GB2339348A - Cooking appliance heating unit - Google Patents

Description

2339348 HEATING UNITS FOR A COOKING APPLIANC Ibis invention relates to

heating units for electrical cooking appliances, especially to heating units for hotplates, grills and oven.

Typical heating units for hotplates comprise one or two ring-shaped elements. They are usually controlled by an electro-mechanical energy regulator. The user rotates a control knob on the cooking appliance to a desired setting. The element or, if two are present, one or both depending on the setting, are connected in circuit. The element(s) then heat up. At the same time as the elements are connected in circuit, a resistance coil is also connected in circuit, and this heats a bi-metallic strip which the coil is wound around or in proximity to. After a period of time, typically fifteen seconds for a mid-point setting of the control, the bi-metallic strip has deflected sufficiently to operate an over-centre switch which disconnects the elements and heating coil. The bi metallic strip then cools and the over-centre strip re-connects the elements and the heating coil, typically after a further period of fifteen seconds. The heat input is maintained in this way, and higher and low inputs result from longer or shorter on relative to off periods.

It follows that operation of a typical hotplate heating element entails a continuous series of on/off switches of the heating elements. A heavy load causes flicker if switched on and off intermittently, and it is for this reason that Electro-ma2netic Compatibility (EMC) Regulations exist. Broadly speaking, the higher the load, the less quickly it can Z 2 be allowed to cycle on and off.

This does have ramifications even in the design of cookers in that the maximum ratings of grill elements have had to have been reduced to enable them to switch as frequently as desired without breaking the EMC regulations.

Small loads such as domestic light bulbs can be switched very rapidly, so rapidly that flicker is not perceptible, and this has been done for dimmers for light bulbs using solid state switching devices such as thyristors and triacs.

The invention provides a heating unit for an electric cooking appliance, which comprises a first heating element, a second heating element rated at a higher power than the first heating element, each heating element being operable at variable settings by varying the amount of time per unit time for which the element is energised, and each increased setting of the higher rated element being accompanied by a reduced setting of the lower rated element to reduce the power increase of the increased setting.

Because the increase of load for an increased setting of the higher rated element is off set by a decrease for the lower rated element, the higher rated element can be switched more rapidly than if it were on its own and hence the heat output can be more finely controlled.

Advantageously, the elements are switched for periods of half cycles of the mains 3 supply in a unit time consisting of a number of cycles, and solid state switches such as thyristors or triacs are preferably used for performing the switching.

The heating unit may comprise a third heating element which can be switched in at the same time as another element is switched out to reduce the power increase. More than three elements can be provided if desired. The power ratings of the heating elements may he in a binary series.

The heating unit may be for a hotplate, but may equally be for a grill or oven element of a cooking appliance.

The invention will now be described in detail by way of example with reference to the accompanying drawings, in which:

Figure I is a perspective view of a cooking appliance; Figure 2 is a diagram showing the architecture of the circuitry connected to the controls of the cooking appliance shown in Figure 1; Figure 3 is a circuit diagram showing triac drive circuitry for some heating elements of the appliance; Figures 4 to 6 are graphical representations of various power levels produced by a hotplate; Figure 7 is a schematic representation of the grill of the cooking appliance; Figure 8 is a schematic representation of four cycles of mains power supply; and Figure 9 is a circuit diagram showing the connection of two heating elements of a hotplate.

The cooking appliance of the invention has a main oven 1, a top oven including a grill 2 and four hotplates 3 to 6, all of which are controlled by rotary controls, one of which has the reference numeral 7, on a control panel 8.

Hitherto, it would have been usual for the mains leads to enter the appliance from the rear and to have been led to the control panel 8, to be attached to electro-mechanical energy regulators mounted on the inside of the control panel 8 connected to the rotary controls 7 on the outside. Further leads would then extend to the heating elements of the hotplates, the grill or the oven.

In the electric cooker shown in Figure 1, the electric supply leads are connected directly to the heating elements of the respective heating units via respective triacs, the C on/off switching of which is controlled by the rotary controls 7. Thus, in place of multiple electro-mechanical regulators, one single micro-controller 9 is provided, controlling the power output of each of the heating units. The required heat input is provided by the rotary controls 7, which connect to potentiometers 10 which provide an analogue voltage representative of the setting of the control. These voltage signals are encoded by rotary encoder I I and combined together by analogue multiplexer 12 for input to the micro-controller 9. Additional thermal inputs from thermocouples 13 are also combined in multiplexer 12. In addition, micro-controller 9 also controls visual display 14 on the appliance via bus 15. If desired the rotary encoder I I could provide an output directly for the micro-controller 9, and the potentiometer 10 and rotary encoder 11 could be replaced by a shaft encoder to provide a direct digital output.

