Patent Publication Number: US-2011062960-A1

Title: Device and method to monitor electrical contact status

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the present invention relates to electromechanical switches generally, and more particularly to a method for determining a status of the electrical contacts of an electromagnetic switch, as well as a device configured for use with such a method; of which the following is a specification, reference being had to the drawings accompanying and forming a part of the same. 
     2. Description of the Related Art 
     Electromagnetic switching devices such as contactors, relays, and other devices are well known and widely used to switch electrical currents. Conventional electromagnetic switches such as those having a moveable armature and a fixed yoke electromagnet as closing elements are commonly used to change the state of electrical contacts. When a current flows through the solenoid coils of the electromagnet, then the resultant magnetic field moves the armature toward the yoke, until the pole faces of the armature and yoke contact one another. When the current through the coils of the electromagnet is switched off, a mechanical resetting device such as a spring, for example, acts to separate the armature from the yoke. Movable electrical contact elements, which are connected to the armature, are moved with respect to stationary electrical contact elements in order to close and open the electrical contacts of the electromagnetic switching device. Such contacts may be of either normally closed or normally open configurations. 
     As shown in  FIG. 1 , a conventional electromagnetic switch  100  is shown, having an electromagnet  101  comprising a magnetic movable core or armature  102  separated by an air gap  104  from a magnetic stationary core or yoke  103  having an electromagnetic triggering solenoid or coil  105 . The armature  102  is movable in the directions indicated by arrow  119 . The movable armature  102  is in operable communication with at least one movable electrical contact  110   a  for making and breaking with a stationary electrical contact  110   b . Although each pair of electrical contacts  110   a ,  110   b  are shown in  FIG. 1  and described herein as in a normally open configuration, it will be understood by those of skill in the art that electrical contacts  110   a ,  110   b  may be of either normally closed or normally open configuration. Additionally, while  FIG. 1  is shown having six pairs electrical contacts  110   a ,  110   b  it will be understood that movable armature  102  may be configured to be in operable communication with any number of electrical contacts. When closed, the contacts  110   a ,  110   b  typically conduct power from a power source  112 , such as for example an AC power supply, to a load  115 , and when the contacts  110   a ,  110   b  open, the power to the load  115  is interrupted. 
     When an electrical current (not shown) is passed through the triggering coil  105  of electromagnet  101 , a magnetic field (not shown) is produced that causes the armature  102  to be magnetically attracted to the yoke  103 . The movement of armature  120  causes at least one face  117  of armature  102  to make contact with at least one face  118  of yoke  103 . The electrical current (not shown) through the triggering coil  105  is conventionally provided by a triggering circuit  120  or other external current source (not shown) connected to the triggering coil  105 . Since the movable contact  110   a  is conventionally driven through a linking element  107  by the movable armature  102 , the magnetic force developed by the electromagnet  101  holds the armature  102  in contact with the yoke  103  and thereby places the normally open electrical contacts  110   a ,  110   b  in an actuated or closed state. Then, when the electrical current (not shown) is cut off, the electromagnet  101  is de-energized, and a return element such as, for example, a spring  106  returns the armature  102  to its initial position thereby causing the at least one face  117  of armature  102  to break contact with the at least one face  118  of yoke  103 , and the electrical contacts  110   a ,  110   b  to change state (i.e., open). 
     During each opening and closing operation of electrical contacts  110   a ,  110   b  when switching currents, electrical arcing occurs in an air gap between the contacts  110   a ,  110   b . The electrical arcing results in material erosion of the switching contacts  110   a ,  110   b  that varies in severity depending at least on the current and voltage load. The material erosion or wear influences the switching behavior of the switching device, and after a sufficient number of switching operations, can cause a failure of the switching device. Additionally, arcing-induced erosion of electrical contacts  110   a ,  110   b  is a significant factor determining the remaining life of, or maintenance interval for, a switching device. It is important to know the contact status, such as for example, the remaining contact material thickness or remaining expected contact life, to enable preventive maintenance, such as replacing the contacts  110   a ,  110   b  or the electromagnetic switching device  100  itself, to avoid unplanned interruption to the system in which the switching device  100  is used. 
