Batangas State UniversityCollege of Engineering, Architecture, and Fine Arts

Electrical Engineering Department

EE 458AC Apparatus and Devices

Transformer CoreTransformer Winding

Members:Agena, KennethAnog, Rajmiko

Atienza, DarwinAustria, Jonas

Bagon, Jerick CaesarBanaag, Mark Kevin

Bautista, Vanessa EllaineBoongaling, Pamela Camille

Bool, MarivetteCantos, Sherlyn

Engr. Ma. Lourdes V. BalansayJanuary 11, 2014

TRANSFORMER

A transformer is a static machine used for transforming powerfrom one circuit to another without changing frequency. This is verybasic definition of transformer.

Electrical transformers are used to "transform" voltage from onelevel to another, usually from a higher voltage to a lower voltage.They do this by applying the principle of magnetic induction betweencoils to convert voltage and/or current levels.

In this way, electrical transformers are a passive device whichtransforms alternating current (otherwise known as "AC") electricenergy from one circuit into another through electromagneticinduction.

History of Transformer

The history of transformer commenced in the year of 1880. In theyear of 1950 400KV electrical power transformer first introduced inhigh voltage electrical power system. In the early 1970s unit ratingas large as 1100MVA were produced and 800KV and even higher KV classtransformers were manufactured in year of 1980.

Working Principle of Transformer

The working principle of transformer is very simple. It dependsupon Faraday's law of electromagnetic induction. Actually mutualinduction between two or more winding is responsible fortransformation action in an electrical transformer.

Faraday’s Law on Electromagnetic Induction

According to these Faraday's law,

"Rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil".

Basic Theory of Transformer

A typical transformer has two or more coils that share a commonlaminated iron core. One of the coils is referred to as the primary(containing Np turns), while the other coil is called the secondary(containing NS turns). A varying current in the primary winding createsa varying magnetic flux in the transformer's core and thus a varyingmagnetic flux through the secondary winding. This varying magneticflux induces a varying electromotive force (emf) or voltage in thesecondary winding.

Types of Transformer

Control Transformer - is generally used in an electronic circuitthat requires constant voltage or constant current with a lowpower or volt-amp rating. Various filtering devices, such ascapacitors, are used to minimize the variations in the output.This results in a more constant voltage or current.

Current Transformer - a reduced current accurately proportionalto the current in the circuit, which can be convenientlyconnected to measuring and recording instruments. A currenttransformer isolates the measuring instruments from what may bevery high voltage in the monitored circuit. They are commonlyused in metering and protective relays in the electrical powerindustry.

Power Transformer - generally used in transmission network.

Isolation Transformer - is a transformer used to transferelectrical power from a source of alternating current (AC) powerto some equipment or device while isolating the powered devicefrom the power source, usually for safety. Isolation transformersprovide galvanic isolation and are used to protect againstelectric shock, to suppress electrical noise in sensitivedevices, or to transfer power between two circuits which must notbe connected. Isolation transformers block transmission of the DCcomponent in signals from one circuit to the other, but allow ACcomponents in signals to pass.

http://en.wikipedia.org/wiki/File:Trenntransformator.jpg

Autotransformer - is an electrical transformer in which there isone winding, a portion of which is common to both the primary andthe secondary circuits. In other words, the primary and secondarycoils have some or all windings in common. It is commonly usedfor the voltage conversion of local power line voltage to someother voltage value needed for a particular piece of electricalequipment.

Dry Type Transformer - provide a safe and reliable power sourcewhich does not require fire proof vaults, catch basins or theventing of toxic gasses. These important safety factors allow theinstallation of dry type transformers inside buildings close tothe load, which improves overall system regulation and reducescostly secondary line losses.

Step-Up / Step-Down Transformer - are designed to rise/reduceelectrical voltage. It converts electrical voltage from one levelor phase configuration up/down to a higher/lower level.

Transformer Core

The core makes up the bulk of a transformer. In an electricalpower transformer there are primary, secondary and may be tertiarywindings. The performance of a transformer mainly depends upon theflux linkages between these windings. For efficient flux linkingbetween these winding one low reluctance magnetic paths common to allwindings, should be provided in the transformer. This low reluctancemagnetic path in transformer is known as core of transformer.

It provides a controlled path for the magnetic flux generated inthe transformer by the current flowing through the windings. It isoften stated as being to increase and concentrate the magnetic fluxthat links the primary and secondary coils.

Core is the low loss link between the primary and secondarycircuits of a transformer, where electrical energy from primary isconverted to magnetic energy in the core and again back to electricalenergy in the secondary.

