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Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2002 The McGraw-Hill Companies Grob Schultz

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Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2003 The McGraw-Hill Companies 14 CHAPTER Magnetic Units

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Topics Covered in Chapter 14 Ampere-turns of Magnetomotive Force (mmf) Field Intensity (H) Permeability ( ) B-H Magnetization Curve

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Topics Covered in Chapter 14 (continued) Magnetic Hysteresis Ohm's Law for Magnetic Circuits Relations between Magnetic Units Comparison of Magnetic and Electric Fields

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Ampere -Turns Ampere-turns = I N I N specifies the amount of magnetizing force or magnetic potential (mmf). I is the amount of current flowing through N turns of wire.

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Magnetomotive Force (At) Ampere-turns = I x N. I N

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Field Intensity (H) Equation: H = mmf/length Units: A·t/m ampere-turns per meter Shorter magnetic circuits produce a greater field intensity.

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Field Intensity (H) length (ampere-turns per meter)H = A·t/m The field intensity in the core is inversely related to length.

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0 1k 2k 3k 4k 5k H in At/m B in Teslas 0.5 B-H Magnetization Curve B meter Soft iron Saturation

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Hysteresis Loop - H+ H + B - B + B max - B max + B R - B R - H C B R is due to retentivity. (Note that H = 0 but B > 0.) H C is the coercive force. (that needed to make B = 0) + H C

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Demagnetization (Also Called Degaussing) B R is reduced to zero.

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Hysteresis Curve When Retentivity is Very Low - H+ H + B - B + B max - B max

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Permeability ( ) Equation: = B/H Unit: T Atm / Permeability is a measure of the ability to concentrate magnetic fields. teslas per ampere-turn per meter

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Permeability drops drastically at saturation. - H+ H + B - B + B max - B max Slope of B/H is small Slope of B/H is large

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Reluctance Reluctance is the opposition to flux. It is comparable with resistance in an electrical circuit. Equation: = mmf / Units: A·t/Wb ampere-turns per weber

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Permeance Permeance is comparable with conductance in an electrical circuit (G = 1/R). Equation: Units: Wb/A·t webers per ampere-turn

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Ohm’s Law for Electrical and Magnetic Circuits Electrical circuits: I = V/R Magnetic circuits: = mmf /

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Increasing reluctance decreases flux. Adding an air-gap increases the reluctance. I As increases, decreases.

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