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After this presentation you will be able to:
Explain the dc circuit analogy to magnetic
circuits
 Represent a magnetic circuit using
reluctances and MMF sources.
 Combine series and parallel reluctance
values to find total reluctance
 Perform calculations to find flux distributions
using dc circuit analogy.

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Riron
Rair gap
I
Length of air
gap=Lag
N
F
V
Length of
Iron=Liron
Pictorial representation
Rag
L ag
0
Riron
A
L iron
iron A
3
I
V
Riron
N
F
Rair gap
F = NI
Electric circuit analogy
Voltage source and coil form MMF source
Iron in magnetic structure has reluctance
Air gap has reluctance
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Problem Statement
N = 200 turns
Core material:
Silicon Iron (1%)
Core cross-sectional
area, A = 0.1 m2
B in air gap 0.8 T
Total core length
L = 50 cm
Air gap length = 0.5 cm
Assume Core R negligible: Find Hair,
, Rair Ftot
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Example Solution
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Example Solution (continued)
Permeability is constant in the air gap
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Example Solution (continued)
Computing reluctance in air gap
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Example Solution (continued)
Computing total MMF
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Problem Statement
N = 200 turns
Core material:
Silicon Iron (1%)
Core cross-sectional
area, A = 0.1 m2
B in air gap 0.8 T
Total core length
L = 50 cm
Air gap length = 0.5 cm
Include the effects of the iron core: Find I, Hair, Hiron
Rair , Riron ,Ftot
,
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Example Solution
Lag
Lag
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Example Solution (continued)
Include effects of iron core
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Example Solution (continued)
Include effects of iron core
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Example Solution (continued)
Include effects of iron core
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Example Solution (continued)
Include effects of iron core
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corresponds to I
F corresponds to E
R corresponds to R
So this gives an “Ohms Law” for magnetic circuits
F
R
All laws and principles from dc circuits are
analogous to magnetic circuits
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Combining Reluctances in series and parallel
Rseries R1 R2 R3 ... Rn
Rparallel
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1
R1
1
R2
1
1
...
R3
Rn
Other rules: Sum of MMFs around loop must be zero (KVL)
Sum of fluxes entering node must equal zero (KCL)
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ET 332a
Dc Motors, Generators and
Energy Conversion Devices
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