Abstrict
Apparatus is described for controlling an electric heater incorporating
at least one electric heating element (2A, 2B, 2C) and energized
from a single voltage alternating current supply (8). The apparatus
includes a control knob (15) for manually selecting any one of a
predetermined number of power settings for the heater from the voltage
supply (8), the power settings varying progressively between a minimum
power setting and a maximum power setting. The apparatus further
includes a controller (10) adapted and arranged to supply to the
electric heater within a predetermined constant time interval in
each power setting a first dissipated power level and a second dissipated
power level. The first dissipated power level comprises a first
selected proportion of half cycles in a predetermined number of
half cycles of the voltage supply and the second dissipated power
level comprises a second selected proportion of half cycles in a
predetermined number of half cycles of the voltage supply. In a
plurality of power settings the second selected proportion is different
to the first selected proportion. There is no more than one change
between the first and second dissipated power levels within the
predetermined constant time interval.
Claims
I claim:
1. Apparatus for controlling an electric heater incorporating at
least one electric heating element and energised from a single voltage
alternating current supply, the apparatus comprising:
means for manually selecting any one of a predetermined number
of power settings for the heater from the voltage supply, the power
settings varying progressively between a minimum power setting and
a maximum power setting; and
control means adapted and arranged to supply in continuing sequence
to the electric heater in a plurality of power settings a first
dissipated power level comprising a first non-zero proportion of
half cycles in a predetermined first plurality of half cycles of
the voltage supply and a second dissipated power level comprising
a second non-zero proportion of half cycles in a predetermined number
of half cycles of the voltage supply, the second proportion being
different to the first proportion and each pair of first and second
proportions occupying a predetermined constant time interval, there
being no more than one change between the first and second dissipated
power levels within the predetermined constant time interval.
2. Apparatus according to claim 1, wherein the predetermined number
of half cycles of the voltage supply, for the first and second selected
proportions of half cycles, is three.
3. Apparatus according to claim 2, wherein the first and second
selected proportions of half cycles in the predetermined number
of half cycles is selected from 0/3, 1/3, 2/3 and 3/3.
4. Apparatus according to claim 3, wherein the heater has a full
or rated power of 1200 to 3000 watts, the at least one heating element
exhibiting a maximum change in electrical resistance of the order
of 10 percent between room temperature and operating temperature.
5. Apparatus according to claim 4, wherein the heater has only
one heating element, the predetermined constant time interval having
a minimum value of 0.3 to 12 seconds.
6. Apparatus according to claim 4, wherein the heater has two heating
elements arranged for parallel operation, the predetermined constant
time interval having a minimum value of 0.2 to 4 seconds.
7. Apparatus according to claim 6, wherein the two heating elements
have substantially the same electrical resistance value, each element
having a full or rated power of 600 to 1500 watts.
8. Apparatus according to claim 4, wherein the heater has three
heating elements arranged for parallel operation, the predetermined
constant time interval having a minimum value of 0.1 to 1.5 seconds.
9. Apparatus according to claim 8, wherein the three heating elements
have substantially the same electrical resistance value, each element
having a full or rated power of 400 to 1000 watts.
10. Apparatus according to claim 3, wherein the heater has a full
or rated power of 1200 to 3000 watts and a heating element comprising
a bright radiating component element exhibiting a large change in
electrical resistance between room temperature and operating temperature
and connected in series with a ballast component element exhibiting
a relatively small maximum change in electrical resistance of the
order of 10percent between room temperature and operating temperature,
and in which about one half of the full or rated power is dissipated
in the bright radiating component element, the predetermined constant
time interval having a minimum value of 2.5 to 50 seconds.
11. Apparatus according to claim 10, wherein the heating element
comprising a bright radiating component element is made of a material
selected from tungsten and molybdenum disilicide.
12. Apparatus according to claim 3, wherein the heater has a full
or rated power of 1200 to 3000 watts and a heating element comprising
a bright radiating component element exhibiting a large change in
electrical resistance between room temperature and operating temperature
and connected in series with a ballast component element exhibiting
a relatively small maximum change in electrical resistance of the
order of 10percent between room temperature and operating temperature,
and in which about two thirds of the full or rated power is dissipated
in the bright radiating component element, the predetermined constant
time interval having a minimum value of 6 to 120 seconds.
13. Apparatus according to claim 12, wherein the heating element
comprising a bright radiating component element is made of a material
selected from tungsten and molybdenum disilicide.
14. Apparatus according to claim 1, wherein the control means comprises
at least one microprocessor-based device.
15. Apparatus according to claim 14, wherein the control means
supplies power to the heater by operation of at least one solid
state switching device.
16. Apparatus according to claim 15, wherein the at least one solid
state switching device comprises a triac.
17. Apparatus according to claim 15, wherein the heater incorporates
a plurality of heating elements, each heating element being supplied
with power by operation of a separate solid state switching device.
18. Apparatus according to claim 1, wherein the means for manually
selecting any one of a predetermined number of power settings is
selected from an analog potentiometer, an encoded digital switch
and a touch control system.
19. Apparatus according to claim 18, wherein the means for manually
selecting any one of a predetermined number of power settings is
selected from an analog potentiometer and an encoded digital switch,
the manually selecting means being of rotary form, operable by a
control knob.
20. Apparatus according to claim 1, wherein the progressive variation
of power settings between the minimum power setting and the maximum
power setting follows a geometric progression.
21. A cooking appliance comprising a heater and apparatus for controlling
the heater, the heater incorporating at least one electric heating
element and being energized from a single voltage alternating current
supply, and the apparatus comprising:
means for manually selecting any one of a predetermined number
of power settings for the heater from the voltage supply, the power
settings varying progressively between a minimum power setting and
a maximum power setting; and
control means adapted and arranged to supply in continuing sequence
to the electric heater in a plurality of power settings a first
dissipated power level comprising a first non-zero proportion of
half cycles in a predetermined first plurality of half cycles of
the voltage supply and a second dissipated power level comprising
a second non-zero proportion of half cycles in a predetermined second
plurality of half cycles of the voltage supply, the second proportion
being different to the first proportion and each pair of first and
second proportions occupying a predetermined constant time interval,
there being no more than one change between the first and second
dissipated power levels within the predetermined constant time interval.
Description This invention concerns apparatus for controlling electric heaters
and particularly, but not exclusively, electric heaters for use
in cooking appliances such as glass-ceramic smooth top cooking appliances.
DESCRIPTION OF PRIOR ART
Heaters in cooking appliances, such as glass-ceramic cooking appliances,
are most generally controlled by well known forms of electromechanical
energy regulators. Although such devices are relatively inexpensive,
they do have limitations. One frequently cited problem resulting
from their use is poor cooking performance at low simmering powers.
As a result of their mode of operation it is difficult to achieve
satisfactory operation to achieve low power dissipations of less
than ten percent of rated power. In the case of heaters having a
heating element of typical wire or ribbon form of relatively low
temperature coefficient of resistance, this is typically only achieved
by cyclically operating heaters such that they have an "on"
time of the order of five or six seconds followed by a long "off"
time typically of the order of one minute.
This type of switching to achieve low power can be detrimental
to cooking processes, resulting in comparatively short periods of
concentrated heat, followed by comparatively long periods of cooling.
The problem is exacerbated by the use of low mass cooking utensils,
such as of aluminum, together with fast-responding heaters and the
relatively low thermal mass of glass-ceramic appliances. Similar
problems are also encountered with recently proposed contact heater
appliances incorporating thin cooking plates of low thermal mass
and high thermal conductivity, and sometimes also with sheathed
forms of heating elements.
The uneven supply of power from a typical electromechanical energy
regulator can be smoothed out to some extent by adopting an arrangement
of series/parallel switching of multiple heating elements but the
complexity of the heater and its controller is undesirably increased,
with only a limited number of power settings available.
One possible solution to the problem would be to consider drastically
reducing the cycling time period of the electromechanical energy
regulator. This is difficult due to the principle of operation of
such a regulator and furthermore the life expectancy of electromechanical
switch contacts in the regulator would be unacceptably short as
a result of the increased switching rate.
Another problem with the use of short cycle times is that international
standards exist which seek to prevent repeated connection and disconnection
to and from a mains supply network of electrical loads which draw
a significant electric current. Resulting changes in current produce
small changes in voltage in the mains network and this results in
an annoying flicker of electric lights connected to the same mains
supply network.
The international standards which determine permitted cycle times,
switching frequencies and associated load powers are incorporated
into European Union law under the auspices of the Electro Magnetic
Compatibility (E.M.C.) Directive and compliance therewith is therefore
mandatory in all European Union countries.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome or minimize
the above mentioned problems.
SUMMARY OF THE INVENTION
The present invention provides an apparatus for controlling an
electric heater incorporating at least one electric heating element
and energized from a single voltage alternating current supply,
the apparatus comprising:
means for manually selecting any one of a predetermined number
of power settings for the heater from the voltage supply, the power
settings varying progressively between a minimum power setting and
a maximum power setting;
control means adapted and arranged to supply to the electric heater
within a predetermined constant time interval in each power setting
a first dissipated power level comprising a first selected proportion
of half cycles in a predetermined number of half cycles of the voltage
supply and a second dissipated power level comprising a second selected
proportion of half cycles in a predetermined number of half cycles
of the voltage supply, the second selected proportion being different
to the first selected proportion in a plurality of power settings,
there being no more than one change between the first and second
dissipated power levels within the predetermined constant time interval.
The first and second dissipated power levels may be the same only
in the maximum power setting for the heater.
The predetermined number of half cycles of the voltage supply,
for the first and second selected proportions of half cycles, may
be three and the first and second selected proportions of half cycles
in the predetermined number of half cycles may be selected from
0/3, 1/3, 2/3 and 3/3.
The heater may have a full or rated power of 1200 to 3000 watts,
the at least one heating element exhibiting a maximum change in
electrical resistance of the order of 10 percent between room temperature
and operating temperature.
For such a heater having only one heating element the predetermined
constant time interval may have a minimum value of 0.3 to 12 seconds.
For such a heater having two heating elements arranged for parallel
operation, the predetermined constant time interval may have a minimum
value of 0.2 to 4 seconds. The two heating elements may have substantially
the same electrical resistance value, each element having a full
or rated power of 600 to 1500 watts.
For such a heater having three heating elements arranged for parallel
operation, the predetermined constant time interval may have a minimum
value of 0.1 to 1.5 seconds. The three heating elements may have
substantially the same electrical resistance value, each element
having a full or rated power of 400 to 1000 watts.
In the case of a heater having a heating element with a full or
rated power of 1200 to 3000 watts and comprising a bright radiating
component element, for example of tungsten or molybdenum disilicide,
exhibiting a large change in electrical resistance between room
temperature and operating temperature and connected in series with
a ballast component element exhibiting a relatively small maximum
change in electrical resistance of the order of 10 percent between
room temperature and operating temperature, and in which about one
half of the full or rated power is dissipated in the bright radiating
component element, the predetermined constant time interval may
have a minimum value of 2.5 to 50 seconds.
In the case of a heater having a heating element with a full or
rated power of 1200 to 3000 watts and comprising a bright radiating
component element, for example of tungsten or molybdenum disilicide,
exhibiting a large change in electrical resistance between room
temperature and operating temperature and connected in series with
a ballast component element exhibiting a relatively small maximum
change in electrical resistance of the order of 10 percent between
room temperature and operating temperature, and in which about two
thirds of the full or rated power is dissipated in the bright radiating
component element, the predetermined constant time interval may
have a minimum value of 6 to 120 seconds.
The control means may comprise at least one microprocessor-based
device and may supply power to the heater by operation of at least
one solid state switching device, such as a triac.
When the heater incorporates a plurality of heating elements, each
heating element may be supplied with power by operation of a separate
solid state switching device.
The means for manually selecting any one of a predetermined number
of power settings may comprise an analog potentiometer, or an encoded
digital switch, or a touch control system. The potentiometer or
encoded digital switch may be of rotary form, operable by a control
knob.
The progressive variation of power settings between the minimum
power setting and the maximum power setting may follow a geometric
progression.
The apparatus of the invention may be applied for control of a
heater in a cooking appliance.
By means of the apparatus of the invention, switching cycle times
for control of a heater can be reduced, compared with those with
an electromechanical energy regulator of the prior art, whilst complying
with current international standards and directives with regard
to disturbance of mains power supply networks.
The invention is now described by way of example with reference
to the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of apparatus for controlling
an electric heater according to the invention;
FIG. 2 is a plan view of a heater, having a single heating element,
for use with the apparatus of the invention;
FIG. 3 is a plan view of a heater, having two heating elements,
for use with the apparatus of the invention;
FIG. 4 is a plan view of a heater, having three heating elements,
for use with the apparatus of the invention;
FIG. 5 is a table demonstrating operation of the apparatus of the
invention at various power settings;
FIG. 6 illustrates application of voltage pulses to a heater having
one, two and three heating elements, in control apparatus according
to the invention; and
FIG. 7 is a plan view of a heater having a heating element comprising
two component elements in series, one of which is a bright radiating
component element.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, apparatus is provided for controlling an electric
heater 1, which incorporates a single heating element 2A, or two
heating elements 2A, 2B arranged for parallel operation, or three
heating elements 2A, 2B, 2C, arranged for parallel operations. The
or each heating element comprises a metal, or metal alloy, such
as iron-chromium-aluminum alloy, such as in wire or ribbon form,
or a thick or thin film material deposited on a support, and exhibits
a maximum change in electrical resistance of the order of 10 percent
between room temperature and the operating temperature.
The heater 1 may be a radiant heater for use in a cooking appliance,
such as behind a plate of glass-ceramic material. FIG. 2 shows an
example of such a heater having a single heating element 2A, of
corrugated ribbon form, supported on a base 3 of insulation material,
such as microporous thermal and electrical insulation material,
in a metal dish 4.
In FIG. 3 an example is shown of a radiant heater having two heating
elements 2A, 2B, of coiled wire form, supported on a base 3 of insulation
material, such as microporous thermal and electrical insulation
material, in a metal dish 4.
In FIG. 4 an example is shown of a radiant heater having three
heating elements 2A, 2B, 2C, of coiled wire form, supported on a
base 3 of insulation material, such as microporous thermal and electrical
insulation material, in a metal dish 4.
In each of the heaters of FIGS. 2, 3 and 4, a terminal block 5
is provided by means of which the heaters may be electrically connected
for energisation an d control. A well-known form of temperature
limiter 6 is also provided.
A peripheral wall 7 of insulation material is provided, the top
of which may contact the rear side of a plate (not shown), such
as a glass-ceramic plate.
Referring once more to FIG. 1, the heater 1 is arranged for energisation
from a single voltage alternating current supply 8 by way of an
isolation switch 9.
A microprocessor-based controller 10, hereinafter referred to as
microcontroller 10, is provided having an oscillator 11 connected
thereto and provided with an input 12 from the mains supply to ensure
synchronisation. A simple form of power supply unit 13 is provided
for the microcontroller. The microcontroller is arranged to control
the heater 1 in burst fire manner, hereafter described, by way of
a solid state switch means such as a triac 14A, in the case of a
heater having one heating element 2A. When the heater 1 has two
heating elements 2A, 2B, control thereof is arranged by way of two
triacs 14A, 14B. In the case of a heater 1 having three heating
elements 2A, 2B, 2C, control thereof is arranged by way of three
triacs 14A, 14B, 14C.
A manually operable selector arrangement 15 is provided, cooperating
with the microcontroller to enable manual selection of any one of
a predetermined number of power settings for the heater from the
voltage supply. Such manually operable selector arrangement 15 can
comprise an analog potentiometer with a suitable control knob, or
a suitably encoded digital switch. Such a switch may comprise several
contacts moving over a suitably encoded section of a printed circuit
track and may be of rotary form. Such a potentiometer or digital
switch is also arranged to operate the isolation switch 9, in an
`off` position, to isolate the heater from the mains voltage supply
8 in such `off` position.
A touch, or other electronic, control system could alternatively
be used as the manually operable selector arrangement 15, and cooperating
with the microcontroller 10.
The control circuit is adapted and arranged to supply to the heater
1 within a predetermined constant time interval in each power setting
a first dissipated power level comprising a first selected proportion
of half cycles in a predetermined number of half cycles of the voltage
supply and a second dissipated power level comprising a second selected
proportion of half cycles in a predetermined number of half cycles
of the voltage supply. The second selected proportion is arranged
to be different to the first selected proportion in a plurality
of power settings. Without consideration of the initial switching
on of the heater and the final switching off of the heater it is
arranged that there is no more than one change between the first
and second dissipated power levels within the predetermined constant
time interval.
It may be arranged for the first and second dissipated power levels
to be the same only in the maximum power setting for the heater,
ignoring in this respect the `off` setting for the heater.
The predetermined number of half cycles of the voltage supply for
the first and second selected proportions of half cycles is arranged
to be three and it is arranged for the first and second selected
proportions of half cycles in the predetermined number of half cycles
to be selected from 0/3, 1/3, 2/3 and 3/3. In this regard it will
be understood that 0/3 represents a situation where the heater is
not energized and therefore the dissipated power level is zero.
3/3 represents a full power condition in which the supply voltage
in full and complete cyclic form is applied to the heater.
As hereinafter demonstrated, the predetermined constant time interval,
which is constantly repeating while the heater is turned on, is
selected according to the full or rated power of the heater and
whether the heater has one, two or three heating elements.
FIG. 5 is a table demonstrating control of a heater 1 having a
single heating element 2A with a full or rated power of about 2100
watts, or less, at 230 volts. A control knob is arranged to be manually
rotatable through about 360 degrees to effectively provide sixteen
power settings varying progressively between a minimum power setting
and a maximum power setting. Such manual control is represented
by reference numeral 15 in FIG. 1. A predetermined constant time
interval of 4 seconds is arranged and the microcontroller 10 (FIG.
1) is programmed to provide, in the first eleven power settings,
first dissipated power levels in the heating element 2A of the heater
1 in which one half cycle in three half cycles of the voltage supply
is applied for selected proportions of the constant time interval
of 4seconds and second power levels in the heating element 2A in
which the heating element 2A is not energized (i.e. no half cycles
in three half cycles of the voltage supply applied) for the remaining
proportions of the constant time interval.
In the twelfth, thirteenth and fourteenth power settings, the microcontroller
is programmed to provide first dissipated power levels in the heating
element 2A of the heater 1 in which one half cycle in three half
cycles of the voltage supply is applied for selected proportions
of the constant time interval of 4seconds. Second dissipated power
levels are provided in the heating element 2A in which two half
cycles in three half cycles of the voltage supply are applied for
the remaining proportions of the constant time interval.
In the fifteenth power setting, the microcontroller is programmed
to provide a first dissipated power level in the heating element
2A in which two half cycles in three half cycles of the voltage
supply are applied for 2.77seconds (i.e. 69.3 percent) of the constant
time interval of 4 seconds and three half cycles in three half cycles
of the voltage supply are applied for the remaining 1.23seconds
(i.e. 30.7 percent) of the constant time interval.
In the sixteenth power setting, which is the full power setting,
the microcontroller is programmed to provide first and second dissipated
power levels in the heating element 2A which are the same, as a
result of applying the voltage supply in full and complete cyclic
form (three half cycles in three half cycles) to the heating element
for the constant time interval, and indeed continuously.
It will be noted from FIG. 5 that no more than one change occurs
between the first and second dissipated power levels within the
constant time interval and this, together with the selected value
for the constant time interval, results in minimum disturbance to
mains power supply networks, particularly with regard to causing
flickering of lights connected to the same supply network and meeting
the standards specified by the Electro Magnetic Compatibility (E.M.C.)
Directive of the European Union.
Furthermore, extremely good performance is achieved in low power
settings of the apparatus, whereby excellent simmering performance
in cooking operations is achieved.
Referring again to FIG. 5, the second column of the table (headed
Power %) shows the dissipated power in the heating element in each
power setting as a percentage of the full or rated power and it
should be noted that the progressive variation of power settings
between the minimum and the maximum follows a geometric progression.
In the third, fourth, fifth and sixth columns in FIG. 5, in addition
to the lists of figures given for the times in seconds in which
the selected proportions of half cycles in three half cycles are
applied in relation to the constant time interval, lists of figures
(headed %) are also given which represent the percentages of the
constant time interval for which the indicated selected proportions
of half cycles in three half cycles of the voltage supply are applied
to the heater. Such percentage figures can be used to determine
specific times in seconds, in the various selected power settings,
for application of the first and second selected proportions of
half cycles in three half cycles of the voltage supply when a different
constant time interval is adopted, which is necessary with heaters
of different full or rated powers or when two or three heating elements
are used instead of one.
For heaters having a single heating element with a full or rated
power between 1200 watts and 3000 watts at 230 volts, a predetermined
constant time interval having a minimum value between 0.3 seconds
(1200 watts) and 12 seconds (3000 watts) is suitable. Specific time
data can be derived from this, by reference to the percentage of
constant time interval data against the various power settings in
FIG. 5.
The following table provides indications of minimum values of predetermined
constant time intervals for heaters having rated powers at 230 volts
between 1200 and 3000 watts and having one, two or three heating
elements, the multiple elements being arranged for parallel operation.
______________________________________ Rated Power of No. of Heating
Minimum Constant Time Heater (Watts) Elements Interval (Seconds)
______________________________________ 3000 1 12 2 4 3 1.5 2400
1 5.2 2 1.5 3 0.5 2100 1 3.5 2 0.8 3 0.3 1800 1 1.9 2 0.3 3 0.2
1500 1 0.8 2 0.2 3 0.2 1200 1 0.3 2 0.2 3 0.1 ______________________________________
In the case of a heater having two heating elements 2A, 2B or three
heating elements 2A, 2B, 2C, it is preferred for the two heating
elements 2A, 2B to have substantially the same electrical resistance
values and be arranged for parallel operation and for the three
heating elements 2A, 2B, 2C to have substantially the same electrical
resistance values and also be arranged for parallel operation. This
means, for example, that for a heater having a full or rated power
of 2400 watts at 230 volts, and having two heating elements, each
element would have a full or rated power of 1200 watts. For a corresponding
heater having three heating elements, each element would have a
full or rated power of 800 watts.
For such heaters a preferred technique for control is illustrated
in FIG. 6. In FIG. 6, section (a) illustrates a burst firing sequence
of one half cycle in three half cycles of the supply voltage applied
to a heater having a single heating element 2A, by way of a triac.
In this respect, reference should be made to FIG. 1 with regard
to heating element 2A and triac 14A.
Also in FIG. 6, section (b) illustrates the burst firing sequence
of one half cycle in three half cycles of the supply voltage applied
to a heater having two heating elements 2A and 2B, by way of two
triacs. In this respect, reference should be made to FIG. 1 with
regard to heating elements 2A, 2B and triacs 14A, 14B. As shown
in section (b) of FIG. 6, the half cycle bursts are applied sequentially
to the heating elements 2A and 2B and because these elements have
substantially the same electrical resistance values, substantially
no mains disturbance occurs, the effect as far as the control circuitry
and mains network are concerned being substantially the same as
if a single heating element were being controlled.
