Introduction to Magnetism and Magnetic Property.pptx

Magnetism and Magnetic Property

1. Syllabus Electrical Machines Part: Magnetism, Magnetomotive Force, Single phase transformer-equivalent circuit and laboratory testing, introduction to three phase transformers. DC generator: principle, types, performances and characteristics. D C Motor: principles, types of motor, performances, speed control, starters and characteristics. A C Machines: three phase induction motor principles, equivalent circuit. Introduction to synchronous machines and fractional horsepower motors .  

2. Magnetism Magnetism: Magnetism is defined as the property of an object to attract certain metallic substances . Non-Ferrous Type : Nickel Cobalt Gadolinium Ferrous Type : Iron Steel alnico Common Magnetic Materials:

3. Key Point: Opposite poles attract; like poles repel. Magnetic Poles North Pole: The end of the magnet that points towards Earth's magnetic north. South Pole: The opposite end that points to Earth's magnetic south . Magnetism If the magnet is suspended to swing freely, it will align itself with the earth's magnetic poles. Magnetic Fields: Magnetic fields are the areas around a magnet where its force can be felt. This field always exists between the poles of a magnet Fig. 1 Magnetic Field Around a Magnet

4. Molecules as Tiny Magnets: Each molecule in an iron bar acts like a small magnet with a north and south pole. In an unmagnetized iron bar, these tiny magnets (molecules) are arranged randomly . Because of this random arrangement, the overall magnetic field cancels out, so the bar doesn’t act like a magnet. Unmagnetized State: When you apply a magnetizing force, like rubbing the bar with a lodestone (a naturally magnetic rock), the molecules start to align . Magnetizing the Bar: All the north poles of the molecules point in one direction, and all the south poles point in the opposite direction. The bar becomes magnetized and behaves like a magnet. Result: This alignment causes the magnetic fields of the molecules to combine, creating a strong overall magnetic field in the bar.

5. Exploring Magnetic Fields We can see a magnetic field using a simple experiment. Visualizing Magnetic Fields: Observation : Lightly tap the sheet to help the filings settle . The Experiment: Setup: Place a bar magnet under a sheet of transparent material (like glass or Lucite™). Iron Filings: Slowly sprinkle iron filings on the sheet. These lines represent the magnetic field. What You See: The iron filings form a pattern of lines from the magnet's north pole to its south pole. The strongest magnetic flux is at the poles (ends) of the magnet. Key Concepts: Magnetic Flux: The lines of force are also called magnetic flux. Properties of the Field: The lines never cross each other; they repel and stay parallel.

6. These ends are called the north and south poles . Magnetic Field Concentration: The magnetic field is strongest at the ends of a magnet. The lines of magnetic force, known as magnetic flux , are most concentrated at the poles. Once this limit is reached, the material is said to be saturated —no more lines of force can be added. Magnetic Saturation: When a magnetic material is exposed to a magnetizing force (like rubbing with a magnet), it builds up magnetic flux. Saturation Point: There is a limit to how many lines of force can fit in a magnet. Dividing a Magnet: Each Piece Becomes a Magnet: If you cut a bar magnet, each piece will have its own north and south poles, forming new magnets. This shows that each molecule in a magnet is like a tiny magnet.

7. Comparison : It is similar to resistance in an electrical circuit. What is Reluctance? Definition: Reluctance is the measure of opposition to the flow of magnetic lines of force through a material. Air has a high reluctance , making it more difficult for magnetic lines of force to pass through. Reluctance in Materials: Soft Iron vs. Air: Soft iron has a low reluctance , meaning it allows magnetic lines of force to pass through easily. The magnetic lines of force follow the path of least reluctance, which is through the soft iron. Example of Reluctance: Distorting Magnetic Field: When a piece of soft iron is placed near a magnet, it distorts the magnetic field. Reluctance in Magnetic Circuits

8. Magnetic Circuits and Electrical Circuit Comparison Unit : Measured in gilberts (F) . Magnetomotive Force (MMF):Definition: MMF is the force that drives the magnetic lines of force (flux) through a magnetic circuit. Comparison: Similar to electromotive force (EMF) or electrical pressure in an electrical circuit . A magnetomotive force of one gilbert will create one maxwell (line of force) in a material when the reluctance of the material is one. Law of Magnetic Circuits: Implication : This indicates that Earth itself acts like a giant magnet. Earth's Magnetic Nature: Observation: A freely suspended magnet (like a compass needle) always aligns in the North-South direction, even when no other magnets are nearby. Earth's magnetic field influences navigational tools, like compasses, by aligning them with the planet's magnetic poles.

