electromagnet fail

[electromateria]

can someone aware me why when I attach various voltages to a coil with a steel core I get no magnetic force? tried multiple voltages and coils.

included an image of the setup im building. want to get the test coil to work before I start winding around the bolt.

[Kleinstein]

The cores look a bit like ferrite. There are quite different ferrites and some are relativel hard and thus need quite some magentic field to react to it.
If there is current flowing, there will be magentic field. It is however possible that the field is too weak to easily observe the force.  With a core like show, that is far from a closed core, it takes quite some current to get a strong field.  Modern permanent magnets and even the already a bit older hard ferrite magnets are quite strom compared to small scale electromagnets. Electromagents also get a bit stronger when large.  A permanent magent may well stick to the core stronger than the magnetic force from the electromagnet is.

[radiolistener]

You're needs to see on current through the coil instead of Voltage... The magnetic flux depends on current and wire winding density

I think there are possible 3 reasons:
1) the coil resistance is too high, so battery Voltage is not enough to make enough current
2) the battery cannot handle high current, it's Voltage drops down and as result current is not enough
3) the coil is bifilar, it has layers with opposite winding directions, so each layer has opposite magnetic flux direction and they cancel each other

Try to use lab PSU and see on the current...

[electromateria]

Thanks for the help guys. I ended up getting the electromagnet working. It's just very weak even with 4 AA batteries.

Its able to attract a very small screw as long as its 1mm away, holding force isn't great. It can move a compass needle that's 3 inches away. The goal is to attract a large suspended nut that's 3/4 inch away.

From what I understand more turns of wire will increase the magnetic force. What if I do a similar amount of turns with wire that's 4x thicker?

[TimFox]

The same number of turns with thicker wire will result in lower wire resistance, which conducts more current, from a fixed voltage..
However, the series resistance of the battery is in series with the wire resistance and reduces that change.
With a given current (determined by the battery voltage, battery resistance, and wire resistance), more turns increases the magnetic force.
If the battery resistance dominates the total resistance, more turns will be a good choice.
Have you tried any quantitative calculations?

[--Oz--]

3) the coil is bifilar, it has layers with opposite winding directions, so each layer has opposite magnetic flux direction and they cancel each other

Bifiler is just 2 wires in parallel, does not mean its wired in a common mode choke , and that has 4 terminals (the ones I have seen).
Everything else you said I agree.

The first large coil looks for use on 110V directly, in other words, has high resistance (impedance for ac). Measure its dc resistance before applying a high voltage.

[electromateria]

The same number of turns with thicker wire will result in lower wire resistance, which conducts more current, from a fixed voltage..
However, the series resistance of the battery is in series with the wire resistance and reduces that change.
With a given current (determined by the battery voltage, battery resistance, and wire resistance), more turns increases the magnetic force.
If the battery resistance dominates the total resistance, more turns will be a good choice.

Thanks for that information, very helpful!

Have you tried any quantitative calculations?

I'm not much of a calculations type of guy (unless it's an absolute must) I try to avoid any kind of math like it's the plague. I really respect the power of math and I'm glad some people are so good at it, but my mind resists learning it.

I know electronics is a field where math can't be avoided, but I'm still doing my best lol.

-

Ended up getting it working check it out:



I'm shocked how much wire was on that bobbin. Good lord I spent like 20-30 minutes with a drill transferring it over. Of course at the end I superglue the final strand, let it go too early and 100 wraps unwind making it all loose 

Is there a big difference between winding the wire on perfectly uniform and just quickly spooling it on? Assuming both are tightly wound.

[electromateria]

The first large coil looks for use on 110V directly, in other words, has high resistance (impedance for ac). Measure its dc resistance before applying a high voltage.

Yea it's from a 120v "monopole motor" I think. Microwave fan motor.

I just tested a coil that's similar size and it measured 50 ohms.

 

[MrAl]

can someone aware me why when I attach various voltages to a coil with a steel core I get no magnetic force? tried multiple voltages and coils.

included an image of the setup im building. want to get the test coil to work before I start winding around the bolt.

