FORM 2
THE PATENT ACT 1970
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION:
"Remaining Life Indicator of the Contact System in an Electrical Switching Device"
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-1956.
(c) ADDRESS: Larsen & Toubro Limited
Electrical & Automation North Wing, Gate 7, Level 0, Powai Campus, Saki Vihar Road, Mumbai 400 072, INDIA
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

Remaining Life Indicator of the Contact System in an Electrical Switching Device
Field of invention
The present invention relates to electrical switching devices, and more particularly to a life indicator system adapted for electrical contacts of the electrical switching devices.
Background of the invention
In electrical switching devices, such as contactors, the contact system comprises mainly two parts namely a fixed contact and a moving contact. In modern contactors, the contacts materials used in the contact systems are silver alloys such' as Silver cadmium oxide (AgCdo), Silver tin oxide (AgSn02) and the like. These contact materials are usually fitted to a copper backing strip. The contactors lead to electrical arcing when the contactor switch is in an ON, a LOAD or an OFF condition. The electrical arcing causes the contact material to erode. The rate at which the contact material decays depends upon many factors like type of supply, load condition, type of operation and the like. The amount of remaining contact material in the contact system decides remaining service life of the contactors which is commonly known as the electrical life of the switching device. Usually the contactors get welded when the point of zero thickness of the contact materials is reached.
When the contactor is control supply turned off, the fixed contacts and the moving contacts are held apart with the help of the armature return spring and the contact spring. When coil of the contactor is energized first the return spring compresses fully, after which the contact spring starts compressing till the moving and the fixed

five times greater than the electrical life. Existing methodologies require the user to turn off the electrical switching device and dismantle the contact system and visually inspect them to deduce its approximate remaining life.
The US patents/patent applications such as for example US6313636, US6359440, and US20010019268 specify methods by which the contact life is measured during turning off of the switching device by a difference in time signal between the start of the magnet system opening and actual contact opening. This requires data to be measured at two points in the system so that the difference in the data can be compared to a reference time to denote the remaining life, one of such point will be to tap into the actual main pole terminals (both incoming and outgoing) and other point can be in the control circuit. By tapping into the actual main poles of the contactor they determine the start of the contact system opening and from the control circuit they were able to determine the magnet opening.
Accordingly, there exists a need for system which overcomes all the drawbacks of the prior art.
Objects of the invention
An object of the present invention is to provide a system which computes remaining lifetime of electrical contacts of an electrical switching device without turning off the electrical switching device.
Another object of the present invention is to provide a system which computes approximate remaining life of electrical contacts of an electrical switching device without dismantling or visually inspecting the contact system of the electrical switching device.

Summary of the invention
A control mechanism adapted for measurement of remaining life of contacts of an electrical switching device is disclosed. The control circuit includes a contactor unit and a remaining lifetime calculation module. The contactor unit is having a plurality of moving contacts and plurality of stationary contacts. The moving contacts are guided through a bridge having a plurality of contact springs positioned therein. The bridge is connecting to a moving magnet having a pair of return springs mounted thereon. The contact springs and the return springs mutually controlling a movement of the moving magnet. The moving magnet is magnetically communicating with a fixed magnet having a coil wound thereon. The coil receiving a main power supply controlled through a freewheeling unit and a coil drive unit. The freewheeling unit is inducing a surge current therein. The remaining lifetime calculation module is having a current sensor, an analog digital converter module, a microcontroller, an electrically erasable programmable read-only memory, a subsidiary power supply unit and a remaining lifetime display unit. The current sensor connects in series with the coil for measuring a surge current. The subsidiary power supply unit facilitates the power to the microcontroller and the remaining lifetime display unit in a turned-off condition of the main power supply. The microcontroller computes the remaining lifetime data through a lookup table. The electrically erasable programmable read-only memory stores the remaining lifetime data and the surge current data. The microcontroller facilitates display of the remaining life of the fixed contacts and the moving contacts through the remaining life display unit.
Brief description of the drawings

