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, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Linear induction drive for operating the disconnector or earth switch of a gas insulated
switch gear
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road. Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTOR
Raysaha Rosy Balaram, Crompton Greaves Limited, C G Global R&D Centre, HVPT, Kanjur Marg (East), Mumbai 400042, Maharashtra, India, an Indian national
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
This invention relates to a linear induction drive for operating the disconnector or earth switch of a gas insulated switchgear
BACKGROUND OF THE INVENTION
Various drives are known to be used to operate an electrical switching device comprising a fixed contact and a moving contact disposed in a housing. In an induction motor or a DC motor based drive for the operation of an electrical switching device, the motor shaft is connected to the moving contact through an insulated drive rod and a mechanical linkage. The mechanical linkage is housed either in a moving contact holder along with the moving contact or in a separate enclosure located outside the housing of the switching device. During operation of the switching device, the rotary movement of the motor is converted into linear motion of the drive rod and the moving contact through the mechanical linkage. Because the drive converts rotary motion into linear motion, energy requirements for operation of the drive is high. Energy is also required in order to overcome frictional forces developed in the mechanical linkage. Therefore, the drive is not energy efficient. Due to friction, wear and tear occurs to the mechanical linkage and it requires periodical maintenance. Life of the mechanical linkage is also reduced thereby requiring periodic replacement thereof. Due to the large number of components and frictional losses, reliability of the drive is reduced and size of the drive is increased. Because of all this, the capital cost and running cost of the drive is also increased.

Linear synchronous motor or permanent magnet motor based drives are also known to be used for the operation of electrical switching devices. Arndt et al describe a magnetic linear drive particularly for an electrical switch. The linear drive comprises an armature having a permanent magnet as a portion of the insulated drive rod and describes a linear reciprocating movement in an air gap formed between two yokes (US6888269 Bl). Kampf et al describe a linear magnetic drive for operating the moving contact of an electrical switch. The linear drive comprises an armature connected to a permanent magnet and a yoke and a spring packet arranged as an elastic element to hold the armature stable. The armature describes a linear reciprocating movement in the air gaps formed in an iron core (US 7482902 B2). Permanent magnet based linear drives are heavy and bulky thereby increasing the material cost of the drives and also the space requirement thereof. As the armature is connected to the permanent magnet and yoke, it has to drive the permanent magnet and yoke also. Therefore, the energy requirement for operation of the linear drive is high. Besides, the performance efficiency of the drive is also reduced. In the case of the linear drive comprising spring packet as an elastic element, reliability of the drive is also reduced as the performance of the spring deteriorates over a period of time thereby requiring periodic replacement thereof.
CN101266893A describes a linear induction drive for operating the moving contact of a high-pressure circuit breaker. The induction drive comprises a linear induction motor having a cylindrical secondary, a contact spring and a permanent magnet. Although end to end connection between the secondary of the motor and insulated drive rod of the circuit breaker is easy due to the cylindrical nature of both, the weight of the cylindrical secondary is high thereby increasing the material cost of the drive and also the energy requirement for operation of the drive.

Furthermore, the spring absorbs and releases energy during contraction and expansion. While releasing the energy, the moving contact may bounce and rub against the fixed contact. Over a period of time, this will cause erosion of the contacts of the circuit breaker. As a result, the isolating gap between the contacts may increase leading to improper contact between the contacts.
There is thus need for linear drives for the operation of an electrical switching device, particularly the disconnector switch or earth switch or disconnector combined earth switch of a gas insulated switch gear which obviate and /or minimise the problems associated with the conventional drives and have advantages like simplicity of construction, improved performance efficiency, low power consumption, improved reliability and reduced weight and cost.
DESCRIPTION OF THE INVENTION
According to the invention there is provided a linear induction drive for operating the disconnector or earth switch of a gas insulated switchgear comprising an enclosure located outside the housing of the switch and a linear induction motor housed in the enclosure, wherein the enclosure comprises a hollow body defined by a front end wall and a rear end wall spaced apart from each other and an enclosing peripheral wall extending between and terminating with the front end wall and rear end wall, the front end wall is mounted to the housing of the switch, with the distal end of the insulated drive rod of the moving contact of the switch protruding into the enclosure through the front wall of the enclosure in a gas tight manner, the linear induction motor is a low speed, single sided type comprising a short primary mounted to the peripheral

