US3749958A

US3749958A - Electrohydraulic crushing apparatus

Info

Publication number: US3749958A
Application number: US00206748A
Authority: US
Inventors: G Ward
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1970-12-30
Filing date: 1971-12-10
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31
Also published as: GB1350600A; SE369041B; ZA718297B; IE35884L; IE35884B1

Abstract

One form of electrohydraulic crushing apparatus comprises a liquid-immersed spark-gap across which is connected a first capacitor pulse-charged by repetitively discharging a second capacitor through the primary winding of a step-up pulsetransformer, the two capacitors being matched in value as reflected by the transformer ratio. In order to maintain the breakdown voltage close to peak value despite erosion of the gap electrodes, the present invention checks that each first halfcycle of current in the transformer primary winding terminates within a predetermined time-range after the start of the halfcycle. If it terminates outside this time-range, electromechanical means are controlled to adjust the electrode gap-width so that the half-cycle terminates within this range. Because of statistical variations in the breakdown voltage, it is arranged that a substantial number of half-cycles must terminate outside the predetermined time-range before the gap-width is adjusted.

Description

United States Patent 1191 Ward July 31, 1973 1 ELECTROHYDRAULIC CRUSHING 3,352,503 11/1967 Maroudas 241/1 APPARATUS Prima Examiner-John Kominski [75] Inventor: Geoffrey Marcus Ward, Reading, g Larson ROSS F Hum Jr et a1 England 73 Assignee: United Kingdom Atomic Energy 57 ABSTRACT Amhomy London England One form of electrohydraulic crushing apparatus com- [22] Fil d; D 10, 1971 prises a liquid-immersed spalrk-gap across which is cor;-

' nected a first capacitor pu se-charged by repetitive y [2]] Appl' 206748 discharging a second capacitor through the primary winding of a step-up pulse-transformer, the two capaci- [30] Foreign Application P i it D t tors being matched in value as reflected by the trans- Dec. 30 1970 Great Britain 61 884/70 maimai" breakdw age close to peak value despite erosion of the gap elec- {52 us. or 313/146 241/1 241/301 Presemimmion checkslhateach 313/232 3 1 6 31 4/69 cycle of current in the transformer primary winding ter- 51 Int. (:1. 1102c 19/00 minms within a predetermined time-range after the 581 Field of Search 241/1, 301; 314/68, star half'cycle-lfit terminates side this time 314/69; 313/146, 232 range, electromechamcal means are controlled to adjust the electrode gap-width so that the half-cycle -ter- $323? ii irelliiiiifio'ili fi if 335323 r111 UNITED STATES PATENTS substantial number of half-cycles must terminate out- Steele al. u side predetermined time range before the gap 3,200,285 8/1965 Kaspanan 314/68 width is ad-usted 3,209,060 9/1965 Borreback 314/68 7 J 3,234,429 2/1966 Schrom 315/111 10 Claims, 5 Drawing Figures /l/fl/Vfl /7A/[ W/Yfi- [Alf/0 *ZJI 6 64/2 #0)? 477%, J/Afll/f/ I 4/? 424) n 4) j T Jlfi/V/f/Zfi/f fill/P I a y 1- ff/W/f/ Z,

film/19725 12 I/I/Vflf/Xi/f //1//7 i/if/fi f/l/Yf am /7 Wm) Mid? 44/2 l 7 I a Q 13 15 /7 ELECTROIIYDRAULIC CRUSIIING APPARATUS CROSS-REFERENCE TO RELATED'APPLICATION The form of apparatus described in the first sentence of the above abstract is the subject of co-pending U.S. application Ser. No. 95,145, filed Dec. 4, I970, by E T Jenkins and G M Ward.

BACKGROUND OF THE INVENTION This invention relates to apparatus for the electrohydraulic crushing of materials, and has one application in conjunction with the apparatus disclosed in our British Patent No 1,225,499 (U.S. application Ser. No 95,145, filed Dec. 4, 1970, by Jenkins and Ward).

