Make an inductance coil - Page 1
Forum section: electronics / projects

author: quentief
author_rank: Newbie
timestamp: March 01, 2020, 10:24:33 pm
content: Hi everyone,I am opening this subject because I would like to ask you some information about the coils designing.Indeed, I am working on a project which requires a coil, with an great deal of inductance, possibly close to 1H, and which can deal with a maximum current of 10 amps.I have already tried with a MOT transformer : I used its primary coil as an inductance and it works perfectly. However the weigh of the transformer bring some drawback and in that respect, I would like to design my own inductance to replace it.I have recovered a magnetic core from an old transformer, smaller than the MOT. I would like to use it to design my own coil. I have tried to wind the magnetic core in different way to make a coil with a suitable inductance but it did not work. I am wondering if my core is not saturated, but I don't have any implement to check that.Therefore, I would like to contact you, see if you could help me. I don't have enough information about the designing of inductance, especially by considering the saturation.

author: jonpaul
author_rank: Super Contributor
timestamp: March 01, 2020, 10:46:33 pm
content: Bonjour monsieur, désolé, impossible pour le bon réponse votre question, avant réception plus des informations.10 ampères AC, DC ou les tous ?Fréquence min, max ?Linéarité ?J'attends pour votre réponses,Bon SoiréeJean-Paul

author: quentief
author_rank: Newbie
timestamp: March 02, 2020, 12:47:15 am
content: Comment savez-vous que je suis français ? Je parle anglais si mal que ça ?Voici ce que je peux vous dire :Il s'agit d'un courant rectifié provenant d'un pont de diode lui même relié au 230 volts du secteur (donc 50Hz). Le courant est limité grâce à une résistance électrique de 23 Ohms, dont courant efficace maximum de 10 ampères. Je cherche à concevoir une inductance de l'ordre de 1 H.

author: Gyro
author_rank: Super Contributor
timestamp: March 02, 2020, 10:04:05 am
content: Quote from: jonpaul on March 01, 2020, 10:46:33 pmYour French is very good!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 02, 2020, 10:21:02 am
content: 1H at 10A saturation current requires a physically massive and heavy inductor, no way around it. I suspect you can't get it much smaller than a microwave oven transformer. What are you actually doing? There's likely a better way which requires less or no inductance.

author: quentief
author_rank: Newbie
timestamp: March 02, 2020, 11:18:24 am
content: I have tried to use this magnetic core to make my own coil but it has been a failure. I suspect my core was saturated, and I have been advised to add an air gap in that respect to avoid the saturation phenomenon, however I do not know how I can design it.I would like to make long electric arcs from the main rectified voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2020, 01:01:44 pm
content: Simple -- get enough capacitors to handle the worst-case peak energy, and use a switching converter to emulate an inductance.1H at 10A is E = 0.5 * (1H) * (10A)^2 = 50J of stored energy, equivalent to a fist-sized electrolytic capacitor. We can calculate the volume of air gap required to store as much energy in an inductor:v = 2 E mu_0 / B^2This is useful because we know B from the materials we will use: transformer steel saturates around 1.2T. Putting that in, we get 87 266 mm^3.Note that v is inverse with B, so we could simply choose a large B and get what we want; the problem is, we can't get B that high using magnetic materials (the best materials saturate up to 2T, which does help), and to push B that high without materials requires massive magnetization, i.e. lots of ampere-turns -- we need orders of magnitude more copper, or incur orders of magnitude higher resistive losses.The resistivity of copper is actually a limiting factor here, and it turns out, a relative permeability of 20-60 is typically best suited to it. If we had a better conductor, we could get by with less, or with a perfect conductor perhaps no core at all. Unfortunately silver is only a few percent better than copper, so we cannot find a solution at room temperature; copper does have a generous tempco and could be cooled to LN2 temperatures, if the refrigeration losses aren't terrible (you might want to check the price on LN2 truckloads and dewars though). Or the perfect conductor does, in fact, exist -- but we still need LN2 temperatures or below to make use of superconductors.If we suppose an air gap of 20 mm, that's a cross-sectional area of 4 363 mm^2, or a 66 mm square. That's about two MOT sized cores, stacked.This is probably a good lesson on the X-Y problem: you have problem X (unknown), and a tentative solution Y (designing a compact inductor that stores a metric shitload of energy). If you could only refine the solution to problem Y, you'd be done; it seems simple enough. But as it turns out, Y is, in fact, impossible (if your volume and cost requirements are completely rigid), so you will have to reconsider problem X instead.Very likely, for example -- my initial suggestion is comically overcomplicated, but with the slightest rearrangement (e.g. using a parallel filter capacitor instead of a series filter inductor), perhaps it becomes a nearly trivial solution. We just don't know until you've given us some information about your underlying application.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: March 02, 2020, 02:33:47 pm
content: To make long arcs, the key is to go DC (or high-frequency AC). With 50/60Hz AC, you are just killing the arcs every time when zero-crossing.The higher the voltage, the longer the arcs get, but once the arc is properly started, a surprisingly low voltage can keep it going.A rectifiedand smoothed(with capacitors) mains would be easiest; using the microwave oven transformer as-is to increase voltage would work as well; upping the voltage could help make bigger arcs, but at some point, you are power-limited and need stronger supply than what mains (or a MOT) can provide.Inductance is only needed to help start the arcs. When touching the pieces, inductor stores energy and releases it at a very high voltage when the pieces are separated, helping the arc form.Remember that all what's discussed here is extremely dangerous and you shouldn't be doing that.

