance in the primary supply without additional circuitry or manual adjustment. CVA transformers run hotter than standard power transformers, for the regulating action is dependent on core saturation, which reduces efficiency somewhat.
Stray field transformer
A Stray field transformer has a significant stray field or a magnetic bypass in its core. It can act as a transformer with inherent current limitation due to its lower tight coupling between the primary and the secondary winding, which is unwanted in other cases. The output and input currents are low enough to prevent thermal overload under each load condition even if the secondary is shortened.
Stray field transformers are used for arc welding. It acts both as voltage transformer and magnetic ballast.
In polyphase transformer the three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Delta, Delta-Y, Delta-Delta and Y-Y. A vector group indicates the configuration of the windings and the phase angle difference between them. If a winding is connected to earth, the earth connection point is usually the center point of a Y winding. If the secondary is a Delta winding, the ground may be connected to a center tap on one winding or one phase may be grounded. There are many possible configurations that may involve more or fewer than six windings and various tap connections.
Example of Y- Y Connection
A resonating transformer used to generate an arc.
A resonant transformer operates at the resonant frequency of one or more of its coils and an external capacitor. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. When the primary coil is driven by a periodic source of alternating current, at the resonant frequency, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are used to generate high alternating voltages, and the current available can be much larger than that from electrostatic machines such as the Van de Graff generator.
1. Tesla coil
2. Electrical breakdown and insulation testing of high voltage equipment and cables. In the latter case, the transformer's secondary is resonated with the cable's capacitance.
Other applications of resonant transformers are as coupling between stages of a super heterodyne receiver, where the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.
A planar transformer
Exploded view: the spiral primary "winding" on side of the PCB
Planar transformers are one of many components on one large printed circuit board.
much thinner than other transformers, for low-profile applications
almost all use a ferrite planar core
Oil cooled transformer
For large transformers used in power distribution or electrical substations, the core and coils of the transformer are immersed in oil which cools and insulates. Oil circulates through ducts in the coil and around the coil and core assembly, moved by convection. The oil is cooled by the outside of the tank in small ratings, and in larger ratings an air-cooled radiator is used. Where a higher rating is required, or where the transformer is used in a building or underground, oil pumps are used to circulate the oil and an oil-to-water heat exchanger may also be used. .
The term 'isolating transformer' is normally applied to mains transformers providing isolation rather than voltage transformation. They are simply 1:1 laminated core transformers. Extra voltage tapings are sometimes included, but to earn the name 'isolating transformer' it is expected that they will usually be used at 1:1 ratio.
Current transformers used in metering equipment for three-phase 400 ampere electricity supply
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 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 through a well-insulated toroidal core wrapped with many turns of wire. Specially constructed wideband CTs are also used, usually with an oscilloscope, to measure high frequency waveforms or pulsed currents within pulsed power systems.
Voltage 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.
While VTs were formerly used for all voltages greater than 240V primary, modern meters eliminate the need VTs for most secondary service voltages. VTs are typically used in circuits where the system voltage level is above 600 V. Modern meters eliminate the need of VT's since the voltage remains constant and it is measured in the incoming supply.
A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses. Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. 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.
Pulse transformers by definition have a duty cycle of less than 1, whatever energy stored in the coil during the pulse must be dumped out before the pulse is fired again.
There are several types of transformer used in radio frequency work.
These are used for high frequency work. The lack of a core means very low inductance. Such transformers may be nothing more than a few turns of wire soldered onto a printed circuit board.
These are mostly tuned transformers, containing a threaded ferrite slug that is screwed in or out to adjust IF tuning. The transformers are usually canned for stability and to reduce interference.
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. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios can be achieved with this technique.
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 to inject feedback into an earlier stage in antique regenerative radio receivers.
Baluns are transformers designed specifically 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 are usually the factors which limit sound when used; electronic circuits with wide frequency response and low distortion are relatively simple to design.
Transformers are also used in DI boxes to convert high-impedance instrument signals to low impedance signals to enable them to be connected to a microphone input on the mixing consol.
Loudspeaker transformers can be used to allow many individual loudspeakers to be powered from a single audio circuit operated at higher-than normal loudspeaker voltages. This application can be seen in industrial public address applications. Such circuits are commonly referred to as constant voltage speaker systems.
The loudspeaker transformers commonly have multiple primary taps, allowing the volume at each speaker to be adjusted in discrete steps.
Output transformer (valve)
Valve amplifiers almost always use an output transformer to match the high load impedance requirement of the valves to a low impedance speaker.
Small Signal transformers
These types of transformer are usually used to convert the voltage to the range of the more common moving-magnet cartridges.
Microphones may also be matched to their load with a small transformer, which is shielded to minimise noise pickup. These transformers are less widely used today, as transistorized buffers are now cheaper.
'Interstage' and coupling TRANSFORMERS
A use for interstage transformers is in the case of push-pull amplifiers where an inverted signal is required. Here two secondary windings wired in opposite polarities may be used to drive the output devices. These phase splitting transformers are not much used today.
Cast resin transformers
Cast-resin power transformers are being widely used for a long time.The advantage of these transformer is easy installation and improve fire behaviour in case of class. This indoor type transformer is totally dry, without cooling oil.
10.Homemade& Obsolete Transformers
Transformers may be wound at home using commercial transformer kits, which contain laminations Or ready made transformers may be disassembled and rewound. These approaches are occasionally used by home constructors, but are usually avoided where possible due to the number of hours required to hand wind a transformer.
It is possible to make the transformer laminations by hand too.Such transformers can be made using laminations cut from scrap sheet steel, paper slips between the laminations, and string to tie the assembly together. The result works, but is usually noisy due to poor clamping of laminations.
Hedgehog transformers are homemade audio interstage coupling transformers.
Enamelled copper wire is wound round the central half of the length of a bundle of insulated iron wire, to make the windings. The ends of the iron wires are then bent around the electrical winding to complete the magnetic circuit, and the whole is wrapped with tape or string to hold it together.
These are sometimes used when the cost of a ready made transformer could not be justified,inductance tends to be on the low side, with consequent loss of bass.
Variocouplers are rf transformers with 2 windings and variable coupling between the windings.
variocouplers were common in 1920s radios for variable rf coupling. The 2 planar coils were arranged to swing away from each other and for the angle between them to increase to 90 degrees, thus giving wide variation in coupling.
These were mostly used to control reaction..
In a design of variocoupler, 2 coils were wound on a 2 circular bands, and housed one inside the other, with provision for rotating the inner coil. Coupling varies as one coil is rotated between 0 and 90 degrees from the other.
These had a higher stray capacitance.
CONCLUSION: From the brief study about transformers I conclude: Energy-efficiency can be improved with better transformer design reducing flux density in a specific core by increasing the core size; increasing conductor cross-section to reduce current density; good balancing between the relative quantities of iron and copper in the core and coils; and so on, or by the adoption of amorphous iron transformers world-wide.
Transformers could emerge as a major focus for energy efficiency initiatives in OECD countries, comparable with electric motors, domestic appliances, etc. They are potentially capable of making a similar contribution to reducing carbon emissions and achieving global-warminggoals.
Higher efficiency copper wound transformer saves
Transformer will be added to top running scheme in JAPANin coming years
Several countries are facing significant growth in electricity demand. They could benefit greatly from installing energy-efficient
transformers. . The energy-efficient transformers initiative could impact the world market, benefiting to economies of the countries also even small improvements in transformer efficiency can result in substantial energy and greenhouse gas savings.