2.4  Generating an arc for a Jacob's Ladder

Figure 2.3: V-I characteristic of a free burning arc in air

Figure 2.3 shows the V-I characteristic of a free burning arc in air, shown in [SEATON1994].

 

The characteristic is negative, which means that for an increasing current the voltage decreases (an ohmic resistor has a positive characteristic: an increase of current means an increase of voltage). To stabilise an electrical circuit with a burning arc, a current - regulating device is needed. One of the easiest ways is to use an ohmic resistor in series with the arc (Figure 2.4a).

[SEATON1994] showed that at least half of the energy provided by the source will be dissipated in the limiting resistor R1. This is not a preferred solution with an arc of several kiloWatts.

 

Figure 2.4 (b) shows another way of stabilising the arc current. Inductor L1 limits the current with only low losses (depend on its quality). To limit the current to 1A in circuit (b) (with VV1 = 230V; 50Hz) the inductance L1 has a value of:

 

(2-1)

An inductor that large is beyond practical possibilities.

Replacing L1 by a capacitor with:

 

(2-2)

would have an equivalent electrical effect. This would be a better solution.

For the circuit of Figure 2.4 (c) with an equivalent output L2 has to have a value of:

 

(2-3)

These requirements lead to a very expensive and heavy inductor design. Alternatively, a capacitor instead of L2:

 

(2-4)

could be assembled from "of - the - shelf" capacitors.

In circuit (c), the capacitor - solution would also lead to lower costs than the inductor - solution.

The circuits (a), (b) and (c) all need a large transformer, a diode - stack and a large smoothing capacitor.

To connect circuit (b) and (c) to the power line, a high effort to compensate the reactive current has to be made due to the fact that the reactive current changes at a variation of the load.

Figure 2.4 (d) is an electronic solution. The 50Hz voltage from the grid is rectified, chopped to several kHz, and the voltage is multiplied (further described in paragraph 3.6). For all electrical parts, off the shelf - components can be used.

 

 

This page is part of a Frameset: Electrodynamic Sculpture: A Thesis by Rafael Bräg.