Method for monitoring the operation of a frequency converter and frequency inverter

Abstract

Method for monitoring the operation of a frequency converter (1) designed to control an electric motor in the form of a synchronous motor (2), the method comprising the steps: - Generating phase voltages (u1, u2, u3) for corresponding phase strands (2.1, 2.2, 2.3) of the electric motor (2), - Determining a voltage rotating field, - Measuring the phase currents that are forming (i1, i2, i3), - Determining a current rotation field as a function of the measured phase currents (i1, i2, i3), and - Calculating a phase difference between the voltage rotating field and the current rotating field and / or calculating a frequency difference between the frequency of the voltage rotating field and the frequency of the current rotating field, - wherein the phase voltages (u1, u2, u3) are generated such that, with fault-free operation of the frequency converter (1), the magnitude of a current vector (I) corresponding to the current rotating field does not fall below a minimum value (Imin), characterized in that - a fault condition of a gate signal generation, the electric motor (2) and / or a current measurement is determined when the phase difference exceeds a phase difference threshold and / or when the frequency difference exceeds a frequency difference threshold.

Description

[0001] The invention relates to a method for monitoring the operation of a frequency converter and a frequency converter.

[0002] DE 11 2015 001 001 T5 relates to a control device for a rotating electric machine with sensorless rotor position detection.

[0003] US patent 2002 / 0117990A1 discloses the operation of a three-phase machine whose position sensorless rotor position detection is achieved by means of an injection method.

[0004] DE 103 32 228 A1 discloses the position sensor-less operation of a permanent magnet synchronous motor, which is protected against falling out of step by monitoring the load angle.

[0005] The invention is based on the objective of providing a method for monitoring the operation of a frequency converter and a frequency converter that enable reliable and cost-effective monitoring of the operation.

[0006] The invention solves this problem by means of a method for monitoring the operation of a frequency converter according to claim 1 and a frequency converter according to claim 6.

[0007] This method is used to monitor the operation of a frequency converter designed to control an electric motor. The electric motor can be, for example, a synchronous motor or an asynchronous motor.

[0008] In this process, phase voltages are conventionally generated for corresponding phase strands or between corresponding phase strands of the electric motor based on associated setpoint values ​​for the phase voltages. The amplitude and frequency of the phase voltages are generated, for example, based on these setpoints, such that a desired speed and / or torque of the electric motor is achieved. Typically, three phase voltages are generated for three corresponding phase strands. Reference is made to the relevant technical literature in this regard.

[0009] As a further procedural step, a voltage rotation field is determined. In particular, the voltage rotation field is calculated as a function of the target values ​​for the phase voltages.

[0010] Regarding the determination of the voltage rotation field, reference should also be made to the relevant technical literature.

[0011] As a further procedural step, at least one phase current is measured that flows in a phase strand and is determined by the phase voltages. For an n-phase electric motor, it may suffice to measure n-1 phase currents, since the nth phase current can be calculated from the other phase currents. Similarly, for a three-phase electric motor, it may suffice to measure two of the three phase currents, since the third phase current can be calculated from the other two.

[0012] As a further procedural step, a rotating magnetic field is calculated as a function of the measured phase current(s). In this respect, reference is also made to the relevant technical literature.

[0013] As a further procedural step, a phase difference between the voltage rotating field and the current rotating field and / or a frequency difference between the frequency of the voltage rotating field and the frequency of the current rotating field are calculated.

[0014] As a further procedural step, an error state is determined if the phase difference or an amount of the phase difference exceeds a phase difference threshold and / or if the frequency difference or an amount of the frequency difference exceeds a frequency difference threshold.

[0015] The phase difference threshold and the frequency difference threshold can be absolute or relative values. They can also accommodate a certain tolerance, particularly potential slippage. For example, the phase difference threshold and the frequency difference threshold can be set within a range of 1% to 10% relative to one phase of the rotating voltage field or relative to one frequency of the rotating voltage field.

