What is a Transformer? What are the Different Types of Transformer?

Editorial Team - everything PE

Jan 9, 2024

A transformer is a static device that works on the principle of electromagnetic induction. It transfers electrical power from one circuit to another with the desired change in voltage and current and without any change in frequency. Transformers are used in different stages of electrical networks such as power generation, transmission, distribution, and utilization.

Working Principle of a Transformer

A transformer works on the principle of mutual induction. When an alternating current is applied to a conductor, a varying magnetic field is induced around it which generates an EMF. This is called self-induced EMF. Now, if another conductor is placed in this varying magnetic field, an EMF is induced in the second conductor by Faraday’s law of electromagnetic induction. If the circuit of the second conductor is closed, a current will flow through it. This is the basic working principle of a transformer.

A transformer has two windings or coils. The winding to which the supply voltage is connected is called the primary winding of a transformer and the winding to which the load is connected is called the secondary winding of a transformer. To maximize the flux linkage between primary and secondary winding, the windings are wound on a low reluctance core.

Basic diagram of a transformerWhen an alternating voltage is applied to the primary of the transformer, an alternating flux Øm is produced in the core and generates an EME, E1. As the secondary coil is inductively coupled to the primary winding through the flux Øm, an EMF Eis generated in the secondary winding. The EMF E1 is called the primary EMF and EMF E is called the secondary EMF and is given by,

thus,

From the above relation, it is evident that the magnitudes of the EMF in the primary and secondary windings of a transformer depend on the number of turns in the primary and secondary winding.

Symbolic representation of transformer

Construction of Transformer

There are two basic parts of a transformer:

• Magnetic core
• Windings or coils

Magnetic Core

The magnetic core provides a low reluctance path for the magnetic flux. Two types of losses occur in the core. They are hysteresis loss and eddy current loss. Together these losses are known as magnetic losses or iron losses. A magnetic core is constructed in such a way that the magnetic or iron losses are as low as possible. Usually, a magnetic core is made up of materials with high permeability such as silicon steel to reduce hysteresis loss.

Eddy current is generated due to the high width of the material. To reduce eddy current loss, the magnetic core is constructed using thin laminations. Laminations as thin as 0.5 or 0.7 mm are put one over the other to form a stack and maintain a minimum air gap between them. Also, these laminations are insulated using varnish. Thus the core is made up of thin laminations of high magnetic material. The transformer core is made up of laminations of different shapes such as E, L, I, C, and U.

Laminated Transformer Core

Winding or Coil

The windings of a transformer are made up of conducting material and are wound over the limbs of the core. Material with less resistivity such as copper is preferred as the winding carries primary and secondary current. The magnetic properties of copper enhance the magnetic flux and aid in the transfer of power from primary to secondary.

Types of Transformers

Transformers may be classified based on various aspects

• Based on construction
• Based on the transformer ratio
• Based on the core material
• Based on application
• Based on the cooling system
• Based on the type of supply

Based on construction: Based on how the transformer winding surrounds the core, transformers may be classified into two types

Core-type transformers: In such transformers, the windings are wrapped around the core. The primary and secondary windings are wound around the two different limbs of the core. It has two cylinders and two horizontal bars forming the frame. The core has excellent magnetic properties and maximum flux linkage between primary and secondary winding. Different types of core plates such as E, I, U, and L shapes may be used as per requirement.

Shell-type transformers: In shell-type transformers, the core surrounds the transformer winding. The primary and secondary windings are wound around the central limb of the core. Such transformers use double magnetic circuits and are suitable for high voltage low current applications due to poor ventilation.

Shell-type transformers use E-I and E-E laminations whereas core-type transformers use L-L and U-I laminations.

Based on transformer ratio: Transformers may be classified based on the number of turns in the primary and secondary.

• Step-Up Transformer: A step-up transformer is a type of transformer that increases the voltage of an alternating current (AC). It has more turns in the secondary winding than in the primary winding, which leads to an increase in voltage.

For a step-up transformer, N2 > N1, V2 > Vand I2 < I1 .