     One typical practice used to prevent such unplanned system interruption is to systematically replace either the contacts  110   a ,  110   b  or the electromagnetic switching device  100  itself, after a predetermined number of operations without examining the actual condition of the contacts  110   a ,  110   b . This results in unnecessary replacement of devices if the contacts are not sufficiently worn, and may result in device and/or system failure if the electrical contacts  110   a ,  110   b  have worn more than anticipated. 
     Therefore, what is needed is a method to more precisely determine the status, such as the remaining thickness of the electrical contacts in order to deduce information related to the residual life of the contacts, since it would enable timely notification to the user, and thus prevent failures that could otherwise occur. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In view of the foregoing considerations, it is desirable to provide a device and method to generate the status of the electrical contacts in an electromagnetic switching device. The generated status may comprise any number of embodiments, including such non-limiting examples as an indication of the residual life of the electrical contacts; an indication of the current thickness of the electrical contacts; a pass/fail indication of the condition of the electrical contacts; or a notification regarding necessary maintenance of the electrical contacts. 
     As used herein, the instant of contact between armature and yoke shall be referred to as closing of the electromagnetic switch. Additionally, the term change of state in reference to a pair of electrical contacts shall refer herein to opening of closed contacts, or alternatively, closing of open contacts. The electrical phase angle difference between the closing of the electromagnetic switch, and the change of state of the electrical contacts is referred to herein as a magnetic lag angle (MLA). 
       FIG. 2  is a graph illustrating a typical MLA for a conventional electromagnetic switch wherein, for example, an AC voltage V c  is applied across normally open electrical contacts and a DC voltage signal V m  is applied across the electromagnetic armature and yoke to sense the close of the electromagnetic switch. When a triggering current through coil causes the electromagnetic switch to close, the point on the AC waveform, herein referred to as an electrical phase angle, of the voltage signal Vc at which the electrical contacts change state (e.g., close) thereby dropping Vc to zero, will typically lead the electrical phase angle at which the armature makes contact with the yoke (i.e., closes). The phase angle difference between the leading electrical phase angle at the electrical switch contacts change of state, and the lagging electrical phase angle at the closing of the electromagnetic switch is the MLA. 
     In accordance with an aspect of the invention, the problem of determining contact status is solved by triggering the armature movement at a substantially consistent electrical phase angle, determining the MLA between electromagnetic switch closing and the electrical contact change of state, and generating a contact status using the MLA. 
     In one embodiment, a MLA value corresponding to a known contact status is predetermined, a moving average of the measured MLA values is determined, and the moving average is compared with the predetermined MLA value to generate a contact status. 
     Other features and advantages of the disclosure will become apparent by reference to the following description taken in connection with the accompanying drawings. 
     The above brief summary sets forth rather broadly the more important features of the present invention in order that the detailed description thereof that follows may be better understood, and in order that the present contributions to the art may be better appreciated. In this respect, before explaining several embodiments of the invention in detail, it will be understood that the invention is not limited in its application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood, that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception, upon which disclosure is based, may readily be utilized as a basis for designing other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as one or more of the features of any drawing may be combined with any or all of the other features of one or more of the remaining drawings in accordance with one or more embodiments of the invention. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate a presently preferred embodiment of the invention, in which: 
         FIG. 1  illustrates a conventional electromagnetic switching device of the kind known in the prior art; 
         FIG. 2  is a graph illustrating voltage signals associated with an electromagnetic switching device; 
         FIG. 3  illustrates a schematic view of an embodiment of the present invention; 
         FIG. 4  illustrates a schematic view of an alternative embodiment; 
         FIG. 5  is a flow diagram of a computer-implemented method according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As used herein, an element or function recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or functions, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the claimed invention should not be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, one of the embodiments of the present invention will now be described. 
