Method for operating a three-phase motor

Abstract

The invention relates to a method for operating a three-phase motor (4) on a multi-phase power grid (5) by means of a soft starter (1), with which one or more phases (a, b, c) of the power grid (5) can be switched by means of a triggering of thyristors (2). In this case, in addition to grid-related triggering criteria, rotor flux-related triggering criteria are also taken into account.

Description

Technical Field

[0001] This invention relates to a method for operating a three-phase electric motor. Similarly, this invention relates to a soft starter. Likewise, this invention relates to a computer program product by which the method can be executed. Background Technology

[0002] In European patent application EP20176295.2, filed May 25, 2020 (which falls under EPC section 54(3)), and in the conference paper entitled “Nannen et al.: Novel predictive start-up algorithm for soft starter driven induction motors,” presented at IECON 2020, 46th Annual Conference of the IEEE Industrial Electronics Society (IES), October 18-21, 2020, Singapore, a model predictive control method for asynchronous motors on soft starters is described, which reduces motor and thyristor losses. Based on measured and / or calculated state parameters, such as grid voltage, grid frequency, rotor speed, and rotor flux, the current and torque directions are predicted using real-time simulation for all possible thyristor triggering combinations. These paths are evaluated based on decision criteria to identify available triggering possibilities. A drawback of this approach is that real-time prediction of motor characteristics requires high computational power, leading to significant additional costs for processors or microcontrollers. Summary of the Invention

[0003] This invention was made in the context of the prior art described above, wherein the technical problem to be solved by this invention is to define a method that, on the one hand, provides similar operational characteristics to model prediction methods, but on the other hand, does not require such high computational power.

[0004] This technical problem is solved by the method according to the invention. Here, it is a method for operating a three-phase motor on a multiphase power grid. Operation is carried out using a soft starter, which allows switching one or more phases of the power grid by triggering thyristors. In addition to at least one grid-related triggering criterion, this operating method also considers at least one triggering criterion related to rotor flux.

[0005] Here, triggering is performed on two-phase or three-phase ground. In the space vector representation, two-phase triggering results in a current vector that is fixed in its orientation and pulsates in its amplitude, while three-phase triggering results in a rotating current vector.

[0006] This invention is based on the following idea: determining whether to perform thyristor triggering based on triggering criteria related to the power grid (e.g., the position of the voltage vector) and triggering criteria related to the rotor flux (e.g., the position of the rotor flux vector). In this respect, the control method according to this invention is fundamentally different from conventional control methods, in which the rotor flux generated in the three-phase motor has no influence on the decision of whether thyristor triggering should be performed.

[0007] The proposed method requires no detailed motor parameters or predictions, but only some angle calculations and comparisons, and is therefore much simpler to implement in industrial products than known model predictive control methods. Preliminary estimates indicate that the method according to the invention requires only about 10% of the computational power of model predictive control methods. Furthermore, regulated operation can even be achieved using the new method.

[0008] According to the present invention, the triggering determination for the triggering probability of using one or more thyristors must satisfy both the grid-related triggering criteria and the rotor flux-related triggering criteria. Each triggering probability is checked in terms of whether it satisfies both the grid-related and rotor flux-related triggering criteria. Only if the triggering probability satisfies both the grid-related and rotor flux-related triggering criteria can it be triggered for practical use.

[0009] According to the present invention, a grid-related triggering criterion is satisfied if the current vectors representing the triggering probability of two or more grid phases in a spatial vector representation lie within a triggering angle region, which is defined by a lower boundary and an upper boundary of the triggering angle region related to grid voltage and / or grid current. The lower boundary of the triggering angle region determines the triggering point where the current flow reaches its maximum length and maximum current amplitude. The lower boundary of the triggering angle region can be predetermined, for example, by a characteristic curve, in a manner similar to the triggering angle α in classical control methods, or it can be adjusted considering the current amplitude. It is also possible to consider the current rotor flux amplitude to limit the maximum possible torque. Since current and torque can be controlled by means of the lower boundary of the triggering angle region, the lower boundary of the triggering angle region functions similarly to the triggering angle α in conventional control methods. The lower boundary of the triggering angle region can be measured using a value from the defined region [-90°, upper boundary of the triggering angle region], which is measured with respect to the grid voltage vector. The upper boundary of the triggering angle region indirectly defines the shortest current flow that can be initiated by triggering. The upper boundary of the trigger angle region can be selected such that even in a power grid where it is difficult to determine zero crossing, the triggering always behaves as follows: the current flows in the desired direction and there is no undesirable triggering with excessively high current amplitude.

[0010] According to the present invention, if the current vectors representing the triggering probability of two or more grid phases in the spatial vector representation lie within the flux and torque angle regions, then a rotor flux-related triggering criterion is satisfied, the boundary values ​​of which are defined with respect to rotor flux. By selecting one of the boundary values ​​of the flux and torque angle regions, the regions in which triggering causes average positive and negative torque can be separated from each other. Depending on the selection of this boundary value, for example, slightly negative torque can be allowed to favor the current that forms a strong rotor flux. Using other boundary values ​​of the flux and torque angle regions, the amount of positive torque and rotor flux linkage generated in the rotor can be defined: the establishment of positive torque may lead to a decrease in rotor flux, and vice versa. Therefore, the selection of this additional boundary value must be carefully examined. This parameter can be used for "flux induction" or "flux regulation" during startup. With the aid of this boundary value, the average absolute rotor flux can be affected. If the motor should be accelerated, the difference between the boundary values ​​of the flux and torque angle regions can be measured using values ​​from the following defined region: [90°, 180°], which is the vector of the rotor flux.

[0011] According to a preferred design of the present invention, based on the rotor flux Grid voltage and stator current The existence of triggering criteria is checked using a spatial vector representation. This spatial vector representation clearly shows the relationships between grid parameters of voltage and current, rotor flux parameters, the trigger angle region, and the flux and torque angle regions. The main difference between this invention and conventional control methods in spatial vector representation is that the rotor flux spatial vector is considered when selecting the triggering time point. Calculations of the grid spatial vector, such as the voltage spatial vector, the rotor flux spatial vector, the trigger angle region, and the flux and torque angle regions can be performed iteratively. Thyristor control can be performed each time the triggering probability falls within the trigger angle region and the flux and torque angle region. This results in a trigger pulse that considers both grid voltage and rotor flux. In this way, positive torque, constrained stator current, and sufficient rotor flux for subsequent triggering can be obtained.

