* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Copyright 2009 Pearson
Mathematical descriptions of the electromagnetic field wikipedia , lookup
Skin effect wikipedia , lookup
Magnetic stripe card wikipedia , lookup
Electromagnetism wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Electromotive force wikipedia , lookup
Electromagnetic field wikipedia , lookup
Giant magnetoresistance wikipedia , lookup
Magnetic monopole wikipedia , lookup
Magnetometer wikipedia , lookup
Earth's magnetic field wikipedia , lookup
Superconducting magnet wikipedia , lookup
Magnetotactic bacteria wikipedia , lookup
Lorentz force wikipedia , lookup
Magnetotellurics wikipedia , lookup
Magnetoreception wikipedia , lookup
Multiferroics wikipedia , lookup
Magnetohydrodynamics wikipedia , lookup
Force between magnets wikipedia , lookup
Friction-plate electromagnetic couplings wikipedia , lookup
Magnetochemistry wikipedia , lookup
Electromagnet wikipedia , lookup
Ferromagnetism wikipedia , lookup
Electrical Machines and Energy Conversion Unit 1 Deck 1 Magnetism Basics Unit 10 Summary Magnetic Quantities Magnetic fields are described by drawing flux lines that represent the magnetic field. Where lines are close together, the flux density is higher. Where lines are further apart, the flux density is lower. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities The unit of flux is the weber. The unit of flux density is the weber/square meter, which defines the unit tesla, (T), a very large unit. j B= Flux density is given by the equation A where B = flux density (T) Flux lines (j j = flux (Wb) A = area (m2) 2 Area (m) Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities Example: What is the flux density in a rectangular core that is 8 mm by 10 mm if the flux is 4 mWb? j B= A B 4 103 Wb 8 10 m10 10 m Electronics Fundamentals Circuits, Devices and Applications - Floyd -3 -3 50 Wb/m2 = 50 T © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities • Magnetic flux lines surround a current carrying wire. • The field lines are concentric circles. • As in the case of bar magnets, the effects of electrical current can be visualized with iron filings around the wire – the current must be large to see this effect. Iron filings Current-carrying wire Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities • Permeability (m) defines the ease with which a magnetic field can be established in a given material. It is measured in units of the weber per ampere-turn meter. • The permeability of a vacuum (m0) is 4p x 10-7 weber per ampere-turn meter, which is used as a reference. • Relative Permeability (mr) is the ratio of the absolute permeability to the permeability of a vacuum. mr Electronics Fundamentals Circuits, Devices and Applications - Floyd m m0 © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities • Reluctance (R) is the opposition to the establishment of a magnetic field in a material. R l mA R= reluctance in A-t/Wb l = length of the path m = permeability (Wb/A-t m). A = area in m2 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities • Recall that magnetic flux lines surround a current-carrying wire. A coil reinforces and intensifies these flux lines. • The cause of magnetic flux is called magnetomotive force (mmf), which is related to the current and number of turns of the coil. Fm = NI Fm = magnetomotive force (A-t) N = number of turns of wire in a coil I = current (A) Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities • Ohm’s law for magnetic circuits is Fm j R • flux (j) is analogous to current • magnetomotive force (Fm) is analogous to voltage • reluctance (R) is analogous to resistance. What flux is in a core that is wrapped with a 300 turn coil with a current of 100 mA if the reluctance of the core is 1.5 x 107 A-t/Wb ? 2.0 mWb Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities • The magnetomotive force (mmf) is not a true force in the physics sense, but can be thought of as a cause of flux in a core or other material. Iron core Current in the coil causes flux in the iron core. What is the mmf if a 250 turn coil has 3 A of current? 750 A-t Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary • Magnetic field intensity is the magnetomotive force per unit length of a magnetic path. H= Fm l or H = NI l H= Magnetic field intensity (Wb/A-t m) Fm = magnetomotive force (A-t) l = average length of the path (m) N = number of turns I = current (A) • Magnetic field intensity represents the effort that a given current must put into establishing a certain flux density in a material. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic Quantities • If a material is permeable, then a greater flux density will occur for a given magnetic field intensity. The relation between B (flux density) and H (the effort to establish the field) is B = mH m = permeability (Wb/A-t m). H= Magnetic field intensity (Wb/A-t m) • This relation between B and H is valid up to saturation, when further increase in H has no affect on B. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary • As the graph shows, the flux density depends on both the material and the magnetic field intensity. Magnetic material Flux density, B, (Wb//m2) Saturation begins Non-magnetic material Magnetic Field Intensity, H, (At/m) Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary As H is varied, the magnetic hysteresis curve is developed. B B B Saturation Bsat H 0 BR Hsat 0 H Hsat H H= 0 0 H Bsat Saturation (a) (b) (c ) (d) B B HC Bsat H H = 0 0 0 BR H Hsat H H HC HC B (e) Electronics Fundamentals Circuits, Devices and Applications - Floyd B B (f ) (g) © Copyright 2009 Pearson Unit 10 Summary Magnetization Curve A B-H curve is referred to as a magnetization curve for the case where the material is initially unmagnetized. The B-H curve differs for different materials; magnetic materials have in common much larger flux density for a given magnetic field intensity, such as the annealed iron shown here. Flux Density, B, (T) 2.0 1.5 Annealed iron 1.0 0.5 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Magnetic Field Intensity, H, (A-t/m) Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Relative motion S When a wire is moved across a magnetic field, there is a relative motion between the wire and the magnetic field. N S When a magnetic field is moved past a stationary wire, there is also relative motion. In either case, the relative motion results in an induced voltage in the wire. N Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Induced voltage The induced voltage due to the relative motion between the conductor and the magnetic field when the motion is perpendicular to the field is dependent on three factors: • the relative velocity (motion is perpendicular) • the length of the conductor in the magnetic field • the flux density Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Faraday’s law Faraday experimented with generating current by relative motion between a magnet and a coil of wire. The amount of voltage induced across a coil is determined by two factors: S 1. The rate of change of the N magnetic flux with respect to the coil. -V+ Voltage is indicated only when magnet is moving. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Faraday’s law Faraday also experimented generating current by relative motion between a magnet and a coil of wire. The amount of voltage induced across a coil is determined by two factors: S 1. The rate of change of the magnetic flux with respect N to the coil. 2. The number of turns of wire in the coil. -V+ Voltage is indicated only when magnet is moving. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Summary Magnetic field around a coil Just as a moving magnetic field induces a voltage, current in a coil causes a magnetic field. The coil acts as an electromagnet, with a north and south pole as in the case of a permanent magnet. South Electronics Fundamentals Circuits, Devices and Applications - Floyd North © Copyright 2009 Pearson Unit 10 Summary DC Generator A dc generator includes a rotating coil, which is driven by an external mechanical force (the coil is shown as a loop in this simplified view). As the coil rotates in a magnetic field, a pulsating voltage is generated. Mechanical drive turns the shaft Brushes Commutator To external circuit Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Magnetic units It is useful to review the key magnetic units from this chapter: Quantity SI Unit Magnetic flux density Flux Permeability Reluctance Magnetomotive force Magnetizing force Tesla Weber Weber/ampere-turn-meter Ampere-turn/Weber Ampere-turn Ampere-turn/meter Electronics Fundamentals Circuits, Devices and Applications - Floyd Symbol B f m R Fm H © Copyright 2009 Pearson Unit 10 Selected Key Terms Magnetic field A force field radiating from the north pole to the south pole of a magnet. Magnetic flux The lines of force between the north pole and south pole of a permanent magnet or an electromagnet. Weber (Wb) The SI unit of magnetic flux, which represents 108 lines. Permeability The measure of ease with which a magnetic field can be established in a material. Reluctance The opposition to the establishment of a magnetic field in a material. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Selected Key Terms Magnetomotive The cause of a magnetic field, measured in force (mmf) ampere-turns. Solenoid An electromagnetically controlled device in which the mechanical movement of a shaft or plunger is activated by a magnetizing current. Hysteresis A characteristic of a magnetic material whereby a change in magnetism lags the application of the magnetic field intensity. Retentivity The ability of a material, once magnetized, to maintain a magnetized state without the presence of a magnetizing current. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Selected Key Terms Induced voltage Voltage produced as a result of a changing (vind) magnetic field. Faraday’s law A law stating that the voltage induced across a coil of wire equals the number of turns in the coil times the rate of change of the magnetic flux. Lenz’s law A law stating that when the current through a coil changes, the polarity of the induced voltage created by the changing magnetic field is such that it always opposes the change in the current that caused it. The current cannot change instantaneously. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 1. A unit of flux density that is the same as a Wb/m2 is the a. ampere-turn b. ampere-turn/weber c. ampere-turn/meter d. tesla Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 2. If one magnetic circuit has a larger flux than a second magnetic circuit, then the first circuit has a. a higher flux density b. the same flux density c. a lower flux density d. answer depends on the particular circuit. Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 3. The cause of magnetic flux is a. magnetomotive force b. induced voltage c. induced current d. hysteresis Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 4. The measurement unit for permeability is a. weber/ampere-turn b. ampere-turn/weber c. weber/ampere-turn-meter d. dimensionless Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 5. The measurement unit for relative permeability is a. weber/ampere-turn b. ampere-turn/weber c. weber/ampere-turn meter d. dimensionless Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 6. The property of a magnetic material to behave as if it had a memory is called a. remembrance b. hysteresis c. reluctance d. permittivity Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 7. Ohm’s law for a magnetic circuit is a. Fm = NI b. B = mH Fm j c. R d. R l mA Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 8. The control voltage for a relay is applied to the a. normally-open contacts b. normally-closed contacts c. coil d. armature Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 9. A partial hysteresis curve is shown. At the point indicated, magnetic flux B a. is zero b. exists with no magnetizing force c. is maximum BR H d. is proportional to the current Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz 10. When the current through a coil changes, the induced voltage across the coil will a. oppose the change in the current that caused it b. add to the change in the current that caused it c. be zero d. be equal to the source voltage Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2009 Pearson Unit 10 Quiz Answers: Electronics Fundamentals Circuits, Devices and Applications - Floyd 1. d 6. b 2. d 7. c 3. a 8. c 4. c 9. b 5. d 10. a © Copyright 2009 Pearson Unit 10 Electrical Machines and Energy Conversion Electronics Fundamentals Circuits, Devices and Applications - Floyd Unit 1 Deck 1 Magnetism Basics © Copyright 2009 Pearson End of Presentation Electrical Machines and Energy Conversion