Protective control arrangement for duty cycle controlled sheathed heating elements
First Claim
1. A protective control arrangement for a heating unit of the type comprising the concentric assembly of a resistive heating element embedded in an insulating material and enclosed within a metallic sheath, the insulating material in its normal state being characterized by a very high resistance and in its combined molten and plasma state being characterized by a relatively low resistance, the resistive element being adapted for energization by an external power supply, the heating unit being susceptible to a discontinuity developing in the resistive element when energized which can result in an electrical art at the discontinuity, the arc being effective to reduce the insulating material proximate to the arc to its relatively low resistance molten and plasma state to create a relatively low resistance current path from the discontinuity to the sheath and ultimately rupture the sheath;
- the arc and rupture if unchecked propagating along the length of the sheath, the period of propagation being the time between initiation and termination of such propagation, the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten and plasma insulating material to return to a relatively high resistance state upon extinguishing the arc by interruption of power to the heating element, said protective control arrangement comprising;
power level selection means for enabling the user to select a power setting for the heating unit from a plurality of available power settings;
switch means adapted to be operatively connected to the heating element for selectively coupling the heating element to the external power supply;
control means responsive to said power level selecting means and operative to control the duty cycle of the heating element for at least one of said power settings by periodically opening said switch means to decouple the heating element from the power supply for idle periods of predetermined duration, the duration of said idle periods and the period between said idle periods depending upon the power level selected by the user; and
timing means operative to monitor the time between decouplings, which time depends upon the power level selected by the user;
said timing means being reset by said control means in response to the occurrence of each power control decoupling;
said control means being responsive to said timing means and further operative to interrupt power to the heating element for a recovery period of predetermined duration whenever the time between decouplings exceeds a predetermined maximum time, said predetermined maximum time being selected to confine the duration of the period of propagation within predetermined limits, said recover period being at least equal to the nominal recovery time;
whereby the propagation of a relatively low resistance current path from the heating element to the metallic sheath through the insulating material along a substantial portion of the length of the heating unit resulting from the occurrence of a discontinuity in the heating element is prevented.
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Accused Products
Abstract
A protective control arrangement for an apparatus incorporating one or more heating units of the type comprising the concentric assembly of a resistive heating element embedded in an insulating material enclosed within a metallic sheath energized in accordance with a duty cycle power control scheme, which prevents the rupture or separation in the sheath caused by an arc at a discontinuity in the heating element from propagating along a substantial portion of the sheath.
In one embodiment, a microprocessor controlled relay associated with each heating element selectively couples and decouples its associated heating element and the power supply. The microprocessor is preprogrammed to control the duty cycle of each heating element by periodically opening the associated relay switch, thereby decoupling the heating element from the power supply for idle periods of predetermined duration. A timer monitors the time between power decouplings. Whenever the time between power decouplings for any heating element exceeds a predetermined maximum time, the microprocessor decouples power from the heating element for a predetermined recovery period. The maximum time is selected so as to confine the propagation time within a predetermined limit.