wall of the enclosure insulated therefrom and connectable to a power supply through a control circuit and a flat secondary linearly, reciprocally disposed in a linear guide mounted to the peripheral wall of the enclosure insulated therefrom, the secondary is parallel to and in the proximity of the primary defining an air gap with the primary, the forward end of the secondary is coupled to the distal end of the insulated drive rod protruding into the enclosure and damper means for preventing over travel of the secondary with the insulated drive rod and the moving contact, the length of the secondary is selected such that the secondary remains in the zone of influence of the primary in all the operating positions of the moving contact.
According to an embodiment of the invention, the damper means comprises a first damper sleeve fixed to the front wall of the enclosure at the inner side thereof with the insulated drive rod slidably disposed therein and a second damper sleeve fixed in the enclosure at a distance from the first damper sleeve with the insulated drive rod slidably disposed therein and a stopper ring disposed between the damper sleeves and mounted on the insulated drive rod adapted to abut against the first damper sleeve and the second damper sleeve in the extreme forward position and extreme reverse position of the secondary of the induction motor with the insulated drive rod and moving contact, respectively and prevent over travel of the secondary with the insulated drive rod and moving contact,
In one embodiment of the invention, the linear induction drive is for the disconnector switch or the earth switch of a gas insulated switch gear and wherein the first damper sleeve and second damper sleeve are at a distance from each other equal to the isolating gap between the contacts of

the respective switch and wherein the length of the secondary is 2 to 2.5 times the isolating gap between the contacts of the respective switch.
In another embodiment of the invention, the linear induction drive is for the disconnector combined earth of a gas insulated switchgear and wherein the first damper sleeve and second damper sleeve are at a distance from each other equal to the sum of the disconnector side isolating gap and earth side isolating gap of the switch and wherein the length of the secondary is 2 to 2.5 times the disconnector side isolating gap or earth side isolating gap of the switch whichever is greater.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs 1 and 2 of the accompanying drawings are schematic views of a linear induction drive for operating the disconnector switch of a gas insulated switchgear in the open and closed positions of the disconnector switch according to an embodiment of the invention, respectively;
Figs 3 is a scrap view of the distal end of the insulated drive rod and the secondary of the linear induction drive of Figs 1 and 2 connected to each other;
Fig 4 is a schematic fragmented view of the secondary and associated linear guide of the linear induction drive of Figs 1 and 2;

Figs 5 and 6 of the accompanying drawings are schematic views of a linear induction drive for operating the earth switch of a gas insulated switchgear in the open and closed positions of the earth switch according to an embodiment of the invention, respectively; and
Figs 7 to 9 of the accompanying drawings are schematic views of a linear induction drive for operating the disconnector combined earth switch of a gas insulated switchgear with the disconnector combined earth switch in the neutral position, disconnector side closed position and earth side closed position, respectively according to an embodiment of the invention.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
As illustrated in Figs 1 to 4 of the accompanying drawings, the disconnector switch A of the gas insulated switchgear (not shown) comprises a fixed contact 2 and a moving contact 3 located in a housing 4 filled with an isolating gas such as nitrogen or SF6 (not shown), 5 is the moving contact holder and 6 is the insulated drive rod of the moving contact. 1 is a linear induction drive for operating the disconnector switch comprising an enclosure 7 located outside the housing 4 and having a hollow body defined by a front end wall 8 and a rear end wall 9 spaced apart from each other and an enclosing peripheral wall 10 extending between and terminating with the front end wall and rear end wall. The front end wall is mounted to the housing 4 with the insulated drive rod of the moving contact protruding into the enclosure in a gas tight manner. 11 is a linear induction motor of low speed, single sided (singly excited) type comprising a short primary 12 mounted to a first insulating material support 13 which in turn is mounted to the peripheral wall of the enclosure. The primary iron core and primary winding are marked 14 and 15, respectively.

The motor also comprises a flat secondary 16 linearly, reciprocally disposed in a linear guide 17 mounted to a second insulating material support 18 which in turn is mounted to the peripheral wall of the enclosure. The secondary is disposed parallel to and in the proximity of the primary defining an air gap (not marked but can be seen in the figures) with the primary. 19 is a first damper sleeve fixed to the front wall of the enclosure at the inner side thereof with the insulated drive rod slidably disposed therein. 20 is a second damper sleeve disposed at a distance from the first damper sleeve and fixed to the second support 18 with the insulated drive rod slidably disposed therein. 21 is a stopper ring disposed between the damper sleeves and mounted on the insulated drive rod adapted to abut against the first damper sleeve and the second damper sleeve in the extreme forward position and extreme reverse position of the secondary with the insulated drive rod and moving contact. The distance between the first damper sleeve and the second damper sleeve is equal to the isolating gap between the contacts of the switch. 22 is a T-shaped connector in crosssection for connecting the insulated drive rod to the secondary. The connector has a threaded socket 23 in the head thereof and an externally threaded edge marked 24. The distal end of the insulated drive rod is externally threaded marked 25 and is engaged and fitted in the socket of the connector in thread engagement therewith. The edge of the connector is engaged in a-threaded hole 26 at the forward end of the secondary and fitted to the secondary in thread engagement therewith.
The insulating material supports are made of any insulating material and for example, they are made of epoxy. The damper sleeves and stopper ring are made of any instulator material and for example they are made of rubber. The secondary is made of a non-magnetic and highly

