What is Transformer│Definition and Working Principle of Transformer

What is Transformer, definition and working principle of Transformer

What is Transformer

A Transformer is a static electromagnetic machine that helps in voltage transformation. They can either be built to step up a voltage or to step down voltage. With the help of electrical induction, the transformer can transfer electrical energy between two or more circuits.

Transformers are used to change the level of current and voltage in an electrical system. However, the voltage and current are in an alternative form. Both the input and output are in electrical form. So if you are working on an electrical system, and you which to either step up or step down the voltage, This machine does the job better.

All part of a transformer are static, in other words, its part does not move. This is a special kind of machine because its operation doesn’t require a direct connection, rather they work through the principle of mutual induction

In a transformer, the voltage and current are inverse in operation. This means that, when you want to step down a high voltage, to a low voltage. However, in doing such, the magnitude of the current will increase as that of the voltage decrease. So, you can simply say that both the voltage and current are inversely proportional to each other(V=IR).

However, during the voltage transformation, from either high to low, or low to high, the input power has to be the same as the output power. That is, no transformation occurs on power. 

A transformer has two important parts, which are the windings and the core. However, there can be more than one windings, and this depends on the types. The core, on the other hand, is a laminated piece of magnetic materials, which acts as a pathway for electrical flux to flow through.

Working principle of a transformer.

A transformer work with the principle of electromagnetic induction. You can picture it as a piece of a conductor connected to a battery. Knowing that when current which is the movement of electrons, passing through a current-carrying conductor, which could be any conducting wire. During the movement of current through this wire, a magnetic field is being produced. However, the direction of this field can be known through the right-hand rule

This field leads to magnetic flux set up around the conductor. A change in magnetic flux with respect to the number of turns of the winding will result in an Electromotive force being produced. This whole process is based on faraday’s law of electromagnetic induction. However, if another piece of a conductor is brought into this magnetic field,  an electromotive force(EMF) is being induced on the conductor. This EMF will then cause the current to flow.

Electromotive force(EMF)  

Although after some time, this current will fade off because there’s no relative movement between both conductors. This type of EMF is known as dynamically induced  EMF. Due to a reduction in the flux density around both conductors, when we remove the battery from the first conductor, EMF is again induced in the first conductor. 

The magnitude of the induced EMF is proportional to the change of the magnetic flux.

The magnitude of the flux increases when using coils on both sides instead of using a single wire. However, this helps to improve the electromagnetic coupling of the machine. 

Winding of a transformer
the winding of a transformer

Again, to improve the electromagnetic coupling, the core is placed between the two windings. The core consists of the yoke and the limb. The limbs are where the winding is wounded. The core provides a path of low reluctance to the magnetic flux. One side of the windings is the primary side, while the other part is the secondary side. 

Type of transformer construction 

There are two types of transformer construction, these types are classified based on the phase. A transformer can either be a single-phase or three-phase. Although the construction of this machine wholly depends on how the primary winding and secondary winding are wounded on the laminated core limbs. These types are;

  • Shell type 
  • Core type 

Core type transformer

The core type transformer can either be a single-phase or three-phase, same with that of shell type. The core effectively helps to transfer the magnetic flux from the primary winding to the secondary winding. However, the core is made up of thin slices of highly permeable magnetic material, with a thickness of about 0.25mm to 0.5mm. 

As said earlier, this machine can be of core type or shell type. The single-phase core type has the windings placed on the core limb at two edges. These windings are cylindrical form. The construction of the three-phase shell type is different from the single-phase core type. Here, both the primary and secondary winding is placed on the same limb of the core. 

Shell type transformer

However, in a single phase shell type, the core surrounds the windings. In another word, the windings are placed on the middle limb of the core. The winding of a shell-type can either be of rectangular form or distributed form. Although just as the core-type, the construction of the three-phase shell type is different from the single-phase shell type. 

It’s important to note that, the core type of transformer is more economical and easy to manufacture compared to the shell-type design. Although when it involves a very high voltage rating, that’s in voltage transmission substation, the shell type transformer design is used. This application of the shell type is due to its better short circuit strength characteristics, making it more reliable. In addition, it also exhibits a better power to weight ratio. 

Step-up and step-down transformers

For a step-up transformer, the number of turns on the secondary side is more than the primary side. Since the emf produced is a function of the number of turns at either side of the transformer, resulting in more voltage on the secondary side compared to that of the secondary. 