[0012] According to a preferred design of the invention, the flux and torque angle regions are shifted by a rotation angle to compensate for the time delay between the trigger decision and the trigger. This avoids the effect that, due to the rotating rotor, the analysis of the positive torque and rotor flux development is based on a different foundation than what actually occurs during triggering. The rotation about a rotation angle performed in the space vector representation aims to compensate for the deviation caused by the rotating rotor as optimally as possible.

[0013] According to a preferred design of the present invention, the three-phase motor is an asynchronous motor, a synchronous motor, or a self-starting permanent magnet motor (PM-Line-Start-Motor, PM = Permanent-Magnet).

[0014] According to a preferred design of the present invention, a period of flux increase is set before checking the power grid-related triggering criteria and the rotor flux-related triggering criteria, during which the rotor flux is generated by the triggering of the thyristor.

[0015] According to a preferred embodiment of the invention, only those triggering possibilities that result in negative torque and thus active braking of the three-phase motor are determined. Therefore, this method can also be used for active braking.

[0016] This technical problem is also solved by a soft starter according to the invention, suitable for performing the steps of the method according to the invention. The soft starter has a trigger signal unit for generating a trigger signal and thyristors. The trigger signal unit can be controlled to generate a trigger signal for one or more of the thyristors. The soft starter also has means suitable for performing the steps of the described method. These means may, for example, be a control unit having a computing unit and a storage unit. A computer program can be loaded into the storage unit and permanently stored there, from where it can be loaded into the computing unit for execution.

[0017] The described technical problem is also solved by a computer program product according to the invention and a computer-readable medium on which the computer program product is stored. The computer program product is configured to execute on at least one processor. The computer program product may be configured as software, such as application software (App) downloadable from the Internet, or as firmware, which may be stored in memory and implemented by a processor or arithmetic logic unit. Alternatively or additionally, the computer program product may also be configured at least partially as hardwired circuitry, such as as an ASIC (Application-Specific Integrated Circuit). The computer program product according to the invention includes commands to cause a soft starter according to the invention to perform the method steps of the described method. Thus, the computer program product is configured to perform a method for operating a three-phase motor by means of a soft starter. In particular, it is configured to consider rotor flux-related triggering criteria in addition to grid-related triggering criteria. Furthermore, the computer program product includes commands to cause the trigger signal unit of the soft starter according to the invention to generate one or more trigger signals for one or more thyristors after a decision has been made regarding the triggering probability. According to the invention, the computer program product is configured to implement and execute at least one embodiment of the described method. Here, the computer program product can combine all the sub-functions of the method together, i.e., it can be constructed monolithically. Alternatively, the computer program product can also be constructed in segments, with each sub-function assigned to a segment implemented on separate hardware. Therefore, the computer program product can be constructed to be implemented partly in the control unit of the soft starter and partly in an external control unit. Furthermore, a portion of the method can be executed in the soft starter device, and another portion of the method can be executed in a control unit, such as a PLC, a manually parameterized device, or a computer cloud, located above the soft starter device. Attached Figure Description

[0018] The features, characteristics, advantages, and ways in which the invention described above is implemented will become clearer and more readily understood in conjunction with the following description of embodiments set forth in more detail with reference to the accompanying drawings. Hereinafter, schematic diagrams are provided:

[0019] Figure 1 This illustrates a traditional soft-starter topology;

[0020] Figure 2 This illustrates a conventional structure for generating control pulses;

[0021] Figure 3 The structure for generating control pulses based on a model prediction method is shown;

[0022] Figure 4 An example of spatial vector representation is shown;

[0023] Figure 5 A diagram illustrating the pulsating current vectors triggered by phases a and b in the coordinate system at coordinate α is shown.

[0024] Figure 6 The diagram illustrates the rotation operation between the spatial vector map in the α-coordinate system and the spatial vector map in the dq coordinate system;

[0025] Figure 7 A diagram illustrating the definition of the trigger angle α is shown;

[0026] Figure 8 The trigger angle region [α] is shown. min α max A diagram illustrating the definition of ];

[0027] Figure 9 An explanation of the problem with zero-crossing detection is shown;

[0028] Figure 10 A spatial vector diagram showing the rotor flux vector and the regions with positive and negative torques is displayed.

[0029] Figure 11 A spatial vector diagram showing the rotor flux vector, voltage vector, and regions with possible triggering is displayed.

[0030] Figure 12 A spatial vector diagram with voltage vectors and trigger windows is shown;

[0031] Figure 13 A spatial vector diagram showing the rotor flux vector, voltage vector, and trigger window is displayed.

[0032] Figure 14 A spatial vector diagram showing the rotor flux vector, voltage vector, and regions that enhance or weaken the rotor flux linkage is displayed.

[0033] Figure 15 It shows that Figure 10 and Figure 14 Information is integrated into a spatial vector diagram;

[0034] Figure 16 A spatial vector diagram with rotor flux vector and flux window is shown;

[0035] Figure 17 A spatial vector diagram showing the rotor flux vector, trigger window, and flux window is displayed.

[0036] Figure 18A spatial vector diagram showing rotor flux vector, voltage vector, trigger window, and flux window is displayed.

[0037] Figures 19 to 24 The time series of the spatial vector diagram over a 3.9 ms time interval is shown, which includes the rotor flux vector, voltage vector, and rotating stator current vector;

[0038] Figures 25 to 28 The time series of the spatial vector diagram over a 6ms time period is shown with rotor rotation, which includes rotor flux vector, voltage vector and rotating stator current vector.

[0039] Figure 29 A spatial vector diagram showing rotor flux vector, voltage vector, trigger window, and flux window is displayed.

