Strongest Electromagnet using 2 AA bateries

[Willbe]

So you derived this with:

mmf= maximum magnetic force
I= current
V= voltage
R= resistance
Rb= internal resistance of batteries
Rw= resistance of wire
T= number of coil turns
D= diameter of wire
K= resistance per inch
Ct= circumference of a turn

Where Vb is the battery voltage = 3v, t is the number of coil turns, Ri is the internal resistance of two cells in series, D is the coil diameter in inches and k is the ohms/inch of the wire

mmf= I*T
I=V/R
mmf= (V/R)*T
mmf= V*T/R
R= Rb + Rw
mmf= V*T/(Rb + Rw)
Rb= Rb1 + Rb2 (since they are in series, right?)
mmf= V*T/(Rb1 + Rb2 + Rw)
Rw= K*Ct*T
mmf= V*T/(Rb1 + Rb2 + (K*Ct*T)

Magnetomotive, not maximum. You plug in values to optimize the equation to the maximum mmf.

I approached it slightly differently, but. . .

if you get the same answer as I did [to several decimal places] when you put your formula into a spreadsheet, then your formula matches mine and hopefully they both match the reality of making coils.
The more decimal places that match, the more likely you didn't get the same answer as me by chance. Three places should be plenty.

Spreadsheets are great for checking calculations and formulas that run to a page or more. You just plug in some numbers you made up and see if both formulas give the same answer. The spreadsheet doesn't make typos, isn't confused by all the scribbling, doesn't care if you ask it to do the same calc. over and over, etc.

Just don't overconstrain a complex formula; e.g., for E=IR you can specify any two of the variables, but not all three.
Sometimes the variables in a complex formula are connected to each other in non-obvious ways, and so you will get nonsense answers if you accidently tell it that 6v = 3a x 7Ω, or if you tell it that the circumference of a circle is equal to 4x the diameter.

Also, for formula troubleshooting purposes, I recommend the final calculation be no more complex than A*B/C, with A, B and C standing for more complex expressons.
This, because in spreadsheets, it's hard to keep all the parentheses straight. If you break down a complex formula into each term [like R] or each two terms [like I*R] then you don't need any parens within your spreadsheet intermediate calculations.

 

[Hero999]

#30 AWG wire in air fuses at about 10A, but it will carry more current if the heat is spread out through the volume of the windings.

That's not true, it will conduct less current before fusing if tightly wound in a coil because there's no where for the heat generated to escape to.

Also the insulation will fail long before the wire will causing all the turns to short together.

 

[Boncuk]

Electromagnet

Hi All,

aren't you missing one point of primary importance with all the highly sophisticated formulas?

The paper clips have to transported from one bin to another!

The strongest e-magnet will blow the job if the clips fall down halfway between the bins because the batteries are exhausted.

Not only strength, but also endurance are the criteria to construct and build the magnet.

Sounds almost like planning to build an aircraft with a thrust to weight ratio of 100:1 and an operating radius of zero! Absolutely no practical use for that.

My approach to this: (number of turns disregarded for the first experiment, 200 windings of 0.4mm enamelled coppper wire wound accurately to make the bobbin should suffice for the beginning), but iron area (soft iron) as big as possible U or E shaped to have the paper clips close the magnetic flux circuit. Over the target bin reverse polarity for a split of a second and the clip drop where they should.

Since the conditions for this "contest" are given very vague you could as well have the magnet pick up the clips, dump them in a small container - and doing this repeatedly you can transport all available paper clips and empty the container overhead the target bin. A container is not an apparatus!

Boncuk

 

[Hero999]

A soft core would do it, as it would only consume current when it's becoming magnetised or demagnetised.

 

[Willbe]

"That's not true, it will conduct less current before fusing if tightly wound in a coil because there's no where for the heat generated to escape to."

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.

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

. . .I think. . .

 

[jpanhalt]

As I recall, the older Mirage almost met the criterion of zero operating radius. They were great for Israel.

