GB2056774A

GB2056774A - Bipolar electrolytic capacitor

Info

Publication number: GB2056774A
Application number: GB7917244A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-05-17
Filing date: 1979-05-17
Publication date: 1981-03-18
Also published as: GB2056774B; PL224316A1; FR2457004A1; DD151383A5; HU180137B; JPS5623735A

Abstract

An electrically freely moveable unformed electrode foil 7 is placed between and is entirely covering and screening formed electrode foils 1,2 of a wound electrolytic capacitor. The unformed electrode foil, upon voltage application, adjusts itself, within the potential drop between the formed electrode foils, to a low positive potential in relation to the negative formed electrode foil, thus leading to a very small potential drop dE between the negative formed electrode foil and its adjacent electrolyte layers. Thereby, harmful cathodic reactions and valve film deterioration are inhibited. The consequent quality improvement makes possible, for example in electrolytic motor start capacitors, to exchange expensive low-ratio stabilized electrode foils for inexpensive high-ratio unstabilized electrode foils with reinforced oxide layers, thus leading to a remarkably cost-reduced and improved capacitor. Preferably the unformed electrode foil contains less than 99.5 % valve metal. One of the formed electrodes may be provided with a reinforced oxide layer obtained by boiling the electrode foil in pure de-ionized water. <IMAGE>

Description

SPECIFICATION Bipolar electrolytic capacitor This invention relates to bipolar electrolytic capacitors having at least two formed electrode foils being provided with terminals.

It is well known that the dielectric in electrolytic capacitors consists of an oxide film which is obtained by anodically oxidizing an electrode foil of a so-called valve metal. Valve metals are aluminium, tantalum, titanium, and niobium, for example, and the dielectric film is also called valve film because it has a high resistance at a voltage beneath the anodizing voltage, also called forming voltage, and a low resistance at reverse voltage direction. Under anodical oxidation, also called formation or preformation, the valve film continues to grow as long as the electric field strength across the valve film remains high enough to maintain ion flow through it.

In the case of aluminium, this minimum field strength amounts to about 10 MV/cm. As soon as the valve film has reached a thickness at which the minimum field strength no longer is exceeded, further film growth stops and the high forming current drops to a low leakage current.

This leakage current is caused by small imperfections of the valve film, due to impurities being present in even the purest available valve metal, and by the fact that the oxide film is never completely insoluble in the forming electrolyte. By dissolving oxide, called deformation, the field strength in the valve film is increased to a value beyond the minimum field strength, and new oxide can be reformed.

A deformation is also going on in electrolytic capacitors in storage without applied voltage, especially at elevated temperatures. But also under working conditions at a working voltage Ew a deformation takes place, because the valve film is then reformed only to this working voltage Ew, whereby a deformation from the forming voltage Ef down to the working voltage Ew cannot be prevented.

The deformation of the valve film results in a considerable deterioration of an electrolytic capacitor, in respect of its electrical properties and useful life. Therefore, it is very important to take advantage of any suitable measure to prevent or to minimize deformation.

From the above-mentioned forming process it becomes obvious, how the forming voltage, at which the valve film has been formed, can be determined. For this purpose, it is only necessary to put the formed electrode foil, or a piece of it, into a suitable electrolyte, together with a counter electrode, apply a DC voltage and increase it smoothly. At a voltage below the forming voltage E, a low leakage current will be observed, whereas a high forming current will result at and beyond the forming voltage E,.

It is well known, for electrolytic capacitors to specify a rated voltage and a surge or maximum voltage. These voltages are defined to be the value of a DC voltage or the resulting peak value of a over-laying AC voltage which may be applied to the capacitor permanently (rated voltage) or which may be applied only shortly and may never be exceeded (surge voltage). The surge voltage is-together with the rated voltage printed on the capacitor or indicated by the manufacturer by referring to relevant technical specifications, and usually amounts to 1.1-1.3, times the rated voltage.

It is well known, for any rated/surge voltage to use a dielectric valve film which has been formed at a forming voltage E, that amounts to at least 1.05 times the surge voltage. For example, an electrolytic capacitor for 50/60 V is preformed at least at 63 V, for 350/400 V at least at 420 V, and for 500/550 V at 600 V. In this way further formation is prevented on applying the surge voltage, which would lead to a loss of capacitance and/or the deterioration of the capacitor. The safety margin of 5 percent or more accounts for the usual manufacturing tolerances.

According to this relation between rated/surge voltage and forming voltage Ef, it is possible to optimize electrolytic capacitors for any surge voltage up to about 600 V. Since the thickness of the dielectric valve film is proportional to the forming voltage E,, high capacitance values result at low forming voltages and lower capacitance values at higher forming voltages.

