CA1062340A - Improper load protection for induction heating appliance - Google Patents
Improper load protection for induction heating applianceInfo
- Publication number
- CA1062340A CA1062340A CA229,507A CA229507A CA1062340A CA 1062340 A CA1062340 A CA 1062340A CA 229507 A CA229507 A CA 229507A CA 1062340 A CA1062340 A CA 1062340A
- Authority
- CA
- Canada
- Prior art keywords
- heater
- coils
- induction heating
- heater coil
- heater coils
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- 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
- H05B6/02—Induction heating
- H05B6/06—Control, e.g. of temperature, of power
- H05B6/062—Control, e.g. of temperature, of power for cooking plates or the like
-
- 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
- H05B3/00—Ohmic-resistance heating
- H05B3/68—Heating arrangements specially adapted for cooking plates or analogous hot-plates
- H05B3/683—Plates having their feeding circuit closed as the kettles, pans or the like are put on
Landscapes
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Cookers (AREA)
- General Induction Heating (AREA)
- Electric Stoves And Ranges (AREA)
- Control Of Resistance Heating (AREA)
Abstract
Abstract of the Disclosure An induction heating apparatus comprising a d.c.
power source, a static power inverter for producing a high-frequency oscillating current and an induction heating unit including a plurality of heater coils jointly connected to the inverter for producing a commutating magnetic field around each of the heater coils is provided with protective circuits for disabling one or more or all of the heater coils from being energized from the inverter in the event an improper load is applied to at least one of the heater coils.
The induction heating apparatus is typically used as a cooking appliance and the improper load which may be applied to each of the heater coils includes a cooking pan, pot or kettle of a non-magnetic material such as aluminum and a rejectable magnetic load such as a kitchen knife, spoon or fork.
power source, a static power inverter for producing a high-frequency oscillating current and an induction heating unit including a plurality of heater coils jointly connected to the inverter for producing a commutating magnetic field around each of the heater coils is provided with protective circuits for disabling one or more or all of the heater coils from being energized from the inverter in the event an improper load is applied to at least one of the heater coils.
The induction heating apparatus is typically used as a cooking appliance and the improper load which may be applied to each of the heater coils includes a cooking pan, pot or kettle of a non-magnetic material such as aluminum and a rejectable magnetic load such as a kitchen knife, spoon or fork.
Description
*he present invention relate~ to an induction heating apparatus which i-q typically utilized for cooking purposes .
The induction heating apparatus to which the pre~ent invention apertainq i8 largely compo~ed of a qtatic power inverter and an induction heating unit which comprises a plurality of heater coils connected between the output terminals of the power in~erter.
The qtatic power inverter compri~es a semiconductor switching circuit consisting of a parallel combination of a silicon controlled rectifier and a diode which are connected in reverse directionq to each other.
Across the semiconductor qwitching circuit i8 connected an o~cillating circuit consisting of a commutating inductor and a commutating capacitor so that a high-frequency oscillating current is produced when the silicon controlled rectifier forming part of the ffwitching circuit is triggered at a predetermined frequency. A resonance current is consequently produced "0 in each of the heater coils which usually are connected in parallel between the output terminals of the qtatlc power inverter or, more exactly, acro 8 the above mentioned commutating capacitor 80 that a commutating magnetic field is induced by each of the heater coils.
The commutating magnetic field produces eddy currents . . .
The induction heating apparatus to which the pre~ent invention apertainq i8 largely compo~ed of a qtatic power inverter and an induction heating unit which comprises a plurality of heater coils connected between the output terminals of the power in~erter.
The qtatic power inverter compri~es a semiconductor switching circuit consisting of a parallel combination of a silicon controlled rectifier and a diode which are connected in reverse directionq to each other.
Across the semiconductor qwitching circuit i8 connected an o~cillating circuit consisting of a commutating inductor and a commutating capacitor so that a high-frequency oscillating current is produced when the silicon controlled rectifier forming part of the ffwitching circuit is triggered at a predetermined frequency. A resonance current is consequently produced "0 in each of the heater coils which usually are connected in parallel between the output terminals of the qtatlc power inverter or, more exactly, acro 8 the above mentioned commutating capacitor 80 that a commutating magnetic field is induced by each of the heater coils.
The commutating magnetic field produces eddy currents . . .
- 2 -' in a cooking pan, pot or kettle placed in the vicinity of each of the heater coils with the result that the material to be cooked is heated directly from the cooking implement in which the material is contained.
In case, in this instance, the load or the cooking implement placed in the vicinity of a heater coil is formed of a non-magnetic metal such as for example aluminum, the current flowing through the semiconverter switching circuit exhibits an unusual mode of oscillation which is causative of destruction of the silicon controlled rectifier due to an overvoltage developed across the silicon controlled rectifier. It is for this reason, desirable that means be provided in the induction heating apparatus so as to have the heater coil disconnected from the power inverter or from a power source or otherwise forcibly de-energized immediately when a non-magnetic load is detected to be applied to the heater coil. Where the induction heating apparatus comprises a plurality of heater coils "0 a~ in the apparatus to which the present invention is directed, moreover, it i~ preferable that such mean~
be further arranged in such a manner as to selectively ~ trip one or more heater coils independently of the : remaining heater coil or coils in case a non-magnetic !~ ~5 load is detected to be applied to the former.
. ~ .
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It is therefore, an object of the present invention to provide an induction heating apparatus provided with protective means adapted to disable a heater coil or coils from being energized when a non-magnetic load is applied to the heater coil or to each of the heater co i 1 s .
It i8 another object Or the present invention to provide an induction heating apparatus provided with protective means capable of detecting a non-magnetic load applied to each of the heater coil8 and disabling the heater coil or coils from being energi~ed indepen-dently of the remaining heater coil or coils in case the former is found to be subjected to a non-magnetic load.
Another drawback inherent in the conventional induction heating apparatus is that the heater coil is effective to produce heat in not only in a cooking pan, pot or kettle but al~o in a kitchen kuife, spoon, fork or any other miscellaneous kitchen utensil which :' ~0 i8 usually not used for heating purposes in cooking.
If such a miscellaneous kitchen utensil happens to be placed over the heater coil which is not used during cooking, the kitchen utensil is heated and may harm the operator finger ir the operator inadvertently ~5 toucho~ the h-ot-d uten il. Such an accid-nt ~-y be ~062340 avoided if each of the heater coils is provided with a manually-operated switch to hold the heater coil de-energized i1` the heater coil is to be unused during cooking. In the absence of a perceivable Sig11 ol` heat generated by the heater coil different from an ordinary heating device which produces heat with flames or glows, however, it is practically impossible to prevent the operator from being burned even though such a switch is provided for each of the heater coils.
~t is, thus, a third object of the pre~ent invention to provide an induction heating apparatus having pro-tective means adapted to disable one or more or all of the heater coils disabled from being energized in tl1e event a kitchen knife, fork or spoon is placed in the vicinity of a heater coil. A kitchen knife, fork, spoon or any other similar implement will be herein referred to generally as a "rejectable magnetic load" whilst a cooking pan, pot, kettle or any other vessel ordinarily used for cooking and heating purpose~ will be referred ~0 to generally as an "acceptable magnetic load". rl`urther-more, the "rejectable magnetic load" and a non-magnetic load such as a cooking pan constructed of aluminum will fall herein under the category of an "improper load".
The above defined "acceptable magnetic load" will also ~5 be referred to as a "proper load" in the description .: .
to follow.
In the induction heating apparatus provided with the protective means responsive to an improper load such as a non-magnetic load or a rejectable magnetic load as above mentioned, there may arise a problem in that the protective means may be objectionably actuated to trip a heater coil or coils if an aceeptable magnetic load which is placed in the vicinity of a heater coil is moved toward and away from the heater coil in the process of cooking. Such a problem will be pronounced in an induction heating apparatus comprising a plurality of heater coils which are jointly connected to a common static power inverter because of the fact that tripping of one of the heater coils will seriously affect the performance of each of the remaining heater coils subjected to acceptable magnetie loads. It is, thus, a fourth object of the present invention to provide an induction heating apparatu~ which i9 free from such a problem.
'O In accordance with the present invention, there i~ provided an induetion heating apparatus which eompri~es a d.c. power supply cireuit, a statie power inverter eonnected to the power supply eireuit for produeing a high-frequeney oseillating current, an ~5 induetion heating unit ineluding a plurality of heater .
coils which are jointly connected to the static power converter for producing a commutating magnetic field around each of the heater coils which are energized from the static power inverter, and protective means responsive to an improper load placed over one of the heater coils for disabling the heater coil from being energized. To achieve the previously mentioned first object of the present invention, the protective means may comprise first switch means connected to the LO induction heating unit, driver means connected to the first switch means for driving the switch means to open when actuated, and second switch means responsive to a non-magnetic load applied to at least one of the heater coils for actuating the driver means. To achieve ~5 the above mentioned second obJect of the present invention, the first switch means may comprise a plurality of switching elements which are respectively connected in series with the heater coils and, li~e-wise, the second switch means may comprise a plurality of switches respectively associated with the heater coils so that only the heater coil or coils being subjected to a non-magnetic load or loads are disabled from being energized from the static power converter.
To achieve that above mentioned third object of the :5 present invention, the protective means may be composed .,~ , ~ ~ 7 -of current detecting means operative to produce an output sigrlal representative of the current flowing in each of the heater coil8 ~ comparing means for comparing the output signals from the current detecting means with predetermined reference signals representative of a range of current to be produced in each of the he~ter coil~ when a rejectable magnetic load is applied to the heater coil, and driver means for disabling the heater coil subjected to the rejectable magnetic load from being energized from the static power inverter.