6 Citations
13 Claims
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1. A protective control arrangement for a heating unit of the type comprising the concentric assembly of a resistive heating element embedded in an insulating material and enclosed within a metallic sheath, the insulating material in its normal state being characterized by a very high resistance and in its combined molten and plasma state being characterized by a relatively low resistance, the resistive element being adapted for energization by an external power supply, the heating unit being susceptible to a discontinuity developing in the resistive element when energized which can result in an electrical art at the discontinuity, the arc being effective to reduce the insulating material proximate to the arc to its relatively low resistance molten and plasma state to create a relatively low resistance current path from the discontinuity to the sheath and ultimately rupture the sheath;
- the arc and rupture if unchecked propagating along the length of the sheath, the period of propagation being the time between initiation and termination of such propagation, the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten and plasma insulating material to return to a relatively high resistance state upon extinguishing the arc by interruption of power to the heating element, said protective control arrangement comprising;
power level selection means for enabling the user to select a power setting for the heating unit from a plurality of available power settings; switch means adapted to be operatively connected to the heating element for selectively coupling the heating element to the external power supply; control means responsive to said power level selecting means and operative to control the duty cycle of the heating element for at least one of said power settings by periodically opening said switch means to decouple the heating element from the power supply for idle periods of predetermined duration, the duration of said idle periods and the period between said idle periods depending upon the power level selected by the user; and timing means operative to monitor the time between decouplings, which time depends upon the power level selected by the user;
said timing means being reset by said control means in response to the occurrence of each power control decoupling;said control means being responsive to said timing means and further operative to interrupt power to the heating element for a recovery period of predetermined duration whenever the time between decouplings exceeds a predetermined maximum time, said predetermined maximum time being selected to confine the duration of the period of propagation within predetermined limits, said recover period being at least equal to the nominal recovery time;
whereby the propagation of a relatively low resistance current path from the heating element to the metallic sheath through the insulating material along a substantial portion of the length of the heating unit resulting from the occurrence of a discontinuity in the heating element is prevented. - View Dependent Claims (2, 3, 4)
- the arc and rupture if unchecked propagating along the length of the sheath, the period of propagation being the time between initiation and termination of such propagation, the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten and plasma insulating material to return to a relatively high resistance state upon extinguishing the arc by interruption of power to the heating element, said protective control arrangement comprising;
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5. A protective control arrangement for a heating unit of the type comprising the concentric assembly of a resistive heating element embedded in an insulating material and enclosed within a metallic sheath, the insulating material in its normal state being characterized by a very high resistance and in its molten state being characterized by a relatively low resistance the resistive element being adapted for energization by an external power supply, the heating unit being susceptible to a discontinuity developing in the resistive element when energized which can result in an electrical arc at the discontinuity, such arc being effective to reduce the insulating material proximate the arc to its relatively low resistance molten and plasma state, thereby providing a relatively low resistance current path from the discontinuity to the sheath ultimately rupturing the sheath, the rupture if unchecked then propagating along the length of the heating unit, the heating unit having associated therewith a nominal propagation time required for the rupture to propagate along a substantial portion of the length of the heating unit;
- the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten insulating material to return to a relatively high resistance hardened state upon extinguishing the arc by interruption of power to the heating element, said control arrangement comprising;
switch means adapted to be operatively connected to the heating element for selectively coupling the heating element to the external power supply; and control means operative to control the duty cycle of the heating element in accordance with the user selected power setting by periodically opening said switch means for idle periods of predetermined duration, thereby decoupling the heating element from the power supply, the duration of and time between idle periods depending upon the selected power setting; counter means operative to repetitively count a predetermined number of cycles of the voltage signal from the external power supply and reset, said predetermined number corresponding to a predetermined time period less than the nominal propagation time; said control means being effective to reset said counter means in response to each opening of said switch means, the time between said openings depending upon the selected power setting; said control means being responsive to said counter means and effective to interrupt power to the heating element for a predetermined recovery period whenever said counter means counts said predetermined number of cycles, the duration of each of said interrupt periods being at least approximately equal to the nominal recovery time for the insulating material, whereby the propagation of a rupture in the sheath along the length of the heating unit resulting from the occurrence of a discontinuity in the heating element is prevented. - View Dependent Claims (6, 7)
- the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten insulating material to return to a relatively high resistance hardened state upon extinguishing the arc by interruption of power to the heating element, said control arrangement comprising;
