Working Principle of Transformer

The working principle of transformer is Faraday’s Law of Induction. To understand the working principle of transformer we should know about Transformer first.

Transformer:

Transformer is a very common word, every other person in the society uses it. Even a person who is not familiar with electrical principles will be familiar with this term. But not all of them know the working principle of Transformer.

Transformer is a device that changes one voltage level to another. It can increase the voltage which is termed as Stepping-up of voltage and the transformer is called as Step-up transformer. It can also decrease the voltage level which is Stepping-down of voltage and the transformer is called as Step-down transformer. When the transformer steps up the voltage level, it steps down the current magnitude and vice versa so that the power remains same. During these changes in voltage level the frequency remains same. The output frequency will be same as input frequency.

There are commonly two (can be more than two) windings in a transformer which are wounded on the same core (a ferromagnetic material). One of the winding is connected to input which is termed as Primary winding and another is connected to the Load (Output) which is known as Secondary winding. Sometimes Primary winding and Secondary winding are called as Input winding and Output winding respectively. The interesting thing to know is that there is no connection path between the windings (as shown in the thumbnail) . The winding wires have very thin layer of insulation.

Working Principle of Transformer:

The question is if there is no connection between Primary and Secondary windings and input is applied on Primary winding only then how the voltage appears on the Secondary winding. The answer is it is due to Mutual Induction. To understand the phenomenon of Mutual Induction we need to go through the Faraday’s Law of Induction.

An Overview of Faraday’s Law of Induction:

It is obvious that the when current flows through the coil it produces a magnetic field. Magnetic flux is the measure of magnetic field passing through an area. Therefore, magnetic field and magnetic flux are related to each other if one of them changes the other will also change.

According to Faraday’s Law of Induction an emf (Electromotive Fore) is induced in a coil when the magnetic flux is changing through it and the emf will remain until the magnetic flux is changing and it will die when the magnetic flux will become constant. It is obvious that the when current flows through the coil it produces a magnetic field. Magnetic flux is the measure of magnetic field passing through an area. Therefore, magnetic field and magnetic flux are related to each other if one of them changes the other will also change.

Mutual Induction (The Working Principle of Transformer):

Mutual Induction is the phenomenon in which magnetic flux of a coil is changed and emf is induced in another coil. Now how do the magnetic flux is changed? The answer is simple, if there is change in current then there will be change in magnetic field as well because magnetic field is due to the current flowing through the coil and if magnetic field is changed then the magnetic flux is also changed. To produce change in DC current we need to create changes in magnitude which is done by installing a Rheostat (or Variable Resistor or Potentiometer) which is a tedious work but in the case of AC we don’t need to do anything as it is alternating current which is always changing.

Now if we need to do mutual induction, we need to apply ac voltage in a coil. For instance, consider the following figure,

The alternating source is connected to Coil A which changes the magnetic field. If we bring Coil B near the magnetic field, then magnetic flux across Coil B will also change. According to Faraday’s Law, emf will be induced in coil B and it will be shown in the voltmeter. There is Air between the coils and coil and magnetic field. It is better to use a ferromagnetic material because the ability to carry magnetic field (permeability) of ferromagnetic material (like iron) is better than air. So, a core can be inserted in between.

Now the emf induced in coil B can be seen in the voltmeter. In this phenomenon by changing the magnetic flux of a coil the emf is induced in another coil which is Mutual Induction.

Mutual Induction in Transformer:

Now let’s get back to transformer. The working principle of transformer is Mutual Induction. We know that we apply AC input at Primary winding. Due to change in magnetic field the magnetic flux also changes. As there is a core on which both the windings are present therefore the flux associated with primary will link to secondary. As Secondary winding has changing magnetic flux and hence the emf is induced in Secondary winding.

Why Transformer works on AC only?

