Post by thread:[EE] Three questions on inductors

Hi, trying to improve my knowledge of inductors. The energy stored in an inductor is proportional to the current that is flowing through it in that certain moment. If the current increases, then (part of it or all?) gets stored as additional energy in the magnetic field, if the current decreases, part of the energy gets released to oppose this. I'd three questions for Your brains: 1) The "(part of it or all?)" already written above part. 2) Suppose I increase the current in an inductor till 1A, then keep it constant for 1 second. What happens during this second? Do I simply waste power? 3) If I simply wasted power (continuing from the previous question), then I suppose I will waste some power even during the "current is increasing" part. Then, when current is increasing (that is, dI/dt is not zero), how much energy I am storing and how much I wasting (as heat, etc..)? What is the formula? I fail to make an analogy with capacitors, as once charged they get (theoretically) infinite impedance. Inductors instead get (theoretical) zero impedance, and this may imply the "waste" problem stated above. As I said, I fail to make an analogy with capacitors on this particular issue. Thank You! Greets, Mario

On 16 May 2010 21:03, Electron <spam_OUTelectron2k4TakeThisOuTinfinito.it> wrote: > > Hi, > trying to improve my knowledge of inductors. > > The energy stored in an inductor is proportional to the current that is flowing > through it in that certain moment. If the current increases, then (part of it or > all?) gets stored as additional energy in the magnetic field, if the current > decreases, part of the energy gets released to oppose this. > > I'd three questions for Your brains: > > 1) The "(part of it or all?)" already written above part. The energy stored is equal to 1/2 x L x I^2. i.e. proportional to the square of the current. If the current increases, then the energy stored increases also. Unless the core is "saturated" in which case no additional energy is stored. > > 2) Suppose I increase the current in an inductor till 1A, then keep it constant > for 1 second. What happens during this second? Do I simply waste power? > With an ideal inductor there is no DC resistance. Energy is only stored when the current is changing and this is the only event that will cause a voltage drop. When the current is stable no energy is being absorbed or (ideally) dissipated. So no power is being wasted. > 3) If I simply wasted power (continuing from the previous question), then I > suppose I will waste some power even during the "current is increasing" part. > Then, when current is increasing (that is, dI/dt is not zero), how much energy > I am storing and how much I wasting (as heat, etc..)? What is the formula? > Power is not being wasted., energy is being stored (or released) only while the current is changing. The formula above E=1/2 x L x I^2 is the one used. While the current is changing, there will be a voltage drop across the inductor equal to V = L x di/dt > I fail to make an analogy with capacitors, as once charged they get (theoretically) > infinite impedance. Inductors instead get (theoretical) zero impedance, and this > may imply the "waste" problem stated above. As I said, I fail to make an analogy > with capacitors on this particular issue. > With the analogy to capacitors, voltage converts to current. So while a capacitor stores energy with a voltage across it, and inductor stores energy when current is passing though it. It's a little harder to visualize, partly because we are more used to dealing with voltage sources, and partly because capacitors tend to be more "ideal" than inductors. Hope this assists RP

