Influence of magnet material on P-90 Inductance

[fieldwrangler]

Folks -

So, I took a lightly wound P-90 coil and measured its inductance (with keeper & pole screws in place) with the Extech (at 120hz.)

Then I placed the usual Alnicos in place (with the proper orientation) and saw the expected increase in the inductance reading.

Then I placed a pair of ceramics with similar dimensions in their place. The reading went down (relative to the coil alone) quite significantly.

I must say, this surprised me... Wish I had uncharged versions of the same bars to test in this way...

Bob Palmieri

[big_teee]

Folks -

So, I took a lightly wound P-90 coil and measured its inductance (with keeper & pole screws in place) with the Extech (at 120hz.)

Then I placed the usual Alnicos in place (with the proper orientation) and saw the expected increase in the inductance reading.

Then I placed a pair of ceramics with similar dimensions in their place. The reading went down (relative to the coil alone) quite significantly.

I must say, this surprised me... Wish I had uncharged versions of the same bars to test in this way...

Bob Palmieri

I figure inductance went down because of the ceramic material and no eddy currents.
You can discharge the magnets with Neos, you may not make them discharged flat, but should see a significant change.
I discharge rod magnets with 2 large ceramic block magnets stuck to my desk. One block up is north the other right beside it is south up.
If you run the rod magnet straight over both magnets when you reach the end of the second magnet they are discharged.
I do this before I put them in the flatwork. It makes them much easier to mount and handle this way.
Then I recharge when I get done.
You can do the same thing with the bar Alnicos, and you can discharge ceramics a lot with Neos.
When your done just charge them back up.
Later,
Terry

[fieldwrangler]

I figure inductance went down because of the ceramic material and no eddy currents.

Down relative to having nothing in that position? I still don't quite get this. I figure that there are a few separate effects involved, and they add & subtract in the different material in different ways in this situation. Maybe magnetic flux, permeability and one or two other factors in very different combinations in these two cases.

Thanks for the reminder that charging & discharging are No Big Deal!

Bob Palmieri

[Mike Sulzer]

Down relative to having nothing in that position? I still don't quite get this. I figure that there are a few separate effects involved, and they add & subtract in the different material in different ways in this situation. Maybe magnetic flux, permeability and one or two other factors in very different combinations in these two cases.

Thanks for the reminder that charging & discharging are No Big Deal!

Bob Palmieri

Yes, eddy currents should make the apparent induction drop, although it would be more correct to say that the measurement is wrong.

Are you using an Extech? Is this a 1000 Hz measurement? If so, what happens at 100 Hz? (The measurements I have made recently show that the inductance value in a 100 Hz measurement should not be significantly affected by eddy currents.)

[fieldwrangler]

Mike -

I've looked at some of your work in this area and have found it to be quite helpful. In fact, that's a large part of the reason I made these measurements at 120Hz. Here's a more detailed report:

Low wind P-90 type coil with keeper & pole screws in place (away from other metal) - 4.920 H, 7.545 ohms

Same w/ ceramics contacting keeper (in proper orientation): - 4.564 H, 7.507 ohms
Same w/ ceramics contacting keeper (in wrong orientation): - 4.941 H, 7.568 ohms

Alnicos (in proper orientation): - 5.130 H, 7.561 ohms
Alnicos (in wrong orientation): - 5.304 H, 7.594 ohms

The interesting thing for me (which has me thinking about factors other than eddy currents) is the difference the orientation makes in both cases. How accurately the induction is being represented by the displayed readings is another thing I leave to brighter minds than mine.

Bob Palmieri

[FunkyKikuchiyo]

I've had the same thing happen. I've just assumed that readings with no magnets are negligible. Not sure, though.

Are you using any different materials? Is your pickup retaining any magnetism when you remove the magnet?

[fieldwrangler]

Normal materials, no residual magnetism to speak of.

[Mike Sulzer]

Mike -

I've looked at some of your work in this area and have found it to be quite helpful. In fact, that's a large part of the reason I made these measurements at 120Hz. Here's a more detailed report:

Low wind P-90 type coil with keeper & pole screws in place (away from other metal) - 4.920 H, 7.545 ohms

Same w/ ceramics contacting keeper (in proper orientation): - 4.564 H, 7.507 ohms
Same w/ ceramics contacting keeper (in wrong orientation): - 4.941 H, 7.568 ohms

Alnicos (in proper orientation): - 5.130 H, 7.561 ohms
Alnicos (in wrong orientation): - 5.304 H, 7.594 ohms

The interesting thing for me (which has me thinking about factors other than eddy currents) is the difference the orientation makes in both cases. How accurately the induction is being represented by the displayed readings is another thing I leave to brighter minds than mine.

Bob Palmieri

Bob, I am sorry I missed 120 Hz the first time.

I agree that there are a number of things going on. First, the permeability of the Alnico is significant, while that of the ceramic is quite a bit lower. So that explains why the inductance goes up, but that is not the whole story.

The inductance going down with the ceramic is the interesting question. It looks like there are two factors at first guess:
1. The low permeability means that there is no tendency to raise the inductance.
2. The permanent magnetic field affects the permeability of the keeper and screws.
I am thinking that the permanent field moves the steel along its B-H curve to a point where the slope is lower. Not all the way to saturation, but just part way in that direction. If I understand "proper orientation" that would mean with both "norths" pointing in (or both "souths"). This would produce a field that goes up through the screws, as you need to magnetize the strings. If you make it so the poles facing in are different, then you get almost no field up the screws, but you still get a field through the keeper, and so you would expect a smaller effect.

So to check this, you look at the alnico results again. The proper orientation has lower inductance than wrong one. This is consistent with the permanent field through the screws altering their permeability.

So maybe this explanation works!

[fieldwrangler]

Mike -

This is almost definitely the core of the issue (pun not consciously intended but not edited out either)!

It also agrees with the way I see the effect change as I slowly move the magnets away from the keeper.

Thanks once again.

Bob Palmieri