magnetic_parameters_electrical machines .pptx

magnetic circuit and its parameters

1. Magnetic Circuits

2. Magnetic Circuit Electromagnetism - relays, solenoids, inductors, chokes, coils, loudspeakers, motors, generators, transformers, and electricity meters etc Magnetic circuit - Structure composed of high permeability magnetic material One or more closed paths containing magnetic flux Flux - Confined to the path by magnetic cores 2

3. Flux - Generated by permanent  magnets or electromagnets Consisting of ferromagnetic materials like iron May include air gaps or other materials  Current passes through a conductor magnetic field is set up around the conductor The quantity of the magnetic field is proportion to the current “North” and “South” poles being determined by the direction of the current flowing through the conductor Direction of the magnetic field is found by Right Hand Rule or Max well's corkscrew rule Magnetic Circuit 3

4. Magnetic Material Magnetic materials - classified based permeability 1.  Dia Magnetic Materials µ < 1, repelled by magnet Ex. Lead, gold, copper, glass, mercury 2. Para Magnetic Materials µ > 1, Force of attraction by a magnet is low Ex.: Copper Sulphate , Platinum, Aluminum 3.  3. Ferro Magnetic Materials µ > > 1, Very much attracted by the magnet Ex. Iron, Cobalt, Nickel 4

5. Types of Magnet Temporary magnets magnetized in the presence of a magnetic field lose their magnetism gradually, when the magnetic field is removed Ex.: Some irons and iron alloys, as well as paper clips and nails Permanent magnets  Retain the magnetic property at all times permanently, do not easily lose their magnetism Naturally-occurring (“rare-earth”) elements, or chemical compounds Ex.: Alnico (an alloy of aluminum, nickel, and cobalt) and ferrites (ceramic-like material made from a mix of iron oxides with nickel, strontium, or cobalt) Electro Magnet Insulated wire wound on a bobbin in many turns and layers in which current is flowing and a soft iron piece placed in the bobbin is called electromagnet Made up of aluminum, nickel, iron, cobalt steel (ALNICO) 5

6. F - Magnetomotive force ( mmf ) in AT/m B – Flux Density in Tesla or wb /m 2 Φ - Total flux in weber N - Number of turns I - Current in amperes S – Reluctance in ampere - turns per weber µ - Permeability of free space in henries per meter (H/m) µ r - Relative permeability a - Magnetic path cross-sectional area in m 2 l - Lengh of magnetic path in m Magnetic Circuit Parameters 6

7. 7 Magnetic Circuit Definitions Magnetic Flux Total number of lines existing on the cross-section of the magnetic core at right angle to the direction of the current Do not have origins or terminating points but exist in continuous loops Magnetomotive Force The “driving force” that causes a magnetic field Symbol, F Definition, F = NI Units, Ampere-turns, (A-T)

8. 8 Magnetic Field Intensity mmf gradient, or mmf per unit length Symbol, H Definition, H = F/l = NI/l Units, (A-t/m) Flux Density The concentration of the lines of force in a magnetic circuit Symbol, B Definition, B = Φ /A Units, ( Wb /m 2 ), or T (Tesla) Reluctance The measure of “opposition” the magnetic circuit offers to the flux Symbol, S Definition,S = F/ Φ Units, (A-t/ Wb )

9. 9 Permeability - Measure of the ease with which magnetic flux lines can be established in the material Relates flux density and field intensity Symbol, μ Definition, μ = B/H Units, ( Wb /A-t-m) Permeability of free space (air) Symbol, μ μ = 4 π x10 -7 Wb /A-t-m Relative Permeability Compares permeability of material with the permeability of free space (air) Symbol, μ r μ r = μ / μ Dimensionless

10. Difference Between Magnetic and Electric Circuit Magnetic Circuit Electric Circuit 1. Closed path for magnetic flux Closed path for electric current 2. Flux Current 3. Magnetomotive force ( mmf ) Electromotive force ( emf ) 4. Reluctance Resistance 5. Permeance = 1/reluctance Conduction = 1/ resistance 6. Permeability Conductivity 7. Reluctivity Resistivity 8. Flux density Current density 9. Magnetic intensity Electric density 10