Method for starting a rotational speed-controlled electric motor of a centrifugal pump

Abstract

The invention relates to a method for starting a rotational speed-controlled electric motor of a centrifugal pump. The centrifugal pump has at least one impeller which is driven by the electric motor and which rotates at a positive rotational speed during a regular pump operation in order to pump a pumped medium from a suction side to a pressure side of the centrifugal pump in an intended manner, and the rotational speed of the motor is controlled in a transmitter-free manner without means for determining the position of the rotor. The invention is characterized in that prior to starting the centrifugal pump, a current measurement is used in order to detect whether the impeller is being rotated by an external flow and if so, in which direction. If a positive rotational speed produced by the external flow is detected, the motor is actuated according to a rotational speed ramp beginning with a rotational speed target value of null, and if a negative rotational speed produced by the external flow is detected, the motor is actuated according to a rotational speed ramp beginning with a negative rotational speed target value.

Description

[0001] Method for starting a speed-controlled electric motor of a centrifugal pump

[0002] The invention relates to a method for starting a speed-controlled electric motor of a centrifugal pump, wherein the centrifugal pump has at least one impeller driven by the electric motor, which rotates at a positive speed in regular pump operation for the intended conveyance of a pumped medium from a suction side to a pressure side of the centrifugal pump, and wherein the speed control of the motor operates without a sensor and without means for determining the position of the rotor.

[0003] If an external medium flows through a centrifugal pump while it is switched off, the external flow can cause the pump impeller and thus the rotor to rotate. This can occur particularly in hydraulic systems with multiple pumps. If the medium flows from the suction side to the discharge side, the impeller is forced to rotate at a positive speed. If, instead, there is reverse flow from the discharge side to the suction side, the impeller rotates at a negative speed.

[0004] For permanent magnet synchronous motors, there are speed control methods, such as field-oriented control, that require knowledge of the current rotor position. However, for cost and maintenance reasons, a dedicated absolute encoder for position and speed measurement is generally not used. If the rotor is set in rotation by an external flow during idle operation, directly switching on the motor with a sensorless control method based on the electromotive force is not easily possible.

[0005] In the past, corresponding pump control systems therefore provided for short-circuiting the stator or stator windings upon detection of external flow and the resulting externally excited rotation of the motor. The resulting phase resistance was utilized in this context, which was used to convert the excess electrical energy resulting from copper losses into heat. This released energy is extracted from the hydraulic system, generating a certain braking torque on the pump, thus slowing the impeller to a standstill. The pump can then be started normally from standstill.

[0006] The disadvantage of such a solution, however, is that the resulting short-circuit current is unregulated and can potentially reach dangerously high levels. This can lead to thermal damage to the motor and irreversible demagnetization of the permanent magnets in the rotor. A further disadvantage is that the braking effect achieved by the short-circuit circuit depends heavily on the characteristics of the machine and thus varies in effectiveness depending on the machine.

[0007] Therefore, an alternative approach is being sought to overcome the aforementioned problems.

[0008] This object is achieved by a method according to the features of claim 1. Advantageous embodiments are the subject of the dependent claims.

[0009] According to the invention, it is proposed to detect an external flow for a corresponding centrifugal pump that uses sensorless motor speed control using a current measurement. By means of the current measurement, in addition to the basic detection of an external flow, the direction of the external flow or the direction of rotation imposed on the motor is to be recorded. If an external flow is detected, then depending on the detected direction of rotation, no speed control is carried out to start the motor as in regular pump operation, but instead the pump is started using speed control. This means that the motor speed is not controlled taking into account an actual speed of the rotor, but instead controlled according to a fixed target speed ramp without feedback of the actual speed or actual rotor position.Regular pump operation is understood to mean active operation of the pump for the intended conveyance of a pumped medium from a suction side to a pressure side of the pump at a positive speed.

