Abstract:
A sensing and switching device, such as an overload relay, is provided which includes a processor configured to make measurements and control operation (e.g., tripping) of the device. The processor regulates measurement of voltage and/or current, and the supply of power to power supplies. The power supplies store charge to provide operational power for the processor and that can be used for tripping and resetting contacts within the device. The processor opens a burden resistor measurement circuit when charge is being stored in the power supplies, and opens switches in the power supplies while closing the burden resistor switch to permit measurements. By alternatively switching for charging of the power supplies and making of measurements, the processor is able to reliably make measurements, control the device, and store sufficient power for operation of the device despite a demanding power budget.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to protective circuitry, such as overload circuits. More particularly, the invention relates to a processor-based overload relay that is self-powered by virtue of power management components, permitting power to be stored for its operation, while performing measurements of voltage and current. 
         [0002]    Overload relays and similar circuits are used in a wide range of settings. For example, applications involving powering electric motors, a motor starter or motor controller is typically coupled to a motor to supply single or three-phase power. The motor drive, in many applications, may synthesize an output waveform to vary the frequency of the drive power so as to permit the motor to be driven at various speeds. The waveform may also be manipulated to control torque, and so forth. Motor controllers, however, do not typically provide for interrupting power to the motor in case of need. Depending upon the circuit configuration, other protective devices typically serve this purpose. Such devices may include fuses that are often positioned upstream of all other circuitry and downstream of a power supply, such as the electric power grid. The fuses may be supplemented by magnetic and thermal overload circuitry. Magnetic overload circuitry typically trips to open the power circuit in response to rapid changes in current. Other overload circuitry may operate more slowly, and may model motor windings or other wiring, to permit opening of the circuitry should longer-term rises in temperature be detected or estimated. 
         [0003]    In the area of overload relays, a number of different configurations have been developed and are presently in use. Such relays can be vary from quite simple electro-mechanical devices to more sophisticated circuitry that incorporates application specific integrated circuits (ASICs), or processors, typically microprocessors. Such ASICs and processors offer a significant benefit in being capable of analyzing current and voltage data and judiciously opening or closing power circuits based upon the analysis. Where possible, sophisticated yet high production (and thus cost effective) processors, including microprocessors and field programmable gate arrays may be used for such purposes. 
         [0004]    One challenge in the use of such circuitry, however, is the ability to provide sufficient power for its operation. Specifically, smaller sizes of overload relays may not be able to provide sufficient power for operation of microprocessor-based control circuitry. In many cases, it is advantageous to power the circuitry from power that is extracted or scavenged from the sensing devices, such as current transformers. However, where power levels required for the processing exceed the available power budget, more costly and less flexible ASICs and other circuitry may be required. 
         [0005]    There is a need, therefore, for improved circuit designs that may permit the use of more sophisticated processing capabilities that are powered by current transformers and similar power scavenging devices. There is a particular need for such circuitry that may permit microprocessors and similar circuits to be used in small electromechanical devices, such as overload relays, that have a reduced power budget. 
       BRIEF DESCRIPTION 
       [0006]    The present invention provides novel circuitry that can be used in overload relays and similar devices configured to respond to such needs. The circuitry may be used in a wide range of settings, but is particularly well-suited to devices where currents and/or voltages are measured and where circuit interruption is powered by power scavenged from the measurement circuits. In accordance with certain aspects of the invention, a processor implements a power management scheme in which a measurement circuit is periodically switched, along with power supply circuitry. The power supply circuitry may include power storage devices, such as capacitors, and more than one power supply circuit may be powered, such as one for operational power and one for tripping and resetting the circuitry. The measurement circuitry may include a burden resistor that is switched into an out of the power supply line from sensors in order to make periodic voltage measurements that are proportional to current. The power may also be provided, in the alternative, from add-on devices or networked option modules which are coupled to the processor, but for which the scavenging power supplies do not have sufficient power. 
