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L-8 MAGNETIC CIRCUITS

L-8 MAGNETIC CIRCUITS. Objectives. To solve various kinds of problems involving series magnetic circuits. To understand losses involved in magnetic circuits. Contents. Series magnetic circuit Fringing effect Useful flux and leakage flux Magnetic hysteresis Losses in magnetic circuit.

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L-8 MAGNETIC CIRCUITS

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  1. L-8MAGNETIC CIRCUITS Dept. of E & E, MIT, Manipal

  2. Objectives • To solve various kinds of problems involving series magnetic circuits. • To understand losses involved in magnetic circuits. Dept. of E & E, MIT, Manipal

  3. Contents • Series magnetic circuit • Fringing effect • Useful flux and leakage flux • Magnetic hysteresis • Losses in magnetic circuit Dept. of E & E, MIT, Manipal

  4. Series Magnetic Circuit • Flux ɸ is the same in all sections if leakage flux is neglected. • Flux density and reluctance in each section may vary, depending on its effective cross-sectional area and material. Rectangular shaped series magnetic circuit with air gap. Dept. of E & E, MIT, Manipal

  5. Series Magnetic Circuit • Equivalent reluctance is the sum of reluctance of different parts/elements. • The resultant MMF is the sum of MMFs in each individual parts/elements Dept. of E & E, MIT, Manipal

  6. Series Magnetic Circuit Ring shaped series magnetic circuit with air-gap Dept. of E & E, MIT, Manipal

  7. Fringing Effect • Air gap is practically an unavoidable part of any magnetic circuit, in which there is a relative movement between different parts. (e.g. in motors, generators, relays, etc.) • Due to increased reluctance of the air gap the flux spreads into the surrounding medium causing flux fringing effect. Dept. of E & E, MIT, Manipal

  8. Fringing Effect • Effectively it increases the cross sectional area of air gap. Dept. of E & E, MIT, Manipal

  9. Useful Flux and Leakage Flux Magnetic leakage : The passage of magnetic flux outside the path along which it can do useful work. Total flux of coil = Useful flux + Leakage flux Dept. of E & E, MIT, Manipal

  10. Magnetic Hysteresis: • Lagging of magnetization or flux density behind the magnetizing force. • Residual magnetism : Magnetic flux remains in the ferromagnetic material even though the magnetizing force is made zero (Br) • Coercive force : The magnetizing force required to remove the residual magnetism from the material (Hc). Hysteresis loop Dept. of E & E, MIT, Manipal

  11. Losses in Magnetic Circuit • Hysteresis loss • The energy dissipated as heat in the process of magnetization and demagnetization which is proportional to the area of hysteresis loop. • ‘Soft' magnetic materials with low hysteresis loss , such as silicon steel, are usually used in cores of electric machines like motors, generators and transformers. Dept. of E & E, MIT, Manipal

  12. Losses in Magnetic Circuit Eddy current loss • The varying flux in the magnetic core induces emf and henceeddycurrent within the material. • Flow in closed loops in planes perpendicular to the magnetic field • Results in loss of power and heating of the material. • Cores of electric machines are laminated to reduce eddy current loss Dept. of E & E, MIT, Manipal

  13. Exercise 1 A steel ring 25cm mean diameter and of circular section 3cm in diameter has an air-gap of 1.5mm length. It is wound uniformly with 700 turns of wire carrying a current of 2A. Calculate (i) magneto motive force (ii) flux density (iii) magnetic flux (iv) reluctance and (v) relative permeability of steel ring. Neglect magnetic leakage and assume that iron path takes about 35% of the total magneto motive force. • 1400 AT • 0.762 Wb/m^2 • 0.538mWb • 2.6×10^6 AT/Wb • 972 Dept. of E & E, MIT, Manipal

  14. Summary • Only one flux path is available in series magnetic circuit and equivalent reluctance is the sum of reluctance of different parts. • Losses occurring in magnetic circuits are classified as hysteresis and eddy current losses. Dept. of E & E, MIT, Manipal

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