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Estimating the Number of Turns of an Inductor
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. But I haven't tried this, and I'm wondering if any other person has, and if it's a not unreasonably accurate[1] way to guesstimate the turns, or if it is prone to a large amount of error. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. So has anyone played around with this contrivance? [1] A not uncommon journalistic contrivance nowadays; seems like these authors just uncan stop not undoing this, and have unremembered to not undo it the old fashioned way, and just say "common". -- @@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/e...s/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 at hotmail.com Don't be ripped off by the big book dealers. Go to the URL that will give you a choice and save you money(up to half). http://www.everybookstore.com You'll be glad you did! Just when you thought you had all this figured out, the gov't changed it: http://physics.nist.gov/cuu/Units/binary.html @@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@ |
Estimating the Number of Turns of an Inductor
"Watson A.Name - "Watt Sun, the Dark Remover"" wrote
in message ... Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I was thinking a solenoid type...obviously you cracked it in half then? I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Ok. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. If you snapped the core back together, the existing winding and your test winding would share a good proportion of the flux, as a result it will act as a good transformer. However, being open to the air, much of the field lines will be lost and you'll have a less than unity coupling coefficient. Depending on the frequency, size and turns you may also encounter trouble measuring it accurately due to parasitic capacitance in the windings. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. This would be much better because you can get a few turns around the core evenly in most cases. Donno about coupling but I imagine it's worse farther from the core, even though the turns still circle it fully. So has anyone played around with this contrivance? No, but it's a good idea if you can work around the coupling problems. If only I had an L meter... Tim -- Just remember, Man was made in God's image. Woman was created out of a rib, which, quite honestly, is a cheaper cut of meat." - toon Website: http://webpages.charter.net/dawill/tmoranwms |
Estimating the Number of Turns of an Inductor
On Fri, 4 Jun 2004 19:54:21 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" wrote: Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. But I haven't tried this, and I'm wondering if any other person has, and if it's a not unreasonably accurate[1] way to guesstimate the turns, or if it is prone to a large amount of error. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. So has anyone played around with this contrivance? [1] A not uncommon journalistic contrivance nowadays; seems like these authors just uncan stop not undoing this, and have unremembered to not undo it the old fashioned way, and just say "common". That is a not-unworthy observation. Given a common ferrite bobbin type inductor, you could apply a reasonably high frequency sinewave to the inductor, then wind a single-turn (or a few, maybe) sense winding over the existing winding, then measure the voltage ratio (excitation/sense) to get the turns ratio. This only works if the sense winding encompasses as much flux as the main winding, which won't be entirely true for a bobbin with air return path. It gets better if you can artificially close the gap between the ends of the bobbin with some sort of ferrite or transformer steel path, sort of a high-permeability c-clamp. Hmmm... maybe it's better to apply an external magnetic field to the thing to get the ratio. That may make it more likely that the sense coil encompasses the same flux as the main coil. Probably so. For a torroid of non-silly permeability, this voltage ratio thing just works. John |
Estimating the Number of Turns of an Inductor
"Watson A.Name - "Watt Sun, the Dark Remover"" wrote
in message Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. Wind turns around it, as you've said. Then drive the unknown core with some voltage at a high enough frequency that it can actually develop some voltage (so you can measure it); detect and measure the voltage at the secondary, (I say detect - depending on what freq. you use. I don't know the freq. response of a typical DVM), and the ratio is the ratio. :-) It shouldn't matter if it's a little lossy, because the turns ratio is the turns ratio, and the DVM is hi-impedance, right? Cheers! Rich |
Estimating the Number of Turns of an Inductor
"Watson A.Name -in message Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. ** One you have got that far you have constructed a transformer. Drive some AC current into the original inductor's winding ( from an audio generator or similar) and measure the AC voltage on it and on the overwind you created. The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. The same method can be used to discover the number of turns in the windings of a toroidal transformer or any transformer where you can place a small overwind. ............. Phil |
Estimating the Number of Turns of an Inductor
"Watson A.Name - "Watt Sun, the Dark Remover"" wrote
in message In fact, I'm willing to bet real money that if you just do the turns ratio by volts, that you'll get an integer answer. Or 1/integer, don't be a smartass. ;-) I'll bet $100.00 it's within +- 20% of the nearest integer (or reciprocal, if you're doing it upside down), $10.00 that it's with +-10%, $5.00 for +- 5%, and if it's within 1%, we should both win. :-) Cheers! Rich |
Estimating the Number of Turns of an Inductor
So my first obvious question is, why would you care? If you want to
