FORM 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 ELECTROMAGNETIC
SWITCHGEAR UNIT

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

The present invention relates to an electromagnetic switchgear unit. More particularly, the invention is concerned about providing weld free configuration in contactor arrangements used in motor controllers, which would avoid welding of the contactors during short circuit condition.

Background and the prior art

Switching devices such as contactors are intended to withstand 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 of spring force. This condition forces moving contact to close ON in molten condition and at this point welding occurs.

In conventional contactors or in switching devices, no special means is provided to prevent blow open at short circuit fault current. To overcome the effect of blow open, typical approach is to use a magnetic force induced by short circuit fault current to keep the contacts closed during high current. One example of such system is disclosed in U.S.P. 4,513,270 in which by the mean of placing two magnetic components provide a compensation force, in this magnetic flux developed in magnetic member when short circuit current flows through a contactor, generating electrodynamics forces to force the movable contacts against stationary contacts so as to prevent the contacts from separation.

One disadvantage of attempting to keep the contacts closed during a short circuit is magnetic saturation of magnetic components that generates the force.

Other way is to have a method and apparatus that could prevent contact welding under fault condition by opening the contacts relatively quickly upon the occurrence of fault condition and maintaining the contacts open until the fault condition dissipates there by allow closer without welding. Example of such system is disclosed in a European patent application No EP09074997A2, in that they placed a U-I shape magnetic material, U shape magnetic material enclose movable contact, this will assist the contacts to open quickly by using magnetic force generated by the fault current and maintains the contacts in an open position and allows the contact surfaces to cool sufficiently and solidify to avoid the welding.

Disadvantage of this method is presence of any magnetic material in contact zone may alter arcing time and performance on electrical duty.

Weld free coordination can also be achieved by electro mechanically actuation to avoid the magnetic saturation such system is disclosed in U.S.P. 6,150,909 in which an electromagnetic drive appliance (lever) act 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.

Disadvantage of this method is electromagnetic drive is actuated by current the flow through terminal which will cause the temperature rise.

So therefore it would be desirable to provide a weld free configuration in contactor arrangements used in motor controllers without using any magnetic material in contact zone to avoid magnetic saturation and blowout force.

The present inventor have found that the moving contact can be restricted from its movement in the upward direction during short circuit condition thereby avoiding formation of arc between the contactors by means of a unique design of the contactor arrangement in the switchgear system such that on rated current Lorentz attraction force in the contactors in not sufficient to overcome copper stiffness but on faulted /short circuit condition force generated is sufficient to provide a unique configuration of the contactors.

Objects of the invention

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

The primary object of the present invention is to develop ability of switching device for self protection during short circuit, which intended to withstand fault current such as contactor.

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

Yet another objet of present invention is to provide weld free configuration with multi break system.

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 electromagnetic switchgear unit comprising :
housing;
bridge means secured in said housing;
contact assembly housed in said bridge means;
said contact assembly comprising
terminal means defining a fixed contact means;
moving contact means;
wherein said fixed contact means comprises a pair of extending limb means having symmetrical flange like profiles at the extended ends comprising cavity means;
wherein each of said limb means having predetermined space of separation in between;
wherein said moving contact means comprising latch means, said latch means being suspended from said moving contact and located so as to align itself substantially symmetrically along axis of the said cavity means;
wherein said limb means being moveable substantially laterally towards each other during short circuit condition so as to operatively trap the said latch means with the said cavity.

Detailed description of the invention

The present invention is based on the objective creating an electromagnetic switchgear unit which is a quarter break system and able to withstand fault current.

The object is achieved in accordance with the invention, by providing a split terminal and a latch on movable contact. When fault current appear, due to ‘Lorentz attraction force’ both the limbs those are part of terminal have the current in same direction try to attract each other. The contact system is design in such a way that on rated current this attraction force is not sufficient to overcome copper stiffness, but on faulted condition force generated is sufficient to attract both limbs. A latch is attached to movable contact, for each making operation this latch moves inside the cavity of terminal limbs, on fault current when both the limbs moves inward that latch is trapped between the limbs there by restricted movement of moving contact. Movement of terminal limbs is comparatively faster than movement of movable contact which is due to repulsion force. Primary attraction force is generated on fixed contact limbs and moving contact action is in response to repulsion; since primary action is dominant in present art, moving contact remains at same condition. By this means, repulsion of movable contact can be avoided.

Brief description of the accompanying drawings

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

Figures 1 and 2 illustrate isometric views of the switchgear unit (in open position) with the housing, incorporating the present invention.

Figure 3 illustrates an isometric view of the switchgear unit only in open position.

Figure 4 illustrates an isometric view of the fixed contact of the switchgear unit.

Figure 5 illustrates an isometric view of the moving contact of the contact assembly.

Figure 6 illustrates an isometric view of the contact assembly in closed position.

Figure 7 illustrates a cross-sectional view of Figure 6 taken along the line 1-1 of Figure 3.

Figures 8a and 8b illustrate different designs of the latch means.

Detailed description of the accompanying drawings

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. Further functioning of the unit has been discussed below to describe the way the system operates. However, such description should not be considered as any limitation of scope of the present unit. 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.

Referring to Fig 1 and Fig 2, the contact assembly along with the housing is shown in its perspective view. The housing 19 is provided with a bridge 20 which houses the contact assembly consisting of the terminal 11, which itself acts as a fixed contact, moving contact 15 and latch 16. The terminal 11 is provided with two sets of limbs 12. Each limb is provided with contact button 13. The moving contact 15 is made up of two plates- 15a and 15b. 15a is a plate made up of mild steel while 15b is copper. The moving contact is provided with a contact button 17 and a provision to hold the latch 16. In order to hold the latch, a cavity 14 is provided in between the two limbs of the fixed contact. A groove 18 is provided on the bridge at both the ends as a provision to house the return spring.