The thickness of the core ranges from 0.23 mm to upwards of 0.36mm. The core cross section can be circular or rectangular, withcircular cores commonly referred to as cruciform construction.Rectangular cores are used for smaller ratings and as auxiliarytransformers used within a power transformer. Rectangular cores use asingle width of strip steel, while circular cores use a combination ofdifferent strip widths to approximate a circular cross-section.

Types of Transformer Cores Laminated Steel Cores Solid Cores Toroidal Cores Solid Cores Air cores

Laminated Steel Cores

Transformers for use at power or audio frequencies typically havecores made of high permeability silicon steel. The steel has apermeability many times that of free space and the core thus serves togreatly reduce the magnetizing current and confine the flux to a pathwhich closely couples the windings.

The effect of laminations is to confine eddy currents to highlyelliptical paths that enclose little flux, and so reduce theirmagnitude. One common design of laminated core is made frominterleaved stacks of E-shaped steel sheets capped with I-shapedpieces, leading to its name of 'E-I transformer'.

Examples of Laminated Steel Cores

Solid Cores

Solid cores, particularly the powdered iron cores used incircuits, have high magnetic permeability as well as electricalresistance. When used in circuits, they tend to work best fortransmission levels above main frequencies. For frequencies that tendto range even higher, such as those beyond the VHF (very highfrequency) band, powdered iron is replaced by ferrites which are non-conductive, magnetic ceramic materials. 

Examples of Solid Core

Comparison of Solid Core and Laminated CoreToroidal Cores

A range of materials are available for use in toroidal cores,including steel, coiled permalloys, powdered iron, or ferrites. Thesecores can be circular in structure, with the rest ofthe transformer built around the core ring—the lack of an opening inthe core ring means no air gaps—or they can be a long strip ofmaterial. The advantage of using a strip is reduced resistance as aresult of properly aligned grain boundaries. With a circular core, thewindings are generally wound around the core, covering the surface inits entirety.

Toroidal cores are more efficient at handling the same kind ofenergy load than steel laminated E shape cores, and can be made

smaller, lighter, and with a lower magnetic field. However, windingstend to be more expensive for toroidal cores. 

In a toroidal core, the primary and secondary coils are oftenwound concentrically to cover the entire surface of the core. Thisminimizes the length of wire needed, and also provides screening tominimize the core's magnetic field from generating electromagneticinterference. Toroidal transformers are more efficient than thecheaper laminated E-I types for a similar power level. Otheradvantages compared to E-I types, include smaller size (about half),lower weight (about half), less mechanical hum (making them superiorin 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 maindisadvantages are higher cost and limited power capacity. Because ofthe lack of a residual gap in the magnetic path, toroidal transformersalso tend to exhibit higher inrush current, compared to laminated E-Itypes.

Air Cores

A physical core is not an absolute requisite and a functioningtransformer can be produced simply by placing the windings near eachother, an arrangement termed an 'air-core' transformer. The air whichcomprises the magnetic circuit is essentially lossless, and so an air-

core transformer eliminates loss due to hysteresis in the corematerial. The leakage inductance is inevitably high, resulting in verypoor regulation, and so such designs are unsuitable for use in powerdistribution. They have however very high bandwidth, and are frequentlyemployed in radio-frequency applications, for which a satisfactorycoupling coefficient is maintained by carefully overlapping theprimary and secondary windings. They're also used for resonanttransformers such as Tesla coils where they can achieve reasonably lowloss in spite of the high leakage inductance.

Sometimes capacitor is connected to the winding to maintain theconstant resonance of the tuning circuit. The flux flows through theair surrounding the winding and air inside hollow cylinder. For properimpedance matching, sometime a guard winding is wound surrounding themain copper winding. This guard winding is further connected withantenna receivers or grounded properly.

(a) Cylindrical Air Core Transformer (b) The flux is linked with both coils through air

Influence of Diameter of Transformer Core

Let us consider the diameter of transformer core as ‘D’, then thecross-sectional area (A) of the core,

Now, the voltage per turnE = 4.44*φm*f = 4.44*A*Bm*fWhere: Bm – maximum flux density of the coreE is proportional to D2

Therefore voltage per turn is increased with increase in diameterof transformer core. If voltage across the winding of transformer isV;

V = eN where: N is the number of turns in thewinding

If V is constant, e is inversely proportional to N. And henceD2 is inversely proportional to N. So if the diameter of the core isincreased, the number of turns in the transformer windings is reduced.Reduction of number of turns results to reduction of height of thecore legs. In-spite of reduction of core legs height, increase in corediameter, results, increased in overall diameter of the magnetic coreof transformer. This increased in steel weight ultimately leads toincreased core losses in transformer.

