Abstrict
A radiant electric heater assembly for a glass ceramic top cooker
includes four radiant electric heating elements and a thermal cut-out
device. Associated with the termal cut-out device is a switching
device for connecting the four heating elements in parallel with
each other to give a first power level setting and for connecting
the heating elements in a lower power configuration in which the
elements are connected as two elements connected in series with
each other and connected in series with a pair of elements connected
in parallel with each other. When the thermal cut-out device detects
a first predetermined temperature the heating elements are connected
in the lower power configuration until such time as the thermal
cut-out device detects a second predetermined temperature, lower
than the first predetermined temperature, when the heating elements
are reconnected in the configuration of the first power level setting.
Claims
I claim:
1. A radiant electric heater assembly for a glass ceramic top cooker
comprising:
a glass ceramic cooking surface;
four radiant electric heating elements;
a thermal cut-out device for protecting the glass ceramic cooking
surface from excessive temperatures; and
means associated with the thermal cut-out device adapted to automatically
connect the four heating elements in parallel with each other to
give a first power level setting, which in the event of the thermal
cut-out device detecting a first predetermined temperature is adapted
to automatically connect the heating elements in a lower power configuration
in which the elements are connected as two elements connected in
series with each other and connected in series with a pair of elements
connected in parallel with other, and which when the thermal cut-out
subsequently detects a second predetermined temperature lower than
the first predetermined temperature is adapted to automatically
reconnect the heating elements in the configuration of the first
power level setting.
2. A radiant electric heater assembly according to claim 1, wherein
the means associated with the thermal cut-out device includes switch
means for selecting between the configuration of the first power
level setting and a second, lower, power level setting in which
the heating elements are connected as two elements connected in
parallel with each other and connected in parallel with two elements
connected in series with each other and which in the event of the
thermal cut-out device detecting the first predetermined temperature
is adapted to automatically connect the heating elements in a lower
power configuration in which the four elements are connected in
series with each other, and which when the thermal cut-out device
subsequently detects the second predetermined temperature is adapted
to automatically reconnect the heating elements in the configuration
of the second power level setting.
3. A radiant electric heater assembly according to claim 2, wherein
the two heating elements which are connected in series with each
other in the lower power configuration of the first power level
setting correspond to the two heating elements which are connected
in parallel with each other in the higher power level configuration
of the second power level setting.
4. A radiant electric heater assembly according to claim 1, wherein
the electric heating elements are in the form of infra-red lamps.
Description FIELD OF THE INVENTION
The present invention relates to radiant electric heater assemblies
such as may be used in electric cookers having a glass ceramic cooking
surface.
DESCRIPTION OF PRIOR ART
Radiant electric heater assemblies are known which comprise a metal
dish containing a base layer of thermal and electrical insulating
material, a peripheral wall of insulating material and a plurality
of heating elements. The heating elements may be of bare resistance
wire or may be in the form of infra-red lamps. A thermal cut-out
device is provided to disconnect all power to the heating elements
in the event of overheating so as to protect the glass ceramic surface
from discoloration or even breakage and avoid heater failure. However,
in certain modes of operation the thermal cut-out device can operate
so frequently that the service life of the heater assembly may be
reduced. In addition the operation of the cut-out and the complete
de-energisation of the heater may create the impression that the
heater has malfunctioned.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a radiant electric
heater assembly which alleviates these problems.
SUMMARY OF THE INVENTION
According to the present invention there is provided a radiant
electric heater assembly for a glass ceramic top cooker comprising:
four radiant electric heating elements;
a thermal cut-out device; and
means associated with the thermal cut-out device adapted to connect
the four heating elements in parallel with each other to give a
first power level setting, which in the event of the thermal cut-out
device detecting a first predetermined temperature is adapted to
connect the heating elements in a lower power configuration in which
the elements are connected as two elements connected in series with
each other and connected in series with a pair of elements connected
in parallel with each other, and which when the thermal cut-out
device subsequently detects a second predetermined temperature lower
than the first predetermined temperature is adapted to reconnect
the heating elements in the configuration of the first power level
setting.
In addition to extending the service life, the radiant electric
heater assembly according to the present invention can also enable
at least one of the heating elements to be energised at all times
giving a visual indication that the heater is on.
The means associated with the thermal cut-out device may include
switch means for selecting between the configuration of the first
power level setting and a second, lower, power level setting in
which the heating elements are connected as two elements connected
in parallel with each other and connected in parallel with two elements
connected in series with each other and which in the event of the
thermal cut-out device detecting the first predetermined temperature
is adapted to connect the heating elements in a lower power configuration
in which the four elements are connected in series with each other,
and which when the thermal cut-out device subsequently detects the
second predetermined temperature is adapted to reconnect the heating
elements in the configuration of the second power level setting.
The two heating elements which are connected in series with each
other in the lower power configuration of the first power level
setting preferably correspond to the two heating elements which
are connected in parallel with each other in the higher power level
configuration of the second power level setting.
The electric heating elements are preferably in the form of infra-red
lamps .
For a better understanding of the present invention and to show
more clearly how it may be carried into effect reference will now
be made, by way of example, to the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view, partly cut away, of a radiant electric
heater assembly arranged beneath a glass ceramic cooking surface;
FIG. 2 is a circuit diagram illustrating one embodiment of a radiant
electric heater assembly according to the present invention;
FIG. 3 is a circuit diagram illustrating another embodiment of
a radiant electric heater assembly according to the present invention;
FIG. 4 is a circuit diagram showing in more detail the circuit
diagram of FIG. 3; and
FIG. 5 shows the switch positions for each power level setting
of the arrangement shown in FIG. 4.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a known radiant electric heater 2 arranged beneath
a glass ceramic cooking surface 1. The heater comprises a dish 3,
for example made of metal, the dish containing a base layer 4 of
thermal and electrical insulation material such as a microporous
material sold under the Registered Trade Mark MICROTHERM. A peripheral
wall 5, for example of ceramic fibre material, extends around the
internal periphery of the dish 3. Four heating elements 6 in the
form of infra-red lamps extend above the base layer 4 and a thermal
cut-out device 7 in the form of a probe-type differential expansion
switch extends between the heating elements 6 and operates to disconnect
the heating elements from a source of electrical energy (not shown)
should the probe of the thermal cut-out device exceed a first predetermined
temperature so as to protect the glass ceramic cooking surface from
excessive temperatures. After the thermal cut-out device has operated
to disconnect the source of electrical energy the temperature will
decrease and at a second predetermined temperature, lower than the
first predetermined temperature, the thermal cut-out device will
operate to connect the source of electrical energy once again.
In the embodiment of the radiant electric heater assembly according
to the present invention illustrated by the circuit diagram of FIG.
2 there are four heating elements 10 in the form of infra-red lamps
which may be rated at, for example, 1000 watts each and which are
arranged in a heater dish having a diameter of for example 300mm.
Different power settings for the heater can be obtained in a number
of ways. For example in one embodiment the different power levels
can be obtained by a switch which connects the lamps in various
parallel and series configurations, the configuration shown in FIG.
2 corresponding to full power. Alternatively, the different power
levels of the heater can be obtained by connecting connections 11
and 12 of the heater shown in FIG. 2 to a suitable control device
such as an electronic phase control or mark-to-space ratio regulator.
The thermal cut-out device incorporates four sets of contacts 13,14,15
and 16. However, in practice, only contacts 13 and/or 14 need to
be incorporated into the thermal cut-out device and the remaining
contacts 15,16 and possibly 14 or 13 can be incorporated into one
or more separate relays which can be triggered by the opening or
closing of the contacts 13 and/or 14. When the heater is first energised
the thermal cut-out device is in its normal position and allows
current to pass to the lamps 10 which in the full power configuration
illustrated are connected in parallel with each other. Contacts
13,14 and 15 are closed in this configuration.
If the temperature of the probe of the thermal cut-out device exceeds
a first predetermined value the cut-out device operates, not to
disconnect all of the heating elements from the source of electric
power, but to open the contacts 13,14 and 15 and to close contacts
16. This connects the lamps in a configuration in which two lamps
are connected in series and are connected in parallel to the remaining
two lamps which are also connected in series. This reduces the power
to substantially 34 per cent of full power and allows the temperature
of the glass ceramic to fall and the temperature of the probe of
the thermal cut-out device to fall to a second predetermined temperature
at which the contacts 16 are opened and the contacts 13,14 and 15
are closed to restore full power to the heating elements.
It is important in this embodiment that the contacts 15 should
open prior to the closing of the contacts 16 and that the contacts
16 should not close until after any arc caused by the opening of
the contacts 15 is extinguished. Similarly the contacts 16 should
open and any arc should be extinguished before the contacts 15 close.
The timing of the opening and closing of the contacts 13 and 14
is not critical.
We have found that, in circumstances where heat is not dissipated
from the cooking surface 1 quickly enough to avoid overheating,
the arrangement whereby the action of the thermal cut-out device
reduces the power of the lamps rather than disconnecting all power
to the lamps reduces the number of times the thermal cut-out device
switches in a given period and thus increases the useful life of
its contacts. Moreover, for any given period during which the heater
is energised the lamps 10 will run at full power for a lower proportion
of the time. This leads to an increased service life for the lamps.
