Method and device for providing a commutation interval

Description

[0001] The present invention relates to a method for determining an initial rotor position of a three-phase electric motor, a control unit, and an electric motor. The invention thus lies particularly in the field of electric motors and the control of electric motors.

[0002] For the reliable operation of a three-phase electric motor, the detection of the rotor position is often advantageous, especially when the motor is stationary or running at low speeds. In particular, knowledge of the rotor position is advantageous for accurate and reliable commutation. Various different methods are known in the state of the art for this purpose, which can be divided into sensor-based and sensorless methods. Sensor-based methods have the disadvantage that they require suitable sensors and, accordingly, the hardware complexity and associated costs are higher than with sensorless methods. Sensorless methods typically rely on the injection of test pulses or measurement pulses into the electric motor, which can lead to undesirable noise. Sensorless methods are often based on the position and current dependence of the stator inductances of the electric motor.The various sensorless process types are described, for example, in the published patent application DE 10 2019 127 051 A1.

[0003] Sensorless methods for determining the initial rotor position typically rely on measuring the voltage at the inductive voltage divider, i.e., at the terminal of the de-energized phase. However, these methods have the disadvantage that in some cases, reliable north / south detection is not possible. This can be particularly the case with high-current motors.

[0004] In order to achieve reliable switching between commutation intervals, DE 10 2019 127 051 A1 proposes a method in which a bipolar pulse width modulation is applied to the terminals of two of the three phases and, when a predetermined voltage threshold is reached, the system switches to the next commutation interval.

[0005] DE 10 2016 123707 A1 describes a control device for a motor. A passively connected phase connection allows access to a voltage applied between a star point and a reference potential. Care must be taken to prevent current flow through the passive phase in order to substantially avoid influencing the voltage divider formed at the star point.

[0006] WO 2009 / 047217 A2 describes a method for operating a three-phase electrical machine. To determine the rotor position, a deviation of a temporal profile of an induced voltage compared to a temporal profile of a pulsed voltage is determined.

[0007] DE 10 2016 123715 A1 describes a control device for a multiphase motor with a control unit. The control unit is configured to apply a pulse-width-modulated voltage pattern to four of five phase connections, resulting in an evaluation signal dependent on the rotation angle of the multiphase motor at the first phase connection. The control unit is further configured to determine the rotation angle and / or a commutation condition of the multiphase motor from the evaluation signal.

[0008] DE 102 21 385 A1 describes a method for starting a brushless DC motor. The phase currents in the winding phases are compared using measuring shunts and then compared using comparators.

[0009] DE 102 20 077 A1 describes a method for starting a brushless DC motor.

[0010] It is an object of the present invention to provide a method with low hardware requirements for the reliable determination of an initial rotor position of a three-phase electric motor, which in particular also allows a reliable north-south detection.

[0011] The object is achieved by the subject matter of the independent claims. Optional embodiments are specified in the dependent claims and in the description.

[0012] In one aspect, the invention therefore relates to a method for determining an initial rotor position of a three-phase electric motor with a drive circuit. The method comprises energizing a first and second phase of the three-phase electric motor with an electrical current that increases or decreases over a first energization interval, and determining an induced voltage in a non-energized third phase of the three-phase electric motor at at least two temporally spaced-apart measurement times during the first energization interval. The method further comprises determining an induction voltage difference of the voltage induced in the third phase between the two temporally spaced-apart induction measurement times, taking into account a voltage change in the drive circuit of the first and / or second phase caused by energizing the first and second phase in the first energization interval.The method also includes determining the initial rotor position based on the determined induction voltage difference.

[0013] In a further aspect, the invention relates to a control unit which is configured to determine an initial rotor position of a three-phase electric motor by means of a method according to the invention.

[0014] In a further aspect, the invention relates to an electric motor comprising a stator and a rotor rotatable relative to the stator, wherein the

[0015] Electric motor is configured to determine an initial rotor position of the electric motor by means of a method according to the invention.

[0016] An electric motor has a stator and a rotor. In particular, the electric motor can be designed as a three-phase electric motor, which has three phases or three windings, each of which can be energized via its own connection. The windings can be arranged in the electric motor such that they at least partially overlap one another. For example, the phases of the electric motor can be connected to one another in a delta connection or a star connection. The terms "electric motor" and "motor" are used synonymously in this disclosure text.

