Note: Descriptions are shown in the official language in which they were submitted.
The present invention generally relates to an
induction heating apparatus and ~lore particularly, to an
induction heating apparatus as applied, for example, to a
cooking oven which is arranged to detect, by the use of a
magnet, whether a container for heating is of magnetizable
material or not. In addition, a thermister is used to
control the temperature oE the container and consequently
that of an object to be heated to an optimum level.
Commonly, in an induction heating apparatus, for
example, in an induction heating cooking oven, if a
container made of non-magnetizable material is used for
heating, various problems such as the abnormal oscillation
of the inverter used in the supply, the impression of an
extremely large current to the heating coil, commutation
failure of control elements, etc. tend to take place due
to rapid variations of coil impedance. As a result, a
magnet member is provided so that the cooking oven can
only be started when a container of magnetizable material
is placed on a top plate of the oven. A thermistor is
also provided for detecting abnormally high temperatures
which give rise to a deterioration of the coil
insulation. The thermistor also controls energization of
the coil so as to maintain the object to be heated at an
optimum temperature.
An object of the present invention is to provide an
improved induction heating apparatus as applied, for
example, to a cooking oven in which there is employed a
magnet member having a bore formed therein, with a
thermistor being provided on the top plate of the
apparatus in a position corresponding to said bore,
Additionally, the curie temperature of the magnet member
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can be set at a predetermined level for a further
improvement in safety.
Another important object of the present invention is
to provide an improved induction heating apparatus in
which there is provided a magnet receptacle for setting or
restricting the vertical travel of the magnet member
through attraction thereo~ toward the container. The
thermistor is disposed in a recess of the magnet
receptacle so as to be pressed against the top plate
thereby protecting the thermistor from possible breakage
and also for simplifying assembly during manufacture.
~ still further object of the present invention is to
provide an improved induction heating apparatus of which
is simple in construction and stable in operation, and
which can be manufactured on a large scale at low cost.
In accordance with an aspect of the invention there is
provided an induction heating apparatus having an
induction heating coil provided below a top plate for
supporting a container of magnetizable material, said
induction heating apparatus comprising a thermistor
element provided on the undersurface of the top plate for
controlling energization of said induction heating coil, a
magnet member movably provided below said top plate so as
to be raised through attraction thereof to said container,
and a switching member provided below said magnet member
for selective energization and de-energization of said
induction heating coil in association with the movement of
the magnet member, said magnet mernber being formed therein
with a bore in a predetermined position corresponding to
said thermistor element so as to accommodate said
thermistor element into said bore upon rising of said
- 2 -
;
magnet member for preventing collislon therebetween.
In accomplishing these and other objects, according to
one preferred embodiment of the present invention, there
is provided an induction heating apparatus having an
induction heating coil disposed below a top plate for
supporting a container of magnetizable materialr and
comprising a thermistor element provided on the
undersurface of the top plate for controlling energization
of the induction heating coil. A magnet member is movably
provided below the top plate so as to be raised through
attraction thereof to the container. A switching member
is provided below the magnet member for selective
energization and de-energization of the induction heating
coil in association with the movement of the magnet
member. The magnet member has a bore located in a
predetermined position corresponding to the thermistor
element so as to accommodate the thermistor element into
the bore upon the rising of the magnet member.
In drawings which illustrate embodiments of the
invention:
Fig. 1 is a fragmentary side sectional view of a main
portion of a conventional induction heating apparatus;
Fig. 2 is a fragmentary side sectional view of a main.
portion of an improved induction heating apparatus
according to one preferred embodiment of the present
invention;
Fig. 3 is a fragmentary side sectional view of a
modification of the embodiment of Fig. 2;
Fig. 4 is an electrical block diagram showing the
circuit of the heating apparatuses o~ Figs. 2 and 3;
Fig. 5 is a fragmentary side sectional view of another
,,
L -~
modification; and
Fig. 6 is a fragmentary side sectional view of a
further modification of the embodiment of Fig. 5.
Before the description of the present invention
proceeds, it is to be noted that like parts are designated
by like reference numerals throughout the figures.
Fig. 1 shows the construction of a conventional
induction heating cooking oven. A thermistor Tm is
affixed at the central portion on the undersurface of a
top plate P of the cooking oven for more accurately
detecting the temperature of a container V. A magnet
member M is movably accommodated for movement only in the
vertical direction, within a magnet support tube Ms, which
is surrounded by an induction heating coil C supported by
a coil support Cs. A microswitch SW secured to a base
plate B of the oven through a fixing plate L in a position
below the magnet member M so as to be switched by the
vertical movement of the magnet member M. The magnet
member M is further accommodated in a cover member Mc to
be spaced from the thermistor Tm by a predetermined
amount, upon contact of the upper edge of the case member
Mc with the undersurface of the top plate P which prevents
the magnet member M from directly contacting the
thermistor Tm when the magnet member M is raised through
attraction thereof to the container V.
