Electrical Transformer related questions with answers #3

1.What are the properties of transformer oil?

Ans.

    ·      High dielectric strength

·      High resistivity and density

·      Low viscosity

·      Higher flash point

·      Low impurity

·      Reasonable cost

·      Less sludge formation.

 

2. Give an expression for the heating time constant of transformer.

Ans.

The heating time constant of transformer is given as below:

Th = G h/S λ

            Where G is weight

             h is specific heat

             λ is the specific heat dissipation.

 

3. How is leakage reactance of winding estimated?

Ans.

            It is estimated by primarily estimating the distribution of leakage flux and the resulting flux leakages of the primary and the secondary windings. The distribution of the leakage flux depends upon the geometrical configuration of the coils and the neighbouring iron masses and also on the permeability of the iron.

 

4. What are the assumptions made for calculation of leakage flux and leakage reactance ?

Ans.

·      The primary and secondary windings have an equal axial length

·      The flux paths are parallel to the windings along the axial height

·      Primary winding mmf is equal to secondary winding mmf

·      Half of the leakage flux in the duct links with each winding

·      The length of the mean turn of the windings are equal

·      The reluctance of flux path through yoke is negligible.

·       The effect of magnetising current is neglected.

 

5. In transformers, why the low voltage winding is placed near the core?

Ans.

        The winding & core are both made of metals and so insulation has to be placed in between them. The thickness of insulation depends on the voltage rating of the winding. In order to reduce the insulation requirement the low voltage winding is placed near the core.

 

6. What are the disadvantages of stepped cores?

Ans.

           With large number of steps a large number of different sizes of laminations have to be used. This results in higher labour charges for shearing and assembling different types of laminations.

 

7. What type of steel is commonly used for the core of transformer?

Ans.

         The hot rolled and cold rolled silicon steel with 3 to 5% silicon are used for the laminations of the core of transformers. The hot rolled silicon steel allows a maximum flux density of 1.45 Wb/m2 and the cold rolled silicon steel permits a maximum flux density of 1.8 Wb/m2.