Chapter 11 – Magnetic Circuits

Introductory Circuit AnalysisRobert L. Boylestad

11.1 - IntroductionMagnetism is an integral part of almost every electrical device

used today in industry, research, or the homeGenerators, motors, transformers, circuit breakers, televisions,

computers, tape recorders and telephones all employ magnetic effects to perform a variety of important tasks

Chinese sailors used compasses, as early as the second century A.D., that relied on a permanent magnet

In 1820, the Danish physicist Hans Christian Oersted discovered that a needle of a compass would deflect if brought near a current-carrying conductor

11.2 - Magnetic FieldsIn the region surrounding a permanent magnet

there exists a magnetic field, which can be represented by magnetic flux lines similar to electric flux lines

Magnetic flux lines differ from electric flux lines in that they don’t have an origin or termination point

Magnetic flux lines radiate from the north pole to the south pole through the magnetic bar

Magnetic FieldsContinuous magnetic flux lines will strive to occupy as small an

area as possibleThe strength of a magnetic field in a given region is directly related

to the density of flux lines in that region If unlike poles of two permanent magnets are brought together the

magnets will attract, and the flux distribution will be as shown below

Magnetic Fields If like poles are brought

together, the magnets will repel, and the flux distribution will be as shown

If a nonmagnetic material, such as glass or copper, is placed in the flux paths surrounding a permanent magnet, there will be an almost unnoticeable change in the flux distribution

Magnetic Fields If a magnetic material, such as soft iron, is placed in the flux path,

the flux lines will pass through the soft iron rather than the surrounding air because the flux lines pass with greater ease through magnetic materials than through air

This principle is put to use in the shielding of sensitive electrical elements and instruments that can be affected by stray magnetic fields

Magnetic Fields

The direction of the magnetic flux lines can be found by placing the thumb of the right hand in the direction of conventional current flow and noting the direction of the fingers (commonly called the right hand rule)

11.3 - Flux Density

In the SI system of units, magnetic flux is measured in webers and has the symbol

The number of flex lines per unit area is called the flux density, is denoted by B, and is measured in teslas

Its magnitude is determined by the following equation:

11.4 - PermeabilityIf cores of different materials with the same physical

dimensions are used in the electromagnet, the strength of the magnet will vary in accordance with the core used

The variation in strength is due to the number of flux lines passing through the core

Magnetic material is material in which flux lines can readily be created and is said to have high permeability

Permeability ()is a measure of the ease with which magnetic flux lines can be established in the material

PermeabilityPermeability of free space 0 (vacuum) is

Materials that have permeability slightly less than that of free space are said to be diamagnetic and those with permeability slightly greater than that of free space are said to be paramagnetic

0 = 4 x 10 -7 WbA • m

Permeability

Magnetic materials, such as iron, nickel, steel and alloys of these materials, have permeability hundreds and even thousands of times that of free space and are referred to as ferromagnetic

The ratio of the permeability of a material to that of free space is called relative permeability

11.5 - ReluctanceThe resistance of a material to the flow of charge (current) is

determined for electric circuits by the equation

The reluctance of a material to the setting up of magnetic flux lines in a material is determined by the following equation

R = lA

11.6 - Ohm’s Law For Magnetic Circuits

Ohm’s law

For magnetic circuits, the effect is the flux The cause is the magnetomotive force (mmf) F, which is the

external force (or “pressure”) required to set up the magnetic flux lines within the magnetic material

The opposition to the setting up of the flux is the reluctance R

Effect =cause

opposition

Ohm’s Law For Magnetic Circuits

Substituting

The magnetomotive force F is proportional to the product of the number of turns around the core (in which the flux is to be established) and the current through the turns of wire

11.7 - Magnetizing Force The magnetomotive force per unit length is called the

magnetizing force (H)

Magnetizing force is independent of the type of core material

Magnetizing force is determined solely by the number of turns, the current and the length of the core

11.8 - HysteresisHysteresis curve is named from the Greek hysterein, meaning

“to lag behind”To an engineer it is important to know the curve of the flux

density of B versus the magnetic force H of a material

Hysteresis

The entire curve represented by bcdefb is called the hysteresis curve The flux density B lagged behind the magnetizing force H during the entire

plotting of the curve. When H was zero at c, B was not zero but had only begun to decline. Long after H had passed through zero and had equaled to –Hd did the flux density B finally become equal to zero

11.9 - Ampère’s Circuital Law

Ampère’s circuital law: The algebraic sum of the rises and drops of the mmf around a closed loop of a magnetic circuit is equal to zero; that is, the sum of the rises in mmf equals the sum drops in mmf around a closed loop

11.10 - The Flux The sum of the fluxes entering a junction is equal

to the sum of the fluxes leaving a junction.a= b + c

b + c = a

both of which are equivalent

11.11 - Series Magnetic Circuits: Determining NI

Two types of problems is given, and the impressed mmf NI must be computed –

design of motors, generators and transformersNI is given, and the flux of the magnetic circuit must be

found – design of magnetic amplifiersTable method

A table is prepared listing in the extreme left-hand column the various sections of the magnetic circuit. The columns on the right are reserved for the quantities to be found for each section

11.12 - Air GapsEffects of air gaps on a magnetic circuit

The flux density of the air gap is given by

Where: g = core > Ag = Acore

The permeability of air is taken to be equal to that of free space. The magnetizing force of the air gap is then determined by

And the mmf drop across the air gap is equal to Hg Ig

11.13 - Series-Parallel Magnetic Circuits

Close analogies between electric and magnetic circuits will eventually lead to series-parallel magnetic circuits similar in many respects to electric circuits encountered previously (in Chapter 7)

11.14 - Determining When determining magnetic circuits with more than one

section, there is no set order of steps that will lead to an exact solution for every problem on the first attempt

Find the impressed mmf for a calculated guess of the flux and then compare this with the specified value of mmf

Make adjustments to the guess to bring it closer to the actual value

For most applications, a value within 5% of the actual or specified NI is acceptable

11.15 - Applications

Recording systems Speaker and microphones Computer hard disks Hall effect sensor Magnetic reed switch Magnetic resonance imaging