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

Abstract

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 in regular pump operation rotates at a positive speed for the intended conveying 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, characterized in that, before the centrifugal pump is started, it is detected by means of a current measurement whether and in which direction the impeller is set in rotation by an external flow.and, upon detection of a positive rotational speed generated by the external flow, the motor is controlled according to a speed ramp starting with a target speed of zero, and upon detection of a negative rotational speed generated by the external flow, the motor is controlled according to a speed ramp starting with a negative target speed.

Description

The invention relates to 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, which rotates at a positive speed in a regular pump operation for the intended delivery of a delivery medium from a suction side to a pressure side of the centrifugal pump, and wherein the speed regulation of the motor operates without transmitter means for determining the position of the rotor.If an external medium flows through a centrifugal pump in the switched-off state, the external flow can cause the pump impeller and thus the rotor to rotate. This can occur especially in hydraulic systems with several pumps. If the medium flows from the suction side to the pressure side, a positive rotational speed is imposed on the impeller. If, instead, a return flow from the pressure side to the suction side occurs, the impeller rotates at a negative rotational speed.In permanent magnet excited synchronous motors, there are speed control methods, for example field-oriented control, which require knowledge about the current rotor position. For reasons of cost and maintenance, however, a dedicated absolute value transmitter for position and rotational speed measurement is generally dispensed with. If the rotor is set in rotation in the idle mode by an external flow, a direct switching on of the motor using a sensorless control method based on the electromotive force is not readily possible.In the past, it was therefore provided in corresponding pump regulations that, when an external flow and an associated externally excited rotation of the motor were detected, the stator or the windings of the stator were switched into a short circuit. In this connection, the resulting phase resistance, which is used to convert the excess electrical energy in the course of copper losses into heat, was exploited. This released energy is extracted from the hydraulic system, whereby a certain braking torque can be generated on the pump and thus the impeller could be braked to a standstill. From the standstill, the pump can then be started regularly.However, a disadvantage of such a solution is that the resulting short-circuit current is not regulated and may possibly become dangerously high. This can lead to thermal damage to the motor and also to irreversible demagnetization of the installed permanent magnets in the rotor. A further disadvantage can be that the achievable braking effect by the short circuit circuit depends strongly on the characteristics of the machine and thus operates differently effectively depending on the machine.An alternative approach is therefore sought to overcome the aforementioned problems.This object is achieved by a method according to the features of claim 1.According to the invention, it is proposed to detect an external flow for a corresponding centrifugal pump which uses sensorless speed regulation of the motor on the basis of a current measurement. By means of the current measurement, in addition to the fundamental determination of an external flow, the direction of the external flow or the direction of rotation of the rotation imposed on the motor is also intended to be detected. If an external flow is detected, then depending on the established direction of rotation for starting the motor, no speed regulation is carried out as in regular pump operation, but instead the pump is started with a speed control. That is to say that the engine speed is not regulated taking into account an actual rotational speed of the rotor, but instead is controlled in accordance with a permanently predefined setpoint rotational speed ramp without feedback of the actual rotational speed or actual rotor position. Regular pump operation is understood to mean active operation of the pump for the intended delivery of a delivery medium from a suction side to a pressure side of the pump at a positive rotational speed.The type of rotational speed ramp, in particular its starting and / or final value, is selected depending on the previously detected rotational speed direction of the rotational speed generated by the external flow. If a positive rotational speed is detected, the engine speed for the rotational speed control is predefined by a rotational speed ramp which initially starts from a rotational speed value of zero and in particular rises linearly in the positive rotational speed range. If, on the other hand, a negative rotational speed direction is detected, the motor is controlled with a rotational speed ramp, the starting value of which lies in the negative rotational speed range and rises into a positive rotational speed range, ideally rises linearly.With the method according to the invention, the rotor field of the rotor rotating due to external flow, which field is caused by the rotor excitation, in particular the permanent magnet excitation, is supposed to be bypassed by the stator field caused by the stator winding, wherein the stator field is generated by the control with the corresponding