[0010] The type of speed ramp, particularly its start and / or end value, is selected depending on the previously detected speed direction of the speed generated by the external flow. If a positive speed is detected, the motor speed for speed control is specified by a speed ramp that initially starts from a speed value of zero and increases linearly in the positive speed range. If, on the other hand, a negative speed direction is detected, the motor is controlled with a speed ramp whose start value is in the negative speed range and increases, ideally linearly, into a positive speed range.

[0011] The method according to the invention is intended to ensure that the rotor field generated by the rotor excitation, in particular the permanent magnet excitation, of the rotating rotor due to external airflow is overtaken by the stator field generated by the stator winding. The stator field is generated by controlling the corresponding speed ramp. If the difference in speed between the two fields is sufficiently small, the rotor falls into step with the stator field, and the motor can continue to operate with speed control as usual.

[0012] This method ensures a successful motor start even without a specific rotor position detection. Advantageously, the current measurement is a short-circuit current generated by the external flow within at least one motor winding of the stator. The motor winding is short-circuited by the installed frequency converter, for example, by closing the respective half-bridges of the frequency converter's integral bridge circuit.

[0013] However, when short-circuiting, care must be taken to ensure that the short-circuit current does not become excessively high, which could result in damage to the frequency converter's circuit breakers or excessive heating of the stator windings. An excessively high short-circuit current can also cause irreversible demagnetization of the rotor magnets. To avoid the aforementioned dangers, a threshold value is defined for the short-circuit current. If the short-circuit current exceeds the threshold value, an external current is detected and all circuit breakers of the inverter are blocked. The short-circuit current can then quickly commutate via the freewheeling diodes due to the high DC link voltage.

[0014] The threshold is set sufficiently low so that even a brief threshold violation is unlikely to cause hardware damage. However, it is necessary to set the threshold high enough to distinguish external flow from normal measurement noise. A suitable threshold can be defined, for example, through one or more training measurements, particularly after the pump has been commissioned. In this case, a situation without external flow is selected, for example, and the short-circuit current is recorded to evaluate the measurement noise.

[0015] It is particularly advantageous if, based on the current measurement carried out, in particular the short-circuit current measurement, the speed of rotation is determined in addition to the direction and the presence of any externally generated rotation. A speed measurement or estimation is made possible, for example, by carrying out two or more measurements of the short-circuit current at different times. By specifically determining the time differences between the short-circuit currents recorded at different times and the phase positions of the measured currents, the frequency and consequently the speed can then be estimated. It is therefore intended that the short-circuit current measurement method is repeated at least once in order to obtain the short-circuit current at two different times. Preferably, however, the method is repeated at least twice, ideally at least three times, in order to be more robust against measurement noise.During the measurements, the time until the threshold is exceeded is always recorded and stored in order to determine the time differences between the exceedance times. For example, if the short-circuit current measurement is repeated three times, three phase currents are available at four different times. Using the well-known Clarke transformation, a transformation into the complex space vector representation is performed, in which the electrical frequency and thus three different speeds can be derived based on the phase positions of the four space vectors and knowledge of the time differences. The estimated speed of the external flow is then calculated from the derived speed values ​​by calculating the mean or median value.

[0016] As already indicated above, between successive short-circuit current measurements, all power switches of the bridge circuit are blocked, allowing the short-circuit current to commutate via the bridge's freewheeling diodes.

[0017] After detection of external flow, the centrifugal pump motor is not speed-controlled, but rather controlled solely at a setpoint speed, i.e., without feedback of a measured, estimated, or otherwise determined actual speed. The setpoint speed specified for the control is derived from the speed ramp, which in turn is determined as a function of the previously detected externally excited speed. This speed control is referred to as open-loop control of the motor, since current control is still performed. The current controller receives setpoints for the current space vector in terms of magnitude and phase position, as well as the measured motor currents, as input variables. The phase position is provided by continuous integration of the speed ramp.The amount of the current space vector to be adjusted is chosen to be constant, but defined sufficiently high to increase the probability of successful motor capture by means of an associated high torque, if necessary.

