Magnetic Circuits, Introduction to electrical machines
- 1. A N O V E R V I E W MAGNETIC CIRCUITS
- 2. OBJECTIVES • At the end of the lecture, the students must be able to: • Describe a magnetic circuit • Discuss magneto-motive force and magnetic field strength • Identify the properties of reluctance • Identify multiple phenomenon present in a magnetic circuit • Discuss the comparison between magnetic & electric circuits
- 3. MAGNETIC CIRCUIT • The complete closed path for the magnetic flux to flow. • Somewhat similar to that of electric circuit but with different quantities.
- 4. MAGNETO-MOTIVE FORCE • The product of current and the number of turns. • Produced when a current flows in a coil of one or more number of turns. MMF = NI (Ampere – turns)
- 5. MAGNETIC FIELD INTENSITY • The amount of MMF per unit length of a magnetic circuit. • Also called as Magnetic Field Strength H = MMF / L (Ampere – turn per meter)
- 6. PERMEABILITY • The measure of the degree in which the lines of force of a magnetic field be able to penetrate a certain medium. • Denoted as μ. μ = μo μr Where: μo = absolute permeability μr = relative permeability
- 7. ABSOLUTE AND RELATIVE PERMEABILITY • Absolute Permeability (μo) • The Permeability of free space • μo = 4π x 10-7 H/m • Relative Permeability (μr) • The Ratio of the permeability of material to the permeability of free space • Unitless
- 8. RELATION OF MAGNETIC FLUX DENSITY TO MAGNETIC FIELD INTENSITY • At any point in the magnetic field, field intensity H is the force maintaining the magnetic flux and producing a particular value of flux density β at that point. • Flux density is directly proportional to field intensity. β = μ H
- 9. RELUCTANCE • The opposition of a magnetic circuit for the flux to flow. • Unit is Reciprocal Henry • Properties: • Directly Proportional to its length. • Inversely Proportional to its cross-sectional area. • Depends on the behavior of the material. R = L / μA
- 10. LAWS ON MAGNETIC CIRCUITS • Ohm’s Law equivalent on Magnetic Circuits MMF = R φ • Series Reluctance RT = R1 + R2 + R3 + … • Parallel Reluctance 1 / RT = 1 / R1 + 1 / R2 + 1 / R3 + …
- 11. SOME PHENOMENON ASSOCIATED IN MAGNETIC CIRCUITS • Leakage Flux • The phenomenon in which flux flows outside the magnetic circuit.
- 12. SOME PHENOMENON ASSOCIATED IN MAGNETIC CIRCUITS • Fringing • The phenomenon in which flux tends to bulge out at the edges of the air gap.
- 13. SOME PHENOMENON ASSOCIATED IN MAGNETIC CIRCUITS • Hysteresis • The name given to the lagging of flux density behind the magnetizing force when a material is taken through a cycle of magnetization. • Eddy Current • The phenomenon in which the magnetic circuit itself tends to cut flux resulting to an induced EMF to be set in the core.