The latter apparatus comprises a spark-gap which, in use, is immersed in liquid, and across which is connected a first capacitor. A second capacitor, charged from a DC source, is repetitively discharged through the primary winding of a step-up pulse-transformer via a triggered thyratron. The pulse thereby produced across the secondary winding pulse-charges the first capacitor to the breakdown voltage of the gap. If the two capacitors are matched in'value, as reflected by the transformerratio, substantially all the energy stored in the second capacitor is transferred to the'first capacitor when the latter reaches its peak voltage, and is, of course, at a higher voltage. Optimum efficiency is therefore obtained if the gap is adjusted to breakdown at peak voltage on the first capacitor. In practice, because the breakdown voltage is not precisely defined, a safety margin is provided by adjusting the gap width so that breakdown occurs shortly before peak voltage is reached. i

It is difficult in use to maintain the desired gap width, because the repetitive discharges gradually erode the gap electrodes. This widens the gap and so increases the breakdown voltage. Eventually the gap ceases to break down at all until readjusted. The latter can be done manually, but in a production crushing process this is unsatisfactory.

SUMMARY OF THE INVENTION According to the present invention, in electrohydrau-.

lic crushing apparatus comprising a spark-gap which in use is immersed in liquid, and means for repetitively applying a high voltage across the gap to cause breakdown thereof, there is provided means controlled by a quantity which varies with the actual voltage atwhich breakdown occurs, for automatically adjusting the gap width so that breakdown occurs substantially within a given voltage range;

Because the breakdown voltage of a spark-gap varies statistically about a mean value, said means is preferably so arranged that a substantial plurality of breakdowns must occur outside the given voltage range before the gap-width is adjusted.

In apparatus in which the voltage is applied as repetitive pulses having a finite rise-time to peak voltage, said quantity may be the time elapsing between the instant of breakdown and a prior datum time for each pulse, and said means may operate to adjust the gap width so that the instant of breakdown occurs within a predeterprimary winding of the transformer, the datum time being the start of said half-cycle, said means operating to adjust the gap so that the end of said half-cycle occurs within a predetermined time-range after said daturn.

The means may include circuit means for generating pulses coincident with the start and end of said halfcycle, and gate means for providing separate outputs when the end-pulse occurs less than a shorter given time-interval after the start-pulse, and when the endpulse occurs more than a longer given time-interval after the start-pulse.

Theoutputs of the two gates may be applied to control the direction of rotation of an electric motor means which drives one or both gap electrodes towards or away from the other to adjust the gap width. For example the two gate outputs may operate relay means which reverse the polarity of the voltage supply to the motor means.

Each gate output is preferably integrated and com pared with a reference level so that a plurality of output a s v ns t r the s single motor or by separate motors. The drive may be use in the present invention;

mined time-range after said datum, which may be the be the duration of the first half-cycle of current in the x arranged, eg by suitable gear trains, to drive one electrode more rapidly than the other. In this way the gap can be maintained at an approximately constant location in the liquid-containing vessel, despite the fact that one electrode, normally thepositive one, may erode more quickly than the other.

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 accompanyingdrawings wherein:

FIG. 1 is a simplified circuit-diagram of the electrohydraulic crushing apparatus disclosed in British Paten FIG. 2 is a graph showing how the breakdown voltage of a spark-gap varies statistically aboutamean value.

FIG. 3 shows waveforms in the circuit of FIG. 1. FIG? 4 isa block schematic diagram of a circuit for FIG. 5 is a simplified perspective view of a motordriven arrangement for varying the electrode separation in the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT In FIG. 1 the same letters and numerals are used to indicate the components as in the specification of British Patent No 1,225,499. Briefly the apparatus comprises a vessel 1 containing a liquid 2, e.g. water, and

two electrodes 3 which form a spark-gap. Connected across the gap is acapacitor Cl. A capacitor C2 is charged from a reservoir C3 and discharged through the primary winding of a step-up pulse-transformer T by triggering a thyratron V; C1 is connected across the primary winding of transformer T, which has a step-up ratioofn. Q

It is preferred to make C1 CZ/rr', ie to match CI and C2 as reflected by the ratio n. Under these conditions substantially all the energy stored in C2 is transferred to C1 when the latter reachesits peak voltage, at which instant the transformer primary current is zero since this is the condition for maximum efficiency. In practice the gap is set to break down slightly before peak voltage, to give a margin of error, and because the breakdown voltage is not a precise value but has a statistical spread about a mean value, as shown for example in FIG. 2, which shows the spread of actual breakdown voltages about a mean gap-setting of 90 kV, the peak voltage in this instance being 100 kV.

