Abstrict
A radiant electric heater arrangement is provided for connection
to a dual voltage supply which provides a first alternating current
supply voltage and a second alternating current supply voltage higher
than the first. The arrangement includes a heater having at least
one heating element and a manually operable multiple discrete position
switch means for manually selecting in sequence a predetermined
number of discrete different power settings for the at least one
heating element from the voltage supply. The arrangement further
includes control means co-operating between the switch means and
the supply whereby in each position of a first sequential series
of discrete positions of the switch means the at least one heating
element is selectively energisable from the first supply voltage
in such a way that at least one selected proportion of half cycles
in a predetermined number of half cycles of the first supply voltage
is or are arranged for application to the at least one heating element,
and further whereby in each position of a second sequential series
of discrete positions of the switch means, following the first sequential
series, the at least one heating element is selectively energisable
from the second supply voltage in such a way that at least one selected
proportion of half cycles in a predetermined number of half cycles
of the second supply voltage is arranged for application to the
at least one heating element. In this way a stepwise increase in
power output from the heater is obtainable in operation from one
position of the switch means to the next through the first and second
sequential series of switch positions.
Claims
I claim:
1. A radiant electric heater arrangement for connection to a dual
voltage supply which provides a first alternating current supply
voltage and a second alternating current supply voltage higher than
the first, the arrangement comprising:
a heater having at least one heating element;
a manually operable multiple discrete position switch means for
manually selecting in sequence a predetermined number of discrete
different power settings for the at least one heating element from
the voltage supply;
control means co-operating between the switch means and the supply
whereby in each position of a first sequential series of discrete
positions of the switch means the at least one heating element is
selectively energisable from the first supply voltage in such a
way that at least one selected proportion of half cycles in a predetermined
number of half cycles of the first supply voltage is or are arranged
for application to the at least one heating element;
and further whereby in each position of a second sequential series
of discrete positions of the switch means, following the first sequential
series, the at least one heating element is selectively energisable
from the second supply voltage in such a way that at least one selected
proportion of half cycles in a predetermined number of half cycles
of the second supply voltage is arranged for application to the
at least one heating element; the arrangement being such that a
stepwise increase in power output from the heater is obtainable
in operation from one position of the switch means to the next through
the first and second sequential series of switch positions.
2. A heater arrangement according to claim 1, wherein the at least
one heating element comprises one or more elements selected from:
coiled bare resistance wire; metal ribbon; infra-red lamp(s); molybdenum
disilicide wire or strip.
3. A heater arrangement according to claim 2, wherein the metal
ribbon is of corrugated form.
4. A heater arrangement according to claim 2, characterised in
that the or each infra-red lamp comprises a tungsten filament inside
an envelope containing a halogenated atmosphere.
5. A heater arrangement according to claim 2, wherein the at least
one heating element is selected from an infra-red lamp and a molybdenum
disilicide wire or strip, each of which is permanently connected
in series with a second heating element.
6. A heater arrangement according to claim 5, wherein the second
heating element is selected from coiled bare resistance wire and
metal ribbon form.
7. A heater arrangement according to claim 5, wherein the second
heating element serves as a ballast resistor to damp inrush current
in the at least one lamp or molybdenum disilicide heating element.
8. A heater arrangement according to claim 1, wherein the dual
voltage supply comprises an alternating current supply having at
least two phases and in which the first alternating current supply
voltage is obtained by connecting to a line for one supply phase
and a neutral line and the second alternating current supply voltage
is obtained by connecting to lines for two supply phases.
9. A heater arrangement according to claim 1, wherein in the first
sequential series of discrete positions of the switch means the
at least one heating element is selectively energisable from the
first supply voltage in such a way that from one switch position
to the next a selectively increased proportion of half cycles in
a predetermined number of half cycles of the first supply voltage
is arranged for application to the at least one heating element
and in the second sequential series of discrete positions of the
switch means, following the first sequential series, the at least
one heating element is selectively energisable from the second supply
voltage in such a way that from one switch position to the next
in the second sequential series a selectively increased proportion
of half cycles in a predetermined number of half cycles of the second
supply voltage is arranged for application to the at least one heating
element.