     In  FIG. 3  a schematic view of an electromagnetic switch  300  of an embodiment is shown, having an electromagnet  301  comprising a movable core or armature  302  separated by an air gap  304  from a stationary core or yoke  303  having an solenoid or triggering coil  305 , and connected to a movable electrical contact  310   a  for making and breaking with a stationary electrical contact  310   b . Although the contacts  310   a ,  310   b  are shown in the Figures and described herein as in a normally open configuration, it will be understood by those of skill in the art that the contacts  310   a ,  310   b  may be of either normally closed or normally open configuration. When closed, the contacts  310   a ,  310   b  typically conduct power from a power source  312 , such as for example an AC power supply, to a load  315 , and when the contacts  310   a ,  310   b  open, the power to the load  315  is interrupted. Electrical current (not shown) through the triggering coil  305  magnetically triggers movement of armature  302  toward yoke  303  and is provided by a triggering circuit  320  connected to the triggering coil  305 . The movement of armature  320  causes at least one face  317  of armature  302  to contact at least one face  318  of yoke  303  (i.e., close the electromagnetic switch). The movable contact  310   a  is driven through a linking element  307  by the movable armature  302 , and the magnetic force developed by the electromagnet  301  holds the movable and stationary contacts  310   a ,  310   b  in an actuated or closed position. When the electrical current (not shown) is cut off, the electromagnet  301  is de-energized, and a return element such as, for example, a spring  306  or gravity returns the armature  302  to its initial position causing the contacts  310   a ,  310   b  to change state or open. 
     Generally, after an initial break-in number of cycles, the MLA of an electromagnetic switching device will decrease over the life of the electrical contacts from an initial value to a minimum value before device failure. The decline or decay in the MLA has been seen to be generally a function of the erosion of the electrical contact material and other variable factors: (a) the remaining thickness of the contacts  310   a ,  310   b;  (b) the closing velocity and acceleration of moveable armature  302 , and (c) general mechanical wear of the electromagnetic switching device  300  parts. By minimizing the effects of the other variable factors as discussed supra, for each electromagnetic switching device, a family of curves or table of values can be empirically developed that indicate the thickness of the switching device electrical contacts for a particular value or range of values of the MLA. 
     In the present invention, the change in the MLA value, over a plurality of energizing operations of the electromagnetic switch  300 , due to the reduction in contact thickness caused by contact erosion is advantageously used to generate a status of electrical contacts  310   a ,  310   b  and hence the anticipated residual life of electromagnet switch  300 . The generated status may comprise any number of embodiments, including such non-limiting examples as an indication of the residual life of the electrical contacts, such as the number of operations remaining; an indication of the current thickness of the electrical contacts; a pass/fail indication of the condition of the electrical contacts; or a notification regarding necessary maintenance of the electrical contacts. In order to use the MLA to provide an indication of electrical contact status, the effect of the aforementioned variable factors, other than the remaining contact  310   a ,  310   b  thickness, causing the change in MLA should be eliminated or greatly reduced. 
     In one embodiment, the influence over time of the aforementioned variable factor of general mechanical wear of the electromagnetic switch  300  parts on the measured values of MLA is diminished by determining the moving average of the measured values of MLA. The MLA moving average value is compared with a predetermined MLA value corresponding to a known contact status. By negating the effects on the measured values of MLA of general mechanical wear of the electromagnetic switch  300  parts, other than the electrical contacts  310   a ,  310   b  themselves, the rolling average value of MLA is used provide a more precise indication of the electrical contacts  310   a ,  310   b  status. 
     According to another aspect, a control unit  330  such as, for example a microcontroller or microprocessor, is in operable communication with the first and second detection circuits  317 ,  318  and the trigger circuit  320 . Control unit  330  comprises an internal memory (not shown) configured to store data, such as for example, in a lookup table, related to a status of the switching device electrical contacts  310   a ,  310   b  for a particular value or range of values of the MLA. The control unit  330  also comprises a processing unit (not shown) configured determine the MLA using the electrical phase angle difference between the closing of the electromagnetic switch, and the change of state of the electrical contacts. The control unit  330  processing unit (not shown) is also configured determine to compare the determined MLA values with the stored lookup table values, in order to determine any number of aspects related to contact status, including such non-limiting examples as the expected residual life of the electrical contacts  310   a ,  310   b , the number of electrical contact operations completed or remaining; the current thickness of the electrical contacts  310   a ,  310   b;  the general condition of the electrical contacts  310   a ,  310   b;  or necessary maintenance of the electrical contacts. 
