GB1568042A - Single layer electrical capacitor - Google Patents

Description

(54) A SINGLE LAYER ELECTRICAL CAPACITOR (71) We, ROBERT BOSCH GmbH, a German company of Postfach 50, 7000 Stuttgart 1, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention is concerned with electrical capacitors and, more particularly, with single layer capacitors that is capacitors in which there is a single layer of dielectric material between adjacent metal layers which are suitable for AC voltage loading.

Single layer capacitors are known which comprise a winding formed from two strips of paper which are each vapour- metallised on one side, the winding being impregnated with mineral oil or polyisobutylene, both of which have a dielectric constant of approximately 2.2.

Similarly capacitors are known in which the winding is formed from a strip of paper which is metallised on both sides and is impregnated with mineral oil. With this arrangement, contiguous metallised layers are, of course, at the same potential. The use of paper strips which are metallised on both sides enables a high specific capacity to be obtained.

It is also known to impregnate capacitors in which the winding is formed of separate metal foil and dielectric material strips with phthalic acid esters as the liquid dielectric tmpregnant. Such capacitors are described in British Specification 1441803, which is primarily concerned with capacitors with a multilayer, for example two- or three-laver.

dielectric construction.

We have now developed an improved single layer capacitor suitable for AC voltage loading. The capicitor has a higher insulation resistance which can withstand prolonged storage and/or operation and can be produced in a simple manner which is well adapted to mass production.

According to the present invention, there is provided a single layer electrical capacitor which comprises a winding of at least one strip of dielectric material which is metallised on both sides, one side of the strip being -unmetallised at one end thereof and the other side being unmetallised at the other end so as to provide end portions which are respectively in electrical contact and insulated from each other, the winding being impregnated with a phthalic acid ester liquid dielectric material.

In a preferred embodiment of the capacitor. the winding is formed from two metallised paper strips.

In order that the invention may be more fully understood a preferred embodiment of single layer capacitor, and the operation thereof. will now be described, by way of example. with reference to the accompanying drawings. in which: Figure 1 is a diagrammatic section through the capacitor. and Figure 2 is a graph showing curves obtained by plotting RC value against storage time.

Figure 1 shows, in diagrammatic form, two dielectric strips 10 and 11 which are metallised on both sides from which the winding of a single layer electrical capacity for AC voltage loading is formed. Metalfree end zones 12 and 13 are provided on the opposite sides of the strips at the ends thereof. When the strips 10 and 11 are coiled to form the winding the contiguous metal layers are in contact so as to short circuit each other so that no electrical field can be built up in the interstices between the strips and the contiguous metal layers have, therefore. the same polarity. External connection to the metal layers 14 and 15 is made by means of taps (not shown) at the respective ends of the winding on the side on which the metal layers 14 and 15 extend up to the end of the strip 10 or 11.

In accordance with the invention, the winding is impregnated with a phthalic acid ester impregnant, preferably dioctylphthalate or dinonylphthalate, instead of the mineral oil or polyisobutylene impregnants conventionally used with single layer capacitors.

Instead of the arrangement shown in Figure 1 comprising two dielectric strips 10 and 11 metallised on both sides, the winding can also be formed from a single such strip or from one such strip and a non-metallised dielectric strip. The dielectric strips can be formed of paper or plastics, the former being preferred since the vapour deposition of metals is more readily effected on paper.

It is also preferred that the winding should comprise at least one paper strip, metallised or non-metallised, so as to facilitate impregnation.

The effect of impregnation with a phthalic acid ester can be seen from Figure 2 which shows curves obtained by plotting the RC value, that is the product of capacity and insulation resistance, in seconds on a logarithmic scale, as the ordinates. against the storage time, t, in hours, as the obscissae, for two otherwise similar capacitors having the construction shown in Figure 1. Curve A was obtained with a capacitor impregnated with mineral oil and curve B with a capacitor impregnated with dioctylphthalate impregnant.

Both capacitors were stored at a temperature of about 70"C without any voltage effect.

