A transformer is a static electromagnetic device used to convert AC electrical power from one voltage level to another level of voltage by either increasing or decreasing the value of current at constant frequency.
Parts of Transformer :
The main parts of transformer are as follows:
- Primary Winding
- Secondary Winding
- Core of transformer
Transformer windings generally consists of several turns of copper (rarely aluminum) coil wound on common magnetic core. Winding turns are isolated from each other.
Winding connected with the AC input source is called primary winding of transformer.
Winding to which an electrical load is connected and output energy is drawn is called secondary windings of transformer.
Core of Transformer:
Metallic part which is made up of steel. Transformer core is made up of very thin steel laminations in order to reduce eddy current losses↗. Transformer core material (iron or ferrite) has high permeability and it provides easy flow of flux from primary to secondary side. It acts as a common magnetic circuit. The below diagram shows the main parts of transformer.
Working Principle of Transformer :
Working principle of transformer is based on the faraday law of electromagnetic induction. This law states that the rate of change of magnetic flux passing through a coil or conductor is directly proportional to the induced emf in that coil or conductor. In case of transformers, emf induced in the secondary coils of transformers is in accordance with the faraday’s law. We can write the equation of the emf in transformers from the faraday’s law is as follows:
Similarly, equation for the emf induced across the secondary coil will be
= Voltage induced across the secondary coil.
= rate of change of magnetic flux within the core.
Transformer consists of two windings (primary and secondary) which are wound around a common magnetic iron core. When we apply alternating voltage at the primary side of transformer, it causes a current (Ip) to flow in the primary coils of transformer. Since current carrying coil( conductor) produces magnetic field around it, therefore alternating current in the primary coil produces alternating magnetic flux. This magnetic flux links with the core of transformer. The flux produce inside the core is going to have a rate similar to the applied voltage.
Alternating magnetic flux in the core also links with the secondary side of transformer and induces voltage in the secondary windings of transformer(since changing magnetic flux induces emf in the coil) according to the the faraday law of electromagnetic induction. Moreover, if load is connected across the secondary side, current begins to flow in the secondary coils of transformer. This is the basic working principle of transformers.
It should be noted that the amount of voltage (emf) induced in the secondary coils of transformer depend on the rate of change of flux which in turn depend on the turns ratio.
Turns ratio of transformer is the ratio of secondary voltage to the primary voltage and is equal to the ratio of number of secondary winding turns (Ns) to the number of turns in the primary windings (Np). Hence primary and secondary emf’s (Ep and Es)will be proportional to Np and Ns respectively. Therefore, the final relationship can be written as:
= Primary voltage.
= Number of Secondary turns.
= Number of Primary turns.
By analyzing the above relation, we can say that if some voltage is fed to the primary coil, we get a different voltage at the secondary coils of transformers depending on the turns ratio. This is the basic purpose of transformer i.e. to step up (increase) and step down (decrease) AC voltages. The transformers which increases or decreases the voltage levels are known as step up and step down transformers respectively.
Step up Transformer:
A transformer which increases the voltage level is known as step up transformer. In that case, induce voltage across the secondary coil is greater than the applied primary voltage. Moreover, the number of secondary winding turns is greater than the number of primary turns in step up transformers.
Step down Transformer:
A transformer which decreases the voltage level is known as step down transformer. In that case, the number of secondary turns is less than the number of primary winding turns. Hence induced voltage across the secondary coil is less than the applied voltage.
Now, let us take an example for better understanding the step up and step down of voltage in transformers. Let assume that we apply AC voltage= 120volt at the primary side of transformer.
Number of primary turns=40
No. of turns in Secondary= 100.
Calculate the voltage induced at the secondary side ?
We can calculate the induced secondary voltage from the turns ratio formula.
The above formula can be written as:
Since we want to calculate the induced voltage at the secondary side.
So we have,
By putting the values, we have
In the above result, induced emf in the secondary coil of transformer is greater than the applied AC voltage which is 120volts. The reason is simple as the number of secondary turns is greater than the number of primary turns (100>40). Hence we have step up voltage induced in the secondary.
Similarly, if we only change the number of secondary turns of the above data and assume
No. of secondary turns= 20
No. of primary turns= 40
AC voltage= 120volts.
Voltage induced at the secondary side ?
By putting the same formula use above, we have
By putting the values, we have
In that case, induced emf at the secondary side of transformer is less than the applied AC primary voltage(60<120). Hence, we have step down voltage due to less number of secondary winding turns.
Transformer is basically magnetically coupled device. The primary and secondary side of transformer is not physically connected and transfer of energy is only occur through magnetic domain(core). The number of primary and secondary winding turns wrap around the iron core matters a lot. Basically, number of turns is the determining factor according to which the voltage is either step up or step down by the transformer.