     The velocity and acceleration of the armature  302  depends substantially upon the electrical closing angle at which the triggering coil  305  is energized. By consistently energizing the triggering coil  305  at substantially the same predetermined electrical angle through each operation of the device  300 , the closing velocity and acceleration of the armature  302  is kept substantially constant. It will be understood that a variety of known triggering circuits  320  may be used to provide an energizing signal to the triggering coil  305 . In a non-limiting example, an electronic switch such as, a triode for alternating current (TRIAC) may be connected in series with the electromagnetic triggering coil  305 . The TRIAC can then be fired at a particular electrical phase angle, which is kept constant throughout the life of the switching device  300 . 
     By maintaining the closing velocity and acceleration of armature  302  substantially constant through each operation, the MLA can be used to generate a more precise indication of the contact  310   a ,  310   b  status. 
     Referring still to  FIG. 3 , an embodiment is shown in which a phase controlled trigger circuit  320  energizes the electromagnetic triggering coil  305 . The trigger circuit  320  is in communication with the control unit  330 . A first detection circuit  317  is also in communication with the control unit  330  for detecting and providing an indication of switch closing between the contacts  310   a ,  310   b . The closing angle of switch closing between the electrical contacts  310   a ,  310   b  is thereby measured and may be stored in the memory (not shown) of control unit  330 . 
     In one embodiment, the first detection circuit  317  senses the instant of closing of contacts  310   a ,  310   b  by detecting the current flow across the contacts. Alternatively, in another embodiment, the first detection circuit  317  senses the instant of closing of contacts  310   a ,  310   b  by detecting the resulting change in voltage, or voltage drop, across the contacts  310   a ,  310   b . It will be understood that the detection of the closing of electrical contacts  310   a ,  310   b  may be accomplished using a number current or voltage detection circuits known in the art. 
     A second detection circuit  318  is in signal communication with the control unit  330  for providing an indication of electromagnetic switch closing between the armature  302  and yoke  301 . In one embodiment, the second detection circuit  318  senses the instant of closing of the moving armature  302  with the yoke  301  by detecting the appearance of a dc voltage (not shown) across a resistance  316  connected in series with a low voltage dc source  319  electrically connected in series with the armature  302  and yoke  301 . The closing angle of the moving armature  302  and yoke  301  is thereby measured and may be stored in the memory (not shown) of control unit  330 . 
       FIG. 4  illustrates an alternative embodiment wherein a detection coil  325  is wound over the triggering coil  305  and is in signal communication with second detection circuit  318 . In this embodiment, at the instant of closing of the electromagnetic switch  300 , an electromotive force (emf) is induced due to the high rate of change of flux, thus causing a rise in voltage in the detection coil  325 . The second detection circuit  318  senses the instant of closing of the electromagnetic switch  300  by detecting the rise in voltage in the detection coil  325 . 
     According to another aspect, the control unit  330  is in operable communication with a communication bus  333  such as for example a serial link, a field bus, a Local Area Network (LAN), or global network. The microcontroller  330  is connected to the communication bus  333  so that information related to the status of an electrical contacts  310   a ,  310   b , stored in the microcontroller  330  internal memory (not shown) can be transmitted on the communication bus  333 . 
     In another embodiment, the switching device comprises a user interface  336  preferably in operable communication with the control unit  330 . The user interface  336 , Non-limiting examples of User Interface  335  include a graphic display screen; an indicator light; an audible signal, and is used to provide or display information related to the status of electrical contacts  310   a ,  310   b , stored in the control unit  330  internal memory (not shown). 
