DESC:FIELD OF THE INVENTION
The present invention relates generally to an electrical switching device. More particularly, the invention relates to residual life determination of a contactor, starters etc. for use in switching motors, lighting & heating elements, capacitors etc. The contactors are generally used in process industries, steel plants, power generation plants, utilities, buildings, agricultural pumps.

BACKGROUND AND THE PRIOR ART
Existing technology in this area calculates the remaining life of a switching device when it is both in closed as well as open positions. The time difference between the contact opening/closing & magnet opening/closing is measured using an analog circuit comprising of a timer and a counter or a microprocessor & remaining life is displayed via appropriate mechanisms. This requires the sensors to be installed in the terminal of the switching device which is generally at a potential upto 1000 V. The life calculated in this case is the instantaneous life of the device.
The patents US6313636, US6359440, US20010019268 and US7109720 discuss the above concepts.
The major disadvantage of the present methodologies is tapping of high voltage signal across the power terminals for calculating the life. They also need two inputs, one from the terminals & the other from the magnet system. Since the life of the contacts measured is instantaneous it will not be a true reflection of the actual life as there can be an anomaly in the instantaneous life calculation because of uneven erosion in the contacts. During every breaking operation, small globules of metal forms which can get attached to the contacts’ surface & can give erroneous remaining life.
The present invention overcomes these drawbacks by calculating the moving average life of the previous few operations by storing in a non volatile memory of a micro-controller. Since the residual life calculation is done by considering only a single input from the coil freewheeling circuit, sensors requirement as well as the computation time will be reduced.
If EEPROM is used (non-volatile memory), which has limited no. of write cycles, it will be updated only when the Residual life is falling beyond the set life band. (for eg. 20%-50% band). To prevent the ambiguous indication of the life at the set life band margins (eg. Between 20%-50% & 50%-80%), suitable programming has been done to prevent erroneous life calculation.

OBJECTS OF THE INVENTION

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

A basic object of the present invention is to provide a method for residual life determination of a contactor, starters etc. for use in switching motors, lighting & heating elements, capacitors and the like.
These and other advantages of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
In an aspect of the present invention, there is provided a method for determining residual life of an electrical switching device, said method comprising steps of:
i. sensing a current peak(s) from a freewheeling path of a coil of a contactor using a sensor means, when power supply to said coil is turned off,
ii. calculating the current peak value(s) from said current sensor output received from step (i) using a analog to digital converter (ADC) of a microcontroller;
iii. storing and updating said current peak value (s) received from step (ii) in a first memory location of a non-volatile memory of said microcontroller;
iv. calculating a moving average of said current peak value (s) obtained from step (iii) and stored in ‘n’ memory location (s) of said non-volatile memory;
v. comparing said moving average value (s) obtained from step (iv) with a look up table;
vi. providing said value obtained from step (v) to a display means for updating said display means.

Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.
Fig. 1 shows the flow chart of the entire residual life calculation sequence
Fig. 2 shows the current sensor input waveform from the coil freewheeling path when the coil drive is used with AC/DC supply
Fig 3 shows the current sensor input waveform from the coil freewheeling path when the coil drive is used with DC supply.
Fig 4 shows the complete block diagram of the circuit.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.
Throughout the drawings, it should be noted that like reference numerals are used to depict the same or similar elements, features and structures.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
Accordingly, present invention provides a method of determining residual life of an electrical switching device.
The fig. 1 shows the flow chart of the complete sequence of residual life calculations. When the coil supply 16 is turned off 1, the energy stored in the coil circulates through the freewheeling path 15 (refer to fig 4). The opening of the electromagnet 14 causes a change in induced voltage in the coil which is observed as a peak 9 & 13 (refer to fig. 2 &3). This current peak is sensed using a current sensor 17 & read using an ADC 18. This value is stored in a non volatile memory location 19. For greater accuracy, recently stored values in ‘n’ memory locations is accessed and averaged 4a & compared with a look up table & the display unit is updated accordingly 7. The look up table is made using thorough experiments done for relating the contact erosion with 9. If there is a limitation in the writing operation of the non volatile memory, the memory updation will be done as follows. After the calculation of peak value 3, it is compared with the previously stored value in the memory location. If both are found to be within the same life band, memory will not be updated. The value thus found will be compared with the look up table and the display unit 22 updated accordingly. To avoid ambiguous display of residual life near the border of two life bands, one more memory location 6 is used to count the no. of consistent stay in the subsequent life band. If it is found to be more than the predefined/set threshold then only the memory location & display unit is updated.
Remaining lifetime calculation of a contactor is done by taking the current sensor output from the freewheeling circuit of the coil. This calculation is done after the contacts are opened. The ADC acquisition and conversion of the microcontroller starts once the coil supply is cut off. The peak of the current sensor output is calculated by the microcontroller ADC and stored in the non-volatile memory of the microcontroller. Then, the moving average is calculated from the ‘n’ no. of values stored in the non-volatile memory, obtained from the series of contactor operations. The non-volatile memory can be FLASH, EEPROM, ROM etc. depending upon the microcontroller used. The moving average calculated, is then compared with the look-up table which was standardized for a particular contactor for different values of contact erosion and the output is given to the LED display unit.
If the non-volatile memory used to store the ADC results has very limited no. of write cycles compared to the electrical life of the contactor like EEPROM, it gets updated only when the ADC result completely crossed the percentage margin as per the look-up table. The effectiveness of EEPROM usage is improved by using another EEPROM location to store the no. of crossings from one life band to another life band. If this value crosses a pre defined threshold, then the EEPROM RLT value as well as the LED indications is updated. This also helps to increase the accuracy and reliability of the display unit.
ADVANTAGES:
The novel features of the invention are as follows:
1. Reliable & robust residual life indication
2. Averaging of the residual life values to improve accuracy
3. Use of only one sensor input for residual life calculation
4. No high voltage input to the circuit
5. No external connection required from the terminals
6. Judicious use of non-volatile memory location
7. Ambiguous display of remaining life is prevented by using suitable programming logic
8. Lesser computational time ,CLAIMS:1. A method for determining residual life of an electrical switching device, said method comprising steps of:
i. sensing a current peak(s) from a freewheeling path of a coil of a contactor using a sensor means, when power supply to said coil is turned off,
ii. calculating the current peak value(s) from said current sensor output received from step (i) using a analog to digital converter (ADC) of a microcontroller;
iii. storing and updating said current peak value (s) received from step (ii) in a first memory location of a non-volatile memory of said microcontroller;
iv. calculating a moving average of said current peak value (s) obtained from step (iii) and stored in ‘n’ memory location (s) of said non-volatile memory;
v. comparing said moving average value (s) obtained from step (iv) with a look up table;
vi. providing said value obtained from step (v) to a display means for updating said display means.
2. The method as claimed in claim 1, wherein energy stored in said coil circulates through said freewheeling path, when power supply to said coil is turned off facilitating opening of a contact(s) of said contactor causing change in induced voltage in said coil, thereby providing said current peak.
3. The method as claimed in claim 1, wherein said current peak value(s) after calculation by said analog to digital converter (ADC), is compared with a previously stored current peak value (s) in said memory location, when said memory having a limited number of write cycle(s).

4. The method as claimed in claim 3, wherein if said calculated current peak value(s) and said previously stored current peak value (s) are in same life band, then said current peak value(s) is compared with said look up table, therefore said display means and said memory is updated.
5. The method as claimed in claim 1, wherein said look up table is standardized for different values of contact erosion of said contactor.
6. The method as claimed in claim 1, wherein a second memory location is provided for storing the number of crossing from one life band to another life band.
7. The method as claimed in claim 1 and claim 6, wherein said memory location and said display means is updated, if number of crossing crosses a pre defined threshold.
8. The method as claimed in claim 1, wherein said non-volatile memory is selected from a group comprising FLASH memory, EEPROM memory, ROM memory.
9. The method as claimed in claim 1, wherein said sensor means is a current sensor.
10. The method as claimed in claim 1, wherein said display means displaying residual life of said electrical switching device.