Transformer – Operation, construction and types

This article will help you understand the basic concepts of electric transformers or power transformers, their construction, the principle of operation, tests conducted on them, and their classification.

Have you ever wondered how electricity generated in the countryside is lighting up your home, powering up your home appliances and electronic gadgets you use and wear? How high-tension lines travelling overhead is converted to low tension and helps you watch live sports on your television? The piece of equipment that does this is called an electric transformer.

Electric transformer-Power Transformer

In the earlier days, DC power was generated close to load stations and distributed. The invention of the transformer resulted in recent advancements in power generation, transmission, and distribution sectors. Transformers made bulk power generation and long-distance AC power transmission possible. Today power is transmitted at up to 765kV with minimum power losses and higher efficiency.

What is an electric transformer?

An electric transformer or power transformer is a piece of equipment that is designed to change the magnitude of AC voltage in a circuit, without altering the frequency, and at a minimum power loss. It is used to step down and step up voltages. Power is delivering from its input side to its output side by the process of electromagnetic induction.

It is used to transmit power generated at a remote location to the consumer efficiently at the required voltage. Transformers are available at various sizes and ratings from those huge ones in a substation to those tiny ones in an electronic board.

Electric transformer
Earliest specimens of ZBD-designed high-efficiency constant-potential transformers manufactured at the Ganz factory in 1885. Source: Wikipedia

Principle of operation of electric transformers

An electric transformer works on the principle of mutual inductance and Faraday’s law of electromagnetic induction. The flow of an alternating current through a coil produces an alternating magnetic field. When another coil is brought in contact with the alternating magnetic field, voltage is induced in that coil. According to Faraday’s law, the magnitude of the induced voltage depends on the rate of change of magnetic flux linking the second coil and the number of turns.

ε =-N dΦ​/dt

In the case of transformers, Since the rate of change of magnetic flux between the coils is almost the same, the induced voltage depends on the number of turns of the coils.

Ideal Transformer

An ideal transformer consists of a primary and a secondary winding, wound around the two vertical limbs of the core. When an alternating voltage is applied to the primary winding of the transformer, a current flow through it, that produces an alternating magnetic field and hence an alternating magnetic flux. The amount of magnetic field produced depends on the number of turns of the coil. This magnetic flux induces an EMF in the secondary coil. The load can be connected to the secondary winding that permits the current flow.

An ideal transformer is an imaginary transformer, having zero losses, infinite permeability to magnetic flux, and 100% efficiency. Since the same amount of flux is linking the transformer primary and the secondary winding, the ratio of applied voltage (Vprimary) and induced voltage (Vsecondary) must be proportional to the ratio of number of turns in the primary to the number of turns (Nprimary) in the secondary winding (Nsecondary).

Vprimary / Vsecondary = Nprimary / Nsecondary

In an Ideal transformer, input power is equal to output power.

Vprimary / Vsecondary = Isecondary / Iprimary

In an actual transformer, the voltage induced per turn is given by the following equation:

E/N = K.Φm.f

where K is a constant, Φm is the maximum value of total flux in Webers linking that turn and f is the supply frequency in hertz.

Ideal Transformer

Step-up Transformer

In step-up transformers, the secondary winding has more turns than that of the primary winding. Also, the voltage at the secondary shall be higher than the primary voltage (depending on the turns ratio). Step-up transformers are used to increase transmission voltage to reduce transmission losses. They can be found in generating stations and are commonly known as power transformers.

Step-down Transformer

In the step-down transformer the number of turns in the secondary side of the transformer is lesser than that of the number of winding on the primary side, and hence the voltage. These transformers are used to reduce the voltage at the distribution side of the power system.

Transformer Ratio

Transformer turns ratio ‘n’, is a number denoting the ratio of the number of turns of the conductor in the primary coil to that of the secondary coil. The transformer ratio is also known as the voltage transformation ratio. This tells about the voltage available at the secondary side of the transformer for an applied primary voltage.

Transformer ratio

NP          – Number of turns of the conductor in the primary coil.

VP           – Applied Primary voltage.

NS           – Number of turns of the conductor in the secondary coil.

VS              – Transformed voltage measured at the secondary.

Read More: Online – Transformer turns ratio calculator

Construction of transformer

Irrespective of design types, the following are the major components of a transformer. Learn more about various parts of an electric transformer.

  • Core
  • Winding
  • Insulation
  • Conservator
  • Transformer oil (in oil-immersed transformers)
  • Buchholz Relay


The transformer core is the part over which primary and secondary are wound. It supports the windings as well as provides a low reluctance path for the magnetic flux linking primary and secondary winding. It is made up of high permeability silicon steel lamination to reduce core losses.


The electric transformers have two sets of windings, a low-tension winding, and a high-tension winding. Several turns of copper conductors bundled together to form transformer windings. The size of copper conductors depends on the load current. Most of the time windings are referred to as primary winding and secondary winding. Normally the winding to which the input voltage is connected is known as the primary winding and the winding to which the load is connected is known as the secondary winding.


Insulation is the most critical part of electric transformers. Windings are insulated from each other and the core. Insulation failures within the transformers are the most serious problem. Hence greater care is taken on the insulation part during transformer design. Varnish, kraft paper, Cotton cellulose, and Pressboard are the most widely used winding insulation materials.

Transformer oil

Not all transformers but in oil-immersed transformers, transformer oil serves the dual purpose of insulation and cooling. It has a high breakdown voltage, high resistivity, and high dielectric strength. It extracts heats from the transformer windings and core and helps in reducing losses and improves the efficiency and life of the transformer.

