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Re: Inductor question
** Reply to note from Tesla List <tesla-at-poodle.pupman-dot-com> 09/05/96 10:25pm -0600
> I am playing around with a variable inductor that has a 4" diameter
> sliding core packed with 20 pounds of welding rods (idea compliments of R.
> Hull) that I plan on using to regulate the current in my 50A controller,
> and I have a few questions.
>
> I don't really have the right wire to wind it with yet, so I just wound it
> with some heavy wire (2 or 4 gauge maybe with fairly thick insulation)
> that I had to see how it works. There aren't a lot of turns in the 13"
> winding length, so the inductance is pretty low, but I noticed that the
> difference between the core being totally out of the winding and totally
> in is less than a factor of 5. Is this reasonable for 20 lb of iron
> welding rods or could something else be screwed up? Using Wheeler's
> formula for the inductnace of an air-core solenoid, I compute a value
> that's pretty close to what I measured with the core out, but I would have
> thought that the iron would have made much more of a difference. Would
> winding a coil with 6 gauge magnet wire (which would give me a much higher
> inductance to start with) make more of a difference?
>
> Thanks in advance.
>
> Steve Roys.
The permeability of different types of steel vary a lot from alloy to alloy.
A permeability of 5 is terrible. 99.9 % pure iron has a max permeability of
around 5000. The very best alloys for this kind of work have a high nickel
content and can reach permeabilitys of 800,000. I'd experiment with different
kinds of steel before you put too much work into winding a nice inductor.
You could try to wind the inductor so it limits the current at just over the 50
amp capacity of your system without a core. That way if have have a core
with a low permeability you have a better chance of being able to get the
current down as low as possible. Ideally you'll want a inductor that gives a
few more amps than your system is capable of with the core out, and zero
current with the core all in.
A co-worker and I were goofing around one day and wound a bunch of #10
THHN around a 3" PVC pipe. We jammed the wires into a wall outlet and
turned on the power. With a peak reading ammeter we pulled 60 amps
before the 20 amp breaker tripped a split second later. We took a piece
of heavy wall steel pipe and inserted it and we were able to hold the current
just under twenty amps. I thought that was great, but after being energized
for just 20 or thirty seconds the pipe was too hot too touch. Very poor core
material! Interestingly enough, adding more steel to the inside of the pipe did
not increase the permeability and at times the permeability would drop. We
were stuffing the steel pipe with steel strapping that was heavily galvanized.
I'm not sure about the permeability of zinc. If it's less than one that may
have been the cause of the drop in permeability.
Eddy currents are currents that flow within a solid magnetic core that produce
heat. I assume you are using flux coated arc rods to reduce this effect?
Another problem is an effect called hysteresis. The reluctance of a core
material is a measure of how fast the residual magnetic flux decays after the
magnetizing force is removed. If it doesn't fall off fast enough, the next half
cycle of magnetic force in the opposite direction has to overcome the residual
flux of the opposite polarity before it can remagnetize the core in the other
direction. (Did anybody follow that?, I almost confused myself.) This causes a
power loss that creates heat in the core material.
I have a thirty pound chunk of laminated steel that I cut from the core of a
burnt up transformer at work that I've been meaning to use for this purpose,
but as with many other things I haven't gotten to it yet. My old Montgomery
Ward stick welder is working fine for right now. That chunk of steel has been
riding around in the back of a company truck for almost a year and is getting
pretty rusty. Now that you've reminded me I just may have to do something with
it.
-Mike McCarty