NRamaswami I do not recall the name of the effect but it has to do with coil geometry and why coils have a rule of thumb for their dimensions. I work more off conceptual aspects and the effects. When a coil makes a magnetic field and the image you get in your head looks like a bubble around the coil it isn't that clean cut. If you made a coil many times longer then it in the diameter the field isnt going to be that nice round bubble.It will stretch out and close in on the coil near the middle portion. You have greater chance to unfortunately cause the self cancellation of the flux on the outer layers when it is elongated depending on the extent and the volume of the flux. Some people have gone to great lengths to design windings that are supposed to prevent that. Gyula might be able to explain it better. I only have short moments between baking formers on my wood stove.

Hi NRamaswami,

I think it would be a very good step for you to obtain an L meter and check the inductances of your single wire, bifilar and trifilar coil constructions. I say this because all the coil calculating formulas or online coil calculators are based on normal enamel-insulated copper wires (i.e. 'magnet' wire) and not based on wires covered with thick insulation layers which introduce space or distance (and capacitance) between adjacent wires, either between turns one above the other or between turns next to each other.

I do think that obtaining an L meter is the quickest way to advance your electromagnet constructions because by knowing the actual primary coil inductance in an assembled setup you can calculate the AC input impedance in advance, this makes the input current estimation possible in advance, you could also see the effect of the secondary load on the primary input coils inductance, how the presence of the secondary load would modify the input impedance of the primaries,  this way you also could quickly browse through some electromagnet coils you have left from earlier experiments etc.

When you have a measured primary coil(s) inductance, and you have the DC resistance of the same coil(s) also measured with your Ohm meter, the input Z impedance can be calculated as I showed earlier, I can help you in that.

With the L meter suggestion I do not mean to toss the ball into your field of course but to indicate for you that there is no calculation readily available for arriving at coil inductance (hence impedance) values when using wires with thick insulation, at least I am not aware of such.
Especially with open core coils this becomes more problematic because manufacturers specify permeability data for their soft iron cores when the core has a closed magnetic path (usualy the permeability data is specified for ring cores made from a given soft iron material).
This means for instance that a normal transformer lamination may have say a permeability of 800 in a closed core, no air gap, and using a certain section of this same lamination as an open core its permeability becomes much less and quasi unpredictable. This is where an L meter can help tremendously.

I understand that you do not have many I laminations but if you are going to use iron rods again, then your electromagnets would become 'ill-behaved' again, heat losses and not readily repeatable results would prevail.

So either you obtain more 6 inch long I laminations or you may wish to consider the steel pellet core I referred to already.

A notice to the I laminations: you can place some 6 inch long piece in a single line and then put a second row onto them which would overlap the gaps and so on, of course this would need a sufficient amount of I laminations. Yes they have sharp edges indeed, they need careful handling.

Ferrite cores would function correctly too but they have become a bit expensive during the years. On ebay you can find 8 to 10mm dia ferrite rods with 180 to 200mm length like this offer http://www.ebay.com/itm/16x-Large-Balun-Ferrite-Rods-10x200mm-/201042983709  but you would need more than 16 such rods to fill up the inner volume of a 45-50cm long coil bobbin with 2.5 inch dia and then you would need still twice as many for the other 2 electromagnets.

More on your questions later on.

Gyula

Hi Gyula:

Thanks for the great hints. I will find out an L meter in a shop that sells these items. The problem is with these shops there is No Guarantee No warranty No return No refund for China made goods. And most of them are china made only. Another aspect is I simply do not know how to use them really. I will buy them and learn to use it from some body. These are not usually bought items and usually not available and we need to do shop hunting. While we can buy ebooks online, any product import requires an import export code number and a lot of formalities at the customs end.

We do have enamelled magnet wires, the place where the I cores are available also has these enamelled magnet wires.

I have a problem with your explanations so far.

If my understanding of your post is correct, by using small gauge wires with high resistance, high DC ohms, it is easy for us to build quadfilar coils which have very high AC impedance and also inductance. As the number of wires increases like bifilar, trifilar,quadfilar,pentafilar etc and by using wires with high ohms we will get to a point where the input AC becomes very low but the magnetism created is very high.

