Transformer Structure

A simple two-winded transformer structure consists of each winding wrapped over a separate soft iron arm or core, which provides the necessary magnetic circuit.

More commonly known as the "transformer core", this magnetic circuit is designed to provide a way for the voltage between the two windings to flow around the magnetic field required for induction.

However, this type of transformer structure,in which the two windings are wrapped in separate arms , is not very efficient, since the primary and secondary windings are well separated from each other.This results in a low magnetic coupling between the two windings, as well as a leakage of magnetic flux in large quantities from the transformer itself.But in addition to this "O" shaped structure, there are different types of "transformer structures" and designs used to overcome these inefficiencies, which produce a smaller, more compact transformer.

Transformer Structure
Transformer Structure

The efficiency of a simple transformer structure can be improved by bringing the two windings into close contact with each other, thereby improving magnetic coupling.Increasing and intensifying the magnetic circuit around the coils can improve the magnetic connection between the two windings, but it also has the effect of increasing the magnetic losses of the transformer core.

In addition to providing a low path of reluctance for the magnetic field, the core is designed to prevent electrical currents circulating inside the iron core.Circulating currents, called vortex currents, reduce the efficiency of the transformer, causing heating and energy losses within the core.

These losses are mainly due to induced voltages in the iron circuit exposed to constantly changing magnetic fields by the external sinusoidal supply voltage.One way to reduce these unwanted power losses is to create the transformer core from fine steel laminations.

In most types of transformer structure, the central iron core is made of a highly permeable material, usually made of thin silicone steel laminations.These fine laminations are combined to provide the necessary magnetic path with minimal magnetic losses.The steel sheet has its own resistance, so it reduces the loss of vortex current by making laminations very thin.

The thickness of these steel transformer laminations ranges from 0.25 mm to 0.5 mm, and since steel is a conductor, laminations and any fastening studs, rivets or bolts are electrically insulated from each other with the coating or use of a very thin insulation varnish.

Transformer's Core

In general, the name for the structure of a transformer depends on how the primary and secondary windings are wrapped around the central laminated steel core.The two most common and basic designs of the transformer structure are the Closed Core Transformer and the Shell Core Transformer.

In the "closed core" type (core form) transformer, the primary and secondary windings are wrapped outside and surround the core ring.In the "shell type" (shell form) transformer, primary and secondary windings pass through the steel magnetic circuit (core), which forms a shell around the windings, as shown below.

Transformer Structure
Transformer Core Structure

In the core design of both types of transformers, the magnetic flux connecting the primary and secondary windings moves completely within the core without loss of magnetic flux by air.In the core type transformer structure, one half of the winding is wrapped around each foot (or limb) of the transformer's magnetic circuit, as shown above.

The coils are not arranged with primary winding on one leg and secondary winding on the other, instead, in order to increase practically all magnetic coupling, half of the primary winding and half of the secondary winding are placed simultaneously on top of each leg. Magnetic force lines pass through both primary and secondary windings at the same time.However, in this type of transformer structure, a small percentage of magnetic force lines flow out of the nucleus, which is called "leakage flux".

Since both primary and secondary windings are wrapped on the same central leg or limb, which has twice the cross-sectional area of the two outer limbs, the shell-type transformer hub overcomes this leakage influx.The advantage here is that the magnetic battery has two closed magnetic paths to flow out of the coils on both the left and right sides before returning to the central coils.

This means that the magnetic flow circulating around the outer limbs of this type of transformer structure is equal to Φ/2.Since magnetic flux has a closed path around the coils, this has the advantage of reducing core losses and increasing overall efficiency.

Transformer Laminations

But you may be wondering how primary and secondary windings for this type of transformer structures are wrapped around these laminated iron or steel cores.Coils are first wrapped on a mold with a cylindrical, rectangular or oval type section in accordance with the structure of the laminated core.In both shell and core type transformer structures, to mount coil windings, individual laminations are stapled or cut from larger steel sheets, and letters are formed in similar thin steel strips "e" , "L" , "U"s and "I"s, as shown below .

Transformer Core Types

Transformer Structure

These lamination stamps form the required core shape when connected.For example, two "E" stamps plus two end closure "I" stamps to give an EI core that forms an element of a standard shell type transformer core .These individual laminations are tightly interlocked during construction to reduce the reluctance of the air gap in the joints, which produce a highly saturated concentration of magnetic flux.

Transformer hub laminations are usually stacked one after the other to create an overlapping connection by adding more lamination pairs to create the correct core thickness.This alternative stacking of laminations also gives the transformer the advantage of reduced flux leakage and iron losses.EI core laminated transformer structure is mostly used in insulation transformers, riser and lowering transformers and auto transformers.