[0018] Once the rotor has been successfully captured, the open-loop control can be switched to closed-loop control, which implements true speed control. Due to the sensorless speed control, the speed or rotor position is estimated in the usual way using an observer, taking into account the current actual motor current and the setpoint voltage. The active speed controller determines the corresponding setpoint current for the current controller based on the speed deviation.

[0019] According to a preferred embodiment, upon detection of a negative speed generated by external flow, the motor is controlled with a speed ramp whose initial negative starting speed value is equal to or greater than the speed value generated by the external flow, estimated by current measurement. It is conceivable to select a value 5%-15% higher in magnitude. It is also conceivable to start the motor at the maximum possible negative speed when a negative speed is detected.

[0020] To reduce the time required to capture the rotor, it may be sufficient to select the initial starting speed of the speed ramp used so that it is smaller in magnitude than the negative speed generated by the external flow. A speed value that is approximately 5% to 15% smaller in magnitude than the negative speed generated by the external flow is conceivable.

[0021] It may also be expedient to define the ramp steepness of the speed ramp sufficiently small. This increases the time window for capture, i.e. for the rotor field to fall into line with the stator field, and increases the chances of success of the rotor capture process. Although this does result in an increase in the required energy expenditure, this is acceptable in view of the improved reliability of the process. It should also be noted that under certain circumstances when the process is carried out, generator operation can occur, i.e. the stator field accelerates the motor against the direction of rotation of the rotor generated by the external airflow. During generator operation, energy is withdrawn from the hydraulic system through recuperation. If this energy is not consumed elsewhere, it would charge the intermediate circuit capacitor.By reducing the ramp steepness in such a constellation, the resulting mechanical power and thus the energy generated by the generator are kept low, so that the DC link capacitor is only charged to a limited extent and overloading can be virtually ruled out. It is also helpful if the setpoint current, in particular the setpoint current amplitude value, is set sufficiently high during speed control in the open circuit. With the associated losses in the frequency converter and motor, additional electrical energy is dissipated at a generator operating point, for example, to prevent or reduce the charging of the DC link capacitor. By reducing the ramp steepness and appropriately defining the setpoint current for speed control, overloading of the DC link during a generator operating point can be reliably avoided.At the same time, care must be taken to ensure that the target current is not set 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.

[0022] Furthermore, it is proposed that, in the case of a positive speed detected as a result of the external flow, the open-loop speed control is switched to a closed-loop speed control, i.e. with real feedback of the actual speed and estimated rotor position, as soon as the motor has been accelerated to a speed above the previously detected positive speed due to the external flow.

[0023] In addition to the method according to the invention, the present invention also relates to a pump, in particular a centrifugal pump, particularly preferably a heating circulation pump, with at least one control module configured to implement the method according to the invention. The pump thus benefits from the same advantages and properties as those already demonstrated above with reference to the method according to the invention. For this reason, a repetitive description is omitted.

[0024] Further advantages and features of the invention will be described in more detail below using a specific embodiment and the following figures. The figures show:

[0025] Figure 1 : a block diagram of the current measurement for detecting an external current,

[0026] Figure 2: the principle diagram for a closed control loop without absolute encoder for speed control,

[0027] Figure 3: a principle representation of the open circuit for the speed control when starting the pump after detection of an external flow,

[0028] Figure 4: the flow chart for starting the engine after detection of an external flow,

[0029] Figure 5: a modification of the starting procedure.

