US2649526A - Liquid resistance electrical apparatus - Google Patents

Description

Aug. 18, 1953 N. R. BENcHEMoUL LIQUID RESISTANCE ELECTRICAL APPARATUS 3 Sheets-Sheet 1 Filed Aug. 5, 1952 Aug. 18, 1953 N. R. BENcHr-:MOUL 2,649,526

LIQUID RESISTANCE ELECTRICAL APPARATUS Filed Aug. 5, 1952 3 Sheets-Sheet 2 fe A 79% 4h02 ec.

74u20 f W Al1g 18, 1953 N. R. BENcHEMoUL 2,649,526

LIQUID RESISTANCE ELECTRICAL APPARATUS Patented Aug. 18, 1.953

OFFICE LIQUID RESISTANCE ELECTRICAL APPARATUS Norbert R. Benchemoul, Paris, ifrance Application August 5, 1952, Serial No. `302;'789

In France August 7, 1951 12 Claims. l

The present invention relates to electrical apparatus which includes a liquid resistance and is capable of use in a variety of ways, e. g. as a rheostat, a means of limiting current strength or of generating current impulses or as a source of heat having inherent thermostatic control. The action of theapparatus is based on the great dif,- ference in resistivity existing between a liquid (for instance water) and its vapour.

Electrical apparatus .according to the invention comprisesone or more enclosed spaces or Ycornf partments each in communication with a receptacle containing an electrically conducting liquid by means of one or more passages oiiering resistance to the flow of the liquid, there being in each compartment two or more electrodes so located as to make contact with liquid in `such compartment. Preferably each compartment is provided with at least two passages, whereby circulation of liquid by means of thermo-Siphon action may take place.

The apparatus is dimensioned-insuch'amanner that, with a current equal to or exceeding a determined threshold value passing from one electrode to another through the liquid in the compartment orv each compartment, .evaporation of the liquid is produced, which is the more complete and rapid the smaller the volume ofthe liquid lying between the electrodes. This evaporation causes a considerable increase in the resistance of the apparatus and in consequence a drop in the value of the current.

The vapour thus generated in the compartment cannot be instantaneously replaced by liquid from the receptacle, because .of the hydraulic lresistance offered to its flow in the communicating passages. However, after a certain time, which is a function mainly of the dimensions of the passages, the compartment is again lled with liquid, the resistance of the apparatus then falls and consequently the value of the current passing through its increases, assuming thata voltage across the electrodes is maintained. If this voltage is of such a value that the current passing through the compartment is equal to or'exceeds the threshold value Ydei-ined above, the phenomenon of evaporation of the. liquid .and its subsequent return tvill be repeated indefinitely, if desired with variations .corresponding yto the resistance and consequently the value of the current.

Thus .the apparatus functions as a generator of electric impulses of a determined frequency. The latter may be modified, for instance, yby varying the effective cross section of the passage or pas- (cl. 201-57) Y* sages .connecting the lreceptacle with the compartment: ,adecreaseofthiscrosszsection causes adecrease ,ofz the frequency and vice versa.

If. Qnthecontrary, the voltageatthe terminals of, the .apparatus has a. value below that, corre- Spending tothe .threshold value of the lcurrent causing evaporation .of the liquid, then the resistance o .f .the yapparat-usY willmaintain a relatively-low andsubstantiallv .constant value. In that .case-the currentpassing through the compartment. causes heating of thev .liquid contained therein and, consequently, circulation by .convection or by thermo-Siphon action Withthat in .the receptacle. thusgraduallvraisins the temperature Gispen liquid;`

This circulation.. Whiehtakes. place the Ymore rapidly the .lower thetemperature of the liquid, capses..1emoval of; the heatsenerated in the compartrn. t. However, in proportion as the temperature of the `liquid in the receptacle rises, this /eiipulatopslows .dov/non. the .one hand and the; supplvofenersvfneeessary for producing the -evaperationlcleereases onthe other hand. When the,temperatureothe liquidreaches a certain eritieal value, theheatliberated in the compartment becomes suoient .for evaporatins, at least partially, the liquid present there.

Trims` a ,cye,1eo .f. current impulses is started andthese besoins :shorterianpl .shorter in proportion asethegternperature continues to rise to an equillbriumyalue. at kwhich `the addition of heat is equal to .thelosses; Conversely, if the temperattirestarts dropping, the cycle of current iinpulses beeomesioneerfand consequently the heat suppliedincreases. The. apparatus may thus constitute a source of heat having inherent ther- Inostatieioontrol.

