Transformer Working Principle


Electricity is one of the greatest discoveries in the history of mankind that has remarkably reshaped the world. Today, we benefit from various conveniences brought by harnessing this fundamental force of nature and transferring it to the far-fetched regions; however, this was not always the case. During the early 1800s, the only current producing devices were voltaic cells, which produced small currents by dissolving metals in acids. In 1830, Faraday and Henry accelerated the research on electricity by linking it to magnetism, which led to the discovery of electromagnetic induction. This discovery revolutionized the world by laying the groundwork for the development of AC generators; however, it wasn’t until 1884 that three Hungarian engineers, Károly Zipernowsky, Ottó Bláthy, and Miksa Déri (ZBD), patented the first commercial transformer that allowed electricity to be transmitted over long distances.

What is a Transformer?

A transformer is an electrical device that employs electromagnetic induction to transfer alternating current from one circuit to another. It is used either to transform low voltage AC to high voltage AC or to obtain a low voltage AC from a high voltage AC.

Components of a Transformer

Despite the fact that transformers can weigh anywhere from a few grams to hundreds of metric tonnes, there are a few fundamental components listed below that are common in their construction.



The transformer core is usually made of materials such as soft iron or CRGO (cold-rolled grain-oriented steel), because they have high permeability, and it is used to provide support to the windings and a controlled path for the magnetic flux generated in the transformer. The core is usually made up of multiple thin laminated sheets or layers, rather than a solid bar. This design aids in the elimination and reduction of heating. To reduce eddy current losses, the core is made up of a stack of thin silicon-steel laminations that are separated by thin layers of varnish.



Windings are the wires coiled around the core. A transformer is primarily composed of two windings: primary and secondary. The coil that draws electricity from the source is known as the primary winding, whereas the coil that supplies energy to the load on the other end of the core is known as the secondary winding. The two coil windings are completely separated from one another, but they are magnetically coupled via the common core, allowing electrical power to be transmitted from one coil to the other. To limit the generation of flux, these two coils are sometimes separated into multiple coils.



Insulation is one of the most significant components of transformers. Insulation protects the transformer from several electrical hazards. The most serious damage to transformers can be caused by insulation failures. Insulation is necessarily required in the several parts of a transformer, such as between the windings and the core, between the windings, each turn of the winding, and all current-carrying elements and the tank. The insulators should have high dielectric strength, strong mechanical qualities, and the capacity to endure high temperatures. In transformers, cellulose insulations are typically used to fulfill these conditions. They retain electrical charge when the transformer is turned on, and thus, isolate transformer components that exist at different voltages. It also serves a mechanical role by supporting the windings and aids the transformer’s thermal stability by forming cooling ducts.

Oil Insulation


In several transformers, transformer oil mainly serves three purposes: insulation between the conducting parts, cooling by better heat dissipation, and detection of faults. Oil insulation is often used along with solid cellulose insulation. It is utilized to cover all of the exposed portions that do not have solid insulation. The oil also penetrates the paper and fills air holes, thereby boosting the quality of the paper insulation. Waste heat is dissipated by the transformer windings and must be eliminated. Transformer oil absorbs heat from the windings and conducts it to the transformer’s outside, where it can be dispersed to the outside air. The oil used in transformers is typically obtained through fractional distillation and subsequent treatment of crude petroleum. There are primarily two types: paraffin-based transformer oil and naphtha-based transformer oil; however, due to their superior fire retardant and moisture-absorbing properties, synthetic oils such as silicone oil are becoming popular.

Bushing Terminals

bushing terminals

Typically present in the high voltage transformers, a transformer bushing terminal is an insulating device that allows a current-carrying conductor to pass through the transformer’s grounded tank without making any electrical contact. They are usually made of porcelain or ebonite and look like a column of circular disks. An electric field is generated by all elements that have an electric charge. When an electrified conductor comes close to a grounded material with an earth potential, it can generate extremely strong field lines, especially if the field lines are forced to curve abruptly around the grounded material. Transformer bushing provides effective insulation around the conducting terminal and the grounded tank of the transformer.

Working Principle of a Transformer

The working principle of the transformer is base on Faraday’s law of electromagnetic induction, which states that “The electromotive force around a closed path is equal to the negative of the time rate of change of the magnetic flux enclosed by the path.”In a transformer, when the current is passed through the primary coil, a magnetic field builds up around it. Since the current is alternating, and the coils are near each other, this changing field extends into the secondary coil, thereby inducing a voltage in the secondary. This process is known as Mutual induction, in which a coil of wire magnetically induces a voltage into another coil located in close proximity to it. Furthermore, transformers acquire their name from the fact that they “transform” one level of voltage or current into another. Transformers can change the voltage and current levels of their power supply without changing its frequency or the amount of electrical power passed from one winding to another via the magnetic circuit. The ratio of the number of actual turns of wire in each coil is critical in determining the type of transformer and the output voltage. The output voltage to input voltage ratio is the same as the number of turns between the two windings. The output voltage of a transformer is greater than the input voltage if the secondary winding has more turns of wire than the main winding. This type of transformer is known as a “step-up transformer.”On the contrary, if the secondary winding has fewer turns than the primary winding, the output voltage is lower. This is known as a “step-down transformer”. Mathematically, this concept can be explained as follow:

Suppose there are {N}_{1} turns in the primary winding and {N}_{1} turns in the secondary winding. An alternating emf {E}_{1} is applied across the primary coil, which produces a current {I}_{1} in the primary circuit and {I}_{2} in the secondary circuit. The current in the coils produces magnetization across the core and sets a corresponding magnetic field inside the core. Due to the magnetization of the core, the field is greater as compared to the field set up by the current in the coils alone. This sets up a greater emf {E}_{2} in the secondary coil, which is directly proportional to the emf in the primary coil. The equation representing this relation is given as:

{E}_{2}= {-}\frac{{N}_{2}}{{N}_{1}}{E}_{1}

The minus sign indicates that {E}_{2} is 180° out of phase with {E}_{1}.

Types of Transformer

Although a transformer is a static electrical component, it has become an essential element in the effectiveness of modern electrical as well as electronic devices. For this reason, several varieties of transformers are available today in the market. Let’s take a look at few types of transformers.

Power Transformer


A power transformer is one of the most common types of transformer that one may come across in their daily life. The power transformer, which converts inbound electricity to a higher or lower voltage for a specified purpose, is a key component in the power grid’s voltage supply. These transformers connect step down and step up voltages in distribution networks without any change in frequency during the power transfer. In an electronic system, a power transformer offers a number of AC supplies of various voltages and appropriate current values from the public electricity supply.

Shell Type Transformer


A shell-type transformer is found in several electrical devices of daily life use, such as television, radio, etc. This transformer has a rectangular shape and is made up of three main components: one core and two windings. The primary and secondary windings of this transformer are both coiled on a single limb of the core, generating concentric cylinders of coils, which sets it apart from other transformers. This configuration offers a significant reduction in flux loss throughout the transformer’s operation. Transformers of this sort are often laminated and do not include any oil for insulation.

Core Type Transformer

Core type

A core type transformer is one that has two windings separately coiled on the two or three limbs of the core. Unlike a shell-type transformer, there is a significant gap between the primary and the secondary winding of a core type transformer. The laminations are sliced in L-shaped pieces, and they are stacked alternatively to eliminate the high reluctance at the joints where laminations are adjoined against each other. To limit leakage flux, the primary and secondary windings are interleaved, with half of each winding is arranged side by side or concentrically on the core’s leg. The primary and secondary windings are separated on the core’s limbs for ease of use. Between the core and the bottom winding, there is an insulation layer that protects the transformer for a short circuit. The core type transformer requires more copper conductor than the shell type transformer because the winding is positioned on separate limbs or legs in the core type transformer.

Toroidal Transformer


The toroidal transformers are used in those electronic or electrical devices in which the space is a concerning attribute. Toroidal transformers are power transformers with a toroidal core on which the primary and secondary coils are wound. As the name suggests, they look like a donut-shaped electrical component. When current travels through the primary coil, it causes an electromotive force (EMF) in the secondary winding, which transfers power from the primary coil to the secondary coil. The toroidal transformer’s distinctive structure allows for shorter coils, which reduces resistive and winding losses and improves overall efficiency. Toroidal power transformers are especially well-suited for vital medical equipment and devices, as exceptional efficiency is critical in medical systems that require low leakage currents, noiseless operation, and long-term reliability. Because these transformers are small and light, they can readily be integrated into medical instruments where space and weight are important design factors.



Mostly used in the low voltage range, an autotransformer is a type of transformer that only contains one winding. The prefix “auto” refers to a single-coil functioning independently (Greek for “self”), rather than to any mechanical system. An autotransformer is similar to a two-winding transformer, but the primary and secondary windings are not connected in the same way. The autotransformer works on the same principle as two winding transformers. It works on the premise of Faraday’s Law of Electromagnetic Induction, which states that anytime the magnetic field and conductors are moved closer together, an emf is induced in the conductors. It’s a transformer with some common turns between the primary and secondary coils. The “Common Section” refers to the piece of the winding that is shared by both the primary and secondary windings. The “Series Section” refers to the piece of the winding that is not shared by both the primary and secondary. Two of the terminals are connected to the primary voltage. The secondary voltage is generated by two terminals, one of which is frequently shared with the primary voltage terminal.

Distribution Transformer


A distribution transformer, also known as a service transformer, is used as a final transformation device in the power distribution, with the basic function to transform high voltage into the voltage suitable for end-use, i.e., 240 V or 440V. These transformers come in several sizes and can be categorized based on several factors including the number of phases, mounting location, voltage class, type of insulation, and basic impulse insulation level. They are usually installed where a high-voltage line carrying thousands of volts meets a utility pole supplying a 240 V or 440 V power supply for household or industrial use, respectively.

