U.S. patent number 3,816,688 [Application Number 05/337,256] was granted by the patent office on 1974-06-11 for safety interlock system for microwave ovens.
This patent grant is currently assigned to Amana Refrigeration, Inc.. Invention is credited to Rex E. Fritts.
United States Patent |
3,816,688 |
Fritts |
June 11, 1974 |
SAFETY INTERLOCK SYSTEM FOR MICROWAVE OVENS
Abstract
A safety interlock system is disclosed for microwave ovens or
other high voltage electrical apparatus incorporating sensor means
for detecting any malfunction. Means are provided for interrupting
line input power or actuating a failure indicating device or a
combination of both. Thermally actuated means, such as bimetallic
or meltable elements, as well as circuit breakers, relays and fuses
are described. The sensor means are associated with each of the
interlocks and do not carry regular equipment load current until
such time as a malfunction of the companion interlock occurs.
Replacement of the faulty equipment by authorized personnel and
manual resetting of the interrupt or indicating means before the
oven is energized again reduce the risk of accidental radiation,
electrical shock or damage to the apparatus.
Inventors: |
Fritts; Rex E. (Cedar Rapids,
IA) |
Assignee: |
Amana Refrigeration, Inc.
(Amana, IA)
|
Family
ID: |
26933577 |
Appl.
No.: |
05/337,256 |
Filed: |
March 1, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
240626 |
Apr 3, 1972 |
3766437 |
Oct 16, 1973 |
|
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Current U.S.
Class: |
219/723; 361/72;
219/720; 340/687 |
Current CPC
Class: |
H05B
6/6417 (20130101); H01H 47/002 (20130101) |
Current International
Class: |
H01H
47/00 (20060101); H05B 6/66 (20060101); H05B
6/76 (20060101); H05b 009/06 (); H02h 005/04 () |
Field of
Search: |
;219/10.55 ;317/4R,4A,16
;340/275,421,274 ;335/160 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Jaeger; Hugh D.
Attorney, Agent or Firm: Rost; Edgar O. Murphy; Harold A.
Pannona; Joseph D.
Parent Case Text
This is a division of application Ser. No. 240,626 filed Apr. 3,
1972 now U.S. Pat. No. 3,766,437 issued Oct. 16, 1973.
Claims
I claim:
1. Microwave oven apparatus comprising:
a conductive enclosure having an access opening;
a door closing said access opening;
a microwave energy generator;
an electrical circuit for energizing said generator including
safety interlock switch means;
said circuit including a voltage source;
sensor means for detecting the malfunction of said interlock switch
with said interlock switch means normally closed and said sensor
means normally open when said apparatus is operative; and
means operatively associated with said sensor means to deenergize
said generator upon detection of an unsafe operating condition with
both said interlock switch and sensor means in closed
positions.
2. Microwave oven apparatus according to claim 1 and warning means
for indicating the occurrence of said unsafe interlock switch and
sensor means condition.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to electrical apparatus and, particularly, to
an interlock system for safe operation of microwave ovens.
2. Description of the Prior Art
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. To provide for safe operation of such
apparatus, 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.
Over extended periods of time, it has been noted that 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. 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.
A need arises, therefore, with the increased population of
microwave ovens for an interlock system which will immediately
disable the circuit upon the failure of any interlock means and
signal such failure for repair by authorized personnel before a
potentially dangerous condition arises.
SUMMARY OF THE INVENTION
In accordance with the invention, 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 disable the oven. The thermal
actuated means are reset or replaced only by qualified service
personnel. Structure is also provided for a manual reset to prevent
operation until such repairs are effected.
Another embodiment provides for the actuation of 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 would provide the
protection desired and the failure warning means or power
interruption means would be actuated permitting 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 later 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.
BRIEF DESCRIPTION OF THE DRAWINGS
Details of the illustrative embodiments of the invention will be
readily understood after consideration of the following description
and reference to the accompanying drawings, wherein:
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
embodiment shown in FIG. 2 disclosing a door actuated
interlock;
FIGS. 4 - 8 are schematic circuit diagrams of illustrative
embodiments of the invention; and
FIGS. 9 and 10 are detailed cross-sectional views of a thermal
actuated limiter device for use in the embodiments of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In 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 failure warning device. A microwave oven 10
incorporating the invention will now be described with reference
being directed to FIGS. 2 and 3. A hollow cavity enclosure 12 is
defined by conductive walls 14 with the dimensions of the enclosure
being selected to excite numerous modes of microwave energy at
desirably a frequency of 2450 MHz which is one of the allotted
frequencies for such devices. Case 16 surrounds the oven enclosure
as well as the high voltage electrical circuits, controls and
microwave energy source. Control panel 18 has a 5-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 drop-down 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 the door 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.
