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Production and design of high Q inductors with 3D printed hardware
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sourcecharge:
Hi there,

I've been working on design and production of inductors of Qs between 200 and 300, and I have been working on trying to squeeze every last bit of efficiency into the design of these things.

So far, I have used a 2inch MPP toroid core( 60u 128al), wound in 24 sections of about 960 turns of 28 AWG polyimide heavy mag wire, using a 1/16 thick electrical tape strip "fence" to hold the beginning and end apart.  Using my 3D printing, I make molds of sections that are screwed together to make these sectional wound toriods as well as a winding bobbin that resembles an "H" to increase the rigidity, reduce the wall thickness, and provide protection of the wire while hand winding. This inductor is measured to have a Q of about 250 to 300 using my mastech LCR meter at 10k hz.  Under resonant load, it drops to about 190 Q (why, is another discussion). 

I am currently designing another inductor to increase the Q by increasing the number of sections to 28, with two 12 degree seperations that are 180 degrees from each other giving the gaps and splitting the inductor in half for a total of 30 sections.  Using my 3D printing, I am going to make the hardware to hold on to the toroid core and maintain the gap between the halfved inductor.  I know that I can increase the Q of the inductor by only using 1 section as a gap, but spliting the inductor in half will allow aditional measurements that I wish to do.

I am wondering about the coating of the toroids though, as the MPP cores from arnolds are diped already.  My question is if there is an extra value in taping the cores with a layer of polyimide tape?

Also, does anyone have any opinion on the inductor core hardware that uses PLA as seperation and covering material as I have not been able to find any datasheets on the electrical properites of PLA for it's dissipation or dielectric breakdown limit.  I could use ABS but I'm not sure if ABS is bettter than PLA.

Thanks.
T3sl4co1l:
To what end?  What are you resonating with/for?

960 turns is an insane amount of wire for most purposes; that's a sizable fraction of a henry, total! :)

If this is for low frequencies, you might try to avoid using inductors, for power transfer, if possible; either shift the frequency up, or use alternate means (capacitors? piezos??).  Something of an X-Y problem.

Why use bobbin sections?  Why not wind freehand, back and forth, covering the toroid evenly (adding tape layers for spacing if needed)?  (The gaps will tend to reduce how much wire you can use, and increase stray fields, probably reducing Q overall.  Maybe it's not much effect?  I'm not sure!)

Why not use a larger core, or a different material?

This may be of interest:



These Q values are derived from the core loss curves; real Q as-wound may vary.  Usually downwards due to copper resistance, but maybe upwards also, depending on relative permeability and winding distribution (i.e., particularly for the #2 material, which has much higher inductivity when the turns are grouped together, instead of spread out evenly).

The Sendust core seems poorer than your MPP core, I'll have to add some MPPs to the plot some time.  (The core you have may well be one of the best picks already.)

Ferrite may be worth consideration too, but it's harder to plot in this way, because airgap is key.


More insulation on the base layer, probably doesn't matter: presumably, you're already operating at such a high frequency that the core looks like a[n electric] short circuit, its resistivity doesn't much matter.

You may also consider using litz wire.  Probably the 28AWG wire is subject to proximity effect, deep within those windings, which I'm guessing go many layers deep.  Even at fairly low frequencies.  Easy enough to try with some, say, 7 to 50 strand wire, of the same total cross section.  (For example, 16 strands x 40AWG would be equivalent I think.)

Regarding large-signal losses, it probably has something to do with initial versus average permeability, the losses that accompany it, and also the change in reactance.  It's pretty common for ferromagnetic materials to have a lower mu_r for small signals, say for fractional A.t/m.  It may be that the lower mu acts like more airgap, and therefore gives lower losses; alternately, it may be that the smaller hysteresis loop thus traveled, happens to enclose a relatively smaller area and therefore there's less loss due to that property.

Conversely, for large signals (which for this many turns, may require hundreds of volts!), mu_r is larger, and while this also gives higher reactance, it comes with a disproportionately larger hysteresis loop, and so the Q is overall lower.

It could well end up the other way around, that despite the core having higher losses, the copper loss was instead dominant, and the higher reactance improves things overall.  That might be the more likely outcome for lower-mu cores, where copper resistance is more significant.  But such cores have less mu_i/mu_avg change, so it would be harder to see as well.

In any case, not much you can do about it, just mentally adjust downward the measurement seen on the meter.  By how much, depends on signal level, number of turns, and material.  So... :-//

Tim
sourcecharge:
1st let me thank you for responding to me in such a very professional and concise questioning.


--- Quote from: T3sl4co1l on August 25, 2019, 10:13:00 am ---To what end?  What are you resonating with/for?

--- End quote ---

Research. Multiphase series resonant converter.


--- Quote from: T3sl4co1l on August 25, 2019, 10:13:00 am ---960 turns is an insane amount of wire for most purposes; that's a sizable fraction of a henry, total! :)

--- End quote ---

It's about 130mH, but compared to my 2000H and 80H toroid inductors, not so much...


--- Quote from: T3sl4co1l on August 25, 2019, 10:13:00 am ---If this is for low frequencies, you might try to avoid using inductors, for power transfer, if possible; either shift the frequency up, or use alternate means (capacitors? piezos??).  Something of an X-Y problem.

--- End quote ---

Higher frequencies mean better switching requirements and lower frequencies have cheaper components with less resistance.


