What is the function of air gap in transformer? What is the effect of adding air gap to 50HZ transformer? Has the ratio changed?

The air gap increases the reluctance of the magnetic core. Transformers generally don't need air gap to make the magnetic resistance as small as possible; Reactor products use reluctance principle to increase excitation current, so the reactor has air gap to obtain inductance or reactance value. The air gap has a certain influence on the transformation ratio, which will make the design value deviate.