author: BravoV
author_rank: Super Contributor
timestamp: March 02, 2020, 03:31:36 pm
content: Quote from: quentief on March 02, 2020, 11:18:24 amHow long ?Have you considered a neon light transformer ? As most are now obsolete and at some places can be dirt cheap.So cheap, that I scored this one last year, for free.An example for 12.000 Volt in action with Jacob's ladder.

author: jonpaul
author_rank: Super Contributor
timestamp: March 02, 2020, 08:16:46 pm
content: Bonjour encore...j'ai appris la française depuis 2015. Très difficile pour l'ingénieur ancient!0/ It is not precise what you seek to accomplish, feed a Tesla coil primary tank? General HV DC? AC? Draw arcs for fun and games? You will get a better response if you give your end objective !1/ The series current limit inductor MUST have an airgap to avoid saturation2/ MOTs have a gap builtin for current limit and safety.3/ Combination of a transformer and series current limit inductor will ALWAYS be much heavier and less efficient than a properly designed transformer with airgap.4/ Beware that MOT in general are capable of LETHAL voltage and current!!!! Danger du mort avec la haute tension! Utiliser tous les precautions!Bon chance et bon journee!Jon

author: quentief
author_rank: Newbie
timestamp: March 02, 2020, 10:26:53 pm
content: Hi everyone,Thank you for all your replies. I just would like to precise that I am using the MOT as a coil, I do not use its secondary to make high voltage. To be honest, I prefer to stay at 230 volts for the moment. Despite the risks, I assume this voltage is safer than the 2000 or more volts produced by a high voltage transformer.The aim of the coil used in my setup is to make a high-voltage spike when I spread the electrodes, to start an arc. Furthermore, the inductance is used to smooth the current and to stabilize the arc. According to my tests, the primary coil of the MOT (again, used as a simple coil) has made very good results. I am not sure about its inductance but by considering the nominal voltage, the frequency of the line and the no load power consumption of the transformer, I have estimate an inductance of about 900 mH.I am now trying to replace this transformer, and I would like to design a coil in that respect. I have made some researches about inductances with iron core but I do not know how to calculate the maximum current before the saturation of the core. I have already read that air gap was used to avoid saturation but I do not know how that works, neither how I can size a gap in that respect.I hope I have not make any mistakes this time, and made my explanations more clear. Please, let me know if some of my sentences are not clear, I have to improve my English, comme vous l'avez fait en Français Jonpaul

