US3735195A - Spark-discharge apparatus for electrohydraulic crushing - Google Patents

Abstract

In existing apparatus of the above kind, a capacitor is discharged through the liquid-immersed gap via a self-triggered or externally triggered switch device. In the present invention an initially uncharged capacitor is connected directly across the gap without any intervening switch device. This first capacitor is pulse-charged to the breakdown voltage of the gap via a stepup pulse transformer by discharging a second capacitor via a triggered switch device through the primary winding thereof. The two capacitors are approximately matched in value, as reflected by the transformer ratio, so that substantially all the energy stored in the second capacitor is transferred to the first capacitor at peak voltage thereon. A principal advantage is that the triggered switch device, eg a thyratron, does not require either to withstand the high gap voltage or to carry the high gap current.

Description

United States Patent [191 Jenkins et al.

SPARK-DISCHARGE APPARATUS FOR ELECTROHYDRAULIC CRUSHING Inventors: Elmer Thomas Jenkins; Geoffrey Marcus Ward, both of c/o United Kingdom Atomic Energy Authority, 11 Charles 11 St., London, S. W. 1, England Filed: Dec. 4, 1970 Appl. No.: 95,145

Related US. Application Data Continuation-impart of Ser. No. 748,695, July 30, 1968.

Foreign Application Priority Data Aug. 7, 1967 Great Britain ..36,097/67 July 29, 1968 Great Britain ..4,890/68 U.S.Cl. ..3l5/1ll, 315/241, 315/219 Int. Cl. ..H05b 37/02 Field of Search ..315/241, 209, 219, 315/229, 228, 226, 310, 311, 360; 241/301, 243

[56] References Cited UNITED STATES PATENTS 3,234,429 2/1966 Schrom ..315/1l1 2,555,305 6/1951 Pity ..321/15 X 3,355,626 1 1/ 1967 Schmidt ..315/241 Primary Examiner-Nathan Kaufman Attorney-Larson, Taylor and Hinds 5 7 ABSTRACT In existing apparatus of the above kind, a capacitor is discharged through the liquid-immersed gap via a selftriggered or externally triggered switch device. In the present invention an initially uncharged capacitor is connected directly across the gap without any intervening switch device. This first capacitor is pulsecharged to the breakdown voltage of the gap via a step-up pulse transformer by discharging a second capacitor via a triggered switch device through the primary winding thereof. The two capacitors are approximately matched in value, as reflected by the transformer ratio, so that substantially all the energy stored in the second capacitor is transferred to the first capacitor at peak voltage thereon. A principal ad vantage is that the triggered switch device, eg a thyratron, does not require either to withstand the high gap voltage or to carry the high gap current.

4 Claims, 3 Drawing Figures F K 2 gwwl/m/ i M ZZ Patented May 22, 1973 2 Sheets-Sheet l SPARK-DISCHARGE APPARATUS FOR ELECTROHYDRAULIC CRUSI-IING CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of copending application Ser. No. 748,695 filed July 30, I968.

BACKGROUND OF THE INVENTION This invention relates spark-discharge apparatus for the electro-hydraulic crushing of materials. The latter technique involves producing a spark discharge in a liquid in which the material is placed, the resulting shock waves causing the material to break up.

The practice hitherto has been to charge a capacitor to a high voltage from a DC source through a resistor,

and to discharge the capacitor through a spark-gap or equivalent switch device connected in series with the liquid-immersed gap. Such arrangements are described, for example, in our U.K. specification No. 1,021,786, in which a self-triggered spark-gap is used, and in U.S. Pat. No. 3,234,429 to Schrom, in which the switch device is an externally triggered rectifier such as an ignitron.

In such arrangements the voltage applied across the immersed gap is held-off, until the breakdown of the latter, across the series switch device. Furthermore, when breakdown of the immersed gap occurs, all-the discharge current from the aforementioned capacitor flows through the switch device. At comparatively low voltages, currents and pulse-repetition rates, the abovedescribed arrangements are usable, but where it is desired to use high voltages, e.g., of the order of 100 kV, at relatively high repetition rates, e.g., 50 cycles/- second, and to achieve instantaneous discharge currents of the order of Megamps, they are not practicable. Switch devices capable of performing the specified duty with a reasonable lifetime are not available.

The present invention provides a form of apparatus which overcomes the above difficulties because in it the high voltage applied to the immersed gap at no time appears across a switch device, and the discharge current in the immersed gap does not flow through a switch device.

