FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2005
COMPLETE SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION
An encapsulated compact vacuum contactor for low power applications
INVENTORS
Rajhans Rupesh Subhashrao and Kulkarni Laxmikant Narharrao, both of Crompton Greaves Limited, Switchgear-6 & Power Quality Business Unit, D2, MIDC, Waluj, Aurangabad 431136, Maharashtra, India, both Indian Nationals
APPLICANTS
CROMPTON GREAVES LIMITED, CG House, Dr Annie Besant Road, Prabhadevi, Mumbai 400025, Maharashtra, India, an Indian Company
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF INVENTION
This invention relates to an encapsulated compact vacuum contactor for low power applications.
This invention also relates to a method of manufacturing an encapsulated compact vacuum contactor for low power applications and low power supply system comprising the vacuum contactor.
The term vacuum contactor as used in the specification includes vacuum switch.
PRIOR ART
Electrical contactors are used for controlling the switching on and off operations of electrical loads in power supply systems and are generally air break, oil break or vacuum break types depending upon the insulating medium used in the switching chambers thereof. A typical vacuum contactor for low power applications is generally unencapsulated and comprises at least one pole comprising a vacuum interrupter comprising a vacuum sealed ceramic switching chamber fitted with seal cups and metallic end plates at the top and bottom ends thereof. A fixed electrode and a moving electrode are located within the switching chamber and are provided with contact tips disposed within a shield which is fitted to the moving electrode or fixed electrode. Alternatively, the shield can also be fitted at floating potential by sandwiching it between a pair of ceramic half chamber members forming the vacuum chamber. The vacuum interrupter is disposed within and fixed to the upper flange of a C-shaped bracket which in turn is disposed within and fixed to a metal enclosure at the upper end thereof. The fixed electrode is fitted in the top end plate and protrudes out through the upper surface of the upper flange of the C-shaped bracket and is provided with a terminal head. The moving electrode is provided with bellows and protrudes down through the bottom end plate and lower flange of the C-shaped bracket and is fixed to one end of a drive insulator whose other end is fitted with a
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drive rod which extends into the metallic framework of an electromagnetic drive at the bottom of the metal enclosure. The drive rod is coupled to the electromagnetic drive and tensioned by a spring disposed over the drive rod. The drive insulator comprises a cylindrical body provided with spaced convolutions along the length thereof. A flexible terminal strip is in contact with the moving electrode. The vacuum interrupter together with the drive insulator provide phase to earth clearance i.e. creepage distance to the vacuum switch for operating personnel safety. Due to the metal enclosure and the C-shaped bracket and configuration of the drive insulator and drive rod and associated compression spring, the vacuum contactor is large and bulky thereby correspondingly increasing the size of the control panel in which it is to be mounted. Depending upon the increase in the kV rating of the switch, the creepage distance and size of the vacuum interrupter increases thereby correspondingly increasing the size of the C-shaped bracket, metal enclosure and control panel and the distance between the flanges of the bracket. Due to the end plates of the vacuum interrupter being fully exposed, the environmental loading of the vacuum contactor increases and it is not touch safe. Besides, in order to provide phase to phase isolation and to prevent flash over in a multiple vacuum contactor, the poles are to be located at sufficient clearances between them. The size of the vacuum switch and metal enclosure and control panel also increases due to the clearance requirement between the poles. Because the end plates of the vacuum interrupter are exposed, the vacuum contactor is not suitable for use in fire prone and polluted atmosphere. Besides, the end plates are prone to rusting and may have to be replaced or the vacuum interrupter as a whole may have to be replaced. This increases maintenance cost of the vacuum contactor. The weight and cost of the vacuum contactor and control panel also increases due to the large and unwieldy size of the switch. The vacuum contactor is assembled by locating the vacuum interrupter within the C-shaped bracket and fixing the C-shaped bracket to the metal enclosure or vice versa. Following this, the drive insulator with the drive rod is assembled to the moving electrode and electromagnetic drive. Consequently assembly and disassembly of the vacuum contactor is time consuming and difficult.