[0040] Figure 30 It shows the rotor flux vector, voltage vector, trigger window, and relative to... Figure 29 The flux window in the middle has been rotated by a rotation angle δ rot A spatial vector image of a flux window;

[0041] Figure 31 It shows the relationship between t delay A diagram illustrating the definition;

[0042] Figure 32 A spatial vector diagram with rotor flux vector and voltage vector is shown, which shows the boundaries of the firing angle region and the rotor flux angle region;

[0043] Figure 33 A flowchart illustrating one embodiment of the method according to the present invention is shown; and

[0044] Figure 34 A structure for generating control pulses according to the present invention is shown. Detailed Implementation

[0045] Figure 1 A typical soft starter topology is shown, which has a soft starter 1 for starting the ASM4 connected between a three-phase power grid 5 and an asynchronous motor (ASM) 4. Each phase a, b, and c of the soft starter 1 has two pairs of thyristors connected in anti-parallel and bypass contacts 3. Current i a i b i c Measurements were taken on the motor side of all three phases a, b, and c. Additionally, soft starter 1 used line-to-line measurements on the grid side. A,B u B,C u C,A In order to calculate the grid voltage u A u Bu C And use line-to-line measurement of u on the motor side. a,b u b,c u c,a In order to calculate the motor voltage u a u b u c A similar topology can be found in Chapter A.1 of the Siemens equipment manual Sanft-starter SIRIUS 3RW50, Siemens AG, Amberg, 09 / 2019, A5E35628455001A / RS-AA / 001, but in that soft starter, only two of the three grid phases can be switched.

[0046] Although the functional range of soft starters has been continuously expanding in recent decades, such as through voltage ramping, current-limiting operation, torque-controlled starting, and speed-controlled starting, the structure used to generate control pulses has remained essentially unchanged and possesses... Figure 2 The structure shown mainly consists of three blocks, which are described below:

[0047] In the first block of the soft starter 1, i.e. in the controller 21, the adjustment structure is based on the measured value 24 (e.g., stator current I). 1,RMS Stator voltage U 1,RMS The firing angle α (also known as the control angle or adjustment angle) is set by the rotor speed n of the motor and / or the motor. The firing angle α can be influenced by input parameters 25, such as the current motor torque, maximum rated current, or speed ramp, to optimize characteristics for the application. The firing angle α defines the RMS value (RMS = Root-Mean-Square) of the voltage applied to each phase a, b, c of the motor. A simple method to generate the firing angle α is a voltage ramp independent of load response; for this, the firing angle α simply increases as a function of time. Here, a sufficiently large initial value of the firing angle α must be used for startup, thereby initializing the motor torque M. M It is already greater than the load torque M L Otherwise, the motor current will not accelerate the rotor and will only generate high losses in the stationary state until the motor torque is large enough to accelerate the rotor.

[0048] The second block of the soft starter 1, namely the control signal block 22, is used to generate a control signal 27 for initiating thyristor triggering, wherein the control signal 27 is preferably generated based on the measured voltage, such as the mains voltage 26 and / or the measured current, preferably at a defined time between thyristor turn-off and thyristor re-triggering when the current is below the minimum.

[0049] The third block of the soft starter 1, namely hardware block 23, includes thyristors and triggering devices, such as trigger signal units; it uses control signals 27 received from control signal block 22 to generate an output voltage.

[0050] Figure 3 The diagram illustrates a structure for generating control pulses using a model predictive control method, as described in European patent application EP20176295.2 and by Nannen et al. The soft starter 1 can be used, for example, in... Figure 1 In the topology shown, the soft starter 1 has a control unit 41, which has a computing unit 46 (also called a processor) and a storage unit 43. The computing unit 46 executes a computer program stored in the storage unit 43, which contains an algorithm for executing the method. When executing the algorithm, the trigger probability for at least one recent time step is determined, the motor characteristics corresponding to the trigger probability are pre-calculated using a motor model, and finally, based on the predicted motor characteristics, a decision is made on whether to select and which trigger probability should be selected. To initialize the algorithm, the system's state variables are measured or calculated. The computing unit 46 receives a series of measured values ​​44 (e.g., motor current I1, motor voltage U1, and rotor speed n) as input values. The grid voltage u... A u B u C The measurement is used to calculate the grid angle. and grid voltage amplitude U grid Motor current i a i b i c and motor voltage u a u b u c It is also used for model-based calculations of rotor flux, along with the measured mechanical rotor speed n and the grid frequency f. grid All system variables are defined. Based on predictions, the possibilities for triggering at future points in time are determined, i.e., what triggering probabilities exist. These probabilities can then be examined and evaluated based on multiple decision criteria 45, such as maximum torque or maximum stator current. If a decision has been made regarding a particular triggering probability, the controller 41 generates one or more control signals 47 to the trigger block 42 of the soft starter 1, which has thyristors and trigger signal units 48 for generating trigger signals for the thyristors. The control signals 47 cause the trigger signal units 48 to generate trigger signals for one or more thyristors, thereby setting the pre-calculated motor characteristics.

[0051] Basic - Spatial Vector Representation

[0052] The present invention is based on spatial vector representation: spatial vector representation provides the possibility of representing the multiphase physical parameters occurring in a multiphase power grid in a simpler two-axis coordinate system with axes α and β as spatial vectors (or simply vectors) rotating around a zero point. The transformation to the α-β coordinate system is performed using the Clarke-Transformation. With the aid of spatial vectors, the physical parameters of the system at each point in time can be represented; in this invention, this is particularly true for magnetic flux, voltage, and current.

[0053] Figure 4 An example of spatial vector representation is shown: In Figure 4 The upper part shows a conventional representation of the time-varying trends of the three-phase AC voltage U and the stator current I (also known as the motor current). The three-phase AC voltage U comes from a typical 50Hz power grid and consists of three separate sinusoidal AC voltages ui with peak values ​​of 325V each, operating at the same frequency. A u B u C The system consists of three separate sinusoidal AC voltages, each fixed at 120° phase angle to the next. The voltage trends are shown over a time interval of 0 to 100 ms. Figure 1 In the configuration shown (with a soft starter and the connected motors), the motor current i of the three motor phases a, b, and c is shown over a time period of 0 to 30 ms. a i b i c It has been generated by a single two-phase triggering at time 12ms in phases a and b.

