Air Core Electro Magnet Construction

Changing wire size would be a pain. You need more than just the dimensions, one usually designs for a given impedance.. the power supply you are using to run it. You can get the same field with a few fat wires and lots of current, or smaller with higher voltage.

George H.

The power supply is two 'D' cell batteries for an initial voltage of 3.2. It has to work down to where the batteries are dead at 2 volts. How do I design the electro-magnet for maximum strength and battery life? I think the magenetic force is a function of only the supply voltage and volume of the copper, but there are many combinations of turns,resistance, and amps to get the same result. I could write a power basic program to show the results based on turns and resistance of each turn, but I thought there was an easier way.

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The magnetic field is a function of current and turns.

if you want it to work down to 2 volts, then calculate the mag field strength to opereate at 2 volts and use a current limiter.

Set the limiter to the field strength you need so basically when the two cells hit 2 volts total the regulator will be fully on. You must also calculate losses. What ever current you get at 2 volts is the current you want to maintain at any voltage above that.

Using a switcher supply for the current regulation would most likely be the most efficient way to go. You need to look for cell voltage switcher chips.

Not sure how you are doing your mag calculations? I suppose you could be using Weber(Wb)/(T)Tesla/Gauss etc ?

Trying to remember the single wire basic math.. ?

W = (I * u) / Pi * R

I = current; u = permittivity of space R = radius

Something like that. It's been a while.

Jamie

I'm pretty sure that changing the wire size would not get you the most field for the power input.

Google "solenoid formula" and see what pops up.

Most people end up using a finite element analysis program. FEMM is free and seems to be pretty good.

Why air core?

--
www.wescottdesign.com

Is this to have some "supermagnet" that you can turn on and off? Unless you need that ability, I'd just pull a magnet out of a hard drive, or a good speaker.

Popular Electronics in the early sixties, I'd guess 1963 or 64 but that is a guess, had what seemed to be a pretty good electro magnet on the cover, details inside. Since those are now available online, one could figure out the issue and download it.

Michael

[about an air-core electromagnet]

The internal resistance of the D cells is an ohm or so when they're at end-of-life, so you need to plan on 2A maximum (and copper resistance means more like 1A will be achievable).

There is no way to maximize strength that also maximizes battery life. You can at best choose a battery technology and working time (NiCd, NiMH, alkaline, and 'standard' D cells will all be different enough to matter). Then, you can increase or decrease the working volume (size of the coil) and solve for the best achievable field.

Adding iron gives a major improvement in field strength (factor of 100). Air core magnets aren't very power-efficient, usually only used for high field when the field desired is higher than the saturation limit for iron, or when the windings can be superconductors.

It's air core because it interacts with a permanent magnet. But look at it this way. Suppose you use a single turn of a certian volume and the resistance was 1 ohm and the voltage is 1 volt for total amp-turns of 1 which produces 1 unit of field and the power input is 1 watt. Now if we use

2 turns of a smaller wire of twice the resistance with the same total volume, the current is reduced 50% and the power input is also reduced 50% but we still get the same field (amp-turns) since amps is cut in half while turns has doubled. So, it appears using more turns of a smaller wire is an advantage in reducing power input without losing field strength for a given voltage. But that only works for a single layer since additional layers will have a larger circumference and more resistance per turn.

. . .

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** No, it is reduced to 25%.

Two turns of double resistance = 4 times resistance.

** Not correct.

The mag field is now half what it was, you have to double the voltage to get the same field. We have been down this path before.

.... Phil

Nope. If the volume of wire is the same but the length is doubled, then the area is halved. Doubled length doubles the resistance, halved area doubles the resistance _again_, and the resistance is quadrupled. The current is reduced to 25% and the power input stays the same.

This is why manufacturers that sell motors with lots of different windings on the same basic mechanical arrangement rate the mechanical arrangement with a "motor constant" that's in units of torque/watt.

It's the same basic issue with solenoids, only you're talking force/watt.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Ok, so using the same volume with a different wire size just changes the impedance and does nothing for efficiency. Suppose we use the same wire size and just add another turn?

In the first case we have 1 turn, 1 ohm, 1 volt, 1 amp, 1 watt, and 1 amp-turn If we add another turn of the same wire, we have 2 turns, 2 ohms,

1 volt, 1/2 amp, 1/2 watt, and the same field. That looks like the efficiency improved by 50% ? But this doubles the volume, so the question is how far can you increase the volume before it's not worth the effort?

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That's where some sort of already-worked solenoid equation, or finite element analysis, would come in handy.

Have you Googled "solenoid equation" yet?

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Yes, there seems to be a lot of ideas about solenoids. This one seems fairly simple. It looks like the solenoid length is important since the field diminishes as the length increases. So, the idea is to add more turns without increasing the length, which means the resistance of each layer is greater since the circumference increases rapidly for a short length. So, there must be some optimum design considering length, diameter, power input, and volume. u is just a constant and be omitted since I'm just looking for a maximum. I'll play around with a basic program to see what I get. I only have two wire sizes to work with #34 and #28, so I have to use some combination of the two..

material.

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The formula B = uNI/L is apparently correct. I simulated a coil using a constant length of #34 wire and rearranged the same wire on different core diameters and lengths. The field increases as the number of turns and length decreases. I wound an experimental coil using 1/4 length with all #34 wire and it produced about the same field with only 1/4 of the power of the old unit since the resistance was 4 times higher for the same result. Quite a difference.

Diameter Length Turns Field

1 1 8290 61 1.4 0 .5 6214 91 2 0.245 4531 136 2.5 0.155 3657 174

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And: "As ferromagnetic material causes noise or distortion in the signal, it should be avoided in high frequency application like signal transmission. Thus air core transformer is introduced, in the application of high frequency radio transmission." From:

S*