Electron wrote: > Hi, > trying to improve my knowledge of inductors. > > The energy stored in an inductor is proportional to the current that is flowing > through it in that certain moment. If the current increases, then (part of it or > all?) gets stored as additional energy in the magnetic field, if the current > decreases, part of the energy gets released to oppose this. > > I'd three questions for Your brains: > > 1) The "(part of it or all?)" already written above part. > > 2) Suppose I increase the current in an inductor till 1A, then keep it constant > for 1 second. What happens during this second? Do I simply waste power? > If the voltage across the inductor is not changing, then "all" of the energy possible is stored. Assuming ideal inductor. If it's a perfect inductor it has zero "real" resistance. Imaginary axis "resistance" is invisible at DC. Thus no power is wasted. IN THE INDUCTOR. Other parts of the circuit will / may dissipate power. The actual power wasted in a non ideal inductor for steady state current is P = I^2 x R, where R is the DC resistance (Real axis Resistance). > 3) If I simply wasted power (continuing from the previous question), then I > suppose I will waste some power even during the "current is increasing" part. > Then, when current is increasing (that is, dI/dt is not zero), how much energy > I am storing and how much I wasting (as heat, etc..)? What is the formula? > Wasted power when current changing is these main parts: P = I^2 x R, where R is the DC resistance (Real axis Resistance). Mainly winding DC resistance. At high frequency the current in a conductor flows closer to surface. Skin effect. This makes "real" resistance higher even at 1MHz. By 400MHz a flat strip will have much lower "real" component (lossy resistance) than a circular wire. High power RF coils may be wound with 4mm hollow copper, possibly silver plated as almost no current flows in the core. Polished copper with lacquer to avoid oxidising is lower loss than tin or tin/lead plated or gold. Radiated RF (large air core coil is higher, on ferrite toroid is lower). Formula is complex and depends on frequency, construction, size Losses in the Magnetic material if not air core. Magnetic/Induction losses: Eddy current with in core and windings, worst for solid conductor, less laminated, lower insulated dust iron or ferrite and least on air core. Coupling to magnetic materials or other nearby coils. Core Saturation Magnetic hysterises Losses are usually lowest with sine wave and highest with Square wave due to frequency dependent eddy current loss, skin effects on conductors, RF emission and such. Interwinding capacitances may not increase losses directly but affect self resonant frequency and actual inductance. > I fail to make an analogy with capacitors, as once charged they get (theoretically) > infinite impedance. Inductors instead get (theoretical) zero impedance, and this > may imply the "waste" problem stated above. As I said, I fail to make an analogy > with capacitors on this particular issue. > > harder to make a "good" inductor that's millihenries. A perfect capacitor with DC (fixed/steady) voltage has no current. A perfect inductor with DC(fixed/steady) current has no voltage across it. For C P = V x 0 = 0 For L P = 0 x I = 0 no power dissipated.

At 12.23 2010.05.16, you wrote: >>Interwinding capacitances may not increase losses directly but affect >>self resonant frequency and actual inductance. > >>From what I've seen (which is not a great deal :-)) self resonant >frequencies can be a lot different for the same rated inductance (between >different inductors). I'm in the process of looking for an SMD power choke, >around 100uH, and the rated maximum SRFs seem to vary between 5 and 12 MHz, >a bit lower than I expected. I suppose unless you take this kind of thing >into account, you could up with performance quite a bit different from that >expected in an ideal situation. Not sure how this would translate to the >stator scenario though (I assume this post is related). Not again! Please if the thread is different do not assume things not explicitly stated.. else only confusion may arise. ;) Cheers, Mario >Maybe a rough sketch >in spice might be helpful for all this, with temperature coeffiecient, >saturation, winding capacitances/resistance etc. > > >-

At 13.30 2010.05.16, you wrote: > >>Not sure how this would translate to the >>>stator scenario though (I assume this post is related). > >> Not again! Please if the thread is different do not assume things not >> explicitly >> stated.. else only confusion may arise. ;) > >Sorry, I meant Bike/CDI related really, but i appreciate it may be nothing >to do with either :-) I was just guessing at why you might be asking.. Inductors have such a wide range of applications that it was just a general question. I have other projects going on as well, not just the CDI one. ;) Also, as inductors knowledge is generally in "shadow" in many hobbyst's minds, I think that making such a question, the eventual answers may be of general interest. In this specific case, the answers were simply great. More than to the point. To be honest I hate to go to the supermarket to get some milk and get a lot of questions like "what do you have to do with it?", "are you sure you really need milk?". I mean, if you ask a very specific question, then you probably need a very specific answer. Of course, even "what do you need to achieve?" is the think to ask, sometimes, but always - not. Sorry for the OT. Greets, Mario

