Alternating Current


Have you heard about Transformers before? Yes. You also might be knowing about the purpose of using transformers. In simple language, we can say that a transformer is a voltage controller device (a device which controls voltage by increasing or decreasing it). Transformers are basically used for the transmission of electrical energy. The main principle operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux.

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A basic transformer consists of two coils that are electrically separate and inductive but are magnetically linked through a path of reluctance. The working principle can be better understood by the above-given figure.


Source: Wikipedia

As shown in the above diagram an electrical transformer consists of two coils named as the primary coil and secondary coil which is wounded on a soft iron core. The soft iron core is laminated to minimize eddy currents. Both the coil in the transformer has high mutual inductance.

A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force.

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Induced EMF

The so produced electro-motive force can be explained with the help of Faraday’s laws of Electromagnetic Induction given:

$$e=-\frac{\mathrm{d} \phi }{\mathrm{d} t}$$

$$e_{p}=-\frac{\mathrm{d} \phi _{p}}{\mathrm{d} t}$$

$$e_{s}=-\frac{\mathrm{d} \phi _{s}}{\mathrm{d} t}$$

By using the above equations we get :


The ratio Ns Np  is called transformation ratio. The transformer formula is given by:


Where, Vp = primary voltage, Vs = secondary voltage, Np = number of turns in the primary, Ns  = number of turns in the secondary.

Types of Transformers

There are two types of transformers:

  • Step-up Transformer
  • Step-down Transformer

Step-up Transformer converts a low voltage into a high voltage. The number of turns in the primary coil is less than the number of turns in the secondary coil, i.e. Np < Ns.

Step-down Transformer converts a high voltage when current decreases into a low voltage when current increases. The number of turns in the primary coil is greater than the number of turns in the secondary coil, i.e. Np > Ns. In short, a transformer carries the operations shown below:

  1. Transfer of electric power from one circuit to another.
  2. Transfer of electric power without any change in frequency.
  3. Transfer with the principle of electromagnetic induction.
  4. The two electrical circuits are linked by mutual induction.

Solved Examples For You

Q1: The number of primary and secondary windings is 100 and 300 respectively. The primary voltage is given by 200V, determine the secondary voltage.

Solution: Given: $$N_{p}= 100$$

$$ N_{s} = 300 $$

$$V_{p}= 200V$$

The transformer formula is given by,


$$V_{s}= V_{p}\times \frac{N_{s}}{N_{p}}$$

$$V_{s} = 200\times \frac{300}{100}$$

$$V_{s}= 600V$$

Q2: Why do we use a soft iron core for the construction of a transformer?

Solution: This is because the hysteresis curve of a soft iron core is extremely thin and thus having less energy loss. That is whatever energy is transferred from primary to secondary coil there will be the minimum loss if we use a soft iron core.

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2 responses to “Representation of AC Current and Voltage by Rotating Vectors – Phasors”

  1. Tanishk says:


  2. sridhar Dasari says:

    Awesome post…

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