The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.
The present invention relates generally to electrical switching devices, and more particularly, to a method and apparatus for monitoring the wellness of contactors and motor starters, especially electromagnetic contactors and motor starters. The present invention measures various currents and voltages in one or both of the switched line and the coil of electromagnetic switching devices to monitor performance and determine indications of impending faults or existing faults of the device.
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In the embodiment depicted, motor starter 10 is a multi-phase motor starter as commonly used in industrial control applications, such as motor control. Motor starter 10 includes a contactor 12 and an overload relay 14. Contactor 12 is an electromagnetic contactor for switching supply current to a load (not shown). Overload relay 14 senses and measures the current to the load, and shuts off or de-energizes contactor 12 if too much current (overload) is flowing to the load, thus protecting the load. Overload relay 14 is shown connected with the contactor 12 at one end and accepts a series of conductors 16a, 16b, and 16c (shown in phantom) at another end through overload relay housing 18. Conductors 16a, 16b, and 16c extend through overload relay 14 and into contactor housing 20 and are secured by lugs 22. It is appreciated, however, that other embodiments of motor starter 10, contactor 12, and/or relay 14 may switch more or fewer lines, and thus may accept more or fewer conductors 16.
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In a preferred embodiment, the physical connection between overload relay 14 and contactor 12 is made with flexing lock tabs 28, which are each connected to a T-shaped retaining projection 30. Retainer projections 30 are insertable into connecting slots 32 within housing wall 34 of contactor 12. Receiving channels 36 of connecting slots 32 terminate in a retaining channel 38 which is narrower than the receiving channel 36 so as to prevent removal of a retaining projection 30 inserted into receiving channel 36 and slid downwardly into retaining channel 38. When a retainer projection 30 has been slid down into retaining channel 38, flexing lock tabs 28 will snap into connecting slots 32 of housing wall 34.
Contactor 12 includes a platform 40 which is integral with and extends substantially transversely to the plane of contactor wall 34. Platform 40 includes supports 42 for supporting flexible coil terminals 44 which extend outwardly from within the contactor 12. When coupled with contactor 12, the overload relay 14 is placed over the platform 40 to make an electrical connection with flexible coil terminals 44. In the embodiment shown, each coil terminal 44 is comprised of three separate conductive leads, while other similar embodiments utilize a number of separate coil terminals per phase connection. In an alternative implementation, each phase connection may have one coil terminal 44 with one conductive lead. Electrical connections may also be integrated with lock tabs 28 or retaining projection 30. In addition, while only two terminals 44 are shown, it is contemplated that other numbers and arrangements of terminals may be utilized. Contactor 12 may include a terminal 44 corresponding to each switched line or may include a number of terminals 44 for monitoring and controlling fewer than all switched lines of the contactor 12. Thus, a variety of electrical connections between contactor 12 and overload relay 14 can be achieved are known.
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A magnetic core 54 surrounded by an electromagnetic coil 56 in a conventional manner is located on a base portion of contactor housing 20. In other embodiments, core 54 and coil 56 may be positioned above contacts 46, 48. Magnetic core 54 is preferably a solid iron member and electromagnetic coil 56 is preferably configured to operate on direct current (DC). It is appreciated, however, that the wellness monitoring aspects of the present invention are also applicable to AC actuating coils, albeit via modified calculations. When energized, magnetic core 54 attracts a magnetic portion or armature 58 of moveable contact carrier 50. Moveable contact carrier 50, along with magnetic armature 58, is guided towards the magnetic core 54 along guide pin 60.
Guide pin 60 is press-fit or molded securely into moveable contact carrier 50 at one end and is slidable along an inner surface of magnetic core 54. The single guide pin 60 is centrally disposed and is utilized in providing a smooth and even path for the armature 58 and moveable contact carrier 50 as they travel to and from the magnetic core 54. Preferably, guide pin 60 and inner surface of magnetic core 54 are manufactured so as to limit friction therebetween. Friction during movement of guide pin 60 and carrier 50 can be a major limiting factor on the useable life of a contactor. Guide pin 60 is partially enclosed by a resilient armature return spring 62, which is compressed as the moveable contact carrier 50 moves toward the magnetic core 54. Armature return spring 62 biases the moveable contact carrier 50 and the armature 58 away from magnetic core 54. Additionally, a bottom portion 61 of guide pin 60 may be used to dampen the end of its downward movement to help reduce bounce and cushion the closure of the armature 58 with magnetic core 54.