author: jonpaul
author_rank: Super Contributor
timestamp: March 03, 2020, 10:18:35 am
content: Bonjour encore Monsieur! Alors, j'ai plus des questions, votre project:1/ Arc is in air or gas or an arc lamp?2/ Carbon arcs like cinema projector?notes:3/ MOT will have 2kV on sec even if that is not used, beware.4/ Most air arcs with carbon or metal electrodes do not need any HV to start, the electrodes are touched momentarily and then gaped open5/ Any current limiter will work, even a big power resistor.6/ if are is in arc lamp eg Xenon or metal halide a different type of HV ignitor is used.7/ All arcs emit powerful and invisible ultraviolet light, capable of corneal or retinal damage, use welders mask and goggles to protect eyesight!8/ Metal or carbon electrode arcs in air cause nitrogen to react and produce noxious compounds, as a gas or fine particles. Professional applications must use a hood, duct with fan to exhaust the fumes, eg cinema projectors. Beware the smoke of the arcs!Nous pouvons contacter a Paris quelquefois, habitez-vous a Troys?Bon journée; Jon

author: quentief
author_rank: Newbie
timestamp: March 03, 2020, 11:36:50 am
content: Bonjour Monsieur1/ The arc is produced through the air, between two iron electrodes (two nails)2/ No, I am trying to establish electric arcs between metal parts. I know it is easier to establish electric arcs between carbon electrodes (I have already tried), but I want to understand how electric arcs can be made between metal parts.3/ I used a modified MOT : I replace its secondary to make a spot welder (the voltage is around 2 volts), so don't worry about that4/ Well according to my tries, when the electrodes are made in metal, a coil is needed to make an electric arc. I don't know if that is because a high-voltage spike is needed to start an arc, or because the current needs to be smoothed, or maybe both, but without a coil with an inductance enough strong, that doesn't work.5/ An electric kettle is used as a power resistance, the R.M.S. current is limited to 10 Amps.6/ No discharged lamp devices is used in my setup.7/ UV glasses are used at ever attempts.8/ Every attempts is done at the outside.Je suis à Troyes et je suis désolé mais je n'ai pas la possibilité de me déplacer jusqu'à Paris pour l'instant. Mais nous pouvons correspondre par mail si vous le souhaitez.

author: chris_leyson
author_rank: Super Contributor
timestamp: March 03, 2020, 12:55:18 pm
content: Maybe you could use something similar to an arc welder ignition transformer.EDIT:https://patentimages.storage.googleapis.com/ea/c6/5d/70433111f93ace/US6156999.pdfgives a little background information on arc welder ignition transformers

author: quentief
author_rank: Newbie
timestamp: March 03, 2020, 01:17:12 pm
content: For the moment, I just just want to know more about inductances, and the saturation phenomenon. I would like to know how I can size an iron core inductance, and especially how I can size a air gap to avoid the saturation phenomenon. I cannot find any information on this subject on the Internet.

author: quentief
author_rank: Newbie
timestamp: March 04, 2020, 09:31:18 am
content: Do you know a relation between the magnetic flux, the reluctance of the magnetic circuit and the maximum current that a iron core coil can tolerate ?

author: jonpaul
author_rank: Super Contributor
timestamp: March 04, 2020, 10:39:09 am
content: Bonjour encore MonsieurSVP Comment vous appelez-vous?1/ Now we understand better. Try other metals besides iron nails, eg copper, tungsten, zinc, etc. all will give different colors and effects. A welding supply store will have professional welders electrodes designed for drawing arcs. These have easier starting with low work function coatings. Still the carbon are the best in general. Much of the work on arcs in air is classic and books back to 1880s...1920s discuss starting, work functions of metals, etc.2/ Starting kick is from the inductive energy stored that is released as a high voltage when a circuit is interrupted. Most carbon arcs can be mechanically started, and perhaps some type of electrodes.3/ The airgap design to avoid saturation is covered in most magnetics courses on graduate level. Bravo to explore this interesitng subject!The formulas and practical design info is easy to find in old engineering textbooks, but perhaps not so easy online. I am sure most are in English and not French!If you PM me I make a list of recommended books.4/ Finally the old steel lamination suppliers had tables and chars for design of mains transformers and inductors, but most of those firms have offshored to cChina and have no good applications. Only old ones exist as printed.5/ Easy solution: Take the MOT or other transformer, remove any varnish and disassemble.Place a plastic or paper spaces as a gap between E and I laminations, perhaps 1 mm thick.Reassemble and clamp together.Test for saturation and repeat if gap is too big (low inductance) or too small (saturates)Bon chance!Jon