US 3925633 A
A circuit for controlling the level of energy transmitted from a high frequency energy source to two or more induction heating loads which permit the level of energy transmitted to the loads to be regulated with minimal switching losses and with low component costs. One important application of the invention is a multi-burner home-type cooking range.
Claims available in
Description (OCR text may contain errors)
United States Patent 11 1 1111 3,925,633
Partridge I Dec. 9, 1975 [5 CIRCUIT FOR CONTROLLING POWER 3,710,062 1/1973 Peters 219/10.49 FLOW FROM A HIGH FREQUENCY 3,770,928 11/1973 Kornrumpf et al. 219/10.77 3,781,503 12/1973 Harnden et al 219/10.49 ENERGY SOURCE To A PLURALITY 0F 3,781,506 12/1973 Ketchum et a1. 219/10.49 HIGH FREQUENCY LOADS 3,786,219 1/1974 Kornrumpf et al. 219/10.49  Inventor; D l F Partridge, 103 Harlem 3,806,688 4/1974 MacKenzie et al. 219/ 10.49 Drive, San Jose, Calif. 95129 B A M Primary Examinerruce Reyno s  filed Sept 1974 Attorney, Agent, or Firm-Gerald L. Moore 21 Appl. No.: 503,781
.  ABSTRACT  US. Cl. 219/10.49; 219/ 10.77; 321/4 A circuit f r on rolling the level of energy transmit-  Int. Cl. H05B 5/04 tcd m a h gh fr q ency nergy so rce t two r  Field of Search 219/10 49, 10,75, 1() 77; more induction heating loads which permit the level of 321/4, 7, 60, 70 energy transmitted to the loads to be regulated with minimal switching losses and with low component  keferences Cited 7 costs One important application of the invention is a UNITED STATES PATENTS multl-burner home-type cooking range. 3,697,716 10/1972 Kornrumpf 219/10.49 I 10 Claims, 16 Drawing Figures 4 51 w I 45 X 111011 1115015101 1 l 2 ENERGY SOURCE 01 5 I I 01111111; I f I 00111101 I l I 1 1 h? I 1 I ---J Y 10111 1115110151101 F 9 l P 1L,
T I I on 011 00111101 I 53 I i 0011 11 I 01111110 I I J I co11111o1 I ,1 CT 1 U.S. Patent Dec. 9, 1975 Sheet 1 of4 3,925,633
L aJ O PRIOR ART TIG. IA
. l I I T: 0A Q7 'IOB IZB I58 |3D T T PRIOR ART PRIcE ART TIC. IB V PRIOR ART Tl FIG. 28 FIG. 5A
US. Patent Dec. 9, 1975 Sheet 3 of 4. 3,925,633
souRcE CURRENT r "1W I Tic. A
- HALF POWER TO con rrrr I I 0 I I I l 0 ZERO POWER TO COIL T FIG. 5c
6| w INDUCTION COIL A LA 7E COMPARATOR 722 74 v 77 W RAMP 52 HIGH FRE c E 69 SIGNAL ENERGY R GENERATOR 76 753 v COMPARATOR I; w w W? muucnoR COIL B c. 1 8 TI 5 CIRCUIT FOR CONTROLLING POWER FLOW FROM A HIGH FREQUENCY ENERGY" SOURCE TO A PLURALITY OF HIGH FREQUENCY LOADS BACKGROUND or THE INVENTION It has long been known that inductive'heating is a very efficient process for utilizing electrical energy for such heating purposes as the cooking of food. Naturally with todays energy shortages, the efficient use of electrical energy becomes even more important than'in the past. Induction heating is recognized as a very efficient way to change electrical energy to thermal energy because the energy is absorbed directly by the container instead of being transferred by conduction, convection and radiation as is, for instance, the case with the normal home cooking range using tubular electrical heaters. For much the same reasons induction heating is also more efficient thangas heating.
