Strongest Electromagnet using 2 AA bateries

[Boncuk]

Just curious, are NiCd batteries still available in the EU?

Haven't been in Germany for more than four years.

They are available in Thailand.

 

[Hero999]

If heat dissipated to the air is proportional to surface area, then the surface area of a cylindrical winding of wire should raise the fusing current of an individual wire conductor, provided that there is good thermal conductivity between the conductors in the winding.

Exactly, the amount of heat that can be dissipated depends on the surface area exposed to the air which is exactly why cable has a lower current capacity when wound into a tight coil than it does when unwound.

Think about it, when it's unwound, all of the cable's surface area is exposed to the air. When you wind it into a tight coil, most of the cable's surface area is not exposed to the air so it won't dissipate the heas as efficiently.

There will also be more thermal inertia in a winding than in an individual conductor, which would delay the time it takes to melt.

That's true but it would melt at a much lower current.

You've also ignored the point that the insulation would fail long before the copper will.

The is true with extension reels, the current rating is usually half of its unwound value when coiled up.
https://en.wikipedia.org/wiki/Extension_lead

An extension reel is an extension lead that rolls up, usually into the socket end, which in some cases has more than one socket on it (often 2 or 4). Another type of extension reel hangs near the plug end and permits the user to draw the cord out by grasping the socket end. It should be noted that these cables can only be used to carry full rated current when fully extended since the portion on the reel constitutes a concentration of the loss power (the result of its series resistance) which is not suitably dissipated unless most of the cable is unreeled to expose it to ambient air.

 

[Willbe]

cable has a lower current capacity when wound into a tight coil than it does when unwound.

I stand corrected.

 

[Trisorion]

It seems that there are high discharge NIMH batteries as well. I found a chart of discharge rates https://www.powerstream.com/Compare.htm. It says there is a NiMH battery capable of 26 amps. Unfortunately I have not been able to find anywhere that sells batteries from that company. I cant even find the product specs for any of their batteries.


How would splitting the electromagnet coil into three sections, then wiring them in parallel effect the circuit?

Specifically, I was thinking of winding three concentric ring electromagnets. In that scenario would the smallest ring produce the most mmf/surface area? Would their magnetic fields cancel out? How are the poles arranged ona ring magnet anyways.

 

[jpanhalt]

Taking the parallel coil proposal to the n^th degree, you could parallel each turn to give effectively a single-turn coil. Or, just use a thin sheet of copper. The problem with that approach is the current needs, heat, and core saturation.

See:https://www.lanl.gov/news/index.php/fuseaction/1663.article/d/200805/id/13276

To quote from that article from a caption near the bottom of the page:

A 3-million-ampere current pulse creates a 240-tesla magnetic field at the loop’s center, generating so much heat and magnetic force that the loop explodes.

That is not what you want to do. Your batteries will limit the current. Make a sufficiently large coil and core that can take it for the 20 s you need, as suggested above by myself and others.

John

 

[Trisorion]

Jpanhalt. I need a very basic answer. Remember, I have no/limited electronics experience. However, I do not want to be told "just do this". Please explain why one is better. Additionally my inexperience limits me from understanding the concepts behind the examples given sometimes. I dont understand what a 3 million amp pulse, or single coil loops have to do with this project. I used your post as an example, and I have said "you" this whole time, but it is not just you; a lot of the posts here are like that to me.

Thanks for the help. Sorry for making it so difficult.

 

[jpanhalt]

1) Why one is better? The battery is your limiting element for power.

2) Neither I nor anyone is trying to tell you "just do this." However, with your limited background, you either have to start with the basics and work up (Is there time?) or take some things as facts from those who have more experience than you.

3) I forget where you are located, if you ever indicated that. It makes a difference as to availability of materials (e.g., NiCd vs. NiMH). What grade level is this project for?

4) Look at this as an example in project management. Divide it into various essential parts and goals. For example, you have two goals: 1) Maximum one-time lift; and 2) Maximum sustained lift and release (20 s). And, there are easily 5 parts to each goal: 1) The battery; 2) The wire size; 3) The number of turns; 4) The core size and shape of coil; and 5) The switch.

Look at each one and decide which will be most difficult to source for, say, the sustained goal. Which will be hardest to change during the course of your development? For example, the battery and switch are easily changed and readily available. So, I wouldn't worry about either of them much right now. For the battery, you have essentially 3 options: alkaline, NiCd, and NiMH. Several of us have recommended the NiCd as having the highest current capability. You've argued against that. Just pick a battery chemistry and move forward. You can try others and compare the results.