[0016] Determining the rotor position of an electric motor in safety-related applications is typically achieved using a so-called safety encoder system. Such a system allows for the determination of the rotor's rotational speed and angular position. However, the encoder system represents a significant space and cost factor. Therefore, it is common practice to forgo such encoder systems in cost-critical installations. This does, however, result in limitations in dynamics and accuracy, which are acceptable in many applications.

[0017] Conventional synchronous or asynchronous motors are driven by a three-phase rotating magnetic field, where the phases are labeled 2.1, 2.2, and 2.3 in the following figures (conventionally also denoted as U, V, and W). The angular velocity ω_mech of the rotor depends on the angular velocity ω_el, or rotational frequency f, of the supplying rotating magnetic field. The rotating magnetic field induces a phase current i1, i2, and i3 in each of the three phases 2.1, 2.2, and 2.3. The angular velocity w_el is given by ω_el = 2πf.

[0018] In synchronous motors, there is a strict proportionality between the angular velocity of the rotating magnetic field or voltage field and the angular velocity of the rotor. In asynchronous machines, the angular velocity of the rotating magnetic field or voltage field is always higher than the angular velocity of the rotor during motor operation. This difference is described by the slip (s) and is necessary for the asynchronous machine to generate torque. During motor operation, the slip is always between 0 and 1. When calculating the mechanical speed, the number of pole pairs (p) must also be taken into account.

[0019] Therefore, the following applies to the synchronous machine: ωmech,SM=2πfp

[0020] The ASM still needs to be updated to include the hatching process. ωmech,ASM=2πfp(1−s)

[0021] According to the invention, a safe encoder system is no longer required, and the frequency of the current rotating field and the frequency of the voltage rotating field are used to monitor the operation of the frequency converter, for example to monitor the angular velocity of the electric motor controlled by the frequency converter.

[0022] The rotating field describes the cycle of voltage in an electric motor. To obtain the current orientation of the current and voltage phasors that describe the rotating field and the rotating field respectively, the current voltage and current values ​​are geometrically added. x→_=23(1 a_ a2_)(x1x2x3) with a_=ej120°

[0023] The value x→_ In addition to amplitude information, it also contains angle information. To calculate the rotational speed, only the derivative of the angle needs to be determined. Instead of voltage, duty cycles can also be used to calculate the angular velocity in a rotating voltage field. Duty cycles and voltage differ only in amplitude, which in turn has no influence on the angular velocity.

[0024] Here, the property can be used that the current and voltage (or duty cycles) include the rotating field frequency and it is possible to determine the speed of the rotor with both values ​​while neglecting slippage.

[0025] This makes it possible to build a dual-channel structure to monitor the rotor's angular velocity. The first channel represents the rotating voltage field, and the second channel represents the rotating current field.

[0026] After determining the fault condition, the frequency converter can be switched off, for example, based on the functions STO, SS1, SLS, SMS and SDI.

[0027] The invention presents a cost-effective solution for sensorless safety technology, particularly for the functions SS1, SLS, SMS, SDI, and SMS. Furthermore, reliable output frequency detection is possible even for asynchronous motors without additional sensors in the power output stage. The invention therefore enables the implementation of speed-related safety functions without an additional feedback system, especially for asynchronous machines.

[0028] The output frequency, i.e., the rotational speed of the electric motor, can be monitored by independently evaluating the phase angle of the current and voltage rotating fields. This monitoring can be performed over a communication channel between a control unit and a power unit, where the voltage rotating field, or the associated voltage space vector, is generated from setpoint values ​​for the phase voltages from the control unit, and the current rotating field, or the associated current vector, is generated from a measured value of the phase currents from the power unit.

[0029] The dual-channel capability, necessary for safety reasons, is achieved by monitoring two separate physical quantities: the angle of the voltage rotating field or voltage vector on the one hand, and the angle of the current rotating field or current vector on the other.

[0030] It is possible for two monitoring units to monitor the angles of the voltage and current vectors and initiate or maintain a safe state if the detected rotational speed is outside of predefined limits.

[0031] No feedback system is required on the motor to measure the rotational speed. This allows for cost savings within the system.

