US3156889A

US3156889A - Rheostat

Info

Publication number: US3156889A
Application number: US202483A
Authority: US
Inventors: Keith E Starner
Original assignee: Aerospace Corp
Current assignee: Aerospace Corp
Priority date: 1962-06-14
Filing date: 1962-06-14
Publication date: 1964-11-10
Anticipated expiration: 1981-11-10

Description

United States Patent Olice 3,156,889 RHEGSTAT Keith E. Starner, Gardena, Calif., assigner to The Aerospace Corporation, Los Angeles, Calif., a corporation of California Filed .lune 14, 1962, Ser. No. 202,483 Claims. (Cl. SSS-151) This invention relates to rheostats and more particularly to high current, low impedance, innitely variable resistance devices.

Prior art rheostats are designed with many characteristics suitable for performing numerous specialized operations. Although forced cooling ot rheostats is known in environments requiring resistance dissipation of large amounts of power and rheostats utilizing mercury pool contacts are known, a combination of these arrangements to provide control of hundreds or thousands of amperes has not been fully exploited. There remains a problem of obtaining infinitely variable low impedance control of large battery currents of up to several thousand amperes.

Therefore, an object of the present invention is to provide an improved high current, innitely variable, liquid contact rheostat.

In accordance with one embodiment of my invention, a plurality of hollow arcuate resistive elements are rotatable to engage mating mercury pools. The mercury electrically couples the resistive elements to a contactor, thus forming a rheostat. Means are provided for pumping through the elements and the contactor respectively, a cooling liuid which results in a capability of this invention for regulating large amounts of current without destructive heating of the conducting resistive elements. A simple arrangement for accomplishing necessary parameter control is attained by sensing the critical parameter and adjusting the positions of the rheostat resistive elements to accomplish any correction necessary. For instance, the load current may be sensed with the rheostat position being adjusted to maintain a desired constant current flow despite impedance variations of the system because of thermal variations of impedance components.

The subject matter which is regarded as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, as to its organization and operation, together with further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a simpliied schematic diagram illustrating one use of the present invention in a parameter sensitive environment requiring substantial current and power regulation;

FlG. 2 is a side elevation view partially in section of one embodiment of my invention; and

FIG. 3 is a cross-section View taken along the line of 3-3 of FIG. 2.

Referring now to the drawing, wherein like numbers refer to similar parts, i show in FIG. 1 a rheostat 10 of the present invention arranged in a circuit diagram. In this circuit an arc plasma generator 12 is arranged to apply heated gas to a test model 14. Ofter such an arrangement will be utilized to test ablation characteristics, wherein the average temperature of the gas impinging on the model 14 is of the order of 3,000 to 5,000 K. The attaining of such a temperature usually requires electron temperatures within the arc of the order of 15,000 to 20,000" K.

The arc generator 12 comprises two

electrodes

15 and 15. The electrode 15 is preferably ol' a refractory metal such as tungsten butin some applications may be a corn- CTL 3,155,889 Patented Nev. 1o, 1964 pacted graphite rod, and the electrode 16 is a subsonic or a supersonic nozzle arrangement formed of a conductive material such as certain copper alloys. Other primary circuit components include a battery power supply 18 having an internal resistance represented schematically by a rheostat 19, a double contact circuit breaker 20 and a fixed ballast resistor 22. The circuit breaker 20 is closed to initiate current flow and is biased open upon release by the action of trip solenoid 23 to terminate current flow. The ballast resistor 22 is provided to limit initial maximum current flow.

The maintaining of a predetermined flow of heated gas from the arc generator 12 turns out to be a relatively complex problem. Gas supplied through line 24 is applied to the arc, preferably through a plurality of choked supersonic nozzles 25 exiting into an insulating housing 26, whereby the particular internal pressure of the system will not materially aliect the rate of gas flow. The gas tiows from the housing 26 through the region of the arc and is heated thereby. Several gases may be used in an arc enerator of this type including air, argon, helium and nitrogen. However, the rate of flow of gas 24 may not be varied over a wide range for a given arc without suffering extinguishing of the arc. Therefore, an additional plenum chamber 28 is provided whereby additional amounts of gas may be introduced to the system through a control valve 29 to regulate the total mass of gas tiow and the temperature of the gas impinging on the model 14. The plenum chamber 28 also operates much in the manner of a muiiler to diiiuse shock gas fronts and dampen the eiects of arc lluctuation and, further, provides a region of low velocity suitable for measuring the stagnation pressure of the gas jet.

