MAGNETIC CIRCUITS

ELE101 / 102 Dept of E & E Engg, MIT Manipal 2

Terminologies

Magnetic flux (Φ):No. of magnetic lines of force created in a magnetic circuitUnit 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 linesB = Φ/AUnit is Weber/sq.m (Tesla)

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 circuitMMF = No. of turns*Current = N IUnit is AT (ampere - Turns)

Magnetic field strength: (H)The magneto motive force per meter length of the magnetic circuit H = (N I) / LUnit is AT / meter

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 70

r 0

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 circuitReluctance = mmf / Magnetic flux

Unit: AT / Wb

A

l;

S

F

A

l

F

Fl

AHence;

l

A

Hl

BA

F

HB;HlmmfF;BAFlux

r

r

rr

0

0

00

Swhere

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

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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

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

Total flux(flux in the iron path)

Useful flux(flux in the airgap)

ELE101 / 102 Dept of E & E Engg, MIT Manipal 9

Magnetic circuits losses

Two types of losses:

Hysteresis losses

Eddy current losses

ELE101 / 102 Dept of E & E Engg, MIT Manipal 10

B-H Curve & Hysteresis loss

a

b

cd

Residualmagnetism

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:

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