EP1763285A1 - Induction hardening apparatus - Google Patents

Abstract

An induction heating assembly hardens metal work-pieces. The assembly has a regulated-operation alternating current rectifier coupled to a hardening induction coil whose inner void accommodates the work-piece. The assembly further has suitable measuring instruments and parameter loggers. The induction hardening assembly is computer-controlled in accordance with the parameters recorded and presents the data on a display.

Description

Background of the invention

The present invention relates to an electrically operated induction hardening plant. The fields of application of such systems are widely spread from hardening applications via welding and sintering processes to special applications such as the heating of materials in vacuum or under inert gas, which would not be possible with conventional gas heating. Induction hardening is a proven process for increasing the quality of precision parts made of steel, cast steel or cast iron. The workpiece to be hardened is brought to red-hot in a very short time (possibly fractions of seconds) and then immediately quenched again. As a quenching agent, water, air, oil or emulsions are used. During hardening, the carbon content in iron plays an important role. For a significant hardness, the carbon content should be at least 0.35%.

Especially complicated shaped metal workpieces are brought only in certain areas by induction to the required hardening temperature and then quenched. Thus arises in this area, e.g. on the track of cams, a hard, wear-resistant steel. The induction hardening is particularly automated, that is used with handling equipment for loading and unloading in mass production, for example, for hardening of gears, camshafts and crankshafts. Induction heating is also used to re-start local areas of a hardened workpiece.

With inductive heating, the heat is generated in the workpiece itself. In principle, all electrically conductive substances can be heated with inductive heating. In practice, mainly metals and alloys are inductively heated due to their good electrical conductivity.

An inductor builds up a magnetic alternating field when it is traversed by alternating current. The workpiece to be machined forms a short-circuited coil. When an alternating current flows through the inductor, a voltage is induced in the workpiece which results in eddy currents induced in the workpiece. This current leads to a heating of the material of the workpiece. The heat does not enter the workpiece from the surface, but rather arises in the layer to be hardened. When warming up Hardening temperature, the structure of the iron lattice changes from cubic-centered to cubic-face centered. The carbon atoms diffuse into the metal lattice. By very fast cooling (quenching), the metal grid folds back, without the carbon atoms can change their position. The carbon atoms are held in the metal grid. The resulting distortion of the space lattice manifests itself macroscopically as an increase in the hardness of the material. By this microstructural transformation can be hardness, brittleness, toughness, internal stresses, etc. influence. The above situation is illustrated for better clarity in Fig. 1a.

The current penetration depth and thus the depth of hardening depends on the operating frequency of the induction system. It decreases with increasing frequency due to the so-called skin effect. Depending on the required hardening depth or current penetration depth, the operating frequency of the induction system is determined. The range of applicable frequencies is divided into low frequency (50 Hz - 500 Hz), medium frequency (500 Hz - 50 kHz) and high frequency (50 kHz - 30 MHz). Penetration depths of between 8 mm in the low-frequency range and approximately up to 0.1 mm in the high-frequency range can thus be achieved. This situation is illustrated for better clarity in Fig. 1b. Not at mains frequency working induction systems must generate these frequencies by means of frequency converters from the mains frequency. The following devices are available for this: Frequency multiplier (static frequency converter), thyristor converter and transistor inverter, RF transistor converter and (tube) high-frequency generator.

The core of every induction heating system is the frequency converter or inverter. The inverter supplies the inductor with the low, medium and high frequency power converted from the mains frequency. The inductor hereby heats the workpieces introduced into it. Semiconductor converters (transistor / thyristor converters) have the following advantages: The use of semiconductor switches eliminates the problems of corrosive effects in the internal cooling circuits. A series resonant circuit converter can be relatively simple and inexpensive. The starting conditions of a series resonant circuit converter are relatively uncritical even in difficult adaptation situations (of the inductor to the converter). Its efficiency can reach a relatively high level even in the partial load range.