10. A heater arrangement according to claim 9, wherein a switch
means having ten positions is provided; a first alternating current
supply voltage of substantially 120 volts and a second alternating
current supply voltage of substantially 240 volts is arranged and
in a first sequential series of six discrete positions the at least
one heating element is selectively energisable from the first supply
voltage such that:
a) in the first position, one half cycle in every seven half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
b) in the second position, one half cycle in every five half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
c) in the third position, one half cycle in every three half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
d) in the fourth position, two half cycles in every four half cycles
of the first supply voltage are arranged for application to the
at least one heating element;
e) in the fifth position, two half cycles in every three half cycles
of the first supply voltage are arranged for application to the
at least one heating element;
f) in the sixth position, the first supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element;
and in a second sequential series of four discrete positions, following
on from the first series and comprising positions seven to ten of
the switch means, the at least one heating element is selectively
energisable from the second supply voltage such that:
i) in the seventh position, one half cycle in every three half
cycles of the second supply voltage is arranged for application
to the at least one heating element;
ii) in the eighth position, two half cycles in every four half
cycles of the second supply voltage are arranged for application
to the at least one heating element;
iii) in the ninth position, two half cycles in every three half
cycles of the second supply voltage are arranged for application
to the at least one heating element;
iv) in the tenth position, the second supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element.
11. A heater arrangement according to claim 1, wherein in one or
more positions of the switch means each sequence of a predetermined
number of half cycles of the first or second supply voltage is separated
into first and second proportions, the second proportion being different
from the first proportion, with the first and second proportions
following each other cyclically in sequence and each in predetermined
ratio of total number of half cycles in each sequence.
12. A heater arrangement according to claim 11, wherein only the
first or the second selected proportion of half cycles in each sequence
results in the application of the supply voltage to the at least
one heating element.
13. A heater arrangement according to claim 12, characterised in
that a switch means having ten positions is provided; a first alternating
current supply voltage of substantially 230 volts and a second alternating
current supply voltage of substantially 400 volts is arranged and
in a first sequential series of seven discrete switch positions
the at least one heating element is selectively energisable from
the first supply voltage such that:
a) in the first position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 40 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 60 percent of each sequence;
b) in the second position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 55 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 45 percent of each sequence;
c) in the third position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 75 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 25 percent of each sequence;
d) in the fourth position, one half cycle in every three half cycles
of the first supply voltage is arranged for continuous application
to the at least one heating element;
e) in the fifth position, two half cycles in every three half cycles
of the first supply voltage are arranged for application cyclically
to the at least one heating element for about 40 percent of a sequence
of a predetermined number of half cycles and one half cycle in every
three half cycles of the first supply voltage is arranged for application
to the at least one heating element during the remaining about 60
percent of each sequence;
f) in the sixth position, two half cycles in every three half cycles
of the first supply voltage are arranged for continuous application
to the at least one heating element;
g) in the seventh position, the first supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element:
and in a second sequential series of three discrete positions,
following on from the first series and comprising positions eight
to ten of the switch means, the at least one heating element is
selectively energisable from the second supply voltage such that:
i) in the eighth position, two half cycles in every three half
cycles of the second supply voltage are arranged for application
cyclically to the at least one heating element for about 40 percent
of a sequence of a predetermined number of half cycles and one half
cycle in every three half cycles of the second supply voltage is
arranged for application to the at least one heating element during
the remaining about 60 percent of each sequence;
ii) in the ninth position, two half cycles in every three half
cycles of the second supply voltage are arranged for continuous
application to the at least one heating element;
iii) in the tenth position, the second supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element.
14. A heater arrangement according to claim 1, wherein the control
means comprises a microprocessor-based control system.
15. A heater arrangement according to claim 14, wherein the control
system is in association with a triac switching element.
16. A heater arrangement according to claim 1, wherein sequential
selection of the switch positions results in a stepwise increase
in power output from the at least one heating element, accompanied
by a corresponding stepwise increase in intensity of visible light
radiation from the at least one heating element.
17. A heater arrangement according to claim 16, wherein the at
least one heating element comprises a lamp.