SUMMARY OF THE INVENTION According to the present invention, in apparatus for the electrohydraulic crushing of material in a liquid by shock waves resulting from repetitive spark discharges produced in said liquid, said apparatus comprising a vessel in which said liquid is contained and a spark discharge gap arranged in said vessel, there is provided the improvement whereby a first capacitor is directly connected by continuously conducting DC connections in parallel with said gap, a secondary winding of a step-up pulse-transformer is connected substantially directly across said first capacitor for applying a pulse to charge said first capacitor to the breakdown voltage of said gap, a second capacitor is connected in series with a triggerable switch across the primary winding of said pulse-transformer to generate said pulse by discharging said second capacitor through said primary winding,

the capacitances of said first and second capacitors being at least approximately matched as reflected by the transformer ratio, means being provided for charging said second capacitor between pulses from a DC source and for repetitively triggering said switch into conduction to initiate said pulses.

The above-mentioned pulses are half-cycles of a sinusoidal waveform produced, when the switch is triggered, by a resonant circuit constituted substantially by the first capacitor (as reflected) and the second capacitor in series with the leakage inductance of the pulsetransformer. Provided the two capacitors are matched as defined, substantially all the energy initially stored in the second capacitor appears a half-cycle later across the first capacitor (ie across the discharge gap), but transformed up to a higher voltage. Ideally, for maximum crushing efficiency, the discharge gap is adjusted to break down when peak voltage is reached on the first capacitor. In practice, in order to provide a safety margin, the gap is desirably adjusted to break down slightly before peak voltage is reached.

Preferably the second capacitor is recharged between pulses from a DC source comprising a reservoir capacitor through a rectifier and an inductor connected in series, so that the second capacitor is recharged to twice the voltage on the reservoir capacitor.

DESCRIPTION OF THE DRAWINGS To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein FIG. 1 is a circuit diagram of apparatus embodying the present invention.

FIG. 2 is a simplified equivalent circuit of the circuit of FIG. 1.

FIG. 3 shows waveforms in the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a vessel 1 contains a suitable liquid 2, e.g., water, in which are immersed a pair of electrodes 3 to form a discharge gap. The vessel and electrodes are shown diagrammatically as such arrangements well known in the electro-hydraulic crushing art. The spacing between the electrodes is preferably variable, as for example, in the specification of Ser. No. 1,021,786. Material to be crushed is immersed in the liquid in the usual way.

Directly connected in parallel with the gap formed by electrodes 3 (without any intervening spark-gap or other switch device) is a first capacitor C1, which is connected across the secondary winding of a step-up pulse'transformer T via a length of high-voltage coaxial cable 4. The primary winding of transformer T is connected in series with a second capacitor C2, an inductor L2, and a diode D1 across a reservoir capacitor C3. C3 is charged from a conventional 3-phase rectifier DC power supply (not shown) via an inductor Ll. A thyratron valve V acting as a switch is connected across C2 and the primary winding of transformer T. Thyratron V is normally biassed off by a DC supply (not shown), but can be triggered into conduction by a pulse applied from a pulse generator unit 5 in a conventional manner.

The values of C1 and C2 are substantially matched as reflected by the transformer ratio, i.e., C1 C2/n, where n is the transformer ratio.

In operation, C2 is charged from C3 via D1 and L2. When thyratron V is triggered into conduction, C2 discharges through V and the primary winding of transformer T. C1, C2 and the leakage inductance I.. of transformer T form a series resonant circuit of which the simplified equivalent circuit is shown in FIG. 2. In FIG. 2 C l is the reflected capacitance of C1, i.e., Cl

= n'*. C1 C2. The resonant circuit thus has a period 211 V L -C2/2.

As C2 discharges, the energy stored therein is transferred via the transformer T to C1, which reaches its peak voltage when C2 is fully discharged. This seesaw action of the circuit is shown in FIG. 3. The discharge of C2 is shown by curve A and the simultaneous charging of C1 by curve B. (It will be noted that these two curves are drawn to different scales.) V is assumed to have been triggered at time t and peak voltage on C1 is reached at time The time-interval t, t, is half the above period of the resonant circuit. i.e., ll-"i= 1 VL -C2/2.

If gap 3 is adjusted to break down at peak voltage on C1, ie at time 2,, the maximum energy is available for the discharge. (In practice the gap width is desirably adjusted to break down slightly before peak voltage is reached, to provide a safety margin. As is well known, the breakdown voltage of such gaps is not a precisely defined value, but varies statistically about a mean value. To reduce the number of failures to break down, the gap width is therefore desirably adjusted so that this mean breakdown voltage is somewhat less than the peak voltage obtainable.)

