GB2307384A

GB2307384A - Damping initial currents using NTC ballast resistance

Info

Publication number: GB2307384A
Application number: GB9523553A
Authority: GB
Inventors: David Aubrey Plumptre
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1995-11-17
Filing date: 1995-11-17
Publication date: 1997-05-21
Anticipated expiration: 2015-11-17
Also published as: GB2307384B; GB9523553D0; DE29619760U1

Description

Infra-Red Heater This invention relates to infra-red heaters for use in glass ceramic top cooking appliances, which heaters incorporate at least one infra-red heating source having substantial positive temperature coefficient of resistance. Such infra-red heating source generally comprises an infra-red lamp, but may also comprise an element of molybdenum disilicide material. A typical infra-red lamp compnses a tungsten filament Inside a sealed quartz or fused silica envelope containing a halogenated atmosphere.

The electrical resistance of such an infra-red heating source is very low at ambient temperatures and this gives rise to high inrush current when the source is energised from a mains supply. The resulting voltage drop on the mains supply is unacceptable to mau1y regulatory authorities.

This problem has been addressed by temporarily or permanently connecting one or more ballast resistance elements in series with the infra-red heating source. Arrangements of this nature are described, for example in EP-A-0176027, EP-A-0206597 and EP-A-0235895. The ballast resistance element is incorporated within the heater, along with the infra-red source, and compnses a coiled bare wire of a material such as iron-chromium-aluminium alloy having a relatively low positive temperature coefficient of resistance. The ballast element also contributes to the heat output of the heater. On switching on, the presence of the ballast resistance element results in lower peak inrush current and reduced disturbance on the mains voltage supply. The ratio of the electrical resistance of the ballast resistance element to that of the infra-red source has been appropriately selected to meet the necessary requirements. For example, for some heaters the ballast resistance element has been selected to have an electrical resistance approximately one half that of the infra-red source at the operating temperature of the latter when such source has comprised a tungsten-halogen lamp. For some other heaters, the ballast resistance element and the infra-red source have been selected of substantially equal electrical resistance.

Until recently, heaters manufactured in this wav have been able to meet switching regulations in force throughout the world.

However the introduction of more stringent regulations will, in future reduce the permitted Inrush currents durmg operation of heaters. To meet these stricter regulations with the arrangements of the prior art would necessitate providing an increased proportion of ballast resistance element than hitherto, in series with the infra-red source. The presence of such a high proportion of ballast element Is disadvantageous m that it results in increased light-up time for the infra-red source, such as a tungsten-halogen lamp, and dimimshes the pnncipal attraction of such a source. namely its virtually mstant visual response on being energlsed.

It Is an obJect of the present invention to overcome or minimise this problem.

The present invention provides an electrical Infra-red heater for a glass ceramic top cooker, which heater comprises: at least one infra-red heating element In a dish-like supporting means, the supporting means being arranged for mounting beneath a glass ceramic cook top, the at least one Infra-red heating element having a substantial positive temperature coefficient of electrical resistance, ballast resistance means electncallv connected In senes with the at least one infra-red heating element, at least dunng initial electrical energislng thereof, for damping inrush current therethrough when the heater Is energlsed from an electrical supply, the ballast resistance means having a negative temperature coefficient of electrical resistance over a first temperature range from 0 C to at least 700"C and having a positive temperature coefficient of electrical resistance over a second temperature range higher than the first temperature range.

The first temperature range may, for example, be from 0 C to 1000 C or from O"C to 9000C, or from 0 C to 800"C.

The ballast resistance means is preferably provided within the heater.

The ballast resistance means preferably comprises a refractory semiconducting material, particularly silicon carbide and more particularly alpha silicon carbide, at least in part. It is suitably provided in the form of at least one elongate rod or wire or strip. An elongate strip of rectangular cross section may advantageously be provided. supported edgewise inside the heater.

The ballast resistance means comprising silicon carbide may include one or more dopants or additions to provide required electrical resistance and/or temperature coefficient of resistance characteristics.

A ballast resistance means comprising silicon carbide may be manufactured of elongate form such that opposite end regions thereof have lower electrical resistance than the remainder of the ballast resistance means. Such opposite end regions provide for connection thereto of connecting leads and attain a substantially lower temperature than the remainder of the resistance means during energisation of the heater. The opposite end regions may, for example. compnse beta silicon carbide or alpha silicon carbide enriched with silicon (such as by having pores therein filled with silicon) , when the remainder of the ballast resistance means comprises alpha silicon carbide.