The main advantages of inductive heating of workpieces are the high degree of efficiency, the high repeatability, the possibility of spot heating without periphery to heat, flexible adaptation options of the system to a variety of workpieces. In addition, a rapid heating and a high throughput of the workpieces realizable.

Further advantages are the uniform heating of the areas to be hardened as well as the short heating times and, as a result, low scale formation on the workpieces. In many cases, no rework is required. Due to the short-term heating, coarse grain formation is avoided by over-heating or overheating. The heat is safe to control. The required temperatures can be easily achieved and adhered to. The delay of the treated workpieces is generally low. Compared to case-hardening, alloyed case-hardened steels can be replaced by cheap tempered steels. Partial curing is usually possible even with the most difficult workpiece shapes. Setting up the hardening machines and inverter / generators can be done directly in the production lines. The space requirement is low, the operation in series operation is simple, the working area is clean and not hazardous to health.

The problem underlying the invention

When setting up an induction hardening system for a new hardening task, a number of parameters must be taken into account: a converter whose output power must be adapted to the new situation by means of adjusting devices, a specifically shaped workpiece with largely unknown material properties, a correspondingly shaped inductor with a mostly unknown complex resistance ( R + jωL + 1 / jωC), an operating current with which the inductor is to be charged (in particular its current, voltage, frequency and power), the residence time of the workpiece in / at the inductor or the conveying speed of the workpiece relative to the inductor, as well the parameters of the inductor cooling (type of cooling medium, temperature, volume flow, etc.) and the workpiece quenching shower (type of quenching medium, temperature, flow rate, etc.), to name but a few. Their definition requires a lot of experience and detailed knowledge of the hardening process and the plant used. This experience and knowledge is often not available to the operators of the induction hardening system to the required extent.

Object of the invention

Therefore, the invention has for its object to provide an induction hardening system, which allows even without special knowledge to set up an induction hardening system for a new curing task at least almost optimally.

Inventive solution

To overcome these disadvantages, the invention teaches an induction heating system, which is defined by the features of claim 1.

Structure, developments and advantages of the inventive solution

An induction heating system according to the present invention is for heat-treating electrically conductive workpieces and is provided with an AC inverter controllable in at least some of its operating parameters by varying variable components, at least one hardness induction coil to be electrically connected to the AC inverter adapted to be treated location of the workpieces has adapted to heat the workpiece at least at this point in a hardening process by means of the hardness induction coil when the Wechselstromumrichter the hardness induction coil charged with electrical power and the workpiece is fed to the hardness induction coil, measuring devices, detect the at least some of the operating parameters of the AC converter, wherein the induction hardening system is adapted to connect to an electronic computer unit, which in Abh Characterized for the change of at least one of the variable components generated by the detected operating parameters and outputs to an output device in order to influence the power output of the controllable AC converter in terms of optimizing the power consumption of the hardness induction coil. This makes it possible, without specific knowledge and experience about the operation of the induction hardening system for new hardening tasks (ie setting the essential operating parameters of the induction hardening system at a new workpiece shape with a suitably designed hardness induction coil) equip and set up.

In this case, the invention has recognized that the parameters of the operating current with which the inductor is charged (current, voltage, frequency, and power), special importance in terms of the efficiency and the power consumption of the induction hardening system belongs. The definition of these parameters is crucial to ensure that the resonant circuit converters work optimally by adapting them to their load, ie the hardness induction coil (and the workpiece being machined). The invention makes use of the knowledge that initially a virtually arbitrary setting of the resonant circuit converter can be used as a starting point to improve the operating parameters, even with unknown electrical properties of the hardness induction coil.

For this purpose, the electronic computer unit is set up and programmed to access a preferably simplified mathematical replacement model of the AC converter, which is kept in the electronic computer unit, wherein the mathematical equivalent model of the AC converter at least the influence of changes at least one of the variable components is adapted to account for at least one operating parameter of the AC drive.

