The Use of an Air Gap

Author: N.H. Crowhurst

With a good transformer core, very little current is needed to produce saturation. This means that passing the plate current through the primary winding will saturate the core unless some means is used to prevent it. Usually, an air gap is used for this purpose. It very considerably reduces the magnetization produced by any current in the primary winding, but it also reduces the primary inductance. The air gap has to be adjusted so that its effect in avoiding saturation is better than its effect in reducing primary inductance.

An air gap prevents the plate current that flows through from causing core saturation

Using too small an air gap, or no air gap at all, means the primary current will cause the core to saturate and thus the inductance will practically disappear. Using too large an air gap amounts to making the transformer almost air cored, which again will result in a very low inductance. There is always an optimum size for the air gap, which will result in the biggest practical value of inductance for a transformer that carries direct current in one of its windings.