However, cooking by induction heating has been slow in being adapted'for household use.-One of the main reasons for the slow progress of induction cooking involves the cost of the controls necessary for regulating the power levels to the cooking units. The power naturally must be supplied at a high frequency, usually in the range of 20,000 Hz. This fact alone dictates the conventional controls cannot be used for regulation of cooking temperatures Silicon controlled rectifiers have offered a possible answer to the control question, but up to now SCRsand the necessary associated circuitry have been found to be too expensive for home cooking ranges. i i
Part of the-expense in applying SCRs to home cookin'g'controls stems from the fact that high quality components have been necessary for efficient and safe operation at the higher-frequency ranges. Naturally safety is of paramount importance. Also switching power losses can be as much as several hundred times the high frequency source. One such multi-load application of this invention is a multi-burner home-type cooking range. I
Still another prime objective of this invention is to provide a new type of high'frequency energy source running at the 6 to IOKW level with a new, concept of dividing and controlling the powerto several different loads (four loads with a standard cooking'range) in a way that is more efficient and less expensive that past controls attempting the same objective.
SUMMARY OF THE INVENTION An inductive heating apparatus comprising a high frequency energy source and two I or inore loads adapted to absorb and be selectively heatedby the high frequency energy source. In the broader sense, the invention includes a first switching means provided for converting unidirectional potential to a high frequency energy source. This' switching means is cycled onv and off at a frequency rate substantially lower than the high second switching means to regulate the energy transmitted from the sourceto the load.
DESCRIPTION OF THE DRAWING FIGS. 1A, 1B and 1C are circuit diagrams of the prior art; p
FIG. 2' shows selected waveforms of FIG. 1;
FIG. 3A is a circuit diagram of a second embodiment of the prior art;
FIGS. 38 and 3C are selected waveforms of the circuit of FIG. 3A;
FIG. 4 shows a first embodiment of the invention;
FIG. 5 shows selected waveforms of FIG. 4;
FIG; 6 is an improvement of the embodiment shown in FIG. 4;
'FIG. 7 shows a waveform of FIG. 6;
FIG. 8 shows another embodiment incorporating the circuits of FIG. 1 and FIG. 4; and
FIG. 9 shows a modification of the embodiment of FIG. 8, which modification is especially applicable to home cooking ranges.
. DESCRIPTION OF PRIOR ART One embodiment of the prior art is shown in FIG. 1A, in which high frequency energy, at a frequency in the order of 20,000 Hertz or higher, is used to energize an induction 'coillS (FIG. 1A) for the magnetic transfer of energy to a vessel 11 for the purpose of heating that vessel by an induction process. By setting up a rapidly changing magnetic field in a manner to encompass vessel 1 1, eddy currents are induced which cause a heating of the vessel walls. In this'manner, any material held by the vesselcan be'rapidly and efficiently heated. The load does not have to be a cooking pot, but such is so shown as one application 'of the present invention. For instance, in some applications the frequency could be well below or above 20 K Hz and the load could be ma- In the embodiment'shown in FIG. 1A, SCRs 13 and 14 convert the direct current energy source 10 into a high freque'ncy'source in which the tum-off time (i.e., the time the SCRs 13 and 14 are reverse biased) is independent of the resonant frequency of the load. The circuit has an effective power factor of 1 since there is no reactive energy feedback to the DC source 10. The gating controls for the SCRs are not shown because such controls are Well known and widely used in the industry.
In the circuit of FIG. 1A, the DC source 10 is connected in series with an interconnecting circuit means including the two SCRs 13 and 14. In parallel with the SCR 14'is the series load circuit represented by the inductance 15 and the capacitor 12. This circuit is capable of generating high frequency energy for transmission to the container 11. If the load is to be used for cooking one suitable frequency'is 20 KHz, which freterial to be heated by being located near the area of the I quency is above the normal hearing level of humans.