As I have discussed above that the working principle of transformer is Mutual Induction. For the phenomenon of Mutual Induction, it is necessary to produce change in magnetic flux. AC is varying in nature, its magnitude changes with time. The change in magnitude causes the change in magnetic field. Due to change in magnetic field magnetic flux changes. Hence, we can do Mutual Induction with AC to run transformer.

Relationship between Primary and Secondary Voltage of Transformer:

I have discussed that Transformer is a device that changes one voltage level to another. The question is that what would be the relationship between applied and obtained voltage and on which thing the level of obtained voltage depends. For instance, why sometimes the obtained voltage is higher than the applied voltage (Step-Up Transformer) and why is sometimes lower (Step-Down Transformer) while the working principle of both the Step-Up and Step-Down Transformer is same.

To answer this question, we need to dive in the Faraday’s Law of Induction

Faraday’s Laws of Induction:

There are two laws of Faraday on Induction.

1. An emf (Electromotive Fore) is induced in a coil when the magnetic flux is changing through it and the emf will remain until the magnetic flux is changing and it will die when the magnetic flux will become constant
2. The magnitude of the induced emf depends on the rate of change of magnetic flux, and it can be given as,

Why the obtained Voltage can be lower or higher than the applied one?

The answer is due to Number of turns in the coil. The relationship between induced emf and the Number of turns is given by Faraday’s Law (eq 1). It means if the number of turns are greater the obtained voltage would be higher otherwise lower than the applied voltage. In the case of Self Induction, the emf induced depends on its own number of turns. But in Mutual Induction the magnetic flux changes in one coil and the emf is induced in another coil, both the coils have no of turns now on which the induced emf will depend?

Turn Ratio:

In Mutual Induction the induced emf depends on the ration of no of turns of both the coil, which is known as Turn Ratio. Turn ratio is simply the ratio between no of turns of primary winding to the number of turns of secondary winding.

We know that source is applied to Primary and by mutual induction emf is induced in Secondary. Now If the No of turns of the Primary winding is greater than the no of turns of Secondary then the emf induced will be lower than the applied one, if the No of turns of the Primary winding is lesser than the no of turns of Secondary then the emf induced will be greater than the applied one and if both are equal then both the Primary voltage and Secondary Voltage will be same.

For Instance,

If,

• There are 100 turns in the primary coil and 50 turns in secondary coil, hence the turn ratio will be 2:1. This means if the voltage is 2V at primary there will be 1V at secondary. This transformer is a Step-Down Transformer.
• There are 100 turns in the primary coil and 500 turns in secondary coil, hence the turn ratio will be 1:5. This means if the voltage is 1V at primary there will be 5V at secondary. This transformer is a Step-Up Transformer.

Hence, we can say,

As we know that if a transformer steps up the voltage than it steps the current and vice versa for constant power. The voltage of a winding is inversely proportional to current flowing through it.

Therefore,

Can we apply any voltage at Primary Winding?

No, we cannot apply any voltage at the Primary winding because the windings can bear specific voltage only. Therefore, rated voltage of transformer is defined. High voltage can damage the insulation (enamel) of windings. The amount of current which the winding can sustain depends on the cross-sectional area of the conductor. If excess amount current is passed through the coil, then it will produce a lot of heat which will damage the coil.

Can a Step-Up Transformer be used as Step-Down Transformer?

Yes, we can use Step-Up Transformer as Step-Down Transformer which can be done by applying voltage to the other side. This is shown in the figure below,

We can use the same transformer for Stepping Up and Stepping Down the Voltage level. But there is one important thing to be noticed. Assume that the rated voltage of a transformer is 220/12. For ease let’s say the winding to the left side can be 220V and the right side one is 12V. We are assuming that the rated voltage is the maximum voltage that the windings can bear.

If we apply 220V to the right side then we will get 12V from the left side, which means that this is step down transformer. Now if apply 12 to the right side then we will get 220V at the right side, we cannot apply 220V to the right side as it can bear only 12V. So, this thing should be in mind that the maximum voltage that a winding can bear. In actual practice windings are selected to bear a little more voltage than the rated one to be on safe side.