Emirates Aviation University - Module 3: Electrical Fundamentals - Topic 3.10: Magnetism PDF

Module 3: Electrical Fundamentals Topic 3.10: Magnetism  INTRODUCTION On completion of this topic you should be able to: 3.10.1 Describe the theory of magnetism. 3.10.2 Describe the properties of a magnet 3.10.3 Describe the...

Module 3: Electrical Fundamentals Topic 3.10: Magnetism  INTRODUCTION On completion of this topic you should be able to: 3.10.1 Describe the theory of magnetism. 3.10.2 Describe the properties of a magnet 3.10.3 Describe the action of a magnet suspended in the earth’s magnetic field. 3.10.4 Describe magnetisation and de-magnetisation 3.10.5 Describe magnetic shielding 3.10.6 Describe the various types of magnetic materials 3.10.7 Identify electromagnets and describe their construction and principles of operation. 3.10.8 Describe the hand clasp rule to determine the magnetic field direction around a current carrying conductor. continued… 30-04-2024 Slide No. 2  INTRODUCTION On completion of this topic you should be able to: 3.10.9 Describe the following regarding magnetism: Magnetomotive force Field strength Magnetic flux density Permeability Hysteresis loop Retentivity Coercive force Reluctance Saturation point Eddy currents 3.10.10 Describe precautions for care and storage of magnets. 30-04-2024 Slide No. 3  INTRODUCTION TO MAGNETISM To understand principles of electricity – knowledge of magnetism is necessary. Magnetism and electricity are closely related. Study of either subject would be incomplete without a basic knowledge of other. Most electrical and electronic equipment could not function without magnetism. Computers Video equipment High-fidelity speakers Electrical motors Generators 2 theories of magnetism: Weber’s Theory Domain Theory 30-04-2024 Slide No. 4  WEBER’S THEORY OF MAGNETISM Assumes all magnetic substances are composed of tiny molecular magnets. Un-magnetised material: Molecular magnets’ forces neutralised by adjacent molecular magnets Eliminates any magnetic effect Magnetised material: Most of its molecular magnets lined up – North 1 direction, South other One effective north pole, and one south pole 30-04-2024 Slide No. 5  WEBER’S THEORY OF MAGNETISM An illustration of Weber's Theory – Steel bar magnetised by stroking. When stroked several times in same direction by magnet – molecules align. Magnetic force from north pole of magnet causes molecules to align themselves. 30-04-2024 Slide No. 6  DOMAIN THEORY More modern theory of magnetism is based on electron spin principle. All matter composed of atoms – each atom contains 1 or more orbital electrons. Electrons orbit in various shells around nucleus – similar to solar system. Along with its orbital motion about sun, each planet also revolves on its axis. It is believed the electron also revolves on its axis as it orbits nucleus of an atom. 30-04-2024 Slide No. 7  DOMAIN THEORY An electron has a magnetic field about it: Effectiveness of field – determined by number of electrons spinning in each direction. If equal numbers spinning in opposite directions – magnetic fields cancel each other. Atom is un-magnetised. 30-04-2024 Slide No. 8  DOMAIN THEORY If more electrons spin in one direction than other – atom is magnetised. An iron atom has 26 protons in nucleus and 26 revolving electrons orbiting nucleus. If 13 electrons spin clockwise and 13 counterclockwise – magnetically neutral If more than 13 electrons spin in either direction – atom is magnetised 30-04-2024 Slide No. 9  MAGNETIC MATERIALS 0.1 kg magnet levitating showing repulsion Magnetism – property of a material which enables it to attract pieces of iron. A material possessing this property is known as a magnet. Materials that are attracted by magnets have the ability to become magnetised. Magnetic materials – iron, steel, nickel and cobalt. Materials which are not attracted by magnets are not able to become magnetised. Non-magnetic materials – paper, wood, glass and tin. 30-04-2024 Slide No. 10  MAGNETIC MATERIALS Ferromagnetic materials are those which are relatively easy to magnetise. Iron Steel Cobalt Alloys - Alnico and Permalloy An alloy is made from combining 2 or more elements, one of which must be a metal. New alloys can be strongly magnetised - capable of lifting 500 times own weight. Ferromagnetic material will become a magnet when placed close to a magnet. 