One cycle of operation of the circuit of FIG. 1A is as follows. When the SCR 13 is caused to conduct to initiate an on and off cycle, sinusoidal current flows through the following path: ground, DC source 10, SCR 13, inductance (load), capacitor 12 andback to ground. The load current-during this period appears as the current in FIG. 2A from timer, to 1 The load current will reduce to near zero when the capacitor 12 becomes charged a maximum amount. The voltage on capacitor 12 will be of a polarity as shown in FIG. 1A and larger in value than DC source 10. Since the voltage on capacitor 12 is larger than the DC source 10, the SCR 13 will become reverse biased and cease conduction. It should be noted that the reverse voltage can remain on SCR 13 for atime that is independent of the natural resonance of the load (inductance 15 and the capacitor 12). Also during the turn-off time of SCR 13, no reverse current flows into the DC source 10. When SCR 13 has been reverse biased for a period long enough to sustain a forward voltage, SCR 14 can be caused to conduct. SCRs are used by way of example but other typesof switching devices such as thyrotrons and mercury arc tubes could be used.
When SCR 14 is caused to conduct, current will again flow in a sinusoidal manner through the load until the polarity of the voltage on capacitor 12 reverses. Whenthe current stops flowing, SCR 14 will be reverse biased by the voltage on capacitor 12. Again the reverse voltage time on SCR 14 is independent of the natural frequency of the load. When SCR 14 has regained its forward blocking ability, the SCR 13 can be turned on and the process repeated.
In FIGS. 1B and 1C are shown other embodiments of the prior art having most components similar to those of FIG. 1A. In FIG. 1B the capacitors 16 and 17 are much larger than the capacitorlZB. The voltage and current waveforms of the SCRs in FIG. 1B and FIG. 1C are the same as for the embodiment of FIG. 1A. The inductance 15A and 15B in FIGS. 3 and 1C are the same as in FIG. 1A. The capacitance 12B of FIG. 1B is the same as the capacitance 12 of I FIG. 1A. The total capacitance in FIG. 1C (12C1 and 12C2) is equal to capacitance 12 ofFIG. 1A. Again the currents and voltages of the SCRs of FIGS 1A, 1B and 1C are all the same. The operation of FIGS. 1B and 1C are similar to FIG. 1A and are well-known in the industry.
Thus operating at 20 KHz (with 10 US turn-off SCRs) the load current and SCR voltage waveforms (for FIGS. 1A, 1B and 1C) are shown in FIG. 2A and FIG. 2B, respectively. Time t to t occurs when SCR 13 is conducting. Time t to 1 occurs when SCR 13 is reverse biased. The time period 2 to t exists while SCR 14 is conducting. Time to r iswhen SCR 14 is reverse biased. Thus the-circuits of FIGS. 1A, 1B and 1C are especially suitable forlow voltage and low Q-type induction heating loads. r
A preferred method. of controlling the power delivered to the load is as follows: the high frequency inverters are turned off and on at a frequency rate substantially lower than the frequency being generated. The preferred low frequency rate is in the order of 0.1 to 10 Hertz. If the low frequency rate is ,1 Hz and 50 percent power is delivered to the load (the cooking pot in preferred application), the high frequency power would be on for one-half second and off for one-half second. If percent power was required then the high frequency inverter would be on for one-fourth second and off for three-fourths second at a low frequency rate of 1 Hz.
FIG. 5A shows a waveform of approximately 80% power being delivered to the load. The inverter is on 4 during time period 54 and off during time period 55. FIG. -5 will be explained in more detail in conjunction with apreferred embodiment. It should be noted that the circuits of FIGS. 1A, 1B and 1C can be turned on at fullfrequency. They do not have to be turned'on at alow frequency and then increased in frequency. In this way. only a low tick sound is heard when the inverter turns on. With normal room noises the tick is not heard. v V
Shown in FIG. 3 is a second embodiment of prior art. The importance of this embodiment involves the obtaining of alower voltage rise time on the switching SCRs. Naturally the faster the rise time the more likely the SCRs will become conductive at an undesirable time and therefore will result'in a crowbarf effect, that is, both the SCRs will be in a conductive, mode vsimultaneously making the circuit useless for inductive heating purposes. The primary difference between the ment of FIG. 1. The SCR 33 is fired periodically at 21 frequency necessary to create high frequency currents in'the load 35. Between the firings of the SCR 33, the SCR 34 is fired to reverse the voltage on the capacitor 36 by current flow through the load inductor.
