Key Factors of Electrical Transformers

principal of mutual induction. A transformer is essentially an a.c. device. It cannot work on d.c. . A transformer changes only a.c. voltages/currents.

Transformer cross-section showing primary and secondary windings is a few inches tall (approximately 10 cm).

There are two types of transformer:-

Step up transformer

Step down transformer

A transformer which increases the a.c. voltage is called step up transformer.

A transformer which decreases the a.c. voltage is called step down transformer.

There are many types of energy loses in transformer. Major sources of energy lose are:-copper loss, iron loss, leakage of magnetic flux, hystresis loss and magnetostriction. We cannot obtain full input power as output power because of energy loses. Output power in a transformer is 90% of the input power.

It is used:-

as voltage regulator

For transmission of a.c.

For welding purposes etc


1) Development of Economical Analysis and Technical Solutions for Efficient Distribution Transformers

M.L.B. Martinez, , H.R.P.M. de Oliveira

Transformers are equipments applied to electrical systems to adjust voltage levels for consumption, distribution, transmission, and generation needs. Actual technology indicates that they are responsible for approximately one third of total network losses. These losses can be accounted as power utility costs, costs to society and to the environment itself. The aim of this paper is to study the relationship between the transformer costs and its losses. A method for collecting cost data and for building cost surfaces – the so called “Production and Total Cost Surfaces” is presented. The economic analysis of transformer designs considers a variation of the reference data of the main constructive parameters of a standard transformer. Therefore, changes are introduced, amongst others, in the core dimensions, such as the column circumscribed area, its diameter and the yoke area. This approach helps the power utilities to purchase equipment according to the forecast demand, decreasing the total network losses.

2) Reducing Heat in a Transformer and increasing its Loading Capacity, by decreasing the Percentage of Harmonics and Reactive Power

Submitted by A.F. Picanço

By M Duta et al

The paper deals with a situation that has taken place while working with average voltage transformers in a cement factory. Due to the adjustable drives in the production flow, both with thyristors and with variable frequency, current harmonics are introduced in the electrical network of the consumer. These harmonics distort the wave forms of the supply voltage and of the current flowing through the distribution equipment of the system. There are presented the results of the measurements obtained at a cement factory belonging to the HEIDELBERG concern in Romania. By using a modern installation for an automated filtering and compensation of the reactive energy, the current gets cleaned of harmonics and the power factor increases, which makes it possible for the transformer to be efficiently loaded at a charge as close as possible to the nominal load.

3) Selecting least-cost, energy efficient distribution transformers

Submitted by C de Salles

The SEEDT project, cooperation between energy agencies, institutes, electrical utilities, transformer manufacturers and academic institutes, presents its new selection guide for energy efficient distribution transformers (EEDT).

The guide describes losses in transformers, technical solutions for improving them and their energy saving potential for the European Union. It continues to describe the cost of losses, and presents methods for economic evaluation of investment decisions in high efficiency. The guide concludes by proposing policy support mechanisms for EEDT.

This guide is for you if you are:

working in an electricity distribution company,

an industry, commercial or public organisation wanting to purchase a distribution transformer,

a facility manager,

a planner or equipment installer in charge of technical planning or preparing the purchase of a distribution transformer in a client’s organisation.

This guide supports you in:

purchasing a distribution transformer for the least lifecycle cost,

achieving further energy and budget savings in your organisation,

Contributing to increasing energy security and reducing greenhouse gas emissions.

The European project SEEDT (Strategies for development and diffusion of energy-efficient distribution transformers), carried out with financial support from the European Commission under the Intelligent Energy – Europe programme and national co-financiers, aims to promote the use of energy-efficient distribution transformers, which can be profitable for investors, and by contributing to European Community energy savings, may help to fulfil EU energy policy targets.

4) Reducing electricity network losses

Submitted by Roman Targosz

Based on a Discussion Webinar.

Losses in transmission and distribution networks represent the single biggest use in any electricity system. In Europe, they consume

between 4 and 10% of electricity generated. What can be done to optimise the electricity system and reduce these losses? Which countries are setting a good example? And what is the role of regulation and policies on this point? Current tariff systems in most European countries are not really favouring network efficiency, and what about the influence of increasingly distributed generation on future network losses?

The following are a few of the major points arising from that discussion.

