2.3.4
INDUCTANCE
MEASUREMENTS WITH
DC
2.
3.
4. 1
W4IEN TO USE DC
Iron
core
inductors
are sensitive to both ac
and dc current
variations.
Quantitative
measurements
of these
effects
can be made with the bridge.
2.
3.4. 2
THINGS
TO WATCH
OUT
FOR
A. GENERAL
PRECAUTIONS
Stray
ac
pickup may
change
sensitive components being measured or
may
give
erroneous
null
detector
readings.
The
ac
impedance
of the dc
external
circuit must be
greater
than
2000
ohms
for
maximum
sensitivity. If cin
isolating inductor is used
to
raise
the ac
impedance
it
may
pick up
stray
signals or may-have
mutual coupling
with
the unknown
inductor being
measured. Either one may cause reading errors.
Such an
inductor
should be well
shielded
magnetically
and be placed to mini-
mize its chance of
interfering.
For low currents isolation may be accom-
plished with a
resistor
instead of the
inductor.
The
parallel
inductance circuit
is recommended because it simplifies dc
current measuring and
bridge
protection
problems.
With
the
series
circuit
the dc
current divides between
the
bridge arms so that
it
is difficult
to meter.
The
series circuit
would also
require
a
more
careful analysis
to avoid exces-
sive current in any of the
bridge arms.
If equivalent
series
inductance
is
required
it can
be
easily
calculated from
the
parallel
inductance
and D (See
Sec.
2,3.2).
If
Q
is too
low to
be read
directly,
connect an external
rheostat
such as
the
ESI
Model DB-655
DEKASTAT® to
the EXT D-Q terminals.
This
extends
the
Q
range
for the L
parallel
circuit.
Q
can be
calculated
from the
equation:
n
-
1
.
59
^
Where;
R
k£2
kc
Storage factor
of
unknown
External
rheostat resistance
in kilohms
Operating
frequency in kilocycles
B.
WITH
DC
SUPPLIED
THROUGH
THE GENERATOR TERMINALS
DC power
can be
supplied
through
the
generator terminals
by using
an
auxiliary ac
generator
instead
of the bridge
generator.
This generator
must
be
isolated from
the
dc
power
supply by a
capacitor
of
at
least
[mi]
250-DA
7/59
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