The at least one infra-red heating element may comprise at least one infra-red lamp or at least one molybdenum disilicide element. Such at least one infra-red lamp may particularly comprise a tungsten filament inside a sealed enclosure containing a halogenated atmosphere.

The dish-like supporting means may comprise a dish, particularly of a metal, having a base layer of thermal and electrical insulating material, such as microporous Insulating material. supported therein.

An element comprising silicon carbide, used as the ballast resistance means in senes with one or more infra-red elements has the following advantageous properties.

Because of its mitial negative temperature coefficient of resistance it is particularly effective for increasing the series resistance of the heater in the cold state and reducing Inrush current othenvise occurring in the infra-red element on energising the heater. Advantageously it exhibits a lower resistance In the hot state and may be arranged to provide very low electrical self-heating in the hot state of the heater. However, as it Is able to withstand high temperatures up to 1 6000C without detriment thereto, it can be used to contribute very usefully to the overall heat generated by the heater, replacing the function of the ballast resistance element or elements of the pnor art in this respect.

The high reslstivity and low specific heat of alpha silicon carbide compared to metals allows the ballast means which compnses it to be able to heat up rapidly and permits an arrangement which does not adversely affect the fast light-up time of the infra-red element, even when the initial inrush current has been reduced to a low level. This is because as the infra-red element heats up and increases in electrical resistance, the ballast resistance means comprising silicon carbide also heats up and reduces in electrical resistance. This reduction in electrical resistance can be arranged to occur at a similar rate to the corresponding increase in the resistance of the infra-red element, the result being the ability to maintain the current flowing through the series combination at a relatively high level, but below the permitted maximum, thereby giving rapid and uniform light-up of the infra-red element. The resulting smoothing of current changes from this arrangement may also be beneficlal in meeting demands of certain regulatory authorities where rate of change of current as well as the maximum current is a consideration.

Silicon carbide has a relatively high thermal conductivity and relatively low thermal expansion and when used in the ballast resistance means the resulting excellent thermal shockwithstanding properties allow it to withstand high heat-up rates of, for example, up to 7000C per second on being switched on from cold. The relatively high thermal conductivity of the material is also useful in ensuring that after switching-off the heater, the ballast means cools sufficiently quickly, by conduction of heat to its relatively cool end regions, for the resistance thereof to increase to a sufficiently high value for effective damping of inrush current to be achieved when the heater is re-energised after a short period of de-energisation.

The physical nature of silicon carbide, particularly its relative brittleness, renders it most suitable for forming straight resistance elements rather than curved or complex-shaped elements although such curved or complex-shaped elements can be provided Conveniently, therefore, one or more ballast resistance means of straight wire, rod or strip form, may be provided extending across a heater, for example diametrically across a circular heater, and may be arranged to pass beneath the one or more infra-red elements.

The ballast resistance means may be permanently connected in series with the at least one infra-red heating element or may be switched out of circuit, for example by short-circuiting, a short period after initial energising of the heater.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure I is a perspective view of an infra-red heater according to the invention, and Figure 2 Is a cross-sectional view of the heater of Figure 1. located beneath a glass ceramic cook top.

Referring to the drawings, an infra-red heater for a glass ceramic top cooker is constructed as follows. A circular metal dish 1 contains a base layer 2 of thermal and electrical insulating material, such as microporous thermal and electrical insulating material. A peripheral wall 3. of well-known form, is provided and arranged to contact the underside of a glass ceramic cook top 10 when the heater is installed in a cooker for operation. The peripheral wall comprises a thermal and electrical insulating material such as, for example, bound ceramic fibres, bound glass filaments, or vermiculite.

An infra-red heating element comprising a circular infra-red lamp 4, having a substantial positive temperature coefficient of electrical resistance, is supported within the dish I above the base layer 2. The lamp 4 suitably comprises a tungsten filament inside a sealed quartz or fused sllica envelope containing a halogenated atmosphere.

By way of example, in a heater of 200 mm diameter, which is required to provide a power of 1800 watts at 240 volts, the circular lamp 4 is suitably of 136 mm inside diameter and rated at 1350 watts at 160 volts. The filament of such a lamp has an electrical resistance when hot of about 24 ohms.