[0017] The rotor position corresponds to the angle of rotation of the rotor relative to the stator. Knowledge of the rotor position can be particularly important for reliable commutation of the motor and efficient operation. The rotor's angle of rotation can range from 0° to 360°. Alternatively, a rotation angle assignment can be performed in a range from 0° to 180°, coupled with a determination of the north-south orientation of the rotor or the rotor magnets.

[0018] The inductive voltage divider can, in particular, correspond to the voltage drop between the terminal of the unpowered phase and ground. The voltage is determined by the ratio of the inductances L 1 and L 2 of the first and second powered phases, respectively, which in turn depends on the rotor position.

[0019] Taking into account a voltage change in the control circuit of the first and / or second phase caused by the current supply to the first and second phase in the first current supply interval when determining the induction voltage difference means that an influence of the voltage change in the control circuit on the induction voltage difference to be determined is determined or estimated and at least partially compensated.

[0020] The invention offers the advantage that potential influences and / or measurement errors when determining the induction voltage difference and the resulting influences and / or measurement errors on the determination of the initial rotor position due to a voltage change in the control circuit can be reduced or avoided. This thus offers the advantage that the reliability of determining the initial rotor position can be improved, since undesirable measurement errors can be reduced or avoided.

[0021] Furthermore, the invention offers the advantage that no additional hardware is required to determine the initial rotor position, taking into account the voltage change induced in the control circuit. In particular, the invention offers the advantage that no separate sensors need to be provided for determining the initial rotor position, thus keeping the manufacturing costs for the three-phase electric motor and the control circuit low.

[0022] Furthermore, the invention offers the advantage that a reliable determination of the initial rotor position is also possible for electric motors with low inductance, and in particular for high-current three-phase electric motors, where, according to conventional methods, the influences that typically occur would lead to pronounced measurement errors and, accordingly, to a greatly reduced reliability of the determination of the initial rotor position. Thus, the invention offers the advantage that the method according to the invention for determining the initial rotor position has universal applicability in various types of three-phase electric motors, and thus the required variety of types of control units and / or methods for determining the initial rotor position for different types of three-phase electric motors can be kept to a minimum.

[0023] Optionally, the first and second phases are energized in such a way that the electrical current increases or decreases strictly monotonically and optionally linearly during the first energization interval. This allows for a reliable determination of the sign of the current change, i.e., whether the current increases or decreases during the energization interval. The sign of the change in the current flow determined in this way can then be used for north-south detection of the rotor position.

[0024] Optionally, the voltage change in the control circuit of the first and / or second phase caused by energizing the first and / or second phase during the energization interval corresponds to a shunt voltage difference, which arises at a voltage drop across a shunt resistor for measuring the phase current in the first and / or second phase between two spaced-apart shunt measurement times. The shunt voltage difference can optionally represent a primary cause of an undesirable change in the induction voltage difference, which can falsify the result of determining the induction voltage difference and accordingly reduce the reliability of determining the initial rotor position. The shunt voltage difference can optionally be very small, for example, in the range of a few millivolts.In many cases, if the induction voltage difference is very large compared to the shunt voltage difference, the influence of the shunt voltage difference on determining the initial rotor position can be negligible. However, if the induction voltage difference is very small or even of the same magnitude as the shunt voltage difference, the shunt voltage difference can have significant undesirable effects on the determined induction voltage difference, which then distort the determination of the initial rotor position. However, by taking the shunt voltage difference into account when determining the induction voltage difference, these undesirable influences can be reduced or avoided.

[0025] Optionally, the induction measurement times comprise a first and a second induction measurement time and the shunt measurement times comprise a first and a second shunt measurement time, wherein the first induction measurement time is separated from the first shunt measurement time and / or the second induction measurement time is separated from the second shunt measurement time by no more than 5 µs. This offers the advantage that the respective shunt voltage measurements and induction voltage measurements are carried out as close to one another as possible and, accordingly, the shunt voltage difference and the induction voltage difference are determined over almost the same time interval. This makes it possible to take into account a shunt voltage difference that is as close as possible to the shunt voltage difference or the respective prevailing shunt voltage actually prevailing during the measurement of the induction voltage difference or the induction voltages.This enables particularly precise compensation of the influences of the shunt voltage difference when determining the induction voltage difference.