18. A radiant electric heater arrangement for connection to a dual
voltage supply which provides a first alternating current supply
voltage of substantially 120 volts and a second alternating current
supply voltage of substantially 240 volts, the arrangement comprising:
a heater having at least one heating element;
a manually operable multiple discrete position switch means for
manually selecting in sequence ten discrete different power settings
for the at least one heating element from the voltage supply;
control means co-operating between the switch means and the supply
whereby in each position of a first sequential series of six discrete
positions of the switch means the at least one heating element is
selectively energisable from the first supply voltage in such a
way that:
a) in the first position, one half cycle in every seven half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
b) in the second position, one half cycle in every five half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
c) in the third position, one half cycle in every three half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
d) in the fourth position, two half cycles in every four half cycles
of the first supply voltage are arranged for application to the
at least one heating element;
e) in the fifth position, two half cycles in every three half cycles
of the first supply voltage are arranged for application to the
at least one heating element;
f) in the sixth position, the first supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element;
and in a second sequential series of four discrete positions of
the switch means, following on from the first sequential series
and comprising positions seven to ten of the switch means, the at
least one heating element is selectively energisable from the second
supply voltage such that:
i) in the seventh position, one half cycle in every three half
cycles of the second supply voltage is arranged for application
to the at least one heating element;
ii) in the eighth position, two half cycles in every four half
cycles of the second supply voltage are arranged for application
to the at least one heating element;
(iii) in the ninth position, two half cycles in every three half
cycles of the second supply voltage are arranged for application
to the at least one heating element; and
(iv) in the tenth position, the second supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element.
19. A radiant electric heater arrangement for connection to a dual
voltage supply which provides a first alternating current supply
voltage of substantially 230 volts and a second alternating current
supply voltage of substantially 400 volts, the arrangement comprising:
a heater having at least one heating element;
a manually operable multiple discrete position switch means for
manually selecting in sequence ten discrete different power settings
for the at least one heating element from the voltage supply;
control means co-operating between the switch means and the supply
whereby in each position of a first sequential series of seven discrete
positions of the switch means the at least one heating element is
selectively energisable from the first supply voltage such that:
a) in the first position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 40 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 60 percent of each sequence;
b) in the second position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 55 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 45 percent of each sequence;
c) in the third position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 75 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 25 percent of each sequence;
d) in the fourth position, one half cycle in every three half cycles
of the first supply voltage is arranged for continuous application
to the at least one heating element;
e) in the fifth position, two half cycles in every three half cycles
of the first supply voltage are arranged for application cyclically
to the at least one heating element for about 40 percent of a sequence
of a predetermined number of half cycles, and one half cycle in
every three half cycles of the first supply voltage is arranged
for application to the at least one heating element during the remaining
about 60 percent of each sequence;
f) in the sixth position, two half cycles in every three half cycles
of the first supply voltage are arranged for continuous application
to the at least one heating element;
g) in the seventh position, the first supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element;
and in a second sequential series of three discrete positions of
the switch means, following on from the first sequential series
and comprising positions eight to ten of the switch means, the at
least one heating element is selectively energisable from the second
supply voltage such that:
i) in the eighth position, two half cycles in every three half
cycles of the second supply voltage are arranged for application
cyclically to the at least one heating element for about 40 percent
of a sequence of a predetermined number of half cycles and one half
cycle in every three half cycles of the second supply voltage is
arranged for application to the at least one heating element during
the remaining about 60 percent of each sequence;
ii) in the ninth position, two half cycles in every three half
cycles of the second supply voltage are arranged for continuous
application to the at least one heating element; and
iii) in the tenth position, the second supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element.
20. A heater arrangement according to claim 19, wherein the control
system is in association with a triac switching element.
Description This invention relates to an arrangement of a radiant electric
heater and a multiple position switch means, which arrangement is
for connection to an alternating current power supply system providing
two different power supply voltages.
BACKGROUND TO THE INVENTION
Radiant electric heaters are well known in which one or more heating
elements are supported, for example, on or above a layer of thermal
insulation material in a metal dish. Heaters of this kind generally
incorporate one or more heating elements selected, for example,
from: coiled bare resistance wire; metal ribbon, particularly of
corrugated form; infra-red lamp(s).
Such heaters are typically incorporated in cooking, or other heating,
appliances having a flat glass-ceramic cooking/heating surface.
Different power levels have been provided by including two or more
heating elements in the heater, with a switch arranged to couple
the elements into different series and/or parallel configurations
in different switch positions.
