DE3528409A1 - Method and device for monitoring the load state of a rotating mechanism which is driven by a field-oriented rotating-field machine - Google Patents

Abstract

The rotating mechanism of a mill, of a roller works, of a conveyor machine etc. is driven using a field-oriented rotating-field machine. In order to protect the mechanism and/or the material being processed, its condition is monitored and the monitoring signal is formed which is dependent at least on the torque acting on the mechanism. The component of the stator current at right angles to the field and, possibly, in addition, the field angle and the field frequency of the rotating-field machine are picked off on the field-oriented control device of the rotating-field machine as equivalent variables for the mechanical opposing torque, the rotation angle and the rotation speed of the mechanism. Thus, for example, the "frozen charge" state of the filling material of a tube mill can be detected without mechanical sensors. <IMAGE>

Description

The invention relates to a method and a device for monitoring the load status of a rotating Mechanics from a field-oriented induction machine is driven.

Electric drives for mechanical systems, e.g. B. rolling mills, Conveyor belts, shaft conveyor systems or mills, usually contain short-circuit monitors, overload monitors and other facilities related to electrical Part of the entire system (e.g. the machine and / or the supplying converter) to protect against malfunctions and monitor destructive errors.

From DE-PS 27 04 764 z. B. a circuit known which is the fundamental vibration performance of a field-oriented Three-phase machine as a product that is field-oriented working control device of the induction machine tapped Setpoints for the electrical moment and the It is allowed to calculate rotor speed, the setpoint for the electrical moment itself the product of the setpoints for the river and the verticals Stator current component can be replaced.

For conveyors, shaft conveyor systems, rolling mills etc. can also in the mechanical part of the system irregular load conditions occur that lead to destruction of the mechanical parts. When editing Machines such as B. Rolling mills or mills can do this  Danger also from the material to be processed and / or threaten it. The detection of such Danger first requires load cells, torque transducers and similar mechanical encoders to pass through intervene in the control system in good time to be able to. For the rotary kiln of a cement plant is proposed in German Offenlegungsschrift 34 09 176, instead of the mechanical torque of the rotary kiln the electric torque of a rotary kiln driving the rotary kiln DC motor from the armature current of the motor to be calculated and processing elements for averaging, Gradient formation and other evaluations to be used for which also by means of a Mechanical tachometer and oven connected Angular step counter the geometric speed and the Angle of rotation of the furnace is used. In Japanese Laid-open specification 58-1 47 625 is the mechanical torque one driven by a three-phase machine rotating machine monitored by mechanical Calculated speed, voltage and frequency of the machine be linked together.

The invention has for its object the load state mechanics, primarily mechanical Wait, the easiest way to monitor.

This is particularly easy if a field-oriented induction machine is used as the drive for the mechanics. Such a three-phase machine requires a flux calculator that calculates the flux vector Ψ or at least its direction angle in the fixed coordinate system that is required to describe the relative movement between the flux or rotor and the stator windings. This flow calculator is preferably designed as a so-called "voltage model". It provides the determinants of the flux as an integral of the voltages (more precisely: the EMF formed from the voltages by subtracting ohmic and inductive voltage drops) and does not require a mechanical encoder. A control device, which is given a reference variable for the field vertical and the field parallel component of the stator current vector, uses this information about the flux to form the control variable with which a current actuator for the stator windings of the induction machine is controlled.

The invention is based on the idea that the mechanical load condition related quantities, initially that is, the one acting on the rotating mechanical parts and mechanical moment transferred to the rotor, next to it possibly also the angle of rotation and / or the speed to replace the rotating mechanics with sizes, that already exists in the field-oriented regulation are formed.

Therefore, it is used as a substitute for the mechanical torque the vertical component of the stator current Three-phase machine monitored. It is assumed that for monitoring the moment of the mechanics the moment of inertia of the rotor on which both this mechanical moment how the electrical moment works doesn't matter. Likewise, it is not considered that the electrical moment of a induction machine for current flow is proportional and at a River decrease only by increasing the stator current component perpendicular to the field can be maintained.

The slip of an asynchronous machine or the load-dependent displacement between the pole wheel angle and the field angle can usually be neglected for monitoring the load state, especially if the induction machine has a high number of pole pairs p (e.g. p = 20, ... 40) . The field angle or field frequency can therefore be recorded as a replacement actual value for the angle of rotation and / or the speed of the mechanics.

The irregular load state mentioned frequently occurs in mechanical part in the form of a dangerous torque Maximum at start-up. For example, for the induction machine a certain speed setpoint is specified during startup be during an actual rotation of the rotor initially due to a strong irregular counter torque Load is braked or prevented. The speed controller then intervenes in the control of the induction machine in the Sense of an increased electrical moment (with field orientation: Increase of the stator current component perpendicular to the field) one leading to the dangerous rise in mechanical Moments leads.