To achieve the previouYly mentioned fourth object of the present invention, the protective means may compri~e, in addition to the components above mentioned, a timing circuit which is adapted to produce an output 8ignal in re8ponse to an improper load which is kept applied to at least one of the heater coil~ for a predetermined period of time. The previously mentioned switch mean~ responsive to a non-magnetic load applied to at least one of the heater coil~ may comprise a 'O piece of permanent magnet fixedly positioned below each of the heater coils, a ~tationary electrical contact fixedly positioned over the permanent magnet and a ~ -movable electrical contact positioncd and-vertically movable between the permanent ms~net and the ~ationary "5 electrical contact and biased to be spaced apart from the ~tationary electrical contact by an attractive force exerted thereon from the permanent magnet.
I`}le featurc~ und advallta~e~ of` the inductiorl heating apparatu~ according to the present invention will become more apparent from the following dexcription taken in con~junction with the accompanying drawings in which like reference numerals indicated corresponding unit~, circuits and elements and in which:
Fig. 1 is a schematic circuit diagram which ~hows a first preferred embodiment of the induction heating apparatus accDrding to the present invention.
~ ig. 2 is a side elevational view showing, partly in section, a preferred example of a magnetically operated switching arrangement incorporated into the embodiment shown in ~ig. 1;
Fig. 3 is a schematic circuit diagram which shows A second preferred embodiment of the induction heating apparatus according to the present invention;
Fig. 4 is a ~chematic circuit diagram showing a modification of the embodiment illustrated in ~ig. 3;
and Fig. 5 is a ~chematlc circuit diagram showing a fourth preferred embodiment of the induction heating apparatu~ according to the present invention.
'5 Reference will now be made to the drawings, first 1C~62340 to l`ig. 1 in which a fir~t preferred embodiment of the induction heating apparatus according to the present invention is illustrated. As shown, the induction heating apparatus largely comprises a d.c. power ~upply circuit 10, a static power converter 12 and an induction heating unit llt. The d.c. power supply circuit 10 is shown to be composed, by way of example, a bridge-type full-wave rectifier 16 having positive and negative output terminals 18 and 18' and a series combination of an a.c. power source 20 and a manually-operated normally-open ~witch 2 connected across the full-wave rectifier 16 ~he full-wave rectifier 16 consists of diodes 16a, 16b, 16c and 16d which are connected in a diametric bridge form between the positive and negative output terminals 18 and 18' of the full-wave rectifier 16. The static power converter 12 ha~ input terminals connected to the output terminals 18 and 18' of the full-wave rectifier 16, A filter inductor 24 and a semiconductor switching circuit 26 are connected in series to terminals 18 and 18'. The semiconductor switching circuit 26 is composed of a sillcon controlled rectifier 28 and a diode 30 connected ln inverse parallel circuit thereto. The-silicon - controlled rectifier 28 ha~ its 8ate eloctrode connected to H gate pulse supply circuit or oscillator 32 ~o as ; .
~ -- 10 --. ' . .
. ' .. ,- , ' ' :
,' ,-' ' . ' ' :
to be repetitiously trigsered at ultrasonic frequency.
Across the switchins circuit 26 is connected a commu-tating network consistillg of` a commutating inductor 31 and a commutating capacitor 36. The static power converter 12 thus arranged has its output terminals connected to the induction heating unit 14. The induction heating unit 14 is shown to comprise a first series combination of a filter capacitor 38 and a heater coil 40 and a second series combination of a filter capacitor 38' and a heater coil 40'. The first and second series combinations of the filter capacitor 38 - and heater coil 40 and the filter capacitor 38' and heater coil 40' are connected in parallel across the output terminals of the converter 12 through normally-open relay contacts 42 and 4~', respectively. Across the first Reries combination of the filter capacitor 38 and the heater coil 40 is connected an additional commutating capacitor 44. Likewise, an additional commutating capacitor 44' is connected across the second series combination of the filter capacitor 38' and the heater coil 40'. These additional commutating capacitors 44 and 44' are intended to compensate for the commutating energy produced by the previously mentioned commutating network as will be described in - ~5 more detail. If desired, however, the additional commutating capacitors 44 and 44' may be dispensed with depending upon the capacitance selected of the commu-tating capacitor 36.
The normally-open relay contacts 42 and 48 associated with the heater coil 40 are operated by a relay 50 and, likewise, the normally-open relay contacts 42' and 48' associated with the heater coil 40' are operated by a relay 50'. The relays 50 and 50' are, in turn, operated by the operation of magnetically operated switch means 54 and 54' which are positioned in the vicinity of the heater coils 40 and 40', re~pec-tively. Each of the switches 54 and 54' is biased into an open condition and is closed in response to a magnetic load placed in the vicinity Or the heater coil associated with the switch means. Thus, the combination of the switching elements 42 and 48, relay 50 and switch mean~
54 consitutes protective means associated with the heater coil 40 and, likewise, the combination of the switching elements 42' and 48', relay 50' and switch means 54' constitute~ protective means associated with the heater coil 40'.
When, in operation, the power 80urce switch 22 is : manually closed by an op~rator, a d.c. voltage is supplied from the full-wave rectifier 16 80 that charges are stored in the commutating capacitor 36 through the _ 12 -filter inductor 24 and the commutating inductor 34.
When the silicon controlled rectifier 28 is then triggered by the gate pulse delivered from the gate pulse supply circuit or oscillator 32, the silicon controlled rectifier "8 is turned on and a commutating current flows in the silicon controlled rectifier 28 and thereafter in the diode 30 through the commutating inductor 34. This causes a voltage drop across the diode 30 so that a voltage is built up across the silicon controlled rectifier 28 in a reverse direction, causing it to turn off. A high frequency oscillating current is thus delivered from the output terminals of the static power converter 12 as the silicon controlled rectifier 28 is repetitiously fired by the train of pulses which are delivered from the gate pulse supply circuit 32. ]f, in this instance, a cooking pan 56 or 56' of a magnetic material such as steel or iron is placed in the vicinity of the heater coil 40 or 40', then the switch means 54 or 54' is caused to close 'O against the biasing force constantly exerted thereon so that the associated relay 50 or 50' is actuated to cause the switching elements 42 and 48 or the switching elements 42' and 48' to close. An energization current A- at the ultrasonic frequency ~ thus flows in the heater coil 40 or 40'. As a consequence, eddy currents are - , _............ . .
~062340 induced in the cooking pan 5~ or 56' by electromaglletic coupling with the heater coil t,o or 40'. Heat is con~equently produced in the cooking pan 56 or 56' by the eddy currents and is directly transferred from the cooki~g pan to the material to be cooked therein.
In case, however, a load constructed of a non-magnetic material -quch as for example an aluminum pan is placed over or in the vicinity of the heater coil 40 or 40', the switch means 54 or 54' will remain open by reason of the biasing force constantly applied thereto so that the a~sociated relay 5b or 50' is maintained in a condition holding the switching elements 42 and Jl8 or the switching element~ 42' and 48' open.
The heater coil 40 or 40' is in this manner disabled from being energized from the power inverter 12 although the power ~ource switch 22 has been closed. The heater coil 40 or 40' i8 kept de-energized until the non-magnetic load is removed therefrom and a load of a magnetic material is applied thereto in substitution ~0 therefor.
.A If the additional commutating capacitors 44 and 44' are removed, the commutating capacitor 36~should be selected to have a capacitance which iY large enough to enable the heater CoilB 40 and 40' to be energized "5 in a ~teady-state condition when both of the heater _ 14 -'~.
coils 40 and ~tO ~ are to be concurrently connected to the converter 12. If only one of the heater coil~ 40 and 40' ls kept de-energi~ed with the other of the hea~er coils energized, then the amount of commutating energy produced only ~y the commutating capacitor 36 become~
exce~sive for the heater coil which is energized. This give~ rise to an undue increase in the oscillation frequency of the converter 12 and results in a rise in current and voltage applied to the silicon controlled rectifier 28, thereby causing a 108s in the switching action of t,he silicon controlled rectifier 28 of the power converter 12. If the switching lo88 of the silicon controlled rectifier 28 is increased to a critical level, then the rectifier 28 will be prevented from being turned off due to high temperature at the PN junctions of the ~witching device 28. Such a problem is, however, eliminated in the embodiment shown in Fig. 1, because the current and voltage applied to the silicon controlled rectifier 28 are maintained sub-'~0 stantially constant irrespective of the number of theheater coils energized becau~e of the additional commutating capacitors 44 and 44' which are connected to the individual heater coils 40 and 40', respectively.
Whell relays 50 and 50' are operated resulting in the "5 operation of their contacts 48 and 48', the commutating ., capacitors 38 and 38' are coupled in parallel circuit through lead 52 in order to adjust the resonance l`requency of` the converter lZ when the heating unit 14 is fully loaded.
Fig. 2 illustrate4 a practical example of the switch means 54 forming part of the fail-safe means provided in the induction heating unit 14 of the embodiment shown in Fig. 1. Referring to Fig. 2, the heater coil 40 i~ wound in a spiral form having an opening 40a formed in the central area thereof and i8 positioned below a flat supporting plate 60 which is cohstructed of a heat-resi4tive, non-magnetic material. Underneath the central opening 40a Or the spiral heater coil 40 i8 positioned a hollow receptacle 62 having an open top end and formed of a non-magnetic material. The receptacle 62 has loosely or vertically movably received therein a piece of permanent magnet 64 which has its top end located in the central opening 40a of the heater coil 40 when the piece of permanent magnet 64 rests in the receptacle 62 by reason of its own gravity. The `- permanent magnet 64 is, thus, held in a lowermost position re~ting in the receptacle in the ab~ence of a magnetic load on the supporting plate 60 and is upwardly moved in response to a magnetic load ~uch as ~5 a cooking pan 56 placed on the upper face of the supporting .
