Switching to wave forms with increased RMS for increasing ac motor speeds avoiding radial stator oscillations

Abstract

A method for operating polyphase AC motor provides the step of generating a polyphase AC voltage (V) by modulating a DC supply voltage (VIN). Further, a polyphase AC motor (EM) is supplied with the generated AC voltage providing a rotating magnetic field in the AC motor. The step of generating the AC voltage (V) involves modulating the DC supply voltage (VIN) using a first modulation scheme (M1), if the rotational speed (n) of the AC motor is below a given threshold speed (T). A second modulation scheme is used otherwise. 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 resonance oscillation of the stator of the AC motor (EM).

Description

[0001] 202401503

[0002] 1

[0003] Description

[0004] Switching to wave forms with increased RMS for increasing AC motor speeds avoiding radial stator oscillations

[0005] Commonly known battery electric vehicles (BEVs) have a traction accumulator supplying a power inverter. The power inverter modulates the DC voltage provided by the accumulator according to a pulse with modulation (PWM) in order to provide a polyphase AC voltage. The polyphase AC voltage is applied to stator windings of an AC motor thereby generating a rotating AC current system in the stator windings, resulting in a rotating magnetic field. The rotor of the AC motor follows this magnetic field, which leads to a rotating movement of the rotor driving the BEV.

[0006] Due to the high power ranges, eg. > 100 kW or > 300 kW, significant forces are exerted on the rotor and the stator. The modulation using the PWM can lead to resonant effects generating undesirable acoustic noise and / or harmonic voltage and current components in the phase current (or in the supply voltage).

[0007] It is an object to the invention to provide a measure to reduce this acoustic noise and / or electrical harmonics.

[0008] This object is met by the method according to claim 1 . Further properties, features, embodiments, applications and advantages are provided by the dependent claims, the description and the figures.

[0009] It is suggested to switch from a first to a second PWM modulation scheme providing a voltage waveform with an increased area (in comparison to a sine), ie. with an increased effective value (provided by the form itself) in case that a speed threshold is exceeded. In this way, a voltage with a lower peak value (providing the same power as a sine) can be used for providing the same traction power (mechanical power). In case that a speed threshold is exceeded, the waveform (formed by modulation) is changed to a waveform with a higher voltage area (area below the voltage amplitude curve), ie. to a waveform with a higher RMS (at the same peak value), which allows to use a voltage signal with reduced peak value in comparison to a sine wave. In this way, the mechanical excitation potentially leading to resonant effects is reduced (due to the reduced peak value). The threshold triggering the selection to the second modulation scheme reflects the mechanical resonance properties of the stator. In particular, the fundamental frequency of a radial 202401503

[0010] 2 resonance oscillation of the stator, ie. a mechanical oscillation of the cylinder formed by the stator. In this way, the modulation scheme is switched to a scheme providing a lower peak value in comparison to a sine (at same power I RMS) before the resonant frequency of the stator is reached. Thus, at a speed corresponding to the resonant frequency, the peak value (and therefore the max. mechanical excitation) is reduced in comparison to a sine wave. The resulting noise I vibrations I harmonics are reduced since a modulation scheme with reduced lower peak value is used at the (critical) speed corresponding to the radial resonance oscillation. The lower peak voltage (at the same RMS) is represented by a lower inverse crest factor. The inverse crest factor is the ratio of the RMS value and the peak voltage. In case of a sine (first modulation scheme), the inverse crest factor is the ratio of the RMS value of a sine (ca. 0,707 x V0) and the peak voltage (V0), ie. is about 0,707. In case of a rectangle (an example of a second modulation scheme), it is the ratio of the RMS value of a rectangle (1 x V0) and the peak voltage (V0), ie. is 1 .

[0011] The larger the area (normalized to the peak voltage V0) below the curve representing a wave form, the higher the inverse crest factor. Briefly, the inverse crest factor (crest factor-1) shows, how large the width or area enclosed by the wave form is, normalized to the peak value. The inverse crest factor (crest factor-1) shows, how large the areal span of a waveform is. The crest factor represents the slenderness of a pertaining wave form; the inverse crest factor shows, how bulky or compact a wave form is. In particular, the inverse crest factor represents the share of phases with high amplitude within a wave form period.

[0012] A method for operating polyphase AC motor representing this idea involves, as a step (a), generating a polyphase AC voltage by modulating a DC supply voltage. The DC supply voltage can be the voltage of a traction accumulator of an electric vehicle drive. 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 closed loop control (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 control variable. The envelope of the pulse pattern corresponds to the desired waveform.