Shown in FIG. 3A in solid lines is a similar waveform as that shown in FIG. 2B. In dotted lines are shown the waveforms resulting because of the effect of the inductors 31 and 32. The much lower dv/dt (voltage rise time) at point A should be noted and, more importantly, is obtained without the use of dv/dt filters which otherwise can cause further power losses in the system,
or slow the operating times of thecont'rols. Under some It should benoted that SCR 34 can be firedj'much sooner after SCR is reverse biased in this embodiment than in the embodiment of FIG. 1A with the same turnoff time requirements. This is possible because the action of the inductors 31 and 32 keeps the SCR 33 reverse biased even when the SCR 34 is conducting. This is accomplished while maintaining the reverse time on the SCR 33 the same (i.e. as in FIG. 1A). Thus the peak current and the di/dt (current rise time) in the SCRs are less for the same power delivered. v I
' This fact is illustrated graphically in FIG. 3C representing an example of the actual current waveforms for the circuit of FIG. 3A. For the same power delivered,
the two currents are shown with and without inductors 31 and 32. Conversely, the load could be returned with the addition of the inductors 31 and 32 to give a similar current waveform as in FIG. 1A. This would then maintain the SCR currents the same, the power delivered the same and as a further advantage, would increase the turn-off time on the SCRs 33 and 34. This is achieved by firing the SCR 34 at time t (FIG. 3B) for delivering the same power and current waveform to the load but with the SCRs having a much longer turnoff time and lower dv/dt than in the previous embodiments of FIGS. 1A, 1B and 1C. The waveform for this embodiment is shown in FIG. 313 as with the dotted line and arrow waveform. The turnoff time is thus increased to the time period t,
Of course the combination of the two approaches can be used, i.e., the combination of the somewhat longer t and somewhat lower di/dt and peak current in thetheir forward blocking ability when they are reverse biased for one-half the period of the high frequency source. This results from the fact that when the SCR 49 is conducting, the SCR 50 is reverse biased momentarily but not for a period of time long enough to regain its forward blocking ability. In this manner when the current from high frequency source 45 reverses direction again, the SCR 50 will conduct with the resulting very low switching losses. This effect can be enhanced by keeping a continuous drive to the gates of the SCRs 49 and 50. The longer the tum-off time of the SCRs 46, 47, 49 and 50, the more easily the continuous conducting effect can be obtained. One theory for this phenomenon is that sufficient carriers remain in the SCRs during the period of current reversal for maintaining conduction capability during the current reversal time, such that on the subsequent forward current cycle the SCR conducts with low switching losses even though DC source. This is important for four reasons. First, the
initial di/dt of the current (i.e., the rate of rise of the current) is lower with the inductors. Secondly, the peak current is less, thirdly the negative di/dt near the end of the pulse is lower (which makes it easier to turn off the SCR) and fourthly the RMS value of the current is lower. These four points in conjunction with the lower dv/dt during turn-off time make this simple circuit very usable at high frequencies. It also should be noted that the current waveforms in either case (i.e., with or without inductors 31 and 32) are sinusoidal in nature wherein most induction heating systems have a step rise in current followed by a sinusoidal relationship. The step rise in current causes a much higher switching loss. The SCRs 33 and 34 can also be built with multiple gates as is now known in the industry. By sequentially firing the individual gates, this type SCR has much higher capabilities for operating in high di/dt and/or high frequency environments than previous SCRs with the same size silicon chip.