rotational speed ramp. With a sufficiently low differential speed of the two fields, the rotor falls in step with the stator field and the motor can continue to be operated in a speed-controlled manner as usual.With the aid of this method, a successful starting process of the motor can be ensured even without specific position detection of the rotor.Advantageously, a short-circuit current generated by the external throughflow is measured as a current measurement within at least one motor winding of the stator. The switching of the motor winding into the short circuit is achieved by the installed frequency converter, for example by closing the respective half-backs of the integral bridge circuit of the frequency converter.In the case of the short-circuit circuit, however, it is to be ensured that the short-circuit current does not become unacceptably high and damage to the power switches of the frequency converter or unacceptable heating of the stator windings occurs. An unacceptably high short-circuit current can also cause irreversible demagnetization of the rotor magnets. In order to avoid the aforementioned risks, a threshold value for the short-circuit current is therefore defined. If the short-circuit current exceeds the threshold value, an external throughflow is detected and all power switches of the inverter are blocked. The short circuit current can then quickly commutation off via the free-wheeling diodes on account of the high intermediate circuit voltage.The threshold value is selected to be sufficiently small so that damage to the hardware is unlikely even if the threshold value is exceeded for a short time. However, it is necessary to choose the threshold value sufficiently high in order to be able to distinguish an external flow from conventional measurement noise. The definition of a suitable threshold value can be established, for example, by one or more training measurements, in particular after the pump has been put into operation, wherein here, for example, a situation without external throughflow is selected and the short-circuit current is detected for evaluating the measurement noise.It is particularly advantageous if, on the basis of the current measurement carried out, in particular the short-circuit current measurement, the rotational speed of the rotation is also determined in addition to the direction and the presence of an 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 of the short-circuit currents detected at different points in time and the phase positions of the measured currents, the frequency and consequently the rotational speed can then be estimated. It is therefore provided that the method for short-circuit current measurement 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 with respect to measurement noise. During the measurements, the time duration until the threshold value is exceeded is always detected and stored in order to determine the time differences of the crossing points in time. For example, in the case of a triple repetition of the short-circuit current measurement, three phase currents are available at four different times. By means of the known Clarke transformation, a transformation into the complex space vector representation is carried out, in which the electrical frequency and thus three different rotational speeds can be derived on the basis of the phase positions of the four space vectors and with knowledge of the temporal differences. The estimated rotational speed of the external flow is then calculated from the derived rotational speed values by averaging or median value formation.As already indicated above, all the circuit breakers of the bridge circuit are blocked between carrying out successive short-circuit current measurements, as a result of which the short-circuit current can be commutated off via the freewheeling diodes of the bridge.After detection of an external flow, the motor of the centrifugal pump is not speed-controlled, but is only controlled with a setpoint speed, i.e. without feedback of a measured, estimated or otherwise determined actual speed. The setpoint rotational speed predefined for the control results from the rotational speed ramp, which is in turn determined as a function of the externally excited rotational speed detected in advance. This speed control is called open-loop motor control / regulation because current control continues to be performed. The current regulator receives as input variables setpoint values for the current space vector in magnitude and phase position and the measured motor currents. The phase position is provided by continuous integration of the rotational speed ramp. The amount of the current space vector to be regulated is selected to be constant, but is defined to be sufficiently high in order, if necessary, to increase the probability of successful capture of the motor by means of a high torque associated therewith.When the rotor is successfully caught, the open loop control / feedback control can be switched to a closed loop control that performs true speed control. On the basis of the sensorless speed regulation, the speed or the rotor position is estimated here in a known manner by means of an observer taking into account the current actual motor current and the setpoint voltage. The active speed regulator determines the corresponding setpoint current for the current regulator as a function of the speed deviation.According to a preferred embodiment, when a negative rotational speed generated by external throughflow is detected, the motor is actuated with a rotational speed ramp whose initial negative starting rotational speed value is equal in magnitude to or greater than the rotational speed value estimated by current measurement