[0026] Optionally, the first and second induction measurement times are spaced apart by at least 10 µs, optionally at least 100 µs, or optionally at least 1 ms. Alternatively or additionally, the first and second shunt measurement times are spaced apart by at least 10 µs, optionally at least 100 µs, or optionally at least 1 ms. This offers the advantage that the time span between the measurement times of the induction voltages and / or the shunt voltages is sufficiently long to achieve a pronounced increase or decrease in the current flow through the inductors and / or the shunt and, accordingly, to obtain a suitable signal amplitude for determining the induction voltage difference.

[0027] Optionally, the voltage change in the first and / or second phase caused by energizing the first and / or second phase during the energization interval is taken into account in the form of a predetermined, estimated shunt voltage difference. In other words, according to an optional embodiment, the determination of the shunt voltage difference can be limited to estimating it rather than measuring it. This optional, alternative embodiment thus offers the possibility of taking the shunt voltage or shunt voltage difference into account by estimating it instead of measuring or determining it. For example, the estimate can be based on calculating the estimated value using other known parameters and / or on experimentally determined and provided data and / or empirical values. For example, the shunt voltage difference can be estimated using the following mathematical relationship: Δ U shunt = U 2 L ⋅ Δ T ⋅ R shunt

[0028] This indicates ΔU snunt the shunt voltage difference, U the supply voltage, L the inductance of the first and second phases (which are assumed to be identical), Δ T the time interval between the first and second induction voltage measurement and R shunt the ohmic resistance of the shunt resistor. Such an estimation of the shunt voltage difference can thus provide a solution that eliminates the need to determine or measure the shunt voltage difference, thus keeping the hardware and / or computational effort to a minimum.

[0029] Optionally, the voltage change in the first and / or second phase caused by energizing the first and / or second phase during the energization interval is taken into account by reducing the induction voltage difference by a value proportional to the shunt voltage difference. For example, the value proportional to the shunt voltage difference can represent the value of the shunt voltage difference itself. According to other embodiments, however, fractions and / or multiples of the shunt voltage difference can also be used for this purpose. The fact that the induction voltage difference is reduced by a value proportional to the shunt voltage difference means that the difference between the induction voltage difference and the shunt voltage difference is used as the determined induction voltage difference for determining the initial rotor position.In this way, the influence of the shunt voltage difference on the determined induction voltage difference can be reduced or even eliminated completely.

[0030] Optionally, the voltage change in the first and / or second phase caused by energizing the first and / or second phase during the energization interval can be taken into account by reducing the induction voltage difference by half the value of the shunt voltage difference. This offers a particularly simple and reliable form of consideration. The change in the shunt voltage, divided by the inverse divider ratio of the inductive voltage divider, essentially affects the measured change in the induction voltage at the inductive voltage divider as an error. Since the divider ratio at the inductive voltage divider is in turn position-dependent, for exact compensation one can actually measure the divider ratio and subtract the shunt voltage difference divided by the measured divider ratio from the induction voltage difference.Since in many embodiments the divider ratio is measured at the inductive voltage divider during rotor position detection, this can be implemented without additional measurement steps. However, to simplify the process and minimize the required computing power, a fixed divider ratio of 1 / 2 can be used, so that half the value of the shunt voltage difference is subtracted from the induction voltage difference. For many electric motors in use, the position dependence of the divider ratio of the inductive voltage divider is only in a range between 1% and 10%. Therefore, a divider ratio of 1 / 2 can represent a very useful approximation that can be used as a basis for a useful consideration of the shunt voltage difference.

[0031] Optionally, the method for determining the initial rotor position further comprises energizing the first and second phases of the three-phase electric motor with an electric current that increases or decreases over a second energization interval, wherein the voltage direction of a voltage applied for energization is opposite to the voltage direction of the voltage for energization in the first energization interval, and determining the induced voltage in the non-energized third phase of the three-phase electric motor at at least two temporally spaced-apart measuring times during the second energization interval.In addition, the method according to this optional embodiment comprises determining the induction voltage difference of the voltage induced in the third phase between the two temporally spaced induction measurement times in the second current supply interval, taking into account a voltage change in the control circuit of the first and / or second phase caused by the current supply to the first and second phase in the second current supply interval, as well as determining the initial rotor position based on the induction voltage differences determined in the first and second current supply interval. This offers the advantage that any measurement errors caused by any interference can be eliminated, provided that these occur equally in both current supply intervals. This can further increase the reliability of determining the initial rotor position.In particular, the use of the two opposing measurements makes it possible to reduce any inaccuracies that may be caused by hysteresis properties of the stator material.