The power output of the various heating units is controlled by progressively increasing or decreasing the number of half cycles of the mains supply in a unit period of say, eight half cycles, by means of triacs in series with the heating elements of the heating units, the switching however always being done at a zero crossing. This provide a recurrent processing period of 10ms (IOOHz). In processing terms this interval is significantly long in respect of the typical instruction speed of the micro-controller of IMHz. However, only processing tasks must be completed well within this period to ensure zero crossings synchronisation is maintained.

The necessary data is passed to the triacs via a bus 15a and register 16.

Referring to Figure 3, the data passes from the re t gister 16 to a driver chip 17 which 6 provides trigger signals for the triacs, four of which are shown as 18-21. The driver chip 17 energises light-emitting diodes 22-25 which causes diacs 26-29 to provide the necessary trigger signals for the triacs 18-21. Once triggered by the diacs, the triacs stay conducting for the remainder of a half cycle but thereafter cease conduction. Only a small current is needed to trigger the triacs, but they can carry a large current between their anode and their cathode, large enough for the heating elements in the cooking appliance. Four of the heating elements are connected between the terminal block 30 and neutral. The live connection is effected at terminal 3 1.

It is a particularly important feature that the micro-controller can be programmed in circuit within the assembled product. This means that only one component needs to be obtained for a range of components, rather than individually programmed components which would add to the cost of a range of appliances. The programming of the micro-controller 9 is to correlate the potentiometer readings with the rotary positions of the controls 7 for each particular heating element and, for this purpose, the micro-controller has access to an external memory 45 which is an EPROM. The EPROM also stores information relevant to the operation of the thermocouple 13 for whatever element is fitted in connected with.

The micro-controller controls the system using a parallel data bus and dedicated control lines. The micro-controller data bus itself is only accessible as the local bus. As the name implies, the local bus is designed to only communicate with nearby devices, namely the external memory 45 and analogue multiplexer modules 12. Where data is 7 to pass further afield, data is passed via register as in the case of the triac array data

16 or switched via a bi-directional buffer drive (not shown) in the case of the display control operations. This design feature is intended to minimised potential EMI transmission or interference by restricting external bus activity to a minimum.

A suitable implementation for the micro-controller is the PIC 16C74A micro chip from the manufacturer Arizona. It comprises 33 110 pins, an 8-bit successive approximation A-D port and an onboard watchdog timer. The part has 4K of program memory with 192 bytes of data ram.

While the use of simple potentiometers connected to the rotary controls and the use of a single micro-controller for the appliance, in conjunction with triacs for controlling the heating elements has been described in relation to a cooking appliance, in fact the concept is extendable to any domestic appliance such as a washing machine, tumble dryer or gas cooking appliance or gas fire employing a heating element.

Reference has also been made to triacs, but equivalent solid state switches e.g. SCRs for switching the mains through to a heating element in response to a triggering impulse may be used instead. The controls 7 need not be rotary controls. For example, a key pad could be supplied.

Referring to Figures 4 to 6 and 9, the control of the heating elements of one hotplate, for example, the hotplate 6 will now be described. The hotplate 6 has two heating 8 elements 32 and 33, both arranged in circular form. The element 32 is rated at 60OW and forms the inner ring and the element 33 is rated at 120OW and forms the outer ring, and these are connected to the live terminal via triacs 34, 35. The hotplate 6 is controlled by a single rotary control.

In the lowest setting of the rotary control, the triacs 34, 35 are controlled so that the element 33 is fully off whereas the element 32 is energised for one half cycle out of a unit time of eight half cycles (Figure 4). This means that the power supplied to the hotplate is 75W. When the control is moved to the next setting, the triac 34 is now triggered so that the element 32 is energised on the first positive half cycle but also on the first negative half cycle i.e. over one complete AC cycle. This corresponds to a power of 150W. The next setting corresponds to three half cycles, and so on up to eight half cycles i.e. the element 32 is on continuously (Figure 5).

