EP0384640A2

EP0384640A2 - Improvements in electric hotplates

Info

Publication number: EP0384640A2
Application number: EP90301604A
Authority: EP
Inventors: Michael H. Buttery
Original assignee: Emaco Ltd
Current assignee: Electrolux Household Appliances Ltd
Priority date: 1989-02-20
Filing date: 1990-02-15
Publication date: 1990-08-29
Also published as: JPH02247996A; GB8903829D0; GB2228396A; EP0384640A3

Abstract

An electric hotplate is described having a laminated structure in which the heating element comprises strips of resistive material R on an insulating substrate. The strips are connected in a series parallel arrangement between conductors A, X, Y, Z and B. The interconnecting conductors X, Y and Z may be of a material having a negative temperature co-efficient of resitivity.

Description

This invention relates to electric hotplates and applies primarily to hotplates for cooking hobs.
Conventionally electric hotplates have been provided with heating elements consisting of a coil of resistive wire, generally of nichrome, which has a very small temperature co-efficient of restivity. As a result, the resistance of the heating element and therefore the power dissipated at any power setting is virtually independent of the temperature of the element.
If contrary to the conventional practice, a heating element is made from a material having a positive temperature co-efficient of resistivity. Thus, the current flow and, therefore, the power dissipated is greater when the element is cold and reduces as the element reaches its operating temperature. This has the advantage that a hotplate using such an element will warm up more rapidly and it is less liable to damage through accidental overheating which would occur, for example, in the case of a cooker hob, if the hotplate is left switched on without a saucepan being placed on it.
One problem that arises in hotplates having heating elements with a positive temperature co-efficient of resistivity is that if the hotplate is cooled over part of its area, for example by a cold saucepan being placed part way thereon, the cooler portions have a lower resistance and, therefore, the current through the element is increased. Consequently, those portions which are not cooled, will become overheated and they are more liable to be burnt out. This problem also manifests itself where the base of the saucepan is warped and only therefore contacts the hot plate over a limited area thereof. An object of the invention is to reduce the probability of this problem occurring.
A further problem with such hotplates is that because the resistance of the element is low on switching on, the initial current may be unacceptably large and may cause damage. One particular embodiment of the invention is directed to this problem.
According to the present invention, etc. An electric hotplate having a heating element comprises a plurality of sections of a material having a positive temperature coefficient of resistivity the sections being connected in parallel in groups, and the groups being connected in series between supply terminals. A number of different hotplates according to the invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 shows the inter-connection of the sections of an element employed in the hotplate according to the invention and Figures 2, 3, 4, 5 and 6 are diagrammatic plan views of three different embodiments of tne invention, showing how the elements are disposed on the hotplates.
Referring to Figure 1, the element of the hotplate comprises not a single conductor, but sixteen conductive sections R all of a material having a substantial positive temperature co-efficient of resistivity, and all having substantially similar resistance when measured at the same temperature. The sections are connected in four groups of four. The sections in each group are connected in parallel and the groups are connected in series with one another. In Figure 1, A represents an input terminal connected for example to an energy regulator. The conductors X, Y and Z connect the elements to connect the respective groups in parallel and B represents an output terminal for connection to the neutral supply line. It will be appreciated that the overall resistance of this assembly is equal to the resistance of any one section.
Initially, when power is applied to the element, a large current flows because the resistance is low and the sections heat up rapidly. By the time the element reaches a desired operating temperature, the resistance is much higher and the current is much reduced, thereby minimizing the risk of an element burning out even if the element is left uncovered at maximum power.
If now a saucepan is placed so as partly to cover the hotplate, those sections which lie under the saucepan are cooled, and their resistance is much reduced. However, because of the series parallel arrangement, current will tend to be diverted through the cooler setions, so reducing local overheating.
Figure 2 is a diagrammatic section in plan of a hotplate according to the invention. The hotplate is a laminated structure comprising a metal plate coated with a ceramic insulating layer on which are deposited resistance films forming the sections of the heating element and conductive films forming the interconnections, The electrical arrangement is as described above with regard to Figure 1.
The resistive sections are composed of a metal/glass mixture, the proportion of metal to glass (typically from 50/50 to 95/5 percent metal/glass) being selected to give a suitable resistivity. Nickel is a particularly suitable metal for use in the resistive sections, although other metals, such as cobalt, and alloys containing nickel and/or cobalt may also be used for this purpose. The conductors are mainly or entirely metallic in composition.
