High or low permeability magnetic core  - Page 1
Forum section: electronics / beginners

author: Maranto
author_rank: Contributor
timestamp: March 23, 2025, 05:22:09 pm
content: Considering a cylindrical rod “I” shaped coil, with a magnetic core, my goal is to be able to use it for electromagnetic induction to create induced voltages in a conductive metal near a distance of 30-40 cm, at a frequency between 5 and 100 khz, between the coil and the metal there is air. The choice of the magnetic core of the coil is between metglas (high permeability and low magnetizing currents) and mpp, sendust powders (low permeability and high magnetizing currents) Now my problem is choosing the right permeability of the magnetic core of the coil, between high or low permeability which of the two is the most suitable for my purpose??

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2025, 06:04:43 pm
content: With a more open I shaped core or other core with a lot of air gap the permeabinity of the core material is not that important. The overall magnetic resistance is to a large part determined by the air part and not much from the core.There is usually no advantage in having a low permeability in this case. On the other side a permeabilty much higher than about the length to diameter ratio is not really needed. In most cases I don't think one would get to really high magnetization. So one would not need high magnetization material either. I would consider ferrite and at the lower frequency end also FeSi transformer material.For a distance of 30-40 cm the size of the core and coils makes quite a difference.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 06:10:51 pm
content: For magnetic circuits containing iron and air, the “reluctance” is proportional to the total path length through air and that through iron divided by the iron’s permeability (linear approximation).

author: Maranto
author_rank: Contributor
timestamp: March 23, 2025, 10:56:10 pm
content: Quote from: Kleinstein on March 23, 2025, 06:04:43 pmthanks kleinstein, so you're telling me that for my setup and purpose, I should choose powders?? like mpp or sendust??

author: Maranto
author_rank: Contributor
timestamp: March 23, 2025, 10:59:31 pm
content: Quote from: TimFox on March 23, 2025, 06:10:51 pmwait a minute Tim I didn't quite understand your message, so as the permeability of the magnetic core material increases the reluctance should decrease? So in this case I should choose a high permeability material like metglas with 35,000?

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 11:58:14 pm
content: Quote from: Maranto on March 23, 2025, 10:59:31 pmYes, higher permeability reduces the reluctance through the iron, but in your case the bulk of the reluctance is in the much longer air path.Inductors wound on a core with a short gap will perhaps depend more on the permeability, but usually the gap length dominates the calculation.

author: Maranto
author_rank: Contributor
timestamp: March 24, 2025, 08:38:00 am
content: Quote from: TimFox on March 23, 2025, 11:58:14 pmso should i choose a low permeability magnetic core?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2025, 08:39:09 am
content: The point is that with a large air gap the permeability if the core is no longer that relevant (would still want µr > ~ 50, so iron powerder could be on the low side). Chances are the geometry and size is way more important. I would consider ferrites.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 24, 2025, 11:56:07 am
content: Based on the few details provided, I don't see any reason to think the core material matters, or that a permeable core is helpful at all. With a gap of 30-40cm, it's likely the only a tiny fraction of flux from the coil is going to intersect the "conductive metal" anyways (unless the core is much longer than the coil, effectively reducing the length of the air gap).

author: Maranto
author_rank: Contributor
timestamp: March 24, 2025, 12:51:38 pm
content: Quote from: Kleinstein on March 24, 2025, 08:39:09 amok yes I understand, in fact studying I found that there is the value of the effective perneability to take into account, and that it decreases with the air spaces. ok so I will opt for the ferrites. but what if for example, instead of a cylindrical coil with an "I" rod I used a toroid? what would happen, the theory states that the field would be concentrated inside and I would not be able to induce any metal at a distance of 30 cm, is it true?

author: Maranto
author_rank: Contributor
timestamp: March 24, 2025, 12:55:43 pm
content: Quote from: mtwieg on March 24, 2025, 11:56:07 amyes you are right, there is an experiment from kainst university where they use dcrs, and in particular the magnetic core is much longer than the windings (turns). however if instead of a cylindrical coil with an "I" rod, I tried to use a toroid? would I be able to induce a voltage in a metal at a distance in the air of 30 cm? the theory states that in a toroid the field is concentrated only inside

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 24, 2025, 09:15:46 pm
content: Toroid is the shape you choose when you want to avoid stray fields completely. Unless the toroid's core is gapped and the "metal" is put in that gap, don't expect anything.

author: Maranto
author_rank: Contributor
timestamp: March 25, 2025, 01:12:17 pm
content: Quote from: mtwieg on March 24, 2025, 09:15:46 pmyes indeed, the toroid is mostly used in transformers for its shape that concentrates the magnetic field inside, so there is little inductive field on the outside. I also did the calculations, and the fact of having to bend the magnetic core circularly makes it difficult for me to wind the coils, that is, it's fine for me if the metal to be induced is positioned at the center of the toroid