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US Patent No 3812314 describes an encapsulated vacuum contactor for high power applications. The vacuum contactor of the above patent is for use in underground electric power distribution systems and comprises an elongated bushing encapsulating the entire switch including the fixed electrode and moving electrode. One end of the bushing is frusto-conical shaped for receiving an electric cable termination housing in water tight relationship therewith. Mounting means comprising an annular steel flange is secured to the bushing in watertight relationship therewith at an intermediate position thereof. The end to end length of the bushing is at least twice as great as the switching chamber in order to increase the end to end flash over voltage rating of the vacuum contactor appreciably beyond its flash over rating in its unencapsulated state. This increases the size of the vacuum contactor. Also the dielectric strength of the bushing is required to be sufficient to prevent electrical puncture thereof when it is subjected to a voltage greater than that required to flash over the switch in its unencapsulated state. A void free seal between the switching chamber and the bushing is achieved by providing a thin resilient material layer between the switching chamber and bushing. This patent also describes a multi terminal high power electrical bushing having a pair of vacuum contactors encapsulated therein. One end of the bushing extends into a steel tank in a gas tight relationship therewith. The tank contains an electrical insulating gas such as freon to insulate the mechanically operable mechanisms of the vacuum contactors.
OBJECTS OF THE INVENTION
An object of the invention is to provide an encapsulated compact vacuum contactor for low power applications whose size is considerably reduced thereby reducing the size of the control panel in which it is to be mounted.
Another object of the invention is to provide an encapsulated compact vacuum contactor for low power applications whose environmental loading is negligible and which is touch safe.
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Another object of the invention is to provide an encapsulated compact vacuum contactor for low power applications which is suitable for use in fire prone and polluted atmosphere.
Another object of the invention is to provide an encapsulated compact vacuum contactor for low power applications whose maintenance cost is reduced.
Another object of the invention is to provide an encapsulated compact vacuum contactor for low power applications whose weight and cost are reduced.
Another object of the invention is to provide an encapsulated compact vacuum contactor for low power applications which is simple in construction and easy to assemble and disassemble.
Another object of the invention is to provide a method of manufacturing an encapsulated compact vacuum contactor for low power applications whose size is considerably reduced thereby reducing the size of the control panel in which it is to be mounted.
Another object of the invention is to provide a method of manufacturing an encapsulated compact vacuum contactor for low power applications whose environmental loading is negligible and which is touch safe.
Another object of the invention is to provide a method of manufacturing an encapsulated compact vacuum contactor for low power applications which is suitable for use in fire prone and polluted atmosphere.
Another object of the invention is to provide a method of manufacturing an encapsulated compact vacuum contactor for low power applications whose maintenance cost is reduced.
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Another object of the invention is to provide a method of manufacturing an encapsulated compact vacuum contactor for low power applications whose weight and cost are reduced.
Another object of the invention is to provide a method of manufacturing an encapsulated compact vacuum contactor for low power applications which is simple in construction and easy to assemble and disassemble.
Another object of the invention is to provide a low power supply system comprising the above vacuum contactor.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided an encapsulated compact vacuum contactor for low power applications, the vacuum contactor comprising at least one pole comprising a tubular housing of 7 - 10 mm thickness provided with spaced convolutions along the length thereof and mounting means and supporting means, the tubular housing being directly mounted on the insulating bracket of the metallic framework of an electromagnetic drive and cast with an electrical grade low viscosity epoxy resin encapsulating a vacuum interrupter, the fixed electrode of the vacuum interrupter protruding out of the tubular housing and provided with a terminal head and the moving electrode of the vacuum interrupter protruding down through the bottom end plate thereof and guided in an insulating material bush fitted to the bottom end plate, a drive insulator comprising an inverted cup shaped portion and a stem portion provided with spaced convolutions along the length thereof, the lower end of the moving electrode being fitted to the stem portion, a drive rod disposed within the inverted cup shaped portion with its one end fixed to the bottom of the inverted cup shaped portion and its other end extending into the metallic framework of the electromagnetic drive and coupled to the electromagnetic drive and compression spring stressed against the drive insulator, the compression spring being located within the inverted cup shaped portion over the drive rod and a flexible terminal strip fixed to the moving electrode and laterally protruding out of the housing and fixed to a lateral arm of the housing.
