Inductor core material reduces its Q? - Page 1
Forum section: electronics / beginners

author: VooDust
author_rank: Regular Contributor
timestamp: September 17, 2021, 06:14:03 am
content: I was experimenting with winding an air core coil. When I moved a metal object inside the core, the inductance slightly increased as expected (and measured by an LCR-Meter).However the same meter also also reported a drastic reduction in Q of the coil. Likewise, in an experiment with a resonating LC tank circuit, the peak-to-peak voltage decreased, too, as I was placing a metallic wrench inside the coil's core.Why is that? Are air coil inductors somehow better than their iron core counterparts? I.e. is an air core conductor preferred when you want high selectivity?

author: Circlotron
author_rank: Super Contributor
timestamp: September 17, 2021, 07:38:01 am
content: You were probably getting eddy current losses in the core because it wasn't laminated, just a solid lump.https://circuitglobe.com/what-is-eddy-current-loss.html

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2021, 08:08:15 am
content: Yes, energy is lost in eddy currents, but also hysteresis, as it takes energy to flip the magnetic poles inside the core.Often an inductor with the correct core can have lower losses, than the equivalent air core, because far fewer turns are required to achieve the same inductance and a shorter winding, means lower resistance and copper losses. Whether to use a core, or not and the material, depends on the frequency. Laminated steel is used at low frequencies, below a few kHz, various ferrite materials, between tens of kHz to MHz, with air cores used above 10MHz or so.

author: Gyro
author_rank: Super Contributor
timestamp: September 17, 2021, 08:10:13 am
content: It's a little known fact that the inductances of the wire induction loops, embedded in road surfaces for vehicle detection, go down when a vehicle is sitting on them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2021, 09:54:27 am
content: What size metal object? Was it mild steel or what?If mild steel, I think you will find, the Q does indeed increase, at certain frequencies; but at frequencies where it experiences skin effect (for, say, a pencil-sized rod, mere 10s of Hz), eddy currents will dominate.At a few Hz, I think you will quite reasonably expect the air-core Q is extremely low (i.e., close to 0), but it should indeed increase in that case.You may have better luck finding a good-quality core, than finding an inductance meter that works at single-digit Hz.Neat fact: at frequencies where skin effect dominates, the Q factor of the core is exactly 1. Skin effect is a diffusion process, which means a Z ~ sqrt(F) response, and a phase shift of 45 degrees, i.e., X = R, so, Q = 1. You can only measure the core Q when it's tightly coupled to the coil (e.g., a closed magnetic circuit with tight windings, like with a toroid or E core), and otherwise you'll measure it in combination with coil resistance and leakage inductance; the sqrt(f) slope may still be sensible over some frequency ranges, or as a subtle component that doesn't seem to fit with the coil parameters as is. (The biggest confounding factor being, the coil itself also experiences skin effect; but we can make coils out of thin enough wire, in myriad strands, to avoid this effect until much higher frequencies.)To be clear, this effect shows up most clearly in a plot of impedance vs. frequency; to measure it, you'd need an impedance meter with not just selectable, but adjustable frequency.Tim

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 17, 2021, 10:43:15 am
content: not only the core material, but also the measurement frequency influences the Q, that was already said before.generally said: the lower the frequency, the more the coil turns into a simple conductor, so does the Q get lower.what LCR meter where you using?air coils are normally used above above 30MHz; from ~100khz to middle MHz range, powdered iron cores are used to achieve resonable high Q.

author: VooDust
author_rank: Regular Contributor
timestamp: September 17, 2021, 11:10:08 am
content: I tried the same experiment with a ferrite bead core, this time Q did increase significantly. In my understanding, the powdered nature of the core material works to counteract eddy currents very effectively. So inductance is increased while introducing only negligible side-effects, thereby increasing Q.It's just that I thought eddy currents was some advanced effects that would never surface in my basic experiments.I measured at 100kHz with my trusty DER DE-5000.Thanks everyone.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2021, 11:47:09 am
content: Eddy currents are a matter of scale -- and skin depth is the measuring stick. So, thickness versus frequency, given material properties.It has nothing to do with, like, power level -- it's a linear effect. Whether it's getting too hot or something (or not, as, in the case of induction heating, we want eddy currents!), will depend on that, but that's simply a matter of available power versus dissipation, nothing at all characteristic of the EM fields.Lots of physical phenomena are like this, they're proportional to some quantity and you can do a quick sanity check whether you'll need to consider them or not.Stray inductance is the same way, with inductance being roughly proportional to wiring length. At very high frequencies, you have to deal with waves in transmission lines (or waves in general); at lower frequencies, you may have to deal with the residuals from that; at very low frequencies, wires look like (plain resistive DC) wires.Specifically with ferrite, the resistivity is a whole lot higher (it is still indeed resistive, give it a try!) -- so the field can penetrate much deeper into it, at a given frequency. It still has hysteresis loss, which also manifests as equivalent resistance, so has a shielding effect; but at this frequency, you need several inches of ferrite to see it, so you're good.As mentioned, the other way is to divide the metal, breaking the eddy loops, at least until a higher frequency; instead of solid iron, try stacked sheets (aligned parallel to the magnetic field -- crossed, it's no different!), or bundled wires. Or even filings or powder, but mind there's a lot of air gap between those, when loose; they need to be packed very tightly (hydraulic press?) to get any useful permeability.Tim