In particular, the electronic computer unit may be configured and programmed to calculate the mathematical equivalent model of the AC drive with any set of presets for the at least one of the variable components of the AC inverter and a seed set of the operating parameters, the seed set of the operating parameters being a first Curing operation results in outputting a new set of defaults for changing at least one of the variable components of the AC drive to improve the output of the controllable AC drive.

Furthermore, the electronic computer unit may be configured and programmed to calculate the mathematical equivalent model of the AC drive with an improved set of specifications for the at least one of the variable components of the AC drive and a measurement set of the operating parameters, wherein the measured value set of the operating parameters from another Hardening process results to issue a new set of prescriptions for the variation of at least one of the variable components of the AC drive to further improve the power output of the controllable AC drive.

Finally, the electronic computing unit may be configured and programmed to discontinue the mathematical replacement model of the AC drive if no relevant change in the setpoint for changing at least one of the variable components of the AC drive occurs.

According to the invention, the variable component of the AC converter may be a resonant circuit capacitor of a resonant circuit, a resonant circuit inductance of a resonant circuit or a transmission ratio of an output transformer containing the resonant circuit inductor, the amplitude of the supply voltage of a semiconductor bridge arrangement, or a switching frequency of the semiconductor bridge arrangement.

In this case, in the induction hardening system according to the invention, the variable component of the AC converter may be to be changed in discrete stages or continuously.

In one embodiment according to the invention, the AC converter can be a LC series resonant circuit with transformer power output which is fed by a bridge circuit be, wherein the resonant circuit capacitor and / or the transmission ratio of the resonant circuit inductance containing output transformer is to be changed in discrete stages or continuously. The Wechselstromumrichter but can also be a parallel resonant circuit or based on another electrical circuit / operating principle. What is essential is that it be possible to construct a mathematical equivalent model of the AC drive that can serve as the basis for calculating an improved set of presets for the at least one of the variable components of the AC drive based on a set of measured operating parameters.

In the induction hardening plant according to the invention, the AC converter is to be operated in or near resonance of the resonant circuit. The fluctuation of the operating frequency around the resonance point depends on the one hand on the principle of the AC converter and on the other hand, whether the power control is done by deliberately detuning the AC inverter from the resonance point.

According to the invention, the AC converter can also be operated such that the AC voltage supplied to the hardness induction coil and the AC current supplied to the hardness induction coil have a phase angle φ ≠ 0 to each other.

Other features, features, advantages, and possible modifications will become apparent from the following description in which reference is made to the accompanying drawings.

Brief description of the drawing

In Fig. 1a, the principle of an induction hardening system according to the invention is schematically illustrated.

In Fig. 1b, the dependence of the current penetration depth / depth of hardness of the operating frequency of the induction system due to the so-called. Skin effect is illustrated schematically.

FIG. 2 illustrates an induction hardening system according to the invention as a schematic block diagram.

In Fig. 2a is a simplified electrical circuit diagram of a Wechselstromumrichter with a powered by a transistor full bridge circuit LC series resonant circuit with transformer power extraction.

In Fig. 2b shows the electrical equivalent circuit diagram of the AC converter of Fig. 2a and its simplified mathematical equivalent model.

Detailed Description of Presently Preferred Embodiments

In Fig. 2, an embodiment of an induction hardening system according to the invention is schematically illustrated, wherein only the components are shown and described here, which are necessary for the understanding of the invention.

The plant has a Wechselstromumrichter 12, which is controlled in this embodiment with respect to its operating parameters output voltage U, output current I, frequency of the output voltage f, output (effective) power P by changing two variable components. How this is done is explained in detail below.

The AC inverter 12 is electrically connected to a hardness induction coil 14. This has a shape adapted to the shape of the workpiece 10 to be treated in order to heat the workpiece 10 in a hardening process. To this end, the AC inverter 12 supplies the hardness induction coil 14 with electric power when / while the workpiece 10 is supplied to the hardness induction coil 14.