and generated by the external throughflow. It is conceivable to select a value which is higher by 5%-15% in terms of amount. It is likewise conceivable, in the case of a detected negative rotational speed, to start the motor at the maximum possible negative rotational speed.In order to reduce the time duration of the method until the rotor is captured, it may be sufficient to choose the initial starting rotational speed of the rotational speed ramp used to be smaller in terms of absolute value than the negative rotational speed generated by the external throughflow. A rotational speed value is conceivable which is between approximately 5%-15% smaller in absolute value than the negative rotational speed generated by the external flow.It may also be expedient to define the ramp slope of the rotational speed ramp to be sufficiently small. As a result, the time window for capturing, i.e. for the rotor field to enter with the stator field, increases, and the prospects of success of the method for capturing the rotor increase. Although this requires an increase in the required energy consumption, this is acceptable with a view to improving the reliability of the method.It should also be noted that in the execution of the method, under certain circumstances, a generator operation can occur, i.e. the motor is accelerated by the stator field against the direction of rotation of the rotor generated by the external flow. During the generator operation, energy is extracted from the hydraulic system by recuperation. This energy, unless otherwise consumed, would charge the DC link capacitor. In such a constellation, the resulting mechanical power and thus the energy generated by the generator are kept low by the reduction of the slope of the ramp, so that the intermediate circuit capacitor can only be charged to a limited extent and an overcharge can actually be ruled out. It is also helpful if the setpoint current, in particular the setpoint current amplitude value, is selected to be sufficiently high during the speed control in the open circuit. With the associated losses in the frequency converter and motor, electrical energy is additionally dissipated at a generator operating point in order, for example, to prevent or reduce charging of the intermediate circuit capacitor. As a result of the reduced slope of the ramp and the suitable definition of the setpoint current for the speed control, overloading of the intermediate circuit during a generator operating point can be reliably avoided. At the same time, it must be ensured that the setpoint current is not selected to be 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.It is also proposed that, in the case of a positive rotational speed detected as a result of the external throughflow, a changeover is made from the rotational speed control in the open circuit into a rotational speed control with a closed circuit, that is to say with true feedback of the actual rotational speed and estimated rotor position, as soon as the motor has been accelerated to a rotational speed above the previously detected positive rotational speed by the external throughflow.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, having at least one control module which is configured to carry out the method according to the invention. The same advantages and properties thus result for the pump as already shown above with reference to the method according to the invention. Repetitive description is omitted for this reason.Further advantages and properties of the invention will be described in more detail below with reference to a specific exemplary embodiment and the following figures. The figures show: FIG. 1 : a block diagram of the current measurement for detecting an external flow, FIG. 2 shows the principle diagram for a closed control loop without absolute sensor for the speed control, FIG. 3 shows a principle illustration of the open circuit for the speed control at the start of the pump after detection of an external throughflow, FIG. 4 : shows the flow chart for starting the engine after detection of an external flow, FIG. 5 shows a modification of the starting method.The method according to the invention will be described below with reference to a centrifugal pump which is driven by means of a permanent magnet excited synchronous motor. The pump has a controller which carries out field-oriented speed control. Since the pump operates without a sensor, i.e. the current position of the rotor is not detected by a sensor, a so-called observer is used which estimates the current rotational speed and rotor position. The direction of rotation of the impeller which is regular in pump operation is referred to here as the direction of rotation with a positive rotational speed.When the pump is at standstill, the impeller of the pump can be set in rotation both in the positive rotational direction (positive rotational speed) and in the negative rotational direction (negative rotational speed) by an external flow. The object of the invention is the reliable detection of such an external flow and the successful start of the pump operation in the presence of an external flow. What is desired is a method which is as reliable as possible and which makes do with a minimum number of parameters. The latter is intended to minimize the development effort with respect to a multiplicity of different pump motors and at the same time to maximize the reliability of the method. The invention can thus be subdivided into two problem positions to be solved, namely the detection of an external flow of the pump, which leads to an impeller movement imposed by an external force, and the starting of the motor in the presence of an external flow.Detection of