     
    
     
       DRAWINGS 
         [0007]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0008]      FIG. 1  is a diagrammatical representation of power supply circuitry in a motor application incorporating a relay in accordance with aspects of the present techniques; 
           [0009]      FIG. 2  is a diagrammatical representation of the circuitry of  FIG. 1  in greater detail, illustrating voltage/current measurement circuitry and power supplies commanded by a processor; 
           [0010]      FIG. 3  is a somewhat more detailed view of the power supplies and measurement circuitry of  FIG. 3  coupled to the processor; and 
           [0011]      FIG. 4  is a diagrammatical representation of a circuit configuration in which power from an add-on device or option module may be used when coupled to the device of the previous figures. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Turning now to the drawings, and referring first to  FIG. 1 , power circuitry  10  is generally illustrated for supplying power to a motor  12 . The power circuitry may be designed for stand-alone operation, or may be part of an overall control system, such as in industrial, commercial, material handling, or any other suitable applications (e.g., coupled to other components and networked to remote monitoring and control equipment). The circuitry includes an overload relay, designated generally by reference numeral  14 , that senses voltage and current provided to the motor and that may open the power supply circuitry based upon actual or anticipated overload conditions. The overload relay  14  may be used with other protective circuitry, indicated generally by reference numeral  16 . Such protective circuitry, which may include fuses, manual or automatic disconnects, and so forth will typically be positioned between the overload relay and a power source, such as the power grid. Three-phase power, in the illustrated embodiment, is provided to the protective circuitry, flows through the overload relay, and is then applied to motor drive circuitry  18  which powers the motor. The motor drive circuitry may include any suitable devices, such as across-the-line starters, soft starters, variable frequency motor drives, and so forth. 
         [0013]    The overload relay  14 , in the illustrated example of  FIG. 1 , utilizes a series of current transformers  20  which are coupled to the three-phase power conductors passing through the device. Any type of current transformer may be used for the application, such as transformers comprising multiple winds of wire positioned about or next to the three-phase power conductors. The coils of the current transformers effectively act as secondary windings of transformers, and carry current induced by fields generated by current through the three-phase power conductors. The current transformers  20  apply the sensed signals to control circuitry  22 , described in greater detail below. The control circuitry  22  takes measurements of current and voltage, and includes a processor that can cause tripping of the device based upon actual or anticipated overload conditions. As also described below, the control circuitry regulates power for measurement and for operation of the device by a power management scheme. In the event of an overload condition, the overload control circuitry  22  can open contacts  24  in the device to interrupt power to the motor. Finally, the control circuit can output signals to energize coils  38  that operate to open or close the contacts of the relay, in a manner well understood to those skilled in the art. 
         [0014]    As best illustrated in  FIG. 2 , the overload relay includes a processor  26  which serves to implement the power management scheme, and to analyze sensed data to determine when a condition exists that may warrant opening of the contacts  24 . The processor is coupled to the current transformers via a rectification circuit  28 . Because the waveform originating in the current transformers will reflect the sinusoidal waveform through the three-phase power conductors, rectification through circuit  28  serves to convert the three-phase AC power to DC power. A current measurement signal indicated by reference numeral  30  is applied to the processor  26  as output by the rectification circuit  28 . Measurements necessary for the decisions implemented by the processor  26  are made by measurement circuit  32 . The measurements made by the measurement circuit  32  are based upon current through a burden resistor in the circuit that is switched into and out of the circuitry as described more fully below. 
         [0015]    As also illustrated in  FIG. 2 , the processor  26  is coupled to an operational power supply  34 , and to a trip/reset power supply  36 . The operational power supply and the trip/reset power supply are both coupled to the rectified power provided by the rectification circuit  28 . In operation, power storage components, such as capacitors, within the operational power supply and the trip/reset power supply are charged under the direction of control signals from the processor  26 , in coordination with measurement by the measurement circuit  32 . The operational power supply  34 , then, supplies power to the processor  26  during operation. The trip/reset power supply  36  stores and supplies power to open or close (i.e., reset) the contacts as commanded by the processor  26 . This power supply, too, is charged under the direction of control signals provided by the processor  26 . 
         [0016]    In the illustrated embodiment, the processor  26  may also be coupled to various options, as indicated generally by reference numeral  40 . Such options may include, for example, modules that may be coupled to, or plugged directly into the relay. Option modules presently contemplated may include inputs and outputs for communicating with the processor, remote reset devices, network interface devices, and so forth. Such option modules may then be coupled to external devices, such as remote control and monitoring equipment. In many applications such option modules may be separately powered, such as by a network link. As described more fully below, when this is the case, power from the network may be used to drive measurement and supply power for the power supply of the device. 
