Hi Ted,The resonance graph was for Terry Fritz MOT used in the Piranha 2 circuit he developed. This was a graph showing resonance in rms volt units. The input was 120Vrms and it was 60Hz. This is of course different than Herwig who is building the same circuit but using a MOT of 230Vrms at 50Hz.
Below, I'll show how I determine L2 and this is not to be some math forum, but simply to show how I go about this:
For Terry's MOT which is 65H at L2 (per Terry), your stating 4.7H, so there is a large error here. I understand your solving for a resonant frequency formula for what L would need given Cres. Looking at L = 1/(4pi^2 x f^2 x C), you should have shown 21.3H (120V, 60Hz) if you were using 330nF for C (but as C is 2X due to the configuration in the circuit, 660nF is real, so 10.7H). This is "not" the correct inductance of L2 (but fine if it's not a transformer).
For L2, we multiply the inductance ratio by L1 inductance. The inductance ratio is the turns ratio squared. So it's "imperative" to determine the turns ratio.
Turns ratio = measured Vsec/Vpri. For Terry's MOT, use eg. 2100/120 = 19.167 (turns ratio). This measurement should be performed at about 1/3 to 1/2 input voltage so that the magnetizing inductance does not inhibit an accurate turns ratio reading. My sec value is just an example.
Inductance ratio = 19.167^2 = 367. To find the primary inductance, we need the primary reactance: Primary reactance = (measured) applied voltage/current.So we need to measure the primary current at 120V applied (with the secondary in open circuit). Assume we measured 1.8A (this is only an assumption and Terry likely measured less for the MOT he used).
Primary reactance = 120/1.8 = 66.67 ohms.Primary Inductance = reactance/(2pif) = 66.67/(6.28 x 60) = 0.1768 = L1 = 176.8mH Secondary Inductance = Inductance ratio x L1 = 367 x .1768 = 64.89 = L2 = 65H
Terry's MOT actually showed 240mH for L1 with a k of 0.963. The transformer model in the simulations (Microsim/Orcad type) require a coupling value, and so we measure that also (which should be measured at expected primary voltage for driving the coil). Granted, this is a linear transformer model and MOT's are far from linear with a poor power factor, but one thing I've learned from simulations is that if you have the model correct, the simulated output is correct (it's getting the model correct that's the bugger as well as how you drive the coil as compared to what you told the model you would do). I showed a graph in an earlier post regarding measured coupling of my particular MOT. Note how coupling changes depending on the driving voltage. One could easily drive 100Vin to 140Vinput. Coupling thus changes 0.983 down to 0.967 and even lower at 140V (if it follows the coupling graphs I've made for NST's. And no reason it shouldn't). This does change the situation significantly in some cases.
http://www.classictesla.com/temp/mot-k.gifAnyway, the #1 difference here is that the "measured" primary current and voltages are used to determine the primary reactance and thus the inductances. And yes, never trust a meter for L on a transformer. You will actually get a near inductance ratio even with the very low inductances measured, but it is still far off enough to be of no use and the inductances themselves are certainly very low and unreal.
Best regards, Bart tesla wrote:
Greetings Team Well in the fist instance I used the resonance graph published recently which showed 60Hz resonance with a capacitor of just under 1.5uF. (close to the value found in an operating MOT, no coincidence here I suspect) I just solved f=1/(4pi^2f^2C) with 60Hz C=1.5E-6 To give that 4.7Hy even if 50Hz environment still close to that value, would not explain the 20 to 65Hy quoted below, I wonder what conditions this L is measured at, I try to measure at high currents in the secondary (ie lots of excitation to test it) so as to test it close to operating conditions. My simple L meters measure around 40 to 100Hy on a secondary but I do not believe those results In the past as I'd said I've measured MOTs (600 watt type) with Pri SC I got 6Hy looking back into the secondary, I'll re-check tonite as it was a while ago and advise results giving excitation voltage and resultant current. Given the magnetising current required I long ago gave up using a meter to measure large iron cored trannies and I'm not sure of the test freq either. Best Ted in NZMessage ----- From: "bartb" <bartb@xxxxxxxxxxxxxxxx>To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx> Sent: Monday, January 05, 2009 2:23 PM Subject: Re: [TCML] MOT MeasurementsHi Ted, Not that high. Terry's MOT measured 65H at L2. My own MOT measured 39.5H (although my MOT is a bit beefy compared). I would expect Herwigs MOT to be no lower than 20H. Your calculated value of 4.7uH is extremely low. How are you basing this value? Herwig is 230V 50Hz power environment. Are you assuming a 120V 60Hz situation? Take care, Bart tesla wrote:Team I noted the Value of Secondary L here of 30.4 Hy I believe this is rather higher than I would expect. Resonance at 1.5uF/60Hz puts this figure down to around 4.7Hy which is in close agreement with experiments I've done (magnetic shunts intact). Measured Secondary L with Pri Shorted (voltage generator source situation as per mains input) and Secondary driven with a large 50Hz AC voltage source and current measured technique Thanks, enjoying the dialogue on this matter Ted in NZ SNIP- I am not so experienced in simulations as you are. Therefore I would like to accept your generous offer to run the simulation with my MOT data: no-load-output-voltage = 2232 Vrms, dc-resistance of secondary winding = 70 Ohms, calculated secondary inductance = 30,4 H, all the other parts were copied from Piranha II. Take care, Herwig_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla---------------------------------------------------------------------------- ---- No virus found in this incoming message. Checked by AVG - http://www.avg.com Version: 8.0.176 / Virus Database: 270.10.2/1872 - Release Date: 1/2/2009 1:10 PM _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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