US3766437A - Safety interlock system for microwave ovens - Google Patents

Safety interlock system for microwave ovens Download PDF

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Publication number
US3766437A
US3766437A US00240626A US3766437DA US3766437A US 3766437 A US3766437 A US 3766437A US 00240626 A US00240626 A US 00240626A US 3766437D A US3766437D A US 3766437DA US 3766437 A US3766437 A US 3766437A
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United States
Prior art keywords
interlock
sensor
sensor means
circuit
interlock switch
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Expired - Lifetime
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US00240626A
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English (en)
Inventor
R Fritts
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Goodman Co LP
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Amana Refrigeration Inc
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/6414—Aspects relating to the door of the microwave heating apparatus
    • H05B6/6417—Door interlocks of the microwave heating apparatus and related circuits
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
    • H01H47/00—Circuit arrangements not adapted to a particular application of the relay and designed to obtain desired operating characteristics or to provide energising current
    • H01H47/002—Monitoring or fail-safe circuits

Definitions

  • ABSTRACT A safety interlock system is disclosed for microwave SWITCH (B) 7y V f Iowa ovens or other high voltage electrical apparatus incor- [22] Filed: Apr. 3, 1972 porating sensor means for detecting any malfunction. Means are provided for interrupting line input power [21] Appl' 240626 or actuating a failure indicating device or a combination of both. Thermally actuated means, such as bime- [52] US. Cl. 317/40 R, 317/40 A, 340/275 tallic or meltable elements, as well as circuit breakers, [51] Int. Cl. H02h 5/04 r lays and fuses are escribed.
  • the sensor means are [58] Field of Search 317/16, 40 R, 40 A; associated with each of the interlocks and do not carry 340/275, 421, 274; 335/160 regular equipment-load current until such time as a malfunction of the companion interlock occurs.
  • Re- [56] References Cited placement of the faulty equipment by authorized UNITED STATES PATENTS personnel and manual resetting of the interrupt or indicating means before the oven is energized again rei2 shall be used to the apparatus.
  • the invention relates to electrical apparatus and, particularly, to an interlock system for safe operation of microwave ovens.
  • a source of microwave energy utilized in microwave ovens is the magnetron which is now well known in the art.
  • Such generators provide for radiation of energy within an enclosure to heat any article disposed therein. Radiation of such energy outside of the enclosure in certain instances may constitute harmful exposure and safety standards have been established by regulatory agencies in this area.
  • Magnetron energy generators conventionally operate from regular line sources of low frequency and low voltage which is stepped up to very high DC rectified voltages of approximately 4,000 to 6,000 volts.
  • interlock switches operatively associated with the oven door and/or manual latches have evolved in the prior art to interrupt the line power and, particularly, to interrupt the radiation of energy until the door to the enclosure is closed. Further, such interlock switches prevent serious damage to the expensive equipment and are intended with associated electrical circuitry to substantially reduce any hazards associated with the high voltage supplies.
  • interlock switches which are continually carrying currents of substantial magnitude fail by welding of the contacts in a closed position to render them completely ineffectual as a safety device.
  • interlock switches In numerous embodiments of microwave ovens plural interlock switches are employed and in some instances additional ones may be employed. It is possible for more than one of these safety switches to fail which further amplifies the problem of providing for safe operation.
  • an interlock switch system is provided with each of the interlock means having an associated companion sensor means which does not carry the full equipment-load current when the oven is operative.
  • the sensor means detect any malfunction of the interlock means and in the illustrative embodiments to be hereinafter described, either actuate failure warning means or interrupt the input power or a combination of both.
  • Thermally actuated latching means are provided in one embodiment to carry the full input voltage in the event that the interlock switch has failed closed by reason of the welding of the contacts.
  • the heating of the thermal means causes associated contacts or a relay to open to immediately disable the oven.
  • the thermal actuated means are replaced only by qualified service personnel. Structure is also provided for a manual reset to prevent operation until such repairs are effected.
  • failure warning means such as, for example, visible or audible signal alarm devices or a combination of both.
  • failure warning means such as, for example, visible or audible signal alarm devices or a combination of both.
  • Such alarms also permit safe operation of microwave ovens by persons deprived of either sight or audio perception.
  • Other combinations involve the interruption of the application of line power upon the sensing of an interlock failure as well as provisions for indicating such failures to prevent further unsafe operation.
  • Meltable elements such as thermal limiters or fuses, as well as relays, circuit breakers and the like, can all be utilized in the disclosed systems.
  • the structure of the invention can be employed singularly with an interlock switch structure. Once the interlock fails, however, the interruption of current or alarm signalling does not occur until the door is opened again. To give the best possible protection it is, therefore, desirable to provide for plural interlock and sensing means. If one interlock fails, the other actuates the failure warning means and permits detection and repair of the defective interlock before further operation is resumed.
