Method for starting speed-regulated electric motor of centrifugal pump

Abstract

The invention relates to a method for starting a speed-controlled electric motor of a centrifugal pump, said centrifugal pump having at least one impeller driven by the electric motor, said impeller rotating at a positive rotational speed in normal pump operation in order to supply a delivery medium from the suction side to the pressure side of the centrifugal pump in a defined manner. The rotational speed control of the motor operates in a sensorless manner without means for determining the position of the rotor, characterized in that whether and in which direction the impeller is rotating as a result of external flow is detected on the basis of a current measurement prior to the start-up of the centrifugal pump, and wherein the rotational speed control of the motor operates in a sensorless manner without means for determining the position of the rotor. When a positive rotational speed due to the external flow is detected, the motor is actuated at a rotational speed target value of zero depending on the rotational speed ramp, and when a negative rotational speed due to the external flow is detected, the motor is actuated at a negative rotational speed target value depending on the rotational speed ramp.

Description

Technical Field

[0001] The present invention relates to a method for starting a speed-regulated electric motor for a centrifugal pump, wherein the centrifugal pump has at least one impeller driven by the electric motor, the impeller rotating at a positive speed during conventional pump operation in order to deliver the transport medium from the suction side to the pressure side of the centrifugal pump as prescribed, and wherein the speed regulation of the motor operates in a sensorless manner without the need for a device for determining the position of the rotor. Background Technology

[0002] If a centrifugal pump is shut down and is subjected to external medium flow, the pump impeller and thus the rotor can be put into rotation by the external flow. This is precisely what can happen in hydraulic systems with multiple pumps. If the medium flows from the suction side to the pressure side, it forces the impeller to rotate at a positive speed. Instead, if a backflow occurs from the pressure side to the suction side, the impeller rotates at a negative speed.

[0003] In permanent magnet synchronous motors, there are speed regulation methods, such as field-oriented regulation, which require knowledge of the current rotor position. However, for cost and maintenance reasons, dedicated absolute sensors for position and speed measurement are often omitted. If the rotor is put into rotation by an external current flow while it is stationary, directly turning on the motor using a sensorless regulation method based on electromotive force is not a simple and feasible approach.

[0004] Therefore, in the past, the corresponding pump regulation was set so that when the external flow and the accompanying externally excited motor rotation were determined, the stator or stator windings were switched to a short circuit. This fully utilizes the generated phase resistance, which is used to convert excess electrical energy into heat during the copper loss process. This released energy is removed from the hydraulic system, thereby generating a certain braking torque on the pump, and thus braking the impeller to a stop. The pump can then be started normally from a stopped state.

[0005] However, a drawback of this solution is that the generated short-circuit current is unregulated and can become dangerously high in certain situations. This can lead to thermal damage to the motor and, similarly, irreversible demagnetization of the permanent magnets mounted in the rotor. Another disadvantage is that the braking effect achievable through short-circuit switching is highly dependent on the machine's characteristics and therefore varies in effectiveness depending on the machine.

[0006] Therefore, it is necessary to find an alternative mode of behavior to overcome the above problems. Summary of the Invention

[0007] This task is solved by the method according to the features of claim 1. Advantageous embodiments are the subject of the dependent claims.

[0008] According to the present invention, for a corresponding centrifugal pump using sensorless speed regulation of a motor, external flow is detected based on current measurement. By means of current measurement, in addition to the basic determination of the external flow, the direction of the external flow or the rotational direction forced upon the motor should also be acquired. If external flow is detected, instead of implementing speed regulation as in conventional pump operation, the pump is started using a speed control unit based on the determined rotational direction for starting the motor. That is, instead of adjusting the motor speed considering the actual rotor speed, it is controlled based on a fixed preset theoretical speed ramp without feedback from the actual speed or actual rotor position. Conventional pump operation is understood as the pump's active operation at a positive speed to deliver the medium from the pump's suction side to the pressure side as specified.

[0009] The type of speed ramp, especially its initial and / or final values, is selected based on the direction of the speed generated by the external flow, which is detected beforehand. If a positive speed is determined, the motor speed for the speed control unit is preset using a speed ramp that initially starts from zero and increases linearly, particularly within the positive speed range. Conversely, if a negative speed direction is determined, the motor is controlled using a speed ramp that starts in the negative speed range and increases linearly into the positive speed range.

