Induction heating apparatus

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, and a protection circuit for the inverter circuit comprising:

• (a) circuit means coupled to the inverter for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the energy in the inverter at the respective times t1 and t2;
• (b) a first peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at time t1;
• (c) a second peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second output signal of opposite polarity to the first output signal and related to the energy in the inverter at time t2;
• and
• (d) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount.

In an induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, a protection circuit for the inverter circuit comprising:

• (a) a snubber capacitor connected in series with a snubber resistor at a junction point, the series combination being connected in parallel with the gate controlled thyristor to comprise means for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the energy in the inverter at the respective times t1 and t2;
• (b) means coupled to said junction for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first voltage supply related to the energy in the inverter at time t1;
• (c) means coupled to said junction for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second voltage supply related to the energy in the inverter at time t1;
• (d) means for taking a ratio of the first voltage supply and the second voltage supply;
• (e) means for preventing gating of the gate controlled thyristor if said ratio deviates beyond a reference value.

In an induction heating apparatus having an inverter circuit, including an induction heating coil operable at operator selected power levels to be loaded by various metallic loads, coupled to a DC power supply for energizing the heating coil, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising:

• (a) means at a given selected power level for producing a gating signal for the gate controlled thyristor after a period of time during which the inverter circuit has been nonconductive;
• and
• (b1) means for producing a signal representative of the length of conduction time of the inverter circuit;
• and
• (b2) means at said selected power level for adjusting the nonconduction time substantially inversely proportional to said signal so as to deliver substantially constant power to the various metallic loads for said selected power level.

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the DC power supply being unregulated to produce a varying voltage output having peaks and valleys, the inverter circuit including a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil at operator selected power settings, an oppositely poled diode coupled in parallel with the gate controlled thyristor, a resistive-capacitive impedance connected in series with the diode which develops a voltage during conduction of the diode that is maintained for a time after conduction of the diode has ceased, said voltage being coupled to the gate of the gate controlled thyristor to provide gate reverse biasing for the thyristor during conduction of the diode and for said time after conduction, comprising a protection circuit for the inverter circuit including:

• (a) circuit means coupled to the inverter for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the enrgy in the inverter at the respective times t1 and t2;
• (b) a first peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at time t1;
• (c) a second peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second output signal of opposite polarity to the first output signal and related to the energy in the inverter at time t2;
• and
• (d) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount;
• and
• a heating coil power control including:
• (e) means for producing at a selected power setting a gating signal for the gate controlled thyristor after a period of time during which the inverter circuit has been nonconductive;
• and (f) means at said selected power setting for decreasing said nonconduction time when conduction time of the inverter circuit increases and for increasing said nonconduction time when the conduction time of the inverter circuit decreases thereby to maintain substantially constant power delivery at said selected power setting.

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, and a protection circuit for the inverter circuit comprising:

• (a) first peak charged means connected to receive energy from the inverter over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at a time t1 which is the start of said second period;
• (b) second peak charged means connected to receive energy from the inverter over substantially the entire range of voltage variation of the DC power supply to produce a second output signal related to the energy in the inverter at a time t2 which is the end of said second period;
• and
• (c) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount.

In an induction heating apparatus having an induction heating coil to which a variety of loads of different magnetic properties can be presented, the heating coil being connected in an inverter circuit coupled to a DC power supply for energizing the heating coil at a plurality of operator selected power settings, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising:

• (a) means operative at a selected power setting for producing a gating signal for the gate controlled thyristor after a period of time T2 during which the inverter circuit has been nonconductive;
• (a1) the inverter including means for responding to the gating signal for initiating a conductive cycle having a duration T1 dependent on the magnetic properties of the load presented to the induction heating coil;
• and
• (b) means operative at the selected power setting for adjusting said nonconduction time T2 dependent upon the conduction time T1 in such a way as to deliver power to each of said variety of loads at a level which is substantially independent of the magnetic properties of the loads.