  • Relay contact means in series with the input voltage source and the warning and sensing means will provide for combined notification of the failure as well as disabling of the circuits at the time of the failure rather than a latter period of time. Further improvements in the embodiments of the invention are described including circuit breakers with current limiting means or fuses which will disable the circuit upon interlock failure. All the foregoing embodiments of the invention may be practiced on existing microwave oven equipments or any electrical apparatus simply and inexpensively.
  • FIG. 1 is a block diagram of a microwave oven electrical circuit embodying the invention
  • FIG. 2 is an isometric view of a portion of a microwave oven embodying the invention
  • FIG. 3 is a side elevation view partially broken away of the embodiemnt shown in FIG. 2 disclosing a door actuated interlock;
  • FIGS. 4 8 are schematic circuit diagrams of illustrative embodiments of the invention.
  • FIGS. 9 and 10 are detailed cross-sectional views of a thermal actuated limiter device for use in the embodiments of the invention.
  • FIG. 1 a block diagram of the electrical circuit is shown for a microwave oven incorporating the safety interlock switch means, means for sensing and means for interruption of line input power or actuation of a oven enclosure as well as the high voltage electrical circuits, controls and microwave energy source.
  • Control panel 18 has a -minute timer switch 20 as well as 30- minute switch 22 mounted on the front face.
  • Control buttons 24, 26 and 28 provide for, respectively, start, stop and light control.
  • the source of electromagnetic energy is the magnetron 30 and its accompanying high voltage electrical circuits have been indicated by block 32. Both of these components are now considered to be well known in the art and need not be further described.
  • Access to the interior of oven enclosure 12 is provided by means of opening 34 closed by means of door assembly 36 which has been illustrated as of the dropdown type with a bottom hinge 38.
  • the door assembly 36 has an inner cover 40 with a window 42 of a high dielectric loss material to assist in maintaining the cleanliness of the oven as well as providing secondary energy absorbing means for any radiated energy escaping around the periphery of the opening.
  • Latch 44 is mounted on door assembly 36 and engages mating slot 46 in a peripheral front wall 48 surrounding the access opening 34.
  • a mechanically actuated latch-locking arrangement 50 is slidably disposed within the control panel 18.
  • An interlock switch 52 is controlled by latch 44 to break the circuit upon any accidental opening of thedoor assembly 36 while the oven is operated. Movement of' the drop-down'type door assembly 36 is controlled by a pair of spring-tensioned counterbalanced arms 54.
  • FIG. 3 the complete door control apparatus, as well as the safety interlock switches 56, which are contacted by the arms 54 are shown in various positions in a side elevation view with a portion of the outer case 16 broken away to show internal structure.
  • Arms 54 are pivotally supported by means of brackets 58 attached to the side' of the door assmebly 36.
  • Spring 60 is attached to the inner end and bottom wall of the oven to provide the appropriate tensioning.
  • Each of the arms 54 has a hook-shaped portion 62 which in the final closed position engages and depresses lever arm 64 to close the interlock switch 56 contacts which are normally in the open position.
  • Terminal connectors 66, 68 and 70 provide for the connection of the door interlock switch to the appropriate electrical circuit for operation of the microwave oven.
  • a right and left door interlock switch is provided and apropriately mounted by means such as bracket 72 attached to oven enclosure wall 14.
  • the latch interlock switch 52 is supported from the oven enclosure wall 14.
  • a conventional three-terminal connector 74 having a grounded lead 76 is connected to the conventional domestic or industrial line voltage source.
  • a terminal board 78 interconnects all of the components to the line voltages.
  • a fan 92 is provided to cool the magnetron energy generator 30 as well as the high voltage components including transformer 31 and DC rectifier circuit 33.
  • the 5-minute timer motor 94 is controlled by means of switch 96 which is actuated by knob 20 on panel 18.
  • Dial light 98 is also provided in the timer arrangement.
  • the 30-minute timer motor 100 is actuated by switch 102 and is controlled by means of knob 22 on the panel.
  • Dial light 104 is associated with this timer component.
  • the mainline voltages are interconnected to the remaining components in the circuit through thermal circuit breaker 1 06, latch interlock switch 52 and door interlocks 84 and 86 to provide the main safety interlock system.
  • the problem which arises in the provision of interlock switches is that over extended periods of time the contacts may become welded closed and energy will be radiated within the enclosure upon the opening of the door.
  • the present invention provides sensing means as well as failure warning means to eliminate any adverse results of interlock malfunction.
  • the systems embodying the invention provide companion sensor means for detecting the positioning of the interlock which are nonequipment-current bearing during normal operation of the oven. Such sensor means detect an interlock malfuction and are operatively associated with the mainline voltage interruption means or failure warning devices such as an audible or visual alarm signal.
  • the left door'interlock 86 is designated with a lever arm 108 together with a companion lever arm 110 which is oriented in the opposite position.
  • interlock switch 108 When interlock switch 108 is opened, sensor switch 110 is closed and vice versa.
  • a failure warning device 112 is shown coupled to the sensor switch. To give the maximum protection it is advisable to provide both door interlock switches with the sensor and interruption or warning means and the right door interlock switch 114 and sensor switch 116 are shown.
  • FIG. 4 an exemplary interlocksensor switch arrangement is shown employing thermally actuated means for interrupting the main line voltages and protecting the microwave oven 124.
  • the conventional on/off switch 126 is provided and main voltage lines 128 and 130 are coupled to the line voltage source 132.
  • Each of the interlock switches 120, 122 has an associated sensor means 134, 136 for de tecting the positioning of the interlock switches.
