GB2112229A

GB2112229A - Electric heaters

Info

Publication number: GB2112229A
Application number: GB08138876A
Authority: GB
Inventors: Edwin Eric Stokes
Original assignee: UNITED GAS IND PLC
Current assignee: UNITED GAS IND PLC
Priority date: 1981-12-24
Filing date: 1981-12-24
Publication date: 1983-07-13
Also published as: GB2112229B

Abstract

A radiant electric fire has a radiant bar (11) the ends of which are connected to be powered via a switch (12) and an energy controller (13) comprising a solid state device. This device may be a triac or a thyristor operating to ON and OFF power periods in cycles of duration between 1/60 second and 30 seconds. The triac preferably switches after bursts of the power waveform at zero voltage positions, in one embodiment using a cycle duration of 2 seconds. A resistance (15) connected in parallel with the element (11) and controller (13) may be another bar of the electric fire or a warning light. <IMAGE>

Description

SPECIFICATION Electric heaters This invention relates to electric heaters of the kind having electrical elements which when energized radiate heat into a space to be heated.

The invention comprises an electric heater having at least one regulated electrical element connected to be operated through an energy regulator which has at least one selectable setting which when selected causes said element to be operated on repeated cycles, each cycle comprising an ON period and an OFF period and each cycle having a duration of between 1/60 second and 30 seconds.

The duration of the cycle is preferably chosen with relation to the temperature inertia of the regulated electrical element so that the temperature variations due to the ON/OFF periods are not noticeable to the casual observer.

In one embodiment the energy regulator comprises a solid state circuit including a triac switch which provides almost infinitely variable ON/OFF adjustment in a cycle duration of between 1/10 second and 5 seconds.

In another embodiment the energy regulator comprises a solid state circuit including a thyristor which provides infinitely variable ON/OFF adjustment in each individual cycle of the waveform, i.e. in cycle duration of 1/60 second when a 60 cycle frequency mains supply is used.

Specific embodiments of the invention are now described with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram for an electric fire, Figure 2 is a representation of a waveform produced using the circuit of Figure 1, and Figure 3 is a representation of a waveform produced using different circuit components.

Referring first to Figures 1 and 2, a radiant electric fire has a 750 watt radiant electric heating bar (1 1 ) of the silica-sheathed coil type.

The ends of the bar are connected into a mains power circuit via an ON/OFF double pole manual switch (12) and an energy controller (13).

The energy controller is a solid state sub-circuit including a triac arranged to be switched to an ON (conducting) condition for periods of time which can be varied. The triac allows bursts of current through, as shown in Figure 2. The heating effect of the current is a function of the ratio between the durations of the ON and OFF periods in any cycle, and can therefore be varied by changing this ratio. In this embodiment the duration of an ON/OFF cycle is 2 seconds. Within the 2 second cycle the ratio between ON and OFF durations can vary from ON all the time to OFF all the time with almost infinitely variable in-between conditions set by knob (14). The variation is not fully infinitely variable because the triac is arranged to switch at or near a zero voltage position of the waveform, i.e. when it crosses the zero line, so reducing interference.

Because a cycle of only 2 seconds is used and the temperature of the bar (11) responds only slowly to the current variations, it is found that the ON/OFF cycle does not produce a cyclic temperature variation detectable to the casual observer or user of the fibre. The apparent effect is that the bar runs at a lower but even temperature. It will be understood that there is such a variation which can be measured, but it is designed to be too small to be easily observable.

The cycles can be longer than 2 seconds, e.g. 5 seconds, and should be chosen in relation to the thermal inertia of the bar (1 1 ) so that the temperature variations are not noticeable. For instance, a bar which comprises a large mass of low radiation coefficient material could be used with a longer cycle, e.g. 30 seconds, because its temperature would tend to vary less. Shorter duration cycles, e.g. down to 1/10 second, could also be used.

Because solid state switching is used, the operation is silent.

In parallel with bar (11) is connected another resistance (15), by-passing the energy regulator so that it is continuously energized when switch (12) is made. Resistance (1 5) may be another bar of the electric fire, or may be a warning light to indicate the fire is switched on.

It will be noted that the bar 1) is stated to be nominally of 750 watts. By adjustment of the knob (14) this can be effectively varied from 0 watts up to 750 watts in steps or continuously.

The knob may be marked with numerals indicating the wattage setting. One problem with previous electric fires has been that the heat output is inflexible. One solution to this problem has been to provide multiple bars, e.g. 3 bars with independent switching. By use of the invention it has become possible to provide a much more flexible heat output from a single bar. The bar (1 1) may therefore in another embodiment be replaced by a single higher wattage bar, e.g. of nominal 2500 watts, which by itself can then be varied to provide a wide range of heat outputs.

The embodiment of Figures 1 and 2 used a triac solid state switch. This may be replaced by a thyristor switch producing the waveform shown in Figure 3. The thyristor switches ON to conduct at point A when a set positive or negative voltage is reached and OFF when the voltage returns to zero. By altering the voltage at which the thyristor switches the point A can be moved in time so that more or less of the available waveform is conducted. The proportion of the waveform which is conducted (that shown cross-matched in Figure 3) determines the heating effect. As before, the proportion of ON to OFF in each cycle determines the heating effect, but the cycle is always the duration of one complete wave, i.e. 1/60 second for a 60 cycle frequency mains supply or 1/50 for a 50 cycle supply.

The fire may have means for protecting the energy controller against high temperatures, e.g.

slots providing a flow of convection air over it, or a heat sink.

Claims

Claims (filed 14/12/82)

1. An electric heater having at least one regulated electrical element connected to be operated through an energy regulator having at least one selectable setting which, when seiected, causes said element to be energized in repeated cycles, each cycle comprising an ON period and an OFF period and each cycle having a duration of between 1/60 second and 30 seconds.

2. An electric heater as claimed in claim 1, wherein the duration of the cycle is chosen with relation to the temperature inertia of the regulated radiant electrical element so that the radiation variations due to the ON and OFF periods are not noticeable to a casual observer.

3. An electric heater as claimed in claim 1 or claim 2, wherein said energy regulator is a solid state device.

4. An electric heater as claimed in any of claims 1 to 3, wherein said energy regulator comprises a triac switch providing variable ON/OFF adjustment in a cycle of duration between 1/10 second and 5 seconds.

5. An electric heater as claimed in claim 4, wherein said triac switch is adapted to change condition at or near a zero voltage condition of the electrical waveform.

6. An electric heater as claimed in claim 4 or claim 5, wherein the cycle duration is 2 seconds.

7. An electric heater as claimed in any of claims 1 to 3, wherein said energy regulator is a thyristor switch providing variable ON/OFF adjustment within each individual cycle of the waveform of the electrical power supply.

8. An electric heater as claimed in claim 7, wherein the cycle duration is 1/60 second.

9. An electric heater as claimed in any of claims 1 to 8, having a resistance element connected in parallel with the energy regulator and regulated element so that it is continuously energized.

10. An electric heater as claimed in claim 9, wherein said resistance element comprises another electric heating element or a warning light.

1 1. An electric heater substantially as described hereinbefore with reference to Figure 1 and Figure 2 or to Figure 1 and Figure 3 of the accompanying drawings.