Preferably, guide pin 60, carrier 50, armature 58, and moveable contacts 48 are configured to allow carrier over-travel. In other words, when moveable contacts 48 fully engage stationary contacts 46, guide pin 60, carrier 50, and armature 58 can continue downward movement a certain distance known as an over-travel. This is achieved by integrating a resilience or flexibility in the connection between moveable contacts 48 and carrier 50. Thus, an increased pressure on the engagement between moveable contacts 48 and stationary contacts 46 is achieved. The time during which guide pin 60, carrier 50, and armature 58 continue downward movement after contact engagement is commonly known as the over-travel time. Contact carrier over-travel distance can be measured by determining the over-travel time. A number of factors can cause over-travel and over-travel time to decrease, such as contact surface wear or erosion, or carrier jam. Once over-travel has decreased to a certain point, the total pressure maintaining engagement of the contacts can reach unacceptable levels, potentially causing contactor failure. Therefore, over-travel time can be an effective indicator of the wellness or remaining useable life of a contactor.
An operation cycle of contactor 12 begins at a contacts open position in which moveable contacts 48 are not in engagement with stationary contacts 46 and no line or phase current is flowing therethrough. A closing operation commences when coil 56 is energized by a DC control voltage causing magnetic core 54 to attract magnetic armature 58 of contact carrier 50. The downward attraction of armature 58 causes carrier 50 and pin 60 to overcome the bias of armature return spring 62. One of the phases of a three phase line current will begin to flow through conductor 16b when moveable contacts 48 first touch stationary contacts 46. Preferably, as described above, contact carrier 50, armature 58, and guide pin 60 will continue to move downward after contacts 46 and 48 have fully engaged until the armature 58 seals against the upper surface of core 54, stopping movement. The over-travel of carrier 50 increases contact engagement pressure to better hold moveable contacts 48 and stationary contacts 46 together.
An opening operation commences when the DC control voltage applied to coil 56 is turned off. Current through coil 56 dissipates, and magnetic core 58 ceases to attract armature 58 strongly enough to overcome the bias of armature return spring 62 as well as the contact force springs 52. Thus, carrier 50, armature 58, and guide pin 60 begin upward movement, and are joined by moveable contacts 48 after the over-travel distance. After moveable contacts 48 and stationary contacts 46 are no longer engaged, line current through conductor 16b will be interrupted. That is, current will flow between moveable contacts 48 and stationary contacts 46 for a very brief time after disengagement due to arcing, but will cease once the arc extinguishes. The bias of spring 62 causes contactor 12 to return to the contacts open position.
In regard to the electrical connection between contactor 12 and overload relay 14, a primary coil connector 64 extends from electromagnetic coil 56 and is electrically connected to coil terminal 44. Coil connector 64 conducts the DC control voltage and current for operating electromagnetic coil 56 from overload relay 14 via terminal 44. In embodiments of the invention in which voltage and current sensing are performed in contactor 12, a current sensor or shunt 68 is included in series with coil 56 and a voltage sensing device or circuit 66 is included in parallel with coil 56. A wire 72 is attached at one end of shunt 68 so that the voltage drop thereacross (as a measure of current flow) can be ascertained. Voltage device 66 has a wire 70 which conducts a measure of the voltage across coil 56. Embodiments in which sensing takes place in the contactor can operate with one or both of shunt 68 and voltage device 66. Thus, in such an embodiment, coil terminal 44 may include two or three leads (not shown) for electrical connection with relay 14—a DC control voltage/current input lead and either or both of a voltage measurement 70 lead and a shunt measurement 72 lead.
In other embodiments, it may be desirable to perform coil current and voltage sensing within overload relay 14. Printed circuit board (PCB) 80 relays power to terminal 44 via a connection 74 therewith. A shunt 76 is inserted between connection 74 and PCB 80, and feedback wire 78 is used to provide a signal indicating the voltage drop across shunt 76 so that current flowing from PCB 80 (and thus current flowing into coil 56) can be measured. Alternatively, shunt 76 may be replaced by a current sensing device capable of directly providing a digital indication of current flow therethrough. It is to be understood that shunt 76 and feedback wire 78 may be implemented as an alternative to shunt 68. For voltage sensing to take place in relay 14, PCB 80 may have voltage sensing circuitry integrated therein to monitor the voltage output to connection 74 and coil 56, rather than using voltage device 66.
Overload relay 14 also contains a magnetic flux concentrating shield 82, made of thin layers of laminated members 84 secured or stamped together. Shield 82 is positioned about the opening through which conductor 16b is inserted. In combination with a magnetic field sensor, such as a Hall Effect sensor 86, flux concentrating shield 82 is used to monitor current flow through conductor 16b. Hall Effect sensor 86 is connected to PCB 80 via leads 88 so that it is positioned over shield 82. Alternatively, other sensors, circuits, or components for monitoring current through conductor 16b may be incorporated so that indications of starts and stops of current flow, as well as the timing thereof, may be used in wellness monitoring, determinations, and calculations. Various well-known alternatives or equivalents (not shown) for measuring the voltage across contacts 46, 48 may be incorporated in lieu of, or in combination with, Hall sensor 86. Such alternatives and equivalents may include voltage detectors or solid state voltage sensors integrated into contactor 12, relay 14, or the three-phase power source (not shown) supplying power.