It is well known to manufacture electrolytic capacitors by winding electrode foil strips together with interleaving strips of absorbent separators (usually paper). The electrode foils may be plain or surface increased, for example by etching. By etching, the specific capacitance, i.e. the capacitance per foil area, can be increased considerably, for example up to 50 times (and the capacitor volume be reduced accordingly) because the dielectric valve films will cling to any surface shape. Low-gain and high-gain electrode foils are used in different applications with different requirements for surface increase.

With regard to their construction, electrolytic capacitors may be polar or bipolar. A polar electrolytic capacitor consists of at least one formed and one unformed electrode foil, each provided with at least one terminal, which are wound up with interleaving strips of absorbent separator. The capacitor roll is then impregnated in a suitable electrolyte and enclosed in a casing. The casing is provided with at least two terminals which are isolated from each other and connected with the electrode foil terminals.

The unformed electrode foil usually also consists of valve metal, and it is then, from oxidation in the air, covered by a thin oxide layer which will act as a valve film corresponding to a forming voltage of less than 5 V, usually of 1-4 V. Therefore, a polar electrolytic capacitor may be operated at a DC voltage without or with an additional AC voltage in such a manner that the peak value of the resulting voltage will keep within the rated or surge voltage limits, and that no reverse voltage exceeding about 4 V will appear.

A polar electrolytic capacitor with multiple parallel capacitor sections is obtained by winding several formed electrode foils together with one common or with several separate unformed electrode foils. Each of these electrode foils has to be provided with at least one terminal.

If polar electrolytic capacitors are to be suitable for frequent fast charging and discharging operation (for example as photoflash capacitors), then an unformed electrode foil has to be used of a specific capacitance Cc meeting, in relation to the peak value Ep of the capacitor surge voltage and to the specific- capacitance Cp of the formed electrode foil, the condition (Ep Cc/Cc + Cp) < 5 V. Only then, the charges flowing-under fast external capacitor dischar ge--internally from the anode to the cathode will charge the unformed electrode foil to less than 5 V. Thus, a gradual forming up of the unformed electrode foil and a capacitance loss of the series circuit "anodic-cathodic capacitance" are avoided. If this condition is met, the electrolytic capacitor is called discharge-proof.

For operation at reversing voltages, bipolar electrolytic capacitors are used in which the unformed electrode foil of polar electrolytic capacitors is exchanged for a second formed electrode foil. Then, two formed electrode foils, both being provided with terminals, are wound together with interleaving absorbent separator strips. After impregnating the capacitor roll, any of the two formed electrode foils can, by turns, serve as anode or cathode. In anodic (cathodic) polarization, the valve film works in its high (low) resistance direction.

When, in a bipolar electrolytic capacitor, the two formed electrode foils have the same or different forming voltages, the capacitor is called reversible or semipolar, respectively. Reversible electrolytic capacitors are suitable for operation at reversing DC and/or AC voltages, semipolar ones at unsymmetric DC and/or AC voltages.

Electrolytic capacitors for AC voltage operation are widely used as motor run capacitors and as cross-over capacitors (in loudspeaker systems) under continuous duty up to about 100 VAC. At higher voltages, electrolytic capacitors are preferably used under intermittent duty as motor start capacitors.

With electrolytic capacitors for AC voltage operation, the rated/surge voltage relates to RMS values. Accordingly, it is a well known rule with these capacitors to form the valve film at a forming voltage Ef of at least 1.05 times the peak value Ep of the surge voltage.

Compared with static capacitors, electrolytic capacitors have the benefit of offering a highly increased capacitance/volume ratio at low costs. They are suffering, however, from the disadvantage of a relative high loss angle (and a high tangent of the loss angle, tand) severely limiting their field of application, especially under AC operation.

An electrolytic capacitor can be considered to be a series combination of a capacitance C and a resistance R, also called equivalent series resistance ESR, and the tangent of the loss angle of this combination, at the frequency f, is tans = R27TfC.

The resistance R consists of several parts and can be approximated by writing.

R = rd + ra + rp.

In this equation, rd corresponds to the dielectric losses in the valve film, whilst ra and rp correspond to the ohmic resistances in the absorbent separator layers and in the etching pores (if etched electrode foil is used), both impregnated with el.ectrolyte.

Accordingly, the capacitor designer is trying, especially with bipolar electrolytic capacitors for AC voltage operation, to use highly porous and absorbent separators, highly conductive electrolytes, and low-ratio electrode foils with wide pore cavities in order to reduce ra and rp, and to develop valve films showing reduced dielectric losses in order to reduce rd.

