WO2000047095A1

WO2000047095A1 - Improvements relating to control of electric heating elements

Info

Publication number: WO2000047095A1
Application number: PCT/GB2000/000446
Authority: WO
Inventors: Robert Henry Hadfield
Original assignee: Otter Controls Limited
Priority date: 1999-02-11
Filing date: 2000-02-11
Publication date: 2000-08-17
Also published as: AU2451900A; GB2346738A; GB2378818B; GB0227124D0; GB2346738B; GB9903106D0; GB2378818A

Abstract

A cordless electrically heated water boiling vessel has an enclosure associated with its heating element (3) for simulating a dry boil condition when water boils in the vessel, this enabling dry boil protection and automatic switch off on boiling to be effected in response to a single thermal sensor. A relay (5) is provided in the base part of the vessel and is arranged to be responsive to the flow of current through the heating element (3), and on/off controls (9, 10) are associated with the relay (5). By virtue of this arrangement, not only does the vessel part not need to have any manually-operable controls since its opeation is fully controlled via the base, but also the base connector is not powered so long as the vessel part of the appliance is not seated on the base since the relay then isolates the base connector from the supply.

Description

Improvements Relating to Control of

Electric Heating Elements

Field of the Invention:

This invention concerns improvements relating to the control of

electric heating elements, particularly though not exclusively electric heating

elements of water boiling vessels such as kettles, jugs, pots, pans, laboratory

equipment etc.

Background of the Invention:

Electric kettles and hot water jugs have conventionally been provided

with an element protector control, for protecting the heating element against

overheating for example as a result of the vessel having been switched on

empty, and a separate steam sensor control, for automatically switching off or

reducing the supply of electricity to the heating element when water boils in

the vessel and steam impinges upon the control. An example of an element

protector control is the X-series control, available from us, which is

substantially as described in GB-A-2 194 099 with reference particularly to

Figures 3A, 3B and 3C thereof and provides two levels of overtemperature

protection, and an exemplary steam control is the J-series control, also

available from us, which is substantially as described in GB-A-2 212 664 with

reference particularly to Figures 3A to 3M thereof. Both of these controls,

and other functionally similar controls, employ thermally-sensitive devices in

the form of bimetallic actuators. Proposals have been made to achieve the functions of heating element

overtemperature protection and steam sensing by means of a single sensor. A

single sensor electronic control is described in GB-A-2 228 634, and in

GB-A-1 143 834 there is described a proposal to incorporate an enclosure

within the water chamber of a water boiling vessel, the enclosure being

arranged to fill with water when the chamber is filled and to be heated by the

vessel heating element so that, when the water boils, the water in the

enclosure is driven therefrom by steam, thereby simulating a dry boil situation

within the enclosure which can be sensed by a bimetallic or other thermal

sensor.

Despite its apparent simplicity and promised advantages, the

arrangement of GB-A-1 143 834 was, so far as we are aware, never put into

production. When we attempted to follow the teachings of GB-A-1 143 834

we found that we could not obtain a satisfactory single sensor control

arrangement with a heating element as described in GB-A-1 143 834. We

were, however, able to devise solutions to the problem of making the

enclosure idea of GB-A-1 143 834 work and these are described in

WO-A-9318631, WO-A-9318632 and WO-A-9511515. The first two of these

involve the utilisation of heating elements of the conventional sheathed type

in which a resistance heating wire is packed into a metal sheath with mineral

insulating material between the wire and the sheath, and the latter makes use

of so-called thick film heating elements which comprise an electrically insulating substrate, commonly of stainless steel with an insulating layer of

glass for example, upon a surface of which a resistance-heating track or layer

is provided. Thick film heating elements are becoming increasingly popular

on account of their clean appearance and potentially high watts density which

gives reduced heating times.

In our British Patent Application No. 9816645.7 there is described a

solution to the problem which can arise, particularly with the proposal of

WO-A-9511515, that the operation of the control may be voltage sensitive.