30-04-2024 Slide No. 11  NATURAL MAGNETS Magnetic stones as found by ancient Greeks are considered to be natural magnets. These stones have ability to attract small pieces of iron – similar to today’s magnets. Magnetic properties were products of nature – Greeks called substances magnetite. Chinese aware of effects of magnetism as early as 2600 B.C. Observed stones (similar to magnetite) when freely suspended pointed North/South. Because of directional quality of these, referred to as lodestones or leading stones. Natural magnets no longer have any practical use. More powerful magnets can easily be produced. 30-04-2024 Slide No. 12  ARTIFICIAL MAGNETS Magnets produced from magnetic materials are called artificial magnets. Generally made from special iron or steel alloys – usually magnetised electrically. Material to be magnetised is inserted into a coil of insulated wire. To magnetise – a heavy flow of electrons is passed through a coil. Artificial magnets are usually classified as permanent or temporary. Depends on ability to retain properties after magnetising force has been removed. 30-04-2024 Slide No. 13  PERMANENT MAGNETS Permanent Magnets – difficult to magnetise but retain a great deal of magnetism. Reluctance – opposition that a material offers to magnetic lines of force. Permanent magnets – produced from materials having high reluctance. 30-04-2024 Slide No. 14  TEMPORARY MAGNETS Made with materials of low reluctance – relatively easy to magnetise. Will retain only a small part of its magnetism once magnetising force is removed. Low reluctance materials – soft iron or annealed silicon steel. Temporary magnets are materials that easily lose most of their magnetic strength. Residual magnetism – Amount of magnetism which remains in a temporary magnet. Retentivity – ability of a material to retain an amount of residual magnetism. 30-04-2024 Slide No. 15  MAGNETIC POLES Magnetic force surrounding a magnet is not uniform. Great concentration of force at each end of magnet and weak force at centre. Proof of this fact can be obtained by dipping a magnet into iron filings. Many filings will cling to ends of magnet while very few adhere to the centre. The 2 ends – regions of concentrated lines of force – called POLES. Magnets have 2 magnetic poles and both poles have equal magnetic strength. 30-04-2024 Slide No. 16  EARTH'S MAGNETIC POLES Earth is a huge natural magnet. Distribution of magnetic force about earth is same as that of a giant bar magnet. Magnetic axis of earth is located about 15° from its geographical axis. This magnetic pole is named the magnetic North Pole. Has polarity of a south magnetic pole as it attracts north pole of a compass needle. Reason for this conflict in terminology – traced to early users of the compass. 30-04-2024 Slide No. 17  LAW OF MAGNETIC POLES If bar magnet is suspended freely on string, it will align itself in a north south direction. Same pole of magnet will always swing toward the North magnetic pole of the earth. Therefore, it is called the north-seeking pole or simply the NORTH POLE. The other pole of magnet is the south-seeking pole or the SOUTH POLE. A practical use of the directional characteristic of the magnet is the compass: Compass - freely rotating magnetised needle indicator that points towards North pole A bar magnet acts as a compass 30-04-2024 Slide No. 18  LAW OF MAGNETIC POLES The law previously stated regarding the attraction and repulsion of charged bodies? Coulomb’s Law5 This law may also be applied to magnetism if pole is considered as a charge. North pole of a magnet will always be attracted to South pole of another magnet. A North pole will show a repulsion to a north pole – same for South poles. The law for magnetic poles is: Like poles repel, unlike poles attract 30-04-2024 Slide No. 19  MAGNETIC LINES OF FORCE Lines are used to represent force existing in the area surrounding a magnet. These lines, called MAGNETIC LINES OF FORCE, do not actually exist. Used to illustrate and describe the pattern of magnetic field. Assumed to emanate from North pole of a magnet and enter the South pole. Lines of force then travel inside the magnet from South pole to the North pole. 30-04-2024 Slide No. 20  MAGNETIC LINES OF FORCE Lines of force change when 2 magnetic poles are brought close together. Mutual attraction or repulsion of poles produces a more complicated pattern. 