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Inductance

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Dave Goldsmith

New Member
Hi, I'm wondering if you make two identical coils, with the only difference being that one is made of copper wire and the other is made of iron, if the inductance of the iron coil will be higher? I can't seem to find any info about this online. I imagine that because an iron core increases the inductance, that the coil being made of iron wire would act like a small iron core that would bump the inductance a bit. I don't have any access to iron wire though, and would need a copper wire of the same dimensions as the iron wire even if I did have iron wire. Thanks for any info
 

MikeMl

Well-Known Member
Most Helpful Member
My guess is that the inductance would be the same as Cu coil, but would have higher losses (lower Q)
 

crutschow

Well-Known Member
Most Helpful Member
Since the iron wire should reduce the magnetic reluctance about the same amount as a similar amount of solid iron, I would expect the inductance to be some higher if it's wound on a magnetic core and significantly more if it's an air core coil.
 

Ratchit

Well-Known Member
Hi, I'm wondering if you make two identical coils, with the only difference being that one is made of copper wire and the other is made of iron, if the inductance of the iron coil will be higher? I can't seem to find any info about this online. I imagine that because an iron core increases the inductance, that the coil being made of iron wire would act like a small iron core that would bump the inductance a bit. I don't have any access to iron wire though, and would need a copper wire of the same dimensions as the iron wire even if I did have iron wire. Thanks for any info

Very confusing question. First you talk about a coil wound with two different metal wires, and then assume that the iron windings are akin to a iron core. The core of a coil does not have a current like the wires do, right? The inductance of a coil is dependent on the geometry, which includes the number of turns, and the permeability.

Ratch
 

Flyback

Well-Known Member
yes its the core material and its geometry that determines inductance, the wire material doesn't make a difference.
 

gary350

Well-Known Member
Do you have a meter that reads inductance? Experement with several materials to see what you get. I am trying to make a 50uH coil using 1 toroid but can not wind enough wire on it to get above 20uh but 2 toroids together with the same number of wire turns increased inductance 3 times to 60uH. Doubling the toroids trippled the inductance. Today I am experementing with Litz wire to see what that does, it is suppose to increase Q by 30% to 40% I wonder what that does to inductance. You can learn a lot by experementing. Buy a good meter.

**broken link removed**
$_12.JPG
 

crutschow

Well-Known Member
Most Helpful Member
yes its the core material and its geometry that determines inductance, the wire material doesn't make a difference.
So you are saying that magnetic wire will have no effect on the magnetic field produced by that wire. I find that hard to believe. :confused:
 

Ratchit

Well-Known Member
So you are saying that magnetic wire will have no effect on the magnetic field produced by that wire. I find that hard to believe. :confused:

Correct, the coil inductance is determined by the coil geometry, number of turns, and the permeability (core material). If you can show that the current carrying wire can change the permeability, then the inductance will change. I have never seen that taken into consideration.

Ratch

Inductance.JPG
 

steveB

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I'm pretty sure the iron wire will change the inductance, but I don't think deriving the formulas is easy.

I think fringe fields will be modified.

I think it is likely that the formulas for toroidal coils will hold accurately and give about the same inductance. The toroid structure minimizes fringe fields.

I'm answering based on intuition, so I could be wrong. Measurements would answer this one quickly.
 

crutschow

Well-Known Member
Most Helpful Member
Correct, the coil inductance is determined by the coil geometry, number of turns, and the permeability (core material). If you can show that the current carrying wire can change the permeability, then the inductance will change. I have never seen that taken into consideration.............
View attachment 90483
It's not normally considered because no one normally uses magnetic wire to wind a coil.

Even if the effect on a coil with a magnetic core might be small, I would certainly think that it would have a noticeable effect on an air core inductor.

Here's a though experiment. Suppose the conductor was a single loop magnetic core. Make a copper core of the same dimensions. I don't believe the inductance of the two would be the same.

But as steveB says, the easiest way to settle that would be to wind two identical coils, one with copper, and one with iron wire, and measure their inductance.
 

Ratchit

Well-Known Member
It's not normally considered because no one normally uses magnetic wire to wind a coil.

Even if the effect on a coil with a magnetic core might be small, I would certainly think that it would have a noticeable effect on an air core inductor.