[0032] Optionally, before the first and / or second current application interval, an electrical voltage is applied to the first and second phases for a predetermined period of time, which is opposite to the voltage applied for current application. This offers the advantage that any influences caused by any remaining magnetization can be reduced or eliminated by the preceding measurement and / or current application. The predetermined period of time is optionally no longer than 100 ms.

[0033] The features and embodiments mentioned above and explained below are not only to be regarded as disclosed in the respective explicitly mentioned combinations, but are also encompassed by the disclosure content in other technically meaningful combinations and embodiments. In particular, the individual aspects of the present disclosure can be combined with one another. Thus, optionally, a method for providing a commutation interval can include the use of magnetization parameters that were determined according to a method according to a further aspect of the present disclosure. Alternatively or additionally, a method for determining an initial rotor position of a three-phase electric motor according to one aspect of the present disclosure can include the use of magnetization parameters that were determined according to a method according to a further aspect of the present disclosure.A method for providing a commutation interval according to one aspect of the present disclosure and a method for determining the initial rotor position of the three-phase electric motor according to one aspect of the present disclosure can also be implemented in a control system of a three-phase electric motor, which optionally use magnetization parameters of the three-phase electric motor determined according to a method according to another aspect of the present disclosure. In other words, the individual aspects of the invention can be combined with one another or used independently of one another.

[0034] Further details and advantages of the invention will now be explained in more detail with reference to the following examples and preferred embodiments with reference to the figures.

[0035] Short description of the characters: The Figures 1A and 1Bshow schematic representations of a three-phase electric motor in star connection ( Figure 1A ) and in delta connection ( Figure 1B ). Figure 1C shows an electric motor and a control unit according to an optional embodiment in a schematic representation. Figures 2A and 2B show conventional measuring and control circuits for a three-phase electric motor. Figure 3 shows an example of a determined induced voltage. Figure 4 shows an example of the induction voltage difference over a full electrical revolution of the rotor relative to the stator. Figure 5 shows an example of the induced voltage in a third, non-energized phase over time, while the first and second phases are energized. Figure 6 shows the time course of the shunt voltage. Figure 7shows a comparison of the measured signal of the uncorrected induction voltage difference with half the value of the shunt voltage difference over the rotation angle of the rotor. Figure 8 shows an example of the difference signal for north-south detection or for determining the initial rotor position.

[0036] In the following figures, identical or similar elements in the various embodiments are designated by identical reference numerals for the sake of simplicity.

[0037] The Figures 1A and 1B show, by way of example, in schematic representations, a three-phase electric motor 310 in star connection ( Figure 1A ) and in delta connection ( Figure 1B) according to optional embodiments. The electric motor 310 has three connections 312.1, 312.2 and 312.3 for the three different phases 312. Each of the three phases 312.1, 312.2 and 312.3 is characterized, for example, by an associated inductance L 1 , L 2 or L 3 and an associated ohmic resistance R M1 , R M2 or R M3. The supply voltage of the electric motor 310 is designated as US and corresponds to a potential difference with respect to a ground potential. Different voltages U 1 , U 2 or U 3 can be present at the three connections 312.1, 312.2 and 312.3 of the three phases, which voltages also represent a potential difference with respect to the ground potential.

[0038] Each of the three phases is connected at one end to a corresponding terminal 312.1, 312.2 and 312.3. In the case of the star connection ( Figure 1A) the other end is connected to a star point 314 of the star connection. In the case of the delta connection ( Figure 1B ) the other end is connected to the terminal 312.1, 312.2 or 312.3 of the next phase.

[0039] Figure 1C shows an electric motor 310 with a control unit 320, which are communicatively connected to one another. The control unit 320 is designed separately from the electric motor 310 and is configured to provide it with control signals and, in particular, to perform the commutation of the electric motor 310. Furthermore, the control unit is configured to determine the initial rotor position of the electric motor 310.

[0040] Figure 2Ashows a conventional measuring and control circuit 3100 for a three-phase electric motor 310. Reference symbols M1 to M6 denote field-effect transistors (FETs) used to apply up to three pulsed voltages to the three-phase electric motor. The arrangement of the OPV IC1 and the resistors R2 to R5 represents an amplifier network that amplifies the voltage drop across a shunt resistor R1 and makes it available to the control unit for further use.

[0041] The resistors R 6 to R 10 serve as resistive voltage dividers, which divide the phase voltages U 1 , U 2 and U 3 to be measured to determine the initial position of the rotor angle at the inductive voltage divider, which are then made available to the control unit.

