Magnetic Circuits and Examples EE201 Denard Lynch Page 1 of 13 Sep 3, 2009 Review of Magnetism Originates at the molecular level: •electrons orbiting a nucleus induce a magnetic “moment” •each “orbit induces its own separate moment •depending on particular molecule, can have a net magnetic moment (i.e. ferromagnetic) This implies and illustrates a fundamental relationship between electricity and magnetismTwo important and related facts: •a moving charge generates a magnetic field, and although not obvious from the above… •relative motion between a magnetic field and a charge exerts a force on the charge(s) (and on that which contains them; i.e. a conductor) So what is this “magnetic field”? •a force field •visualized as “lines of magnetic flux” that appe ar to follow rules: oform closed loops from one “pole” to anotherodirectional (by convention, from north to south p ole of magnet) orepel each other * ofollow the path of least resistance* o*these last two rules conflict with one another! •Remember… the field consists of continuous lines, but are really a visual aid •Magnetic flux can also be visualized as flowing, similar to water or electric current (symbol: Φ ) •We are often interested in its density (symbol B) Depending on the material, the magnetic moments from each set of orbiting electrons in a molecule can reinforce or cancel each other to give the molecule a net magnetic moment. Various molecular arrangements lead to three types of materials: oParamagnetic – only very slightly receptive to a magnetic field (e.g. air, glass, wood, paper, plastics etc.) oDiamagnetic (anti-magnetic) – form opposite dipoles in response to an imposed magnetic field (bismuth, pyrolytic graphite), but only a weak response oFerromagnetic – net magnetism at the molecular leve l; get together into polarized “domains”. They are normally ori ented randomly, but may be aligned temporarily orpermanently. (E.g. iron, cobalt etc.) We are interested in this last category because it is relatively easy * to establish magnetic flux, Φ, in these materials in response to a magnetic field. •this “relative ease” is somewhat temperature dependent. Each material has a “Curr ie temperature”, above which they behave like paramagnetic materials.
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• electrons orbiting a nucleus induce a magnetic “moment”
• each “orbit induces its own separate moment
• depending on particular molecule, can have a net magneticmoment (i.e. ferromagnetic)
This implies and illustrates a fundamental relationship between
electricity and magnetism
Two important and related facts:
• a moving charge generates a magnetic field, and although not obvious from the above…
• relative motion between a magnetic field and a charge exerts a force on the charge(s)
(and on that which contains them; i.e. a conductor)
So what is this “magnetic field”?
• a force field• visualized as “lines of magnetic flux” that appear to follow rules:
o form closed loops from one “pole” to another
o directional (by convention, from north to south pole of magnet)
o repel each other *
o follow the path of least resistance*
o *these last two rules conflict with one another!
• Remember… the field consists of continuous lines, but are really a visual aid
• Magnetic flux can also be visualized as flowing, similar to water or electric current
(symbol:Φ)
• We are often interested in its density (symbol B)
Depending on the material, the magnetic moments from each set of orbiting electrons in amolecule can reinforce or cancel each other to give the molecule a net magnetic moment.
Various molecular arrangements lead to three types of materials:
o Paramagnetic – only very slightly receptive to a magnetic field (e.g. air, glass, wood,
paper, plastics etc.)
o Diamagnetic (anti-magnetic) – form opposite dipoles in response to an imposed
magnetic field (bismuth, pyrolytic graphite), but only a weak response
o Ferromagnetic – net magnetism at the molecular level; get together into polarized“domains”. They are normally oriented randomly, but may be aligned temporarily or
permanently. (E.g. iron, cobalt etc.)
We are interested in this last category because it is relatively easy* to establish magnetic flux,
Φ, in these materials in response to a magnetic field.
• this “relative ease” is somewhat temperature dependent. Each material has a “Currietemperature”, above which they behave like paramagnetic materials.
Why is this (ferromagnetic) ability of interest to us?