In designing such heaters various different and possibly conflicting
requirements must be accommodated. The resistances of the different
heating elements must be chosen so that differing combinations of
the elements produce power levels which together form an appropriately
distributed sequence between zero and full power. In particular
it has been found generally desirable to include a very low power
level, of the order of 5 percent of full power. Where possible the
intensity of visible light radiation produced by the energised elements
at each power setting should be indicative of the power level at
that setting; in addition it may be desirable that one or more elements
should be energised sufficiently to produce some visible radiation
for as many power settings as possible, to provide assurance to
the user that the heater is functioning. This is particularly the
case for any infra-red lamp heating element that may be included
in the heater; the presence of an infra-red lamp element in an appliance
typically results in a premium price, so the user is likely to expect
that element to be visible in use.
DESCRIPTION OF PRIOR ART
U.S. Pat. No. 5,171,973 describes a radiant electric heater arrangement
which utilises series/parallel switching of at least two heating
elements and a diode. When a heating element comprising an infra-red
lamp is incorporated, this has usually to be operated permanently
connected in series with a further heating element which serves
as a ballast resistor to damp the inrush current of the lamp. In
such an arrangement, therefore, three heating elements are present
in the heater. When used with a multiple position switch and a dual
voltage supply (for example 120 V/240 V), as many as ten heat settings
can be obtained with a good spread of heating power over the range
of settings. The available range includes a very satisfactory low
power level of the order of 5 percent of full power.
This prior art arrangement does, however, suffer from disadvantages.
When used with an infra-red lamp heating element and two coiled
wire heating elements it is found that the illumination of the lamp
is not visible against the radiance of the other heating elements
in some lower power positions of the switch. Furthermore no change
in brightness of the lamp may be experienced between some adjacent
power positions of the switch.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome or minimise
these disadvantages by providing a radiant electric heater arrangement
which is operated from a dual voltage supply and may be used with
a single heating element (or single combination of two heating elements
in the case of an infra-red lamp where a permanently series-connected
ballast element is required) in association with a multiple position
(for example 10 position) switch to provide an excellent distribution
of heating powers and a stepwise visible gradation of element brightness
across the range of switch positions.
SUMMARY OF THE INVENTION
The present invention provides a radiant electric heater arrangement
for connection to a dual voltage supply which provides a first alternating
current supply voltage and a second alternating current supply voltage
higher than the first, the arrangement comprising:
a heater having at least one heating element;
a manually operable multiple discrete position switch means for
manually selecting in sequence a predetermined number of discrete
different power settings for the at least one heating element from
the voltage supply;
control means co-operating between the switch means and the supply
whereby in each position of a first sequential series of discrete
positions of the switch means the at least one heating element is
selectively energisable from the first supply voltage in such a
way that at least one selected proportion of half cycles in a predetermined
number of half cycles of the first supply voltage is or are arranged
for application to the at least one heating element;
and further whereby in each position of a second sequential series
of discrete positions of the switch means, following the first sequential
series, the at least one heating element is selectively energisable
from the second supply voltage in such a way that at least one selected
proportion of half cycles in a predetermined number of half cycles
of the second supply voltage is arranged for application to the
at least one heating element; the arrangement being such that a
stepwise increase in power output from the heater is obtainable
in operation from one position of the switch means to the next through
the first and second sequential series of switch positions.
The at least one heating element may comprise one or more elements
selected from: coiled bare resistance wire; metal ribbon, particularly
of corrugated form; infra-red lamp(s), particularly comprising a
tungsten filament inside a quartz or fused silica envelope containing
a halogenated atmosphere; molybdenum disilicide wire or strip.
Where a heating element comprising an infra-red lamp or a molybdenum
disilicide wire or strip is employed, this may for some applications
require the provision in the heater of a second heating element,
suitably of coiled bare resistance wire or metal ribbon form, permanently
connected in series with the infra-red lamp or molybdenum disilicide
wire or strip. Such second heating element serves as a ballast resistor
to damp inrush current in the lamp or molybdenum disilicide heating
element.
The dual voltage supply may comprise an alternating current supply
having at least two phases and in which the first alternating current
supply voltage is obtained by connecting to a line for one supply
phase and a neutral line and the second alternating current supply
voltage is obtained by connecting to lines for two supply phases.
