Inductive cooking range and cooktop

• US 5,648,008 A
• Filed: 11/23/1994
• Issued: 07/15/1997
• Est. Priority Date: 11/23/1994
• Status: Expired due to Term

• Current Assignee: Maytag Corporation (Whirlpool Corporation)
• Original Assignee: Maytag Corporation (Whirlpool Corporation)
• Inventors: Lee, Jong Hak, Barritt, William D.
• Primary Examiner(s): Leung, Philip H.
• Application Number:
  US08/344,505
• Time in Patent Office: 965 Days
• Field of Search: 219/620, 219/625, 219/626, 219/627, 219/661, 219/663, 219/664, 219/665, 219/667, 363/49, 363/21
• US Class Current: 219/626
• CPC Class Codes: H05B 6/062 for cooking plates or the like

Abstract

An induction cooking apparatus and method for inductively heating cookware is provided. An analog/digital circuit, including a microprocessor, generates a plurality of gating pulses for operating the power inverter circuit and monitors the operation of the power inverter circuit. The microprocessor provides signals to start and stop the inverter circuit, and determines a period of uninterrupted power inverter circuit operation corresponding to a desired cooking temperature. Analog circuitry is provided for sustaining operation of the power inverter circuit after the power inverter circuit is started, and for compensating for variations in cookware materials.

Claims

1. An induction cooking apparatus, comprising:

an electrical power source for providing, from an AC powerline, a DC power signal including a plurality of powerline half cycles;

a power inverter circuit, including an L/C network with a work coil coupled to said DC power signal for inducing heating current in metallic cookware and a switching means for intermittently connecting said L/C network with said DC power signal; and

an analog/digital control circuit coupled to said power inverter circuit, said analog/digital circuit including a user input for selecting a cooking temperature to be generated in said cookware, digital means generating a start signal for initiating operation of said power inverter circuit and for generating a stop signal for stopping operation of said power inverter circuit to define a period of operation of said power inverter circuit for generation of said cooking temperature, said digital means being coupled to said user input, and an analog circuit including gate means for generating gating pulses for operation of said power inverter circuit and feedback means coupled with said work coil for generating trigger signals to sustain generation of said gating pulses by said gate means after said power inverter circuit is started by said digital means.

2. The apparatus of claim 1, wherein said switching means comprises an insulated gate bipolar transistor coupled to said work coil to intermittently establish conduction of electrical current of a first polarity through said work coil when said insulated gate bipolar transistor is gated by said gating pulses.

3. The apparatus of claim 1, wherein said digital means generates said start signal to start gating pulse generation by said gate means at a low voltage of said dc power signal.

4. The apparatus of claim 3, wherein said digital means generates said stop signal for stopping said analog circuit from generating said trigger signals.

5. The apparatus of claim 1, wherein a timing of the generation of said trigger signals is based on a voltage phase shift across said work coil.

6. The apparatus of claim 1, wherein said digital means generates an output for adjusting duration of gating pulses during a start-up period for said power inverter circuit.

7. The apparatus of claim 1, wherein said digital means, in response to said user input for selecting a cooking temperature, controls generation of gating pulses for a selected number of powerline half cycles corresponding to a desired cooking temperature.

8. The apparatus of claim 7, wherein said digital means comprises:

counting means for counting powerline half cycles which occur after said power inverter circuit is started, said digital means generating a stop signal to interrupt said generation of said gating pulses when a counted number of powerline half cycles equals or exceeds said selected number of powerline half cycles, and

storing means for storing a number of powerline half cycles associated with a maximum cooking temperature generated by said power inverter circuit, said digital means generating a start signal to reinitiate operation of said power inverter circuit after occurrence of said maximum number of powerline half cycles during cooking.

9. The apparatus of claim 1, wherein said digital means comprises a microprocessor.

10. The apparatus of claim 1, wherein said analog/digital circuit further comprises means for detecting the presence of acceptable cookware near said work coil during a first period of a powerline half cycle of said DC power signal.

11. The apparatus of claim 10, wherein said analog circuit generates a pan-no pan signal related to the current through the work coil, said digital means generates a window signal for said pan-no pan signal defining a short portion of each powerline half cycle following its zero voltage crossing as a pan checking period, said analog circuit generating, during said window signal, an input for use by said digital means to generate a stop signal for said power inverter circuit if no acceptable cookware is present.

12. The apparatus of claim 10, wherein said means for detecting the presence of acceptable cookware comprises:

a commutating capacitor coupled between said electrical power source and said work coil, said commutating capacitor generating a voltage related to a recovery time for said commutating capacitor when said switching means is OFF;

means for sampling said voltage corresponding to said recovery time; and

a cookware detector circuit including a storage capacitor for accumulating a voltage corresponding to said voltage of said commutating capacitor during a plurality of sampling periods.