However, these voltages are alternating, and since voltage and current are inversely proportional to each other, the magnitude of the current at the secondary side becomes smaller compared with that on the primary side. So the magnitude of the current reduces as the voltage increases. 

Similarly, when the number of turns on the primary side is more than that of the secondary, this type of transformer is called a step-down transformer. This voltage at the secondary side is lesser than that on the primary side. Moreover, the magnitude of the current is the opposite. Most importantly, the magnitude of the transformer power remains the same for both step up or step down transformer. 

Lamination of the transformer

Currents and voltages are applied to the input side of the transformer, which is always the primary side. However, the construction of this machine is done to reduce losses, such as eddy current losses, hysteresis losses, Iron losses, copper losses, stray loss, and dielectric losses. In as much as the core provide a low reluctance path for magnetic flux, it also subjected to circulating electrical current. 

However, these circulating electrical currents are known to be eddy currents. These currents in the transformer produce heat and electrical losses within the core. As such, it reduces its efficiency. Moreover, these eddy currents produced are due to the constant subjecting of the emf that is induced on the core. 

Although lamination of the transformer core helps to reduce the eddy currents produce. Lamination is off highly permeable material, made up of silicon steel. However, these laminations are electrically insulated from each other, using an oxide layer on the surface. 

Based on the discussed construction of the transformer. However, due to the spacing between the primary winding and the secondary winding, this will considerably reduce the efficiency and magnetic coupling in the transformer. Nevertheless, this can be solved by placing both the primary and secondary winding on the middle limb as shown in the diagram below.

single phase core type transformer
the middle limb of the transformer

A transformer is a special kind of static electrical machine because it can transform power from the primary side to the secondary side without any metallic connection between the two circuits.

Application of transformers

Distribution transformer
Application of transformers

Transformers finds application in several electrical segments, which includes;

  • Power generation: power transformer, directly located next to a power plant. This type of transformer mainly does the function of stepping up voltage produced by the power generator, in the Generator Step-up Unit (GSU).
  • Utilization of electrical power: instrument transformer, and measuring transformer. The instrument transformer helps to transform or isolate the current or voltage level in an electrical power system. It’s important to note that the instrument transformer is of high accuracy class. They are classified as voltage transformers(VTs) and current transformers(CTs).

However, despite their several applications, transformers are placed and classified based on different categories. These categories are based on the following. 

  1. Type of supply 
  2. Construction 
  3. Voltage level
  4. Their use and instrument
  5. Core medium 

It’s important to note that, these windings are of different layers and they are insulated from each other. However, based on their medium of cooling, transformers are classified as Air core transformer and Iron core transformer. Since no moving parts, there won’t be any cause for lubrication. 

What is the difference between the iron core and the air core?

  • For the air-core transformer, the flux linkage between the primary winding and secondary winding of the transformer is through the air. However, in the air-core transformer, the mutual inductance effect is less compared to that of the iron core. 
  • The air-core has completely no or less eddy current and hysteresis losses, although this is dominant in iron core type.
  • Compared to the iron core, the reluctance path provided by the core yoke is high for the air-core transformer. However, the iron-type provides a less reluctance path for linkage of magnetic flux, as a result of the magnetic and conductive properties of the iron.
  • In terms of physical construction, for iron core transformer, to create a better magnetic flux linkage, both primary winding, and secondary winding can be placed on multiple iron plates. Most importantly, the iron core transformer has more applications due to its high efficiency. 


Basically, a transformer helps to either step up or step down voltages. It requires an applied voltage to get an induced voltage. The parts of a transformer are static, as such, it’s referred to as a static electrical machine. Since no metallic connection between the two circuits, a transformer works with the principle of electromagnetic induction. However, a transformer can either be an indoor or outdoor transformer. 

You can learn more on Electrical/Electronic Engineering, by checking our Electrical/Electronic Engineering category, you can find this under Academic page. Thanks for coming around.

FAQs on Transformers

Why is high voltage winding not put near the core?

Compare to low voltage(LV) winding, much current flows through the high voltage(HV) winding. Electrical heating is a function of the current, so the more the current, the high the tendency to result in electrical heat. However, with the high voltage(HV) winding placed on the core limb, more insulation will be required. Moreover, low voltage (LV) winding won’t require much insulation compared to the high voltage(HV) winding…

Why is a transformer rated in kVA and not kW?

There are several losses in a transformer, among others are the iron loss and copper loss. These two losses are independent of the phase difference between the voltage and current which is the power factor of a transformer. However, KVA which is the unit of apparent power, this the phasor sum of both the real and reactive power…

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