Question

Why the primary and secondary coils of a transformer are preferably wound on the same core?

Answer 1

For a single phase power transformer on a two-limb transformer as shown in the drawing below, it is usual to split the primary and secondary windings into two halves. Half the primary and half the secondary are placed on one winding, and the other halves are placed on the other winding. These windings are later connected in series and/or parallel, depending on the design.



The reason for this is to achieve tight coupling between the primary and secondary on each winding. This tight coupling reduces leakage flux and inductance between primaries and secondaries, which gives less voltage drop and tighter regulation when load is applied to the secondary. It also ensures that power dissipated in each set of windings is evenly balanced.

One exception to this may be in a step-up or step-down transformer with a high voltage difference between primary and secondary. Here the primary may be placed on one limb and the secondary on the other limb, so to give maximum electrical clearance between primary and secondary.

Another exception may be if there is a need for a controlled amount of inductance between the primary and secondary windings, such as in some switchmode power supply designs. The core for such a transformer is likely to have a gap placed in it.

Answer 2

The primary and secondary coils of a transformer are preferably wound on the same core so as to obtain tight coupling between the primary and secondary on each winding.

Due to this tight coupling flux leakage and inductance between primaries and secondaries is reduced, which gives less voltage drop and tighter regulation when load is applied to the secondary.

It also ensures that equal amount of power dissipated in each set of windings.