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Author Topic: Self running coil?  (Read 302262 times)

gotoluc

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Re: Self running coil?
« Reply #495 on: April 08, 2010, 01:32:33 AM »
I'm still looking at inductances with various setups - I noticed something which I cant explain, I'm hoping one of you can.

I wound a new toroid (Finemet nanocrystalline - small, about 14mm in diameter).  Once wound it has an inductance of 250mH.  As I approach my large 2" dia magnet, its inductance INCREASES to a peak, and then begins falling off as expected.  The maximum inductance increase is found when the toroid approaches the north or south pole of the magnet, with the toroid parallel with the magnet (inner hole facing the magnets pole).  The increase goes from 250mH to 500mH before falling off.

What mechanism is causing the toroid to GAIN inductance by the presence of the permanent magnets field?  I thought I understood the lowering of inductance, I just dont understand the gain.

Hi void109,

I think Inductance should be measured when there is no magnetic fields moving around. A moving magnet will induce an electrical current in the coil and interfere with the sampling frequency your inductance meter is sending through the coil to give you a reading.

That's what I think.

Luc


void109

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Re: Self running coil?
« Reply #496 on: April 08, 2010, 02:36:09 AM »
Hi,

Mind answering some questions?   
Do you have the exact type of the core? 
What if you approach the core with a much smaller magnet in the same way?
What if you approach the core tangentially with a small or with the big magnet?  (Tangentially= simple let the magnet attract to side of the core on its outside perimeter.)

The answer to your question is probably in the way you place the core in parallel with the big magnet, flux surely is concentrated or collected first in the core more or less equally, without any chance for saturation. Then at closer and closer the core cannot 'collect' more flux and starts saturate: inductance falls.

rgds, Gyula

Hi Gyula,

The core is MP1305LF3T

http://www.metglas.com/products/page5_2_2_3_3.htm

A much smaller magnet does not increase the inductance, but it does reduce the inductance as expected.  I have various magnet sizes, I'll try with several more to see if there's a tipping point.

The increase does not seem to occur if approached tangentially  - but for maximum increase, you have to approach as I described.  My original numbers were wrong, it was from memory.  More precisely the inductance of the core is 25mH outside of the field.  After securing it to the magnet with electrical tapes using plastic guitar pics as spacers for an air gap, it now has a measured inductance of 71mH.  The highest I could get is 71.9, but its VERY touchy - it falls off very rapidly if I move it in any direction more than 1mm.

@Luc - I have it fixed in place, it is a constant inductance while within the magnets field.

With it fixed in place, I have tried finding its resonant point both with a 10nF and 100uF  capacitors.  No luck - I do the math, no dice - I tried to "eye ball" it on the scope, and I cant find resonance - it appears that resonance may be just outside of my function generators range as I can see the ringing of the circuit after a pulse, and it appears a bit higher than 4Mhz.


gotoluc

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Re: Self running coil?
« Reply #497 on: April 08, 2010, 02:41:32 AM »
Hi Luc,

I am not sure on a correct answer for your last question above, it must have a relationship with the MOSFET switching on just or very near there in time.

When you replace the pulse coil with the sig gen, and supply the same amplitude at the same frequency, then the MOSFET capacitances are surely the same like with the pulse coil. The resonant pulse coil is a pure resistance in the oscillator mode, it is just like you place a several tens of kOhm resistor between the gate-source electrodes.
And when you connect the sig gen instead of the coil, it also has a pure resistance, though it is 50 Ohm, not many kOhms, this does not matter because the original resonant voltage is still there from the generator with the correct amplitude and frequency.
And when you slightly mistune the generator, then the amplitude stays the same but now the small frequency change causes a different phase relationship with the drain circuit that did not change. The FET gate-drain capacitance will have also a slighly different reactance now at this newer freq, alltogether these explain why the drain - gate waveshapes now can be shifted wrt each other. 
Maybe this latter could be done with the pulse coil in place if the gate-drain capacitance could be made a bit variable.  Have you placed ,say, a 20-30pF (or some more) capacitor between the gate-drain in a running oscillator to see if the waveforms shift away wrt each other?  It is possible that the gate-drain FET capacitance ought to be reduced and NOT increased: this can be possible only to use a big value tuneable inductance between the gate-drain but in series with a big uF capacitor to block DC. The latter method is called neutralization of the unwanted capacitance in an active device between its input and output electrodes, not often used in oscillators though, normally the active device is chosen for the oscillator with the smallest Crss capacitance.