I already replied to this thread but it seems to have disappeared so i'll just list the things that help make the attractions stronger.

1.  Use magnetically active metal that is made for this.  That's materials like transformer metal silicon steel.
A steel bolt or nail has permeability of maybe 100 at best, while transformer laminations have permeability of 1000 or better.  The metal acts almost like an amplifier for the field strength so the best metal works best. 
2.  Distance is very important. The closer you get the pole faces to the object the better.  The field strength falls off sharply with distance so close is much better.
3.  Complete the magnetic path as well as you can.  If you can get a core in a shape like a "C" and have the nut in the center opening you get more strength.  That means you will get both a push and a pull.  You can also instead place a second pole piece on the opposite side and energize it so that it is the opposite pole of the original.
4.  If you can increase the number of batteries in series that will increase the current, but the wire must be able to put up with the extra current without getting too hot.  The higher the current, the stronger the pull.

Just by changing the type of metal used would make a big difference.  This is like using a smaller value resistor in a circuit to make the current increase.  The better metal means less resistance in the magnetic path so stronger pull.

[electromateria]

If you can get a core in a shape like a "C" and have the nut in the center opening you get more strength.  That means you will get both a push and a pull. 

Thanks for all the info, much appreciated. I don't really understand this part. If the bolt is centered in between the N / S wouldn't it cancel out because of similar push / pull forces?

[B J]

Good catch, electromateria.                 If you had a bar magnet, with the N $ S fields out the ends, not the sides like a lot of them are today,  then you could do it by having one end in the electromagnet  field, and the other end sticking out away from the field area.  Lets say you have the N end in the field.  The electro magnet is such that when a current is flowing, the top is N  and bottom is S.     Now with the bar magnet in the center between the two end poles of the  "C"  shaped magnet, with no current, the bar will be at rest.  As the current is increased, the top gets stronger N and the bottom stronger S.  The two N poles will repel, and the N & S poles  will attract.  Now you will have a downward  force.      ------------       Also remember that the field strength is the number of turns,  times the current.      If you double the turns, with the same current, the field strength is double.         If have the same number of turns, and half the current, yep 1/2 the force.     Don't think about applied voltage, rather the current.

[radiolistener]

From what I understand more turns of wire will increase the magnetic force. What if I do a similar amount of turns with wire that's 4x thicker?

Not exactly. The magnetic force depends on winding density and current.

If you want to increase magnetic force you can decrease distance between adjacent turns and use the same turns count. If you adding more turns, your wire will be more longest and it's resistance will be higher, so if you apply the same Voltage, it leads to lower current.

[MrAl]

Thanks for all the info, much appreciated. I don't really understand this part. If the bolt is centered in between the N / S wouldn't it cancel out because of similar push / pull forces?

I think you maybe right there, i was thinking in terms of a mechanism that had one of the poles connected to the moving piece such as with a metal hinge, where when the coil was energized the moving piece would hinge toward the open pole end.
That would probably mean that the 'nut' would have to be connected to one pole with a hinging mechanism.
It may work with one pole closer to the nut than the other though you could try that.

The voltage is important though because the voltage applied to the coil produces the current, and the current produces the magnetic field.  So the higher the voltage the higher the current and thus the stronger the field.
Of course the more turns the stronger the field too up to the point where the current starts to decrease and then you have to apply more voltage to overcome the larger resistance.

There is an interesting formula about the resistance of the coil vs the resistance of the battery but as i was saying it's hard to get the coil to match the battery very well so you end up just doing whatever is most practical.

What may be more beneficial is to have both a north and south pole on one side, separated by some distance.  The moving piece will then try to complete the magnetic path by moving toward both pole faces.
In a typical electromagnet 'pick up' application the idea it so use a C shape where the two ends of the C shape touch the piece to be picked up.  When the piece touches the two ends of the C shape, it completes the magnetic path and that is the strongest position possible so it picks up the most weight that way.
You would have to experiment with the spacing between poles vs the size of the nut.
A real electromagnet for this would have the coil in the middle of a circular shape with a 'cup' shape over the electromagnet so that the cup open end contacts the piece to be picked up as well as the inside coil pole.  When the piece touches both the inside coil face and the outside open cup edges it sticks with the most strength.

