EP0938388B1 - Mill train with at least one roll stand with three-phase current driving system - Google Patents

Abstract

PURPOSE: A mill train with at least one roll stand with three-phase current driving system is provided which an output data of a motor can be more freely designed than the existing mill train. CONSTITUTION: A mill train comprises: one roll stand for rolling the stock; one AC motor operatively coupled to the roll stand to drive same; and a controller for controlling the motor, wherein the motor speed is adjustable between a minimum speed (nmin) and a maximum speed (nmax); wherein the ratio of the maximum speed (nmax) to the minimum speed (nmin) lies within the range 2.0 and 8.0; and wherein the motor has a power output between the minimum speed (nmin) and the maximum speed (nmax) that deviates from the continuous output of the motor at the minimum speed.

Description

The invention relates to a rolling train with at least one roll stand for rolling rolled material, the roll stand being driven by at least one three-phase motor.

The invention relates to rolling mills with a large speed setting range according to US Pat. No. 4,882,923, which is incorporated by reference.

The rolling mill described in US Pat. No. 4,882,923 has electric motors for driving at least one of the roll stands, the ratio of the maximum to the minimum rolling speed being at least 3.0 but not more than 10.0. For the drive motor disclosed in US Pat. No. 4,882,923, which is not particularly regulated, it is expected that it will have a constant power output according to its nominal power in the speed range described. In the case of a three-phase motor, however, a control range of constant power output can only be achieved with an additional effort which is greater the larger the control range.

It is an object of the invention to provide a rolling mill with three-phase motors, in which the performance data of the motors allow a more revealing design of the rolling programs than with the conventional rolling mills, in particular a more optimal design than with the rolling mill described in US Pat. No. 4,882,923. Furthermore, it is desirable to keep the additional expenses, ie the costs for a large ratio of maximum rolling speed to minimum rolling speed, lower than in conventional rolling mills and also to improve the utilization of the installed drive equipment and its efficiency.

The object is achieved by a rolling train with at least one roll stand for rolling rolled material, the roll stand being driven by at least one three-phase motor, the speed of which for rolling the rolled material can be set between a minimum speed and a maximum speed depending on the requirements of the rolling process by means of a controller is, the ratio of maximum speed to minimum speed is between 2.0 and 8.0, wherein the three-phase motor can deliver a first continuous power at the minimum speed and a second continuous power can be given at the maximum speed, whereby the three-phase motor in A continuous output that is greater than the first continuous output and greater than the second continuous output can be output between the minimum and the maximum rpm.

While the rolling mill according to US Pat. No. 4,882,923 is fixed to a mode of operation at nominal power, which the electric motors driving it are able to deliver as a continuous power (continuous rated output) at minimum speed, the power output in the rolling mill according to the invention is not kept constant at this value , but a variable power is specified. In this way it is possible to work in a particularly flexible and optimal manner on different rolling stock, e.g. in terms of different dimensions, temperature or steel quality.

The rolling mill according to the invention is particularly advantageous in an embodiment as a multi-stand rolling mill.

In an advantageous embodiment of the invention, the regulation of the three-phase motor thus allows a continuous power output that is greater than the continuous output of the three-phase motor at a minimal speed in a wide speed range and in particular is greater than the continuous power according to Section 4 of the IEEE Standard 11-1980.

While the electric motors according to US Pat. No. 4,882,923 are operated at nominal power, the advantageously designed rolling mill is operated with three-phase motors, which for selected rolling programs in the range between minimum and maximum speed over any time above their continuous or nominal power (continuous rated output) of US 4,882,923 can run.

If the power reserve offered according to the invention is not used or is only partially used, it is thus possible to make both the three-phase motors and the converters feeding them smaller and less expensive than in a rolling mill according to US Pat. No. 4,882,923.

In a further advantageous embodiment of the invention, the continuous power that can be output by the three-phase motor increases essentially linearly between the minimum speed and an average speed.

According to a first alternative, the continuous power that can be output by the three-phase motor drops substantially continuously from the average speed to the maximum speed. According to a second alternative, the power which can be output by the three-phase motor initially remains essentially constant above the average speed and only drops towards the maximum speed.