7% of the energy is lost in the network

The world average loss in the electric network system is 8.8%. However, this figure includes countries like India and Brazil, where the losses are high due to so-called “non-technical losses” – electricity which could not be invoiced and is mainly lost via illegal network connections.

5) Transformer field emissions (1)

Submitted by Stefan Fassbinder.

Who’s afraid of magnetic fields? And if so, who are the culprits? Probably those whose basic principle of working is magnetic fields – or rather not that much? This transformer has a magnetic stray flux of 37.8µT when measured right on top of the cover, where people should not walk anyway for a number of reasons.



A TRANSFORMER is based on the principle of mutual induction. (Mutual induction:-It is the phenomenon by virtue of which an e.m.f. (electromotive force) is induced across a coil (secondary coil) when a changing current flows through the neighbouring coil.) i.e. whenever amount of magnetic flux linked with coil changes, an e.m.f. is introduced in the neighbouring coil.


A Transformer consists of a rectangular soft iron core made of laminated sheets, well insulated from one another. Two coils P1P2 and S1S2 are wound on the same core, but are well insulated from each other. Note that both the coils are insulated form the core. The source of alternating e.m.f. (to be transformed) is connected to P1P2, the primary coil and a load resistance R is connected to S1S2, the secondary coil through an open switch S. Thus there can be no current through the secondary coil so long as the switch is open.

For an ideal transformer, we assume that the resistance of the primary and secondary windings are negligible. Further, the energy losses due to magnetic hystresis in the iron core is also negligible.

Well designed high capacity transformers may have energy losses are as low 1%.

Let the alternating e.m.f. supplied by the a.c. source connected to the primary coil be

E=E0SinWt …1

As we have assumed the primary coil to be a pure inductance, the sinusoidal primary current Ip lags the primary voltage Ep by 90 degree. The primary’s power factor, cosÑ„ = cos90⁰ = 0. Therefore, no power is dissipated in primary.

The alternating primary current induces an alternating

magnetic Ñ„B in the iron core. Because the core extends through the secondary winding, the induced flux also extends through the turns of secondary.

According to FARADAY’S LAW OF INDUCTION, the induced

e.m.f. per turn (Eturn) is same for both the primary and secondary. Also the voltage Ep across the primary coil is equal to the e.m.f. induced in the primary, and the voltage Es across the secondary is equal to the induced e.m.f. induced in the secondary coil. Thus,

Eturn = dфB = Ep = Es

dt np ns

Here, np; ns represent total number of turns in primarily and secondary coils respectively

Or Es =Epnp/ns …2

If ns > np; Es > Ep, the transformer is a step up transformer. Similarly, when ns < np; Es < Ep the device is called step down transformer.

ns/ np = K represents TRANSFORMATION RATIO.

Note that this relation is based on three assumptions:-

The primary resistance and current are small.

There is no leakage of flux links both, the primary and secondary coils.

The secondary coil is small.

Now, the rate at which the generator/source transfers energy to the primary = IpEp. The rate at which the primary then transfers energy to the secondary (via the alternating magnetic field linking the two coils) is IsEs.

As we assume that no energy is lost along the way, conservation of energy requires that

IpEp = IsEs

Therefore Is = IpEp/Es …3

Is = Ipnp/ns = Ip/

For a step up transformer, Es > Ep;

K >1

Therefore Is < Ip

Step-down transformer: (many turns :few turns).

i.e. secondary current is weaker when secondary voltage is higher i.e. whatever we gain in voltage; we lose in current in the same ratio.

The reverse is true for a step down transformer,

From equ. 3,

Ip = Is (ns/np) = Es/R (ns/np)

Using equ. 2, we get

Ip = 1/R.Ep/(ns/np) (ns/np)

Ip =1/R.Ep (ns/np)2 …4

This equ. Has the form

Ip = Ep/Req,

Where the equivalent resistance Req is

Req = (ns/np)2R …5

Thus Req is the value of load resistance as seen by the source/generator i.e. the source /generator produces the current Ip and the voltage Ep as if it were connected to the resistance Req.

Note: – For maximum transfer of energy from an e.m.f. device to a resistive load, the resistance of e.m.f. device and load must be equal.

We can match the impedance of the two devices by coupling them through a transformer with a suitable turn ratio (ns/np).

Thus impedance matching is yet another important function of the transformer.


