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Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
there is no file unfortunately. its just a LUA-script. multiple scripts currently, because i am trying stuff. and some of the data analysis was shifted to python. its a mess. once i know what i am doing (or what must be done) i can clean things up. i currently look at the frequency dependence of the projectile force.
And i am thinking about bringing the frequency dependence into the model, but thats simply not possible. Simple frequency-stuff is easy with laplace sources, however their coefficients are not allowed to be time variant unfortunately. So i cant implement any dependence on projectile position or current..
Edit: I am extracing parameters @200Hz now. (Including core losses). AC-solving the issue is just sooooo sloow.. its already taking up 6h with a lower resolution density and its still at 75% -.- I just want to know if this AC-stuff (and core losses) is actually changing things nor not. So for now.. single frequency only. Then i need to utilize parallelisation. This could run 10x faster...
Yes, it would. I'm just trying to make sense out of FEMMs output. For a simulation the needed 3 parameters are coil inductance, coupling and L/R for the eddy current. I believe eddies are an important part of power loss. The high ESR as measured with the multimeter and also the experimentally found large damping are indicators. Eddies might not so much contribute to the force. Inductance and coupling depend on projectile position, L/R probably not.
Anyway, there are 2 ways to get parameters for the simulation. One is to extract them somehow out of FEMM, the other one are multimeter measurements. A measurement at a single frequency will only tell you 2 of the 3 parameters. A measurement at a second, different frequency is needed to get the complete set. You'd need to the measurements at all relevant projectile positions. What you also need, are the equations to get from the multimeter output to coupling and L/R and the equations for the simulation itself using these parameters.
What is kind of bad is that the unsaturated version at 1kHz predicts an Rp of 92Ohm for 200Turns. Not what i measured. (far off)
That is more than measured even for an empty coil. Alone considering DC coil resistance should give a lower value.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
The LCR-Meter measures either in parallel or series mode. See the picture below.
These measurements are worthless if you have a combination of ESR and CoreLoss. Because you measure either only CoreLoss or ESR. I tried to calculate CoreLosses from the phase shift (displayed by the LCR-Meter) And this is actually in the right ballpark.
You are right, the empty coil has allready a lower parallel resistance. However this measurement does not mean anything, Since the LCR-Meter assumed a dominant parallel resistance wher there was only a dominant ESR (without projhectile). To all you really know is that the phase shift was 85°. I mean.. all the LCR_Meter knows is the 85°.. so everything else it displays is just math. My math is more complex and therefore more correct.
I dont know why even cheap LCR-Meters dont do a DC-Measurement for the ESR to at least to display the right parallel resistance... quite pathetic. instead you have a "series mode" and a "parallel mode" which is never ever the case.
Anyway.. for me the only way to get the parameters is by simulation. Measuring them is way to unprecise. And my goal issnt really to match reality but to match a full blown FEM-Simulation with easier means and quick simulation time (once the data is collected). The side product is btw a good matching to reality. Curiously the refernce model does predict mechanics perfectly but electrical stuff poorly. The simulation efficiency is allways lower than measured.
Edit: The imulation for f=200Hz finished. Its results are bad. Much wose than expected. Also the core loss is estimated way too small. I dont really know whats going on. But the DC-Simulation had way better matching. And i can understand that actually, because its not a sinusoidal excitation in a coilgun, so results are different and there is actually magnetisation inside the projectile (which issnt there at AC 200Hz). So what i do now is to re-run the data extaction at 100mHz. This quasi-static frequency should be close do DC and still give me a CoreLoss i can work with. It should also simulate way faster. So lets see what comes out. The good thing is that is takes the whole AC-simulation out of the picture. There is no need for such results when they dont apply. A coilgun is even, if the frequency is quite high, a quasi static problem as it seems. Actually, in magnetically speeking this LC-Oscillation is already vcery much the worst case. In a real coilgun the magnetisatin is only in one direction and way more constant.