[0032] According to the invention, the phase voltages are generated such that, with fault-free operation of the frequency converter, the magnitude of a current vector corresponding to the rotating current field does not fall below a minimum value. This minimum value is typically chosen such that, with fault-free operation of the entire system consisting of the frequency converter and electric motor, measurement of the rotating current field is still (or just barely) possible under all relevant operating points. The minimum value can be determined by means of simulation, test series, etc.

[0033] In other words, a non-torque-generating current is applied (accepting the associated additional losses) to enable monitoring of the frequency converter / electric motor even at unfavorable operating points.

[0034] In a synchronous machine, conventional monitoring and alignment of the current and voltage phase sequences is not always possible because the current or phase sequence of the machine becomes zero at certain operating points. According to the invention, an additional current component in the form of a magnetizing current (a non-torque-generating current) is introduced, thereby ensuring continuous alignment of the current and voltage phase sequences. This allows for the reliable detection of all faults, from gate signal generation and the motor to current measurement. It is irrelevant whether this additional current component is positive or negative.

[0035] The voltage rotating fields or voltage vectors and current rotating fields or current vectors according to the invention can be considered in a coordinate system which rotates with the stator frequency (voltage).

[0036] As an extension, if the torque-generating current is large enough, the additional current component, or magnetizing current, can be reduced again. This can reduce losses.

[0037] The additional current component, which fundamentally enables the measurement of the current phase sequence, can be set by a non-safety-relevant controller and evaluated by a safe hardware and software solution.

[0038] If the current phase sequence can no longer be evaluated, for example because the underlying currents are too small or no longer match the voltage phase sequence, an error is detected and an appropriate response is taken.

[0039] In a synchronous machine, the range for the permissible angle between the current rotating field and the voltage rotating field can be adjusted, since in a synchronous machine the current rotating field and the voltage rotating field can also be in phase.

[0040] According to one embodiment, the phase voltages are generated in such a way that a torque-generating component of the current vector has an magnitude such that a predetermined or desired torque is achieved, wherein a current component in the form of a non-torque-generating component of the current vector has an magnitude such that the magnitude of the current vector does not fall below the minimum value.

[0041] The electric motor is a synchronous motor.

[0042] According to one embodiment, the voltage rotation field is determined as a function of the target values ​​for the phase voltages. Reference should also be made to the relevant technical literature in this regard.

[0043] According to one embodiment, the phase voltages for the corresponding phase strands of the electric motor are generated by means of pulse width modulation with variable duty cycles, whereby the voltage rotation field is determined as a function of the duty cycles of the pulse width modulation. Reference is also made to the relevant technical literature in this regard.

[0044] According to one embodiment, after determining the fault state, fault treatment is carried out; in particular, the generation of phase voltages and thus also of the rotating field is prevented.

[0045] According to one embodiment, a safe torque-off function is performed after the fault condition has been determined.

[0046] The frequency converter according to the invention is designed to carry out the method described above.

[0047] According to one embodiment, the frequency converter has a control unit designed to regulate the phase currents.

[0048] According to one embodiment, the control unit is designed to use the phase voltages as the control variable for regulating the phase currents.

[0049] The invention is described in detail below with reference to the drawings. These show: Fig. 1. Highly schematic representation of a drive system with a frequency converter and an electric motor controlled by the frequency converter and Fig. 2 schematically a trajectory of a current space vector as a function of a torque.

[0050] Fig. Figure 1 shows a highly schematic drive system with a frequency converter 1 and an electric motor 2 controlled by the frequency converter 1.

[0051] The frequency converter 1 is designed to generate three phase voltages u1, u2, u3 for corresponding phase strands 2.1, 2.2, 2.3 or between corresponding phase strands 2.1, 2.2, 2.3 of the electric motor 2 and to measure the resulting phase currents i1, i2 and i3. Reference is also made to the relevant technical literature in this regard.

[0052] The frequency converter 1 has a control unit 3, for example in the form of a microcontroller, wherein the control unit 3 controls the operation of the frequency converter, in particular regulating the phase currents i1, i2, i3.

