Patent Document

This is a divisional application of Ser. No. 09/057,730 filed Apr. 8, 1998, now U.S. Pat. No. 6,005,760 which is a continuation of U.S. Ser. No. 09/264,367 filed Mar. 8, 1999 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a single protection device, including a transformer, an input circuit, and a switch circuit, suitable for addition to an electrical power system that includes a control panel electrically interconnected with a starter to control an electrical device. It is desirable to monitor the performance of devices such as electric motors and to control them through remotely located control panels. While this has long been possible, it has been costly to install the required sensors and controls. 
     Springer, U.S. Pat. No. 4,885,655 discloses an integrated capacitor-start, induction-run electric motor starter and protection circuit specifically suitable for a water pump. The circuit requires physical electrical connection to the power cables to sense the phase angle between the voltage and the current applied to the motor to indicate when it is operating without a load. When such a condition occurs, the circuit triggers an activator coil to de-energize the motor by opening a switch member. The integrated starter and protection circuit is suitable only for induction machines. 
     Libert, U.S. Pat. No. 4,887,018, discloses a start-up circuit for gradually starting a multiphase motor and which also includes detector circuits for detecting various fault conditions and for disconnecting power from the motor in response to fault conditions. Integrating the motor starter and protection circuit into one device makes it unsuitable for addition to existing power systems to detect fault conditions within power cables. 
     Flückiger, U.S. Pat. No. 5,359,273, discloses a load control circuit for controlling the load of an asynchronous motor. The circuit includes a pair of capacitors, one of which can be connected in parallel with the other by a switch. Control over whether one or two capacitors is supplied as a load to the motor is provided by comparing signals from a current sensor and a voltage sensor in a comparator. Depending upon the relative size of the compared signals, the switch is turned off or on. 
     Domshy, et al., U.S. Pat. No. 3,593,078, discloses a starting and operating control circuit that includes voltage sensors and current sensors to start a motor and limit the power supplied to it. 
     One design of a starter includes an integrated starter and protection circuit to detect fault conditions of an associated electrical device and, in response to detecting a fault condition, disable the electrical device. When a starter with an integrated starter and protection circuit is used to control devices in an environment in which the electrical devices are not dependent upon other machines, or their operation does not impact other devices, the inclusion of the integrated starter and protection circuit is an effective way to protect the associated electrical device. However, when a starter including such an integrated protection circuit is used in a system where the associated electrical device is interdependent with other electrical devices, use of such a starter to independently enable and disable the electrical device may result in devastating effects to the entire system. In other words, where several electrical devices combine to produce a result, as in systems that includes a computerized control panel or system controller, it is desirable for the control panel or system controller to maintain control over the status of the electrical devices to minimize potential problems. 
     However, many starter designs do not include an integrated protection circuit and also include no-fault detection. To provide fault detection for electrical devices, a current sensor surrounding the power cable to the electrical device may be used to sense the electrical load current. The current sensor may be electrically interconnected between a control panel and the power cable to provide a signal to the control panel representative of the current within the power cable. Suitable current sensors are known, such as the current sensor sold by Veris Industries, Inc., under the trademark HAWKEYE 700. It is desirable to locate such a current sensor within a housing where a starter for the electrical device is located. 
     A relay is typically electrically interconnected between a remotely located control panel and a starter to receive a control signal from the control panel and in response to provide an indication to the starter. Like the current sensor, such a relay may be located within a starter housing. 
     The relay and current sensor in the past have been separate individual devices, each requiring a mounting location that may not be available within the starter housing due to limited space. Additionally, it takes additional time to install two separate devices and requires stocking two separate replacement parts for use in the event of device failure. Also, troubleshooting likely requires the inspection of both devices. 
     What is desired, therefore, is a protection device that is suitable for addition to existing electrical systems that operate in an environment including a control panel and a starter controlling the operation of an associated electrical device. The protection device should be compact, inexpensive, and fast to install. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks and shortcomings of the prior art by providing a protection device for monitoring current in a power cable to an electrical device and for controlling a starter for the electrical device in response to a remotely located system controller. The protection device includes a transformer magnetically linked with the power cable connected to the electrical device. The transformer produces a voltage signal in response to the presence of a changing current within the power cable. An input circuit located proximate to the transformer has an output terminal and is electrically connected to the transformer so as to receive the voltage signal. The input circuit produces, in response to receiving the voltage signal, one of a first signal representative of the changing current and a first circuit condition at the output terminal of the input circuit representative of the changing current in the power cable. A switch circuit has a first terminal for sensing one of a second signal and a second circuit condition of the remotely located system controller. The switch circuit has a second terminal to provide one of a third signal or a third circuit condition, in response to sensing one of the second signal and the second circuit condition. The third signal and the third circuit condition is effective to control the starter when the starter is electrically connected thereto. All of the transformer, the input circuit, and the switch circuit are located in a single unitary package. 