I could draw up a quick picture of these ideas if you think that would help.
These ideas are all related to the idea of completing the magnetic path as well as possible.

The idea of using the right metal however has a profound effect on the strength.  You can actually measure the difference using a linear hall effect sensor.
The idea of completing the magnetic path also has a very profound effect on the strength when the piece can touch both pole pieces, but it could also help for an application where it never actually comes into contact with the two pole faces.

There is a 'best' coil cross section shape too, but if you use good transformer metal it matters less because most of the field is inside the core material so it is almost like all the turns are concentrated at the very front of the electromagnet.

[electromateria]

Thanks for all the info, much appreciated. I don't really understand this part. If the bolt is centered in between the N / S wouldn't it cancel out because of similar push / pull forces?

I think you maybe right there, i was thinking in terms of a mechanism that had one of the poles connected to the moving piece such as with a metal hinge, where when the coil was energized the moving piece would hinge toward the open pole end.
That would probably mean that the 'nut' would have to be connected to one pole with a hinging mechanism.
It may work with one pole closer to the nut than the other though you could try that.

The voltage is important though because the voltage applied to the coil produces the current, and the current produces the magnetic field.  So the higher the voltage the higher the current and thus the stronger the field.
Of course the more turns the stronger the field too up to the point where the current starts to decrease and then you have to apply more voltage to overcome the larger resistance.

There is an interesting formula about the resistance of the coil vs the resistance of the battery but as i was saying it's hard to get the coil to match the battery very well so you end up just doing whatever is most practical.

What may be more beneficial is to have both a north and south pole on one side, separated by some distance.  The moving piece will then try to complete the magnetic path by moving toward both pole faces.
In a typical electromagnet 'pick up' application the idea it so use a C shape where the two ends of the C shape touch the piece to be picked up.  When the piece touches the two ends of the C shape, it completes the magnetic path and that is the strongest position possible so it picks up the most weight that way.
You would have to experiment with the spacing between poles vs the size of the nut.
A real electromagnet for this would have the coil in the middle of a circular shape with a 'cup' shape over the electromagnet so that the cup open end contacts the piece to be picked up as well as the inside coil pole.  When the piece touches both the inside coil face and the outside open cup edges it sticks with the most strength.

I could draw up a quick picture of these ideas if you think that would help.
These ideas are all related to the idea of completing the magnetic path as well as possible.

The idea of using the right metal however has a profound effect on the strength.  You can actually measure the difference using a linear hall effect sensor.
The idea of completing the magnetic path also has a very profound effect on the strength when the piece can touch both pole pieces, but it could also help for an application where it never actually comes into contact with the two pole faces.

There is a 'best' coil cross section shape too, but if you use good transformer metal it matters less because most of the field is inside the core material so it is almost like all the turns are concentrated at the very front of the electromagnet.

Sounds like some interesting ideas, but I don't fully understand! Pics are always good 

I thought of this while reading your comment (attached). a square shaped core with a notch cut out. The nut goes in the notch area so it gets simultaneously pushed and pulled at the same time. Where N and S are marked is bare core in those spots. Don't mind my crappy PS skills just imagine its wound properly. Can the magnetic field work that way? I don't have any understanding of how the fields interact or conflict with each other.

I have 10-15 microwave transformers sitting around, eventually I'll try building some powerful electromagnets with them. I'm curious about chopping and welding that layered steel, or putting a square piece of it into a metal lathe and turning it into a rod then bending it etc.

You can actually measure the difference using a linear hall effect sensor.

That's very interesting. Would you hook the sensor to an aduino or something like that?

[electromateria]

Not exactly. The magnetic force depends on winding density and current.

If you want to increase magnetic force you can decrease distance between adjacent turns and use the same turns count. If you adding more turns, your wire will be more longest and it's resistance will be higher, so if you apply the same Voltage, it leads to lower current.