.
plate 60. The supporting plate 60 has fixedly attached to the lower face thereof a stationary electric contact 66 which is held in position over the central opening 40a of -the spiral heater coil 40, whilst the permanent magnet 64 has fixedly secured to the top end thereof a movable electric contact 68 which, together with the permanent magnet 64, is vertically movable into and out of contact with the stationary contact 66 on the supporting plate. The movable contact 68 is biased toward a position spaced apart from the stationary contact 66 by the weight of the assembly Or the permanent magnet 64 and the movable contact 68. The stationary and movable contacts 66 and 68 thus arranged are connected to leads 70 and 70' which are connected across the previously mentioned relay 50. The switch means 54!
associated with the heater coil 40' shown in Fig. 1 i8 constructed and arranged entirely similarly to the switch means 54 and, thuff, the description thus far made in connection with the switch means 54 wholly applies 0 to the switch mean~ 54!.
The operation of the switch means 54 will now be - described with concurrent reference to FigH. 1 and 2.
In the absence of a load placed on the supportin$ plate 60, the permanent magnet 64 is held ~eceived in the "5 receptacle 62 so that the movable contact 68 carried on the permanent magnet 64 is spaced apart f`rom the stationary contact 66 on the supporting plate 60 whetller or not the power ~ource ~WitC}I 22 of tl~e induction heating apparatus may be open or clo~ed.
Such a condition i~ maintained even when a load con-~tructed of a non-magnetic material ~uch as a cooking pan formed of aluminium is placed on the supporting plate 60 because of the fact that the permanent magnet 64 i8 held in the receptacle by reason of weight of its own plu~ the weight Or the movable contact 68.
The normally-open switching element 42 mechanically connected to the relay 50 i~ thu~ held open ~o that the heater coil 40 associate~ with the switch mean~
54 i~ disabled from being en~rgized although the power ~ource switch 22 may have been closed and the power inverter 12 initiated into action producing an o~ci-llating current. When, however, a load formed of a magnetic material such as a cooking pan 56 of iron or steel is placed on the supporting plate 60, the permanent magnet 64 which has been resting in the receptacle 62 is attracted by and moved toward the supporting plate 60 through the central opening 40a of the ~piral heater coil 40 against the bia~ing force exerted thereon by the weights of the magnet 64 and the "5 movable contact 68. As a con~equence, the movable .
contact 64 carried on the permanent magnet 64 i~ brought into abuttillg engagement with the stationary contact ~6 on the underside of the supporting plate 60 with the result that the leads 70 and 70' are connected together through the stationary and moval,le contacts 66 and 68. The relay connected between the leads 70 and 70' is now energized to drive the normally-open switching element 42 into closed condition so that the heater coil 40 associated with the switch means 54 is energized with the resonance current if the power source switch 22 i8 closed. The heater coils 40 and 40' are in this manner tripped independently of each other by the switch means 54 and 54' respectively associated therewith in the event a load of a non-magnetic material is placed in the vicinity of each of the heater coils 40 and 40'.
The induction heating unit 14 of the embodiment thus far described has been assumed to comprise two heater coils 40 and 40' which are connected in parallel "0 between the output terminals of the static power inverter 1~. This is, however, merely for the purpose of illustration and it will be apparent that the~induction heating apparatus according to the pre~ent invention may compri~e more than two heater coils which are connected either in parallel or in series between the output terminals of the static power inverter. In whicllsoever form the heater coils may be arranged, it is preferable that each of the heater coils be connected in series to a filter capacitor and a normally-open switching element controlled by a relay or otherdriver means connected to switch means of the nature which has heen described with reference to ~ig. 2.
It will also be apparent that the advantages of the switch means shown in Fig. 2 can be exploited, if desired, even in an induction heating apparatus using only one heater coil.
Fig. 3 illustrates a second preferred embodiment Or the induction heating app~ratus according to the present invention. The embodiment herein shown is characterized, inter alia, by provision of protective means by which heater coils are prevented from being energized in the event a "rejectable magnetic load"
is detected.
The induction heating apparatus shown in Fi~. 3 comprises a d.c. power supply circuit 10 and a static power converter 12, both of which are constructed and arranged entirely similarly to their respective counter-part~ in the e~bodiment shown in Fig. l and each of which is therefore illustrated in a block form. The "5 induction heating apparatus further comprises an .. . .
:.
- 20 _ ' induction heating unit 14a which is now shown to comprise two heater coils 40 and 40' connected in parallel between the output terminals of the static power inverter 12 over switching elements 72 alld 72', respec-tively, These switch elements 72 and 72' may he arrangedto ~e manually operated so as to selectively enelgi~e the heater coils 40 and 40' or may be arranged similarly to the normally-open switches 42 and 42' provided in the embodiment shown in Fig. 1 so as to trip each of the heater coils 40 and 40' when a non-magnetic load i9 applied thereto as previously described.
The induction heating apparatus thus co~tructed is provided with protective means which comprises current detecting means such~as current transformers 74 and 74' having respective primary circuits connected in series with the heater coils 40 and 40' so that currents proportional to the currents produced in the heater coils 40 and 40' are induced in respective secondary circuits of the current tran~former~ 74 and 74'. The secondary coils of the current transformers 74 and 74~ are connected in parallel to a comparator 76 which i~ adapted to produce an output signal when the current fed to the comparator from the secondary circuit of at least one of the current transformers 74 and 74' i~ within a predetermined range. The range :
- 21 _ within which the comparator is made operative to produce the output signal is so selected as to contain all the possih~e magnitudes of the current whicl- may ~e produced in the secondary circuit of each Or the current trans-formers 74 and 74' when a "rejectable magnetic load"
~uch as a kitchen knife, fork or spoon of a magnetic material is placed in the vicinity of the heater coil.
The range is thus lower than the magnitude of a current which may be produced in the secondary circuit each of the current transformers 74 and 74' when a relatively large-sized cooking and heat1ng utensil such as a cooking pan is placed in the vicinity of the heater coil. When each of the switching elemenOs 72 and 72' is open, there is apparently no current flo~ing through each of the heater coils 40 and 40'. Wh~n, however, both of the switching elements 72 and 72' are closed and a load of a magnetic material is applied to one of the switching elements, a small current is produced in the other of the switching elements 72 and 72'. The lower limit 'O of the above mentioned range to enable the comparator 76 to produce an output signal should therefore be selected to be higher than the magnitude of the current induced in the ~econdary circuit of the current trans-former as a result Or the small current thus produced in the heater coil. If, furthermore, the switching 106Z3~0 elements 72 and 72' are utilized as manually-operated selector ~witche~ as previously mentioned and are therefore inoperable to re~pond to a non-magnetic load placed over each of the heater coil~ 40 and 40', the comparator 76 may be preferably arranged in ~uch a manner that the upper limit of the above mentioned range be lower than the magnitude of the current induced in the secondary circuit of each of the current trans-formers 74 and 74' when an overcurrent i8 produced in the as~ociated one of the heater coil~ 40 and 40' because of a non-magnetic load placed in the neighbour-hood of the heater coil. The comparator 76 ha~ an output terminal connected to a driver circuit 78, ~uch as for example a normally-open relay, which has a mechanical oùtput connected by a mechanical linkage 80 to the power source switch 22 provided in the previously mentioned d.c. power supply circuit 10.
The driver circuit 78 is adapted to cause the power source switch 22 to open in response to an output signal delivered from the comparator 76 to the driver circuit 78. The static power inverter 12 Mnd accordingly the induction heating unit 14a are thus disconnected ; from the power ~upply circuit 10 and as u con~equence the heater coils 40 and 40' are forcibly and automati-; 25 cally deenergized when a relatively small-sized , , "rejectable magnetic load" is placed in the vicinity of one of the heater coils 40 and 40'. The comparator 76 i8 held inoperative to produce the output signal if every one of the heater coils 40 and 40' is main-tained in an unloaded condition or loaded with an "acceptable magnetic load" such as a cooking pan formed of a magnetic material or if at least one of the switching elements 72 and 72' is open. While, in the embodiment shown in Fig. 3, the driver circuit 78 has been assumed to have a mechanical output connected to the power source switch 22, the driver circuit 78 may be arranged to have an electrical output connected to the gate pulse supply circuit 32 of the static power inverter 12 (see ~ig. 1) 80 that the g~te pulse supply circuit 32 is disabled from triggering the silicon controlled rectifier 28 for causing the inverter 12 to be inoperative when the comparator 76 is actuated to produce an output signal. In this instance, the power source switch 22 is kept closed even though the heater coils 40 and 40' have been tripped and, for this reason, the heater coil~ 40 and 40' can be made operative for a second time without manipulating the power source switch 22 immediately when the "rejectable magnetic load" is removed from the heater coil 40 or 40'. As an alternative, an arrangement may be made 80 that one ~' .
_ 24 -.;
~06Z340 of the switching elements 72 and 72' i-~ actuated to open independently of the other switching element when the former i~ subjected to a "rejectable magnetic load".
Fig. 4 illustrates an embodiment incorporating an example of such an arrangement.
In the embodiment shown in Fig. 4, the induction heating unitl also designated in its entirety by ll~a, i9 arranged entirely similarly to its counterpart of the embodiment illustrated in Fig. 3 and is thus composed of a parallel combination of heater coils 40 and 40' connected over switching elements 72 and 72' between the output terminal~ of the static power inverter 12, the detailed construction of which ha~ been shown in Fig. 1. The switching elements 72 and 72' are, in this instance, assumed to be of the normally-olo~ed type for the reason which will become apparent as the description proceeds. Similarly to the induction heating unit 14a of the embodiment shown in Fig. 3, the induction heating unit 14a of the embodiment herein ehown i8 provided with current detecting means such as current transformers 74 and 74' which have respective primary circuits connected in series with the heater coils 40 and 40'. The current transformers 74 and 74' have secondary circuits connected to comparators 76 and 76'~ re~pcctively, which are s~parate from each .
