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Biddle MEGGER - Page 53

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The
control
coil
B
is
connected
in
series
with
a
fixed
resist-
ance
R.
Coils
A
and
B
are
mounted
on
the
moving
system
at
a
fixed
angle
to
each
other,
and
are
so
connected
that
when
current
is
supplied
they
develop
opposing
torques
and
tend
to
turn
the
moving
system
in
opposite
directions.
The
pointer,
there-
fore,
comes
to
rest
where
the
two
torques
are
balanced.
With
either
perfect
insulation
or
nothing
at
all
connected
to
the
testing
terminals,
no
current
will
flow
in
coil
A.
Coil
B,
however,
receives
current
and
will
take
a
position
opposite
the
gap
in
the
C-shaped
iron
core.
The
corresponding
position
of
the
pointer
over
the
scale
is
marked
Infinity.
When
a
resistance
is
connected
to
the
Earth
and
Line
ter-
minals,
a
current
will
flow
in
the
deflecting
coil
A
and
the
corresponding
torque
will
draw
coil
B
away
from
the
Infinity
position
into
a
field
of
gradually
increasing
magnetic
strength
until
a
balance
is
obtained
between
the
forces
exerted
by
the
two
coils.
In
this
manner
coil
B
acts
like
a
restraining
spring.
As
shown
in
Figure
46,
the
control
coil
is
actually
in
two
parts
forming
an
astatic
system,
B
being
the
main
part
that
develops
control
torque
as
a
result
of
the
permanent
magnet
field.
B’
is
a
coil
mounted
outside
of
the
permanent
magnet
field,
and
in
series
with
the
control
coil,
but
connected
in
op-
position
so
that
stray
fields
are
neutralized.
In
other
words,
any
external
fields
which
may
ordinarily
tend
to
displace
the
pointer
from
its
infinity
calibration,
will
produce
equal
and
opposing
torques
in
the
two
parts
of
the
control
coil,
thereby
making
the
moving
system
free
of
any
errors
from
that
cause.
Scales—
Calibration
By
connecting
resistances
of
different
known
values
to
the
Earth
and
Line
terminals
and
marking
the
corresponding
posi-
tion
of
the
pointer
in
each
case,
a
scale
calibrated
in
resistance
is
obtained.
In
this
manner
the
scales
of
all
“Megger”
Insula-
tion
Testers
are
individually
calibrated
and
drawn.
See
Appendix
(page
56)
for
“Instructions
for
Checking
the
Calibration
of
Megger
Insulation
Testers—All
Types.”
If
the
Earth
and
Line
terminals
are
short
circuited,
the
pointer
simply
moves
to
Zero.
The
ballast
resistance
R’
offers
ample
protection
against
excessive
current
in
coil
A.
With
no
current
supplied
to
the
ohmmeter,
the
pointer
will
relax
and
float
towards
Infinity
as
already
explained.
[51]