[0042] The shunt resistor R 1 is used to measure the individual phase currents, i.e., the currents supplied to or flowing into the respective phase of the three-phase electric motor. The shunt voltage drop across the shunt resistor R 1 is typically small compared to the other voltages dropped in the control circuit and can, for example, be in the double-digit millivolt range.

[0043] Figure 2Bshows another conventional measuring and control circuit 3102 for a three-phase electric motor 310, which has a plurality of shunt resistors, namely the shunt resistors R 1 and R 2 , which are each connected in series with an FET M 1 or M 3 for energizing one of the phases of the three-phase electric motor. Even though the three-phase electric motor 310 has three phases, two shunt resistors R 1 and R 2 are generally sufficient in such a configuration to determine the respective phase current, since the third phase current of the motor 310 can be calculated via the node set from the measured currents in the two other phases. In such a control circuit 3102, the voltage drops across both shunt resistors R 1 and R 2 can optionally be taken into account when determining the induction voltage difference. The consideration orCompensation of the shunt voltage difference is optionally only carried out during those current application intervals in which a shunt resistor R 1 or R 2 is actually present in the current path. If, however, the current is applied in such a way that the current is discharged to ground via the outer right-hand path, i.e. via the FET M 5 , no compensation is required in this case. Accordingly, it may be useful to take the shunt voltage difference into account for some current application intervals, but not for others. Optionally, this can also be taken into account when averaging over several current application intervals to improve the signal.

[0044] In the following, a method according to an optional embodiment for determining an initial rotor position of a three-phase electric motor by means of a control circuit as in Figure 2A presented, described and its background explained.

[0045] Figure 3 shows an exemplary curve of a determined voltage induced in the de-energized, third phase of a three-phase electric motor 310 when the first and second phases are energized, depending on the rotor position or the angle of rotation of the rotor in degrees (horizontal axis) over a full electrical revolution of the rotor relative to the stator. The vertical axis shows an induction voltage difference, which arises by determining the induced voltage in the third, de-energized phase or at the inductive voltage divider at at least two temporally spaced-apart measurement times during the first energization interval, while the first and second phases are energized with an increasing or decreasing current during the first energization interval. The graph in Figure 3shows that the induction voltage difference depends significantly on the rotor position and, in particular, changes its sign depending on the rotor position. Therefore, determining the induction voltage difference is useful for determining the initial rotor position of the three-phase electric motor 310 and the north-south orientation of the rotor. The pole position of the rotor can be determined based on the sign of the induction voltage difference, which results from determining the induction voltage difference at the prevailing rotor position to be determined.

[0046] In Figure 4As an example, the curve of the induction voltage difference over a full electrical revolution of the rotor relative to the stator for a three-phase electric motor 310 with a low dependence of the inductance on the current is shown. The low dependence of the inductance on the current means that the curve of the induction voltage difference no longer shows a change in sign and, accordingly, a reliable determination of the north-south orientation of the rotor and the initial rotor position is no longer possible based on determining the sign of the induction voltage difference (without further correction). As in Figure 4As can be seen, the values ​​of the induction voltage difference range from approximately 0.015 V to approximately 0.07 V and assume only positive values. This can be further amplified by the fact that in some motors the amplitudes of the voltage difference are even smaller, thus further increasing the probability of a failure to change sign.

[0047] Although the course of the induction voltage difference appears in Figure 4 qualitatively similar to the course in Figure 3to correspond and the deviation to consist mainly of a vertical offset, however, this means that the initial rotor position cannot be reliably determined based on this during operation. This is because when determining the initial rotor position, an average value of the induction voltage difference cannot be calculated, since this would require determining the induction voltage difference over a full electrical revolution or at least a large part of it. However, this is not possible when the rotor is stationary and for determining the initial position of the rotor, since the rotor position naturally does not change when the rotor is stationary.

[0048] According to the embodiment explained, the cause of the displacement is taken into account and the cause is taken into account when determining the induction voltage difference to determine the initial rotor position.