duplicate the inductor, you already know the inductance, and you can measure saturation effects and even loss, with some ingenuity. But playing along with your request, if you can wind turns around the existing coil, you also have made a transformer. To the extent the two windings share a common magnetic field, they will be coupled. You can, in fact, measure the leakage inductances and come up with quite a good model, and I suppose from that you can deduce the number of turns fairly accurately, especially if the coupling is good (and the leakage inductance small compared with the coupled inductance) as it would be with a ferrite toroid or a pot core or such. Cheers, Tom "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ... Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. But I haven't tried this, and I'm wondering if any other person has, and if it's a not unreasonably accurate[1] way to guesstimate the turns, or if it is prone to a large amount of error. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. So has anyone played around with this contrivance? [1] A not uncommon journalistic contrivance nowadays; seems like these authors just uncan stop not undoing this, and have unremembered to not undo it the old fashioned way, and just say "common". -- @@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/e...s/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 at hotmail.com Don't be ripped off by the big book dealers. Go to the URL that will give you a choice and save you money(up to half). http://www.everybookstore.com You'll be glad you did! Just when you thought you had all this figured out, the gov't changed it: http://physics.nist.gov/cuu/Units/binary.html @@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@ |
Estimating the Number of Turns of an Inductor
If it is an air-cored inductor, calculate the number of turns from its
measured dimensions. This won't work with a ferrite core because its material permeability is not known. Although if the core is a simple rod the effective permeability is roughly 25 regardless of material permeability. With a high permeability core, 100 or more, effective permeability becomes a function only of the very long 'air gap'. So inductance stops increasing with increasing core material permeabilty. But why would you want to know the number of turns if the coil is already wound! ---- Reg. |
Estimating the Number of Turns of an Inductor
On a sunny day (Fri, 4 Jun 2004 19:54:21 -0700) it happened "Watson A.Name -
\"Watt Sun, the Dark Remover\"" wrote in : Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. But I haven't tried this, and I'm wondering if any other person has, and if it's a not unreasonably accurate[1] way to guesstimate the turns, or if it is prone to a large amount of error. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. So has anyone played around with this contrivance? [1] A not uncommon journalistic contrivance nowadays; seems like these authors just uncan stop not undoing this, and have unremembered to not undo it the old fashioned way, and just say "common". If you can add turns, put ten turns, and 400Hz 1V for example. Measure voltage on original winding. If 30V it is 10 x 30 = 300 turns. JP |
Estimating the Number of Turns of an Inductor
What am I missing here? If you know the inductance of the original
coil, there are formulas that will tell you the number of turns. Wind a coil according to the formula, measure the inductance, and tweak the number of turns to get as close as you need to be. Bill ==================== Watson A.Name - "Watt Sun, the Dark Remover" wrote: Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. But I haven't tried this, and I'm wondering if any other person has, and if it's a not unreasonably accurate[1] way to guesstimate the turns, or if it is prone to a large amount of error. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. So has anyone played around with this contrivance? |
Estimating the Number of Turns of an Inductor
On Sat, 05 Jun 2004 03:16:28 GMT, "Rich Grise"
wrote: It shouldn't matter if it's a little lossy, because the turns ratio is the turns ratio, As long as the same flux traverses all the turns. John |
Estimating the Number of Turns of an Inductor
On a sunny day (Sat, 05 Jun 2004 08:55:27 -0700) it happened John Larkin
wrote in : On Sat, 05 Jun 2004 03:16:28 GMT, "Rich Grise" wrote: It shouldn't matter if it's a little lossy, because the turns ratio is the turns ratio, As long as the same flux traverses all the turns. John pepepepepepepedantic |
Estimating the Number of Turns of an Inductor
"Phil Allison" wrote in message ... "Watson A.Name -in message Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. ** One you have got that far you have constructed a transformer. Drive some AC current into the original inductor's winding ( from an audio generator or similar) and measure the AC voltage on it and on the overwind you created. The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. The same method can be used to discover the number of turns in the windings of a toroidal transformer or any transformer where you can place a small overwind. After reading several followups so far, I'm getting the picture that it would be easier to measure the voltage ratio. Rich suggested using a DVM, but IIRC their AC bandwidth is limited, and drops off above a few kHz or so. Rectifying the AC is an alternativce, but then it's not accurate if the .6V diode drop is a considerable part of the rectified DCV. An O'Scope seems the best way to measure, if it can be calibrated. Actually, come to think of it, all that's needed is the ratio, not the absolute V values. One thing that I had in mind when I originated this idea was that, say for instance, I'm measuring a trigger transformer for a xenon tube, where the number of turns could be thousands. If I wound a few tens of turns on it, the V ratio could be a hundred or more. That might be a bit more difficult to measure than the inductance. Thanks to all for the thoughtful responses. I'm going to have to try a few experiments to see how these work. ............ Phil |
