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#61
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Bill Turner wrote:
You are speaking in *practical* terms, which is fine. It's true that at relatively low frequencies, well below the self-resonant point, coils appear to have constant inductance. No argument there. The discussion came about because someone asserted that inductance was a constant, REGARDLESS of frequency, and that is just not true. I disagree. The inductive component of the impedance remains essentially constant through resonance. What is non ideal about the inductor is that it does not exhibit just inductance, but a parallel combination if inductance and capacitance. Ignoring the capacitance and calling the effect variable inductance is just not as accurate a way to describe what is going on. -- John Popelish |
#62
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Bill Turner wrote:
That will work, no doubt. My point was that it takes some serious engineering and careful testing; you can't just wrap some wire on a form and expect it to work correctly across a wide range of frequencies. This is a generality I can agree with. -- John Popelish |
#63
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Bill Turner wrote:
That will work, no doubt. My point was that it takes some serious engineering and careful testing; you can't just wrap some wire on a form and expect it to work correctly across a wide range of frequencies. This is a generality I can agree with. -- John Popelish |
#64
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Bill Turner wrote:
On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote: Write on the whiteboard 100 times: Inductance does not change with frequency...reactance changes with frequency. __________________________________________________ _______ Not true. Inductance and reactance are related by the formula XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and vice versa. How could you possibly define it otherwise? But the impedance of a coil near resonance is not well described as an XsubL. It is a combination of XsubL and XsubC, including their different phase shifts. You cannot just measure the magnitude of impedance of a coil and assume that you are measuring pure XsubL. You have to prove that this is the case by some other measurement, like the phase relationship between voltage and current. -- John Popelish |
#65
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Bill Turner wrote:
On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote: Write on the whiteboard 100 times: Inductance does not change with frequency...reactance changes with frequency. __________________________________________________ _______ Not true. Inductance and reactance are related by the formula XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and vice versa. How could you possibly define it otherwise? But the impedance of a coil near resonance is not well described as an XsubL. It is a combination of XsubL and XsubC, including their different phase shifts. You cannot just measure the magnitude of impedance of a coil and assume that you are measuring pure XsubL. You have to prove that this is the case by some other measurement, like the phase relationship between voltage and current. -- John Popelish |
#66
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Bill Turner writes:
On Sun, 07 Dec 2003 15:54:05 +0000, John Devereux wrote: No, you are talking about the *reactance* ("reactive impedance"). We have been talking about the *inductance* ! They are not the same thing. __________________________________________________ _______ No one ever said they were the same thing. They are related to each other by the formula XsubL = 2 pi F L. That is a direct, linear relationship. The important thing here is the "subL". It applies only to the inductive part of the overall reactance. Are you saying that formula is correct as some (low) frequency but incorrect at another (high) frequency? No, it is always correct. It is practically the *definition* of inductance so it had better be! I'll say it another way: Inductance and reactance are directly related to each other by the (2 pi F) factor. Given one (inductance or reactance) you can calculate the other. There is no other way. No. Because the "reactance" (without the sub-L) now has both inductive *and* capacitive terms. When you measure the *overall* reactance of a real life coil you are measuring the effect of *both* terms. You cannot measure this combined reactance and then just plug the number into a formula which ignores the capacitive part. You have to use the general formula which include the self capacitance. Ignoring the coil resistance (i.e. we have infinite Q) the correct formula is something like: Xtotal = 1 -------------- |1/Xc| - |1/Xl| Where Xc = 1/(2 pi F C) and Xl = 2 pi F L. Hopefully you can see how Xtotal behaves as you describe, even with constant L. -- John Devereux |
#67
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Bill Turner writes:
On Sun, 07 Dec 2003 15:54:05 +0000, John Devereux wrote: No, you are talking about the *reactance* ("reactive impedance"). We have been talking about the *inductance* ! They are not the same thing. __________________________________________________ _______ No one ever said they were the same thing. They are related to each other by the formula XsubL = 2 pi F L. That is a direct, linear relationship. The important thing here is the "subL". It applies only to the inductive part of the overall reactance. Are you saying that formula is correct as some (low) frequency but incorrect at another (high) frequency? No, it is always correct. It is practically the *definition* of inductance so it had better be! I'll say it another way: Inductance and reactance are directly related to each other by the (2 pi F) factor. Given one (inductance or reactance) you can calculate the other. There is no other way. No. Because the "reactance" (without the sub-L) now has both inductive *and* capacitive terms. When you measure the *overall* reactance of a real life coil you are measuring the effect of *both* terms. You cannot measure this combined reactance and then just plug the number into a formula which ignores the capacitive part. You have to use the general formula which include the self capacitance. Ignoring the coil resistance (i.e. we have infinite Q) the correct formula is something like: Xtotal = 1 -------------- |1/Xc| - |1/Xl| Where Xc = 1/(2 pi F C) and Xl = 2 pi F L. Hopefully you can see how Xtotal behaves as you describe, even with constant L. -- John Devereux |