Use of the battery power supply 1S provides a desirable source of ripple-free current not readily attained when using alternating current or rectifier-type power supplies. However, batteries supply current at an essentially constant voltage and desired smooth current variation is accomplished by a change of the system resistance. For arc generator operation requiring a programmed change of current, the ohmic variation is produced by the rheostat 10 of my invention. ln one arrangement, a circuit current signal is generated in a shunt around ammeter 30 and applied to a reference network 32 which generates an error signal 34. The error signal 34 is applied to a control iield 36 of a selsyn-type positioning motor 38 which is drivingly coupled to the rheostat 10. Any error signal 34 will energize the motor 3S, to reposition the rheostat 10 in accordance with a reference voltage applied within the network 32. The reference voltage may be programmed to simulate various phenomena.

In order to produce transient ablation of the test model 14 for simulation of nose cone re-entry heat transfer rates, programmed variation of the current supplied to the arc generator 12 will be a standard operating procedure. It is well known in the art that the arc operating voltage will remain essentially constant over a wide range of current 'low. Thus the power dissipated in the arc and applied to the model 14 is directly proportioned to the current controlled by my invention. By way of example, when the battery power supply 1S is comprised of 320 12.5 volt storage batteries arranged in a series parallel array to give a fixed supply voltage of 250 volts, a current of 1500 amperes can flow through the arc generator 12. During such operation a voltage of about volts directly across the electrodes 15 and 16 is typical.

With the rheostat 10 initially positioned to provide zero ohms the resistance of the rheostat 19 and fixed ballast resistance 22 will develop the remaining 150 volt drop of the circuit. By increasing the resistance of rheostat 10 the current can be made to decrease thus changing the entrasse sa power of the are and the heat-transfer rate to the model i4. Using specially designed batteries, a typical value of total battery internal resistance in such a series parallel array is 4.65 (10)*3 ohms leaving 9.53 (10)-2 ohms to 'be made up by the ixed ballast resistor 22 to limit the vwill be compensated for automatically by operation of the rheostat i in accordance with the error signal 3d.

In the event that excess currents do flow in the circuit,

rthey will also be sensed in the ammeter shunt and applied to an overcurrent relay 39, which will operate the trip solenoid 23 in a manner well known in the circuit breaker art. 0bviously, the error signal 34 may be replaced by a manual operation, when the reference network 32 includes an ammeter dial. Also, programming may be manually effected. However, a more sophisticated arrangement as indicated often will provide a smoother transition.

Referring new to FiG. 2, I show in detail the rheostat of my invention which is capable or continuously conducting many hundreds of amperes while at the same time having a controlled and infinitely variable impedance. Empirical data indica-te that a stepped variation of the impedance in the circuit adversely affects the operation of the arc jet and often produces inaccurate test data. Therefore, I have provided a plurality of rotatable annular nichrome tubular resistive elements dil. Using nichrome elements having a peripheral length of about 20" and a diameter of 1A with a wall 4thickness of .005, the resistive elements 40 will each continuously conduct about 400 amperes when they are forced-circulation cooled by room temperature water at 60 psi. and at a iiow rate therethrough of l0 per second. The iiow rate of l0 per second is approximately 12 gallons per minute when using eight resistive elements -as shown in FIG. 2. A pump 4l easily supplies this flow rate in a completely pressurized system, and by enlarging the pump al the output water may be dumped in arrangements where no large storage capacity is available. Also, in using the present invention, Where distilled water is unavailable scaling should be removed periodically.

