# Lecture 4 by Moeen Ghiyas Chapter 11 – Magnetic Circuits 21/01/20161.

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### Transcript of Lecture 4 by Moeen Ghiyas Chapter 11 – Magnetic Circuits 21/01/20161.

Lecture 4 by Moeen Ghiyas Chapter 11 Magnetic Circuits 21/01/20161 TODAYS LECTURE CONTENTS Review Ohms Law For Magnetic Circuits Magnetizing Force Hysteresis Amperes Circuital Law (Applying KVL) The Flux (Applying KCL) Series Magnetic Circuits Ohms Law For Magnetic Circuits Ohms law for magnetic circuit Where the magnetomotive force F is proportional to the product of the number of turns N around the core (in which the flux is to be established) and the current I through the turns of wire Obviously, an increase in the number of turns N or the current I through the wire will result in an increased pressure on the system to establish flux lines through the core. Magnetizing Force The magneto-motive force per unit length is called the magnetizing force (H). In equation form, But from Ohms law for magnetic circuits, we know Substituting above, we have 21/01/20164 Magnetizing Force The applied magnetizing force has a pronounced effect on the resulting permeability of a magnetic material. 21/01/20165 Magnetizing Force Also the flux density and the magnetizing force are related by the following equation: This equation indicates that for a particular magnetizing force, the greater the permeability, the greater will be the induced flux density. 21/01/20166 7 Hysteresis 21/01/ Hysteresis 21/01/ Hysteresis 21/01/ Hysteresis 21/01/ Hysteresis 21/01/ Hysteresis Domain Theory of Magnetism The atom, due to its spinning electrons, has magnetic field associated. In nonmagnetic materials, the net magnetic field is zero since the magnetic fields due to the atoms oppose each other. In magnetic materials such as iron and steel, however, the magnetic fields of groups of atoms in the order of are aligned, forming very small bar magnets. 21/01/ Hysteresis Domain Theory of Magnetism This group of magnetically aligned atoms is called a domain. Each domain is a separate entity; that is, each domain is independent of the surrounding domains. For an un-magnetized sample of magnetic material, these domains appear in a random manner, such as shown in fig. The net magnetic field in any one direction is zero. 21/01/ Hysteresis 21/01/ Amperes Circuital Law KVL 21/01/ Amperes Circuital Law (KVL) 21/01/ The Flux (KCL) Magnetic circuit problems are basically of two types: In one type, is given, and the impressed mmf NI must be computed (problem encountered in the design of motors, generators, and transformers). In the other type, NI is given, and the flux of magnetic circuit must be found (problem encountered primarily in the design of magnetic amplifiers and is more difficult since the approach is hit or miss. For magnetic circuits, the level of B or H is determined from using the B-H curve. 21/01/ Series Magnetic Circuits Ex ample For the series magnetic circuit of fig: a) Find the value of I required to develop a magnetic flux of = 4 x Wb. b) Determine and r for the material under these conditions. 21/01/ Series Magnetic Circuits a) Find the value of I required to develop a magnetic flux = 4 x Wb Solution 21/01/ Series Magnetic Circuits a) Find the value of I required to develop a magnetic flux = 4 x 10 4 Wb Solution Using B H curves of fig, we can determine magnetizing force H: . H = 170 At / m 21/01/ Series Magnetic Circuits b) Determine and r for the material under these conditions. 21/01/ Series Magnetic Circuits Summary / Conclusion Review Ohms Law For Magnetic Circuits Magnetizing Force Hysteresis Amperes Circuital Law (Applying KVL) The Flux (Applying KCL) Series Magnetic Circuits 21/01/201624