At 13.45 2010.05.16, you wrote: >Oli Glaser wrote: >> >>Not sure how this would translate to the >> >> Sorry, I meant Bike/CDI related really, but i appreciate it may be nothing >> to do with either :-) I was just guessing at why you might be asking.. >> >> >> >Because people here might know the answer and he wants to know? > >Maybe asking here is easy if you are not sure what to ask Google >or Wikipedia. They are easy when you sort of half know the answer or >have simply forgotten the details but you understand it. You need at >least the correct terminology. I use Wikipedia and - as a more general rule - Google a lot, I actually think I consumed it during the years. However, sometimes you have a doubt and even if you read tens of pages that explain everything about inductors, they repeat the same things but none of them clear the little, sometimes strange, doubt you have in your mind. Then and only then I resort to ask things here, and I do it with an ease directly proportional to my belief that bringing up the matter and causing the production of answers may be of general interest for many other readers, who maybe didn't even need to know, but that will be enriched by the answers anyway - without even needing to ask. :D

Electron wrote: > trying to improve my knowledge of inductors. Then Google and find a tutorial first. Surely there must be many basic introduction to inductors out there. Do your homework first, then ask here about remaining issues. > The energy stored in an inductor is proportional to the current that > is flowing through it in that certain moment. No. Go look it up. > If the current > increases, then (part of it or all?) All unless you hit saturation. > gets stored as additional energy > in the magnetic field, > I'd three questions for Your brains: Phew. Good thing they are not for my foot or left elbow. Those aren't very good at answering questions. > 2) Suppose I increase the current in an inductor till 1A, then keep > it constant for 1 second. What happens during this second? Do I > simply waste power? In a ideal inductor, no. However, most (all you can hold in your hand) inductors have a non-zero resistance. During the 1 second of constant current this resistance will dissipate power. > 3) If I simply wasted power (continuing from the previous question), > then I suppose I will waste some power even during the "current is > increasing" part. Then, when current is increasing (that is, dI/dt > is not zero), how much energy I am storing and how much I wasting > (as heat, etc..)? What is the formula? Model the real inductor as a ideal inductor in series with a resistor and it will become obvious. > I fail to make an analogy with capacitors, as once charged they get > (theoretically) infinite impedance. Huh? Go read a tutorial on capacitors too. > Inductors instead get > (theoretical) zero impedance, and this may imply the "waste" problem > stated above. As I said, I fail to make an analogy with capacitors on > this particular issue. This is too confused to try to correct here. Go read some introductory material on inductors and capacitors and then come back after having done your homework. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000.

At 15.23 2010.05.16, you wrote: >Electron wrote: >> trying to improve my knowledge of inductors. > >Then Google and find a tutorial first. Surely there must be many basic >introduction to inductors out there. Do your homework first, then ask here >about remaining issues. This is what I did*. You like to be uselessly mean, as always. >> The energy stored in an inductor is proportional to the current that >> is flowing through it in that certain moment. > >No. Go look it up. No amount of good technical advice is worth having someone like you in a list. Proof is that I got other answers which cleared up my doubts and even went beyond, and in doing do have also taught it to many other people who don't even bother to ask questions here, maybe just because they "fear" you may give them one of your "silly" (sorry if you use your (tm) adjective) answers. * just to name one: http://www.allaboutcircuits.com/vol_1/chpt_15/2.html You remain an uselessly mean person anyway. I wonder if you treat people the same way also when you have them face to face, or if you do this only through the keyboard. But I don't really care. Welcome to my killfile Olin, it's the right tool for the job.

Electron wrote: > To be honest I hate to go to the supermarket to get some milk and get > a lot of questions like "what do you have to do with it?", "are you > sure you really need milk?". You are paying them and not asking for advice, and they just want to sell you milk, not spend time on advice about how to use it. > I mean, if you ask a very specific > question, then you probably need a very specific answer. Maybe, but often enough we have people here asking the wrong question in the first place. > Of course, > even "what do you need to achieve?" is the think to ask, sometimes, > but always - not. Sorry for the OT. First, people are here for their own entertainment and other reasons. Nobody is really here to help you with your specific problem, although doing that serves their ends so they do help. Second, anytime you don't like the free advice you receive here, you can have a full refund. Third, often enough we get people here asking about specific details to the wrong approach. The right answer is to find the true problem one or two levels up, then discuss how to solve that. That's how it works, get over it. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000.