Gnrtheother hand, the same device may be ntilisedmoresimply, as a current limiter. As vlong .as the. current remains suiciently Weak, .circulationproeeedsnormally .as lone `as .the consumpton of enersvin the apparatus remains below the-critcal consumption. When the consumpticn, ;i. e. Ethe@value .ofthe lcurrent passing, exceeds the threshold value, the impulse device operates such .a manner as to limit the effective current to a well-determined quantity. No interruption ofthe V,current .occur-s. If the overload is only temporary, the system immediately returns to .a ,Stablestate IL, on .the contrary, the overload-continues, ther apparatus may be used to ensureV the protection of -diierent electric circuits by limiting the value of the current pass- 111g.

Finally, inthe case when the voltage applied to the terminals of the apparatus varies from a value exceeding the threshold marking of the start of evaporation to a Value below this threshold, the apparatus is applicable as a rheostat of inherently variable resistance.

Such cases of voltage variation are frequent in practice. It is known, for instance, that the back-electromotive force of an electric motor, for instance an asynchronous motor, is proportional to its speed. In particular, when stationary, the back-electromotive force is Zero, and at the moment of starting, if full voltage is applied, there is a risk of damaging the motor. With such types of motors and, in particular, with motors having wound rotors, starting rheostats are therefore used, the object of which is to limit the intensity of the rotor current and consequently of the current drawn by the stator from the supply line.

On the other hand, when a rheostat is connected in series with the rotor windings, it is possible to maintain the starting torque at a suitable value, whilst at the same time limiting the current. A number of automatic devices have been developed which function to vary the resistance of the rotor circuit during starting, by reducing it to a minimum value after a certain time by which the motor has reached its `operating speed. Such devices are, in general, expensive, comprise a multiplicity of electrical circuits which increase the possibility of breakdown and do not always operate under conditions of perfect safety.

On the other hand, liquid rheostats are known, which possess numerous advantages, but which frequently give rise to complaints about the instability of the resistance between the electrodes, the instability being due to variations in the concentration of the electrolyte with time.

Apparatus according to the present invention permits avoidance of these various inconveniences and production of an automatic starting device not expensive in its purchase price, extremely unsusceptible to deterioration and variations in the resistance of which are suiiiciently independent of possible alteration in the conductivity of the liquid.

It is an outstanding advantage that apparatus according to the invention, in all its applications, operates automatically, i. e. without actuation of any movable member, thermo-dynamic and hydraulic phenomena alone ensuring its operation. It is possible to provide movable members for making a preliminary adjustment, but subsequent operation of the device is not dependent on movement of such members.

In order that the invention may be clearly understood and readily Icarried into effect it will now be described more fully with reference to the accompanying drawings which illustrate, by way of example, two forms of apparatus suitable to constitute a starting rheostat for a threephase asynchronous motor, and wherein:

Figure 1 is a longitudinal section of one form of the apparatus,

Figure 2 is a cross-section on the line II-II of Figure l,

Figure 3 illustrates the control plate for the passages,

Figures 4 and 5 show the character of the curves of variation in the resistance vof the apparatus as a function of time for two different cases.

Figure 6 illustrates in longitudinal section a modified construction of apparatus, showing diagrammatically connections thereof to a threephase asynchronous motor, and

Figure '7 is a section on the line VII-VII of Figure 6.

Referring to the drawings, the rheostat represented in Figures 1, 2 and .3 comprises a cylindrical case I of insulating material pierced by a number of radial ports 2. This case is closed at one end by a base Ia and is open at the other end. Let into its wall so as to be flush therewith are three electrodes 3 having the shape of cylindrical segments occupying about 60 and having the same internal radius as the cylindrical cavity of the case. These electrodes are mutually displaced at and are connected respectively to current-carrying terminals 4 xed to the outside of the base Ia.

In the case I there is mounted a cylindrical rotor 5, also made of insulating material, following generally the shape of the cavity of the case and provided with three electrodes 6 of substantially the same span and disposed angularly in the same way as the electrodes 3. These electrodes 6 are placed at the bottom of shallow recesses 1 provided on the surface of the rotor. Thus the electrodes 6 are situated respectively at a slight distance from the electrode 3 and, with the latter, form the sides of spaces `or compartments 8 having a relatively small volume.

The rotor 5 is traversed by a series of ducts 9 parallel to the axis of rotation and communicating at one end 9a with the compartments 8. The rotor 5 is integral with a shaft I0 which passes through the base Ia of the case, a sealing gasket II being clamped around the shaft I0 by means of a threaded collar I2 screwed into the base. A control knob, fixed on the shaft I0 by means of a screw I4, allows adjustment of the angular position of the rotor.