Estimating the Number of Turns of an Inductor
"Reg Edwards" wrote in message ... If it is an air-cored inductor, calculate the number of turns from its measured dimensions. This won't work with a ferrite core because its material permeability is not known. Although if the core is a simple rod the effective permeability is roughly 25 regardless of material permeability. With a high permeability core, 100 or more, effective permeability becomes a function only of the very long 'air gap'. So inductance stops increasing with increasing core material permeabilty. Thanks for the interesting info. I would expect the core to be more of a bobbin. But when it's covered, it's not always certain. But why would you want to know the number of turns if the coil is already wound! If I don't know the number of turns to begin with, do you expect me to UNwind the coil to find the number of turns? As I said, the coil is usually covered or potted in epoxy. ---- Reg. |
Estimating the Number of Turns of an Inductor
"Bill Jeffrey" wrote in message ... What am I missing here? If you know the inductance of the original coil, there are formulas that will tell you the number of turns. Wind a coil according to the formula, measure the inductance, and tweak the number of turns to get as close as you need to be. Bill ==================== Okay, I have two identical adjustable core coils, one with the slug all the way in and the other all the way out. The Out one measures 100 uH and the In one measures 180 uh. I put both into a box, each with terminals to the outside, so that the physical coil can't be seen. Then I give them to you along with the inductance of each, and you tell me that, by your formulas, the Out one has a different number of turns than the In one???? Watson A.Name - "Watt Sun, the Dark Remover" wrote: Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. But I haven't tried this, and I'm wondering if any other person has, and if it's a not unreasonably accurate[1] way to guesstimate the turns, or if it is prone to a large amount of error. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. So has anyone played around with this contrivance? |
Estimating the Number of Turns of an Inductor
One way might be to in-case the coil in epoxy resin and saw it in half and simply count the windings.
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Estimating the Number of Turns of an Inductor
"Watson A.Name - "Watt Sun, the Dark Remover"" wrote in message ... Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. The voltage ratio of a transformer is the same as the turns ratio. Therefore, wind ten turns around the inductor and measure the voltage at the output when a known voltage is applied to the inductor. Norm Strong |
Estimating the Number of Turns of an Inductor
The voltage ratio of a transformer is the same as the turns ratio.
Therefore, wind ten turns around the inductor and measure the voltage at the output when a known voltage is applied to the inductor. Norm Strong ==================== Agreed. That's about the best he can manage. But what is not known is the coefficient of coupling between the two coils. They are not wound in the same volume of space or anywhere near to it. One is entirely outside the other. If the outside coil has a coefficient of coupling of 0.5 with the inside coil then it is equivalent to a coil with only half the number of turns. The arithmetic is simple. But what the coeff of coupling might be is anybody's guess without knowledge of ALL dimensions of BOTH coils. Ask your dentist if you could borrow his X-ray machine for the day. Even then a hefty treatise involving higher mathematics on how to calculate the coefficient of coupling between two coils would be essential. All one knows is that the turns error, possibly very large, must lie on the low side of the true value. Its just occurred to me that with access to a precision X-ray machine or electron microscope it may be possible actually to count the number of turns. Try NASA. How many Henrys is the thing anyway? === Reg. |
Estimating the Number of Turns of an Inductor
On Sat, 5 Jun 2004 21:11:09 +0000 (UTC), "Reg Edwards"
wrote: The voltage ratio of a transformer is the same as the turns ratio. Therefore, wind ten turns around the inductor and measure the voltage at the output when a known voltage is applied to the inductor. Norm Strong ==================== Agreed. That's about the best he can manage. But what is not known is the coefficient of coupling between the two coils. They are not wound in the same volume of space or anywhere near to it. One is entirely outside the other. If the outside coil has a coefficient of coupling of 0.5 with the inside coil then it is equivalent to a coil with only half the number of turns. The arithmetic is simple. But what the coeff of coupling might be is anybody's guess without knowledge of ALL dimensions of BOTH coils. Ask your dentist if you could borrow his X-ray machine for the day. Even then a hefty treatise involving higher mathematics on how to calculate the coefficient of coupling between two coils would be essential. All one knows is that the turns error, possibly very large, must lie on the low side of the true value. Its just occurred to me that with access to a precision X-ray machine or electron microscope it may be possible actually to count the number of turns. Try NASA. How many Henrys is the thing anyway? === Reg. A lot of our larger battery charger transformer designs have primaries wound outside with the secondaries closest to the core. It is quite a common technique, even on some of the smaller stuff we use. Peter -- Peter & Rita Forbes Engine pages for preservation info: http://www.oldengine.org/members/diesel |
Estimating the Number of Turns of an Inductor
A lot of our larger battery charger transformer designs have primaries
