How Do You Design an Electromagnet to Attract a Disk from 15cm Away?

[sbotzer]

Hello,

I need to build an electromagnet in order to pull a disk magnet from a distance of a 15cm.

I don't know how many turns and how large to make the electromagnet. I need it to be in the shape of a solenoid. What are the formulas needed in order to do this?

Thanks in advance,
Shay

 

[Bob S]

Certainly, you need to have iron or a ferrite inside the solenoid. Is this an ac or dc electromagnet? If ac, you might consider punched transformer "E"s. You need iron outside as well as inside the solenoid to concentrate the field lines outside one end of the solenoid. If dc, then soft iron is the best choice. How is the disk magnet magnetized? What is its diameter? Is there any iron around the disk? This might make a difference on the choice of solenoid design or polarity. Also, If the disk is brittle, then it would have to be cushioned as it accelerates toward and strikes the solenoid.

 

[Carl Pugh]

15 cm is an awfully long distance for an electromagnet.
What are you trying to do?

 

[Danger]

Shay, you didn't mention anything about how heavy this disc magnet is, how much load it might have to move, the environment that it will be in (temperature, fluids, etc.), all of which will make a significant difference to how strong your solenoid must be. Carl is correct that 15cm is quite a reach.
Is there no other approach that you could take, such as using air pressure or a mechanical linkage?

 

[sbotzer]

Certainly, you need to have iron or a ferrite inside the solenoid. Is this an ac or dc electromagnet? If ac, you might consider punched transformer "E"s. You need iron outside as well as inside the solenoid to concentrate the field lines outside one end of the solenoid. If dc, then soft iron is the best choice. How is the disk magnet magnetized? What is its diameter? Is there any iron around the disk? This might make a difference on the choice of solenoid design or polarity. Also, If the disk is brittle, then it would have to be cushioned as it accelerates toward and strikes the solenoid.

Thanks Bob,

I'm going to put an iron core in it. The disk is magnetized on it's flat faces. There is no other metal near the system. I'm going to cushion it in order for it not to brake, it's a disk with a diameter of 50mm and a width of 5mm.

I still don't know how to calculate the electromagnet needed, do I use magnetic moments, or inductance and magnetic circuits?

Thanks again,
Shay

 

[sbotzer]

15 cm is an awfully long distance for an electromagnet.
What are you trying to do?

I'm trying to pull the disk from 15cm away

 

[sbotzer]

Shay, you didn't mention anything about how heavy this disc magnet is, how much load it might have to move, the environment that it will be in (temperature, fluids, etc.), all of which will make a significant difference to how strong your solenoid must be. Carl is correct that 15cm is quite a reach.
Is there no other approach that you could take, such as using air pressure or a mechanical linkage?

The disk magnet will weigh with the load 400grams. It will be in normal environment (air).
I can't use any mechanical approach, have to do it with electromagnets.

Thanks again,
Shay

 

[Danger]

I just had another thought here. It's probably completely inappropriate, but I'll mention it anyway.
Does the entire attraction mechanism have to be 15cm away, or just the coil? The reason that I ask is because it occurred to me that you could use a pull-in solenoid rather than a simple electromagnet. If that's possible, the core would be much closer to the disc until energized, then retract into the coil and pulled the field with it. Any chance of that helping?

 

[sbotzer]

I just had another thought here. It's probably completely inappropriate, but I'll mention it anyway.
Does the entire attraction mechanism have to be 15cm away, or just the coil? The reason that I ask is because it occurred to me that you could use a pull-in solenoid rather than a simple electromagnet. If that's possible, the core would be much closer to the disc until energized, then retract into the coil and pulled the field with it. Any chance of that helping?

I'm not sure i understand what you are proposing.

 

[Danger]

Neither am I, unfortunately.
I'll try better to explain what I'm talking about, but I really don't know whether or not it could work. I'm going to log out now, pull up one of my essentially useless drawing programmes, and see if I can post a picture. If my wife hadn't stashed my old G3's where I can't get at them, I'd fire it up in Illustrator in a couple of minutes. This might take a while.

 

[Danger]

"[URL=[PLAIN]http://img24.imageshack.us/i/000001q.png/[/URL]

Okay. This has got to be the most useless drawing that I've ever produced, but it's the best that I can do with what I have available.

 

[sbotzer]

"[URL=[PLAIN]http://img24.imageshack.us/i/000001q.png/[/URL]

Okay. This has got to be the most useless drawing that I've ever produced, but it's the best that I can do with what I have available.

Danger, you're great! thanks for the effort! I understand what u mean now. but still, there can't be any interaction between all the 15cm.

Thanks again,
Shay

 

[vk6kro]

Not useless at all. (not beautiful, though :) )
Depending on the application, the disk could even be glued to the armature and pulled in as shown probably against a spring's tension.

Again depending on the application, you could have a motor rotating a threaded rod with a nut on it that couldn't rotate. The rod could move along the threaded rod as the motor turns and pull the disk with it.

Wish we knew the application, though. What are you actually doing?

 

[Danger]

there can't be any interaction between all the 15cm.

Hmmm...
I'm afraid that I really don't know very much about electricity in general, and magnetism in particular. From what I've found so far, the only ways to increase the strength are to add more coils or crank up the juice.
I wonder if there might be some way to focus the field rather than simply increase it. Something along the line of winding the coil in a conical fashion rather than cylindrical, or using a tapered core? Maybe covering the sides and rear of the coil with some material that the field can't penetrate, so it has to go forward? I'm really out of my league here, and I'm sure that the experts are going to laugh themselves silly over this suggestion. On the other hand, I have no dignity to start with, and I try to never overlook any possibility regardless of how stupid it might be.

 

[vk6kro]

The idea is to have as much of the magnetic path Iron as possible.

So, you would start with a big U-shaped piece of soft Iron and wind a heap of turns of wire onto each arm of the U-shape.
After that, you would need a lot of luck to attract something 15 cm away.

Maybe you could get behind the disk and push it with the actuator Danger drew above. Do this by attaching something non magnetic (wood, plastic etc) to the Iron bit that goes into the coil.

I wonder how a bank of Neodymium magnets would go if they were mounted in a circle on a disk and this was brought up opposite the disk 15 cm away. Might be a problem getting them to let go if they did work.