  • Circuit interruption means are coupled to the main line voltage connections 128 and 130 and comprise thermally actuated latching switches 138 and 140, each having an associatedthermal element 142, 144.
  • the safety interlock system operates in the following manner. Interlock switches 120, 122 are in the open position and sensor means 134, 136 are then in'the closed position. When the on/off switch 126 is actuated and the oven door is closed, interlock switches 120, 122 close and sensor means 134, 136 open. Hence, when the equipment load is-energized the interlock switches carry the main line voltagesand the sensor means are noncurrent-carrying. In the event of an interlock malfuction such as the welding of the contacts and the sensor means 134, 136 remain closed, voltage will be applied through thermal elements 142 or 144,
  • thermal limiters 146, 148 together with heaters 150, 152 which will result in disintegration of an element in the thermal limiters upon an interlock malfunction.
  • similar components have been designated by similar reference numerals.
  • an interlock failure which results in the line power being applied through the sensor means 134, 136 is applied to a heater or resistor which will rapidly disintegrate its associated thermal limiter element and break the circuit.
  • a newthermal limiter would be installed at the time the necessary repairs are made.
  • thermal limiter protector for utilization in this embodiment for components 146, 148, 150 and 152 is disclosed in a copending patent application, Ser. No. 239,480 filed Mar. 30, 1972, now US. Pat. No. 3,717,793 issued Feb. 29, 1973 by Donald E. Peterson. Such devices have been illustrated in FIGS. 9 and in-the before and after firing condition.
  • thermal limiter devices 154 comprise a conductive lead 156 encased in a sealing compound 158 secured at one end of a conductive case 160 while lead 162 is disposed at the opposing end and is conductively secured to the case 160.
  • a pellet 164 of a non-conductive material which rapidly disintegrates at a predetermined temperature is disposed in contact with a disc member 166 to urge spring 168 against disc member 170.
  • a metallic star contact member 172 is in contiguous relationship with disc member 170 and the interior walls of the conductive case 160 as well as the inner end of lead 156.
  • a trip spring 174 urges the contact member 172 in the opposite direction and is maintained in position by a ceramic bushing 176, though which lead 156 is disposed.
  • Electric isolation means such as insulating tape 178 has disposed adjacent its outer wall thermal means 180 such as a heater wire or electrical resistor which is connected to the branch circuit with the sensor means 134, 136 while the leads 156, 162 are connected to the main voltage lines 128, 130.
  • the thermal limiter leads carry the main equipment current and the means disposed in thermal contact with the thermal limiter are actuated upon failure of the interlocks.
  • the fired condition of the thermal limiters is illustrated which disables the oven circuits upon an interlock malfunction.
  • the pellet 164 has been disintegrated and lead 156 no longer contacts star contact member 172.
  • Spring 168 is fully extended with disc member 166 now positioned against the interior of the connecting lead 162. The intervening space prevents the conduction of any current and effectively disables the circuit.
  • failure warning means 182 and 184 in series with the interlock position sensor means 134 and 136.
  • Such warning devices may be of any nature, as for example, lights or audible sounding devices or a combination of both. Such devices will also assist in the safe operation of microwave ovens by persons deprived of either sight or sound perception.
  • no power-interrupt means have been illustrated, however, it is realized that in the practice of the invention a combination of both the interrupt and failure warning means may be desirable.
  • FIG. 7 a combination of interrupt and warning means are illustrated including relays 186, 188 connected in parallel across each of the interlock failure warning devices 182, 184.
  • the contacts of the relays are in series with the line voltage source and connections 128, 130.
  • An alternative method of providing the same protection comprises a single relay with two sets of coils and two sets of contacts. Such a structure would conserve space and be less expensive. Latching or locking means may also be provided to maintain the relay contacts open in order that the equipment cannot be operated until the defective equipment is replaced.
  • FIG. 8 another alternative embodiemnt of the in- 'vention is illustrated incorporating means for interrupting the application of the line voltages.
  • a circuit breaker 190 or fuse is connected in the main line voltage connections 128, 130.
  • the sensor means 134, 136 apply sufficient current through the circuit breaker upon an interlock malfunction to cause the opening of the contacts or a melting of the fuse element.
  • Resistances 192 and 194 may also be provided in series with the sensor means to limit the current flowing through these branches of the circuit and thereby protect the equipemnt from damage.
  • a safety interlock system for electrical apparatus comprising:
  • an electrical circuit including a connected voltage source and interlock switch means actuated by predetermined means;
  • circuit deenergizing means comprise thermally actuated means operatively associated with said sensor means means coupled in series with said sensor means and to deenergize said circuit upon the detection of an said voltage source. unsafe operating condition with both said interlock 3.
  • cirswitch and sensor means in closed position; and cuit deenergizing means comprise a thermally disinte- 10 warning means for indicating the occurrence of said gratable element coupled in series with said sensor unsafe operating condition. means.
  • cuit deenergizing means comprise a thermally disinte- 10 warning means for indicating the occurrence of said gratable element coupled in series with said sensor unsafe operating condition. means.
  • said 4. A safety interlock system for electrical apparatus warning means are audible. comprising: 6. The system according to claim 4 wherein said an electrical circuit including a connected voltage warning means are visible.
  • source and interlock switch means actuated by pre-