[0054] Figure 4 The lower part shows the space vector representation of voltage and current over a time interval of 12ms to 21ms with a time step of 1ms. Vector scaling is irrelevant to the analysis here. The voltage vector is shown with a thicker line width. The current vector is shown with a thinner linewidth. At time t = 12 ms, no current is flowing yet; therefore, only the voltage vector can be observed. In comparing the graphs at time points t=12ms and t=13ms, the first thing to note is the voltage vector... It has rotated a little further. This can also be seen in subsequent time steps. Based on the three-phase AC voltage, it is the rotating voltage vector. Its frequency f grid =50Hz proportional rotation: voltage vector With angular frequency ω=2πf grid =2π50Hz=2π / (20ms) rotates counterclockwise around the zero point, which is located at the center of the time step window. Voltage vector The magnitude and consequently vector length remain constant in an ideal power grid, where there is no impedance or voltage drop on the lines. In the space vector representation, at time t = 13 ms, it can also be seen that the initial current flow causes the current vector... The current vector initially has only a small magnitude. This can be seen from the spatial vector representation for subsequent time steps. Its length continues to increase while its direction remains unchanged; this is typical for two-phase triggering. At time t = 16 ms, the current vector... It has reached its maximum value, and from this point onwards, its length decreases in each time step until the current flow ends at time point t = 21 ms.

[0055] This example illustrates how the relationship between different physical parameters (here: voltage and current) can be visually represented using spatial vector representations.

[0056] Basic - Two-phase triggering

[0057] In the case of a soft starter, different triggering methods can be used, resulting in different motor characteristics. Triggering can be performed either on two-phase ground (phases a&b, b&c, c&a) or three-phase ground (phases a&b&c). For example... Figure 4 As already shown, two-phase triggering results in a current vector that is fixed in its orientation and pulsates in its amplitude.

[0058] If the thyristors in two of the three grid phases a and b are triggered, then these phases a and b will conduct current, while the thyristor in the third phase c will remain current blocked. Since the sum of the currents I in all three phases a, b, and c must always be zero, the current flowing to the motor in one of the conducting phases a and b must be the same as the current flowing out of the motor in the other conducting phase; therefore, the current amplitudes in the two conducting phases a and b are exactly the same, only with opposite signs.

[0059] Figure 5 This involves two-phase triggering, phases a and b. Figure 5 In the left-hand diagram, within the fixed α-β coordinate system, it is clear that this condition leads to [condition] along path I. c =0 pulsating current vector Wherein, current vector Although its length changes, its direction remains unchanged. Therefore, only the current vector... absolute value | i a +i b|Changes occur. Therefore, in the case of two-phase triggering, a fixed path 7 is generated, on which a pulsating current space vector can be generated; in Figure 5 In the diagram on the left, these paths 7 are associated with I. a =0, I b =0 and I c =0 indicates that path I a =0 extends along the β axis, and path I b =0 forms a 30° angle with the α axis; each of paths 7 forms a 60° angle with the next adjacent path 7. Conversely, in the case of three-phase triggering, the current also forms a rotating space vector (similar to the case of voltage). Figure 5 The diagram on the right shows the current amplitude i in the two conducting phases a and b at time t. a i b and the current vector generated by two-phase triggering absolute value | i a |+|i b |

[0060] Basics - Electric Motor Modeling and Rotation Operation

[0061] This invention is generally applicable to three-phase motors connected to a soft starter. Here, the three-phase motor can be configured as an asynchronous motor, a synchronous motor, or a self-starting permanent magnet motor (PM-Line-Start-Motor).

[0062] The modeling of asynchronous motors has been elaborated in detail in European patent application EP20176295.2, mentioned in the introduction of this specification, and in Nannen et al. All symbol definitions made there are also used in this specification. The motor model equations described in these publications (assumed here) are:

[0063]

[0064]

[0065] Subscript index 1 identifies stator-related parameters, and subscript index 2 identifies rotor-related parameters. Superscript index S indicates that the parameter is related to the stator fixed coordinate system. The motor model uses stator current. Blondel's coefficient (leakage coefficient), inductances L1 and L2, voltage Resistors R1, R2, and linked rotor flux Main inductor L H (English: mutual inductance), rotor speed Ω L and motor torque M M.

[0066] In European patent application EP20176295.2 and in Nannen et al., a coordinate transformation is introduced here, which rotates the coordinates in the α-β coordinate system by a rotation angle.

[0067]

[0068] Here, rotation angle The rotor flux vector is selected such that, after rotation is complete, the rotor flux vector... The positive direction of the d-axis in the dq coordinate system is indicated. The value d describes the magnetic flux density of the excitation in the rotor, and q is an expression for the torque generated by the rotor. For clarity: all vectors have been rotated by an angle in the α-β coordinate system. Correspondingly, for the current, we obtained:

[0069]

[0070] This rotational operation forms the basis of the entire theory of field-oriented control of the motor on the converter, and is implemented here so that the process to be considered can be more easily understood. In this regard, rotor flux orientation is discussed. In equations (3) and (4), the index K indicates that the indexed parameter is related to any coordinate system.

[0071] Figure 6 The rotation operation is explained: In Figure 6 The left side shows the result after rotating by a certain angle. Previous magnetic flux vector The position indicates that the magnetic flux vector has been rotated by a rotation angle. As referenced above Figure 5 The two-phase triggering described herein results in a current vector with a fixed direction but pulsating amplitude. These fixed directions of the current vector obtained under the three possible two-phase triggering combinations a&b, b&c, and a&c are... Figure 6 The fixed directions 7 are drawn with dashed lines; in this specification, these fixed directions 7 are also referred to as paths, in which current vector pulsations generated by two-phase triggering occur.

[0072] Through this rotation operation 6, the magnetic flux vector And path 7 was brought to Figure 6 The position shown on the right: magnetic flux vector The direction pointing towards the d-axis.

[0073] In the dq coordinate system, a first region 8 with a trigger for positive torque and a second region 9 with a trigger for negative torque can be distinguished. The first region 8 includes a half-space with a positive q value delineated by a dotted line, and the second region 9 includes a half-space with a negative q value delineated by a dashed line. The first region 8 and the second region 9 are separated from each other along the d-axis.

[0074] The diagram obtained through rotation operation 6 allows for the division of the current into a magnetic field current corresponding to the current vector component along the d-axis and a torque current corresponding to the current vector component along the q-axis. In this diagram, the dq coordinate system is aligned with the rotor flux vector at each time point. The entire rotor magnetic flux is located in the d direction.

