Patent Publication Number: US-9842714-B2

Title: Detecting current leakage in a heating element

Description:
FIELD OF THE INVENTION 
     The present disclosure relates generally to dishwashing appliances, and more particularly to protecting heating elements of dishwashing appliances. 
     BACKGROUND OF THE INVENTION 
     Modern dishwashing appliances (e.g. dishwashers) typically include a tub defining a wash chamber where, for instance, detergent, water, and heat can be applied in order to clean food and/or other materials from dishes and other articles being washed. Various cycles may be included as part of the overall cleaning process. For example, a typical, user-selected cleaning option may include a wash cycle and rinse cycle (referred to collectively as a wet cycle), as well as a drying cycle. A pre-wash cycle may also be included as part of the wet cycle, and may be automatic or an option for particularly soiled dishes. 
     It is common to provide dishwashers with rod-type, resistive heating elements in order to supply heat within the wash chamber during one or more of the dishwasher cycles (e.g. during the drying cycle). Generally, these heating elements include an electric resistance-type wire that is encased in a magnesium oxide-filled, metallic sheath. 
     Such dishwasher heating elements can be exposed to harsh environments that may cause premature failure of the heating elements. For instance, chlorine attack, calcium buildup and/or power surge events can cause premature failure of a dishwasher heating element. Such premature heating element failure may cause a violent ignition due at least in part to high current arcing or sheath rupture. Heating element failure generally follows a measurable increase in current leakage. Ground fault detection can be used to detect current leakage and to prevent such failure. To enable ground fault detection, a means must be provided for electrical dispersion from the heating element to earth ground. However, coupling a heating element sheath to ground can place the heating element at risk of failure due to lightning strikes. Additionally, it can be difficult to detect current leakage at various fault points on the heating element due at least in part to the voltage drop across the heating element. 
     Thus, it is desirable to provide a system for detecting current leakage with a high level of sensitivity that provides protection from high voltage surges caused by lightning strikes. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description, or may be learned from the description, or may be learned through practice of the embodiments. 
     One example aspect of the present disclosure is directed to a heating element protection system for a dishwashing appliance. The protection system includes a resistive heating element having a metallic sheath that is coupled to ground. The heating element includes a line side terminal coupled to a line conductor and a neutral side terminal coupled to a neutral conductor. The protection system further includes a first relay coupled to the line conductor. The protection system further includes a second relay coupled to the neutral conductor. The protection system further includes a control system in operative communication with the first and second relays. The control system is configured to monitor for a leakage current flowing from the heating element to ground by controlling a sequence of operations of the first relay and the second relay such that a magnitude of the leakage current is increased. 
     Another example aspect of the present disclosure is directed to a method of monitoring current leakage in a heating element having a sheath coupled to ground. The heating element is further coupled to a line conductor having a line relay and a neutral conductor having a neutral relay. The method includes applying an alternating current signal to the heating element. The method further includes configuring the line relay and the neutral relay in a first state, wherein during the first state, the line relay is closed and the neutral relay is open. The method further includes identifying a current flowing through the heating element to ground. Configuring the line relay and neutral relay in the first state provides an increase in the magnitude of a leakage current flowing through the heating element to ground. 
     Yet another example aspect of the present disclosure is directed to a dishwashing appliance. The dishwashing appliance includes a tub defining a wash chamber. The dishwashing appliance further includes a rack assembly disposed within the wash chamber of the tub. The rack assembly is configured for supporting articles for washing within the wash chamber of the tub. The dishwashing appliance further includes a resistive heating element comprising a resistance-type wire and a metallic sheath coupled to ground. The dishwashing appliance further includes a control system in operative communication with the heating element. The control system is configured to detect a leakage current flowing from the heating element to ground by controlling a sequence of operations of the heating element such that a magnitude of the leakage current is increased. 
     Variations and modifications can be made to these example embodiments of the present disclosure. 