30-04-2024 Slide No. 21  PROPERTIES OF MAGNETIC LINES OF FORCE (1) Although being imaginary, magnetic lines of force have many important properties: They are continuous and will always form closed loops They will never cross one another Lines that are close together indicate a STRONG magnetic field Lines that are farther apart indicate a WEAK magnetic field 30-04-2024 Slide No. 22  PROPERTIES OF MAGNETIC LINES OF FORCE (2) No actual movement occurs although they are considered to have direction. Flow from North to South pole outside Flow from South to North pole within material They pass through all materials, both magnetic and nonmagnetic. They always enter or leave a magnetic material at right angles to the surface. 30-04-2024 Slide No. 23  PROPERTIES OF MAGNETIC LINES OF FORCE (3) Parallel magnetic lines of force traveling in the same direction repel one another. Parallel magnetic lines of force traveling in opposite directions tend to unite with each other and form into single lines traveling in a direction determined by the magnetic poles creating the lines of force. They tend to shorten themselves They existing between two unlike poles cause poles to be pulled together 30-04-2024 Slide No. 24  DC MAGNETISERS Employ large coils through which a current is applied for a short duration. Current flowing through the coil produces a magnetic field. Field is usually directed by use of iron cores and pole pieces. Magnets are placed in the gap between the pole pieces. Only practical for magnetising Alnico or small sections of Ceramic materials. These materials have a low magnetising force requirement 30-04-2024 Slide No. 25  CAPACITOR DISCHARGE MAGNETISERS Employ capacitor banks that are charged, and then discharged through a coil. Generates a very short pulse of very high electric current. A few milliseconds long, with currents of a few hundred to over 100,000 amps. Electrical pulse is used to cause a brief but very strong magnetic field. Extremely high magnetising fields can be achieved. 30-04-2024 Slide No. 26  DEMAGNETISATION Removal of a field (demagnetisation) may be accomplished in several ways: Heating material above its curie temperature – 770ºC for a low carbon steel Subjecting component to a reversing and decreasing magnetic field Can be accomplished by pulling a component from a coil with AC passing through it. 30-04-2024 Slide No. 27  DEMAGNETISATION Placing the magnet in an alternating magnetic field with an intensity above the material's coercivity and then either slowly drawing the magnet out or slowly decreasing the magnetic field to zero. This is the principle used in commercial demagnetizers to demagnetize tools or parts and erase credit cards and hard disks, and degaussing coils used to demagnetize CRTs. 30-04-2024 Slide No. 28  DEMAGNETISATION Hammering or jarring: the mechanical disturbance tends to randomize the magnetic domains. This will leave some slight residual magnetization. 30-04-2024 Slide No. 29  CONFIGURATION TYPES OF MAGNETISATION 30-04-2024 Slide No. 30  MAGNETIC SHIELDING Sensitive mechanisms of electric instruments can be influenced by magnetic fields. Necessary to employ some means of directing flux around the instrument. Flux will deviate from an air path to pass more readily through a ferromagnetic path. This behaviour of flux suggests an easy way to magnetically shield an area. If magnetic material (e.g. soft iron) is placed in a magnetic field – flux is redirected. Magnetic material has greater permeability than air. Material with a high permeability is used for magnetic shielding. 30-04-2024 Slide No. 31  RELUCTANCE AND PERMEABILITY Difference between a permanent and a temporary magnet – reluctance. Permanent magnet has high reluctance Temporary magnet has low reluctance Magnets are also described in terms of the permeability of their materials. Permeability – ability of a material to act as a path for magnetic lines of force. Permanent magnet – produced from material with high reluctance, low permeability. Temporary magnet – produced from material with low reluctance, high permeability. 