1. Topics Magnetic Field Electromagnetism Electromagnetic Devices
Magnetic Hysteresis
Electromagnetic Induction
DC Generators
DC Motors

2. Flux Lines Always go from N to S
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.
Flux Lines Always go from N to S

3. The Magnetic Field
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.

4. Earth’s Magnetic Poles

5. 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.
Current-carrying wire
Iron filings

6. 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)(ampere-turn)
N = number of turns of wire in a coil
I = current (A)

7. Magnetic Quantities Lets calculate’s the magnetomotive force (mmf)
N= turns in the coil I = 5A
Fm = NI
Fm = magnetomotive force (A-t)(ampere-turn)
N = number of turns of wire in a coil
I = current (A)

8. 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.
Current in the coil causes flux in the iron core.
Iron core
What is the mmf if a 250 turn coil has 3 A of current?
750 A-t

9. Factors Influencing Electromagnetic Strength in an Coil
Number of turns of Wire
Amount of Current
Reluctance of the material in the core (conductance)

10. 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.

11. 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

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

14. Relays
A relay is an electrically controlled switch; a small control voltage on the coil can control a large current through the contacts.
Structure
Schematic symbol

15. Relays
A relay is an electrically controlled switch; a small control voltage on the coil can control a large current through the contacts.
Alternate schematic symbol
Structure
Schematic symbol

16. Magnetic Quantities
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)
j = flux (Wb)
A = area (m2)

17. Magnetic Flux: Magnetic Flux Density Denoted by Greek letter (Φ, phi)
Flux and Flux Density
Magnetic Flux:
Denoted by Greek letter (Φ, phi)
Unit of Measurement: Weber (Wb)
1 Weber = 108 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

18. Flux and Flux Density 1 Dot = 100 lines of Flux
Calculate the flux density for (a) and (b)

19. Magnetic Quantities Example: Solution: Follow up:
What is the flux density in a rectangular core that is 8.0 mm by 5.0 mm if the flux is 20 mWb?
Solution:
Follow up:
Express this result in gauss.

20. 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 G = 1T
This instrument is used to
Measure flux density.

21. Operation of a typical magnetic switch.

22. Electromagnetic Field
Current flow

23. Left Hand Rule
Lines of Force
Current flow

24. What is the flux density in a rectangular core that is 7. 0 mm by 4
What is the flux density in a rectangular core that is 7.0 mm by 4.0 mm if the flux is 42 mWb?

25. 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.

26. Magnetic Quantities
Permeability (µ - mu) is the ease in the establishment of a magnetic field in a material.
Vacuum (mo) = 4π*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.
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))
m = permeability (Wb/A-t m).
A = area (m2)

27. What is the Relative Permeability of a ferromagnetic material whose absolute permeability
is 480 x Wb/At x m?
𝜇 𝑟 = 𝜇 𝜇 0
𝜇 𝑟 = 480× 10 −6 4𝜋× 10 −7 =381.97
is 560 x 10-6 Wb/At x m?
445.63
is 720 x 10-6 Wb/At x m?
572.95

28. Reluctance R= reluctance in A-t/Wb l = length of the path (meter(s))
m = permeability (Wb/A-t m).
A = area (m2)
4. Determine the reluctance of a material with a length of 3.5 cm and a cross-sectional area of 0.1 m2 if the absolute permeability is 120 x 10-7 Wb/At*m.
 2.9× 𝐴𝑡 𝑊𝑏
Determine the reluctance of a material with a length of 0.25 m and a cross-sectional area of 0.15 m2 if the absolute permeability is 110 x 10-7 Wb/At*m.
 1.5× 𝐴𝑡 𝑊𝑏

29. 1. Ohm’s law for electromagnetic circuits: because the flux (φ) is analogous (comparable in
certain aspects) to________, the mmf (Fm ) is similar to __________ and Reluctance (R) is
analogous to resistance.
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 105 At/Wb?
There is 0.1 ampere of current through a coil with 400 turns.
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 105 At/Wb?

30. Summary
Magnetic field intensity is the magnetomotive force per unit length of a magnetic path. or
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.

31. 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.

32. 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

33. 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 equals the number of turns in the coil times the rate of change of the magnetic flux.

34. Lenz’s law
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 current that caused it.

35. 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.

36. DC Generator A dc generator includes a rotating coil,
Mechanical drive turns the shaft
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.
Wire Loop
Brushes
Commutator
To external circuit

37. Wound Rotor core

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

40. Brushless DC Motor
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.
Hall sensor
Permanent magnet rotor
(Courtesy of Bodine Electric Company)

41. Summary Brushless DC Motor Series wound dc motor
A series wound dc motor has the field coil(s) in series with the armature.
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.

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

43. Magnetic units
It is useful to review the key magnetic units from this chapter:
Quantity SI Unit Symbol
Magnetic flux density
Tesla
Weber
Weber/ampere-turn-meter
Ampere-turn/Weber
Ampere-turn
Ampere-turn/meter B f m R Fm H
Flux
Permeability
Reluctance
Magnetomotive force
Magnetizing force

44. Key Terms Magnetic field
Magnetic flux
Weber (Wb)
Permeability
Reluctance
A force field radiating from the north pole to the south pole of a magnet.
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 108 lines.
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.

45. Key Terms Magnetomotive force (mmf)
Solenoid
Hysteresis
Retentivity
The cause of a magnetic field, measured in ampere-turns.
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 ability of a material, once magnetized, to maintain a magnetized state without the presence of a magnetizing current.

46. Key Terms Induced voltage (vind)
Faraday’s law
Lenz’s law
Voltage produced as a result of a changing magnetic field.
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.
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.

47. 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

48. 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.

49. Quiz 3. The cause of magnetic flux is a. magnetomotive force
b. induced voltage
c. induced current
d. hysteresis

50. Quiz 4. The measurement unit for permeability is a. weber/ampere-turn
b. ampere-turn/weber
c. weber/ampere-turn-meter
d. dimensionless

51. Quiz 5. The measurement unit for relative permeability is
a. weber/ampere-turn
b. ampere-turn/weber
c. weber/ampere-turn meter
d. dimensionless

52. 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

53. Quiz
7. Ohm’s law for a magnetic circuit is a. b. c. d.
Fm = NI
B = mH

54. Quiz 8. The control voltage for a relay is applied to the
a. normally-open contacts
b. normally-closed contacts
c. coil
d. armature

55. 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

56. 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

57. Quiz
Answers:
1. d
2. d
3. a
4. c
5. d
6. b
7. c
8. c
9. b
10. a