     By locating the sensor circuit, which is a combination of the transformer and the input circuit, and the switch circuit proximate to one another within the single package, significant advantages are realized. Due to the limited available space within a starter housing, the single package is more desirable. The expense of manufacturing a single package is frequently less than manufacturing two separate packages. The time required to install both the switch circuit and the sensor circuit is reduced by only requiring installation of one combined package. A reduction in the number of backup parts and troubleshooting time is also realized. 
     In a preferred package a mounting bracket includes a slide arrangement to support the transformer so that its position can be adjusted easily to the best location with respect to the power cable. 
    
    
     The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an electrical system that includes a protection device according to the present invention, a control panel, and a starter controlling the operation of an associated electrical device. 
     FIG. 2 is a pictorial representation of the protection device of FIG. 1, showing a transformer, an input circuit, and a switch circuit. 
     FIG. 3 is an electrical schematic diagram of a transformer and input circuit which is a portion of a first embodiment of the present invention, suitable to provide a full scale 0 volt to 5 volt output signal. 
     FIG. 4 is an electrical schematic diagram, of a transformer and input circuit which is a portion of a second embodiment of the present invention suitable to provide an open circuit or short circuit condition. 
     FIG. 5 is an electrical schematic diagram of a transformer and input circuit which is a portion of third embodiment of the present invention, suitable to provide a 4-20 ma output signal. 
     FIG. 6 is an electrical schematic diagram of a switch circuit including a relay, which is a portion of one embodiment of the present invention. 
     FIG. 7 is an electrical schematic diagram of a switch circuit including a triac, which is a portion of another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an electrical system  10  includes a control panel  20  that utilizes a digital computer to provide effective control of many associated electrical devices. The centralized control panel  20  determines the effects on the entire system  10 , or a portion of the system  10 , of enabling or disabling an electrical device. For example, such associated electrical devices may include motors, pumps, fans, valves, generators, switches, lights, etc. One type of control panel  20  is generally known as a programmable logic controller, such as those sold by Allen Bradley. 
     A starter  22 , designed to start (energize) and stop (de-energize) remotely located electrical devices, is electrically connected to an associated electrical device  24  by three power cables  23   a ,  23   b , and  23   c . Each starter  22  is usually located within an individual starter housing  26  which is a part of a substation. Most substations are not large, so it is desirable to reduce the size of the housing  26 , so as to maximize the number of housings  26  that may be located within the substation. Accordingly, the housing  26  is normally designed to be only slightly larger than the enclosed starter  22 , and so there is only limited space within the housing  26  in which to place additional devices, such as protection devices. 
     Referring to FIG. 2, a protection device  35  includes both a current sensor, including a transformer  34  and an input circuit  50 , and a switch circuit  60  within the single package  30 . The package  30  is preferably slidably mounted on a support  31  mounted within the housing  26 . By placing the transformer  34 , input circuit  50 , and switch circuit  60  proximate to one another, within the single package  30 , it is considerably easier to locate the device  35  within the limited space of the starter housing  26 . Additionally, installing only the single package  30  requires less installation time than installation of separate devices to perform each of the desired functions, and the expense of manufacturing, packaging and shipping a single device is less than for two separate devices. A reduction in the number of backup parts and troubleshooting time is also realized. 
     The package  30  defines a central opening  32  through which the power cable  23   c  is routed. Surrounding the central opening  32  is a toroidal sensing transformer  34  to sense the changing current within the power cable  23   c . The toroidal sensing transformer  34  is preferably a wire-wrapped magnetically permeable toroidal core, normally made of iron, encircling the respective power cable. Thus, the wire wound on the toroidal sensing transformer  34  is the secondary winding, while the power cable  23   c , or a parallel shunt current divider (not shown), is the primary winding of the toroidal sensing transformer  34 . Changing current in the power cable  23   c  induces a changing electromagnetic field around the power cable  23   c , which in turn induces a magnetic flux in the magnetically permeable core. The magnetic flux in the core induces in the wire windings on the toroidal core a voltage representative of the current in the power cable. An exemplary sensing transformer has the following construction: core material made by Arnold Engineering, of Norfolk, Nebr., of 0.012 silectron, 3% silicon steel, grain oriented, with an outside diameter of 1.375 inches, an inside diameter of 1.125 inches, strip width of 0.500 inches, strip thickness of 0.012 inches, an epoxy powder coating of 0.010 to 0.030 inches thick, a nylon overcoat wound on the metal core, and a #35 AWG size wire coated with a heavy polyurethane wound 1,800 turns as a secondary winding. 