If I were to look at the 5 elements, I would consider the magnet core as perhaps the most difficult item to source and change once incorporated into the design. So, I would start looking around for a suitable core. You know up front some of its general characteristics. It needs to be big, but not overly massive. Can you find a U-shaped core? Can you bend a 2.5cm dia. steel rod? Can you find a suitable cast iron core? Maybe you have an old microwave transformer, can disassemble the core, and convert it to a E-core? Or, maybe there is an electronics surplus store in your area and you can scrounge around to find something.

Those are all just suggestions. The message is to get organized, develop a "critical path" and get underway at solving the problem.

For the single lift, you can consider using capacitor discharge to increase your current by switching the cells to series. So, I would focus on the sustained carry first.

John

 

[neon]

mags

you allready have iron paper clipsif you wrap this same iron with wires you get a magnet. AAA battery can deliver a lot of amps if shorted for a very short period. experiment with that.

 

[mashersmasher]

use this

**broken link removed**

if the rules only state that the batteries must be AA then you should charge the alkaline aa's to 1.7v

you need to use some lateral thinking bud

 

[audioguru]

Alkaline batteries blow up or leak when their capacitance is charged. The "charge" lasts for a very short time even if it is not used.

 

[mashersmasher]

Alkaline batteries blow up or leak when their capacitance is charged. The "charge" lasts for a very short time even if it is not used.

that's been working great for me. i've been doing it with my robot's batteries for about a month since the ni cad batteries i was using became horribly unreliable. they don't last as long as when they come out of the factory but they last for a decent amount of time even after multiple charging. overcharging could be a bit (or allot) more damaging to the battery but if he only needs it once then it should last that long. the chemicals in alkaline batteries are practically harmless and easy to clean up too.

 

[jpanhalt]

... since the ni cad batteries i was using became horribly unreliable. they don't last as long as when they come out of the factory ...

Have you tried "sparking" them? You can search on that or PM me, if you wish to avoid getting too far OT.

John

 

[mashersmasher]

aww i wish i found that before i threw most of them out >.< thanks for that! i have a welder and i think i have them somewhere. i found a good instructable too

 

[jpanhalt]

Just use a capacitor. It self-limits the amount of high current. A welder may momentarily weld to the battery so you could end up with an uncontrolled amount of current. John

 

[audioguru]

I zapped some old Ni-Cad battery cells that were shorted. They didn't last long before they shorted again.

 

[Trisorion]

Thank you for the help jpanhalt. I live in Maryland, US but currently I am in Indonesia and I will be in Thailand next week. The competition is 5 months away, so I have some time. I was thinking of going to an iron art shop to get the core.

I want to try putting in some switches to do one electromagnet for maximum lifiting and parellel electromagnets for fast moving. I have a feeling you will still be right, but I have a few months, so I want to try a few prototypes.

I am way out of my leauge on calculating the size of the core as to avoid saturation. I found a good website for this but the concepts are too advanced for me. I will try to post it when I get back to my computer.

When I start building I will make a new thread with pictures, test results, and a summary of what has been discussed so far.

Some questions:

My concern is that a switch might add too much resistance. Do you think 5 switches would add an appreciable ammount? Is there a certain style of switch that is better?

Is there a type of solder that would be good for this application? Low resistance and able to withstand the heat.

I have some old PC heatsinks. Would It be helpfull to dremel them up and use thermaly conductive epoxy to secure them to the upper side of the electromagnet? Or would the wire still fuse at the same current because the lower side would have no help.

 

[rahulan999]

I dont like complex formulas for building a school-level battery operated electromagnet!. Just take 5-flat I-plates of a small audio transformer.Attach them all to make a core..wind a paper on it ititially...then carefully,and tightly,adjacently wind 150 turns of 30SWG insulated wire...concentrate fully when winding upto completion..over! You have got a powerful electromagnet without liquidnitrogen or complex electrical formulas. Just use 2 fully charged Ni-Cd AA batteries for operation.

 

[Trisorion]

I know what you mean Rahulan999, but I enjoy doing it this way. And it insures I dont wrap a core with 30 gauge wire for 2 hours, then plug in a battery and it all goes up in a puff of smoke because I did not understand what I was doing.

 

[neon]

I think if the core of the electromagnet is a "U" shape with the ends fairly close together then it will be a strong magnet. Maybe make the core like a speaker where the center pole is surrounded by the other pole so it has a lot of space to hold many paper clips and it has both poles of the magnet to be very strong.

what has the shape do it with it.Explain?

 

[neon]

a battery even a "D" type can supply a lot of power into a short. fortunately temperature does not follow the current it takes times to heat up things. buy that time the battery has no longer the original power. Conclusion it will heat up faster and cool off much slower