[0010] Using the method according to the invention, the rotor field of the rotor rotating due to external current flow, caused by rotor excitation, especially permanent magnet excitation, should be surpassed by the stator field caused by the stator windings, wherein the stator field is generated by manipulation with a corresponding speed ramp. When the speed difference between the two fields is sufficiently small, the rotor enters synchronization with the stator field, and the motor can continue to operate in a speed-regulated manner as usual.

[0011] This method ensures a successful motor start-up process even without the specific location of the rotor being captured.

[0012] Advantageously, as a current measurement, the short-circuit current generated by the external current flow in at least one motor winding of the stator is measured. The switching from motor winding to short circuit is achieved by the installed frequency converter, for example by closing the corresponding half-bridge of the integrated bridge circuit of the frequency converter.

[0013] However, during short-circuit switching, care must be taken to ensure that the short-circuit current does not become unacceptably high, which could damage the inverter's power switches or cause unacceptable overheating of the stator windings. Similarly, an unacceptably high short-circuit current could cause irreversible demagnetization of the rotor magnets. To avoid these dangers, a threshold is defined for the short-circuit current. If the short-circuit current exceeds the threshold, an external short-circuit current is detected, and all power switches of the inverter are latched. Due to the high intermediate loop voltage, the short-circuit current can be quickly commutated via the freewheeling diode.

[0014] The threshold is chosen to be small enough that damage to the hardware is unlikely even if it is exceeded for a short period. However, it is necessary to choose a threshold high enough to distinguish external current flow from common measurement noise. The appropriate threshold can be determined, for example, through one or more training measurements, especially after pump commissioning, where, for example, a case without external current flow is selected, and short-circuit current is collected to evaluate measurement noise.

[0015] Particularly advantageous is that the rotational speed is determined, in addition to the direction and presence of the externally generated rotation, based on the performed current measurement, especially the short-circuit current measurement. Rotational speed measurement or estimation is achieved, for example, by performing two or more measurements of the short-circuit current at different time points. By specifically determining the time difference of the short-circuit current acquired at different time points and the phase position of the measured current, the frequency can be estimated, and consequently, the rotational speed. Therefore, the method for short-circuit current measurement is set to be repeated at least once to obtain the short-circuit current at two different time points. However, preferably, the method is repeated at least twice, ideally at least three times, for greater robustness to relative measurement noise. The duration of time exceeding a threshold is always acquired and stored during measurement to determine the time difference exceeding the time point. For example, in a triple repetition of the short-circuit current measurement, three phase currents at four different time points are available. A transformation to a complex space vector representation is performed using the known Clarke transform, in which the electrical frequency and thus three different rotational speeds can be derived from the phase positions of the four space vectors and, given the time differences. The estimated rotational speed of the incoming flow is then calculated from the derived rotational speed values ​​by using the average or median.

[0016] As stated above, between the successive short-circuit current measurements, all power switches of the bridge circuit are locked out, allowing the short-circuit current to be commutated via the bridge's freewheeling diodes.

[0017] Upon detection of external flow, the centrifugal pump motor is not regulated at its speed, but rather controlled only at its theoretical speed—that is, without feedback of the actual speed, whether measured, estimated, or otherwise determined. The theoretical speed preset for control is derived from a speed ramp, which in turn depends on the previously detected speed of the external excitation. This speed control is referred to as motor control / regulation in open loop, as current regulation continues. The current regulator receives the theoretical value of the current space vector in terms of magnitude and phase position, along with the measured motor current, as input parameters. The phase position is provided through continuous integration of the speed ramp. The magnitude of the current space vector to be regulated is chosen to be constant, but defined sufficiently high to increase the likelihood of successfully capturing the motor if necessary, through the accompanying high torque.

[0018] If the rotor is successfully captured, the open-loop control / regulation can be switched to closed-loop regulation, which implements true speed control. Based on sensorless speed regulation, the speed or rotor position is estimated in a known manner by means of an observer, taking into account the current actual motor current and theoretical voltage. The active speed regulator determines the corresponding theoretical current for the current regulator based on the speed deviation.

