GB2330255A

GB2330255A - Radiant electric heater control giving fast heat-up to visible radiance

Info

Publication number: GB2330255A
Application number: GB9721322A
Authority: GB
Inventors: Richard Charles Scott
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1997-10-09
Filing date: 1997-10-09
Publication date: 1999-04-14
Also published as: GB9721322D0

Abstract

On initial connection of an AC power supply to a radiant heater 2, particularly for a glass-ceramic top cooking appliance, the heater 2 is supplied with a first preselected proportion of AC half cycles in a predetermined number of half cycles (eg. 3 in 3, or 2 in 3) for a predetermined time, and the heater 2 is then supplied with a second preselected proportion of half cycles in a predetermined number of half cycles (eg. 2 in 3 or 1 in 3). The second preselected proportion results in operation of the heater, at its rated power, and the first preselected proportion is greater than the second so that the heater is initially operated at above its rated power for the predetermined time, whereby fast heat-up to visible radiance is effected. The predetermined time period may be a maximum of 10 seconds. A microprocessor 10 may be provided to control the heater 2. The microprocessor 10 may vary the predetermined time period on the basis of the previous history of operation of the heater 2, so that if the heater is still warm or hot from a previous use, the predetermined time is reduced.

Description

Radiant Electric Heater Assembly This invention relates to a radiant electric heater assembly particularlx but not exclusively for use with a cooking appliance such as a glass-ceramic smooth top cooking appliance.

It has been a constant endeavour to develop radiant electric heaters for glass-ceramic cooking appliances which have a fast heat-up to visible radiance the aim being to approach the performance of gas cookers. The best results to date have been achieved with electric heaters incorporating tungsten-halogen lamps as heat sources. but such lamps are expensive and efforts have therefore been directed towards improving the light-up times of coiled wire and ribbon form heating elements. which typically comprise iron-chromium-aluminium alloy Reduction in mass and/or increase in surface area of heating elements have provided some improvements in this regard and heater assemblies have also been proposed in which combinations of heating elements have been provided whereby one element is initially encrgised for a fast light-up and then thereafter complected in series with another element for continued operation of the heater.

It is an object of the present invention to provide a radiant electric heater assembly in which accelerated heat-up to visible radiance is achieved b electronic control means.

The present invention provides a radiant electric heater assembly comprising: a radiant heater having a rated power and adapted to be energised from an altemating current voltage supply control means adapted and arranged to supply to the heater upon energising a first preselected proportion of half cycles in a predetermined number of half cycles of the voltage supply for a predetermined time period and thereafter a second preselected proportion of half cycles in a predetermined number of half cycles of the voltage supply.. the second preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply resulting in operation of the heater at its rated power the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply being greater than the second and resulting in operation of the heater with a power dissipation higher than the rated power for the predetermined time period hereby fast heat-up to visible radiance of the heater is effected upon energising.

The predetermined number of half cycles of the voltage supply for the first and second preselected proportions of half cycles may be three.

The first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply may be 3/3 while the corresponding second preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply may be either 2/3 or 1/3. preferably 2/3.

AlternativelS the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply may be 2/3 while the corresponding second preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply may be 1/3.

The predetermined time period during which the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is supplied to the heater ma be a maximum of 10 seconds.

The control means may comprise a microprocessor-based device and may supply power to the heater by operation of a solid state switching device such as a triac.

The control means may be adapted to vary the predetermined time period on the basis of previous history of operation of the heater stored in the microprocessor-based device.

The heater may incorporate a single heating element or a plurality of heating elements connected in parallel for non-independent operation.

The or each heating element may be of coiled wire or ribbon form.

B means of the invention. a radiant electric heater assembly is provided having a control means employing burst fire control technology to provide a boost power level to the heater for a predetermined short time period upon energising the heater followed by automatic reduction to the rated power level of the heater. Desirably fast visual response of the heater to radiance is thereby achieved each time the heater is switched on.