External FlowIn the presence of an external flow, the rotor rotates driven by the pump impeller through which flow takes place. As a result, voltages are induced in the winding of the stator. For the detection of the external flow, the short-circuit current is used as a measurement variable for the rotor speed and rotational direction when the pump is deactivated. The principle for detection is shown in the diagram representation of FIG. 1.For this purpose, the winding of the stator is temporarily switched into the short circuit at the time t=0 via the bridge circuit of the frequency converter, in particular the B6 bridge in the case of a 3-phase motor (block 10). If the rotor is rotating (e.g. due to the presence of an external flow), a short-circuit current builds up. In this case, 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 in order to prevent damage to the circuit breakers of the inverter or impermissible heating of the stator winding or even a current which leads to irreversible demagnetization of the magnets.In order to ensure this, a threshold value I S,ef of the current I S is defined as the only parameter for detecting the external throughflow. This is to be chosen such that a. it is large enough to be able to unambiguously distinguish from measurement noise in order to be able to exclude erroneous detection. b. it is small enough that the hardware and / or the motor are not damaged.In block 20, a check is made as to whether the short-circuit current I S exceeds the defined threshold value I S,ef before a timeout, in this case 100 ms. If this is the case, then a presence of an external flow is assumed and the method is continued in block 30. If the motor instead remains de-energized, i.e. the threshold value is not exceeded, a stationary rotor can be concluded (block 70) and the motor can be started normally from the standstill (block 80).The time t1 required until the threshold value I S,ef is exceeded in block 20 is stored. Subsequently, the pulse width modulation of the frequency converter is switched off (block 30), as a result of which all power switches (for example. MOSFETs) of the bridge circuit of the frequency converter block and the short-circuit current can quickly commutation off via the free-wheeling diodes on account of the high intermediate circuit voltage. In addition, the measured short-circuit current I s,k is stored, the index k representing the number of the measurement carried out.The procedure with the short-circuit measurement (block 40) and the deactivation of the PWM and the storage of the measured value (block 30) is repeated three more times with the determined timing, so that the three phase currents in total at four different points in time are known. These can be converted into complex space pointers by means of the known Clarke transformation (block 50). The electrical frequency and finally three rotational speeds can be calculated from the phase positions of the four space vectors with knowledge of the time difference ΔT (block 50).In principle, although two indicators and two measured values would be sufficient to be more robust with respect to measurement noise, four indicators are measured or calculated. The median value from the set of estimated rotational speeds is used as the estimated value for the rotational speed (block 50).The signed rotational speed is then obtained byThe implemented method still takes into account a detection of the zero crossing at the angle argument Δφ=φ t- φt-(ΔTsc+ΔTPWM,aus)(Definitionsbereich [0.2π]).After the procedure of FIG. 1 determines a bypass flow as well as the speed imposed on the rotor, the motor may be spun up at block 60 with a modified speed ramp tuned to the estimated speed.starting the engine in the presence of a bypass flow (block 60)If an external flow has been detected using the method described above, the procedure for starting the motor is established as a function of the estimated rotational speed and rotational direction. In principle, a start attempt is made only up to a defined maximum rotational speed in terms of absolute value.The rotor position is not known at the beginning of the starting process. In the method described above for detecting the external flow, phase positions of the short-circuit current are indeed determined. However, this is a transient transient transient current and not the steady-state short-circuit current, which is located in the d-axis of the rotor-oriented coordinate system if the phase resistance is ignored. Furthermore, the phase position is only calculated temporally after the sampling on the basis of the computation-intensive atan2calculation in a slower task. This means that the phase position of the current at the time at which it was calculated is already out of date and cannot be used for regulation. However, since in the case of an external flow, a steady-state operating point can be assumed, it can be assumed that the rotational speed remains largely constant during the computing time which is relatively short with respect to the mechanical time constants.To start up the pump, the motor is then controlled in an open circuit. This means that the motor current is regulated, but the rotational speed is controlled. This difference will be briefly discussed with reference to FIG. 2 which shows the closed loop speed control scheme during regular pump operation. The speed regulator 104 generates a setpoint value I Soll for the motor current as a control variable as a function of a setpoint speed n soll and the actual speed _i or the resulting control deviation. The current regulator 100 adjusts the motor to the setpoint value I Soll wherein the estimated value of the rotor position φ≅and the phase currents I ist are