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Electric Ovens (AREA)
  • Control Of High-Frequency Heating Circuits (AREA)
  • Constitution Of High-Frequency Heating (AREA)
  • Rotary Switch, Piano Key Switch, And Lever Switch (AREA)
US00240626A 1972-04-03 1972-04-03 Safety interlock system for microwave ovens Expired - Lifetime US3766437A (en)

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US24062672A 1972-04-03 1972-04-03

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US (1) US3766437A (ja)
JP (1) JPS4915040A (ja)
AU (1) AU476734B2 (ja)
BE (1) BE797670A (ja)
CA (1) CA995309A (ja)
CH (1) CH571687A5 (ja)
DK (1) DK138620B (ja)
FR (1) FR2178942B1 (ja)
GB (1) GB1407306A (ja)
NL (1) NL7304194A (ja)
ZA (1) ZA731338B (ja)

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS50152333A (ja) * 1974-05-27 1975-12-08
US3959984A (en) * 1974-11-29 1976-06-01 Lennox Industries Inc. Motor protection apparatus using fusible links
US4101750A (en) * 1977-05-31 1978-07-18 Whirlpool Corporation Door interlock system for microwave oven
US4117294A (en) * 1975-06-12 1978-09-26 U.S. Philips Corporation Safety arrangement for microwave ovens
US4232210A (en) * 1977-06-22 1980-11-04 Tokyo Shibaura Electric Co., Ltd. Output power control system for microwave ovens
US4663508A (en) * 1984-12-20 1987-05-05 Matsushita Electric Industrial Co., Ltd. Combination thermal and mechanical safety switch for microwave heating oven
US4853607A (en) * 1986-09-29 1989-08-01 Black & Decker Inc. Non-isolated thermally responsive battery charger
EP0338315A2 (fr) * 1988-04-21 1989-10-25 Moulinex Four à micro-ondes
US20160270154A1 (en) * 2015-03-11 2016-09-15 Lg Electronics Inc. Cooking appliance and control method of the same
US10396542B2 (en) * 2015-12-08 2019-08-27 Dartpoint Tech. Co., Ltd. Bath safety control system and bath safety control method