The RC value with mineral oil impregnation (curve A) drops from 50 seconds to 8 seconds within 250 hours, that is substantially by an order of magnitude. On the other hand, the value with dioctylphthalate impregnation (curve B) increased from the somewhat higher initial value of 65 seconds to more than double. namely to 160 seconds.

Thus, after 250 hours. the dioctylphthalate impregnated capacitor had 20 times the insulation resistance of a similar capacitor provided with mineral oil impregnation.

This difference remained during a continuation of the experiment for a further 800 hours storage time.

WHAT WE CLAIM IS: 1. A single layer electrical capacitor which comprises a winding of at least one strip of dielectric material which is metallized on both sides, one side of the strip being unmetallised at one end thereof and the other side being unmetallised at the other end so as to provide end portions which are respectively in electrical contact and insulated from each other, the winding being impregnated with a phthalic acid ester liquid dielectric material.

2. A capacitor according to claim 1. in which the strip is formed of paper.

3. A capacitor according to claim 1 or 2, in which the winding is formed of two metallised paper strips.

4. A capacitor according to claim 1 or 2, in which the winding is formed of a strip metallised on both sides and an unmetallised dielectric strip.

5. A capacitor according to any of claims 1 to 4, in which the phthalic acid ester is dioctylphthalate or dinonylphthalate.

6. An electrical capacitor substantially as herein described with reference to Figure 1 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. to the end of the strip 10 or 11. In accordance with the invention, the winding is impregnated with a phthalic acid ester impregnant, preferably dioctylphthalate or dinonylphthalate, instead of the mineral oil or polyisobutylene impregnants conventionally used with single layer capacitors. Instead of the arrangement shown in Figure 1 comprising two dielectric strips 10 and 11 metallised on both sides, the winding can also be formed from a single such strip or from one such strip and a non-metallised dielectric strip. The dielectric strips can be formed of paper or plastics, the former being preferred since the vapour deposition of metals is more readily effected on paper. It is also preferred that the winding should comprise at least one paper strip, metallised or non-metallised, so as to facilitate impregnation. The effect of impregnation with a phthalic acid ester can be seen from Figure 2 which shows curves obtained by plotting the RC value, that is the product of capacity and insulation resistance, in seconds on a logarithmic scale, as the ordinates. against the storage time, t, in hours, as the obscissae, for two otherwise similar capacitors having the construction shown in Figure 1. Curve A was obtained with a capacitor impregnated with mineral oil and curve B with a capacitor impregnated with dioctylphthalate impregnant. Both capacitors were stored at a temperature of about 70"C without any voltage effect. The RC value with mineral oil impregnation (curve A) drops from 50 seconds to 8 seconds within 250 hours, that is substantially by an order of magnitude. On the other hand, the value with dioctylphthalate impregnation (curve B) increased from the somewhat higher initial value of 65 seconds to more than double. namely to 160 seconds. Thus, after 250 hours. the dioctylphthalate impregnated capacitor had 20 times the insulation resistance of a similar capacitor provided with mineral oil impregnation. This difference remained during a continuation of the experiment for a further 800 hours storage time. WHAT WE CLAIM IS:

1. A single layer electrical capacitor which comprises a winding of at least one strip of dielectric material which is metallized on both sides, one side of the strip being unmetallised at one end thereof and the other side being unmetallised at the other end so as to provide end portions which are respectively in electrical contact and insulated from each other, the winding being impregnated with a phthalic acid ester liquid dielectric material.

2. A capacitor according to claim 1. in which the strip is formed of paper.

3. A capacitor according to claim 1 or 2, in which the winding is formed of two metallised paper strips.

4. A capacitor according to claim 1 or 2, in which the winding is formed of a strip metallised on both sides and an unmetallised dielectric strip.

5. A capacitor according to any of claims 1 to 4, in which the phthalic acid ester is dioctylphthalate or dinonylphthalate.

6. An electrical capacitor substantially as herein described with reference to Figure 1 of the accompanying drawings.