What is a Single Phase Transformer : Construction and Its Working

A single-phase transformer is an electrical device that accepts single-phase AC power and outputs single-phase AC. This is used in the distribution of power in non-urban areas as the overall demand and costs involved are lower than the 3-phase distribution transformer. They are used as a step-down transformer to decrease the home voltage to a suitable value without a change in frequency. For this reason, it is commonly used to power electronic appliances at residences. This article discusses an overview of a single-phase transformer.

What is a Single Phase Transformer?

Definition: A transformer is a device which converts magnetic energy into electrical energy. It consists of two electrical coils called as a primary winding and secondary winding. The primary winding of a transformer receives power, while the secondary winding delivers power. A magnetic iron circuit called “core” is commonly used to wrap around these coils. Though these two coils are electrically isolated, they are magnetically linked.


An electric current when passed through the primary of a transformer then a magnetic field is created, which induces a voltage across the secondary of a transformer. Based on the type of application, the single-phase transformer is used to either step-up or step-down the voltage at the output. This transformer is typically a power transformer with high-efficiency and low losses. The single-phase transformer diagram is shown below.

Single-Phase-Transformer
single-phase-transformer

Principle of Single Phase Transformer

The single-phase transformer works on the principle of Faraday’s Law of Electromagnetic Induction. Typically, mutual induction between primary and secondary windings is responsible for the transformer operation in an electrical transformer.

Working of Single Phase Transformer

A transformer is a static device that transfers electric power in one circuit to another circuit of the same frequency. It consists of primary and secondary windings. This transformer operates on the principle of mutual inductance.

When the primary of a transformer is connected to an AC supply, the current flows in the coil and the magnetic field build-up. This condition is known as mutual inductance and the flow of current is as per the Faraday’s Law of electromagnetic induction. As the current increases from zero to its maximum value, the magnetic field strengthens and is given by dɸ/dt.

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This electromagnet forms the magnetic lines of force and expands outward from the coil forming a path of magnetic flux. The turns of both windings get linked by this magnetic flux. The strength of a magnetic field generated in the core depends on the number of turns in the winding and the amount of current. The magnetic flux and current are directly proportional to each other.

Working-of-Single-Phase-Transformer
working-of-single-phase-transformer

Source: Wikimedia

As the magnetic lines of flux flow around the core, it passes through the secondary winding, inducing voltage across it. The Faraday’s Law is used to determine the voltage induced across the secondary coil and it is given by:

N. dɸ/dt

where,

‘N’ is the number of coil turns

The frequency is the same in primary and secondary windings.

Thus, we can say that the voltage induced is the same in both the windings as the same magnetic flux links both the coils together. Also, the total voltage induced is directly proportional to the number of turns in the coil.

Let us assume that the primary and secondary windings of the transformer have single turns on each. Assuming no losses, the current flows through the coil to produce magnetic flux and induce voltage of one volt across the secondary.

Due to AC supply, magnetic flux varies sinusoidally and it is given by,

ɸ = ɸmax Sin ωt

The relationship between the induced emf, E in the coil windings of N turns is given by,

E=N (d∅)/dt

E=N*ω*ɸmax cosωtφ

Emax=Nωɸmax

Erms=Nω/√2*ɸmax=2π/√2*f*N*ɸmax

Erms=4.44 fNɸmax

Where,

‘f’ is the frequency in Hertz, given by ω/2π.

‘N’ is  the number of coil windings

‘ɸ’ is s the amount of flux in Webers

The above equation is the Transformer EMF Equation. For emf of a primary winding of a transformer E, N will be the number of primary turns (NP), while for the emf, E of a secondary winding of a transformer, the number of turns, N will be (NS).

Construction of Single Phase Transformer

A simple single-phase transformer has each winding being wound cylindrically on a soft iron limb separately to provide a necessary magnetic circuit, which is commonly referred to as “transformer core”. It offers a path for the flow of the magnetic field to induce voltage between two windings.

As seen in the figure above, the two windings are not close enough to have an efficient magnetic coupling. Thus, converging and increasing the magnetic circuit near the coils can enhance the magnetic coupling between primary and secondary windings. Thin steel laminations shall be employed to prevent power losses from the core.

Based on how the windings are wound around the central steel laminated core, the transformer construction is divided into two types

Core-type Transformer

In this type of construction, only half of the windings are wound cylindrically around each leg of a transformer to enhance magnetic coupling as shown in the figure below. This type of construction ensures that magnetic lines of force flow across both the windings simultaneously. The main disadvantage of the core-type transformer is the leakage flux that occurs due to the flow of a small proportion of magnetic lines of force outside the core.

Core-Type-Transformer
core-type-transformer

Shell-type Transformer

In this type of transformer construction, the primary and secondary windings are positioned cylindrically on the center limb resulting in twice the cross-sectional area than the outer limbs. There are two closed magnetic paths in this type of construction and the outer limb has the magnetic flux ɸ/2 flowing. Shell type transformer overcomes leakage flux, reduces core losses and increases efficiency.

Shell-Type-Transformer
single-phase-transformer-shell-type

Applications

The applications of a single-phase transformer are mentioned below.

  • To step-down long-distance signals to support both residential and light-commercial electronic devices
  • In television sets for voltage regulation
  • To step-up power in home inverters
  • To supply power to non-urban areas
  • To isolate two circuits electrically as primary and secondary are placed far from each other

FAQs

1). What does a single-phase mean?

A single-phase system or circuit which generates or uses single alternating voltage

2). Do houses use single-phase supply?

Generally, homes are supplied with single-phase supply

3). On which principles does the single-phase transformer operate?

Faraday’s Law of Electromagnetic Induction and Mutual Induction

4). What is the transformer “Turns Ratio”?

NP/NS = VP/VS = n = Turns Ratio

5). Give two uses of a single-phase transformer

  • In television sets for voltage regulation
  • To step-up power in home inverters

Thus, a single-phase transformer is appropriate for lighter electrical devices. It is less expensive and highly preferred to supply power to non-urban areas. This article emphasizes the transformer working principle, construction, and applications of a single-phase transformer. The reader can learn in-depth about single-phase transformer from this article.

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