Contact arcing simulation - Page 1
Forum section: electronics / beginners

author: brandonl
author_rank: Newbie
timestamp: June 06, 2020, 02:50:12 pm
content: Hi all,My main goal here is the simulation of different snubbers or transient suppression of AC inductive loads. There is a good post on this here:https://www.eevblog.com/forum/projects/snubbing-relay-contacts/Based on the the attached circuit and waveform (without the snubber attached), when the voltage controlled switch opens there is a large spike in voltage but the current through the inductor goes to zero. The simulation I don't believe is taking into consideration the arcing that could occur as the contact is opening and the surface area of contact is becoming smaller (I believe this is called constriction). I would like to add the contact arcing to the simulation but am unsure how to approach this.Things I have tried:- googling for a spice model or article on contact arcing simulation - articles are behind a paywall, and probably over my head- could a sparkgap across the switch be a approximation? I believe microcap12 has a model that I may be able to bring in to LTspiceIt might be a big ask looking for an easy to understand model for a complex phenomenon but Ill ask anywayThanks All

author: Jwillis
author_rank: Super Contributor
timestamp: June 06, 2020, 03:22:04 pm
content: Theirs is really no simple way of determining the nature of contact arcing in a switch or relay . Many factors are involved like the material of the contacts, a gases surrounding the contact , area of the contacts , voltage , current ect. ect.You are correct about the constriction but you also need to factor in the blow off force when the metal ions at the contact point become gaseous ions because the temperatures of the arc can reach thousands of degrees . In simple terms this causes a bounce of the contacts. Snubbers are used to slow the rapid rise in voltage across the contact points.I you want a more in depth in site on nature of contact arcing yo may want to look at Electrical Contacts: Principles and Applications edited by Paul G. Slade . Available on Google Books .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2020, 05:58:19 pm
content: SPICE handles some nonlinearities alright, but changes as dramatic as air breakdown are... challenging.The instant of breakdown can be less than a nanosecond, in which time the voltage around the spark nearly equalizes; this change propagates outward from the area, down wires and contacts, radiating into nearby conductors and space, all the while carrying displacement charge back to the spark, supplying it with current which maintains it. Eventually the current drops below some threshold (more accurately, the spark becomes too cool to maintain ionization; the threshold is a complex function of past current, temperature, and contact and gas condition and composition), and the gap becomes insulating again.For a typical inductive load like a motor, this process repeats multiple times, the initial current in the motor's inductance discharging into wiring capacitance until breakdown voltage is reached (creating a ramp-like waveform, which is actually a fragment of an LC ringdown waveform). Eventually, the current drops to the point where breakdown voltage is no longer reached, and it rings down in a linear manner at the resonant frequency.The repeated sparking waveform is often separated from its rampy, ringy origin, as just the impulses are of interest, and the nature of surrounding wiring tends to act as a high-pass filter. This is the origin of fast-transient tests like IEC 61000-4-4, which use a tone-burst pattern of fast, high voltage pulses.You could construct a basic behavioral spark gap model by... hmm...- Start with a two-terminal SUBCKT. This will be your spark gap.- When the terminal voltage is above a threshold, begin breakdown.- While the terminal current is above a threshold, maintain breakdown. This gives you hysteresis.- Both current and voltage are sensitive to the absolute value.- Breakdown is a state variable, which is filtered in a nonlinear manner. This way, if current crosses through zero, the spark does not immediately extinguish. It has some thermal inertia.- Breakdown occurs rapidly (fractional ns), but deionization takes longer (10s, 100s ns). Hence the nonlinear filter. Use a back-to-back pair of diodes, with series resistors, to charge/discharge a capacitor. This will be the nonlinear filter for the state variable.- The state variable finally goes into a switch, or a dependent resistance, or a general nonlinear dependent source, in any case having the effect we expect: as the state variable rises, the effective resistance falls.We can build upon this as a starting point. For example, we might not use a hard threshold, but a sigmoidal threshold, or a ReLU (rectified linear), and use the additional range in state to control voltage drop (which is somewhat smaller at higher currents), or to extract outputs like light emission.We could replace the state threshold with a nonlinear current function, so that current goes up as voltage goes up (leakage, field emission), and so that it has an apparent threshold (using an exponential or other high-order function -- can be tuned to the geometry as well). The current feeds back into the state as described above, giving positive feedback.We could add noise to the sensed voltage, so that leakage current flows very erratically, particularly around threshold; this gives an expected time dependence to breakdown, which is physically realistic (breakdown occurs once sufficient emission causes avalanche breakdown).If glow or corona discharge is desirable to simulate as well (say for low pressure gas discharges), multiple thresholds need to be implemented. This, along with a slower time constant, would be a reasonably complete model for a neon lamp.If the emission current is simulated, and replaced with a vacuum tube triode or tetrode, a thyratron can be made....I do appreciate, this description probably isn't all that obvious to someone not very familiar with SPICE, or maybe even someone who is. It's a very high level, qualitative description, and will take some hours or days to implement and test. It's, obviously, much faster to merely describe it than to go out and build it, and the hope is also that by giving a description, one can gain more from the detailed behavior of the phenomena, which will be helpful both to empirical study and in reference to other models.Which would be my main recommendation: as far as I know, breakdown models are a solved problem, they're out there (alas, I don't have a recommendation handy). SPICE tends to be a somewhat write-only language, it can be opaque to parse; I wouldn't expect too much may be learned from reading an existing model, but perhaps this can also be used as a guide to that.As for the surrounding conductors -- to recreate realistic waveforms, you will need to place the spark gap in a realistic environment. Perhaps a transmission line between the spark gap and inductor would be enough to give reasonable results, using a lossy TL, or termination, or a modest size lumped equivalent, to represent electrical and radiation losses to the wave. This finally should give reasonable waveforms, such that you will be able to observe the rapid-fire sawtooth source waveform which EFT is designed to emulate.But at the same time, you will have answered, in a manner, your own question -- by constructing a network representative of the environment around the contacts, you can also compute the exact answer for your snubber: Rsnub = sqrt(L/C) to dampen the resonance, where L is the load inductance (at the resonant frequency) and C is the wiring capacitance (or the TL's equivalent). Use Csnub > 2.5*C.If you try enough wiring networks and loads, you will eventually come to this truth:1. Arcing can't always be prevented (though its repetition can be significantly reduced).2. There is no, and there can be no, slam-dunk snubber value -- it depends on the load. And the snubber itself has impedance, which if the contacts are switching AC, is problematic by itself (leakage current!).3. A parallel R+C snubber only acts to dampen the rising edge (spark turning off, deionization); it does nothing to the falling edge, indeed it makes it sharper if anything (sourcing more current to the spark). For which we might intuit the complementary network: an L||R in series. Ferrite beads are a handy example -- though you need quite a lot of them to have any effect at all on EFT, the voltage is just too high. The unfortunate fact is this: inductors rated for load current, and with enough turns around enough core to handle the voltage, are expensive; so we just don'tbother, even though it would be greatly helpful.Which I really should've opened with instead, cutting to the chase; but the idea of constructing ones' own spark gap model is an interesting one, not at all without merit (even given other models already exist; which are themselves worthy of study, so some background is desirable), and so I wish you the best of luck on this surprisingly complex problem!Tim