[0075] One of the greatest advantages of representation in the dq coordinate system is that it is applicable to motor torque M. M Equation (2) is greatly simplified and becomes a simple multiplication:

[0076]

[0077] Ψ 2d I is the d-component of the linked rotor flux in the dq coordinate system. 1q It is the q component of the stator current in the dq coordinate system: only the components of the rotor flux and the stator current that are perpendicular to each other contribute to the motor torque.

[0078] Fundamentals - Converting the classic method with firing angles into vector representation

[0079] The commonly used methods for controlling soft starters in modern industry are based on the firing angle α. This firing angle can be related to the grid voltage position or the phase current. Commonly used algorithms are used as the starting point for deriving the firing algorithm according to the present invention, which requires relatively little computational power. Figure 7 The conventional definition of the firing angle α is explained, which determines the firing time relative to the mains voltage, specifically relative to the zero-crossing 10° of the mains voltage. Therefore, the firing of the thyristor is always at a fixed distance α from the immediate preceding zero-crossing 10° of the mains voltage. Figure 7 The current pulse i(t) caused by the corresponding trigger is also shown in dashed lines; the sign of the current pulse i(t) corresponds to the sign of the voltage u(t) at the corresponding trigger time point.

[0080] This traditional definition of the firing angle α makes it difficult to integrate more criteria for firing decisions: because the precise firing time that must occur is determined, additional criteria can only affect whether firing occurs or not. Since this restrictive definition does not allow for the inclusion of other criteria (e.g., rotor flux as an additional firing criterion besides grid angle / grid voltage zero crossing), this definition is therefore... Figure 8 The following modification is shown: Instead of a specific trigger time point (trigger angle α), a corner region (the so-called trigger angle region 11) is now defined, and the trigger time point should be located within this trigger angle region 11. The trigger angle region 11 is also called a trigger sector or trigger window. The trigger time point can be selected from the value region [α]. min α max The value range is selected from the lower boundary value α. min Minimum trigger angle and upper boundary value α max The trigger angle is used to define the trigger point. By introducing the trigger angle region 11, more alternative options are provided for selecting the trigger time point, as not only the time point but also the angle region and the resulting time period are provided for triggering. Compared to the traditional trigger angle α reference point "voltage zero crossing," the reference point of the trigger window is shifted by 90 degrees. Of course, the width of the trigger window 11 is zero (α). min =α max The design scheme is also possible, that is, it is possible to have only a single trigger point.

[0081] Due to the introduction of a trigger window, not only single current pulses (such as...) Figure 7 It is possible to (as shown by i(t)) and a set of current pulses 120 (such as...) Figure 8 (As shown by the dashed line) is also possible. For example... Figure 8 As shown, when choosing the minimum angle α min In this case, a current pulse 120 with the maximum possible current amplitude is obtained. The larger the firing angle α is chosen, that is, the closer the firing angle is to the upper boundary value α... max The amplitude of the generated current pulse 120 becomes smaller; when selecting the maximum angle α max In this case, a current pulse 120 with the minimum possible current amplitude is obtained. This relationship can be explained by essentially treating the asynchronous motor as a load: very simply, an asynchronous motor can be represented as a series connection of an inductor and a resistor; that is, an asynchronous motor always has an ohmic-inductive effect. This causes the current flowing due to the applied voltage to always lag behind that voltage in time. To generate a control pulse, this means that triggering must always be implemented before the relevant voltage crosses zero by 10. The closer the triggering occurs to the voltage crossing zero by 10, the larger the current generated.

[0082] By selecting the upper boundary value α of the trigger angle region 11 max This ensures that even if the voltage zero crossing detection fails to accurately identify the zero crossing, no unwanted trigger pulses will be generated; such unwanted trigger pulses could damage the soft-start motor system due to their very high current amplitude. Figure 9 The example illustrates the potential consequences if the zero-crossing of the grid voltage is not correctly identified: the amplitude of the sinusoidal grid voltage u(t) is plotted over time t. Thyristor triggering is performed at a fixed angular distance α to the zero-crossing 10 of the grid voltage u(t). Due to high harmonic loads or distortions in the region before and after the zero-crossing 10, a non-sinusoidal voltage region 51 may occur. Because of this non-sinusoidal voltage region 51, the theoretical zero-crossing 10' of the ideal grid voltage u'(t) is delayed by a delay period 52; correspondingly, the measurement of the trigger angle α does not begin at the theoretical zero-crossing 10', but at the actual measured zero-crossing 10. Therefore, due to the delay of the final voltage zero-crossing 10, subsequent triggering Z does not occur before the updated voltage zero-crossing 10, but after it: thus, a very small control angle is set instead of a very large control angle, and thus a second current pulse i2(t) is generated, with an amplitude several times larger than the previous first current pulse i1(t). This unfavorable scenario can be mitigated by correspondingly selecting the upper boundary value α of the trigger angle region 11. max This is used to exclude trigger pulses that result in an acceptable current amplitude, even if there is uncertainty about the grid voltage crossing zero in the system.

[0083] Flux-oriented starting algorithm for induction motors used in soft-starter control

[0084] The above foundations and relationships form the basis for explaining this invention. From equation (5), the following conclusion can be drawn: if the stator current I... 1q A positive value produces positive torque. A negative value produces negative torque. This can be used to assess the likelihood of triggering.

[0085] For further derivation, and for simplicity, let's now assume the existence of a d-component Ψ so large that it produces torque. 2d The rotor flux Ψ2 (see Equation 5) provides torque that can put the motor into measurable rotational motion, but the rotor does not rotate. Furthermore, it is known how the theoretically possible two-phase triggering paths 7 extend in the α-β coordinate system. Therefore, it can be precisely determined which triggering method can produce positive torque (see Equation 5). Figure 10 The half-space outlined by the dashed line (8), and which trigger can be used to generate negative torque (see...). Figure 10 The half-space outlined by the midpoint line (9).

[0086] However, the situation is that the power grid is described in space vector representation by a voltage vector rotating at the grid voltage angular frequency. Therefore, not all triggering possibilities are possible; only those within the theoretically possible region are feasible. The theoretically possible region in space vector representation is a half-space around the voltage vector from +90° to -90°. For illustration, see [link to relevant documentation]. Figure 11 :in Figure 10 The spatial vector diagram complements the exemplary voltage vector. And the possible trigger is located in area 13. Figure 11 In, with Figure 10 In contrast, region 13 has been added, which contains the possible triggering scenarios (paths) based on the location of the power grid.

