Switching to waveforms with increased QMW to increase AC motor speeds while avoiding radial stator vibrations

Abstract

A method for operating a multiphase AC motor comprises the step of generating a multiphase AC voltage (V) by modulating a DC supply voltage (VIN). A multiphase AC motor (EM) is then supplied with the generated AC voltage, which produces a rotating magnetic field in the AC motor. The AC voltage generation step involves modulating the DC supply voltage (VIN) using a first modulation scheme (M1) when the speed (n) of the AC motor is below a given threshold speed (T). Otherwise, a second modulation scheme is used. The second modulation scheme provides an AC voltage (V) with a higher inverse crest factor than the first modulation scheme. The threshold speed (T) is lower than a fundamental frequency of a radial resonant vibration of the stator of the AC motor (EM).

Description

[0001] Conventional battery electric vehicles (BEVs) have a traction battery that powers an inverter. The inverter modulates the direct current (DC) supplied by the battery using pulse-width modulation (PWM) to provide a multi-phase alternating current (AC) voltage. This AC voltage is applied to the stator windings of an AC motor, creating a rotating AC system within the windings, which in turn generates a rotating magnetic field. The rotor of the AC motor follows this magnetic field, causing the rotor to rotate and propel the BEV.

[0002] Due to the high power outputs, e.g., > 100 kW or > 300 kW, considerable forces act on the rotor and stator. Modulation using PWM can lead to resonance effects that generate unwanted acoustic noise and / or harmonics of the voltage and current components in the phase current (or in the supply voltage).

[0003] One object of the invention is to provide a measure for reducing these acoustic noises and / or electrical harmonics.

[0004] This problem is solved by the method according to claim 1. Further properties, features, embodiments, applications and advantages will become apparent from the dependent claims, the description and the figures.

[0005] It is proposed to switch from a first to a second PWM modulation scheme, which provides a voltage waveform with a larger area (compared to a sine wave), i.e., with a higher RMS value (provided by the waveform itself), if a speed threshold is exceeded. In this way, a voltage with a lower peak value (providing the same power as a sine wave) can be used to deliver the same traction power (mechanical power). If a speed threshold is exceeded, the waveform (formed by modulation) is changed to a waveform with a higher voltage area (area under the voltage amplitude curve), i.e., a waveform with a higher RMS value (at the same peak value), which allows the use of a voltage signal with a reduced peak value compared to a sine wave.In this way, the mechanical excitation that could potentially lead to resonance effects is reduced (due to the reduced peak value). The threshold that triggers the selection of the second modulation scheme reflects the mechanical resonance characteristics of the stator. Specifically, it is the fundamental frequency of a radial resonant vibration of the stator, i.e., a mechanical vibration of the cylinder formed by the stator. Thus, the modulation scheme switches to one that provides a lower peak value before the stator's resonant frequency is reached, compared to a sine wave (at the same power / mW). At a rotational speed corresponding to the resonant frequency, the peak value (and therefore the maximum mechanical excitation) is thus reduced compared to a sine wave.The resulting noise / vibrations / harmonics are reduced because, at the (critical) rotational speed corresponding to the radial resonant vibration, a modulation scheme with a reduced lower peak value is used. The lower peak voltage (at the same QMW) is represented by a lower inverse crest factor. The inverse crest factor is the ratio of the QMW value to the peak voltage. In the case of a sine wave (first modulation scheme), the inverse crest factor is the ratio of the QMW value of a sine wave (approximately 0.707 × V0) to the peak voltage (V0), i.e., it is approximately 0.707. In the case of a square wave (an example of a second modulation scheme), it is the ratio of the QMW value of a square wave (1 × V0) to the peak voltage (V0), i.e., 1.

[0006] The larger the area (normalized to the peak voltage V0) under the curve representing a waveform, the higher the inverse crest factor. In short, the inverse crest factor (peak factor) indicates -1 The inverse crest factor (peak factor) is the width or area enclosed by the waveform, normalized to its peak value. -1 The crest factor indicates the area covered by a waveform. The crest factor represents the slenderness of a corresponding waveform; the inverse crest factor indicates how voluminous or compact a waveform is. In particular, the inverse crest factor represents the proportion of high-amplitude phases within a waveform period.