9. Magnetization and Demagnetization Magnetization is needed to restore weakened magnets or create magnetic tools. Demagnetization is necessary when unwanted magnetism occurs. Testing Material Suitability: Result : If the object retains minimal magnetism after a few minutes, it is not suitable as a permanent magnet. Bring the material close to a strong magnet. Use the magnetized item to pick up iron filings or paper clips.

10. Magnetization Methods Caution : Hammering can reduce the magnetic field strength of an existing magnet. Hammering Method : Hammering a rod across the field lines (East-West) can demagnetize it. Hammer a rod aligned with a magnetic field (North-South) to magnetize it. About 20 passes can achieve the best results. Stroking: Single Touch: Draw a magnet over the rod along its length. Divided Touch: Use two magnets in a mirroring action to produce a stronger magnet. Caution: Ensure correct polarity to avoid creating magnets with two like poles (consequent poles). Cooling Method: Heat a bar above its Curie point and then cool it in alignment with the Earth's magnetic field. Demagnetization: Allow the bar to cool in an East-West orientation, shielded from magnetic influences.

11. Electricity Method: The field strength is proportional to the current. Pass a current through a coil to produce a magnetic field. If the current flows clockwise at one end of the coil, it produces a south-seeking pole; the other end will produce a north-seeking pole. A higher current for a short duration is most efficient.

12. Magnetic Shielding Magnetic shielding does not "block" magnetic flux but redirects it. The flux lines can pass through all materials, but some materials offer less resistance, allowing easier passage. Shielding Mechanism: Flux Lines: Magnetic lines of force can pass through different materials, but they prefer paths with higher permeability. Instruments can be shielded by surrounding them with materials like soft iron that offer an easier path for magnetic flux. Figure 10-14 : An instrument surrounded by a soft iron path, which redirects the flux lines away from the instrument. Ferromagnetic Materials: (e.g., iron, steel) have high permeability, offering the least opposition to flux. Paramagnetic Materials: (e.g., nickel, cobalt) have slightly higher permeability than nonmagnetic materials but less than ferromagnetic materials. Diamagnetic Materials: (e.g., bismuth) have a permeability less than one, offering more resistance to flux lines. Shielding is essential in protecting sensitive instruments from magnetic interference, ensuring accurate readings and functionality.

13. Types and Characteristics of Magnets Natural vs. Artificial Magnets: Natural Magnets : Example : Lodestones Practical Use: Limited; not considered in modern applications. Artificial Magnets: Types: Permanent Magnets: Retain magnetism after magnetizing force is removed. Temporary Magnets: Lose most magnetism quickly when the external force is removed. Key Points: Permanent Magnet Materials: Common Materials: Alnicos: Made from Aluminum-Nickel-Cobalt. Ferrites (Ceramics): Strontium-Iron based. Neomagnets : Neodymium-Iron-Boron composition. Samarium-Cobalt: Known for strong magnetic properties. Special Alloys: Alnico: Considered one of the best for permanent magnets. Other Alloys: Remalloy ™ and Permendur ™ are also noted for excellent magnetic qualities.

14. Types and Characteristics of Magnets Magnetic Retentivity: Retentivity: Ability of a magnet to retain its magnetism. Soft Iron: Easily magnetized, but loses magnetism quickly after removal of the magnetizing force. Residual Magnetism: The small amount of magnetism that remains, important in applications like generator operation. Applications: Instruments and Electrical Devices: Magnets are critical in various devices such as generators, motors, and transformers, with their shape and material influencing their effectiveness.

15. Oersted's Discovery and Its Implications Key Insight: The magnetic field generated by the current has no direct connection with the conductor's material, as seen with nonmagnetic copper. Oersted's Experiment: Observation: A compass needle is deflected when brought near a current-carrying conductor. Significance: The deflection demonstrated that an electric current creates a magnetic field around a conductor. Figures 10-16 and 10-17: Demonstrate the magnetic field's expansion with increasing current flow. Magnetic Field Around a Conductor: Magnetic Field Representation: Illustrated as concentric circles around the wire, showing how the field radiates outward from the conductor.

16. As current flow increases, the magnetic field expands, with new lines of force forming further out from the conductor. Current-Field Relationship : The magnetic field exists as long as the current flows . The strength of the magnetic field increases with the current. The understanding of the magnetic field surrounding a current-carrying wire became crucial in designing electrical devices and systems. Foundation for Electromagnetism: Oersted's discovery laid the groundwork for electromagnetism, leading to the development of electromagnets and numerous modern technologies.