Here's a though experiment. Suppose the conductor was a single loop magnetic core. Make a copper core of the same dimensions. I don't believe the inductance of the two would be the same.

But as steveB says, the easiest way to settle that would be to wind two identical coils, one with copper, and one with iron wire, and measure their inductance.

If it made any difference, I would think they would use ferromagnetic wire to wind motor coils or other high inductance coils. The motor application is where they want the maximum magnetic field possible.

Ratch
 

steveB

Well-Known Member
Most Helpful Member
Ratch,

I dont' think that's a valid argument. First, the difference (if there is one) would be nowhere near that of using a ferromagnetic core. Second, why add all that wire resistance? Iron is not that good of a conductor.

The difference may not be that much, but it still may be significant (more than +/- 10 %).

Generally, iron wires are not a good idea.

The main reason why I think there might be a difference is that fringe fields (on an air core inductor) are somewhat sensitive to small changes, and differences in fringe fields can modify inductance. The more the inductor mitigates the effect of fringe fields (i.e. toroids and long/narrow solenoids, or use of high permeability cores), the less significant this effect would be.
 
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Ratchit

Well-Known Member
Ratch,

I dont' think that's a valid argument. First, the difference (if there is one) would be no where that of using a ferromagnetic core. Second, why add all that wire resistance? Iron is not that good of a conductor.

The difference may not be that much, but it still may be significant (more than 10 %).

Generally, iron wires are not a good idea.

The main reason why I think there might be a difference is that fringe fields are somewhat sensitive to small changes, and differences in fringe fields can modify inductance. The more the inductor mitigates the effect of fringe fields (i.e. toroids and long/narrow solenoids), the less significant this effect would be.

I said that if it made any difference, motor manufactures would use iron wires. They don't, so I assume it doesn't help any. I believe the fringe fields would also be present in copper. Iron wires much be susceptible to hysteresis and eddy current losses.

Ratch
 

steveB

Well-Known Member
Most Helpful Member
I said that if it made any difference, motor manufactures would use iron wires. They don't, so I assume it doesn't help any. I believe the fringe fields would also be present in copper. Iron wires much be susceptible to hysteresis and eddy current losses.

Ratch
The question is if the inductance would be higher, not whether it would help any. You said that if it made a difference, manufacturers would use it. That's nonsense. It's not practical to use iron wires, and any small difference in inductance would not outweigh the large drawbacks of doing so. So your argument holds no water.

Your original argument is a good one.

For the fringe fields, yes of course they are there for the copper too. My point is that the field distribution in the fringe field area is sensitive to changes (geometry, permeability etc) for air core inductors. The different fringe field would change the inductance. The only question is how much? Is it significant? I'm not sure, but perhaps it is for some inductor designs, and probably would not be for toroids and long/narrow solenoids.
 

Ratchit

Well-Known Member
The question is if the inductance would be higher, not whether it would help any. You said that if it made a difference, manufacturers would use it. That's nonsense. It's not practical to use iron wires, and any small difference in inductance would not outweigh the large drawbacks of doing so. So your argument holds no water.

If, and only if the inductance were higher, it would make a stronger magnet and a more powerful motor for the size. I don't know what the difference in inductance would be, if any. Evidently, it is not worth while using iron because they don't do it.

Your original argument is a good one.

For the fringe fields, yes of course they are there for the copper too. My point is that the field distribution in the fringe field area is sensitive to changes (geometry, permeability etc) for air core inductors. The different fringe field would change the inductance. The only question is how much? Is it significant? I'm not sure, but perhaps it is for some inductor designs, and probably would not be for toroids and long/narrow solenoids.

The link I gave in a previous post said coil geometry was complicated.

Ratch
 

steveB

Well-Known Member
Most Helpful Member
If, and only if the inductance were higher, it would make a stronger magnet and a more powerful motor for the size. I don't know what the difference in inductance would be, if any. Evidently, it is not worth while using iron because they don't do it.
Copper losses in the windings are a very big deal in motor design. Copper has 6 times better conductivity than iron. So, of course it's not worth using iron, and that's why they don't do it.

No one is saying that the inductance increase (if there is one) would be enough to counteract other detriments of that approach. The fundamental question is does the inductance increase. You gave a good answer about that. Others did too. I threw in my intuitive feeling about it. But, measurements will resolve the answer quickly.