• Step-Down Transformer: A step-down transformer is a type of transformer that reduces the voltage of an alternating current (AC). It has fewer turns in the secondary winding compared to the primary winding. The primary function of a step-down transformer is to decrease the voltage while maintaining the same frequency as the input AC.

For a step-down transformer, N1> N2, V1 > Vand I1 < I2 .

Based on Core Material: Transformers transfer electrical energy from one circuit to another through electromagnetic induction over the core. The material used for the core influences the flux density generated in the core. Accordingly, there are several types of transformers based on the core material used.

• Iron Core Transformer: Such transformers use a stack of soft iron plates as their core material. Transformers employing iron cores benefit from the excellent magnetic properties of iron. Both primary and secondary coils are wound around iron plates, creating a path for magnetic flux. The excellent conductivity and magnetic properties of iron assure minimum resistance to flux linkage. Therefore, iron core transformers are widely used due to their higher efficiency compared to air core transformers.
• Ferrite Core Transformer: Such transformers use ferrite as core material. These transformers are used for high-frequency applications such as RF circuits and SMPS. The ferrite cores ensure minimum losses with high frequency signals ensuring overall system efficiency. Ferrite core transformers are flexible in shapes and sizes and can be custom-made for specific needs. The most commonly used core is the E-shaped ferrite core.
• Air Core Transformer: Such transformers use air as the core material where the primary and secondary winding are not wound to any magnetic material. The magnetic flux generated due to the current in the primary winding flows through the air. The mutual inductance produced by such transformers is low compared to iron and ferrite core transformers.

Based on Application: A transformer may be used as a power transformer, distribution transformer, instrument transformer, or autotransformer based on the specific application for which it is used.

• Power Transformer: A power transformer converts low voltage, high current electricity to high voltage low current electricity. They are usually employed at generating stations and transmission substations to step down the power to a desired level suitable for distribution to consumers.
• Distribution Transformer: Distribution transformers convert high grid voltage to lower voltage for end users and form the final phase in the power distribution system. They are either single-phase or three-phase transformers and are available in various sizes to cater to various capacities.
• Instrument Transformer: An instrument transformer is also known as an isolation transformer. It is used to isolate the secondary winding from the primary winding when the primary has a high voltage and high current supply. This transformer safeguards the measuring instrument or the energy meter that is connected to the secondary of the transformer.

For a step-down transformer, N1= N2, V= V2,  and I1 = I2

• Autotransformer: An autotransformer is a transformer with a single winding that acts as both primary and secondary winding. Due to the single winding configuration, it makes possible power transmission through electrical and magnetic means. They are widely used in applications that need precise voltage control such as laboratories and test benches.

Based on Cooling System: Based on the cooling methods used in transformers, transformers are classified as

• Dry-type Transformer: A dry-type transformer is a type of transformer that uses air or non-flammable solid insulation instead of liquid as a cooling medium. Unlike oil-immersed transformers, dry-type transformers do not require a liquid cooling agent such as mineral oil. Instead, they rely on natural air convection or forced air circulation to dissipate heat generated during operation.
• Oil-immersed Transformer: An oil-immersed transformer, also known as an oil-filled transformer or liquid-filled transformer, is a type of transformer that uses oil as both a cooling and insulating medium. The oil provides effective heat dissipation from the transformer's core and windings while also serving as an electrical insulator between the different components.

Based on Type of Supply: Based on the type of supply, transformers are classified as single-phase and three-phase transformers.

• Single-Phase Transformer: A single-phase transformer is a type of transformer designed to work with a single-phase alternating current (AC) power supply. In a single-phase system, the voltage alternates in a sinusoidal manner, completing one full cycle in a given period. The standard residential power supply in many places around the world is a single-phase system.
• Three-Phase Transformer: A three-phase transformer is a type of transformer designed to work with a three-phase alternating current (AC) power supply. In a three-phase system, three sinusoidal waveforms of electric current or voltage are offset in time, creating a continuous and balanced power flow. Three-phase power systems are commonly used in industrial and commercial applications due to their efficiency in power generation, transmission, and distribution.