JPS5851396B2 - Method and device for obtaining constant power in induction heating/melting furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to a method and apparatus for obtaining constant power in an induction heating/melting furnace, and in particular to a method and apparatus for obtaining constant power in an induction heating/melting furnace, and in particular to a method and apparatus for obtaining constant power in an induction heating/melting furnace where the conductance of the load varies. , by appropriately adjusting the phase angle of a constant current parallel root circuit inverter in response to changes in load conductance.

Electrical devices for supplying suitable current to parallel resonant circuits (tank circuits) are well known.

Normally, three-phase power at the mains frequency is rectified to obtain direct current.

The harmonic ripples are removed by the action of a smoothing choke and the direct current is fed to a solid state inverter consisting of a thyristor full-wave rectifier bridge.

This power inverter converts direct current into single-phase alternating current with a controllable frequency.

Its output (roughly rectangular current) is fed into a parallel resonant load circuit to generate a sinusoidal voltage.

In induction heating and melting furnaces, both the load inductance and resistance value change significantly.

A previously proposed control method for such loads has been to control the DC voltage. Examples of this type of control can be found in US Pat. Nos. 3,551,632 and 3,942,090.

Furthermore, a method has already been proposed in which an inverter is controlled by giving a constant turn-off time to a thyristor consisting of a bridge circuit.

US Pat. No. 3,882,370 is representative of this type of control.

These devices degrade the power factor of the power supply line, thus increasing the line capacity (KVA) for a given power when variable loads are present. Although controlling an inverter by maintaining a turn-off time is effective for loads whose conductance is approximately constant throughout the average operating cycle, such control methods are effective for inductive loads whose conductance varies widely. As mentioned above, when it is necessary to keep the power constant, the line capacity (
KVA) becomes excessively large and the line power factor lags excessively.This invention efficiently controls inverter circuits, especially in circuits that supply power to loads whose conductance changes significantly. Therefore, it is suitable for applications where the power supplied to the load can be kept almost constant regardless of changes in conductance.

The improved control herein also effectively isolates the controlled bridge network for rectifying a three-phase power supply from any conductance variations in the load.

That is, in the present invention, when the power of the furnace is controlled to be constant using the converter and the inverter, it is possible to control the phase angle of the inverter based on the conductance fluctuation of the load in a load fluctuation region that cannot be controlled by the converter. It is a purpose.

It is an object of this invention to provide an improved method of operating a constant current parallel resonant circuit to provide constant power to the resonant circuit despite variations in conductance.

Another object of the invention is to deliver constant power to an induction heating and melting furnace so that the power level is not affected during the induction heating and melting cycle.

Yet another object of the invention is to provide an induction heating and melting furnace with a constant current parallel resonant circuit to effectively isolate variations in load conductance from the input power source.

Yet another object of the invention is to provide a device for implementing the above.

In order to achieve the above object, the configuration of the device according to the present invention is such that the induction heating is controlled by a controllable converter and a parallel resonant circuit inverter that supplies power to the induction heating/melting furnace, and is capable of responding to a wide range of conductance fluctuations. - In a power control device for a melting furnace, the DC current of the controllable converter is kept constant by adjusting the phase angle of the inverter in response to fluctuations in the conductance in a load fluctuation range that cannot be controlled by the controllable converter. 1. A power control device for an induction heating/melting furnace, comprising means for maintaining the phase angle constant, and limiting means for limiting the phase angle to a predetermined minimum value.

Further, the method according to the present invention has a configuration in which electric power is controlled to be constant by a controllable converter and a parallel resonant circuit inverter connected to an induction heating/melting stream whose conductance changes, wherein the controllable converter adjusting the phase angle of the inverter in response to fluctuations in the conductance of the furnace in a range of load fluctuations that cannot be controlled by the controllable converter;
and limiting the minimum value of the phase angle.

Other objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments of the invention, illustrated in the accompanying drawings.

In the drawings, like parts are designated by the same reference numerals.

FIG. 1 shows a controllable AC/DC converter or rectifier 12 that provides direct current through a choke or reactor 16 to a DC/AC (direct/alternating current) inverter 14 .