Thank you for the series 10kOhm test, I simply wanted to create an extreme value supply inner impedance for the oscillator. If you found more or less the same 'willingness' for oscillation with the 10kOhm like without it, then it is ok.

Will ponder on these...

rgds,  Gyula

Hi Gyula,

thanks for taking the time to post your explanation.

I did experiment some days back of adding a small pf capacitance between the drain and source and it has an effect on the pulse coil and main coil. It reduces the amplitude of the main coil and makes the pulse coil (top) fatter and rounder. It also causes a small advance timing shifts to the pulse coil and resulting in a small drop in current.

I don't believe the above to be a true benefit as it's kind of clear that a lower capacitance mosfet is the way to go. Yesterday I ordered some IRF510 which have a little lower capacitance then the IRFBC20 but a much lower gate resistance. I also bought 2 other 20v mosfet models irlml2502 and FDC637AN on ebay which have a low gate trigger voltage to see how they would performs also.

I attached all the pdf data sheet below for reviewing.

Luc

gotoluc

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Re: Self running coil?
« Reply #498 on: April 08, 2010, 02:51:08 AM »
Hi Gyula,

The core is MP1305LF3T

http://www.metglas.com/products/page5_2_2_3_3.htm

A much smaller magnet does not increase the inductance, but it does reduce the inductance as expected.  I have various magnet sizes, I'll try with several more to see if there's a tipping point.

The increase does not seem to occur if approached tangentially  - but for maximum increase, you have to approach as I described.  My original numbers were wrong, it was from memory.  More precisely the inductance of the core is 25mH outside of the field.  After securing it to the magnet with electrical tapes using plastic guitar pics as spacers for an air gap, it now has a measured inductance of 71mH.  The highest I could get is 71.9, but its VERY touchy - it falls off very rapidly if I move it in any direction more than 1mm.

@Luc - I have it fixed in place, it is a constant inductance while within the magnets field.

With it fixed in place, I have tried finding its resonant point both with a 10nF and 100uF  capacitors.  No luck - I do the math, no dice - I tried to "eye ball" it on the scope, and I cant find resonance - it appears that resonance may be just outside of my function generators range as I can see the ringing of the circuit after a pulse, and it appears a bit higher than 4Mhz.

Thanks void109 for confirming this was happening with fixed positions.

In that case can anyone think of how we could use this close to 3 times increase in inductance to or benefit?

Thanks for sharing your findings

Luc

ADDED

void109, can you make a drawing of the position of your magnet an toroid so I can be clear on this. Thanks

ADDED

Never mind, I reread your post and it is clear but I tried it with my toroid and I don't get any increases that stay. Why the difference ???

gotoluc

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Re: Self running coil?
« Reply #499 on: April 08, 2010, 04:16:39 AM »

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I have contacted Stefan about this user. All 14 of his posts are this kind of stuff ::)

I don't have a feeling his stay here will be a long one ;)

Luc

gotoluc

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Re: Self running coil?
« Reply #500 on: April 08, 2010, 04:25:55 AM »
I'm still looking at inductances with various setups - I noticed something which I cant explain, I'm hoping one of you can.

I wound a new toroid (Finemet nanocrystalline - small, about 14mm in diameter).  Once wound it has an inductance of 250mH.  As I approach my large 2" dia magnet, its inductance INCREASES to a peak, and then begins falling off as expected.  The maximum inductance increase is found when the toroid approaches the north or south pole of the magnet, with the toroid parallel with the magnet (inner hole facing the magnets pole).  The increase goes from 250mH to 500mH before falling off.

What mechanism is causing the toroid to GAIN inductance by the presence of the permanent magnets field?  I thought I understood the lowering of inductance, I just dont understand the gain.

Hi void109,

I have a small 14mm toroid on hand and wound it with a very short wire giving me 750uH and can confirm that at a certain position I was able to make the inductance higher, The max I go it to was 925uH.

