Question about Effects of metal on Inductors

[lcolbur1]

Quick question for the community: suppose you had a large piece of metal with a crossover mounted near it. How would the proximity of each affect the crossovers, specifically inductors? Would it have any affect at all? Would the inductors magnetize the metal? Would it change impedance values?

[Heli-Tim]

It would depend on the permeability of the metal involved, and the strength of the inductance/ inductive reactance. You can design magnetically coupled inductors that were specifically engineered to achieve this effect.

Perhaps another member would be able to help more if they knew the specifics of your situation. I know it is beyond what I am able to offer advice on .

[Steve Manning]

What Heli-Tim said. Knowing the specifics of the metal, distance, etc., would determine your answer. If your talking something like aluminum, no issues, soft iron, then you could have problems.

[bvbellomo]

Magnet fields fall off at the cube of the sphere of the distance of the magnet. The implication of this is "how near" makes a huge difference, and moving only 1mm further away might completely eliminate an otherwise significant effect.

Other than the obvious solution of not putting metal near your inductors, any significant interference should show up in your measurements. I'd expect magnetic metal too close to an inductor to raise the magnetic permeability of the effective 'core' of the inductor and therefore raise its inductance, although I've never tested this.

For those with more practical experience, am I correct in saying if you can't measure a change in inductance, you don't have to worry about any possible interference?

[sdl2112]

I can’t say how close as a rule of thumb but metal will effect the inductors, especially near the center of aircore inductors. Even non magnetic metals will effect it. I ran a test with an inductor placed on lead and aluminum sheets. The magnetic field will produce eddy currents which create an opposing flux that reduces the inductance. If the metal were ferrous it would increase the permeability and increase the inductance. In both cases I assume the addition or subtraction would be nonlinear.

http://www.htguide.com/forum/showthread.php?41225-Modula-MT-XE-build&p=579192&viewfull=1#post579192

[wolf_teeth]

I can’t say how close as a rule of thumb but metal will effect the inductors, especially near the center of aircore inductors. Even non magnetic metals will effect it. I ran a test with an inductor placed on lead and aluminum sheets. The magnetic field will produce eddy currents which create an opposing flux that reduces the inductance. If the metal were ferrous it would increase the permeability and increase the inductance. In both cases I assume the addition or subtraction would be nonlinear.

Yes, this^^^ is the best answer. If you want documented experiments, Troels did some with the aluminum-tubed capacitors, and they reduced his values somewhat. Aluminum will reduce the value. I used steel-laminate coils in a Minimus 7 cabinet, and had to oversize the inductance to counteract the loss.

I've also had steel laminate cores too close to a driver motor virtually nuke its value to nothing or very little. The motor presents a magnetic field on the core of the coil, and that's all it takes.

Iron or ferrous additions will increase value, but saturate more readily due to not being the right kind of special ferrous silicon-coated laminated material. I've seen more than once where a P-core or sintered/ferrite or even iron-rod cored coil will readily saturate with not much input power, and I have virtually stopped using them except in lower current conjugate filters like mid-dome Fs compensation and such. I would recommend avoiding them on woofers in general.

Later,
Wolf

[lcolbur1]

So how far away would I need to mount crossovers from steel plates used to weight down the cabinets? I have two steel plates stacked on top of one another at the bottom of each of my floorstanding cabinets. The plates total about 1"x3"x8". I don't need to mount the crossovers directly to the plates (and as stated above, clearly shouldn't) but I still need to mount the crossovers somewhere in the cabinet, preferably near the bottom.

[xandresen]

You can measure the coil inductance with an inductance meter while moving the coil closer to the steel plates.

Keep in mind that a related problem occurs if the inductors of the crossover are close to each other. They magnetically couple to each other, adding an electrical effect not present in the crossover design. I don't know how to measure that one easily. The usual suggestion is to orient inductors at right angles to each other to minimize the magnetic coupling.

[lcolbur1]

Here's a dumb follow-up question: would brick/concrete/granite/stone have any effect on the inductance? I am guessing no but wanted to get the community's thoughts on it.

[wolf_teeth]

That depends on the stone, I would wager. There are some stones with flecks of metal in them, some even contain iron or steel. I don't know however if it's enough ferrous or conductive material to make a difference.

Later,
Wolf

[wolf_teeth]

xandresen-

I have read some places that people can connect headphones to a secondary coil, and have it's coil approach the first that is playing the source. You can supposedly hear the coupling effects.

Also, orienting at right angles really isn't enough. I've seen measured coupling by way of a speaker hooked to the secondary coil, and orthogonally is the best method. They should never 'face' or its axis looking directly at another coil if at all possible. ie- Both on edge placed at right angles in an "L" layout, or one laid flat and the other looking to roll across it.

Later,
Wolf