the resource handbook of electronics, 8353ch 09

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Whitaker, Jerry C. Inductors and Magnetic Properties

The Resource Handbook of Electronics.Ed. Jerry C. Whitaker

Boca Raton: CRC Press LLC, 2001

2001 by CRC PRESS LLC


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Chapter

9Inductors and Magnetic Properties

9.1 Introduction

The elemental magnetic particle is the spinning electron. In magnetic materials, such

as iron, cobalt, and nickel, the electrons in the third shell of the atom are the source of

magnetic properties. If the spins are arranged to be parallel, the atom and its associ-

ated domains or clusters of the material will exhibit a magnetic field. The magnetic

field of a magnetized bar has lines of magnetic force that extend between the ends,

one called the north pole and the other the south pole, as shown in Figure 9.1a. The

lines of force of a magnetic field are calledmagnetic flux lines.

9.1.1 Electromagnetism

A current flowing in a conductor produces a magnetic field surrounding the wire as

shown in Figure 9.2a. In a coil or solenoid, the direction of the magnetic field relative

to the electron flow ( to +) is shown in Figure 9.2b. The attraction and repulsion be-

tween two iron-core electromagnetic solenoids driven by direct currents is similar to

that of two permanent magnets.

The process of magnetizing and demagnetizing an iron-core solenoid using a cur-

rent being applied to a surrounding coil can be shown graphically as a plot of the mag-

netizing field strength and the resultant magnetization of the material, called a hyster-

esis loop(Figure 9.3). It will be found that the point where the field is reduced to zero, a

small amount of magnetization, calledremnance, remains.

9.1.2 Magnetic Shielding

In effect, the shielding of components and circuits from magnetic fields is accom-plished by the introduction of a magnetic short circuit in the path between the field

source and the area to be protected. The flux from a field can be redirected to flow in a

partition or shield of magnetic material, rather than in the normal distribution pattern



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between north and south poles. The effectiveness of shielding depends primarily upon

the thickness of the shield, the material, and the strength of the interfering field.

Some alloys are more effective than iron. However, many are less effective at high

flux levels. Two or more layers of shielding, insulated to prevent circulating currents

from magnetization of theshielding, areused in low-level audio,video,anddata appli-

cations.

9.2 Inductors and TransformersInductors are passive components in which voltage leads current by nearly 90 over a

wide range of frequencies. Inductors are usually coils of wire wound in the form of a

cylinder. The current through each turn of wire creates a magnetic field that passes

through every turn of wire in the coil. When the current changes, a voltage is induced

Figure 9.1 Theproperties of magnetism: (a) lines offorce surrounding a bar magnet, (b)relation of compass poles to the earths magnetic field.

Figure 9.2 Magnetic field surrounding a current-carrying conductor: (a) Compass atright indicates thepolarity and direction of a magnetic field circling a conductor carrying

direct current. Iindicates the direction of electron flow. Note: The convention for flow ofelectricity is from+ to, the reverse ofthe actualflow. (b) Directionof magnetic field foracoil or solenoid.

(a) (b)

(a) (b)



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in the wire and every other wire in the changing magnetic field. The voltage induced

in the same wire that carries the changing current is determined by the inductance of

the coil, and the voltage induced in the other wire is determined by the mutual induc-

tance between the two coils. A transformer has at least two coils of wire closely cou-

pled by the common magnetic core, which contains most of the magnetic field within

the transformer.

Inductorsand transformersvarywidely insize,weighingless than 1 g ormore than 1

ton, and have specifications ranging nearly as wide.

9.2.1 Losses in Inductors and TransformersInductors have resistive losses because of the resistance of the copper wire used to

wind the coil. An additional loss occurs because the changing magnetic f ield causes

eddy currents to flow in every conductive material in the magnetic field. Using thin

magnetic laminations or powdered magnetic material reduces these currents.

Losses in inductors are measured by the Q, or quality, factor of the coil at a test fre-

quency. Losses in transformersaresometimesgiven as a specific insertionloss in deci-

bels. Losses in power transformers are given as core loss in watts when there is no load

connectedandas a regulation in percent, measured as therelativevoltage drop foreach

secondary winding when a rated load is connected.