In this respect, one has to have in mind that etching electrode foils is conducted in acids or brine solutions. After rinsing the foil, it is dried, and under this drying process, the abovementioned thin oxide layer develops by oxidation in the air. A reinforcing of this oxide layer by boiling the foil in purest de-ionized water leads to a remarkable saving of about 50 % of electrical forming energy input and to a considerable increase of capacitance output. However, on the other hand, the dielectric losses in the valve film are increased. Due to the cost-savings, electrode foils with a reinforced oxide layer are used in electrolytic capacitors for high voltage (beyond 100 V) for DC operation and smoothing purposes, whereas electrode foils with a thin oxide layer are used in electrolytic capacitors for low voltage (below 100 V).

When using the above electrode foils in bipolar electrolytic capacitors, difficulties arise under AC operation, especially at high AC voltages, for example with motor start capacitors at high AC voltages. These difficulties consist of unstable capacitance, fast increasing loss angle under operation, overheating, deformation, break-downs, and explosions. These difficulties are especially remarkable with electrode foils having a reinforced oxide layer.

Accordingly, attempts have been made to reduce the dielectric valve film losses by using electrode foils with extremely thin oxide layers which are obtained by treating the electrode foil, immediately after etching and rinsing and before the drying process, in a solution containing phosphates. Electrode foils treated in this way are called stabilized.

In spite of the cost-increasing use of stabilized low-ratio electrode foils without reinforced oxide layer, bipolar electrolytic capacitors are still suffering, especially under AC voltage operation, as for example motor start capacitors, from disadvantages with regard to quality and economy.

The object of this invention is therefore to provide an improved construction of bipolar electrolytic capacitors, especially for AC voltage operation, in which the abovementioned disadvantages are reduced so far as to make possible the use of cost-saving high-ratio unstabilized electrode foils with reinforced oxide layers.

Other aspects, objects, and advantages of the invention will be apparent from a study of the disclosure, the drawing, and the appended claims of the invention.

According to the invention, a bipolar electrolytic capacitor having at least two formed electrode foils, being provided with terminals, is characterized by having at least one further unformed electrode foil being placed between and entirely covering and screening the formed electrode foils against each other and electrolytically integrating the formed electrode foils to a series circuit "formed-unformed-formed electrode foil" and the unformed electrode foil being electrically freely moveable within the potential drop between the formed electrode foils.

The electrolytic series circuit may be obtained by placing the formed electrode foils in transverse or in longitudinal position against each other. In transverse position, the formed electrode foils are placed across each other and separated by two unformed electrode foils and absorbent separators. In longitudinal position, the formed electrode foils are consecutively wound together with one unformed electrode foil and absorbent separators, all extending throughout the entire capacitor roll. These alternative configurations will be further explained and described by referring to the drawing.

The basic refiections of the invention are the presumption that the above-mentioned well known efforts of using highly conductive electrolytes and stabilized low-ratio electrode foils without reinforced oxide layer are only suitable to reduce, but not to remove, the difficulties in question, and the apprehension that cathodic reactions at the negative formed electrode foil have an important influence in bipolar electrolytic capacitors by gradually and permanently harming the valve film properties, especially at high working voltages and under AC voltage operation and its heat generation.

Influencing cathodic reactions might be the reduction of the valve film by cathodically developed hydrogen in statu nascendi or by the reducing effect of ammonia which is a widely used ingredient of electrolytes for electrolytic capacitors, a valve film deterioration from increasing pH in its adjacent electrolyte layers caused by hydrogen withdrawal, the deposition of heavy metal traces (always being present in even the purest available electrolyte ingredients) which will, upon deposition on the valve film, eliminate the valve film function, as it is well known, and other reactions of cathodically reducing or decomposing the electrolyte ingredients at high working voltages and at high temperatures.

Under DC operation, the potential drop across an electrolytic capacitor will arise almost entirely across the valve film of the positive formed electrode which is in its high resistance direction, whereas almost no potential drop will arise across the conducting electrolyte and the low resistance directioned valve film of the negative formed (or unformed) electrode. Accordingly, the potential drop between the negative electrode and its adjacent electrolyte layers is very small and cathodic reactions will be of no influence.On charging and discharging the capacitor, however, and likewise on reversing the polarity and under AC voltage operation, the charge flow involved will cause, at sufficient high working voltages, a sufficient high potential drop across the electrolytic series resistance and, accordingly, between the negative electrode and its adjacent electrolyte layers, and cathodic reactions at the negative electrode will start and influence on and gradually deteriorate the valve film properties.