         [0017]      FIG. 3  illustrates the voltage/current measurement circuit  32  and power supplies  34  and  36  in somewhat greater detail in accordance with a presently contemplated implementation. As shown in  FIG. 3 , the measurement circuit  32  includes a burden resistor  42  and a switch  44 . Switch  44  may be any suitable switch, such as a transistor. As will be appreciated by those skilled in the art, the output of the rectification circuit  28  which serves as the input to the voltage measurement circuit  32  functionally resembles a current source with a somewhat variable voltage. The burden resistor  42  permits measurement of the line current by measuring a proportional current through the burden resistor to ground upon closing of switch  44 . Opening and closing of switch  44  is controlled by processor  26  as described more fully below. Because current through the burden resistor is used to measure a voltage proportional to current, the burden resistor, if left in the circuit, represents a drain of power. By commanding switch  44  to open, the processor stops this drain and can use available power to charge components within the operational power supply  34  and the trip/reset power supply  36 . 
         [0018]    The processor  26  also controls power supplies  34  and  36  by appropriately charging components within those power supplies via switches. In the embodiment illustrated in  FIG. 3 , for example, power supply  34  includes a capacitor  46  which is charged to supply operational power for the device. Capacitor  46  is coupled to a linear regulator  48  which conditions and regulates the output power for operation of the processor. A switch  50 , which again may be a transistor, is opened and closed by signals from processor  26 . Similarly, power supply  36  includes a pair of capacitors  52  and  54  separated by a diode  56 . Charging of the capacitors is regulated by operation of a switch  58  on the controller processor  26 . In the present embodiment, capacitors  46 ,  52  and  54  provide bulk storage for charge that can be drained for operation of the circuitry in the case of capacitor  46 , and for tripping (opening the contacts) of the device and resetting the device in the case of capacitors  52  and  54 . The reset capacitor may be used, for example, for automatic reset of the contacts. 
         [0019]    The processor  26  may be provided with electronically erasable programmable read-only memory, flash memory, or any other suitable memory circuitry. Programming for analyzing the current and voltage signals, and any other signals collected by the processor is stored within this memory. Moreover, for certain types of memory, reprogramming of the device may be performed by altering the programming stored within this memory, such as via an option module of the type described above with reference to  FIG. 2 . In operation, the processor  26  closes switch  44  to make voltage measurements (proportional to current) at intervals when switches  50  and  58  are open. Once data has been collected for the measurement, then, switch  44  may be opened, and switches  50  and  58  may be closed to store power collected by the current transformers by charging capacitors  46 ,  52  and  54 . In the presently contemplated embodiment, for example, the switches are alternatively opened and closed to perform measurements and store power, with a measurement period occurring every 1 ms. Other intervals and periods for alternative measurement and charging may, of course, be used, and durations for measurement and charging need not be equal. Certain functions may also be set by other means, such as resets, trip classes, and so forth may be set by appropriate dip switches (not shown). These functions may be implemented by virtue of the use of the processor to control operation of the device. 
         [0020]    In certain applications where option modules are coupled to the circuitry described above and separately powered, such as through a network, this option module power may be used instead of scavenged power from the current transformers.  FIG. 4  represents exemplary circuitry for this type of alternative power configuration. As shown in  FIG. 4 , the power supplies  34  and  36  are essentially identical to those shown in  FIG. 3 . However, a pair of diodes  60  prevents power from an option module from being transmitted back to the upstream circuitry. Similarly, a pair of diodes  62  isolates the option power supply. Power supplies  34  and  36  may be powered by either the power supply or the current transformers, depending upon if the power supply that provides the higher voltage. 
         [0021]    As compared to the circuitry shown in  FIG. 3 , that of  FIG. 4  includes a solid state switch  64  that controls operation of switch  50 . This configuration may be preferred such that switch  50  may be normally on (such as a JFET), allowing for cold start, that is, when no power is available to place the switch in a conductive state to charge the power supply. A resistor  66  is provided to hold the switch on (i.e., pull the switch down). In a presently contemplated embodiment, capacitors  46 ,  52  and  54  have values of 4.7 μF, 680 μF, and 680 μF, respectively, although differently sized capacitors may be employed depending upon the power needs and the power budget of the circuitry. 
         [0022]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.