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Introduction To Transformer & Its Working Principle, Design Aspects & Safety Precautions

What Is Transformer? Its Working Principle, Functional Design Aspects & Safety Precautions 

What is Transformer?

Transformer is a static electrical device which transfers electrical energy from one circuit to another circuit using the principles of electromagnetic induction, either increasing or decreasing the input voltage & current.

Electrical power transmission over immense distances from generating stations to remote areas of civilization has only been possible because of “Transformer”.

Introduction To Transformer & Its Working Principle, Design Aspects & Safety Precautions

As the name suggests, transformer actually transforms the potential magnitude of the electrical power supply, which ultimately improvises power flow in transmission lines and reduces power losses. Transformers are economical, robust and efficient static machines.

Working principle:

The transformer is purely an AC device since it works on the principle of mutual induction. Faraday’s law of electromagnetic induction is the main driving force of transformer.

Functionally, a transformer has two coils generally known as the primary and secondary coil.

Primary Coil

Primary coil or Primary winding refers to the input coil and electrical power input is connected to primary terminals. Changing sinusoidal electrical source produces a magnetic field around the coil. Since source signal is executing electrical SMH with a certain frequency, magnetic field tends to have the same response.

Secondary Coil

Secondary coil or Secondary winding refers to the output coil. The secondary coil is placed near to the primary coil, which experiences the changing magnetic flux & hence induces a voltage. This induced voltage is the output voltage of the transformer.

Primary & Secondary coil of transformer

Mutual Induction

The phenomenon of producing Induced EMF (Electro-Motive Force) in the secondary coil with respect to the changing current in the primary coil is known as mutual induction.

The power flow through a transformer is not by electrical connections but it is through magnetic coupling using the idea of mutual induction.

Apart from basic principle, most of the studies related to transformer are directed towards its efficiency enhancement, ergonomics, operations, and safety. Transformers are expansive devices, so utilities take special measures to ensure their health status.


To apprehend the design of any transformer, we must have clear understating of factors that affect transformer’s efficiency. Because every machine is designed to maximize efficiency.


In case of the transformer, the difference between power inward and outward flow determines the efficiency. Let us have a look at general induction equation.Transformer Induction Equation Here,  ε is induced EMF and is changing magnetic field with respect to time.

To have more and more changing magnetic field, we must have higher and higher supply frequency. But certainly, supply frequency has an upper limit 50 or 60Hz. 

So in case of a transformer, the rate of changing magnetic flux does not majorly contribute towards its efficiency.

Also, the flux in a transformer is always constant irrespective of operating load as long as its Voltage and Frequency are constant. That is the reason transformers are designed for a fixed frequency.

Medium For Magnetic Flux

Moreover, the flow of magnetic lines from the primary winding to secondary is a key factor in the design of a transformer. If both windings are linked through the air, a large amount of power will be consumed in air gap because air has a very low permeability of magnetic flux. So, ferromagnetic materials with high permeability are used as a linkage medium between both windings. Such linkage is called “Core” of the transformer as shown below.

Transformer CoreHow to Select A Ferromagnetic Material For The Core

Next riddle to solve is, “how to select a ferromagnetic material for the core?” The answer lies in a phenomenon called as “hysteresis loss”.

As mentioned earlier that the input of primary winding is AC. So when the polarity of supply reverses, dipoles of ferromagnetic material also change their alignments and shifts 180°. Energy consumed in the polarity reversal of these dipoles is called hysteresis loss. To minimize this loss we use silicon steel core (given in the figure below). The decrease in hysteresis loss of a transformer reduces power dissipation as heat, hence improving the efficiency.Silicon laminated steelAccommodation of Eddy Current Loss

Next factor to accommodate in design is the “eddy current” loss. Using a good ferromagnetic material as a core for winding’s flux linkage causes another problem. Since the core is placed under the influence of changing magnetic, an EMF is also induced in the core.

Since the core is electrically a short path, therefore induction of an EMF will cause a current flow, which is called Eddy current. This current flow produces heat as per

H =i2Rt

To tackle this problem in transformer design, we refer to basic Ohm’s Law.


To reduce eddy current through a closed loop of silicon steel core, we must increase its resistance high enough to operate transformer in sustainable thermal limits. To increase the electrical Resistance of the core without affecting its permeability we again refer to the basic concept:Resistance Equationρ is constant since we can’t change core material. However, we can change its geometry. We decrease core area (A). This process is called as laminating the core pieces.

Instead of a solid core, we divide the core into small sheets and laminate them with synthetic epoxy resins. This only increases the electrical resistance of core without affecting the permeability, thus reduces the eddy current loss significantly.

