Hi all,
I've been unable to get sensible results from simulating the voltage spike inductors generate when their magnetic fields collapse. The only possibility I can think of for this failure is that the standard inductor models have insufficient parameters for this purpose and require an extended parameter set. Like many of y'all I use LTspice, btw. Any ideas?
you are doing something wrong in your simulation. THe spice stadard inductor model works fine for that.
Connect a pulsed current source to an inductor.. the voltage will spike when the current shuts off
Mark
Show your circuit. LTspice's inductor model is plenty good for this.
Jeroen Belleman
Add some shunt resistance and/or capacitance to avoid singularities. That's realistic anyhow.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The standard inductor model deals only with current and voltage.
If you are seeking a model that reflects behavior when you force an externally-induced field you'll need to write your own subcircuit. ...Jim Thompson
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Okay, gentlemen, thanks for the suggestions. I've found the problem - seems to be the box running LTS on Linux under WINE. No probs on the Windows installations, so nothing you fine people can assist me with. I'll post to a more appropriate Linux/WINE forum see if anyone there can figure it out. Thanks!
That's even more reason to post your circuit! I'm runnning LTSpice under Wine in Linux. I see inductive spikes just fine.
Jeroen Belleman
I don't blame LT or Wine on reflection. I had an update for my Fedora installation a few days ago and something definitely got corrupted. I lost all my menus and have had to invoke applications from the command line in a terminal. I'd put money on these two things being connected, as various other things have been behaving rather oddly since as well.
Actually, it will always spike, going both up and down. If you set the risetime to zero, you'll even get a divide by zero error! (But then, SPICE doesn't *do* discontinuous functions, so that should be no surprise.)
A real inductor has RLC elements around it, equivalent to loss and parasitic capacitance and stuff. This limits the voltage, even if an ideal CCS were applied.
A real "current source" exhibits finite capacitance and resistance, and a finite compliance range, so tends to saturate for large voltage changes.
The common case of e.g. a solenoid driver or flyback SMPS, is better seen as an impedance mismatch situation.
If the drive impedance were always low, the voltage would always be well-defined (but the current will rise arbitrarily -- for a pure inductive load that is). If the drive impedance is switched*, then the voltage is well-defined when the switch is on (low resistance, load exposed to a constant voltage source), and only the current is well-defined when the switch is off (high resistance, load exposed to constant current source; that current being zero, usually).
*A quite reasonable model of a switch, is something which goes from a high resistance state to a low resistance state fairly quickly.Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
I've noticed the "parallel capacitance" of the inductor makes a BIG difference to the level of back emf produced; smaller = greater voltage. Is this "parallel capacitance" in LTS the same thing as *interwinding capacitance* in real life?
Not to be too pickey, but we don't consider inductors to have "back emf". That term is usually reserved for motors.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Correct. You can estimate it tolerably well by treating you windings as layers of wire with capacitance between the layers.
Single layer windings have much lower parallel capacitance - typically around 1pF - which can be estimated fairly accurately if you are good enough at calculus.
-- Bill Sloman, Sydney
r
You might reserve it for motors. I've always understood it to be the voltag e generated by an inductor that acts against a reduction of the current thr ough an inductor. It's electronics for dummies, but I did first year physic s in the class that wasn't expected to have master calculus, which was pure anti-Tasmanian prejudice, and we got all sorts of over-simplified explanat ions.
Tulane might have had other problems.
-- Bill Sloman, Sydney
So what's your preferred term for the same effect in a solenoid, then? I'm guessing "flyback" perhaps, although this seems to be more of an American English term than anything else.
Flyback, kickback, elldeeeyedeetee maybe. I just don't hear "back emf" much around here.
A solenoid is interesting because the armature moves and modulates the value of L as it does. Relays, too.
No big deal, as Humpty Dumpty said.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
In the fundamental inductor equation: V = L * dI/dt
V is defined as the EMF.
(I don't have a curly 'E' so I put 'V' there.)
Faraday's law says it doesn't matter if the dPhi/dt is from relative motion (as in a motor) or varying magnitude (as in a transformer); indeed, Relativity itself says the two are perfectly equivalent.
A motor is just a spinning transformer!
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Is twice really the operable factor though? You'd have removed additional capacitance between the windings of the first and second coils, so I thought you'd get a smaller increase.
Perhaps this depends on the insulation thickness? If you have thin insulation, most of the capacitance will be from wires touching, not indirectly to the next layer - whereas thicker insulation will mean that the proportion of capacitance from close contact is less.
Your thoughts?
Interwinding-capacitance causes eddy currents in the wire and lowers the Q of the inductor. This is why Litz wire is used to wind antenna loopsticks.for higher Qs and narrower bandwidths. LTS apparently considers the capacitance of the inductor to be entirely external, and not between the windings. I tried it both ways and noticed no difference.
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The current that charges the inter-winding capacitances runs through the wi ndings. If you bank wound an inductor, so it had lower interwinding capacit ance it would be self-resonant at a higher frequency, so the inductive reac tance would be higher and the winding resistance less important.
Wrong. Litz wire is better because at higher frequencies, current tends to flow on the surface of a wire. Litz wire has a lot more surface area spread over a number of parallel windings. If you look at the manufacturers notes on Litz wire, you will find that progessively finer-filament Litz wire is recommended for progressively higher frequency applications.
Interwinding capacitance doesn't come into it.
no > difference.
I've split an inductor into close-coupled halves, each half having a quarte r of the original inductance and twice the parallel capacitance, and it did make very little difference. Treating the interwinding capacitance as exte rnal parallel capacitance does seem to an adequate approximation at frequen cies where the single loop inside the inductive core is resistive enough no t to matter.
-- Bill Sloman, Sydney
Nor any other evidence that a comprehensive and detailed education in elect ronic engineering is valued. It's well-known that it's unwise to sound more sophisticated than the boss, and John Larkin went to Tulane, and didn't pa y all that much attention to what his lecturers were telling him.
-- Bill Sloman, Sydney
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