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A thickness of 7-10 mm for the tubular housing encapsulating the vacuum interrupter ensures proper adhesion of the housing to the vacuum interrupter and the bond between the two remains unaffected during vibrations or the like, under operating conditions of the vacuum contactor. Preferably, the thickness of the tubular housing is 8 mm and it is cast with epoxy polyester resin.
Preferably, the mounting means comprises a flange provided at the bottom of the tubular housing and having metallic inserts embedded therein spacedly, the inserts being formed with tapped holes therein and the supporting means comprises a projection provided at the upper end of the tubular housing and having a metallic insert embedded therein, the metallic insert being formed with a tapped hole therein.
The tubular housing may be made of any electrical grade low viscosity epoxy resin such as epoxy polyester, epoxy vinyl ester, polyurethane, epoxy acrylic ester or acrylic or combination thereof.
The vacuum contactor of the invention comprises a single pole, two poles or three poles or multipoles.
According to the invention there is also provided a method of manufacturing an encapsulated compact vacuum contactor for low power applications, the vacuum contactor comprising at least one pole comprising a tubular housing as described above and gravity cast with a casting composition comprising electrical grade low viscosity epoxy resin in combination with hardner, filler and primer followed by curing the casting by heating at 130 to 170 °C.
The hardner may be amine or amide type anhydride, peroxide such as methyl ethyl ketone peroxide or tertiary butylcumyl peroxide. The filler may be silica, mica, calcium carbonate, quartz, aluminium hydroxide, titania, chopped glass or
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aramid fibre or combination thereof. The primer may be of silicone or polythene based.
Preferably the casting composition comprises epoxy polyester resin in combination with quartz, titania, calcium carbonate and aluminium hydroxide as fillers and methyl ethyl ketone peroxide as hardner and silicone based primer and the casting is cured by heating at 130 - 170°C in an electric oven.
According to the invention there is also provided a low power supply system comprising the contactor described above.
The following is a detailed description of the invention with reference to the accompanying drawings, in which :
Figs 1 and 2 are, respectively, front view and back view of an encapsulated compact vacuum contactor for low power applications according to an embodiment of the invention;
Figs 3 and 4 are side views of the vacuum contactor of Fig 1;
Figs 5 and 6 are, respectively, plan view and bottom view of the vacuum contactor of Fig 1;
Fig 7 is crosssection of the vacuum contactor of Fig 1 at line A-A in
Fig l;
Figs 8, 9 and 10 are side view, front view and bottom view of a tubular housing of the vacuum contactor Fig 1, respectively; and
Fig 11 is front view of the drive insulator of the vacuum contactor of
Fig l.