13. The apparatus of claim 12, wherein said cookware detector circuit supplies a cookware detector signal to said digital means related to said accumulated voltage, and said digital means compares said cookware detector signal with a predetermined reference value for determining whether acceptable cookware is present near said work coil.

14. The apparatus of claim 1, wherein said feedback means further comprises means for providing varying trigger signal durations to said gate means for generating varying duration gating pulses for operation of said power inverter circuit.

15. The apparatus of claim 14, wherein said means for providing varying trigger signal durations for said gating pulses comprises a phase detector circuit, coupled between said work coil and said gate means, for detecting a voltage phase difference across said work coil and generating trigger signals with durations determined from the voltage phase difference.

16. The apparatus of claim 1, wherein said analog means further comprises means for adjusting a cooking power output of said power inverter circuit to compensate for variation in cookware materials.

17. The apparatus of claim 16, wherein said means for adjusting a cooking power output comprises:

a current transformer having a primary coil and a secondary coil, said primary coil being coupled between said electrical power source and said work coil for sensing the amount of current flowing from said source of electrical power when said switching device of said power inverter circuit is ON; and

a level shift circuit coupled to said secondary coil of said current transformer for rectifying and filtering a voltage received from said secondary coil and producing a DC output used by said gate means to increase the ON time of said switching device when a current sensed by said current transformer decreases, and to decrease said ON time of said switching device when said current sensed by said current transformer increases.

18. The apparatus of claim 1, further comprising over- heat protection means for detecting an over-temperature near said work coil.

19. The apparatus of claim 18, wherein said over-heat protection means comprises a transistor for establishing an analog ground to stop said gate means from generating gating pulses upon detection of an over-temperature.

20. The apparatus of claim 1, further comprising a zero crossing circuit for generating a zero crossing pulse at each occurrence of zero voltage during each powerline half cycle, said zero crossing pulses being coupled to said digital means as an input.

21. An induction cooking apparatus having an inverter circuit comprising:

a DC power supply for supplying a plurality of DC power half cycles from an AC powerline;

an inverter circuit having an induction heating coil connected at one end to said DC power supply;

an insulated gate bipolar transistor connected to the other end of said induction heating coil for conducting current of a first polarity through the heating coil;

a diode connected in parallel with said transistor for conducting current of an opposite polarity through said induction heating coil;

a zero crossing detector for generating zero crossing reference signals of the AC powerline;

a microprocessor for supplying an initial inverter start signal to said inverter circuit, said microprocessor receiving said zero reference signals from said zero crossing detector and starting said inverter circuit at a low voltage generated by said DC power supply, selecting a number of powerline half cycles of inverter operation corresponding to a user-desired cooking temperature, and generating signals to interrupt operation of the inverter circuit to obtain the user-desired cooking temperature and to prevent unacceptable operation; and

an analog circuit referenced to a voltage of said DC power supply for sensing the presence of cookware made of proper material, and for generating timing signals for sustaining compensated acceptable operation of said inverter circuit after said inverter circuit is started by said microprocessor.

22. The apparatus of claim 21, wherein said analog circuit comprises a phase detector including a phase comparator circuit with one input resistively connected to the collector of the said insulated gate bipolar transistor and the other input resistively connected to the connection between said DC power supply and said induction heating coil, and having an output related to the phase difference produced by the inductance of the induction heating coil during inverter operation for generating said timing signals.

23. The apparatus of claim 22, wherein said analog circuit further comprises a gate generator having a comparator with a first input coupled to an output of said microprocessor and to said output of said phase detector for starting and sustaining operation of said inverter circuit by switching said insulated gate bipolar transistor.

24. The apparatus of claim 23, wherein said analog circuit further comprises:

a current transformer having a primary winding connected in series with said DC power supply, and having a secondary winding;

a rectifier/filter, coupled to said secondary to produce a DC voltage signal related to the amount of current flowing from said DC power supply through said induction heating coil; and

a bias control device coupled to said rectifier/filter and to a second input of said gate generator comparator for receiving said DC voltage signal to generate an offset bias voltage at said second input of said gate generator comparator corresponding to an amount of current flowing from said DC power supply through said induction heating coil, said offset bias voltage adjusting operation of said gate generator comparator and conduction time of said insulated gate bipolar transistor to adjust power supplied to said cookware based on a material from which said cookware is made.

25. The apparatus of claim 22, wherein said analog circuit further comprises a pan detection circuit coupled to the output of said DC power supply, said phase detector and said microprocessor, and wherein a commutating capacitor is coupled to the output of said DC power supply, said commutating capacitor generating a voltage related to said recovery time for said DC power supply, and wherein said microprocessor and said phase detector supply signals to said pan detector circuit for selectively sampling said signal related to the recovery time, said pan detector circuit including a storage capacitor for accumulating a voltage corresponding to the voltage of said commutating capacitor during a plurality of sampling periods, said pan detector circuit supplying said accumulated voltage as a pan detector output to said microprocessor, and said microprocessor comparing said pan detector output to a predetermined reference value for determining whether acceptable cookware is present near said induction heating coil.

