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

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

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

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

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.

Magnetomotive Force (A•t) Ampere-turns = I x N.

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

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

B-H Magnetization Curve B meter 0.5 0.4 Saturation B in Teslas 0.3 0.2 0.1 Soft iron 0 1k 2k 3k 4k 5k H in A•t/m

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

Demagnetization (Also Called Degaussing) BR is reduced to zero.

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

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

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

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

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

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

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