1. MAGNETIC CIRCUITS

2. ELE101 / 102 Dept of E & E Engg, MIT Manipal 2
Terminologies
Magnetic flux (Φ):
No. of magnetic lines of force created in a magnetic circuit
Unit is Weber (Wb)
Flux Density: (B)
No. of magnetic lines of force created in a magnetic circuit per unit
area perpendicular to the direction of flux lines
B = Φ/A
Unit is Weber/sq.m (Tesla)

3. ELE101 / 102 Dept of E & E Engg, MIT Manipal 3
Terminologies
Magneto motive force: (F)
Force which drives or tends to drive the magnetic flux through a
magnetic circuit
MMF = No. of turns*Current = N I
Unit is AT (ampere - Turns)
Magnetic field strength: (H)
The magneto motive force per meter length of the magnetic circuit
H = (N I) / L
Unit is AT / meter

4. ELE101 / 102 Dept of E & E Engg, MIT Manipal 4
Permeability [μ]
A property of a magnetic material which indicates the ability of
magnetic circuit to carry electromagnetic flux.
Ratio of flux density to the magnetizing force, μ = B / H
Unit: henry / meter
Permeability of free space or air or non magnetic material
Relative permeability [μr] :
Terminologies
mH /104 7
0
−
×= πµ
rµµµ 0=

5. ELE101 / 102 Dept of E & E Engg, MIT Manipal 5
Terminologies
Reluctance [S]
It is the opposition of a magnetic circuit to setting up of a
magnetic flux in it.
‘S’ is called the reluctance of the magnetic circuit
Reluctance = mmf / Magnetic flux
Unit: AT / Wb








==






=






===
=====
A
l
;
S
F
A
l
F
F
l
A
Hence;
l
A
Hl
BA
F
HB;HlmmfF;BAFlux
r
r
rr
µµ
µµ
φ
µµ
φ
µµφ
µφ
0
0
00
Swhere

6. ELE101 / 102 Dept of E & E Engg, MIT Manipal 6
Magnetic circuit
The complete closed path followed by any group of magnetic lines
of flux is referred to a magnetic circuit
VI
R
Equivalent electrical circuit

7. ELE101 / 102 Dept of E & E Engg, MIT Manipal 7
Analogy with Electric circuits
Electric circuit Magnetic circuit
Similarities:
Emf, volt mmf, ampere Turn
Current, ampere flux, weber
Resistance, ohm Reluctance, A / Wb
Current density, A / m2
flux density, T or Wb / m2
Conductivity Permeability
Difference:
 Current actually flows flux is created, but does
not flow
 Circuit may be open or Circuit is always closed
closed

8. ELE101 / 102 Dept of E & E Engg, MIT Manipal 8
Leakage flux
Leakage Flux : The flux that does not follow the intended
path in the magnetic circuit
Leakage flux factor λ
Fringing: Effect of spreading out of flux in the air gap
Totalflux(fluxinthe iron path)
Useful flux(flux in the airgap)
=

9. ELE101 / 102 Dept of E & E Engg, MIT Manipal 9
Magnetic circuits losses
Two types of losses:
Hysteresis losses
Eddy current losses

10. ELE101 / 102 Dept of E & E Engg, MIT Manipal 10
B-H Curve & Hysteresis loss
a
b
c
d
Residual
magnetism
Hysteresis loss:
Magnetic hysteresis: Lagging
of magnetization or flux
density behind the
magnetizing force.
Hysteresis loss
= ή (Bmax)1.6
J / m / cycle
Where ή is Steinmetz
hysteresis co-efficient
B-H Curve:

11. ELE101 / 102 Dept of E & E Engg, MIT Manipal 11
Eddy current loss
Eddy current: The varying flux in the Magnetic core
induces emf and hence Current
Eddy current results in Loss of power, Heating of the material