[0013] The generated AC voltage is supplied to a polyphase AC motor. The resulting polyphase AC current in the AC motor (in particular in its stator) provides a rotating magnetic field in the AC motor. The rotor mechanically follows this field (due to magnetic forces. The polyphase AC motor can be a synchronous electric motor 202401503

[0014] 3

[0015] (electrically excited synchronous machine, permanently excited synchronous machine, or an asynchronous machine).

[0016] The step of generating the AC voltage (ie. the step of modulating) involves modulating the DC supply voltage using a first modulation scheme or a second modulation scheme depending on the rotational speed of the AC motor. If the rotational speed of the AC motor is below a given threshold speed, the first modulation scheme is used. If the rotational speed of the AC motor corresponds to or exceeds the threshold speed, the second modulation scheme is used. The second modulation scheme provides a waveform, which is more bulky I 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 the first modulation scheme. The inverse crest factor is the RMS-value of the pertaining waveform or voltage divided by the peak amplitude thereof. The inverse crest factor can also be defined by the (root of) the peak-to-average power ratio (PAPR). The RMS value of a voltage is the effective value of the voltage. The RMS value can be defined by the 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 inverse of the length of the waveform period. The peak amplitude is the maximum absolute value of the amplitude within the waveform period. In case the first modulation scheme provides a sine as a waveform, the waveform provided by the second modulation scheme has an inverse crest factor greater than 1 I sqrt

[0017] (2) (corresponding to the inverse crest factor of a sine wave). In case the first modulation scheme provides a triangle wave as a waveform, the waveform provided by the second modulation scheme has an inverse crest factor greater than 1 I sqrt

[0018] (3) (corresponding to the inverse crest factor of a triangle wave). The second modulation scheme can adapted to provide a square wave or a PWM signal as waveform (= envelope). The inverse crest factor of a PWM signal is equal to the square root of the duty cycle, wherein the duty cycle is defined as the ratio of the ON-time within a time period divided by the length of a time period. Thus, the second modulation scheme can be a PWM signal with a duty cycle greater than 50% (in case of a sine as waveform of the first modulation scheme) or greater than 3314% (in case of a sine as waveform of the first modulation scheme). These inverse crest factors refer to individual phases, and, in particular, do not refer to an signal provided by the addition of a plurality of phase voltages.

[0019] The threshold speed is lower than a fundamental frequency of a radial resonance oscillation of the stator (magnetic core + windings) of the AC motor. The radial 202401503

[0020] 4 resonance oscillation refers to a resonance effect resulting in a mechanical oscillation mode with a (mainly) radial amplitude. The radial resonance oscillation refers to the cylindrical body provided by the magnetic material (and the stator windings), in particular the iron sheets providing the magnetic material of the stator core.

[0021] In the following, additional definitions are given for the wave forms or AC voltage provided by the first I second modulation scheme. The area below the curve representing a single phase of the AC voltage (eg. voltage between a phase L1 and the neutral connector N) provided by the first modulation scheme is preferably smaller than the area below 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 such that the voltage level (or amount of the peak voltage) does not impact this comparison. The area below the pertaining curves corresponds to the effective voltages. Since the effective voltages refer to the same normalization, this corresponds to the inverse crest factor (referring to the same, ie. normalized peak voltage).

[0022] Preferably, the second modulation scheme provides more mechanical power in the AC motor than the first modulation scheme in case of equal peak values of both modulation schemes. In other words, the second modulation scheme allows the AC motor to provide more mechanical power than the first modulation scheme, wherein the modulation schemes are both based on the same peak value (or mutually normalized otherwise). An alternative definition is that, for providing the same mechanical power by the AC motor, the second modulation scheme provides a lower peak amplitude (in the resulting envelope) than the first modulation scheme. For providing the same effective voltage (corresponding to the same mechanical power), the second modulation scheme has a peak value lower than the peak value of the first modulation scheme. In a plurality of embodiments, the duty cycle of a PWM modulation defines the peak amplitude (of the AC voltage, ie. of the envelope). This allows a further definition according to which the maximum duty cycle of the second modulation scheme is lower than the maximum duty cycle of the first modulation scheme for providing the same mechanical power / effective voltage. The maximum duty cycle corresponds to the peak voltage.

[0023] In further embodiments, the peak value of the AC voltage (in particular the maximum duty cycle of a PMW modulation) is reduced when the modulation scheme is changed from the first to the second modulation scheme. This occurs when passing 202401503

[0024] 5 the threshold speed (from low to high). The reduction of the peak value is possible since the effective voltage of the second modulation scheme is higher than the effective voltage of the first modulation scheme (when referring to the same peak voltage).