conductive material and is for example, made of aluminium or copper. Aluminium is a preferred material for the secondary as it is cheap, light weight and easy to handle. Preferably, the secondary comprises a back up iron plate.
During operation of the linear induction drive of the invention, the primary winding 14 of the motor 11 is connected to a power supply which may be an AC supply or DC supply (not shown) through a control circuit (not shown). In case of a DC power supply, the primary winding is connected to the DC supply through an inverter (not shown) to convert the DC supply to an AC supply. In order to close the contacts of the disconnector switch, an alternating current is allowed to flow in the primary winding in one direction. Due to the excitation of the primary winding, a time varying magnetic field is produced in the axial direction of the primary winding. The time varying magnetic field induces a voltage in the secondary of the linear induction motor to generate an eddy current in the secondary. The interaction between the eddy current and the time varying magnetic field produces an electromagnetic thrust on the secondary axially of the primary in the forward direction and moves the secondary linearly in the forward direction along with the insulated drive rod and moving contact of the disconnector switch to close the moving contact against the fixed contact (Fig 2).
In order to move the movable contact linearly in the reverse direction and open the contacts, the flow of current in the primary winding is reversed. As a result, an electromagnetic thrust is produced on the secondary axially of the primary in the reverse direction to move the secondary linearly in the reverse direction along with the insulated drive rod and the moving contact to disconnect and open the contacts of the disconnector switch (Fig 1). The forward and reverse

movements of the insulated drive rod along with the secondary and moving contact are restricted by the stopper ring by abutting against the first damper sleeve and the second damper sleeve respectively. The backup iron plate serves to amplify the magnetic field produced in the primary winding and correspondingly increase the electromagnetic thrust on the secondary.
The linear induction drive 1 as illustrated in Figs 5 and 6 of the accompanying drawings for operating the earth switch B of a gas insulated switchgear (not shown) is similar to that of Figs 1 to 4. The earth switch comprises a fixed contact 27. Figs 5 and 6 show the open and closed positions of the earth switch, respectively.
In the case of the disconnector switch or earth switch of a gas insulated switchgear it has been found out by us that the length of the secondary should be ideally 2 to 2.5 times the isolating gap between the contacts of the disconnector switch or the earth switch as the case may be, for the secondary to remain in the zone of influence of the primary in all the operating positions of the moving contact of the disconnector switch or earth switch and to ensure that the contacts are closed and opened instantly, effectively and efficiently.
The linear induction drive 1 connected to the insulated drive rod of the disconnector combined earth switch C of a gas insulated switchgear (not shown) as illustrated in Figs 7 to 9 of the accompanying drawings is similar to the linear induction drive of Figs 1 to 4. The disconnector side fixed contact and earth side fixed contact of the disconnector combined earth switch are marked 28 and 29, respectively. The disconnector side closed position, neutral and earth side closed position of the disconnector combined earth switch are seen in Figs 7 to 9 respectively. In

the case of the disconnector combined earth switch it has been found out by us that the length of the secondary should be ideally 2 to 2.5 times the disconnector side isolating gap (DG) or earth side isolating gap (EG) of the disconnector combined earth switch, whichever is greater, for the secondary to remain within in the zone of influence of the primary in all the operating positions of the moving contact of the disconnector combine earth switch and to ensure that the contacts are closed and opened and are in the neutral position instantly, effectively and efficiently,
According to the invention the mechanical linkage, permanent magnets and associated yokes and spring packets and cylindrical secondary all have been eliminated. The linear induction drive of the invention comprises a few component parts and is simple in construction. The weight and cost of the linear induction drive have been substantially reduced. Leakage flux in a slow speed single sided linear induction motor comprising short primary and flat secondary are substantially reduced. Therefore, the electromagnetic thrust generated by the motor is very high thereby increasing the performance efficiency of linear induction drive. The motor requires low power for its operation. As it is ensured that the secondary is always within the zone of influence of the primary, the secondary and the insulating drive rod with the moving contact can be easily moved' in both the forward and reverse directions to close and open the contacts and to take the moving contact to the neutral position instantly, effectively and efficiently. Very effective current control is achieved in the primary with the control circuit. Therefore, the speed of the secondary and the insulated drive rod with the moving contact can be easily controlled by the controlling the power in the primary winding. Because of all this, the reliability and efficiency of the induction drive are also improved.

