Inductance Measurement

[Larry Kirkland]

I have a General Radio 1650A Impedance Bridge. It has two positions for
measuring unknown inductance, Lp (parallel inductance) and Ls (series
inductance). The book is not clear as to the differences. I can get
significant differences when measuring the same coil on the two settings.

The book says that Ls equals Lp within 1% when the Q of the coil is
greater than 10. The coils I'm measuring have a Q of less than 10 at
the bridge frequency of 1 Khz (2 or 3). Which scale should I use? What
am I missing here?

Thanks
Larry

 

[John Popelish]

Larry Kirkland wrote:

I have a General Radio 1650A Impedance Bridge. It has two positions for
measuring unknown inductance, Lp (parallel inductance) and Ls (series
inductance). The book is not clear as to the differences. I can get
significant differences when measuring the same coil on the two settings.

The book says that Ls equals Lp within 1% when the Q of the coil is
greater than 10. The coils I'm measuring have a Q of less than 10 at
the bridge frequency of 1 Khz (2 or 3). Which scale should I use? What
am I missing here?

Thanks
Larry

Parallel resistance is normally caused by core losses. Air core coils
generally have their losses dominated by the wire series resistance.
You should be able to compare the DC resistance to Rs and see if they
agree fairly closely. If so, then the series resistance approximation
is the way to go. If there is a big discrepancy between DC resistance
and Rs then parallel loss will probably give a better measure of
inductance.

--
John Popelish

 

[Fred Abse]

On Fri, 28 Jan 2005 21:28:42 +0000, Larry Kirkland wrote:

I have a General Radio 1650A Impedance Bridge. It has two positions for
measuring unknown inductance, Lp (parallel inductance) and Ls (series
inductance). The book is not clear as to the differences. I can get
significant differences when measuring the same coil on the two settings.

The book says that Ls equals Lp within 1% when the Q of the coil is
greater than 10. The coils I'm measuring have a Q of less than 10 at the
bridge frequency of 1 Khz (2 or 3). Which scale should I use? What am I
missing here?

Any combination of reactance (inductive or capacitive) and resistance may
be represented as either a perfect reactance and resistance in series, or
a perfect reactance and resistance in parallel. The values of both
reactance and resistance in each case are *not* equal. The values of
reactance in each case become close when Q>10.

With a Q between 2 and 3, the values of series and parallel reactance,
hence inductance will differ significantly.

Which value to use? It all depends... In the case of small, air-cored
coils, it is usual to use the series case. With big, iron-cored coils, the
parallel case is usual.

Google for "hay maxwell bridge" - plenty of information.

Also try googling for "series parallel impedance conversion"

Omit the quotes in both cases.

--
Then there's duct tape ...
(Garrison Keillor)