Inverter 14 powers a parallel resonant load circuit 17 comprising an induction heating and/or melting furnace 18 and a tuning capacitor 19 .

The inverter firing control circuit 20 provides the firing order of the inverter switching means, which may be thyristors, transistors, or functionally equivalent known elements.

Converter ignition control circuit 21 controls the bridge rectifier of converter 12, thereby controlling the DC output.

The output DC current of the converter is kept constant by constant power control of the inverter 14 by the inverter ignition control circuit 20.

The inverter 14 supplies a current I with a substantially rectangular waveform to the parallel resonant load circuit 17, thereby generating a voltage vT therein with a substantially sinusoidal waveform.

Figure 2 shows the relationship between voltage waveform and current waveform,
As shown, these are offset by an electrical angle φ so that the current leads the voltage.

Since the inverter current I is continuous and symmetrical, the inverter current II basic component ■.

is shifted by the same electrical angle φ with respect to the voltage vTFc, and the cosine function of this electrical angle φ is defined as the fundamental operating coefficient, ie simply the power factor of the load circuit.

The converter 12, choke 16, inverter 14, and circuit 21 shown in FIG.
Furnace power can be controlled by boiling.

In Figure 2, tl is the point at which the current waveform changes from fully conducting through one electrical path to fully conducting through alternating electrical paths (indicated by t2).
This represents the point at which the switch begins.

The difference between these two points in time represents the commutation time of the inverter circuit.

The positive to negative commutation must be completed before the voltage waveform crosses zero.

FIG. 3 shows a typical power cycle observed when inductively melting ferromagnetic metals using the well-known constant voltage control method.

After a slight increase from the initial power value when the induction heating and melting furnace is filled with material, the output power decreases due to a sharp decrease in the conductance of the load as the filling material heats up through the cue transition point. , levels off at an intermediate value until dissolution begins.

As the filler dissolves, the conductance of the load increases and the power gradually increases.

The power variation is directly due to the filler conductance variation.

Conventional devices that provide either constant voltage control or constant turn-off time control cannot adequately account for conductance variations observed during actual heating or melting.

These control methods are appropriate for loads that do not exhibit variations in conductance during the operating cycle, but for loads that exhibit large variations in conductance, these measures for controlling the inverter are If it is necessary to keep the line constant, the result will be an excessively large line capacity and an excessively delayed line power factor.

The present invention, which changes the phase angle between voltage and current, overcomes the drawbacks of the prior art.

Specifically, the control device of the present invention detects the voltage vT across a parallel resonant circuit (tank circuit) and the current ■ flowing through the resonant circuit, generates a signal proportional to these, and generates a signal proportional to the voltage vT across the parallel resonant circuit (tank circuit) and a predetermined reference signal. compare.

This reference signal is chosen such that the power of the load is at a desired and predetermined value.

By suitably comparing the detected power value with the desired power value, a correction signal is generated indicating the degree of deviation from a predetermined reference value, and by suitably adjusting the firing point of the inverter switching means, the reference value is adjusted. can be adjusted to the value.

Furthermore, this causes the power factor angle to vary in a manner that substantially eliminates deviations from the optimal power state caused by variations in load conductance.

The relationship between current, voltage, and phase angle is best shown in FIG.

FIG. 4 is a graph showing the variation in tank circuit voltage VT and the variation in the in-phase (active) component of the load current as a function of the phase angle φ therebetween.

Vector -F shows the fundamental component of the load current, which is constant in magnitude.

Therefore, the vector locus over the Oo to 9000 phase angle range is a quadrant.

The current vector IF consists of two components: an in-phase or active component (2) and an inactive component (2).

The relative values of (2) and (2) are determined by the phase angle φ over the entire range of FIG.

The voltage V1 is determined by the intersection of the tangent from the arc of the vector IF and the ordinate of the graph of FIG.

Therefore, the power of the tank circuit can be easily determined as the product of vT and ■8.

Operating according to the principles of the invention, this product (ie, power) remains constant regardless of phase angle.