The open end of the case I is closed by a circular plate I5 provided with two rows of holes: the holes I6 are in line with the compartments 8 on the one hand, whilst the holes I1 may be brought opposite the ends Sib of the ducts 9 on the other hand, the holes Il being substantially of the same diameter as the ducts 9. The perforated plate I5 which is provided with an adjusting knob t8, is retained over the mouth of the case l by means of a threaded ring I9.

The whole assembly described above is arranged in a receptacle or expansion vessel 29, into which it is inserted through a circular opening 20a arranged in the wall of this vessel. The case I, which is provided with a flange Ib, is fixed to the receptacle 20 by means of a clamping ring 22 screwed on to a threaded nipple 23 which is integral with the receptacle 2li, a sealing washer 2| being interposed. A hood 24 is arranged between the control knob I3 and the base la of the case; its purpose is, on the one hand, to prevent axial displacement of the shaft IB and, on the other hand, to prevent any accidental contact with the current-carrying terminals 4.

The receptacle 20 has in its upper part a neck 25 closed by a threaded plug 26 for the introduction by a conducting liquid 21. In addition it is provided with cooling ribs 28.

The operation of the apparatus described above takes place in the following manner:

The conducting liquid 2l lls the compartments 8, which are in communication with the receptacle 20 through the ports 2, the holes I6, the ducts 9 and the holes I1. If a three-phase voltage is applied to the terminals 4, a current will pass through the electrolyte contained in the compartments 8, causing heating, which will be a function of the square of the value of this current. If this value is sunciently high, the liquid contained in the compartments 8, or a part of it, evaporates, causing the expulsion of the remainder of the liquid into the receptacle 20. Thereby the resistance between the electrodes rises considerably, thus reducing the current.

Following this the liquid returns gradually to the compartments 8 at a rate which, in view of the frictional loss of head taking place in the various passages connecting these compartments with the receptacle, depends largely upon the eilective cross section of these passages; it is therefore possible to regulate this rate by modifying this cross section. In the apparatus illustrated the passages 2 and I6 have a constant cross section; as against this the effective cross section of the ducts 9 may be adjusted by means of the perforated plate i5, the holes I1 of which may be brought to a greater or less degree opposite the ends 9b of these ducts.

After the liquid has filled the compartments 8 the same phenomenon repeats itself, if the current when the compartments are full has a value exceeding the threshold value. If on the contrary the voltage applied to the terminals 4 decreases and the current then drops to a value below the threshold value at which the evaporation of the liquid takes place, the resistance of the apparatus will remain relatively low.

In order to utilise the apparatus shown on Figures l, 2 and 3 as a starting rheostat, for instance for an asynchronousr motor, it is su'icient to connect vthe terminals 4 to the slip ring f this motor. For this purpose the electrodes 6 are short-circuited by means of a three-armed metal plate Ba shown chain-dotted in Figure 2, so that the apparatus forms a liquid rheostat connected in star.

At starting, when the full voltage is applied to the stator of the motor and its back-electronictive force is zero, an augmented voltage is induced at the terminals of the rotor and therefore at the terminals 4 of the apparatus. This produces evaporation of the liquid and, consequently, the insertion of a comparatively high resistance inseries with the rotor. The volume of the liquid in the compartments S being very small, the evaporation is practically instantaneous. In practice this evaporation occurs before the starting current has reached its maximum theoretical value, i. e. during the transient phase immediately following upon the closing of the circuit. As the motor gains speed, its rotor voltage decreases proportionately and approaches zero at full speed.

As has been seen above, after evaporation and expulsion of the liquid from the compartments 8, their refilling takes a longer or shorter time depending upon the loss of head caused in the passages connecting the compartments 8 with the receptacle 20. Thus the resistance of the rheostat decreases gradually to a minimum value when the compartments are filled. Figure 4 shows the character of the curve of resistance variation as a function of time under these conditions.

However, it is possible for the rotor Voltage, with a given resistance having a value' intermediate between the maximum and the minimum, to be still higher than the value corresponding to the threshold of vcurrent intensity and a fresh evaporation occurs before the liquid has completely filled the compartments. In thatv 6,.' case the curve assumes the character shown in Figure 5..

When the motor reaches its operating speed for a given opposing torque, the rotor voltage becomes low and the consumption in the rheostat, the resistance of which is very low, can be regarded as negligible. Nevertheless the rheostat maybe short-circuited by any known automatic means, which preferably should re-i-nsert the rheostat into the circuit in case of an excessive increase of the opposing torque and, consequently, of slip. In that case the increase of slip results a corresponding increase of the rotor voltage, and the current passing through the rheostat becomes higher than the evaporation threshold value, so that the phenomena described above reproduce themselves. On account of the automatic increase of the rotor resistance with increasing opposing tor'que the motor can deliver a higher driving torque and this prevents the motor from falling out of step. Thus, thanks to the' automatic rheostat according to the invention, the stability of the motor is greatly increased, even down to very low speeds.