.,~ , --other. Each of the comparators 76 and 76 l is con-structed and arranged essentially ~imilarly to the comparator 76 incorporated into the embodiment il lu~t-rated in Fig. 3 and i~ thus adapted to produce an output signal when the magnitude of the current induced in the secondary circuit of the current transformer 74 or 74 ' associated with the comparator 76 or 76 ', respectively, is within a range which i~ predetermined in ~uch a manner as has been described in connection with the comparator 76 shown in Fig. 3. The comparator0 76 and 76 ' have output terminal~ respectively connected to driver circuits 78 and 78 ' each comprising a normally-open relay by way of example. The driver circuits 78 and 78 ' have mechanical outputs connected to the previouffly mentioned normally-closed switching elements 72 and 72 ' by mechanical linkages 80 and 80 ', re~pec-tively. Each of the driver circuits 78 and 78 ' is adapted to cause the associated one of the normally-cloffed switching elements 72 and 72 ' to open when supplied with an output signal from the comparator 74 or 74 ' connected to the driver circuit 78 or 78 ', ; re~pectively. If, in this instance, each of the driver -i circuits 78 and 78 ' is con~tituted by a relay alone, then each of the comparators 76 and 76 ' and accordingly each of the driver circuits 78 and 78' would be :' .
~ .
de-energized when the associated one of the switching elements 7 and 72' is caused to open and, thus, might allow the normally-closed switching element 72 or 72' to close immediately after the switching element is made open~ To prevent this from occurring, each of the driver circuits 78 and 78' is provided with memory or self-holding means such as for example a monostable multivibrator connected to the normally-open relay so that the relay is maintained closed for a predetermined period of time after the as~ociated one of the switching elements 72 and 72' has been made open and consequently the comparator 76 or 76' associated with the switching element has been rendered inoperative. If preferred, the memory of ~elf-hold means of this nature may be incorporated into each of the comparator circuit~ 76 and 76' so that each of the driver circuits 78 and 78' is kept energized from the associated one of the comparator circuits 76 and 76' for a predetermined period of time after the switching element associated ~0 with the comparator has been tripped. Each of the switching elements 72 and 72' may constitute the normally-closed contact forming part of the relay provided in each of the driver circuits 78 and 78', if desired. In this instance~ a manually-operated ~ 5 switch (not shown) may be connected in series between :, .
.
each of the heater coil~ 40 and 40' and the output terminale of the power inverter 12 in addition to each of the switching elements 72 and 72' so that the heater coils 40 and 40' are selectively disconnected from the inverter 12 by the operator~ Such additional switche~
may however be dispen~ed with because each of the comparators 76 and 76' is so arranged as to produce an output signal in response to a condition in which the magnitude of the current flowing in the associated heater coil 40 or 40' iY less than a predetermined level which apparently is greater than the magnitude of the current to be produced in the absence of a load : on the heater coil 40 or 40'.
Each of the embodiments of the present invention thu~ far described with reference to Figs. 1 to 4 is adapted to have one or more or all of the heater coils de-energized in response to application Or an improper load (viz., a non-magnetic load or a rejectable magnetic load such as a kitchen knife, fork or spoon to at leaYt one of the heater coils and to allow the heater coil or coils to be energized only when an acceptable magnetic load is applied to the coil or each of the coils. As - pointed out previously, however, a problem is encountered in such an arrangement because there is a po~sibility that the heater coil or coils may be 4bjectionably - 28 _ .
, de-energized even when the coil or each of the coils is , subjected to an acceptable magnetic load if alld when the acceptable load such as a magnetic cooking pan i~
moved toward or away from the heater coil or each of the heater coils during cooking. Tllis problem will be pronounced e~pecially when a plurality of heater coils are arranged to be jointly energized by a common static power inverter as in each of the embodiment herein shown because of the fact that tripping of one of the heater coils seriously affect the power output of each of the remaining coil~. Fig. 5 illustrates an embodiment of the induction heating apparatus provided with means adapted to solve such a problem.
Referring to Fig. 5, the induction heating apparatus comprises, in addition to a d.c. power supply circuit 10 and a static power inverter 12 which are constructed -and arranged entirely similarly to their counterparts shown in Fig. 1, an induction heating unit 14b which now shown to comprise ~ first ~erie~ comblnation of a filter capacitor 38 and a heater coil 40 and a second oerie~ combination of a filter capacitor 38~ and a heater 40'. ~he first. and ~econd 8-erle8 combination of the filter capacltors 38 and 38' and the heater coils 40 and 40' are connected in parallel between the output terminals Or the static power inverter 12 or, more .. . .
~ 29 -.~ :
- .,. .. . :
. -" ' ' ; ' :, ~
specifically, across the commutating capacitor 36 of the power inverter 12 over switching elements 8 and 82', respectively. The heater coils 40 and 40' are, furthermore, connected in parallel to each of ~he filter capacitors 38 and 38' over a serie~ combination of switching elements 84 and 84', as shown. The switching elements 82 and 84' associated with one heater coil 40 are operatively connected by a mechanlcal or magnetic linkage 86 to a switch control circuit 90 and likewise the switching elements 82' and 84' associated with the other heater coil 40 are operatively connected by a nlechanical or magnetic linkage 88 to the above mentioned switch control circuit 90. The switch control circuit 90 has a first set of input terminals connected through a first timer 92 across a normally-open switch 94 as~ociated with the heater coil 40 and a second set of input terminals which are ~imilarly connected through a second timer 92' across a normally-open switch 94' associated with the heater coil 40'. The normally-open switches 94 and 94' are arranged, by~y ofexample, similarly to the previously described switch means 54 and 54' incorporated into the embodiment shown in Fig. 1. Each of the switche~ 94 and 94' may therefore be preferably constituted by the magnetically actuated switching arra~gement ~hown in Fig. 2, thus compri~ing ' .
~ 3 -1062340 -`
a piece of permanent magnet 64 fixedly positioned below each of` the heater coils /~0 and 40', a ~tationary electrical contact 66 fixedly positiotled underneath each of the heater coils 40 and 40' and above the permanent magnet 64, and a movable electrical contact 68 which i~ vertically movable between the permanent magnet ~4 and each of the heater coils 40 and 40'.
Each of the switche~ 94 and 94' is thus biased to ~e normally open and is actuated when a magnetic kitchen implement such as a cooking pan 56 or 56' formed of steel or iron is placed in the vicinity of the heater coil 40 or 40', Each of the timers 92 and 92' includes, by way of example, an integrating circuit (not shown) and is thus operative to produce an output signal when kept supplied with an input signal for a predetermined period of time.
The output signal thus delivered from each of the timers 92 and 92' is fed into the switch control circuit 90 and actuates the switch control circuit 90 into a condition causing the switching elements 82 and 84 or the switching elements 82' and 84' to open. As a Consequence~ the heater coil 40 or ko ~ is de-energized when the switch 94 or 94' associated therewith i8 kept open for a certain period of time which i~ prescribed on the timer 92 or 92', respectively. When, thus, the . ~ .
' .
~062340 magnetic load positioned in the vicinity of one of -the heater coils, say the heater coil 40 for example, and consequently the switch 94 associated with the heater coil 40 is made open instantaneously or for a short while, the switch control circuit 90 is kept inoperative in the absence of an output signal from the timer 92 associated with the heater coil 40. If, however, the magnetic load which has been placed in the vicinity of the heater coil 40 i8 moved away from the heater coil 40 for a period of time longer than the time period prescribed on the timer 90, the timer 92 is made operative to produce an output signal so that the ~witch control circuit 90 is actuated to open the switching elements 82 and 84 to open. As a consequence, the heater coil 40 in an unloaded condition is disconnected from the power inverter 12 whether the switching elements 82' and 84' associated with the other heater coil 40' are open or closed. When one of the heater coils 40 and 40' i9 energized and the other thereof i8 de-energized as in the condition above described, either of the switching elements 84 and 84' i8 open so that the former heater coil i8 connected to the power inverter 12 through the filter capacitor 38 or 30' associated with the heater coil. When, however, both of the heater coils 40 and 40' are energi~ed . - 32 -simultaneously with all of the switching elements 8 and 82~ and the switching elements 84 and 84~ kept closed, the heater coils 40 or 40 ' are connected in parallel to the power inverter through the filter capacitors 38 and 38 ~, respectively. The switches 84 and 84 ~ are thus adapted to prevent each of the heater coils 40 and 40' from being subjected to an increased load impedance when one of the heater coils 40 and 40 ' is energized with the other of the heater coils de-energized. The switching elements 84 and 84 ~ may therefore be dispensed with if such a consideration need not be paid.
If desired, each of the switches 94 and 94' arranged similarly to the switch means 54 and 54 ~ of '!15 the embodiment shown in Fig. 1 may be replaced with suitable protective means responsive to placement of a rejectable magnetic load in the vicinity of each of the heater coils 40 and 40'. The protective means may be arranged similarly to the protective means incorporated 'O into each of the embodiments shown in Figs. 3 and 4, comprising a current transformer having a primary c$rcuit connected in series with each of the heater coils 40 and 40' and a comparator having an input terminal connected to the secondary circuit of the current transformer and arranged es~entially ~imilarly , ., .
.: - 33 -to each of the comparators 76 and 76' shown in Eig. 3 or 4. The comparator is connected through each of _ the above mentioned timers 92 and 92' to the switch control circuit 90. Each of the heater coils 40 and 40' provided with the protective means thus arranged is kept energized when an acceptable magnetic load such as a cooking pan 56 or 56' positioned over the heater coil 40 or 40' is moved relative to the heater coil and consequently the current flowing through the heater coil is diminished instantaneously or only for a short while to such a level as would be produced in the heater coil if a rejectable magnetic load is applied to the heater coil 40 or 40'.
