Transformers – Construction, Working, Types, Necessity


Electrical transformers are machines that transfer electricity from one circuit to another with varying voltage level, but with no change in frequency. Today, they are designed to use AC supply, which means that supply voltage fluctuations are affected by current fluctuations. As a result, an increase in current will result in an increase in voltage and vice versa. It works on the principle of the Faraday’s Law of Electromagnetic Induction, which states that “the voltage magnitude is directly proportional to the rate of flux change.”

Necessity of a Transformer-

Transformers help improve power systems safety and efficiency by increasing and lowering voltage levels as and when needed. These are used in a wide range of residential and industrial applications, mainly and perhaps most significantly in long distance power distribution and regulation.

Electrical power is typically produced at 11Kv. AC power is distributed over long distances at very high voltages for economic reasons, say220 kV or 440 kV. In the generating stations, therefore, a step-up transformer is applied. Now, for safety reasons, the voltage at different substations is stepped down to different rates by step down transformer to feed the electricity to the different locations and thus the power is used at 400/230 V.

If (V2 > V1) the output side voltage is increased and is known as the Step-up transformer.

If (V2 < V1) the output side voltage is reduced and is known as the Step-down transformer.

Transformer Construction-



  •   VP  –  is the Primary Voltage
  •   VS  –  is the Secondary Voltage
  •   NP  –  Number of Primary Windings
  •   NS  –  Number of Secondary Windings
  •   Φ (phi)  –  is the Flux Linkage

Remember that the two coil windings are not connected electrically, but are only magnetically linked. To increase or decrease the voltage applied to the primary winding, a single-phase transformer can operate. When a transformer is used with respect to the primary to “increase” the voltage at its secondary winding, it is called a Step-up transformer. It is called a step-down transformer when it is used to “decrease” the voltage on the secondary winding in relation to the primary.

Furthermore, there is a third state in which a transformer on its secondary generates the same voltage as its primary winding. In other words, in terms of voltage, current and power transfer, its output is identical. This type of transformer is called an “Impedance Transformer” and is used specifically either match impedance or to separate adjacent electrical circuits.

Through changing the number of coil turns in the primary winding (NP) compared to the number of coil turns on the secondary winding (NS), the difference in voltage between the primary and secondary windings is achieved.

Since the transformer is essentially a linear device, there is now a ratio between the number of primary coil turns divided by the number of secondary coil turns. This ratio, called the transformation ratio, more commonly referred to as a “turns ratio” transformer, (TR). This turns ratio value determines the transformer’s function and the corresponding voltage on the secondary winding.

Compared to secondary winding, it is necessary to know the ratio of number of wire turns on the primary winding. The turns ratio, which has no units, is in order to compare the two windings and is written with a colon like 3:1(3-to-1). It implies in this case that there will be 1 volt on the secondary winding, 3 volts on the primary winding 3 volts-to-1 volt. Then we can see that if the ratio between the number of turns changes the resulting voltages will also have to change with the same ratio, and that is true.

All transformers are about “ratios.” The primary-to-secondary ratio, the input-output ratio, and any given transformer’s turn ratio will be the same as its voltage ratio. In other words, ” voltage ratio =turns ratio ” for a transformer. Generally speaking, the actual number of wire turns on any winding is not important only the ratio of turns and this relation is given as:

Transformers Turns Ratio –


Working Principle of Transformers-

An electrical transformer uses the Faraday’s electromagnetic induction law to work –”Change in the rate of flux connection is directly proportional to the induced EMF in a conductor or coil.”

The physical basis of a transformer is the mutual induction between two circuits which are connected by a common magnetic flux. It typically has 2 windings: primary and secondary. Such windings share a laminated magnetic core, and the reciprocal induction between these circuits leads to the movement of electricity from one point to another.

Depending on the amount of connected flux between the primary and secondary windings, different flux contact rates can occur. A low reluctance path is common to both windings in order to ensure maximum flux relation, i.e. maximum flux passing through and linking from the primary to the secondary winding. This leads to increased work performance productivity and forms the transformer’s cores.

Applying alternating voltage to the primary side windings produces an alternating flux in the core. It links all windings in the primary as well as the secondary side to induce EMF. When there is a load connected to the secondary section, EMF in the secondary winding causes a current, known as the load current.

This is how electrical transformers transmit AC power from one (primary) circuit to another (secondary) by converting electrical energy from one value to another, changing the rate of voltage but not the frequency.

Types of Transformer-

According to the number of turns in the windings

  • Step up
  • Step down

The position of the core windings

  • Core type
  • Shell type

According to the basis of the supply

  • Single phase
  • Three phase

According to types of services

  • Power transformer
  • Distribution transformer
  • Instrument transformer
    • Current transformer
    • Potential transformer
    • Auto-transformer

According to the basis of cooling

  • Air Natural (AN)
  • Air ForceD (AF)
  • Oil Natural Air Natural (ONAN)
  • Oil Natural Air Forced (ONAF)
  • Oil Forced Air Forced (OFAF)
  • Oil Natural Water Forced (ONWF)
  • Oil Forced Water Forced (OFWF)

EMF Equation of the Transformer-

EMF Equation is


Where the voltages are E1 and E2, and N1, N2 is the number of turns respectively in the primary and secondary windings.

Also Read – Construction of single-phase transformers

Also Read – Construction and principle of operation of current transformer

Also Read – Main parts of Transformer

Also Read – Civil construction for installing of transformer

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