--- Quote from: T3sl4co1l on August 25, 2019, 10:13:00 am ---Why use bobbin sections?  Why not wind freehand, back and forth, covering the toroid evenly (adding tape layers for spacing if needed)?  (The gaps will tend to reduce how much wire you can use, and increase stray fields, probably reducing Q overall.  Maybe it's not much effect?  I'm not sure!)

--- End quote ---

I have tried freehand winding, and making mulitple inductors with tolerances between them to be somewhat resembeling each other requires a technique that dictates a more repetitive productive procedure.   It's true that I "could" do this free hand, but doing it over and over would be very, well, hard....The stray parasitic capictance is what I was asking about, but I'm thinking that the 12 degrees seperations for the gaps would be large enought to decrease this capacitance.  My concern is about the dissipation, dielectric constant, and the dielectric breakdown of PLA vs ABS.  One of the problems with progressive winding and using tape to cover the previous layer would be in production, as this would be extremely difficult to do, and you would know this if you have ever wound a toroid by hand, let alone, multiple toroids.


--- Quote from: T3sl4co1l on August 25, 2019, 10:13:00 am ---Why not use a larger core, or a different material?

--- End quote ---
MPP cores are the most efficient cores on the market, aside from no core and using LN2, but this is not an opition for my "lab."  MPP cores are extremely expensive and the last time I checked on the price of a 4 inch MPP core, which BTW would fit in my toroid winder, was about 50 bucks a core.



--- Quote from: T3sl4co1l on August 25, 2019, 10:13:00 am ---This may be of interest:



These Q values are derived from the core loss curves; real Q as-wound may vary.  Usually downwards due to copper resistance, but maybe upwards also, depending on relative permeability and winding distribution (i.e., particularly for the #2 material, which has much higher inductivity when the turns are grouped together, instead of spread out evenly).

The Sendust core seems poorer than your MPP core, I'll have to add some MPPs to the plot some time.  (The core you have may well be one of the best picks already.)

Ferrite may be worth consideration too, but it's harder to plot in this way, because airgap is key.


More insulation on the base layer, probably doesn't matter: presumably, you're already operating at such a high frequency that the core looks like a[n electric] short circuit, its resistivity doesn't much matter.

You may also consider using litz wire.  Probably the 28AWG wire is subject to proximity effect, deep within those windings, which I'm guessing go many layers deep.  Even at fairly low frequencies.  Easy enough to try with some, say, 7 to 50 strand wire, of the same total cross section.  (For example, 16 strands x 40AWG would be equivalent I think.)

Regarding large-signal losses, it probably has something to do with initial versus average permeability, the losses that accompany it, and also the change in reactance.  It's pretty common for ferromagnetic materials to have a lower mu_r for small signals, say for fractional A.t/m.  It may be that the lower mu acts like more airgap, and therefore gives lower losses; alternately, it may be that the smaller hysteresis loop thus traveled, happens to enclose a relatively smaller area and therefore there's less loss due to that property.

Conversely, for large signals (which for this many turns, may require hundreds of volts!), mu_r is larger, and while this also gives higher reactance, it comes with a disproportionately larger hysteresis loop, and so the Q is overall lower.

It could well end up the other way around, that despite the core having higher losses, the copper loss was instead dominant, and the higher reactance improves things overall.  That might be the more likely outcome for lower-mu cores, where copper resistance is more significant.  But such cores have less mu_i/mu_avg change, so it would be harder to see as well.

In any case, not much you can do about it, just mentally adjust downward the measurement seen on the meter.  By how much, depends on signal level, number of turns, and material.  So... :-//

Tim

--- End quote ---

Q, depends on resistance, resistance of the wire in both AC and DC, resistance of the core, and resistance of the parasitic capacitance of the wire.

The AC resistance of the 28 AWG wire at 10khz is nil.  Litz wire is expensive and would not contrubute to increasing the Q at such low frequencies.  The MPP cores from arnolds magnetics are "the most efficient cores on the market." 

Parasitic capacitance of the wire can be limited by the winding technique and can include progressive winding and sectional winding techiques.  Progressive winding that is the same from one inductor to another, can be done using toroid winders.  My DIY toroid winder is able to use about 3 inch cores and higher.  Secontional winding allows the production of multiple inductors with relativly the same winding technique characteristics.   I already own 24, 2inch MPP cores that I bought at a electronics depot for a reduced price.

Core resistance can be calculated by "Legg's equation"

R(core) = u(r) x L x F (a x B(max) x f + c x f x e x f^2)   (edited due to mistake, see later post)
a, c, and e are listed within the arnold's datasheets and therefore the core resistance and be approximated for testing purposes
Notice as the permablity increases, the core resistance increases..That is the main drawback for using solid iron cores.

BTW, do you have much experience with PLA or ABS 3d printed hardware for inductor cores?
ch_scr:
Kerry Wong has tested the dielectric strength of PLA, also linked in some papers http://www.kerrywong.com/2019/01/06/pla-dielectric-strength-measurement/
Any chance we get to see some pictures / renders of your coil winding aids?
sourcecharge:

--- Quote from: ch_scr on August 25, 2019, 02:00:33 pm ---Kerry Wong has tested the dielectric strength of PLA, also linked in some papers http://www.kerrywong.com/2019/01/06/pla-dielectric-strength-measurement/
Any chance we get to see some pictures / renders of your coil winding aids?

--- End quote ---


45kV/mm, nice, now all I need is the dielectric constant and the dissipation factor.

Here are screen captures of one of the sectional mold wedges, the core hardware and the H winding bobbin.

I can't upload the freecad files, but anyone really can make these with the right dimensions for individual toroid cores using freecad.

Did you want some boring pics of the wound core?
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