Control system for a synchronous machine and method for operating a synchronous machine

Abstract

The invention relates to a control system and to a method for operating a synchronous machine. In particular, the synchronous machine is controlled on the basis of a rotor angle that was determined by means of a sensorless rotor-angle detection method. In order to check the reliability of the rotor angle determined without sensors, the difference value between the rotor inductances in the q direction and in the d direction is monitored. If said difference value falls below a limit value, this indicates possible instabilities in the determination of the rotor angle.

Claims

1. A method for operating a synchronous machine, the method comprising: determining (S1) a rotor angle of the synchronous machine via a sensor-free rotor angle detection; determining (S2) a difference value between a rotor inductance of the synchronous machine in a pole axis direction and a rotor inductance of the synchronous machine in a pole gap direction in a coordinate system, fixed to a rotor, of the synchronous machine; discarding (S3) the determined rotor angle when an amplitude of the difference value between the rotor inductance of the synchronous machine in the pole axis direction and the rotor inductance of the synchronous machine in the pole gap direction is below a predefined threshold value; discarding the determined rotor angle when the amplitude of the difference between the rotor inductance of the synchronous machine in the pole axis direction and the rotor inductance of the synchronous machine in the pole gap direction is smaller than a previously determined difference; and driving (S4) the synchronous machine using the determined rotor angle when the amplitude of the difference between the rotor inductance of the synchronous machine in the pole axis direction and the rotor inductance of the synchronous machine in the pole gap direction is greater than the predefined threshold value.

2. The method as claimed in claim 1, wherein determining the rotor angle includes applying high-frequency test voltage signals in the pole axis direction, measuring system responses to the applied high-frequency test voltage signals, and estimating an angle based on the measured system responses.

3. The method as claimed in claim 2, wherein determining the difference value includes applying high-frequency test voltage signals in a direction differing from the pole axis direction, measuring the system responses to the applied high-frequency signals, and estimating an angle using the measured system responses.

4. The method as claimed in claim 3, wherein the high-frequency test voltage signals for determining the rotor angle and the high-frequency test voltage signals for determining the difference value have different operating frequencies.

5. The method as claimed in claim 1 further comprising producing a modified determined rotor angle from the determined rotor angle when the amplitude of the difference between the rotor inductance of the synchronous machine in the pole axis direction and the rotor inductance of the synchronous machine in the pole gap direction lies below the predefined threshold value.

6. A control system (10) for a synchronous machine (5b), comprising: a control apparatus (2) configured to perform field-oriented control of the synchronous machine (5b) according to the method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is explained in more detail below on the basis of the exemplary embodiments given in schematic figures of the drawings. In the figures:

(2) FIG. 1: shows a schematic illustration of a control system for a synchronous machine according to one embodiment;

(3) FIG. 2: shows a schematic illustration of a characteristic diagram of the dependence of the inductance difference between q-axis and d-axis inductance of a synchronous machine on the useful energization values; and

(4) FIG. 3: shows a schematic illustration of a flowchart for a method for operating a synchronous machine according to one embodiment.

DETAILED DESCRIPTION

(5) FIG. 1 shows a schematic illustration of a control system 10 for an electric drive unit 5 having an inverter 5a, which feeds a synchronous machine 5b with three-phase current.

(6) The synchronous machine 5b may be for example a three-phase synchronous machine. However, it is also possible in principle to provide another number of phases for the synchronous machine. In this case, the control of the synchronous machine 5b in the electric drive unit 5 plays a central role. To provide a required torque using a synchronous machine 5b, a rotating electrical field, which rotates synchronously with the rotor, is generated in the stator of the machine. To generate this field, the current angle of the rotor is required for the control.

(7) The control system 10 therefore comprises a control apparatus 2 that performs field-oriented control of the synchronous machine 5b or of the inverter 5a of the drive unit in the d-axis, q-axis coordinate system fixed to the rotor. To this end, the control apparatus 2 is fed with a setpoint torque Ts and refers to the instantaneous useful energization values Iq, Id in the d-axis, q-axis coordinate system fixed to the rotor, which values are provided by a first transformer apparatus 1. The first transformer apparatus 1 to this end measures the phase currents Ip of the synchronous machine 5b and transforms the phase currents Ip into the useful energization values Iq, Id.

