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The soak impregnation method has been generally employed from time to time for want of a better method but considerable improvement in impregnation technique has occurred during the past five or six years.
One of the first improvements to be employed was the vacuum soak impregnation method. In this process, the capacitor windings are placed into a vessel of heated electrolyte, the vessel is closed and a vacuum pump reduces the pressure inside the vessel. Obviously the reduction in pressure inside the vessel reduces the surface tension of the electrolyte and results in a proportionate lowering of the boiling point temperature. This condition is met by adjusting the vacuum pressure and electrolyte temperature to a point below the boiling point of the electrolyte being used. As a general rule, the maximum vacuum pressure which can be employed is approximately 15 inches of mercury for the high voltage types of non-aqueous electrolytes. This is approximately equivalent to an internal pressure in the capacitor windings of some six or seven pounds which materially improves and accelerates the saturation of the separator medium.
The vacuum soak method, while an Improvement, is limited also to the use of certain separator absorbencies and certain electrolyte viscosities as well as certain types of electrolytes.
The greatest improvement in impregnation technique is found in the centrifugal impregnation method. By the employment of centrifugal force the electrolyte may be forced into the capacitor windings with almost unlimited pressure without any lowering of the boiling point temperature of the electrolyte.
Another method of impregnating the separator material is the so-called hand pasting procedure. In this process the anode plate, the cathode plate and the separator materials are cut to size and the tabs formed or folded on the plates or foils. The cathode plate is placed on a flat surface, a separator is placed on top of the foil and electrolyte is pasted or smeared on and into the separator. This is done manually with a knife or scraper-like implement, much the same way that plaster would be applied to a surface. On top of the separator is then placed the anode foil or plate and in turn the other separator on top of the anode foil. This second separator is then manually pasted with electrolyte and the whole laminated section wound or folded into a concentric roll, the cathode foil forming the outside and last turn when finished. After winding or rolling, the section remains more or less round but is sometimes pressed flat in a press.
The rolling or winding operation is sometimes accomplished with the aid of a simple hand operated winding mandrel. In this case, one end of the completed pasted assembly is fastened to a spindle or mandrel, which is in turn rotated by a hand operated crank, and wound into a concentric roll which is then pulled off the mandrel.
This method has been extensively employed in past years but was generally limited to electrolytes of very high viscosities which, at room temperatures, become semi-hard masses. The type separator employed was limited to the gauze or cloth class because the paper or cellulose separator material does not possess sufficient mechanical strength to stand the scraping or abrasive treatment. The gauze separator used in this process was frequently starched stiff to prevent wrinkling during the pasting operation.
Another method, frequently employed, consists in passing anode and cathode foils, properly interleaved with separators, through vessels of hot electrolyte, to hand or power operated winding heads or rotating spindles. The foils and separators are supplied in rolls and pass continuously through the bath of electrolyte to the winding position where windings are made of a predetermined size for a desired capacity. The foils are then cut off and the completed winding removed. In this case, tabs are cut and folded on the foils after the winding has been completed and removed from the mandrel. All types of separators are used by this impregnation method althou'gh the cellulose separator is somewhat limited in handling since it must have sufficient mechanical strength, when saturated with electrolyte, to withstand the mechanical force necessary to pull it through the electrolyte bath. Electrolytes are also limited to certain viscosities because otherwise too heavy a coating of electrolyte might tend to accumulate at the winding position if the electrolyte solidified, on cooling, to too great an extent.
Other variations of this method are to pass only the cathode foil and the separators through the bath of electrolyte, allowing the anode foil to enter the winding position dry. This allows anode foils to be cut to predetermined sizes or lengths beforehand. And sometimes the separator material alone is passed through the electrolyte, saturated, then rewound into rolls. These rolls of saturated separator material are placed on a winding machine and wound, with the cathode and anode foils, into capacitors.
The sketches below show diagrammatic illustrations of winding machines for dry and impregnating winding methods:
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In this method, the dry capacitor windings are placed in a bowl shaped vessel. This vessel is rotated at high speed and the electrolyte fed into it until the windings are completely immersed. After the bowl has spun or rotated a few minutes it is found that the electrolyte has thoroughly penetrated and saturated the separator and winding. The explanation of this thorough saturation follows.
The method involves characteristics unique in that by it a pressure pattern is developed which is not uniform on all sides of a winding, as would be the case of the equalized force under a strictly static pressure. This pressure pattern varies from point to point. It is this unbalanced force, due to variation in pressure, that produces a high rate of penetration of electrolyte produced by a positive flow of solution straight through a winding in the direction of reduced pressure. The fluid pressure builds up with increased speed of rotation and increased diameter of the bowl. Thus, it becomes determinable and may be established, but in all cases is greatest toward the rim or outer wall of the revolving bowl, and decreases gradually toward the center. The pressure on the windings is the same on the top and bottom at corresponding points; likewise, the same on the right and left sides at corresponding points, but varies according to the above mentioned pressure gradient measured radially toward the center. This results in a tremendous hydrostatic "squeeze" from all sides, but much greater toward the axis than away from it. Thus, a peculiar directional pressure pattern develops, differing characteristically from any other method. The time required for complete impregnation of a capacitor winding will depend on the difference in pressures along a given distance, the penetrability of the separator medium, the tightness of winding, the specific gravity of the electrolyte and the viscosity of the electrolyte at impregnation temperatures.
The derivation of the centrifugal force which produces the referred to pressure patterns is given below:
F = Wr OMEGA2 / g
F = 1.118Wr * (R.P.M.)2 * 10-5
Where the electrolyte is being considered W can be taken as the density of the solution in grams per cubic centimeter.
It is important to note that the centrifugal force varies as the first power of the radial dimension but as the second power of the speed of rotation and as the product of the two.
It is readily seen that if a capacitor winding is rotated, in a bowl-like vessel, while submerged in an electrolyte, the pressure pattern which results in penetration of the electrolyte, is a function of the length of the winding and the other factors mentioned. A cross sectional, diagrammatic illustration of the position of the winding in a centrifuge follows:
The electrolyte is illustrated as being in the position taken at the time the bowl is rotating.
In practice the bowl rotates inside a protective shell or housing and is heated in order that the electrolyte can be maintained at proper impregnation temperature. The capacitor windings are generally held in perforated aluminum baskets to facilitate handling and subsequent drainage of surplus electrolyte.
The above illustration shows a typical installation of a modern type of centrifuge such as is employed for the centrifugal impregnation of dry electrolytic capacitor windings.
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