Page 493 - Basic Electrical Engineering

Figure 5.14 An iron-core coil produces more flux than an air-core coil for the same amount of
magnetizing force: (a) iron-core coil; (b) air-core coil; (c) coil with iron core and an air gap

Now, if we make a cut in the magnetic material to create an air gap as
shown in Fig. 5.14 (c), the flux produced for the same ampere turns will be

somewhat less as in the case of Fig. 5.14 (a), because the total reluctance of
the flux path is now increased. The flux has to cross the air gap whose
reluctance is very high as compared to iron. The flux produced will be

calculated as

The magnetic circuits of electrical machines, transformers, electromagnetic
relays, and other electrical equipment are of different shapes and sizes as

shown in Fig. 5.15. The current-carrying coil providing the required ampere
turns are placed at various convenient locations as shown.