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Submerged Electromagnets

02/20/2007 9:10 AM

I have been working on designing a submersible platform, held underwater by electromagnets. When the platform is submerged is exerts about 3 kg of bouyancy. When power is cut to four electromagnets at the base of the platform, the platform rises to the surface. Functionally it works well, but I am having trouble preventing corrosion of the electromagnets. I experimented with some magnets made of 400 Series Magnetic Stainless Steel, but passivation is not perfect and they still show signs of rust. I have tried nickel coating, painting and plastic coating but the nickel corrodes, the paint seems to crack around the edges of the magnet, and the plastic adds too much gap for the magnets to work. Does anyone have any suggestions or previous experience with submerging electrical components?

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#1

Re: Submerged Electromagnets

02/20/2007 11:38 PM

tin coating will work. It does not corrode and prevents fouling. A small area like that is not a danger to the environment that ships hulls are.

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#2

Re: Submerged Electromagnets

02/21/2007 12:17 AM

A slight radius on the edge of the magnet will reduce the stress on the paint.

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#3

Re: Submerged Electromagnets

02/21/2007 4:10 AM

Are you using AC or DC to energize the magnets. If you are using AC or full wave rectified DC then the magnetic field will not be constant and there will be vibrations in the magnet along with a host of other problems that might be causing the cracking.

I havn't actually tried this but it might be worth using a regulated DC supply and seeing if the cracking still takes place.

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#4

Re: Submerged Electromagnets

02/21/2007 8:36 AM

I've used electromagnets underwater quite a bit.

Try potting with a slowish (>2hr) curing epoxy. If you pull a partial vacuum on the part, the coating will be very thin. Look up vacuum forming of composite structures for instructions in setting up the vacuum curing things.

Any boat repair shop, surf board shop, hobby shop that builds RC planes, or anyone else that regularly uses composites, could probably help you do this for very little.

If you don't want to mess with vacuum, use slower (>5hr) curing epoxy and just let the excess drip off (hang it business end up or sideways).

Spray coating fast cure epoxy could work as well.

Make sure whatever epoxy you use is good for the water you are in (salt/chlorine?), and the material you are coating. The epoxy section on www.mcmaster.com has pretty extensive material properties.

can you tell I like epoxy?

The other option might be making a case with replaceable non-stainless steel at the end of the magnet. As long as the magnet is flush on the inside, you should get good propagation of the field. The steel will corrode, but could be easily replaceable.

If the 3kg of buoyancy isn't critical, think about adding a little ballast and lowering the current through the magnet. That's a considerable amount of force for electromagnets to hold underwater, since you will have some sort of coating/case that will make a gap.

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#5

Re: Submerged Electromagnets

02/21/2007 8:53 PM

Is it salt or fresh water, what sort of depth, are magnets the only answer?

Could you not use water ballast to hold fast?

The amount of bouyancy seems a fairly small amount to have to counteract.

How big is this structure? Where is it located? Indoors or out of doors?

Some more info would go a long way to giving you a better answer.

Vacuum impregnation could be your answer, also using glass insulated wire.

Well there's my two penny worth.

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#6

Re: Submerged Electromagnets

02/22/2007 12:13 AM

Could you have it ceramic coated? they use this process in automobiles? for heat dissipation and strengthening

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#7
In reply to #5

Re: Submerged Electromagnets

02/22/2007 8:26 AM

The platform is a 6" Diameter Cylinder about 7 inches tall. The tank in which the platform is used is indoors and contains about 15" of fresh water. Im sure magnets are not the only answer, but I chose them because the release needs to be as quiet as possible. Water ballast would probably work, but Im not sure that this would allow the platform to rise quick enough. I think encapsulating the magnets in place is going to be my best bet.

Thanks All for the Input.

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#8
In reply to #7

Re: Submerged Electromagnets

02/22/2007 4:13 PM

I just had an idea, Yes dangerous I know. Can you put the magnets on the underside of the water container and the metal keepers in the water, these could be epoxy coated. Is you container metal or glass/plastic? Do you heat the water? add any chemicals? All these would alter how to go about this release problem. Try a good nights sleep it often helps.

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#9
In reply to #8

Re: Submerged Electromagnets

02/22/2007 4:38 PM

It would take an impractical amount of power for the magnets to work through the plastic tank. Plus the whole system needs to be self contained so it can be easily removed from the tank. The water may, in some cases, be heated to around 80F. Any additives to the water will be non toxic. ie: non toxic paint, or powdered milk.

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Anonymous Poster
#10

Re: Submerged Electromagnets

02/23/2007 5:14 AM

Try Magnet Schultz of america they can get 2 types of IP 68 (Water proof) 50 mm diameter 15 micron nickel plated 24v DC electro amgnet and electro permanent magnets that have been used on under water vehicles.

regards

Tim Lloyd

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#11
In reply to #4

Re: Submerged Electromagnets

02/23/2007 9:42 AM

Sounds good, Thanks!

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#12
In reply to #10

Re: Submerged Electromagnets

02/23/2007 9:42 AM

Thanks a lot, Ill contact them.

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#13

Re: Submerged Electromagnets

02/23/2007 6:45 PM

We had similar problem, but the components were rather small in size. I don't know how big your are magnets, but we solve the problem, for several years at least, Coating it by dipping the component in a molten Phosphor-Bronze bath. not cosmetically nice, but the job was fairly well done. You will also be facing some mechanical problems that we didn't have. my nickle worth of thoughts your considaration...

Wangito.

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