The operating time was given as 20 seconds, 0.0056 hour. That means that a 600 mAH battery should in theory be able to provide 100 A for 20 seconds. Of course, that will not be attained in practice, but expecting something like 30 to 40C for a short time (maybe destroying the battery) is possible. Thus, a 600 mAH should be able to give 18 to 24 A for 20 seconds. Anyone who has seen a NiCd catch fire from a short knows that the current can be truly surprising.

John

Edit: Of course, the internal resistance for the alkaline battery will limit the current to something less than that.

 

[Boncuk]

As I recall, the older Mirage almost met the criterion of zero operating radius. They were great for Israel.

That doesn't only apply to the Mirage. The CAS (close air support) FB type Fiat G91 had a weapon load capacity of zero and an operating radius of zero.

It was replaced by the Alpha-Jet with double capacity in the late 1990s.

That thing had a dynamic air resistance factor of 0.1 (clean). Adding external wing tanks and a fuselage tank that factor increased to 0.7!. With or without external tanks the (no load) range didn't change by one mile, but helped to increase the amount in the fuel bill!

Hans

 

[Boncuk]

As I recall, the older Mirage almost met the criterion of zero operating radius. They were great for Israel.

That doesn't only apply to the Mirage. The CAS (close air support) FB type Fiat G91 had a weapon load capacity of zero and an operating radius of zero.

It was replaced by the Alpha-Jet with double capacity in the late 1990s.

That thing had a dynamic air resistance factor of 0.1 (clean). Adding external wing tanks and a fuselage tank that factor increased to 0.7!. With or without external tanks the (no payload) range didn't change by one mile, but helped to increase the amount in the fuel bill!

Hans

 

[Boncuk]

"That's not true, it will conduct less current before fusing if tightly wound in a coil because there's no where for the heat generated to escape to."

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.

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

. . .I think. . .

No problem! If the copper of a coil starts melting there are distinct signals prior to the melting process, such as the well known indian smoke signals combined with the odour of burning enamel.

This coil has to serve for a short period only and so the job will be done long before that could possibly happen.

Even three layers of tightly wound copper wire have enough surface to radiate heat to the ambient air - at least for a limited time. An additional safety precaution could be reducing the current after initial magnetization.

That way the OP could not only build the strongest, but also the longest lasting electro-magnet.

I'm just afraid the batteries won't supply an additional water cooling circuit.

And one more question: How do you normally wind coils other than cylindrical?

 

[audioguru]

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.

 

[Willbe]

And one more question: How do you normally wind coils other than cylindrical?

Depending on my blood alcohol level, scramble-wound.

 

[Trisorion]

I will be out of the country for a few weeks, but I will try to pick up and test some batteries.

Should I add heat sinks to the electromagnet? If I made them out of iron perhaps they would add more surface area for clips. Or I could make them out of copper and put them on top (and out of the way). Would copper heatsinks interfere with the performance?

Should I really expect to be getting 20 amps out of NiCd battery. According to this chart https://www.powerstream.com/Wire_Size.htm I would need about 16 gauge wire.


I am also worried that the magnetized soft iron core (ferromagnetic) would continue to hold steel (ferromagnetic) paper clips when I tried to drop them. Could I reverse the flow of electricity for 1/4 second to make the paper clips drop?

On another note, what does the magnetic field of a ring magnet look like? I was thinking of arranging two concentric ring electromagnets.


Edit: I have searched around a bit for NiCd AA batteries and I have noticed something. Duracell and Energizer do not make them. Duracell and energizer do make NI-MH batteries. Are they equal?

 

[Willbe]

Could I reverse the flow of electricity for 1/4 second to make the paper clips drop?

Maybe add another coil that bucks the first one?

 

[jpanhalt]

Sanyo makes NiCds. See:

https://www.batteriesamerica.com/newpage8.htm#Rechargeable Battery
https://www.cheapbatterypacks.com/?sid=772203&pgid=loosecells&chem=NICD

NiMH are great batteries, but will not give the brute current that a NiCd will give.

