change the permeability of the magnetic material upon
The value of inductance that an inductor (coil) has
which the coil is wound.
depends on many things. Among them is the number of
turns of wire, the ratio of the length of inductor to its
Theory
diameter, and the material used in the core. Various
formulas have been developed for calculating the
To understand the theory of magnetic amplifiers, a
self-inductance of inductors. The following formula is
knowledge of magnetism and magnetic circuits is
essentially correct and can be used to calculate
necessary. To provide this knowledge, inductance and
self-inductance:
p e r m e a b i l i t y a r e d i s c u s s e d i n t h e f o l l ow i n g
2
L + 1.256N A 0-8
paragraphs, but only to the depth necessary to
1
INDUCTANCE.--Inductance is the property of
where
an electric circuit that opposes any CHANGE IN THE
L = inductance in henries
CURRENT through a circuit. That is, if the current
increases, a self-induced voltage opposes this change
N = number of turns
and delays the increase. If current decreases, a
A = area of the core in square centimeters
self-induced voltage tends to aid or prolong the current
= permeability of the core material
flow and delays the decrease. Inductance is caused by
the buildup or collapse of a magnetic field around a
1 = length of the core in centimeters
conductor (fig. 7-14).
From the foregoing formula, you can see that the
The current flow through a conductor always
permeability of the core of the inductor affects the
p r o d u c e s a m a g n e t i c fi e l d t h a t s u r r o u n d s t h e
inductance (opposition to current change) of the coil.
conductor. Therefore, even a perfectly straight
A change in the permeability of the core is of concern
conductor has some inductance. When the current flow
to you as you study the magnetic amplifier.
c h a n g e s , t h e m a g n e t i c fi e l d c h a n g e s ; a n e m f
(electromotive force) or voltage is induced in the
PERMEABILITY.--The permeability () of a
conductor to oppose the current change. These effects
substance is a measure of the ease with which it
are summarized by Lenz's Law, which states that THE
conducts magnetic lines of force when compared with
INDUCED EMF IN ANY CIRCUIT IS ALWAYS IN
air. When air is taken as the standard, its permeability is
S U C H A D I R E C T I O N A S TO O P P O S E T H E
1 . O n t h i s b a s i s , t h e p e r m e a b i l i t y va l u e s o f
EFFECT THAT PRODUCED IT. For all practical
ferromagnetic materials, such as iron and steel, range
purposes, inductance has no effect on STEADY
from approximately 60 to 6,000.
CURRENT (dc).
Table 7-2 shows B, H, and for four common
magnetic materials.
MAGNETIC
Permeability of magnetic materials decrease to
FIELD
nearly that of air if the current flow through the
CONDUCTOR
CURRENT
i n d u c t o r i s s u ffi c i e n t t o s a t u r a t e ( c o m p l e t e l y
GOING IN
magnetize) the core material. The relationship of
permeability to magnetization can be seen in view A
and B of figure 7-15.
A
Basic Magnetic Amplifier
A magnetic amplifier consists of a controlled
variable inductance in series with an ac power supply
S
N
and the load. The control action involves changes in the
magnetic permeability of the inductance coil with
resulting changes in inductance, inductive reactance,
and impedance of the load circuit. As a result, changes
+
B
are made in the current flowing in the load and the
ASf07014
Figure 7-14.--Magnetic field of a current-carrying conductor.
voltage available from the output.
7-8