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3829809A (en) * 1973-03-26 1974-08-13 Micro Devices Corp Thermal limiter construction and electrical switch and system utilizing the same
JPS5339883Y2 (ja) * 1974-04-27 1978-09-27
JPH06187556A (ja) * 1992-10-22 1994-07-08 Fuji Electric Co Ltd 自動販売機およびその扉スイッチ装置
JP3288234B2 (ja) * 1996-11-29 2002-06-04 株式会社東芝 加熱調理器

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2718630A (en) * 1954-08-18 1955-09-20 Norman Bauch Automatic door locking mechanism with delayed alarm means for dispensing machines
US2855235A (en) * 1956-07-06 1958-10-07 Koutesy Kontrol Company Door lock assembly for rest room control apparatus

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3321604A (en) * 1964-02-03 1967-05-23 Sunbeam Corp Electronic oven
DE2017422C3 (de) * 1970-04-11 1979-12-06 Arburg Maschinenfabrik Hehl & Soehne, 7291 Lossburg Selbsttätige Sicherheitsschaltung zur Überwachung des Leistungsschaltgliedes eines elektrischen Stromverbrauchers
US3656023A (en) * 1970-09-21 1972-04-11 Joseph M Hadfield Protective circuit
US3699300A (en) * 1971-11-30 1972-10-17 Norris Industries System for detecting safety switch failure

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2718630A (en) * 1954-08-18 1955-09-20 Norman Bauch Automatic door locking mechanism with delayed alarm means for dispensing machines
US2855235A (en) * 1956-07-06 1958-10-07 Koutesy Kontrol Company Door lock assembly for rest room control apparatus

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS50152333A (ja) * 1974-05-27 1975-12-08
JPS5344066B2 (ja) * 1974-05-27 1978-11-25
US3959984A (en) * 1974-11-29 1976-06-01 Lennox Industries Inc. Motor protection apparatus using fusible links
US4117294A (en) * 1975-06-12 1978-09-26 U.S. Philips Corporation Safety arrangement for microwave ovens
US4101750A (en) * 1977-05-31 1978-07-18 Whirlpool Corporation Door interlock system for microwave oven
US4232210A (en) * 1977-06-22 1980-11-04 Tokyo Shibaura Electric Co., Ltd. Output power control system for microwave ovens
US4663508A (en) * 1984-12-20 1987-05-05 Matsushita Electric Industrial Co., Ltd. Combination thermal and mechanical safety switch for microwave heating oven
AU571363B2 (en) * 1984-12-20 1988-04-14 Matsushita Electric Industrial Co., Ltd. Microwave heating oven
US4853607A (en) * 1986-09-29 1989-08-01 Black & Decker Inc. Non-isolated thermally responsive battery charger
EP0338315A2 (fr) * 1988-04-21 1989-10-25 Moulinex Four à micro-ondes
FR2630613A1 (fr) * 1988-04-21 1989-10-27 Moulinex Sa Four a micro-ondes
EP0338315A3 (fr) * 1988-04-21 1991-08-21 Moulinex Four à micro-ondes
US20160270154A1 (en) * 2015-03-11 2016-09-15 Lg Electronics Inc. Cooking appliance and control method of the same
US10251216B2 (en) * 2015-03-11 2019-04-02 Lg Electronics Inc. Cooking appliance and control method of the same
US10396542B2 (en) * 2015-12-08 2019-08-27 Dartpoint Tech. Co., Ltd. Bath safety control system and bath safety control method

Also Published As

Publication number Publication date
CH571687A5 (ja) 1976-01-15
DE2315276B2 (de) 1976-09-30
GB1407306A (en) 1975-09-24
DK138620C (ja) 1979-04-09
BE797670A (fr) 1973-07-31
NL7304194A (ja) 1973-10-05
AU5263573A (en) 1974-08-29
ZA731338B (en) 1973-12-19
CA995309A (en) 1976-08-17
JPS4915040A (ja) 1974-02-09
FR2178942B1 (ja) 1976-05-21
FR2178942A1 (ja) 1973-11-16
DE2315276A1 (de) 1973-10-18
AU476734B2 (en) 1976-09-30
DK138620B (da) 1978-10-02

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