author: floobydust
author_rank: Super Contributor
timestamp: June 06, 2020, 11:45:02 pm
content: Don't forget a real world inductor has distributed winding capacitance, I would include that on your load.LTSpice does have a neon bulb model, and more involvedspark gapand spark ignition models there.

author: brandonl
author_rank: Newbie
timestamp: June 10, 2020, 02:41:20 am
content: Thank you all for the reply!T3Sl4co1l, it will definitely take me a while to digest everything you posted here. In the meantime the link floobydust posted had a spark gap model I could use. I plan to study this while considering the information you had provided. For now I just used the model as is and added it to the circuit, as well as some winding capacitance. I've attached the waveform as well as a scope trace from a jobsite that has bad contact arcing on a set of pilot relays (likely due to a bad suppressor). These relays are connected to a much larger contactor than what is modeled in the simulation but I think the simulation waveform is looking closer to real life than without the sparkgap. I plan to get some values off the actual contactor and update the simulation, as well Id like to figure out why the decay is so long in the simulation vs real.

author: floobydust
author_rank: Super Contributor
timestamp: June 10, 2020, 05:51:07 am
content: If the load is an AC contactor, their inductance step changes with the armature closing the magnetic circuit as they pull in. You can measure the inductance pushed in (at your desk with your finger) and at rest. It's also why their inrush current is very high I thought 20X holding current. I went down to 6nF and the sim decay time seemed better.In the field I see small 10mm MOV's used across the coil to limit pilot arcing.

author: brandonl
author_rank: Newbie
timestamp: June 10, 2020, 10:07:16 am
content: The values for the inductor I got from checking a smallish Fuji SH-4 relay. The inductance I measured with the relay pushed in. I also measured the capacitance at 600nF but I am not sure if is a valid way to measure. I measured at the coil terminals with a De-5000 lLCR. The winding capacitance that you changed to 6nF, is this the parallel capacitance of the inductor or did you add a series capacitor?The scope trace is from a much larger 3 ph motor contactor. It currently has a rc snubber across it I plan to replace today. One thing that this manufacturer does that has my spidey sense tingling is, the rc snubber is just a resistor and cap connected with wires and inline crimp connectors not a sealed unit. This for one, places the snubber "far away" from the actual coil. Typically on other manufacturers equipment they will use snubbers that plug directly in to the relay or right at the coil. I wonder if being further from the coil would make the snubber less effective.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2020, 10:39:43 pm
content: The snubber belongs at the switch, actually. All the wiring around the switch is just stray inductance. Lead length on the snubber is far less important.That basic spark gap model, looks to hit most of the points I made; it has a basic, maybe not very aggressive, ionization model; it has a single (linear, not asymmetrical) time constant; and it doesn't seem to have much of a voltage threshold, though I'd have to have a good think to see how exactly it behaves. I don't like that it has a singularity, i.e. if voltage goes to zero, current goes to zero, V(ion) goes to zero, and R_ion divides by it. Simple solution is putting a tiny offset (uV, mV?) into V(ion) so it can never hit true zero. (And we might even physically excuse such a step by saying it's ambient ions or cosmic rays or something.)It seems to be quite snappy, but it doesn't seem to have a high enough breakdown voltage -- or your snubber is too low an impedance -- to allow a dramatic recreation, like the scope shot illustrates.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: June 11, 2020, 01:26:07 am
content: you can try to change resistance over time to simulate mechanical power on/off. Just use resistor instead of mechanical switch and change it's resistance from low to very high value over time. Something like that: