Abstract:
A modular overload relay provides a low cost analog base unit performing overload detection functions and one or more add-on units communicating with the base unit through an electric conductor on a side of the base unit to which the add-on module may be attached. The add-on units augment the circuitry of the base unit through additional analog or microprocessor-based circuitry communicating with various points within the analog circuitry of the base unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0001]    Overload relays are current sensitive relays, normally used in conjunction with an electomechanical contactor, that may be used to disconnect power from equipment, for example, from a three-phase motor, when an overload condition exists.  
           [0002]    In a typical installation, the contactor provides three contacts, one associated with each of the three phases of power, closed by an electromagnetically operated armature. The overload relay includes current sensing elements that are wired in series with the three phases passing through the contactor. In this way, the overload relay can monitor current flowing in the three phases through the contactor, and based on current magnitude and duration, may interrupt the current flow through the contactor armature circuit to open the contactor contacts when an overload occurs. For this purpose, the overload relay includes a set of latching contacts which can be used to control the contactor coil and/or provide a signal indicating an overload conditions.  
           [0003]    Generally, an overload relay provides a different protective function than that of a circuit breaker which may also be wired in series with the contactor and overload relay.  
           [0004]    Simple overload relays make use of mechanical elements, such as bimetallic strips for current sensing, communicating with contacts for providing a switched output. It is known, however, to construct overload relays from solid-state analog circuit components using current transformers for the current sensing element. Power for the solid-state analog overload relays is obtained through separate wiring or may be tapped from the secondary windings of the current transformers. The solid-state analog circuitry may be implemented in an application specific integrated circuit (ASIC) making the solid-state overload re-lay inexpensive and reliable.  
           [0005]    It may be desirable to incorporate additional features into an overload relay, for example, to allow it to check for three-phase current imbalance, ground faults, or motor jam conditions. Remote operation and monitoring of the overload relay, for example, may also be desirable. These latter features may be implemented by providing dedicated wiring, to communicate, for example, a reset signal to the overload relay, or by providing the overload relay with a network connection, allowing serial digital data to pass between the overload relay and a separate controller.  
           [0006]    When one or more of these additional functions is required, the analog circuitry of the overload relay is normally replaced with a microprocessor or microcontroller-based circuit. Measured current from the current transformers of the overload relay may be converted to digital values by an analog to digital converter and the base and supplemental protective functions are implemented in the microcontroller&#39;s firmware. The microcontroller may further implement a network interface allowing the overload relay to communicate with external sources for external control and readout of the overload relay function.  
           [0007]    The use of a microcontroller in an overload relay substantially increases the cost of the overload relay, both because of the cost of the microcontroller but also because of the ancillary circuitry needed to support the microprocessor including power processing circuits, clock circuits, start-up circuits, memory and other interface circuits, and so-called “glue” logic circuits. Accordingly, additional overload relay functions are normally available only in feature rich, high priced overload relays. Mid-tier overload relays for users who need only a single additional feature, for example, represent a relatively low volume (fragmented by the number of different mid-tier products that are possible) making manufacturers reluctant to address this market.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a),modular overload relay in which the base overload relay module uses low cost analog circuitry. A side-mounted connector allows an added-function module to be attached to the base module to augment its capabilities. Analog communication between the added function modules and the base modules limits the cost of each. Many of the added function modules may also be implemented using low cost analog circuitry, but the high volumes obtainable with the shared low-cost base-module can make even high tier functionality, where the added function modules include microcontroller circuits, cost effective.  
           [0009]    Specifically, the present invention provides a multi-function overload relay. A first portion of the multi-function overload relay is a base overload relay module having a first housing supporting first and second terminals where the second terminals are receivable by a contactor. Current conductors conduct current from the first terminals to the second terminals and solid-state analog circuitry monitors the current in the current conductors to produce a signal proportional to the current. The overload relay module includes an electrical connector extending from the wall of the housing and communicating with different points of the solid-state analog circuitry.  
           [0010]    A second portion of the multi-function overload relay is an added-function module having a second housing attachable to the wall of the overload relay module. A second electrical connector located on the added-function module joins with the electrical connector of the overload module when the added-function module is attached to the wall of the housing. Ancillary circuitry within the added-function module communicates with the solid-state analog circuitry to augment its function.  
           [0011]    Thus, it is one object of the invention to provide for a physical separation of the functions that maybe performed by an overload relay, allowing a variety of mid-tier overload relays of different functions to be offered in, a cost-effective basis. The analog interface between components allows division of functions to be accomplished with minimal interface cost.  
           [0012]    It is another object of the invention to provide a base module for an overload relay that is competitive with the lowest cost overload relays of comparable performance, so as to provide a base unit that achieves low cost through high sales volume in multiple market tiers. Eliminating microcontroller circuitry in the base module makes this possible.  
           [0013]    The ancillary circuitry may provide functions selected from the group consisting of motor jam detection, current imbalance detection, and ground fault current detection.  
           [0014]    Thus it is another object of the invention to provide for a variety of additional features that may leverage the current sensing capabilities of the base module.  
           [0015]    The ancillary circuitry may provide remote reset or trip of the overload relay.  
           [0016]    Thus it is another object of the invention to provide remote resetting as an optional feature, thereby reducing the cost of the overload relay base module.  
           [0017]    The second housing of the added-function module may include a network connector and the ancillary circuitry may provide an interface to a serial digital network connected at the network connector.  
           [0018]    Thus it is an object of the invention to provide for optional network connection to an overload relay, without increasing the price of the base module of the overload relay.  
           [0019]    The second housing of the added-function module may further include third terminals providing an interface for input and output signals, and the ancillary circuitry may provide an interface to a serial network allowing reading of the input signals at the third terminals and writing of the output signals at the third terminals.  
           [0020]    Thus it is another object of the invention to make additional use of the complex circuitry required for a network interface to provide compact input/output capabilities at the overload relay.  
           [0021]    The second housing may include captive machine screws received in corresponding holes in the first housing to attach the first housing to the second housing.  
           [0022]    Thus it is another object of the invention to provide for an attachment method which does not increase the cost burden of the base module (which needs only molded holes) and yet which is robust against the high vibration environment of the overload relay unit when mounted on a contactor.  
           [0023]    The first and second electrical connectors may communicate signals selected from the group consisting of an overload relay reset signal, a ground signal, a burden voltage signal, a thermal capacity utilization signal, an overload relay power supply signal, any applicable voltage reference signals, and overload relay trip and reset signals.  
           [0024]    Thus it is another object of the invention to establish a set of core analog signals that may be communicated between the overload relay and the added-function module avoiding the need for a complex serial interface appropriate only for more expensive microcontroller circuitry in the base module.  
           [0025]    The first terminals may be screw terminals receiving wires and the first housing may provide access to screws of the screw terminals along the first wall and receive the wires at a second wall perpendicular to the first wall, and the second housing may attach to the first housing at a third wall perpendicular to both the first and second walls.  
           [0026]    Thus it is another object of the invention to provide for an attachment location the overload relay module that does not interfere with attachment of wires or pre-existing wire pathways used for the overload relay.  
           [0027]    The second housing of the added-function module may further include third terminals providing connections to ancillary circuitry.  
           [0028]    Thus it is another object of the invention to provide additional contact points to be added to the overload relay via the added function modules.  
           [0029]    The input terminals may be connectors mating with the second connectors, the second connectors providing screw terminals for receiving wires.  
           [0030]    Thus it is another object of the invention to accommodate tight clearance wiring conditions by allowing pre-wiring of the second connectors using screw terminals and then connection of the second connectors to the first connectors on the added-function module.  
           [0031]    A kit may be provided having the overload relay base module described above and at least two added-function modules.  
           [0032]    Thus it is another object of the invention to simplify the stocking and manufacturing of overload relays having different functions.  
           [0033]    The kit may include a second overload relay module omitting the electrical connector extending from a wall of the housing.  
           [0034]    Thus it is another object of the invention to provide the ability to provide a base module not compatible with the added function modules, for very low cost applications.  
           [0035]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    [0036]FIG. 1 is a fragmentary, exploded, perspective view of an overload relay of the present invention showing joining of a base module and an added-function module and the base module to a contactor;  
         [0037]    [0037]FIG. 2 is a fragmentary view of the base module of FIG. 1 in an embodiment omitting a connector between the base module and the added-function module;  
         [0038]    [0038]FIG. 3 is a block diagram of the analog solid-state circuitry of the overload relay showing development of interface signals for the added-function modules;  
         [0039]    [0039]FIG. 4 is a simplified block diagram of a first added-function module making use of an application specific integrated circuit implementing analog circuitry such as may provide remote reset and other protected features; and  
         [0040]    [0040]FIG. 5 is a figure similar to that of FIG. 4 showing an added-function module making use of a microcontroller for network communication, local input and output, and other control advanced protection functions.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0041]    Referring now to FIG. 1, a modular overload relay system  10  includes a base module  12 . The base module  12  has a housing  14  with a top wall  16  from which bayonet terminals  18  extend upwardly to be received by corresponding screw clamp terminals (not shown) of a contactor  20 . Three bayonet terminals  18  are provided for three conductors of a three-phase power system.  
         [0042]    Each of the bayonet terminals  18  is associated with a corresponding internal conductor  60  (shown in FIG. 3) within the base module  12  which leads to a corresponding screw clamp or cage clamp (screwless) terminal  22 . The screw clamp terminals  22  are accessible from a front wall  24  of the housing  14  and allow the attachment lines  26  which may be received at a bottom wall  28  of the housing  14 .  
         [0043]    A side wall  30  of the housing  14  perpendicular to the top wall  16 , the bottom wall  28 , and the front wall  24 , supports an outwardly facing electrical connector  32 . The side wall  30  also has screw holes  34 .  
         [0044]    An added-function module  40  having a side wall  42  may attach to the side wall  30  of the base module  12  so that a second electrical connector  44  (not visible in FIG. 1) may mate with electrical connector  32  when the added-function module  40  is attached to the base module  12 . Screws  36 , captive in a housing  38  of an added-function module  40 , may be threaded into the screw holes  34  of the side wall  30  of the base module  12 . Alternatively snaps attached to or molded into housing  38  may engage corresponding holes in the side wall  30  of the base module  12 . In either case, the cost burden of the attachment means on the base module  12  is minimized.  
         [0045]    The mounting of the added-function module  40  at the side wall  30  provides clear access to the top wall  16 , the front wall  24 , and bottom wall  28  simplifying wiring of the base module  12 .  
         [0046]    A toggle switch array  46  may be exposed on a side wall  43  of the housing  38  of the added-function module  40  opposite the side wall  42  to allow optional programming of the functions of the circuitry contained in the added-function module  40 . A bottom wall  48  of the added-function module  40  includes electrical connectors  50  (not visible in FIG. 1) receiving pins from mating electrical connectors  52  having screw clamp or cage clamp terminals for receiving wiring  56 . The wiring  56  may be thus attached to the electrical connectors  52  prior to connection of the electrical connectors  52  to the added-function module  40  in this way accommodating possibly reduced clearance resulting from the use of the added-function module  40 .  
         [0047]    Referring now to FIG. 3, each of the bayonet terminals  18  communicate with a conductor  60  (which may be an extension of the bayonet terminals  18 ) passing through the housing  14  of the base module  12  to screw clamp terminals  22 . A primary winding of a current transformer  62  is placed in series with each of these conductors  60  to allow measurement of the current passing through each of these conductors without significant power loss.  
         [0048]    Leads  66  attached to the secondary windings each of the current transformers  62  received by a monitoring circuit  74  thereby simulating a burden resistor to determine a total RMS current delivered through each of the conductors  60  connected as a load across the three phase power, for example, as a composite load a Y or Delta configuration. The monitoring circuit  74  produces a burden resistor voltage  78  (or optionally three burden voltages associated with each of conductors  60 ) providing an instantaneous representation of total current flow.  
         [0049]    Leads  66  attached to the secondary windings each of the current transformers  62  are also received by a power supply  68  taps a small amount of power from each of the conductors  60  to produce direct current (DC) power  70  referenced with respect to an internal ground  72  and used for operation of the circuitry of the base module  12  and optionally the added-function module  40 . The amount of power tapped off conductors  60  for this purpose is minor and does not affect the overall calculation of overload currents or is compensated by the monitoring circuit  74  components.  
         [0050]    The burden voltage  78  from the monitoring circuit  74  is received by an integrator/comparator  80  which produces a thermal capacity value (TCU)  82  being a combination of current and duration that models the amount of thermal capacity remaining in an associated motor that may be connected to lines  26 . Integrator/comparator  80  applies a threshold determined by a potentiometer  84  which may be part of the ‘burden resistor network’ so as the potentiometer is adjusted, it actually adjusts the burden resistance. Because the secondary current stays relatively constant at a given primary current, adjusting the potentiometer  84  effectively alters the FLA setting of the overload relay. The potentiometer  84  is exposed at the front wall  24  of the housing  14  and provides a threshold level to integrator/comparator  80  to generate a set signal  94  that is communicated to a latched contact set  86  when the threshold has been exceeded indicating potential overload on the attached motor.  
         [0051]    The latched contact set  86  provides a set of normally open contacts  88  and normally closed contacts  90  and receipt of the set signal closes the normally open contacts  88  and opens the normally closed contacts  90  which may be in series with the coil of the contactor  20  to break the electrical circuit of lines  26  under overload condition. The latched contact set  86  and the normally open contacts  88  and normally closed contacts  90  may be realized with solid-state elements such as transistors and need not be a mechanical latched relay as is understood in the art.  
         [0052]    A front panel reset button  92  is provided to apply reset signal  96  to the latched contact set  86  to re-open the normally open contacts  88  and close the normally closed contacts  90 .  
         [0053]    The set signal  94 , the reset signal  96 , the TCU value  82 , the burden voltage  78 , the ground  72  and power  70 , and any other voltage to be monitored may be provided to the electrical connector  32  for use by the added-function module  40 . In this way various points within the analog circuitry implementing the component  68 ,  74 ,  80  and  86  may be accessible outside of the housing  14 . All this circuitry may be realized by a low cost ASIC.  