DESCRIPTION OF THE INVENTION In FIG. 4 is shown a first embodiment of the present invention which embodiment is an improvement over the prior art just described. In this embodiment there are provided the individual load switching means or controls 43 and 44 in cooperation with the on-off switching means or control 45 for allowing independent regulation of the energy levels transmitted through interconnecting circuit means to the induction heating coils A and B, respectively. The control 43 uti lizes a forward conducting SCR 46 and a reverse parallel connected SCR 47 having a gating control 48 for regulating the firing of the SCRs 46 and 47. Similarly, the control 44 includes the SCRs 49 and 50 and the gating control 53. Thus the SCRs 46, 47, 49 and 50 are standard and relatively low cost type SCRs capable of efficient operation in 'low frequency ranges. The high frequency energy source 51 supplies AC current adapted to maintain each SCR in a conducting mode once a gating signal is supplied to each respective gating terminal T.
Stated otherwise, the SCRs 46, 47, 49 and 50 are not required to turn off at the high frequency source rate (i.e., in the order of 20 Khz or higher) but at a much lower rate (i.e., in the order of l to 10 Hertz), and more importanty are not subjected to high switching losses because once they are turned on, they do not recover the SCR 49 was momentarily reverse biased.
The control 52 serves as a low frequency on-off control for the high frequency energy source 51. Such controls are well known and a specific description of the control is not necessary. The control 52 cycles the source 51 on and off (preferably at approximately a l Hertz rate) with a preferred conduction cycle of percent on and I0 percent off. For instance, FIG. 5A indicates current conduction from the source 51. Note that'the source 51 is in the on-mode during the period 54 and is turned off during the period 55. The control 52 can be of a type which will turn off a high frequency oscillator (not illustrated) in the source 50. Thus there is supplied to the controls 43 and 44 a high frequency voltage waveform 56 having a profile indicated by the waveform 57 of FIG. 5A. For purpose of the present invention it is necessary that the source 51 be turned off for a period 55 having a time duration sufficient to turn off the SCRs 46, 47, 49 and 50 automatically by a process commonly referred to as starvation and in that sense, the controls for the individual loads are automatically turned off each time the high frequency source turns off. That is, the voltages on the SCRs are turned off for a sufficient period of time to reset each SCR automatically to the nonconducting mode. It should be noted that it is not essential that the SCRs be suppplied for conduction on both half cycles of the energy signals 56, therefore the SCRs 47 and 50 could be deleted with the result being that a less than full 360 degree current source is supplied to the induction coil. As will be described later in detail, the SCRs 47 and 50 can also be replaced with a diode. Further, the SCRs 46 and 47 and the SCRs 49 and 50 can be replaced with a TRIAC with substantially equal results.
Thus for the time period 55 (FIG. 5A) no current is supplied to any of the induction coils because the SCRs are turned off by the fact that the source is turned off as previously described. For the regulation of the energy level to the coil A, the power control 43 can be regulated in a manner to serve as a means to turn on at any time interval following the turning on of the source 51 by the control 52 thus for the time interval after the high frequency source is turned back on but before the control 43 turns on, energy flow from the source to the load is impeded. Thus, as indicated in FIG. 5B, the source 51 supplies the input signal for the time duration 54, however, because the gating control 43 is regulated not to turn on during the period 58, and only to initiate to the coil A approximately one-half of the power av ail- It can be seen from the foregoing that the SCRs 46,
I 47, 49 and 50 can be used as the control elements for thehigh frequency signal without theattendant power losses normally associated with the application of such SCRs in the range of Kilohertz. In addition economically priced SCRs can be used as the variable control component because operation in the l Hertz frequency range only is required. For a better power factor and therefore more efficient operation, capacitors X and Y can be added in series with coil A and coil B where the natural resonant frequency of the coils and the capacitor is near or at the frequency of the high frequency source 51. The basic control technique is the same as for the previous art embodiments with the added advantage of independently controlling two loads from r the same source.