In practice, the use of a rheostat according to the' invention maires it possible to substitute for the usnal slip-torque characteristics' of asynchronous motors with reduced stability ranges and corresponding each to a given rotor resistance, a single characteristic with a relatively wide stabilityrange. This allows, in particular, satisfactory utilisation of asynchronous motors for driving machines subject to jerks' (for instance traction machines, rolling mills,- etc.)

In ord-er to put the problem precisely, it is necessary to take into consideration that the apparatus establishes a relation between the different' electrical values of the system (voltage, current intensity, apparent resistance). This relation is a relation of effective values, as certain of thesey values oscillate around a mean value.

In effect the' arrangement' o'f the compartments is such that the liquid tends to return and to ill them' completely, whilst the reaction, proceeding' with` increase of heat energy beyond the threshold, tends to` evaporate the liquid between the electrodes and to drive it out. These twoeffects oppose each other' andl tendv to bring the system back into a relatively stable state close to the energy' threshold. In other words, the energy dissipated in the system 2m- W-Rt R must remain close to a certain value, the moment a.- tendency to exceed' this value appears. (In the above formula, R=the electrical resistance orleredl by the apparatus, i=current and V is the voltage across the liquid in the compartments 8..)

In the case of an asynchronous motor, at the initial moment of starting the voltage is at a maximum and the current passing through the rheostat is considerably higher than the determined threshold value. From thisit follows that the voltage-resistance complex will tend to values connected with this threshold condition.

TheconsequenceV ofthis is important. In fact, because of the value of the relation being substantially constant, a decrease of the resistance will correspond to a decrease of voltage, i. e. to an increase of the speed of the motor.

audace' Thus the changes in the effective values of the several variables are consistent with the starting process.

It should be noted that effective values are involved, because the phenomenon is actually a phenomenon of impulses, the value of the resistance oscillating around the value which is imposed upon it by the speed of the motor and/or the threshold conditions. It follows from this that, depending upon the circumstances of the case, the variations'of the resistance affect the character of the curve in Figure 4 or in Figure 5. In this connection it should be mentioned that the threshold value is reached in a time of the order of the period of a cycle of the current.

In the apparatus represented in Figures 1, 2 and 3 a two-fold type of regulation has been provided for: on the one hand variation of the area of the opposed surfaces of the electrodes 3 and 6, which allows of adjustment of the initial value of the liquid resistance (OA in Figures 4 and 5) on the other hand the regulation of the eiective cross section of the orices 9b, which allows of adjustment of the time of iilling the compartments after evaporation and, in particular, of obtaining a curve having the character of that in Figure 4 or, alternatively, that in Figure 5.

The advantage of such a device lies in its great simplicity and its perfectly automatic functioning, as well as in the fact that the liquid resistance does not act as a stable resistance, but as a resistance reacting to excessive current intensities in such a way as to limit these.

On the other hand, in using the apparatus as a motor-starting rheostat as above, a relation, which can be perfectly determined, automatically connects the rotor voltage, and consequently, the motor speed, with the resistance of the rheostat. The relation maintains itself independently of the electrical fluctuations of the system. As it is not dependent, at least within a Sulliciently wide range, upon the value of the conductivity of the liquid, the same rheostatic device charged with the same solution suffices for a whole range of motors having different outputs and characteristics. From this result advantages in the manufacture of automatic rheostats, advantages which have a bearing on the time of manufacture, the purchase price, the maintenance facilities and the replacement of such apparatus.

Modifications may be made in the apparatus described above, particularly by substituting equivalent mechanical or electrical details, without going beyond the scope of the present invention. In particular, instead of arranging the electrodes on two coaxial cylindrical surfaces, they may be disposed radially and the electrodes may be ilat; thus, instead of adjusting the area of the opposed surfaces of the electrodes, their distance from each other is varied. In addition, the liquid 21 in the receptacle 2l) may be subjected to a definite pressure, in which case the plug 26 closes vthe receptacle hermetically. This has the elfect of limiting the maximum value of the resistance by opposing the expulsion of the liquid out of the compartments during evaporation and by accelerating its return after evaporation. A similar result may be obtained by providing a thin and long hermetically closed neck 25, in which case the level of the liquid, whilst at rest, is at the base of the neck, so that it traps in the latter a small column of air.

A further modication is illustrated in Figures 6 and 7 which also show the apparatus as ap- 8. plied to an asynchronous motor having a wound rotor for starting purposes.