In case, in this instance, the load or the cooking implement placed in the vicinity of a heater coil is formed of a non-magnetic metal such as for example aluminum, the current flowing through the semiconverter switching circuit exhibits an unusual mode of oscillation which is causative of destruction of the silicon controlled rectifier due to an overvoltage developed across the silicon controlled rectifier. It is for this reason, desirable that means be provided in the induction heating apparatus so as to have the heater coil disconnected from the power inverter or from a power source or otherwise forcibly de-energized immediately when a non-magnetic load is detected to be applied to the heater coil. Where the induction heating apparatus comprises a plurality of heater coils "0 a~ in the apparatus to which the present invention is directed, moreover, it i~ preferable that such mean~
be further arranged in such a manner as to selectively ~ trip one or more heater coils independently of the : remaining heater coil or coils in case a non-magnetic !~ ~5 load is detected to be applied to the former.
. ~ .
~ 3 --, .
It is therefore, an object of the present invention to provide an induction heating apparatus provided with protective means adapted to disable a heater coil or coils from being energized when a non-magnetic load is applied to the heater coil or to each of the heater co i 1 s .
It i8 another object Or the present invention to provide an induction heating apparatus provided with protective means capable of detecting a non-magnetic load applied to each of the heater coil8 and disabling the heater coil or coils from being energi~ed indepen-dently of the remaining heater coil or coils in case the former is found to be subjected to a non-magnetic load.
Another drawback inherent in the conventional induction heating apparatus is that the heater coil is effective to produce heat in not only in a cooking pan, pot or kettle but al~o in a kitchen kuife, spoon, fork or any other miscellaneous kitchen utensil which :' ~0 i8 usually not used for heating purposes in cooking.
If such a miscellaneous kitchen utensil happens to be placed over the heater coil which is not used during cooking, the kitchen utensil is heated and may harm the operator finger ir the operator inadvertently ~5 toucho~ the h-ot-d uten il. Such an accid-nt ~-y be ~062340 avoided if each of the heater coils is provided with a manually-operated switch to hold the heater coil de-energized i1` the heater coil is to be unused during cooking. In the absence of a perceivable Sig11 ol` heat generated by the heater coil different from an ordinary heating device which produces heat with flames or glows, however, it is practically impossible to prevent the operator from being burned even though such a switch is provided for each of the heater coils.
~t is, thus, a third object of the pre~ent invention to provide an induction heating apparatus having pro-tective means adapted to disable one or more or all of the heater coils disabled from being energized in tl1e event a kitchen knife, fork or spoon is placed in the vicinity of a heater coil. A kitchen knife, fork, spoon or any other similar implement will be herein referred to generally as a "rejectable magnetic load" whilst a cooking pan, pot, kettle or any other vessel ordinarily used for cooking and heating purpose~ will be referred ~0 to generally as an "acceptable magnetic load". rl`urther-more, the "rejectable magnetic load" and a non-magnetic load such as a cooking pan constructed of aluminum will fall herein under the category of an "improper load".
The above defined "acceptable magnetic load" will also ~5 be referred to as a "proper load" in the description .: .
to follow.
In the induction heating apparatus provided with the protective means responsive to an improper load such as a non-magnetic load or a rejectable magnetic load as above mentioned, there may arise a problem in that the protective means may be objectionably actuated to trip a heater coil or coils if an aceeptable magnetic load which is placed in the vicinity of a heater coil is moved toward and away from the heater coil in the process of cooking. Such a problem will be pronounced in an induction heating apparatus comprising a plurality of heater coils which are jointly connected to a common static power inverter because of the fact that tripping of one of the heater coils will seriously affect the performance of each of the remaining heater coils subjected to acceptable magnetie loads. It is, thus, a fourth object of the present invention to provide an induction heating apparatu~ which i9 free from such a problem.
'O In accordance with the present invention, there i~ provided an induetion heating apparatus which eompri~es a d.c. power supply cireuit, a statie power inverter eonnected to the power supply eireuit for produeing a high-frequeney oseillating current, an ~5 induetion heating unit ineluding a plurality of heater .
coils which are jointly connected to the static power converter for producing a commutating magnetic field around each of the heater coils which are energized from the static power inverter, and protective means responsive to an improper load placed over one of the heater coils for disabling the heater coil from being energized. To achieve the previously mentioned first object of the present invention, the protective means may comprise first switch means connected to the LO induction heating unit, driver means connected to the first switch means for driving the switch means to open when actuated, and second switch means responsive to a non-magnetic load applied to at least one of the heater coils for actuating the driver means. To achieve ~5 the above mentioned second obJect of the present invention, the first switch means may comprise a plurality of switching elements which are respectively connected in series with the heater coils and, li~e-wise, the second switch means may comprise a plurality of switches respectively associated with the heater coils so that only the heater coil or coils being subjected to a non-magnetic load or loads are disabled from being energized from the static power converter.
To achieve that above mentioned third object of the :5 present invention, the protective means may be composed .,~ , ~ ~ 7 -of current detecting means operative to produce an output sigrlal representative of the current flowing in each of the heater coil8 ~ comparing means for comparing the output signals from the current detecting means with predetermined reference signals representative of a range of current to be produced in each of the he~ter coil~ when a rejectable magnetic load is applied to the heater coil, and driver means for disabling the heater coil subjected to the rejectable magnetic load from being energized from the static power inverter.
To achieve the previouYly mentioned fourth object of the present invention, the protective means may compri~e, in addition to the components above mentioned, a timing circuit which is adapted to produce an output 8ignal in re8ponse to an improper load which is kept applied to at least one of the heater coil~ for a predetermined period of time. The previously mentioned switch mean~ responsive to a non-magnetic load applied to at least one of the heater coil~ may comprise a 'O piece of permanent magnet fixedly positioned below each of the heater coils, a ~tationary electrical contact fixedly positioned over the permanent magnet and a ~ -movable electrical contact positioncd and-vertically movable between the permanent ms~net and the ~ationary "5 electrical contact and biased to be spaced apart from the ~tationary electrical contact by an attractive force exerted thereon from the permanent magnet.
I`}le featurc~ und advallta~e~ of` the inductiorl heating apparatu~ according to the present invention will become more apparent from the following dexcription taken in con~junction with the accompanying drawings in which like reference numerals indicated corresponding unit~, circuits and elements and in which:
Fig. 1 is a schematic circuit diagram which ~hows a first preferred embodiment of the induction heating apparatus accDrding to the present invention.
~ ig. 2 is a side elevational view showing, partly in section, a preferred example of a magnetically operated switching arrangement incorporated into the embodiment shown in ~ig. 1;
Fig. 3 is a schematic circuit diagram which shows A second preferred embodiment of the induction heating apparatus according to the present invention;
Fig. 4 is a ~chematic circuit diagram showing a modification of the embodiment illustrated in ~ig. 3;
and Fig. 5 is a ~chematlc circuit diagram showing a fourth preferred embodiment of the induction heating apparatu~ according to the present invention.
'5 Reference will now be made to the drawings, first 1C~62340 to l`ig. 1 in which a fir~t preferred embodiment of the induction heating apparatus according to the present invention is illustrated. As shown, the induction heating apparatus largely comprises a d.c. power ~upply circuit 10, a static power converter 12 and an induction heating unit llt. The d.c. power supply circuit 10 is shown to be composed, by way of example, a bridge-type full-wave rectifier 16 having positive and negative output terminals 18 and 18' and a series combination of an a.c. power source 20 and a manually-operated normally-open ~witch 2 connected across the full-wave rectifier 16 ~he full-wave rectifier 16 consists of diodes 16a, 16b, 16c and 16d which are connected in a diametric bridge form between the positive and negative output terminals 18 and 18' of the full-wave rectifier 16. The static power converter 12 ha~ input terminals connected to the output terminals 18 and 18' of the full-wave rectifier 16, A filter inductor 24 and a semiconductor switching circuit 26 are connected in series to terminals 18 and 18'. The semiconductor switching circuit 26 is composed of a sillcon controlled rectifier 28 and a diode 30 connected ln inverse parallel circuit thereto. The-silicon - controlled rectifier 28 ha~ its 8ate eloctrode connected to H gate pulse supply circuit or oscillator 32 ~o as ; .
~ -- 10 --. ' . .
. ' .. ,- , ' ' :
,' ,-' ' . ' ' :
to be repetitiously trigsered at ultrasonic frequency.
Across the switchins circuit 26 is connected a commu-tating network consistillg of` a commutating inductor 31 and a commutating capacitor 36. The static power converter 12 thus arranged has its output terminals connected to the induction heating unit 14. The induction heating unit 14 is shown to comprise a first series combination of a filter capacitor 38 and a heater coil 40 and a second series combination of a filter capacitor 38' and a heater coil 40'. The first and second series combinations of the filter capacitor 38 - and heater coil 40 and the filter capacitor 38' and heater coil 40' are connected in parallel across the output terminals of the converter 12 through normally-open relay contacts 42 and 4~', respectively. Across the first Reries combination of the filter capacitor 38 and the heater coil 40 is connected an additional commutating capacitor 44. Likewise, an additional commutating capacitor 44' is connected across the second series combination of the filter capacitor 38' and the heater coil 40'. These additional commutating capacitors 44 and 44' are intended to compensate for the commutating energy produced by the previously mentioned commutating network as will be described in - ~5 more detail. If desired, however, the additional commutating capacitors 44 and 44' may be dispensed with depending upon the capacitance selected of the commu-tating capacitor 36.
The normally-open relay contacts 42 and 48 associated with the heater coil 40 are operated by a relay 50 and, likewise, the normally-open relay contacts 42' and 48' associated with the heater coil 40' are operated by a relay 50'. The relays 50 and 50' are, in turn, operated by the operation of magnetically operated switch means 54 and 54' which are positioned in the vicinity of the heater coils 40 and 40', re~pec-tively. Each of the switches 54 and 54' is biased into an open condition and is closed in response to a magnetic load placed in the vicinity Or the heater coil associated with the switch means. Thus, the combination of the switching elements 42 and 48, relay 50 and switch mean~
54 consitutes protective means associated with the heater coil 40 and, likewise, the combination of the switching elements 42' and 48', relay 50' and switch means 54' constitute~ protective means associated with the heater coil 40'.