Accordingly, the basic idea of this invention consists in providing a suitable concept by which the potential drop between the negative formed electrode foil and its adjacent electrolyte layers will be reduced, even at high working voltages, sufficiently to avoid cathodic reactions influencing the valve film properties.

This is achieved, in the electrolytic capacitor according to the invention, by the unformed electrode foil which is electrically freely moveable within the potential drop between the formed electrode foils. If this unformed electrode foil consists of a non-valve metal, it will not have any valve film and, accordingly, it will, in relation to the negative formed electrode foil, only be charged to a very low positive potential according to the electrolytic polarization. If it consists of a valve metal, it will always be covered with a thin oxide layer, which acts as a valve film corresponding, in the case of aluminium, to a forming voltage of 1-4 V, and then it can assume, in relation to the negative formed electrode foil, only a positive potential of not exceeding 1-4 V, provided that it is electrically freely moveable, i.e. electrically disconnected.

The small potential drop AE between the unformed and the negative formed electrode foil (for example AE = 1-4 V) will under charge flow conditions lead to a very small potential drop dE < AE between the negative formed electrode foil and its adjacent electrolyte layers, thus preventing harmful cathodic reactions and deterioration of its valve film properties.

In the electrolytic capacitor according to the invention, the cathodic reactions take place at its unformed electrode foil as they do in any polar electrolytic capacitor. A deterioration of its va!ve film properties, however, is not harmful but advantageous because the series circuit "formedunformed electrode foil" will become more discharge-proof and its series capacitance will increase. Furthermore, in the electrolytic capacitor according to the invention the potential drops AE and dE are further reduced and the valve film protection is increased.

irrespective of these explanations, the experimental proof of this concept is leading to unexpected remarkable advantages, as will be explained later on with reference to the examples.

In order to avoid any formation of the unformed electrode foil under duty conditions, its specific capacitance Cc has to meet, in relation to the peak value Ep of the capacitor surge voltage and to the specific capacitance Cp of the formed electrode foils, the condition (Ep CC/Cc + Cp) Hz 10 V, if it consists of a valve metal.

In a first embodiment of the invention, the unformed electrode foil consists of a material which is difficult to form and which contains less than 99.9 %, preferably less than 99.5 %, valve metal. In this case, it may assume, under AC voltage operation, a voltage of maximum 10 V without suffering from- any unwanted formation.

In order to achieve an entire covering and screening of the formed electrode foils by the unformed electrode foil while winding the capacitor roll, when a lateral drift away of the foil strips is not always completely avoidable, it is advantageous, if the latter is wider than the former. By complete screening, an increased capacitance output, a reduced loss angle, and an improved heat conduction towards the ends of the capacitor roll are achieved.

Since the electrically freely moveable unformed electrode foil must not be connected under any circumstance, it is advantageous if it is not provided with a terminal.

The electrolytic capacitor according to the invention, having an electrolytical series circuit of at least two formed electrode foils and at least one unformed electrode foil being electrically freely moveable within the potential drop between the formed electrode foils and being disconnected accordingly, is clearly different from known polar electrolytic capacitors having several parallel capacitor sections and one common unformed electrode foil which serves as a current feeder and must be provided with a terminal and must be connected externally.

In a further embodiment of the invention, the electrolytic capacitor consists of at least three formed electrode foils provided with terminals and one unformed electrode foil without a terminal. This embodiment makes possible at least two bipolar capacitor sections in one common capacitor roll, for example, in a motor start capacitor for intermittent operation an additional motor run capacitor section for continuous operation.

In a still further embodiment of the invention, the electrolytic capacitor consists of a series circuit of two bipolar capacitor sections, each consisting of a series circuit "formed-unformedformed electrode foil", within the same capacitor roll, which are separated from each other by a sheet of insulating material, for example a flexible thermoplastic film of low porosity, where the two capacitor sections constitute a series circuit ' ''formed-unformed-formed--formed-unformed- formed electrode foil". This embodiment is especially suitable with motor start capacitors for operation at high voltages when an electrolyte with sufficient high maximum voltage would suffer from too low conductivity and would lead to poor capacitor properties and overheating.

In an especially advantageous development of the invention for AC operation, the electrolytic capacitor contains at least two formed electrode foils and is characterized by the sum of the forming voltages Ef in at least one voltage direction having, in relation to the peak value Ep of the capacitor surge voltage, a value ZE, < 1.05 Ep, preferably a value according to 0.5 Ep < EEf < Ep. In other words, the electrolytic capacitor according to this development of the invention is characterized by having, compared with known electrolytic capacitors of a distinct voltage, a reduced, preferably a very reduced, forming voltage Ef.