[0087] By including voltage vector This shows all physically possible triggering possibilities at this point in time, specifically those located in region 13. However, it is unclear how large the maximum amplitude of the resulting current becomes. To define the degree of constraint for the maximum current here, a definition of the triggering angle α, close to that in conventional methods, is used. However, unlike defining a fixed triggering angle α in conventional methods, as already combined... Figure 8 As described above, the minimum firing angle α is now determined. min and maximum trigger angle α max The firing angle region 11, which is related to the voltage vector It is in a fixed angular relationship and moves together with the voltage vector at an angular velocity ω. U =2πf grid Rotation, the angular velocity is related to the grid frequency f grid Directly proportional (see) Figure 12 For the purpose of explanation: In Figure 12 In this context, the trigger probability will be those whose paths lie within the trigger angle region 11, that is, only those paths i. b =-i c The corresponding triggers are phase b and phase c.

[0088] The allowed triggering area is limited to the minimum and maximum trigger angles α. min and α max After limiting the firing angle region 11, the torque region should also be limited. If the purpose is to accelerate the motor, all firing possibilities in the first region 8 are attractive because they produce positive torque, while all possibilities in the second region 9 are unattractive because they produce negative torque.

[0089] In addition to torque standards, it is also essential to ensure sufficient magnetic flux remains in the motor after triggering during three-phase motor operation. For example, if along... Figure 11 path i a =-i c Triggering the current vector will generate positive torque because the trigger is located in half-space 8, where a positive torque +M can be generated. M However, along path i a =-i c Pulsating current vector It will be largely related to the magnetic flux vector The opposite direction will result in a significant reduction in magnetic flux in the rotor. For simplicity, this consideration is based on a single, specific trigger; in fact, statistically, it is the superposition of multiple individual triggers that leads to the reduction in magnetic flux.

[0090] Figure 14 It is shown where the magnetic flux will increase and where it will decrease, neglecting rotor resistance R2 and iron loss: there is region 14 surrounded by dashed lines, where the triggering probability of rotor flux linkage is strengthened; and there is region 15 surrounded by dotted lines, where the triggering probability of rotor flux linkage is weakened.

[0091] Figure 15 Will Figure 10 and Figure 14 The information is summarized in a diagram: Regions 8 and 9 (where positive torque +M is generated) M Or negative torque -M M ) as well as regions 14 and 15 (where the magnetic flux increases) Or the magnetic flux decreases. They overlap with each other, resulting in the four distinct sectors 16 to 19 shown. Figure 15 Regions 8, 9, 14, 15, 16, 17, 18, and 19 depicted in the diagram are located in relation to the rotor flux vector. A fixed angular relationship, and together with it, an angular velocity ω Ψ =2πnp rotation, this angular velocity is directly proportional to the mechanical rotor speed n and the number of pole pairs p of the three-phase motor.

[0092] If the purpose of the soft starter is to accelerate the motor, the selected current vector should be located in the sector with positive torque, i.e., in sectors 16 and 19. This is because the rotor flux decreases continuously due to the rotor resistance R2; if the flux generated by triggering is on average greater than the lost flux, then it is usable. Figure 16 The flux and torque angle region 12 is shown, also known for simplicity as the flux window, which takes into account the characteristic of the rotor flux vector. The relevant flux and torque angle region 12 is determined by the rotor flux vector. Measured angle γ start and γ end To limit. The flux and torque angular region 12 is based on Figure 15 The relationship is determined by combining the following preset examples: statistically, the magnetic flux generated by triggering should, on average, be greater than the magnetic flux lost.

[0093] Therefore, the defined flux and torque angle region 12 includes sectors with positive torque and sectors with increased flux or only a slight decrease in flux. The two defined regions, namely the trigger angle region 11 and the flux and torque angle region 12, should be combined into a single decision criterion. Therefore, if triggering probability 7 exists both within the trigger angle region 11 and within the flux and torque angle region 12, then triggering should be performed. Figure 17 In the example shown, this is not the case for any triggering possibility; therefore, triggering should not be performed. Now, a certain amount of time can be waited for the voltage vector to... Continue rotating. For example, after approximately 5 ms, this corresponds to a 90° rotation in the spatial vector diagram with an angular frequency ω = 2π / (20ms) AC voltage, see as... Figure 18 In the case of voltage vector It has already rotated another 90°, and conversely, the rotor flux vector Its amplitude and direction remain unchanged (the motor is stationary, with no iron loss or rotor loss). Now, path 7 of the trigger combination b&c is located in both sector 11 and sector 12. Therefore, this trigger combination b&c can be executed to generate a positive torque +M. M To increase rotor magnetic flux to a certain extent And it does not exceed the maximum current amplitude.

[0094] The method shown here can be repeatedly executed with a fixed step size. It is triggered whenever one of the trigger combinations a&b, b&c, or c&a is located within sectors 11 and 12. Therefore, a simple standard check is sufficient to generate the control pulse.

[0095] When triggering is executed, it is possible that while the current is still flowing, another triggering possibility arises, in which the still-blocking third thyristor pair participates in this additional triggering possibility. In this case, the trigger will also be executed immediately. Therefore, this will result in three-phase triggering a&b&c, which produces a rotating current vector compared to two-phase triggering. Figures 19 to 24 The text describes this three-phase triggering scenario. t = 3.4 ms Figure 23 And t=3.9ms Figure 24 It is shown that when the current vector When the next triggering possibility arises during the motion, the current vector is "locked" at that position. If the relevant phase current is considered, this is the point where the thyristor blocks in one of the phases due to the current crossing to zero.

[0096] Rotary electric motor

[0097] All considerations so far have been based on the simplified assumption that the rotor is stationary. However, this assumption no longer holds once the rotor is in motion, after the first triggering. Therefore, the motor speed must be considered. Fixed path of the current vector 7 (see...) Figure 5 The α-β diagram in the figure is unaffected by the rotational speed because it depends only on the non-rotating stator. The fundamental relationship of the firing angle also remains unchanged. However, during current flow, the rotor flux vector... and stator current vector The position changes in the spatial vector representation. More precisely: not only spatial vectors and The position changes, and the angular position of these two space vectors relative to each other also changes. This deviation leads to a deviation in the assessment of the expected torque and the effect on the rotor flux. This deviation occurs in... Figures 25 to 28 This is illustrated by example.