The base layer 2 of insulating material is dished such that an elongate ballast resistance element 5 of rectangular strip form can pass diametrically across the heater beneath the infrared lamp 4. The ballast resistance element 5 is suitably inserted into and across the heater through openings provided in side walls of the dish and through supports comprising ceramic components in the form of inserts 6 provided diametrically opposed in the undished peripheral regions of the base layer 2. Such inserts 6 may suitably be installed during formation of the base layer 2 in the dish 1.

The ballast resistance element 5 is capable of withstanding a temperature of at least 7000C and has a negative temperature coefficient of electrical resistance over a first temperature range from 0 C to at least 7000C and a positive temperature coefficient of resistance over a second temperature range higher than the first temperature range. It is a refractory semiconducting material which may be ceramic in nature and suitably comprises an exposed central region 5A of alpha silicon carbide, which may be in a pure form or contain a proportion of beta silicon carbide and/or small amounts of Impurities or dopant material, and may have end regions 5B of lower electrical resistance than the central region. A possible strip material for the ballast resistance element 5 is recrystallised alpha silicon carbide supplied by Kanthal AB, having the reglstered trade name Hot Rod. This matenal has a 30% porosity and the opposite end regions 5B are filled or enriched with silicon to provide reduced electrical resistance at these end regions. As an alternative, the strip-form element 5 could compnse a central region 5A of dense recrystallised or reaction bonded alpha silicon carbide having end regions SB of lower resistivitv beta silicon carbide reaction bonded thereto. In the case of the 200 mm diameter heater of the present example, the strip-form ballast resistance element 5 suitably has an exposed central region of about 100 mm in length and opposite lower resistance end regions each of about 60 mm in length. The element 5 is suitably 2 mm thick and 8 mm wide and is supported edgewise in the heater with the 8 mm wide faces substantially normal to the plane of the base of the heater to ensure optimum thermal radiation from the element 5 towards the glass ceramic cook top during operation of the heater.

The resistance element 5 suitably has an end to end electrical resistance in the cold state of about 30 ohms. The electrical resistance when hot is about 6 ohms at 800-9009C about 6.5 ohms at l000"C and about 8 ohms at 1100-I l500C.

Electncal leads (not shown) are connected to the ends 7. 8 of the ballast resistance element 5. for example by means of friction-gnp connectors or b, unsung small threaded fasteners passing through holes at the ends 7. 8 of the element S and fitted with suitable nuts. To facilitate connection, the ends 7 8 of the element 5 may be provided with metallic coatings of good electrical conductivity, such as of aluminium or an alloy such as Kovar.

The ballast resistance element S is electrically connected in senes with the infra-red lamp 4 and the combination thereof its arranged for connection to a mains electncity supply, for example of 240 volts, through a thermal limiter 9, of well-known form, which has a sensor rod extending across the heater and is designed to interrupt the voltage supple to the elements 4. 5 at a predetermined temperature to prevent thermal damage to the glass ceramic cook top dunng operation of the heater in a cooker.

Operation of the specifically exemplified heater is as follows.

On switching on from cold, the electrical resistance of the senes combination of the infra-red lamp 4 and the silicon carbide ballast resistance element 5 is about 32.4 ohms, the current being about 7.4 amps with a supply voltage of 240 volts. Inrush current which would othenvise occur in the infra-red lamp is consequentlv damped. At this stage the silicon carbide ballast element 5 is dissipating about 1643 watts and the infra-red lamp 4 only about 131 watts. Under these conditions the silicon carbide element 5 is heating up faster than the lamp 4. As soon as the silicon carbide element 5 begins to heat up, its resistance starts falling and the current increases from its initial level of about 7.4 amps up to a maximum of about 14 amps. At this stage and with the continulng increase in electrical resistance of the lamp 4 the temperature of the lamp rapidly increases and very qulckly overtakes the temperature of the silicon carbide element 5. After a few seconds the electrical resistance of the series combination of the element 5 and lamp 4 reaches a value of about 30 ohms, with a current of about 8 amps and a total power disslpation of about 1920 watts.

As the glass ceramic cook top heats up, the silicon carbide element 5 and the infra-red lamp 4 continue to rise In temperature. Although the resistance of the tungsten filament In the infrared lamp 4 does not significantly change further, the resistance of the silicon carbide element 5 now Increases from about 6 ohms to about 8 ohms as a result of its positive temperature coefficient of resistance characteristic at such a temperature. Thus in final steady state operation, the series combination has a resistance of about 32 ohms and the power dissipation of the heater is 1800 watts.