For example, for a phase angle φ1, the common mode current is 0.5 and the tank circuit voltage is 2.0 on the scale of FIG.

When the phase angle changes to φ2, the common mode current becomes 0.8,
The tank circuit voltage will be 1.25.

In the third example for the phase angle φ3 shown in FIG. 4, the common mode current is 0.4 and the tank circuit voltage is 2.5.

Therefore, in all cases, the power is kept at a constant value of 1.0 on the scale of FIG.

The phase angle φ is determined by the power factor (cosφ) according to the following relationship cosφ2K
(G”/2) or φ-cos !(K(G'/2)) (where K
is a constant), so that it is directly related to the conductance G as long as it is a function of the conductance G of the load.

Therefore, the phase angle φ can be used as a controllable variable to counteract the effects of uncontrolled variable conductance fluctuations so that the power to the load remains constant during the operating cycle.

The relationship between phase angle, tank circuit voltage, and common mode current versus load conductance is shown in FIG. 5 for constant DC voltage and current.

Therefore, certain parameters for a given conductance can be easily calculated from FIG.

For example, the phase angle φ to obtain constant power for a conductance of 0.5 (unit method) is determined by the curve φ and the horizontal axis scale of 0.
It is determined by the intersection with a line extending perpendicularly from 5.

It becomes 45°. Under these conditions, to maintain the desired constant power, the tank circuit voltage would be 1.4 (units), but the common mode current would be 0.7C (units).

The product of current and voltage in this state is power 1,
This corresponds to the purpose of maintaining the power of 1.

As the conductance changes, the value of the phase angle φ necessary to guarantee constant power also changes, and its absolute value can be ascertained from FIG.

FIG. 6 is a block diagram of an inverter firing control circuit 20 implementing the above-described method of the present invention such that the phase angle of current and voltage changes in response to variations in load conductance to provide constant power. It shows.

Feedback signals representative of tank circuit voltage and current are applied to calculation circuitry 22.

The computational network 22 consists of a voltage transducer 24 and a voltage squared transducer 26 which receive their feedback signals and output a signal proportional thereto.

The electric power transducer 24 receives both the tank circuit voltage and the load current and generates a signal proportional to the electric power.
Voltage squared transducer 26 receives only the tank circuit voltage.

The outputs from these transducers 24 and 26 are applied to a calculation circuit 28 which generates an error signal representing the phase angle as a function of the conductance of the load.

That is, in the calculation circuit 28, the inductance G (=-=P/V2
=VI/V2), and also obtains the phase angle φ corresponding to the conductance from the glass shown in FIG. 5, and outputs this as an error signal.

This error signal is applied to the phase angle adjustment circuit 30 as a first manual input.

The feedback signal from the load is also input to a phase angle detector 32 which provides a second input to a phase angle adjustment circuit 30 representing the phase angle between the load current and the tank circuit voltage. ing.

A reference input signal selected to obtain the appropriate power value at the load forms the third input to the phase angle adjustment circuit 30.

The error signal from calculation circuitry 22 is added or subtracted from the reference phase angle and the sum of these two inputs is compared to the detected phase angle in the load circuit.

A corrected output from the phase angle adjustment circuit 30 is provided to a comparator circuit, which directly indicates any error in phase angle from the proper phase angle necessary to guarantee the desired power level. .

That is, when the conductance of the load fluctuates, it is necessary to change the phase angle to eliminate the fluctuation.

Computation circuitry 22 generates an appropriate correction factor (error signal), which is first summed with a constant reference phase angle input and then compared to the actual phase angle of the current and voltage at the load.

This provides a corrected output to the comparator 34.

As an example, assume that a phase angle φ of 3000 is required to obtain an appropriate power value to be supplied to the load, which is determined from the function of G as shown in FIG. 5, and as a function of T.

A voltage corresponding to 30° is applied as a reference phase angle input to the phase angle adjustment circuit 30.

The phase angle detection circuit 32 detects this phase angle of 30° between voltage and current defined at the load, while the calculation circuitry 22 outputs a correction factor, which would be zero if the conductance did not change.

