Transformer Basics & Its Working Principle

What is a Transformer?

The transformer is a static device that transfers electrical energy from one circuit to another circuit without altering the frequency. The transfer of energy in the transformer happens through the electromagnetic induction process.

The transformer raises or lowers the voltage according to the turns ratio of the transformer. The current also changes in the same proportion of the voltage. Thus, the power remains the same. The frequency also remains unchanged. In this post, we shall learn about transformer basics and its working principle.

Why voltage is required to be raised or lowered?

The beauty of the alternating voltage is that it can easily be generated, transmitted, and raised or lowered by a transformer. The voltage is raised for long transmission to reduce the line losses.  The line loss depends on the current and resistance of the conductor. The line loss varies proportionally to the current in the circuit. By raising the voltage the current gets reduced for transfer of the same amount of electrical power and as a result, the line loss reduces. At receiving end, the voltage is lowered to the utilization level through a step-down transformer. 

Working Principle of Transformer

The transformer functions on the principle of Faraday’s Law of Electromagnetic Induction. The law states that if a conductor is placed in a varying magnetic field, the voltage is induced in the conductor. When the alternating voltage is fed to the primary, the flux varying in magnitude and direction gets linked to the primary and the secondary winding of the transformer.

Transformer Theory

The magnitude of induced EMF depends on the rate of change of the flux and the number of turns in a coil. According to Lenz’s Law, the induced EMF in a coil always opposes the applied voltage.


The induced EMF in a coil can be expressed as;

e = – N dФ/dt

The transformer has primary and secondary winding which is wound on the magnetic core. When the primary winding is fed alternating voltage the transformer draws magnetizing current to set up magnetic flux in the core. The varying flux gets linked to the primary and the secondary winding of the transformer. 

Ideally, the flux produced in the primary must get linked to the secondary without any leakage. However, practically all the flux produced in the primary does not link to the secondary, and some parts of the flux are lost. The magnetic core made of Cold Rolled Grain Oriented (CRGO) is more efficient for transferring the flux to the secondary winding. Also, the hysteresis loss of the CRGO  core is less.  

If the transformer is ideal, the flux generated by the primary winding gets linked to the secondary and primary and there is no leakage flux. However, this is an ideal condition and practically some of the flux gets leaked and links to the other parts of the transformer. The flux links in the core material induce emf which further causes heat loss in the core because of eddy current. The loss is called the eddy current loss. The magnetic core is laminated to reduce the eddy current loss.

The primary winding is fed sinusoidal voltage. The sinusoidal voltage is alternating voltage and the current flow in the primary set up magnetic flux in the core. The flux travels through the core and gets linked to the secondary and primary. The varying flux induces EMF in the primary and secondary. The magnitude of the EMF induced in the primary and secondary is as given below.

EMF Equations of Transformer

Ep = 4.44Np fФm
Es = 4.44Ns fФm

Ep     = Induced EMF in the primary winding
ES     = Induced EMF in the secondary winding
Np     = number of turns in the primary winding
Ns     =  number of turns in the secondary winding
f        =  Frequency
Фm   =  Flux in the core

The transformer core can carry the flux up to its maximum rated capacity. The CRGO core can carry flux up to 1.9 Tesla. If the flux increases above 1.9 Tesla the core may get saturated and the output voltage may get distorted.

The transformer functions pretty well as long as the flux remains constant. The flux density depends on the ratio of voltage/ frequency.

Turns ratio & voltage transformation ratio of  Transformer

EMF equation of the transformer is as given below.

Ep = 4.44Np fФm  ——-(1)
Es = 4.44Ns fФm ———(2)

Dividing equation (2) By (1) we get,

Es/Ep = Ns/ Np ———(3)

Voltages Es and Ep are almost equal to the primary and secondary terminal voltage if we ignore the winding resistance and the leakage reactance. 

Es= Vs and Ep= Vp

Vs/Vp = Ns/ Np = K  —-(4)

K is the voltage transformation ratio. 

The input power of the transformer is almost equal to the output power.

Vs x Is  = Vp x Ip

Vs/Vp   = Ip/Is  ———(5)

From equations (4) and (5) we get

Vs/Vp = Ns/Np = Ip/Is = K

The turns ratio is the ratio of primary turns to secondary turns. 

Turns ratio = Np/Ns= 1/K

The transformer has its use for stepping up or stepping down the voltage. The transformer which raises the primary voltage is called the step-up transformer, and the transformer which lowers the voltage is called step down transformer.  

In a step-down transformer, the number of turns in the primary is more than the number of turns in the secondary.

In a step-up transformer, the number of turns in the primary(Np) is less than the number of turns in the secondary(Ns).

Transformer Parts

The transformer has three main parts.

  • Primary Winding 
  • Magnetic Core 
  • Secondary Winding 

Primary Winding

A primary winding has its connection to an external source. and it receives electrical energy. The primary winding set up the magnetic flux in the core when connected to an alternating voltage supply.

Magnetic Core

The magnetic core provides a low reluctance path to magnetic flux. The magnetic flux links to the primary and secondary winding and induces the voltage in both winding.

Secondary Winding 

The secondary winding wounds on the same core. The flux is set up by primary links to the secondary winding and induces a voltage in it.

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