Unexpectedly it has been found that an electrolytic capacitor according to this development of the invention having at least one electrode foil being formed at a voltage Ef below the peak value Ep of the capacitor surge voltage, under AC voltage operation at the surge voltage, did not gradually form up to the peak value Ep of the surge voltage and deteriorate, as it would be expected by a suitably competent and knowledgeable person.

To explain this unexpected observation, it may be assumed that the ion flow through the valve film under formation is going on with a field-strength-depending finite velocity. Then it becomes understandable that an additional valve film growth demands a field-strength-depending minimum duration and that short-timed; limited voltage peaks will be unable to cause a gradually forming up, in spite of frequent repetitions.Suppose, for example, a forming voltage Ef = 0.85 Ep Then it will be acrsin 0.85 = 58.2 ", and at a frequency of 50 Hz with a cycle duration of 20 ms the forming voltage Ef will be exceeded only for a duration of 3.53 ms and, according to the observations made, this is not sufficient to cause any further formation.-Of course, suitable measures have to be taken in order to avoid a too high DC voltage remaining upon suddenly switching off the AC voltage; or the capacitor has to be discharged rapidly, for example by means of a discharging resistor.

Tests have also proved that electrolytic capacitors according to this development of the invention show a highly improved stability under duty conditions compared with known electrolytic capacitors. This observation may be explained by assuming that the abovementioned deformation is prevented or highly reduced by reducing the forming voltage E, to or below the peak value Ep of the working voltage.

Electrolytic capacitors according to this development of the invention have the advantages, on the one hand, of inhibiting valve film property deterioration at the negative formed electrode foil, and, on the other hand, of inhibiting valve film deformation at the positive formed electrode foil which had been impossible with known electroyltic capacitors. Besides these technical advantages, remarkable economic advantages are achieved by this development. For example, reducing the forming voltage Ef from 1.05 Ep to 0.85 Ep corresponding to 19 %, a capacitance increase is obtained in excess of 1 9 % with etched electrode foil because of its porous structure, for example of 25-30 %, resulting in a cost-saving of 25-30 % of electrode foil as well as separator and electrolyte.Since the electrical forming energy input is increasing progressively with increasing forming voltage, it results with this example a saving of 35-50 % of electrical consumption besides corresponding savings of chemicals, materials, and time, under formation.

Furthermore, the increased capacitance output results in further time-savings under manufacture as well as in a smaller size of electrolytic capacitors according to this development of the invention.

In a semipolar design of this development, having the peak values Ep, and Ep2 of the capacitor surge voltages, the electrolytic capacitor consists of one formed electrode foil being formed at a forming voltage Ef, < 1.05 Ep1 and another formed electrode foil being formed at a forming voltage E,2 < 1.05 Ep2.

In a further embodiment of this invention the electrolytic capacitor contains at least one formed electrode foil being provided, before formation, with a reinforced oxide layer as it may be obtained by boiling the electrode foil in purest de-ionized water.

It has been found that the valve film properties of the negative formed electrode foil are so strongly inhibited from deterioration by the presence of the unformed electrode foil according to the invention that cost-increasing low-ratio stabilized electrode foils can be exchanged for inexpensive high-ratio electrode foils with reinforced oxide layers, even in motor start capacitors for higher voltages.

Taking advantage of the above-mentioned issues of the invention by using high-ratio electrode foils with reinforced oxide layers and by reducing the forming voltage E,, it is made possible to achieve in bipolar electrolytic capacitors for AC voltage operation, for example in motor start capacitors, a total cost reduction of 60-70 % as far as the capacitor roll is concerned.

Under certain conditions of switching on and off electrolytic capacitors for AC voltage operation, for example motor start capacitors, transient over-voltages up to ten times the rated capacitor voltage may be produced and consequent reduced service life may result. The unformed electrode foil according to the invention will, however, result in a remarkable impedance increase at higher frequencies. Tested at 5 kHz, known motor start capacitors showed a tans of 20-30 %, those according to the invention, however, 70 %, thus leading to the benefit of highly damped transient oscillations and consequent increased service life and reliability.

The invention will be further represented and illustrated by referring to the drawing showing in Fig. 1, 2, and 7 examples of suitable capacitor roll constructions, in Fig. 3 and 4 potential relations, in Fig. 5 and 6 electrodes and their adjacent electrolyte layers, and in Fig. 8 a capacitor roll according to the invention.