Transformer loss heats the transformer and raises its temperature. For this reason,

transformers are rated in watts or volt-amperes and with a temperature code designat-

ing the maximum hotspot temperature allowable for continued safe long-term opera-tion. For example,class A denotes 105C safe operating temperature.Thevolt-ampere

rating of a power transformer must be always larger than the dc power output from the

rectifier circuit connected because volt-amperes, the product of the rms currents and

Figure 9.3 Graph of the magnetic hysteresis loop resulting from magnetization and de-magnetization of iron. The dashed line is a plot of the induction from the initial magneti-zation. Thesolid line shows a reversalof thefieldand a return to theinitialmagnetizationvalue. Ris theremainingmagnetization (remnance) whenthe field isreduced tozero.



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rms voltages in the transformer, are larger by a factor of about 1.6 than the product of

the dc voltages and currents.Inductorsalso have capacitance between thewires of thecoil, whichcauses thecoil

to have a self-resonance between the winding capacitance and the self-inductance of

the coil. Circuits are normally designed so that this resonance is outside of the fre-

quency range of interest. Transformers are similarly limited. They also have capaci-

tance to the other winding(s), which causes stray coupling. An electrostatic shield be-

tween windings reduces this problem.

9.2.2 Air-Core Inductors

Air-core inductors are used primarily in radio frequency applications because of the

need for values of inductance in the microhenry or lower range. The usual construc-

tion is a multilayer coil made self-supporting with adhesive-covered wire. An inner

diameter of 2 times coil length and an outer diameter 2 times as large yields maxi-mum Q, which is also proportional to coil weight.

9.2.3 Ferromagnetic Cores

Ferromagnetic materials have a permeability much higher than air or vacuum and

cause a proportionally higher inductance of a coil that has all its magnetic flux in this

material. Ferromagnetic materials in audio and power transformers or inductors usu-

ally are made of silicon steel laminations stamped in the forms of letters E or I (Figure

9.4). At higher frequencies, powdered ferric oxide is used. The continued magnetiza-

tion and remagnetization of silicon steel and similar materials in opposite directions

does not follow the same path in both directions but encloses an area in the magneti-

zation curve and causes a hysteresis loss at each pass, or twice per ac cycle.

All ferromagnetic materials show the same behavior; only the numbers for perme-ability, core loss, saturation flux density, and other characteristics are different. The

properties of some common magnetic materials and alloys are given in Table 9.1.

9.2.4 Shielding

Transformers and coils radiate magnetic fields that can induce voltages in other

nearby circuits. Similarly, coils and transformers can develop voltages in their wind-

ings when subjected to magnetic fields from another transformer, motor, or power cir-

cuit. Steel mounting frames or chassis conduct these fields, offering less reluctance

than air.

The simplest way to reduce the stray magnetic field from a power transformer is to

wrap a copperstripas wideas thecoilofwirearoundthe transformerenclosingall three

legs of the core. Shielding occurs by having a short circuit turn in the stray magnetic

field outside of the core.



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Figure 9.4 Physical construction of a power transformer: (a) E-shaped device with thelow- andhigh-voltage windings stacked asshown,(b) constructionusinga boxcore withphysical separation between the low- and high-voltage windings.

(a)

(b)



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Table 9.1 Properties of Magnetic Materials and Magnetic Alloys (From[1]. Used with permission.)


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9.3 References

1. Whitaker, Jerry C. (ed.), The Electronics Handbook, CRC Press, Boca Raton, FL,1996.

9.4 Bibliography

Benson, K. Blair, and Jerry C. Whitaker, Television and Audio Handbook for Techni-cians and Engineers, McGraw-Hill, New York, NY, 1990.

Benson, K.Blair,AudioEngineeringHandbook, McGraw-Hill,New York,NY, 1988.Whitaker, Jerry C., Television Engineers Field Manual, McGraw-Hill, New York,

NY, 2000.

9.5 Tabular Data

Table9.2 MagneticPropertiesof Transformer Steels (From[1]. Usedwithpermission.)



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Table 9.3 Characteristics of High-Permeability Materials (From[1]. Used with permission.)


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Table 9.4 Characteristics of Permanent Magnet Alloys (From[1]. Used with permission.)


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Table 9.5 Properties of Antiferromagnetic Compounds (From[1]. Used with permis-sion.)

Table 9.6 Saturation Constants for Magnetic Substances (From[1]. Used with permis-sion.)



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Table 9.7 Saturation ConstantsandCurie Pointsof Ferromagnetic Elements (From[1].Used with permission.)

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