[0007] A method for operating a multi-phase AC motor that represents this idea includes, as step (a), generating a multi-phase AC voltage by modulating a DC supply voltage. The DC supply voltage can be the voltage of a traction battery in an electric vehicle drive system. The modulation is provided by switching the DC supply voltage on and off according to a pulse pattern. The pulse pattern can be provided by a control loop (vector control) for an AC motor. The DC supply voltage is switched by an inverter. The modulation can involve switching an inverter using a duty cycle as the control variable. The envelope of the pulse pattern corresponds to the desired waveform.

[0008] The generated alternating voltage is fed to a multi-phase AC motor. The resulting multi-phase alternating current in the AC motor (especially in its stator) provides a rotating magnetic field within the motor. The rotor mechanically follows this field (due to magnetic forces). The multi-phase AC motor can be a synchronous electric motor (electrically excited synchronous machine, permanent magnet synchronous machine, or asynchronous machine).

[0009] The AC voltage generation step (i.e., the modulation step) involves modulating the DC supply voltage using either a first or second modulation scheme, depending on the AC motor's speed. If the AC motor's speed is below a given threshold speed, the first modulation scheme is used. If the AC motor's speed is equal to or above the threshold speed, the second modulation scheme is used. The second modulation scheme provides a waveform that is more voluminous / less slender than the waveform provided by the first modulation scheme. The inverse crest factor of the waveform (or AC voltage) provided by the second modulation scheme is higher than that of the first modulation scheme.The inverse peak factor is the QMW value of the waveform or voltage in question divided by its peak amplitude. The inverse peak factor can also be defined by the peak-to-average power ratio (PAPR). The QMW value of a voltage is its root mean square (RMS) value. The QMW value can be defined by the square root of the interval (over a waveform period) of the square of the amplitude as a function of time and can be multiplied by a factor representing the reciprocal of the length of the waveform period. The peak amplitude is the maximum absolute value of the amplitude within the waveform period.If the first modulation scheme provides a sine wave, the waveform provided by the second modulation scheme has an inverse crest factor greater than 1 / √2 (corresponding to the inverse crest factor of a sine wave). If the first modulation scheme provides a triangle wave, the waveform provided by the second modulation scheme has an inverse crest factor greater than 1 / √3 (corresponding to the inverse crest factor of a triangle wave). The second modulation scheme can be adapted to provide a square wave or a PWM signal as its waveform (envelope). The inverse crest factor of a PWM signal is equal to the square root of the duty cycle, where the duty cycle is defined as the ratio of the on-time within a time interval to the length of that time interval.Thus, the second modulation scheme can be a PWM signal with a duty cycle greater than 50% (in the case of a sine wave as the waveform of the first modulation scheme) or greater than 33⅓% (in the case of a sine wave as the waveform of the first modulation scheme). These inverse crest factors refer to individual phases and specifically not to a signal generated by the addition of multiple phase voltages.

[0010] The threshold speed is lower than the fundamental frequency of a radial resonant vibration of the stator (magnet core + windings) of the AC motor. Radial resonant vibration refers to a resonance effect that results in a mechanical vibration mode with a (predominantly) radial amplitude. This radial resonant vibration is related to the cylindrical body provided by the magnetic material (and the stator windings), specifically the iron laminations that form the magnetic material of the stator core.

[0011] The following are additional definitions for the waveforms or AC voltages provided by the first / second modulation scheme. The area under the curve representing a single phase of the AC voltage (e.g., voltage between a phase L1 and the neutral terminal N) provided by the first modulation scheme is preferably smaller than the area under the curve representing a single phase of the AC voltage provided by the second modulation scheme. This refers to curves normalized to the same peak value, so the voltage level (or peak voltage magnitude) does not affect this comparison. The area under the respective curves corresponds to the RMS voltages. Since the RMS voltages are referenced to the same normalization, this corresponds to the inverse crest factor (referenced to the same, i.e.,normalized peak voltage).