Instrument Transformer

Instrument Transformers

An instrument transformer is a device that is used in conjunction with measuring instruments to effectively measure high voltage, current, power, energy, or power factor. In other words, instrument transformers are step-down transformers that come with protective relays. They are used to extend the range of AC ammeters or voltmeters. Instrument transformers are mainly of two types: current transformer and potential (voltage) transformer. A current transformer (CT) is a current measuring device that produces a low-level current in its secondary coil, proportional to a high-level current in the primary coil. Because the primary winding is linked in series with the current-carrying conductor supplying a load, the current transformer is frequently referred to as a “series transformer.” On the contrary, voltage or potential transformers are connected in parallel and function as a small load to the source being monitored while maintaining a precise voltage ratio and phase relationship to enable accurate secondary connected metering.

Single-phase Transformer


In a single-phase transformer, there is only one primary and one secondary winding involved that transforms a single-phase AC input to a single-phase AC output of higher or lower voltage; however, input and output remain out of phase. These transformers are often used to supply power for residential lighting, outlets, air-conditioning, and heating elements. Single-phase transformers can be made even more resourceful by splitting the primary and secondary windings in half and rejoining them in series or parallel configurations.

Three-phase Transformer


A three-phase transformer consists of three primary and three secondary windings coiled around a single core. Since three-phase power distribution is more efficient in transferring electricity to far places, three-phase transformers are more often used than single-phase transformers.  Nonetheless, three single-phase transformers can be coupled together to form a three-phase transformer. There are two connection configurations for three-phase power: Delta configuration and Star (Wye-“Y”) configuration. The three conductors in a delta connection are linked end to end in a triangle or delta configuration. In a star configuration, all conductors radiate from the center, indicating that they are all connected at one place. At the same voltage, a three-phase system utilizes less conductor material than equivalent single-phase, two-phase, or direct-current systems. Three circuit wires in a three-phase system carry three alternating currents (of the same frequency) that reach their instantaneous peak values at different times.

Pulse Transformer


In the electronics domain, a pulse transformer is often utilized to generate and transmit high-velocity, constant-amplitude electrical pulses. A pulse transformer is a device that converts the voltage and current of a pulse signal while preserving the original shape of the pulse at the output. Pulse transformers are segregated into two main types based on their application: power pulse transformer and signal pulse transformer. To alter the power-level voltages from one range to another, power pulse transformers are utilized. The fundamental design of these transformers can be either 1-phase or 3-phase, or it can alter depending on the linked winding. Signal pulse transformers are employed for transmitting data from one type of circuit to another via electromagnetic induction. As a result, they are typically employed to increase or decrease the voltage from one surface of a power transformer to another.

Audio Transformers

Audio Transformers

An audio transformer is an electromagnetic device that isolates an input circuit from an output circuit while also filtering a signal passing through it. It is called an audio transformer because it is made to convert the signals that have a frequency spectrum in the audible range, i.e, 20Hz to 20Khz. Since audio transformers work in the audible signal range, where there is a lot of noise from things like mains power and inverting power supplies in the surroundings, they usually feature magnetic shielding to protect their circuits from noise coupling. Audio transformers are available in both step-up and step-down configurations, but instead of being configured to create a specific voltage output, they are mostly used for impedance matching. Audio transformers have several functions in audio devices; for example, increasing the output level of a microphone, decreasing the output level of instruments to match the microphone, etc.

Losses in a Transformer

A “loss,” in any electrical device, can be defined as the difference between input and output power. Theoretically, an ideal transformer does not have any loss of power or energy; however, a transformer, being a static machine, has some energy losses. These losses can be categorized as follow:

Core Loss or Iron Loss

Iron losses are mostly caused by the alternating flux within the transformer’s core. This type of loss is mostly determined by the magnetic characteristics of the material within the transformer’s core, and it can be further classified as:

  • Hysteresis Loss: The reversal of magnetism in the transformer core causes hysteresis loss. This loss largely depends on the core material and is determined by the volume and grade of iron, the frequency of magnetic reversals, and the value of flux density. It can be mathematically calculated by the Steinmetz formula:
    {W}_{h}= η {B}_{max}^{1.6}{ƒV} watts
    Where {W}_{h} is hysteresis loss, η is Steinmetz coefficient of the material, {B}_{max} is the maximum flux density, ƒ is the function of magnetic reversal, and V is the volume of the core.
  • Eddy Current Loss: The primary winding of a transformer receives AC, which creates an alternating magnetizing flux. This flux causes induced emf in the secondary winding. Nonetheless, some of this flux is absorbed by other conducting parts such as the steel core, iron body, or transformer, resulting in induced emf and a small circulating eddy current in those parts. This loss of flux is known as eddy current loss.

Copper Loss

The phrase “copper loss” refers to the loss of energy as heat produced by the transformer’s electrical components. Copper is commonly used in transformer windings. The ohmic resistance offered by the copper wires to the current causes the heat to produce. The mathematical description of the rate at which resistance in a circuit converts electric energy into heat energy is given by Joule’s Law of Heating as follow:

{Q} = {I}^{2}{RT}

Where Q is the heat dissipated, I is current, R is the resistance, and T denotes time.

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