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2
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7 ----_.‘._
+
<
Q
A
magnetized
transformer
is
usually
noisy
and
micro-
phonic
compared
to
when
it
is
not
magnetized.
This
noise
is
the
most
objectional
characteristic
of
a
magnetized
transformer
because
it
affects
low level
signals.
The
distortion
of
high level signals
described
here
is
not
usually an
operating
problem
because
the
signal
is
seldom
so
large,
but
high level
distortion
is
useful
in
determining
the
condition
of
the
core
and
in
monitoring
the
degaussing process.
Notice
from
Figure
"1-19A
that
as
long as
the
core
IS
not
magnetized
the
transformer
works
on
a
loop
around
center-I ine
#1
and
thesignal
can
be
of
maxi-
mum
amplitude.
If
the
core
becomes
permanently
magnetized,
the
transformer
works
on
a
loop
around
center-line
#2
(or
equivalent
for
opposite
polarity),
and,
if
the
same
maximum
signal
current
is
applied,
the
core
becomes
saturated
at
one
of
the
peaks
of
the
signal.
B,
C,
and
D
of
Figure
111-19
show
a
primary
signal
current,
corresponding
core
magnetization,
and
corresponding
induced
secondary
voltage
for
a
magnetized-core
transformer
with
large signal
input.
Notice
that
the
permanently
magnetized
core
saturates
sooner
for
one
polarity
of
signal
and
later
for
the
other
polarity.
To
demagnetize
(degauss)
the
transformer,
it
is
neces-
sary
to
pass a relatively large ac
current
through
the
primary
and
to
very slowly
decrease
this
current.
The
degaussing
current
is
initially large
enough
to
mag-
netize
the
core
in
the
direction
of
each
corresponding
polarity
excursion
of
this
current,
and
overrides
the
initial
magnetization.
The
object
of
decreasing
the
current
slowly
is
so
that
each
successive
magnetization
is
slightly less
than
the
preceding
one
of
the
opposite
polarity.
As
the
current
is
decreased,
the
core
is
less
magnetized
until
it
is
no
longer
magnetized
at
all.
Figures 111-20A,
B,
and
C
illustrate
the
secondary
waveforms
as
the
transformer
becomes
demagnetized.
IMPORTANT:
Use a
LOW
FREQUENCY
degaussing
current,
1 Hz
or
so,
to
avoid
inducing
excessively high
voltage
in
the
secondary,
and
monitor
the
secondary
with
a high impedance
oscilloscope.
The
Model
190's
secondary
is
protected
with
a
neon
lamp,
but
the
Model AM-1
could
suffer
insulation
breakdown
if
the
operator
is
not
careful.
DISTORTION
AT
ONLY
EVERY
2!!:!Ef!
ZERO
~SSING
INDICATES
PRIMARY
CURRENT
SATURATES
CORE
IN
ONE
DIRECTION
ONLY.
OONTlNUE
TO
INCREASE
PRIMARY
CURRENT
UNTlL
WAVEFORM
A
~-----"~----r-------"
......
-----,.....--
RESEMBLES
B.
B
c
DlSlORTION
AT
EVERY
ZERO
CROSSING
INDICATES
CORE
SATURATES
FOR
BOTH
POLARITY
EXCURSIONS
OF
-'-:...----.:.....;:-----..,-c.----~---__;.,.__-
DEGAUSSI
NG
CURRENT.
Figure
111-20.
DEGAUSSING
WAVEFORMS
111-11
VERY
SLOWLY
DECREASE
DEGAUSSING
CURRENT
UNTI
L
WAVEFORM
APPEARS
UNOISTORTEO,
IF
WAVEFORM
GOES
BACK
TO
A,REPEAT
PROCEDURE
USING
HIGHER
INITIAL
CURRENT_
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