30-04-2024 Slide No. 32  SATURATION POINT Unmagnetised Magnetised Material Material Ferromagnetic materials become magnetised when magnetic domains are aligned. Materials can be magnetised by: Placing material in a strong external magnetic field Passing electrical current through material Some or all of the domains can become aligned. The more domains that are aligned – the stronger the magnetic field in material. When all domains are aligned – material is said to be magnetically saturated. At saturation – no additional amount of magnetisation force will increase flux. 30-04-2024 Slide No. 33  HYSTERESIS If an alternating magnetic field is applied to material, its magnetisation will trace out a loop called a hysteresis loop. Magnetic hysteresis is the lag of an effect after its cause. Changes of flux density (B) lag behind the changes in magnetising force (H). Different quality magnetic materials have different shape hysteresis loops. Hard materials (high retentivity) have wide loops. Soft materials (low retentivity) have narrow loops. 30-04-2024 Slide No. 34  COERCIVITY AND REMANENCE IN PERMANENT MAGNETS Desirable properties are typically stated in terms of remanence and coercivity. When ferromagnetic material is magnetised in one direction, it will not relax back to zero magnetisation when the imposed magnetising field is removed. Amount of magnetisation it retains at zero driving field is called its remanence. Remanence is also known as retentivity. It must be driven back to zero by a field in the opposite direction. Amount of reverse driving field required to demagnetise it is called its coercivity. 30-04-2024 Slide No. 35  COERCIVITY AND REMANENCE IN PERMANENT MAGNETS Coercivity is also known as coercive force. Both coercivity and remanence are quoted in Tesla. Permanent magnets use materials with high remanence and high coercivity. These materials are sometimes said to be "magnetically hard“. "Magnetically soft" materials used for transformer cores and coils for electronics. 30-04-2024 Slide No. 36  RETENTIVITY / REMANENCE / RESIDUAL MAGNETISM Remanence and Retentivity are the same: Amount of magnetic flux density a material retains when the magnetising force is removed after achieving saturation (value of B at point b on hysteresis curve). Residual Magnetism – amount of magnetic flux density that remains in a material when magnetising force is zero. Residual magnetism and retentivity are same when material has been magnetised to saturation point. May be lower than retentivity value when magnetising force did not reach saturation. 30-04-2024 Slide No. 37  HYSTERESIS 30-04-2024 Slide No. 38  ELECTROMAGNETIC FIELDS Magnets are not the only source of magnetic fields. 1820 – discovered current flowing through a wire causes a compass to deflect. Indicates that current in a wire generates a magnetic field. Magnetic field exists in circular form around wire. Intensity of field is directly proportional to amount of current carried by wire. Strength of field is strongest close to wire and diminishes with distance. In most conductors – magnetic field exists only as long as current is flowing. 30-04-2024 Slide No. 39  LEFT-HAND GRASP RULE Direction of magnetic field dependent on direction of electrical current in the wire. Simple rule for remembering direction of magnetic field around a conductor. Left-Hand Rule Grasp conductor in left hand with the thumb pointing in the direction of current. Fingers will circle conductor in direction of magnetic field. This rule is used for Electron Flow only. 30-04-2024 Slide No. 40  MAGNETIC FIELD PATTERNS Magnetic field around conductors carrying current Imagine you are looking at conductors end on. Cross indicates tail of arrow / feather / conductor (heading away from you). Point indicates front of arrow / conductor (coming towards you). Using Left-Hand Rule, magnetic field direction is worked out. 30-04-2024 Slide No. 41  PARALLEL CONDUCTORS When 2 conductors placed in parallel – will attract or repulse. When current is in same direction – magnetic fields combine. 