The Wechselstromumrichter 12 are a plurality of measuring devices A1 .. An in the form of voltage, current, Frequenzaufnehmern or the like. Assigned to detect relevant operating parameters of the Wechselstromumrichters 12 and possibly implement in a further processible format / data signal.

Finally, the induction hardening system is connected to an electronic computer unit ECU. This generates depending on the detected operating parameters for the change of one of the variable components Cs, Np and outputs them to an output device 16 to affect the power output of the controllable Wechselstromumrichters 12 in terms of optimizing the power consumption of the hardness induction coil 14.

This is achieved in the present embodiment of the invention in that the electronic computer unit ECU is set up and programmed to access a simplified mathematical equivalent model of the AC converter 12. This replacement model is kept in the electronic computer unit ECU and is based on the actual circuit and operation of the AC inverter 12 so modeled; in that the influence of changes of at least one of the variable components of the AC converter 12 on at least one operating parameter of the AC converter 12 is taken into account. In other words, the replacement model is able to determine how the AC drive 12 with connected hardness induction coil 14 behaves in operation during the hardening of a workpiece 10 without specifying in detail the data of the hardness induction coil 14 (or the AC drive 12). should be known.

In the present exemplary embodiment, the induction hardening plant has an AC converter 12 with an LC series resonant circuit with transformer power output coupled to a transistor full bridge arrangement T1... T4. The primary resonant circuit capacitor Cp and the gear ratio Np / Ns of an output transformer T containing the resonant circuit inductance Lp are to be changed in discrete steps. The transistors T1 .. T4 can be so-called FETs ( F eld effect T ransisor ) whose series resistance is indicated in the switched-on state with R _DSON . The (complex) line resistances, which may be particularly significant on the secondary side, are taken into account as Rz, Lz (see FIG. 2a).

• ein Schwingkreiskondensator Cp, Cs des Schwingkreises,
• eine Schwingkreisinduktivität Lp, Ls eines Schwingkreises oder
• ein Übersetzungsverhältnis Np/Ns eines Ausgangstransformators T,
• die Amplitude der Speisespannung einer Halbleiterbrückenanordnung T1 .. T4, oder
• eine Schaltfrequenz der Halbleiterbrückenanordnung T1 .. T4.

Basically, the following components of the AC inverter are variably designed:

• a resonant circuit capacitor Cp, Cs of the resonant circuit,
• a resonant circuit inductance Lp, Ls of a resonant circuit or
• a gear ratio Np / Ns of an output transformer T,
• the amplitude of the supply voltage of a semiconductor bridge arrangement T1 .. T4, or
• a switching frequency of the semiconductor bridge arrangement T1 .. T4.

Instead of changing the variable component (s) of the AC drive in discrete stages, this can also be infinitely possible. In the present example, the AC converter is to be operated at least almost in resonance of the resonant circuit. In this case, the operating frequency is ω = ω _res = 2 • pi • f = 1 / (L '• C') ^½ .

• (A) Dazu ist die elektronische Rechnereinheit ECU mit einer Computer-Softwareprogramm-Komponente ausgestattet, die das mathematische Ersatzmodell des Wechselstromumrichters 12 mit einem beliebigen Vorgaben-Satz aus einem für die variablen Komponenten des Wechselstromumrichters gültigen Wertebereich und einem Startwerte-Satz der Betriebsparameter durchrechnet. Dabei stammt der Startwerte-Satz der Betriebsparameter aus den Messeinrichtungen A1 .. An in Form von Spannungs-, Strom-, Frequenzwerten während eines ersten Härtungsvorganges und dient dazu, einen neuen Vorgaben-Satz für die Veränderung einer der variablen Komponenten des Wechselstromumrichters im Sinne einer Verbesserung der Leistungsabgabe des steuerbaren Wechselstromumrichter auszugeben.
• (B) Dieser neue, verbesserte Vorgaben-Satz wird dem Wechselstromumrichter zugeführt, indem seine variablen Komponenten entsprechend gestellt werden. Anschließend wird ein neuer Härtungsvorgang durchgeführt und die dabei erhaltenen Betriebsparameter aus den Messeinrichtungen A1.. An in Form von Spannungs-, Strom-, Frequenzwerten erfaßt. Mit diesem erhaltenen Messwerte-Satz der Betriebsparameter wird das mathematische Ersatzmodell des Wechselstromumrichters erneut durchgerechnet, um einen neuen Vorgaben-Satz für die Veränderung wenigstens einer der variablen Komponenten des Wechselstromumrichters im Sinne einer weiteren Verbesserung der Leistungsabgabe des steuerbaren Wechselstromumrichters auszugeben.