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The encapsulated compact vacuum contactor 1 as illustrated in the accompanying drawings comprises three poles each marked 2 (Figs 1, 2, 3, 4, 5, 7, 8, 9 and 10). Each of the poles comprises a tubular housing 3 provided with spaced convolutions 4 along the length thereof and a flange 5 at the bottom thereof. The flange is provided with metallic inserts 6 embedded therein spacedly. The inserts are formed with tapped holes 7 therein. The tubular housing is directly mounted on the insulating bracket 8a of the metallic framework 8 of an electromagnetic drive 9 with flush countersunk screws 10 secured through the insulating bracket 8a and tightened in the tapped holes 7 in the metallic inserts 6. The electromagnetic drive is of known construction and its armature and plunger are marked 11 and 12 respectively. The housing is cast with an electrical grade low viscosity epoxy resin encapsulating a vacuum interrupter 13 comprising a vacuum sealed ceramic switching chamber 14 fitted with seal cups 15a and 15b and metallic end plates 16a and 16b at the top and bottom thereof respectively. A fixed electrode 17 and a moving electrode 18 are located within the switching chamber and provided with contact tips 19 and 20 respectively disposed within a shield 21a which is fitted to the moving electrode. Alternatively the shield can be fitted to the fixed electrode or can be fitted at floating potential by sandwiching it between a pair of ceramic half chambers (not shown) forming the switching chamber. The fixed electrode is fitted in the top end plate 16a and protrudes out of the top end of the housing and is provided with terminal head 17a. The moving electrode 18 is provided with bellows 21b and protrudes down through the bottom end plate 16b. A bush 21d made of an insulating material such as nylon is fixed on the bottom end plate and guides the moving electrode while it is moving up and down. 22 is a drive insulator comprising an inverted cup shaped portion 23 and a stem portion 24 provided with spaced convolutions 25 along the length thereof (Figs 7 and 11). The lower end of the moving electrode 18 is fitted to the stem portion 24 of the drive insulator 22 by screwing the screw portion 25a at the upper end of the drive insulator into the tapped hole 25b at the lower end of the moving electrode (Fig 7). 26 is a drive rod disposed within the inverted cup shaped portion 23 of the drive insulator with its one end fixed to the bottom of the inverted
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cup shaped portion 23 (Figs 7 and 11). The other end of the drive rod extends into the metallic framework 8 and is coupled to the electromagnetic drive 9 in known manner. The details of the metallic framework 8 and electromagnetic drive 9 are not described in detail as such are known and are not part of the invention. The drive rod is stressed against the drive insulator by compression spring 27 located within the cup shaped portion 23 of the drive insulator over the drive rod (Fig 7). 28 is a flexible terminal strip whose one end is fixed to the screw portion 25a against the lower end of the moving electrode by tightening nut 25c against the said one end of the flexible terminal strip (Fig 7). The other end of the flexible terminal strip is laterally protruding out of the tubular housing through a slot 29 in the tubular housing and is fixed to a lateral arm 30 of the tubular housing by bolts 33 engaged in tapped holes 34 in the lateral arm (Fig 7). Electrical connections are taken from the terminal 17a of the fixed electrode and from the flexible terminal strip 28 by cables or busbars (not shows). The cables or busbars are fixed to the flexible terminal strips by the bolts 33. Tapped hole in the terminal 17a for fixing the cables or busbars with screws (not shown) is marked 17b. The handles of the vacuum contactor are marked 34a, 34a. Metallic base plate marked 35 is for mounting the vacuum contactor in a control panel (not shown) with bolts (not shown) to be secured through bolt holes 35a in the base plate. 35b is projection on the tubular housing and having a metallic insert 35c fitted therein. The metallic insert 35c is formed with a tapped hole 35d. The tubular housing is supported in the control panel by a screw (not shown) secured through the sidewall of the control panel and tightened in the tapped hole 35d. Terminal blocks of the vacuum contactor are marked 36a, 36a and terminals in the blocks are marked 36b. Slots for the insulated wires are marked 37. 38a, 38b are protrusions provided with interlocking slots 39 for interlocking adjoining vacuum contactors. The vacuum contactor works in known manner.
A typical compact encapsulated vacuum contactor of 7.2 kV according to Fig 1 was gravity cast as follows:
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A casting composition was prepared by mixing 35 - 40 parts by weight of epoxy polyester resin, 55 - 60 parts by weight of mixture of quartz, titania, calcium carbonate and aluminium hydroxide and 0.5 parts by weight of silicone based primer under vacuum at 1-100 torr and at 130°-170°C for 2 - 4 hrs. The vacuum was interrupted and 0.75 - 5 parts by weight of methylethyl ketone peroxide was added to the above mixture and mixing continued under vacuum at 1 - 100 torr and at 130°C for 15 minutes. The vacuum interrupter was located in a mould and both were cleaned with carbon tetra chloride to remove oil and grease particles. Areas in the mould not required to be gravity cast with the casting composition were masked with silicone grease. The mould and vacuum interrupter were preheated at 60°C for 1 Vi hrs and placed in a vacuum gravity casting plant. The casting composition was poured into the mould to form an encapsulation of a tubular housing of 8 mm around the vacuum interrupter. On completion of pouring, the mould was removed from the casting plant and kept at ambient temperature for ½ hrs. The same was cured in an electric oven at 130 — 170°C for 10-20 hrs. The vacuum contactor housing was demoulded and deburred and cleaned with xylene.