When, in operation, the power 80urce switch 22 is : manually closed by an op~rator, a d.c. voltage is supplied from the full-wave rectifier 16 80 that charges are stored in the commutating capacitor 36 through the _ 12 -filter inductor 24 and the commutating inductor 34.
When the silicon controlled rectifier 28 is then triggered by the gate pulse delivered from the gate pulse supply circuit or oscillator 32, the silicon controlled rectifier "8 is turned on and a commutating current flows in the silicon controlled rectifier 28 and thereafter in the diode 30 through the commutating inductor 34. This causes a voltage drop across the diode 30 so that a voltage is built up across the silicon controlled rectifier 28 in a reverse direction, causing it to turn off. A high frequency oscillating current is thus delivered from the output terminals of the static power converter 12 as the silicon controlled rectifier 28 is repetitiously fired by the train of pulses which are delivered from the gate pulse supply circuit 32. ]f, in this instance, a cooking pan 56 or 56' of a magnetic material such as steel or iron is placed in the vicinity of the heater coil 40 or 40', then the switch means 54 or 54' is caused to close 'O against the biasing force constantly exerted thereon so that the associated relay 50 or 50' is actuated to cause the switching elements 42 and 48 or the switching elements 42' and 48' to close. An energization current A- at the ultrasonic frequency ~ thus flows in the heater coil 40 or 40'. As a consequence, eddy currents are - , _............ . .
~062340 induced in the cooking pan 5~ or 56' by electromaglletic coupling with the heater coil t,o or 40'. Heat is con~equently produced in the cooking pan 56 or 56' by the eddy currents and is directly transferred from the cooki~g pan to the material to be cooked therein.
In case, however, a load constructed of a non-magnetic material -quch as for example an aluminum pan is placed over or in the vicinity of the heater coil 40 or 40', the switch means 54 or 54' will remain open by reason of the biasing force constantly applied thereto so that the a~sociated relay 5b or 50' is maintained in a condition holding the switching elements 42 and Jl8 or the switching element~ 42' and 48' open.
The heater coil 40 or 40' is in this manner disabled from being energized from the power inverter 12 although the power ~ource switch 22 has been closed. The heater coil 40 or 40' i8 kept de-energized until the non-magnetic load is removed therefrom and a load of a magnetic material is applied thereto in substitution ~0 therefor.
.A If the additional commutating capacitors 44 and 44' are removed, the commutating capacitor 36~should be selected to have a capacitance which iY large enough to enable the heater CoilB 40 and 40' to be energized "5 in a ~teady-state condition when both of the heater _ 14 -'~.
coils 40 and ~tO ~ are to be concurrently connected to the converter 12. If only one of the heater coil~ 40 and 40' ls kept de-energi~ed with the other of the hea~er coils energized, then the amount of commutating energy produced only ~y the commutating capacitor 36 become~
exce~sive for the heater coil which is energized. This give~ rise to an undue increase in the oscillation frequency of the converter 12 and results in a rise in current and voltage applied to the silicon controlled rectifier 28, thereby causing a 108s in the switching action of t,he silicon controlled rectifier 28 of the power converter 12. If the switching lo88 of the silicon controlled rectifier 28 is increased to a critical level, then the rectifier 28 will be prevented from being turned off due to high temperature at the PN junctions of the ~witching device 28. Such a problem is, however, eliminated in the embodiment shown in Fig. 1, because the current and voltage applied to the silicon controlled rectifier 28 are maintained sub-'~0 stantially constant irrespective of the number of theheater coils energized becau~e of the additional commutating capacitors 44 and 44' which are connected to the individual heater coils 40 and 40', respectively.
Whell relays 50 and 50' are operated resulting in the "5 operation of their contacts 48 and 48', the commutating ., capacitors 38 and 38' are coupled in parallel circuit through lead 52 in order to adjust the resonance l`requency of` the converter lZ when the heating unit 14 is fully loaded.
Fig. 2 illustrate4 a practical example of the switch means 54 forming part of the fail-safe means provided in the induction heating unit 14 of the embodiment shown in Fig. 1. Referring to Fig. 2, the heater coil 40 i~ wound in a spiral form having an opening 40a formed in the central area thereof and i8 positioned below a flat supporting plate 60 which is cohstructed of a heat-resi4tive, non-magnetic material. Underneath the central opening 40a Or the spiral heater coil 40 i8 positioned a hollow receptacle 62 having an open top end and formed of a non-magnetic material. The receptacle 62 has loosely or vertically movably received therein a piece of permanent magnet 64 which has its top end located in the central opening 40a of the heater coil 40 when the piece of permanent magnet 64 rests in the receptacle 62 by reason of its own gravity. The `- permanent magnet 64 is, thus, held in a lowermost position re~ting in the receptacle in the ab~ence of a magnetic load on the supporting plate 60 and is upwardly moved in response to a magnetic load ~uch as ~5 a cooking pan 56 placed on the upper face of the supporting .
.
plate 60. The supporting plate 60 has fixedly attached to the lower face thereof a stationary electric contact 66 which is held in position over the central opening 40a of -the spiral heater coil 40, whilst the permanent magnet 64 has fixedly secured to the top end thereof a movable electric contact 68 which, together with the permanent magnet 64, is vertically movable into and out of contact with the stationary contact 66 on the supporting plate. The movable contact 68 is biased toward a position spaced apart from the stationary contact 66 by the weight of the assembly Or the permanent magnet 64 and the movable contact 68. The stationary and movable contacts 66 and 68 thus arranged are connected to leads 70 and 70' which are connected across the previously mentioned relay 50. The switch means 54!
associated with the heater coil 40' shown in Fig. 1 i8 constructed and arranged entirely similarly to the switch means 54 and, thuff, the description thus far made in connection with the switch means 54 wholly applies 0 to the switch mean~ 54!.
The operation of the switch means 54 will now be - described with concurrent reference to FigH. 1 and 2.
In the absence of a load placed on the supportin$ plate 60, the permanent magnet 64 is held ~eceived in the "5 receptacle 62 so that the movable contact 68 carried on the permanent magnet 64 is spaced apart f`rom the stationary contact 66 on the supporting plate 60 whetller or not the power ~ource ~WitC}I 22 of tl~e induction heating apparatus may be open or clo~ed.
Such a condition i~ maintained even when a load con-~tructed of a non-magnetic material ~uch as a cooking pan formed of aluminium is placed on the supporting plate 60 because of the fact that the permanent magnet 64 i8 held in the receptacle by reason of weight of its own plu~ the weight Or the movable contact 68.
The normally-open switching element 42 mechanically connected to the relay 50 i~ thu~ held open ~o that the heater coil 40 associate~ with the switch mean~
54 i~ disabled from being en~rgized although the power ~ource switch 22 may have been closed and the power inverter 12 initiated into action producing an o~ci-llating current. When, however, a load formed of a magnetic material such as a cooking pan 56 of iron or steel is placed on the supporting plate 60, the permanent magnet 64 which has been resting in the receptacle 62 is attracted by and moved toward the supporting plate 60 through the central opening 40a of the ~piral heater coil 40 against the bia~ing force exerted thereon by the weights of the magnet 64 and the "5 movable contact 68. As a con~equence, the movable .
contact 64 carried on the permanent magnet 64 i~ brought into abuttillg engagement with the stationary contact ~6 on the underside of the supporting plate 60 with the result that the leads 70 and 70' are connected together through the stationary and moval,le contacts 66 and 68. The relay connected between the leads 70 and 70' is now energized to drive the normally-open switching element 42 into closed condition so that the heater coil 40 associated with the switch means 54 is energized with the resonance current if the power source switch 22 i8 closed. The heater coils 40 and 40' are in this manner tripped independently of each other by the switch means 54 and 54' respectively associated therewith in the event a load of a non-magnetic material is placed in the vicinity of each of the heater coils 40 and 40'.
The induction heating unit 14 of the embodiment thus far described has been assumed to comprise two heater coils 40 and 40' which are connected in parallel "0 between the output terminals of the static power inverter 1~. This is, however, merely for the purpose of illustration and it will be apparent that the~induction heating apparatus according to the pre~ent invention may compri~e more than two heater coils which are connected either in parallel or in series between the output terminals of the static power inverter. In whicllsoever form the heater coils may be arranged, it is preferable that each of the heater coils be connected in series to a filter capacitor and a normally-open switching element controlled by a relay or otherdriver means connected to switch means of the nature which has heen described with reference to ~ig. 2.
It will also be apparent that the advantages of the switch means shown in Fig. 2 can be exploited, if desired, even in an induction heating apparatus using only one heater coil.
Fig. 3 illustrates a second preferred embodiment Or the induction heating app~ratus according to the present invention. The embodiment herein shown is characterized, inter alia, by provision of protective means by which heater coils are prevented from being energized in the event a "rejectable magnetic load"
is detected.
The induction heating apparatus shown in Fi~. 3 comprises a d.c. power supply circuit 10 and a static power converter 12, both of which are constructed and arranged entirely similarly to their respective counter-part~ in the e~bodiment shown in Fig. l and each of which is therefore illustrated in a block form. The "5 induction heating apparatus further comprises an .. . .
:.
- 20 _ ' induction heating unit 14a which is now shown to comprise two heater coils 40 and 40' connected in parallel between the output terminals of the static power inverter 12 over switching elements 72 alld 72', respec-tively, These switch elements 72 and 72' may he arrangedto ~e manually operated so as to selectively enelgi~e the heater coils 40 and 40' or may be arranged similarly to the normally-open switches 42 and 42' provided in the embodiment shown in Fig. 1 so as to trip each of the heater coils 40 and 40' when a non-magnetic load i9 applied thereto as previously described.
