Chapter 12

Chapter 8Magnetism and Magnetic Circuits1The Nature of a Magnetic FieldMagnetic fieldForce fieldMagnetic fluxFlux lines (lines of force)Show direction and intensity of the field

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The Nature of a Magnetic Field (contd.)Unlike poles attractLike poles repel3Ferromagnetic MaterialsMagnetic materialsMaterials attracted by magnetsIron, nickel, cobalt, and their alloysProvide an easy path for magnetic flux

4Ferromagnetic Materials (contd.)Nonmagnetic materialsHave no effect on a magnetic field5Application: loudspeaker

Figure 12-46ElectromagnetismRight hand rule for a uniform length conductorUsed to indicate field directionPlace right hand around conductorThumb points in direction of currentFingers point in direction of the field7

8Electromagnetism (contd.)Rule for coilsCurl fingers of right hand around coil in direction of the currentThumb will point in direction of the field

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10Magnetic Flux and Flux Density

Equation 12-111Magnetic Flux and Flux Density (contd.)Example: For the magnetic core shown, the flux density at cross section 1 is B1 = 0.4TDetermine B2

12Magnetic Flux and Flux Density (contd.)

13Magnetic CircuitsFound in motors, generators, speakers, transformersMagnetic fields can be created by electric currentsOr electric currents and permanent magnetsExample: speaker application in Figure 12-414Magnetic Circuits (contd.)

Figure 12-10 Audio speaker, conceptual representation15Magnetic Circuits (contd.)Magnetic stripe containing information Used in bank ATM cards, library cards, etc.Magnetic patterns encode informationReader sees varying magnetic fieldInduces a voltage in the pickup windingVoltage is amplified and sent to decoding circuitryMRI machine uses superconductor coilsCreate intense magnetic field16

Magnetic Circuits (contd.)17Air Gaps, Fringing, and Laminated CoresMost practical magnetic circuits:Have air gaps essential to their operationFringing occurs at air gapsResults in slight weakening of the field in the gapCan be neglected for short gapsOr estimate the effect by increasing each cross-sectional dimension of the core by gap length18Air Gaps, Fringing, and Laminated Cores (contd.)LaminationCore is created with thin sheets of stacked iron or steelStacking factorRatio of actual area of ferrous material to physical area of the coreUse to determine cores effective area19Series Elements and Parallel ElementsSeries magnetic circuit

20Series Elements and Parallel Elements (contd.)Parallel elements

21Magnetic Circuits with DC ExcitationTwo basic problemsDetermine current required to produce a given fluxCompute flux produced by a given current22MMF: The Source of Magnetic FluxCurrent through a coil creates magnetic fluxThe greater the current or number of turns, the greater the fluxMagnetomotive force (mmf)Measured in ampere-turnsDenoted by the symbol

23MMF: The Source of Magnetic Flux (contd.)Equation defining mmf

Equation 12-224Reluctance : Opposition to Magnetic FluxOpposition that circuit presents to flux

Where =material permeability

Equation 12-325Reluctance : Opposition to Magnetic Flux (contd.)Permeability measures ease of establishing magnetic flux in a materialFerromagnetic materials have high permeabilityNonmagnetic materials have low permeability

26Ohms Law for Magnetic CircuitsFlux does not flow like current

Equation 12-4

27Magnetic Field Intensity and Magnetization CurvesMagnetic field intensity, HAlso called magnetizing forceMeasures mmf per unit length of a circuit

Equation 12-5Equation 12-628Magnetic Field Intensity and Magnetization Curves (contd.)Electric circuit analogyNI is an mmf sourceHl is an mmf drop

29The Relationship Between B and HFor air gaps:

Equation 12-7

Equation 12-930B-H CurvesCurves obtained experimentallyAvailable in handbooks

31Force Due to an ElectromagnetElectromagnetsUsed in relays, doorbells, lifting magnets

Where Bg is the flux density in the gap in teslas Ag is gap area in square meters F is force in Newtons

Equation 12-1332

33Properties of Magnetic Materials

34Magnetizing a Specimen

35Magnetizing a Specimen (contd.)SaturationAll domain fields line upCannot magnetize it much further36Measuring Magnetic FieldsHall effectVoltage proportional to field strength B

37Measuring Magnetic FieldsHall effectThe Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor when the current flows through the conductor in a magnetic field.38Measuring Magnetic Fields (contd.)Hall effect gaussmeters use this principle

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Thank you40