In the above conventional arrangement, however, there
is the disadvantages that the assembly of the oven is
extremely inefficient. The thermistor Tm must be affixed
to the undersurface of the top plate P and wiring for the
thermistor Tm has to be carried out by applying current
collecting paste, or the like, onto the top plate P. In
.1 ~
addition, there is a possibility that the cover member Mc
can strike against the undersurface of the top plate P and
breaks. This would result in the breakage of the
- thermistor Tm.
Fig. 2 shows a main portion of an improved induction
heating apparatus HA according to one preferred embodiment
of the present invention~ The heating apparatus HA
generally includes a top plate 1 for holding a container
or vessel V, a heat sensing element or thermistor 2
attached to the central portion on the undersurface of the
top plate 1, a magnet support tube 4 provided between the
central portion of the undersurface of the top plate 1 and
a bottom plate 6 of the apparatus housing and a magnet
member 3, for example, of a cylindrical shape. The ma~net
member 3 has a bore 3a formed therein and is movably
supported in the support tube 4 for movement in the
vertical direction. A microswitch 5 is provided in the
support tube 4 in a position below the magnet member 3 at
a predetermined height from the bottom plate 6 and is
secured to the bottom plate through a support plate 5Q so
as to be selectively turned ON or OFF by the vertical
movement of the magnet member 3. An induction coil or
heating coil 8 is supported by a coil support 7 and
surrounds the support tube 4 in a position adjacent to the
top plate 1. The thermistor 2 is supported and pressed
against the undersurface of the top plate 1 by one end of
a support rod R which extends through the bore 3a is
secured at its other end to the base plate 6. Thermistor
2 is arranged to be accommodated into the bore 3a of the
cylindrical magnet member 3 when the magnet member 3 is
raised towards the top plate 1. The curie temperature of
~ T `
the magnet member 3 is set to be lower than the
temperature at the undersurface of-the top plate 1 when
the temperature of the induction heating coil 8 has
reached its allowable limit.
In Fig. 3, there is shown a modification of the
induction heating apparatus HA of Fig. 2. In the modified
induction heating apparatus HB of Fig. 3, the support rod
R is replaced by a support rod RB which supports the
thermistor 2 at its one end, and is movably received at
its other end in a cylindrical bore 9a of a support tube 9
suitably secured to the base plate 6. A spring member Sp
is disposed around the support rod RB in a position
between the upper end of the rod RB and the upper edge of
the support tube 9 for normally urging the rod RB upwardly
in Fig. 3 so as to achieve a close contact of the
thermistor 2 to the undersurface of the top plate 1.
Since other constructions and functions of the
modified heating apparatus HB of Fig. 3 are generally
similar to those of the arrangement of Fig. 2, a detailed
description thereof is omitted here for brevity.
Fig. 4 shows an electrical block diagram for the
induction heating apparatus according to the present
invention. A first rectification and smoothing circuit 12
and the primary winding of a transformer 13 are connected
across the terminals of an AC power source E. The
induction heating coil ~ and the emitter and collector of
an output transistor 16 are connected in series between
the positive and negative terminals of the first
rectification and smoothing circuit 12. A protecting
diode 17 is connected in the reverse direction between the
collector and emitter of the output transistor 16. A
eapacitor 18 is eonneeted in parallel to the protecting
diode 17. A second rectification and smoothing circuit 14
is coupled to the secondary winding of the transformer
13. The positive terminal of the seeond reetifieation and
smoothing eireuit 14 is eonneeted to the positive terminal
of a monostable multi~vibrator 19 through the microswitch
S, and the negative terminal of the circuit 14 is coupled
to the ground terminals of the monostable multi-vibrator
19 and an astable multi-vibrator 20 which is coupled to
the monostable multi-vibrator 19. The output terminal of
the monostable multi-vibrator 19 is connected to the base
o~ the output transistor 16. The thermistor 2 is eoupled
to a temperature detecting eircuit 21 so as to control,
for example, the base input signal ~or the output
transistor 16 or output signal of the transistor 16 by the
output signal of the temperature deteeting eureuit 21.
By the above arrangement, the heating operation is
started by turning ON a starting switch (not shown), with
the eontainer V placed on the top plate 1.
If the eontainer V is of magnetizable material, the
magnet member 3 is raised through the attraetion thereof
towards the eontainer V, and thus, the mieroswitch 5,
whieh is normally open through depression by the magnet
member 3, is elosed to apply positive voltage to the
positive terminal of the monostable multi-vibrator 19.
The monostab].e multi-vibrator 19 produces pulses of a
width of predetermined time, rising at the timing of the
pulse periodieally developed from the astable
multi-vibrator 20, and by rendering the output transistor
16 eonductive with the output pulse of the monostab].e
multi-vibrator 19, the induction heating eoil 8 is
energized for the induction heating operation.
As the induction heating is continued, the teMperature
of the container V is raised, with a consequent increase
of the resistance value of the thermistor 2, and when the
temperature of the container V has reached the
predetermined level, and the resistance of the thermistor
2 has been decreased down to a predetermined value, the
temperature detecting circuit 21 functions to cut off the
input signal or output signal, etc. of the output
transistor 16.