The metal and glass are finely powdered and thoroughly mixed in the required proportion with a screen printing medium to produce a viscous ink which can be readily screen printed in the required position on the hotplate. The conductors may be applied by similar means. After screen printing, the printed hotplate is dried at a moderate temperature and is then fired at a high temperature to bind the resistive sections and the conductors to the ceramic substrate. Connections are made to the supply conductors and a further protective layer may be applied.
The conductive and resistive pattern is shown in Figure 2. The outermost conductor A and the innermost conductor B are connected to a supply and between them are intervening conducting rings X, Y and Z between which the resistive sections are connected concentrically and in parallel groups. There are four parallel resistive sections between each consecutive pair of conductors. Because of the concentric arrangement, a saucepan partially covering the hotplate will reduce the temperature of some sections in each of the groups and the increased current will be diverted through these cooler sections. Conversely, if a single continuous element were employed, the whole of the increased current would pass through the hottest part of the element, increasing the risk of a burn out in that region.
Further emebodiments of this invention are shown in Figures 3 and 4. In the embodiment shown in Figure 3, two annular conductors C, D are connected to the sections R by a plurality of radial conductors F, G. Radial conductors F connect each group of four sections R to the outer annular conductor C and radial conductors G connect each group of sections R to the inner annular conductor D.
The embodiment shown in Figure 4 is broadly similar to that of Figure 3 but has three annular conductors: outer and inner annular conductors C, D are as before but there is an intermediate annular conductor E. In Figure 4, there are only two sections R in each group but there are twice as many groups and therefore the number of sections in Figure 4 is the same as that in Figure 3. The groups of sections are split into inner and outer sets by the intermediate annular conductor E.
Each group of sections R in the outer set is connected to the outer annular conductor C by radial conductor F, and is also connected to the intermediate annular conductor E by radial conductors H. Each group of sections R in the inner set is connected to the intermediate annular conductor E by radial conductors I, and is also connected to the inner annular conductor D by radial conductors G.
In the embodiment shown in Figures 3 and 4, the annular conductors C, D, E are connected to a suitable supply at terminals T. Each embodiment is suitable for use with an energy regulator and the Figure 4 embodiment is suitable for use with a switched power supply.
It will be appreciated that the annular conductors C,D,E need not be situated on the plate but could instead take the form of a wired ring main.
In a modification of the invention, the intermediate conductors X, Y and Z or the radial conductors F, G, H and I are made of a material which has a strong negative temperature co-efficient of resistivity, such that its resistance is comparable with that of the heating sections at room temperature but falls to a much smaller value, preferably negligibly low, but at most one-quarter of the resistance of a section at the operating temperature.
This modification increases the resistance of the elements appreciably at room temperature and therefore reduces the otherwise objectionably large initial current on first switching on.
The embodiment shown in Figures 5 and 6 is of laminated construction similar to that of Figure 2, and includes a spiral arrangement of four conductive sections R. The four conductive sectsions R are connected as shown between outer (F1) and inner (F2) line feeder tracks and an inner neutral feeder track (M1) connected to live and neutral terminals L and M respectively. This embodiment again overcomes localised overheating using conductive sections R made from material possessing a substantial positive temperature coefficient of resistivity and possessing substantially similar resistance when measured at the same temperature.

Claims

1. An electric hotplate having a heating element comprising a plurality of sections of a material having a postive temperature coefficient of resistivity the sections being connected in parallel in groups, and the groups being connected in series between supply terminals.

2. An electric hotplate according to claim 1, in which the heating element comprises four groups each of four sections, the sections having substantially the same resistance.

3. An electric hotplate according to claim 1 or claim 2 in which the groups are arranged concentrically.

4. An electric hotplate according to any preceding claim, having a laminated structure and in which the heating element sections each comprise a film of resistance material applied to an insulating substrate.

5. An electric hotplate according to any preceding claim in which the groups of element sections are interconnected by conductors having a negative temperature co-efficient of resistivity such that at operating temperatures, the interconnecting conductors having a resistance which is small in relation to that of the element sections.