supplied to the current regulator 100 as input variables for the actual value, and the stator voltage U Soll is output as the actuating variable. The latter is then generated by the inverter 101 of the frequency converter for supplying power to the motor 102. Since the pump operates without generator, the current actual rotational speed ξ and the rotor position φξare estimated with the aid of an observer 90 on a model basis with the aid of the input variables actual current I ist and voltage setpoint value U Soll. As a result, the motor can be operated efficiently, since only the current necessary to achieve the desired rotational speed n Soll is adjusted. The setpoint voltage U Soll represents the most important input variable. To a good approximation, it is proportionally dependent on the rotational speed, for which reason the control method with the observer 103 used only works reliably at higher rotational speeds, since here the induced countervoltage in the voltage setpoint value is dominant, and thus a sufficiently good signal-to-noise ratio is ensured.In the case of a detected external flow, a switchover is now made to a speed control for the pump start, i.e. a closed-loop control, the principle of which is explained with reference to FIG. 3. The setpoint rotational speed n Soll is specified via a ramp 105 and the current is controlled by the current controller 100 to a setpoint value I Soll which is fixed in terms of absolute value, the said setpoint value being selected such that it is sufficiently large to enable capture of the rotor and to prevent the rotor from dropping out during the further course of the start. The current is, just as in the closed circuit, controlled in its phase position with reference to a reference φ≅ soll but here a phase position φ≅ soll is used which results from the integration of the rotational speed setpoint values n Soll. This phase position φ≅ soll does not correspond to the rotor position for the first time.The starting point and steepness of the ramp 105 are adapted to the purpose to be achieved, namely to overrun the rotor field caused by the permanent magnet excitation with the stator field caused by the stator winding and to fall with the latter when the differential speeds of the two fields are sufficiently low. The amplitude of the current is regulated and the torque-forming portion of the current is adjusted as a function of the load point. The ramp slope is made relatively small to increase the likelihood of successful entry drops. The procedure is shown in the schematic diagram of FIG. 4.For the selection of the specific ramp 105, a case distinction 61 is initially made for rotational speed ntestimated in block 50 of FIG. 1. At a positive speed _n due to the external flow, the ramp 105a for the open circuit according to FIG. 3 is started with an initial speed setpoint value of zero. The speed ramp 105 moderately increases in the positive speed range. The transition from the open-loop speed control (FIG. 3 ) to the closed-loop speed control (FIG. 2 ) is realized when the rotor has been accelerated to a speed above the estimated speed nt due to the external flow.In the case of a negative rotational speed as a result of the external throughflow, the start of the ramp 105 bis set to a negative rotational speed. Advantageously, but not necessarily, it is necessary to set the start to an initial nominal rotational speed value which is greater in terms of amount than the initial estimated rotational speed, here for example to a value +500.This increases the time window in order to ensure successful incidence of the rotor field with the stator field.The reduced slope of the ramp 105 a, 105 bused has a further function in addition to increasing the time window for the rotor field to enter the stator field when an opposing external flow is present. Since it is intended to accelerate counter to the direction of rotation of the rotor, a generator operating point can be established at which energy can be extracted from the hydraulic system by recuperation. This energy, unless otherwise consumed, would charge the DC link capacitor. If this charge is excessively heavily charged, damage can occur. Generally, the generator case efficiency is defined as follows, where P mech is the mechanical power extracted from the system.For operating cases with P mech >P v positive generator efficiencies result and the intermediate circuit is charged, whereby the voltage present at the intermediate circuit capacitor increases. Such an operating case should be excluded as far as possible or at least be present over such a short period of time that the voltage applied to the capacitor remains within the permissible limit values.This is realized by selecting the ramp to be small, as a result of which P mech remains small and at the same time ensuring that the losses are sufficiently large, that is to say P mech< P v applies, as a result of which energy is extracted from the intermediate circuit. This is realized by a sufficiently large adjusted phase current I Soll in the open circuit, which leads to current heat losses in the motor and switching losses in the inverter 101. Moreover, a higher motor current also increases the success prospects for capturing the rotor, as the possible torque for capturing the rotor is increased.A slightly modified approach is shown in FIG. 5. In contrast to the variant of FIG. 4, a slightly modified ramp 105b' is to be used here in the case of an estimated negative rotational speed π. According to the modified ramp 105b', the initial starting value of the setpoint rotational speed is not set to a value greater in terms of amount than the estimated value nt, but instead to a value 0.9*nt that is less by 10%.