(8) The control apparatus 2 outputs drive voltages Udq in the d-axis, q-axis coordinate system, fixed to the rotor, of the synchronous machine 5b to a second transformer apparatus 4, which performs a corresponding transformation of the drive voltages Udq into phase drive voltages Up for the synchronous machine 5b. Both the first transformer apparatus 1 and the second transformer apparatus 4 refer to the time-dependent rotor angle 0 of the rotor of the synchronous machine 5b in relation to the stator of the synchronous machine 5b for the transformation. This rotor angle 0 is generated by an observer 8, which in turn is able to refer to a determined rotor angle s of a position sensor 6 and/or to a rotor angle difference that is determined by an angle estimation algorithm 7 depending on measured system responses of the synchronous machine 5b.

(9) The position sensor 6 may record for example electrical operating parameters of the synchronous machine 5b, for example by recording the voltage at the neutral point of the synchronous machine 5b. The observer 8 may have for example a Kalman observer, a Luenberger observer, a Hautus observer or a Gilbert observer for assisting in and checking the plausibility of the angle observation 0.

(10) To feed the angle estimation algorithm 7, a summing element 3 is provided between the control apparatus 2 and the second transformer apparatus 4, by way of which summing element 3 for example test voltage pulses ud, uq at a particular operating frequency c are able to be modulated up to the drive voltages Udq. These test voltage pulses ud, uq may be fed in at an input connection 3a by a controller apparatus 9 that is able to counteract the observed angle difference of the angle estimation algorithm 7.

(11) The control system 10 is likewise suitable for any other kind of sensor-free rotor angle determination method, that is to say, as an alternative to feeding in test voltage pulses ud, uq, other methods may also be used to allow system responses to be measured. By way of example, system responses may be used for rotor angle determination purposes by measuring voltages at the neutral point at suitable times using pulse width-modulated phase driving. It may also be possible to use test signal methods for rotor angle determination purposes, wherein test signals having a high operating frequency are modulated up to the drive signal. In general, any sensor-free determination method that is based on a difference between rotor inductance Ld in the pole axis direction and the rotor inductance Lq in the pole gap direction of the synchronous machine 5b is suitable for feeding the angle estimation algorithm 7 for evaluating the angle difference.

(12) The system response of the synchronous machine 5b depends inter alia on the useful energization, whose value influences the difference between rotor inductance Ld in the pole axis direction and the rotor inductance Lq in the pole gap direction. The longitudinal currents Id and transverse currents Iq of a permanently excited synchronous machine behave in a manner depending on the rotor inductance Ld in the pole axis direction and the rotor inductance Lq in the pole gap direction and the applied voltage Ud or Uq, as follows:
dId/dt=Ld.sup.1.Math.UdR.Math.Ld.sup.1.Math.Id+Lq.Math.Ld.sup.1.Math.e.Math.Iq
dIq/dt=Lq.sup.1.Math.UqR.Math.Lq.sup.1.Math.Iq+Ld.Math.Lq.sup.1.Math.e.Math.IdLq.sup.1.Math.u_p

(13) This applies for the angular velocity e of the rotor of the synchronous machine 5b, the ohmic resistance R and the claw-pole rotor voltage u_p. Furthermore, it is assumed that the pole shoe or pole shoes are not operated in saturation, that is to say that the relationship between current and magnetic flux is linear and the respective inductance is not dependent on the current strength.

(14) At a high useful energization of the synchronous machine 5b, there may however be progressive saturation of the rotor core of the synchronous machine 5b, such that the relationship between current and magnetic flux does not exhibit nonlinearity. In particular, for some 2-tuples of the useful energization Id, Iq in the coordinate system fixed to the rotor, it may be the case that the difference between rotor inductance Ld in the pole axis direction and the rotor inductance Lq in the pole gap direction disappears. Sensor-free rotor angle determination methods that depend on this inductance difference in order to obtain meaningful measured values may lose relevance at such operating points.