In a further embodiment of the invention, it is provided that the three-phase motors can briefly output a higher output than the continuous output at the minimum and maximum speed, in particular 1.5 to 2.5 times, in the intended speed range. The range of 1.5 to 1.75 times for cold rolling and the range of 1.75 to 2.25 times for hot rolling is particularly advantageous. In this way, increased safety is achieved for the rolling operation with its sudden loads and its requirements for accelerations and speed corrections. As a short-term service in Above sense is to be understood as the power that the three-phase motor during the rolling of a rolling stock, for. B. can briefly release a rolled strip without damage.

The invention can be used with asynchronous motors or synchronous motors.

In a particularly advantageous embodiment, the ratio of maximum speed to minimum speed is between 2.0 and 5.0. The area above a speed ratio of 5.0 enables a particularly flexible operation of the rolling mill, but in contrast, particularly large speed ratios lead to the need to provide larger additional expenses for converters and motors. Under this boundary condition, the range 2.0 to 5.0 has proven to be particularly advantageous for the ratio of minimum speed to maximum speed (see FIG. 3).

In a further advantageous embodiment, the continuous output at maximum speed is equal to or greater than the continuous output at minimum speed.

In a further embodiment of the invention, the three-phase motor is operated in the speed range between the minimum speed and the maximum speed in an analogous manner with a short-term power which is greater than the short-term power of the three-phase current at a minimum speed and which is greater than the short-term power of the three-phase motor at maximum speed, the short-term power at maximum speed is equal to or less than the short-term power at minimum speed.

It is provided in the context of the invention that the three-phase motor in training as a synchronous motor is regulated by a variable displacement angle between the internal voltage and the stator current to the optimal active load delivery. This is a particularly favorable regulation of a rolling mill drive possible in the specified speed range. However, it is provided that above the minimum speed to increase the output, the internal voltage e is first increased as a function of the speed and, after the converter has reached the voltage limit, the phase position of the stator current is regulated up to the maximum value. In this way, the physical conditions of a synchronous motor are used particularly well.

In a further embodiment of the invention it is provided that the phase position of the current is set by means of the control such that an identical phase angle is formed between the stator current i _S and the electromotive internal voltage e as between the stator current i _S and a fictitious internal voltage u. The fictitious internal voltage u is defined as the counter-voltage to cover the vectorial sum of the power loss-independent voltages, which is composed of the internal voltage e and the inductive voltage drop i _S ^· x _S. X _{S is} the reactance of the stator winding of the three-phase motor. This results in the optimal regulation desired according to the invention with its advantageous effects and the possibility of advantageous, greatest possible power output with reduced effort and improved efficiency. The electric motor designed as a synchronous motor can also be designed to be optimally small, ie with a small moment of inertia.

In a further advantageous embodiment of the invention it is provided that the inner displacement angle ϕ _i , which results as the angle between the vectors of the internal voltage e and the stator current i _S , is set to be less than or equal to 40 °, in particular less than or equal to 35 °. The synchronous motor works reliably, particularly when the displacement angle is set to be less than or equal to 35 °. It is also provided that the internal voltage drop to at most by appropriate dimensioning of the motor $\sqrt{\text{2}}$ · E is determined, the internal voltages e and u, based on the corresponding value at the minimum speed, by a factor increased by a maximum of 1.22. This measure contributes to the best use of the equipment and the safe operation of the synchronous motor.

In an analog application of the invention it is provided that when the drive motor is designed as an asynchronous motor, the motor is operated with a voltage reserve to cover the voltage drops at the highest speed and the highest load, the internal voltage e above the minimum speed with increasing speed corresponding to the respective existing voltage reserve is increased. This results in optimal utilization of the installed power and an improvement in efficiency for asynchronous motors and their converters.

FIG 1

die maximal verfügbare Leistung von Drehstrommotoren bei konstantbleibender Spannung und konstant gehaltener innerer Spannung e,

FIG 2

die Leistungscharakteristik bei einer Regelung entsprechend der Erfindung für einen Asynchronmotor,

FIG 3

Aufwendungen (Expenses) in Abhängigkeit des Verhältnisses von maximaler zu minimaler Drehzahl am Beispiel eines Antriebsmotors für ein Kaltwalzwerk,

FIG 4

ein Vektordiagramm für den Punkt der höchsten Drehzahl für einen Synchronmotor mit großem Drehzahlregelbereich bei einer Regelung entsprechend der Erfindung,

FIG 5

ein Vektordiagramm für den Punkt der höchsten Drehzahl für einen Synchronmotor mit großem Drehzahlregelbereich bei einer Regelung entsprechend der Erfindung im Vergleich mit einer Regelung mit einem Leistungsfaktor von 1 und

FIG 6

die Leistungscharakteristik bei einer Regelung entsprechend der Erfindung für einen Synchronmotor.