It might be thought that the next setting would be for the element 33, which hitherto has remained fully off, to be energised for a first half cycle out of each unit period of eight half cycles, but this would result in an increased power of 150W by virtue of the 120OW energised for 10 milliseconds each period of 80 milliseconds. Such a rapid switching of such an element would cause undesirable fluctuation on the mains, and could breach Electro-magnetic Compatibility (EMC) Regulations. These are designed to prevent fluctuations of the mains supply which could cause visible fluctuation of e.g.

lighting circuits in the vicinity of the appliance.

9 For this reason, the next increased setting after the element 32 has been fully energised is, referring to Figure 6, a first half cycle of energisation of the 120OW element 33 via the triac 35, but in conjunction with a switching off of the lower rated element 32 for the same first half cycle (of every eight cycles) as the element 33 is energised for. In this way, the increased power dissipated is only 60OW, not 120OW, i.e. exactly the same as for the first element.

The next increased power setting is produced by the element 33 being energised for the first two half cycles as of each eight half cycle unit in combination with the element 32 now ceasing to operate over those two first half cycles. 'Mis carries on in step for further increases in power i.e. the element 33 is energised for a further half cycle and the element 32 ceases to be energised for that half cycle. This carries on until element 33 is fully on and element 32 is fully off.

Further increases in power are then produced by element 33 staying fully on and element 32 being increased in power from one half cycle per eight half cycles, two half cycles per eight half cycles, three half cycles etc. up to the maximum of eight half cycles per 80 milliseconds i.e. both elements 32 and 33 fully on.

In this way, the maximum load being switched is 60OW throughout the entire range of use of the appliance, and the 180OW of the hotplate can be progressively increased in the 75W steps over twenty four discrete levels.

The advantage of the method of controlling the hotplate is that the heating is far more even than if an electro-mechanical regulator was supplied, since this would supply full power for a short time, cool, supply full power again, cool etc. Further, there are now a large number of very accurate power input levels, the lowest of which has a lower value than would have been obtainable with an electro-mechanical regulator, making it possible e.g. to melt chocolate in a saucepan.

Study of EMC flicker requirements backed by empirical testing indicated that a switching period of eight half cycles was a viable minimum for a load change of 600W.

The operation of the hotplate heating unit has been described in relation to a window of 80 milliseconds. Other lengths of window could be chosen. For example, the window length could be 160 milliseconds. In this case, the lowest setting would be two half cycles of element 32, extending up to sixteen half cycles before element 33 was brought in. This would not provide any problem from an EMC point of view, since the rate of switching would be slower e.g. the lowest setting would recur every milliseconds instead of 80 milliseconds. However, the element itself might now flicker in brightness if viewed, and this could be a disadvantage for an open element beneath a ceramic hob. However, it would not be a disadvantage for an element beneath a sealed hob i.e. a solid hotplate.

Further, the arrangement is readily extendable to a third heating element, for example, contained in the same hotplate as 32 and 33. In this case, the ratings could be 40OW, 80OW and 120OW to give a maximum rating of 2.4kW. The lowest two elements would be switched as for the elements 32 and 33. When both elements 32 and 33 were fully energised, the third element would switch in for a first half cycle, and the middle rated element would switch off for that half cycle so that the step would be no more than when the lowest element was switched on for one half cycle. Further, the arrangement could be extended to further elements, the power ratings of which were in the ratio 1:2:4:8:16 etc.

Further, it would be possible for the elements to switch for other than complete half cycles. Thus, the elements could switch for quarter or other multiples of a cycle, provided that the difference between the load added from one element and subtracted from another element was below the desired maximum, and still comply with the EMC flicker requirements. However, means would have to be provided to remove the r.f.

interference which would be caused by non-zero-crossing switching.

While the switching of the elements 32 and 33 has been described in relation to a hotplate, it applies to any other heating unit of the cooking appliance.

Referring to Figures 7 and 8, the operation of the grill contained in the top oven 2 will now be described. The grill consists of eight elements 36 to 43 each connected across the mains in series with a respective triac (not shown). The individual elements 36 to 43 may be metal sheathed or may be glass e.g. silica glass with a coil wound around the outside or contained inside each tube. The elements are mounted on a support 44.

12 The grill is capable of providing four power levels.

The maximum power level is clearly when all eight elements 36 to 43 are fully energised. The rating of each element is 40OW, and this provides a maximum power of 3.2kW.