Buchholz Relay

Transformer Buchholz relay

Buchholz relay is a protection device used in electric transformers. It is an oil actuated relay used to sense the faults occurring inside the main tank of an oil-immersed transformer. It can sense short circuits, oil leakage, overheating of transformer coils, etc.

Read more about Buchholz relay detail: Buchholz relay – Principle of operation

Who invented the electic transformer? 
In 1884, three Hungarian engineers, Károly Zipernowsky, Ottó Bláthy, and Miksa Déri, designed the first high-efficiency transformer. This transformer was called ZND transformer. It led to new developments in the transformer design. The first three-phase transformer was designed by Mikhail Dolivo-Dobrovolsky.

Losses in an electric transformer

The losses occurring in a transformer are classified into winding loss and core loss. Winding loss occurs due to resistance offered by the conductor. It is proportional to the square of the current flowing through it. Using thick copper conductors minimize the resistance to current flow and reduce winding loss. Core losses are due to eddy currents, formed in the transformer core, and the hysteresis effect. Core losses, also known as iron losses, are always constant and are independent of load. Using laminated soft iron core and thick conductors can help in reducing core losses and improving transformer efficiency.

Equivalent Circuit of transformer

It is a theoretical circuit that represents a transformer and its physical behavior. This circuit shown below represents various electrical parameters of the transformer. From this circuit, various losses and voltage drops can be easily calculated.

Equivalent circuit of power transformer.

VP           – Primary voltage or applied voltage

IP            – Primary current

RP           – Resistance offered by the primary winding

XP               – Reactance offered by the primary winding

IC            – Current component contributing to core losses

RC               – Resistive component contributing to core losses

IM           – Magnetizing current

XM          – Magnetizing reactance

Vs           – Secondary voltage or applied voltage

Is             – Secondary current

Rs           – Resistance offered by the secondary winding

Xs           – Reactance offered by the secondary winding


The above equivalent circuit is a generalized form of an equivalent circuit for an ideal transformer with a transformer ratio of 1:1 and without referring to either primary or secondary side.

Learn more: Equivalent circuit and Phasor diagram of a transformer

Voltage regulation of the transformer

How accurate the voltage transformation occurs in the transformer when the load varies from no load to full load is dictated by the voltage regulation of the transformer. It is calculated using the following formula:

Voltage regulation of transformer


               Esec-noload – Voltage measured at the secondary at no load.

               Esec-fullload – Voltage measured at the secondary at full load.

Read more about voltage regulation

Classification of transformer

Transformers are classified into various types depending on various parameters such as type of supply, their application, type of construction, cooling method, operational voltage, duty type, the shape of the core, etc.

Classification based on the type of power supply: Three-phase transformer, single-phase transformer.

Classification based on the type of construction: Core type transformer, Shell type transformer.

Classification based on cooling method:  Dry-type or natural air-cooled, Oil cooled- Oil Natural Air Natural (ONAN), Oil Natural Air Forced (ONAF), Oil Forced Air Natural (OFAN), Oil Forced Air Forced (OFAF), Oil and Water-cooled – Oil Natural Water Forced (ONWF), Oil Forced Water Forced (OFWF)

Classification based on purpose: Distribution transformer, potential transformer, current transformer, isolation transformer, radio frequency transformer, tesla coil.

Learn more: Types of electric transformers.

Distribution transformers

Distribution transformers are electric transformers having low voltage rating. They are found close to power centers. It steps down the transmission voltages to levels that can be used by the consumer.

Potential transformers

Potential transformers (PT) are used to step down high voltages to measurable levels in order to facilitate measurement and controllability. They are connected in parallel to the transmission lines and measuring devices are connected to their secondary.

Current transformers

Current transformers (CT) are used to step down high currents to measurable levels in order to facilitate measurement and controllability. They are connected in series to the load and measuring devices are connected to their secondary.

Testing of transformer

Electric transformers are subjected to the following tests:

  1. Winding resistance test.
  2. Insulation resistance test.
  3. Transformer resistance test.
  4. No-load test – Open circuit test.
  5. Short circuit impedance test – Short circuit test.
  6. Temperature rise test.
  7. Polarity checks.
  8. Dielectric test for transformer oil.
  9. Noise Level tests

Learn More: Open circuit test & Short circuit test in transformers

Why are electric transformers used in the power system?

An electric transformer can be considered as the most important component in a power transmission and distribution network. It performs the duty of improving transmission efficiency and reducing losses and transmission costs. Basically, the transformer step-up/step down voltages. Power station generates power at a voltage of 11kV to 28kV at 50Hz. To reduce transmission losses, the voltage is stepped up to 220kV or more and transmitted. At the distribution substation, it is again stepped down to 33kV or 11kV upon the requirement and supplied to industries. It is again stepped down at the domestic consumer end to low voltage loads of the consumer.

Read More: Why is electricity transmitted at high voltages?

By stepping up the voltage, the load current flowing through the transmission lines is reduced. Reduction in load current results in the reduction of copper loss (I2R loss) and the size of the conductor used for power transmission. Hence, the cost of power transmission as well as its efficiency is improved. Hence electric transformers improve the system efficiency, reliability and reduce power transmission costs.


  1. Toroidal Line Power Transformers. Power Ratings Tripled:
  2. Lane, Keith (2007) (June 2007). “The Basics of Large Dry-Type Transformers”. EC&M. Retrieved 29 January 2013.
  3. Electromechanical systems, electric machines, and applied mechatronics By Sergey Edward Lyshevski.
  4. Electric Transformers By Eric Egbert Wild · 1940

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