Referring now to the drawing, Fig. 1 shows, in a schematic manner, a capacitor roll construction which is especially suitable to explain the invention. The formed electrode foils 1 and 2 are provided with the terminals 3 and 4 and are placed in transverse position across each other and separated by two unformed electrode foils 7 and 7' and absorbent separators 5, 6 and 5', 6' of one or several layers each. The formed electrode foils are entirely covered and screened against each other by the unformed electrode foils 7 and 7' being placed between them. After winding up the sandwich as indicated by the arrow, and after impregnation, the formed electrode foils 1 and 2 are electrolytically integrated by the unformed electrode foils 7 and 7' to a series circuit "formed-unformed-formed electrode foil".According to what formed electrode foil surface is considered, the series circuit is "1-7-2" or "2-7'-1", respectively.

The unformed electrode foils 7 and 7' are electrically disconnected and therefore electrically freely moveable within the potential drop between the formed electrode foils 1 and 2 under operation, and are therefore self-adjusting to a low positive potential of less than + 10 V, for example of 1-4 V, in relation to the potential of the negative unformed electrode foil. Thus, the potential drop between the negative formed electrode foil and its adjacent electrolyte layers is strongly reduced and any deterioration of its valve film properties by cathodic reactions is inhibited.

Fig. 2 shows, in a schematic manner, a preferred simple construction according to the invention. The formed electrode foils 1 and 2 are provided with the terminals 3 and 4 and are placed in longitudinal position against each other, and after consecutively winding, as indicated by the arrow, together with one or several layers 5 and 6 of absorbent separators and with the unformed-electrode foil 7, the latter will be placed between and entirely covering and screening the formed electrode foils 1 and 2 against each other and will be, upon impregnation, electrolytically integrating the formed electrode foils 1 and 2 to a series circuit "formed unformedformed electrode foil".The unformed electrode foil 7 is electrically disconnected and therefore electrically freely moveable within the potential drop between the formed electrode foils 1 and 2 and is, under voltage application, self-adjusting to a low positive potential of less than + 10 V, for example of 1-4 V, in relation to the potential of the negative formed electrode foil.

Fig. 3 and 4 illustrate the potential drop, under operation, between the electrode foils. In these Figures, the potential of the unformed electrode foil 7 is supposed zero.

In Fig. 3 the terminal 3 is supposed positive, the terminal 4 negative. The unformed electrode foil 7 will adjust to a low positive potential of less than + 10 V in relation to the negative formed electrode foil2 which will show an according low negative potential in relation to the unformed screening electrode foil 7. The small potential drop AE < 10 V between the electrode foils 2 and 7 will inhibit any deterioration of the valve film properties of the negative formed electrode foil 2.

Fig. 4 illustrates the potential drop on reverse polarization. The small potential drop AE < 10 V is now obtained between the electrode foils 1 and 7 and will inhibit any deterioration of the valve film properties of the negative formed electrode foil 1.

The Fig. 5 and 6 illustrate anions and cations appearing, under operation, in the electrolyte layers adjacent to the electrode foils.

In Fig. 5, the electrode foil 1 is supposed positive, the electrode foil 2 negative. The high potential drop across the section 1-7 (according to Fig. 3) will result in reforming the positive formed electrode foil 1 (anode) by anions entering its valve film where necessary. In this section 1-7, the unformed electrode foil 7 is negative (cathode) and, because of the high potential drop 1-7, cathodic reactions are not inhibited and may influence and deteriorate the properties of the very thin valve film of the unformed electrode foil 7.This is no disadvantage but will improve the series circuit 1-7 to become more discharge-proof and will increase its series capacitan ce.-The low potential drop across the section 2-7 (according to Fig. 3) of AE < 10 V will result in a very low potential drop dE < AE between the negative formed electrode foil 2 and the electrolyte layers adjacent to it, thus inhibiting cathodic reactions and any deterioration of the valve film properties of the negative formed electrode foil 2.In this section 2-7, the unformed electrode foil 7 is positive (anode) and wil adjust to a low positive potential, according to the electrolytic polarization if it consists of a non-valve metal, or according to its valve film voltage and specific capacitance if it consists of a valve metal, and its valve film will then be reformed accordingly.

In Fig. 6, the corresponding relations are illustrated upon reverse polarization. Further discussion is not necessary after the explanations of Fig. 5.