[0019] According to a preferred embodiment, upon detecting a negative speed generated by an external current flow, the motor is controlled using a speed ramp, the initial negative starting speed of which is equal to or greater than the speed value estimated by current measurement as generated by the external current flow. A value 5%-15% higher in magnitude can be envisioned. Similarly, it is envisioned that the motor be started at the maximum possible negative speed upon detection of a negative speed.

[0020] To reduce the time it takes for the method to capture the rotor, it may be sufficient to choose an initial starting speed of the speed ramp used that is less than the negative speed caused by the external flow. A speed value that is approximately 5%–15% smaller than the negative speed caused by the external flow can be envisioned.

[0021] Furthermore, defining the slope of the speed ramp as sufficiently small may be appropriate. This increases the time window for capturing, i.e., for the rotor and stator fields to synchronize, and increases the likelihood of the method successfully capturing the rotor. Although this necessitates accepting an increase in required energy consumption, this is acceptable given the improved reliability of the method.

[0022] It should also be noted that in some cases during the implementation of the method, power generation operation may occur, i.e., the motor is accelerated against the direction of rotor rotation caused by external current flow through the stator field. During power generation operation, energy is removed from the hydraulic system by recovery. If this energy is not otherwise consumed, it will charge the intermediate circuit capacitor. By reducing the ramp angle, the mechanical power generated in such a configuration, and therefore the energy generated by power generation, is kept low, so that the intermediate circuit capacitor is charged only in a limited manner and overcharging can be effectively eliminated. Also helpful is that during speed control in the open loop, the theoretical current, especially the theoretical current amplitude, is selected to be sufficiently high. Using the losses associated with it in the inverter and motor, electrical energy is additionally eliminated at the power generation operation point in order to, for example, prevent or reduce the charging of the intermediate circuit capacitor. By reducing the ramp angle and appropriately defining the theoretical current for speed control, overload of the intermediate circuit can be reliably avoided during the power generation operation point. At the same time, care must be taken not to select the theoretical current too high in order to avoid damage to the electronics and / or the motor, for example, due to overheating or demagnetization of the permanent magnets.

[0023] Furthermore, it is suggested that, in the case of a positive speed detected due to external flow, once the motor is accelerated to a speed higher than the previously detected positive speed due to external flow, the speed control should transition from open-loop control to closed-loop speed regulation, i.e., with real feedback of the actual speed and the estimated rotor position.

[0024] In addition to the method according to the invention, the invention also relates to a pump, particularly a centrifugal pump, especially a heating circulation pump, having at least one control module configured to perform the method according to the invention. This pump thus exhibits the same advantages and characteristics as those already demonstrated above according to the method according to the invention. For this reason, repeated descriptions are omitted. Attached Figure Description

[0025] Further advantages and features of the invention will be described in more detail below with reference to specific embodiments and the accompanying drawings. In the drawings: Figure 1 A block diagram for detecting external through-current is shown. Figure 2 A schematic diagram of a closed-loop control circuit without an absolute sensor for speed regulation is shown. Figure 3 The diagram illustrates the open-loop principle for speed control during pump startup after detecting external flow. Figure 4 The flowchart illustrates the process of starting the motor after detecting an incoming flow. Figure 5 The modified startup method is shown. Detailed Implementation

[0026] The method according to the invention will subsequently be described with reference to a centrifugal pump driven by a synchronous motor with permanent magnet excitation. The pump has a control unit that implements field-oriented speed regulation. Since the pump operates in a sensorless manner, i.e., the current position of the rotor is acquired without sensor sensors, a so-called observer is used to estimate the current speed and rotor position. The impeller's usual direction of rotation during pump operation is referred to herein as the direction of rotation with positive speed.

[0027] When a pump is stopped, the pump impeller can be rotated not only in the positive rotational direction (positive speed) but also in the negative rotational direction (negative speed) by an external flow. The objective of this invention is to reliably detect this external flow and to successfully start the pump in its presence. A method that is as reliable as possible and manages a minimum number of parameters is desired. The latter, given the large number of different pump motors, should minimize development costs while maximizing the reliability of the method. Therefore, this invention can be divided into two problem-solving objectives: detecting the external flow to the pump that causes impeller movement forced by an external force, and starting the motor in its presence.