The following explains how the electronic computer unit ECU determines, in dependence on the acquired operating parameters, specifications for the variation of one of the variable components Cs, Np in order to optimize the power output of the AC converter.

• (A) For this purpose, the electronic computer unit ECU is equipped with a computer software program component which contains the mathematical equivalent model of the AC inverter 12 with any one of the variable components of the AC converter valid range of values and a starting value set of the operating parameters. In the form of voltage, current, and frequency values during a first hardening process, the starting value set of the operating parameters A1.An arrives at a new set of specifications for the change of one of the variable components of the AC converter in the sense of a To improve the output of the controllable AC drive issue.
• (B) This new, improved set of prescriptions is fed to the AC drive by setting its variable components accordingly. Subsequently, a new curing process is performed and the operating parameters obtained from the measuring devices A1 .. An detected in the form of voltage, current, frequency values. With this obtained measured value set of the operating parameters, the mathematical equivalent model of the AC converter is re-calculated to output a new set of specifications for the change of at least one of the variable components of the AC inverter in the sense of further improving the power output of the controllable AC converter.

This process (B) described above is repeated until there is no relevant change in the target rate for the change of at least one of the variable components of the AC inverter.

• Ein Wechselstromumrichter hat eine vereinfachte Schaltung wie in Fig 2a, 2b gezeigt und hat außerdem folgende Basisdaten:
  • P_max: 90 kWatt
  • U_max: 300 Volt
  • I_max: 450 Ampere
• Die variablen Komponenten des Wechselstromumrichters sind das Übersetzungsverhältnis Np/Ns des Transformators T mit einem Wertebereich von 4/1, 6/1, 8/1, 10/1, 13/1 und 16/1 als möglichen Einstellungen sowie der Sekundärkondensator Cs des Schwingkreises mit einem Wertebereich von 8 ... 51 * 0,33 microFarad als möglichen Einstellungen.
• Die Schwingkreisgüte Q ergibt sich zu $$\mathrm{Q}\mathrm{=}{\mathrm{\omega }}_{\mathrm{res}}\mathrm{•}\mathrm{Lʹ}\mathrm{/}\mathrm{Rʹ}\mathrm{=}{\left(\mathrm{Lʹ}\mathrm{/}\mathrm{Cʹ}\right)}^{\mathrm{½}}\mathrm{•}\mathrm{1}\mathrm{/}\mathrm{Rʹ}$$
  

The mode of operation of the invention is further clarified on the basis of a concrete exemplary embodiment:

• An AC converter has a simplified circuit as shown in Figures 2a, 2b and also has the following basic data:
  • P _max : 90 kW
  • U _max : 300 volts
  • I _max : 450 amps
• The variable components of the AC converter are the gear ratio Np / Ns of the transformer T with a value range of 4/1, 6/1, 8/1, 10/1, 13/1 and 16/1 as possible settings and the secondary capacitor Cs of the resonant circuit with a value range of 8 ... 51 * 0.33 microFarad as possible settings.
• The resonant circuit quality Q is given by $$\mathrm{Q}\mathrm{=}{\mathrm{\omega }}_{\mathrm{res}}\mathrm{•}\mathrm{L\text{'}}\mathrm{/}\mathrm{R\; \text{'}}\mathrm{=}{\left(\mathrm{L\text{'}}\mathrm{/}\mathrm{C\; \text{'}}\right)}^{\mathrm{½}}\mathrm{•}\mathrm{1}\mathrm{/}\mathrm{R\; \text{'}}$$
  