In 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 assembly 36. A bearing member 59
of a low-friction, long-wearing material, such as Nylon, provides a
riding surface for each arm. 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 may be provided and appropriately
mounted by means such as bracket 72 attached to oven enclosure wall
14. Similarly, the latch interlock switch 52 is supported from the
oven enclosure wall 14.
Referring again to FIG. 1, the electrical circuit of a microwave
oven will be described. 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. Light
switch 80 controlled by the previously-described button 28 actuates
an oven light 82 to illuminate the interior of the oven enclosure.
A right and a left door interlock switch and sensor arrangement 84,
86 in accordance with the teachings of the invention is
interconnected to start-stop switch 88 which is also actuated by
the panel mounted buttons 24, 26 previously described. Conventional
means for the distribution of the microwave energy, such as a mode
stirrer, is actuated by means of motor 90. 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 five-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 106, 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. To achieve these objective 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 malfunction and are
operatively associated with the mainline voltage interruption means
or failure warning devices such as an audible or visual alarm
signal. In FIG. 1 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. When interlock switch 108 is
opened, sensor switch 110 is closed and vice versa. In addition, 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.
Referring now to FIG. 4, an exemplary interlock-sensor 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 detecting 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 associated
thermal 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 voltages and the sensor means are noncurrent-carrying. In
the event of an interlock malfunction such as the welding of the
contacts and the sensor means 134, 136 remain closed, voltages will
be applied through thermal elements 142 or 144, dependent on the
respective defective interlock switch, causing heating. At a
predetermined temperature switch contacts 138 or 140 open thereby
disabling the complete circuit. The first interlock switch to fail
and its associated sensor means will disable the circuit. It is
desirable that the contacts remain open until the required repairs
are made to prevent unsafe operation. For this reason latching
means are provided in the thermally actuated switches 138, 140
require a manual resetting after the necessary repairs have been
effected. The plural safety interlock system incorporating sensor
and detecting means with each interlock switch is preferable in
view of the fact that input power interruption is automatic when
the door is opened and a malfunction condition exists.
Referring now to FIG. 5, an alternative system is disclosed
employing 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. In this and the remaining
alternative embodiment descriptions, similar components have been
designated by similar reference numerals. In this embodiment 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. In this embodiment a new
thermal limiter would be installed at the time the necessary
repairs are made.
An exemplary 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 March 30,
1972, now U.S. Pat. No. 3,717,793 issued Feb. 20, 1973 by Donald E.
Peterson. Such devices have been illustrated in FIGS. 9 and 10 in
the before and after firing condition. Such 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. In normal operation,
the thermal limiter leads carrying the main equipment current and
the means disposed in thermal contact with the thermal limiter are
actuated upon failure of the interlocks.
In FIG. 10 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.
Referring to FIG. 6, another feature of the invention will be
described relating to the provisions of 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. In this
embodiment, 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.
In 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. In the event of a malfunction of the
interlock, the alarm is presented and power is also interrupted to
deenergize the oven. 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.
In FIG. 8 another alternative embodiment of the invention is
illustrated incorporating means for interrupting the application of
the line voltages. A circuit breaker 190 or fuse is connected in
the mainline 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 equipment from damage.
While the foregoing illustrative embodiments of the invention have
been described with relation to microwave ovens, the interlock
systems are equally applicable to any high voltage electrical
supplies, transmitters or other electrical equipment requiring the
complete functioning of interlocks to protect the operator as well
as the equipment. In the designing of the components for the
various systems a slight delay may be provided in the operation of
the interlock and sensor means to avoid false alarms by having the
interlock open before the sensor means closes. Similarly, the
interlock should close after the sensor means opens. Numerous
substitutions in the components may be practiced by those skilled
in the art without departing from the spirit and scope of the
invention. It is intended, therefore, that the foregoing detailed
description of the illustrative embodiments be considered broadly
and not in a limiting sense .
* * * * *