[0028] Detecting incoming flow In the presence of an external through-current, the rotor is driven to rotate by the pump impeller through which the through-current flows. This induces a voltage in the stator windings. To detect the external through-current, the short-circuit current is used as a measurement parameter for the rotor speed and direction of rotation, with the pump stopped. The principle used for detection is... Figure 1 As shown in the chart.

[0029] Therefore, at time t=0, the stator windings are temporarily switched to a short circuit via the inverter's bridge circuit, especially in the case of a 3-phase motor via bridge B6 (box 10). If the rotor is rotating (e.g., due to external current flow), a short-circuit current is generated. It must be ensured here that: a. The short-circuit current is large enough to be detected accurately.

[0030] b. It should not become too large in order to prevent damage to the inverter's power switches or unacceptable temperature rise in the stator windings, or even to prevent currents that could cause irreversible demagnetization of the magnets.

[0031] To ensure this, the current I is defined as the sole parameter used to detect external cross-current. S Threshold I S,ef The threshold should be chosen such that... a. It is large enough to be clearly distinguished from measurement noise so that false identifications can be eliminated.

[0032] b. It is small enough that the hardware and / or motor will not be damaged.

[0033] Check in box 20 for the short-circuit current I. S Does the defined threshold I exceed the timeout (100ms in this case)? S,ef If this is the case, then the process proceeds from the presence of an external current flow and continues in box 30. If the motor remains without current instead, i.e., the threshold is not exceeded, then it can be deduced that the rotor is stationary (box 70) and the motor can be started normally from a stopped state (box 80).

[0034] Until the threshold I is exceeded in box 20 S,ef The required time t1 is stored. Subsequently, the inverter's pulse width modulation (box 30) is turned off, thereby locking all power switches (e.g., MOSFETs) in the inverter's bridge circuit and allowing the short-circuit current to quickly commutate via the freewheeling diode due to the high intermediate loop voltage. Additionally, the measured short-circuit current I is stored. S,k , where index k represents the number of the measurement performed.

[0035] The program, including short-circuit measurement (box 40), PWM deactivation, and measurement value storage (box 30), is repeated three times with a determined timing, resulting in a total of three phase currents at four different time points known. These can be converted into complex space vectors using the known Clarke transform (box 50). From the phase positions of the four space vectors, the electrical frequency can be calculated, and ultimately, the three rotational speeds can be calculated, given the time difference ΔT (box 50). While in principle two vectors and two measurements would suffice, four vectors are measured or calculated for greater robustness against measurement noise. The median value from the group of estimated rotational speeds is used as an estimate (box 50).

[0036] The rotational speed with a sign is obtained as follows: .

[0037] The implementation method also considers the angle of argument. Identification of zero crossings in the domain [0, 2π]

[0038] In accordance with Figure 1 After the procedure determines the external flow and the forced speed for the rotor, the motor can be accelerated in block 60 using a modified speed ramp that is coordinated with the estimated speed.

[0039] Start the motor in the presence of external through-flow (box 60) If an external flow is detected using the methods described above, the behavior for starting the motor is determined based on the estimated rotational speed and direction of rotation. By principle, a starting attempt is only made up to the maximum speed defined in the model.

[0040] The rotor position is not known at the start of the startup process. Although the phase position of the short-circuit current is determined in the method described above for detecting external through-current, it involves transient stabilizing currents and is not a stabilized short-circuit current existing without considering the phase resistance in the d-axis of the rotor orientation coordinate system. Furthermore, due to the computationally intensive atan2 calculations, the phase position is calculated in a slower task only after sampling. This means that the phase position of the current at the time it is calculated is outdated anyway and cannot be used for regulation. However, since it is possible to start from the steady-state operating point in the case of external through-current, it is possible to assume that the rotational speed remains substantially constant during a relatively short calculation period relative to the mechanical time constant.