Sounds like ohms law could help?

If so, would you mind sharing an example how I could use it to try to find the ideal power setup for an electromagnet? Assuming I only know resistance of the coil (50 ohms).

Good catch, electromateria.                 If you had a bar magnet, with the N $ S fields out the ends, not the sides like a lot of them are today,  then you could do it by having one end in the electromagnet  field, and the other end sticking out away from the field area.  Lets say you have the N end in the field.  The electro magnet is such that when a current is flowing, the top is N  and bottom is S.     Now with the bar magnet in the center between the two end poles of the  "C"  shaped magnet, with no current, the bar will be at rest.  As the current is increased, the top gets stronger N and the bottom stronger S.  The two N poles will repel, and the N & S poles  will attract.  Now you will have a downward  force.      ------------       Also remember that the field strength is the number of turns,  times the current.      If you double the turns, with the same current, the field strength is double.         If have the same number of turns, and half the current, yep 1/2 the force.     Don't think about applied voltage, rather the current.

The wire I used is super thin, 1/15th of a mm thick. There's probably a thousand + turns. How do I know what kind of voltage and current it can handle without doing destructive testing?

 

[WattsThat]

What exactly are you trying to achieve? Is the goal to build an electromagnet that will pick up a known object from a certain distance? Or, are you just exploring? Some info there will help get you better explanations rather than just bits and pieces of information.

The applied current, the number of turns and the core material determines the force created. Ohms law will tell you how many volts must be applied to known resistance to achieve a given current. Google “electromagnet design calculator” for more info you’ll ever want or need on the topic.

Given your 50 ohm coil example, if you need 2 amps to flow, you need to apply 100 volts. That’s straight from Ohms law that says:

E (voltage) = I (current) * R (resistance)

To design an electromagnet, you typically start with the force needed, the voltage available and work from there.

[MrAl]

Thanks for all the info, much appreciated. I don't really understand this part. If the bolt is centered in between the N / S wouldn't it cancel out because of similar push / pull forces?

I think you maybe right there, i was thinking in terms of a mechanism that had one of the poles connected to the moving piece such as with a metal hinge, where when the coil was energized the moving piece would hinge toward the open pole end.
That would probably mean that the 'nut' would have to be connected to one pole with a hinging mechanism.
It may work with one pole closer to the nut than the other though you could try that.

The voltage is important though because the voltage applied to the coil produces the current, and the current produces the magnetic field.  So the higher the voltage the higher the current and thus the stronger the field.
Of course the more turns the stronger the field too up to the point where the current starts to decrease and then you have to apply more voltage to overcome the larger resistance.

There is an interesting formula about the resistance of the coil vs the resistance of the battery but as i was saying it's hard to get the coil to match the battery very well so you end up just doing whatever is most practical.

What may be more beneficial is to have both a north and south pole on one side, separated by some distance.  The moving piece will then try to complete the magnetic path by moving toward both pole faces.
In a typical electromagnet 'pick up' application the idea it so use a C shape where the two ends of the C shape touch the piece to be picked up.  When the piece touches the two ends of the C shape, it completes the magnetic path and that is the strongest position possible so it picks up the most weight that way.
You would have to experiment with the spacing between poles vs the size of the nut.
A real electromagnet for this would have the coil in the middle of a circular shape with a 'cup' shape over the electromagnet so that the cup open end contacts the piece to be picked up as well as the inside coil pole.  When the piece touches both the inside coil face and the outside open cup edges it sticks with the most strength.

I could draw up a quick picture of these ideas if you think that would help.
These ideas are all related to the idea of completing the magnetic path as well as possible.

The idea of using the right metal however has a profound effect on the strength.  You can actually measure the difference using a linear hall effect sensor.
The idea of completing the magnetic path also has a very profound effect on the strength when the piece can touch both pole pieces, but it could also help for an application where it never actually comes into contact with the two pole faces.

There is a 'best' coil cross section shape too, but if you use good transformer metal it matters less because most of the field is inside the core material so it is almost like all the turns are concentrated at the very front of the electromagnet.

