Question

Which of the following statements is true:
Statement ( A ) : Flux leakage in a transformer can be minimized by winding the primary and secondary coils one over the other.
Statement ( B ) : Core of the transformer is made of soft iron
(A) A is true but B is false
(B) Both A and B are true
(C) A is false but B is true
(D) Both A and B are false

Answer 1

Hint:By winding the primary and secondary coils one over the other, it helps most of their flux to take part in main flux without getting leaked.Soft iron has low coercivity and high retentivity, which makes it the best material for making a transformer core.

Complete step by step answer:
Let us first understand what is leakage flux in transformers.
Leakage Flux in Transformer:
In an ideal transformer, all the flux is linked with both the primary and secondary windings but in practical cases, it is impossible to link all the flux in the transformer with both the primary and secondary windings. Although maximum flux will be linked with both the windings through the core of the transformer but still there will be a small amount of flux which will be linked either one of the windings but not both together. This flux which doesn’t get linked to the main flux through the transformer core is called leakage flux.

On interwinding the two windings i.e. the primary and secondary coils are winded one over the other, the distance between the two windings is reduced which helps in maximizing the flux interaction of the two windings. Hence, mutual flux of the two windings is increased by bringing them in close proximity and leakage flux is reduced but not zero.Therefore, statement A is true.
Soft iron has high permeability so it provides complete linkage of magnetic flux of the primary coil to the secondary coil. It has high coercivity and low retentivity and the hysteresis loss of energy in soft iron is less. Therefore, it is the best material for making the core of a transformer.Therefore, statement B is true.
Hence,both the statements A and B are true.

Note:Coercivity: The minimum value of magnetizing intensity that is required to bring the material to its original state is defined as coercivity. When in a ferromagnetic substance we apply an increasing magnetic field intensity in reverse direction, then at some point ${H_c}$, the substance loses its magnetizing properties. This reverse magnetic field intensity ${H_c}$ is called coercivity.
Retentivity: The ability of the magnetic field to remain in the magnetic material even after removing the external source is known as retentivity.