[0098] Figures 25 to 28 Applicable to n = 250min -1 (Rated speed 1470 min) -1 The mechanical rotation speed of the rotor triggers the voltage vector. With the power grid frequency f grid Directly proportional angular velocity ω U =2πf grid Rotation, magnetic flux vector An angular velocity ω that is directly proportional to the number of pole pairs p and the rotational speed n of the mechanical rotor. Ψ =2πnp rotation, where the two angular velocities do not coincide; conversely, the current vector axis 7 used for two-phase triggering, i.e., the so-called path, points in a fixed direction. Figures 25 to 28 As can be seen, due to the rotation of the rotor, in addition to the voltage vector... In addition to rotation, magnetic flux vector It also rotates. Correspondingly, the current vector... The fixed relationship between the start of triggering no longer applies to the entire duration of current flow, but rather varies with time. This results in considerations of the development of positive torque and rotor flux being based on a different foundation than what actually occurs during triggering. To compensate for this effect, the initially determined region for possible triggering is rotated by a rotation angle δ. rot The purpose of this rotation is to compensate for the deviation caused by the rotating rotor as optimally as possible.

[0099] With respect to rotation angle δ rot The determination of can be as follows, where other systematic determinations are also conceivable: It is assumed that the time-averaged condition of the current pulse approximately represents the average value of the ratio over the entire current pulse, which can be used as a reference. For example... Figure 30 As shown, now based on the last current pulse, the time interval t between the trigger time and the maximum current value is determined. delay Now we can make a good approximation by assuming that the time interval t delay The situation under evaluation is the same as that of the previous trigger pulse. In other words: if in the previous trigger pulse, the time interval between the trigger and the maximum current was equal to t... delay If so, then they will undoubtedly be very similar or identical in subsequent trigger pulses. Correspondingly, such as Figure 30 As shown, the flux and torque angle region 12 can be pre-rotated with respect to the time interval t. delay The corresponding rotation angle δ rot This is to minimize the influence of the rotating rotor on the triggering decision.

[0100] The rotation angle δ can be determined in this method using the following equation (6). rot , where Ω L Rotor speed:

[0101]

[0102] If the rotation angle δ rot If determined in the manner described, most of the changes that occur due to the rotating rotor can be compensated.

[0103] Influencing characteristics by affecting the position of the switching boundary.

[0104] To date, the determination of the two trigger regions is quite vague, and the rationale for the basic approach is explained through theoretical considerations. Conversely, in practical applications, a systematic approach is needed to determine these values ​​so that the actual operating characteristics are as consistent as possible with the desired operating characteristics. Therefore, the boundaries must be systematically defined. Figure 32 The boundary α is shown in the figure. min α max γ start γ end The following systematically elaborates on its impact on the method.

[0105] upper boundary α of the trigger angle region max :

[0106] This boundary is of decisive importance; it indirectly defines the shortest current flow through the trigger. This angle α maxIt should be selected to ensure that even in power grids where zero-crossing determination is difficult (due to high harmonic loads or distortions in the regions before and after the zero-crossing), the triggering always behaves as follows: the current flows in the desired direction, and there is no undesirable triggering with excessively high current amplitude.

[0107] Lower boundary α of the trigger angle region min :

[0108] This boundary defines the trigger point where the current flow reaches its maximum length and amplitude. This angle α... min The firing angle α can be predetermined, for example, via a characteristic curve, similar to the firing angle α in classical control methods, or it can be adjusted considering the current amplitude. It is also conceivable to consider the current flux amplitude to limit the maximum possible torque. This is achieved by using the angle α... min It can control both current and torque; the effect of this angle is similar to that of the trigger angle α in traditional control methods. α min Values ​​can be taken from the following defined regions: [-90°, a max ], its relation to voltage vector To measure.

[0109] For the boundary γ of negative torque start :

[0110] This boundary defines the limit for negative torque. Based on the angle γ... start The choice of current, for example, allows for a slightly negative torque to facilitate the formation of a strong rotor flux.

[0111] Magnetic flux regulation boundary γ end :

[0112] This angle γ end It was determined how long the trigger would last to generate positive torque +M. M Depending on how this boundary is determined, triggering can either be performed for an extended period, thereby reducing rotor flux linkage, or triggering can be performed only if the average rotor flux linkage does not decrease. This is achieved using the angle γ. end The determination of this parameter actively determines how much rotor flux remains in the rotor and exists in the next triggering phase. This parameter can be used for "flux guidance" or "flux regulation" during startup. This is achieved using the angle γ. end This can affect the average absolute rotor flux. If the motor should be accelerated, then the difference γ end -γ start Values ​​can be taken from the following defined region: [90°, 180°], which have relation to the magnetic flux vector. To measure.

[0113] Exemplary adjustment structure for influencing switching boundaries

[0114] As explained in the previous chapter "Affecting Characteristics by Influencing the Position of the Switching Boundary", the angle α of the trigger window or flux window is defined. min α max γ start γ end It can be used to affect startup.

[0115] Controlled operation is also possible using this invention. The representation of the above relationships so far is based on the assumption that the goal is to accelerate the rotor shaft; for this, a positive torque needs to be generated. The same argument can also be applied to braking situations, except that the window for possible triggering is arranged in the region for negative torque.

[0116] Method transfer – combining with traditional methods

[0117] The method described herein has advantages over traditional control methods based solely on the firing angle α, but it also presumably has some drawbacks. Therefore, besides being specifically used to determine the control signal, this invention can also be used to supplement existing methods. It is possible that this method is more suitable for specific speed ranges than other methods. Therefore, the new method presented herein can also be combined with known methods during startup. In a preferred design, startup can begin with the new method; subsequently, during startup, a switch to traditional control is determined based on quality criteria (e.g., speed) to combine the advantages of both methods.