For example, in the USA the two-phase supply in that country may
be used to provide a first supply voltage of 120 volts and a second
supply voltage of 240 volts. In some European countries, such as
Germany, where a supply voltage of two phases and a neutral line
is available, a first supply voltage of, for example, 230 volts
and a second supply voltage of, for example, 400 volts may be provided.
In one embodiment of the invention, in the first sequential series
of discrete positions of the switch means the at least one heating
element is selectively energisable from the first supply voltage
in such a way that from one switch position to the next a selectively
increased proportion of half cycles in a predetermined number of
half cycles of the first supply voltage is arranged for application
to the at least one heating element and in the second sequential
series of discrete positions of the switch means, following the
first sequential series, the at least one heating element is selectively
energisable from the second supply voltage in such a way that from
one switch position to the next in the second sequential series
a selectively increased proportion of half cycles in a predetermined
number of half cycles of the second supply voltage is arranged for
application to the at least one heating element.
In an example of such an embodiment, a switch means having ten
positions is provided; a first alternating current A supply voltage
of substantially 120 volts and a second alternating current supply
voltage of substantially 240 volts is arranged and in a first sequential
series of six discrete positions the at least one heating element
is selectively energisable from the first supply voltage such that:
a) in the first position, one half cycle in every seven half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
b) in the second position, one half cycle in every five half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
c) in the third position, one half cycle in every three half cycles
of the first supply voltage is arranged for application to the at
least one heating element;
d) in the fourth position, two half cycles in every four half cycles
of the first supply voltage are arranged for application to the
at least one heating element;
e) in the fifth position, two half cycles in every three half cycles
of the first supply voltage are arranged for application to the
at least one heating element;
f) in the sixth position, the first supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element;
and in a second sequential series of four discrete positions, following
on from the first series and comprising positions seven to ten of
the switch means, the at least one heating element is selectively
energisable from the second supply voltage such that:
i) in the seventh position, one half cycle in every three half
cycles of the second supply voltage is arranged for application
to the at least one heating element;
ii) in the eighth position, two half cycles in every four half
cycles of the second supply voltage are arranged for application
to the at least one heating element;
iii) in the ninth position, two half cycles in every three half
cycles of the second supply voltage are arranged for application
to the at least one heating element;
iv) in the tenth position, the second supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element.
The heater arrangement of the invention may be embodied such that
in one or more positions of the switch means each sequence of a
predetermined number of half cycles of the first or second supply
voltage is separated into first and second selected proportions,
the second proportion being different from the first proportion,
with the first and second proportions following each other cyclically
in sequence and each in predetermined ratio of the total number
of half cycles in each sequence. With this arrangement, only the
first or the second selected proportion of half cycles in each sequence
may be such as to result in application of the supply voltage to
the at least one heating element.
In an example of such an embodiment, a switch means having ten
positions is provided; a first alternating current supply voltage
of substantially 230 volts and a second alternating current supply
voltage of substantially 400 volts is arranged and in a first sequential
series of seven discrete switch positions the at least one heating
element is selectively energisable from the first supply voltage
such that:
a) in the first position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 40 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 60 percent of each sequence;
b) in the second position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 55 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 45 percent of each sequence;
c) in the third position, one half cycle in every three half cycles
of the first supply voltage is arranged for application cyclically
to the at least one heating element for about 75 percent of a sequence
of a predetermined number of half cycles, substantially no supply
voltage being arranged to be applied to the at least one heating
element during the remaining about 25 percent of each sequence;
d) in the fourth position, one half cycle in every three half cycles
of the first supply voltage is arranged for continuous application
to the at least one heating element;
e) in the fifth position, two half cycles in every three half cycles
of the first supply voltage are arranged for application cyclically
to the at least one heating element for about 40 percent of a sequence
of a predetermined number of half cycles and one half cycle in every
three half cycles of the first supply voltage is arranged for application
to the at least one heating element during the remaining about 60
percent of each sequence;
f) in the sixth position, two half cycles in every three half cycles
of the first supply voltage are arranged for continuous application
to the at least one heating element;
g) in the seventh position, the first supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element:
and in a second sequential series of three discrete positions,
following on from the first series and comprising positions eight
to ten of the switch means, the at least one heating element is
selectively energisable from the second supply voltage such that:
i) in the eighth position, two half cycles in every three half
cycles of the second supply voltage are arranged for application
cyclically to the at least one heating element for about 40 percent
of a sequence of a predetermined number of half cycles and one half
cycle in every three half cycles of the second supply voltage is
arranged for application to the at least one heating element during
the remaining about 60 percent of each sequence;
ii) in the ninth position, two half cycles in every three half
cycles of the second supply voltage are arranged for continuous
application to the at least one heating element;
iii) in the tenth position, the second supply voltage in full and
complete cyclic form is arranged for application to the at least
one heating element.