     These and other features, aspects and advantages of various embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the related principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed discussion of embodiments directed to one of ordinary skill in the art are set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  depicts a front view of an example dishwashing appliance according to example embodiments of the present disclosure; 
         FIG. 2  depicts a cross-sectional view of the example dishwashing appliance according to example embodiments of the present disclosure; 
         FIG. 3  depicts an example heating element protection system implemented in the dishwashing appliance according to example embodiments of the present disclosure; 
         FIG. 4  depicts an example circuit implementation of a heating element protection system according to example embodiments of the present disclosure; 
         FIG. 5  depicts an example sequence of operations for detecting current leakage according to example embodiments of the present disclosure; 
         FIG. 6  depicts an example sequence of operations for detecting current leakage in a heating element according to example embodiments of the present disclosure; and 
         FIG. 7  depicts a flow diagram of an example method of monitoring current leakage in a heating element. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Example aspects of the present disclosure are directed to monitoring current leakage in a dishwashing appliance (e.g. dishwasher) heating element. As used herein, current leakage (also referred to as leakage current) can be defined as current that flows through a protective ground conductor to ground. As used herein, “ground” refers to an electrical ground or other reference point or common. In particular, a dishwasher can include a resistive heating element. The heating element can include a resistance-type wire encased in a magnesium oxide-filled, metallic sheath. The sheath can be coupled to ground or other reference potential via a ground conductor. The heating element can include a line side terminal and a neutral side terminal. A line conductor can be coupled between the line side terminal and a positive terminal of an alternating current power supply. A neutral conductor can be coupled between the neutral side terminal and a negative terminal of the power supply. The line conductor can be further coupled to a line relay, and the neutral conductor can be further coupled to a neutral relay. In example embodiments, when the line and neutral relays are closed, a current can flow through the resistance wire of the heating element to the neutral conductor, causing the heating element to increase in temperature. The line relay and the neutral relay can be used to break the circuit at the line conductor and the neutral conductor respectively. 
     In example embodiments, a sequence of operations of the line relay and the neutral relay can be controlled to monitor leakage current from the heating element to ground. In particular, during an energize cycle of the heating element, both the line relay and the neutral relay can be closed to provide power to the heating element. The neutral relay can then be configured to open for a predetermined time period. For instance, the neutral relay can be configured to open for a time period in a range of about 20 milliseconds to about 70 milliseconds. As used herein, the term “about,” when used in reference to a numerical value, is intended to refer to within 30% of the numerical value. It will be appreciated that the relay can be configured to open for various other suitable time periods, such as any suitable time period sufficient to detect a leakage current. 
     With the neutral relay open and the line relay closed, current will not flow through the resistance wire of the heating element to the neutral conductor, but the resistance wire will have a high potential relative to ground. This configuration can cause an increase in the magnitude of any leakage current flowing through resistance wire to ground. The increased leakage current can be more easily detected. In example embodiments, if a leakage current is detected having a magnitude above a leakage threshold, both the line relay and the neutral relay can be configured to open to break the circuit at the line conductor and the neutral conductor and to cease operation of the dishwashing appliance. 
       FIGS. 1 and 2  depict one embodiment of a domestic dishwashing appliance  100  that may be configured in accordance with aspects of the present disclosure. As shown in  FIGS. 1 and 2 , the dishwashing appliance  100  may include a cabinet  102  having a tub  104  therein defining a wash chamber  106 . The tub  104  may generally include a front opening (not shown) and a door  108  hinged at its bottom  110  for movement between a normally closed vertical position (shown in  FIGS. 1 and 2 ), wherein the wash chamber  106  is sealed shut for washing operation, and a horizontal open position for loading and unloading of articles from the dishwasher. As shown in  FIG. 1 , a latch  112  may be used to lock and unlock the door  108  for access to the chamber  106 . 
     As is understood, the tub  104  may generally have a rectangular cross-section defined by various wall panels or walls. For example, as shown in  FIG. 2 , the tub  104  may include a top wall  160  and a bottom wall  162  spaced apart from one another along a vertical direction V of the dishwashing appliance  100 . Additionally, the tub  104  may include a plurality of sidewalls  164  (e.g., four sidewalls) extending between the top and bottom walls  160 ,  162 . It should be appreciated that the tub  104  may generally be formed from any suitable material. However, in several embodiments, the tub  104  may be formed from a ferritic material, such as stainless steel. 
     As particularly shown in  FIG. 2 , upper and lower guide rails  114 ,  116  may be mounted on opposing side walls  164  of the tub  104  and may be configured to accommodate roller-equipped rack assemblies  120  and  122  configured for supporting articles for washing within the wash chamber of the tub. Each of the rack assemblies  120 ,  122  may be fabricated into lattice structures including a plurality of elongated members  124  (for clarity of illustration, not all elongated members making up assemblies  120  and  122  are shown in  FIG. 2 ). Additionally, each rack  120 ,  122  may be adapted for movement between an extended loading position (not shown) in which the rack is substantially positioned outside the wash chamber  106 , and a refracted position (shown in  FIGS. 1 and 2 ) in which the rack is located inside the wash chamber  106 . This may be facilitated by rollers  126  and  128 , for example, mounted onto racks  120  and  122 , respectively. As is generally understood, a silverware basket (not shown) may be removably attached to rack assembly  122  for placement of silverware, utensils, and the like, that are otherwise too small to be accommodated by the racks  120 ,  122 . 
