Method for operating electric motor

Abstract

A method for operating an electric motor having a stator and a rotor is characterized in that an injected signal is used to estimate the rotor position and / or speed of the electric motor over the entire speed range in which the electric motor is operated.

Description

Technical Field

[0001] The invention relates to a method having the features of the preamble of claim 1 . Background Art

[0002] In a permanent magnet synchronous motor, it is very important how the various components through which the magnetic field flows are positioned relative to each other. This also involves precise knowledge of the angular position of the rotating components, because as the motor rotates, the changing position (angular position) of the magnets incorporated in the rotating rotor relative to the windings incorporated in the stator must always be precisely known in order to correctly control the electric motor. The changing angular position of the rotor must always be precisely known in order to determine the orientation of the rotor components (e.g. the rotor magnets, which are usually designed as permanent magnets) relative to the stator components (e.g. the stator magnets, which are usually designed as electromagnets / stator windings) and to be able to adjust the control of the motor accordingly.

[0003] Therefore, the control of such an electric motor is achieved by applying a rotating magnetic field to the windings of the motor. Depending on the rotor position angle, the rotating magnetic field must be adjusted via a closed-loop control system. Typically, a rotor position sensor is used to measure the position of the rotor and transmit the determined rotor position angle to the electric motor control system.

[0004] However, in order to save costs and installation space, sensorless closed-loop control systems are known, which do not require physical rotor position sensors. Only current sensors are used here, which are indispensable for field-oriented closed-loop control in any case. This closed-loop control concept, which is particularly common in three-phase permanent magnet synchronous machines, is based on converting three-phase AC variables into a two-axis coordinate system that rotates synchronously with the rotor flux of the machine. In this coordinate system, which is usually referred to as, for example, the d / q coordinate system, the three-phase currents i_u, i_v, i_w of the stator winding are represented by a two-dimensional current vector with components i_q and i_d. For an ideal sinusoidal rotor flux and an ideal sinusoidal phase current, the original AC variables i_u, i_v, i_w are mapped into constant variables i_q, i_d due to the coordinate system that rotates synchronously with the rotor flux.

[0005] In the case of field-oriented current control, the voltage values ​​or current values ​​of the phases of the stator of the synchronous machine are transformed in a known manner into a two-dimensional coordinate system, the mutually perpendicular axes of which are usually called the d-axis ("direct axis") and the q-axis ("orthogonal axis"). This coordinate system rotates relative to the stator of the synchronous machine and is stationary relative to the rotor of the synchronous machine. The transformation itself is called the Park transformation; the two-dimensional coordinate system into which it is transformed is called the Park coordinate system. The Park transformation can be carried out via the intermediate step of the also known Clarke transformation, which transforms the voltage values ​​or current values ​​of the phases of the stator of the synchronous machine into a two-dimensional orthogonal coordinate system stationary relative to the stator.

[0006] As already mentioned above, when the electric motor is operated without sensors, the rotor position sensor which is usually used to determine the current angle of the rotor is omitted. For example, the current sensor signal and the measured or estimated phase voltages are used to infer the rotor position and speed of the motor via a model. Below a speed threshold for the absolute speed, it is necessary to input a so-called injection signal which supports the identification of the rotor position and speed within this speed range.

[0007] WO 2020 001 681A1 describes an electric motor having a stator and a rotor that can rotate relative to the stator, and a control system that can output current pulses to the electric motor, wherein the current pulses cause the rotor to rotate in a first rotational direction and at a first rotational angle, and thereby cause an induced voltage received by the control system, and the control system determines the rotational direction and / or rotational position of the rotor relative to the stator via the induced voltage.

[0008] DE 10 2018 120 421 A1 discloses a method for sensorless closed-loop control of a permanent magnet excited synchronous electric motor, in which the system is described in a fixed αβ coordinate system of the electric motor. The system comprises an electromagnetic model and a mechanical model of an electric motor with a drive train. For the model, differential inductances are stored in the form of a lookup table, each of which depends on the current of the electric motor. The lookup table can be retrieved for calculations. Based on the electromagnetic model and the mechanical model, the speed and angle of the electric motor are estimated by a Kalman filter, primarily via the mechanical model. The electrical model can be used to provide an internal torque to the torque equation in order to determine changes in speed or angle.

