# Core and winding dimensions for an electromagnet and bifiliar winding

I am making a powerful electromagnet for repulsion for my project. I made some electromagnets with iron core diameter of 1 cm, 1.5 cm and 2 cm for each 6 cm and 8 cm length. Different gauge enameled copper wire I used is 0.42 mm, 0.70 mm and 1.0 mm. Winding width is 1 cm for each electromagnet. The end result is an electromagnet with 2 cm x 8 cm dimension with 1.0 mm wire gauge is the most powerful of all. But still not enough for what I want.

Now I am going to experiment with using 4 cm, 6 cm, and 8 cm diameter iron core of 8 cm length.

My question is: is there any rule/what is rule to decide winding dimensions in relation with core dimensions? As I used 1 cm winding diameter with wire of 1.0 mm and 2 cm diameter core. Will increasing the winding diameter say using 1.5 cm winding instead of 1 cm on an increased 4 cm diameter core will make a more powerful electromagnet?

I am also thinking to make an electromagnet with this same dimension as of my current best electromagnet (i.e. 2 cm x 8 cm with 1.0 mm wire gauge) using bifiliar winding (winding with two wires together and attach the ends) to compare both. As I read winding this way decreases overall resistance of wire, input more amperes & increases power of electromagnet. Any comment about this ?

I find this one online simulator for air core online electromagnet calculator. It's very helpful for finding resistance of coil and number of turns for cored EM too. Is there any such simulator available for electromagnets with core?

I found this calculator on the web. It seems to use the correct formula and that is: -

Force exerted = $(NI)^2 \times \mu_0 \times\dfrac{A}{2g^2}$

Where

• N is number of turns
• I is amps
• A is cross sectional area of coil
• $\mu_0$ is permeability = $4\pi\times 10^{-7}$
• g is the gap between the solenoid and the thing it attracts to

Note that the permeability of the material you wind it on does produce more flux density but the permability of the gap is the totally dominant part of the equation and material permeability is not included because it is so very minor.

An an electrical analogy, having something with high conductance (ferrite) in series with something with low conductance (air) still means something with low conductivity i.e. air is dominant except when the gap becomes small as in gapped ferrite transformers or inductors (which you haven't got)

Doubling the turns or doubling the current produces 4 times the force but doubling the gap quarters the force. Doubling the diameter of the solenoid produces four times the area and hence 4 times the force too.

Increasing the wire diameter allows more current to flow (for a given power supply voltage) BUT only if the supply can give the extra current. Ideally, wind with silver wire (costly) thus keeping the resistance low and feed as much current in as you can. Make the coil diameter as big as you can too. As many turns as you can manage also.

Bifilar winding won't help - this will be a dc solenoid with maybe a low frequency excitation and skin effect will be almost negligible - better to use two wires in series because N doubles rather than split current into two parallel paths (in effect this is just one turn still).

• How series winding is done? I read from some source that dividing total length of wire into smaller parts like 30 feet into two 15 feet.And winding those parallel(attach ends) reduces overall resistance & more current input.Another thought is to winding each part on top of other.Like wind 15 feet 1st and then 2nd 15 feet over 1st and attach ends,it's referred as parallel winding too.I am thinking to follow these,starting with using two parallel wires which I referred bifiliar winding in OP.What winding in series are you referring. – enterprize Dec 30 '13 at 13:54
• Please refer any suggestion about dealing with the heat of the coil. – enterprize Dec 30 '13 at 14:38
• Parallel winding will not help on a dc or low frequency electromagnet. AMPS times TURNS are the magneto motive force behind magnetism. Parallel turns are wasted turns because they only count once. Series winding is just my words for "more turns in series rather than in parallel". Don't wind Bifilar - you'll be wasting your time. Look at the formula - more turns (series), more amps, larger diameter is what you need. Wind the turns close and in layers - heat will be a problem but it's hard to say to what extent. – Andy aka Dec 30 '13 at 14:51

If parallel winding is done the greater ampage produced will be flowing through windings closer to the centre core, thus creating greater magnetism. One length of wire that is 40ohm and wound round a 3cm diameter and 4cm length cylinder core will project around 4cm from the core in windings bulk. If this wire is halved into two lengths of 20ohm and the ends twisted together the resulting bulk of wire will be the same but the reststance will be 10ohm generating more current.