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I have recently noticed some toroids wound with what I consider to be huge space left over, and I am asking if there is any good reason to do this. (or even an acceptable reason would do). My thoughts are that if toroids can "whine", that you want to wrap the wires relatively tight to the core so that they don't move, not set them up perfectly for a host of resonant vibrations. I am also thinking that the DCR is needlessly increased, and therefore efficiency reduced.

Possible reasons for me might be better conductivity of the copper through less deformation / work hardening and therefore removal of the need/desire to anneal. Is leakage inductance affected?

In this pic, all have needlessly large gaps between the wires and the core except last one, which is conventionally tight-to-the-core (though thicker). You should be able to see that for the first toroid, there is space both inside and outside.

enter image description here


EDIT:

Here is a close up of what I mean, just to be clear:

enter image description here

And here's another one. Doesn't this look much to easy to deform?

enter image description here

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    \$\begingroup\$ Leakage, uniformity of field. If you only have 4 turns and you do them 2 by two, in opposite parts, the EMI would increase, despite the core. \$\endgroup\$ Oct 26, 2021 at 18:42
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    \$\begingroup\$ I want to make it clear that I'm talking about the space between the wire and the core itself (radially), not between one turn of wire and another turn of wire (laterally). \$\endgroup\$ Oct 26, 2021 at 18:54
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    \$\begingroup\$ @MicroservicesOnDDD That clarification is very helpful. Wind tightly. \$\endgroup\$
    – jonk
    Oct 26, 2021 at 19:01
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    \$\begingroup\$ I'm just guessing (or this would be an answer), but based on the fact that the windings seem to be evenly (i.e. circularly) curved as they wrap around a core of more-or-less rectangular cross section, I suspect this is done for ease of manufacture. \$\endgroup\$
    – TimWescott
    Oct 26, 2021 at 19:38
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    \$\begingroup\$ @MicroservicesOnDDD Loose wiring will allow axial forces to more readily compress the wiring "hoops." So tight is good. I think Tim expressed my thoughts, too, about why you see it open like that: automated winder design issues. \$\endgroup\$
    – jonk
    Oct 26, 2021 at 19:53

3 Answers 3

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The only advantage I can think of is keeping the core cooler. Powdered metal cores can exhibit thermal runaway, so keeping them cooler by keeping the copper losses away and increasing the convection around the core could be useful.

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  • \$\begingroup\$ They are power inductors, but the material was unspecified. Thanks for your insight, as I hadn't thought of that. \$\endgroup\$ Oct 27, 2021 at 4:23
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I saw toroid inductances and Common Mode Filters with a lot of windings and no space between 2 adjacent coils.

I saw toroid inductances and Common Mode Filters with a few windings and noticeable space between 2 adjacent coils.

It depends on the value of the target inductance that the manufacturer wants to make.

All of your depicted Common Mode Filters and the inductance work in the kHz range, that is, low frequency.

They are mainly used at the input if AC/DC converters as EMI filters to reduce conducted emissions, also known as common mode interferences.


Air gap between the core and the windings:

In Common Mode Filters and inductances the gap between the core and the windings shouldn't exist at all as far as EMI or functional performance is concerned.

Manufacturers want to confine the magnetic flux H inside the core and not in the air.

The only reason I can think of is the following: The wire was pretty thick and wasn't that easy to bend.

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    \$\begingroup\$ See OP's comment below the question, "I want to make it clear that I'm talking about the space between the wire and the core itself (radially), not between one turn of wire and another turn of wire (laterally)." \$\endgroup\$
    – Transistor
    Oct 26, 2021 at 20:16
  • \$\begingroup\$ Did you look at my second picture? \$\endgroup\$ Oct 26, 2021 at 21:47
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    \$\begingroup\$ I added a note to the bottom of my answer. \$\endgroup\$ Oct 27, 2021 at 4:13
  • \$\begingroup\$ I did think that wire thickness might be the issue, but then many inductors have much thicker wire and yet are tightly wound, so I'm 50-50 on that possibility. \$\endgroup\$ Oct 27, 2021 at 4:26
  • \$\begingroup\$ The lowest resistance wire is the wire with the least work-hardening to it, implying that the less bending, the better the conductivity of the wire. The conductivity can be restored by annealing the wire, but I don't know if the manufacturers do that. But the loosely-wound toroids would definitely have less work-hardening to them. That might be offset, though, by the greater length of the wire. So, once again there's no clear answer. Plus one for adding the second part. \$\endgroup\$ Jan 18, 2022 at 18:32
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enter image description here

Figure 1. This one (from your montage) seems to use circular windings on a core with a rectangular cross-section (with chamfered corners). This results in radial gaps between the wire and the core towards the centre of the outer and inner surfaces but near tangential contact at the chamfers.

enter image description here

Figure 2. This one has a squarer, tighter winding and doesn't exhibit the radial gaps to the same extent.

I think the effect is likely to be determined by the winding equipment, the bobbin that has to pass through the core and the amount of tension that can be applied to the wire during winding.

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