First, the variable components of the AC drive are set to (any preset value (Np / Ns) _is : = 4/1 and Cs _is : = 30 * 0.33 microFarad.) Then, a first hardening process is performed Operating parameters from the measuring devices A1 .. An determined in the form of voltage and current values In the example, U _is : 75 volts and I _is : 450 amps determined This starting set is used as input for the mathematical replacement model, in which the optimum transmission ratio (Np / Ns) _{opt of} the transformer T results in $${\left(\mathrm{np}\mathrm{/}\mathrm{ns}\right)}_{\mathrm{opt}}\mathrm{=}{\left(\mathrm{np}\mathrm{/}\mathrm{ns}\right)}_{\mathrm{is}}\mathrm{•}{\left({\mathrm{I}}_{\mathrm{is}}\mathrm{/}{\mathrm{U}}_{\mathrm{is}}\mathrm{•}{\mathrm{I}}_{\mathrm{Max}}\mathrm{/}{\mathrm{U}}_{\mathrm{Max}}\right)}^{\mathrm{½}}$$

With the present data, an (Np / Ns) _opt = 8 is determined.
This new value is fed (with the secondary capacitor Cs unchanged) to the AC converter, in which the transmission ratio Np / Ns of the transformer T is set to 8/1 automatically or manually.

With these values, a new hardening process is started and again a starting value set of the operating parameters from the measuring devices A1 .. An is determined in the form of voltage and current values. In the example, U _is : 140 volts and I _is : 306 amps determined. This start value set is again used as input for the mathematical replacement model. With the available data, the optimum transmission ratio (Np / Ns) _{opt of} the transformer T of 9.32 (= about 10) is determined.

This new value T = 10 is fed (with the secondary capacitor Cs unchanged) to the AC converter, in which the transmission ratio Np / Ns of the transformer T is set to 10/1 automatically or manually.

With these values, a new hardening process is started and again a starting value set of the operating parameters from the measuring devices A1 .. An is determined in the form of voltage and current values. In the example, U _is : 185 volts and I _is : 243 amps determined. This start value set is again used as input for the mathematical replacement model. With the available data, the optimum transmission ratio (Np / Ns) _{opt of} the transformer T of 9.35 is determined.

Since nothing is changed in the setting of the transmission ratio (Np / Ns) of the transformer T of 10/1, this is also no relevant change of the setpoint for the change of the gear ratio (Np / Ns) _{opt of} the transformer T of the AC inverter. Consequently, the setting of the variable components in terms of an optimal (= maximum) power output of the AC drive is achieved.

If a change in the frequency of the AC drive is sought to vary, for example, the depth of hardness for the workpiece, this can be done by changing the capacitor C ', which also for this purpose, the electronic computer unit ECU is equipped with a computer software program component, the mathematical equivalent model of the AC converter 12 with any set of requirements from a valid for the variable components of the AC inverter range of values and a starting set of operating parameters. It uses the relationships that apply to the mathematical replacement model: $${{\mathrm{f}}_{\mathrm{New}}\mathrm{=}{\mathrm{f}}_{\mathrm{is}}\mathrm{•}\mathrm{/}{\mathrm{C}}_{\mathrm{is}}\mathrm{/}{\mathrm{C}}_{\mathrm{New}}\mathrm{)}}^{\mathrm{½}}$$

It is further understood that the above relationships, which explicitly apply to series resonant converters, can also be modified accordingly for parallel resonant converters and then in which the electronic computer unit ECU can be used as a computer software program component.