Therefore, no correction signal is generated.

If the conductance of the load decreases, the established
That is, a larger phase angle is required to supply the required power to the load.

This increase in phase angle φ as a function of the new conductance G is calculated by calculation circuitry 22 and a signal indicative thereof is provided to phase angle adjustment circuit 30 .

The phase angle detection circuit 32 continues to detect 3000 phase angles.

Therefore, the signal from calculation circuitry 22 has a reference phase angle of 30°.
When added to and compared with the detected 3000 phase angle, a correction signal representative of the error is input to comparator 34 to change the point at which the inverter thyristor begins firing.

By changing the firing timing of the thyristor, the second
The deviation between the zero crossing points of the inverter current and the tank circuit voltage shown in the figure becomes larger, increasing the phase angle φ.

Once the phase angle is adjusted to the required larger value due to the reduced conductance, the calculation circuitry 22
The sum of the correction signal from the phase angle input signal and the reference phase angle input signal will rebalance the detected phase angle signal from the phase angle detector 32 until the conductance changes further.

The phase angle limiting circuit 36 sets a minimum phase angle, which is adjusted to the minimum turn-off time of the inverter thyristor so that the calculation circuit does not fire the bridge thyristor with a phase angle φ that is too small. It is.

The phase angle limiting circuit 36 receives the tank circuit voltage feedback signal and provides an output signal to the comparator 34 having a constant predetermined value representing the minimum allowable phase angle.

If the output from phase angle adjustment circuit 30 is greater than the output of phase angle limiting circuit 36, the output from phase angle adjustment circuit 30 causes comparator 34 to generate a pulse that adjusts the start of the firing time of the thyristor.

If this minimum allowable value is exceeded, phase angle limiting circuit 36 overrides the output signal from phase angle adjusting circuit 30 and comparator 34 generates a pulse representative of the minimum turn-off time.

The pulse from the comparator 34 is amplified in a pulse amplifier 38 which sends the appropriate firing signal to the gate electrode of the thyristor.

An optional phase angle limiting circuit 40 may be provided for setting the maximum phase angle, which is shown in dotted lines in FIG.

If, for example, the phase angle increases to a value that exceeds the maximum voltage rating for the circuit element, the maximum phase angle limiting circuit 40 supplies an override signal to the comparator 34 so that the phase angle It can be varied between predetermined minimum and maximum values chosen to ensure sufficient turn-off time for the devices and avoid excessive voltages across the inverter devices.

In operation, the embodiment shown in FIG. 6 provides a simple yet highly efficient method of delivering constant power to a load circuit.

For a given operating mode, the required power can be easily determined.

Therefore, appropriate values of 8 and vT can be ascertained for a given load conductance from the graph of FIG.

Thereby, an appropriate phase angle φ corresponding to this power can be determined, and a signal proportional to this phase angle is supplied as a reference phase angle input to the phase angle adjustment circuit 30.

During the operating cycle, phase angle detection circuit 32 monitors the actual phase angle of the voltage and current of the load and computation circuitry 22
calculates the phase angle φ as a function of conductance.

When the conductance changes, the phase angle is no longer at a suitable value to deliver the established required power to the load, and an error signal is calculated.

This error signal is added to the reference angle and compared with the detected value to output a signal to the comparator 34.

This signal adjusts the point at which the bridge thyristor begins firing to change the phase angle relationship between the predetermined minimum and maximum values allowed by the phase angle limiting circuits 36 and 40.

FIG. 1 is a block diagram illustrating another embodiment of an ignition control circuit for carrying out the method of the present invention.

The circuit of FIG. 7 is a closed loop regulator 50 with constant power delivered to load circuit IT.

This closed loop regulator 50 determines the power of the load, and
In order to generate an output signal proportional thereto, an electric current transducer 52 is provided which inputs the load current feedback signal and the tank circuit voltage feedback signal.

This power signal is sent as an input signal to a power (KW) regulator 54, which also receives a reference signal proportional to the desired predetermined power to be delivered to the load.

Power regulator 54 compares the detected power signal from power (KW) transducer 52 with a reference signal and outputs an error signal that is input to phase angle detection circuit 56 .