The drive insulator with the drive rod housed in the inverted cup shaped portion thereof was assembled to the moving electrode and then the drive rod was assembled to the electromagnetic drive in known manner. Following this, the vacuum contactor was fitted on the insulating bracket of the electromagnetic drive.
The overall size of the typical 7.2 kV encapsulated compact vacuum contactor of the invention was 0.016m3, whereas the average overall size of equivalent conventional non-encapsulated vacuum contactors was found to vary from 0.03 to 0.073 m. Thus, the encapsulated vacuum contactor of the invention is reduced in size and compact.
According to the invention the C-shaped bracket and metal enclosure have been eliminated. The drive rod and associated compression spring are located
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within the inverted cup shaped portion of the drive insulator. The vacuum contactor is directly mounted on the metallic framework of the electromagnetic drive. The convolutions on the housing and the drive insulator reduce the overall end to end linear distance of the vacuum contactor but at the same time provide effective phase to earth clearance ie creepage distance. Due to the vacuum interrupter including the end plates being totally insulated from one another, the phase to phase clearance between poles is reduced considerably and the poles are closer to one another centre to centre. As a result of all this, the size of the vacuum contactor is considerably reduced and it is compact correspondingly reducing the size of the control panel in which it is to be mounted. Because the end plates are totally covered within the housing, environmental loading on the switch is negligible and it is touch safe and prevents flash over between the poles. The contactor is, therefore, suitable for use in the fire prone and polluted atmosphere. The end plates are not prone to rusting as they are covered within the housing. This increases the life of the vacuum interrupter even in highly polluted environment and eliminates the need for replacement of vacuum interrupter due to rusting of the end plates. This also reduces the maintenance cost of the contactor. Because of the compact and light construction of the vacuum contactor, the control panel size and weight reduces as also the mechanical strength of the control panel needed to mount the vacuum contactor therein. The housing is cast with the vacuum interrupter embedded or encapsulated therein as single unit. The stem portion of the drive insulator assembled to the moving electrode and the drive rod housed in the inverted cup shaped portion of the drive insulator and assembled to the electromagnetic drive is directly fitted on the metallic framework of the electromagnetic drive. Therefore, the compact vacuum contactor is simple in construction and easy to assemble and disassemble. The compact vacuum contactor of the invention does not use end to end resin encapsulation in a linear fashion as in the case of the US Patent No 3812313 thereby realizing reduction in size of the vacuum contactor and in the casting material cost. Besides, the tubular housing eliminates the void free seal of the US patent and eliminates the bushing thereof. It also does not use electrical gas insulation as in the case of the US patent.
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There may be modifications or variations in the construction of the vacuum contactor of the invention as illustrated in the drawings without deviating from the scope thereof. For instance, the mounting means and supporting means of the vacuum contactor of the invention may be of different construction / configuration. The curing temperature of the housing of the vacuum contactor can vary depending upon the casting composition. The terminal strip and supporting lateral arm configuration may be different. Instead of the electro magnetic drive, a different operating linkage or mechanism can be used in the vacuum contactor of the invention. Such variations of the invention are to be construed and understood to be obvious variations falling within the scope of the invention.