In the foregoing embodiment (i.e., FIG. 4), there remains the losses of operating the source 51 during the periods when no energy is supplied to either of the coils A or B. Such operation has also been encountered in previous attempts to utilize high frequency energy sources for such applications as induction heating. Itis thepurpose of the following additional embodiment to further limit the losses in such circuits and provide an even higher efficiency of operation in the control device just described. For this purpose, the circuit of FIG. 6 is provided having a high frequency energy source 60 for supplying energy to the induction coil A and induction coil B for the heating of the containers 61 and 62. As described before, there is supplied a pair of SCRs 64 and 65 for regulating the-energy supplied to the induction coil A and SCRs 66 and 67 for regulating the energy supplied to the induction coil B. However, in this circuit the high frequency energy source. 60, while preferably being cycled on and offat a duty cycle of 90 percent on and 10 percent off, is only turned on when it is desired to turn on the induction coil demanding the highest energy settingv and therefore being turned on first. For this purpose there is provided a pair of control resistors .68 and 69 being supplied current from a L source 70 passing through each resistor in parallel and through a diode 71 to ground. The output'signal from sulting in noenergy transfer to the associated container the tap 72 is supplied to the negative terminal of a comparator 74 while that of tap 75 is supplied to a negative terminal of a comparator 76. At the positive terminals of the comparators 74 and 76 is supplied a ramp signal generator 77 with such ramp signal appearingas that shown in FIG. 7 and indicated by the. waveform 78. It can thus be seen that depending upon the setting of the tap 72, the comparator 74 will supply an output signal for firing the SCRs 64 and 65 sometime during the period 81 (FIG. 7). However, because the Zener diode 71. Y in functioning to always provide a voltage drop thereacross biases the output voltage from the rheostat 68 by the voltage level V, actually the comparator 74 cannot supply a signal during the period 80 and can only s upply a signal during the period 79. Thus, by controlling the rheostat 68 the firing angle of the SCRs 64 and 65 can be regulated. In this way by controlling the voltage of the Zener diode 71 the minimum 10 percent off and 90 percent on time isset by the high frequency source 60.
Similarly, by regulation of the rheostat 69 through adjustment of the tap 75, the comparator 76 can be made to fire any time within the time period 79 to set the SCRs 66 and 67 in the conducting mode. The output signals from the comparators 74 and 76 are fed to an Or gate 82 connected to the high frequency energy source 80. Any time the Or gate 82 supplies an output or up signal, the high frequency source is turned on.
Thus it can be seen that until one of the comparators 74 or 76 is turned on, the energy source will remain off. Also, as in the previous embodiment the energy source is always turned off a percentage of the time as indicated by the time duration in FIG. 7. During this time duration the SCRs 64, 65, 66 and 67 if turned on previously, will be turnedoff or set in the non-conducting mode by the process known as starvationexplained previously. Thus, the'energy level can be regulated to the coils A and B by manipulation of the rheostats 68 and 69 to set the level of energy supplied to the respective induction coil. However, the source will not be turned on during each duty cycle until the power setting of the induction coil set to the highest power setting is reached. Thus, the high frequency source only supplies output power when needed and all attendant losses normally occurring during operation of the source 60 even though no energy is being supplied to the induction coil such as those losses resulting from internal resistance or other attendant losses with the control components within the source are eliminated. It
is of course, obvious that more than two loads could be supplied by high frequency source 60. Again, capacitors can be added in series with the loads for a better power factor. The logic circuits of FIG. 6 are shown by way of explaining the operation of the logic require-. ments and are not given necessarily as optimum circuits (i.e., in general, comparatorssuch as comparator 74 would not be used to turn on SCRs 64 and 65 but insupplied by the high frequency gating control 88. The high-frequency source 84 thus is similar to that shown in FIG. 1 and works in conjunction with the loads A andB. i
In this instance, the load A energy level is controlled by an SCR 91 which receives a gating signal from the gating control A. A diode 92 is provided for reverse conduction. Similarly, an SCR 94 controlled by a gating control B and regulates the energy level to the load B. A diode 93 is provided for reverse conduction around the SCR 94.