The invention is explained in more detail with reference to exemplary embodiments, from which further details, also essential to the invention, can be found, as well as from the subclaims. In detail shows:

FIG. 1

the maximum available power of three-phase motors with constant voltage and constant internal voltage e,

FIG 2

the performance characteristic in a control according to the invention for an asynchronous motor,

FIG 3

Expenses depending on the ratio of maximum to minimum speed using the example of a drive motor for a cold rolling mill,

FIG 4

a vector diagram for the point of highest speed for a synchronous motor with a large speed control range in a control according to the invention,

FIG 5

a vector diagram for the point of highest speed for a synchronous motor with a large speed control range in a control according to the invention in comparison with a control with a power factor of 1 and

FIG 6

the performance characteristics in a control according to the invention for a synchronous motor.

A speed control range with an approximately constant power output is often required for drive motors for applications in the rolling mill sector. For example, for drive motors of reels for winding rolled strip, a high speed with a small torque is required for the initial diameter, but a low speed with a high torque is required for the final diameter. For constant strip tension, this means an almost constant output for the entire speed range of the winding process. For reel plants, e.g. B. in cold rolling mills, this speed control range, d. H. the ratio of minimum speed to maximum speed during operation is 1: 5. In some cases, however, the largest possible range of approximately constant power is also desirable for drive motors of rolling stands, in order to have adequate scope for designing the rolling programs. Roll passes with low reductions or narrow strips require low torques, but allow high rolling speeds. Conversely, rolling passes with high reductions or wide strips with high torque requirements can only be rolled at a correspondingly limited speed. For such applications, a speed range up to 1:10 may be desirable (US 4,882,923).

From the direct current machine, it is known that a range of constant power can be realized relatively simply by weakening the field of excitation and in such a way that the flux Φ of the motor after reaching the speed for full power is reduced by adjusting the field current with increasing speed becomes that the electromotive internal voltage e of the machine remains constant: e = Φ ^· n.

If the internal voltage e remains constant, the motor can also operate with the same armature current of the DC motor constant voltage are operated and the power, which is formed by the product of the armature current i and the internal voltage e, remains constant: P = i · e.

In principle, for physical reasons, the three-phase machine cannot have a working range of constant power with a constant terminal voltage and constant current. It is possible to set the effective flow of the three-phase machine by taking appropriate precautions in the regulation of the machine, i.e. also to keep the internal voltage e of the three-phase machine constant over a certain speed range, as in the case of the direct-current machine, and thus to increase the speed beyond a minimum speed n _N increase.

The problem is the inductive voltage drops. The voltage requirement of the three-phase machine is determined not only by the internal voltage e generated by an electric motor, but above all by the voltage drops at the reactance x _{S of} the stator winding of the machine. (The voltage drops across the ohmic resistors and at the reactance of the rotor winding - in the case of an asynchronous machine - play a less important role in comparison.)

The reactance x _S results from the effective inductance L _{S of} the stator winding and the frequency of the motor. For them $${\text{x}}_{\text{S}}{\text{= 2π f}}^{\text{·}}{\text{L}}_{\text{S}}$$

F is the operating frequency of the engine. The operating frequency f changes with the speed of the engine. The inductive voltage drop depends on the stator current. It is calculated from i _S ^· x _S and adds vectorially to the internal tension e of the machine.

The voltage drop i _S ^· x _S is therefore dependent not only on the load but also on the frequency and thus on the speed.

If the voltage of the motor and the feeding converter is selected so that all voltage drops are covered at a minimum speed and a desired maximum short-term power output P _S , the stator current must be reduced accordingly at increasing speeds due to the increasing reactance, and it results in constant voltage and constant regulated internal voltage e an available short-term power P _S according to FIG 1. The available short-term power drops very sharply with the speed. In the case of larger speed ranges, it can even _fall below the continuous power P _min available at the minimum speed.

