EE ELECTRICAL MATERIAL SCIENCE

Transcription

1 EE ELECTRICAL MATERIAL SCIENCE Module I Hard and soft magnetic materials and applications - Ferrites - Magnetic materials used in electrical machines, instruments and relays Arun Xavier VAST

2 2 Module 1(9 hours) Conducting materials: Review of metallic conduction on the basis of free electron theory- Fermi-Dirac distribution - Variation of conductivity with temperature and composition - Contact potential - Materials for electric resistances, brushes of electrical machines, lamp filaments, fuses and solders. Semiconductors: Compound semiconductors - Basic ideas of amorphous and organic Semiconductors Magnetic materials: Classification of magnetic materials - Ferromagnetism - Hysteresis curve - Ferromagnetic domains - Curie - Weiss law - Hard and soft magnetic materials and applications - Ferrites - Magnetic materials used in electrical machines, instruments and relays.

3 3 Why do Domains Form? A magnetic domain is region in which the magnetic fields of atoms are grouped together and aligned. Domains form to minimize (and in some cases to completely eliminate) demagnetization fields (H D ). They are not random structures.

4 4 Magnetic Domains In reality, a ferro- or ferrimagnet is comprised of many regions ( domains ) with mutual alignment of the individual atomic magnetic dipole moments. These domains are not necessarily aligned with respect to each other. The overall magnetization of the material (M) is the vector sum of the magnetization vectors for all of the individual domains. If not magnetized, the overall magnetization is simply zero.

5 5 Once a magnetic material is saturated, decreasing H again does not return M (or B) to the same position. For no external magnetic field, a remanent induction (±Br) will remain. A negative field, the Coercive Field (±Hc), must be applied to eliminate all Br.

6 6 Magnet Types Magnets are categorized depending on the shape of the magnetic hysteresis loop. Soft magnet = Narrow in H Hard magnet = Broad in H

7 7 Magnet Types The area of the loop represents energy lost in moving the domain walls as the magnet is poled from one extreme to the other and back again. Energy may also be lost due to local electric currents generated within the material caused by the external field. AC electric field causes a magnetic field, and vice versa.

8 8 Soft and Hard Magnetic Materials

9 9 Soft Magnets Strong induction for a relatively weak external field. High saturation field (Bs), High permeability (μ), low coercive field (Hc) Therefore a low energy loss per poling cycle. Applied when rapid, lossless switching is required; usually subjected to ac magnetic fields: Transformer cores

10 10 Hard Magnets High saturation induction, remanence, and coercively. High hysteresis losses It is hard to repole a hard magnet. Standard and high energy hard magnets. Standard are simple tungsten steel; FeNiCu alloys

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12 12 Magnetic Hard Drives The magnetic disk has a soft magnet (easy to pole and repole with little energy loss. The read/write head is a hard magnet, or an electromagnet. Concept is the same as for an audio tape or video tape. Magnetics have thus far ruled for computer hard drives. Flash (solid state, Si based) is coming on strong Ferroelectrics are also increasingly being applied

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14 14 Applications

15 15 Transformer

16 16 Transducers and Inductors

17 17 Hard Disk

18 18 Recording Tape Sound current sent through ring-shaped electromagnet Split in ring develops north and south poles Nearby tape region becomes magnetized

19 19 Playing Back Tape Tape moves under the ring-shaped electromagnet Fluctuating magnetism in ring induces current in playback coil

20 20 Recording Details Louder Sound Deeper Magnetization Higher Pitch Closer Magnetic Reversals Stereo Two Separate Magnetic Tracks/Heads Noise Reduction High Pitch Expansion Pitch Control Tape Speed Control Sound Degradation Magnetization Damage

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22 22 Ferrites

23 23 Ferrites Ferrites are the most useful ferrimagnetic materials. Ferrites are ceramic material containing compounds of iron oxides[ hematite (Fe 2 O 3 ) or magnetite (Fe 3 O 4 )]. Ferrites are non-conducting magnetic media so eddy current and ohmic losses are less than for ferromagnetic materials.

24 24 Application of Ferrites Ferrites for Permanent Magnets Hard Ferrites Ferrites for Transformers and inductors Soft Ferrites Data Storage Rectangular Loop ferrites Microwave applications Ferrite cores are used in electronic inductors, transformers, and electromagnets. Ferrite powders are used in the coatings of magnetic recording tapes. Most common radio magnets & loudspeakers, are ferrite magnets.

25 25 Electrical Materials Electrical materials used in the construction of all commercial electrical machines may be broadly classified into three groups Conducting Insulating Magnetic Materials

26 26 Magnetic Materials properties for machines Size of the magnetic frame for a machine will depend upon the flux density Higher the value of flux density, lower will be the size of the machine for a particular output. Thus the best magnetic materials for electrical machines are those, that can be worked at higher flux densities.

27 27 Magnetic Materials properties for machines High magnetic permeability, so that even a weak current flowing in the electromagnet can set up large flux in its core. High electrical resistivity in order to decrease the eddy current losses of the magnetic material. The hysteresis loop of the magnetic material should be narrow and must have a small area, in order to reduce the hysteresis loss.

28 28 Regards Arun Xavier