Accommodation Of Other Losses

Collectively aforementioned factors are called Iron losses of a transformer, other losses are called Copper losses and stray losses. Copper losses are load dependent so power transformers are designed to have maximum efficiency near full load because maxima of efficiency curve is located where both iron and core losses are same. However, distribution transformers have maximum efficiency near 50~70% of full load.

These are only basic design requirements of any transformer, all other design considerations are a function of operational conditions.

Functional design aspects:

Operating Voltage

Transformers can be designed for a wide range of operating voltage, from few volts to several kilovolts.

On basis of operating voltages, two design aspects are accommodated in transformers: insulation and step up or step down function.

Higher and higher the operating voltage is, more and more insulation will be required to isolate high potential parts to ground parts. Mostly, oil impregnated cellulose paper is used for winding insulation. Painted ceramic or glass insulated bushings are used for terminal insulations.

Voltage induction on secondary coil is a function of the turn ratio of windings, so to achieve rated design voltage suitable turn ratio of wingding is ensured. Low voltage winding is placed near to core and high voltage winding is placed over it concentrically to enhance insulation level of HV winding.

Fixed Output Voltage

Since input voltage is not always constant. It depends on many variables like seasons, type of transmission, the design of power systems, frequency and type of faults, percentage loading and unloading of system etc.

As input voltage can vary, so is the output voltage because they are a function of each other and turn ratio.

To get fix output irrespective of the input fluctuations, we use Tap-changer in the design of transformer. Tap changing system changes the turn ratio w.r.t to the output voltage. Tap changer can be designed to operate ONLOAD or OFFLOAD. The Tap-changer system is shown in the figure below.

Tap Changer

Mostly it is connected to the high voltage winding to reduce electrical arcs during operation.

Cooling System

As we discussed that power flow cause enormous heat. So cooling system is also an important aspect of transformer design.

Heat generation in the transformer is a function of its MVA (Mega Volt-Ampere) rating. Therefore Low rating transformers are only air cooled and hence called Dry type Transformers. Examples are given in figure below:

Small Dry type Transformer

But in high MVA rating transformers, cooling through the air is not enough because air has a very low coefficient of convective heat transfer. So, oil is used for cooling of such transformers. Core and windings are placed in a metallic tank filled with oil. This oil can be pumped to a heat exchanger or it can flow through natural density difference through cooling fins. it can be seen in the fig. below:

Oil Transfomer

On basis of cooling, a transformer can be ONAN (oil natural air natural), ONAF (oil natural air force) or OFAF (oil force air force).

Core Design

The core design is also a functional aspect. Transformers have majorly two types of cores (core form and shell form). It depends on phases and flux linkages of the transformer.Core Form & Shell Form

Operating variables:

  • Electrical Operating variables of a transformer are, sub-transient impedance, transient impedance, zero sequence impedance, short-circuit impedance, MVA rating, and primary/secondary potential.
  • Core saturation is also a major issue during operation because beyond saturation knee point of the core, transformer takes 4 to 5 times more current for per unit increment of flux magnitude in core hence produces a lot of heat and can damage windings. As core saturation is proportional to V/f, so these two parameters are kept as constant as possible.
  • Connections can be made as per user requirements like delta, star, Scott-T etc.
  • Power factor is not considered as a variable in transform operation because power factor of a transformer greatly depends upon its load.
  • Auto-transformers, used in most welding plants are same in design and principle with slight tap connection adjustments.
  • Instrument transformers are also widely used CTs, PTs, CVTs, LVDTs etc. all have same operation principle with different design modification to serve a specified cause.

Safety precautions and Protections:

The health of a transformer is proportional to the health of its insulting material.


One of the major issues for transformer health is heating. Because heating of cellulose paper produces CO2, CO, ozone and compounds of acids, which can further cause damage to oil and results in serious problems partial discharge.

How To Avoid Heating?

To avoid Transformer from such circumstances, certain safeties are incorporated in the design.

Buchholz relay

Buchholz relay is one of them. It operates when temperature and thermal expansion of oil reaches a certain limit and isolate the transformer.


Silica Gel

Silica gel is a hygroscopic material which absorbs humidity (water) from its surroundings.


The hot oil in the transformer is cooled using air that comes through the breather. Hygroscopic Silica gel is used to capture the moisture from the air that enters the tank through the breather.

Lightning Strokes

Insulation of transformer can also be damaged by excessive potential pressure like switching and lighting stroke during thunder-storm.

How To Protect From Lightning? 

To protect the Transformer from lightning Strokes, proper surge arrestors are placed at primary and secondary feeders. Surge arrestors are made of Zinc Carbide and they offer very minimal resistance across live part and ground when high voltage strokes happen.



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