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False indications of current starting or stopping time can also occur due to the complexity in current rise and fall rates and variations in transience. False indications of current starting time can make it appear that contacts have strong erosion or contactors are close to failure. These false indications generally do not occur for each operation of a contactor, however. Thus monitoring through a number of operations can allow a user to average timing data, create trend lines, or to disregard statistically inconsistent or ignorable data, such as extreme outliers. Such practices are effective at eliminating the effects of false indications of openings or closings of contacts 46, 48.
Next, a processing unit of the overload relay or of another external device subtracts the phase current begin time ta, tb, tc from the coil current minimum time tmin to determine contact carrier over-travel time 114. The coil current peak time tmax may also be subtracted from the coil current minimum time tmin to determine armature pull-in time 116. Optionally, armature pull-in time may be averaged and included in a calculation to determine mean carrier closing speed as an indication of carrier/armature/guide pin friction. The system may also subtract the peak coil current value Imax from the minimum coil current value Imin to determine a coil current differential 118. Over-travel time, armature pull-in time, and coil current differential are metrics by which the wellness, or remaining usable life, of the contactor can be determined as well as existing faults
These wellness metrics 114, 116, 118 may then be averaged over a chosen number of contactor cycles 120. The longer the period chosen to average values, the less will be the impact of false start or stop indications. However, a longer averaging period can also lead to decreased precision if only averaged values are compared to thresholds. Therefore, a user should select an appropriate averaging period based upon the type of contactor used and the desired precision.
Most contactors and motor starters have manufacturer test data indicating over-travel, over-travel time, armature pull-in, and/or coil current differential thresholds. These thresholds can be absolute values or can represent percentage decreases from new contactor parameters. Once these thresholds are reached, it can reasonably be expected that a fault is imminent. Tested threshold data usually varies by contactor type, use, and model. Therefore, a controller, such as the overload relay or another external device, may be programmed to store the threshold over-travel time, armature pull-in time, and/or coil current differential value for the contactor in use. These thresholds are compared with the determined actual over-travel times, armature pull-in times, and/or coil current differential values, averaged values, or trends 122. If the wellness metric value (or values) being compared exceeds the corresponding threshold 126, a signal or indication of impending or existing fault is issued 128. When a measured coil current differential does not fall within the coil current differential threshold, it is likely that a fault such as contact weld or carrier jam has already occurred. When a measured over-travel time or pull-in time is not within the corresponding threshold, a fault is likely to occur. The indication of impending or existing fault may take the form of a warning light or alarm, a user alert, or an automatic shutdown for contactor replacement. If the wellness metric (or metrics) does not exceed the corresponding threshold 124, then the contactor is permitted to continue operation cycles. The monitoring described above may take place for each operation or cycle of a contactor, after a given number of cycles, or upon a set timing period.
Contact carrier over-travel time 114 may be used as a direct indication of contact remaining life or of the extent of contact surface erosion. Essentially, over-travel time is a parameter that measures the contact force spring compression after contacts engage. As contact surfaces erode, the over-travel distance decreases, resulting in the after-engagement compression force decreasing. The contactor will fail when the total contact force, including magnetic attraction and after-engagement compression, falls below a certain limit. Therefore, contactor remaining life, or “wellness,” has a roughly proportional relationship to over-travel time.
In practice, variations will exist in the detected carrier over-travel times, due in part to variations in detection of current start times. Thus, averages over multiple cycles to establish trend lines for a contactor can be very beneficial in predicting impending faults and future extent of wear and erosion, etc. In general, a threshold over-travel time value can reliably be set at about 70% of new contactor over-travel time for determining potential contactor failure, as measured against a decreasing actual over-travel time. As stated above, however, the most appropriate threshold values may vary by contactor and application. Also, since contact erosion and mass loss can occur unequally in the movable contacts or the stationary contacts, and can vary among the contacts for each phase, measuring the over-travel time for all phases is preferable.