[0012] Preferably, if both modulation schemes have the same peak values, the second modulation scheme provides more mechanical power to the AC motor than the first modulation scheme. In other words, the second modulation scheme allows the AC motor to provide more mechanical power than the first modulation scheme, where both modulation schemes are based on the same peak value (or are otherwise mutually normalized). An alternative definition is that, if the AC motor provides the same mechanical power, the second modulation scheme provides a lower peak amplitude (in the resulting envelope) than the first modulation scheme. To provide the same RMS voltage (corresponding to the same mechanical power), the second modulation scheme has a peak value that is lower than the peak value of the first modulation scheme.In many implementations, the duty cycle of a PWM modulation defines the peak amplitude (of the AC voltage, i.e., the envelope). This allows for a further definition where the maximum duty cycle of the second modulation scheme is lower than the maximum duty cycle of the first modulation scheme in order to provide the same mechanical power / RMS voltage. The maximum duty cycle corresponds to the peak voltage.

[0013] In further embodiments, the peak value of the AC voltage (in particular the maximum duty cycle of a PWM modulation) is reduced when the modulation scheme is changed from the first to the second modulation scheme. This occurs when the threshold speed (from low to high) is exceeded. The reduction of the peak value is possible because the RMS voltage of the second modulation scheme is higher than the RMS voltage of the first modulation scheme (relative to the same peak voltage).

[0014] In further embodiments, the first modulation scheme generates a synthesized sine wave (or triangle wave) as a multiphase alternating voltage. The second modulation scheme preferably generates a waveform with a higher RMS value than a comparable sine wave (i.e., with the same peak value or otherwise mutually normalized). The second modulation scheme can be a full-block mode (FBM). The second modulation scheme can be a space vector pulse-width modulation (SVPWM), provided it is designed to provide a waveform with a higher inverse crest factor than a sine wave, e.g., a square or trapezoidal wave. The first modulation scheme can be a space vector modulation (SVPWM), in particular an SVPWM designed to provide a sine or triangle wave.Furthermore, the first modulation scheme can be a generalized discontinuous pulsewidth modulation (GDPWM), specifically a GDPWM that provides a waveform in the form of a sine wave or a triangle wave. The GDPWM can be a modulation scheme that includes different discontinuous pulse modulations (DPMW), which can be selected according to an optimization goal (e.g., minimized harmonics in the DC input signal or in the output signal / phase signal, maximized efficiency, etc.) depending on the current operating point. Additionally, a DPMW modulation can be used. Furthermore, the first modulation scheme can be a Synch3PWM / Synch5PWM / Synch7PWM designed to provide a sine wave or triangle wave.Other schemes can also be used for the first modulation scheme, which provide a setting for generating a (synthetic) sine wave or a triangle wave.

[0015] In further embodiments, the electric machine is operated in field weakening mode when the speed of the AC motor is equal to or higher than a predetermined threshold speed. In particular, the second modulation scheme is used to modulate the DC supply voltage if the AC motor is operated in field weakening mode; otherwise, the first modulation scheme is used. As a variant of the described method, the second modulation scheme is used to modulate the DC supply voltage if the AC motor is operated in field weakening mode, provided (as a second condition) that the speed of the AC motor is equal to or higher than the given threshold speed; otherwise, the first modulation scheme is used.

[0016] Another variant of the aforementioned method stipulates that the second modulation scheme is used for modulation in high-power mode, provided that the speed of the AC motor is equal to or higher than the specified threshold speed. In lower-power mode, the modulation scheme can be independent of the AC motor speed. Optionally, the DC supply voltage in high-power mode can be modulated depending on the threshold speed. According to this option, in low-power mode, the DC supply voltage is modulated using the first modulation scheme, regardless of the AC motor speed (i.e., whether the speed threshold is exceeded or not).It may be provided that in a low-power mode, the DC supply voltage is modulated using different modulation schemes, selected according to a different threshold speed, which differs from the threshold speed related to the fundamental frequency of the stator's radial vibration. The high-power mode can be defined by an operating condition in which an (electrical or mechanical) power threshold is exceeded. The high-power mode can also be defined by an operating condition in which a torque threshold is exceeded. This refers to the electrical power consumed by the AC motor, the mechanical power supplied by the AC motor, or the torque supplied by the AC motor. The low-power mode can be defined by an operating condition in which the (electrical or mechanical) power threshold is not exceeded.The low-power mode can be defined by an operating state in which the torque threshold is not exceeded.

