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The
Magnetic Circuit:
The classic iron filings experiment demonstrates that
a magnetic field or flux exists in the space around
a conductor carrying current. In this experiment the
field has the appearance of spaced circular lines. This
is simply the effect on the iron filings sprinkled on
the paper. There is no sharply defined limit to this
field nor does it exist in lines. The field around a
coil can be thought of as "flow" of flux in the area
surrounding a coil. The size of this flux flow is determined
by the product of current (I) and the number of turns
(N) in the coil, plus the magnetic conductivity of the
area surrounding the coil. The property of flux conduction
is called permeability (p), the force N x I required
to create the flow is called magnetomotive force (mmf).
| Definitions |
| B |
Core
Flux Density (gauss) |
|
|
|
| E |
Voltage
Applied rms, peak (volts) |
| N |
Turn
Count of the Winding |
| F |
Frequency
(Hz) |
| t |
Time
(Seconds) |
| L |
Inductance
(Henry) |
| I |
Current
(Amps) |
| Ae
|
Core
area (cmˆ2) |
| Le
|
Magnetic
Path Length (cm) |
 I |
Change
in Current (Amps) |
|
t |
Change
in Time (Seconds) |
 i |
Material
Initial permeability |
|
e |
Effective
permeability |
|
g |
Gap
permeability |
| Lg |
Gap
Length (cm) |
|
Flux
Density:
The following formulas are based on integrating Faraday's
Law and conversion to metric units for various applied
voltages extensively used in magnetic design.
| Core
Flux Density (B) |
| Sine
Wave |
 |
| Symmetrical
Square Wave (No Off Time) |
 |
| Symmetrical
Square Wave (With Off Time) |
 |
| Unidirectional
Pulse |
 |
| Half
Wave Rectified (AC Only) |
 |
| Full
Wave Rectified (AC Only) |
 |
| Current |
 |
|
Magnetomotive
Force
| Magnetomotive
Force (oersteds) |
 |
|
|
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Dividing
the mmf by the length of the magnetic circuit (Le) and
the unit length is centimeters, the mmf will be Oersteds
(H). The most common method of expressing mmf is:
Inductance:
| Inductance
defined as: |
 |
|
|
|
e: Instantaneous voltage across coil (volts)
L: Constant of proportionality (henries)
(di/dt): Rate of change of current (amps per second)
Inductance defined related to magnetic core geometries:
A core's effective permeability depends upon its geometry,
the core gap, material permeability and flux density
and magnetizing force of actual circuit use and test
measurement. Core manufacturers supply their materials
based on an inductance factor (AL). The AL is usually
specific in milihenries per 1000 turns. This factor
is only guaranteed at low sinewave flux densities (5
gauss typical).
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