HK1097688A - Gradient induction heating of a workpiece - Google Patents

Description

Gradient induction heating of workpieces

Technical Field

The present invention relates to controlled gradient induction heating of a workpiece.

Background

It is advantageous to heat certain workpieces to a temperature gradient along the workpiece dimension. For example, cylindrical aluminum workpieces or billets that are subjected to an extrusion process, typically the billet is first drawn through a cross-section at one end of the extruder that is heated to a higher temperature than the cross-section at the other end of the billet. This is so because the extrusion process itself is exothermic and heats the billet as it passes through the extruder. If the billet is heated uniformly along its longitudinal axis throughout its cross-section, the other end of the billet will be overheated prior to extrusion and will undergo sufficient thermal deformation to make extrusion impossible.

A method of achieving gradient induction heating of a conductive billet, such as an aluminum alloy billet along its longitudinal axis, is to wind the billet with a plurality of separate, ordered coil induction coils. Each coil is connected to a current source at the supply line frequency (e.g. 50 or 60 hz). The current flowing through each solenoidal coil creates a longitudinal magnetic flux field around the coil that penetrates and inductively heats the billet. In order to achieve gradient heating along the longitudinal axis of the billet, the intensity (power) of the supply current to each coil from one end of the billet to the other is generally successively smaller and smaller. A silicon controlled rectifier may be used in series with the induction coil to make the current in the coil sequence adjustable.

The use of the frequency of the power supply line is advantageous to simplify the current source, but such an arrangement limits the range of sizes of billets that can be heated in large quantities. Penetration depth of induced currentThe degree (unit: meter) is determined by the formula: 503(ρ/. mu.F)^1/2Where ρ is the billet resistivity, in units: ohm-meter; mu is relative magnetic permeability of the billet and is dimensionless; f is the applied field frequency. The permeability of a non-magnetic billet such as aluminum is 1. The resistivity of aluminum at 500 ℃ was 0.087 micro-ohm-meter. Thus, from the equation, the penetration depth can be calculated to be about 19.2 millimeters, or about 0.8 inches, at a frequency of 60 hertz. Induction heating of the billet is in practice accomplished by a "soaking" process rather than trying to inductively heat the entire cross section of the billet at one time. That is, the induction field penetrates a portion of the billet cross section and allows the induced heat to radiate (crack) to the center of the billet. Typically, an induction field penetration depth of one-fifth the radius of the cross-section of the billet is considered to be an effective penetration depth. Thus, an aluminum billet with a 4 inch radius has an optimum penetration depth of 0.8 inch at 60 hertz current. Thus, the range of billet sizes that can be effectively inductively heated at a single frequency is limited.

It is an object of the present invention to provide a method and apparatus for induction heating of billets with a current frequency gradient that is easily varied for different workpiece sizes.

Disclosure of Invention

In one aspect, the invention is a method and apparatus for inductively heating or melting a workpiece with a multiple coil gradient. Each of the plurality of induction coils is connected to a power supply having a tuning capacitor connected across the inverter input. Each inverter has a pulse width modulated ac output that is controlled in synchronization with the pulse width modulated ac outputs of the other power sources via control lines between all power sources.

Other aspects of the invention are set forth in the specification and the appended claims.

Drawings

The accompanying drawings illustrate one or more non-limiting embodiments of the present invention, and together with the description and claims, serve to explain the principles of the invention. The invention is not to be limited in form or content by the illustrated drawings.

FIG. 1 is a simplified schematic diagram of one example of a gradient induction heating or melting apparatus of the present invention.

FIG. 2 is a simplified schematic diagram of one of the multiple power supplies for the gradient induction heating or melting apparatus of the present invention.

Fig. 3 is a graph of typical results of load coil current as inverter output voltage varies according to one example of a gradient induction heating or melting apparatus of the present invention.

Detailed Description

Figure 1 shows an example of a gradient induction heating apparatus 10 of the present invention. In this particular non-limiting example, the workpiece is a billet 12. The dimensions of the billet in fig. 1 are exaggerated to show the ordered induction coils 14a to 14f around the workpiece. The workpiece may be any type of electrically conductive workpiece requiring gradient heating along its length, in which case, for convenience, the workpiece will be referred to as a billet and the gradient heating will be done along the long axis of the billet. In other examples of the invention, the workpiece may be an electrically conductive material placed inside a crucible or susceptor that is heated to transfer heat to another material. In these examples of the invention, the induction coil is disposed about the crucible or susceptor to provide gradient heating to a material placed within the crucible or susceptor.