Optimum Design of Cross-Section of Transformer Core

One important criteria for design of transformer core is that itmust not be saturated during transformer’s normal operation mode.Voltages of transformer depend upon its total magnetizing flux.

The idea shape of cross – section of a transformer core iscircular. For making perfect circular cross section, each and everysuccessive lamination steel sheet should be cut in different dimensionand size. This is absolutely uneconomical for practical manufacturing.In reality, manufacturers use different groups or packets ofpredefined number of same dimension lamination sheets. The group orpacket is a block of laminated sheets with a predefined optimum height(thickness). The core is assembly of these blocks in such a successivemanner as per their size from core central line that it gives anoptimum circular shape of the cross – section.

Transformer Core Construction

Generally, the name associated with the construction of atransformer is dependant upon how the primary and secondary windingsare wound around the central laminated steel core. The two most commonand basic designs of transformer construction are the Closed-coreTransformer and the Shell-core Transformer. In the "closed-core" type(core form) transformer, the primary and secondary windings are woundoutside and surround the core ring. In the "shell type" (shell form)transformer, the primary and secondary windings pass inside the steelmagnetic circuit (core) which forms a shell around the windings.

In both types of transformer core design, the magnetic fluxlinking the primary and secondary windings travels entirely within thecore with no loss of magnetic flux through air. The coils are notarranged with the primary winding on one leg and the secondary on theother but instead half of the primary winding and half of thesecondary winding are placed one over the other concentrically on eachleg in order to increase magnetic coupling allowing practically all ofthe magnetic lines of force go through both the primary and secondarywindings at the same time. However, with this type of transformerconstruction, a small percentage of the magnetic lines of force flowoutside of the core, and this is called "leakage flux".

Figure 1 - Schematic of three-phase core-form construction Figure 2 – Schematic of single-phase core-form construction.

Transformer Lamination Core Types

These lamination stampings when connected together form therequired core shape. For example, two "E" stampings plus two endclosing "I" stampings to give an E-I core forming one element of astandard shell-type transformer core. These individual laminations aretightly butted together during the transformers construction to reduce

the reluctance of the air gap at the joints producing a highlysaturated magnetic flux density.

Transformer core laminations are usually stacked alternately toeach other to produce an overlapping joint with more lamination pairsbeing added to make up the correct core thickness. This alternatestacking of the laminations also gives the transformer the advantageof reduced flux leakage and iron losses. E-I core laminatedtransformer construction is mostly used in isolation transformers,step-up and step-down transformers as well as auto transformers.

Transformer Core Losses

The ability of iron or steel to carry magnetic flux is muchgreater than it is in air, and this ability to allow magnetic flux toflow is called permeability. Most transformer cores are constructedfrom low carbon steels which can have permeabilities in the order of1500 compared with just 1.0 for air. This means that a steel laminatedcore can carry a magnetic flux 1500 times better than that of air.However, when a magnetic flux flows in a transformers steel core, twotypes of losses occur in the steel. One termed "eddy current losses"and the other termed "hysteresis losses".

Hysteresis Loss

Transformer Hysteresis Losses are caused because of the frictionof the molecules against the flow of the magnetic lines of forcerequired to magnetise the core, which are constantly changing in valueand direction first in one direction and then the other due to theinfluence of the sinusoidal supply voltage. This molecular frictioncauses heat to be developed which represents an energy loss to thetransformer. Excessive heat loss can overtime shorten the life of theinsulating materials used in the manufacture of the windings andstructures. Therefore, cooling of a transformer is important.

Also, transformers are designed to operate at a particular supplyfrequency. Lowering the frequency of the supply will result inincreased hysteresis and higher temperature in the iron core. Soreducing the supply frequency from 60 Hertz to 50 Hertz will raise the

amount of hysteresis present, decreased the VA capacity of thetransformer.

Eddy Current Loss

Transformer Eddy Current Losses on the other hand are caused bythe flow of circulating currents induced into the steel caused by theflow of the magnetic flux around the core. These circulating currentsare generated because to the magnetic flux the core is acting like asingle loop of wire. Since the iron core is a good conductor, the eddycurrents induced by a solid iron core will be large. Eddy currents donot contribute anything towards the usefulness of the transformer butinstead they oppose the flow of the induced current by acting like anegative force generating resistive heating and power loss within thecore.

