Re: [MEG_builders] Inductance Changes with Magnet
| With respect to Stan's excellent work with his core, he noted
| very little change in inductance with and without magnets inserted.
| The problem is that he is using a cut core, which has a small
| air gap between the core halves even when the edges appear to be
| fitting perfectly. Even this small a gap adds enough reluctance
| to dwarf the inherent reluctance of the core. Hence, even if the
| magnetic bias from the NIB magnets changes the permeability significantly, the change in
total reluctance is still small.
| Note that when he was measuring the core halves by themselves
| the difference with and without magnets was smaller than that of
| the assembled core.
This is true - and while it would be interesting to see comparisons between cut and uncut cores, Naudin used a cut core for his MEG.
- Here are some selected figures from my lab book:
Jan 5th 2003.
Did a whole lot of inductance measurments on the AMCC 1000 core with
two secondaries coils.
The coil under test had 496 turns of 1.51mm dia wire.
Measured DC resistance: 2.4 ohms.
I took measurments in sets of 3 differetnt core biases sep up with
This was acomplished with stacks of magnets -- either none, 3 or 6
stacks. These are Neodynium magnets with a nominal field strength of
1T. The stacks either sum up to 25% or 50% of the AMCC 1000 core
cross sectional area and were positioned right along the edge of the
cut core and comprised of 13 of 25mm x 3mm x 9mm magnets.
Tests included measurments with a total air gap of 0.25mm. All
measurments are in Heneries.
Bias T gap mm Inductance H
0.0 0.25 0.738
0.25 0.25 0.700
0.50 0.25 0.620
The reduction of inductance with increacing bias was also observed
for no air gap.
At 0.5T bias the same coil has a measured inductance of 2.45 - 3.7 H.
The varyation is due to the dificulty of clamping the cores together
and reliably and reducing the air gap to minimum.
Also of note:
In numerous tests on several occasion - and in multiple
configurations - the inductance of the same coil redudes reliably to
0.181 - 0.185 H when one of the control coils is shorted -- the
control coil is a 40 turn coil of 2.36mm dia wire with a calculated
DC resistance 0.0047 ohms (I have yet to make up calibrated low ohms
bridges to reliably measure these low values!... same goes for custom
teflon caps with vanishingly small ESR!!).
All measurments were taken with a DMM and will soon be repeated with
bridges, signal generator and scope at a range of frequencies.
PS... the AMCC-1000 (7.1Kg) is more than just a little bigger than
the core Bearden used, AMCC-320 (2.2Kg)!