In some cases, there may be an impending danger to the mechanical parts or the material to be processed solely at the level of the mechanical counter-torque that occurs and therefore the amount of the electrical moment of the Drive ("drive torque") can be recognized. In other Use cases is the absolute amount of the occurring Moments not alone or not at all crucial, rather there are other parameters, by linking the counter torque with speed and / or the angle of rotation of the mechanics even then recognized can, if they are not at all causative with these Related parameters.

This is one of the particularly advantageous application Tube mill, in particular a gearless tube mill, shown.  

In Fig. 1, the cross section through the inside of the grinding tube of a tube mill is shown at 1 , the hatched area indicating to what degree of filling the grinding tube is filled with filling material. If the grinding tube is deflected by an angle α compared to a starting position marked by α 0 = 0 ° without the individual parts of the filling material moving in a mutual manner, the center of gravity of the filling material becomes point 3 with respect to its starting position marked with 2 raised and for a further raising of the center of gravity, an electrical torque ("drive torque") must be applied by the electric drive, which is proportional to the mass of the filling material, the distance of the center of gravity 3 from the axis of rotation 4 and also proportional to sin α . If the filling material is sufficiently loose and therefore individual parts of the filling material can move against each other to a certain extent as a result of gravity, then at a maximum value α m of the angular deflection α parts of the filling will detach and fall downwards in the grinding tube.

In the case of an extremely loose and ideally flowable state of the filling material, this separation angle α m is close to 0 ° and the electrical moment of the drive is determined by the inertia of the grinding tube and the internal friction of the filling. The result is an approximately constant value of the torque which is dependent on the degree of filling, parameters of the filling and the speed of the drive.

In the other extreme case, in which the filling is practically baked, frozen, glued, pressed or sintered into a rigid body ("frozen and charge"), the filling is raised with a torque proportional to sin α from the drive to the angular deflection α = 90 ° to crash afterwards as a whole.

With this crash it will break and thereby increase the grinding effect of the mill, but means their impact on the grinding tube wall is considerable mechanical stress on the inner lining of the pipe can be mechanically destroyed.

It is therefore desirable to monitor the mechanical state of the filling material and to stop the mill, to reverse it or to take other measures if, in the case of an unfavorable state of the filling material, separation angles a m can occur which are above a critical value α c .

In Fig. 2 the course of the electrical torque of the drive is a function of the rotational angle α shown, when the mill is operated with a filling degree of 45% (curve 5) and 30% (curve 6) at constant speed. In the assumed, loose state of the filling material, a maximum value of the torque is reached at approximately α m = 45 °, at which falling filling material parts prevent the center of gravity of the filling from being raised further. This maximum value of the torque depends on the nature of the respective filling. In the case of a “frozen filling”, however, the torque continues to increase in accordance with curve 7 .

In a special embodiment of the invention, the maximum value of the mechanical moment of the tube itself is not detected. Rather, the angle deflection α m , at which the respective maximum value of the electric drive torque is reached, serves as a measure of the loose state of the filling. This angular deflection α m is largely independent of the individual parameters of the filling, as the comparison of curves 5 and 6 shows.

Accordingly, on the one hand the angular deflection α of the mechanics relative to the initial position α 0 and on the other hand the electrical moment Mel of the drive itself can be detected. So should z. For example, if the contents of the product are monitored to determine whether mechanical damage to the mill is imminent if a critical angle α c is exceeded, the condition α m ≦ ωτ α c is monitored, and if this is observed an undisturbed grinding operation is possible. In this case, a status signal can be set if the drive torque does not increase due to the maximum value being reached. If the angle signal α has reached the critical value α c , the status signal can then be used to determine whether the maximum value associated with the angle α m has already been assumed beforehand, i.e. whether the filling is sufficiently loose, or with increasing deflection, the torque and thus the torque Risk of destruction of the grinding tube continues to grow. Conversely, of course, if the torque drops after the maximum value, the corresponding instantaneous value α m of the angular deflection α can also be read and queried for the condition α c ≦ ωτ α m .

The use of a field-oriented induction machine enables this monitoring to be carried out simply by using the vertical stator current component, the field angle and its frequency, which are required for field-oriented operation anyway, as substitute values for mechanical torque, angular deflection α and speed of the grinding tube.

An advantageous device for carrying out the method and preferred developments of the invention are characterized in the subclaims and explained using two exemplary embodiments and two further figures. Show it:
Fig. 1 shows the cross-section has already been explained by a partially filled grinding pipe,
Fig. 2 shows the profile already explained the torque depending on the angular deflection,
Fig schematically the structure of an apparatus for carrying out the method in a pipe mill, whereby not made use of the first features of a field-oriented induction machine. 3,
Fig. 4 shows a preferred embodiment of this device.

