magnet

1. Chapter 8
Magnetism and Magnetic Circuits

2. The Nature of a Magnetic Field
• Magnetic field
– Force field
• Magnetic flux
– Flux lines (lines of
force)
• Show direction and
intensity of the field

3. The Nature of a Magnetic Field
(cont’d.)
• Unlike poles attract
• Like poles repel

4. Ferromagnetic Materials
• Magnetic materials
– Materials attracted by
magnets
– Iron, nickel, cobalt, and
their alloys
– Provide an easy path for
magnetic flux

5. Ferromagnetic Materials
(cont’d.)
• Nonmagnetic materials
– Have no effect on a magnetic field
– Ex: copper, aluminium, gold, etc

6. • Application: loudspeaker
Figure 12-4

7. Electromagnetism
• Right hand rule for a uniform length
conductor
– Used to indicate field direction
– Place right hand around conductor
– Thumb points in direction of current
– Fingers point in direction of the field

9. Electromagnetism (cont’d.)
• Rule for coils
– Curl fingers of right hand around coil in
direction of the current
– Thumb will point in direction of the field

11. Magnetic Flux and Flux Density
• Weber
– Unit of flux
• Tesla
– Unit of flux density
1 𝑇 =
1 𝑊𝑏
𝑚2
Equation 12-1

12. Magnetic Flux and Flux Density
(cont’d.)
• Example:
• For the magnetic core
shown, the flux density at
cross section 1 is B1 = 0.4T
• Determine B2

13. Magnetic Flux and Flux Density
(cont’d.)

14. Magnetic Circuits
• Found in motors, generators, speakers,
transformers
• Magnetic fields can be created by electric
currents
– Or electric currents and permanent magnets
• Example: speaker application in Figure 12-4

15. Magnetic Circuits (cont’d.)
Figure 12-10 Audio speaker, conceptual representation

16. Magnetic Circuits (cont’d.)
• Magnetic stripe containing information
– Used in bank ATM cards, library cards, etc.
– Magnetic patterns encode information
– Reader sees varying magnetic field
• Induces a voltage in the pickup winding
• Voltage is amplified and sent to decoding circuitry
• MRI machine uses superconductor coils
– Create intense magnetic field

17. Magnetic Circuits (cont’d.)

18. Air Gaps, Fringing, and
Laminated Cores
• Most practical magnetic circuits:
– Have air gaps essential to their operation
• Fringing occurs at air gaps

19. • Fringing occurs at air gaps
– Results in slight weakening of the field in the
gap
• In a gapped inductor, airgap fringing flux induces
eddy currents in coil conductors in the vicinity of
the airgap, producing unwanted power loss and
heat in the coil.
• Can be neglected for short gaps
• Or estimate the effect by increasing each cross-
sectional dimension of the core by gap length

20. Air Gaps, Fringing, and
Laminated Cores (cont’d.)
• Lamination
– Core is created with thin sheets of stacked
iron or steel
• Stacking factor
– Ratio of actual area of ferrous material to
physical area of the core
– Use to determine core’s effective area

21. Series Elements and Parallel
Elements
• Series magnetic circuit

22. Series Elements and Parallel
Elements (cont’d.)
• Parallel elements

23. Magnetic Circuits with DC
Excitation
• Two basic problems
– Determine current required to produce a given
flux
– Compute flux produced by a given current

24. MMF: The Source
of Magnetic Flux
• Current through a coil creates magnetic
flux
– The greater the current or number of turns,
the greater the flux
• Magnetomotive force (mmf)
– Measured in ampere-turns
– Denoted by the symbol

25. MMF: The Source
of Magnetic Flux (cont’d.)
• Equation defining mmf
Equation 12-2

26. Reluctance : Opposition to
Magnetic Flux
• Opposition that circuit presents to flux
Where µ=material permeability
Equation 12-3

27. Reluctance : Opposition to
Magnetic Flux (cont’d.)
• Permeability measures ease of
establishing magnetic flux in a material
– Ferromagnetic materials have high
permeability
– Nonmagnetic materials have low permeability

28. Ohm’s Law for Magnetic Circuits
• Flux does not flow like current
Equation 12-4

29. Magnetic Field Intensity and
Magnetization Curves
• Magnetic field intensity, H
– Also called magnetizing force
– Measures mmf per unit length of a circuit
Equation 12-5
Equation 12-6

30. Magnetic Field Intensity and
Magnetization Curves (cont’d.)
• Electric circuit analogy
– NI is an mmf source
– Hl is an mmf drop

31. The Relationship Between B
and H
• For air gaps:
Equation 12-7
Equation 12-9

32. B-H Curves
• Curves
obtained
experimentally
– Available in
handbooks

33. Force Due to an Electromagnet
• Electromagnets
– Used in relays, doorbells, lifting magnets
• Where Bg is the flux density in the gap in teslas
Ag is gap area in square meters
F is force in Newtons
Equation 12-13

35. Properties of Magnetic Materials

36. Magnetizing a Specimen

37. Magnetizing a Specimen
(cont’d.)
• Saturation
– All domain fields line up
– Cannot magnetize it much further

38. Measuring Magnetic Fields
• Hall effect
– Voltage proportional to field strength B

39. Measuring Magnetic Fields
• Hall effect
– The Hall effect is the production of a voltage
difference (the Hall voltage) across an
electrical conductor when the current flows
through the conductor in a magnetic field.

40. Measuring Magnetic Fields
(cont’d.)
• Hall effect gaussmeters use this principle

41. Transformer
• A transformer is a device that changes ac electric power at
one voltage level to ac electric power at another voltage
level through the action of a magnetic field.
• There are two or more stationary electric circuits that are
coupled magnetically.
• It involves interchange of electric energy between two or
more electric systems
• Transformers provide much needed capability of changing
the voltage and current levels easily.
– They are used to step-up generator voltage to an appropriate
voltage level for power transfer.
– Stepping down the transmission voltage at various levels for
distribution and power utilization.

42. Transformer Classification
• In terms of number of windings
– Conventional transformer: two windings
– Autotransformer: one winding
– Others: more than two windings
• In terms of number of phases
– Single-phase transformer
– Three-phase transformer
• Depending on the voltage level at which the winding is operated
– Step-up transformer: primary winding is a low voltage (LV)
winding
– Step-down transformer : primary winding is a high voltage (HV)
winding

43. Primary and Secondary Windings
A two-winding transformer consists of two windings interlinked
by a mutual magnetic field.
– Primary winding – energized by connecting it to an input
source
– Secondary winding – winding to which an electrical load is
connected and from which output energy is drawn.
Primary winding Secondary winding

44. Ideal Transformers
• No iron and copper losses
• No leakage fluxes
• A core of infinite magnetic permeability and of infinite
electrical resistivity
• Flux is confined to the core and winding resistances are
negligible
An ideal transformer is a lossless device with an input winding
and an output winding. It has the following properties:

45. Ideal Transformers
An ideal transformer is a lossless device with an input winding
and an output winding.
The relationships between the input voltage and the output voltage,
and between the input current and the output current, are given by the
following equations.
 
 
 
 
a
ti
ti
tv
tv
p
s
s
p
In instantaneous quantities
fM

46. Ideal Transformers
Np: Number of turns on the primary winding
Ns: Number of turns on the secondary winding
vp(t): voltage applied to the primary side
vs(t): voltage at the secondary side
a: turns ratio
ip(t): current flowing into the primary side
is(t): current flowing into the secondary side
 
 
 
 
a
N
N
ti
ti
tv
tv
s
p
p
s
s
p

a
I
I
V
V
p
s
s
p
In rms quantities

47. Thank you