Air gap (magnetic)

Air gap in magnetic circuits is a term used to define an intentional gap left in the magnetic material.
In stationary devices, like inductors and transformers, the air gap is used for a few purposes:

to minimize the magnetic saturation of their cores due to the direct current that might be flowing through the coils. Without saturation the inductance of a choke stays constant regardless of the DC current flowing;
counter-intuitively, if a DC magnetization is present in an inductor, an increased air gap actually incrementally increases the effective inductance;
in a shunt reactor an air gap is used for two reasons:
* with an ungapped core the reluctance is small, so very little reactive power is obtained with the disproportionate effect of the iron loss;
* an increase of the gap reduces the ratio of the total loss to the reactive power, with the limiting factor being the increased heating due to the copper loss.

The total gap is frequently made of a series of small gaps to limit the effect of eddy currents in the core.
When one of the circuit-forming parts of the machine is moving with respect to another, the gap is an obvious mechanical necessity and is typically detrimental to the performance of the machine, since extra power is required to overcome the added reluctance. However, a larger air gap in a synchronous generator is associated with higher short circuit ratio, an often desirable trait.