This is a particularly European problem, since supply voltages in Europe can

vary from one country to another within a range of from 210 volts to 250

volts. According to the solution described in GB 9816645.7 abovementioned,

the resistance heating track of a thick film heating element has a portion

designated to register with an enclosure as described in WO-A-9511515 and

formed of a material having a negative temperature coefficient of resistance,

the remainder of the heating element track being formed of a material having a

positive temperature coefficient of resistance. As is explained in

GB 9816645.7, this arrangement can compensate for differences in supply

voltage insofar as the operation of the enclosure and its attendant bimetallic

sensor is concerned.

The single bimetal control principle as described in the foregoing thus

enables a single bimetallic sensor, responsive to the temperature of a

particular part of the heating element underlying an enclosure, particularly a thick film heating element, to function both to protect the heating element

against overheating and also to switch off the electricity supply to the heating

element when water boils in the vessel. The vessel still has to have an on/off

switch and this can be arranged by use of a manually resettable control to

which the appliance (vessel) manufacturer can affix his styled rocker knob.

However, this enforces design limitations as regards the location of the rocker

knob (on/off switch) and requires an element temperature control having the

facility to be switched both on and off.

Objects and Summary of the Invention:

It is the principal object of the present invention to overcome or at

least substantially reduce the abovementioned problem.

According to the present invention, a single sensor control in

accordance with the teachings of any of our above-mentioned proposals

further includes a relay connected in series with the heating element and the

control, the relay operation being arranged to be dependent upon the flow of

electric current to the heating element, for example by provision of a current

transformer in the circuit, and the relay furthermore being manually operable.

A water boiling vessel in accordance with the teachings of the present

invention including a single bimetallic control and an electromagnetic relay

switch would thus operate as follows:

(1) Start - the relay switch is de-energised, its contacts are open,

and no current flows through the heating element. (2) The contacts of the relay are manually closed (by an "On"

switch or push button. Current flows, via the current transformer, to

the heating element. The output of the transformer serves to energise

the relay coil, holding the contacts closed, so the "On" switch may be

released.

(3) When the water boils, the single bimetal control switches off

the element. No current flows, so the output of the current transformer

drops to zero and the relay is de-energised, opening its contacts.

(4) The water ceases boiling and the single bimetal control resets

automatically. No current flows because the relay contacts are open.

(5) The cycle may be repeated by closing the relay contacts again.

The boil cycle may be interrupted at any time by manually breaking

the circuit, for example by manually opening the relay contacts or by

operating a separate "Off switch.

(6) In the event of switching on dry or boiling dry, the normal

functions of the element protector control serve to protect the element

by switching it off. In this case the relay circuit will de-energise and

will require the user to switch on to reset.

The relay and switches may be situated on the vessel, but preferably

the vessel is cordless and the relay and switches are situated within the

cordless base. This gives a very simple appliance, in which the vessel proper

has only the element and its protector, and as with the above single bimetal control arrangements, no steam duct or separate steam control is required. In

addition, however, no rocker or manual control of any type is required on the

vessel proper, giving complete freedom of design. This is particularly

appropriate for metal vessels, where the provision of plastics mouldings and

rocker knobs spoils an otherwise clean design. By situating the controls

within the cordless base a further advantage arises in that as soon as the vessel

is lifted from the base, the circuit is broken and the relay is de-energised.

Thus the connector part in the base is not connected to the supply and

additional safety is achieved. In addition, even if the vessel is replaced on the

base, it will have to be manually switched on. The control system additionally

means that the vessel cannot be left switched on but unplugged (or plugged

into a switched off socket outlet) so that accidental dry switch on is less likely.

Of course, any type of cordless connection system could be utilized, such as

for example the 360° cordless connection system made and sold by us as the

CS4/CP7 system which is substantially as described in WO- A-9406185. We

believe that such a construction is inventive in itself, in that it provides a

cordless appliance connector which cannot be switched on without the

appliance proper being in place.

The invention is not restricted to utilization of bimetallic temperature

sensitive controls and any kind of thermally responsive switch could be

employed. Additionally, sensors could be utilized comprising PTC (positive

temperature coefficient of resistance) materials as disclosed for example in our British Patent Application No. 9807385 J which describes and claims a

thick film heating element having associated therewith a PTC material for

determining the current supply to the coil of an electromagnetic relay switch.