[0053] Frequency converter 1, for example, includes a conventional inverter for generating the phase voltages u1, u2, and u3. Frequency converter 1 also includes conventional current sensors, for example, in the form of shunt resistors. The shunt resistors can be arranged, for example, in an emitter path. The phase currents i1, i2, and i3, or phase currents, are measured by means of the current sensors. Furthermore, frequency converter 1 can include a Safe Torque Off (STO) circuit, by means of which an STO state can be initiated.

[0054] For this purpose, control unit 3 determines a voltage rotating field as a function of the generated or to-be-generated phase voltages u1, u2, u3. Furthermore, control unit 3 determines a current rotating field as a function of the measured phase currents i1, i2, i3 and calculates a phase difference between the voltage rotating field and the current rotating field and / or calculates a frequency difference between the frequency of the voltage rotating field and the frequency of the current rotating field. Control unit 3 determines an error condition if the phase difference exceeds a phase difference threshold and / or if the frequency difference exceeds a frequency difference threshold.

[0055] If a fault condition has been identified, fault handling is carried out by inducing a STO state through appropriate control of the inverter.

[0056] Fig.Figure 2 schematically shows a trajectory T of a current space vector I as a function of a changing torque.

[0057] According to the invention, the phase voltages u1, u2, u3 are generated such that a torque-generating component Iq of the current vector I has a magnitude such that a predetermined torque is established, while a non-torque-generating component Id of the current vector I has a magnitude such that the magnitude of the current vector I does not fall below the minimum value Imin. Imin is, for example, selected such that the determination of the current rotation field as a function of the measured phase currents i1, i2, i3 is reliably possible with fault-free operation.

[0058] This ensures that the current phase sequence can be determined under all relevant operating conditions, provided there is no malfunction of the frequency converter and / or the electric motor.

Claims

[1] Method for monitoring the operation of a frequency converter (1) designed to control an electric motor in the form of a synchronous motor (2), the method comprising the steps: - Generating phase voltages (u1, u2, u3) for corresponding phase strands (2.1, 2.2, 2.3) of the electric motor (2), - Determining a voltage rotating field, - Measuring the phase currents that are forming (i1, i2, i3), - Determining a current rotation field as a function of the measured phase currents (i1, i2, i3), and - Calculating a phase difference between the voltage rotating field and the current rotating field and / or calculating a frequency difference between the frequency of the voltage rotating field and the frequency of the current rotating field, - wherein the phase voltages (u1, u2, u3) are generated such that, with fault-free operation of the frequency converter (1), the magnitude of a current vector (I) corresponding to the current rotating field does not fall below a minimum value (Imin), characterized by , that - a fault condition of a gate signal generation, the electric motor (2) and / or a current measurement is determined when the phase difference exceeds a phase difference threshold and / or when the frequency difference exceeds a frequency difference threshold. [2] Method according to claim 1, characterized by , that - the phase voltages (u1, u2, u3) are generated such that a torque-generating component (Iq) of the current vector (I) has an magnitude such that a predetermined torque is established, wherein a non-torque-generating component (Id) of the current vector (I) has an magnitude such that the magnitude of the current vector (I) does not fall below the minimum value (Imin). [3] Method according to any one of the preceding claims, characterized by , that - the generation of the phase voltages (u1, u2, u3) for the corresponding phase strands (2.1, 2.2, 2.3) of the electric motor (2) is carried out by means of pulse width modulation with variable duty cycles, whereby the determination of the voltage rotating field is carried out as a function of the duty cycles of the pulse width modulation. [4] Method according to any one of the preceding claims, characterized by , that - after determining the fault condition, fault handling is carried out, in particular the generation of the phase voltages (u1, u2, u3) is prevented. [5] Method according to any one of the preceding claims, characterized by , that - a Safe Torque Off function is performed after the fault condition has been determined. [6] Frequency converter (1), characterized by , that - the frequency converter (1) is designed to carry out the method according to one of the preceding claims. [7] Frequency converter (1) according to claim 6, characterized by , that - the frequency converter (1) has a control unit (3) designed to control the phase currents (i1, i2, i3). [8] Frequency converter (1) according to claim 7, characterized by , that - the control unit (3) is designed to use the phase voltages (u1, u2, u3) as a control variable for the control of the phase currents (i1, i2, i3).

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