Referring to FIG. 3, the grinding tube 1 of the tube mill is driven with an electric machine 10, which is powered by a power actuator 11 in response to the output signal of a command variable regulator 12. The speed preferably serves as the reference variable, for which a setpoint n * can be entered and a corresponding actual value n can be tapped at a tachometer generator 8 . The angular deflection α then corresponds to the integral of the actual speed value n and can be formed by means of an angle detector 15 designed as an integrator. A device for state detection according to the invention is indicated at 19 , which derives a state signal from a suitable operating parameter describing the electrical moment of the drive when the electrical moment practically no longer increases. This is indicated schematically in FIG. 3 in that a slightly smoothed actual or setpoint value for the electrical torque Mel of the drive is differentiated and the sign of the derivative is formed as a status signal by means of a threshold value element.

An evaluation circuit supplies the angle signal α by linking with the state signal sign (DMELs / dt), the monitor signal A. This linkage occurs in the variant shown in FIG. 3 in that a switch 21 at the input of the integrator 15 remains closed as long as the drive torque Mel increases when starting. Consequently, the angular deflection α ≦ ωτ α m is present at the integrator output and a differential amplifier fed with the critical value α c indicates that the state of the filling material still permits a further increase in the angular deflection.

However, when Mel reaches the maximum value, the switch 21 is opened by the output signal of the state detection 19 and the integrator output 15 remains at the value α = α m . A status memory, not shown, ensures that the switch 21 remains open, so that positive values present at the output of the differential amplifier 22 indicate that the state of the filling material is still more or less far from a critical state and the normal operation of the mill is maintained can. However, negative values at the output of the differential amplifier 22 indicate how far the critical angle α c has already been exceeded and corresponding measures in the control of the drive are urgently required.

The state detection 19 shown only schematically in FIG. 3 does not necessarily have to monitor the reaching of the maximum value for the drive torque by means of its derivation. According to the embodiment of FIG. 4 can be, for. B. the parameter Mel is supplied via a transmission element 25 with a low smoothing time constant to the one input of a differential amplifier 26 , wherein a rectifier 27 can be provided to compensate for the reversal of the sign of the drive torque and its derivation when the sense of rotation is reversed in the mill. At the same time, the parameter Mel is fed to the other input of the differential amplifier 26 via a smoothing element 28 with a larger time constant. The smoothing member 28 may e.g. B. be designed as an integrator with a resistive return line and also contain a downstream rectifier 29 to take into account the sense of rotation.

Since the less smoothed parameter rises more than the mean value formed by the stronger smoothing, the differential amplifier 26 delivers a positive signal until the conditions are reversed at the angle α m . A limit value detector 30 supplies the monitoring signal B corresponding to the polarity at the output of the differential amplifier 26 .

According to FIG. 3, a three-phase machine, in particular a synchronous machine, is used as the drive motor 10 and is fed by a converter 11 (for example a direct converter or converter with a DC link). Its control rate 18 a is from a field-oriented control device 18 with control variables that define a control vector i * for the three-phase system i at the output of the converter. In such a control device 18 existing control loops and control sections ensure that the current actual value system i is practically the same as that of the stator-oriented control vector i , so that the actual value and setpoint of the drive torque Mel can be used practically equivalent. The field angle ϕ required for field orientation is supplied by a flow computer 13 .

The electrical drive torque is determined by the command variable i ϕ 2 * for the component perpendicular to the field (that is to say for the drive torque Mel ), which is tapped at a speed control of the drive at the output of the speed controller 12 and - neglecting the influence of flow changes - as a measure for the drive torque serves.

In the same way, the direction of the flux vector of the induction machine can also be detected as the angular deflection of the grinding tube in the angle detector 14 . The direction of the flow vector is advantageous in the form of the stand-oriented components Ψ . cos ϕ and Ψ . sin ϕ or the standardized components cos ϕ , sin ϕ . If the zero crossings of a component are counted, their number m gives a digital value of the angular deflection according to α = m . π / p + f / 2 p , which has a sufficient angular resolution at the usual high number of pole pairs ( p = 20 ... 40). This can be increased if the zero crossings of the other stator-oriented flow components are also counted.

For this purpose, the angle detector 14 in FIG. 4 contains the zero point detectors 15 a , 15 b , the output signals of which change their state at each zero crossing. If these status sequences are read into subsequent shift registers 16 a , 16 b , their memory contents can be converted into a suitable form for the further processing of the angular deflection in a subsequent decoder.

In the exemplary embodiment in FIG. 4, the counted number of zero crossings, ie the memory contents of the shift registers 16 b and 16 b describing the angle α , is read into a comparator 31 and compared there with the critical angle α c corresponding to a critical filling state. This comparator 31 releases the state signal of the state detection 19 , which is input to the other input of the AND gate, via a downstream AND gate 32 as soon as the critical limit angle α c is reached or exceeded.