[0025] In additional embodiments, the first modulation scheme generates a synthetized sine wave (or triangle wave) as polyphase AC voltage. The second modulation scheme preferably generates a wave form with a higher effective value than a comparable sine wave (ie. with the same peak value or mutually normalized otherwise). The second modulation scheme can be a full block mode (FBM). The second modulation scheme can a SVPWM (space vector pulse width modulation) as far as it is adapted to provide a waveform with a higher inverse crest factor than a sine, eg. a rectangular or a trapezoid wave. The first modulation scheme can be space vector modulation (SVPWM), in particular SVPWM adapted to provide a waveform of a sine or a triangle wave. Further the first modulation scheme can be a generalized discontinuous pulsewidth modulation (GDPWM), in particular GDPWM providing a waveform in form of a sine wave or a triangle wave. The GDPWM can be a modulation scheme comprising distinct discontinuous pulse with 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 I phase signal, maximized efficiency, ... ), depending on the current operating point. In addition, a DPMW modulation. Further, the first modulation scheme can be a Synch3PWM I Synch5PWM I Synch7PWM adapted to provide a sine wave or triangle wave. For the first modulation scheme, other schemes provided with a setting to generate a (synthetical) sine wave or a triangular wave can apply.

[0026] In further embodiments, the electric machine is operated in field weakening mode if the rotational speed of the AC motor is equal to or higher than a given threshold speed. In particular, the second modulation scheme is used for modulating the DC supply voltage, in case that the AC motor is operated in field weakening mode, wherein the first modulation scheme is used otherwise. As a variant of the described method, the second modulation scheme is used for modulating the DC supply voltage, in case that the AC motor is operated in field weakening mode, provided that (as a second prerequisite) the rotational speed of the AC motor is equal to or higher than the given threshold speed, wherein the first modulation scheme is used otherwise. 202401503

[0027] 6

[0028] A further variant of the mentioned method provides that the second modulation scheme is used for modulation in case that a high power mode applies, provided that the rotational speed of the AC motor is equal to or higher than the given threshold speed. In a lower power mode, the modulation scheme can be independent of the rotational speed of the AC motor. It is an option that the DC supply voltage is modulated depending on the threshold speed in the high power mode. According to this option, in a lower power mode, the DC supply voltage is modulated using the first modulation scheme independent from the rotational speed of the AC motor (ie. if the speed threshold is exceeded or not). It can be provided that, in a low power mode, the DC supply voltage is modulated with different modulation schemes selected according to another threshold speed distinct to the threshold speed referring to the fundamental frequency of the radial oscillation of the stator. The high power mode can be defined by an operating state, in which a (electrical or mechanical) power threshold is exceeded. The high power mode can be defined by an operating state, in which a torque threshold is exceeded. This refers to the electrical power consumed by the AC motor, the mechanical power provided by the AC motor or a torque provided by the AC motor, respectively. The low power mode can be defined by an operating state, 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.

[0029] The fundamental frequency of the stator preferably reflects the impact of the stator slots to the radial resonance oscillation. Preferably, the fundamental frequency of the stator preferably reflects the impact of the number of the stator slots, and / or the positions of the slots and / or the geometry of the slots. In particular, the fundamental frequency corresponds to a mechanical resonance oscillation of the stator, in which the stator slots mark the position of an oscillation node. This can refer to each slot or to an n-th slot, n being a divider of the number of slots in the stator. In addition, the fundamental frequency of the stator can reflect the impact of mechanical elements attached to the stator, in particular housing elements, cooling elements, bearings or other. The impact is particularly given in case that the vibration of the stator is transferred to the mechanical element and back, such that the vibration and in particular the resonance is given by a vibrating system comprising the stator as well as the mechanical element mechanically attached thereto.

[0030] Further implementations include a software code adapted to realize the method according to one of the preceding claims when running on a programmable inverter controller. In particular, software realizing a machine control for the AC motor (eg. 202401503

[0031] 7 realizing a vector control) can be provided on the controller, wherein the software code realizing the method can be a part of the machine control software or can be a software part interacting (controlling) the machine control software, in particular by selecting the modulation scheme as described herein, the machine control software controlling the modulating process according to the selected modulation scheme. In this case, the step of modulation is carried out by the machine control software, wherein the selection according to the speed (eg. involving comparing the speed with the threshold) is performed by the software code. The software code or the machine control software controlled by the software code generate a pulse pattern, which realizes the step of modulating the DC supply voltage. Further, the operation of a power stage of an inverter can controlled according to the pulse pattern, which results from the step of modulating.