     Such a sensing transformer with a core of magnetically permeable material, such as iron, generates a voltage signal reasonably accurately representative of the current in the power cable over a certain normal load range. However, iron and other magnetically permeable materials have hysteresis and other nonlinear responses to changing magnetic fields that result in a nonlinear relationship between current in the power cable and the voltage signal produced in a transformer coil having such a core. The nonlinearity of such responses is especially significant with large variations in load current and frequency. To provide a more linear measurement of power, “air core” transformers have been designed using wire wrapped on a core made of material having a low magnetic permeability, such as one of plastic or nylon. Without a magnetically permeable core, however, the transformer winding generates relatively small voltage levels in response to power cable currents. An exemplary air core transformer has the following construction: core of nylon, outside diameter of 1.375 inches, inside diameter of 1.125 inches, strip width of 0.500 inches, and a #35 AWG size wire coated with a heavy polyurethane, wound 4,000 turns as a secondary winding. Examples of circuitry suitable for use with an “air core” transformer are disclosed in U.S. patent application Ser. No. 08/300,732, assigned to the same assignee, and incorporated herein by reference. 
     The ends of the secondary winding  40   a  and  40   b  of the transformer  34  are electrically connected to an input circuit  50 . The input circuit  50  is designed to convert the voltage signal received from the transformer  34  to either a signal representative of the changing current in the power cable or a circuit condition at the output terminal  41   a  and  41   b  representative of the changing current in the power cable. The signal or circuit condition is provided to transmission lines  54  and  56  which are connected to the control panel  20 . For example, the signal could be a current signal, voltage signal, or some sort of frequency modulation, amplitude modulation, or digital encoding. The circuit condition, for example, could be a short circuit, open circuit, or other suitable type of condition. The input circuit  50  can be designed and constructed in any manner, so long as it converts the voltage signal output from the transformer  34  to an appropriate corresponding signal or circuit condition. Several exemplary input circuit designs are described below. A light emitting diode  58  is electrically connected to the input circuit  50  and is illuminated when current is sensed within the power cable. A potentiometer  59  allows adjustment of a threshold level within the input circuit  50  of the sensed voltage from the transformer  34 . 
     The use of the control panel  20  or system controller provides automated control over the electrical system  10 . The control panel  20  receives the signal from the input circuit  50  or determines the circuit condition of the input circuit  50  via a pair of transmission lines  54  and  56 . The control panel  20  in response to receiving the signal or determining the circuit condition of the input circuit  50  analyzes the signal or circuit condition to determine information such as power consumption, overcurrent, overvoltage, undercurrent, undervoltage, frequency, spikes, harmonics, etc. From this information the control panel  20 , among other things, may determine that the electrical device  24  should be disabled or enabled. For example, if the current sensor indicates that a motor (not shown) for a pump is malfunctioning, then the control panel  20  may have that motor deactivated. If deactivation of that motor would also impact another device, such as an auger within a storage tank supplying fluids to the pump, then the control panel  20  may also deactivate the motor for the auger. 
     The control panel  20  is electrically connected to a switch circuit  60  by a pair of transmission lines  61  and  63 . The switch circuit  60  is located proximate to the transformer  34  and input circuit  50 . The switch circuit  60 , transformer  34 , and input circuit  50  are all enclosed within the single package  30 . The package is preferably mounted within the starter housing  26 . The switch circuit  60  includes any suitable switching device, for example, a triac or a relay, as will be described below. The triac or relay is powered by a 24 volt AC or DC signal through the transmission lines  61  and  63 . The power on the transmission lines  61  and  63  closes the circuit through the switch circuit  60  and maintains a short circuit between the output terminals  67   a  and  67   b  of the switch circuit  60 . When power ceases to be supplied to the switch circuit  60 , the output terminals  67   a  and  67   b  of the switch circuit  60  are electrically isolated from each other (open circuit). With the output terminals of the switch circuit  60  in an open circuit condition when the transmission lines  61  and  63  are not powered, a safety feature for the starter  22  is provided in the event of power failure to the control panel as will be described below. Alternatively, the switch circuit  60  could be designed to be controlled by any type of suitable signal or circuit condition. 
