Chapter 7 Magnetism Electromagnetism MECH1100 Topics Magnetic Field Electromagnetism Electromagnetic Devices Magnetic Hysteresis Electromagnetic Induction DC Generators DC Motors

Chapter 7 Magnetism Electromagnetism MECH1100 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. Magnetic Quantities Flux Lines Always go from N to S

Chapter 7 Magnetism Electromagnetism MECH1100 Magnetic fields are composed of invisible lines of force that radiate from the north pole to the south pole of a magnetic material. Field lines can be visualized with the aid of iron filings sprinkled in a magnetic field. The Magnetic Field

Chapter 7 Magnetism Electromagnetism MECH1100 Earth’s Magnetic Poles

Chapter 7 Magnetism Electromagnetism MECH1100 Magnetic flux lines surround a current carrying wire. The field lines are concentric circles. Magnetic Quantities 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. Current-carrying wire Iron filings

Chapter 7 Magnetism Electromagnetism MECH1100 F m = NI 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. F m = magnetomotive force (A-t)(ampere-turn) N = number of turns of wire in a coil I = current (A) Magnetic Quantities

Chapter 7 Magnetism Electromagnetism MECH1100 F m = NI Lets calculate’s the magnetomotive force (mmf) N= 450 turns in the coil I = 5A F m = magnetomotive force (A-t)(ampere-turn) N = number of turns of wire in a coil I = current (A) Magnetic Quantities

Chapter 7 Magnetism Electromagnetism MECH1100 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. 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 Magnetic Quantities Iron core

Chapter 7 Magnetism Electromagnetism MECH1100 Factors Influencing Electromagnetic Strength in an Coil 1.Number of turns of Wire 2.Amount of Current 3.Reluctance of the material in the core (conductance)

Chapter 7 Magnetism Electromagnetism MECH1100 The Hall effect is an occurrence of a very small voltage that is generated on opposite sides of a thin current-carrying conductor or semiconductor (the Hall element) that is in a magnetic field. The Hall effect is widely employed by various sensors for directly measuring position or motion and can be used indirectly for other measurements.

Chapter 7 Magnetism Electromagnetism MECH1100 The Hall Effect Hall sensors are commonly used to measure parameters associated with rotating devices (e.g. wheels and shafts) –internal combustion engine ignition timing – tachometers – anti-lock braking systems –brushless DC electric motors to detect the position of the permanent magnet

Chapter 7 Magnetism Electromagnetism MECH1100

Chapter 7 Magnetism Electromagnetism MECH1100 A solenoid is a magnetic device that produces mechanical motion from an electrical signal. Solenoids One application is valves that can remotely control a fluid in a pipe, such as in sprinkler systems. Stationary core Sliding core (plunger) Spring

Chapter 7 Magnetism Electromagnetism MECH1100 A relay is an electrically controlled switch; a small control voltage on the coil can control a large current through the contacts. Relays StructureSchematic symbol

Chapter 7 Magnetism Electromagnetism MECH1100 A relay is an electrically controlled switch; a small control voltage on the coil can control a large current through the contacts. Relays StructureSchematic symbol Alternate schematic symbol

Chapter 7 Magnetism Electromagnetism MECH1100 The unit of flux is the weber (Wb). The unit of flux density is the weber/square meter, which defines the unit tesla, (T), a very large unit. Flux density is given by the equation where B = flux density (T)  = flux (Wb) A = area (m 2 ) Magnetic Quantities

Chapter 7 Magnetism Electromagnetism MECH1100 Flux and Flux Density Magnetic Flux: –Denoted by Greek letter (Φ, phi) –Unit of Measurement: Weber (Wb) –1 Weber = 10 8 lines of flux –Normally use µW = 100 lines of flux Magnetic Flux Density –Denoted by B –Unit of Measure: Tesla (T) –1 Tesla = 1 Weber/ SQ meter

Chapter 7 Magnetism Electromagnetism MECH Dot = 100 lines of Flux Calculate the flux density for (a) and (b) Flux and Flux Density

Chapter 7 Magnetism Electromagnetism MECH1100 What is the flux density in a rectangular core that is 8.0 mm by 5.0 mm if the flux is 20  Wb? Magnetic Quantities Express this result in gauss.

Chapter 7 Magnetism Electromagnetism MECH1100 Gauss Meter Although the Tesla (T) Is the SI unit for Flux Density, another unit Called the gauss (G) From the GCS (centimeter-gram-second) System, is used 10 4 G = 1T This instrument is used to Measure flux density.