Claims

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, which in a regular pump operation for the intended delivery of a delivery medium from a suction side to a pressure side of the centrifugal pump rotates at a positive speed, and wherein the speed regulation of the motor operates without encoder 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 throughflow, and when a positive speed of rotation generated by the external throughflow is detected the motor is controlled according to a speed ramp starting with a speed setpoint value of zero and when a negative speed of rotation generated by the external throughflow is detected the motor is controlled according to a speed ramp starting with a negative speed setpoint value.Method according to Claim 1, characterized in that a short-circuit current generated by the external throughflow is measured within at least one motor winding as a current measurement by at least one phase of the motor windings being switched into the short circuit by means of the bridge circuit of the frequency converter.Method according to Claim 2, characterized in that a threshold value for the short-circuit current is defined and, if the threshold value is exceeded, external throughflow is detected, wherein the threshold value level is preferably defined in such a way that a differentiation of a short-circuit current generated by external throughflow from measurement noise is possible, but hardware damage due to an excessively high current intensity is avoided.Method according to claim 3, characterised in that one or more times, in particular when the pump is put into operation, a short-circuit current measurement is carried out in the absence of an external flow in order to evaluate the measurement noise of the individual pump.Method according to one of the preceding claims, characterized in that the rotational speed of the pump impeller generated by the external throughflow is furthermore determined.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 taking into account the phase position of the measured short-circuit currents and the time difference between the measurement points, wherein the time required from switching the short circuit until the threshold value is reached is stored in order to record the time difference.Method according to one of the preceding claims, characterized in that the motor is controlled after detection of an external flow in the open circuit by the motor current being controlled to a setpoint value, the rotational speed being controlled using a rotational speed ramp.Method according to Claim 7, characterized in that, when controlled in the open circuit, the current controller receives as input variables a current amplitude setpoint value, the measured actual currents and a reference for the phase position of the currents calculated by integration of the setpoint rotational speed according to the rotational speed ramp.Method according to Claim 7 or 8, characterized in that the motor is controlled in the open circuit to a constant setpoint current amplitude value, the selected setpoint value being selected such that the losses exceed the power generated in generator form to be expected, but do not lead to thermal or other overloading of the pump assembly.Method according to one of the preceding claims, characterized in that, when a negative rotational speed generated by external throughflow is detected, the motor is actuated according to a rotational speed ramp whose initial negative rotational speed is greater than or equal in terms of absolute value to the negative rotational speed generated by the external throughflow.Method according to one of the preceding claims, characterized in that, when a negative rotational speed generated by external throughflow is detected, the motor is actuated according to a rotational speed ramp whose initial negative rotational speed is smaller in terms of absolute value by 5%-15% than the negative rotational speed generated by the external throughflow.Method according to one of the preceding Claims 7 to 11, characterized in that, in the case of a positive rotational speed detected as a result of the external throughflow, a changeover is made to closed-loop rotational speed control if the engine has been accelerated to a rotational speed above the detected positive rotational speed.Pump, in particular a centrifugal pump, particularly preferably a heating circulation pump, having at least one controller configured to carry out the method according to one of the preceding claims.

Images