Does this not go against the Magnetic field strength = Amperes x number of turns formula. Only when amperes go up the magnetic field strength would go up. Here you are reducing the amperes, number of turns remain the same, but the inductance increases for the same core and same material indicating greater magnetization. It certainly happens any way whether the coil consumes less amperage or more amperage but how come for the same coil it happens is a mystery when the amperage consumed is less.

Also please advise  if my understanding above is right or wrong on using small guage wires. It will take some time for me to buy the I cores, arrange them and put duct tapes around them, buy the L meter etc but you are certainly correct that reading at diffrent times gives different results.. May be it is way the gaps inside the material changes. We have iron powder and we can dump them and pack the pipes but the boys are agaist it as it is usually makes them suffer cuts in hands hands however safe we are in handling them.

I will get the L meter and get back to you but I would be very obliged if you indicate whether we should go in for small wires with high DC resistance or large wires with low DC resistance ( which one has the higher AC impedance and hence inductance when coiled as bifilar or trifilar or quadfilar wire). Please let me know.  Without the L meter I cannot make further experiments, I would be very grateful and obliged if you could clear these theoretical doubts. Thank you so much.

Hi all,

This thread has grown a lot. 20 pages in 15 days!!  I post here a little detail from a Buforn patent to get relaxed among so much technical info:

Hi NRamaswami,


On the choice of LCR meter types,  I would like to mention that the measuring frequency in the L ranges is to be considered: the closer this frequency to the operating frequency of a coil with core, the more accurate the measurement will be. This is important for coils with low frequency cores like normal laminations.  In your case the operating frequency for the coils is the 50Hz of the mains voltage but most LC meters have 100Hz 120Hz or higher built-in test frequencies. Here is a meter which has 80Hz test frequency in the 200mH and 2H ranges and 26Hz for the 20H range, so this is a good trade-off, although I do not know the price: http://www.tradeindia.com/fp23040/LCR-Meter-KM-954MK-II.html and here is the data sheet: http://www.signalhawk.in/LCR/KM_954MK-II.pdf

There is the LCR-4070 type which has 200Hz test frequency on all the L ranges from 2mH to 20H: http://www.tradeindia.com/fp747683/Digital-LCR-Meter-Model-LCR-4070.html 

Of course there are many other types but perhaps the KM-954 MK-II would serve the purpose


You wrote: 
"I'm still unable to understand why the multicore wires perform well only when they are wound two times and do not perform well when the layers are more than 2." 

I am unsure here, a possible explanation would be that in the first case (when you fill up the gap left between the turns of the 'forward' winding with the turns of the backwards winding if I got you correctly) you eventually have a bifilar coil in series aiding phase connection for the two windings.  While in the second case you do not have gaps between the turns of the first layer winding and you place the second layer onto the top of the first layer and so on. You need to clarify what you mean exactly when you say for the second case: 'they do not perform well'.  What was the number of turns for the two cases, what dia had the bobbins etc


You wrote:

"If my understanding of your writing is correct, greater inductance will increase the magnetism but would also consume more amperage and greater impedance would reduce the amperage consumption but would lead to lesser magnetism in the coil. Am I right?"

No I did not write or mean the second assumption (i.e. consume more current). It is okay that greater inductance (more turns and correct core) would increase magnetism  i.e. insure more flux for an electromagnet but this does not involve consuming more amperage. It is also okay that a greater impedance for a coil (i.e. again more turns and correct core) inherently reduces current input if you stay with the same AC input voltage amplitude (220V in this case) but magnetism would be reduced only in a lesser degree: the explanation comes from the AmperTurns you also mentioned, this means the multiplication of the coil current with the number of coil turns, you increase the number of turns which inreases impedance hence the input current reduces (provided the input voltage stays the same), however their product changes but a little. This is a trade-off game in a sense because inreasing the number of turns inherently increases DC resistance too.

I suggest you to avoid using too thin dia wires which have higher DC resistances versus the thicker dia wires. High DC Ohms increase heat losses and make the coils burn down on the long run.

More on your questions later.

Gyula