Section (c) of FIG. 6 illustrates the burst firing sequence of
one half cycle in three half cycles of the supply voltage applied
to a heater having three heating elements 2A, 2B and 2C, by way
of three triacs. Reference should again be made to FIG. 1 with regard
to heating elements 2A, 2B, 2C and triacs 14A, 14B, 14C. As in the
case of the heater with two heating elements, the half cycle bursts
are applied sequentially to the three heating elements 2A, 2B and
2C and because these elements have substantially the same electrical
resistance values, substantially no mains disturbance occurs, the
effect as far as the control circuitry and mains network are concerned
being substantially the same as if a single heating element were
being controlled.
The apparatus of the invention is also applicable where the single
heating element 2A of FIG. 1 comprises two component elements 2A1
and 2A2 connected in series as illustrated in FIG. 7. A bright radiating
component element 2A1, comprising a tungsten filament inside an
envelope, such as of quartz, containing a halogenated atmosphere,
is connected in series with a coiled wire resistance element 2A2,
such as of iron-chromium-aluminum alloy. The tungsten filament of
the bright radiating component element has a large temperature coefficient
of resistance of positive sign, such that it exhibits a large change
in electrical resistance between room temperature and its operating
temperature. The coiled wire component element 2A2, which could
be replaced by an element of similar composition but of different
form, such as a ribbon element, exhibits a relatively small change
in electrical resistance (10percent maximum) between room temperature
and its operating temperature. It is connected in series with the
bright radiating component element 2A1 to serve as a ballast resistance
therefor, in well known manner, to damp inrush current through the
component element 2A1 when the heater is switched on. Instead of
the bright radiating element 2A1 being of tungsten-halogen form,
it could comprise another form, such as an element of molybdenum
disilicide.
The remainder of the heater of FIG. 7 is similar to the heaters
of FIGS. 2, 3 and 4, with regard to the supporting dish 4, insulation
3, peripheral wall 7, terminal block 5 and temperature limiter 6.
The heater of FIG. 7 is connected to control apparatus and a power
supply as shown in FIG. 1, the series combination of component elements
2A1 and 2A2 of FIG. 7 being equivalent to the element 2A in FIG.
1.
The heater of FIG. 7 may be designed to have a full or rated power
typically between 1200 watts and 3000 watts, typically at 230 volts.
A heater may be designed such that about one half of the rated power
is dissipated in the bright radiating component element 2A1, with
the remaining power being dissipated in the series-connected component
element 2A2. Alternatively a heater may be designed such that about
two thirds of the rated power is dissipated in the component element
2A1 and one third in the component element 2A2.
The control apparatus of FIG. 1 operates with the heater of FIG.
7 in similar manner as previously described with reference to the
heater of FIG. 2, the predetermined constant time interval and the
times for application of the first and second selected proportions
of half cycles in a predetermined number of half cycles of the voltage
supply being appropriately selected. For a heater rated at 1200
watts and having about one half of this power dissipated in the
bright radiating component 2A1, the predetermined constant time
interval may have a minimum value of the order of 2.5 seconds, while
for a similar heater rated at 3000 watts, the predetermined constant
time interval may have a minimum value of the order of 50 seconds.
Minimum values for the constant time interval may be readily established
pro-rata for heaters rated at powers between 1200 and 3000 watts.
Although these minimum values for the predetermined constant time
interval may seem long, particularly for heaters of the higher rated
powers, they are in fact about one third of the corresponding minimum
switching time intervals possible to meet mains disturbance regulations
with the electromechanical energy regulators of the prior art and
therefore representing a very real improvement over the prior art
regulators.
For a heater of FIG. 7 rated at 1200 watts and having about two
thirds of this power dissipated in the bright radiating component
element 2A1, the predetermined constant time interval may have a
minimum value of the order of 6 seconds, while for a similar heater
rated at 3000 watts, the predetermined constant time interval may
have a minimum value of the order of 120 seconds. These minimum
values are about one quarter of the corresponding minimum switching
time intervals possible to meet mains disturbance regulations with
the electromechanical energy regulators of the prior art.
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