A power factor = 1 can be achieved by means of a synchronous machine with corresponding regulation of the phase position of the stator current. This means that the power required in the feeding converter can be reduced compared to an asynchronous machine. However, it is disadvantageous that for a certain converter power and operation with power factor = 1 there is also a power drop similar to FIG. 1 for the synchronous machine.

In the case of a regulated three-phase machine, the voltage requirement that increases with the speed can be covered by appropriate voltage provisions in the feeding frequency converter. The disadvantage here, however, is that very large voltage reserves and thus power reserves in the converter and in the motor are required and that these are used very poorly with constant power output above the minimum speed. The degree of utilization can drop below 50% for drives with very large speed control ranges.

This poor utilization of the converter and motor, which is accompanied by higher power losses, leads to high investment costs and also increases operating costs. FIG 3, reference numeral 1 illustrates the example of a conventional three-phase asynchronous motor for a cold rolling mill, how the Increase expenses depending on the ratio of maximum to minimum speed.

To avoid these disadvantages, it is proposed:

1. for the asynchronous motor

Voltage reserves are provided to cover the internal voltage drops for loading the motor with the short-term load P _{S (max)} at the maximum speed.

The asynchronous motor is operated by appropriate determinations in the regulation of the motor voltage and the regulation of the phase position of the current in the range above the minimum speed, but not with constant internal voltage e and constant power, but in such a way that in the speed range in which the intended voltage reserve is not is required to cover the inductive voltage drops, the internal voltage e above the minimum speed n _min while the magnetic flux is still full is increased further with increasing speed in accordance with the available reserves, except for the internal voltage e _D , which with the maximum usable converter voltage at the Speed n _D , is reached. This results in a power characteristic of non-constant power for a constant stator current, for example according to FIG. 2. The asynchronous motor is therefore operated according to the invention in a wide speed range above its continuous power P _(min) at a minimum speed, with a maximum of the power P _D (or briefly with P _SD ). The power P _D results as a design power at the speed n _D using this method.

In FIG. 2, line e shows the course of the internal voltage e at the continuous power P and the line e _{S shows} the course of the short-term power P _S.

• a) In einem weiten Drehzahlbereich ergibt sich eine Leistungsreserve, die zur Gestaltung von Walzprogrammen genutzt werden kann. Die Ausnutzungsgrade von Motor und Umrichter und deren Wirkungsgrade werden entschieden verbessert.
• b) Bei Betrieb mit einer Leistungsanforderung unterhalb der verfügbaren Leistung ergibt sich in einem weiten Drehzahlbereich eine Reduzierung des aufgenommenen Stromes in der Relation von benötigter Leistung zu verfügbarer Leistung. Die thermische Belastung von Drehstrommotor und Umrichter wird reduziert. Dadurch ergibt sich die Möglichkeit, unter Berücksichtigung der geforderten Kurzzeitleistung und entsprechend der zu erwartenden Lastspiele und der resultierenden geringen effektiven Strombelastung einen kleineren Drehstrommotor zu wählen und auch Komponenten des Umrichters zu verkleinern.

The solution offers the following advantages:

• a) In a wide speed range there is a power reserve that can be used to design rolling programs. The degree of utilization of the motor and converter and their efficiency are decidedly improved.
• b) When operating with a power requirement below the available power, the current consumed is reduced over a wide speed range in relation to the power required and the power available. The thermal load on the three-phase motor and converter is reduced. This results in the possibility, taking into account the required short-term power and in accordance with the expected load cycles and the resulting low effective current load, to select a smaller three-phase motor and also to reduce the size of the converter components.

2. for the synchronous motor

It is proposed to use a synchronous motor for applications that require a large ratio of minimum to maximum rolling speed and to optimize the active load delivery by a variable displacement angle between the internal voltage e and the stator current i _S. For this purpose, it is proposed to adjust the phase position of the current i _S by regulating the phase position of the stator current such that there is an equal phase angle between the stator current i _S and the internal voltage e generated by an electric motor (= inner displacement angle ϕ _i ) as between the stator current i _S and a fictitious inner tension u. The internal voltage u is defined as the counter-voltage to cover the vectorial sum of the voltages independent of the power loss, which is composed of the internal voltage e and the inductive voltage drop i _S ^· x _S , ie the product of the stator current i _S and the motor reactance x _S.

