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    Single Phase Transformer – Construction and Working Principle

    Single Phase Transformer – Construction and Working Principle - Construction of Single Phase TransformerA single phase transformer consists of two winding ...

    Single Phase Transformer – Construction and Working Principle

    ElectronElectronics & ElectricalDigital Electronics

    Construction of Single Phase Transformer

    A single phase transformer consists of two windings viz. primary winding and secondary winding put on a magnetic core. The magnetic core is made from thin sheets (called laminations) of high graded silicon steel and provides a definite path to the magnetic flux. These laminations reduce the eddy-current losses while the silicon steel reduces the hysteresis losses.

    The laminations are insulated from each other by enamel insulation coating. The thin laminations are stacked together to form the core of the transformer. The air-gap between the laminations should be minimum so that the excitation current being minimum.

    For a single phase transformer, there are two types of transformer constructions viz. the core type and the shell type.

    Core Type Transformer Construction

    In core type construction of the transformer, the magnetic circuit consists of two vertical lags (called limbs) and two horizontal sections called yokes. To minimise the effect of leakage flux, half of each winding is placed on each limb (see the figure).

    The low-voltage winding is placed next to the core while the high-voltage winding over the low-voltage winding to reduce the insulation requirements. Therefore the two windings are arranged as concentric coils and known as cylindrical winding.

    The laminations of the core type transformer are of U-I shape as shown in the figure.

    Shell Type Transformer Construction

    In the shell type construction of transformer, the magnetic circuit consists of three limbs, both the primary and secondary windings are placed on the central limb and the two outer limbs complete the low reluctance flux path. The each winding is sub-divided into sections viz. the low voltage (LV) section and the high-voltage (HV) section, which are alternatively put one over the other in the form of sandwich (see the figure). Therefore, such windings are called sandwich winding or disc winding.

    The core of the shell type transformer is made up either U-T shape or E-I shape (see the figure).

    Working Principle of Single Phase Transformer

    The working of the transformer is based on the principle of mutual inductance between two coils wound on the same magnetic core.

    When an alternating voltage (V1) is applied to the primary winding, an alternating magnetic flux (Φm) sets up in the core and links with the secondary winding, i.e. the magnetic flux links both the windings of the transformer magnetically. This magnetic flux induces EMF E1 in the primary winding and E2 in the secondary winding according to Faraday’s law of electromagnetic induction.

    According to Lenz’ law,

    $$\mathrm{Primary\:EMF,E_{1}=-N_{1}\frac{d\phi_{m}}{dt}\:\:\:\:\:...(1)}

    SecondaryEMF, E 2 =− N 2 d ϕ m dt ...(2)

    SecondaryEMF,E2=−N2dϕmdt...(2)

    Therefore, E 2 E 1 = N 2 N 1 ...(3) E2E1=N2N1...(3)

    From the above equations, it is clear that the induced EMFs in the primary and secondary windings depends upon the number of turn of the winding.

    If 𝑁1 > 𝑁2, then 𝐸1 > 𝐸2 i.e. the primary EMF is greater than the secondary EMF, the transformer is called as step-down transformer.

    If 𝑁2 > 𝑁1, then 𝐸2 > 𝐸1 i.e. the primary EMF is less than the secondary EMF, the transformer is called as step-up transformer.

    If a load is connected across the terminals of the secondary winding, the secondary EMF causes a current I2 to flow through the load. In this way, a transformer transfers AC power from one circuit to another circuit with a change in voltage level without any electrical connection between both the circuits i.e. the power from input circuit to output circuit transfers magnetically. During this transfer of electrical power, the frequency does not change.

    Manish Kumar Saini

    Updated on 05-Jul-2021 09:21:29

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    Single Phase Transformer : Working Principle, Construction & Applications

    This Article Provides a Clear Insights on Single Phase Transformer, Its Operation, Working Principle and Its Various Applications

    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

    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.

    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

    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∅)/dtE=N*ω*ɸmax cosωtφEmax=NωɸmaxErms=Nω/√2*ɸmax=2π/√2*f*N*ɸmaxErms=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.

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    Construction of single Phase transformer

    Construction of single Phase transformer with Examples on Electrical Machine, Single Phase Transformer, Construction of single Phase transformer, Losses in transformer, Open-Circuit and Short-Circuit Test, Three-Phase Transformer, Auto transformer etc.

    Construction of Single - Phase Transformers

    A single-Phase transformer consists of primary and secondary winding. The core of the transformer is made of thin sheets (called laminations) of high grade of silicon. These laminations are provided in the transformer to reduce eddy-current loss, and the silicon steel reduces hysteresis loss. The laminations present in the transformer are insulated from one another by heat resistant enamel coating. L - Type and E- type laminations are used for constructions.

    There are two basic types of transformer constructions:

    Core type construction.

    Shell type construction.

    Core type Construction

    In the core type transformer, the magnetic circuit consists of two vertical legs or limbs with two horizontal sections, called yokes. To minimize the leakage flux, half of each winding is placed on each leg of the core. The low voltage winding is placed next to the core, and the high voltage winding is placed around the low voltage winding to reduce the insulating material required. Thus, the two winding are arranged as concentric coils. Such type of winding is called as concentric winding or cylindrical winding.

    Shell type Construction

    In the shell type transformer, both the primary and secondary winding are wounded on the central limb, and the low reluctance path is completed by the outer limbs. Each winding is subdivided into sections. Low voltage (lv) and High voltage (hv) subsections are alternatively placed in the form of sandwich that is why this winding is also called sandwich or disc winding.

    The core is made up of two types of laminations. The laminations for the core type are U, and I shaped. Firstly the U- shaped laminations are stacked together for the required length. Half of the prewound low voltage coil is placed around the limbs. The lv coil is further provided with insulation. Then half of the prewound hv coil is placed around the lv coil. The core is then closed by the I-shaped laminations at the top.

    Ideal Transformer

    An Ideal transformer is an imaginary transformer which has the following properties:

    Its primary and secondary winding resistances are negligible.

    The core has infinite permeability (µ) so that negligible mmf is required to establish the flux in the core.

    Its leakage flux and leakage inductances are zero. The entire flux is confined to the core and links both windings.

    There are no losses due to resistance, hysteresis and eddy currents. Thus, the efficiency is 100 percent.

    Figure: Ideal iron-core transformer

    The transformer that has zero primary and zero secondary impedance, is called an ideal transformer. The applied voltage V1 in the primary is equal to the induced voltage E1. Similarly, the induced voltage E2 is equal to the output voltage V2 of the secondary.

    Figure: No-load Phasor diagram of an ideal transformer.

    For an ideal transformer, if a = transformation ratio = turn ratio

    Then,

    The equation-2 states that the demagnetizing, ampere-turns of the secondary are equal and opposite to the magnetizing mmf of the primary of an Ideal transformer.

    Applications of Transformers

    The level of voltage and current can be changed in electrical power systems.

    Transformer known as instrument transformer is used to measure the voltage and current.

    In combined ac/dc power systems, the transformers are used to convert hvac to hvdc.

    To isolate one circuit from another, since primary and secondary are not connected.

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