In this embodiment the gating control 88 is regulated to turn on the SCRs86 and 87m a high frequency rate and thereafter is timed to turn off the high frequency source for a period of time sufficient to shutoff the SCRs 91 and 94 bythe process of starvation. As previously, explained, the duty cycle of the source 84 is preferably 90 percent and 10 percent off at a 1 Hertz rate. Thus, by regulating thegating control A, the energy level to load A is controlled by permitting the SCR 91 to turn on at any time during the l Hertz cycling of the source 84. Similarly, the SCR 94 is controlled to regu- 9 1 late the energy level supplied to the load B Thus, it can be seen that the embodiment of FIG. 8 operates in the same manner as that previously described to supply high frequency energy at regulated energy levels by use of components associated with each load which need be capable of operating only at a .1 I-Iertz rate.
Using diode 92 instead of an SCR in the same relative position reduces the number of SCRs of the'circuit but increases the voltage rating of SCR 91 under some circuit conditions. One SCR and one diode has theadded advantage of being able to reverse the relative positions of the diode and the SCRs and of having common cathods of the two or more regulating SCRs. With either two SCRs back to back, or one SCR and one diode, the high frequency source can be turned on at a low frequency rate with a duty cycle equal to the longer of the duty cycles of gating control A or control B. Also the SCR and diode pairs could be replaced with a TRIAC.
It should be noted that more than two loads can be controlled from the one main high frequency source. Sinceit is less expensive to have one highfrequency source of 10. Kw instead of four at 2.5 Kw the circuit shown in FIG. 9 approachedthe cost of one high power system with four separately controlled loads by the addition of the inexpensive back to back SCRs, SCR and diode or one TRIAC.
The difference of the cost of the low frequency control components compared to two SCRs running at the high frequency being required to turn off at a kHz rate is'in the order of 10 to l or more depending on the circuit configuration used. Further the switching loss in the configuration of the present patent application is much less than if each load has its own set of high frequency SCRs.
If control A turns on while control B is still off, the operation of circuit of FIG. 8 is the same as that of FIG. 1. If control A is on, the operation is the same with the overall load current being at a higher level. There will be a small difference in the current pulse width if load B has a natural resonance somewhat different than load A. The circuit shown in FIG. 8 has an advantage over the circuits of FIGS. 4 and 6 in that the load in FIG. 8 determines the resonance and supplies the commutation means of the high frequency source whereas in FIGS. 4 and 6 a much more complex high frequency source is required to generate high frequency voltages independent of the load.
It should be noted that the circuit of FIG. 8 can also be modified in the same manner that the circuit of FIG. 1 was modified to obtain that of FIG. 3A, i.e., with the additions of inductors 31 and 32. In circuits such as FIG. 8 (i.e., two or more load), with inductors added as in FIG. 3A, and if they are not much smaller than the inductance of the individual loads, then means may be necessary to control the power to the loads to account for the different power delivered to the loads when two or more loads are on at the same time. If one load only is running 50% of the time on and delivering X amount of power and thereafter another load is turned on, during the time that both loads are running the total amount of power delivered to the first load can change from the value X to an extent that correction should be made due to higher voltages and/or higher current and- /or wider current pulses.
It should also be noted that when two or more loads are used, the control means can be changed to deliver power to one load at a time, (i.e., full load to one load in a two-load system would be power delivered to the 10 load a little less than one-half of the time. In a threeload system, full load would be power delivered for a little less than one-third of the time).
