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Registered Member #567
Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147
I'm planning to build an arc furnace. I came across this. while looking for some documention. It describes using an inductor as a reactance to act as a current limiter or somesuch, which puzzles me. Is anyone able to direct me to a decent explanation on how this works out? The wikipedia article is unhelpful. Also, how would I go about calculating a suitable reactance for 240V/50Hz mains, and is there a way of making it adjustable?
Also, thick a wire ought I use for the reactance? The article states 12 guage is sufficient, but I'm somewhat sceptical; 10 amps is a lot of current.
EDIT: How much metal can I reasonably expect to melt on the arc furnace?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
It's called inductive reactance. It comes from the fact that an inductor has a voltage/current relationship of V = L*dI/dt. For a sinusoidal current when you take the derivative you end up with V = 2*pi*f*L*I (plus a 90degree phase shift). So that 2*pi*f*L is called the reactance, which is like a resistance in that it can limit current flow, except no power is actually dissipated inside the inductor because the voltage and current are out of phase.
Once you have decided on a current to use in your setup you must set V_line / (2*pi*f*L + R) = I_desired, where R is any additional resistance in the arc furnace setup.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
The use of a reactance, for arc furnaces,gas discharge lampa, mercury and fluorescent, is done because all these have a negative resistance characteristic. This means the resistance falls as current flows through the plasma. If connected to a constant voltage source (ie your 220 volt 50 Hz mains) current would approach a short circuit. The reactor provides a high striking voltage because of the self induction as the current fluctuates as the arc starts. once the arc is hot, it drops in terminal voltage (the voltage across the two electrodes) and the current is the equal to the vector difference between the line voltage and the arc voltage. divided by the impedance of the reactor. Arc devices sometimes are used on DC, in this case a resistance must be used (wastes energy) and often a reactor is used in series as well, for the self induction reasons mentioned above.
Your reactor could be made of #12 magnet wire wound on an old transformer core. If I wanted an inductor that would draw 10 amps with 170 volts 50 Hz across it. it would have an inductance of 57 millihenries. Incidently a capacitor of 200 microfarads would have that reactance too. The arc would be regulated by the capacitor, same as the inductor. The capacitor would have to oil or paper rated at 330 VAC. Ballpark guess for the inductor 60 turns wound on a core with center leg 5 CM^2. To adjust the reactance you could open up the gap.
As to how much heat in furnace? The numbers above would give 500 watts of power. The total power consumed at 10 amps, 220 volts (2200 VoltAmps) with the capacitor balast would be about 540 watts. The wider you set the gap of the inductor, the hotter the inductor would get.
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
Do not use a capacitor. An inductor also serves to limit dI/dt. If you limit current with a capacitor, your furnace will draw large pulses of current when the line voltage exceeds the strike voltage of the arc. This will put out a rather unseemly amount of RFI from line frequency to daylight. The behavior might stabilize once the ambient temperature inside the furnace is high enough, but I wouldn't bet on it.
You should probably use a series inductor (rewound MOT works well) and a capacitor across the mains to keep the inevitable hash from propagating to everybody else in the neighborhood.
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Reactance and arc furnaces