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8. A protective control arrangement for an appliance employing a plurality of heating units, each heating unit being of the type comprising the concentric assembly of a resistive heating element embedded in an insulating material and enclosed within a metallic sheath, the insulating material in its normal state being characterized by a very high resistance and in its molten and plasma state being characterized by a relatively low resistance, the resistive element being adapted for energization by a voltage signal from an external alternating current power supply, the heating unit being susceptible to a discontinuity developing in the resistive element when energized which can result in an electrical arc at the discontinuity, the arc being effective to reduce the insulating material proximate to the arc to its relatively low resistance molten and plasma state to create a relatively low resistance current path from the discontinuity to the sheath and ultimately rupture the sheath, the arc and rupture if unchecked propagating along the length of the sheath, the period of propagation being the time between initiation and termination of such propagation, the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten and plasma insulating material to return to a relatively high resistance state upon extinguishing the arc by interruption of power to the heating element, said control arrangement comprising:
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a plurality of sheathed heating units, each comprising the concentric assembly of a metallic sheath, insulating material within said sheath and a resistive heating element embedded in said insulating material; power level selection means for enabling the operator to select one of a plurality of power settings for each of said heating units; a plurality of pilot relays, each being connected in electrical series with an associated one of said plurality of heating elements;
to form a plurality of pilot relay and heating element combinations;each of said pilot relay and heating element combinations being electrically connected in parallel with each of said other relay and heating element combinations, thereby forming a parallel array of such combinations; a master relay serially operatively coupled to said parallel array effective to couple said parallel array to the external power supply; a commutating diode coupled in parallel with said master relay; control means for controlling the switching of said master and pilot relays in response to said power settings selected by the operator for each of said heating units; said control means being operative to switch said pilot relays in synchronization with the voltage signal from the external power supply by performing said switching in the following sequence; (a) said master relay is opened during a first half-cycle of the voltage signal during which said commutating diode is forward biased; (b) the state of said pilot relay is changed during the next occurring half-cycle of the voltage signal; and (c) said master relay contact is closed during the third successive half-cycle of said power signal; counter means operative to repetitively count a predetermined number of cycles of the voltage signal from the external power supply and reset; said control means including means responsive to said counter means operative to open said master relay means for a recovery period of predetermined duration whenever said counter means counts said predetermined number of cycles, said predetermined number of cycles being selected to confine the duration of the period of propagation within predetermined limits; said counter means being arranged to reset regardless of its count in response to said control means whenever said master relay is opened, the time between openings of said master relay being a function of the power settings selected for said heating elements, thereby preventing the propagation of a relatively low resistance current path from the heating element to the metallic sheath of the heating unit through the molten insulating material along a substantial portion of the length of the heating unit resulting from the occurrence of a discontinuity in the heating element. - View Dependent Claims (9, 10, 11)
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12. A protective method for sheathed resistance heating units of the type having associated therewith a nominal breakdown time required for an arc created by a discontinuity in the heating element to create a relatively low resistance current path from the discontinuity to the sheath employed in an apparatus of the type in which power control is accomplished by periodically decoupling the heating unit from the power supply, the period between decouplings being determined according to the power level setting selected by the operator from a plurality of available power settings, the method comprising the steps of:
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monitoring the time duration of the period between successive power control decouplings for each heating unit, which duration varies according to the selected power level setting; and interrupting power to the heating element for a predetermined interrupt period whenever the time between successive power decouplings at least equals the predetermined breakdown time, the interrupt period being of sufficient duration to extinguish the arc and restor a high resistance between the discontinuity and the sheath.
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13. A protective method for limiting the propagation of a disruption along the length of the sheath resulting from a discontinuity in the heating element for heating units of the type comprising the concentric assembly of a resistive heating element adapted for energization by an external power supply, embedded in an insulating material and enclosed within a metallic sheath adapted for electrical grounding, the insulating material having in its normal state a very high resistance and in its combined molten and plasma state a relatively low resistance, the heating unit being susceptible to a discontinuity developing in the resistive element which can result in an electrical arc at the discontinuity, the arc being effective to reduce the insulating material proximate to the arc to its relatively low resistance molten and plasma state to create a relatively low resistance current path from the discontinuity to the sheath, and ultimately rupture the sheath, the arc and resultant if unchecked propagating along the length of the sheath, the period of propagation being the time between initiation and termination of such propagation, the insulating material having associated therewith a nominal recovery time required to allow the resultant relatively low resistance molten and plasma insulating material to return to a relatively high resistance state following extinguishing of the arc by interruption of power to the heating element, such heating units being in an appliance employing duty cycle power control such that power to the heating element is periodically interrupted for certain power settings, the time between power control interrupting a function of the power level selected, the method comprising the steps of:
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timing the period between power control interruptions for each heating element; resetting the timing upon the occurrence of each power control interruption; and interrupting the application of power to the heating element for an interrupt period of duration at least equal to the nominal recovery time whenever the time between interruptions of power to the heating element equals a predetermined time, the predetermined time being selected to confine the duration of the period of propagation within a predetermined limit.
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Specification