I would recommend making three pairs of air-core coils. Each pair would be an iron and copper matched set. Identical wire diameters should be used, and identical solid cores with permeability essentially that of free space/air.

1. Tightly wound single layer toroid.
2. Tightly wound Long and narrow single layer solenoid
3. Tightly wound Short and wide single layer solenoid

My prediction is that 1 and 2 will show little difference and 3 will show some difference (> +/- 10 % let's say as a threshold). I could be wrong, and maybe they all show differences, or maybe none show any significant difference. I wish I had some spare time to do it myself.
 
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Ratchit

Well-Known Member
Copper losses in the windings are a very big deal in motor design. Copper has 6 times better conductivity than iron. So, of course it's not worth using iron, and that's why they don't do it.

No one is saying that the inductance increase (if there is one) would be enough to counteract other detriments of that approach. The fundamental question is does the inductance increase. You gave a good answer about that. Others did too. I threw in my intuitive feeling about it. But, measurements will resolve the answer quickly.

I would recommend making three pairs of air-core coils. Each pair would be an iron and copper matched set. Identical wire diameters should be used, and identical solid cores with permeability essentially that of free space/air.

1. Tightly wound single layer toroid.
2. Long and narrow single layer solenoid
3. Short and wide single layer solenoid

My prediction is that 1 and 2 will show little difference and 3 will show some difference (> 10 % let's say as a threshold). I could be wrong, and maybe they all show differences, or maybe none show any significant difference. I wish I had some spare time to do it myself.

Inductance is proportional to the relative permeability. I don't know if iron windings will increase the permeability and thereby the inductance at all or not. Evidently not, because it is not used.

Ratch
 

steveB

Well-Known Member
Most Helpful Member
Inductance is proportional to the relative permeability. I don't know if iron windings will increase the permeability and thereby the inductance at all or not. Evidently not, because it is not used.

Ratch

Inductance is proportional to relative permeability? Correct. Remember those are simplified formulas that we use, but still you are correct.

Iron windings will increase permeability, or effective permeability really, very slightly. This is a second order effect, and I'm not referring to this at all. So, for all practical purposes, I agree with you.

My argument, which I already said may not be correct and is just based on intuition and not calculation, is that the simple formulas we use for inductance (such as the typical one you referenced) ignores fringe field. The fringe field does have an effect on inductance, particularly for air-core inductors. The fringe field is usually ignored, because it is not easy to calculate. Many structures have insignificant effect from the fringe field, and hence the simple formula works well. Again, toroids and long/narrow solenoids are of this type.

So saying "evidently not, because it is not used" does not hold water. Experiment will hold water here. Your answer may be perfectly correct, and my intuition may be completely wrong. But, your comment here is not the proof of that.
 

Ratchit

Well-Known Member
Inductance is proportional to relative permeability? Correct. Remember those are simplified formulas that we use, but still you are correct.

Iron windings will increase permeability, or effective permeability really, very slightly. This is a second order effect, and I'm not referring to this at all. So, for all practical purposes, I agree with you.

My argument, which I already said may not be correct and is just based on intuition and not calculation, is that the simple formulas we use for inductance (such as the typical one you referenced) ignores fringe field. The fringe field does have an effect on inductance, particularly for air-core inductors. The fringe field is usually ignored, because it is not easy to calculate. Many structures have insignificant effect from the fringe field, and hence the simple formula works well. Again, toroids and long/narrow solenoids are of this type.

So saying "evidently not, because it is not used" does not hold water. Experiment will hold water here. Your answer may be perfectly correct, and my intuition may be completely wrong. But, your comment here is not the proof of that.

If the manufacturers don't do it, that is a pretty good indication that the experiment has been run quite extensively before and decided upon.

Ratch
 

steveB

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Most Helpful Member
If the manufacturers don't do it, that is a pretty good indication that the experiment has been run quite extensively before and decided upon.

Ratch
Again, they don't do it for a lot of reasons. I doubt inductance increase from the wire is of any concern at all. Conductivity is a huge concern, especially when magnetic cores increase inductance much more effectively. Also, putting feromagnetic material in the stator winding could have other detriments. None of this is relevant to the very basic question of whether the inductance might be higher. Again, the experiment will tell, and you might be proved right when the experiment is done.
 
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