Referring to Fig 3, a contact assembly is shown in perspective view. The assembly has a terminal 11 which itself acts as a fixed contact. The terminal 11 has a pair of extending limbs 12 with a pair of contact buttons 13. A pair of movable contacts 15 is provided with a pair of contact buttons 17 and a provision to hold the latch 16. The terminal 11 houses a cavity 14 to hold the latch.

Fig 4 shows the layout of the terminal 11. This terminal itself acts as the fixed contact. The terminal consists of two limbs 12, each capable of lateral movement. The limbs 12 have flange like profile 23 at their extended ends which comprises cavities 14 to hold the latch 16 at the time of short circuit. Under ideal conditions, the two limbs always orient themselves as depicted in this figure. The flange profile is substantially L shaped. Limbs 12 are separated by an appropriate space in between each other. The force of attraction between the two limbs 12 is considerable only at the time of short circuit.

Fig 5 illustrates the strategic location of the latch 16 with respect to the moving contact 15 and the terminal 11. The latch is made to pass through the cavity of the moving contact 15. It aligns itself symmetrically along the axis of the cavity 16 of the terminal 11. Latch 16 comprises rounded head portion 16a which cooperates with the cavity 14 to engage itself during short circuit condition. The latch 16 can also be substantially T shaped (16b) (referring to figure 8a) or having a polygonal shaped head portion 16(c) (referring to figure 8b).

Fig 6 shows the same contact assembly in closed position during short circuit with the latch 16 of the moving contact 15, held firmly between the two limbs 12 of the terminal 11. Under ideal closed condition, latch 16 will orient itself symmetrically in the cavity 14 of the terminal 11. This is due to fact that at normal current the force of attraction between the limbs 12 of the terminal 11 is insufficient. However during short circuit, the force of attraction is enough for the sufficient and equal lateral movement of the two limbs 12 which results in the trapping of the latch 16 within the cavity 14. The force of repulsion between the contacts during short circuit is thus less than the force of attraction between the two limbs. Thus, during short circuit, the fixed contact buttons 13 remain in contact with the moving contact buttons 17.

Fig 7 illustrates the cross-sectional view of the Fig 3 taken along the line 1-1. This figure illustrates the position of the latch within the cavity at the time of short circuit. During short circuit, the latch is held between the two limbs 12 of the terminal 11 due to the equal displacement of the limbs on account of very high Lorentz force of attraction.

Under ideal conditions, the current flowing in the split terminals 12 is not sufficient enough to cause the lateral movement of the limbs 11. Short circuit fault current condition generates an extremely high constriction force across the contact surfaces in contactors. Such high constriction forces often overcome the contact biasing forces and leads to the blow open of the contacts. Because of the rapid decrease of the arc pressure difference across the movable 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 the limbs 12 of the terminal 11 being far greater than the force of repulsion between the moving contact 15 and fixed contact, the limbs 12 move laterally and the latch is trapped within the cavities 14 of the limbs 12. This prevents the opening of the contacts during short circuit and helps in achieving weld free configuration. These limbs 12 of the terminal 11 then resume back to their original position once the fault current subsidies and the latch 16 is able to move freely again. Thus the self protection of the contactor is ensured during short circuit with the present invention. The switchgear unit of the present invention is also applicable in multiple break system in the contactors.

WE CLAIM:

1. An electromagnetic switchgear unit comprising :
housing;
bridge means secured in said housing;
contact assembly housed in said bridge means;
said contact assembly comprising
terminal means defining a fixed contact means;
moving contact means;
wherein said fixed contact means comprises a pair of extending limb means having symmetrical flange like profiles at the extended ends comprising cavity means;
wherein each of said limb means having predetermined space of separation in between;
wherein said moving contact means comprising latch means, said latch means being suspended from said moving contact and located so as to align itself substantially symmetrically along axis of the said cavity means;
wherein said limb means being moveable substantially laterally towards each other during short circuit condition so as to operatively trap the said latch means with the said cavity.

2. An electromagnetic switchgear unit as claimed in claim 1 wherein said flange profile is substantially L shaped having chamfered surface.

3. An electromagnetic switchgear unit as claimed in claim 1 wherein said cavity means provided on the chamfered surface of said flange profile is adapted to trap the latch means during short circuit condition.

4. An electromagnetic switchgear unit as claimed in claim 1 wherein said latch means comprising substantially round head portion adapted to co-operate with said cavity means during short circuit condition preventing said moving contact means to move in the upward direction.

5. An electromagnetic switchgear unit as claimed in claim 4 wherein said head portion of latch means is optionally polygonal.

6. An electromagnetic switchgear unit as claimed in claim 1 wherein said latch means is optionally substantially T shaped.

7. An electromagnetic switchgear unit as claimed in claim 1 wherein said limbs comprising contact buttons.

8. An electromagnetic switchgear unit as claimed in any of the preceding claims wherein Lorrentz force of attraction between said limbs is greater than the repulsive force between said moving contact means and said fixed contact means.

9. An electromagnetic switchgear unit as claimed in any of the preceding claims adapted to be used in multi break system in contactors.

10. An electromagnetic switchgear unit as herein substantially described and illustrated with reference to the accompanying drawings.