[0009] Sensorless operation of electric machines has not yet been established in electric vehicles. The reason for this is that sensorless operation has been shown to work well and in a stable manner at higher speeds. For speeds close to zero revolutions, this operation is only possible with the addition of an injection signal. However, the selection of a suitable injection signal constitutes a great challenge, since the influence of the injection signal on the system is not always positive. On the one hand, the injection signal may induce unwanted noise, and on the other hand, it is difficult to find a stable and robust combination in terms of frequency and amplitude when selecting the injection signal.

[0010] However, the use of safety-certified rotation angle sensors for the rotation angle position of the rotor also helps in discussing safety-critical scenarios in functional safety. For example, for pump drives, compressors or fans, sensorless operation of the corresponding motors is already state of the art. A common approach for starting motors is to switch from purely open-loop controlled starting of the electric motor to sensorless closed-loop control of the electric motor by specifying a rotating magnetic field. This approach avoids the use of injection signals at speeds close to zero.

[0011] The unpublished German patent application DE 10 2022 102 634.5 discloses a start sequence for an electric motor in a P1 hybrid, which starts with a speedless open-loop controlled start and then switches to a sensorless start closed-loop controlled operation, wherein the speed signal of the internal combustion engine is also used to check whether the desired speed has been reached.

[0012] Switching from open-loop control operation to sensorless closed-loop control operation inevitably results in strong pulses in the current signal or voltage signal, since the angle signal required to control the electric motor generally has a discontinuity between open-loop control operation and closed-loop control operation. During startup, the current angle signal is, for example, assigned to a specific angle position in the rotor-fixed coordinate system, for example a specific angle position in the d-direction. The angle position that occurs in sensorless closed-loop control operation differs from this, since the q-direction component is then added to the d-direction component.

[0013] As already mentioned above, when the electric motor is operated without sensors, the rotor position sensor which is usually used to determine the current angle of the rotor is omitted. For example, the current sensor signals and the measured or estimated phase voltages are used to determine the rotor position and speed of the motor via a model and / or using anisotropy. The use of the estimated speed and rotor position in the closed-loop control of the electric motor means that the estimated variables must be able to be determined in a stable and robust manner at all possible operating points of the electric motor.

[0014] Below a predetermined speed threshold for absolute speed (see Figure 1 , "Prior Art"), it is necessary to input a so-called injection signal in order to estimate the rotor position and speed, which enables the rotor position and speed to be identified in this speed range, since the evaluation of the induced voltage on the motor alone does not work reliably in this range due to the reduced signal-to-noise ratio. Specifically, this means that if the speed decreases and tends to zero, the term used for the evaluation becomes less dominant compared to the other terms of the equation and eventually disappears.

[0015] The injection signal generally has a negative impact on the acoustics of the system and may be heard as unwanted noise, for example in the interior of a vehicle.

[0016] EP 2 144 362 B1 discloses an injection method designed for small absolute velocities.

[0017] The not yet published German patent application DE 10 2022 110 304.8 shows a method for initializing the rotor position angle even at low absolute speeds.

[0018] The possibility of switching between different methods is investigated in the German patent application DE 10 2022 112 712.5 which has not yet been published.

[0019] The as yet unpublished German patent application DE 10 2022 103 221.3 describes the operation of an electric motor in an open-loop controlled manner at a low absolute speed during the startup phase. The open-loop control operation generates a target angle. During this startup, another target angle is also calculated according to the sensorless closed-loop control algorithm, but this information is not yet used for operation. By comparing the two target angles, it can be determined whether an angle correction is necessary. During the startup, the sensorless closed-loop control algorithm reliably finds the angle and speed that correspond to the actual system, so that this information is then used when the sensorless closed-loop control is activated.

[0020] Using a motor model and thereby sensorless closed-loop control with induced voltage (injection-based approach) represents a complex software product. This increased resource requirement has a negative impact on memory requirements and required computation time and may overwhelm available hardware.

[0021] Combining several methods with open-loop control of transient transitions between methods places an even greater burden on resources. In addition, conventional model-based methods require accurate model parameters of the electric motor at all relevant temperatures. Determining these parameters also requires a lot of effort. Summary of the invention

[0022] The present invention is based on the object of reducing this resource requirement.

[0023] This object is achieved by a method having the features according to claim 1 .

[0024] According to the method for operating an electric motor having a stator and a rotor according to the invention, the rotor position and / or speed of the electric motor is estimated using an injected signal over the entire speed range in which the electric motor may be operated.

[0025] Thus, without a rotor position sensor, a parameter-independent closed-loop control of the electric motor is performed based on the injected signal over the entire operating range.