This state signal is set in the state detection 19 of FIG. 4 by the polarity detector 30 to a critical value as long as the drive torque Mel increases. It disappears as soon as Mel reaches the maximum value, irrespective of the height of the maximum value itself. The AND gate 32 therefore shows the critical state at its output if the maximum has not yet been reached at the critical angle α c .

As can be seen from FIG. 2, the acceleration of the torque to a relative maximum value and the subsequent decrease in the torque are repeated several times, as long as the filling material is not sufficiently ground and homogeneous due to the progressive grinding process. Due to its structure, however, the condition detection does not respond to small changes in the center of gravity, which can be attributed to random relative movements in the filling material and can be superimposed on the smooth course shown in FIG. 2. The first relative maximum of the drive torque reported by the condition detection is also its absolute maximum. Relative maxima occurring later, which are also detected by the differential amplifier 26 and the polarity detector 30 , therefore play no role in the detection of the critical state. Therefore, a memory (flip-flop 33 ) is advantageously arranged at the output of the state detection 19 , which is set to the critical state when starting up and is set to the uncritical state when the first maximum is reached and is retained in this state.

Claims (11)

1. A method for monitoring the load state of a rotating mechanism, which is driven by a field-oriented controlled induction machine, characterized in that the value of the vertical component of the stator current vector of the induction machine on the control device of the induction machine is tapped as a substitute for the mechanical counter-torque acting on the rotating mechanics and is monitored. 2. The method according to claim 1, characterized characterized in that as a replacement size for the angle of rotation of the mechanics on the control device Field angle of the induction machine tapped and monitored becomes. 3. The method according to claim 1, characterized characterized in that as a replacement size for the speed of the mechanics on the control device Field frequency of the induction machine tapped and monitored becomes. 4. The method according to claim 1, characterized characterized that the value of the vertical field Component tapped at the output of a controller which is the control deviation of the field frequency is supplied by a frequency setpoint. 5. The method according to claim 2, characterized characterized in that the field angle stand-oriented Cartesian components of the field vector formed the zero crossings of one or both Components counted and the number of zero crossings can be used as a substitute value for the angle of rotation.   6. The method according to the combination of claims 1 and 2, characterized in that at Reaching a maximum for the counter torque substitute size set a status signal and the status signal with the rotation angle substitute for a monitoring signal is linked. 7. The method according to claim 6, characterized characterized that with a low Smoothing time constant is the value of the counter-torque equivalent and with a larger smoothing time constant Mean value of the counter-torque equivalent quantity is formed and that reaching a maximum value of the counter torque Substitute size at the polarity reversal of the difference between the two Values is recognized. 8. Application of the method for recognizing a critical state when the mechanics starts, in particular for identifying a "frozen charge" state of the filling of a tube mill, characterized by the following steps:
a) The angular deflection of the field vector in relation to an initial position is recorded during the start-up;
b) by means of a signal formed from the component of the stator current perpendicular to the field, a status signal associated with a critical load state is formed as soon as the component perpendicular to the field reaches a maximum value; and
c) from the value of the status signal and the value of the angular deflection, the angular deflection present when the maximum value is reached is formed as a measure of the load state of the mechanics. 9. Device for monitoring the load state of a rotating mechanics by a field-oriented Three-phase machine is driven, in a control device a flux calculator the field angle of the induction machine determined and entered the control device Command values for the parallel and perpendicular to the field vector Component of the stator current vector using the Field angle in stand-oriented control variables of a Stator windings of the inverter feeding the induction machine are transferred by a monitoring device for generating a at least from the mechanical acting on the mechanics Counter-torque derived monitoring signal, wherein Inputs of the monitoring device to the control device are connected and the monitoring device the actual value as a substitute value for the counter torque or setpoint of the component of the stator current vector perpendicular to the field is fed. 10. The device according to claim 9, characterized characterized in that the monitoring device one at the entrance for the vertical field Component connected device for condition detection, an angle detector connected to the flow computer to evaluate the field angle and one to the Condition detection device and angle detector connected evaluation device to form the monitoring signal by linking the angle signal with contains the status signal. 11. The device according to claim 10, characterized in that
a) the device for state detection contains a transmission element with a low smoothing time constant and a smoothing element with a higher smoothing time constant as well as a polarity detector for the difference between the signals supplied by the transmission element and the smoothing element and a state memory that can be reset by the polarity detector;
b) that the angle detector contains a counting device for the zero crossings of the stand-oriented Cartesian components of the field vector picked up at the flow computer; and
c) that the evaluation device contains a query device fed by a critical limit value for the angular deflection, the output signal of the angle detector and the output signal of the state memory, which provides the monitoring signal by comparing the predetermined critical limit value with the angle signal coinciding with a change in the state signal stored in the state memory.