Transformer Winding Arrangements

Transformer windings form another important part of a transformer structure, since they are conductors carrying mainstream wrapped around laminated parts of the nucleus.Two windings will be available, as shown in a single-phase two-winded transformer.The winding, which is connected to the voltage source and forms the magnetic flux, is called the primary winding, and the second winding is called the second winding, in which a voltage is induced as a result of mutual induction.

If the secondary output voltage is lower than the primary input voltage, the transformer is called the "Lowering Transformer".If the secondary output voltage is greater than the primary input voltage, it is called the "Step Transformer".

The type of wire used as a conductor carrying mainstream in a transformer winding is copper or aluminum.Although aluminum wire is lighter and generally cheaper than copper wire, a larger cross-sectional area should be used to carry the same amount of current as copper, so it is mainly used in applications of larger power transformers.

Small kVA power and voltage transformers used in low voltage electrical and electronic circuits tend to use copper conductors because they have higher mechanical strength and smaller conductive size than equivalent aluminum types.The disadvantage is that these transformers can be much heavier when completed with their cores.

Transformer windings and coils can be classified as concentric coils and sandwich coils in general.In the construction of a core-type transformer, the windings are usually arranged concentricly around the core limb, as shown above, where the higher voltage primary winding is wrapped over the lower voltage secondary winding.

Sandwich or "pancake" coils consist of flat conductors wrapped in a spiral form and are called in this way due to the arrangement of conductors into discs.Alternative discs are made spiral from the outside to the center in an intermittent arrangement, where individual coils are stacked together and separated by insulating materials such as plastic sheet paper.Sandwich coils and windings are more common in the shell-type core structure.

Helisel Windings, also known as screw wraps, are another very common cylindrical coil arrangement used in low voltage high current transformer applications.Windings consist of large cross-section rectangular conductors wrapped on the side with ins isolated wires constantly wrapped parallel along the length of the cylinder, along with adjacent turns or suitable intermediates placed between discs to minimize circulation currents between parallel wires.The coil moves outward as a spiral that resembles a corkscrew.

Insulation used to prevent conductors from short-circuiting in a transformer is usually a thin layer of varnish or emulate in an air-cooled transformer.This fine varnish or easing paint is painted on the wire before wrapming it around the wire.

In larger power and distribution type transformers, conductors are isolated from each other using oil-impregnated paper or cloth.All cores and windings are immersed and closed in a protective tank containing transformer oil.Transformer oil acts as an insulator and also a cooler.

Transformer Point Direction

We can't just take a laminated core and wrap one of the coil configurations around it.We can only find out that secondary voltage and current can be the primary voltage and the current out of phase.Two coil windings have a pronounced orientation of one over the other.Both coils can be wrapped clockwise or counterclockwise around the core, so "dots" are used to identify a specific end of each bandage to track their relative orientation.

This method for determining the direction or winding direction of a transformer is called the "dot rule".Then the windings of a transformer are wrapped so that the polarity of the transformer is defined as the relative polarity of the secondary voltage relative to the primary voltage, as shown below, while the correct phase relationships are found between the winding voltages.

Transformer Structure Using Point Routing

Transformer Structure

The first transformer shows its two "points" side by side on two windings.The current separated from the secondary point is "in the same phase" with the current entering the primary side point.Therefore, the polarity of the voltages at the dotted ends is also in the same phase, so when the voltage is positive at the dotted end of the primary coil, the voltage on the secondary coil is also positive at the dotted end.

The second transformer shows two points at opposite ends of the windings; this means that the primary and secondary coil windings of the transformer are wrapped in opposite directions.The result is that the current coming out of the secondary point is 180 o "out of phase" with the current entering the primary point.Therefore, the polarity of the voltages at the dotted ends is also out of phase, so when the voltage is positive at the dotted end of the primary coil, the voltage on the corresponding secondary coil will be negative.

Then the structure of a transformer can be in such a way that the secondary voltage can be "in-phase" or "out of phase" according to the primary voltage.Transformers with several different secondary windings electrically isolated from each other are important to know the point polarity of each secondary winding so that they can be connected to each other as serial auxiliary (secondary voltage is collected) or serial opposite. (secondary voltage difference) configurations.

The ability of a transformer to adjust the rotational rate is often desirable to compensate for the effects of changes in the primary feed voltage, the regulation of the transformer or changing load conditions.The voltage control of the transformer is usually carried out by changing the rotation rate and therefore the voltage ratio, so that part of the primary winding on the high voltage side is cut to allow easy adjustment.Since the voltage per circuit is lower than the lower-voltage secondary side, the stage is preferred on the high voltage side.

