Method and apparatus for variable phase induction heating and stirring

An induction furnace comprising:

• a crucible;
• a plurality of induction coils surrounding the crucible;
• first and second inverters each having switch means with known turn-off-time characteristics for generating an alternating polarity output voltage across a load, the first inverter being connected to a first set of the induction coils, and the second inverter being connected to a second set of the induction coils;
• means associated with the first inverter for monitoring the current and detecting the zero-crossing of the current in the first set of induction coils, and for generating a control signal in response to such detection of zero-crossing, said means generating the control signal at an interval following detection of the zero-crossing which is greater than the turn-off-time characteristic of the switch means;
• means for supplying the control signal to the switch means of the inverters to change the polarity of the inverter voltage in response to said signal,
• means associated with the second inverter for introducing a selectable delay of said control signal to the switch means of the second inverter to produce a relative phase difference between the output voltage of the first and second inverters.

A method of controlling the power supplied to an induction furnace by two or more inverters, each having switch means responsive to a control signal for generating an alternating polarity voltage across a load, said switch means having known turn-off-time characteristics, and in which furnace a first inverter is connected to supply power to a first set of induction coils, and each other inverter is connected to supply power to a different associated set of induction coils, comprising the steps of:

• monitoring the current in the induction coils supplied by the first inverter,
• detecting the zero-crossings of said current,
• generating a switch means control signal after a delay interval of predetermined duration following detection of each zero-crossing of the current, the duration of the delay interval being greater than the turn-off time of the switch means,
• supplying the control signal to the switch means of each of the inverters to supply power to each of the induction coils which is essentially in phase coincidence with the power supplied to each other coil,
• operating the induction coils in phase coincidence until a significant portion of a metal charge in the furnace melted, then
• delaying the control signals supplied to the switch means of the inverters other than the first inverter by a selected interval to create a relative phase difference between the induction coils sufficient to produce stirring wave motion in a melted metal charge.

A method of controlling the power supplied to an induction furnace by two inverters, each having switch means responsive to a control signal for generating an alternating polarity voltage across a load, said switch means having known turn-off-time characteristics, in which furnace there are four induction coils vertically separated from each other around a crucible, a first inverter is connected to supply power to a first pair of the induction coils, and the second inverter is connected to supply power to a second pair of the induction coils, the first pair of coils being connected in parallel to each other but wound in opposite directions around the crucible, the second pair of coils being connected in parallel to each other but wound in opposite directions around the crucible, and one of the second pair coils being located between the first pair coils such that the coils wound in the same direction are adjacent to each other, comprising the steps of:

• monitoring the current in the first pair induction coils,
• detecting the zero-crossings of said current,
• generating a switch means control signal after a delay interval of predetermined duration following detection of each zero-crossing of the current, the duration of the delay interval being greater than the turn-off time of the switch means,
• sending the control signal to the switch means of both inverters to supply the second pairs of induction coils with power that is essentially in phase coincidence with the power supplied to the first pair,
• operating the induction coils in such phase coincidence until a portion of a metal charge in the crucible has melted, then
• delaying the control signals supplied to the switch means of the second inverter by a selected interval to create a relative phase difference between the induction coils sufficient to produce stirring wave motion in the melted metal charge.

A method of controlling the power supplied to an induction furnace by two inverters, each having silicon controlled rectifiers (SCR) responsive to a gate signal for generating an alternating polarity voltage across a load, and said SCRs having a known turn-off-time, in which furnace there are four induction coils vertically separated from each other around a crucible, a first inverter is connected to supply power to a first pair of the induction coils, and the second inverter is connected to supply power to a second pair of the induction coils, the first pair of coils being connected in parallel to each other but wound in opposite directions around the crucible, the second pair of coils being connected in parallel to each other but wound in opposite directions around the crucible, and one of the second pair coils being located between the first pair coils such that the coils wound in the same direction are adjacent to each other, comprising the steps of:

• monitoring the current in the first pair of induction coils,
• detecting the zero-crossings of said current,
• generating a gate signal after a delay interval following the generation of each signal pulse,
• comparing the power supplied to the first pair of induction coils to a preset power level,
• reducing the duration of the delay interval when the power supplied is less than the preset power level,
• limiting the minimum duration of the delay interval to a duration greater than the turn-off time of the SCRs,
• supplying the gate signals to alternating SCRs in each inverter to change the polarity of the inverter voltages at essentially the same time to supply power to the second pair of induction coils which is essentially in phase coincidence with the power supplied to the first pair,
• operating the induction coil pairs essentially in such phase coincidence with each other until a portion of a metal charge in the crucible has melted, then
• progressively delaying the gate signals supplied to the SCRs of the second inverter by a selected interval to create a phase difference between the induction coils sufficient to produce stirring wave motion in the melted metal charge.