[0017] The fundamental frequency of the stator preferably reflects the influence of the stator slots on the radial resonant vibration. More preferably, the fundamental frequency of the stator reflects the influence of the number of stator slots and / or their positions and / or their geometry. In particular, the fundamental frequency corresponds to a mechanical resonant vibration of the stator, where the stator slots mark the position of a vibration node. This can refer to each slot or to an nth slot, where n is a divisor of the number of slots in the stator. Furthermore, the fundamental frequency of the stator can reflect the influence of mechanical elements attached to the stator, in particular housing elements, cooling elements, bearings, or other components.The influence is particularly evident if the vibration of the stator is transmitted to the mechanical element and back, so that the vibration and, in particular, the resonance is provided by a vibration system that includes the stator as well as the mechanically attached mechanical element.

[0018] Further implementations include software code designed to implement the method according to any of the preceding claims when executed on a programmable inverter controller. In particular, software implementing machine control for the AC motor (e.g., vector control) can be provided on the controller, wherein the software code implementing the method can be part of the machine control software or can be a software component that interacts with (controls) the machine control software, in particular by selecting the modulation scheme as described herein, wherein the machine control software controls the modulation process according to the selected modulation scheme. In this case, the modulation step is performed by the machine control software, the selection being based on the rotational speed (e.g.,This process (including comparing the rotational speed with the threshold value) is carried out by the software code. The software code, or the machine control software controlled by the software code, generates a pulse pattern that implements the step of modulating the DC supply voltage. Furthermore, the operation of a power stage of an inverter can be controlled according to the pulse pattern resulting from the modulation step.

[0019] As an application of the method, an electric vehicle drive can be provided with an inverter designed to execute the method described herein. In particular, the inverter includes a controller designed to provide a pulse pattern that reflects the modulation step described herein. The controller may include the aforementioned software code. The electric vehicle drive preferably includes an AC motor connected to the inverter. The inverter is designed to execute the method described herein in order to supply the AC motor with the AC voltage (resulting from the modulation by the power stage). Fig. Figure 1 shows an example of an electric drive. Fig. Figure 2 shows a diagram illustrating features of the procedure described herein, in particular two different modulation schemes and the selection of one of the modulation schemes.

[0020] Fig. Figure 1 shows an electric drive with an inverter I that supplies an electric machine, represented as a multi-phase AC motor EM, with alternating current V. A DC supply voltage VIN, provided by a battery ES, supplies the DC side of the inverter I. The inverter I and the AC motor EM have the same number of phases, i.e., three phases, as shown in Figure 1. Fig. Figure 1 shows the first phase of inverter I, which has two switching elements 11 and 21 (corresponding to a first half-bridge) connected in series via connection point A1. A second phase of inverter I has two switching elements 12 and 22 (corresponding to a second half-bridge) connected in series via connection point A2. A third phase of inverter I has two switching elements 13 and 23 (corresponding to a third half-bridge) connected in series via connection point A3. Connection points A1-A3 provide the multiphase alternating voltage V. This voltage V induces a multiphase current in the stator windings (not shown) to generate a rotating magnetic field in the electric machine.

