What is a Transformer? 

Ever since electricity has been created, people have started using it in its entirety. We see many types of Transformers in the neighborhoods. Some are smaller in size and some are very large. They are used according to their requirements. Many of us have seen the electrical transformer but they do not know what work they are engaged in.

An electrical transformer is a static electrical device that transfers electricity from one place to another place keeping the frequency constant. The circuit which receives energy from the source is known as primary winding or input side or sending end. The circuit which delivers energy to load or the circuit which is connected to the load side is known as secondary winding or output side or receiving end. The concept of or the principle working of the transformer was invented by Mikael Faraday in 1831 then later on William Stanley redesigned that transformer for the use of electrification in 1885. It does not have any moving parts and has higher efficiency than the rest of the electrical machines. In industries sometimes engineers call a transformer with another short name 'TRAFO'. 

Working of Transformer 

They work on the principle of Faraday’s law of electromagnetic induction which is stated as emf induced is proportional to the rate of change of flux concerning time. 

E dϕ dt


E = Induced EMF  

ϕ = Magnetic Flux

To remove the proportionality constant from the above equation, we know that magnetic flux generated in the core is dependent upon a number of turns which is denoted by ‘N’. So, we get 

E=N dϕ dt

The negative sign appears due to the concept of Len’z law. Now if we consider the primary side is connected to the source and the secondary side is connected with the load so the equation can be written as, 

E 1 E 2 = N 1 N 2 = I 2 I 1 =a


  • E1= Induced voltage in the primary side 
  • E2= Induced Voltage in the secondary side 
  • N1= Primary turns 
  • N2= Secondary turns 
  • I1= Primary current 
  • I2= Secondary current 
  • a = Turn ratio.  

When a source or input (AC supply) is applied to the primary coil, magnetic flux is generated in the core (due to the magnetic field derived by the primary current) . This induces emf in the secondary of the coil. In this manner, the TRAFO transfers energy from one circuit to another circuit without any physical contact by means of electromagnetic induction. 

Need and Importance of Transformer 

For transmitting power, the electric power system needs an apparatus that can transmit power to a long distance with loss of energy so here there are two constraints: one is Transmitting power to the long-distance, and the second one is with minimal loss. If we talk about long-distance transmitting then it is possible with higher transmitting voltages than can be achieved by the TRAFO, secondly, the loss of the TRAFO is designed to work approximately on 95 % efficiency so here both the constraints are fulfilled. 

Types of Electrical Transformer 

Based on Voltage levels: There are only two possibilities.  

(1) Step-up transformer is used in Generation and Transmission systems which comprises 400, 230, 132 kV. In this transformer, secondary coil has more number of turns than primary coil. 

(2) The step-down transformer is used in the Distribution System which comprises 66, 33, 11 kV. In the step-down transformer, secondary voltages are less than the primary voltage. The step-down transformer has a greater number of turns in the primary coil. 

Based on connection: There are two types.  

(1) 3-phase which are used for voltage rating 415 V. 

(2) single-phase which are used for voltage rating 230 V and below are also termed two-winding TRAFO. 

Based on Core OR Magnetic-core: 

  • Air core transformer: It comprises covering both primary and secondary coil in a non-magnetic strip where the flux is linked between the primary & secondary windings in the air. A mutual inductance of the air core is not as much of the iron core, i.e. Which means that the reluctance that is presented to the generated flux is high in the air medium. But the hysteresis & eddy current losses are eradicated in the air-core type transformer. 
  • Iron core transformer: In this, both primary coil and secondary coil are covered in several iron plate bundles which offers a perfect linkage path to the flux which offers low reluctance to linkage flux owing to the conductive and the magnetic property of iron. They are very much in use due to their higher efficiency than air core type transformers. 

Based on Connection: Autotransformer. In a regular transformer, the primary and secondary are connected in the same direction but in autotransformers, both the windings are connected together in series. So as tapping equipped on secondary changes voltage level changes. Autotransformers are mainly used for voltage adjustments. 

Based on Usage: 

  • Distribution Transformer: All the transformers which are of voltage rating 33 kV and below are called distribution transformers. They are small in size. It has a low magnetic loss. They are not always fully loaded. Generally, this type of transformer is of low efficiency approximately between 50-70 %. 
  • Power Transformer: All the transformers which are of voltage rating greater than 33 kV are called a Power transformer. Power transformers are big in size and almost fully loaded. Generally, Power transformers are of high efficiency approximately between 91-99 %. They have insulation levels. They are categorized based on the MVA ratings. Small Power transformers are 500-750 KVA. Medium Power transformers are of 750 KVA to 1000 MVA. Large Power transformers are greater than 100 MVA. Generally, Power transformers are designed for transmitting power for heavy loads. Mostly all power transformers are oil-immersed types. 
  • Protection Transformers: This type of transformer is used for the purpose of protection OR Measurement and they are called Protection or Instrument Transformers. They are equipped everywhere in the system with any type of transformer. They are of two types: current transformer and potential transformer or Voltage transformer. A current transformer is the most important apparatus in power system protection. A current transformer is capable of measuring high currents and based on that all the relays operate. All protection transformers are equipped with power transformers in the switchyards. 

Based on Place of Use: They are classified into two Indoor (which are properly covered with roof similarly like process industry) and Outdoor (which is nothing but the distribution transformers) 

Based on Cooling: Self-air cooled OR Dry-type, Air Natural (AN), Air Force (AF) OR Air Blast type, Oil Cooled Air Natural (OCEAN), Oil Cooled Air Forced (OCAF), Oil Cooled Water Forced (OCWF), Oil Forced Water Forced (OFWF) and Oil Natural Water Forced (ONWF). 

There are several other transformer types like Audio transformer, Resonant transformer Phase transformer, Ideal transformer, isolation transformer 


  • E=N dϕ dt
  • E 1 E 2 = N 1 N 2 = I 2 I 1 =a

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

  • Bachelors in Technology (Electrical Engineering)
  • Masters in Technology (Electrical Engineering)
  • Bachelors in Science (Physics)
  • Masters in Science (Physics)

Want more help with your electrical engineering homework?

We've got you covered with step-by-step solutions to millions of textbook problems, subject matter experts on standby 24/7 when you're stumped, and more.
Check out a sample electrical engineering Q&A solution here!

*Response times may vary by subject and question complexity. Median response time is 34 minutes for paid subscribers and may be longer for promotional offers.

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in
EngineeringElectrical Engineering

Electrical Machines


Three Phase Transformer

Transformer Homework Questions from Fellow Students

Browse our recently answered Transformer homework questions.

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in
EngineeringElectrical Engineering

Electrical Machines


Three Phase Transformer