Preferably the at least one heating element comprises an infra-red
lamp, optionally connected in series with a further heating element
of coiled bare resistance wire or metal ribbon form. Such infra-red
lamp may particularly comprise a tungsten filament inside a quartz
or fused silica envelope containing a halogenated atmosphere.
The control means may comprise a microprocessor-based control system,
preferably in association with a triac switching element.
By means of the invention, sequential selection of the switch positions
may result in a stepwise increase in power output from the at least
one heating element, accompanied by a corresponding stepwise increase
in intensity of visible light radiation from the at least one heating
element, particularly when such heating element comprises a lamp.
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 top plan view of a radiant electric heater for use
in a radiant electric heater arrangement of the invention;
FIG. 2 is a circuit diagram of a radiant electric heater arrangement
according to the invention;
FIG. 3 is a diagrammatic representation of voltage waveforms applied
to the radiant electric heater of FIG. 1 in the radiant electric
heater arrangement of FIG. 2;
FIG. 4 is a graph showing heater power as a function of switch
position in respect of an embodiment of radiant electric heater
arrangement according to the invention;
FIGS. 5A, 5B, 5C and 5D are top plan views of alternative forms
of radiant electric heaters for use in place of the heater of FIG.
1 in the radiant electric heater arrangement of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, a radiant electric heater 1, for use with
a glass-ceramic cooking appliance, comprises a circular lamp heating
element 2, permanently connected in series with a ballast resistance
element 3 in the form of a coiled bare wire heating element and
supported inside a metal dish 4, of well-known construction, containing
a base layer 5 of thermal and electrical insulation material, such
as well-known microporous thermal and electrical insulation material.
The lamp heating element 2 suitably comprises a tungsten filament
supported in a tubular quartz enclosure containing a halogenated
gas atmosphere. The two heating elements 2 and 3 are designed such
that, for example, two thirds of the total combined power of the
elements is produced in the lamp 2 and one third in the coiled wire
element 3 when energised from a voltage supply. The heater is provided
with a peripheral wall 6 of thermal insulation material and such
that when the heater is located beneath a well-known form of glass-ceramic
cook top (not shown), in a cooking appliance, the peripheral wall
contacts the under side of the cook top.
A well-known form of temperature limiter 7 has a rod-like sensor
which traverses the heater above the heating elements. The limiter
7 is arranged to interrupt the power supply to the heating elements
at a predetermined temperature of the glass-ceramic cook top to
prevent thermal damage to the cook top.
A terminal connector 8 is provided on the heater and the heater
is arranged for connection to a power supply by means of terminals
9 and 10.
Referring now to FIG. 2, the heater 1 of FIG. 1 is connected by
way of a manually-operable ten-position switch, having seven sets
of contacts A, B, C, D, E, F, G. a microprocessor-based control
system (hereafter referred to as `microcontroller`) 11 and a triac
12, to a dual voltage alternating current supply arranged to provide
120 volts or 240 volts by selection. The ten position switch is
suitably of well-known rotary form.
The arrangement is such that as the switch is operated from an
off position through the ten consecutive discrete operating positions
the power at the heater 1 increases incrementally up to a maximum
in position ten. Such increase in power is visible to the user as
a gradual stepwise increase in brightness of the heater lamp 2 (FIG.
1) as the switch is moved from one position to the next from the
off position towards the highest position. This is achieved as hereafter
described.
Table 1 shows which contacts of the ten position switch are closed
for each user-selected position of the switch.
The arrangement is such that in switch positions 1 to 6, operation
of the heater is by way of the 120 volts supply and in switch positions
7 to 10, operation is by way of the 240 volts supply. The microcontroller
11 operates in conjunction with the triac 12 in such a way that
in increasing order from switch position 1 to switch position 6,
a selectively increased proportion of half cycles in a predetermined
number of half cycles of the 120 volts supply is applied to the
heater 1. Furthermore, in increasing order from switch position
7 to switch position 10, a selectively increased proportion of half
cycles in a predetermined number of half cycles of the 240 volts
supply is applied to the heater 1.
