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Re: 18,000. AMP high energy Cap bank



Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

Original poster: FIFTYGUY@xxxxxxx
In a message dated 12/11/04 9:46:06 PM Eastern Standard Time, tesla@xxxxxxxxxx writes:
The capacitors weigh about 100 lbs each. Caster
oil fill caps. Rated 18,000. AMP High Energy pulse capacitors. 3 capacitors in parallel is 54,000. AMPS.
How does one measure these very high current levels to any degree of accuracy?
How does one limit these current levels? If a energy discharge cap system has an ultimate safe rating of (say) 20kA, how does one make sure that current isn't exceeded during an experiment that vaporizes a specimen?
-Phil LaBudde


.
Hi Phil,

You can measure huge currents by using a specially constructed wideband current transformer, a related device called a Rogowski Coil, or via a specially constructed ultra-low inductance current shunt called a Current Viewing Resistor (CVR). Most of the time, a current transformer or Rogowski coil is preferred since they allows your measurement circuitry to be electrically isolated from the discharge circuit. Hall effect current transformers are another option, but these tend to have lower maximum current ratings that the above options.

Wideband current transformers from Stangenes Industries are available up to 200 kA, and from Pearson Electronics up to 500 kA. Custom versions go higher - see:
http://www.pearsonelectronics.com/datasheets/standard-current-monitor.htm
http://www.stangenes.com/curtransformers.html#3


Rogowski coils require an integrator circuit (a passive RC circuit or, for best accuracy, a wideband operational amplifier integrator). Commercially available Rogowski coils can measure currents into the multi-megAmp range. It's also possible to make a homemade Rogowski coil and IC integrator circuit. However, then the challenge becomes calibrating the unit. For examples of commercial Rogowski Coils see:
http://www.rocoil.cwc.net/
http://www.titan-psd.com/html/CommComponentsRogowski.htm


Standard CVR's are also available to handle multi-megAmp currents and these devices have the advantage of being able to measure down to DC. For examples, see:
http://www.tandmresearch.com/


As far as limiting the peak current - the peak discharge current is often limited by inserting "hard" inductance (i.e., an inductor that's robust enough that it doesn't get destroyed) into the discharge path. In this case the maximum current will always be below the level limited by energy conservation:

Imax = Vcap*Sqrt(C/L)

However, simple inductive limiting also may cause excessive ringing which can overstress the capacitor's dielectric during voltage reversals. By adding suitable (high-energy capable!) resistance in the current loop, you can critically damp the circuit (R > 2*Sqrt(L/C). However, this significantly reduces the peak discharge current. By reducing the loop resistance below the critically damped value thereby permitting a bit of ringing, you can increase the peak current while still keeping the worst case capacitor voltage reversal within the voltage reversal spec of the capacitor bank (typically 20% for many pulse capacitors).

Best regards,

-- Bert --
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