The motor, indicated as a whole at 30, consists of stator windings 3l connected in delta to terminals 32 and rotor windings 33 connected in start and, at their outer extremities, to slip rings. By means of brushes 34 in contact respectively with these slip rings, the rotor windings are connected to the terminals 4 of a liquid resistance apparatus according to the invention- This apparatus comprises, as in that above described, a receptacle 20 into which there project three conducting rods 35 which constitute extensions of the terminals 4. The lower extremities 35a of these rods extend into cylindrical cavities 36a of a conducting member 36 immersed in electrically conducting liquid 21 which is contained in the receptacle 20. This member is supported by the conducting wall of thel receptacle 23, which is earthed at 40.

The lower extremities 35a of the rods 35 and the walls of cavities 36u constitute respectively the two electrodes, whereas free spaces respectively subsist between the extremities 35a and the walls of the cavities 36a, forming the compartments 8. By means of passages 31 and 42 formed in the member 36 each of these compartments communicates with the liquid space in the receptacle 20, so that the liquid 21 may circulate through the compartments by convection or by thermo-Siphon action.

On each of the rods 35 there is threaded a tube 38 of insulating material preventing passage of current between the upper portions of the rods via the liquid 21. Each of these insulating tubes is enlarged at its lower end to constitute insulating bush 39 fitting into one of the cavities 35a and is screwed on to a rod 35 so as to permit the longitudinal displacement of each bush relatively to its rod. The bushes 39 are perforated by small channels 4| which, in co-operation with the passages 31, ensure circulation of liquid in the compartments 8 by thermo-siphon action. By means above-described it is possible so to regulate the volume of the compartments 8 that, with the co-operating surfaces or the walls 36a and of the lower exposed portions of the rods 35a, the value of liquid resistance subsisting between these members may be determined.

The arrangement shown in Figures 6 and '1, 1n which the terminals are disposed in the manner shown, operates in a manner similar to the liquid resistance described with reference to Figures 1, 2 and 3, including, if desired, the application of pressure to the Surface of the liquid 21.

What I claim is:

l. An electrical apparatus comprising at least one small volume compartment, a larger volume receptacle adapted to contain an electrically conducting liquid, connecting means between said compartment and said receptacle including at least two passages located at different levels and adapted to offer resistance to the ilow of the liquid, and at least one pair of opposed electrodes 1n said compartment, so located as to make -contact with liquid in such compartment.

2. An electrical apparatus according to claim 1, wherein the electrodes are movable relatively to one another, whereby the electrical resistance .therebetween is made to vary.

3. An electrical apparatus according to claim 1,

further comprising means for adjusting the cross includes, at its uppermost part, an exhaust passage.

5. An electrical apparatus according to claim 1, further comprising a stationary hollow cylinder of insulating material and an inner rotary cylinder also of insulating material bearing on the inner surface of the hollow cylinder, the compartment being constituted by a recess in that surface of one of the cylinders by means of which it bears on the other cylinder and the electrodes being Xed respectively to the two cylinders.

6. An electrical apparatus according to claim 1, further comprising an insulated stem having an electrically conducting extremity located inside said receptacle and forming one of said electrodes, and an electrically condu-cting member located inside said receptacle and having a, recessed wall surrounding the conducting extremity of said stem and constituting the other one of said electrodes.

7. An electrical apparatus according to claim 1 having a plurality of pairs of electrodes and further comprising a plurality of electrically insulated terminals on said receptacles, electrically connecting means respectively between one electrode of each of said pairs and one of said terminals, and interconnecting means between the other electrodes of each of said pairs.

8. An electrica1 apparatus according to claim 1, having a plurality of pairs of electrodes and further comprising a plurality of electrically insulated stems having each an electrically conducting extremity located inside said receptacle and forming each one electrode of said pairs, and an electrically conducting member located inside said receptacle and including a wall having a plurality of recesses surrounding each the conducting extremity of one of said stems and constituting each the other electrode of said pairs.

9. An electrical apparatus according to claim 6, further comprising a hollow electrically insulating plug movable along said stem and adapted to fill the space between said stem and said recessed wall.

10. An electrical apparatus according to claim 9, further comprising narrow duct means located inside said plug, parallel to said stem.

1l. An electrical apparatus according to claim 6, further comprising at least two ducts located at dii-ferent levels and with different inclinations inside said Iconducting member for connecting the inner part of said recessed wall to said receptacle.

12. An electrical apparatus according to claim 1, further comprising means for hermetically sealing said receptacle containing conducting liquid.

NORBERT R. BENCI-IEMOUL.

No references cited.

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