Efficiency of a transformer is defined as the ratio of output power to the input power.

i.e. η = Output power/input power

= IsEs / IpEp …6

In an ideal transformer, where there is no power loss, η =1(i.e. 100%). However, practically there are many energy losses. Hence efficiency of a transformer in practice is less than 1(i.e. less than 100%).

Note that a transformer is essentially an a.c. device.

It cannot work on d.c. A transformer changes a.c. voltages/currents. It does not affect the frequency of

a.c. When a.c. voltage is raised n times, the corresponding alternating current reduces to 1/n times.


Following are the major sources of energy loss in the transformer:

COPPER LOSS is the energy loss in the form of heat in the copper coils of a transformer. This is due to joule heating of conducting wires. These are minimised using thick wires.

IRON LOSS is the energy loss in the form of heat in the iron core of the transformer. This is due to the formation of eddy currents in iron core. It is minimised by taking laminated cores.

LEAKAGE OF MAGNETIC FLUX occurs inspite of best insulations. Therefore, rate of change of magnetic flux linked with each turn of S1S2 is less than the rate of change of magnetic flux linked with each turn of P1P2. It can be reduced by winding the primary and secondary coil one over the other.

HYSTERESIS LOSS: This is the loss of energy due to repeated magnetisation and demagnetisation of the iron core when a.c. is fed to it. The loss is kept minimum by using a magnetic material which has a low hysteresis loss.

MAGNETOSTRICTION: – i.e. humming noise of a transformer. Therefore, output power in a transformer is roughly 90% of the input power.


A transformer in almost all a.c. operations e.g.

In voltage regulators for T.V., refrigerator, computer, air conditioner etc.

In the induction furnaces.

A step down transformer is used for welding purposes.

In the transmission of a.c. over the long distances. The loss of power in the transmission lines is I2R, where I is strength of current and R is the resistances of the wires. To reduce the power loss, a.c. is transmitted over long distances at extremely high voltages. This reduces I in the same ratio. Therefore, I2R becomes negligibly low. As I has been reduced sufficiently, I2R remains negligible even when R is not very small. This means we can use even thin line wires of large resistance R instead of thick ones. This saves a lot of material (copper). Therefore, cost of transmission is reduced considerably.

Normally, at the generating station, we use a step up

transformer, which raises a.c. voltage to about 132000V. Thin line wires carry the power to the receiving station. The voltage is reduced in steps using a number of step down transformers. For the domestic consumers, the voltage is 220V, 50Hz.

Transformer types

Circuit symbols

Transformer with two windings and iron core.

Step-down or step-up transformer. The symbol shows which winding has more turns, but not usually the exact ratio.

Transformer with three windings. The dots show the relative configuration of the windings.

Transformer with electrostatic screen preventing capacitive coupling between the windings.

A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.

Power transformers

Laminated core

Laminated Core Transformer

This is the most common type of transformer, widely used in appliances to convert mains voltage to low voltage to power electronics

Widely available in power ratings ranging from mW to MW

Insulated lamination minimizes eddy current losses

Small appliance and electronic transformers may use a split bobbin, giving a high level of insulation between the windings

Rectangular core

Core laminate stampings are usually in EI shape pairs. Other shape pairs are sometimes used

Mu-metal shields can be fitted to reduce EMI (electromagnetic interference)

A screen winding is occasionally used between the 2 power windings

Small appliance and electronics transformers may have a thermal cut out built in

Occasionally seen in low profile format for use in restricted spaces

Laminated core made with silicon steel with high permeability


Toroidal Transformer

Doughnut shaped torodial 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 thus threaded through the ring during winding), and tape for insulation.

Toroidal transformers compared to EI core transformers:

Lower external magnetic field

Smaller for a given power rating

Higher cost in most cases, as winding requires more complex and slower equipment

Less robust

Central fixing is either

bolt, large metal washers and rubber pads

bolt and potting resin

Over-tightening the central fixing bolt may short the windings

Greater inrush current at switch-on


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. 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 V across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230 V supply to drive 115 V equipment, or reversed to drive 230 V equipment from 115 V. 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 (two-winding) transformer of the same rating. Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 33 kV and 66 kV sub-transmission networks.

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. The difference is usually slight enough to allow reversal where the actual voltage level is not critical. This is true of isolated winding transformers too.