Edit2: The low frequency quasi static data extraction worked fine and it becomes closer to the reference simulation. Still i am not satisfied, because i still need to correct the Amp*Turns by a magic factor of [1-sqrt(Kp)] with Kp being the weird projectile coupling factor. I have no explaination for that, even if that worked really fine. I now extract a full blown parameter set.. mostly everything i could ever extract... including magnetic energy in the projectile, current inside the projectile , average magnetisation and Lorentz forces. I want to check out specially how the magnetisation can be low pass filtered and used to extract a correction factor out of it. Who knows. Maybe it works. At this stage its guessing, i admit.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Ok. Sry for double post but its kind of important. The new dataset gave me some nice stuff to play around and i can see e.g. that lowpass filtering the B-Field and just putting it in some way into the coil model does actually add new detail to the force curve so that we are even closer to the reference simulation. This being said, i must say i mean it only feature-wise. The general shape is more correct.. but quantitive values are very wrong. This is due to my more or less trial&error approach of building the model. So maybe making it based on math would make more sense.. but honestly i am not capable of that.
..so thats why this double post:
1) Eddy currents, Lorentz-Force I have extracted the resistance that the projectile poses as a core loss (resistor in parallel to the inductor). Given that the projectile and the coil are a shorted transformer i can calculate what current should be inside the projectile. That current (is in the kA-range) should create a Lorentz-force. I do even have a value for the Lorentz force but thats caluclated only for f=100mHz.. so the first 2 questions: 1a) how would you think the core losses behave with frequency? The higher the frequency, the less skindepth, the thinner the sheet of conductive material. Skindepth is ~1/sqrt(f).. so resistance should increase with sqrt(f).. maybe even more than that? 1b) how do Lorentz forces scale with frequency? Given the reference point of 100mHz.. can i extrapolate what Lorentz force there would be for higher frequency?
I am looking for a solution to substract around 70N (of 280N) off the pull force here. Not all of it must be Lorentz force but it can also be....
2) B-Field smoothing. The Eddy current inhibit the B-Field from changing rapidly creating a sort of "momentarily constant magnetzisation" for the projectile so that force is not F ~ I^2 anymore at all times even below saturation. If the current starts at zero and there is allready a magnetisation present in the projectile the force should be inherently larger like it would be if we would shoot a permanent magnet. My model gives me data about how magnetized the projectile is (should be; average field density inside´the projectile volume in Tesla). This however only applies to steady state (100mHz). 2a) Can we somehow derive from the lorenz force or whatever how to low pass filter the magnetisation? 2b) how to get this calculation into the model? Given a "current magnetised"-level and a "should be magnetized"-level how does it actually alter the acting force?
I think those are 4 complicated question that need dicussion. As for sharing the simulation and stuff... i can do if you wish, however this is a) hard to set up and get it running and b) without having the reference simulation you dont know if the stuff you try is correct. Maybe i could make screenshots of the reference case... so adding another question: shall i put together a .zip?
These measurements are worthless if you have a combination of ESR and CoreLoss. Because you measure either only CoreLoss or ESR. I tried to calculate CoreLosses from the phase shift (displayed by the LCR-Meter) And this is actually in the right ballpark.
I've obtained an equation for the complex resistance of the coil:
Rcoil = RcoilDC + j * w * Lcoil * (1 + w * k^2/(j*Rp/Lp-w))
The model is a primary coil coupled to a secondary Lp (projectile) loaded with a resistor Rp. On the primary side, there is a series resistance RcoilDC to account for the coils internal resistance. The part in brackets on the right side introduces an extra negative imaginary part, which implies a reduction of inductance and a positive real part, which shows up as an extra ESR. From a measurement at a single frequency, you can't distinguish between these contributions, but you can if there are several measurements made at at least 2 different frequencies.
Still i am not satisfied, because i still need to correct the Amp*Turns by a magic factor of [1-sqrt(Kp)] with Kp being the weird projectile coupling factor. I have no explaination for that, even if that worked really fine.
In the limit of a very low frequency, the equation won't change the measured inductance (imaginary part) so it is no surprise, that you need to include that in your simulation explicitly. The physical reason for this is, that the voltage induced in the projectile is so low, that it won't cause much current and affect the inductance of the primary.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Hi uspring I have doubts your formula applies: the reduction of inductance with higher frequencies could also be a consequence of skin depth so that the amount of active iron decreases. I am not sure a "projectile inductance" can be applied to a coilgun.. this is more something that applies to induction launchers. Of course there must be something like that because the eddy currents are modeled by a transformer but i wouldnt put much effort in measuring the problem since i can extract the eddy current equivalent resistor from the FEM simulation. However i am not sure about its general freuqency dependence.
The magic factor [1-sqrt(Kp)] does not correct the inductance in the spice simulation but the Amp*Turns.
