F O R M 2

THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)

1. Title of the invention: A WELD FREE ELECTRICAL SWITCHING DEVICE DURING SHORT CIRCUIT CONDITION

2. Applicant(s):

(a) NAME : LARSEN & TOUBRO LIMITED
(b) NATIONALITY : An Indian Company
(c) ADDRESS : L & T House, Ballard Estate, Mumbai 400 001,
State of Maharashtra, India

3. 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 generally to electrical switching devices. Particularly, the present invention relates to control devices like contactors which would be able to withstand the abnormal condition of short circuit in an electrical circuit without imparting abnormality to the switching device, mainly contact welding. More particularly, the invention is concerned about providing weld free configuration in contactor arrangements which would avoid welding of the contactors during short circuit condition.

BACKGROUND OF THE INVENTION

Switching devices such as contactors are intended to withstand stand the short circuit current until any protection device will clear the fault. However, in contactors in some applications, particularly in motor controllers, a short circuit fault current generates repulsion force between moving and fixed contacts due to constriction force. Because of this repulsion force, moving contact moves in the upward direction and arcing takes place. When contacts depart from each other, constriction force reduces to zero and during this condition actuator force tries to keep moving contact in ON condition biased by means on spring force This condition forces moving contact to close ON in molten condition and at this point welding occurs.

Weld free coordination can be achieved by electro mechanical actuation because magnetic forces increase with square of current. So when fault occurs, higher value of force is acquired. This condition is free from magnetic saturation. Such a system is disclosed in US 6,150,909.

US 6,150,909 discloses an electromagnetic switchgear unit has an electromagnetic drive appliance which acts as an auxiliary magnet system and by means of which the contact force is first increased on the occurrence of a short-circuit current, before the contacts are abruptly torn apart. For this purpose, the drive appliance actuates a lever rotatably supported on the contact bridge carrier, which lever displaces the contact bridge carrier and, by this means, compresses a contact compression spring and a further compression spring.

US ‘909 in which an electromagnetic drive appliance (lever) acts as an auxiliary magnet system and by means of which the contact force is first increased on the occurrence of short-circuit current, before the contacts are abruptly torn apart.

The disadvantage of the above mentioned prior art method and apparatus is that electromagnetic drive is actuated by the current flow through the terminal which will cause temperature rise.

So therefore it would be desirable to provide a method and apparatus without using any magnetic material in contact zone to avoid magnetic saturation and blowout force.

Thus there is a need to provide an improved arrangement in switching device likewise contractors that would provide weld free feature in switching devices during short circuit current.

OBJECTS OF THE INVENTION

A basic object of the present invention is to overcome the drawbacks/disadvantages of the prior art.

One of the main objects of the invention is to develop an ability of switching device such as contactor, for self protection during short circuit, which is primarily intended to withstand the fault current.

Another object of the present invention is to provide weld free configuration without affecting the arcing time and the electrical life of the product.

These and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided an electrical switching device, said device comprising
housing;
plurality of contact assemblies located in said housing
said each contact assembly comprising :
a bridge means ;
a plate means housed on said bridge means ;
a bridge guide means to support the motion of said bridge means;
a latch means being engaged with said plate means;
a moving contact means operatively connected with said latch means,
a spring means operatively biased with plate means;
a fixed contact in proximity to said moving contact means;
wherein said latch means having substantially a T-shaped modular profile comprising a modular upper portion having plurality of slot means adapted to accommodate said plate means within said slots and a modular lower portion having substantially tapered edge operatively connected with said modular upper portion;
wherein said moving contact means having a split substantially at its centre to define a pair of moving contact terminal on either side of said split, said each moving contact terminal having a substantially L-shaped modular profile comprising :
plurality of moving contact guide means adapted to guide the path of said latch means;
a moving contact button means and
spring holder means substantially placed at the center of said moving contact means adapted to hold said contact spring .

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an electrical switching device that solves the problem of welding at the time of short circuit, by opposing the opening of contacts in closed position during fault current condition. The object is achieved in accordance with the invention, by providing split movable contact and a tapered edge latch inserted between two contacts which is placed on top of the bridge. In normal operating condition, both movable contacts carry the current in same direction and are apart from each other by a latch. In fault condition, movable and fixed contact buttons repel each other due to constriction force. At the same time, due to ‘Lorentz attraction force ‘split moving contacts are attracted towards each other and the tapered edges of the latch create friction forces between them and the split moving contact. This avoids further upward motion of movable contact. By this means, repulsion of movable contact can be avoided.

Key features of the present invention:

• Easy in construction only movable contact is split into two ,fixed contact remains as single unit.
• Bridge housing includes latch part.
• Magnetic attraction and friction forces are opposing welding in contactors
• In this concept arcing chamber does not contain any magnetic element so normal operations are not affected.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

Other features as well as the advantages of the invention will be clear from the following description.
In the appended drawings:

Figure 1 illustrates the perspective view of the contact assembly in closed with the housing, incorporating the present invention.

Figure 2 illustrates the perspective view of the contact assembly in open position with the housing, incorporating the present invention.

Figure 3 illustrates a perspective view of latch used in this concept.

Figure 4 illustrates the perspective view of the movable contacts.

Figures 5(a) and 5(b) show the perspective views of the contact assembly (in open position) with the housing, incorporating the present invention.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWING

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and illustrate the best mode presently contemplated for carrying out the invention.