FIG. 1 is a block diagram of a control circuit constructed in accordance with the present invention adapted for computing a remaining lifetime of electrical contactors of an electrical switching device;
FIG. 2 illustrates a current surge produced from a freewheeling circuit of the contactor during a turn off condition of the electrical contactors of FIG. 1; and
FIG. 3 shows a flow chart illustrating a preferred method of calculation of a remaining life of the electrical contactors of FIG. 1.
Detailed description of the present invention
The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.
Referring to FIG. 1, a block diagram of a control mechanism 100 adapted for computing a remaining lifetime of an electrical contactor of an electrical switching device is shown. The control circuit 100 includes a contactor unit 12 and a remaining contact system service life calculation module 14. The contactor unit 12 includes a plurality of moving contacts 16 and a plurality of fixed contacts 18 that are positioned along a bridge 20. The bridge 20 includes a plurality of contact springs 22 adapted to assist in movement of the moving contact 16. The bridge 20 connects to a moving magnet 24. A first return spring 26 and a second return spring 28 are respectively connected to either of the ends of the moving magnet 22. The movement of the bridge 20 is adapted to be controlled by the first return spring 26, the second return spring 28 and the contact springs 22 in this one preferred

embodiment. The moving magnet 24 magnetically communicates with a fixed magnet 30. The fixed magnet 30 is wound by a coil 31 -
A main control supply 32 is adapted to be fed to the coil 31 and controlled by a coil drive unit 34. The control circuit 100 includes a freewheeling unit 36 adapted to induce a current surge. The coil drive unit in conjunction with freewheeling unit 36 controls the voltage supplied by the control voltage supply 32. The remaining contact system service life calculation module 14 includes a current sensor 38 that is connected in series with the coil 31. The current sensor 38 measures the magnitude of the surge current that is further fed to an analog digital converter (ADC, hereinafter) module 40. The current sensor 38 measures the current through the freewheeling circuit 36 of the coil drive 34 and converts it to a voltage output that is read by the ADC module 40. The ADC module converts the analog voltage output of the current sensor 38 into digital terms adapted to be read by a microcontroller 42. The control circuit 100 includes an electrically erasable programmable read-only memory (EEPROM, hereinafter) 44 that is configured to store the surge values.
The microcontroller 42 is independently powered and operated through a separate power supply unit 46. The separate power supply unit 46 is configured to incorporate a super capacitor (not shown) for the functioning of the microcontroller 42 and a remaining life time (RLT, hereinafter) display unit 48 even when the contactor control supply 32 is turned off. The microcontroller 42 is configured to compute the RLT data of the moving contacts 16 and fixed contacts 18 that is stored in EEPROM 44. The microcontroller 42 is adapted to calculate the remaining life the moving contacts 16 and fixed contacts 18 through a look up table. An output of the microcontroller 42 is configured to be fed to a remaining life display unit 48 that gives appropriate indication of the remaining life of the moving contacts 16 and fixed contacts 18.

Referring to FIG. 2, a graphical representation 200 of the current surge produced during turn off condition of the contactor unit 12 is shown. In a hold on phase 50 of the contactor unit 12, the current is freewheeling that drops down in a turn-off condition 52 of the control supply 32. Specifically, when the control supply voltage 32 to the coil drive 34 is turned off, the energy stored in coil 31 starts to decay due to inductance through the freewheeling unit 36. In such situation, the magnetic force between the magnets 24, 30 becomes less than the force of the contact springs 22. This makes the moving magnet 24 to open due to de-energisation of the coil 31. In the drop-off phase, the movement of the magnet 24 causes movement of the bridge 20 that is dependent on the contact springs 22 and the return springs 26, 28 wherein the speed of the contact opening depends on the compression of the contact springs 22. In the drop off phase of the contactor unit 12, a current surge 54 is produced in the freewheeling unit 36.
Referring to FIG. 3, a preferred method 300 adapted for calculation of a remaining life of the contactor by the remaining contact system service life calculation module 14 is shown. In a first step 301, the current sensor 38 outputs a voltage that corresponds to the freewheeling current. In next step 302, the microcontroller 42 is actuated to read the output of the current sensor 38 during the instance when the contactor unit 12 is turned off. In next step 303, the microcontroller 42 reads the values of the current sensor 38 and stores the peak values as remaining life time ( RLT, hereinafter) values in the EEPROM 44 before shutting down of the contactor unit 12. In next step 304, the microcontroller 42 calculates the average RLT values of last 10 operations in the next turn operation of the contactor unit 12. In further step 305, the microcontroller 42 checks the look-up table for calculating the remaining life time. In last step 306, the microcontroller gives a signal to the user about the remaining life time through a remaining life display unit 48.