         [0054]    Referring still to FIG. 3, the above components  62 ,  68 ,  74 ,  80  and  86  provide a base functionality of an overload relay system  10  such as may be sold independently of the added-function module  40  and stocked or manufactured as part of a kit. Referring to FIG. 2, it will further be understood that for sales only with this basic functionality, the electrical connector  32  and its associated wiring may be eliminated further reducing the cost of the base module  12  while supporting high volumes for the components  62 ,  68 ,  74 ,  80  and  86  to lower their costs.  
         [0055]    Referring now to FIG. 4, the added-function module  40  when attached to the base module  12  may receive each of the signals  70 ,  72 ,  78 ,  82 ,  94  and  96  through electrical connector  44 . These signals are routed to a a second set of analog solid state circuitry possibly realized by a second ASIC  100  implementing analog circuit blocks but without the microcontroller features of executing a program and whose functions will vary depending on the added function to be provided. Generally, the ASIC  100  will provide current integration with different time constants for jam detection, and comparator circuits for ground fault and current imbalance detection. More specifically, the ASIC  100  may monitor the burden voltage  78  to detect current use indicative of jam condition to provide a set signal  94  through electrical connector  44  to the latched contact set  86 . By expanding the burden voltage  78  to three separate values obtainable from each of the current transformers  62  separately, the ASIC  100  may detect current imbalance in the phase or ground fault conditions, again asserting the set signal  94  when appropriate. In this regard, the ASIC  100  may also communicate with the toggle switch array  46  and possibly potentiometers (not shown) to set necessary variable parameters and determine its operating mode. Analog circuits for these functions are well known in the art and implementation in this invention will be understood to those of ordinary skill in the art from the above description.  
         [0056]    The ASIC  100  may also communicate with electrical connectors  50  joined with wiring  56  via electrical connectors  52  to enable additional functions and applications. For example, one of the wires  56  may provide an external reset signal or trip signal which may be routed by the ASIC  100  to reset signal  96  to provide for resetting or tripping of the overload relay of the base module  12 . Such a reset signal or trip signal may come from a remote electrical pushbutton or from a central controller.  
         [0057]    Power for the ASIC  100  may be obtained by wiring  56  from an external source via electrical connectors  52  or from the power  70  of the base module  12 .  
         [0058]    Referring now to FIG. 5, a more advanced added function module  40  is contemplated which includes a microcontroller  102  providing internally a processor, a read only memory and sonic interface circuitry. The microcontroller  102  communicates with a clock circuit  104  and start up circuit  114 , and via an internal bus  106 , communicates with toggle switch array  46 , interface circuit  108 , and interface circuit  110 . Interface circuit  108  communicates with electrical connector  44  and each of signals  70 ,  72 ,  78 ,  82 ,  94  and  96  and thus may include analog to digital and digital to analog circuits as well as level shifting circuits understood in the art, on an as needed basis. Interface circuit  110  (which may consist of one or more devices) communicates with the first and second electrical connectors  52  and  52 ′ and thus with wiring  56  and may include protection, level shifting, and voltage or current limiting circuits understood in the art. Again, power can be obtained either through external wiring  56  or from the electrical connector  44  and processed by power circuitry  112  to be suitable for the microcontroller  102 .  
         [0059]    The microcontroller  102  executes a stored internal program to provide a variety of functions required in higher performance applications, for example, communication over a network implemented on wiring  56  running a DeviceNet protocol connected to electrical connectors  52 ′. With such a network connection, the added-function module  40  may allow a reading of any of the values from electrical connector  44  and a transmitting of those values to a remote location. For example, burden voltage  78 , TCU  82  and power  70  may all be monitored. Alternatively or in addition, the network on wiring  56  may transmit signals converted by the microcontroller  102  to set signals  94  and reset signal  96  based on a remote command. Microcontrollers  102  pre-programmed to implement the well-known CAN protocol used in DeviceNet, and thus to permit bi-directional transfer of serial digital data, are available from a number of commercial vendors. Other networks such as Profibus, Modbus, and ASI may be used in lieu of DeviceNet.  
         [0060]    The microcontroller  102  may also provide generalized input and output signals through electrical connectors  52 , for example, monitoring the outputs of the latched contact set  86  being normally open contacts  88  and normally closed contacts  90  on the base module  12  or other local I/O functions including auxiliary contacts on the contactor and a possible circuit breaker attached to the system.  
         [0061]    It will be understood that the use of the microcontroller  102  requires additional support circuitry and thus substantially increases the cost of the combined overload relay system  10 , however, this allows the overload relay system  10  to meet performance requirements required of high tier units and may ultimately further lower the cost of the base module  12  through higher volumes.  
         [0062]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.