It will be convenient to describe the operation of the lower power settings as if the grill consisted of 2 "halves" i.e. the set of elements 36 to 39 and the set of elements 40 to 43. Thus, for the lowest power setting, for each half, each of the four elements is driven in succession for a successive half cycle of the mains waveform shown in Figure 8. Thus, the element 36 is switched by its triac to be in circuit for the first half cycle of each 80 millisecond unit time of eight half cycles, the elements 37 to 39 being out of circuit. Similarly the element 40 is in circuit and the elements 41 to 43 out of circuit. On the next half cycle the elements 37 and 41 are in circuit and the elements 36, 38, 39, 40, 42, 43 are out of circuit. On the next half cycle the elements 38 and 42 are in circuit the others being out of circuit and on the fourth half cycle the elements 39 and 43 are in circuit, the others being out of circuit. This process repeats so that on the fifth half cycle out of each unit period of 80 milliseconds the elements 36 and are energised. It will be seen that the heating will be entirely uniform, since each element is on for the same amount of time, and that there is no problem with EMC regulations, because the output remains constant at 40OW for each "half' of the element i. e. 800W.

13 The next power level is produced by triggering the elements 36, 38 and 37, 39 in synchronism with each other, similarly the elements 40, 42 and 41, 43. Thus, on the first half cycle, elements 36 and 38 are energised and on the second half cycle elements 37 and 39 are energised. This provides a net 80OW per grill half, and the heating is again even.

The third power level is produced by three of the four elements of each half being driven in rolling succession, providing an output of 1.2kW for each half and 2.4kW for the whole grill, Thus, the element 36 is initially off while the elements 37 to 39 are on. Similarly, the element 40 is off and the elements 41 to 43 are on. Then on the next half cycle the element 37 is off and so is the element 41, on the next half cycle the element 38 is off together with the element 42, all other elements being on, and on the fourth half cycle the elements 39 and 43 are off.

If desired, even lower settings could be produced from the grill. For example, instead of the elements 36, 37, 38, 39 being energised in successive half cycles, the element 36 could be energised in a first half cycle, the element 37 in a sixth half cycle, the element 38 in an eleventh half cycle and the element 39 in a sixteenth half cycle, with the same applying to elements 40-43. Progressive increase could then be obtained by energising those elements for two half cycles out of each group of four half cycles.

With the improved method of controlling the grill, the grill can now be used safely to warm food or plates.

14 The grill could consist of more than eight elements or less than eight. Further, the physical arrangement of the elements could be different, and each element need not be in the form of a bar, for example, one element could be formed by two or more individual elements connected together in series, or could be in circular form. Instead of controlling the elements in half cycles (e.g. for the lowest power setting each element is energised for a half cycle), the elements could be controlled in multiples of a half cycle e.g. a whole cycle. Thus, the elements could each be on for a successive whole cycle in a longer unit period of eight whole cycles for the lowest power setting, and so on for the other power settings. While the elements would usually be the same power rating, and this would be necessary for the control by half cycles method described to avoid drawing an asymmetric current, for control by whole cycles the elements could have power ratings somewhat different from each other. The unit period shown in Figure 8 could be longer (for the illustrated arrangement of eight elements).

Claims (8)

1. A heating unit for an electric cooking appliance, which comprises a first heating element, a second heating element rated at a higher power than the first heating element, each heating element being operable at variable settings by varying the amount of time per unit time for which the element is energised, and each increased setting of the higher rated element being accompanied by a reduced setting of the lower rated element to reduce the power increase of the increased setting.

2. A heating unit as claimed in Claim 1, in which each heating element is operable at a discrete number of heating settings obtained by increasing the number of half cycles per unit time for which the element is energised.

3. A heating unit as claimed in Claim 2, in which the heating elements are energised and de-energised at the zero crossings of the AC mains supply.

4. A heating unit as claimed in any of Claims 1 to 3, in which the second heating element is rated at twice the power of the first heating element.

5. A heating unit as claimed in any one of Claims I to 4, in which each heating element is in series with a solid state switch.

6. A heating unit as claimed in any one of Claims I to 5, in which the solid state 16 switch is a triac.

7. A heating unit for an electric cooking appliance substantially as herein described with reference to and as shown in the accompanying drawings.

8. An electric cooking appliance employing at least one heating unit as claimed in any one of Claims I to 7.