It is thus not necessary to break the circuit to cause operation. The

current sensing component may also use a reduction in the load current below

a set value to de-energise the relay. When applied to a PTC protected

element, the reduction of current caused as the PTC resistance rises can be

used. This has an advantage in that it is not necessary to maintain the PTC at

a high temperature (using energy) to hold the element off, which means that

the whole element will be cold and hence safer. In one implementation on a

thick film printed element, the PTC resistor would be located under the

enclosure to sense both boil and dry boil. The rise in resistance, giving rise to

a fall in element current, would de-energise the relay. In this application a

PTC having a knee characteristic (giving a rapid rise in resistance above a set

temperature) would be preferred. Since PTC thick film printing materials are

not readily available, the PTC sensor could be a discrete surface mount

component. A normal linear characteristic could also be made to work and

would lead to an element with no additional control components assembled to

it, which would be a cheap solution. NTC and PTC tracks provided on the

heating element to give a sort of thermal runaway may be used, where the

power of the element as a whole is transferred from the NTC to the PTC portions as the PTC resistance rises on sensing a rise in temperature, leading

to an overall drop in current.

The teachings of the present invention could also be applied to a

conventionally constructed vessel with a steam tube and a steam switch,

allowing a retrofit to an existing design. In this case the steam switch need

only be a simple self-resetting thermal cut-out, with no manual actuator. A

sealed contactstat could be used. The lack of a manual rocker and linkage

would have safety and constructional advantages, and would simply the

design of switches of a suitably sealed design to resist condensation.

A further advantage arises because the invention can be implemented

using standard, readily available, components. No additional contacts are

needed on the cordless system, as were required by the electronic kettle

(GB 2 228 634) so the standard CS4/CP7 or CS2 described in

GB-A-2 241 390 may be used. The element protector control can be of any

auto-reset type, such as our X2 or X3 controls. The latching relay is readily

available, and together with the current transformer, costs around £1. This

will replace around 60p of components, together with the necessary cover

mouldings normally found on a conventional kettle. It has the constructional

advantages claimed for the single bimetal control, including lack of steam

sealing and spillage problems.

The teachings of the present invention are applicable to vessels

incorporating any kind of electric heating element, though in the case of die cast elements or elements of the mechanical Blitzkocher construction,

modification of the heating element to provide a location which is normally

cooled by the presence of water, and which has a heater between it and the

control actuator may be necessary. This can be achieved by providing a sump

in the water-side surface of a die cast element, situated very close to the inner

side of part of the sheathed heating element which is enclosed in the casting,

and providing an apertured cover over the sump. A similar modification of a

Blitzkocher type heating element could be effected.

The above and further features of the present invention are set forth

with particularity in the appended claims and, together with the advantages

thereof, will be well understood from consideration of the following

description given with reference to the accompanying drawings.

Description of the Drawings:

Figure 1 is a schematic circuit diagram of a heating element control

circuit embodying the teachings of the present invention; and

Figures 2A, 2B and 2C show, respectively, a perspective view of the

underside of an exemplary die cast heating element, a view similar to Figure

2A but with a bimetallic control shown affixed to the heating element, and a

perspective view of the upper (water) side of the heating element of Figures

2A and 2B showing a sump-defining pocket formed in the surface thereof.

Detailed Description of the Embodiments: Referring to Figure 1 , the circuit shown is for a cordless electric water

heating appliance, such as a kettle or hot water jug, comprising a vessel part

and a base part and wherein a heating element in the vessel part is arranged to

be powered via the base through the intermediacy of co-operating connectors

provided on the two parts which operatively engage when the vessel part is set

down on the base part. In Figure 1 , the co-operating connectors are shown at

1 and 2, the vessel part is to the right hand side of the connectors 1 and 2, and

the base part is to the left hand side of the connectors 1 and 2.

The vessel part has a heating element 3 and an element protector

control 4. The heating element can for example be a thick film heating

element provided with an enclosure as described in WO-A-9511515. The

element protector control 4 can be an X-series control as described in

GB-A-2 194 099 for example, the X-series controls providing dual protection

for the heating element as is well known. The connector set 1, 2 can for

example be of the 360° kind described in WO-A-9406185 which enables the

vessel part to be seated upon the base part irrespective of the relative

rotational orientation of the two parts.