So we can design “magnetic circuits” to route flux in certain ways and to certain places; thesame way we route electric current or hydraulic fluid. We route this to certain places for one of
two reasons:
1) The force it can provide
a.
Speakers, sound reproduction b. Motors, generators
c. Door bells, electromagnets2) its magnetizing effect
a. HDDs, magnetic tapes
b. Transformers, inductors
How can we create a magnetic field, or a magnet?
Back to moving charges…
Moving charges, like a current flowing in a conductor, create a magnetic field. The induced“lines of flux” are formed in concentric circles in a plane perpendicular to the direction of
current flow.
The direction of the flux, Φ, can be determined using a “right-hand-rule” (RHR)
Note: diagrammatic convention: into page/board = ⊗; out of page/board =
Looking at a current carrying conductor from one end, the density of the magnetic field, B, is
If we route current through a cylindrical coil of wire, we will end up with a fairly uniformmagnetic field inside the coil. The coil, in effect, becomes a magnet (electromagnet), which is a
source of magnetomotive force (mmf), symbol: ℑ.
A source of magnetomotive force (mmf), which could be an electromagnet or a permanentmagnet, will cause an amount of magnetic flux, Φ, to ‘flow’. The amount of flux that floes
depends on the magnetic ‘resistance’ of the circuit or path. Again, we can use a right-hand-rule(RHR) to determine the north pole of the source of mmf. (For an electromagnet consisting of acoil of wire, if you wrap the fingers of your right hand around the coil in the direction of current
flow, your thumb points in the direction of the flux flow, and the north pole.)
The ‘resistance’ of the path is proportional to the length, l , inversely proportional to the area, A,
and is know as the reluctance, ℜ. Similar to electrical resistance where, the magnetic
reluctance, ℜ =
l
µ A, where µ is the permeability of the material and is the product of the
relative permability, µr , and the permability of free space, µ0 (µ=µr µ0).
Also similar to the electric circuit case, there is a fundamental relationship between the driving
force, ℑ, the reluctance of the path, ℜ, and the amount of flux, Φ, that flows in the circuit. This
relationship is know as Ohm’s Law for Magnetic Circuits: ℑ=Φℜ, which will be a fundamental
too for analysis of magnetic circuits.
Ω’s Law for Magnetic Circuits: ℑ = Φℜ, where:
ℑ = magnetomotive force (mmf) = (number of turns)(current in Amperes) = NI
Φ = total magnetic flux in Webers, Wb
ℜ =
l
µ A= reluctance in Amp-turns per Weber (A-t/Wb) or rels, and
l = average path length in meters A = cross-sectional area in m
2
µ = µr µ0, and µr is unit-less and specific to the material,
B is the flux density in Webers per square meter (Wb/m2) or Teslas, (T)
(1T=10,000Gauss)H is the magnetizing force or magnetic field intensity in Amp-turns/m (A-t/m),
and substituting these expressions for B and H in the last expression, we have:
B = µ H
Take the expression for flux density: B =
Φ
A, or Φ = BA. Recalling Ω’s Law and that the mmf,
ℑ = NI = Φℜ, we can do another manipulation:
NI = Φ( ) ℜ( ) = BA( )l
µ A
⎛
⎝ ⎜⎞
⎠ ⎟ =
B
µ l = Hl
or NI = Hl , which is a variation of Kirchhoff’s Voltage Law for electric circuits, which
can also be stated as: NI = Hl ∑∑ , or the sum of the mmf rises around any loop in a magnetic
circuit must equal the sum of the magnetizing force drops. This relationship will prove very
useful in analyzing magnetic circuits!
Let us re-examine the relationship between the flux density, B (which is proportional to Φ) and
the magnetizing force, H (proportional to NI ) given as B= µ H . µ is the slope of the relationship
between B and H and is typically not linear for magnetic materials. It is usually given ingraphical form in a “B-H curve”.