Chapter 7 Magnetism Electromagnetism MECH1100 Operation of a typical magnetic switch.

Chapter 7 Magnetism Electromagnetism MECH1100 Electromagnetic Field Current flow

Chapter 7 Magnetism Electromagnetism MECH1100 Left Hand Rule Current flow Lines of Force

Chapter 7 Magnetism Electromagnetism MECH1100 What is the flux density in a rectangular core that is 7.0 mm by 4.0 mm if the flux is 42  Wb?

Chapter 7 Magnetism Electromagnetism MECH1100 Permeability (  ) 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. Magnetic Quantities Relative Permeability (  r ) is the ratio of the absolute permeability to the permeability of a vacuum. The permeability of a vacuum (  0 ) is 4  x weber per ampere-turn meter, which is used as a reference.

Chapter 7 Magnetism Electromagnetism MECH1100 Reluctance ( R ) is the opposition to the establishment of a magnetic field in a material. (similar to resistance) R = reluctance in A-t/Wb l = length of the path (meter(s))  = permeability (Wb/A-t m). A = area (m 2) Magnetic Quantities Permeability ( µ - mu ) is the ease in the establishment of a magnetic field in a material. Vacuum (  = 4*10 -7 Wb/At*M (webers/amp-turn*meter) Relative permeability is compared to a vacuum. Relative permeability, also Has no units of measurement because of the ratio permeabilities.

Chapter 7 Magnetism Electromagnetism MECH1100 What is the Relative Permeability of a ferromagnetic material whose absolute permeability is 480 x Wb/At x m? What do we know? What is the Relative Permeability of a ferromagnetic material whose absolute permeability is 560 x Wb/At x m? What is the Relative Permeability of a ferromagnetic material whose absolute permeability is 720 x Wb/At x m?

Chapter 7 Magnetism Electromagnetism MECH Determine the reluctance of a material with a length of 3.5 cm and a cross-sectional area of 0.1 m 2 if the absolute permeability is 120 x Wb/At*m. 5. Determine the reluctance of a material with a length of 0.25 m and a cross-sectional area of 0.15 m 2 if the absolute permeability is 110 x Wb/At*m. Reluctance R = reluctance in A-t/Wb l = length of the path (meter(s))  = permeability (Wb/A-t m). A = area (m 2)

Chapter 7 Magnetism Electromagnetism MECH Ohm’s law for electromagnetic circuits: because the flux (φ) is analogous (comparable in certain aspects) to________, the mmf (F m ) is similar to __________ and Reluctance ( R) is analogous to resistance. 2. How much Flux is established in a magnetic path of a coil with 500 turns and 0.3A with a Reluctance of 2.8 x 10 5 At/Wb? 3.There is 0.1 ampere of current through a coil with 400 turns. a) what is the mmf? B.) What is the reluctance of the circuit if the flux is 250µWb? 4.How much Flux is established in a magnetic path of a coil with 350 turn and 0.2A with a Reluctance of 3.5 x 10 5 At/Wb?

Chapter 7 Magnetism Electromagnetism MECH1100 Summary Magnetic field intensity is the magnetomotive force per unit length of a magnetic path. H= Magnetic field intensity (Wb/A-t m) F m = magnetomotive force (A-t) l = average length of the path (m) N = number of turns I = current (A) or Magnetic field intensity represents the effort that a given current must put into establishing a certain flux density in a material.

Chapter 7 Magnetism Electromagnetism MECH1100 Relative motion When a wire is moved across a magnetic field, there is a relative motion between the wire and the magnetic field. 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.

Chapter 7 Magnetism Electromagnetism MECH1100 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: Induced voltage the flux density the length of the conductor in the magnetic field the relative velocity (motion is perpendicular)

Chapter 7 Magnetism Electromagnetism MECH1100 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: Faraday’s law 1.The rate of change of the magnetic flux with respect to the coil. Voltage is indicated only when magnet is moving.

Chapter 7 Magnetism Electromagnetism MECH1100 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: Summary 1.The rate of change of the magnetic flux with respect to the coil. 2.The number of turns of wire in the coil. Faraday’s law Voltage is indicated only when magnet is moving.