     Additionally, the dishwashing appliance  100  may also include a lower spray-arm assembly  130  that is configured to be rotatably mounted within a lower region  132  of the wash chamber  106  directly above the bottom wall  162  of the tub  104  so as to rotate in relatively close proximity to the rack assembly  122 . As shown in  FIG. 2 , a mid-level spray-arm assembly  136  may be located in an upper region of the wash chamber  106 , such as by being located in close proximity to the upper rack  120 . Moreover, an upper spray assembly  138  may be located above the upper rack  120 . 
     As is generally understood, the lower and mid-level spray-arm assemblies  130 ,  136  and the upper spray assembly  138  may generally form part of a fluid circulation assembly  140  for circulating water and dishwasher fluid within the tub  104 . As shown in  FIG. 2 , the fluid circulation assembly  140  may also include a pump  142  located in a machinery compartment  144  located below the bottom wall  162  of the tub  104 , as is generally recognized in the art. Additionally, each spray-arm assembly  130 ,  136  may include an arrangement of discharge ports or orifices for directing washing liquid onto dishes or other articles located in rack assemblies  120  and  122 , which may provide a rotational force by virtue of washing fluid flowing through the discharge ports. The resultant rotation of the lower spray-arm assembly  130  provides coverage of dishes and other dishwasher contents with a washing spray. 
     The dishwashing appliance  100  may be further equipped with a controller  146  configured to regulate operation of the dishwasher  100 . The controller  146  may generally include one or more memory devices and one or more microprocessors, such as one or more general or special purpose microprocessors operable to execute programming instructions or micro-control code associated with a cleaning cycle. The memory may represent random access memory such as DRAM, or read only memory such as ROM or FLASH. In one embodiment, the processor executes programming instructions stored in memory. The memory may be a separate component from the processor or may be included onboard within the processor. 
     The controller  146  may be positioned in a variety of locations throughout dishwashing appliance  100 . In the illustrated embodiment, the controller  146  is located within a control panel area  148  of the door  108 , as shown in  FIG. 1 . In such an embodiment, input/output (“I/O”) signals may be routed between the control system and various operational components of dishwashing appliance  100  along wiring harnesses that may be routed through the bottom  110  of the door  108 . 
     Typically, the controller  146  includes a user interface panel/controls  150  through which a user may select various operational features and modes and monitor progress of the dishwasher  100 . In one embodiment, the user interface  150  may represent a general purpose I/O (“GPIO”) device or functional block. Additionally, the user interface  150  may include input components, such as one or more of a variety of electrical, mechanical or electro-mechanical input devices including rotary dials, push buttons, and touch pads. The user interface  150  may also include a display component, such as a digital or analog display device designed to provide operational feedback to a user. As is generally understood, the user interface  150  may be in communication with the controller  146  via one or more signal lines or shared communication busses. 
     Additionally, as shown in  FIG. 2 , a portion of the bottom wall  162  of the tub  104  may be configured as a tub sump portion  152  that accommodates a filter assembly  154  configured to remove particulates from the fluid being recirculated through the wash chamber  106  during operation of the dishwashing appliance  100 . For example, fluid collected within the tub sump portion  152  of the bottom wall  162  may be passed through the filter assembly  154  and then diverted back to the pump  142  for return to the wash chamber  106  by way of the fluid recirculation assembly  140 . 
     Moreover, as shown in  FIG. 2 , the dishwashing appliance  100  may also include a heating element  200  provided in operative association with the tub  104  for providing heat energy during a wash, rinse, and/or drying cycle to, for example, heat the fluid introduced into wash chamber  106  and/or to assist with drying articles. As will be described in greater detail below, heating element  200  may be configured (e.g. using controller  146 ) to operate in a manner that facilitates the monitoring of current leakage from heating element  200  to ground. 
     It should be appreciated that the present subject matter is not limited to any particular configuration, model, or style of dishwashing appliance. The exemplary embodiment depicted in  FIGS. 1 and 2  is simply provided for illustrative purposes only. For example, different locations may be provided for the user interface  150 , different configurations may be provided for the racks  120 ,  122 , and other differences may be applied as well. 
       FIG. 3  depicts an example heating element protection system  202  implemented in a dishwashing appliance  100  according to example embodiments of the present disclosure. Heating element protection system  202  can include a resistive heating element  200 . As indicated above, heating element  200  can include a resistance-type wire encased in a magnesium oxide-filled, metallic sheath. It will be appreciated that other suitable materials may be used to fill the sheath, such as for instance, various suitable ceramics. The sheath is coupled to earth ground  204 , or other reference potential. In particular, heating element  200  can be a resistive heating element that converts electricity into heat by providing resistance to the applied signal as the signal flows through the resistance wire of the heating element. Heating element  200  can further be coupled to a control system  206 . Control system  206  can include, for instance, controller  146 , and/or various other suitable circuit configurations for detecting current leakage. 