At the same time it is determined that the inner displacement angle ϕ _i as the displacement angle between i _S and e is not greater than 35 °, that the inductive voltage drop i _S ^· x _S not bigger than $\sqrt{\text{2}}$ ^· E is and that the internal voltage e generated by the electric motor and the internal voltage u above the minimum speed n _{min are} each increased by a factor F of a maximum of 1.22.

4 illustrates the definition for the limit case of 35 °. As shown in FIG. 4, it is assumed that the external terminal voltage u _{S is} chosen to be at least so large that the ohmic voltage drop i _S ^· r _{S is also} covered and that the excitation current i _{e is set} so large that the demagnetizing effect of the reactive stator current component i _{SR is} compensated and a required fictitious magnetizing current i _µ results.

Based on the definition described, it can be calculated how large the relative stator reactance of the motor x _s may be. $${\text{x}}_{\text{S}}\text{=}\frac{{\text{n}}_{\text{min}}}{{\text{n}}_{\text{Max}}}\text{•}\frac{{\text{P}}_{\text{(Min)}}}{{\text{P}}_{\text{S (max)}}}\text{•}\sqrt{\text{2}}$$ Example: $$\begin{gathered} {\mathit{\text{n}}}_{\text{Max}}{\text{= 8}}^{\text{·}}{\text{n}}_{\text{min}}\text{(Range 1: 8)} \\ {\text{P}}_{\text{S (max)}}{\text{= 2.0}}^{\text{·}}{\text{P}}_{\text{(Min)}} \\ {\text{x}}_{\text{S}}\text{= 0.088} \end{gathered}$$

N _{min is} the minimum speed, n _max the maximum speed, P _(min) the continuous power at n _min and P _{S (max)} the desired short-time power at n _max .

The available maximum active power according to FIG. 4 results from the product of the active component of the stator current i _SA and the internal voltage e. With the maximum displacement angle ϕ _i of 35 ° while increasing the internal stresses u and e by the factor F = 1.22, the following power is available: $${\text{P = cosϕ}}_{\text{i}}{}^{\text{·}}{\text{i}}_{\text{S}}{}^{\text{·}}{\text{F}}^{\text{·}}{\text{e}}_{\text{(Min)}}{\text{= 0.82}}^{\text{·}}{\text{i}}_{\text{S}}{}^{\text{·}}{\text{1.22}}^{\text{·}}{\text{e = i}}_{\text{S}}{}^{\text{·}}{\text{e}}_{\text{(Min)}}$$ where e _{(min) is} the internal stress e at the minimum speed n _min . With the maximum speed n _max , 100% of the short-term power P _{S (min)} available at the minimum speed n _{min are} thus available.

At cos ϕ = 1, the greatest possible active power is given based on the power consumed. In an advantageous embodiment of the invention and surprisingly for the person skilled in the art, however, the setting cos ϕ = 1 is deviated. A certain minimum effective load delivery is thus achieved in a desired very large speed range. With this advantageous embodiment, the best possible power factor is achieved, which makes it possible to make do with a smaller and therefore less expensive drive.

5 shows which operating conditions would result at cos ϕ = 1 with the same active load delivery, and illustrates the great advantages which a surprising deviation for the person skilled in the art from the setting cos ϕ = 1 entails. Accordingly, for better comparability, the vectors from FIG. 4 and the corresponding vectors for the case cos ϕ = 1 are shown.

At cos ϕ = 1, the voltage U and the stator current i _{S are} in phase. To achieve this, the internal voltage e (EMF) would have to increase considerably. The active component of the stator current i _SA could be reduced in the same ratio (same power output), but because of the increased internal displacement angle praktisch _i the stator current i _S would remain practically unchanged and thus the inductive voltage drop i _S ^· x _{S would} remain unchanged. If the EMF is greater than the stator voltage u _S , there are certain operating conditions, e.g. B. with rapid relief, uncontrollable current increases that can destroy the converter that feeds the drive. The rise in EMF would also require a larger flow, which would require a larger machine with a stronger excitation system. The excitation system would also have to be able to compensate for the sharply increased demagnetizing component i _{SR of} the stator current. The advantageous deviation from the setting cos ϕ = 1 under certain conditions allows it to use smaller motors compared to the prior art and at the same time protect the destruction of the converter, for. B. to prevent rapid relief.