[0041] To increase the pump's speed, the motor is regulated in an open-loop configuration. This means that while the motor current is adjusted, the speed is controlled. This difference is based on... Figure 2 A brief discussion is given, illustrating the speed regulation scheme in the closed loop during conventional pump operation. The speed regulator 104 depends on the theoretical speed n. Soll The theoretical value I for motor current is generated by the actual rotational speed n or the resulting adjustment deviation. Soll As an adjustment parameter, the current regulator 100 adjusts the motor to the theoretical value. I Soll The current regulator 100 is supplied with an estimated value of the rotor position φ̃ and a phase current I as input parameters for the actual value. ist And as an adjustment parameter, the output stator voltage U Soll The latter is then generated by the inverter 101 of the frequency converter to supply energy to the motor 102. Since the pump operates in a sensorless manner, the current actual speed n and rotor position φ̃ are obtained by means of the observer 90 using the input parameter actual current I. ist and theoretical voltage value U Soll The value is estimated based on the model. Therefore, the motor can operate efficiently because only adjustments are needed to achieve the desired speed n. Soll Required current. Theoretical voltage. U Soll This represents the most important input parameter. It depends proportionally to the rotational speed under good approximation, which is why the adjustment method with the observer 103 used works reliably at higher speeds, because the induced reverse voltage at this theoretical voltage value is dominant, and thus ensures a sufficiently good signal-to-noise ratio.

[0042] Upon detection of external flow, the pump now switches to speed control for startup, i.e., open-loop regulation. The principle is based on... Figure 3Explanation. Theoretical rotational speed n Soll Here, the current is preset by ramp 105 and adjusted by current regulator 100 to a fixed theoretical value I. Soll This theoretical value is chosen to be large enough to capture the rotor and prevent it from desynchronizing during further startup. The current, as in the closed loop, is referenced to the reference quantity φ̃ in terms of its phase position. Soll Adjusted, however, the phase position φ̃ is used here. Soll The phase position is determined by the theoretical value of the rotational speed n. Soll The phase position φ̃ is obtained by integration. Soll Initially, it did not correspond to the rotor position.

[0043] The starting point and gradient of ramp 105 are matched to the desired outcome: to utilize the stator field induced by the stator windings to surpass the rotor field induced by permanent magnet excitation, and to achieve synchronization when the speed difference between the two fields is sufficiently small. The amplitude of the current is adjusted here, and the share of the current in generating torque depends on the load point setting. The ramp gradient is measured to be relatively small to increase the likelihood of successful synchronization. The behavior is as follows: Figure 4 The principle is shown in the presentation.

[0044] To select the specific slope 105, first consider the following... Figure 1 The estimated rotational speed n in box 50 is used to differentiate cases 61. In the case of a positive rotational speed n due to external crossflow, ramp 105a is used for open-loop operation according to... Figure 3 It starts with the theoretical initial speed of zero. The speed ramp 105 moderately increases in the positive speed range. If the rotor is accelerated above the estimated speed n due to external flow, speed regulation from the open loop is achieved. Figure 3 Speed ​​regulation in closed loop ( Figure 2 The transition of ).

[0045] In the case of negative speed due to external crossflow, the initiation of ramp 105b is placed at a negative speed. Here, it is advantageous, but not necessary, to place the initiation at the initial theoretical speed value, which is magnitude greater than the initially estimated speed, for example, n+500. This increases the time window to ensure that the rotor field and stator field successfully synchronize.

[0046] The reduced slope of ramps 105a and 105b, in addition to increasing the time window for synchronizing the rotor and stator fields, also serves another function in the presence of reverse external current flow. Since acceleration should be performed against the rotor's rotation direction, a generator operating point can be set, where energy can be removed from the hydraulic system through recovery. If this energy is not otherwise consumed, it will charge the intermediate circuit capacitor. Damage may occur if this intermediate circuit capacitor is forcibly charged without proper authorization. Efficiency in generator operations is typically defined as follows.

[0047] Where P mech This refers to the mechanical power removed from the system.

[0048] For P mech >P v The operating condition shows a positive power generation efficiency and that the intermediate circuit is charged, resulting in a rise in the voltage applied to the intermediate circuit capacitor. Such an operating condition should be eliminated as much as possible, or at least allowed to exist for such a short period of time, so that the voltage applied to the capacitor remains within the permissible limits.