Your results are quite impressive. Can you give me some details on your 2" magnet. Is this a Neo, ceramic, how thick is it, grade, how close is the toroid?

I noticed if I tilt the toroid a little I got the highest results. Is it the same for you?

Thanks for sharing

Luc
« Last Edit: April 08, 2010, 04:53:51 AM by gotoluc »

void109

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Re: Self running coil?
« Reply #501 on: April 08, 2010, 04:40:27 AM »
Hi - it is a neo 2" diameter .5" thick.  The toroid is about dead center on a pole side - roughly 12mm or so from the magnets surface

void109

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Re: Self running coil?
« Reply #502 on: April 08, 2010, 04:54:02 AM »
I tried to find the same with many other toroids I had previously wound of various unimpressive cores - couldnt see the effect using another unaffixed 2" magnet.  Perhaps its a property of the core.

gotoluc

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Re: Self running coil?
« Reply #503 on: April 08, 2010, 05:19:34 AM »
Hi - it is a neo 2" diameter .5" thick.  The toroid is about dead center on a pole side - roughly 12mm or so from the magnets surface

I also have a 2" Neo but it's 1" and N52. Quite strong so I have it about 2" away in center.

When you move it away from the center, how far down does the 25mH drop to?

Thanks

Luc

gotoluc

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Re: Self running coil?
« Reply #504 on: April 08, 2010, 05:22:18 AM »
Perhaps its a property of the core.

Yes, that's why I would like to know how far down the inductance drops when the toroid is away from the center of the magnet.

Maybe these cores would be even better for what I'm doing ???

Anyone care to comment

Luc

void109

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Re: Self running coil?
« Reply #505 on: April 08, 2010, 05:42:57 AM »
I also have a 2" Neo but it's 1" and N52. Quite strong so I have it about 2" away in center.

When you move it away from the center, how far down does the 25mH drop to?

Thanks

Luc

If its closer to the magnet (not optimal height), its down in the low micro-henry range.  In sub optimal positions but at proper height, its around 45-50mH.

void109

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Re: Self running coil?
« Reply #506 on: April 08, 2010, 05:52:28 AM »
Short video demonstrating what I'm seeing with the core:

http://www.youtube.com/watch?v=PuzSkKlnCzc


gotoluc

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Re: Self running coil?
« Reply #507 on: April 08, 2010, 06:15:15 AM »
Short video demonstrating what I'm seeing with the core:

http://www.youtube.com/watch?v=PuzSkKlnCzc

Excellent video demo ;)

Thanks for taking the time to do this and share what you have found.

Going to have to get one of those cores to test this.

Luc

void109

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Re: Self running coil?
« Reply #508 on: April 08, 2010, 06:19:02 AM »
Thanks - sorry for the quality, been awhile since I've uploaded something.

I don't see immediately how this effect is useful.  I would like to understand WHY though, because if its a new phenomenon - and new information, it may lead to something useful. :)  And if its not a new phenomenon then someone can tell me so and I can stop wasting time thinking about it  ???

gotoluc

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Re: Self running coil?
« Reply #509 on: April 08, 2010, 06:51:52 AM »
I'm no expert but if you can close to triple the inductance of a coil by introducing the center of a permanent magnet that sounds good to me.

One use I can think of is a PM attraction motor. The magnet is attracted to the Finemet toroid core (free work) and when it reaches TDC the coil is energized to release the magnets attraction to the core. Since you can close to triple the inductance of a coil using this Finemet core when the magnet is in the ideal location that it needs to be anyways for best results with this magnet motor design to work, the bonus will be that the coils electromagnetic field will be close to 3 times stronger without using more wire length which adds resistance and longer magnetic field reactance time.

It all looks like a win win situation for this kind of thing to me!

@Gyula, what do you think? Sounds like an ORBO ;D

Luc

ADDED

One thing you may want to check is to what inductance the same length of wire can produce on a regular ferrite core of the approximate same size. If it's as high as you can get the Finemet using the magnet then maybe it's not all that great but if it's the same 25mH then I think this is very good. Please let us know of your results.