In this particular application the resisitive elements 40 each have a mam'mtlm resistance of about 0.2 ohm and are arranged each to conduct as much as 400 amperes without destructive heating. The coolant water flows in the direction of arrows 4t2 through a coupling d3 into a rotatable central support pipe d4, through insulating coupling pipes 46, and elbows 47, through the nichrome resistive elements 40 and elbows 49 to a coolant header d8 and out through a coupling 50. The coolant header 48 also functions as a current collector bus bar `for coupling together a plurality of the resistive elements d0. In one arrangement the coupling pipes d6 are of Teiion and the other coupling elements are chrome-plated copper tubes and elbows. Chrome plating is resistive to mercury vapor corrosion. In order that high pressure water may be pumped through this system7 a rotatable seal is provided in the form of O-rings 5l and 52 at each of the couplings 43 and 50 respectively. Also, the pipe 44 is provided with a plug 53 for directing the water through the resistive elements 40.

As shown in FIG. 2, the mating contactor element comprises a mercury well contacter arrangement 54 defining a series of depressions or wells 55 which cont-ain pools 56 of mercury to couple electrically the resisitive elements 40 to the contacter 5d. In order that the resistive elements l0 may be serially coupled, the contactor 5d is divided into a plurality of sections by insulating inserts or washers Thus, the flow ot current as illustrated by dashed arrows 59 may be from left to right (FIG. 2) through the illustrated rheostat lil, from a terminal oil to the irst section of the contacter 54, through the first pair of resistive elements d0 and elbows d@ to the bus bar d8, through a second pair of resistive elements 40 to the second section of the contacter 5d, through a third pair of the resisitive elements di), etc. The return of the current through the central resistive elements dil to the central section of the contactor 5d is assured by the insertion of an insulating coupling 62 which divides the bus bar 4S into `a plurality of sections.

Finally, the current iiows from the last section of the contacter 5d to the output terminal 64. Obviously, the current ilow may be arranged to pass between the contactor sections and theV bus bar sections a greater number oi times than that illustrated herein. Similarly, additional resistive elements may be stacked in parallel when it is necessary to provide increased current capacity.

it is preferable to arrange 4for the water coolant to iiow in the direction of the arrows d2 (FIG. 2, 3) so that those portions of resistance elements 42 which are immersed in the mercury pools 56 are first to be cooled by the water to thus minimize heat transfer to the mercury. In this Yway the creation of noxious and corrosive mercury vapors can be avoided. For most effective and safest operation the water inlet temperature should be approximately 10 C. with a flow rate sufficient to keep outlet coolant temperature at or below 80 C. Should it be necessary to operate at higher coolant temperatures, in a manner well known the coolant system could be pressurized to increase the boiling point of the water and thus eliminate the possibility or" localized boiling in resistance tubes i2- As will be seen in viewing FIGS. 2 and 3 additional cooling for the mercury pools 56 is provided by having the area beneath wells SS iacketed. In this jacketed area the pump il also forces the circulation of coolant iiuid in a direction from left to right in FIGURE 2.

As discussed in connection with FIG. l, regulation oi the angular position of the resistive elements d0 is controlled by the motor 3S. As shown in FIG. 2 the motor 38 is coupled `to the rotatable pipe 44 through corrosion resistant reduction gearing 66. As a safety measure, eX- cessive temperatures in the rheostat l@ are prevented and the elements i0 are protected from high exit water temperatures or water pressure failure by a combined temperature and pressure actuated relay 67 which is also coupled to the trip solenoid 23 and will energize itin the event of high temperature or low pressure. Protective controls of this type are well known in the heating industry and need not be explained in detail here. An additional safety feature is provided in the form of a Enger or stop 68 positioned on the outer surface of the rotatable equipment (the trailing edge of at least one elbow d'7) to engage an abutment 69 (FIG. 3) in the well 55 and thus prevent substantial loss of contact between the mercury pool 56 and the elements 40, when a maximum amount of resistance has been provided.