[0049] This is achieved by selecting a smaller slope, thus P mech Keep it small while simultaneously handling sufficiently large losses, i.e., P mech <P v This is applicable, thereby removing energy from the intermediate loop. This is achieved by using a sufficiently large regulated phase current I in the open loop. Soll This phase current results in current-induced heat loss in the motor and switching losses in the inverter 101. Furthermore, the higher motor current also improves the prospects for successful rotor capture because the possible torque used to capture the rotor is increased.

[0050] A slightly modified method in Figure 5 Presented in the middle. With Figure 4 Unlike other variations, a slightly modified ramp 105b' should be used here for the estimated negative speed n. According to the modified ramp 105b', the initial starting value of the theoretical speed is not placed on a value that is greater than the estimated value n, but instead it is placed on a value that is 10% smaller, 0.9*n.

Claims

1. A method for starting a speed-regulated electric motor of a centrifugal pump, wherein the centrifugal pump has at least one impeller driven by the electric motor, the impeller rotating at a positive speed during normal pump operation to deliver a transport medium from the suction side to the pressure side of the centrifugal pump as prescribed, and wherein the speed regulation of the motor operates in a sensorless manner without the need for a device for determining the position of the rotor. Its features are, Before starting the centrifugal pump, at least one current measurement is used to detect whether and in which direction the impeller is rotating due to external flow. If a positive speed is detected due to the external flow, the motor is started to be operated at a theoretical speed of zero according to the speed ramp. If a negative speed is detected due to the external flow, the motor is started to be operated at a theoretical speed of negative according to the speed ramp.

2. The method according to claim 1, characterized in that, As the current measurement, the short-circuit current generated in at least one motor winding due to the external current is measured by switching at least one phase of the motor winding to a short circuit using the bridge circuit of the inverter.

3. The method according to claim 2, characterized in that, A threshold is defined for the short-circuit current, and an external current flow is detected when the threshold is exceeded. The threshold height is preferably defined such that it is possible to distinguish the short-circuit current caused by the external current flow from the measurement noise, but hardware damage caused by excessive current intensity is avoided.

4. The method according to claim 3, characterized in that, Short-circuit current measurements are performed once or multiple times, especially during the commissioning of the pump, in the absence of external through-current, in order to assess the measurement noise of a single pump.

5. The method according to any one of the preceding claims, characterized in that, Furthermore, the rotational speed of the pump impeller, resulting from the external flow, is determined.

6. The method according to claim 5, characterized in that, The short-circuit current measurement is repeated at least once, preferably at least twice, ideally at least three times, and the rotational speed can be determined by taking into account the phase position of the measured short-circuit current and the time difference between the measurement points, wherein, in order to acquire the time difference, the time required from switching the short circuit until the threshold is reached is stored.

7. The method according to any one of the preceding claims, characterized in that, After detecting an external current flow, the motor is adjusted in an open-loop manner by adjusting the motor current to the theoretical value and controlling the speed using a speed ramp.

8. The method according to claim 7, characterized in that, In the open-loop regulation, the current regulator obtains the theoretical value of the current amplitude, the measured actual current, and a reference for the phase position of the current calculated by integrating the theoretical speed according to the speed ramp as input parameters.

9. The method according to claim 7 or 8, characterized in that, In the open-loop configuration, the motor is adjusted to a constant theoretical current amplitude value, wherein the selected theoretical value is chosen such that the losses exceed the expected power generation, but do not cause thermal overload or other overloads to the pump unit.

10. The method according to any one of the preceding claims, characterized in that, Upon detecting a negative rotational speed due to external flow, the motor is controlled according to a rotational speed ramp, the initial negative rotational speed of which is greater than or equal to the negative rotational speed due to the external flow.

11. The method according to any one of the preceding claims, characterized in that, Upon detecting a negative speed due to external flow, the motor is controlled according to a speed ramp, the initial negative speed of which is 5%-15% smaller than the negative speed due to the external flow.

12. The method according to any one of claims 7 to 11, characterized in that, If a positive speed is detected due to the external flow, and the motor is accelerated to a speed above the detected positive speed, then the process transitions to speed regulation with a closed loop.

13. A pump, particularly a centrifugal pump, especially a heating circulation pump, having at least one control unit configured to perform the method according to any one of the preceding claims.

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