Contact carrier (or armature) pull-in time 116 may be used as an indication that the speed of the carrier, armature, and guide pin during a closing or opening operation is decreased or that the carrier, armature, and/or guide pin are experiencing too much friction. Friction in the contact motion can result simply from wear between the magnetic core and guide pin or between the contact carrier and contactor housing. In other instances, friction can be due to the accumulation of debris generated by contact erosion or arcing. Over the course of many operations, a contactor will inevitably wear, regardless of the cause, and the armature pull-in time will increase. Pull-in times of a contactor will generally increase more drastically the closer a contactor gets to a failure point, after which time contacts cease to close or open altogether. While pull-in times may be compared to threshold values as discussed above, another more relative method for using pull-in times to predict failure incorporates the use of means and/or trend lines. Contactors will experience quite noticeable increases in pull-in time (by factors of almost 100%) just prior to failure. Thus, a trend line indicating a sudden jump in pull-in time can positively predict impending failure.
Coil current differential 118 can be used as an indication of carrier jam or contact weld. That is, as a contactor approaches failure, coil current differential can decrease by as much as 40% or more. Decreased coil current differential (i.e. a decrease in the range of coil current values during operation) indicates either that the carrier and armature are not fully returning to a contacts open position and/or that the contacts are welding. As coil current differential decreases appreciably from the new contactor value, a failure becomes more and more imminent. Thus, coil current differential may be used as an indicator for carrier jam or contact weld.
Conversely, detected coil current differentials within acceptable ranges can be assumed to mean that contacts are opening and closing properly, independently of the detection of line currents and voltages. Similarly, issues relating to coil temperature, such as contactor overheating, are also evidenced by changes in coil current differential values. Detecting normal coil current peaks, minimums, and differentials therebetween can indicate that the coil is operating under normal temperature conditions and sensitivities. Thus, coil current differential measurements may be used in a variety of ways to monitor coil temperature characteristics.
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Other applications of the wellness monitor of the present invention can operate as mirror contacts or instead of mirror contacts, can provide real time updating of contactor on and off timings to optimize the performance of Point on Wave control, and can detect re-ignition during contactor switching. That is, due to the ability of the present invention to monitor line current and voltage start and stop times and contact closing and opening start and stop times, positive indications of contact closure and full opening and closing cycles can be achieved for increased control and to monitor for system problems not necessarily caused by wear of the contactor.
In particular, the present invention finds application in augmentation or replacement of safety interlocks or mirror contacts. By not relying upon mechanical implementations for determining contact closure and opening, the present invention avoids many of the problems associated with mirror contacts. Therefore, an indication of contact closure derived from coil current, such as from an indication of the cessation of carrier movement (coil current minimum) after a full range of motion (coil current differential), can be used to gate or interlock the commencement of line current. The gating or interlocking of the commencement of three-phase current flow may be performed by external components as known in the art.
In addition, the present invention has been described thus far with particular reference to one embodiment of a particular contactor type with an overload relay attached thereto. However, it is appreciated and contemplated that the present invention may be embodied in many contactor embodiments in other applications, such as a contactor which does not include an attached relay. Likewise, the present invention may be embodied in contactors of configurations and types other than that discussed herein.
Moreover, reference has been made to multiple parameters, predictors, and indicators for determining contactor wellness. For example, contact over-travel, armature pull-in time, and coil current differential are discussed as useful for estimating future faults or remaining useful life, etc. However, it should be recognized that no single one of these parameters individually is necessary to predict wellness, that all are inter-compatible in determining wellness, and that other components, parameters, predictors, and indicators not explicitly mentioned herein may also be used in conjunction with the present system and method.
Therefore, a contactor embodying the invention includes a pair of moveable contacts, a pair of stationary contacts, and an electromagnet arranged to switch the contacts between open and closed positions. A coil current sensor is included to output signals indicative of electromagnet current during operation and a line current sensor is included to output signals indicative of current through the contacts. A controller is connected to receive these signals and determine a fault indicator therefrom.
A method for predicting contactor faults is also presented. The method includes the steps of measuring line current, measuring coil current, and determining a contactor performance indicator from one or both measured currents. The performance indicator is compared to a threshold value in order to predict imminence of a fault.
In addition, a switching apparatus is disclosed, which includes a contactor, having a DC actuating coil, connected to a relay. The relay controls operation of the contactor and contains a circuit which receives inputs from the contactor and causes at least one of armature pull-in time, over-travel time, and coil current differential to be evaluated. The circuit then causes an indication of contactor fault likelihood to be generated, based upon the outcome of the evaluation.
The present invention also encompasses a method for manufacturing a contactor wellness monitor. The method includes providing a contactor having an electromagnetic coil, arranging electrical components to acquire coil current signals, and establishing electrical connections to conduct the signals toward a processing unit. The processing unit is programmed to monitor coil current, determine one or both of armature pull-in time and coil current differential, and generate a contactor wellness predictor.
As such, the present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.