In use the gap width increases as the electrodes erode, so that unless some correction is applied to maintain the gap width, the breakdown voltage thereof gradually increases to a value exceeding the peak voltage and the apparatus ceases to function.

The present invention provides this correction automatically. The embodiment to be described utilises for this purpose a property of the primary current waveform shown in FIG. 3. FIG. 3(a) shows this current waveform, and the voltage waveform across CI, for the ideal condition in which the instant of breakdown (:1) occurs at peak voltage (:2). Component values in this embodiment are assumed to be:

C1 0.024 uF L (the leakage inductance of transformer T) 40 p.11

Supply voltage on C3 kV.

As explained in the aforementioned Patent, the voltage on C2 prior to the first discharge is 5 kV. Thereafter the resonant charging of C2 via D1 and L2 raises it to 10' kV after each discharge. A peak voltage of approximately l00 kV is obtained across Cl and the peak current in the primary winding is about 2,000 A.

Breakdown (11), peak voltage (t2) and zero current (t3) all coincide after a half-cycle of oscillation of the resonant circuit formed by L in series with C2 and Cl, where Cl is the reflected capacitance of C1. C1 n 'CI C2. Accordingly the oscillation period is with the above component values.

If the start of the half-cycle, t is taken as a datum, the time-interval to the end of the current half-cycle (t3) is approximately 27 psec.

FIG. 3( b) shows the current waveform when the gap breaks down before peak voltage on C1. In efiect C1 (and hence C1) is short-circuited at the breakdown time 11, which doubles the series capacitance and hence prolongs the current half-cycle beyond 27 usec at time :3. The time to 3 increases as the time to t1 diminishes, relative to the datum t,,.

FIG. 3(c) shows that if the gap breaks down afier peak voltage, t3 coincides with 12 after 27 psec, as in FIG. 3(a). FIG. 3(d) shows that the same is true if the gap fails to break down at all. The effects shown in FIGS. 3(0) and 3(d) are for comparison only, and are not utilised in the present invention.

The quantity used to control the gap width in the present embodiment is the time to :3, which, as shown in FIG. 2(b), increases as the breakdown voltage is reduced relative to the peak voltage, ie as the time to :1 decreases. With a peak voltage of I00 kV, the embodiment operates to maintain the breakdown voltage within a range of about 85-98 kV. This voltage range corresponds approximately to values of t lying within the time-range 28-30 psec after the datum t The circuit used in this embodiment is shown in FIG. 4. The half-cycle current waveform is developed across resistor R1 in FIG. 1 and is fed to a filter circuit 4 which removes any high-frequency oscillations present on the signal. Thecleaned-up signal is applied to a Schmitt trigger circuit 5 which produces a rectangular pulse whose length equals the duration of the current pulse, i.e., the time to :3. This rectangular pulse is fed to a differentiating circuit 6 which produces short pulses coincident with its start and end.

The start pulse from circuit 6 triggers a monostable circuit 7 which produces an output pulse of adjustable duration. Let it be assumed that the duration is set to about 28 usec. This pulse is fed to open a gate 8 which also receives the end pulse from circuit 6. If these pulses coincide, an output from gate 8 ,is fed to monostable circuit 9 which produces a 10 msec output pulse.

The start pulse also triggers a monostable circuit 10 which produces an output pulse of adjustable duration. Let it be assumed that the duration is set to about 30 sec. This output pulse is connected to close a gate 11 to which the end pulse from circuit 6 is also fed. If the end pulse arrives at gate 11 after the end of the pulse from circuit 10, an output from the gate triggers monostable circuit l2,'which produces a 10 msec output pulse.

Thus, with the above-assumed pulse durations, circuit 9 is triggered if the gap breaks down above about 98 kV and circuit 12 is triggered if the gap breaks down below about kV. If the gap breaks down between these voltages, neither circuit is triggered. In practice these pulse durations are adjusted until satisfactory consistent operation of the apparatus is obtained.