wound outside with the secondaries closest to the core. It is quite a common technique, even on some of the smaller stuff we use. Peter ================================= Of course they are. They do it all the time. Ever since the Victorian Age. But the primary-to-secondary coefficient of coupling, with the leakeage reactances, is accurately KNOWN from the start of the design. Certainly not the last. It's fundamental. But being at least acquainted with the things, I would have thought you already knew that. But students should not take me too seriously. I'm really a kindly person. === Reg |
Estimating the Number of Turns of an Inductor
"Reg Edwards" wrote in message
... Even then a hefty treatise involving higher mathematics on how to calculate the coefficient of coupling between two coils would be essential. I bet I could dig up some Fraday's law stuff from my physics textbook. It'd be a nasty integral to evaluate but would get you there. Tim -- "Just remember, Man was made in God's image. Woman was created out of a rib, which, quite honestly, is a cheaper cut of meat." - toon Website: http://webpages.charter.net/dawill/tmoranwms |
Estimating the Number of Turns of an Inductor
On Sat, 5 Jun 2004 13:17:08 +1000, "Phil Allison"
wrote: ** One you have got that far you have constructed a transformer. Drive some AC current into the original inductor's winding ( from an audio generator or similar) and measure the AC voltage on it and on the overwind you created. The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. As long as the same flux traverses all the turns. John |
Estimating the Number of Turns of an Inductor
"John Larkin" On "Phil Allison" ** One you have got that far you have constructed a transformer. Drive some AC current into the original inductor's winding ( from an audio generator or similar) and measure the AC voltage on it and on the overwind you created. The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. As long as the same flux traverses all the turns. ** That is not a very helpful remark. The suggestion was that the overwind be around the existing coil of the inductor * PLUS * there is no load on the overwind so leakage inductance is irrelevant. ................ Phil |
Estimating the Number of Turns of an Inductor
"Reg Edwards" But the primary-to-secondary coefficient of coupling, with the leakeage reactances, is accurately KNOWN from the start of the design. ** Leakage reactance is irrelevant with a no load test. ............ Phil |
Estimating the Number of Turns of an Inductor
As has been pointed out in other postings to the thread, the
coefficient of coupling is important. Whatever flux from the primary (driven winding) does not couple to the secondary will not induce voltage in the secondary, and the measured turns ratio will be low as a result. However, by measuring the inductance of the primary when the secondary is open and again when it is shorted, and doing the same with the secondary, you can find the leakage inductances and therefore the coefficient of coupling, fairly accurately. (The second measurement is really a check for consistency.) No need for xrays. You could further improve the accuracy, I suppose, by including a resistance value for each winding; ideally it would be the AC resistance at the operating frequency. It will probably make for easier calculations if you load the secondary very lightly for the measurement. But I'm still not seeing any need to know the number of turns, other than for idle curosity. "I need to know because I want to"?? Cheers, Tom "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ... .... After reading several followups so far, I'm getting the picture that it would be easier to measure the voltage ratio. Rich suggested using a DVM, but IIRC their AC bandwidth is limited, and drops off above a few kHz or so. Rectifying the AC is an alternativce, but then it's not accurate if the .6V diode drop is a considerable part of the rectified DCV. An O'Scope seems the best way to measure, if it can be calibrated. Actually, come to think of it, all that's needed is the ratio, not the absolute V values. One thing that I had in mind when I originated this idea was that, say for instance, I'm measuring a trigger transformer for a xenon tube, where the number of turns could be thousands. If I wound a few tens of turns on it, the V ratio could be a hundred or more. That might be a bit more difficult to measure than the inductance. Thanks to all for the thoughtful responses. I'm going to have to try a few experiments to see how these work. ............ Phil |
Estimating the Number of Turns of an Inductor
"Jan Panteltje" wrote in message
s.com... On a sunny day (Sat, 05 Jun 2004 08:55:27 -0700) it happened John Larkin wrote in : On Sat, 05 Jun 2004 03:16:28 GMT, "Rich Grise" wrote: It shouldn't matter if it's a little lossy, because the turns ratio is the turns ratio, As long as the same flux traverses all the turns. John pepepepepepepedantic Well, it does make a difference. I learned something today! Guess I can go back to bed. :-) Cheers! Rich |
Estimating the Number of Turns of an Inductor
On Sun, 6 Jun 2004 13:23:28 +1000, "Phil Allison"
wrote: The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. As long as the same flux traverses all the turns. ** That is not a very helpful remark. But it's true. There's not a lot of sense pretending you can measure something if you can't. The suggestion was that the overwind be around the existing coil of the inductor * PLUS * there is no load on the overwind so leakage inductance is irrelevant. Leakage inductance means exactly that the same flux does *not* thread all turns. So the unloaded voltage induced into the sense winding will be less volts/turn than the main coil. This is the likely situation for a drum core with a large air return path; some of the return flux will sneak back *inside* the sense coil. As the sense winding gets bigger in diameter, its signal level tends to zero, loaded or not. John |
Estimating the Number of Turns of an Inductor