Electron wrote: >> Then Google and find a tutorial first. Surely there must be many >> basic introduction to inductors out there. Do your homework first, >> then ask here about remaining issues. > > This is what I did*. You certainly didn't give any clue to that. To me it sounded like you hadn't looked up anything. You've been around here long enough to know that showing you've done your homework is as important as doing it. > You like to be uselessly mean, as always. I like people to do their homework, then help those that have tried to help themselves. >>> The energy stored in an inductor is proportional to the current that >>> is flowing through it in that certain moment. >> >> No. Go look it up. > > No amount of good technical advice is worth having someone like you > in a list. Oh get over yourself. Just telling you that your statement is incorrect should have been useful information. I could have told you the answer, but I felt you would learn more by looking it up. This should not be difficult since you now know what specifically to look up. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000.

>>> The energy stored in an inductor is proportional to the current that >>> is flowing through it in that certain moment. >>No. Go look it up. > No amount of good technical advice is worth having someone like you in a list. > Proof is that I got other answers which cleared up my doubts ... Be very wary of being so dismissive of any known competent person's technical input. Olin's style may be lacking, and he sometimes gets the wrong idea of what someone is saying in his enthusiasm to 'correct', BUT the technical basis for his utterances is almost invariably sound and it is highly advisable to sort the wheat from the chaff and understand what the technical point was that he was making. It's very usually a valid one. You said: >>> The energy stored in an inductor is proportional to the current that >>> is flowing through it in that certain moment. He said: >>No. He was, of course, correct. Although, as you tell us, you have looked extensively at Wikipedia etc before asking questions here, you have managed to form a fundamentally incorrect impression. Olin sought to correct this. To ignore his input, no matter how badly it is put, is to allow yourself to become blind to technical truth and to resist correction. He said: >>Go look it up. That's not terrible advice. Gargoyling your exact phrase +"The energy stored in an inductor is" Produced a large number of good quality hits. The first 5 or 6 give the actual formula or are obviously directly relevant. Many others will be too. The top result says Electronics Forum -> Energy Stored In The Inductor. The energy stored in an inductor is given by the formula = (L*I*I)/2. http://www.dutchforce.com/~eforum/index.php?showtopic=31528... - Cached ie energy stored is proportional to current squared. An extremely important thing to know about the internet is that you get good and bad information. Pages which LOOK escellent may be rubbish. Checking a few sites against each other is always wise. Unfortunately, the sight that you cite www.allaboutcircuits.com/vol_1/chpt_15/2.html has some good material BUT also some misleading material and also some rubbish. Unfortunately, I'd go so far as to say that despite its helpfulness and the effort that they have gone to that that is effectively a bad page for beginners. They NOWHERE explicitly mention the relationship between inductor current energy and current, although they easily could and certainly should have given the detail they go into. They make one correct related which may mislead in what it says ie " The more rapidly current is decreased, the more voltage will be produced by the inductor, in its release of stored energy to try to keep current constant." This may have mislead you in coming to your conclusion. Unfortunately you were wrong and Olin was correct. You need to be able to both expect that and to pick up on his "clues" even if you think that his mother wears army boots. So - > ... they "fear" you may give them one of your "silly" ... answers. Learn to take the wheat and throwaway the chaff. Expect the technical content to very very largely be correct. (just like technical content from me :-) - very very largely correct, sometimes rubbish for whatever reason just to keep you alert :-) ). > * just to name one: > http://www.allaboutcircuits.com/vol_1/chpt_15/2.html Good attempt, as above. . It shows you tried. Compare several sources and find ones which expressly address the point being studied. ie in this case energy stored is not actually addressed even though they imply a relationship. > You remain ... > I wonder if you ... Your anger is understood BUT please read the FAQ and list rules and use the appropriate channels. Nobody is above proper behavior requirements, but there are appropriate ways to complain set out in the rules. Personal slanging matches on line often end up with the person complaining getting more hurt than the person being complained about. Really. > Welcome to my killfile Olin, it's the right tool for the job. That would probably be a mistake on your part if you want to learn. ADMIN HAT ON: ALL: If you MUST continue anything on the personal aspects of this and other matters DO IT IN [OT]. Even there it may, at this rate, end up causing you to be subject to admin action if it relates to mothers wearing army boots or anything approaching ad hominem attack. Hat off ... Russell