[0030] The method according to the invention will now be described using a centrifugal pump driven by a permanent magnet synchronous motor. The pump has a controller that performs field-oriented speed control. Since the pump operates without a sensor, i.e. the current position of the rotor is not detected by a position sensor, a so-called observer is used which estimates the current speed and rotor position. The regular direction of rotation of the impeller during pump operation is referred to here as the direction of rotation with positive speed. When the pump is at a standstill, the impeller of the pump can be set in rotation by an external flow in both the positive direction of rotation (positive speed) and the negative direction of rotation (negative speed). The aim of the invention is the reliable detection of such an external flow and the successful starting of pump operation when an external flow is present.The goal is to achieve the most reliable method possible, requiring a minimum number of parameters. This should minimize development effort while simultaneously maximizing the reliability of the method, given the wide variety of different pump motors. The invention can therefore be divided into two tasks to be solved: the detection of an external flow through the pump, which leads to an impeller movement imposed by an external force, and the starting of the motor when an external flow is present.

[0031] Detection of external flow

[0032] When external flow is present, the rotor rotates, driven by the pump impeller through which the flow is passing. This induces voltages in the stator winding. To detect external flow, the short-circuit current is used as a measure of the rotor speed and direction of rotation when the pump is deactivated. The detection principle is shown in the diagram in Figure 1.

[0033] For this purpose, the stator winding is temporarily short-circuited at time t=0 via the frequency converter's bridge circuit, particularly the B6 bridge in a 3-phase motor (block 10). If the rotor is rotating (e.g., due to the presence of external current), a short-circuit current builds up. It must be ensured that a. the short-circuit current is large enough to be detected with sufficient accuracy. b. it does not become too large to prevent damage to the inverter's power switches, excessive heating of the stator winding, or even a current that leads to irreversible demagnetization of the magnets.

[0034] To ensure this, a threshold value / s, the current / s, is defined as the only parameter for detecting the extraneous flow. This threshold value must be selected so that a. it is large enough to be clearly distinguished from measurement noise, thus ruling out false detection. b. it is small enough to prevent damage to the hardware and / or the motor.

[0035] Block 20 checks whether the short-circuit current / s exceeds the defined threshold / s, ef, before a timeout, in this case 100 ms. If this is the case, an external current is assumed to be present, and the process continues in block 30. If the motor remains de-energized, i.e., the threshold is not exceeded, it can be concluded that the rotor is stationary (block 70), and the motor can be started normally from standstill (block 80).

[0036] The time t1 required until the threshold value Is.ef is exceeded is stored in block 20. The pulse width modulation of the frequency converter is then switched off (block 30), which blocks all power switches (e.g., MOSFETs) in the frequency converter's bridge circuit, allowing the short-circuit current to commutate quickly via the freewheeling diodes due to the high intermediate circuit voltage. In addition, the measured short-circuit current l s ,k, where the index k represents the number of the measurement performed.

[0037] The procedure for measuring the short circuit (block 40), deactivating the PWM, and saving the measured value (block 30) is repeated three more times using the determined timing, so that the three phase currents are known at four different points in time. These can be converted into complex space vectors using the well-known Clarke transformation (block 50). From the phase positions of the four space vectors, knowing the time difference ΔT, the electrical frequency and ultimately three speeds can be calculated (block 50). In principle, two vectors and two measured values ​​would be sufficient; however, to be more robust against measurement noise, four vectors are measured and calculated. The median value from the set of estimated speeds is used as the estimated speed (block 50).

[0038] The signed speed is then given by

[0039] The implemented method also takes into account the detection of the zero crossing in the angle argument (p = p t - <Pt-^T sc +^T PWM , aus ) (domain [0,2TT]).

[0040] After an external flow and the speed imposed on the rotor have been determined by the procedure according to Figure 1, the motor can be ramped up in block 60 with a modified speed ramp adjusted to the estimated speed.

[0041] Starting the engine in the presence of external flow (Block 60)

[0042] If an external flow is detected using the procedure described above, the procedure for starting the motor is determined based on the estimated speed and direction of rotation. Due to the principle of principle, a start attempt is only made up to a defined maximum speed.

