GB2064239A - Power supply for electric cooker - Google Patents

Abstract

In order to limit the disturbance to the mains supply caused by periodic energisation and deenergisation of a resistive load such as the heater of the grill of an electric cooker, the heater is split into two heating elements which are energised alternately each for a period of time set by a user operable control 2. The mark to space ratio of the energisation of the partial loads 1a and 1b is varied in dependence upon the setting of the control 2. In order to provide fast response when the apparatus is first turned on, circuitry 16 overrides the proportion of control and energises both loads 1a and 1b simultaneously for a predetermined period of time long enough to bring the heater up to its working temperature. <IMAGE>

Description

SPECIFICATION Power control apparatus The present invention relates to the control of resistive loads such as electric heating elements for use particularly, but not exclusively, in electric cookers.

In our British Patent No. 1464093 we describe and claim a heating arrangement for use in an electric cooker in which the desired heating level is stored as a binary number in a counter and a triac in series with the heating element is appropriately gated in dependence on the contents of the counter. User operable controls such as touch pads are associated with count-up and count-down inputs of the counter so that the set heating level can be increased or decreased by the user. In order to speed the response of the heating element to a change in the setting, the control circuitry was arranged so that following the user changing the desired heating level, for a period of time the user-set heating level is overridden and instead the maximum power available in the control range is applied to the heater.

In our copending Application No. 7940618, we describe and claim an electrical heating arrangement comprising at least one electrical heating element and control cricuitry for controlling the power supply to the heating element, the control circuitry comprising a useroperable control, means for causing the power supplied to the heating element to have a value in a predetermined control range, the value being dependent upon the setting of the user-operable control, and means for causing the power supplied to the heating element to exceed the maximum value within the control range for a period following an increase in the set power.

Because of the problems of RFI generation particularly with large resistive loads such as cooker heating elements, it is preferred to control the power to such loads on a "burst fire" basis rather than by means of phase control. Burst firing involves the supply ofa given, integral number of half cycles of the supply waveform to the load per unit time. Although operation in the symmetrical burst fire mode largely avoids RFI problems, it means that there are variations in the power drawn from the mains supply over intervals of time considerably greater than the period of the supply waveform. Associated with these variations in power drawn are fluctuations of the line voltage brought about by the connection and disconnection of the load to the supply lines.In a number of countries there are regulations or standards (for example British Standard No. 5406 and the proposed EEC SENELEC regulation) which seek to reduce the effects of these fluctuations by defining an upper limit on the number of changes in line voltage which a device is permitted to cause, based on curves determined by various psychological, biological and other considerations which relate the maximum permissible number of energisations and de-energisations of a device to the fractional fluctuation in line voltage which they cause.The "nuisance value" of a device is roughly dependent on both the frequency and the magnitude of the fluctuations in line voltage which it causes so that under the draft SENELEC rule a device which causes relatively little change in line voltage is permitted to cause more fluctuations per unit time of the line voltage than a device which produces more substantial fluctuations in line voltage.

According to the present invention there is provided a power control apparatus comprising: a resistive load comprising at least two partial loads; and control circuitry for cyclically energising the partial loads in time interleaving relationship with one another.

In the embodiment described below with reference to the accompanying drawing, the load is the heater of the grill of an electric cooker, this being a split load made up of two heating elements disposed in side-by-side relation. The control circuitry is arranged to control respective triacs in series with the two heating elements so that these are operated in the burst fire mode. A user-operable control is provided to vary the duty cycle of the energisation of each of the heating elements. The heating elements are so designed that the rated output of the heater as a whole is produced when the heating elements are each operated with a 50% duty cycle, with the on periods of one heating element coinciding with the off periods of the other. For lower settings the heating elements are energised for shorter, nonoverlapping equal periods.

In this way the fluctuations in line voltage caused by the grill heater as a whole tend to even out and be less marked.

