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L11 magnetic circuits

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L11 magnetic circuits

  1. 1. MAGNETIC CIRCUITS
  2. 2. ELE101 / 102 Dept of E & E Engg, MIT Manipal 2 Terminologies Magnetic flux (Φ): No. of magnetic lines of force created in a magnetic circuit Unit is Weber (Wb) Flux Density: (B) No. of magnetic lines of force created in a magnetic circuit per unit area perpendicular to the direction of flux lines B = Φ/A Unit is Weber/sq.m (Tesla)
  3. 3. ELE101 / 102 Dept of E & E Engg, MIT Manipal 3 Terminologies Magneto motive force: (F) Force which drives or tends to drive the magnetic flux through a magnetic circuit MMF = No. of turns*Current = N I Unit is AT (ampere - Turns) Magnetic field strength: (H) The magneto motive force per meter length of the magnetic circuit H = (N I) / L Unit is AT / meter
  4. 4. ELE101 / 102 Dept of E & E Engg, MIT Manipal 4 Permeability [μ] A property of a magnetic material which indicates the ability of magnetic circuit to carry electromagnetic flux. Ratio of flux density to the magnetizing force, μ = B / H Unit: henry / meter Permeability of free space or air or non magnetic material Relative permeability [μr] : Terminologies mH /104 7 0 − ×= πµ rµµµ 0=
  5. 5. ELE101 / 102 Dept of E & E Engg, MIT Manipal 5 Terminologies Reluctance [S] It is the opposition of a magnetic circuit to setting up of a magnetic flux in it. ‘S’ is called the reluctance of the magnetic circuit Reluctance = mmf / Magnetic flux Unit: AT / Wb         ==       =       === ===== A l ; S F A l F F l A Hence; l A Hl BA F HB;HlmmfF;BAFlux r r rr µµ µµ φ µµ φ µµφ µφ 0 0 00 Swhere
  6. 6. ELE101 / 102 Dept of E & E Engg, MIT Manipal 6 Magnetic circuit The complete closed path followed by any group of magnetic lines of flux is referred to a magnetic circuit VI R Equivalent electrical circuit
  7. 7. ELE101 / 102 Dept of E & E Engg, MIT Manipal 7 Analogy with Electric circuits Electric circuit Magnetic circuit Similarities: Emf, volt mmf, ampere Turn Current, ampere flux, weber Resistance, ohm Reluctance, A / Wb Current density, A / m2 flux density, T or Wb / m2 Conductivity Permeability Difference:  Current actually flows flux is created, but does not flow  Circuit may be open or Circuit is always closed closed
  8. 8. ELE101 / 102 Dept of E & E Engg, MIT Manipal 8 Leakage flux Leakage Flux : The flux that does not follow the intended path in the magnetic circuit Leakage flux factor λ Fringing: Effect of spreading out of flux in the air gap Totalflux(fluxinthe iron path) Useful flux(flux in the airgap) =
  9. 9. ELE101 / 102 Dept of E & E Engg, MIT Manipal 9 Magnetic circuits losses Two types of losses: Hysteresis losses Eddy current losses
  10. 10. ELE101 / 102 Dept of E & E Engg, MIT Manipal 10 B-H Curve & Hysteresis loss a b c d Residual magnetism Hysteresis loss: Magnetic hysteresis: Lagging of magnetization or flux density behind the magnetizing force. Hysteresis loss = ή (Bmax)1.6 J / m / cycle Where ή is Steinmetz hysteresis co-efficient B-H Curve:
  11. 11. ELE101 / 102 Dept of E & E Engg, MIT Manipal 11 Eddy current loss Eddy current: The varying flux in the Magnetic core induces emf and hence Current Eddy current results in Loss of power, Heating of the material

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