[0021] A controller C, which may be part of the inverter I or an external device (as shown), is connected to the switching elements 11-23 of the inverter. The controller C provides control signals at the control inputs of the switching elements 11-23, which are symbolically represented as arrows and may be implemented as gate ports if the switching events are implemented as MOSFETs. The controller C is designed to perform the procedures described herein and is specifically designed to operate the switches according to selectable modulation schemes (first, second modulation scheme M1, M2). Since the modulation scheme is selected according to the speed of the AC motor, the controller C is designed to provide the speed, for example, from values ​​used in a motor controller, or it is designed to receive a signal representing the speed.Furthermore, the controller C is designed to provide the current torque generated by the AC motor EM, for example, from values ​​used within a motor controller, or it is designed to receive a signal representing the torque, such as a desired torque. The procedure described herein can be performed depending on the torque. Motor control software can be used within the controller C to provide a closed-loop control or vector control for the AC motor EM. Additionally, software code designed to execute the procedure described herein can be part of the controller or even the motor control software.

[0022] Fig. Figure 2 shows a diagram representing the peak amplitude A of AC voltages V (i.e., the envelope of a PWM signal) as a function of the rotational speed n of the AC motor. The linear relationship of the amplitude A is a simplification; more complex relationships may also exist. Furthermore, three curves are shown depicting the envelope / shape of the AC voltage V at different rotational speeds. It can be assumed that the curves shown in the line diagram of Fig. 2. The peak amplitude A shown is given relative to the rotational speed n for a torque or power (as a function of n).

[0023] At rotational speeds below the threshold speed T, the amplitude A increases monotonically with the rotational speed. At a low rotational speed n1, the amplitude A1 is lower than the amplitude A2 at a rotational speed n2 that is higher than the rotational speed n1 (but below the threshold speed T). For n1, the corresponding waveform (i.e., the envelope of a modulated DC voltage, in particular the envelope of a PWM-modulated AC voltage) is shown as C1. The waveform (1) is a sine wave with a peak amplitude of A1. The area F1 under the curve C1 (for the depicted wave period) corresponds to the signal power for n1. For a higher rotational speed n2 (higher than the lower rotational speed n1, below the threshold rotational speed T), the waveform C2 applies. The waveform C2 (occurring at rotational speed n2) has the same shape as the waveform C1 (occurring at rotational speed n1). However, the amplitude A2 of the waveform C2 is higher than the amplitude A1 at rotational speed n1.Waveform C2 exhibits a higher RMS voltage than waveform C1. The area F2 under curve C2 (for the depicted wave period) corresponds to the signal power for n2 and is larger than the area F1. The modulation schemes used at n1 and n2 (i.e., below T) are the same. In particular, the modulation schemes below T result in the same waveforms (in ). Fig. 2: sinusoidal). The inverse crest factor (ratio of the QMW value, see areas F1, F2, and the peak voltage A1, A2) of waveforms C1 and C2 is identical (due to the same waveform). As in Fig. As can be seen in Figure 2, the peak amplitude increases with the rotational speed, while the waveform (especially its shape) remains the same.

[0024] When the threshold speed T is exceeded, the modulation scheme changes. The resulting change in the waveform is evident when curve C3 (representing the waveform resulting from the modulation for n > T) is compared with curve C1 or curve C2. At speed n3 (above T), full-block modulation is used, generating a square waveform as shown by curve C3. The amplitude A3 of curve C3 is lower than the amplitude A2. However, the area F3 covered by curve C3 is larger than the area F2 covered by curve C2. Since the area reflects the power of the signal, curve C3 provides more power than C2. Simultaneously, the amplitude A3 of curve C3 is lower than the amplitude C2.The mechanical excitation, defined by the (peak) amplitude and resulting from curve C3, is lower than the mechanical excitation resulting from curve C2, even though curve C3 provides more power (and a higher rotational speed) than curve C2. In this explanation, the term "curve" corresponds to the term "modulation," since the modulation defines the curve, i.e., the envelope of the resulting alternating voltage. The block mode (see curve C3) is maintained as the modulation scheme for rotational speeds > n3 until the maximum rotational speed n_max is reached. Fig. Figure 2 shows a first modulation scheme M1 for speeds < T and a second modulation scheme M2 for speeds □ T. For speeds n = 0 ... T, modulation scheme M1 is used, and for speeds n = T ... n_max, modulation scheme M2 is used. Other embodiments can provide more than two modulation schemes, with the modulation scheme(s) for speed n > T having a higher inverse crest factor than the modulation scheme(s) for lower speeds. Regarding Fig. 2. The inverse crest factor can be defined by the ratio of the QMW value and the peak voltage, where the QMW value corresponds to the area F1, F2 or F3, normalized by the duration of the waveform period D, and the peak voltage corresponds to the respective associated amplitude A1, A2 and A3.

