Page 517 - Basic Electrical Engineering

Example 5.10 An electromagnetic-type relay shown in Fig. 5.31 has the

following particulars. The mean length of the flux path through the iron is 20
cm and the length of air gaps is 1 mm each. The exciting coil has 8000 turns

and carries a current of 50 mA when excited. The cross-sectional area of the
2
core is 0.5 cm . The permeability of iron is 500. Calculate the flux density
and the magnetic pull produced in the armature (i.e., on the moving part).

Figure 5.31

Solution:

Total reluctance of the flux path = Reluctance of the path through the iron +
Reluctance of the two air gaps. The length of each of the air gaps, l = 1 mm.
g
We have to take into account 2l as the total air gap to the flux path.
g

Total reluctance