17. When the current stops, the magnetic field collapses, and the magnetism around the conductor disappears. Magnetic Field Behavior with Current: Steady Direct Current: A constant direct current creates a stationary magnetic field around the conductor. Similarly , doubling the number of loops in the wire also doubles the field strength Field Strength Adjustment : The strength of the magnetic field can be increased by either increasing the current flow or the number of loops (turns) in the wire. Doubling Current or Loops: Doubling the current approximately doubles the magnetic field strength.

18. If the bar is free to move, it will be drawn into the coil toward the center, where the magnetic field is strongest (Figure 10-18). Role of the Core Material: Iron Core Attraction: A soft iron bar is attracted to the poles of a permanent magnet and similarly to the magnetic field of a current-carrying coil. The lines of force extend through the soft iron, magnetizing it by induction and pulling the bar toward the coil. Some devices, like solenoids or solenoid switches/relays, operate based on the principle that an iron core held away from the center of a coil will be rapidly pulled into a central position when the coil is energized. This movement can complete a circuit, allowing the solenoid to act as a switch. Applications of Electromagnets: Usage in Devices: Electromagnets are integral components in electrical instruments, motors, generators, relays, and other devices. Principle of Operation in Solenoids:

19. Magnetic Field around a Current-Carrying Conductor Compass Needle and Magnetic Field: Demonstration Setup: A compass needle is placed at a right angle, about one inch from a current-carrying conductor (Figure 10-19). Without current flow, the compass needle aligns with the Earth's magnetic pole. Effect of Current Flow: When current flows through the conductor, the compass needle realigns itself perpendicular to the radius from the conductor. The needle turns so that the direction of its magnetic field lines aligns with the circular magnetic field lines around the conductor.

20. Magnetic Field around a Current-Carrying Conductor Reversing Current Flow: If the current flow through the conductor is reversed, the compass needle also reverses direction, indicating that the magnetic field has reversed its direction as well. Direction of Magnetic Field: Circular Magnetic Field: As the compass needle is moved around the conductor, it maintains a position at right angles to the conductor, confirming the circular nature of the magnetic field around the current-carrying conductor. If you grasp the conductor with your left hand, with your thumb pointing in the direction of current flow, your fingers will naturally curl around the conductor in the same direction as the magnetic field lines. Left-Hand Rule for Determining Field Direction: Procedure: The direction of the magnetic field lines can be determined using the left-hand rule (Figure 10-20).

21. Magnetic Field around a Current-Carrying Conductor Current Flow Indication: The direction of current flow (from negative to positive) is often represented in illustrations with a dot on the conductor when electrons flow toward the observer and a plus sign when current flows away (Figure 10-21). Understanding Magnetic Lines of Force: Although the lines of force seem to have a directional tendency (clockwise or counterclockwise), they do not actually move or revolve around the conductor.

22. Magnetic Field around a Current-Carrying Conductor Left-Hand Rule Application: If you grasp the loop with your left hand, with your fingers following the direction of the current, your thumb will point to the magnetic north pole of the loop. Magnetic Field in a Single Loop: When a wire is bent into a loop and an electric current flows through it, the left-hand rule still applies (Figure 10-22).

23. Magnetic Field around a Current-Carrying Conductor

24. Magnetic Field around a Current-Carrying Conductor

25. Magnetic Field around a Current-Carrying Conductor Formation of a Coil: When a wire is coiled into many loops, it forms a coil. In this coil, the lines of force form a pattern through all the loops, leading to a high concentration of flux lines through the center of the coil (Figure 10-24). Magnetic Field in Multiple Loops (Coil Formation): Two Loops: If the wire is coiled into two loops, many of the lines of force become large enough to encompass both loops. The lines of force pass through the loops in the same direction, circle around the outside of the coils, and re-enter at the opposite end (Figure 10-23).

26. Magnetic Field around a Current-Carrying Conductor Polarity Reversal: When direct current flows through the coil, the core is magnetized with the same polarity as the coil would have without the core. Reversing the current flow reverses the polarity of the electromagnet. Concentration of Magnetic Field with a Soft Iron Core: Effect of a Soft Iron Core: In a coil made of multiple loops, many lines of force are dissipated between the loops. However, placing a soft iron bar inside the coil concentrates the lines of force in the center of the coil, due to the higher permeability of soft iron compared to air (Figure 10-25). Electromagnet Formation: This combination of a coil with a soft iron core is called an electromagnet. The ends (poles) of the coil exhibit characteristics similar to those of a bar magnet. The soft iron core increases the magnetic flux and concentrates the flux lines.