Referring to FIGS. 1-3, in operation, the contact material on the moving contacts 16 and stationary contacts 18 erodes during the course of time that proportionally decreases the compression strength of the contact spring 22. This leads the force exerted by the contact spring 22 to gradually reduce. The force exerted by the contact spring 22 becomes the least after a particular instance. In such situation, an opening velocity of the moving magnet 24 is directly proportional to the force of the contact springs 22 and the force on the return springs 26, 28 remains constant over the entire life of the contacts 16, 18. In this situation, a flux path linked between the moving magnet 24 and the fixed magnet 18 starts to break.
In operation, a rate of change of magnetic flux in the contactor unit 12 is proportional to the EMF induced in it as per the Faraday's law of induction. Also, the rate of change of flux in the coil 31, fixed magnet 12 and moving magnet 14 become high when the magnet opening velocity is high. This causes an EMF to be induced in the coil 31 according to Lenz's law. The EMF induced is always higher in magnitude when the contacts 16, 18 are new. In comparison, the EMF induced is relatively low when the contacts 16, 18 are eroded. Accordingly, the induced EMF in the coil 31 causes a current surge to flow through the coil 31 and the freewheeling unit 36 in the coil drive 34 respectively. Thus, the peak reached by the current surge is directly proportional to the magnet opening velocity that in turn is directly related to the remaining contact life of the contacts 16,18.
In operation, the percentage of the remaining contact life and the corresponding current surge produced during turn off condition of the contactor unit 12 can be standardized for different values of erosions of the contacts 16, 18 based on which look up table is made which is stored in the microcontroller 42 and the current surge value output is compared by the current sensing unit 38 to deduce the remaining service life of the contacts 16, 18. The microcontroller 42 is configured to calculate the remaining life of the contacts 16, 18 during every turn off cycle of the contactor

be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.

We claim:
1. A control mechanism for measurement of a remaining life of electrical contacts of an electrical switching device, the control circuit comprising:
a contactor unit having a plurality of moving contacts and plurality of stationary contacts, the moving contacts guiding through a bridge having a plurality of contact springs positioned therein, the bridge connecting to a moving magnet having a pair of return springs mounted thereon, the contact springs and the return springs mutually controlling a movement of the moving magnet, the moving magnet magnetically communicating with a fixed magnet having a coil wound thereon, the coil receiving a main power supply controlled through a freewheeling unit and a coil drive unit, the freewheeling unit inducing a surge current therein; and
a remaining lifetime calculation module having a current sensor, an analog digital converter module, a microcontroller, an electrically erasable programmable read-only memory, a subsidiary power supply unit and a remaining lifetime display unit, the current sensor connecting in series with the coil for measuring the surge current, the subsidiary power supply unit facilitating the power to the microcontroller and the remaining lifetime display in a turned-off condition of the main power supply, the microcontroller computing the remaining lifetime data through a lookup table in the turned-off condition of the main power supply, the electrically erasable programmable read-only memory storing the remaining lifetime data and the surge current data, the microcontroller facilitating display of the remaining life of the fixed contacts and the moving contacts through the remaining life display unit.