[0049] When using a control circuit according to Figure 2AThe reason for the shift is that, in order to determine the initial rotor position, a first and second phase of the three-phase electric motor 310 are energized with an electric current that increases or decreases over a first energization interval, and the current change, in turn, leads to a voltage change in the voltage drop across the shunt resistor R 1. This voltage drop influences the determined induction voltage difference between two spaced-apart induction measurement times during the first energization interval and leads to the observed shift.Particularly in three-phase electric motors where the current only slightly changes the magnetic field when energizing the first and second phases, resulting in only a small induction voltage and induction voltage difference, the voltage change of the shunt voltage can result in a shunt voltage difference at two spaced shunt measurement points, which are as close as possible to the induction measurement points in the energization interval, with a similar characteristic, size, or amplitude to the induction voltage difference. In such a case, the voltage change of the voltage drop across the shunt resistor or the shunt voltage difference can significantly shift the induction voltage difference and, in particular, also result in the induction voltage difference no longer changing its sign depending on the rotor position.

[0050] In Figure 5The curve of the induced voltage in a third, non-energized phase is shown over time as an example, while the first and second phases are energized with an increasing electrical current. The supply voltage is 5 V. The illustrated time window of approximately 25 µs can be regarded as an exemplary energization interval, in which the induced voltage in the third phase, i.e., at the inductive voltage divider, increases by approximately 100 mV. If the two induction measurement times are set at the beginning and end of the illustrated time window or the exemplary energization interval, an induced voltage difference of approximately 100 mV is obtained.

[0051] Figure 6 shows for the same time window Figure 5 the time course of the shunt voltage, ie the voltage which, when energized, appears across the shunt resistor R 1 ( Figure 2A) drops. This shows that the shunt voltage increases by about 130 mV over the current application interval and thus the shunt voltage difference between the beginning and the end of the current application interval itself is greater than the induction voltage difference ( Figure 5 ). Since the shunt voltage or shunt voltage difference, divided by the inverse divider ratio of the inductive voltage divider, has an error in the measured voltage change at the inductive voltage divider, this leads to an inaccurate measurement of the induction voltage difference if the shunt voltage difference is not taken into account. Accordingly, according to the embodiment described, the shunt voltage difference is taken into account when determining the induction voltage difference.

[0052] According to an optional embodiment, the shunt voltage difference can be taken into account when determining the induction voltage difference by measuring the divider ratio of the inductive voltage divider, which in turn depends on the rotor position, and then subtracting the shunt voltage difference divided by the measured divider ratio from the induction voltage difference. This offers the advantage of precisely considering and compensating for the influences of the shunt voltage difference on the induction voltage difference. Since the divider ratio at the inductive voltage divider can be measured during the determination of the initial rotor position, this is possible in some optional embodiments without additional effort.

[0053] According to a further optional embodiment, the shunt voltage difference is taken into account in a different way when determining the induction voltage difference. According to this optional embodiment, a fixed divider ratio of 2 is assumed or used for the correction or consideration. This appears to be a useful approximation, since the fluctuations in the divider ratio with the rotor position of the three-phase electric motor typically only range from 1% to 10% of the total amplitude. This approximation can reduce the computing power required to account for the shunt voltage difference.

[0054] Figure 7 shows an example of a comparison of the measured signal of the uncompensated or uncorrected induction voltage difference 3700, as already described in Figure 4shown, with half the value of the shunt voltage difference 3702 over the rotation angle of the rotor. It can be seen that the halved shunt voltage difference 3702 contains a periodic component with half the period of an electrical revolution. This would be eliminated if full compensation without approximation, as described above, were used. However, according to some embodiments, the resulting residual error is unproblematic for the reliability of the method for determining the initial rotor position, so that the approximation with regard to the fixed divider ratio can also lead to consistently usable results.

[0055] In principle, three usable intervals are available for north-south detection, from which, according to an optional embodiment, the most suitable one can always be selected for the application, i.e. the one at which the difference between the signal at the inductive voltage divider, i.e. the induction voltage difference, and the error signal, i.e. the shunt voltage difference, is the greatest, so that the behavior of this difference around the zero crossing is irrelevant for the determination of the initial rotor position.

[0056] Figure 8 shows an example of the difference signal for north-south detection or for determining the initial rotor position, which corresponds to a difference between the induction voltage difference 3700 and the shunt voltage difference 3702. In comparison to the Figure 4 The signal of the (uncorrected) induction voltage difference shown in Figure 1, which is useless for a reliable determination of the initial rotor position, has the corrected signal in Figure 8 It has excellent symmetry and excellent signal quality with reliable zero crossing and is therefore very well suited for the reliable determination of the initial rotor position.