Sounds like some interesting ideas, but I don't fully understand! Pics are always good 

I thought of this while reading your comment (attached). a square shaped core with a notch cut out. The nut goes in the notch area so it gets simultaneously pushed and pulled at the same time. Where N and S are marked is bare core in those spots. Don't mind my crappy PS skills just imagine its wound properly. Can the magnetic field work that way? I don't have any understanding of how the fields interact or conflict with each other.

I have 10-15 microwave transformers sitting around, eventually I'll try building some powerful electromagnets with them. I'm curious about chopping and welding that layered steel, or putting a square piece of it into a metal lathe and turning it into a rod then bending it etc.

You can actually measure the difference using a linear hall effect sensor.

That's very interesting. Would you hook the sensor to an aduino or something like that?

Yes you could use an Aruduino or just a power source like 5v and a digital meter to measure the output of the hall device.

In the drawing, Fig 1 shows two halves of a "C" core magnetic core.  The two halves are butt together and the coil is the orange rectangle.  When the coil is energized the two halves stick together with a lot of force because the metal is the right type and the two faces are very very close together, actually touching.  That's the strongest.  When the two halves are separated a little it's weaker.

Fig 2 shows just one half of the C core and an iron or steel plate.  The plate is very close to the two pole faces so it is held very tight.  Moving it farther away decreases the force.

Fig 3 shows one half of the C core and a hex nut some distance from the two pole faces while Fig 4 shows the same thing but the nut is closer.  There is some force on the nut in both figures because the nut is in the magnetic path but in Fig 4 there is more pull than in Fig 3 because the nut is closer so the path distance is shorter.
You can see indication of the flux and if you follow it around it forms a closed loop just like current in a wire.  That is more or less how we get the magnetic/electric analogy between flux and current.
As the nut moves closer to the C core pole faces, the magnetic resistance (often called reluctance) decreases so the pull strength goes up.
If the nut were to touch the two pole faces it would be held tight.

Now the subject of wire diameter comes up because the wire diameter and applied voltage creates the current flow and the current determines the strength of the electromagnet in that the more current the more strength.
If you use many many turns of very fine wire then you need a lot of voltage to push enough current through the wire, but too much current and the wire burns up.
You could test a piece of wire separately to see how hot it gets but when it is in a coil it gets hotter on the inside because it's harder for the heat to get out from the inside.  The best way would be to slowly increase the voltage and thus the current and wait for the temperature to reach near equilibrium and see what the temperature is on the turns on the outside of the coil.  Maybe turn up the current little by little every 10 minutes or something, then measure the outside temperature.  You could check the temperature rating of the wire with the specs from the manufacturer.  Wire temperature ratings vary widely so you may have a really good quality wire or not so good.  The best wire can handle some pretty high temperatures.

Here is the diagram...

[EPAIII]

I did not read every response so this may have been said already.

Without going into too much of that dreaded math, the basic equation for the magnetic field strength generated by a coil is current multiplied by the number of turns. By using a very small wire gauge, there is a lot more resistance in the wire for a given number of turns so with a given Voltage you get less current (Ohm's law). You can find tables that show the resistance for different gauges of copper wire. This is usually given in Ohms per 1000 feet.

Most high power magnets try to have a balance between the wire size and the overall size and weight of the coil. A larger wire size may mean that fewer turns will fit if the volume is limited.

You may get better results with a larger wire gauge (smaller gauge number). Again that pesky math comes into play as if you want the same length of wire more of the larger gauge wire will be required because every layer is thicker.

And you asked about messy windings vs. neat ones. Messy windings will also take up more space and therefore require longer lengths of wire for the same number of turns. That is why most coils that are simply for producing a magnetic field are wound in a specific pattern, usually with the coils in each layer up against each other. Tight packing gives a better design. And it costs less too.

Another thing to consider is all core materials have a saturation point. Trying to induce a greater field after that point has been reached will be very difficult. To paraphrase the guy in Jaws, you gonna need a bigger core.