Since I am utilizing mercury contacts, I prefer to en-V ,shows the annular configuration of one resistive element 40, the central location of the central pipe 44, the insulating pipe 46 and the relative location of the bus bar header 48. The electrical current flow is illustrated by the dashed arrow 59 and the cool-ant water ow is illustrated by the solid arrow d2. FIG. 3 also illustrates insulating support straps 7d which engage the elements 40 by means of loops to maintain them a iixed radial distance from central pipe iid. To maintain the arcuateV re- :greased sistance elements di) fixed longitudinally of central pipe it and to generally rigidize the assembly, aligned hollow spacers 77 are interposed between adjacent straps 7 4i, and tie rods '76 are secured through the spacers 77 and the holes in the straps pill. These tie rods 76 and spacers 7'7 will withstand any mechanical vibrations and magnetic forces that might tend to move the resistance elements relative to each other and relative to the mercury pools. Using no more than about 20 resistive eleLA ents itl with no more than 400 amperes flow through each element, additional cross bracing is usually unnecessary.

As is also illustrated in FIG. 3 each of the resistive elements itl is provided with a substantial arcuate gap 73 which may be positioned in the region of mercury pool 56 when the equipment is not in use. In this manner, corrosion of the nichrome tubing caused by extended contact with mercury may be eliminated.

While I have shown and described a particular embodiment of the present invention, further modifications may occur to those skilled in this art. For instance, although I have described in some detail one invironment requiring the infinitely variable reheostat of my invention, this invention may be used in numerous applications for controlling large battery currents and the like. I desire it understood, therefore, that my invention is not limited to the particular form disclosed and I intend by the appended claims to cover all such modifications which do not depart from the true spirit and scope of my invention.

I claim:

l. An infmitely variable, high current rheostat for regulating hundreds of amperes in an environment sensitive to relatively small current variations, comprising:

a plurality of parallel hollow annular resistive elements adaptable for conducting large electric currents and substantial coolant liuid;

a centrally disposed rotatable support pipe coupled to one end of each of said elements by means of an insulated hollow coupling, respectively;

a bus bar header both electrically and hydraulically coupled to the opposite ends of said elements;

a contacter defining upwardly opening arcuate wells arranged to encompass an arcuate portion of each of said elements respectively;

a mercury pool within each of said wells for electrically coupling said contactor to said elements;

insulating means for dividing said contactor into a plurality of sections so that the only coupling therebetween is through a plurality of said elements;

means for detecting the current flow through the rheostat; and

means for rotating uniformly said elements relative to.

said contactor to vary the impedance between said mercury pools and said header and thereby regulate the voltage between said contacter sections while maintaining a programmed current flow of the order of hundreds of amperes through the rheostat.

2. An infinitely variable, high current rheostat for use in a circuit arranged to conduct a programmed current iiow of the order of hundreds of amperes, comprising:

a centrally disposed rotatable support pipe arranged to support one end of each of said elements;

means for electrically insulating said elements from said pipe;

a contactor defining upwardly opening arcuate wells arranged to encompass an arcuate portion of each of said elements respectively;

insulating means for dividing said header into a plurality of sections so that the only coupling therebetween is through a plurality of said elements;

means for forcing coolant iiuid through said elements so that it flows from the region of said pools toward said header;

means for detecting the current flow through the rheostat; and

means for rotating uniformly said elements relative to said contacter to vary the impedance between said mercury pools and said header and thereby vary the Voltage between said header sections while maintaining a controlled current flow through the rheostat.

3. An infinitely variable, high current rheostat for use in a circuit arranged to conduct a programmed current iiow of the order of hundreds of amperes, comprising:

a plurality of parallel hollow annular resistive elements adaptable for conducting large electric currents and substantial coolant fluid;

a centrally disposed rotatable support conduit arranged to support one end of each of said elements;

means for electrically insulating said elements from said conduit;

a contacter delining upwardly opening arcuate Wells arranged to encompass an arcuate portion of each of said elements respectively;

insulating means for dividing said contacter into a plurality of sections so that the only coupling therebetween is through a plurality of said elements; and

means for forcing coolant ud through said elements so that it iiows from the region of said pools toward said header to thereby minimize the temperature variation of said pools during prolonged operation of the rheostat.