The outputs from circuits 9 and 12 are fed to integrators l3 and 14 respectively. The latter each comprise a capacitor in parallel with a variable resistor. The latter are set so that after receipt of an adjustable number of pulses in a given time from circuits 9 or 12, e.g., 14 consecutive pulses at 32 pulses/sec, the voltage on the capacitor exceeds the threshold level of one of the relevant Schmitt circuit 15 or 16 and triggers it. The outputs of the latter are fed to amplifiers 17 and 18 respectively which are normally biassed-off, but which, when turned-on by the Schmitt outputs, energise relays 19 and 20 respectively. When relay 19 is energised, 25 V is applied to a DC motor 21 which rotates in a direction to drive the electrodes together and so reduce the gap. When relay 20 is energised, +25 V is applied to motor 21 which rotates in the opposite direction to separate the electrodes and so increase the gap as described hereafter.

Referring now to FIG. 5, the vessel 1 is suitably made of a tough insulating material such as monocast nylon.

The electrodes 3/1 'and 3/2 are inserted through thevessel walls. Leakage of water from the vessel past the electrodes is prevented by conventional O-ring seals (not shown). The outer end of electrode 3/1 is secured to a worm wheel 22 which runs in a thread formed on the inside of a sleeve 23 made of an insulating material such as polymethylmethacrylate. The inner end of sleeve 23 is secured to a bevel gear 24 which engages a bevel gear 25. The latter is secured to an insulating shaft 26 driven from a reversible electric motor .21 via a gear-train 28. y

Bevel gear 24 runs in a split bearing (not shown) which is located in a metal housing 29. Housing 29 also 5 forms the electrode terminal to which the live (positive) side of capacitor C1 is connected, and for this reason has a smooth-contoured outer surface to prevent sparking. Electrical contact between housing 29 and electrode 3/ l is made by a split metal bush (omitted for clarity) secured to the inside of the housing. The electrode is a sliding fit in the bush. Housing 29 includes an extension 31 of semi-circular cross-section which is secured to the similarly shaped end of a circular bar 32 by a screw 33. Bar 32 passes through and is secured to the outer wall of the enclosure 34 within which the vessel and electrodes are mounted. Bar 32 forms the electrical connection to Cl. I

A rectangular section strip 35 of insulating material is secured to the outer end of worm 22 and slides in a grooved member 36 which is also secured to the outer wall of the enclosure 34.

In the present embodiment electrode 3/2 is fixed.

In operation the reversible motor 21 is controlled by relays l9 and 20 (FIG. 4) as described above. Rotation of the motor in either direction causes a corresponding rotation of sleeve 23. Worm 22 is prevented from rotating the sleeve 23 by the engagement of strip 35 with member 36. Worm 22 therefore moves axially along sleeve 23 in a direction depending on the direction of rotation of the motor. Electrode 3/1 is accordingly driven towards or away from electrode 3/2 to reduce or increase the gap between them.

In a modification of FIG. a driving mechanism similar to that shown for electrode 3/] is provided for electrode 3/2. The two mechanisms can be driven either by the same motor 21 or by two separate motors. In this way the gap between the two electrodes can be maintained at approximately the same position in vessel 1 as the electrodes erode. Since the positive electrode, eg electrode 3]], normally erodes quicker than the negative electrode, the gear train ratios in the drives to the two electrodes can be designed to drive electrode 3/l more rapidly than electrode 3/2 in order to compensate for the different rates of erosion.

Because the first pulse appearing across C1 is of reduced voltage (C2 being charged to only 5 kV) the initial gap setting is done manually. Thereafter the abovedescribed circuit operates to maintain the breakdown voltage within the desired range.

The invention is not limited to the particular circuit shown in FIG. 4. Nor need the time to breakdown be derived from the duration of the primary current halfcycle. For example the interval between triggering the thyratron V (or corresponding device) and the occurrence of breakdown as detected by the resulting lightflash or shock-wave may be measured instead, using piezo-electric or photo-electric detectors respectively.

I claim: I

1. Electrohydraulic crushing apparatus comprising a spark-gap which in use is immersed in liquid, and means for repetitively applying a high voltage across the gap to cause repetitive breakdown thereof, wherein there is provided: means for deriving a signal coincident with the instant of breakdown voltage; means for comparing said parameter of said signal with predetermined limiting values thereof corresponding to upper and lower limits of a predetermined breakdown voltage range, and for generating an output signal if said parameter of said derived signal is outside either of said limits, said output signal being characteristic of whether said parameter is outside the upper or lower limit; and means responsive to said output signal for adjusting the gap width in a sense to prevent the generation of said output signal and thereby maintain the actual breakdown voltage substantially within said upper and lower limits.