Claims (10)

An induction hardening plant for the heat treatment of electrically conductive workpieces (10), with an AC converter (12) which can be controlled with regard to at least some of its operating parameters (U, I, f, P) by varying variable components (Cs, Np), - At least one with the Wechselstromumrichter (12) to be electrically connected to the hardness induction coil (14) adapted to the shape of at least one point to be treated of the workpieces (10) adapted to the workpiece (10) at least at this point in a Curing process by means of the hardness induction coil (14) when the AC converter (12) supplies the hardness induction coil (14) with electric power and the workpiece (10) is supplied to the hardness induction coil (14), - Measuring devices (A1 .. An), which detect at least some of the operating parameters of the AC converter (12), wherein - The induction hardening system is adapted to connect to an electronic computer unit (ECU) which generates in response to the detected operating parameters for the change of at least one of the variable components (Cs, Np) and outputs to an output device (16) to influence the power output of the controllable AC converter (12) in the sense of optimizing the power consumption of the hardness induction coil (14). The induction hardening system of claim 1, wherein the electronic computer unit (ECU) is arranged and programmed to access a preferably simplified mathematical replacement model of the AC drive (12) maintained in the electronic computer unit (ECU), the mathematical equivalent model of the AC drive (12) at least the influence of changes of at least one of the variable components (Cs, Np) is adapted to take into account at least one operating parameter of the AC converter (12). The induction hardening apparatus according to claim 1 or 2, wherein the electronic computer unit (ECU) is arranged and programmed to provide the mathematical equivalent model of the AC inverter (12) with an arbitrary set of the at least one of the variable components (Cs, Np) of the AC inverter (12) and calculating a seed set of the operating parameters, the seed set of the operating parameters resulting from a first hardening operation, a new set of constraints for changing at least one of the variable components (Cs, Np) of the AC inverter (12) in terms of improving the power output of the controllable AC inverter (12) output. The induction hardening apparatus according to claim 3, wherein the electronic computer unit (ECU) is set up and programmed to provide the mathematical equivalent model of the AC drive (12) with an improved set of prescriptions for the at least one of the variable components (Cs, Np) of the AC drive (12) and a set of measurements To calculate operating parameters, wherein the measured value set of the operating parameters resulting from a further hardening process to a new set of specifications for the change of at least one of the variable components (Cs, Np) of the Wechselstromumrichters (12) in the sense of further improving the power output of the controllable AC converter (12). The induction hardening apparatus according to claim 3 or 4, wherein - the electronic computer unit (ECU) is set up and programmed to discontinue the mathematical replacement model of the AC drive (12) if no relevant change in the set of rates for the change of at least one of the variable components (Cs, Np / Ns) of the AC drive (12) occurs. The induction hardening plant according to any one of the preceding claims, wherein the variable component of the AC drive (12) a resonant circuit capacitor (Cs) of a resonant circuit, a resonant circuit inductance (Lp, Ls) of a resonant circuit or a gear ratio (Np / Ns) of an output transformer (T) containing the resonant circuit inductance, the amplitude of the supply voltage of a semiconductor bridge arrangement (T1 .. T4), or a switching frequency of the semiconductor bridge arrangement (T1 .. T4). The induction hardening apparatus according to any one of the preceding claims, wherein the variable component (Cs, Np / Ns) of the AC inverter (12) is to be changed in discrete steps or continuously. The induction hardening plant according to any one of the preceding claims, wherein the AC inverter (12) is a bridge circuit powered LC series resonant circuit with transformer power extraction, wherein the resonant circuit capacitor and / or the transmission ratio of an output transformer containing the resonant circuit inductance is to be changed in discrete steps or steplessly. The induction hardening apparatus according to any one of the preceding claims, wherein the AC inverter (12) is to be operated in or near resonance of the resonant circuit. The induction hardening apparatus according to any preceding claim, wherein the AC inverter (12) is to be operated such that the AC voltage supplied to the hardness induction coil (14) and the AC current supplied to the hardness induction coil (14) have a phase angle (φ ≠ 0) to each other to have.

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