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