Electrical Engineering ⇒ Topic : Air Gaps in Magnetic Circuits

Air Gaps in Magnetic Circuits

In many practical magnetic circuits, air gap is indispensable. For example, in electromechanical conversion devices like electric motors and generators, the magnetic flux must pass through stator as well as rotor. This necessitates to have a small air gap between the stator and rotor to permit mechanical clearance.

                                                                  figure (a)

The magnitude of AT required for air gap is much greater than that required for iron part of the magnetic circuit. It is because reluctance of air is very large compared to that offered by iron. Consider a magnetic circuit of uniform cross-sectional area a with an air gap as shown in Fig.(a). The length of the air gap is l_g and the mean length of iron part is I_t,. The flux density B = (Φ/a) is constant in the magnetic circuit.

Now relative permeability μ_r of iron is very high (> 6000) so that reluctance of iron part is very small as compared to that of air gap inspite of the fact that  I_t>l_g . In fact, most of ampere-turns (AT) are required in a magnetic circuit to force the flux through the air gap than through the iron part.

In some magnetic circuits, we neglect reluctance of iron part compared to the air gap/gaps. This assumption leads to reasonable accuracy.