"John Larkin" "Phil Allison" The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. As long as the same flux traverses all the turns. ** That is not a very helpful remark. But it's true. ** It is *unhelpful* because it is so damn ambiguous. There's not a lot of sense pretending you can measure something if you can't. ** It makes less sense to scorn a perfectly practical test method. The suggestion was that the overwind be around the existing coil of the inductor * PLUS * there is no load on the overwind so leakage inductance is irrelevant. Leakage inductance means exactly that the same flux does *not* thread all turns. So the unloaded voltage induced into the sense winding will be less volts/turn than the main coil. This is the likely situation for a drum core with a large air return path; some of the return flux will sneak back *inside* the sense coil. ** The overwind is to be around the existing coil, wound in parallel and on top of it, touching it - is that hard to comprehend ? A further ( rather obvious) condition is that the inductor coil current for the test be low enough to not generate a significant voltage drop across the coil's resistance - or you calculate that drop and take it into account. As the sense winding gets bigger in diameter, its signal level tends to zero, loaded or not. ** I just took a small mains toroidal ( 30VA) and with the primary energised at 230 volts passed a one turn loop through the core and measured 0.102 volts rms across the ends. The loop could be made as open as you liked or tight wrapped as you liked with NO change in the measured voltage. The primary magnetising current was only 1.5 mA and the primary resistance was 94 ohms - so a negligible primary drop of 140 mV. So I make the primary turns to be 2255 ( +/- the AC voltmeter's 0.3 % error, or about 7 turns) ............. Phil |
Estimating the Number of Turns of an Inductor
In article ,
Tom Bruhns wrote: As has been pointed out in other postings to the thread, the coefficient of coupling is important. Whatever flux from the primary (driven winding) does not couple to the secondary will not induce voltage in the secondary, and the measured turns ratio will be low as a result. However, by measuring the inductance of the primary when the secondary is open and again when it is shorted, and doing the same with the secondary, you can find the leakage inductances and therefore the coefficient of coupling, fairly accurately. That method of measuring the leakage inductance (by shorting windings) gives a hint towards a possible experimental method.... Short the sec with an ammeter and treat the thing as a CT. After all, CT's have a current-ratio that is quite close to the turns-ratio, even though the coupling can be poor (as in a CT with a bar primary). This is because the leakage inductance (and R-primary) can be regarded as being in series with a constant current stimulus source. The major source of error is then the sideways current due to the shunt loss. So perhaps do a short-circuit current-ratio test, then measure the sideways shunt-current taken by just the primary, at the same equivalent voltage. -- Tony Williams. |
Estimating the Number of Turns of an Inductor
"Phil Allison" wrote in message
... "John Larkin" Leakage inductance means exactly that the same flux does *not* thread all turns. So the unloaded voltage induced into the sense winding will be less volts/turn than the main coil. This is the likely situation for a drum core with a large air return path; some of the return flux will sneak back *inside* the sense coil. ** I just took a small mains toroidal ( 30VA) and with the primary energised at 230 volts passed a one turn loop through the core and measured 0.102 volts rms across the ends. The loop could be made as open as you liked or tight wrapped as you liked with NO change in the measured voltage. The difference, of course, is that in a toroid, all the flux is constrained to the core, so it'll work every time. As Mr. Larkin pointed out, since the winding in question is on a bobbin, and would go in a cup core or pot core, you would, in fact, lose leakage flux. So the problem does become kinda non-trivial. But I'm thinking some kind of temporary core, a la amprobe or some UI core from the junk box, but then you're getting into Rube Golberg stuff. Cheers! Rich |
Estimating the Number of Turns of an Inductor
Watson A.Name - "Watt Sun, the Dark Remover" wrote:
"Bill Jeffrey" wrote in message ... What am I missing here? If you know the inductance of the original coil, there are formulas that will tell you the number of turns. Wind a coil according to the formula, measure the inductance, and tweak the number of turns to get as close as you need to be. Bill ==================== Okay, I have two identical adjustable core coils, one with the slug all the way in and the other all the way out. The Out one measures 100 uH and the In one measures 180 uh. I put both into a box, each with terminals to the outside, so that the physical coil can't be seen. Then I give them to you along with the inductance of each, and you tell me that, by your formulas, the Out one has a different number of turns than the In one???? No, I'm saying that you take the slug all the way out, and the bobbin off the pot core/cup core, so you have an air core coil. Measure the inductance and plug it into the formula. (You did say that it's wound on a bobbin, which usually implies that you can get the bobbin off the ferrite.) There are many formulas for calculating inductance. All of them admit to being approximations - but that's all you need. For example: "For a coil of rectangular cross-section, of thickness t inches, length l inches and mean diameter (average of inside and outside) d inches, Hazletine's formula is L = 0.8d^2N^2 /(12d + 36l + 40t) uH" Now if your entire coil, including the ferrite, is potted in epoxy, it is a different situation. But I don't see that in any of your posts. Bill |
Estimating the Number of Turns of an Inductor
"Phil Allison" wrote in message ...