Chapter 7 Magnetism Electromagnetism MECH1100 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. Summary Magnetic field around a coil

Chapter 7 Magnetism Electromagnetism MECH1100 A dc generator includes a rotating coil, DC Generator Mechanical drive turns the shaft 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. Brushes Commutator To external circuit Wire Loop

Chapter 7 Magnetism Electromagnetism MECH1100 Wound Rotor core

Chapter 7 Magnetism Electromagnetism MECH1100

Chapter 7 Magnetism Electromagnetism MECH1100 A dc motor converts electrical energy to mechanical motion by action of a magnetic field set up by the rotor. DC Motor Mechanical output Brushes The rotor field interacts with the stator field, producing torque, which causes the output shaft to rotate. The commutator serves as a mechanical switch to reverse the current to the rotor at just the right time to continue the rotation. Commutator

Chapter 7 Magnetism Electromagnetism MECH1100 A brushless dc motor has rotating field and a permanent magnet rotor. An electronic controller periodically reverses the current in the field coils. This causes the stator field to rotate, and the permanent magnet rotor moves to keep up with the rotating field. Brushless DC Motor Permanent magnet rotor Hall sensor (Courtesy of Bodine Electric Company)

Chapter 7 Magnetism Electromagnetism MECH1100 Summary Brushless DC Motor A series wound dc motor has the field coil(s) in series with the armature. Series wound dc motor The series wound motor has very high starting torque. It can run too fast if a load is not connected; therefore it is always used with a load.

Chapter 7 Magnetism Electromagnetism MECH1100 A shunt wound dc motor has the field coil(s) in parallel with the armature. Shunt wound dc motor The magnetic flux is constant because of the parallel arrangement. Unlike the series wound motor, torque tends to be nearly constant for different loads.

Chapter 7 Magnetism Electromagnetism MECH1100 Magnetic flux density Flux Magnetizing force Magnetomotive force Permeability Tesla Weber Weber/ampere-turn-meter Ampere-turn/Weber Ampere-turn Ampere-turn/meter B   R FmFm H Reluctance It is useful to review the key magnetic units from this chapter: Quantity SI Unit Symbol Magnetic units

Chapter 7 Magnetism Electromagnetism MECH1100 Magnetic field Magnetic flux Weber (Wb) Permeability Reluctance The lines of force between the north pole and south pole of a permanent magnet or an electromagnet. The SI unit of magnetic flux, which represents 10 8 lines. A force field radiating from the north pole to the south pole of a magnet. Key Terms The measure of ease with which a magnetic field can be established in a material. The opposition to the establishment of a magnetic field in a material.

Chapter 7 Magnetism Electromagnetism MECH1100 Magnetomotive force (mmf) Solenoid Hysteresis Retentivity An electromagnetically controlled device in which the mechanical movement of a shaft or plunger is activated by a magnetizing current. A characteristic of a magnetic material whereby a change in magnetism lags the application of the magnetic field intensity. The cause of a magnetic field, measured in ampere-turns. Key Terms The ability of a material, once magnetized, to maintain a magnetized state without the presence of a magnetizing current.

Chapter 7 Magnetism Electromagnetism MECH1100 Induced voltage (v ind ) Faraday’s law Lenz’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. Voltage produced as a result of a changing magnetic field. Key Terms 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.

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz 1. A unit of flux density that is the same as a Wb/m 2 is the a. ampere-turn b. ampere-turn/weber c. ampere-turn/meter d. tesla

Chapter 7 Magnetism Electromagnetism MECH1100 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.

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz 3. The cause of magnetic flux is a. magnetomotive force b. induced voltage c. induced current d. hysteresis

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz 4. The measurement unit for permeability is a. weber/ampere-turn b. ampere-turn/weber c. weber/ampere-turn-meter d. dimensionless

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz 5. The measurement unit for relative permeability is a. weber/ampere-turn b. ampere-turn/weber c. weber/ampere-turn meter d. dimensionless

Chapter 7 Magnetism Electromagnetism MECH1100 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

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz 7. Ohm’s law for a magnetic circuit is a. b. c. d. F m = NI B =  H

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz 8. The control voltage for a relay is applied to the a. normally-open contacts b. normally-closed contacts c. coil d. armature

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz 9. A partial hysteresis curve is shown. At the point indicated, magnetic flux a. is zero b. exists with no magnetizing force c. is maximum d. is proportional to the current

Chapter 7 Magnetism Electromagnetism MECH1100 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

Chapter 7 Magnetism Electromagnetism MECH1100 Quiz Answers: 1. d 2. d 3. a 4. c 5. d 6. b 7. c 8. c 9. b 10. a