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WE CLAIM
1) An encapsulated compact vacuum contactor for low power applications, the vacuum contactor comprising at least one pole comprising a tubular housing of 7-10 mm thickness provided with spaced convolutions along the length thereof and mounting means and supporting means, the tubular housing being directly mounted on the insulating bracket of the metallic framework of an electromagnetic drive and cast with an electrical grade low viscosity epoxy resin encapsulating a vacuum interrupter, the fixed electrode of the vacuum interrupter protruding out of the tubular housing and provided with a terminal head and the moving electrode of the vacuum interrupter protruding down through the bottom end plate thereof and guided in an insulating material bush fitted to the bottom end plate, a drive insulator comprising an inverted cup shaped portion and a stem portion provided with spaced convolutions along the length thereof, the lower end of the moving electrode being fitted to the stem portion, a drive rod disposed within the inverted cup shaped portion with its one end fixed to the bottom of the inverted cup shaped portion and its other end extending into the metallic framework of the electromagnetic drive and coupled to the electromagnetic drive and compression spring stressed against the drive insulator, the compression spring being located within the inverted cup shaped portion over the drive rod and a flexible terminal strip fixed to the moving electrode and laterally protruding out of the housing and fixed to a lateral arm of the housing.
2) An encapsulated compact vacuum contactor as claimed in claim 1, wherein the tubular housing is 8 mm thick and is cast with epoxy polyester resin.
3) An encapsulated compact vacuum contactor as claimed in claim 1 or 2, wherein the mounting means comprises a flange provided at the bottom of the tubular housing and having metallic inserts embedded therein spacedly, the inserts being formed with tapped holes therein and the supporting means comprises projection provided at the upper end of the tubular housing and having a metallic insert embedded therein, the metallic insert being formed with a tapped hole therein.
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4) An encapsulated compact vacuum contactor as claimed in any one of claims 1 to 3, which comprises single pole, two poles or three poles or multipoles.
5) A method of manufacturing an encapsulated compact vacuum contactor for low power applications, the vacuum contactor comprising at least one pole comprising a tubular housing as claimed in any one of claims 1 to 3 and gravity cast with a casting composition comprising electrical grade low viscosity epoxy resin in combination with hardener, filler and primer followed by curing the casting by heating at 130 to 170 °C.

6) A method as claimed in claim 5, wherein the casting composition comprises epoxy polyester resin in combination with quartz, titania, calcium carbonate and aluminium hydroxide as fillers and methyl ethyl ketone peroxide as hardner and silicone based primer and the casting is cured by heating at 130 - 170°C in an electric oven.
7) A low power supply system comprising the vacuum contactor as claimed in any one of claims 1 to 4.
Dated this 30th day of March 2006

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ABSTRACT
An encapsulated compact vacuum contactor for low power applications. The vacuum contactor (1) comprises at least one pole (2) comprising a tubular housing (3) of 7 - 10 mm thickness provided with spaced convolutions (4) along the length thereof and mounting means (5) and supporting means (35b). The tubular housing is directly mounted on the insulating bracket (8a) of the metallic framework (8) of an electromagnetic drive (9). The tubular housing is cast with an electrical grade low viscosity epoxy resin encapsulating a vacuum interrupter (13). The fixed electrode (17) of the vacuum interrupter protrudes out of the tubular housing and is provided with a terminal head (17a). The moving electrode (18) of the vacuum interrupter protrudes down through the bottom end plate (16b) thereof and is guided in an insulating material bush (21d) fitted to the bottom end plate. A drive insulator (22) comprises an inverted cup shaped portion (23) and a stem portion (24) provided with spaced convolutions (25) along the length thereof. The lower end of the moving electrode is fitted to the stem portion of the drive insulator. A drive rod (26) is disposed within the inverted cup shaped portion with its one end fixed to the bottom of the inverted cup shaped portion and its other end extending into the metallic framework of the electromagnetic drive and coupled to the electromagnetic drive and compression spring (27) stressed against the drive insulator. The compression spring is located within the inverted cup shaped portion over the drive rod. A flexible terminal (28) is fixed to the moving electrode and laterally protruding out of the housing and fixed to a lateral arm (30) of the housing (Fig 7).