The invention is now described by way of example with reference to the accompan!ing drawings in which: Figure l is a schematic representation of a radiant electric heater assembly according to the invention; and Figure 2 is a plan view of an electric heater for use in the assembly of Figure 1.

Referring to Figure 1 a radiant electric heater assembly includes a radiant heater 1, which may be a heater for use in a cooking appliance, such as behind a plate of glass-ceramic material in such an appliance. An example of such a heater is shown in Figure 2, in which a heating element 2. such as of coiled wire or corrugated ribbon fonn. for example of iron-chromium aluminium alloy, is supported on a base 3 of insulation material. such as microporous thermal and electrical insulation material in a metal dish 4 A terminal block 5 is provided by means of which the heater may be electrically connected for energisation and control such connection being by -a) of a well-known form of temperature limiter 6.

Although a single heating element 2 is shown in Figure 2. a plurality of heating elements could be provided, connected in parallel for non-independent operation.

The heater 1 is arranged for energisation from an altcmating current supply 8 by ay 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 alternating current supply to ensure synchronisation. A simple fonn of power supply unit 13 is provided for the microcontroller 10.

The microcontroller 10 is arranged to control the heater 1 in burst fire manner; as hereafter described, by way of a solid state snitch means such as a triac 14.

A manually operable switch arrangement 15 is provided. cooperating with the microcontroller 10, and also being arranged in its off position to operate the isolation switch 9, to isolate the heater 1 from the mains voltage supply 8.

When the switch arrangement 15 is actuated to energise the heater 1 the microcontroller 10 supplies to the heater 1. b way of the triac 14 a first preselected proportion of half cycles in a predetermined number of half cycles of the voltage supply 8. for a predetermined short time period. and thereafter, continuously, a second preselected proportion of half cycles in a predetermined number of half cycles of the voltage suppl. The second preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is so arranged that it results in operation of the heater 1 at its rated powder. The first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is arranged to be greater than the second and results in operation of the heater 1, upon energisation, with a power dissipation higher than the rated power during the predetermined short time period.

Accelerated heat-up to visible radiance of the heater I is thereby achieved upon energisation, which is visually appealing to the user as well as contributing to the operating efficiency of the heater for cooking purposes.

The first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply 8 is preferably 3/3. This means that during the predetermined time period following energising. the voltage supply in full and complete cyclic form is applied to the heater. Thereafter, the second preselected proportion of half cycles in the predetermined number of half c cycles of the voltage supply, and which is preferably 2/3 (although it could be 1/3). is automatically applied to the heater 1 by the microcontroller 10. such that the heater 1 operates at its rated power.

It would also be possible to arrange for operation of the heater whereby the first preselected proportion of half cs cles is 2/3 and the second preselected proportion is 1/3.

The predetermined time period during which the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is supplied to the heater may be of any desired Icngth with the proviso that during it the temperature of the heating element should not rise to a level whereby damage to the element and/or its surroundings occurs. or the life of the element is unacceptabl reduced. The predetermined time period can be readily determined for a particular heater by experiment, but will generally not exceed 10 seconds.

In a specific example, the heater shown in Figure 2 had a diameter of 165 nun and incorporated a heating element 2 comprising a well known fonn of corrugated ribbon and was arranged to be energised from a 230 volt altemating current supply b' way of the circuit of Figure 1. The heater had a rated powder of 1200 watts. being that powder dissipated by the heating element hen supplied with two half cycles in ever, three half cycles of the voltage supply. If the heater mere to have been energised in this wa directly on switching on. clearly visible radiance of the heating element would only have occurred after about 2.7 seconds.