The induction heating apparatus thus co~tructed is provided with protective means which comprises current detecting means such~as current transformers 74 and 74' having respective primary circuits connected in series with the heater coils 40 and 40' so that currents proportional to the currents produced in the heater coils 40 and 40' are induced in respective secondary circuits of the current tran~former~ 74 and 74'. The secondary coils of the current transformers 74 and 74~ are connected in parallel to a comparator 76 which i~ adapted to produce an output signal when the current fed to the comparator from the secondary circuit of at least one of the current transformers 74 and 74' i~ within a predetermined range. The range :
- 21 _ within which the comparator is made operative to produce the output signal is so selected as to contain all the possih~e magnitudes of the current whicl- may ~e produced in the secondary circuit of each Or the current trans-formers 74 and 74' when a "rejectable magnetic load"
~uch as a kitchen knife, fork or spoon of a magnetic material is placed in the vicinity of the heater coil.
The range is thus lower than the magnitude of a current which may be produced in the secondary circuit each of the current transformers 74 and 74' when a relatively large-sized cooking and heat1ng utensil such as a cooking pan is placed in the vicinity of the heater coil. When each of the switching elemenOs 72 and 72' is open, there is apparently no current flo~ing through each of the heater coils 40 and 40'. Wh~n, however, both of the switching elements 72 and 72' are closed and a load of a magnetic material is applied to one of the switching elements, a small current is produced in the other of the switching elements 72 and 72'. The lower limit 'O of the above mentioned range to enable the comparator 76 to produce an output signal should therefore be selected to be higher than the magnitude of the current induced in the ~econdary circuit of the current trans-former as a result Or the small current thus produced in the heater coil. If, furthermore, the switching 106Z3~0 elements 72 and 72' are utilized as manually-operated selector ~witche~ as previously mentioned and are therefore inoperable to re~pond to a non-magnetic load placed over each of the heater coil~ 40 and 40', the comparator 76 may be preferably arranged in ~uch a manner that the upper limit of the above mentioned range be lower than the magnitude of the current induced in the secondary circuit of each of the current trans-formers 74 and 74' when an overcurrent i8 produced in the as~ociated one of the heater coil~ 40 and 40' because of a non-magnetic load placed in the neighbour-hood of the heater coil. The comparator 76 ha~ an output terminal connected to a driver circuit 78, ~uch as for example a normally-open relay, which has a mechanical oùtput connected by a mechanical linkage 80 to the power source switch 22 provided in the previously mentioned d.c. power supply circuit 10.
The driver circuit 78 is adapted to cause the power source switch 22 to open in response to an output signal delivered from the comparator 76 to the driver circuit 78. The static power inverter 12 Mnd accordingly the induction heating unit 14a are thus disconnected ; from the power ~upply circuit 10 and as u con~equence the heater coils 40 and 40' are forcibly and automati-; 25 cally deenergized when a relatively small-sized , , "rejectable magnetic load" is placed in the vicinity of one of the heater coils 40 and 40'. The comparator 76 i8 held inoperative to produce the output signal if every one of the heater coils 40 and 40' is main-tained in an unloaded condition or loaded with an "acceptable magnetic load" such as a cooking pan formed of a magnetic material or if at least one of the switching elements 72 and 72' is open. While, in the embodiment shown in Fig. 3, the driver circuit 78 has been assumed to have a mechanical output connected to the power source switch 22, the driver circuit 78 may be arranged to have an electrical output connected to the gate pulse supply circuit 32 of the static power inverter 12 (see ~ig. 1) 80 that the g~te pulse supply circuit 32 is disabled from triggering the silicon controlled rectifier 28 for causing the inverter 12 to be inoperative when the comparator 76 is actuated to produce an output signal. In this instance, the power source switch 22 is kept closed even though the heater coils 40 and 40' have been tripped and, for this reason, the heater coil~ 40 and 40' can be made operative for a second time without manipulating the power source switch 22 immediately when the "rejectable magnetic load" is removed from the heater coil 40 or 40'. As an alternative, an arrangement may be made 80 that one ~' .
_ 24 -.;
~06Z340 of the switching elements 72 and 72' i-~ actuated to open independently of the other switching element when the former i~ subjected to a "rejectable magnetic load".
Fig. 4 illustrates an embodiment incorporating an example of such an arrangement.
In the embodiment shown in Fig. 4, the induction heating unitl also designated in its entirety by ll~a, i9 arranged entirely similarly to its counterpart of the embodiment illustrated in Fig. 3 and is thus composed of a parallel combination of heater coils 40 and 40' connected over switching elements 72 and 72' between the output terminal~ of the static power inverter 12, the detailed construction of which ha~ been shown in Fig. 1. The switching elements 72 and 72' are, in this instance, assumed to be of the normally-olo~ed type for the reason which will become apparent as the description proceeds. Similarly to the induction heating unit 14a of the embodiment shown in Fig. 3, the induction heating unit 14a of the embodiment herein ehown i8 provided with current detecting means such as current transformers 74 and 74' which have respective primary circuits connected in series with the heater coils 40 and 40'. The current transformers 74 and 74' have secondary circuits connected to comparators 76 and 76'~ re~pcctively, which are s~parate from each .
.,~ , --other. Each of the comparators 76 and 76 l is con-structed and arranged essentially ~imilarly to the comparator 76 incorporated into the embodiment il lu~t-rated in Fig. 3 and i~ thus adapted to produce an output signal when the magnitude of the current induced in the secondary circuit of the current transformer 74 or 74 ' associated with the comparator 76 or 76 ', respectively, is within a range which i~ predetermined in ~uch a manner as has been described in connection with the comparator 76 shown in Fig. 3. The comparator0 76 and 76 ' have output terminal~ respectively connected to driver circuits 78 and 78 ' each comprising a normally-open relay by way of example. The driver circuits 78 and 78 ' have mechanical outputs connected to the previouffly mentioned normally-closed switching elements 72 and 72 ' by mechanical linkages 80 and 80 ', re~pec-tively. Each of the driver circuits 78 and 78 ' is adapted to cause the associated one of the normally-cloffed switching elements 72 and 72 ' to open when supplied with an output signal from the comparator 74 or 74 ' connected to the driver circuit 78 or 78 ', ; re~pectively. If, in this instance, each of the driver -i circuits 78 and 78 ' is con~tituted by a relay alone, then each of the comparators 76 and 76 ' and accordingly each of the driver circuits 78 and 78' would be :' .
~ .
de-energized when the associated one of the switching elements 7 and 72' is caused to open and, thus, might allow the normally-closed switching element 72 or 72' to close immediately after the switching element is made open~ To prevent this from occurring, each of the driver circuits 78 and 78' is provided with memory or self-holding means such as for example a monostable multivibrator connected to the normally-open relay so that the relay is maintained closed for a predetermined period of time after the as~ociated one of the switching elements 72 and 72' has been made open and consequently the comparator 76 or 76' associated with the switching element has been rendered inoperative. If preferred, the memory of ~elf-hold means of this nature may be incorporated into each of the comparator circuit~ 76 and 76' so that each of the driver circuits 78 and 78' is kept energized from the associated one of the comparator circuits 76 and 76' for a predetermined period of time after the switching element associated ~0 with the comparator has been tripped. Each of the switching elements 72 and 72' may constitute the normally-closed contact forming part of the relay provided in each of the driver circuits 78 and 78', if desired. In this instance~ a manually-operated ~ 5 switch (not shown) may be connected in series between :, .
.
each of the heater coil~ 40 and 40' and the output terminale of the power inverter 12 in addition to each of the switching elements 72 and 72' so that the heater coils 40 and 40' are selectively disconnected from the inverter 12 by the operator~ Such additional switche~
may however be dispen~ed with because each of the comparators 76 and 76' is so arranged as to produce an output signal in response to a condition in which the magnitude of the current flowing in the associated heater coil 40 or 40' iY less than a predetermined level which apparently is greater than the magnitude of the current to be produced in the absence of a load : on the heater coil 40 or 40'.
Each of the embodiments of the present invention thu~ far described with reference to Figs. 1 to 4 is adapted to have one or more or all of the heater coils de-energized in response to application Or an improper load (viz., a non-magnetic load or a rejectable magnetic load such as a kitchen knife, fork or spoon to at leaYt one of the heater coils and to allow the heater coil or coils to be energized only when an acceptable magnetic load is applied to the coil or each of the coils. As - pointed out previously, however, a problem is encountered in such an arrangement because there is a po~sibility that the heater coil or coils may be 4bjectionably - 28 _ .
, de-energized even when the coil or each of the coils is , subjected to an acceptable magnetic load if alld when the acceptable load such as a magnetic cooking pan i~
moved toward or away from the heater coil or each of the heater coils during cooking. Tllis problem will be pronounced e~pecially when a plurality of heater coils are arranged to be jointly energized by a common static power inverter as in each of the embodiment herein shown because of the fact that tripping of one of the heater coils seriously affect the power output of each of the remaining coil~. Fig. 5 illustrates an embodiment of the induction heating apparatus provided with means adapted to solve such a problem.
Referring to Fig. 5, the induction heating apparatus comprises, in addition to a d.c. power supply circuit 10 and a static power inverter 12 which are constructed -and arranged entirely similarly to their counterparts shown in Fig. 1, an induction heating unit 14b which now shown to comprise ~ first ~erie~ comblnation of a filter capacitor 38 and a heater coil 40 and a second oerie~ combination of a filter capacitor 38~ and a heater 40'. ~he first. and ~econd 8-erle8 combination of the filter capacltors 38 and 38' and the heater coils 40 and 40' are connected in parallel between the output terminals Or the static power inverter 12 or, more .. . .