#68
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On Sun, 07 Dec 2003 04:31:46 -0800, Bill Turner
wrote: On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen wrote: One can still argue that the inductance and inductive reactance are as well as the capacitance and the capacitive reactance are still there as separate entities, but we can not measure them separately from terminals of the coil. Thus, this is an artefact of the measurement method. Not only can you *not* measure them separately, they can not be physically separated either, since the parasitic capacitance is always present between adjacent windings. I would not call it an artifact of the measurement method, but rather an artifact of the coil itself. The problem with circuits containing both inductances and capacitances is that in one kind of reactance, there is a +90 degree phase shift and the other with -90 degree phase shift. Thus, when these are combined, they partially cancel each other, producing different magnitudes and some phase shift between -90 and +90 degrees. If only the resultant magnitude is used (and the resultant phase is ignored), this would give the false impression that the inductance changes with frequency. Instead of using the resultant reactance on some specific frequency, the inductance could be measured in a different way. When a DC current I is flowing through and inductance L, the energy stored in the inductance is W = I*I*L/2. This could be used to determine the inductance L. One way to measure the energy W would be to cut the DC current through L and after disconnecting I, dissipate the energy in some kind of integrating load across L. Even if there is a significant capacitance across L, no energy is initially stored in C, since during the steady state condition, the current I would be flowing through L, but there would be no voltage difference between the ends of L (assuming R=0), thus all energy in this parallel resonance circuit is stored in L. After disconnecting the DC current I, the energy would bounce back between L and C, but finally it would be dissipated by the external load. The same energy would be dissipated in the external load even if C did not exist (assuming zero losses). Thus using this measurement method, the value of L would be the same regardless if C is present or not. Thus, getting a frequency dependent L, is a measurement artifact in the method that you are using. Paul OH3LWR |
#69
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On Sun, 07 Dec 2003 04:31:46 -0800, Bill Turner
wrote: On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen wrote: One can still argue that the inductance and inductive reactance are as well as the capacitance and the capacitive reactance are still there as separate entities, but we can not measure them separately from terminals of the coil. Thus, this is an artefact of the measurement method. Not only can you *not* measure them separately, they can not be physically separated either, since the parasitic capacitance is always present between adjacent windings. I would not call it an artifact of the measurement method, but rather an artifact of the coil itself. The problem with circuits containing both inductances and capacitances is that in one kind of reactance, there is a +90 degree phase shift and the other with -90 degree phase shift. Thus, when these are combined, they partially cancel each other, producing different magnitudes and some phase shift between -90 and +90 degrees. If only the resultant magnitude is used (and the resultant phase is ignored), this would give the false impression that the inductance changes with frequency. Instead of using the resultant reactance on some specific frequency, the inductance could be measured in a different way. When a DC current I is flowing through and inductance L, the energy stored in the inductance is W = I*I*L/2. This could be used to determine the inductance L. One way to measure the energy W would be to cut the DC current through L and after disconnecting I, dissipate the energy in some kind of integrating load across L. Even if there is a significant capacitance across L, no energy is initially stored in C, since during the steady state condition, the current I would be flowing through L, but there would be no voltage difference between the ends of L (assuming R=0), thus all energy in this parallel resonance circuit is stored in L. After disconnecting the DC current I, the energy would bounce back between L and C, but finally it would be dissipated by the external load. The same energy would be dissipated in the external load even if C did not exist (assuming zero losses). Thus using this measurement method, the value of L would be the same regardless if C is present or not. Thus, getting a frequency dependent L, is a measurement artifact in the method that you are using. Paul OH3LWR |
#70
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On Sun, 7 Dec 2003 19:13:36 +0000, John Woodgate
wrote: Low-frequency iron-cored coils are quite another matter; the inductance varies with frequency, voltage, temperature, previous history and the state of the tide on Europa. I assume that you are referring to DC biased iron cores (without an air gap) or some high permeability ferrites with a strong DC bias current. These do indeed show a variation of inductance depending on the DC bias current. Paul OH3LWR |
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