"Reg Edwards" But the primary-to-secondary coefficient of coupling, with the leakeage reactances, is accurately KNOWN from the start of the design. ** Leakage reactance is irrelevant with a no load test. You'll get a lot of disagreement with that; even experiments will disagree with you. (One way to think about it is that the applied primary voltage is split between the leakage inductance and the perfectly-coupled inductance of the model. There's no drop across the secondary's leakage inductance, but there for sure is across the primary's leakage inductance.) But as Tony W. so kindly pointed out, it's much less important if you use a short-circuit (current ratio) test. Cheers, Tom |
Estimating the Number of Turns of an Inductor
On Sun, 6 Jun 2004 15:31:40 +1000, "Phil Allison"
wrote: "John Larkin" "Phil Allison" The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. As long as the same flux traverses all the turns. ** That is not a very helpful remark. But it's true. ** It is *unhelpful* because it is so damn ambiguous. There's not a lot of sense pretending you can measure something if you can't. ** It makes less sense to scorn a perfectly practical test method. The suggestion was that the overwind be around the existing coil of the inductor * PLUS * there is no load on the overwind so leakage inductance is irrelevant. Leakage inductance means exactly that the same flux does *not* thread all turns. So the unloaded voltage induced into the sense winding will be less volts/turn than the main coil. This is the likely situation for a drum core with a large air return path; some of the return flux will sneak back *inside* the sense coil. ** The overwind is to be around the existing coil, wound in parallel and on top of it, touching it - is that hard to comprehend ? Not a bit. But the outer coil has - surprise! - a bigger diameter than the inner, so more of the return flux is flowing *inside* the sense coil, in the direction that reduces the induced voltage. Given a typical drum/bobbin type inductor, I'd guess that the resulting error might be in the 50% sort of turf; the actual error depends on the geometry of the windings, and how close the sense winding can actually get, given the insulation or epoxy or whatever cited. A further ( rather obvious) condition is that the inductor coil current for the test be low enough to not generate a significant voltage drop across the coil's resistance - or you calculate that drop and take it into account. Up to saturation - and an drum core will usually vaporize before it saturates - the voltage ratio, whatever it is, will be independent of drive level; the coupling is linear. A high drive *frequency* will limimize the effects of copper loss, although it can be approximately accounted for. As the sense winding gets bigger in diameter, its signal level tends to zero, loaded or not. ** I just took a small mains toroidal ( 30VA) and with the primary energised at 230 volts passed a one turn loop through the core and measured 0.102 volts rms across the ends. The loop could be made as open as you liked or tight wrapped as you liked with NO change in the measured voltage. With a closed, high-permeability core, voltage ratios can track turns ratios to a part per million, as in a precision AC ratio box. Since virtually all the flux is concentrated in the steel, any loop of any size pretty much slices the same amount of flux. A torroid is ideal for close coupling. That's not the case with a system dominated by air gap, because the flux is scattered all over in space. The primary magnetising current was only 1.5 mA and the primary resistance was 94 ohms - so a negligible primary drop of 140 mV. So I make the primary turns to be 2255 ( +/- the AC voltmeter's 0.3 % error, or about 7 turns) Transformar manufacturers routinely use DVM-looking gadgets that indicate turns ratio, and can easily and accurately resolve whole or half turns. But only when leakage inductance is low, as for a closed, high-mu core with tightly-coupled windings. John |
Estimating the Number of Turns of an Inductor
|
Estimating the Number of Turns of an Inductor
It's interesting, when I learned this stuff ( I won't tell you when, but my
then-new text was published in 1935!), albeit in the context of utility/power engineering, about the LAST thing we learned was the tricks and conventions about turns ratios, etc. Just looking at a question in my book: "Assuming a coil of thus & so dimensions surrounding a core of this & that dimension & type of material, calculate: 1) The flux in the core, 2) the flux in the air," etc. Follow-up question: "Assuming a second identical coil placed elsewhere on the core, calculate induced voltage if only the flux in the iron passes through the second coil," etc. The whole text was written like that. The concept of a "perfect transformer" was introduced much later, and only in certain contexts. For utility purposes, perfect transformers are undesirable! Kind of strange by today's standards; we were taught all the painful details right up