(15) One exemplary such rotor angle determination method is illustrated below. To determine the angle error , the following test signal [ud, uq] is applied to the synchronous machine 5b:
[ud,uq]=uc.Math.cos(ct).Math.[cos(), sin()]

(16) Since the electrical behavior of the synchronous machine 5b is able to be described as a purely inductive load at high frequencies, the following current vector [id, iq] is given as system response to the test signal [u.sub.d, u.sub.q]
[id,iq]=(uc/c).Math.sin(ct).Math.[cos()/Ld, sin()/Lq]

(17) To extract angle information from these currents through signal processing, the connection between the measured phase currents in the coordinate system, fixed to the stator, of the synchronous machine 5b and the currents in the coordinate system fixed to the rotor is required. Depending on the estimated d-axis direction of the synchronous machine 5b, the following is given for the currents in the coordinate system fixed to the rotor:
[id,iq]=uc/(4cLq Ld).Math.[(LqLd)(sin(ct2)+sin(ct+2))+2(Lq+Ld)(sin(2ct),
(LqLd)(sin(ct2)+sin(ct+2))+2(Lq+Ld)(sin(2ct)]

(18) To extract the desired terms containing the angle difference information from the measured values of the currents [id, iq] in the coordinate system fixed to the rotor, following high-pass filtering, a convolution at the operating frequency c and subsequent low-pass filtering, it is possible to obtain that term that contains the angle difference information :
[id,iq]=u.sub.c/(4cLqLd).Math.[(LqLd)(sin(2)),(LqLd)(cos(2)+(Lq+Ld)]

(19) As is easily able to be seen, the meaningfulness of the measured phase currents with regard to the angle difference information depends on the difference in the inductances Lq and Ldthe smaller the difference, the more inaccurate the determination of the angle difference .

(20) To counteract this phenomenon, it is advantageous to select the operating points of the synchronous machine 5b as far as possible such that the difference in the inductances Lq and Ld remains as large as possible, that is to say that the synchronous machine 5b is not operated at saturation where possible, including at high torques.

(21) As is able to be seen through the above embodiments, the difference LqLd between the q-axis inductance (rotor inductance in the pole gap direction) and the d-axis inductance (rotor inductance in the claw-pole rotor axis direction) is decisive for determining the angle information of the rotor angle. This difference in the inductances may be determined by operating the sensor-free rotor angle detection method briefly at an angle differing from 0 degrees for diagnostic purposes. In this case, the high-frequency test voltage signal is applied and evaluation thereof is performed no longer in the d-axis direction, but rather in a direction differing from the d-axis direction. In particular, any desired number of different test angles may be used for determining the difference value in the rotor inductances. Thus, for example, n test angles may be used, having an equidistant angle spacing of 360:n degrees. By way of example, a set of test angles of 0 degrees, 15 degrees, 30 degrees, etc. may be used. Test angles of 0 degrees, 45 degrees, 90 degrees, etc. are likewise possible for example.

(22) If the difference value between the rotor inductances of the synchronous machine 5b is diagnosed in the same way as for the sensor-free rotor angle detection, then the frequency ranges for the operating frequency of the applied test voltage pulses should have a sufficiently large spacing from one another. By way of example, the test signal for the sensor-free rotor angle determination may be implemented at a frequency of approximately 1 kHz. In this case, the test voltage signals are applied in the d-axis direction. To check the difference value in the rotor inductances, in this case for example a frequency of approximately 10 kHz may be used, the test voltage signals being applied with a varying direction differing from the d-axis direction. Synchronous sensor-free rotor angle determination and simultaneous checking of the difference value in the rotor inductances are thereby able to be performed. As an alternative, it is also possible to switch back and forth alternately between an angle determination mode for determining the rotor angle and a diagnostic mode for determining the difference value in the rotor inductances.

