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Navrit Bal

Sat Dec 04 2010, 12:04PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I was never even considering having these in series, mainly due to the potential for more things to go wrong amongst other things. I'll make sure to use some sandpaper then. Unless I can get my hands on some scrap copper, probably from my various friends' or if I have to, the dump. I've just looked on ebay for cooper tube, which might be best out of these

?

Don't both copper and aluminium form oxides when exposed to air? With aluminium, doesn't the oxide layer form extremely quickly ? Great joke signatures Barry

I know I've already asked, but is it a good idea to use titanium coated silicon steel as a projectile?

Pinky's Brain

Sat Dec 04 2010, 01:37PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Navrit Bal wrote ...

Barry, if I can get my hands on some copper pipe then I will exactly what you did, as it is probably the cheapest solution to getting bus bars, or at least nearly equivalent.

Copper is expensive enough and copper sheet is used enough that it's unlikely to be much cheaper than sheet per weight.

sheet

flats (thicker than 5 mm is a waste of copper)

Barry

Sat Dec 04 2010, 04:42PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

It's that age-old problem of engineering that is never solved: "Which is best?" Haha, first you have to define "best". Both choices of copper pipe will work fine for a busbar. Watch the shipping prices, they add up quickly. Maybe you can save time and shipping cost at your local ironmonger. As for me, I like to see these things in real life if possible before choosing.

Scrap copper is great. Careful if it's not straight. Copper turns brittle when worked and a pipe can be impossible to straighten if bent even twice. Best choice is a straight unused pipe. My mistake was to use copper pipe that came coiled from the mfr; even though it was "new" it took a lot of work to make it sufficiently straight before flattening.

Navrit Bal wrote ...

Don't both copper and aluminium form oxides when exposed to air? With aluminium, doesn't the oxide layer form extremely quickly ? Great joke signatures Barry

Yeah, both materials form oxide layers in air. I'm not sure how quickly. Is the Al oxide conductive? I think a star washer is a good gripping idea to dig through to bare metal in either material.

I assisted one student using aluminum busbars, and he took no special care in polishing and assembly, other than using star washers in connections. He built a 400v railgun and it worked great. You probably don't need to spend a lot of extra time nor effort there, just use common sense for good tight connections.

Pinky's Brain wrote ...

(thicker than 5 mm is a waste of copper)

Pinky, can you tell us more? I would like to understand this better, thanks.

I'll tend to use "A busbar resistance less than 10% of the capacitor's ESR is a waste of copper." The R calculation is a matter of cross-sectional area and length and bulk resistivity. Any combination of thickness and width that gives the desired cross-section will work great. Even then, a larger cross-section is not a waste if you desire extra conductivity for possible future projects. Or just want it to look massively cool, lol.

Navrit Bal wrote ...

I know I've already asked, but is it a good idea to use titanium coated silicon steel as a projectile?

Sounds good to me. The coating doesn't help anything, but it's so thin that the extra mass doesn't hurt anything either.

The real problem is that we don't know its magnetic properties. It would be nice to have the B-H curve for comparison with other projectiles, or at least know its maximum permeability. But alas, no, if these things are important specifications then we must shop at specialty suppliers. I would love to have a pocket device to measure permeability of random small objects while I wander in a hardware store. No idea if it's even possible to build one.

You mentioned "silicon steel". Here is a chart of magnetic properties that shows maximum permeability varies widely according to percentage of silicon. But it varies widely over a really good range, several times better than the usual cheap iron rods that I use.
So I'd say go for it!

Cheers, Barry
"Are you going to come quietly, or do I have to use earplugs?" - Spike Milligan

Navrit Bal

Sat Dec 04 2010, 04:43PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I have taken a few measurements with my capacitors and it looks like if I cut this bar

in half then I could have a bit left over and its 10 pounds overall. Before I order, I'll see what I can do about getting at least 1.5 times more but as aluminium, as I might be able to get this for free from my school.

Neuuubeh

Sat Dec 04 2010, 06:01PM

Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33

If you want to go for a low-resistance bus bar, you have to consider your pulse lengths and the skin effect - depending on frequency, the current tends to flow on the "outer shell" of the conductor as this represents the least resistance, due to interaction of inductances (in the peripheral areas there is less interference from eddy currents as compared to the center I believe). Now surely this depends on your pulse lengths, as well as the conductor material. There are plenty of calculators on the web for this, for example here :

. Most are for copper I think, you can also check the formulas on wiki :).

One step further in this direction would be having a coil wound of litzwire.

Thing is, both measures are only really relevant with extreme currents and very fast pulses, and I dont think this is of that much importance in the reluctance coilguns most of us can build. It still might be interesting to try out :)

Navrit Bal

Sat Dec 04 2010, 06:33PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Since the frequency only looks like it will be between 150 and 200 Hz, the skin effect probably won't be a problem.

Also, I've just ordered the silicon steel for just under 5 pounds which I think is a great price for silicon steel. I'll order the copper unless my school is willing to supply the amount of aluminium required.

Pinky's Brain

Sat Dec 04 2010, 06:46PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Barry wrote ...
Pinky, can you tell us more? I would like to understand this better, thanks.

At 5 mm you are pretty much at skin depth for a 1 KHz signal, so better to add the weight in width (or diameter for a pipe) than thickness (assuming the cost per weight is equal).

klugesmith

Mon Dec 06 2010, 07:41PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Navrit Bal wrote ...
...Also, I've just ordered the silicon steel for just under 5 pounds which I think is a great price for silicon steel.

IIRC, the main purpose of silicon in electrical steel (as discovered some 100 years ago)
is not to improve the magnetic properties (high saturation & permeability, low coercive force & hysteresis loss).
It is to increase the electrical resistivity as much as possible, to minimize eddy current loss, without degrading the magnetic properties or physical workability. So Si would be of little value in a DC electromagnet, or the rotor of a conventional rotating-field alternator.
Anybody know better? Radiotech? Carl P.?

Navrit Bal

Mon Dec 06 2010, 07:57PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Either way, wouldn't it be best to minimize eddy currents, isn't that a potential large power loss? Also, I think it can't really be a bad idea and I've never seen any coilgun experiments using silicon steel, so lets see what happens

besides, I can always swap to iron rods...

Barry

Wed Dec 08 2010, 05:56PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Navrit Bal wrote ...

Either way, wouldn't it be best to minimize eddy currents, isn't that a potential large power loss? Also, I think it can't really be a bad idea and I've never seen any coilgun experiments using silicon steel, so lets see what happens

besides, I can always swap to iron rods...

It's always a good idea to minimize eddy current losses. For example in one of my early coilguns, I got a 24% increase in exit velocity by changing from a brass firing tube to plastic.

I don't have figures on eddy currents in projectiles but I'm sure there is some effect. Other people, like James Paul, have tried a variety of projectiles to reduce eddy current losses.

It would be a great experiment to find out how your silicon steel bits compare to similarly-sized iron rods. I can't wait.

I am going to try slotting the projectile on my next large-bore coilgun and report back then.

Cheers, Barry
*<[:{) Hey, look, it's Santa!!!

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