The base part has an electromagnetic relay 5 comprising contacts set 6

and operating coil 7, a current transformer 8 coupled to the relay coil 7 and

arranged so that the relay coil 7 is energised so long as current is flowing in

the heating element 3 of the vessel. An "on" switch 9 is connected across the relay contacts 6 and an "off

switch 10 is connected in series with the relay contacts 6.

The contacts of the element protector control 4 are normally closed,

but will open when water is boiled in the vessel, by virtue of the provision of

the enclosure aforementioned, and also if the heating element 3 is powered

without the vessel having been filled. The relay coil 7 is energized via current

transformer 8 when the heating element 3 is powered. Otherwise, because the

"off switch 10 has been operated or the element protector control 4 has

operated to open its contacts or the vessel part has been lifted off its base part,

no current flows in the heating element 3 and the relay coil 7 is de-energised.

When relay coil 7 is de-energised the relay contacts 6 are open. The "on"

switch 9 enables the open relay contacts 6 to be bypassed so as to cause the

heating element 3 to be powered, so long as the vessel is seated on its base

and the element protector 4 has not operated, and operation of the "on" switch

9 will, subject to these conditions, cause the relay coil 7 to be energised

thereby closing relay contacts 6. The "off switch 10, which can be integrated

with the "on" switch 9 in an appropriate toggle switch, enables the circuit

through the base and to the vessel to be broken. Otherwise, the operation and

advantages of the circuit will be clear from the explanations provided

herebefore.

Whilst the teachings of the present invention are, in principle,

applicable to vessels incorporating heating elements of all the commonly used kinds, some modification of die cast elements or elements of the mechanical

Blitzkocher type construction, comprising a sheathed heating element

clamped or clenched to the underside of a metal plate, may be necessary. This

is because there is no location on such elements which is normally cooled by

the presence of water, and which has a heater between it and the control

actuator. To overcome this we propose to provide a sump in the water-side

surface of the die cast element, situated very close to the inner side of part of

the sheathed heating element which is enclosed in the casting. On a test

sample this was done by machining a pocket in the upper surface of the

heating element. The particular element that we used has a platform built

(cast) up on its lower surface close to the heating element proper and an X2

control is mounted on this platform. Heat from the heating element proper is

conducted to the platform sideways and lost to the water upwards, which

gives a suitable running temperature for normal use and gives a rapid

temperature rise when no water is present. The pocket abovementioned was

machined from the upper surface of the heating element, effectively hollowing

out the platform from above. A cover, formed of moulded plastics material

for example, was secured over the pocket, and one or more suitably sized

holes left to allow water to enter and steam to leave. The size of the hole(s) is

determined by trial and error and depends on the heating element

configuration and power density. Since the pocket is sunk below the top

surface of the heating element, the cover may be made flush, giving a neat appearance. A similar construction may be applied to heating elements of

mechanical Blitzkocher type. Where such elements are used with metal

bodied vessels, a hole might be cut in the vessel bottom and a sump welded in

place underneath. The sump may be provided with a thermal link to the

heating element, for example a copper loop pressed onto the outside of the

element sheath or brazed to it, and the thermal control may be placed in

thermal contact with the sump to create an equivalent construction to the die

cast element described above. It is conceivable that a sump could be designed

which has a restricted opening such that a separate cover is not required.

Figures 2A, 2B and 2C of the accompanying drawings show an

exemplary die cast heating element modified as above described. Figure 2A

shows the heating element as comprising a die cast metal body 20 having a

sheathed, spiral heating element 21 incorporated into the underside thereof,

the upper surface of the heating element, as shown in Figure 2C, being planar.