27. Magnetic Field around a Current-Carrying Conductor Using the Left-Hand Rule: To determine the polarity of an electromagnet, the left-hand rule is applied similarly as with a coil without a core. Grasp the coil with your left hand so that your fingers curve in the direction of electron flow (from minus to plus), and your thumb will point in the direction of the north pole (Figure 10-26). Determining Electromagnet Polarity with the Left-Hand Rule:

28. OTHER MAGNETO-ELECTRICAL PHENOMENA Magnetomotive Force (MMF):Definition : Magnetomotive force (MMF), or magnetic potential, is the property of certain substances that generates a magnetic field. It is analogous to electromotive force (EMF) or voltage in electrical circuits. Conversion : To convert from ampere-turns to gilberts, multiply by 1.26 . To convert from gilberts to ampere-turns, multiply by 0.796 . Unit of Measurement: The standard unit for MMF is the ampere-turn (AT) , which represents the magnetic potential generated by a steady current of one ampere flowing in a conducting material in a vacuum. Another unit sometimes used is the gilbert (G) . The gilbert is slightly smaller than the ampere-turn. Permanent Magnets and MMF : Permanent magnets, as well as planets like Earth with their own magnetic fields, also exhibit magnetomotive force (Figure 10-27).

29. OTHER MAGNETO-ELECTRICAL PHENOMENA Distinction : Field strength focuses on the force per unit current, while flux density focuses on the force per unit area of the magnetic field. Field Strength: Magnetic Field Strength: One way to describe the intensity of a magnetic field is through magnetic field strength , measured in amperes per meter . Magnetic Flux Density: Another method is magnetic flux density , measured in teslas (T) , which represents the magnetic force per unit area. Flux density is measured in Newton-meters per ampere .

30. OTHER MAGNETO-ELECTRICAL PHENOMENA It is proportional to the current (amperes) in the wire. Magnetic Field and Flux : Magnetic Field Lines: Magnetic fields can be visualized as lines of force. Field Strength: The density of these lines corresponds to the strength of the magnetic field. Magnetic Flux: Definition: The total number of magnetic field lines passing through an area. Unit: Measured in tesla meters squared (T·m²) . Flux Density and Distance: The magnetic flux density decreases with distance from a current-carrying wire.

31. OTHER MAGNETO-ELECTRICAL PHENOMENA Magnetic Flux Density : Measures the strength of the magnetic field at a specific point. Unit: The tesla (T) is the standard unit. 1 tesla = 10,000 gauss (G ) . Teslas are typically used in industrial electromagnetics, while gauss is more common for smaller applications like in aviation. Effect of Environment: Permeability can change with temperature or magnetic field intensity. Permeability: A measure of how easily magnetic flux can pass through a material. Unit: 1.257 × 10⁻⁶ henry per meter (H/m) in a vacuum .

32. Hysteresis Loop and Magnetic Properties As magnetizing force increases, magnetic flux density also increases until the material becomes magnetically saturated (Point A). Hysteresis Loop Overview: Magnetizing Process : Begins at the origin with a non-magnetized material. Retentivity (Point B ): When magnetizing force is reduced to zero, some magnetic flux remains in the material, demonstrating its retentivity. Reverse Saturation (Point D ): Increasing the reversed magnetizing force leads to opposite polarity saturation. Coercivity (Point C ): Applying a reverse magnetizing force removes the remaining flux, known as coercivity . Return to Initial State (Point E ): Removing the reverse force leaves some residual flux due to retentivity. Increasing the original magnetizing direction once again builds up to saturation, but does not pass through the origin because of the retained flux.

33. Hysteresis Loop and Magnetic Properties Retentivity : The ability of a material to retain some magnetism after the magnetizing force is removed. Coercive Force: The intensity of an external magnetic field needed to demagnetize a ferromagnetic material after it has been magnetized. Reluctance : The opposition within a magnetic circuit to magnetic flux, similar to electrical resistance in an electrical circuit. Saturation Point : The state when no further increase in magnetizing force can increase the magnetization of the material. The magnetic flux density levels off, and the magnetization curve shows a plateau. Eddy currents are induced in conductors when there is a change in the magnetic field. They can generate heat and reduce the efficiency of devices that rely on changing magnetic fields Formation and Impact: Eddy Currents and Their Effects: Mitigation Techniques : Using materials with high magnetic permeability and low electrical conductivity helps minimize eddy currents. Laminations in magnetic cores also suppress these currents.

34. Temperature Considerations: Most magnets maintain their properties well under typical conditions but may lose magnetism if heated above specific temperatures (usually around 400°C). Care and Storage of Magnets Handling and Environment: Magnets should be handled carefully to avoid drops or shocks. They should be stored in dry, room-temperature environments. Storage Tips: Storing magnets with opposite poles together is recommended. Using a keeper across the poles can help maintain their strength.

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