4. An infinitely variable, high current rheostat for use in a circuit arranged to conduct a programmed current iiow of the order of hundreds of amperes, comprising:

a plurality of parallel hollow annular resistive elements adaptable for conducting large electric currents and substantial coolant lluid;

a rotatable support conduit arranged to support one end of each of said elements;

means for electrically insulating said elements from said conduit;

a bus bar header both electrically and hydraulically coupled selectively to the opposite ends of said elements;

a first terminal;

a first mercury pool for electrically coupling said first terminal to one of said elements;

a second terminal;

a second mercury pool for electrically coupling said second terminal to another of said elements;

means for forcing coolant iiuid through said elements so lthat it flows from the region of said pools toward said header to thereby minimize the temperature Variation of said pools during prolonged operation of the rheostat; and

lateral support means between said elements for preventing substantial reaction thereof to magnetic forces therebetween.

5. An infinitely variable, high current rheostat for use in a circuit arranged to conduct a programmed current flow of the order of hundreds of amperes, comprising:

means for electrically insulating said elements from `said conduit;

7 a bus bar header both electrically and hydraulically coupled to the opposite ends of said elements; a rst terminal; a rsvt mercury pool for electrically coupling said irst terminal to one of said elements; a second terminal;

a second mercury pool for electrically coupling said second terminal to another of said elements; and means for forcing coolant lluid through said elements so that it ows from the region of said pools toward said header to thereby minimize the temperature variation of said pools during prolonged operation of the rheostat.

References (liteit in the file of this patent UNITED STATES PATENTS Von Brockdort Y Feb. 26, Smith lluly 20, Emmet May 27, Sykes Feb. 26, Kennen Sept. 2, Richardson Feb. 24,

FOREIGN PATENTS Germany Feb. 28,

Claims

1. AN INFINITELY VARIABLE, HIGH CURRENT RHEOSTAT FOR REGULATING HUNDREDS OF AMPERES IN AN ENVIRONMENT SENSITIVE TO RELATIVELY SMALL CURRENT VARIATIONS, COMPRISING: A PLURALITY OF PARALLEL HOLLOW ANNULAR RESISTIVE ELEMENTS ADAPTABLE FOR CONDUCTING LARGE ELECTRIC CURRENTS AND SUBSTANTIAL COOLANT FLUID; A CENTRALLY DISPOSED ROTATABLE SUPPORT PIPE COUPLED TO ONE END OF EACH OF SAID ELEMENTS BY MEANS OF AN INSULATED HOLLOW COUPLING, RESPECTIVELY; A BUS BAR HEADER BOTH ELECTRICALLY AND HYDRAULICALLY COUPLED TO THE OPPOSITE ENDS OF ELEMENTS; A CONTACTOR DEFINING UPWARDLY OPENING ARCUATE WELLS ARRANGED TO ENCOMPASS AN ARCUATE PORTION OF EACH OF SAID ELEMENTS RESPECTIVELY; A MERCURY POOL WITHIN EACH OF SAID WELLS FOR ELECTRICALLY COUPLING SAID CONTACTOR TO SAID ELEMENTS; INSULATING MEANS FOR DIVIDING SAID CONTACTOR INTO A PLURALITY OF SECTIONS SO THAT THE ONLY COUPLING THEREBETWEEN IS THROUGH A PLURALITY OF SAID ELEMENTS; MEANS FOR DETECTING THE CURRENT FLOW THROUGH THE RHEOSTAT; AND MEANS FOR ROTATING UNIFORMLY SAID ELEMENTS RELATIVE TO SAID CONTACTOR TO VARY THE IMPEDANCE BETWEEN SAID MERCURY POOLS AND SAID HEADER AND THEREBY REGULATE THE VOLTAGE BETWEEN SAID CONTACTOR SECTIONS WHILE MAINTAINING A PROGRAMMED CURRENT FLOW OF THE ORDER OF HUNDREDS OF AMPERES THROUGH THE RHEOSTAT.