[0057] In the following, a method according to an optional embodiment for determining an initial rotor position using a control circuit according to Figure 2A explained by example.

[0058] First, a first and second phase of the three-phase electric motor are energized with an electric current that increases or decreases over a first energization interval.

[0059] In a further step, an induced voltage is then determined in a non-energized third phase of the three-phase electric motor at a first induction measurement time during the energization interval and, at the same time or as short a time interval as possible, a measurement of the voltage drop across the shunt resistor R 1 of the control circuit, which is referred to as the shunt voltage drop, is taken.

[0060] After a sufficient time has elapsed during the current application interval to cause a current change, for example 1 ms, a second value of the induced voltage is measured in the first current application interval at a second induction measurement time and, at the same time or as short a time interval as possible, the shunt voltage drop across the shunt resistor R 1 is measured a second time.

[0061] The induction voltage difference is then determined by determining the difference between the first and second measured values ​​of the induced voltage. To account for the voltage change in the control circuit, half the difference between the two measured shunt voltage drops, i.e., the halved shunt voltage difference, is subtracted. This provides a corrected induction voltage difference, which is suitable for reliably determining the initial rotor position. Accordingly, the initial rotor position can be determined based on the determined induction voltage difference, which corrects for the voltage change induced in the control circuit.

[0062] According to another optional embodiment, instead of the halved shunt voltage difference, a difference between the two shunt voltages multiplied by an actually measured inverse divider ratio of the inductive voltage divider can also be used.

[0063] Another optional option for determining the voltage drop in the control circuit to be used for compensation can be based on measuring only the induced voltage or the induction voltage difference in a first current application interval. Then, in a further current application interval, applying the identical voltages again and measuring the shunt voltage difference in the second current application interval at the corresponding measurement times at which the induction voltage difference was measured in the first current application interval. This offers the possibility of determining the induction voltage difference and the shunt voltage difference with only one analog-to-digital converter.

[0064] Another optional possibility is to estimate the shunt voltage difference, as already explained above in the general description section.

[0065] Another optional way to improve the signal can involve using multiple current application intervals in which the applied voltage is alternately inverted and the resulting voltage differences are summed and / or averaged, for example. For this purpose, the induction voltage differences and the shunt voltage differences can be summed and / or averaged.

[0066] The described methods are not limited to compensating for errors resulting from a voltage drop across one or more shunt resistors. Rather, these or similar methods can also be used to compensate for other errors and / or asymmetries in the control circuit, such as between half-paths to the positive and negative supply voltage. For this purpose, the resulting errors can be measured and / or, if the asymmetry is known, calculated similarly to the described estimate. List of reference symbols

[0067] 310 Three-phase electric motor 312 Phase of the three-phase electric motor 312.1, 312.2, 312.3 First, second or third phase of the three-phase electric motor 314 Star point 320 Control unit U 1 , U 2 , U 3 Voltage at the connection of the first, second or third phase L 1 , L 2 , L 3 Inductance of the first, second or third phase R M1 , R M2 , R M3 Ohmic resistance of the first, second or third phase i L1 , i L2 , i L3 Current flow in the first, second or third phase US Supply voltage M 1 to M 6 Field-effect transistors of the control circuit R 1 (R 2 ) Shunt resistors R 2 to R 5 Circuitry for current measurement R 6 to R 10 Ohmic resistors of the resistive voltage divider IC 1 OPV for current measurement 3100Control circuit 3102Control circuit 3700Induction voltage difference signal 3702Shunt voltage difference signal

Claims

1. A method for determining an initial rotor position of a three-phase electric motor (310) with a control circuit (3100), the method comprising: - energizing a first and second phase (312.1, 312.2) of the three-phase electric motor (310) with an electric current increasing or decreasing over a first energization interval; - determining an induced voltage in a non-energized third phase (312.3) of the three-phase electric motor (310) at least at two temporally spaced measurement times during the first energization interval; - determining an induction voltage difference (3700) of the voltage induced in the third phase (312.3) between the two temporally spaced induction measurement times, taking into account a voltage change in the control circuit (3100) of the first and / or second phase (312.13312.1, 312.2) caused by the energization of the first and second phase in the first energization interval; and - determining the initial rotor position based on the determined induction voltage difference (3700); characterized in that - the voltage change in the control circuit (3100) of the first and / or second phase (312.13312.1, 312.2) caused by the energization of the first and second phase in the first energization interval is taken into account in the form of a predetermined, estimated shunt voltage difference, or - the voltage change in the control circuit (3100) of the first and / or second phase (312.1, 312.2) caused by the energization of the first and / or second phase (312.1, 312.2) in the energization interval corresponds to a shunt voltage difference (3702) which arises at a voltage drop across a shunt resistor (R1) for measuring the phase current in the first and / or second phase (312.1, 312.2) between two spaced-apart shunt measurement times, and wherein the voltage change in the first and / or second phase (312.1, 312.2) caused by the energization of the first and / or second phase (312.1, 312.2) in the energization interval is taken into account in that the induction voltage difference (3700) is reduced by a value proportional to the shunt voltage difference (3702).