The duration of the on periods of the two heating elements is determined by generating two ramp signals which are inverted relative to one another and are each compared with a reference signal, representing the user set-point; the circuitry is arranged so that one of the heating elements is energised during each cycle of the ramps until one of the ramps exceeds the reference signal while the other heating element is energised for the remainder of the cycle after the other of the ramps exceeds the reference level. Of course, instead of generating two mutually inverted ramps, a single ramp could be compared with two, mutually inverted reference signals in order to determine the on periods of the heating elements.

The invention further provides power control apparatus comprising a heating element and control circuitry operable in one mode to vary the power supply to the heating element in dependence upon the value of a set point signal and in a second mode for supplying to the heating element a power level substantially greater than that applied to the heating element during operation in the first mode with the set point signal at its maximum value.

The invention will be further described with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram, partially in block form, of one embodiment of the present invention; and Figs. 2A-2D are timing diagrams illustrating the operation of the embodiment of Fig. 1.

The circuitry shown in Fig. 1 is for controlling the power supplied to two heating elements 1 a and 1 b which are arranged in side-by-side relationship in the grill of an electric cooker in dependence upon the setting of a user-operable control, namely a potentiometer 2. To this end, the heating elements 1 a and 1 b are connected in series with respective triacs 3a and 3b across the mains supply and appropriate gating pulses for turning the triacs 3a and 3b on are generated by means of the control circuitry 4 surrounded by dotted lines in Fig. 1.

The control circuitry 4 is energised from the mains supply via a voltage dropping resistor 5, a diode 6 and a voltage regulator 7.

In the control circuitry 4, the voltage picked off at the wiper of potentiometer 2 is applied to one input of a comparator 8, whose other input is connected to the output (A in Fig. 2A) of a free running ramp generator 9 having a period of several seconds. When the ramp voltage exceeds the reference voltage from potentiometer 2, the output of comparator 8 changes state and triggers operation of control logic 10. This control logic 10 incorporates a zero crossing detector and appropriate gating circuitry so that at each zero crossing of the mains supply following the change in state of the output of comparator 8 until the end of that cycle of the ramp, the control logic 1 0 delivers a gating pulse via output amplifier 11 to the gate of triac 3a to energise the heating element 1 a, which is thus energised for the remainder of that half cycle of the mains supply.

The output of the ramp generator 9 is also applied to an amplifier 12 which is arranged to invert the ramp so that the ramp falls with time as indicated by B in Figure 2A. The inverted ramp is applied to one input of comparator 13, the other input of which receives the voltage picked off from the potentiometer 2. Control logic 14, which is connected to control logic 10, is arranged so that until the inverted ramp B falls below the reference level, gating pulses are delivered via output amplifier 1 5 to the triac 3b to energise the heating element 1 b. For the remaining mains half cycles in each cycle of the ramp, the triac 3b is turned off.

Thus during each cycle of the ramp, the heating elements 1 a and 1 b are energised for equal periods at opposite ends of each ramp cycle as indicated in Fig. 2D. As indicated by Fig. 2C, turning the potentiometer 2 to the maximum usersettable value extends the periods of energisation of the heating elements 1 a and 1 b so that one is turned off as the other is turned on.

As in our copending Application No. 7940618, in order to improve the response time of the heating elements when started from cold, the heating elements 1 a and 1 b are wound so that they produce their rated output, each of 2 kW when energised at less than the nominal mains voltage so that when energised continuously for a long period of time at the mains voltage they are overrun and each produce approximately twice their rated output. The circuitry generally designated 1 6 makes the elements 1 a and 1 b be overrun when the circuitry is first energised. A capacitor 1 7 is initially discharged and when the control circuitry is first energised, charges slowly over a period of a minute or two via a resistor 1 8.