     Control system  206  can be configured to control the operation of heating element  200 . In example embodiments, control system  206  can be configured to control a sequence of operations associated with heating element  200  to monitor and detect current leakage, for instance by configuring various relays and/or switches coupled to heating element  200  to open and close in accordance with example embodiments of the present disclosure. As will be described below, control system  206  can further be configured to protect heating element  200  from overvoltage surges caused by, for instance, lightning strikes. 
       FIG. 4  depicts an example circuit configuration  210  of a heating element protection system. For instance, circuit configuration  210  can correspond to heating element protection system  202 . Circuit configuration  210  includes an AC power supply  212  and a heating element  200  having a resistance-type wire  205  encased in a metallic sheath  207  coupled to earth ground  204 . Although  FIG. 4  depicts a 120 volt power supply, it will be appreciated that various other suitable power supplies can be used. Circuit configuration  210  further includes a control system  206  coupled between power supply  212  and heating element  200 . As indicated above, control system  206  can be configured to control the operation of heating element  200 . In particular, control system  206  can control the operation of heating element  200  by sending command signals to line relay  214  and neutral relay  216  to cause the relays to open and close. 
     In this manner, control system  206  can control a sequence of operations of line relay  214  and neutral relay  216 . The sequence of operations can be controlled to facilitate detection of a leakage current present in circuit configuration  210 . In particular, a leakage current can be detected by comparing current at the line conductor with current at the neutral conductor. If the two currents are equal, it can be assumed that there is no leakage current flowing from heating element  200  to earth ground  204 . A leakage current can be identified when the line conductor current is different than the neutral conductor current. In particular, the leakage current can be equal, or nearly equal, to the difference between the line conductor current and the neutral conductor current. 
     In example embodiments, circuit configuration  210  can further include metal oxide varistors  217 . Varistors  217  can be used in conjunction with gas discharge tube  218  to suppress overvoltage surges in circuit configuration  210 . Because the sheath of heating element  200  is coupled to earth ground, heating element  200  can be susceptible to failure due to lightning strikes. Varistors  217  and gas discharge tube  218  can suppress overvoltage surges due to such lightning strikes. Circuit configuration  210  can further include fuse  219 . Fuse  219  can “blow” causing a break in the circuit if the current flowing through fuse  219  exceeds a fuse threshold. Accordingly, fuse  219  can further protect circuit configuration  219  from excessive current. 
       FIG. 5  depicts an example sequence of operations  221  of heating element  200  according to example embodiments of the present disclosure.  FIG. 5  depicts a relevant portion of circuit configuration  210 , including heating element  200 , line relay  214 , neutral relay  216 , and earth ground  204 .  FIG. 5  further depicts a ground fault  220  located proximate the neutral conductor. As depicted in  FIG. 5 , the sequence of operations  221  can include state  222  and state  224 . During state  222 , both line relay  214  and neutral relay  216  can be closed, and current can flow through the resistance wire of heating element  200 , which can cause an increased temperature of heating element  200 . When configured in state  222 , leakage current caused by ground fault  220  can be difficult to detect due to the voltage drop across the resistance wire of heating element  200 . 
     To facilitate improved detection of the leakage current, line relay  214  and neutral relay  216  can subsequently be configured in state  224 . During state  224 , line relay  214  can be closed and neutral relay  216  can be opened. As described above, when configured in state  224 , no current flows through the resistance wire of heating element  200  to the neutral conductor, but heating element has a potential of 120 volts relative to ground  204 . If there is a ground fault present in heating element  200  (e.g. ground fault  220 ), the magnitude of the leakage current flowing from the resistance wire of heating element  200  to ground  204  will be increased (compared to the leakage current present in state  222 ). Such increased leakage current can be more easily detected. In particular, such leakage current can be detected regardless of the position of the ground fault on heating element  200 . 
     In example embodiments, line relay  214  and neutral relay  216  can be configured in state  224  for a predetermined period of time. For instance, the predetermined period of time can be in a range of about 20 milliseconds to about 60 milliseconds. It will be appreciated that other suitable periods of time can be used, such as any period of time in which a current leakage can be detected. Subsequent to the predetermined period of time, line relay  214  and neutral relay  216  can be configured in a different state, for instance, in accordance with a previously scheduled cycle, or as otherwise desired by a user. 
