Question

How do the leakage flux effect the operation of a transformer

How are they minimised

In the preceding discussion, it has been assumed that all the flux linked with primary winding also links the secondary winding. But, in practice, it is impossible to realize this condition.

It is found, however, that all the flux linked with primary does not link the secondary but part of it i.e. Φ_L1 completes its magnetic circuit by passing through air rather than around the core, as shown in Fig a

Fig a

This leakage flux is produced when the m.m.f. due to primary ampere-turns existing between points a and b, acts along the leakage paths.

Hence, this flux is known as primary leakage flux and is proportional to the primary ampere-turns alone because the secondary turns do not link the magnetic circuit of Φ_L1. The flux Φl1 is in time phase with /,. It induces an e.m.f. eL1in primary but none in secondary.

Similarly, secondary ampere-turns (or m.m.f.) acting across points c and d set up leakage flux Φl₂ which is linked with secondary winding alone (and not with primary turns).

This flux Φl₂ is in time phase with I₂ and produces a self-induced e.m.f. in secondary (but none in primary).

At no load and light loads, the primary and secondary ampere-turns are small, hence leakage fluxes are negligible.

But when load is increased, both primary and secondary windings carry huge currents.

Hence, large m.m.f.s. are set up which, while acting on leakage paths, increase the leakage flux.

As said earlier, the leakage flux linking with each winding, produces a self-induced e.m.f. in that winding.

Hence, in effect, it is equivalent to a small choker or inductive coil in series with each winding such that voltage drops in each series coil is equal to that produced by leakage flux.

In other words, a transformer with magnetic leakage is equivalent to an ideal transformer with inductive coils connected in both primary and secondary circuits as shown in Fig. b

Fig b

Such that the internal e.m.f. in each inductive coil is equal to that due to the corresponding leakage flux in the actual transformer.

Following few points should be kept in mind :

(1) The leakage flux links one or the other winding but not both, hence it in no way contributes to the transfer of energy from the primary to the secondary winding.

(2) The primary voltage V₁ will have to supply reactive drop I₁X₁ in addition to I₁R₁ Similarly :-E_b will have to supply I₂R₂ and I₂X₂.

(3) In an actual transformer, the primary and secondary windings are not placed on separate legs or limbs as shown in Fig. b because due to their being widely separated, large primary and secondary leakage fluxes would result.

These leakage fluxes are minimised by sectionalizing and interleaving the primary and secondary windings as in Fig. b