The voltage across the capacitor 1 7 is compared by a comparator 1 9 with a reference voltage and until the voltage across capacitor 1 7 exceeds the reference voltage, a transistor 20 is turned on which pulls the inverting inputs of comparators 8 and 13 down to the zero volt rail. This has the effect that until the output of comparator 1 9 changes state, both elements 1 a and 1 b are energised throughout each cycle of the ramp as indicated in Fig. 2B thereby dissipating a mean power of 4 kW and rapidly heating to operating temperature. Once the capacitor 1 7 has charged, the transistor 20 is turned off and proportional control of the energisation of the heating elements, in dependence upon the setting of the potentiometer 2, takes place.The heating elements 1 a and 1 b are not overrun during proportional operation because in those circumstances each is energised with a maximum duty cycle of 50%.

Numerous modifications of the above described embodiment will be apparent to those skilled in the art. For example, the reference voltage used to control the energisation of the heating elements need not be produced by the potentiometer 2 but could be produced in a number of other ways, for example by means of a digital to analog converter connected to the output of a touch-controlled counter as in our British Patent No. 1464093.

Furthermore, the circuitry 1 6 could be arranged so that instead of overriding the proportional operation only when the circuitry is first energised, it is actuated when the user-set level is increased.

Equally, as in our British Patent No. 1464093, an additional circuit could be provided to de-energise the heating elements 1 a and 1 b for a predetermined period following a reduction in the user-set level.

Although the invention has been described with reference to a partially analog implementation, it will be appreciated that the circuitry could be entirely digital, and be implemented in large part by a microprocessor which could also control the various other heating elements of the electric cooker.

The invention is not exclusively confined to split loads, i.e. two partial loads under control of the same user-operable control. The invention could also be applied to a number of independent loads within the same device, such as the hot plates of an electric cooker.

Although the invention has been described with reference to open-loop control of the energisation of the heating elements so that the user setting determines the power supplied to the heating elements, it is also possible for the invention to be applied to closed loop control, for example where the user-operable control sets a desired temperature and a suitable temperature sensor is provided to monitor the output of the heating elements.

Claims (12)

1. A power control apparatus comprising: a resistive load comprising at least two partial loads; and control circuitry for cyclically energising the partial loads in time interleaving relationship with one another.

2. Apparatus according to Claim 1 wherein the control circuitry is arranged to vary the power delivered to the partial loads in dependence upon the setting of a user operable control.

3. Apparatus according to Claim 2 wherein the control circuity is arranged to vary the duty cycles of energisation of the partial loads in dependence upon the setting of the user operable control.

4. Apparatus according to Claim 3 wherein the control circuitry includes a ramp generator and a comparator for comparing the output of the ramp generator with a reference signal for determining the period of energisation of one of the partial loads.

5. Apparatus according to Claim 3 or 4 wherein there are two partial loads and means are provided for producing either two ramp signals which are inverted relative to one another and one reference signal or two reference signals which are inverted relative to one another and one ramp signal and the apparatus includes a pair of comparators for comparing the ramp or ramps with the reference signals or signal for determining the periods of energisation of respective ones of partial loads.

6. Apparatus according to any one of Claims 2 to 5 wherein the control circuitry is arranged selectively to be operable in a mode in which the partial loads are energised at a mean power substantially above that settable by the user operable control.

7. Apparatus according to Claim 6 wherein the arrangement is such that the operation of the circuitry in said mode occurs when the apparatus is initially turned on and/or when the user-set power is increased.

8. Apparatus according to Claim 6 or 7 wherein during operation in said mode, the partial loads are energised simultaneously.

9. An electric cooker comprising a heater including a pair of resistive heating elements, a user operable control, and control circuitry for cyclically energising the heating elements in time interleaving relationship with one another to apply to the partial loads a power level dependent upon the setting of the user operable control.

1 0. Power control apparatus comprising a heating element and control circuitry operable in one mode to vary the power supply to the heating element in dependence upon the value of a set point signal and in a second mode for supplying to the heating element a power level substantially greater than that applied to the heating element during operation in the first mode with the set point signal at its maximum value.

1 Apparatus according to Claim 10, wherein the arrangement is such that operation in said second mode occurs only when the control circuitry is initially energised.

12. An electric cooker incorporating power control apparatus according to any one of Claims 1 to 8 and/or Claim 10 or 11.

1 3. Power control apparatus constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.