[0032] As an application of the method, an electric vehicle drive can be provided with an inverter which is adapted to carry out the method described herein. In particular, the inverter comprises a controller adapted to provide a pulse pattern reflecting the step of modulating as described herein. The controller can encompass the software code mentioned above. The electric vehicle drive preferably comprises an AC motor connected to the inverter. The inverter is adapted to carry out the method described herein in order to supply the AC voltage (resulting from the modulation by the power stage) to the AC motor.

[0033] Figure 1 shows an example of an electric drive.

[0034] Figure 2 shows a diagram for exemplifying features of the method described herein, in particular two distinct modulation schemes and the selection of one of the modulation schemes.

[0035] Figure 1 shows an electric drive with an inverter I, which supplies AC current V to an electrical machine shown as polyphase AC motor EM. A DC supply voltage VIN provided by an accumulator ES supplies the DC side of the inverter I. The inverter I and the AC motor EM have the same number of phases, ie. three phases as shown in Figure 1 . A first phase of the inverter I has two switching elements 11 , 21 (equivalent to a first half bridge) which are connected in series via connecting point A1 . A second phase of the inverter I has two switching elements 12, 22 (equivalent to a second half bridge) which are connected in series via connecting point A2. A third phase of the inverter I has two switching elements 13, 23 (equivalent to a third half bridge) which are connected in series via connecting point A3. The connecting 202401503

[0036] 8 points A1 - A3 provide the multiphase AC voltage V. This voltage V generates a polyphase current in the stator windings (not shown) for generating a rotating magnetic field in the electrical machine.

[0037] A controller C, which can be part of the inverter I or an external device (as depicted), is connected to the switching elements 11 - 23 of the inverter. The controller C provides controlling signals to the control inputs of the switching elements 11 - 23, which are depicted symbolically as arrows and can be realised as gate ports in case the switching events are realised as MOSFETs. The controller C is adapted to carry out the methods mentioned herein and, in particular, is adapted to operate the switches according to selectable modulation schemes (first, second modulation scheme M1 , M2). Since the modulation scheme is selected according to the rotational speed of the AC motor, the controller C is adapted to provide the rotational speed, for example from values used within a motor control, or is adapted to receive a signal representing the rotational speed. In addition, the controller C is adapted to provide the current torque generated by the AC motor EM, for example from values used within a motor control, or is adapted to receive a signal representing the torque, for example a desired torque. The method described herein can be carried out in dependency of the torque. Motor control software can be used within the controller C for providing a closed loop control or vector control for the AC motor EM. Further, software code adapted to realise the method described herein can be part of the controller or even of the motor control software.

[0038] Figure 2 shows a diagram depicting the peak amplitude A of AC voltages V (ie. the envelope of a PWM-signal) in dependency of the rotational speed n of the AC motor. The linear dependency of amplitude A is a simplification; more complex dependencies can apply. Further, three curves are given showing the envelope I course of the AC voltage V at distinct speeds. It can be assumed that the peak amplitude A vs. the speed n shown in the line chart of Fig. 2 is given for a torque or power (as a function of n).

[0039] For speeds below threshold speed T, amplitude A monotonically increases with the speed. At a low speed n1 , the amplitude A1 is lower than the amplitude A2 at a speed n2 higher than speed n1 (but below the threshold speed T). For n1 , the corresponding wave form (ie. the envelope of a modulated DC voltage, in particular the envelope of a PWM-modulated AC voltage) is shown as C1 . Waveform C1 is a sine wave with a peek amplitude of A1 . The area F1 below the curve C1 (for the depicted wave period) corresponds to the signal power for n1 . For a higher speed 202401503

[0040] 9 n2, (higher than low speed n1 , below threshold speed T), the waveform C2 applies. The waveform C2 (occurring at speed n2) has the same shape as waveform C1 (occurring at speed n1 ). However, amplitude A2 of waveform C2 is higher than the amplitude A1 at a speed n1 . The waveform C2 has a higher effective voltage than waveform C1 . The area F2 below the curve C2 (for the depicted wave period) corresponds to the signal power for n2 and is larger that area F1 . The modulation schemes used at n1 and n2 (ie. used below T) are the same. In particular, the modulation schemes below T result in the same waveform shapes (in Fig. 2: sinusoidal). The inverse crest factor (ratio of RMS value, see areas F1 , F2, and the peak voltage A1 , A2) of waveforms C1 and C2 are identical (due to the same waveform shape). As can be seen in Fig. 2, the peak amplitude increases with the speed, wherein the waveform (in particular its shape) remains the same.

