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Chapter 12: Magnetic circuits
• Magnetomotive Force– The “driving force” that causes a magnetic field– Symbol, F – Definition, F = NI– Units, Ampere-turns, (A-t)
• Flux, – Total number of lines
• Magnetic Field Intensity/ Magnetizing force/ magnetic field strength It is the magnetomotive force per unit length at any given point.
Symbol, H
Definition, H = F/l = NI/lUnits, (A-t/m)
• Flux Density
The concentration of the lines of force in a magnetic circuit (Number of lines per unit area)
Symbol, BDefinition, B = Φ/AUnits, (Wb/m2), or T (Tesla)
• Reluctance
The measure of “opposition” the magnetic circuit offers to the fluxThe analog of Resistance in an electrical circuit
Symbol, RDefinition, R = F/ΦUnits, (A-t/Wb)
• Permeability
Measure for establishing flux in a material. Relates flux density and field intensity ()
Symbol, μDefinition, μ = B/HUnits, (Wb/A-t-m)
Permeability of free space (air) - μ0
μ0 = 4πx10-7 Wb/A-t-m
• Relative Permeability
Compares permeability of material with the permeability of free space (air)
Symbol, μr
μr = μ/μ0 Dimensionless
B-H Curves
B-H Curves for Sheet steel, Cast steel, and Cast Iron
Principal Sections of a B-H Curve
F = MMF is analogous to Electromotive force (EMF) = E
= Flux is analogous to I = Current
= Reluctance is analogous to R = Resistance
P = Permeance
1
= Analogous to conductance R
1G
Analogy Between Magnetic and Electric Circuits
Ampere’s Circuital Law
Algebraic sum of mmfs’ around a closed loop in a magnetic circuit
= 0Similar to KVLSince = NI, NI = HN1I1 + N2I2 = Hironiron + Hsteelsteel + Hgg
Series Magnetic Circuits
Solve a circuit where is known –
First compute B using /ADetermine H for each magnetic section from B-H curvesCompute NI using Ampere’s circuital lawUse computed NI to determine coil current or turns as required
Series-Parallel Magnetic Circuits
• Use Ampere’s Law• Find B and H for each section• Calculate the required entity
Home Work
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