Polyphase transformers

Example of Y Y Connection

For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single 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 groupindicates the configuration of the windings and the phase angle difference between them. If a winding is connected to earth (grounded), 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 (high leg delta) or one phase may be grounded (corner grounded delta). A special purpose polyphase transformer is thezig-zag transformer. There are many possible configurations that may involve more or fewer than six windings and various tap connections.

Resonant transformers

A resonant transformer operates at the resonant frequency of one or more of its coils and (usually) 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, such as a square or sawtooth waveat 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 orWimshrust Machine.


Tesla Coil

Oudin Coil(or Oudin resonator; named after its inventorPaul Oudin)

D’Arsonval apparatus

Ignition coil or induction coil used in the ignition system of a petrol engine

Flyback transformer of a CRT television set or video monitor.

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 superheterodyne receiver, where the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.

Constant voltage transformer

By arranging particular magnetic properties of a transformer core, and installing a ferro-resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage relatively constant for varying primary supply without additional circuitry or manual adjustment. CVA transformers run hotter than standard power transformers, because regulating action depends on core saturation, which reduces efficiency somewhat. The output waveform is heavily distorted unless careful measures are taken to prevent this. Saturating transformers provide a simple rugged method to stabilize an AC power supply.

Ferrite Core

Ferrite core power transformers are widely used in switched mode power supplies (SMPSes). The powder core enables high frequency operation, and hence much smaller size to power ratio than laminated iron transformers.

Ferrite transformers are not usable as power transformers at mains frequency.

Planar transformer

A planar transformer

Exploded view: the spiral primary “winding” on one side of the PCB (the spiral secondary “winding” is on the other side of the PCB)

Manufacturers etch spiral patterns on a printed circuit board to form the “windings” of a planar transformer. (Manufacturers literally wind pieces of wire on some core or bobbin to form the windings of other kinds of transformers).

Some planar transformers are commercially sold as discrete components—the transformer is the only thing on that printed circuit board. Other planar transformers are one of many components on one large printed circuit board.

much thinner than other transformers, for low-profile applications (even when several PCBs are stacked)

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.[1] Formerly, indoor transformers required to be fire-resistant used PCB liquids; since these are now banned, substitute fire-resistant liquids such as silicone oils are instead used.

Cast resin transformers

Cast-resin power transformers encase the windings in epoxy resin. These transformers simplify installation since they are dry, without cooling oil, and so require no fire-proof valut for indoor installations. The epoxy protects the windings from dust and corrosive atomospheres. However, because the molds for casting the coils are only available in fixed sizes, the design of the transformers is less flexible, which may make them more costly if customized features (voltage, turns ratio, taps) are required.

Isolating Transformer

Most transformers isolate, meaning the secondary winding is not connected to the primary. But this isn’t true of all transformers.

However 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 tappings are sometimes included, but to earn the name ‘isolating transformer’ it is expected that they will usually be used at 1:1 ratio.

Instrument transformers

Current transformers

Current transformers used in metering equipment for three-phase 400 ampere electricity supply

A current transformer (CT) 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 (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. The CT is typically described by its current ratio from primary to secondary. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or have several tap points to provide a range of ratios. Care must be taken that the secondary winding is not disconnected from its load while current flows in the primary, as this will produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer.

Specially constructed wideband CTs are also used, usually with an oscilloscope, to measure high frequency waveforms or pulsed currents within pulsed power systems. One type provides a voltage output that is proportional to the measured current; another, called a Rogowski coil, requires an external integrator in order to provide a proportional output.

Voltage transformers

Voltage transformers (VT) or potential transformers (PT) 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 V or 120 V at rated primary voltage, to match the input ratings of protection relays.

The transformer winding high-voltage connection points are typically labeled 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. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is essential for proper operation of metering and protection relays.

While VTs were formerly used for all voltages greater than 240 V 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.

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 relatively constant amplitude). 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. 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. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors.

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. Generally speaking, the larger this product, the larger and more expensive the transformer.

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.

RF transformers

There are several types of transformer used in radio frequency (RF) work. Steel laminations are not suitable for RF.

Air-core transformers

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.

Ferrite-core transformers

Widely used in intermediate frequency (IF) stages in superheterodyne radio receivers. 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.

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. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) 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 (called a tickler winding) to inject feedback into an earlier (detector) 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

Transformers in a tube amplifier. Output transformers are on the left. The power supply toroidal transformer is on right.

Audio transformers are usually the factor which limit sound quality 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 (e.g. bass guitar) to low impedance signals 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

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