Simulation model of an Arc

[Grollor33]

Hi all,

I'm using the circuit simulation package Simplorer to try to determine dynamic behavior of a circuit due to an arc. The arc will be modeled on the output of a rectifier bridge. I was wondering if anyone had a relatively simple approach to modeling this DC arc. I have researched the different theoretical arc models, but I'm not exactly sure how I would implement - or what the proper input parameters would be. Any help would be much appreciated! Thanks a lot!

Jweav33

 

[logbook]

I understand that arcs have a negative resistance characteristic which might be tricky to simulate. Neverthless, if you have some maths to simulate the arc you can use a polynomial voltage source (for example) to simulate the arc.

What information do you have?

 

[Warpspeed]

As Logbook says, an unconstrained arc is basically a negative resistance, so what happens is largely dependant upon the impedance of what supplies power to the arc, and how physically large the arc can grow.

An arc in air will basically pull the terminal voltage down to very roughly around 15 volts, and provided more than a few hundred mA can flow, a continuous arc will be sustained.

It is a negative resistance, because it will grow to whatever size it can, with however much current is available from the source. A very scary thing to contemplate in low impedance circuits.

 

[Grollor33]

Thanks logbook, warpspeed,

actual implementation of the negative resistance shouldn't be a problem, its the value itself that i'm unsure of.

I'm not sure i understand where the 15V comes from, but i'll looko into it. Thanks again

 

[Warpspeed]

My post was totally from memory, but looking up one of my reference books, "Noise reduction techniques in electronic systems" by Henry Ott (of Bell Labs), he goes into arc formation in some detail.

Apparently the minimum sustainable arc voltage varies for various contact materials:

Silver 12v
Gold 15v
Palladium 16v
Platinum 17.5v

The arc just grows fatter if more current is available, but essentially it grows, and the resistance of the plasma path falls until roughly around the above voltages are reached, where the process then becomes self limiting.

Initially the required voltage to puncture the air may be vastly higher, but once an plasma arc path is formed, things happen quite differently.

It is basically why an arc welder can be made stable with fairly low voltage if a current limited source is used.

You probably really need to ask this question of a plasma physicist rather than a humble engineer.