Transformer Primary Stage Changes

Transformer Structure

In this simple example, primary stage changes are calculated for a feed voltage change of 5% ±, but any value can be selected.Some transformers may have two or more primary or two or more secondary windings for use in different applications that provide different voltages from a single core.

Transformer Core Losses

The ability of iron or steel to carry magnetic flux is much greater than in the air, and the ability to allow this magnetic battery to flow is called permeability.Most transformer cores are made of low-carbon steels, which can have a permeability of 1500, compared to only 1.0 for air.

This means that a steel laminated core can carry a magnetic flux 1500 times better than air.However, when a magnetic flux flows through the steel core of a transformer, two types of losses occur in steel.One is called "vortex current losses", the other is called "hysteresis losses".

Hysteresis Losses

Transformer Hysteresis Losses are caused by the friction of molecules against the flow of magnetic force lines, which are necessary to magnetize the nucleus, which constantly changes value and direction in one direction and then in the other direction due to the effect of sinusoidal. supply voltage.

This molecular friction causes the development of heat, which represents a loss of energy in the transformer.Excessive heat loss can shorten the life of insulation materials used in the manufacture of windings and structures over time.Therefore, it is important to cool a transformer.

In addition, transformers are designed to operate at a certain feeding frequency.Lowering the supply frequency will result in increased hysteresis and higher temperature in the iron core.Therefore, reducing the feed frequency from 60 Hertz to 50 Hertz will increase the amount of hysteresis present, reducing the VA capacity of the transformer.

Vortex Current Losses

Transformer Vortex Current Losses are caused by the flow of circulatory currents induced into the steel caused by the flow of magnetic flow around the nucleus.These circulatory currents are produced because the nucleus acts as a single wire cycle according to magnetic flux.Since the iron core is a good conductor, the vortex currents caused by a solid iron core will be large.

Vortex currents do not contribute to the usefulness of the transformer, instead they oppose the flow of induced current, acting as a negative force that produces resistant heating and loss of power within the nucleus.

Lamination of iron core

Transformer Structure

Vortex current losses in a transformer core cannot be completely eliminated, but the thickness of the steel core can be greatly reduced and controlled.Instead of having a large solid iron core as the magnetic core material of the transformer or coil, the magnetic path is divided into many finely pressed steel shapes called "laminations".

Laminations used in the construction of a transformer are very finely insulated metal strips that are combined to produce a solid but laminated core, as we see above.These laminations are ins isolated from each other with a layer of varnish or paper to increase the total resistance to increase the effective resistance of the belly and thus limit the flow of vortex currents.

The result of all this insulation is that the unwanted induced vortex current in the nucleus greatly reduces the power loss, and therefore the magnetic iron circuitry of each transformer and other electromagnetic machines is all laminated.The use of laminations in a transformer structure reduces vortex current losses.

Energy losses caused by heat due to both hysteresis and vortex currents in the magnetic pathway are commonly known as "transformer core losses".Because these losses occur in all magnetic materials as a result of changing magnetic fields.Transformer core losses will always be present in a transformer when the primary winding is energized, even if there is no load due to the secondary winding.In addition, the combination of hysteresis and vortex flow losses is commonly referred to as "transformer iron losses", since the magnetic flux that causes these losses is constant in all loads.

Copper Losses

However, there is another type of energy loss associated with the transformer called "copper losses".Transformer Copper Losses are mainly due to the electrical resistance of primary and secondary windings.Most transformer coils are wrapped using copper wire, which has a resistance value in Ohm (Ω), and as we know from the Ohm Law, the resistance of the copper wire will oppose any magnetization current that passes through it.

When an electric charge is connected to the secondary winding of a transformer, large electrical currents begin to flow in both primary and secondary windings, and losses of electrical energy and electricity (I 2 R) occur as heat.Usually copper losses vary according to load current, are almost zero in loadless state and maximum at full load when current flow is at maximum.

The volt-amperage (VA) value of a transformer can be increased with better design and structure to reduce these core and copper losses.A transformer with high voltage and current ratings requires large cross-section conductors to help minimize copper losses.Increase the rate of heat dissipation (better cooling) with forced air or oil, or improve its insulation so that it can withstand higher temperatures, thereby increasing the VA rating of the transformer.

Then we can describe an ideal transformer as follows:

  • No hysteresis cycle or loss of Hysteresis → 0
  • Infinite resistance of the core material, which gives zero vortex current losses, → 0
  • Zero I 2 *R copper losses → zero winding resistance 0