1. MAGNETIC CIRCUITS
- a review -

2. Ampere’s law
Line integral of magnetic field intensity H around a closed path is equal to the total current enclosed by the path i r H r

3. B-H Relation B exists whenever there is H
The strength of B depends on the permeability of the medium
Wb/m2 or T
o = permeability of free space (4 10-7 H/m)
r = relative permeability of medium
 1 for air
Several hundreds to thousands for ferromagnetic materials

4. Magnetic equivalent circuit
N turns l r i

5. Magnetic equivalent circuit
N turns r l
= F A
= magnetomotive force i 
Magnetic flux density inside the core,
The flux crossing the cross section area A of the core, Wb

6. Magnetic equivalent circuit
Magnetomotive force
Reluctance F  R

7. Magnetic equivalent circuitlc Rc Rg lg N i
if Rg>> Rc

8. Magnetic equivalent circuit
In electrical machine with ferromagnetic material, c >> o
It is permissible to assume that ALL MMF drop appear across the air gap
Since magnetic path crosses the airgap twice, half of the MMF appear at each air gap
if Rg>> Rc

9. Magnetic equivalent circuit a’  
/2
-/2 -
Ni / 2
-Ni / 2