It will be seen that the current in thyratron V at time t is substantially zero. Moreover even the peak current in V need not be large, since it is the current over the integrated time-interval t t which transfers the energy from C2 to C1.

To suppress transients (not shown in FIG. 3) which would otherwise be communicated to transformer T at the instant when Cl begins to discharge through gap 3, with the attendant risk of internal breakdown, a lowvalue inductor L3 is connected between cable 4 and the gap. L3 with its parallel self-capacitance behaves as a filter at the ringing frequency of the gap discharge, which is about ZMc/s, but presents only a low impedance to the pulse B from transformer T.

Component values in one embodiment are as follows:

C2 3.0 pF L3 280 H C3 =30 12F V CVl I54 thyratron n=approx llzl Short-circuit leakage inductance (L) approx 50 H. With these values the duration of the half-period to t, is approximately 27 ,usec.

The transformer T is wound on one limb of a 4 thou laminated silicon iron C-core having a cross-sectional area of 2% X 2% inches and a window 11 X 3 inches. To minimize the leakage inductance, the single-layer primary winding consists of 32 turns of four electrically parallel windings, using rectangular-section copper conductors each 40 X 100 thou. Each turn consists of four such conductors per groove of the former, giving a conductor total cross-section of 0.08 X 0.2 inches per turn. The double-layer secondary winding has 351 turns of 20 SWG copper wire, 172 turns on the first layer and 179 on the second. Conical formers are used to assist high-voltage insulation, as is known practice, and the transformer is oil-immersed.

Inductor L3 is an air-cord, air-spaced coil of 18 SWG copper wire buried in grooves in a 4 inch diameter methylmethacrylate former.

The above described apparatus can produce 100 kV discharges from a kV DC power supply, taking into account the voltage doubling achieved during the charging of C2 as described below, at discharge repetition frequencies of between 1 and 50 per second. The transformer ratio is llz l rather than 10:1 to compensate for transformer losses and for shunt losses due to the finite conductivity of the water. At a repetition frequency of 35 per second, the average power supplied as sparks (not the power level during each spark) is about 4 kW. It will be seen that although the peak voltage obtainable across the gap in kV, and the peak gap current of the order of Megamps, the peak voltage across the thyratron V is only 10 kV and the peak current therein only about 2,000 amps.

During the first charge cycle after switching on, C2 is only charged to the voltage of C3, but after the first discharge L2 causes the voltage on C2 to swing to approximately twice the voltage on C3, at which value it is held by the actionof diode D1. Diode D2 and low value resistor R2 are connected across thyratron V and act, as is conventional in such charging circuits, 'to clamp C2 to earth in preparation for its recharge from C3. Otherwise the voltage on C2 could tend to build up over successive pulses to greater than twice the voltage on C3.

As stated above, for optimum operation the gap between electrodes 3 is adjusted to break down approximately when the charging current into C1 has fallen to zero at time 1,, as this condition provides the maximum transfer of energy from C2 into the discharge. Should the discharge be delayed beyond the point of zero current, e.g., due to maladjustment of the gap width, the secondary winding of transformer T acts as a saturating inductor, and has a time-constant with C1 and the water gap which maintains the voltage on C1 at more than 50 percent of full voltage for about 50 secs, as shown at B in FIG. 3. This increases the likelihood of breakdown occurring even after time To facilitate adjustment of the gap between electrodes 3 as described, a small resistor R1 is connected in series with the transformer primary winding and the voltage across this is displayed on a cathode-ray tube when adjusting the gap after the first discharge.

The discharge voltage, and hence discharge energy, can be varied by adjusting the DC power supply to capacitor C3, using a variable AC input to a three-phase transformer forming part of the DC power supply.

In addition to preventing the build-up of voltage on C2 as described, diode D2 and resistor R2 form a protective circuit to dissipate the energy fed back from the secondary side of the transformer should a short-circuit exist between the two electrodes 3.

It will be seen that the present invention enables high gap voltages to be used, and high discharge currents obtained, at high pulse recurrence frequencies, without the need for a switch device capable of withstanding these voltages and currents.