30-04-2024 Slide No. 42  ELECTROMAGNETIC FIELDS Magnetic field around a single loop If our single strand of wire is coiled up into a loop, magnetic field is strengthened. Lines of force which go around the conductor join up and strengthen each other. By coiling the wire, its inductance increases. 30-04-2024 Slide No. 43  EFFECT OF TURNS The field around a coil of wire If more turns added to loop to create a coil, magnetic field is further strengthened. Fields now contract to drive a field throughout the entire length of the coil. This field is identical in form to the field obtained from a bar magnet. 30-04-2024 Slide No. 44  LEFT- HAND GRASP RULE Left-Hand grip rule can be applied in a different way to determine polarity of a coil. Wrap fingers around the coil in direction of current flow (electron flow). Thumb will point to the North pole of the electromagnet. Electromagnetic coils are used extensively in all types of industry: Relays Contactors Solenoids 30-04-2024 Slide No. 45  ELECTROMAGNETIC FIELD STRENGTH Strength of an electromagnet depends upon 4 factors: Type of core material Size and shape of core material Number of turns on coil SMALL AMOUNT Amount of current in coil OF INDUCTANCE LARGE AMOUNT OF INDUCTANCE GREATEST LESS LEAST LESS GREATER LESS GREATEST SHAPE OF CORE 30-04-2024 Slide No. 46  MAKE YOUR OWN ELECTROMAGNET Materials required: Iron Nail Insulated wire Scotch Tape Scissors 1.5 volt Battery (any size) Wind insulated wire tightly around nail. Tape one end of wire to battery –. Keep other end of wire within reach of battery +. Electromagnet is ready. Touch battery + with wire and test on some paper clips. Remove wire from battery +. If nail is made from pure iron – paper clips will drop off slowly. If nail contains some steel – nail becomes a permanent magnet. 30-04-2024 Slide No. 47  ELECTROMAGNETS – BASIC RELAY A – Electromagnet B – Spring to retract contact when magnet is not energised C & D – Controlled circuit – ON when magnet is energised OFF when not energised E – Power to energise electromagnet 30-04-2024 Slide No. 48  ELECTROMAGNET OPERATION Electromagnet used with a contact arrangement (as in a relay or contactor). Contact arm is attracted when electromagnet is powered. 30-04-2024 Slide No. 49  ELECTROMAGNETS 30-04-2024 Slide No. 50  MAGNETOMOTIVE FORCE Also known as MMF/mmf or magnetic potential. This is represented by H on the B-H curve. MMF – flux producing ability of an electric current in a magnetic circuit. Similar to electromotive force in an electric circuit. Standard unit of mmf is the ampere-turn (AT). 30-04-2024 Slide No. 51  MAGNETOMOTIVE FORCE Standard definition of MMF involves current passing through an electrical conductor. Dependent on how much current flows in the turns of coil: The more current, the stronger the magnetic field The more turns of wire, the more concentrated the lines of force. 30-04-2024 Slide No. 52  MAGNETOMOTIVE FORCE Calculate the mmf for a coil with 1000 turns and a 5 mA current. NI = 5 AT Calculate the mmf for a coil with 150 turns and a 500 mA current. NI = 75 AT 30-04-2024 Slide No. 53  MAGNETIC FIELD STRENGTH Field intensity depends on the length of the coil. When an electromagnetic coil is stretched to twice its length – force is halved. 30-04-2024 Slide No. 54  MAGNETIC FIELD STRENGTH Base unit of magnetic field strength - the ampere-turn per meter (A. t/m). H = MMF / Length. 30-04-2024 Slide No. 55  FIELD INTENSITY / FIELD STRENGTH Calculate the field intensity of an 80 turn, 20 cm coil, with 6A of current 2400 At/m. The coil used previously with the same current is now wound around an iron core 40 cm in length. Find the field intensity 1200 At/m. 30-04-2024 Slide No. 56  MAGNETIC FLUX Magnetic flux is a measure of the amount of magnetic field passing through a surface. Base unit of magnetic flux is the weber (1 weber per square metre is 1 tesla). A weber (Wb) is defined only in terms of a change in the flux in a magnetic circuit. 1 Wb - amount of flux change required in 1 sec to induce 1 Volt in a single conductor. 30-04-2024 Slide No. 57  MAGNETIC FLUX DENSITY Number of magnetic lines of force cutting through a plane of a given area at a right angle is known as the magnetic flux density (B). Unit of flux density (or magnetic induction) – tesla. One tesla is equal to 1 Newton/(Am) A 14 kg speaker is approx. 