Fig. 7 shows, in a schematic manner, a capacitor roll construction according to the invention with two capacitor sections. The formed electrode foils 1, 2 and 8 are provided with the terminals 3, 4, and 9 and are separated by one or several layers of absorbent separators 5 and 6 and by one unformed electrode foil 7. After being wound up as indicated by the arrow, and after impregnation, the unformed electrode foil 7 behaves and acts in relation to the formed electrode foils as it has been described and explained under Fig. 2. Suppose, for example, the foils 1 and 2 have the same capacitance. Between their terminals 3 and 4 a motor start capacitor section is available for intermittent operation. The foil 8 is supposed to have a lower capacitance and may be of plain or low-ratio electrode foil for further reducing the loss angle for continuous operation.This capacitor section is then available between the terminals 3 and 9 or 4 and 9, and the generated heat will in this configuration dissipate over the entire capacitor roll and can easily be led off by it and by the common capacitor casing, without local overheating.

Fig. 8 shows a partially unwound capacitor roll 10 of an electrolytic capacitor according to the invention. The terminals 3 and 4 are connected with the formed electrode foils 1 and 2 (of which only the latter is visible at the unwound end of the roll) being wound together with absorbent separators 5 and 6 and the unformed electrode foil 7 according to Fig. 2, the unformed electrode foil 7 integrating electrolytically, after impregnation, the formed electrode foils 1 and 2 to a series circuit "1-7-2" according to Fig. 2. The impregnated capacitor roll 10 may be encased within a suitable casing with external terminals to which the terminals 3 and 4 are electrically connected.

The invention will be further explained by the following two examples referring to bipolar electrolytic capacitors for AC voltage operation, electrolytic motor start capacitors, which are especially suitable to demonstrate the advantages of the invention.

Example 1 For a motor start capacitor 360 yF 110/140 VAC (Ep = < 2 140 = 198 Vp) electrode foil of the foil type 2035 was formed at a forming voltage Ef = 150 VDC = 0.76 Ep. Two foil strips of 6 93 cm2 were provided with terminals and, according to Fig. 2, wound up with aborbent paper separators and an unformed electrode foil of the foil type Kappa 204.

The capacitor roll was impregnated with 'an electrolyte consisting of 100 ml ethylene glycol, 70 g boric acid, 20 ml aqua ammonia of 25 % by weight of NH3, 4 g diammonium phosphate, (NH4)2HPO4, which ingredients were mixed and boiled until a specific resistivity was obtained of 80 Ohm cam at 80 "C.

After impregnation, the capacitor roll was enclosed in a casing of 36 mm diameter and 80 mm length.

The foil type 2035 is a high-gain aluminium electrode foil provided, by boiling in de-ionized water before formation, with a reinforced oxide layer, manufactured by ALUMINIUM WALZWERKE SINGEN, D-7700 Singen am Hohentwiel (German Federal Republic). This foil type is unsuitable for the manufacture of high-quality motor start capacitors, when manufactured according to the prior art.

The foil type Kappa 204 is a high-gain aluminium cathode foil for polar electrolytic capacitors, manufactured by BECROMAL, 1-20100 Milano (Italy).

By placing, according to invention, an electrically freely moveable unformed electrode foil between the formed electrode foils, harmful cathodic reactions at the negative formed electrode foil and any deterioration of its valve film properties are inhibited, thus making possible the use of cost-saving high-gain electrode foils with reinforced oxide layers.

By reducing the forming voltage (in this example by at least 27 % compared with the prior art), the deformation of known capacitors is avoided and further very remarkable cost-savings are achieved.

The high quality standard of capacitors according to this example becomes evident from the following favourable results of the life test conducted at the capacitor surge voltage: Average of 10 samples C yF tand % Initial measurement 363 2.8 After 30,000 duty cycles of 1.2 sec 140 VRMS 50 Hz and 1.2 min 0 V, corresponding to 359 3.2 1.7 % intermittence, at 55 "C ambient temperature Example 2 For a motor start capacitor 80 tLF 220/275 VAC (Ep = /2 275 = 389 Vp) electrode foil of the foil type 2035 was formed at a forming voltage Ef = 330 VDC = 0.85 Ep.Two foil strips of 649 cm2 were provided with terminals and, according to Fig. 2, wound up with absorbent paper separators and an unformed electrode foil of the foil type Kappa 204 and impregnated and encased according to example 1.

By placing, according to the invention, an electrically freely moveable unformed electrode foil between the formed electrode foils, harmful cathodic reactions at the negative formed electrode foil and any deterioration of its valve film properties are inhibited also in this example of increased voltages, making possible the use of cost-saving high-gain electrode foils with reinforced oxide layers.

By reducing the forming voltage (in this example by at least 1 9 % compared with the prior art) a considerable cost-saving- is obtained and the deformation of known capacitors is avoided.