26. The apparatus of claim 21, wherein said analog circuit further comprises a pan detection circuit connected between said inverter circuit and said DC power supply, and connected to said microprocessor, for generating a signal corresponding to a recovery time for said DC power supply during a period of non-conduction of said insulated gate bipolar transistor.

27. A method for heating metallic cookware with a work coil of an inductive heating device, comprising the steps of:

generating a plurality of gating pulses for operating a power inverter circuit wherein a start pulse is generated by digital circuitry and subsequent pulses are generated by analog circuitry;

starting said power inverter circuit at substantially zero applied voltages of powerline half cycles;

adjusting pulse durations of a plurality of gating pulses during a start period to start said power inverter circuit at reduced power;

determining during an initial portion of each powerline half cycle whether acceptable cookware is positioned near said work coil, wherein if after said first initial portion it is determined that acceptable cookware is present, then said power inverter circuit is operated through a remainder of said half cycle, and wherein if at the end of said first initial portion it is determined that no acceptable cookware is present, then said power inverter is commanded by said digital circuitry to stop; and

adjusting pulse durations of said gating pulses supplied to said power inverter to compensate for cookware materials other than that of a predetermined standard for said cookware material.

28. The method of claim 27, wherein during said initial portion of each powerline half cycle, a signal for determining the absence of acceptable cookware is generated during a plurality of power inverter cycles during the periods when said gating pulses are absent.

29. The method of claim 27, further comprising the step of selecting a number of powerline half cycles of operation of the power inverter circuit corresponding to a desired cookware temperature during which said power inverter circuit will run uninterrupted.

30. The method of claim 29, further comprising the step of interrupting said generation of said gating pulses by said analog circuitry when an actual number of powerline half cycles is greater than a selected number of powerline half cycles to temporarily stop said power inverter circuit.

31. The method of claim 30, further comprising the step of restarting said power inverter circuit following a predetermined number of powerline half cycles corresponding to a maximum cookware temperature.

32. In a cooking range having an induction heating burner connectable to any AC powerline including an inverter circuit for induction heating of a cooking utensil, and having a control circuit providing a range user with control of the induction burner, including the temperature generated by the cooking utensil, the improvement comprising:

a digital control portion in communication with the range user providing a signal to start the inverter circuit with an initially reduced power output, a signal to interrupt operation of the inverter circuit in response to the absence of acceptable cookware at the burner, and signals to interrupt operation of the inverter circuit for variable intermittent times to control the temperature generated by the cookware and thereafter restart the inverter circuit during cooking, and

an analog control portion generating triggering signals for continuous operation of the inverter circuit once started and in the absence of said interrupting signals from the digital control portion, and signals to compensate for cookware made of materials with differing electromagnetic properties during cooking and to interrupt, through said digital control portion, operation of said inverter circuit if no acceptable cookware is present at the burner.

33. The cooking range of claim 32 wherein the digital control portion comprises a programmed microprocessor having as inputs:

a signal generated at each zero voltage occurrence by the AC powerline,

a signal related to the user-selected cookware temperature,

a signal from said analog circuit portion indicating the absence of acceptable cookware, and

a signal at each cycle of operation of said inverter circuit,

said microprocessor providing in response thereto;

a soft start output for initiating operation of said inverter circuit by said analog circuit at reduced power,

a window signal for a small initial portion of each powerline half cycle to permit application of the analog circuit signal if no acceptable cookware is present at the burner assembly,

break signals to interrupt operation of the inverter circuit following a programmed number of powerline half cycles corresponding to the user-selected cookware temperature and to interrupt operation of the inverter circuit at any time acceptable cookware is not present at the burner, and

start signals to restart the inverter circuit, after its interruption at said programmed number of powerline half cycles corresponding to the user-selected cookware temperature, at a number of powerline half cycles corresponding to a maximum user-selectable temperature;

said analog circuit portion comprising,

a generator for gating pulses for operation of the inverter circuit,

a detector circuit connected with the inverter circuit for generation of triggering signals for said generation of gating pulses,

a compensator circuit coupled with the inverter circuit and providing a compensator signal to said generator for gating pulses that adjust a gating pulse duration for differing cookware materials, and

a switch across the output of the generator for gating pulses to interrupt operation of the inverter circuit in response to outputs of said microprocessor.

34. The cooking range of claim 32 further comprising an over temperature sensor adjacent the burner and an overheat detection circuit to remove application of said triggering pulses from said generator for gating pulses and terminate operation of the inverter circuit in the event of burner overheating.