[0043] The rotor position is unknown at the beginning of the start-up process. Although the previously described method for detecting the external flow determines the phase position of the short-circuit current, this is a transient transient current and not the steady-state short-circuit current, which, if the phase resistance is neglected, is located along the d-axis of the rotor-oriented coordinate system. Furthermore, the phase position is only calculated after sampling in a slower task due to the computationally intensive atan2 calculation. This means that the phase position of the current is outdated at the time it was calculated and cannot be used for control purposes.However, since a stationary operating point can be assumed in the case of external flow, it can be assumed that the speed remains largely constant during the calculation time, which is relatively short with respect to the mechanical time constants.

[0044] To start up the pump, the motor is then controlled in an open loop. This means that the motor current is regulated, but the speed is controlled. This difference is briefly discussed in Figure 2, which shows the closed-loop speed control scheme during regular pump operation. The speed controller 104 generates a constant speed depending on a setpoint speed n. S oii and the actual speed n or the resulting control deviation as a manipulated variable, a setpoint Isoii for the motor current. The current controller 100 regulates the motor to the setpoint Isoii, with the estimated value of the rotor position being provided to the current controller 100 as input variables for the actual value. and the phase currents st are supplied, and the stator voltage Usoii is output as the manipulated variable. The latter is then generated by the inverter 101 of the frequency converter to supply power to the motor 102. Since the pump operates without a sensor, the current actual speed n and the rotor position are determined model-based using an observer 90 with the aid of the input variables actual current st and voltage setpoint U S0 n is estimated. This allows the motor to be operated efficiently, since only the current required to achieve the desired speed nsoii is controlled. The desired voltage Usoii represents the most important input variable. It is, to a good approximation, proportional to the speed, which is why the control method with the observer 103 used only functions reliably at higher speeds, since here the induced counter voltage dominates in the voltage setpoint, thus ensuring a sufficiently good signal-to-noise ratio.

[0045] In the event of detected external flow, the pump is switched to speed control for start-up, i.e., open-loop control, the principle of which is explained using Figure 3. The setpoint speed nsetpoint is specified via a ramp 105, and the current is controlled by the current controller 100 to a fixed setpoint Isoii, which is selected such that it is sufficiently large to enable the rotor to catch and prevent the rotor from falling out of step during the further start-up process. The current is controlled in its phase position with reference to a reference Vsetpoint, just as in a closed loop, but in this case a phase position <p soü which results from the integration of the speed setpoints nsoii. This phase position <p soü does not initially correspond to the rotor position.

[0046] The starting point and steepness of ramp 105 are adapted to the desired purpose, namely, to allow the stator field generated by the stator winding to overtake the rotor field generated by the permanent magnet excitation and, at sufficiently small differential speeds between the two fields, to fall into step with it. The amplitude of the current is regulated, and the torque-generating component of the current is adjusted depending on the load point. The ramp steepness is dimensioned relatively small to increase the probability of a successful fall into step. The procedure is shown in the schematic diagram in Figure 4.

[0047] To select the specific ramp 105, a case distinction 61 is first made for the speed n estimated in block 50 of Figure 1. At a positive speed n due to the external flow, ramp 105a for the open loop according to Figure 3 is started with an initial speed setpoint of zero. Speed ​​ramp 105 rises moderately in the positive speed range. The transition from open-loop speed control (Figure 3) to closed-loop speed control (Figure 2) is realized when the rotor has been accelerated to a speed above the estimated speed n due to the external flow.

[0048] In the case of a negative speed due to the external flow, the start of ramp 105b is set to a negative speed. It is advantageous, but not necessary, to set the start to an initial speed setpoint that is greater in magnitude than the initially estimated speed, in this case, for example, to a value of n+500. This increases the time window to ensure successful synchronization of the rotor field with the stator field.

[0049] The reduced ramp steepness of the ramp 105a, 105b used serves another function besides increasing the time window for the rotor field to fall into contact with the stator field in the presence of an external flow in the opposite direction. Since acceleration is intended to occur against the direction of rotation of the rotor, a generator operating point can occur at which energy can be extracted from the hydraulic system through recuperation. This energy, if not otherwise consumed, would charge the intermediate circuit capacitor. If this is charged to an inadmissibly high level, damage can occur. In general, the efficiency in the generator mode is defined as follows: where Pmech is the mechanical power extracted from the system.