The maximum value of the EMF of 1.22 and the maximum angle ϕ _i of 35 ° enable the inductive stator voltage drop i _S ^· x _{S to be} at a maximum $\sqrt{\text{2}}$ ^· E can be (height h of the triangle is 1). The limitation of the EMF (and thus the limitation of the inductive voltage drop and the limitation of the angle ϕ _i ) are based on practical considerations. With the particularly advantageous choice of a maximum angle of 35 ° it is achieved that the demagnetizing component of the stator current i _SR remains significantly below the active component i _SA . The repercussions on the magnet system and thus on the size of the machine remain within acceptable limits when the decision is made.

It is proposed to use the intended voltage reserve (maximum 22%) in order to increase the internal voltage e above the minimum speed while the magnetic flux is still full, proportional to the speed, except for the internal voltage e _D , which corresponds to the lower speed range maximum usable converter voltage at speed n _{D is} reached. As a result, with a constant stator current, a performance characteristic of non-constant power output is obtained, for example, as shown in FIG 6. Even with the synchronous machine, the rated power P _D (design rating) results from this method at the speed n _D , which in a wide speed range above the continuous power P _(min) at the minimum speed n _min .

The example according to FIG. 6 is based on the fact that, over a control range of 1: 8, at least twice the continuous power P _min at the minimum speed n _{min is} temporarily available.

The solution described allows the power reserve used to be chosen as small as possible in accordance with the factor F and to be used in the best possible way.

3 (reference numeral 2) shows an example of the course of the expenditure as a function of the ratio of maximum speed n _max to minimum speed n _min for a synchronous motor according to the present invention in comparison to the course designated by reference number 1 for an asynchronous motor according to the known State of the art.

The course of the expenditure as a function of the ratio of maximum speed n _max to minimum speed n _min for a synchronous motor according to reference number 2 also makes it clear that the invention can be used particularly economically for a speed range from 1: 2 to 1: 5.

Claims (19)

1. Rolling-mill train comprising at least one rolling mill stand for the rolling of rolling stock,

   - wherein the rolling mill stand is driven by at least one three-phase motor whose rotational speed for rolling the rolling stock is adjustable by means of a regulator between a minimum rotational speed (n_min) and a maximum rotational speed (n_max) as a function of the characteristics of the rolling process,

   - wherein the ratio of maximum rotational speed (n_max) to minimum rotational speed (n_min) is between 2.0 and 8.0,

   - wherein a first continuous output (P_min) can be delivered by the three-phase motor at the minimum rotational speed (n_min) and a second continuous output (P_max) can be delivered by the three-phase motor at the maximum rotational speed (n_max),