[0041] When passing the threshold speed T, the modulation scheme changes. The resulting change of the waveform can be seen when comparing curve C3 (depicting the waveform resulting from the modulation for n > T) to curve C1 or to curve C2. At speed n3 (above T), a full block modulation is used which creates a rectangular waveform according to curve C3. Curve C3 has an amplitude A3 lower than amplitude A2. However, the area F3 covered by curve C3 is larger than area F2 covered by curve C2. Since the area reflects the power of the signal, curve C3 provides more power than C2. At the same time, the amplitude A3 of curve C3 is lower than the amplitude C2. The mechanical excitation, which is defined by the (peak) amplitude, resulting from curve C3 is lower than the mechanical excitation resulting from curve C2 eventhough curve C3 provides more power (and a higher speed) than curve C2. The term “curve” in this discussion corresponds to the term “modulation” since the modulation defines the curve, ie. the envelope of the resulting AC voltage. As modulation scheme, the block mode (see curve C3) is maintained for speeds > n3 until the maximum speed n_max is reached. Fig. 2 depicts a first modulation scheme M1 for speeds < T and a second modulation scheme M2 for speeds > T. Modulation scheme M1 is used for speeds n = 0 ... T and modulation scheme M2 is used for speeds n = T ... n_max. Other embodiments can provide more than 2 modulation schemes, 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 RMS value and the peak voltage, the RMS value being equivalent to the area F1 , F2 or F3 normalized by the duration of the waveform period D, and the peak voltage being equivalent to the pertaining amplitude A1 , A2 and A3, respectively. 202401503

[0042] 10

[0043] In Fig. 2, the peak voltage A1 , A2 is increased with increasing speed n (while maintaining the modulation scheme M1 ) until the threshold speed T is reached. When passing the threshold speed T, the areal shape of the waveform resulting from the modulation scheme is changed to a bulkier, more extensive (wider) waveform. In this way, the area covered by the waveform C1 - C3 can be extended eventhough the amplitude is reduced (see C3 - C2) at the same time, while the mechanical power of the drive is constantly increased (with increased speed n).

Claims

20240150311Patent Claims1 . Method for operating polyphase AC motor, comprising:(a) Generating a polyphase AC voltage (V) by modulating a DC supply voltage (VIN) and(b) Supplying a polyphase AC motor (EM) with the generated AC voltage providing a rotating magnetic field in the AC motor, wherein the step of generating the AC voltage (V) involves modulating the DC supply voltage (VIN) using a first modulation scheme (M1 ), if the rotational speed (n) of the AC motor is below a given threshold speed (T) and using a second modulation scheme otherwise, 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 resonance oscillation of the stator of the AC motor (EM).

2. Method according to claim 1 , wherein the area (F1 , F2) below a curve (C1 , C2) representing a single phase of the AC voltage provided by the first modulation scheme is smaller than the area (F3) below a curve (C3) representing a single phase of the AC voltage provided by the second modulation scheme, both curves (C1 , C2; C3) being 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 case of equal peak values of both modulation schemes.

4. Method according to claim 1 , 2 or 3, wherein, when passing the threshold speed (T) and changing the modulation scheme from first to second modulation scheme, the peak value of the AC voltage is reduced.

5. Method according to one of the preceding claims, wherein the first modulation scheme generates a synthetized sine wave as polyphase AC voltage and wherein the second modulation scheme generates a wave form with a higher effective value than a comparable sine wave.

6. Method according to one of the preceding claims, wherein the second modulation scheme is selected from the list: full block mode, space vector pulse width modulation, synchronous 3-Step Pulse width modulation, generalized pulse width modulation or discontinuous pulse width modulation.202401503127. Method according to one of the preceding claims, wherein the electric machine is operated in field weakening mode if the rotational speed (n) of the AC motor is equal to or higher than a given threshold speed (T).

8. Method according to one of the preceding claims, wherein the fundamental frequency of the stator reflects the impact of the stator slots to the radial resonance oscillation.

9. Method according to claim 8, wherein the fundamental frequency of the stator reflects the impact of mechanical elements attached to the stator.

10. Method according to one of the preceding claims, wherein the DC supply voltage (VIN) is modulated depending on the threshold speed (T) in a high power mode and wherein, in a lower power mode, the DC supply voltage (VIN) is modulated using the first modulation scheme (M1 ) independent from the rotational speed (n) of the AC motor or is modulated with different modulation schemes selected according to another threshold speed distinct to the threshold speed referring to the fundamental frequency of the radial oscillation of the stator.11 . Software code adapted to realize the method according to one of the preceding claims when running on a programmable inverter controller.

12. Electric vehicle drive having an inverter (I) and an AC motor (EM) connected therewith, the inverter (I) being adapted to carry out the method according to one claims 1 - 10.

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