[0118] Based on rotor flux

[0119] All the evaluations were based on the use of linked rotor flux. However, compared to the stator current, the linked rotor flux... In industrial environments, direct measurement using corresponding sensors is not possible. The simplest and most likely way to determine rotor flux linkage is by using the modeling described above (see equation (16) in Nannen et al.):

[0120]

[0121] Therefore, stator current is required. and rotor speed Ω L Stator current It can be easily measured: measure the currents of two or three phases, and then calculate the current vector using the Clarke transform. Mechanical speed can theoretically be acquired via suitable measurement techniques. However, this is not typical in industrial environments where soft starters are used, as the cost and overhead involved in installation make their use uneconomical. Therefore, determining the speed without additional sensors seems highly advantageous. Several model-based possibilities exist for determining speed, models developed over the past few decades for sensorless control of three-phase motors. An overview is provided in the following monograph: Vas, Peter: Sensorless Vector and Direct Torque Control, Monographs on Electrical and Electronic Engineering, Volume 42, Oxford: Oxford University Press, 1998, ISBN-13: 978-0198564652. Modeling alternatives to rotor flux is also possible.

[0122] Figure 33 A flowchart illustrating one embodiment of the method according to the present invention is shown. In the first step S1, a grid-related triggering criterion and a rotor-related triggering criterion are defined. If the triggering probability is within the triggering angle region 11, the grid-related triggering criterion is satisfied, the triggering angle region 11 having a relationship with respect to the voltage vector. The lower boundary α of the trigger angle region min and the upper boundary α of the trigger angle region max If the triggering probability lies within the flux and torque angle region 12, then the rotor-related triggering criterion is met, and the region boundary value γ of the flux and torque angle region 12 is [value missing]. start and γ end The rotor flux is defined as follows: In the first step S1, the thyristor is also triggered to rotate the rotor, so that the rotor flux is not equal to zero.

[0123] In the second step S2 following the first step S1, it is checked whether the grid-related triggering criteria are met. If the triggering probabilities of two or more grid phases a, b, and c are located within the triggering angle region 11, then the grid-related triggering criteria are met.

[0124] In the third step S3 following the second step S2, it is checked whether the rotor flux-related triggering criteria are met. If the triggering probability of two or more grid phases a, b, c is within the flux and torque angle region 12, then the rotor flux-related triggering criteria are met.

[0125] In step S4, following step S3, if both the grid-related triggering criteria and the rotor flux-related triggering criteria are met, then the corresponding thyristors are triggered when there is a triggering probability in two or more grid phases a, b, and c. After step S4, the process returns to step S2, and steps S2 to S4 are repeated from there.

[0126] Soft starter topology

[0127] The basis for all considerations is Figure 1 The topology shown is illustrated. In principle, it is also conceivable to use this method for each additional thyristor-based topology.

[0128] Possible structures for generating control pulses according to the present invention are as follows: Figure 34 As shown, in the first block of the soft starter 100, the control unit 61 receives rated values ​​65 (e.g., desired motor torque, maximum rated current, or speed ramp) and measured values ​​64 (e.g., motor stator current I1, stator voltage U1, torque M, and / or rotor speed n). Based on the received rated values ​​65 and the received measured values ​​64, the control unit 61 sets one or more of the setting values ​​68, namely α. min ,、α max γ start γ end This allows the set values ​​68 to be achieved. These setting values ​​68 are transmitted to the second block of the soft starter 100, namely the computing unit 62, which has a processor and a storage unit. The processor executes a computer program stored in the storage unit, which contains an algorithm for performing the method according to the invention. The computing unit 62 generates a control signal 67 based on the obtained setting values ​​68.

[0129] The third block 63 of the soft starter 100 includes thyristors and a triggering device, such as a trigger signal unit. A control signal 67 received from the computing unit 62 causes the trigger signal unit to generate a trigger signal for one or more of the thyristors, thereby turning on one or more thyristors.

Claims

1. A method for operating a three-phase motor (4) on a multiphase power grid (5) by means of a soft starter, wherein the soft starter is capable of switching one or more power grid phases (a, b, c) of the power grid (5) by triggering thyristors (2). in, In addition to power grid-related triggering standards, rotor flux-related triggering standards are also considered. Among them, the triggering determination for the triggering probability of using one or more thyristors (2) must meet both the power grid-related triggering criteria and the rotor flux-related triggering criteria. In this process, two-phase or three-phase triggering is performed. In space vector representation, two-phase triggering results in a current vector that is fixed in orientation and pulsates in amplitude, while three-phase triggering results in a rotating current vector. Wherein, if the current vectors representing the triggering probabilities of two or more grid phases (a, b, c) in the spatial vector representation are located within the triggering angle region (11), then the grid-related triggering criterion is satisfied, wherein the triggering angle region (11) is defined by the lower boundary of the triggering angle region related to the grid voltage and / or grid current ( ) and the upper boundary of the trigger angle region ( To define, and Wherein, if the current vectors of the triggering probability of two or more grid phases (a, b, c) in the space vector representation are located within the flux and torque angle region (12), then the rotor flux-related triggering criterion is satisfied, and the region boundary value of the flux and torque angle region (12) is ( The definition is based on rotor flux.

2. The method according to claim 1, wherein, According to the rotor magnetic flux ( ), grid voltage ( ) and stator current ( The spatial vector representation is used to check the existence of the triggering criteria.

3. The method according to claim 1, wherein, The flux and torque angular region (12) was shifted by a rotation angle ( This is to compensate for the time delay between the trigger decision and the triggering.

4. The method according to any one of claims 1 to 3, wherein, The three-phase motor is an asynchronous motor, a synchronous motor, or a self-starting permanent magnet motor.

5. The method according to any one of claims 1 to 3, wherein, Before checking the triggering criteria related to the power grid and the triggering criteria related to the rotor flux, a period of flux increase is set during which the rotor flux is generated by the triggering of the thyristor.

6. The method according to any one of claims 1 to 3, wherein, Only the triggering probability of the three-phase motor (4) that results in negative torque and thus active braking is determined.

7. A soft starter adapted to perform the steps of the method according to any one of claims 1 to 6.

8. A computer program product comprising commands that cause a soft starter according to claim 7 to perform the method steps according to any one of claims 1 to 6.

9. A computer-readable medium on which a computer program product according to claim 8 is stored.

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