Note: Descriptions are shown in the official language in which they were submitted.
1062340
*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
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1062340
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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1062340
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
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~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
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106Z340
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
.
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106Z340
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
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1062340
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
1062340
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
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106Z340
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
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106Z340
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
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1062340
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
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~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
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1062340
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
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106Z340
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
.
16 -
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106Z340
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
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106Z340
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
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1062340
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
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106Z340
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
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:.
- 20 _
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106Z340
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 _
1062340
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
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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
,
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106Z340
"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
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_ 24 -
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~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
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106Z340
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
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106Z340
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
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1062340
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
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,
1062340
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
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1062340
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
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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
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~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
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106Z340
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
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106Z340
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'.
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