However, according to the invention it was arranged for the heater to be initially energised in such a way that the voltage supply in full and complete cyclic waveform was supplied thereto (i.e. three half cycles in ever' three half cycles of the voltage suppl ). This resulted in a power dissipation of 1800 watts in the element. Clearly visible radiance of the ribbon heating element occurred after about 1.4 seconds and it was arranged that after about 2 seconds the controller reduced the powder dissipation in the element to the rated power. for further continuous operation, two half cycles in ever; three half cycles of the voltage supply being supplied to the element during this period of continuous operation.

In a further example. the heater had a diameter of 180 nlm and incorporated a heating element 2 comprising a well known form of coiled wire construction and was arranged to be energised from a 230 volt altemating current suppl by va) of the circuit of Figure 1. The heater had a rated powder of 1200 watts, being that power dissipated by the heating element when supplied with two half cycles in ever!; three half cycles of the voltage supply. If the heater were to have been energised in this way directl on switching on. clearly visible radiance of the heating element would onl have occurred after about 6.6 seconds.

However. according to the invention it was arranged for the heater to be initially energised in such a way that the voltage supply in full and complete cclic fonn (i.e. three half cycles in ever three half cycles of the voltage supply) was supplied thereto. This resulted in a power dissipation of 1800 watts in the element. Clearly visible radiance of the coiled wire heating element occurred after about 3.8 seconds and it was arranged that after 6 seconds the controller reduced the power dissipation in the element to the rated powder, for further continuous operation, two half cycles in ever' three half cycles of the voltage suppl!- being supplied to the element during this period of continuous operation.

The predetermined time period can be automatically varied to take account of whether the heating element in the heater is cold when energised or is being reenergised a short period of time after being de-energised and therefore still ama or hot. In the latter case a shorter predetermined time period can be employed in comparison with the former case. The microcontroller 10 can be programmed to store information obtained from previous operation of the heater and from the history of operation can predict what predetermined time period is appropriate at the point of the next energisation of the heater.

Claims

Claims 1. A radiant electric heater assembl) comprising: a radiant heater having a rated powder and adapted to be energised from an altemating current voltage supply; control means adapted and arranged to supply to the heater upon energising a first preselected proportion of half cycles in a predetermined number of half cycles of the voltage supply for a predetermined time period. and thereafter a second preselected proportion of half cycles in a predetermined number of half cycles of the voltage supply, the second preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply resulting in operation of the heater at its rated power. the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply being greater than the second and resulting in operation of the heater with a power dissipation higher than the rated power for the predetermined time period whereby fast heat-up to visible radiance of the heater is effected upon energising.
2. An assembly according to claim 1, in which the predetermined number of half c -cles of the voltage supply for the first and second selected proportions of half cycles is three.
3. An assembl according to claim 2, in which the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is 3/3 while the corresponding second preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is either 2/3 or 1/3.
4. An assembly according to claim 3, in which the corresponding second preselected proportion of half cycles in the predetermined number of half c!-cles of the voltage supply is 2/3.
5. An assembly according to claim 2, in which the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage suppl} is 2/3, while the corresponding second preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is 1/3.
6. An assembly according to any one of the preceding claims, in which the predetermined time period dunng which the first preselected proportion of half cycles in the predetermined number of half cycles of the voltage supply is supplied to the heater is a maximum of 10 seconds.
7. An assembl according to any one of the preceding claims in which the control means comprises a microprocessor-based device.
8. An assembly according to claim 7. in which the control means supplies power to the heater by operation of a solid state switching device.
9. An assembly according to claim 8, in which the solid state switching device comprises a triac.
10. An assembl) according to claim 7, 8 or 9 in which the control means is adapted to var) the predetermined time period on the basis of previous history of operation of the heater stored in the microprocessor-based device.
It. an assembles according to any one of the preceding claims. in which the heater incorporates a single heating element, or a pluralit) of heating elements connected in parallel for non-independent operation.
12. An assembly according to claim I I, in which the or each heating element is of coiled wire or ribbon form.
13. A radiant electric heater assembly constructed and arranged substantiall as hereinbefore described with reference to the accompanying drawings.