The high quality standard also of capacitors according to this example becomes evident from the following favourable results of the life test conducted at 85 % of the capacitor surge voltage: Average of 25 samples C pF tans % Initial measurement 80.7 2.8 After 30,000 duty cycles of 1.2 sec 234 VRMS 50 Hz and 1.2 min 0 V, corresponding to 78.8 3.0 1.7 % intermittence, at 55 "C ambient temperature It is a widely used practice to manufacture electrolytic motor start capacitors from three formed electrode foils constituting a series circuit "formed-formed-formed electrode foil" as above-mentioned. In this construction, the voltage is reduced to one half, by voltage division, and the difficulties arising from cathodic reactions are reduced accordingly, but not removed.To obtain the capacitance C in this construction, formed electrode foil of the capacitance 1 6 C and the forming voltage 0.5 E, is needed compared with 4 C and Ef for a simple "formed-formed electrode foil" construction which, accordingly, is much cheaper to manufacture-, though the capacitance output is at least doubled at 0.5 Ef. Nevertheless, this expensive practive has had to be used because, before this invention, no other means have been known by which the difficulties arising from cathodic reactions could have been removed.

From the above two examples to this invention it becomes obvious that the known constructions "formed-formed electrode foil" and even "formed-formed-formed electrode foil" easily can be exchanged for the "formed-unformed-formed electrode foil" construction according to the invention throughout the voltage range in which suitably highly conductive electrolytes can be prepared, i.e. up to a peak voltage of about 450 V.

Beyond this voltage, the series circuit ''formed-unformed-formed--formed-unformed-formed electrode foil" construction as disclosed is still cheaper to manufacture than the "formedformed-formed electrode foil" construction, and it will inhibit harmful cathodic reactions.

This invention has been described as applied particularly to motor start capacitors. It is also applicable to other bipolar electrolytic capacitors. Accordingly, - it is intended that the scope of the invention be limited only by the following claims.

Claims

1. Bipolar electrolytic capacitor with at least two formed electrode foils being provided with terminals, characterized by having at least one further unformed electrode foil being placed between and entirely covering and screening the formed electrode foils against each other and electrolytically integrating the formed electrode foils to a series circuit "formed-unformed-formed electrode foil", the unformed electrode foil being electrically freely moveable within the potential drop between the formed electrode foils.

2. Electrolytic capacitor as claimed in claim 1, characterized by the specific capacitance Cc of the unformed electrode foil meeting, in relation to the peak value Ep of the capacitor surge voltage and the specific capacitance Cp of the formed electrode foils, the condition (EpQ/Cc + Cp) < 10 V.

2. Electrolytic capacitor as claimed in claim 1, characterized by the specific capacitance Cc of the unformed electrolde foil meeting, in relation to the peak value Ep of the capacitor surge voltage and the specific capacitance Cp of the formed electrode foils, the condition (Ep CC/Cc + Cp) < 10 V.

3. Electrolytic capacitor as claimed in claim 1, characterized by the unformed electrode foil consisting of a material which is difficult to form and containing less than 99.99 %, preferably less than 99.5 %, valve metal.

4. Electrolytic capacitor as claimed in claim 1, characterized by the unformed electrode foil being wider than the formed electrode foils.

5. Electrolytic capacitor as claimed in claim 1, characterized by the unformed electrode foil not being provided with a terminal.

6. Electrolytic capacitor as claimed in claim 1, characterized by at least three formed electrode foils being provided with terminals and by at least one unformed electrode foil not being provided with a terminal.

7. Electrolytic capacitor as claimed in claim 1, characterized by a series circuit of two bipolar capacitor sections, each consisting of a series circuit ''formed-unformed-formed electrode foil'', within the same capacitor roll, and being separated from each other by a sheet of insulating material, the two capacitor sections constituting a series circuit ''formed-unformed-formed- formed-unformed-formed electrode foil".

8. Electrolytic capacitor as claimed in any preceeding claim, for AC voltage operation, characterized by the sum of the forming voltages Ef in at least one voltage direction having, in relation to the peak value Ep of the capacitor surge voltage, a value TEf < 1.05 Ep, preferably a value according to 0.5 Ep < EEf < Ep.

9. Electrolytic capacitor as claimed in claim 8 and in semipolar construction, having the peak values Ep, and Ep2 of the capacitor surge voltages, characterized by the formed electrode foils having the forming voltages Ef, < 1.05 Ep, and Ef2 < 1.05 Ep2.

10. Electrolytic capacitor as claimed in any preceeding claim, characterized by at least one of the formed electrode foils being provided, before formation, with a reinforced oxide layer of the kind being obtained by boiling the electrode foil in purest de-ionized water.

CLAIMS (26 Jul 1980)