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Construction of single-phase transformers

Construction of single-phase transformers

Construction of single-phase transformers- A single-phase transformer consists of two winding, called primary and secondary windings mounted on a magnetic core. To confine flux to a definite path magnetic flux is used. Transformer cores are made from thin sheets (called lamination) of high-grade silicon steel. The laminations reduce eddy-current loss and the silicon steel reduces hysteresis loss. The laminations are insulated from each other by heat resistant enamel insulation coating. L-type and e-type laminations are used. The laminations are built up into stack and the joints in the laminations are staggered to minimize air-gaps (which require large exciting currents). The laminations are tightly clamped.

These are two basic types of transformer constructions, the core type and the shell type.

IDEAL TRANSFORMER:

                An ideal transformer has the following properties:

  1. Its primary and secondary winding resistances are negligible.
  2. The core has infinite permeability(µ) so that negligible mmf is required to establish the flux in the core.
  3. Its leakage flux and leakage inductances are zero. The entire flux is constricted to the core and links both windings.
  4. There are no losses due to resistance, hysteresis and eddy currents. Thus, the efficiency is 100 percent.

                It is to be noted that practical (commercial) transformer has none of these properties in spite of the fact that its operation is close to ideal.

                An ideal iron-core transformer is shown in figure below. It consists of two coils wound in the same direction on a common magnetic core. The winding connected to the supply V1, is called the primary. The winding connected to the load, Z1, is called the secondary.

(Ideal Iron-Core Transformer)

Since the ideal transformer has zero primary and zero secondary impedance, the voltage induced in the primary E1 is equal to the applied voltage V1. Similarly, the secondary voltage V2, is equal to the secondary induced voltage E2. The current I1 drawn from the supply is just sufficient to produce mutual flux ɸM and the required magneto-motive force (mmf) I1,T1, to overcome the demagnetizing effect of the secondary mmf I2,T2, as a result of connected load.

By Lenz’s law E1, is equal and opposite to V1, Since E2, and E1, are both induced by the same mutual flux, E2, is in the same direction’s E1, but opposite to V1, The magnetizing Iµ lags V1 by 90˚ and produces ɸM in phase with Iµ,.E1, and E2 lag ɸM by 90° and are produced by ɸM .V2, is equal in magnitude to E2, and is opposite to V1. Figure below shows the no-load phasor diagram of the ideal transformer.

(No Load phasor diagram of An Ideal Transformer)

For an ideal transformer, if

a = transformation ratio = turn ratio

then, a = T1/T2 = E1/E2 V1/V2 = I2/I1 ……(1)

            I1 T1 = I2 T2                               …….(2)

            E1 I1 = E2 I2 = S2 = S1               ……..(3)

            V1 I1 = V2 I2 = S2 = S1                   ………(4)

Equation (2) states that the demagnetizing ampere turns of the secondary are equal and also opposite to the magnetizing mmf of the primary of an ideal transformer.

Equation (3) shows that the volt-amperes (apparent power) drawn from the primary supply is equal to the volt-amperes (apparent power) transferred to the secondary without any loss in and ideal transformer. In other words,

input volt-amperes = output volt-amperes

Also,      V1I1/1000 = V2I2/1000

                (kVA)1 = (kVA)2   ………….(5)

Or,

input kilovolt amperes output kilovolt amperes

                Thus, the kVA input of an ideal transformer is equal to the kVA output. That is, kVA is the same on both the sides of the transformer.

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