FIG. 9shows a variation of FIG. 8 in that inductors 97 and 98 are added and function similar to the inductors 31 and 32 in FIG. 3A, with the load being a fourload configuration using TRlACs instead of back-toback SCRs or SCRs in antiparallel relationship with diodes. The low cost TRlACs have been found to have a very low forward drop (with respect to the current frequency and magnitude). In some applications, the TRIAC has the lowest losses and costs of any of the low frequency switching means described. The circuit of FIG. 9 is especially applicable to home cooking ranges. Thus the operation of FIG. 9 is the same as FIG. 8 but with four loads instead of two. TRIACs are used instead of back-to-back SCRs and diodes. The addition of inductors 97 and 98 are for the attendant reasons .as described for FIG. 3A. The basic power control method can be as described for the circuit'at FIG. 6. Two of the loads can be connected between points A and B instead of points B and C to reduce the ripple current in DC source 96. It should be noted that whether the four loads are as shown in FIG. 9 or with two loads between points A and B they are still dynamically in parallel through the DC sources 96 when the TRIACs are conducting. Further, other configurations as shown in FIGS. 1B and. 1C could be used while still keeping all the loadsdynamically in parallel. In the multiload configurations the series tuned induction heating loads are tuned to resonate at approximately the same frequency when loaded with a cooking pot or other load. Another advantage of the multi-load approach is that one load can be made at a much higher power level than the other three (or one load very high and one load very low) at very little (if any) increase in cost. If this was done with a standard system (i.e., four controllers for four loads) the increase in cost would be substantial. The described preferred embodiments of the multiload configurations are low in cost and more efficient than known induction heating systems. They are much more efficient than standard gas or electric stoves. The invention has been shown in the multi-load configuration for home cooking ranges. It is understood that the concept could be used for other multi-load applicatrons.
While the invention has been particularly shown and described with reference to several preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
1. An inductive heating apparatus for supplying energy to one or more induction heating coil loads comprising:
a high frequency energy source suitable for heating said load by an inductive heating effect;
on-off switching means to cycle said energy source on and off at a frequency rate substantially lower than the high frequency rate of the energy source; and interconnecting circuit means connecting said source and said load for supplying energy to the load, said interconnecting circuit means including individual switching means for impeding energy flow from the source to the load, said individual switching means being capable of impeding energy flow after each v necting circuit individual switching means initiates energy flow to the load. 4. An inductive heating apparatus as defined in claim 3 wherein said switching means of the interconnecting circuit means automatically impedes energy flow from the source to the load each time said on-off switching means turns the energy source on.
5. Aninductive heatng apparatus as defined in claim 4 wherein said on-off switching means does not turn on the source until the time the individual switching means of the interconnecting circuit means first set to turn on after the energy source is cycled off actually turns on. 6-.2An inductive heating apparatus as defined in claim 5 including power factor correcting means to correct for a low power factor of the load.
7. An inductive heating apparatus as defined in claim 6 wherein said high frequency energy source includes a DC source, two SCRs and two inductors all in series connection.
8. An inductive heating apparatus as defined in claim 7 wherein a plurality of loads are supplied fromsaid 12 high frequency source and the loads are four cooking positions on ahome-type' cooking range.
9."A control for regulating energy flow from a high frequency energy source comprising:
a plurality of loads suitable to be energized by the high frequency source; 7 I
- interconnectingcircuit means connecting each load to the source for energization of each load-sepa- Y rately;
on-off switching means operable to cycle on and off and periodically interrupt the flow of energy from the source to the loads at a frequency substantially less than the frequency of the source;
individual switching means in the interconnecting.
a switching means control for setting the individual switching means so it ceases to impede the flow of energy from the source to the associated load during the time interval between the on-off switching means turning on but before the on-off switching means again turns off thereby to enable regulation of the flow of energy from the source to each individual load.
'10. A control for regulating energy flow from a high frequency energy source as defined in claim 9 wherein the frequency of the energy'source is in the, general range of 20,000 Hertz and the operating frequency of the on-off switching means is in the range of 0.1 tel 10 Hertz.