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 (DC) that might be flowing through the
coils.[1] Without saturation the inductance (and thus the blocking capability) of a
choke stays constant regardless of the DC current flowing;[2]
counter-intuitively, if a DC magnetization is present in an inductor, an increased (up to some limit) air gap actually incrementally increases the effective
inductance;[3]
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.[5]
When one of the circuit-forming parts of the machine is moving in respect to another (for example, the
rotor of an
alternator or motor rotates while the
stator is stationary), 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.[1] However, a larger air gap in a
synchronous generator is associated with higher
short circuit ratio, an often desirable trait.[6]