front, and later allowed to throw out the ones that didn't apply. I think it's done the other way around today. Oh, if anyone cares, book cited is "Alternating Current Machinery," Bryant & Johnson, 1935. "John Larkin" wrote in message ... On Sun, 6 Jun 2004 15:31:40 +1000, "Phil Allison" wrote: "John Larkin" "Phil Allison" The turns ratio and the (unloaded) voltage ratio you measure are in exact proportion. As long as the same flux traverses all the turns. ** That is not a very helpful remark. But it's true. ** It is *unhelpful* because it is so damn ambiguous. There's not a lot of sense pretending you can measure something if you can't. ** It makes less sense to scorn a perfectly practical test method. The suggestion was that the overwind be around the existing coil of the inductor * PLUS * there is no load on the overwind so leakage inductance is irrelevant. Leakage inductance means exactly that the same flux does *not* thread all turns. So the unloaded voltage induced into the sense winding will be less volts/turn than the main coil. This is the likely situation for a drum core with a large air return path; some of the return flux will sneak back *inside* the sense coil. ** The overwind is to be around the existing coil, wound in parallel and on top of it, touching it - is that hard to comprehend ? Not a bit. But the outer coil has - surprise! - a bigger diameter than the inner, so more of the return flux is flowing *inside* the sense coil, in the direction that reduces the induced voltage. Given a typical drum/bobbin type inductor, I'd guess that the resulting error might be in the 50% sort of turf; the actual error depends on the geometry of the windings, and how close the sense winding can actually get, given the insulation or epoxy or whatever cited. A further ( rather obvious) condition is that the inductor coil current for the test be low enough to not generate a significant voltage drop across the coil's resistance - or you calculate that drop and take it into account. Up to saturation - and an drum core will usually vaporize before it saturates - the voltage ratio, whatever it is, will be independent of drive level; the coupling is linear. A high drive *frequency* will limimize the effects of copper loss, although it can be approximately accounted for. As the sense winding gets bigger in diameter, its signal level tends to zero, loaded or not. ** I just took a small mains toroidal ( 30VA) and with the primary energised at 230 volts passed a one turn loop through the core and measured 0.102 volts rms across the ends. The loop could be made as open as you liked or tight wrapped as you liked with NO change in the measured voltage. With a closed, high-permeability core, voltage ratios can track turns ratios to a part per million, as in a precision AC ratio box. Since virtually all the flux is concentrated in the steel, any loop of any size pretty much slices the same amount of flux. A torroid is ideal for close coupling. That's not the case with a system dominated by air gap, because the flux is scattered all over in space. The primary magnetising current was only 1.5 mA and the primary resistance was 94 ohms - so a negligible primary drop of 140 mV. So I make the primary turns to be 2255 ( +/- the AC voltmeter's 0.3 % error, or about 7 turns) Transformar manufacturers routinely use DVM-looking gadgets that indicate turns ratio, and can easily and accurately resolve whole or half turns. But only when leakage inductance is low, as for a closed, high-mu core with tightly-coupled windings. John |
Estimating the Number of Turns of an Inductor
In
sci.electronics.design,sci.electronics.repair,sci. electronics.components,alt.binaries.schematics.ele ctronic, "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote: "Reg Edwards" wrote in message ... But why would you want to know the number of turns if the coil is already wound! If I don't know the number of turns to begin with, do you expect me to UNwind the coil to find the number of turns? You didn't answer the question. WHY do you want to find the number of turns? Okay, I'll answer for you. Reverse engineering. You want to make one or more coils exactly like it. Of course, not only do you need the number of turns (and the exact layout of the turns), you also need to know exactly what the magnetic core material is - you can either ask the manufacturer (of either the core or the coil), or measure its physical size and test all its magnetic properties. An inductor is one of the easier components to make in the "home laboratory" and it's good to know you can look up formulas and stuff (the ARRL handbook, at least older editions, has some useful coolbook formulas for cylindrical coils) for those times when you need it ASAP and can't wait for overnight delivery, but otherwise it's still cheaper to buy than to build. As I said, the coil is usually covered or potted in epoxy. ---- Reg. ----- http://mindspring.com/~benbradley |
Estimating the Number of Turns of an Inductor
OK, let's suggest something different.