1T Earths magnetic field between 30-60μT Quantity Symbol SI Units CGS Units Flux Density B tesla gauss (Magnetic Induction) 30-04-2024 Slide No. 58  MAGNETIC FLUX DENSITY Flux density is a measure of force applied to a particle by the magnetic field. The gauss is CGS (Centimeters/Grams/Seconds) unit for flux density and is commonly used by US industry (1 Tesla = 10000 Gauss). One gauss represents one line of flux passing through one square centimeter of air oriented 90 degrees to flux flow. 30-04-2024 Slide No. 59  EDDY CURRENTS Eddy currents flowing in a conductor Eddy currents are produced in any conductor (magnetic or non-magnetic): Which is moving so as to "cut through" a magnetic field, or Is surrounded by a changing magnetic field A ‘proper’ complete circuit is not necessary for currents to flow. Microscopic currents flow within conductors – known as eddy currents. Such currents can cause magnetic and heating effects. Depending on application, eddy currents may or may not be useful. 30-04-2024 Slide No. 60  EDDY CURRENTS When AC is passed through coil, magnetic field is generated in and around coil. 30-04-2024 Slide No. 61  EDDY CURRENTS When brought in close proximity to conductive material – changing magnetic field generates current flow in material. Induced current flows in closed loops in planes perpendicular to magnetic flux. Eddy currents produce their own magnetic fields. Named so because they resemble eddy currents seen swirling in streams. 30-04-2024 Slide No. 62  CARE OF MAGNETS Keeper Personnel wearing pacemakers should not handle magnets. Keep magnets away from sensitive electronic equipment. Ensure personnel are trained in handling of magnets. Do NOT knock or heat – results in loss of some of its effective magnetism. Care must be exercised when handling instruments containing magnets. Horseshoe magnet should always be stored with a keeper (a soft iron bar). Bar magnets to be stored in pairs with a north pole and a south pole together. 30-04-2024 Slide No. 63  TERMINOLOGY REVIEW Describe the following: Magnetomotive Force - MMF Flux producing ability of an electric current in a magnetic circuit Field Strength Amount of MMF available to create a magnetic field for each unit length of a magnetic circuit Magnetic Flux Density Number of magnetic lines of force cutting through a plane of a given area at a right angle Permeability Ability of a material to act as a path for magnetic lines of force. Retentivity Amount of magnetic flux density a material retains when the magnetising force is removed after achieving saturation 30-04-2024 Slide No. 64  TERMINOLOGY REVIEW Describe the following: Hysteresis Loop If an alternating magnetic field is applied to material, its magnetisation will trace out a loop called a hysteresis loop Magnetic hysteresis is the lag of an effect after its cause. Reluctance Opposition that a material offers to magnetic lines of force Coercive Force Amount of reverse driving field required to demagnetise a material Saturation Point The point at which no additional amount of magnetisation force will increase flux Eddy Currents Circular induced currents generated in a conductor (of any type) by a moving magnetic field, or equivalent Slide No. 65 30-04-2024  CONCLUSION Now that you have completed this topic, you should be able to: 3.10.1 Describe the theory of magnetism. 3.10.2 Describe the properties of a magnet 3.10.3 Describe the action of a magnet suspended in the earth’s magnetic field. 3.10.4 Describe magnetisation and de-magnetisation 3.10.5 Describe magnetic shielding 3.10.6 Describe the various types of magnetic materials 3.10.7 Identify electromagnets and describe their construction and principles of operation. 3.10.8 Describe the hand clasp rule to determine the magnetic field direction around a current carrying conductor. continued… 30-04-2024 Slide No. 66  CONCLUSION Now that you have completed this topic, you should be able to: 3.10.9 Describe the following regarding magnetism: Magnetomotive force Field strength Magnetic flux density Permeability Hysteresis loop Retentivity Coercive force Reluctance Saturation point Eddy currents 3.10.10 Describe precautions for care and storage of magnets. 30-04-2024 Slide No. 67  This concludes: Module 3: Electrical Fundamentals Topic 3.10: Magnetism