We claim:

1. In apparatus for the electrohydraulic crushing of material in a liquid by shock waves resulting from repetitive spark discharges produced in said liquid, said apparatus comprising a vessel in which said liquid is contained and a spark discharge gap arranged in said vessel, the improvement whereby a first capacitor is directly connected by continuously conducting DC connections in parallel with said gap, a secondary winding of a step-up pulse-transformer is connected substantially directly across said first capacitor for applying a pulse to charge said first capacitor to the breakdown voltage of said gap, and a triggerable switch device is connected across a series combination of a second capacitor and the primary winding of said pulsetransformer for generating said pulse by discharging said second capacitor through said primary winding, the capacitances of said first and second capacitors being matched as reflected by the transformer ratio, and means being provided for charging said second capacitor between said pulses from a DC source and for repetitively triggering said switch device into conduction to initiate said pulses the arrangement being such that the maximum voltage on the first capacitor is obtained when the current charging said first capacitor has fallen to zero and the said gap being adjusted to break down at approximately said maximum voltage.

2. In apparatus for the electrohydraulic crushing of material in a liquid by shock waves resulting from repetitive spark discharges produced in said liquid, said apparatus comprising a vessel in which the liquid is contained and a spark discharge gap arranged in said vessel, the improvement comprising an arrangement including a first capacitor directly connected by continuously conducting DC connections in parallel with said gap, a step-up pulse-transformer including a secondary winding connected substantially directly across said first capacitor for applying a pulse to charge said first capacitor, a second capacitor connected in series with the primary winding of said pulse-transformer, a triggerable switch device connected across the series combination of said first capacitor and said primary winding of said pulse-transformer for generating said pulse by discharging said second capacitor through said primary winding, and means for charging said second capacitor between said pulses from a DC source and for repetitively triggering said switch device into conduction to initiate said pulses, the arrangement being such that the maximum voltage on said first capacitor is obtained when the current charging said first capacitor has fallen to zero and the said gap being adjusted to break down at approximately said maximum voltage.

3. Apparatus as claimed in claim 2 wherein said DC source comprises a reservoir capacitor, and wherein a rectifier and an inductor are connected in series between said reservoir and said second capacitor for charging said second capacitor to twice the voltage on said reservoir capacitor.

4. Apparatus as claimed in claim 2 wherein said triggerable switch device is a thyratron.

Claims (4)

1. In apparatus for the electrohydraulic crushing of material in a liquid by shock waves resulting from repetitive spark discharges produced in said liquid, said apparatus comprising a vessel in which said liquid is contained and a spark discharge gap arranged in said vessel, the improvement whereby a first capacitor is directly connected by continuously conducting DC connections in parallel with said gap, a secondary winding of a step-up pulse-transformer is connected substantially directly across said first capacitor for applying a pulse to charge said first capacitor to the breakdown voltage of said gap, and a triggerable switch device is connected across a series combination of a second capacitor and the primary winding of said pulse-transformer for generating said pulse by discharging said second capacitor through said primary winding, the capacitances of said first and second caPacitors being matched as reflected by the transformer ratio, and means being provided for charging said second capacitor between said pulses from a DC source and for repetitively triggering said switch device into conduction to initiate said pulses , the arrangement being such that the maximum voltage on the first capacitor is obtained when the current charging said first capacitor has fallen to zero and the said gap being adjusted to break down at approximately said maximum voltage.

2. In apparatus for the electrohydraulic crushing of material in a liquid by shock waves resulting from repetitive spark discharges produced in said liquid, said apparatus comprising a vessel in which the liquid is contained and a spark discharge gap arranged in said vessel, the improvement comprising an arrangement including a first capacitor directly connected by continuously conducting DC connections in parallel with said gap, a step-up pulse-transformer including a secondary winding connected substantially directly across said first capacitor for applying a pulse to charge said first capacitor, a second capacitor connected in series with the primary winding of said pulse-transformer, a triggerable switch device connected across the series combination of said first capacitor and said primary winding of said pulse-transformer for generating said pulse by discharging said second capacitor through said primary winding, and means for charging said second capacitor between said pulses from a DC source and for repetitively triggering said switch device into conduction to initiate said pulses, the arrangement being such that the maximum voltage on said first capacitor is obtained when the current charging said first capacitor has fallen to zero and the said gap being adjusted to break down at approximately said maximum voltage.

3. Apparatus as claimed in claim 2 wherein said DC source comprises a reservoir capacitor, and wherein a rectifier and an inductor are connected in series between said reservoir and said second capacitor for charging said second capacitor to twice the voltage on said reservoir capacitor.

4. Apparatus as claimed in claim 2 wherein said triggerable switch device is a thyratron.

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