[0025] In Fig.2. The peak voltage A1, A2 is increased with increasing rotational speed n (while maintaining the modulation scheme M1) until the threshold rotational speed T is reached. Upon exceeding the threshold rotational speed T, the surface shape of the waveform resulting from the modulation scheme is changed to a more voluminous, extended (wider) waveform. In this way, the area covered by the waveform C1-C3 can be expanded, although the amplitude is simultaneously reduced (see C3-C2), while the mechanical power of the drive is continuously increased (with increasing rotational speed n).

Claims

[1] Method for operating a multi-phase alternating current motor, comprising: (a) Generating a multi-phase alternating voltage (V) by modulating a direct current supply voltage (VIN) and (b) Supplying a multi-phase AC motor (EM) with the generated alternating voltage, which produces a rotating magnetic field in the AC motor, where The step of generating the AC voltage (V) involves modulating the DC supply voltage (VIN) using a first modulation scheme (M1) when the speed (n) of the AC motor is below a given threshold speed (T), and using a second modulation scheme, wherein the second modulation scheme provides an AC voltage (V) with a higher inverse crest factor than the first modulation scheme and wherein the threshold speed (T) is lower than a fundamental frequency of a radial resonant vibration of the stator of the AC motor (EM). [2] Method according to claim 1, wherein the area (F1, F2) under a curve (C1, C2) representing a single phase of the alternating voltage provided by the first modulation scheme is smaller than the area (F3) under a curve (C3) representing a single phase of the alternating voltage provided by the second modulation scheme, wherein both curves (C1, C2; C3) are normalized to the same peak value. [3] Method according to claim 1 or 2, wherein the second modulation scheme provides more mechanical power in the AC motor than the first modulation scheme in the case of equal peak values ​​of both modulation schemes. [4] Method according to claim 1, 2 or 3, wherein when the threshold speed (T) is exceeded and when the modulation scheme is changed from the first to the second modulation scheme the peak value of the alternating voltage is reduced. [5] Method according to any of the preceding claims, wherein the first modulation scheme generates a synthesized sine wave as a multiphase alternating voltage and wherein the second modulation scheme generates a waveform with a higher RMS value than a comparable sine wave. [6] Method according to any one of the preceding claims, wherein the second modulation scheme is selected from the following list: full block mode, space pointer pulse width modulation, synchronous 3-step pulse width modulation, generalized pulse width modulation or discontinuous pulse width modulation, [7] Method according to any of the preceding claims, wherein the electric machine is operated in field weakening mode when the speed (n) of the AC motor is equal to or higher than a predetermined threshold speed (T). [8] Method according to any of the preceding claims, wherein the fundamental frequency of the stator reflects the influence of the stator slots on the radial resonant vibration. [9] Method according to claim 8, wherein the fundamental frequency of the stator reflects the influence of mechanical elements attached to the stator. [10] Method according to one of the preceding claims, wherein the DC supply voltage (VIN) is modulated in a high-power mode depending on the threshold speed (T) and wherein the DC supply voltage (VIN) is modulated in a low-power mode using the first modulation scheme (M1) independently of the speed (n) of the AC motor or is modulated with different modulation schemes selected according to a different threshold speed which differs from the threshold speed which relates to the fundamental frequency of the radial vibration of the stator. [11] Software code designed to implement the method according to any of the preceding claims when executed on a programmable inverter controller. [12] Electric vehicle drive with an inverter (I) and an associated AC motor (EM), wherein the inverter (I) is designed to carry out the method according to one of claims 1-10.

Images