1. Measure the diameter of the wire in the existing coil. 2. Measure the DC resistance of the existing coil, and calculate the length of the wire needed to generate that resistance. 3. Figure out how many turns will use up that length of wire. You did say ESTIMATE, did you not? ??!!?? "Watson A.Name - "Watt Sun, the Dark Remover"" wrote in message ... Suppose that I have an inductor that's covered with epoxy or similar that prevents me from seeing or finding out how many turns of wire are on the core. The core is open, so that it's uncovered and most of the magnetic field is outside outside of the inductor. Obviously it's a bobbin type core. I have measured the inductor with an inductance meter, so I know what the inductance and other parameters are. Suppose I take some wire, say roughly small if the inductor is small, and wind it around the inductor, over the existing windings so that it's within the magnetic field. I wind enough wire onto the inductor so that I get about 1/9, or 1/16 or 1/25 the inductance in the new coil. Since the inductance is the square of the turns, I can say that if I have wound 10 turns and the inductance is 1/16th that of the original coil, then the turns ratio is 4 to 1, so the original coil is about 40 turns. Obviously the Real WOrld kicks in, and things may not always be exactly as they should be. But I haven't tried this, and I'm wondering if any other person has, and if it's a not unreasonably accurate[1] way to guesstimate the turns, or if it is prone to a large amount of error. I guess it would also apply to a toroid if there is enough room to loop some wire thru the center hole, but this hole may be filled or covered up. So has anyone played around with this contrivance? [1] A not uncommon journalistic contrivance nowadays; seems like these authors just uncan stop not undoing this, and have unremembered to not undo it the old fashioned way, and just say "common". -- @@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/e...s/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 at hotmail.com Don't be ripped off by the big book dealers. Go to the URL that will give you a choice and save you money(up to half). http://www.everybookstore.com You'll be glad you did! Just when you thought you had all this figured out, the gov't changed it: http://physics.nist.gov/cuu/Units/binary.html @@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@ |
Estimating the Number of Turns of an Inductor
On Sun, 6 Jun 2004 16:30:39 -0400, "BFoelsch"
wrote: It's interesting, when I learned this stuff ( I won't tell you when, but my then-new text was published in 1935!), albeit in the context of utility/power engineering, about the LAST thing we learned was the tricks and conventions about turns ratios, etc. Just looking at a question in my book: "Assuming a coil of thus & so dimensions surrounding a core of this & that dimension & type of material, calculate: 1) The flux in the core, 2) the flux in the air," etc. Follow-up question: "Assuming a second identical coil placed elsewhere on the core, calculate induced voltage if only the flux in the iron passes through the second coil," etc. I had to take a year of Electrical Machinery in college, including labs with big transformers and motors and stuff. I learned a lot from it. The whole text was written like that. The concept of a "perfect transformer" was introduced much later, and only in certain contexts. For utility purposes, perfect transformers are undesirable! Is that because they conduct short circuits too well? John |
Estimating the Number of Turns of an Inductor
On Sun, 06 Jun 2004 14:32:46 -0700, John Larkin
wrote: [snip] I had to take a year of Electrical Machinery in college, including labs with big transformers and motors and stuff. I learned a lot from it. [snip] John Same here. Now-a-days I think they just point at a motor and grunt, judging from what little today's engineering graduates seem to know :-( ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |
Estimating the Number of Turns of an Inductor
"John Larkin" wrote in message ... On Sun, 6 Jun 2004 16:30:39 -0400, "BFoelsch" wrote: It's interesting, when I learned this stuff ( I won't tell you when, but my then-new text was published in 1935!), albeit in the context of utility/power engineering, about the LAST thing we learned was the tricks and conventions about turns ratios, etc. Just looking at a question in my book: "Assuming a coil of thus & so dimensions surrounding a core of this & that dimension & type of material, calculate: 1) The flux in the core, 2) the flux in the air," etc. Follow-up question: "Assuming a second identical coil placed elsewhere on the core, calculate induced voltage if only the flux in the iron passes through the second coil," etc. I had to take a year of Electrical Machinery in college, including labs with big transformers and motors and stuff. I learned a lot from it. The whole text was written like that. The concept of a "perfect transformer" was introduced much later, and only in certain contexts. For utility purposes, perfect transformers are undesirable! Is that because they conduct short circuits too well? Primarily. In olden times transformers were designed for the best possible "regulation." After power systems got stiff enough, however, the concept was changed to allow transformers to limit fault current, and the old measure of "regulation" was replaced with the modern day "impedance." Extra credit: What is the definition of impedance, in the transformer sense? Answer: The percentage of rated primary voltage which will produce rated current through a short-circuited secondary. John |
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