1
\$\begingroup\$

I want to model a real-life scenario when a mechanical switch is rapidly opened when passing current through an inductive load. Below is a model and corresponding plots for the switch control (Vn001), switch current I(S1) and a plot to observe the voltage spike occurring at the inductor terminal V(vspike). Initially the switch is ON and after 0.2 seconds the switch is turned OFF.

enter image description here

I set the switch on resistance to 1 Ohm and the off resistance to 10Meg. According to the theory I can understand why the inductor current causes such huge voltage spike when the switch is opened.

But in real life I don't think this crazy voltage makes sense and goes that high.

What happens in reality that we never see like 10 MegaVolts? Or how can we model this to make it more realistic?

edit:

Adding some capacitance for the switch resulted more realistic results:

enter image description here

\$\endgroup\$
2
\$\begingroup\$

The moment the switch opens its contact, an arc is immediately forming and dissipating the energy formerly stored in the inductor. You won't get this in a sim. You also have contact capacitance that lowers as the contacts move further apart.

So that's roughly what happens and I'll leave it to you to figure out how to model it.

\$\endgroup\$
  • \$\begingroup\$ I added parallel 10uF cap(just a guess) to the switch, now results are more realistic. I guess in real that capacitance is time dependent and maybe in series I dont know. I added the plots to my edit. \$\endgroup\$ – atmnt Jun 25 '17 at 17:59
  • 1
    \$\begingroup\$ What is capacitance across that light switch on your wall, when the contacts have separated only 1micron? with 4mm by 4mm contact sizes? C = E0*Er*area/distance = 9e-12Farad/meter * 4mm * 4mm / 0.001mm = 9e-12 * 16 = 150 picroFarad. \$\endgroup\$ – analogsystemsrf Jun 25 '17 at 20:56
  • \$\begingroup\$ @analogsystemsrf I guess you estimated C1 as 150pF. Should that be in series or in parallel to the switch for the switching action? I added as parallel but have no idea of whether it is correct. \$\endgroup\$ – atmnt Jun 25 '17 at 22:25
  • \$\begingroup\$ @user134429 -- in parallel (in series won't work) \$\endgroup\$ – ThreePhaseEel Jun 25 '17 at 23:53
  • \$\begingroup\$ @ThreePhaseEel I see but 150pF parallel cap as C1 in my second model produces again unrealistically huge voltages like a ringing voltage from -10KV to +10KV. In real I would expect max a few hundred volts. What do you think? Am I wasting my time? Is it more complicated than what Im trying to do? \$\endgroup\$ – atmnt Jun 26 '17 at 0:12
1
\$\begingroup\$

There are standards that recommend peak voltage, rise-time, fall-time and ringing characteristics for representative transients that equipment should withstand in various locations and situations. ANSI / IEEE C62.41 and C62.45 are the ones that I think may be applicable. C62.41 describes a 6 kV ringing impulse with a 0.5 microsecond rise time with the second ringing peak at 60% voltage and 100 kHz ringing for the first cycle after the initial impulse. There also unidirectional impulses with 1.2 and 8 microsecond rise times with the voltages falling to 50% voltage after 50 and 20 microseconds respectively. I have not seen a recent version of that standard, but I don't think it will have changed drastically.

Here is a link that might be helpful. It is out of date, but it should give you an idea of what is involved. If you search you should be able to find something similar that is more recent. If you have the necessary resources, you should read the referenced standards.

https://www.progress-energy.com/assets/www/docs/business/tvs-lightning-equip-protection.pdf

Here is a link to some more recent information:

http://www.exportyellowpages.com/Uploads/CompanyBrochure2/863802ac-7ac9-46db-9128-988a55d3c594.PDF

\$\endgroup\$
0
\$\begingroup\$

But in real life I don't think this crazy voltage makes sense and goes that high.

What happens in reality that we never see like 10 MegaVolts? Or how can we model this to make it more realistic?

Your right it's not realistic. If it were realistic you'd be able to generate insanely high voltages. Voltages are limited by how fast electrons can move in a wire after the magnetic field starts to drive them when the inductor is switched.

The main problem is all materials have parasitic resistance an inductance. If you wish to simulate these, then you need to find out what they are for your application. If its a PCB trace, then it has parasitic inductance and resistance, these can be found with PCB trace calculators. If it's a wire, you can look up the wire gauge and find out the inductance per foot and resistance per foot. So the parasitics of the wires connecting components matter in some simulations

Inductors also have parasitic inductance and resistance, these can usually be found in the datasheet. ESR can also be measured with an ohm meter for an approximation for an inductor. In LT spice these are fields that can be modified by right clicking on the inductor.

Everything also has mutual capacitance. Any two metal surfaces have mutual capacitance between them. The wires in an inductor coil also have a small amount of capacitance between them. Below is an example of modeling the parasitics of inductor in fine detail. One thing to realize is the more detailed a model is, the more time it will take to simulate, so go with a more generalized model unless you absolutely have to simulate the details. Models will never reflect what is actually going on in the real world, its your job to know the difference between what the simulation is capable of and what the real world is doing.

Fig. 1. HF equivalent circuit of single-layer solenoid air-core inductors with a shield.  

Source: Stray capacitance of single-layer solenoid air-core inductors

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.