FIG. 3 shows the voltage waveforms at the heater 1 according to
the switch position. Referring to Table 1 and FIG. 3, in switch
position 1, contacts B, C and G are closed and one half cycle in
every seven half cycles of the 120 volts supply is applied to the
heater 1. Contacts C serve to isolate the heater 1 from the supply
voltage in the off position. In switch position 2, contacts B, C
and F are closed and one half cycle in every five half cycles of
the 120 volts supply is applied to the heater 1. In switch position
3, contacts B, C, F and G are closed and one half cycle in every
three half cycles of the 120 volts supply is applied to the heater
1. In switch position 4, contacts B, C and E are closed and two
half cycles in every four half cycles of the 120 volts supply are
applied to the heater 1. In switch position 5, contacts B, C, E
and G are closed and two half cycles in every three half cycles
of the 120 volts supply are applied to the heater 1. In switch position
6, contacts B, C, D, E and F are closed and the 120 volts supply
in full and complete cyclic form is applied to the heater 1, the
triac 12 being short-circuited by closure of the switch contacts
D, thereby avoiding power dissipation of the triac in this switch
position.
In switch positions 7 to 10, the power supply is changed from 120
volts to 240 volts by closure of switch contacts A instead of switch
contacts B.
Accordingly, in switch position 7, contacts A, C, F and G are closed
and one half cycle in every three half cycles of the 240 volts supply
is applied to the heater 1. In switch position 8, contacts A, C
and E are closed and two half cycles in every four half cycles of
the 240 volts supply are applied to the heater 1. In switch position
9, contacts A, C, E and G are closed and two half cycles in every
three half cycles of the 240 volts supply are applied to the heater
1.
In switch position 10, contacts A, C, D, E and F are closed and
the 240 volts supply in full and complete cyclic form is applied
to the heater 1, the triac 12 being short-circuited by closure of
the switch contacts D, thereby avoiding power dissipation of the
triac in this switch position.
With the resulting heater arrangement there is a smooth stepwise
increase in power output and visible brightness of the heater as
the switch is operated through its discrete operating positions
up to position 10. The effect is illustrated in FIG. 4 which is
a graphical plot of the heater power in watts against position of
the switch, the heater being designed to dissipate 1800 watts at
the full supply voltage of 240 volts.
In spite of the use of burst firing techniques, in which selected
numbers of half cycles in chosen numbers of complete half cycles
of the voltage supply are applied to the heater, there is no objectionable
flicker from the heater lamp 2 in any of the switch positions. only
in position 1 of the switch is there no visible illumination of
the heater lamp 2, there being a distinct and different visible
level of illumination in each of the remaining switch positions.
Instead of the ballast resistance element 3 comprising a coiled
bare wire resistance element, it could comprise a known form of
corrugated ribbon resistance element.
The ballast resistance element 3 may be able to be dispensed with
where the inrush current on energising the lamp 2 is permitted and
can be tolerated. Such a heater, containing only a lamp 2, is illustrated
in FIG. 5A, this heater being otherwise the same as the heater of
FIG. 1, previously described.
The invention is also applicable to heaters which do not employ
a heating element in the form of a lamp. FIG. 5B illustrates a heater
in which a heating element in the form of bare coiled resistance
wire 20 is used in place of the lamp 2 of FIG. 5A.
FIG. 5C illustrates a heater in which a heating element in the
form of a known corrugated ribbon 200 of a metal alloy is used in
place of the lamp 2 of FIG. 5A.
FIG. 5D illustrates a heater in which a heating element in the
form of a known strip 202 of molybdenum disilicide material permanently
connected in series with a bare coiled resistance wire 203 is used
in place of the lamp 2 of FIG. 5A.
In some European countries a multi-phase alternating current supply
voltage is available in which by connecting between one phase line
and a neutral line a 230 volts supply is obtainable and by connecting
between two phase lines a 400 volts supply is obtainable. The circuit
arrangement of FIG. 2 and used with the heaters of FIGS. 1, 5A,
5B, 5C, 5D can be applied to such a voltage supply instead of the
120/240 volts supply. When used with a 230/400 volts supply, with
switch contacts B and C closed and switch contacts A open, a supply
voltage of 230 volts is obtained. With switch contacts A and C closed
and switch contacts B open, a supply voltage of 400 volts is obtained.