     A pair of wires  70  and  72  are connected between the output terminals  67   a  and  67   b , respectively, of the switch circuit  60  and starter terminals  74  and  76 . The starter terminals  74  and  76  permit exterior control over the operation of the starter  22 . For most starters  22 , when the terminals  74  and  76  are short circuited (electrically connected together) the starter  22  energizes and operates the associated electrical device  24 . Alternatively, when the terminals  74  and  76  are open circuited (isolated from each other), the starter  22  de-energizes, or otherwise ceases the operation of the associated electrical device  24 . Accordingly, the open or short circuited circuit conditions applied between the output terminals  67   a  and  67   b  of the switch circuit  60  connected to the wires  70  and  72  are suitable to control the starter  22 . The switch circuit  60  may alternatively be constructed to provide whatever signal or circuit condition is necessary to control the particular starter  22 , which may include a voltage signal, a current signal, digital signal, etc. A light emitting diode  64  is electrically connected to the switch circuit  60  and is illuminated when the transmission lines  61  and  63  are powered. 
     Referring to FIG. 3, an electrical schematic diagram of a current sensor  300  suitable to provide a full scale 0 volt to 5 volt output signal is shown. The transformer  34  encircles a power cable  23   c , producing a voltage between the ends  40   a  and  40   b  of its secondary winding. The ends  40   a  and  40   b  of the transformer secondary winding are connected to the input circuit  50  which includes a full wave rectifier  312 , connected to a variable resistance  314  and associated capacitors  316  and  318 , to scale the output of the full wave rectifier  312  to the desired range. The preferred range to interface with conventional control panels  20  is 0 volts when no current within the power cable  23   c  is sensed to 5 volts when the maximum desired level within the power cable is sensed. 
     Referring to FIG. 4, current sensor  320  provides either an open circuit or short circuit at its output terminals  41   a  and  41   b  depending on whether the voltage signal produced in response to the current sensed by the transformer  34  surpasses a predetermined threshold level. A variable resistor  326  sets the threshold level. 
     Referring to FIG. 5, current sensor  340  provides a 4-20 ma variable output signal at its output terminals  41   a  and  41   b . When the current sensed in the power cable  23   c  is 0 then the sensor circuit  340  puts out a 4 ma signal. When the current sensed in the power cable  23   c  is equal to a desired maximum, when the variable resistor  341  is correctly set, then the sensor circuit  340  puts out a 20 ma signal. 
     Referring to FIG. 6, an electrical schematic of a switch circuit  60  is shown. A voltage or current signal from the control panel  20  is provided to the input terminals  66  and  68  of the switch circuit  60 . When a non-zero signal is received by the switch circuit  60  a light-emitting-diode  64  is illuminated to indicate that the switch circuit  60  is energized. A diode  368 , a resistor  370  and a capacitor  372  rectify the signal received at input terminals  66  and  68  if it is an alternating signal. The voltage imposed across the capacitor  372  is the input to the direct current relay  374 . If the signal received at input terminals  66  and  68  is a direct voltage or current signal, then the signal will also pass through to the relay  374 . Accordingly, the switch circuit  360  is suitable to receive both an alternating signal or direct signal. The relay  374  is energized by a high voltage signal at the input terminals  66  and  68  and thereby its output contacts  67   a  and  67   b  are shorted. When the high voltage signal provided to the relay  374  is below a threshold level, the output contacts  67   a  and  67   b  to the relay  374  open, open-circuiting the output contacts  67   a  and  67   b . The output contacts  67   a  and  67   b  are connected to the terminals  74  and  76  of a remotely located starter  22  (not shown) via wires  70  and  72  (FIG.  1 ). In other words, the starter  22  is spaced apart from the switch circuit  60 . 
     Referring to FIG. 7, an alternative switch circuit  60   a  includes a pair of input terminals  66  and  68 , a resistor  406 , a diode  407 , and a capacitor  408  to permit the use of either an alternating signal or a direct signal as the input to the input terminals  66  and  68 . An opto-isolator  410  isolates the high voltage to the input terminals  66  and  68  from the output terminals  67   a  and  67   b  for safety. A triac  412 , which is a switching device, is energized with a low voltage on the gate  413  of the triac  412  to close the triac  412  creating a short circuit between the output terminals  67   a  and  67   b . A “snubber circuit” includes a resistor  418  and capacitor  420  connected in parallel across the output terminals  67   a  and  67   b . In general, the ‘snubber circuit’ prevents false triggering of the triac  412  that may occur when driving an inductive load. Output terminals  67   a  and  67   b  are thus short circuited or open circuited (by the operation of the triac  412 ) with the result that the terminals  67   a  and  67   b  exhibit a circuit condition to the starter  22  indicative of whether the electrical device  24  controlled by the starter  22  should be operating. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Technology Category: 5