Transformer — its working, construction, types, and uses

What is the Transformer?

Transformers are electrical devices that convert the voltage to a higher or lower value while ideally keeping the power constant.

They are an integral part of an electrical system and their application can be observed in almost all areas of electrical engineering ranging from the electrical power systems to common household appliances.

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Why do we need them?

With the development of AC power sources, the need for transformers was also born. In the early times, DC power transmission was done in the resulting in greater losses and poor efficiency. With the invention of the transformer, this issue is resolved and AC power transmission became prominent.

However, by stepping up the transmission voltages using a transformer, this problem was solved. An increase in voltage is accompanied by a decrease in current to keep the power constant in a transformer.

And with power losses being directly proportional to the square of the current, results in a decrease in current by a factor of 10, consequently reducing losses by a factor of 100. Indeed, without transformers, we would not have been able to use electric power as we use it now.

That is why we generate electricity at voltages up to 11 to 25 kV and then step-up these voltages to 132,220 or 500 kV for transmission with minimum losses and then we later step-down the voltage for safe residential and commercial use.

Construction of a Transformer:

A transformer consists mainly of a core, windings, and a tank, however, bushings, breathers, radiators, and conservators are also present in some transformers.

Core: A transformer core is made of soft iron or silicon steel which provides a low reluctance path (magnetic field lines can easily pass through them).

Transformer cores are laminated to reduce eddy current losses, the laminations are usually 2.5mm to 5mm thick and are insulated from each other and the windings by a coating of oxide, phosphate or varnish. The Core is constructed with the laminations in different shapes such as E, L, I, C, and U.

shell and core type laminations

In shell-type transformers, the core surrounds or covers the windings like a shell.

In core-type transformers, the windings are wrapped around the two limbs or rectangles of the core.


A single-phase 2 winding transformer has generally 2 windings, primary and secondary windings, which are made from high quality stranded copper. The windings are coiled around the core and have completely no electrical contact with each other.

They can also be called High voltage and Low voltage windings respectively, with the high voltage winding having greater insulation than the low voltage winding.

Working principle:

transformer working principle
By BillC at the English language Wikipedia, CC BY-SA 3.0, Link

The basic operating principle of a transformer is the work of mutual induction between the primary and secondary windings which are linked by a common magnetic flux through the transformer core. The core provides a path of low reluctance for the magnetic flux to pass through.

The winding connected to the source can be considered as a primary winding and the current it is carrying can be considered to have a magnetic field of its own.

This magnetic field is created across the core and is changing directions due to alternating currents, and now according to Faraday’s law of electromagnetic induction:

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

This change in the magnetic field induces a voltage on the secondary coil which is proportional to the number of turns on the windings. This can be further understood by the following equation:

E = N dϕ /dt


E = Induced EMF

N = the number of turns

dϕ = Change in flux

dt = Change in time

Once the secondary winding is connected to a load, the circuit will be completed and current will start to flow through it.

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Transformer turns ratio:

Both the windings on a transformer I.e. primary and secondary have a specific number of turns. The ratio of the number of turns on the primary winding to the number of turns on the secondary winding is known as the turns ratio.

transformer turns ratio

Ideal Transformer:

An ideal transformer is a transformer which gives a power output that is exactly equal to the power input. This means that it does not have any type of loss.

Ideal transformers do not exist and are only used to simplify transformer calculations. Their voltage ratio can be modeled by these simple equations:

ideal transformer

How an Ideal Transformer is different from a Real Transformer?

In actuality, we have transformers that consist of some power losses; hence the output power is never equal to the input power of the transformer.

Real transformers have some value of winding resistance, leakage flux, and also have copper and core losses which we discussed here.

Transformer Equivalent Circuit:

transformer equivalent circuit

An equivalent circuit of a transformer is a simplified representation of a transformer comprising of the resistances and reactances.

An equivalent circuit helps us in performing transformer calculations as basic circuit analysis can now be applied to a transformer.

Read our latest article to learn more about equivalent circuit.


Transformer efficiency is the ratio of transformer output power to the input power.

It is given by

transformer equivalent circuit efficiency


transformer equivalent circuit efficiency


transformer equivalent circuit efficiency

As the output power will always be less than the input power, transformer efficiency will always lie between 0–100% while an ideal transformer will have an efficiency of 100%.

To calculate the transformer efficiency from an equivalent circuit we just add the copper losses and core losses to the efficiency equation to get the following equation:

transformer equivalent circuit efficiency

Voltage Regulation:

It is also important to know that because a transformer has series impedances within it, it will have voltage drops across them as well. This will result in varying output voltage with the varying load even if the input voltage is kept constant.

The quantity that compares the output voltage at no load to the output voltage at full load is known as voltage regulation.

It can be calculated from the following equation:

voltage regulation


voltage regulation where

It should be noted that an ideal transformer will have a voltage regulation of 0%.

Transformer types and their applications

Step-up transformer: ​These transformers increase the lower voltage level on the primary side to a higher voltage value on the secondary side. In this case, the secondary winding has a greater number of turns than the primary one.

Step-up transformer

These are mainly used in generating stations where the generated voltage of about 11 kV is stepped up to 132 kV or more for transmission

Step-down transformer: ​Step-down transformers reduces the high voltage at the primary side to a lower voltage value on the secondary side. In this case, the primary winding has a greater number of turns.

Step-down transformer

Step-down transformers are used at grid stations to decrease the high transmission voltages to a suitable lower value for distribution and utilization. They can also be found on our mobile chargers.

Other types include Power transformers, Distribution transformers, Core type transformers, Single and three phase transformers, Indoor and outdoor transformers. You may check our previous blog focusing on transformer types and their applications.

Limitations of a transformer:

It is also important to note here that a transformer will only operate in AC. This is because a Direct Current (DC) will produce a constant magnetic field instead of a changing magnetic field and hence no emf will be induced in the secondary winding.

One of AllumiaX’s recent initiatives is a corporate sponsorship for the GeneralPAC platform which provides tutorials for power systems protection, automation, and controls. Here, you will find the video series of Transformers. In this series they will be going over the Introduction to the Delta Wye Transformer Connection, Introduction to Wye Wye Transformer Connection, Introduction to Delta Wye Transformer Connection and Circulating Current and Voltages, Open Phase Condition in Transformer Analysis, Difference Between Core Form and Shell Form Power Transformer.

Let us know if you have any queries regarding this topic and do provide us with your feedback in the comments.

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About The Author

Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.

Abdur Rehman is the CEO and co-founder of and creator of GeneralPAC by AllumiaX. He has been actively involved in various roles in the IEEE Seattle Section, IEEE PES Seattle, IEEE Region 6, and IEEE MGA.



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