   - wherein a continuous output (P) that is greater than the first continuous output (P_min) and greater than the second continuous output (P_max) can be delivered by the three-phase motor in the rotational-speed range between the minimum rotational speed (n_min) and the maximum rotational speed (n_max).
2. Rolling-mill train according to Claim 1, characterized in that the continuous output (P) deliverable by the three-phase motor increases substantially linearly between the minimum rotational speed (n_min) and a mean rotational speed (n_D) that is between the minimum rotational speed (n_min) and the maximum rotational speed (n_max).
3. Rolling-mill train according to Claim 2, characterized in that the continuous output (P) deliverable by the three-phase motor decreases substantially continuously between the mean rotational speed (n_D) and the maximum rotational speed (n_max).
4. Rolling-mill train according to Claim 2, characterized in that the continuous output (P) deliverable by the three-phase motor remains substantially constant between the mean rotational speed (n_D) and the maximum rotational speed (n_max) in the vicinity of the mean rotational speed (n_D) and then decreases towards the maximum rotational speed (n_max).
5. Rolling-mill train according to one of the preceding claims, characterized in that a short-term output (P_S) that is greater than 1.5 times the first and the second continuous outputs (P_min, P_max) can be delivered by the three-phase motor between the minimum rotational speed (n_min) and the maximum rotational speed (n_max).
6. Rolling-mill train according to Claim 5, characterized in that a short-term output (P_S) that is greater than 1.75 times the first and the second continuous outputs (P_min, P_max) can be delivered by the three-phase motor between the minimum rotational speed (n_min) and the maximum rotational speed (n_max).
7. Rolling-mill train according to Claim 6, characterized in that a short-term output (P_S) that is greater than 2.0 to 2.5 times, in particular greater than 2.25 times, the first and the second continuous outputs (P_min, P_max) can be delivered by the three-phase motor between the minimum rotational speed (n_min) and the maximum rotational speed (n_max).
8. Rolling-mill train according to one of the preceding claims, characterized in that the three-phase motor is designed as an overloadable three-phase motor.
9. Rolling-mill train according to one of the preceding claims, characterized in that the three-phase motor has a temperature monitor and that the output deliverable by the three-phase motor is limited if a permissible limit temperature is exceeded by the temperature of the three-phase motor.
10. Rolling-mill train according to one of the preceding claims, characterized in that the second continuous output (P_max) is at least as great as the first continuous output (P_min).
11. Rolling-mill train according to one of the preceding claims, characterized in that a short-term output (P_S) that is greater than the short-term output (P_S(min)) of the three-phase motor at minimum rotational speed (n_min) can be delivered by the three-phase motor between the minimum rotational speed (n_min), and the maximum rotational speed (n_max) and in that, at maximum rotational speed (n_max), the short-term output (P_S(max)) of the three-phase motor is maximally as great as the short-term output (P_S(min)) of the three-phase motor at minimum rotational speed (n_min).
12. Rolling-mill train according to one of Claims 1 to 11, characterized in that the three-phase motor is designed as an asynchronous motor.
13. Rolling-mill train according to Claim 12, characterized in that the asynchronous motor can be fed from a converter that is designed in such a way that it can vary frequency, voltage and phase position of the voltage at the output, in that the asynchronous motor can be operated with a voltage reserve to take account of the internal voltage drops at maximum rotational speed (n_max) and maximum load (P_max), and in that the internal voltage (e) can be increased further above the minimum rotational speed (n_min) at full magnetic flux with increasing rotational speed in accordance with the voltage reserve respectively available until an internal voltage (e_D) is reached that, for the maximum usable voltage of the converter, is reached at a mean rotational speed (n_D) at which the continuous output (P) deliverable by the asynchronous motor is a maximum.
14. Rolling-mill train according to one of Claims 1 to 11, characterized in that the three-phase motor is designed as a synchronous motor.
15. Rolling-mill train according to Claim 14, characterized in that the synchronous motor can be operated by means of a static exciter and by means of a converter that is designed in such a way that it can vary frequency, voltage and phase position of the voltage at the output and in that the synchronous motor can be regulated to deliver the optimum active-power load by means of a variable displacement angle (ϕ_i) between the internal voltage (e) and the stator current (I_s) of the synchronous motor.
16. Rolling-mill train according to Claim 15, characterized in that the internal voltage of the motor can be regulated by the regulator of the three-phase motor by means of the static exciter and the converter in such a way that, above the minimum rotational speed (n_min), the internal voltage (e) can be increased further in accordance with the available voltage reserve at full magnetic flux as a function of rotational speed up to the internal voltage (e_S) that, at the maximum usable voltage of the converter, is reached at a mean rotational speed (n_D) at which the continuous output (P) deliverable by the synchronous motor is a maximum.
17. Rolling-mill train according to Claim 15 or 16, characterized in that the phase position of the stator current (I_s) can be adjusted by means of the regulation of the phase position of the stator current (I_s) of the synchronous motor in such a way that a phase angle (ϕ_i) that is appropriate for the rotational-speed and load requirement and that is increased to a maximum value is formed between the stator current (I_s) and the internal voltage (e) generated by the electric motor.
18. Rolling-mill train according to Claim 15, 16 or 17, characterized in that the phase position of the stator current (I_s) can be adjusted by means of the regulation of the phase position of the stator current (I_s) of the synchronous motor in such a way that, at the maximum rotational speed (n_max), an identical phase angle is formed between the stator current (I_s) and the internal voltage (e) generated by the electric motor to that formed between the stator current (I_s) and an imaginary internal voltage (u), wherein the internal voltage (u) is formed as countervoltage to take account of the vectorial sum of the voltages that are independent of the active-power losses.
19. Rolling-mill train according to one of Claims 15 to 18, characterized in that the inner displacement angle (ϕ_i) at the maximum rotational speed (n_max) is limited to a value less than or equal to 40°.