The induction coils 14a to 14f are schematically shown in fig. 1. In practice the coils are tightly wound solenoid coils and are positioned close to each other and separated from each other as required to prevent short circuits between the coils, which can be achieved by placing insulating material between the coils. Other coil configurations are also contemplated within the scope of the present invention.

Pulse Width Modulation (PWM) power supplies 16a through 16f are capable of providing different root mean square values of current (power) to the induction coils 14a through 14f, respectively. Each power source may include a rectifier/inverterPower supply, wherein a low-pass filter capacitor (C)_F) Across the output of the rectifier 60 and the tuning capacitor (C)_TF) Connected across the input terminals of the inverter 62, as shown in fig. 2, and disclosed in U.S. patent No. 6,696,770 entitled "induction heating or melting power supply using tuning capacitors," the entire contents of which are incorporated herein by reference. L in FIG. 2_fcIs a line-selectable filter, L_clrIs a current limiting reactor. The output of each power supply is a pulse width modulated voltage to each induction coil.

FIG. 2 further illustrates details of a typical power supply, where the non-limiting main power supply (designated lines A, B and C) powering each power supply is 400 volts, 30 hertz. The inverter 62 comprises a full bridge inverter using IGBT switching devices. In other examples of the invention, the inverter may also be configured as a resonant inverter or an inverter using other types of switching devices. The microcontroller MC provides a means for control and indication functions of the power supply. Most relevant to the present invention is the microcontroller controlling the gate control circuits of the four IGBT switching devices in the bridge circuit. In this non-limiting example of the invention, the gating circuitry is represented by a Field Programmable Gate Array (FPGA), and the gating signals are provided to the gates G1 through G4 by fiber optic connectors (represented by dashed line 61 in FIG. 2). The induction coil connected to the power supply output terminal as shown in fig. 2 is denoted as a load coil L_load. Coil L_loadOne of the induction coils 14a to 14f of fig. 1 is shown. The resistance element R in fig. 2 represents the resistance of the heated billet 12 inserted into the coil as shown in fig. 1.

In operation, the duration, phase and/or amplitude of the inverter pulse width modulated output of each power source 16a to 16f may be varied to achieve a desired billet induction heating gradient. FIG. 3 shows the generation of currents I in three adjacent load coils, respectively₁、I₂And I₃Power supply voltage output (V)₁、V₂And V₃) Typical illustration of variations of (c). The desired heating profile can be programmed into one or more programs executed by a host computer that communicates with the microcontroller in each power supply. The induction coil hasMutual inductance; to prevent low frequency beat oscillations, all coils should operate at substantially the same frequency. With the flexibility provided by using inverters with pulse width modulated outputs, all inverters are synchronized. That is, the output frequencies and phases of all inverters are typically synchronized.

Two diagonally-positioned switching devices (e.g., S in fig. 2) as energy flows from the output of each inverter to its associated induction coil₁And S₃Or S₂And S₄) In conducting, a voltage is applied across the load coil. Other times the coil is shorted, current flows through a switching device and an anti-parallel diode (e.g., S in FIG. 2)₁And D₂，S₂And D₁，S₃And D₄And S₄And D₃). This minimizes the energy extracted from adjacent coils.

Referring back to fig. 1, synchronously controlling the power output of multiple power supplies is used to minimize circuit interference between adjacent coils. Series control loop 40 represents a non-limiting means of synchronously controlling the power output of multiple power sources. In this non-limiting example of the invention, the series control loop 40 may include a fiber optic cable connector (FOL) that connects all power sources in series. The control input ("control input" in fig. 1) of the control connector connected to each power supply may be a Fiber Optic Receiver (FOR), and the control output ("control output" in fig. 1) from the control connector of each power supply may be a Fiber Optic Transmitter (FOT). One of the controllers of the multiple power supplies, such as the controller of power supply 16a, can be programmably selected as the master controller. The "control output" of the master controller of power supply 16a outputs a normal synchronization pulse 20 to the "control input" of the slave controller of power supply 16 f. If the slave of power supply 16f is in a normal operating state, it passes a normal synchronization pulse to the slave of power supply 16e, and so on, until the normal synchronization pulse returns to the master "control input" of power supply 16 a. In addition, each controller generates an independent pulse width modulated signal for each inverter in the plurality of power sources. If an abnormal condition occurs in any one of the power supplies, the affected controller can output an abnormal operation pulse to the controller of the next power supply. For example, the normal sync pulse is about 2 microseconds, while the abnormal operation pulse is about 50 microseconds. The abnormal operation pulse is processed by an upstream controller of the power supply to shut down or modify the induction heating process. Typically the full transmission time delay of the synchronization pulse from the master controller to the master controller is negligible. If one of the controllers fails, the synchronization signal will not return to the master controller, which will cause an abnormal state procedure to be performed, such as stopping the generation of subsequent normal synchronization pulses.