The body 20 of the heating element has integral mounting posts 22 and the

heating element cold tails 23 and 24 can be seen. Also shown are integral cast

metal pads 25, 26 and 27 which serve for the mounting to the heating element

of an X2 control 30 in the manner shown in Figure 2B. As shown in Figure

2C, the metal pad 26 is hollow thus defining a sump 35 as aforementioned

which can be provided with an apertured cover to define a single sensor

enclosure as previously explained herein. Having thus described the invention by reference to specific

embodiments, it is to be well understood that the embodiments are exemplary

only and that modifications and variations thereto are possible without

departure from the spirit and scope of the invention. For example, whereas

the arrangement of Figure 1 employed an electromechanical relay, the same

function could be performed electronically, for example by means of a triac

circuit.

Claims

1. An electrically heated water boiling vessel wherein an enclosure is

associated with the heating element so as to simulate a dry boil condition upon

boiling of water in the vessel, a thermal sensor being provided responsive to

said dry boil condition, and wherein means responsive to current flow through

the heating element is provided in circuit with the thermal sensor and the

heating element, the last-mentioned means being manually operable.

2. A vessel as claimed in claim 1 wherein said current responsive means

comprises an electromagnetic relay comprising a set of switch contacts and a

coil, the state of the switch contacts being determined by whether or not the

relay coil is energised and the state of energisation of the relay coil being

determined by whether or not current is flowing through the heating element.

3. A vessel as claimed in claim 2 wherein manually-operable on/off

switch means are associated with said relay.

4. A vessel as claimed in claim 1 wherein said current sensitive means

comprises an electronic circuit or device.

5. A vessel as claimed in any preceding claim configured as a cordless

appliance comprising a vessel part, a base part and co-operating electrical connectors on the vessel and base parts enabling the vessel part to be powered

via the base part when the two parts are operatively co-located with their

electrical connectors operatively coupled together.

6. A vessel as claimed in claim 5 wherein the vessel part includes the

heating element, the enclosure and the thermal sensor, and the base part

includes the current responsive means and associated manually-operable

control(s).

7. A vessel as claimed in any of the preceding claims wherein the heating element is a thick film heating element.

8. A vessel as claimed in any of claims 1 to 6 wherein the heating

element comprises a die cast mass incorporating a heating element proper and

the enclosure is defined by a sump formed in the die cast mass.

9. A vessel as claimed in any of claims 1 to 6 wherein the heating

element comprises a metal plate having a sheathed heating element affixed to

the underside thereof and the enclosure is defined by a sump formed in the

metal plate.

10. A vessel as claimed in claim 9 including a thermally conductive

element extending between the sump and the sheathed heating element.

11. A vessel as claimed in claim 9 or 10 wherein said plate comprises the

base of a metal body of the vessel.

12. A vessel as claimed in any of the preceding claims wherein the thermal

sensor comprises a bimetallic element.

13. A vessel as claimed in claim 12 wherein said bimetallic element is an

automatically-resetting bimetallic element.

14. A cordless electrical appliance comprising an appliance proper and a

base and wherein co-operating electrical connectors are provided on the two

parts for powering the appliance proper through the base when the former is

operatively seated on the latter, and wherein means are provided in the

appliance to ensure that the base connector cannot be powered if the appliance

proper is not seated on the base.

15. A cordless electrical appliance wherein means are provided in the base

part of the appliance for disconnecting the base connector from the electrical

supply if the appliance part is not operatively in place on the base part.

16. A cordless electrical water heating appliance comprising a vessel, a

base and co-operating electrical connectors on the vessel and the base for

powering a heating element in the vessel via the base when the vessel is seated

on the base with said electrical connectors operatively connected, and wherein

operating controls for the appliance are provided exclusively on the base.

17. An electrically heated water boiling vessel substantially as herein

described with reference to Figure 1 of the accompanying drawings.

18. An electrically heated water boiling vessel as claimed in claim 17

wherein the heating element is substantially as herein described with reference

to Figures 2 A, 2B and 2C of the accompanying drawings.

19. An electric heating element substantially as herein described with

reference to Figures 2 A, 2B and 2C of the accompanying drawings.

20. In or for an electrically heated water boiling vessel, an electric heating

element formed with a sump defining an enclosure for simulating a dry boil

condition when water is boiled in the heating element.

21. An electrically heated water boiling vessel having a sump formed in

the bottom thereof in close thermal contact with a heating element of the

vessel, the sump defining an enclosure for simulating a dry boil condition

when water is boiled in the vessel.