Hysteresis
Hysteresis is a property of magnetic material that causes some residual magnetism to remain in
a material after it has been exposed to an external source of mmf. Once a ferromagneticmaterial is subjected to a magnetic field, the domains align. If the magnetic field is removed,
most of the domains will return to (approximately) their original orientation. Dependent on thetype of material, some domains may stay oriented in the induced direction, giving the sample
some residual magnetism of its own. The amount of (reverse) field strength needed to return
the magnetic moment of the sample back to zero is called the coercive force.
Magnetic Circuit Problems
Magnetic circuit problems faced by a designer are generally one of two types:
1. the NI (mmf) is known, and we need to find the flux, Φ or B, or
2. the desired flux, Φ, or flux density, B, is known and we want to find the required NI.
The parallel between magnetic circuits and (particularly) electric circuits is very useful for
analyzing and designing magnetic circuits!!
Channeling Flux: Gaps and Fringing; Laminations and Magnetic Force of Attraction
Fringing:
Recall that two opposing properties of magnetic ‘lines’ of flux is that they will “avoid” each
other by spreading out as much as possible, but that they also take the path of least resistance,so they will “get together” to follow a lower reluctance path. This means that when they have
been channeled through a ferromagnetic material (easy path) and then come to a paramagnetic
(e.g. air) gap, they will start to “spread out” to avoid each other again over the length of the gap,which will affect the density, B. To estimate the effective area of the path in an air gap:
•
add the length of the gap, g, to the length and width of a rectangular or square cross-section:
A Eff − gap
= l + g ( ) w+ g ( )
• add the length of the gap, g, tothe radius of a circular (or
elliptical) cross-section:
A Eff − gap
= π r + g ( )2
Laminations:
Magnetic cores are often laminated to reduce heating and losses from eddy currents (thosecurrents induced in a conductor by a varying magnetic field). For laminated cores, the effectivearea is the nominal area multiplied by the Stacking Factor (S.F.).
StackingFactor = A
Eff
AOverall
For a rectangular cross-section, for example, A Eff. = (S.F.)(length X width)
Note: the Stacking Factor is sometimes given
as a percentage: e.g. S.F. = .95 or 95%.
Overlapping Areas:Where two legs of a magnetic circuit cometogether, the areas may not be the same or
oriented to exactly match. In these cases, the
flux will almost entirely pass through theoverlapping area. Although the lines of flux will have to make some path adjustment when
transitioning from one piece to the other, this happens quickly enough so an assumption that it
happens immediately at the transition is a reasonable one.
In any magnetic circuit, there is a ‘magnetic force of attraction’. This attractive force exists
anywhere in the circuit, but is most useful where there is a physical break or separation between
one piece and another. This makes it possible to lift or hold ferromagnetic loads with a magnet
(or electromagnet). The magnetic force of attraction is given by:
F Mag − Attr
=
B gap
2 A gap
2µ 0
.
The designation ‘gap’ in this formula should really be interpreted as ‘interface’, as this forceexists at any interface (or other point in the circuit), regardless of whether an actual gap exists
or not. Also note, it depends only on the flux density, B, and the area of the contact , not on the
length of any ‘gap’. The gap length, however, may indirectly influence the force by increasingthe reluctance of the magnetic path and thus reducing the total flux.
Remember, when dealing with overlap situations, use the smallest common area as the ‘Agap_ ’.
SUMMARYElectric Magnetic
E = IR ℑ = Φℜ
R =ρ l A
ℜ = l µ A
Φ = BA
KVL
Σ V loop = 0 Σℑ loop = 0
Σ V loop = Σ IR Σ NI loop = Σ Hl
KCL
Σ I node = 0 ΣΦ node = 0
Σ I in = Σ I out ΣΦ in = ΣΦ out
B – H Curves
Where R typically stays reasonably constant with varying current, ℜ (reluctance) does not
usually stay constant with varying levels of flux!