Generally it is difficult to connect the flat secondary to the cylindrical insulated drive rod. However, the connector of the invention facilitates easy connection or assembly and disconnection or disassembly between the secondary and drive rod thereby reducing the assembly and disassembly time also. The damper sleeves and stopper ring only absorb energy and limit the movement of the secondary with the drive rod and moving contact when the stopper ring abuts against the sleeves. As a result, the moving contact of the respective switch does not bounce while making and breaking the contacts and consequent disadvantages are avoided.
It is understood that the linear induction drive of the invention can be used for operating the movable contact of any other electrical switching device. It is also understood that the configuration and damper means comprising damper sleeves and stopper ring can be different to limit the movement of the secondary and moving contact. The linear induction motor can be mounted in the enclosure differently. Such variations of the invention which are obvious to a person skilled in the art are to be construed and understood to be within the scope of the invention. The scope of the invention is defined by and encompassed within the appended claims and should be construed and understood on the basis of the claims.

We claim:
J. A linear induction drive for operating the disconnector or earth switch of a gas insulated switchgear comprising an enclosure located outside the housing of the switch and a linear induction motor housed in the enclosure, wherein the enclosure comprises a hollow body defined by a front end wall and a rear end wall spaced apart from each other and an enclosing peripheral wall extending between and terminating with the front end wall and rear end wall, the front end wall is mounted to the housing of the switch, with the distal end of the insulated drive rod of the moving contact of the switch protruding into the enclosure through the front wall of the enclosure in a gas tight manner, the linear induction motor is a low speed, single sided type comprising a short primary mounted to the peripheral wall of the enclosure insulated therefrom and connectable to a power supply through a control circuit and a flat secondary linearly, reciprocally disposed in a linear guide mounted to the peripheral wall of the enclosure insulated therefrom, the secondary is parallel to and in the proximity of the primary defining an air gap with the primary, the forward end of the secondary is coupled to the dista! end of the insulated drive rod protruding into the enclosure and damper means for preventing over travel of the secondary with the insulated drive rod and the moving contact the length of the secondary is selected such that the secondary remains in the zone of influence of the primary in all the operating positions of the moving contact.
2. The linear induction drive as claimed in claim 1, wherein the damper means comprises a first damper sleeve fixed to the front wall of the enclosure at the inner side thereof with the insulated drive rod slidably disposed therein and a second damper sleeve fixed in the enclosure at

a distance from the first damper sleeve with the insulated drive rod slidably disposed therein and a stopper ring disposed between the damper sleeves and mounted on the insulated drive rod adapted to abut against the first damper sleeve and the second damper sleeve in the extreme forward position and extreme reverse position of the secondary of the induction motor with the insulated drive rod and moving contact, respectively and prevent over travel of the secondary with the insulated drive rod and moving contact.
3. The linear induction drive as claimed in claim 2, which is for the disconnector switch or the earth switch of a gas insulated switch gear and wherein the first damper sleeve and second damper sleeve are at a distance from each other equal to the isolating gap between the contacts of the respective switch and wherein the length of the secondary is 2 to 2.5 times the isolating gap between the contacts of the respective switch.
4. The linear induction drive as claimed in claim 2, which is for the disconnector combined earth of a gas insulated switchgear and wherein the first damper sleeve and second damper sleeve are at a distance from each other equal to the sum of the disconnector side isolating gap and earth side isolating gap of the switch and wherein the length of the secondary is 2 to 2.5 limes the disconnector side isolating gap or earth side isolating gap of the switch whichever is greater.
5. The linear induction drive as claimed in claim 3 or 4, wherein the secondary is made of aluminium or copper.

6. The linear induction drive as claimed in claim 5, wherein the secondary includes a backup iron plate.
7. The linear induction drive as claimed in claim 1, wherein the forward end of the secondary is detachably connected to the distal end of the insulated drive rod.
8. The linear induction drive as claimed in claim 7, wherein the forward end of the secondary is detachably connected to the distal end of the insulated drive rod with a T-shaped connector comprising a threaded socket in the head thereof and an externally threaded edge, the distaf end of the insulated drive rod is externally threaded and fitted in the socket in the head of the T-shaped connector in thread engagement therewith and the secondary is formed with a threaded hole at the forward end thereof and the threaded edge of the T-shaped connector is fitted in the hole in thread engagement therewith.
9. The linear induction drive as claimed in claim 1, wherein the power supply is an AC supply.
10. The linear induction drive as claimed in claim 9, wherein the AC supply is a 3-phase supply.

11. The linear induction drive as claimed in claim 1, wherein the power supply is a DC supply and the control circuit is connected to the DC supply through an inverter.