(23) If, in the diagnosis of the difference value between the rotor inductances of the synchronous machine, it is established that the rotor inductance in the d-axis direction (pole axis direction) and the rotor inductance of the synchronous machine in the q-axis direction (pole gap direction) drops below a predefined threshold value (moves towards zero), then it is not possible to provide a meaningful determination of the rotor angle of the synchronous machine 5b, for the reasons described above. Therefore, in this case, the determined rotor angle is discarded and thus does not enter into the control procedure for driving the synchronous machine. Otherwise, there is the danger of the sensor-free rotor angle detection method determining an erroneous rotor angle, and this erroneous rotor angle may lead to instabilities in the driving of the synchronous machine 5b.

(24) By contrast, if the difference value between the rotor inductances in the q-axis direction and d-axis direction lies above the predefined threshold value, then reliable sensor-free rotor angle detection is possible. In this case, the determined rotor angle may be used to drive the synchronous machine.

(25) FIG. 2 shows a schematic illustration of a characteristic diagram K of the dependence of the inductance difference between the d-axis and q-axis inductances of a synchronous machine on the useful energization value Id and Iq. In this characteristic diagram K, the amplitudes of the system responses from the previously described test voltage pulses are illustrated as a difference value in the inductancesfor example as isobars L1, L2, L3 and L4. In this case, the isobar L1 for example shows a relatively small inductance difference, whereas the isobars L2 to L4 each exhibit constantly increasing inductance differences. For a predefined torque, various sub-combinations of the useful energization values Id and Iq may be selected. This is indicated for example by the dashed lines T1, T2 and T3. An operating point trajectory A1 that is established using a maximum torque per ampere (MTPA) method is provided for torque-dependent operating points, for example.

(26) However, as may be seen from the profile of the operating point trajectory A1, this runs through an operating point area having a low inductance difference, which is indicated here by the region L1.

(27) If the method described above establishes that the synchronous machine is being operated in the region L1, then reliable sensor-free rotor angle determination is not possible here. Therefore, if the difference value in the rotor inductances drops below a predefined threshold value, it is possible to switch to another torque-dependent operating point trajectory, for example the operating point trajectory A2. The driving of the synchronous machine is thereby able to be stabilized by way of the information about the difference value between the rotor inductances of the synchronous machine.

(28) FIG. 3 shows a schematic illustration of a flowchart as underlies a method for operating a synchronous machine according to one embodiment. In step S1, a rotor angle of the synchronous machine is determined by way of a sensor-free rotor angle detection method. This may be performed for example by way of the rotor angle detection method described above.

(29) In step S2, a difference value between the rotor inductance of the synchronous machine in the d-axis direction (pole axis direction) and a rotor inductance of the synchronous machine in the q-axis direction (pole gap direction) is determined. The difference value is determined in this case in a coordinate system (d-axis q-axis coordinate system), fixed to the rotor, of the synchronous machine. The difference value may be determined in the manner described above by applying high-frequency test voltage signals in a direction differing from the d-axis direction.

(30) The determined difference value in the rotor inductances is then checked. If the difference value in the rotor inductances lies below a predefined threshold value, then the determined rotor angle is discarded in step S3 and does not enter into the control for driving the synchronous machine.

(31) By contrast, if the difference value in the rotor inductances is greater than the predefined threshold value, then the determined rotor angle may be classified as reliable and enter into the driving of the synchronous machine.

(32) As an alternative or in addition to comparing the difference value in the rotor inductances with a predefined threshold value, it is also possible to compare the determined difference value in the rotor inductance with a previously determined difference value in the rotor inductances. If it is established in the process that the difference value in the rotor inductances decreases, then this may already indicate the onset of instability in the rotor angle detection. Accordingly, in the case of a decreasing difference value in the rotor inductances, corresponding countermeasures may already be initiated in order to maintain the stability of the control for the synchronous machine.

(33) In summary, the present invention relates to a control system and to a method for operating a synchronous machine. In particular, the synchronous machine is driven in this case on the basis of a rotor angle that was determined by way of a sensor-free rotor angle detection method. To check the reliability of the rotor angle determined in a sensor-free manner, the difference value between the rotor inductances in the q-axis direction and d-axis direction is monitored. If this difference value lies below a threshold value, then this indicates possible instabilities in the determination of the rotor angle.