35. The cooking range of claim 34 wherein said overheat detection circuit provides an input to the microprocessor, said microprocessor generating a warning signal to warn the range user.

36. In an induction cooking apparatus, comprising a DC voltage source of a filtered plurality of AC powerline half cycles, a power inverter including an induction heating coil, a semiconductor switch providing, upon operation, an electric current path from said DC voltage source through said induction heating coil, and a control for switching said semiconductor switch ON and OFF to provide induction heating of cookware adjacent said induction heating coil, the improvement comprising a capacitor connected across said DC voltage source at the input of the induction heating coil, a pan-present circuit connected with said capacitor and generating a no pan voltage from the voltage of said capacitor including first window means for said no pan voltage permitting generation of said no pan voltage for only a small initial portion of each powerline half cycle following each zero voltage event, second window means permitting generation of said no pan voltage only during the OFF periods of said semiconductor switch, and an integrator to accumulate said no pan voltage only during a plurality of semiconductor switch OFF periods in the small initial portions of each powerline half cycle, said no pan voltage being thereby generated from recovery of said DC voltage source from electric current through the inductor heating coil and being applied to said control to interrupt operation of said semiconductor switch.

37. In an induction cooking apparatus including a DC power supply, a power inverter circuit including an induction heating coil and a switching circuit for providing electric current pulses from said DC power supply through said induction heating coil, and a control for operating said switching circuit and generating with said induction heating coil an induction heating field, the improvement wherein said switching circuit for said induction heating coil comprises an insulated gate bipolar transistor, and said control comprises analog circuitry connected with said induction heating coil to generate, once started, operating signals of variable duration for said insulated gate bipolar transistor based on the phase of a voltage of the induction heating coil.

38. The apparatus of claim 37, wherein said analog circuitry comprises a phase comparator having one input connected with one end of the induction heating coil and the other input connected with the other end of the induction heating coil to provide an output only when said one end is at a voltage less than said other end, said output providing a variable duration operating signal for controlling said insulated gate bipolar transistor.

39. The apparatus of claim 37, further comprising a temperature sensor located adjacent said induction heating coil for providing, in the event of unacceptable temperatures, an output to interrupt the operating signals of said analog circuitry to stop operation of the induction heating coil.

40. The apparatus of claim 37, further comprising a pan detection circuit for determining the presence of acceptable cookware adjacent the induction heating coil from the recharging rate of the DC power supply and wherein the operation signals of the analog circuitry gates the pan detection circuit so that it only receives recharging rate signals.

41. A method for heating metallic cookware using an interrupted flow of electric current through an inductive work coil, comprising

providing a signal to start the interrupted flow of electric current through said inductive work coil by the application of DC voltage pulses to the inductive work coil during a working period;

developing phase differences of voltage across the induction work coil resulting from the interrupted flow of electric current,

developing trigger signals from the phase differences of voltage across the induction work coil and generating therefrom electric current gating signals controlling the interrupted flow of electric current through the inductive work coil, said electric current gating signals providing a continuous interrupted flow of electric current through said inductive work coil so long as the DC voltage pulses are applied until a stop signal is provided.

42. The method of claim 41 further comprising the steps of generating a zero crossing signal upon the initial voltage rise of each DC voltage pulse and using said zero crossing signal for starting the flow of electric current through said inductive work coil during each DC voltage pulse.

43. The method of claim 41 further comprising the step of counting the number of trigger signals and developing a pan check period during an initial portion of each DC voltage pulse.

44. The method of claim 43 further comprising the step of developing a pan check signal from the DC voltages of said DC voltage pulses during the pan check period and using the pan check signal to remove the application of the DC voltage pulse in the absence of an acceptable cookware.

45. The method of claim 41 further comprising controlling the number of DC voltage pulses applied to the inductive work coil to control the heating of the metallic cookware.

46. The method of claim 45 further comprising the steps of selecting a maximum member of DC voltage pulses for generation of a maximum cooking temperature, and repetitively applying no more than the maximum number of DC voltage pulses to the inductive work coil during the working period.

47. The method of claim 46 further comprising the steps of selecting a cooking temperature for the metallic cookware, selecting a corresponding number of DC voltage pulses, less than said maximum number of DC voltage pulses corresponding to said maximum temperature, to generate said selected cooking temperature, and repetitively applying said corresponding number of DC voltage pulses to said inductive work coil during the working period.

48. The method of claim 41 further comprising the steps of generating a compensation signal from the current flowing through the inductive work coil during the application of the DC voltage pulses, and using said compensation signal to control the current through the inductive work coil and compensate for differences in the metallic cookware.

49. The method of claim 48 further comprising the step of using said compensation signal to vary the duration of said electric current gating signals.