During operation of the heater, the amount of heat generated in the infra-red lamp is maintained substantially greater than 50% of the heat generated in the heater as a whole.

The heater of the invention is mtended to meet the stringent switching (PST) specifications of official regulatory authorities. For this purpose the silicon carbide resistance element 5, used in senes with one or more infra-red lamps 4, is arranged to operate with its minimum electrical resistance at about 900"C.

On switching off the heater, the temperature of the silicon carbide element 5 falls quickly particularly as a result of its high thermal conductivity. which ensures that heat is efficiently conducted to the cooler end regions of the element. This means that if the heater is switched on agaln after a relatively short period, which occurs during normal controlled switching operation of the heater, the electrical resistance of the silicon carbide element 5 will have nsen sufficiently for it to perform its required ballasting function to damp inrush current through the infra-red lamp 4.

Instead of the silicon carbide element 5 being permanently connected in series with the infrared lamp 4, it could be arranged to be switched out of clrcuit after an initial predetermined period of energisation of the heater. Arrangements suitable for achieving this are known, being described for example in EP-A-0235895.

The one or more Infra-red lamps 4 need not be circular. but could be straight or of other desired shapes.

Instead of the one or more infra-red lamps 4, one or more infra-red heating elements, such as comprising molybdenum disilicide, having a substantial positive temperature coefficient of resistance, could be considered.

Although a silicon carbide resistance element 5, of straight strip form has been described. other shapes may be considered. However, a straight form may be generally preferred because of the difficulty in forming this relatively brittle material mto more complex shapes.

Claims

Claims 1. An electrical infra-red heater for a glass ceramic top cooker, which heater comprises: at least one infra-red heating element in a dish-like supporting means, the supporting means being arranged for mounting beneath a glass ceramic cook top, the at least one infra-red heating element having a substantial positive temperature coefficient of electrical resistance; ballast resistance means electrically connected in series with the at least one infra-red heating element, at least during initial electrical energising thereof, for damping inrush current therethrough when the heater is energised from an electrical supply, the ballast means having a negative temperature coefficient of electrical resistance over a first temperature range from 0 C to at least 7000C and having a positive temperature coefficient of electrical resistance over a second temperature range higher than the first temperature range.

2. An infra-red heater according to claim I, in which the first temperature range is from 0 C to 1000 C.

3 An infra-red heater according to claim 2, in which the first temperature range is from 0UC to 9000C.

4. An infra-red heater according to claim 3 in which the first temperature range is from 0"C to 800"C. t. An infra-red heater according to any preceding claim, in which the ballast means is provided within the heater.

6. An infra-red heater according to any preceding claim, in which the ballast means comprises a refractory semiconducting material.

7. An infra-red heater according to claim 6, in which the ballast means comprises silicon carbide.

8. An infra-red heater according to claim 7, in which the silicon carbide comprises alpha silicon carbide, at least in part.

9. An Infra-red heater according to claim 7, or 8, in which the ballast means is in the form of at least one elongate rod or wire or strip.

10. An infra-red heater according to claim 9, in which the strip comprises an elongate strip of rectangular cross section supported edgewise inside the heater.

11. An infra-red heater according to any of claims 7 to 10 in which the silicon carbide includes one or more dopants or additions to provide required electrical resistance and/or temperature coefficient of resistance characteristics.

12. An infra-red heater according to any of claims 7 to 11 in which the ballast resistance comprising silicon carbide Is of elongate form and such that opposite end regions thereof have lower electrical resistance than the remainder of the ballast resistance means.

13. An infra-red heater, according to claim 12, in which the opposite end regions compnse beta silicon carbide, or alpha silicon carbide enriched with silicon, and the remainder of the ballast resistance means compnses alpha silicon carbide.

14. An infra-red heater according to any preceding claim, in which the at least one infra-red heating element comprises at least one infra-red lamp or at least one molybdenum disilicide element.

IS. An infra-red heater according to claim 14, in which the at least one infra-red lamp compnses a tungsten filament inside a sealed enclosure containing a halogenated atmosphere.

16. An Infra-red heater according to any preceding claim, in which the dish-like supporting means comprises a dish having a base layer of thermal and electrical insulating material supported therein.

17. An infra-red heater according to claim 16, in which the dish comprises a metal.

18. An infra-red heater according to claim 16, or 17, in which the insulating matenal compnses microporous thermal and electrical insulating material.

19. An electrical infra-red heater constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.