Awww somehow this form of communication issnt suited for such a complex problem If you dont even have the simulation how could i explain what the problem is. But explaining the simulation alone is half an hour of vocal communication alone.
I have doubts your formula applies: the reduction of inductance with higher frequencies could also be a consequence of skin depth so that the amount of active iron decreases.
You're right. That would make Rp/Lp frequency dependent. But possibly you could still extract that from a meter measurement. I have no idea, though, how you would include this dependence into a LTSpice simulation. Possibly you can get away by disregarding its frequency dependence in the LTSpice simulation and just use the value at the major frequency component of the coil, e.g. 200Hz.
I am not sure a "projectile inductance" can be applied to a coilgun.. this is more something that applies to induction launchers. Of course there must be something like that because the eddy currents are modeled by a transformer but i wouldnt put much effort in measuring the problem since i can extract the eddy current equivalent resistor from the FEM simulation. However i am not sure about its general freuqency dependence.
As said above, possibly a FEM run at 200Hz will suffice. Once you have an Lp/Rp, a coupling and the iron effect on the inductance, the latter two as a function of projectile position, you can start a LTSpice simulation. I've yet to work out the equations for LTSpice and the force. The force is a result from these equations since they also describe the electrical energy in the circuit.
Yeah, this is difficult stuff. I haven't even touched saturation effects. Edit: Sorry, didn't reply to any of your questions. Will do so tomorrow.
1a) how would you think the core losses behave with frequency? The higher the frequency, the less skindepth, the thinner the sheet of conductive material. Skindepth is ~1/sqrt(f).. so resistance should increase with sqrt(f).. maybe even more than that?
I'd expect a sqrt(f) dependence from some frequency upwards. At 0.1Hz there is probably not much skin effect, so it this frequency is outside the sqrt(f) dependence region.
1b) how do Lorentz forces scale with frequency? Given the reference point of 100mHz.. can i extrapolate what Lorentz force there would be for higher frequency?
The Lorentz forces depend on the current. At low f the current is proportional to f, i.e. Vinduced/Rprojectile. Vinduced is given by the rate of change of flux through the projectile, thus it is proportional to f. At higher f, the current will flatten out. The current will create a field opposing the incoming flux, thus reducing the flux in the projectile. The boundary between proportionality and flattening out is at 2*pi*f = Rprojectile/Lprojectile.
2a) Can we somehow derive from the lorenz force or whatever how to low pass filter the magnetisation? 2b) how to get this calculation into the model? Given a "current magnetised"-level and a "should be magnetized"-level how does it actually alter the acting force?
Phicoil is the flux in the coil and dPhicoil/dt is its voltage. The first term is the flux caused by coil current, the second term the flux caused by magnetisation and the third caused by coupling. The effect of magnetisation due to projectile current is lumped into M(x).
This model assumes a constancy of Rprojectile. This is actually false due to skin effects. But as pointed out in my previous post, this might be a good approximation if Rprojectile is calculated at the coils operating frequency. f2(x) might be almost a constant close to ur.
I'm not sure if all of this holds in the case of saturation.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Sry i has driven away by a different part of the project (cap charger - updates soon in the other thread)
I havent worked much on it, but i made a standalone-version of the simulation including the reference simulation data and now i can all share it with you. Its not pretty but it shows the concept (problems)
Download, if you have LTSpice: ]simulation.zip[/file]
Update: somewhat more descriptive net names in few instances and an added usecase - the typical SCRdesign indicated by the word _monkey on the netnames.
]simulation.zip[/file]
All in all it matches extremely good now. There is obviously an issure with the frequency dependence of the projectile magnetisation, which is hardcoded right now but i think this may be solvable. The problem is that the simulation assumes instantaneous magnetisation thus giving an instantaneous force responce and an according inductance change. The truth is that the magnetisation issnt instantaneous due to eddy currents leading to a smaller inductance change than we think (i modeled it by applying a square root to the inductance change due to projectile presence) and it of course leads to a smaller force.
This frequency dependence is obvious in the _monkey-Design where the force is calculated totally wrong due to my hardcoded -inductance change correction which applies only for the specific frequency with the 470uF capacitor and the coil. While the same coil with a 220uF capacitor in the _monkey-design has different properties. hard to explain maybe i should do a video explaining the stuff, but it would be horrible.
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Lets's Play: Model the Multistage-Coilgun
If you dont even have the simulation how could i explain what the problem is. But explaining the simulation alone is half an hour of vocal communication alone.