     It will be appreciated that the teachings of the present disclosure can be implemented at various times during the operation of a dishwasher. For instance, such teachings can be implemented at the beginning of (or immediately prior to) an energize cycle of heating element  200 , and/or at the end of (or immediately after) the energize cycle. As another example, such teachings can be implemented upon the opening of the dishwasher door at any point during the operation of the dishwasher. It will be further appreciated that the teachings of the present disclosure may be implemented at various other suitable times and/or in response to various other suitable triggers. 
     As indicated above, once a leakage current is detected, the leakage current can be compared to a leakage threshold. For instance, the leakage threshold can be in the range of about 15 milliamps to about 30 milliamps. The leakage threshold can be comprise various other suitable current amounts, such as for instance, an amount of current in the range of about 10 milliamps to about 100 milliamps. If the detected leakage current is greater than the leakage threshold, the operation of the heating element can be ceased. In example embodiments, the operation of the heating element can be ceased by configuring line relay  214  and neutral relay  216  to open. In alternative embodiments, the operation of heating element  200  can be ceased through a software operation implemented by a controller associated with heating element  200 , such as controller  146  of  FIG. 2 . 
     According to alternative embodiments, various other suitable sequences can be used to facilitate detection of current leakage. For instance,  FIG. 6  depicts an example sequence of operations according to an example embodiment of the present disclosure.  FIG. 6 , like  FIG. 5 , depicts a relevant portion of circuit configuration  210 , including heating element  200 , line relay  214 , break relay  216 , and earth ground  204 . In particular,  FIG. 6  depicts a sequence of operations  230  for detecting current leakage at the beginning of an energize cycle of heating element  200 . As shown, in state  232  both line relay  214  and neutral relay  216  are open. Accordingly, no current flows through heating element  200 . To initiate monitoring for current leakage, line relay  214  and neutral relay  216  can then be configured in state  234 . During state  234 , line relay  214  is open and neutral relay  216  is closed. Configuring the relays  214  and  216  in state  234  can be useful, for instance, if the power supply (e.g. power supply  212 ) reverses polarity. In such scenario, the neutral conductor becomes analogous to the line conductor and vice versa. 
     Line relay  214  and neutral relay  216  can then be configured in state  236 . During state  236 , line relay  214  is closed and neutral relay  216  is open. State  236  is analogous to state  224  in  FIG. 5 . As in  FIG. 5  above, the relays  214  and  216  can be configured in state  234  and state  236  each for a predetermined period of time. For instance, the predetermined period of time can be in the range of about 20 milliseconds to about 60 milliseconds. After the predetermined period of time corresponding to state  236 , the energize cycle can begin by configuring line relay  214  and neutral relay  216  in state  238 . During state  238 , both relays are closed and current can flow from the line conductor to the neutral conductor through heating element  200 . 
       FIG. 7  depicts a flow diagram of an example method ( 300 ) of detecting current leakage according to example embodiments of the present disclosure. Method ( 300 ) can be implemented using any suitable system, including, for example, heating element protection system  202  of  FIG. 3 . In addition,  FIG. 7  depicts steps performed in a particular order for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that the various steps of any of the methods disclosed herein can be omitted, adapted, and/or rearranged in various ways. 
     At ( 302 ), method ( 300 ) can include applying an AC signal to a heating element. At ( 304 ), method ( 300 ) can include configuring a line relay coupled to the heating element and a neutral relay coupled to the heating element in a first state. In particular, during the first state, the line relay can be closed and the neutral relay can be opened. Such configuration can facilitate detection of leakage current by increasing a magnitude of the leakage current (relative to the magnitude of a leakage current when the line relay and the neutral relay are both closed). It will be appreciated that the first state can include various other suitable relay arrangements, such as for instance, an arrangement wherein the line relay is open and the neutral relay is closed. 
     At ( 306 ), method ( 300 ) can include identifying a current flowing through the heating element resistance wire to ground (e.g. leakage current). As indicated above, the leakage current can be identified by determining the difference between the current flowing through the line conductor and the current flowing through the neutral conductor. 
     At ( 308 ), method ( 300 ) can include comparing the identified leakage current with a leakage threshold. The leakage threshold can be a value in the range of about 15 milliamps to about 30 milliamps. At ( 310 ), method ( 300 ) can include ceasing the operation of the heating element if the identified current is greater than the leakage threshold. As indicated above, the operation of the heating element can be ceased by configuring both the line relay and the neutral relay to open. In alternative embodiments, the operation of the heating element may be ceased using a software operation implemented by a controller associated with the heating element. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.