Minimum and maximum phase angle limiting circuits 58 and 60, respectively, are provided similarly to the embodiment of FIG. To disable.

If the error signal is within the tolerance range, an output pulse is generated from the phase angle control circuit 56 and amplified by a pulse amplifier 62 before being applied to the gate electrode of the bridge thyristor.

The closed loop regulator 50 of FIG. 7 differs in operation from the device of FIG. 6 insofar as the phase angle between tank circuit voltage and current is not calculated but compared to a predetermined reference value.

Rather, the phase angle is varied either larger or smaller to meet the demands imposed on the firing control circuit by the reference power input to power regulator 54.

However, the actual effect is the same in that when the conductance of the load changes, a constant current ■ is supplied to the load by changing the phase angle relationship of voltage and current accordingly to the firing of the bridge thyristor.

According to the present invention described above, since the phase angle of the inverter is adjusted in response to changes in conductance, ■ smooth, continuous and highly accurate constant power control is possible, and ■ without changing the frequency. (If the frequency changes, the heating characteristics will change depending on the load, which is undesirable.) The power can be kept constant, and it also has the following advantages: ■ Since it is not mechanical, it can be controlled instantaneously, has fewer breakdowns, is easy to maintain, and is economical. play.

Although this invention has been described with reference to several preferred embodiments, those skilled in the art will appreciate that various other substitutions, modifications, changes, and omissions can be made without departing from the spirit of the invention. That should be obvious.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a control circuit in this invention;
Figure 2 is a graph showing the load current and voltage waveforms across the tank circuit, Figure 3 is a graph showing the power fluctuations typically experienced in induction heating and melting furnaces, and Figure 4 is a graph showing the tank circuit voltage and voltage waveforms across the tank circuit as a function of phase angle. A graph showing the relationship between the common-mode and reactive components of the load current, Figure 5 shows the relationship between the phase angle, tank circuit voltage, and common-mode current when the conductance changes (
FIG. 6 is a block diagram showing the ignition circuit used in the control circuit of the present invention and the limiting circuit used therewith, and FIG. 7 is a graph showing the ignition circuit according to the present invention. A block diagram showing another embodiment,
It is. 14...Inverter, 17...Tank circuit, 18...Furnace, 20...Inverter ignition control circuit, 22...Calculation circuit Net, 30・
... Phase angle adjustment circuit, 32 ... Phase angle detection circuit, 36. 40. 58°60... Phase angle limiting circuit, 34... Comparator. The same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. The induction heating and melting furnace is controlled by a controllable converter and a parallel resonant circuit inverter that supplies power to the induction heating and melting furnace and is responsive to a wide range of conductance fluctuations.
In a power control device for a melting furnace, the DC current of the controllable converter is kept constant by adjusting the phase angle of the inverter in response to fluctuations in the conductance in a load fluctuation range that cannot be controlled by the movable control converter. 1. A power control device for an induction heating/melting furnace, comprising: means for maintaining the phase angle constant; and limiting means for limiting the phase angle to a predetermined minimum value. 2. A power control device for an induction heating and melting furnace according to claim 1, wherein a closed loop power regulator is provided to automatically maintain the power at a predetermined value. 3. The power control device for an induction heating/melting furnace according to claim 1, wherein the phase angle is limited to a predetermined maximum value. 4. The power control device for an induction heating/melting furnace according to claim 21, wherein the phase angle is limited to a predetermined maximum value. 5. A method of controlling electric power to a constant level using a controllable converter and a parallel resonant circuit inverter connected to an induction heating/melting furnace whose conductance changes, the method comprising: The method comprises the steps of: adjusting the phase angle of the inverter in response to variations in conductance to maintain a constant DC current of the controllable converter; and limiting a minimum value of the phase angle. how to. 6. The method of claim 5, wherein the power is automatically maintained at a predetermined value. 7. The method of claim 5, wherein the phase angle is limited to a predetermined maximum value. 8. The method of claim 6, wherein the phase angle is limited to a predetermined maximum value.