Transformer Windings

Transformer windings form another important part of a transformerconstruction, because they are the main current-carrying conductorswound around the laminated sections of the core. The conductingmaterial used for the windings depends upon the application, but inall cases the individual turns must be electrically insulated fromeach other to ensure that the current travels throughout every turn.For small power and signal transformers, in which currents are low andthe potential difference between adjacent turns is small, the coilsare often wound from enamelled magnet wire. Larger power transformersoperating at high voltages may be wound with copper rectangular stripconductors insulated by oil-impregnated paper and blocks ofpressboard.

Each strand is individually insulated, and the strands arearranged so that at certain points in the winding, or throughout thewhole winding, each portion occupies different relative positions inthe complete conductor. The transposition equalizes the currentflowing in each strand of the conductor, and reduces eddy currentlosses in the winding itself. The stranded conductor is also moreflexible than a solid conductor of similar size, aiding manufacture.

In a single-phase two winding transformer, two windings would bepresent as shown. The one which is connected to the voltage source andcreates the magnetic flux called the primary winding, and the secondwinding called the secondary in which a voltage is induced as a resultof mutual induction. If the secondary voltage is less than that of theprimary the transformer is called a "step-down transformer", and ifthe secondary voltage is greater than, it is called a "step-uptransformer".

The most important requirements of transformer winding are:

1. The winding should be economical both as regards initial cost, with a view to the market availability of copper, and the efficiency of the transformer in service.

2. The heating conditions of the windings should meet standard requirements, since departure from these requirements towards allowing higher temperature will drastically shorten the service life of the transformer.

3. The winding should be mechanically stable in respect to the forces appearing when sudden short circuit of the transformer occur.

4. The winding should have the necessary electrical strength in respect to over voltages.

The different types of windings are classified and briefly discussed below:

Concentric windings: o Cross-over

o Helical

o Disc.

Sandwich windings 

Concentric windings

These windings are used for core-type transformers. Each limb iswound with a group of coils consisting of both primary and secondaryturns which may be concentric cylinders. The l.v. winding is placednext to the core and h.v. winding on the outside. But the two windingscan be sub-divided, and interlaced with high tension and low tensionsection alternately to reduce leakage reactance. These windings can befurther divided as follows:

a. Cross-over windings. Cross-over windings are used forcurrents up to 20 A so they are suitable for h.v. winding of smalltransformers. The conductors are either cotton covered round wires orstrips insulated with paper. Cross-over coils are wound over formersand each coil consists of a number of layers with a number of turnsper layer. The complete winding consists of a number of coilsconnected in series. Two ends of each coil are brought out, one frominside and one from outside. The inside end of a coil is connected tothe outside end of the adjacent coil.

b. Helical winding. A helical winding consists of rectangularstrips wound in the form of a helix. The strips are wound in parallelradially and each turn occupies the total radial depth of winding.Helical coils are well suited for l.v. windings of large transformers.They can also be used for h.v. windings by putting extra insulationbetween layers in addition to insulation of conductors.

c. Continuous disc winding. This type of winding consists of anumber of flat strips wound spirally from inside (radially) outwards.The conductor is used in such lengths as are sufficient for completewinding or section of winding between tappings. The conductor caneither be a single strip or a number of strips in parallel, wound onthe flat. This gives a robust construction for each disc. The discsare wound on insulating cylinders spaced from it by strips along thelength of cylinder. The discs are separated from each other with pressboard sectors attached to the vertical strips. The vertical andhorizontal spacers provide ducts for free circulation of oil which isin contact with every turn.

Sandwich windings

Sandwich coils are employed in transformers of shell type. Bothhigh and low voltage windings are split into a number of sections.Each high voltage section lies between the voltage sections.

The advantages of sandwich coils is that their leakage can beeasily controlled and so any desired value of leakage reactance can behad by the division of windings.

The type of wire used as the main current carrying conductor in atransformer winding is either copper or aluminum. While aluminum wireis lighter and generally less expensive than copper wire, a largercross sectional area of conductor must be used to carry the sameamount of current as with copper so it is used mainly in larger powertransformer applications.

References:

http://en.wikipedia.org/wiki/Transformer#Windings

http://www.electronics-tutorials.ws/transformer/transformer-construction.html

http://www.electrical-engineering-assignment.com/transformer-windings

http://www.thomasnet.com/articles/electrical-power-generation/transformer-cores

http://www.electrical4u.com/core-of-transformer-and-design-of-transformer-core/

http://www.electrical4u.com/electrical-power-transformer-definition-and-types-of-transformer/

http://www.electricityforum.com/products/trans-s.htm