~ 29 -.~ :
- .,. .. . :
. -" ' ' ; ' :, ~
specifically, across the commutating capacitor 36 of the power inverter 12 over switching elements 8 and 82', respectively. The heater coils 40 and 40' are, furthermore, connected in parallel to each of ~he filter capacitors 38 and 38' over a serie~ combination of switching elements 84 and 84', as shown. The switching elements 82 and 84' associated with one heater coil 40 are operatively connected by a mechanlcal or magnetic linkage 86 to a switch control circuit 90 and likewise the switching elements 82' and 84' associated with the other heater coil 40 are operatively connected by a nlechanical or magnetic linkage 88 to the above mentioned switch control circuit 90. The switch control circuit 90 has a first set of input terminals connected through a first timer 92 across a normally-open switch 94 as~ociated with the heater coil 40 and a second set of input terminals which are ~imilarly connected through a second timer 92' across a normally-open switch 94' associated with the heater coil 40'. The normally-open switches 94 and 94' are arranged, by~y ofexample, similarly to the previously described switch means 54 and 54' incorporated into the embodiment shown in Fig. 1. Each of the switche~ 94 and 94' may therefore be preferably constituted by the magnetically actuated switching arra~gement ~hown in Fig. 2, thus compri~ing ' .
~ 3 -1062340 -`
a piece of permanent magnet 64 fixedly positioned below each of` the heater coils /~0 and 40', a ~tationary electrical contact 66 fixedly positiotled underneath each of the heater coils 40 and 40' and above the permanent magnet 64, and a movable electrical contact 68 which i~ vertically movable between the permanent magnet ~4 and each of the heater coils 40 and 40'.
Each of the switche~ 94 and 94' is thus biased to ~e normally open and is actuated when a magnetic kitchen implement such as a cooking pan 56 or 56' formed of steel or iron is placed in the vicinity of the heater coil 40 or 40', Each of the timers 92 and 92' includes, by way of example, an integrating circuit (not shown) and is thus operative to produce an output signal when kept supplied with an input signal for a predetermined period of time.
The output signal thus delivered from each of the timers 92 and 92' is fed into the switch control circuit 90 and actuates the switch control circuit 90 into a condition causing the switching elements 82 and 84 or the switching elements 82' and 84' to open. As a Consequence~ the heater coil 40 or ko ~ is de-energized when the switch 94 or 94' associated therewith i8 kept open for a certain period of time which i~ prescribed on the timer 92 or 92', respectively. When, thus, the . ~ .
' .
~062340 magnetic load positioned in the vicinity of one of -the heater coils, say the heater coil 40 for example, and consequently the switch 94 associated with the heater coil 40 is made open instantaneously or for a short while, the switch control circuit 90 is kept inoperative in the absence of an output signal from the timer 92 associated with the heater coil 40. If, however, the magnetic load which has been placed in the vicinity of the heater coil 40 i8 moved away from the heater coil 40 for a period of time longer than the time period prescribed on the timer 90, the timer 92 is made operative to produce an output signal so that the ~witch control circuit 90 is actuated to open the switching elements 82 and 84 to open. As a consequence, the heater coil 40 in an unloaded condition is disconnected from the power inverter 12 whether the switching elements 82' and 84' associated with the other heater coil 40' are open or closed. When one of the heater coils 40 and 40' i9 energized and the other thereof i8 de-energized as in the condition above described, either of the switching elements 84 and 84' i8 open so that the former heater coil i8 connected to the power inverter 12 through the filter capacitor 38 or 30' associated with the heater coil. When, however, both of the heater coils 40 and 40' are energi~ed . - 32 -simultaneously with all of the switching elements 8 and 82~ and the switching elements 84 and 84~ kept closed, the heater coils 40 or 40 ' are connected in parallel to the power inverter through the filter capacitors 38 and 38 ~, respectively. The switches 84 and 84 ~ are thus adapted to prevent each of the heater coils 40 and 40' from being subjected to an increased load impedance when one of the heater coils 40 and 40 ' is energized with the other of the heater coils de-energized. The switching elements 84 and 84 ~ may therefore be dispensed with if such a consideration need not be paid.
If desired, each of the switches 94 and 94' arranged similarly to the switch means 54 and 54 ~ of '!15 the embodiment shown in Fig. 1 may be replaced with suitable protective means responsive to placement of a rejectable magnetic load in the vicinity of each of the heater coils 40 and 40'. The protective means may be arranged similarly to the protective means incorporated 'O into each of the embodiments shown in Figs. 3 and 4, comprising a current transformer having a primary c$rcuit connected in series with each of the heater coils 40 and 40' and a comparator having an input terminal connected to the secondary circuit of the current transformer and arranged es~entially ~imilarly , ., .
.: - 33 -to each of the comparators 76 and 76' shown in Eig. 3 or 4. The comparator is connected through each of _ the above mentioned timers 92 and 92' to the switch control circuit 90. Each of the heater coils 40 and 40' provided with the protective means thus arranged is kept energized when an acceptable magnetic load such as a cooking pan 56 or 56' positioned over the heater coil 40 or 40' is moved relative to the heater coil and consequently the current flowing through the heater coil is diminished instantaneously or only for a short while to such a level as would be produced in the heater coil if a rejectable magnetic load is applied to the heater coil 40 or 40'.
Claims
1. An induction heating apparatus comprising a d.c.
power supply circuit, a static power inverter connected to the power supply circuit for producing a high-frequency oscillating current, an induction heating unit including a plurality of heater coils which are jointly connected to the static power inverter for producing a commutating magnetic field around each of the heater coils which are energized from the static power inverter, and protective means responsive to an improper load applied to at least one of said heater coils for disabling the heater coil from being energi-zed from the power inverter, said protective means including a timing circuit operative to provide an output signal in response to an improper load kept applied to at least one of said heater coils for a predetermined period of time for disabling each of the heat-er coils when the improper load is applied thereto for said prede-termined period of time.
power supply circuit, a static power inverter connected to the power supply circuit for producing a high-frequency oscillating current, an induction heating unit including a plurality of heater coils which are jointly connected to the static power inverter for producing a commutating magnetic field around each of the heater coils which are energized from the static power inverter, and protective means responsive to an improper load applied to at least one of said heater coils for disabling the heater coil from being energi-zed from the power inverter, said protective means including a timing circuit operative to provide an output signal in response to an improper load kept applied to at least one of said heater coils for a predetermined period of time for disabling each of the heat-er coils when the improper load is applied thereto for said prede-termined period of time.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP49069313A JPS50160849A (en) | 1974-06-17 | 1974-06-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1062340A true CA1062340A (en) | 1979-09-11 |
Family
ID=13398933
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA229,507A Expired CA1062340A (en) | 1974-06-17 | 1975-06-17 | Improper load protection for induction heating appliance |
Country Status (8)
| Country | Link |
|---|---|
| JP (1) | JPS50160849A (en) |
| AU (1) | AU475059B2 (en) |
| CA (1) | CA1062340A (en) |
| DE (2) | DE2526877C3 (en) |
| FR (1) | FR2275105A1 (en) |
| GB (1) | GB1515131A (en) |
| NL (1) | NL164453C (en) |
| SE (1) | SE408120B (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3224738C1 (en) * | 1982-07-02 | 1984-02-09 | Aeg-Elotherm Gmbh, 5630 Remscheid | Device for inductive heating of a workpiece by means of several inductors |
| AU543894B2 (en) * | 1982-09-13 | 1985-05-09 | Tokyo Shibaura Denki Kabushiki Kaisha | Electromagnetic induction heating apparatus |
| GB2198618B (en) * | 1986-11-25 | 1990-06-13 | Ti Creda Ltd | Improvements in or relating to induction heating circuits for cooking appliances |
| DE3827856A1 (en) * | 1988-08-17 | 1990-02-22 | Voss Hermann | Method and arrangement for monitoring an electric line |
| DE4038560A1 (en) * | 1990-12-04 | 1992-06-11 | Licentia Gmbh | Power control of microwave and induction heating cooker - involves microprocessor limiting load on inverter by switching of microwave oscillator and regulation of induction energy |
| ES2321262B1 (en) * | 2007-03-13 | 2010-03-04 | Bsh Electrodomesticos España S.A. | COOKING FIELD AND PROCEDURE FOR THE DETECTION OF A KITCHEN BATTERY PIECE. |
| KR102626705B1 (en) | 2018-10-10 | 2024-01-17 | 엘지전자 주식회사 | Induction heating device having improved switch stress reduction structure |
| EP4033153A1 (en) * | 2021-01-20 | 2022-07-27 | Electrolux Appliances Aktiebolag | Domestic appliance with stand-by function |
-
1974
- 1974-06-17 JP JP49069313A patent/JPS50160849A/ja active Pending
-
1975
- 1975-06-16 SE SE7506891A patent/SE408120B/en unknown
- 1975-06-16 DE DE2526877A patent/DE2526877C3/en not_active Expired
- 1975-06-16 NL NL7507140.A patent/NL164453C/en not_active IP Right Cessation
- 1975-06-16 FR FR7518779A patent/FR2275105A1/en active Granted
- 1975-06-16 DE DE2559519A patent/DE2559519C3/en not_active Expired
- 1975-06-17 CA CA229,507A patent/CA1062340A/en not_active Expired
- 1975-06-17 AU AU82174/75A patent/AU475059B2/en not_active Expired
- 1975-06-17 GB GB25743/75A patent/GB1515131A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| NL164453C (en) | 1980-12-15 |
| FR2275105B1 (en) | 1980-04-04 |
| DE2526877B2 (en) | 1977-12-15 |
| SE408120B (en) | 1979-05-14 |
| DE2559519A1 (en) | 1977-03-10 |
| NL7507140A (en) | 1975-12-19 |
| FR2275105A1 (en) | 1976-01-09 |
| DE2526877A1 (en) | 1976-01-08 |
| AU8217475A (en) | 1976-08-12 |
| AU475059B2 (en) | 1976-08-12 |
| DE2559519C3 (en) | 1980-02-21 |
| JPS50160849A (en) | 1975-12-26 |
| DE2559519B2 (en) | 1979-06-21 |
| SE7506891L (en) | 1975-12-18 |
| GB1515131A (en) | 1978-06-21 |
| NL164453B (en) | 1980-07-15 |
| DE2526877C3 (en) | 1978-08-24 |
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