John

Where are you located? The major internet hobby shops also carry them.

 

[Trisorion]

Rechargeable Nickel Metal Hydride Cylindrical Cell
https://www.electro-tech-online.com/custompdfs/2008/08/240AAHPDF.pdf
Max recommended discharge current: 7.2 amp
Internal Resistance: 20 miliohm

Rechargeable Nickel Cadmium Cylindrical Cell
https://www.electro-tech-online.com/custompdfs/2008/08/100aas.pdf
Max recommended discharge current: 3 amp
Internal Resistance: 18 miliohm


Ni-MH>Ni-Cd?


Location: Maryland, US

 

[Boncuk]

Rechargeable Nickel Metal Hydride Cylindrical Cell
https://www.electro-tech-online.com/custompdfs/2008/08/240AAHPDF-1.pdf
Max recommended discharge current: 7.2 amp
Internal Resistance: 20 miliohm

This battery has a capacity of 2,400mA/h

Rechargeable Nickel Cadmium Cylindrical Cell
https://www.electro-tech-online.com/custompdfs/2008/08/100aas-1.pdf
Max recommended discharge current: 3 amp
Internal Resistance: 18 miliohm

This battery has a capacity of 1,000mA/h

Ni-MH>Ni-Cd?

Comparing a battery with 2.4 times the capacity of another you must come up with a result like that! (assuming an NI-MH battery can deliver 2.4 the current of a NiCad.)

Using the factor 2.4 to calculate for NiCad you'll end up with the same result: 7.2A. (3.0 * 2.4)

Considering the internal resistance of 20 milliΩ of the NI-MH versus the internal resistance of 18 milliΩ of the NI-Cad guess which one of both can supply higher current.

It's not a disadvantage to be able to read datasheets correctly.

Boncuk

 

[jpanhalt]

The NiCd may have a lower "recommended" current. That surely takes into account longevity and heat. Thus, if you were going to build an industrial paper-clip electromagnet for moving paper clips, 8 hours a day, 7 days a week, then follow the advice.

This is a competition. The goal is to win. If the battery bursts into flames afterward (just kidding), it will add to the excitement of winning.

Thus, to reiterate, I would use a parallel connection, very low resistance coil, and 2 sets of batteries -- one for testing and one for competition.

If you have any doubts about the current delivery potential of a NiCd, short a freshly charged one with a paper clip and compare that to a NiMH of the same physical size. The NiMH will have higher amp-hour capacity, last longer, but will probably not get the clip as hot. NiCds come in different types. I am not certain of the type that gives the most peak current. The descriptions should help.

John

 

[Boncuk]

Hi John,

the problem was the comparison of a high capacity NI-MH (2,400mA/h) to a medium capacity Ni-Cad (1,000mA/h).

For the NI-MH the recommended max discharge current was given 7.2A, for the much lower Ni-Cad it was given with 3.0A.

This is like comparing a Ferrari to a VW buggy.

Taking the factor 2.4 (2,400/1,000) into consideration a comparable Ni-Cad (2,400mA/h) would also have a recommended max discharge current of 7.2A. (3A of the 1,000mA/h to 7.2 for the 2,400mA/h.

Looking at the internal resistance of both batteries the Ni-Cad wins with 18mΩ against the NI-MH with 20mΩ.

Hans

 

[jpanhalt]

Hans,
I agree with your comment. What I was suggesting is that in a competition, one will get out of the realm of usual practices, which are based on longevity and re-use.

I couldn't find the specific statement about NiMH's internal resistance increasing with discharge rate. All of the common sources (e.g, Energizer, Panasonic) stop the comparison at discharge rates of about 3C.

I did find in Wikipedia a statement to support a maximum discharge rate for NiCds of 20C:

The maximum discharge rate for a NiCd battery varies by size. For a common AA-size cell, the maximum discharge rate is approximately 18 amps.

That statement is consistent with my experience using NiCads for motor power and NiMH for electronics in model aircraft. Two AA's in parallel should give 36A, for a short period.

John

 

[mneary]

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