Nevertheless, for the embodiment of the invention described with
reference to FIG. 2 it is vital that the contacts 15 should open
before the contacts 16 close and that the contacts 16 should open
before the contacts 15 close.
In the embodiment of the radiant electric heater assembly according
to the present invention illustrated by the circuit diagram of FIG.
3 there are four heating elements 10 as with the embodiment of FIG.
2. The configuration shown in FIG. 3 corresponds to full power while
the contacts 20,21 of the thermal cut-out device are closed.
If the temperature of the probe of the thermal cut-out device exceeds
a first predetermined value the thermal cut-out device operates
to open the contacts 20,21 and thus to connect two of the lamps
(the lower pair in the Figure) in parallel with each other and the
remaining lamps (the upper pair) in series with the parallel pair.
This reduces the power to substantially 18 per cent of full power
and allows the temperature of the glass ceramic to fall and the
temperature of the probe of the thermal cut-out device to fall to
a second predetermined temperature at which the contacts 20, 21
close to restore full power to the heating elements.
With this embodiment, in addition to increasing the service life
of the lamps and the contacts, the timing of the opening and closing
of the contacts is not crucial because no short circuit configuration
can exist. The contacts 21 carry the current of three lamps whereas
the contacts 20 carry the current of only two lamps.
In practice it is not necessary for both contacts 20,21 to be incorporated
into the thermal cut-out device and either set or both sets of contacts
could be incorporated into a separate relay.
Other configurations are also possible.
Although for simplicity we have described above the power being
reduced only from full power to a lower power configuration, and
indeed for heater control by phase control or mark-to-space ratio
regulators no further configuration may be needed. However, for
heater control by multi-position switch it is also possible to connect
the heating elements so that in the event of the thermal cut-out
device having detected a temperature which exceeds the first predetermined
value and with the multi-position switch set to any intermediate
power setting the heating elements are connected in a lower power
configuration or the same power configuration where the heater power
setting is low. This is illustrated in FIG. 4 in which the same
reference numerals as those used in FIG. 3 are employed to denote
the same components. FIG. 4 shows a more complete circuit diagram
in which the contacts 20, 21 are used to connect the lamps 10 in
a lower power configuration at a number of power level settings
if the temperature of the probe of the thermal cut-out device exceeds
a first predetermined value. FIG. 4 also shows contacts 22, 23,
24, 25, 26, 27 and 28 which are incorporated into a multi-position
switch and which in co-operation with diodes 29 and 30 are used
to connect the lamps in six different configurations giving six
different power levels plus an off setting. The power levels are
level 1 with all four lamps connected in series with each other
and with both diodes (7 per cent of full power), level 2 with all
four lamps connected in series (12 per cent of full power), level
3 with two pairs of lamps connected in series with each other and
with a diode, the two pairs being connected in parallel such that
the diodes conduct in alternate half cycles of the power supply
(20 per cent of full power), level 4 with two pairs of lamps connected
in series with each other, the two pairs being connected in parallel
and supplied via parallel opposite-poled diodes (34 per cent of
full power), level 5 with two lamps (the lower pair in FIG. 4) connected
in series, the two lamps being connected in parallel with the remaining
two lamps (67 percent of full power) and level 6 in which all four
lamps are connected in parallel (100 per cent of full power). FIG.
5 shows which of the contacts 22-28 should be open and which should
be closed to give any particular power level. At levels 1 and 2
it is considered unnecessary to switch to a lower power configuration
should the temperature of the probe of the thermal cut-out device
exceed the first or second predetermined values because this situation
should only arise if the level is changed from a high level to a
lower level while the temperature is in excess of one of the said
values and will quickly fall to below the second predetermined value.
At level 3 the contacts 20, 21 co-operate to connect the lamps so
that two lamps (the upper pair in FIG. 4) are connected in series
with each other and with the diode 30 in the event of an excessive
temperature, the remaining two lamps being de-energised. Similarly
at level 4 the contacts 20, 21 co-operate to connect two lamps (again
the upper pair in FIG. 4) in series with each other in the event
of an excessive temperature with the remaining two lamps being de-energised.
At level 5, the contacts 20, 21 co-operate to connect the four lamps
in series with each other in the event of an excessive temperature,
while level 6 has already been described in connection with FIG.
3. It can be seen that the lamps which carry a higher current in
normal operation at level setting 5 (the upper pair in FIG. 4) are
the same as the lamps which carry all the current at level setting
6 with the limiter contacts open (though they are then connected
in series). This facilitates arrangement of the lamps to provide
an aesthetically pleasing appearance at different power settings
and states of limiter operation. |