     While the various embodiments have been described generally with reference to single phase circuits, it will been seen that the embodiments are not so limited and are equally useful with other voltage configurations. For example, in the case of a three-phase circuit, the MLA is determined in each of the three phases. In one embodiment, the MLA values are determined separately for each phase and then are compared by the control unit  330  and the phase having the minimum value (i.e., indicative of the greatest erosion of the contacts  310   a ,  310   b ) is considered for the contact  310   a ,  310   b  status determination. Additionally, while the embodiments herein have been shown having six sets of contacts  310   a ,  310   b , it will be understood that the embodiments are not so limited, and may be configured with a single pair of contact  310   a ,  310   b , or any other convenient number of contact pairs  310   a ,  310   b.    
       FIG. 5  is a flow diagram of a computer-implemented method according to an embodiment of the invention. Each block, or combination of blocks, depicted in the block diagram can be implemented by computer program instructions. These computer program instructions may be loaded onto, or otherwise executable by, a computer or other programmable apparatus to produce a machine, such that the instructions, which execute on the computer or other programmable apparatus create means or devices for implementing the functions specified in the block diagram. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture, including instruction means or devices which implement the functions specified in the block diagrams, flowcharts or control flow block(s) or step(s). The computer program instructions may also be loaded onto a computer or other programmable apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the block diagrams, flowcharts or control flow block(s) or step(s). 
     Accordingly, blocks or steps of the flowchart illustration supports combinations of means or devices for performing the specified functions, combinations of steps for performing the specified functions and program instruction means or devices for performing the specified functions. It will also be understood that each block or step of the flowchart, and combinations of blocks or actions depicted in the flowchart, can be implemented by a special or general-purpose hardware-based computer system that is configured to perform the specified functions or steps, or combinations of special purpose hardware and computer instructions. 
     Referring now to  FIG. 5 , a flow chart illustrates an embodiment of the present invention for determining a status of an electrical contact of an electromagnetic switch having  3 -poles, designated pole a, pole b, and pole c, respectively, in an electrical system having three-phases, designated phase A, phase B, and phase C, respectively. It will be understood that the process shown in  FIG. 5  is not so limited, and may be also used to generate a status of an electrical contact of an electromagnetic switch in other types of electrical systems, such as a single-phase electrical system, and for other types of switches, such as a single-pole switch. 
     At step  502  the method begins by energizing the electromagnetic switch  300  at a substantially constant electrical phase angle. At step  503 , the MLA values are determined for each electrical switch contact, pole a, pole b, and pole c, corresponding to each electrical system phase A, phase B, and phase C, respectively. 
     At step  504 , each of the MLA values determined in step  503  are compared. For example, at  504   a  the MLA value for the electrical switch contact  310   a ,  310   b  of pole a is compared with the MLA value for the electrical switch contact  310   a ,  310   b  of pole b; at  504   b  the MLA value for the electrical switch contact  310   a ,  310   b  of pole b is compared with the MLA value for the electrical switch contact  310   a ,  310   b  of pole c; and at  504   c  the MLA value for the electrical switch contact  310   a ,  310   b  of pole a is compared with the MLA value for the electrical switch contact  310   a ,  310   b  of pole c. 
     At step  505 , the lowest MLA value determined in step  504  is selected. At step  507 , the MLA value selected in step  505  is used to determine the moving average of selected MLA values from previous switch  300  operations. At step  508 , the moving average value determined in step  507  is compared with a predetermined threshold value. At step  510 , the contact status of the device is generated based on the comparison of the MLA value determined in step  507  and the predetermined threshold value. 
     With respect to the above description, it should be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, form function and manner of operation, assembly and use, are deemed readily apparent and illustrated in the drawings and described in the specification are intended to be encompassed only by the scope of appended claims. 
     In addition, while the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be practical and several of the preferred embodiments of the invention, it will be apparent to those of ordinary skill in the art that many modifications thereof may be made without departing from the principles and concepts set forth herein. Hence, the proper scope of the present invention should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications and equivalents. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.