2. The method according to claim 1, wherein the energization is carried out in such a way that the electrical current in the first energization interval increases or decreases strictly monotonically and optionally linearly.

3. The method according to claim 1 or 2, wherein the induction measurement times comprise a first and a second induction measurement time and the shunt measurement times comprise a first and a second shunt measurement time, wherein the first induction measurement time is temporally spaced apart from the first shunt measurement time and / or the second induction measurement time is temporally spaced apart from the second shunt measurement time by no more than 5 µs.

4. The method according to claim 3, wherein the first and the second induction measurement time are spaced apart from each other by at least 10 µs, optionally at least 100 µs, optionally at least 1 ms, and / or wherein the first and the second shunt measurement time are spaced apart from each other by at least 10 µs, optionally at least 100 µs, optionally at least 1 ms.

5. The method according to any one of claims 1 to 4, wherein the voltage change in the first and / or second phase (312.1, 312.2) caused by the energization of the first and / or second phase (312.1, 312.2) in the energization interval is taken into account in that the induction voltage difference (3700) is reduced by a value proportional to the shunt voltage difference (3702).

6. The method according to any one of claims 1 to 5, wherein the voltage change in the first and / or second phase (312.1, 312.2) caused by the energization of the first and / or second phase in the energization interval is taken into account in that the induction voltage difference (3700) is reduced by a value corresponding to half of the shunt voltage difference (3702).

7. The method according to any one of claims 1 to 6, further comprising: - energizing the first and second phases (312.1, 312.2) of the three-phase electric motor (310) with an electric current that increases or decreases over a second energization interval, wherein the voltage direction of a voltage applied for energization is opposite to the voltage direction of the voltage for energization in the first energization interval; - determining an induced voltage in a non-energized third phase of the three-phase electric motor (310) at least at two temporally spaced apart measurement times during the second energization interval; - determining the induction voltage difference of the voltage induced in the third phase between the two temporally spaced apart induction measurement times in the second energization interval, taking into account a voltage change in the control circuit of the first and / or second phase (312.1, 312.2) caused by the energization of the first and second phase (312.1, 312.2) in the second energization interval; - determining the initial rotor position based on the induction voltage differences determined in the first and second energization intervals.

8. The method according to any one of claims 1 to 7, wherein before the first and / or second energization interval, an electrical voltage is applied to the first and second phases (312.1, 312.2) for a respective predetermined period of time, which voltage is opposite to the voltage to be applied for the energization.

9. The method according to claim 8, wherein the predetermined period of time is not longer than 100 ms.

10. A control unit for a three-phase electric motor, characterized in that - the control unit is designed to receive phase voltages measured by a measuring and control circuit of the three-phase electric motor at the phases of the three-phase electric motor and a measured voltage drop across a shunt resistor for measuring individual phase currents and to apply up to three clocked voltages to the three-phase electric motor by means of the measuring and control circuit; and - to determine an initial rotor position of the three-phase electric motor according to a method according to any one of the preceding claims using the voltage drop received from the measuring and control circuit and the phase voltages received from the measuring and control circuit and through energizing the phases by the measuring and control circuit.

11. A system for determining an initial rotor position of a three-phase electric motor (310), the system comprising: - a measuring and control circuit (3100); and - a control unit according to claim 10; characterized in that - the measuring and control circuit (3100) is designed to measure phase voltages at phases of the three-phase electric motor and a voltage drop across a shunt resistor for measuring individual phase currents through the phases of the three-phase electric motor (310) and to provide them to the control unit and to apply up to three clocked voltages to the three-phase electric motor.

12. An electric motor (310) comprising a stator, a system for determining an initial rotor position of a three-phase electric motor (310) according to claim 11 and a rotor, which is rotatable relative to the stator.