In a particular embodiment, the ten position switch is arranged
such that in positions 1 to 7, operation of the heater is by way
of the 230 volts supply and in switch positions 8 to 10, operation
is by way of the 400 volts supply.
Table 2 shows which contacts of the ten position switch are closed
for each user-selected position of the switch.
The microcontroller 11 is configured to operate with the triac
12 and the ten position switch as follows, for operation of the
heater 1.
In switch position 1, contacts B and C are closed and one half
cycle in every three half cycles of the 230 volts supply is applied
cyclically to the heater 1 for 40 percent of a total cycle time
period comprising a sequence of a predetermined number of half cycles
and substantially no voltage is applied to the heater 1 during the
remaining 60 percent of the total cycle time period of each sequence.
Contacts C serve to isolate the heater 1 from the supply voltage
in the off position.
In switch position 2, contacts B, C and G are closed and one half
cycle in every three half cycles of the 230 volts supply is applied
cyclically to the heater 1 for 55 percent of a total cycle time
period comprising a sequence of a predetermined number of half cycles,
substantially no voltage being applied to the heater 1 during the
remaining 45 percent of the total cycle time period of each sequence.
In switch position 3, contacts B, C and F are closed and one half
cycle in every three half cycles of the 230 volts supply is applied
cyclically to the heater 1 for 75 percent of a total cycle time
period comprising a sequence of a predetermined number of half cycles,
substantially no voltage being applied to the heater 1 during the
remaining 25 percent of the total cycle time period of each sequence.
In switch position 4, contacts B, C, F and G are closed and one
half cycle in every three half cycles of the 230 volts supply is
continuously applied to the heater 1.
In switch position 5, contacts B, C and E are closed and two half
cycles in every three half cycles of the 230 volts supply are applied
cyclically to the heater 1 for 40 percent of a total cycle time
period. During the remaining 60 percent of each total cycle time
period, one half cycle in every three half cycles of the 230 volts
supply is applied to the heater 1.
In switch position 6, contacts B, C, E and F are closed and two
half cycles in every three half cycles of the 230 volts supply are
continuously applied to the heater 1.
In switch position 7, contacts B, C, D, E, F and G are closed and
the 230 volts supply in full and complete cyclic form is applied
to the heater 1.
In switch positions 8 to 10, the power supply voltage is changed
from 230 volts to 400 volts by closure of switch contacts A instead
of switch contacts B.
Accordingly, in switch position 8, contacts A, C, E and G are closed
and two half cycles in every three half cycles of the 400 volts
supply are applied cyclically to the heater 1 for 40 percent of
a total cycle time period comprising a sequence of a predetermined
number of half cycles. During the remaining 60 percent of each total
cycle time period (i.e. each sequence), one half cycle in every
three half cycles of the 400 volts supply is applied to the heater
1.
In switch position 9, contacts A, C, E and F are closed and two
half cycles in every three half cycles of the 400 volts supply are
continuously applied to the heater 1.
In switch position 10, contacts A, C, D, E, F and G are closed
and the 400 volts supply in full and complete cyclic form is applied
to the heater 1.
In switch positions 7 and 10, the closure of contacts D results
in short-circuiting of the triac 12, to avoid power dissipation
of the triac in these switch positions.
With the resulting heater arrangement, there is a smooth stepwise
increase in power output and visible brightness of the heater 1
as the switch is operated through its discrete operating positions
up to position 10. For the heater of FIG. 1, in which two thirds
of the total combined power of the heating elements 2 and 3 is produced
in the lamp 2 and one third in the coiled wire element 3 when energised,
the percentage of full power output for the heater in each of the
ten switch positions, when operating from the 230/400 volts supply
is approximately as follows:
The heater arrangement of the invention for 230/400 volts operation
is particularly advantageous in that since the switching element
is a triac, with no moving contacts to wear out, and since changes
in current between discrete power levels are small, a much faster
switching rate (typically ten times faster) than with conventional
electromechanical energy regulators hitherto used, can be employed
without infringing strict European switching regulations with respect
to causing disturbances on mains voltage supplies. |