Referring to Fig 1 and Fig 2, the contact assembly along with the latch is shown in its perspective view. Bridge 11 is provided with the bridge guide 11(a) to support motion of bridge in the main housing. Bridge 11 also houses a plate 12 to hold the latch 14 on both incoming and outgoing side which is inserted in between moving contact 16. Moving contact guide 16(a) guides the path of latch to make sure that it is inserted fully in between the moving contact 16 at the time of over travel. Contact spring 13 provides an over travel in contactor and the present invention can restrict the further compression of contact spring after over travel when fault current occurs. In the present invention contact spring is a compression spring, however, any other spring can be used and such use is not to be considered as departure from the scope of the present invention. The spring is biased with said plate 12 and compression of the spring moves the said plate 12 which correspondingly moves the latch means. Plate 15 is a holding plate which holds spring 13 and moving contact 16. Moving contact button 17 and fixed contact button 18 repel each other when fault occurs. Fixed contact is shown as 19. Fig 2 is the same contact assembly in open condition.

Referring to Fig 3, latch is shown in perspective view. The latch is substantially T shaped and has tapered edge 22 which opposes the moving contact movement in upward direction and creates frictional forces. A slot 21 is provided in the latch so that it can fix itself in plate 12.

Fig 4 shows the layout of a L shaped moving contact in which, 16(a) is the moving contact guide to guide the latch movement in between the split movable terminals. Spring holder 16(b) holds the contact spring.

Referring to Fig 5(a) and 5(b), the contact assembly along with the housing is shown in its perspective view. The figures include the housing 23 incorporating the bridge 11 and the bridge guide 11(a) to support its motion. Bridge 11 also houses a mild steel plate 12 which is used to hold the latch 14. The figure also shows the splitted moving contact 16 which is housed within the bridge. The bridge assembly also consists of box clamp assembly 20 , arc chute assembly 25 and the exit plate 24.

Under normal operation i.e. with normal operating current, both split moving contacts 16 are apart from each other by latch 14 and carry current in the same direction. Attraction force is not sufficient enough to cause any lateral movement of the split moving contacts 16. Short circuit fault condition generates an extremely high constriction force across the contact surfaces 17 and 18 in contactors. Such high constriction forces often overcome the contact biasing forces and leads to the blow open of the contacts. After the contacts are blown open, together with the increasing force created by the biasing springs when further compressed, the contacts will reclose within a few milliseconds, and usually well before the fault current has returned to current zero which can result in permanent welding of the contacts.

In other words, contact separation between the short circuit conditions leads to arcing between the moving and the fixed contact which can cause the contacts to melt. If the contacts were to close together before the molten metal cools and solidifies, the fixed and moving contacts will become firmly and permanently welded together. Thus in the present invention the contacts are prevented from moving away from each other during short circuit current.

During short circuit current, the Lorentz force of attraction between two moving contact 16 is being far greater than force on normal rated current, so both moving contacts move inward and hold latch 14. At the same time, due to short circuit, repulsion force occurs between contacts (17 and 18), which then tries to repel open the moving contact. However the tapered edge of latch 21 restricts the motion in the upward direction. Thus the self protection of contactor is ensured during short circuit with the present invention.

The invention has been described in a preferred form only and many variations may be made in the invention which will still be comprised within its spirit. The invention is not limited to the details cited above. The structure thus conceived is susceptible of numerous modifications and variations, all the details may furthermore be replaced with elements having technical equivalence. In practice the materials and dimensions may be any according to the requirements, which will still be comprised within its true spirit.

WE CLAIM:

1. An electrical switching device, said device comprising
housing;
plurality of contact assemblies located in said housing
said each contact assembly comprising :
a bridge means ;
a plate means housed on said bridge means ;
a bridge guide means to support the motion of said bridge means;

a latch means being engaged with said plate means;
a moving contact means operatively connected with said latch means,
a spring means operatively biased with plate means;
a fixed contact in proximity to said moving contact means;

wherein said latch means having substantially a T-shaped modular profile comprising a modular upper portion having plurality of slot means adapted to accommodate said plate means within said slots and a modular lower portion having substantially tapered edge operatively connected with said modular upper portion;

wherein said moving contact means having a split substantially at its centre to define a pair of moving contact terminal on either side of said split, said each moving contact terminal having a substantially L-shaped modular profile comprising :
plurality of moving contact guide means adapted to guide the path of said latch means;
a moving contact button means and
spring holder means substantially placed at the center of said moving contact means adapted to hold said contact spring .

2. Device as claimed in claim 1 wherein said bridge adapted to hold the latch.

3. Device as claimed in claim 1 further comprising a holding means substantially placed above said moving contact means adapted to hold said spring means and moving contact means.

4. Device as claimed in claim 1 wherein said plate with tapering ends means holds the latch means on both incoming and outgoing side.

5. Device as claimed in claim 1 wherein said tapered edge of latch accommodates inside said split defined by said moving contact means.

6. Device as claimed in claim 5 wherein said tapered edge of latch restricts the motion of moving contact in upward direction.

7. Device as claimed in claim 6 wherein said tapered edge is adapted to create frictional forces to restrict the motion of moving contact in upward direction.

8. Device as claimed in claim 1 wherein said split of said moving contact means is provided to accommodate said modular lower portion.

9. Device as claimed in claim 1 wherein plurality of slot means comprising a pair of slot to accommodate said plate means.

10. An electrical switching device as herein substantially described and illustrated with reference to the accompanying drawings.