[0026] In a preferred embodiment of the invention, the method is applied to the effective voltage limit.

[0027] In another preferred embodiment of the present invention, anisotropy exists, ie Ld is not equal to Lq.

[0028] In a further preferred embodiment of the invention, the effective voltage is obtained from the difference between the available intermediate circuit voltage and the voltage amplitude of the superimposed injection signal.

[0029] In another preferred embodiment of the invention, an injection signal is used in combination with field weakening to achieve higher speeds beyond the effective voltage limit.

[0030] In another preferred embodiment of the invention, a portion of the maximum driving frequency is used as injection frequency.

[0031] In another preferred embodiment of the invention, an integer factor of the maximum drive frequency is used as the injection frequency.

[0032] In a further preferred embodiment of the invention, at most 16 kHz / 4 is used as injection frequency.

[0033] In a further preferred embodiment of the invention, at least 1 kHz to 4 kHz is used as injection frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Advantages and advantageous embodiments of the invention are the subject matter of the following figures and their description.

[0035] In the accompanying drawings, in detail:

[0036] Figure 1 A comparison is shown between using a previously common method (prior art) and using the method according to the invention. DETAILED DESCRIPTION

[0037] Typically, injection-based methods are used only in operating ranges where induced voltage-based methods are unreliable (see Figure 1 , “Prior Art”). In order to reduce resources, the present invention proposes to expand the operating range of the injection-based method.

[0038] In the case of acoustically well-damped systems, advantages such as parameter independence and therefore robustness of the injection method and low computational effort clearly outweigh disadvantages such as undesirable noise, marginal efficiency losses and field weakening that occur in comparative systems even at slightly lower speeds.

[0039] According to the invention, the injection-based method will be used up to the effective voltage limit (see Figure 1, "new"). The main advantage is the increased robustness against parameter uncertainties, since the method is parameter independent. Only sufficient anisotropy is required, i.e. Ld must not be equal to Lq. The voltage amplitude of the superimposed injection signal must be maintained. The effective voltage can be determined by subtracting the amplitude from the available intermediate circuit voltage. When using the injection method, it may be necessary to weaken the magnetic field faster than using conventional methods (prior art) in order to enable operation at higher speeds.

[0040] Figure 1 The effective area without field weakening is shown. If higher speeds are required, the field is weakened. In the traditional case this is a combination of the back electromotive force (back EMF) method (back electromotive force) with field weakening, while for the new method this is a combination of the injection method with field weakening. For clarity, Figure 1 This is not shown in FIG.

[0041] The method can be used, for example, in hydraulic pumps, since pumps arranged in oil have very good acoustic damping. Acoustically undesirable noise is negligible over the entire operating range.

[0042] The electromagnetic compatibility of the components, in particular at the excitation frequency of the injection signal, is provided, as is the functionality of the electric motor.

[0043] The choice of injection frequency is crucial. On the one hand, the maximum drive frequency (PWM frequency) is limited in the system, for example 16 kHz, and on the other hand, the frequency cannot be too low to avoid causing mechanical vibrations.

[0044] In the pump example, very good results can be achieved with 1 / 4 of the PWM frequency, here for example 4 kHz. At 2 kHz, the function is still guaranteed, but there is a stronger acoustic impairment.

[0045] For speeds above 10% of rated speed, no measurable loss in efficiency could be detected.

Claims

1. A method for operating an electric motor having a stator and a rotor, characterized in that The injected signal is used to estimate the rotor position and / or speed of the electric motor over the entire speed range over which the electric motor is operated.

2. The method according to claim 1, characterized in that: The method is used up to the effective voltage limit.

3. The method according to any one of claims 1 or 2, characterized in that There is anisotropy, that is, Ld is not equal to Lq.

4. The method according to any one of the preceding claims, characterized in that The effective voltage results from the difference between the available intermediate circuit voltage and the voltage amplitude of the superimposed injection signal.

5. The method according to any one of the preceding claims, characterized in that An injection signal is used in combination with field weakening to achieve higher speeds beyond the effective voltage limit.

6. The method according to any one of the preceding claims, characterized in that A fraction of the maximum driving frequency is used as the injection frequency.

7. The method according to any one of the preceding claims, characterized in that An integer factor of the maximum drive frequency is used as the injection frequency.

8. The method according to any one of the preceding claims, characterized in that At most 16 kHz / 4 is used as the injection frequency.

9. The method according to any one of the preceding claims, characterized in that At least 1 kHz to 4 kHz is used as the injection frequency.

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