Therefore, not only is the object to be cooked, which
is accommodated in the container V, maintained at an
optimum heating temperature, but also the abnormal heating
of the container V is positively prevented. Moreover, in
the case where there are some problems with the thermistor
2 or in the temperature detection circuit 21 the further
rising of the temperature of the container V causes the
temperature of the magnet member 3 to reach the curie
temperature. The magnet member 3 then loses its magnetism
and fall5 downward to open the microswitch 5. The
monostable multi~vibrator 19 stops producing pulses and
energization of the induction heating coil 8 is cut off.
Therefore, a further improvement is acheived for the safe
operation of the heating apparatus. On the other hand, if
the container V is of non-magnetizable material, the
magnet member 3 does not ascend and the microswitch 5
remains open and no heat is generated.
Since a bore 3a for accommodating the thermistor 2 is
provided in the magnet member 3, the magnet member never
strikes against the thermistor 2, thereby eliminating the
possibility of the magnet breaking the thermistor.
Furthermore, since the curie ternperature of the magnet
member is set to be lower than the al~owable temperature
limit at the undersurface of the top plate 1, an abnormal
temperature rise due to heating, especially heating
without objects, may be positively prevented even if the
thermistor is inoperative.
Referring to Fig. 5, there is shown a further
modification of the arrangement of Fig. 2. In this
modified induction heating apparatus ~C of Fig. 5, the
magnet support tube 4 and thermistor support rod R are
dispensed with and the magnet member 3 is movably
accommodated in a magnet receptacle or casing D which is
supported by a plate 5~-1 laterally extending from the
upper edge of the support plate 5~ which also supports a
microswitch 5. This arrangement restricts the lateral
movement of the magnet member 3. The magnet receptacle D
has a configuration which conforms with the external shape
of the magnet member 3 and has a recess Do at its center
corresponding to the bore 3a of the magnet member 3. In
the recess Do, there is disposed a resilient thermistor
support material 2f, for example, of expanded silicon
rubber or the like on which the thermistor 2 is mounted.
A heat collecting plate 2h is disposed on the thermistor 2
for effectively conducting the heat of the top plate 1 to
the thermistor 2. Moreover, since the height from the
upper surface of the magnet receptable D to the upper
surface of the heat collecting plate 2h is arranged to be
higher than the height from the upper surface of the
receptable D to the undersurface of the top plate 1, the
resilient thermistor support material 2f is normally
compressed between the undersurface of the top plate 1 and
1 9 ~
the bottom wall of the recess Do, and thus, the heat
collecting plate 2h and the thermistor 2 are pressed
against the undersurface of the top plate 1. The
microswitch ~ is disposed below the magne-t member 3 by the
support plate 5Q and is activated by the movement of the
magnet member 3 in a similar manner as in the arranyement
of Fig. 2.
In the arrangement of Fig. 5, when the container V of
magnetizable material is placed at a predetermined
position on the top plate 1, the magnet member 3 is
attracted by the container V and attached to the inner
face of the upper portion of the magnet receptacle D. The
microswitch contacts are closed to start the heating.
If, however, a container V of non-magnetizable
material is placed on the top plate 1, the magnet member 3
is not attracted, and the microswitch 5 is kept open, and
the heating coil 8 is not energized.
Since the magnet receptacle D remains stationary, with
only the magnet member 3 moving vertically within the
receptacle D, neither the magnet member 3 nor the magnet
receptacle D is brought into contact with the top plate 1
and the heat sensing element or thermistor 2, and thus,
not only are the impact noises reduced to a large extent,
but the top plate, magnet receptacle, thermistor, etc. are
completely free from the danger of breakage.
Furthermore, troublesome assembly procedures for
installing the heat sensing element, such as bonding,
application of electrically conductive paste, etc. are
eliminated by virtue of elastically supporting the
thermistor.
Referring to Fig.~6, there is shown another
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modification of the arrangement of Fig. 5. In the
modified heating apparatus HD of Fig. 6, the magnet
receptacle D of Fig. 5 is arranged -to be integrally formed
with the coil suppor~ 7D. The thermistor 2 is disposed in
the central recess Do in a sirnilar manner as in the
arrangement of Fig. 5. This configuration simplifies
construction and facilitates manufacture.
It should be noted here that, in the foregoing
embodiments, althouyh the present invention has been
mainly described with reference to an induction heating
apparatus as applied to cooking ovens, the concept of the
present invention is not limited in its application to the
cooking ovens alone, but may readily be applied to
induction heating apparatuses for industrial purposes in
general.
As is clear from the foregoing description, in the
induction heating apparatuses HC and HD according to the
present invention, since the magnet member and magnet
casing or the like are arranged not to be brought into
contact with the top plate, possible breakage of the top
plate, magnet receptable, heat sensing element, etc. i5
positively prevented. Moreover, owing to the arrangement
in which the heat sensing element is adapted to be pressed
against the top plate through the elastic supporting of
the heat sensing element by the magnet receptable, it
becomes unnecessary to bond the heat sensing element to
the top plate or to apply electrically conductive paste.