2. Apparatus as claimed in claim 1 comprising means for receiving the characteristic output signals from said comparing means and arranged to operate said gapwidth adjusting means only after receiving a substantial plurality of said output signals of either one said characteristic.

3. Apparatus as claimed in claim 1 wherein the means for repetitively applying the high voltage comprises means for generating high-voltage pulses having a finite rise-time to peak voltage, and wherein said parameter of said derived signal is the time elapsing between a datum time for each pulse and the occurrence of said derived signal, and said upper and lower limits define between them a predetermined time-range after said datum.

4. Apparatus as claimed in claim 3 wherein said datum is the start of each high-voltage pulse.

5. Apparatus as claimed in claim 3 wherein said means for repetitively applying the high voltage comprises a first capacitor connected directly across said spark-gap, a second capacitor chargeable from a DC source and repetitively dischargeable through the primary winding of a step-up pulse-transformer, the secondary winding of said transformer being connected across said first capacitor whereby the first capacitor is pulse-charged thereby and the first and second capacitors being substantially matched in value as reflected by the transformer ratio, and wherein said parameter is the duration of the first half-cycle of current in the primary winding of transformer and the datum time is the start of said half-cycle, whereby the gap is adjusted so that the end of said half-cycle occurs within a predetermined time-range after said datum.

6. Apparatus as claimed in claim 5 further comprising circuit means for generating pulses coincident with the start and end of said half-cycle, and gate means for providing separate outputs when the end-pulse occurs less than a shorter given time-interval after the startpulse, and when the end-pulse occurs more than a longer given time-interval after the start-pulse.

7. Apparatus as claimed in claim 6 wherein the outputs of the two gates are applied to control the direction of rotation of an electric motor means which drives at least one gap electrode towards or away from the other to adjust the gap width.

8. Apparatus as claimed in claim 7 wherein the two gate outputs are applied to operate relay means which reverse the polarity of the voltage supply to the motor means.

9. Apparatus as claimed in claim 6 wherein each gate output is integrated and compared with a reference level so that a plurality of output pulses must issue from a given gate before the gap-width is adjusted.

10. Apparatus as claimed in claim 7 wherein one electrode is fixed and the other driven towards and away from it by a single reversible motor.

i l I t t v i 'EN TE STATES PATENT OFFICE 7 CERTIFICATE OF CORRECTION I Patent'No. 3,749,958 Dated JulyBl, 1973 -Inven'tor(s) Y GEOFFREY MARCUS WARD f It is certified that error appears in the above-identified patent and that said Letters Patentare hereby. corrected as shown below:

I In Clairh e't line 60,1 add" the following after: "breakdown" ian a nf ba a e hi va es w th Few? Q breakdown-"e.

Signed Ediea this--luth day 05. May 197 L) Attest:

LEDWARD MELETCHERJH." '0 MARSHAL-L DANE" Attes ting Officer 7 Gommissioner.v of. Patents uscoMM-oc scanea FORM PO-1050 (16-69) u.s. c oyihunzur rm aimc 0mg: III! o-uhsu UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,749,958 Dated July 31 1973 Inventor s) "GEOFFREY MARCUS WARD It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below;

In Claim 1, at line 60, add the following after "breakdown" -and having a parameter which varies with the actual breakdownm Signed and sealed this ll th day of May 197R.

(SEAL) Attest:

EDWARD M.FLETCHEIR,JR. V c. MARSHALL DANN Attesting Officer Commissionerof Patents F ORM PO-I 050 (10-69) USCOMM-DC. 60376-P69 u.s. GOVERNMENT PRINTING OFFICE: i985 0-366-334

Claims

2. Apparatus as claimed in claim 1 comprising means for receiving the characteristic output signals from said comparing means and arranged to operate said gap-width adjusting means only after receiving a substantial plurality of said output signals of either one said characteristic.

6. Apparatus as claimed in claim 5 further comprising circuit means for generating pulses coincident with the start and end of said half-cycle, and gate means for providing separate outputs when the end-pulse occurs less than a shorter given time-interval after the start-pulse, and when the end-pulse occurs more than a longer given time-interval after the start-pulse.