[0050] For operating cases with PmeciPPv, positive generator efficiencies are achieved and the DC link is charged, causing the voltage across the DC link capacitor to rise. Such an operating case should be avoided if possible, or at least occur for a period of time short enough to ensure that the voltage across the capacitor remains within the permissible limits.

[0051] This is achieved by choosing a small ramp, which keeps Pmech small and at the same time ensures that the losses are sufficiently large, i.e. Pmech

Claims

Claims 1. A method for starting a speed-controlled electric motor of a centrifugal pump, wherein the centrifugal pump has at least one impeller driven by the electric motor, which in a regular pumping operation for the intended conveyance of a pumped medium from a suction side to a pressure side of the centrifugal pump rotates at a positive speed, and wherein the speed control of the motor operates sensorless without means for determining the position of the rotor, characterized in that before the start of the centrifugal pump, it is detected on the basis of at least one current measurement whether and in which direction the impeller is set in rotation by an external flow,and upon detection of a positive speed generated by the external flow, the motor is controlled according to a speed ramp starting with a speed setpoint of zero, and upon detection of a negative speed generated by the external flow, the motor is controlled according to a speed ramp starting with a negative speed setpoint.

2. Method according to claim 1, characterized in that as current measurement a short-circuit current generated by the external flow within we- at least one motor winding is measured by short-circuiting at least one phase of the motor windings using the bridge circuit of the frequency converter.

3. Method according to claim 2, characterized in that a threshold value for the short-circuit current is defined and, when the threshold value is exceeded, an external flow is detected, wherein preferably the threshold value level is defined in such a way that a distinction between a short-circuit current generated by an external flow and measurement noise is possible, but hardware damage due to an excessively high current is avoided.

4. Method according to claim 3, characterized in that one or more times, in particular when starting up the pump, a short-circuit current measurement is carried out in the absence of external flow in order to evaluate the measurement noise of the individual pump.

5. Method according to one of the preceding claims, characterized in that the speed of the pump impeller generated by the external flow is also determined.

6. 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 speed can be determined taking into account the phase position of the measured short-circuit currents and the time difference between the measuring points, wherein the time required from the switching of the short-circuit until the threshold value is reached is stored to record the time difference.

7. Method according to one of the preceding claims, characterized in that the motor is controlled in an open circuit after detection of an external flow by regulating the motor current to a setpoint and controlling the speed using a speed ramp.

8. Method according to claim 7, characterized in that the current controller, in the case of open-loop control, receives as input variables the current amplitude setpoint, the measured actual currents and a reference for the phase position of the currents calculated by integrating the setpoint speed according to the speed ramp.

9. Method according to claim 7 or 8, characterized in that the motor is controlled in the open circuit to a constant target current amplitude value, wherein the selected target value is chosen such that the losses exceed the expected generator-generated power, but do not lead to thermal or other overloading of the pump unit.

10. Method according to one of the preceding claims, characterized in that upon detection of a negative speed generated by external flow, the motor is controlled according to a speed ramp whose initial negative speed is greater than or equal to the negative speed generated by the external flow.

11. Method according to one of the preceding claims, characterized in that upon detection of a negative speed generated by external flow, the motor is controlled according to a speed ramp whose initial negative speed is 5%-15% smaller than the negative speed generated by the external flow.

12. Method according to one of the preceding claims 7 to 11, characterized in that when a positive speed is detected as a result of the external flow, a transition is made to a closed-loop speed control when the motor has been accelerated to a speed above the detected positive speed.

13. Pump, in particular centrifugal pump, particularly preferably heating circulation pump, with at least one controller configured to carry out the method according to one of the preceding claims.

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