In the above non-limiting example of the invention, six power supplies and induction coils are used. In other examples of the invention, other numbers of power supplies and induction coils may be used without departing from the scope of the invention.

Examples of the invention include the specific electrical components mentioned. One of ordinary skill in the art may implement the invention using elements that are not necessarily of the same type but that produce the desired state of the invention or achieve the desired results of the invention. For example, single elements may be substituted for multiple elements and vice versa.

The foregoing examples do not limit the scope of the present disclosure. The scope of the present disclosure is further set forth in the appended claims.

Claims (16)

1. Apparatus for gradient induction heating or melting of a workpiece, the apparatus comprising:

a plurality of induction coils arranged in sequence around the workpiece;

a power supply for powering each of the plurality of induction coils, the power supply including an inverter having an adjustable pulse width modulated ac output connected to its associated induction coil; and

and a control line connected between the power supplies for synchronously controlling the pulse width modulated AC output of the power supplies.

2. The apparatus of claim 1 wherein at least one of the inverters has a tuning capacitor connected across the inverter input.

3. The apparatus of claim 1 wherein the plurality of induction coils are tightly wound solenoid induction coils and are in close proximity to each other and are insulated apart to prevent short circuits between adjacent coils.

4. The apparatus of claim 1, wherein the workpiece comprises an electrically conductive material disposed within a crucible.

5. The apparatus of claim 1, wherein the workpiece comprises a susceptor.

6. Apparatus for gradient induction heating or melting of a workpiece, the apparatus comprising:

two or more induction coils arranged in sequence around the workpiece;

an inverter for each of the two or more induction coils, each inverter including at least four solid state switching devices, each inverter having a pulse width modulated ac output connected to its associated induction coil;

a controller connected to each of the inverters to control switching devices of the inverters; and

and a control line connected between the inverters to synchronously control the outputs of the inverters.

7. The apparatus of claim 6 wherein at least one of the inverters has a tuning capacitor connected across the inverter input.

8. The apparatus of claim 6 wherein the plurality of induction coils are tightly wound solenoid induction coils and are in close proximity to each other and are insulated apart to prevent short circuits between adjacent coils.

9. The apparatus of claim 6, wherein the workpiece comprises an electrically conductive material disposed within the crucible.

10. The apparatus of claim 6, wherein the workpiece comprises a susceptor.

11. A method of heating or melting a workpiece by an induction gradient, comprising the steps of:

providing pulse width modulated ac power from the output of the plurality of inverters to the plurality of induction coils to induce a magnetic field in each of the plurality of induction coils, each of the plurality of induction coils being individually connected to the output of one of the plurality of inverters;

placing a workpiece in a magnetic field region generated by each of a plurality of induction coils; and

varying the pulse width modulated ac power output of each of the plurality of inverters.

12. The method of claim 11, further comprising the step of inserting a tuning capacitor across the input of at least one of the plurality of inverters.

13. The method of claim 11, further comprising the step of synchronizing pulse width modulated ac power from a plurality of inverter outputs.

14. The method of claim 13, further comprising the step of: control signals are sent serially between the plurality of inverters to synchronize pulse width modulated ac power from the outputs of the plurality of inverters.

15. The method of claim 14, wherein the control signal comprises a master signal generated in one of the plurality of inverters for serial transmission to the remaining plurality of inverters.

16. The method of claim 15, further comprising the step of: one of the plurality of inverters continuously generates an abnormal control signal to the one of the plurality of inverters that generates the master control signal.