Electric motor control

Abstract

A method for controlling an AC electric motor by means of a voltage inverter with a plurality of electrical switches having on-off states, which together define a plurality of non-zero vectors arranged perpendicularly around a common origin, includes, by means of one or more controllers, calculating a three-phase voltage reference vector that is applied to the AC electric motor by means of the voltage inverter. The method also includes calculating the switching times of the voltage inverter by means of one or more controllers using a first subset of the plurality of non-zero vectors, wherein the first subset contains fewer than all of the plurality of non-zero vectors and no adjacent non-zero vectors, to synthesize the three-phase voltage reference vector.Additionally, the procedure includes applying the three-phase voltage reference vector to the AC electric motor using the voltage inverter.

Description

introduction

[0001] The present disclosure relates to the field of electric motor controls.

[0002] Electric motors are used in many applications, including generating traction power for powering electric vehicles. When controlling an AC electric motor, especially a high-voltage electric motor, significant unintended leakage currents generated during motor operation can reduce motor efficiency. Furthermore, unintended leakage currents can flow through various motor components, such as the bearings, which can compromise motor durability. A motor control method that reduces unintended leakage currents can therefore be beneficial. Description

[0003] A method for controlling an AC electric motor by means of a voltage inverter having a plurality of electrical switches with on-off states that together define a plurality of non-zero vectors angularly arranged around a common origin, comprising, by means of one or more controllers: calculating a three-phase voltage reference vector applied to the AC electric motor by means of the voltage inverter. The method also comprises, by means of one or more controllers, calculating switching times of the voltage inverter using a first subset of the plurality of non-zero vectors, wherein the first subset includes fewer than all of the plurality of non-zero vectors and no adjacent non-zero vectors, to synthesize the three-phase voltage reference vector.In addition, the method comprises applying the three-phase voltage reference vector to the AC electric motor by means of the voltage inverter.

[0004] As an additional feature, the method may further comprise calculating, by means of one or more controllers, switching times for the voltage inverter using a second subset of the plurality of non-zero vectors, the second subset comprising fewer than all of the plurality of non-zero vectors and comprising no adjacent non-zero vectors, to synthesize the three-phase voltage reference vector, the second subset of the plurality of non-zero vectors being different from the first subset of non-zero vectors, and applying, by means of the voltage inverter, the three-phase voltage reference vector to the AC electric motor. The first subset of the plurality of non-zero vectors and the second subset of the plurality of non-zero vectors may not have any common members.

[0005] The AC electric motor can be a three-phase electric motor, and the voltage inverter can be an H-bridge two-level voltage inverter with six electrical switches and six non-zero vectors. Alternatively, the voltage inverter can be a multi-level voltage inverter with 24 non-zero vectors.

[0006] Furthermore, applying the three-phase voltage reference vector to the AC electric motor may include applying the voltage reference vector by means of pulse width modulation, and the pulse width modulation may be a state vector pulse width modulation.

[0007] Alternatively, each non-zero vector in the first subset of the plurality of non-zero vectors may result in a first resultant voltage at a neutral point of the AC electric motor. As a further alternative, each non-zero vector in the second subset of the plurality of non-zero vectors may result in a second resultant voltage at the neutral point of the AC electric motor. The first resultant voltage may be different from the second resultant voltage. The method may also include switching from using the first subset of non-zero vectors to using the second subset of the plurality of non-zero vectors in response to a voltage of a shaft of a rotor of the AC electric motor.

[0008] A second method for controlling an AC electric motor by means of a voltage inverter having a plurality of electrical switches with on-off states that together define a plurality of non-zero basis vectors angularly arranged around a common origin comprises calculating, by means of one or more controllers, a rotational state vector to be applied to the AC electric motor by means of the voltage inverter. The method further comprises controlling, by means of one or more controllers, switching of the voltage inverter using a first subset of the plurality of non-zero basis vectors, wherein the first subset includes fewer than all of the plurality of non-zero basis vectors and no adjacent non-zero basis vectors to synthesize the rotational state vector.Furthermore, the method includes applying the rotational state vector to the AC electric motor by means of pulse width modulation using the voltage inverter.

[0009] The method may additionally include, by means of one or more controllers, controlling the switching of the voltage inverter using a second subset of the plurality of non-zero basis vectors, wherein the second subset includes fewer than all of the plurality of non-zero basis vectors and includes no adjacent non-zero basis vectors to synthesize the rotational state vector, wherein the second subset of the plurality of non-zero basis vectors differs from the first subset of non-zero basis vectors, and, by means of the voltage inverter, applying the rotational state vector to the AC electric motor using pulse width modulation. The first subset of the plurality of non-zero vectors and the second subset of the plurality of non-zero vectors may not have any common members.

[0010] The method may further include calculating, by means of one or more controllers, the switching times of the voltage inverter using all non-zero basis vectors to synthesize the rotational state vector, and applying, by means of the voltage inverter, the rotational state vector to the AC electric motor using pulse width modulation. The first subset of non-zero basis vectors may be used in a first operating state of the AC electric motor, and all non-zero basis vectors may be used in a second operating state of the AC electric motor. The first operating state of the AC electric motor may be a lower motor speed or a lower motor torque than the second operating state of the AC electric motor.

[0011] In the method for controlling an AC electric motor, each non-zero vector in the first subset of the plurality of non-zero vectors may result in a first resultant voltage at a neutral point of the AC electric motor, and each non-zero vector in the second subset of the plurality of non-zero vectors may result in a second resultant voltage at the neutral point of the AC electric motor. The first resultant voltage may be different from the second resultant voltage. Furthermore, the use of the first subset of non-zero vectors may be changed to the use of the second subset of the plurality of non-zero vectors in response to a voltage of a shaft of a rotor of the AC electric motor.

[0012] A motor vehicle includes a source of stored electrical energy and an alternating current electric motor. The motor vehicle also includes a voltage inverter coupled to the source of stored electrical energy and the alternating current electric motor for supplying switched electrical energy from the source of stored electrical energy to the alternating current electric motor. The voltage inverter includes a plurality of electrical switches with on-off states that together define a plurality of non-zero vectors angularly arranged about a common origin. The motor vehicle additionally includes one or more control devices.The one or more controllers are programmed together with the following instructions: calculating a three-phase voltage reference vector to be applied to the AC electric motor via the voltage inverter; calculating switching times for the voltage inverter using a first subset of the plurality of non-zero vectors, the first subset including fewer than all of the plurality of non-zero vectors and no adjacent non-zero vectors, to synthesize the three-phase voltage reference vector; applying the three-phase voltage reference vector to the AC electric motor via the voltage inverter.

[0013] The one or more controllers may additionally be programmed together with the following instructions: By means of one or more controllers, calculating switching times for the voltage inverter using a second subset of the plurality of non-zero vectors, the second subset including fewer than all of the plurality of non-zero vectors and no adjacent non-zero vectors, to synthesize the three-phase voltage reference vector, the second subset of the plurality of non-zero vectors being different from the first subset of non-zero vectors; by means of the voltage inverter, applying the three-phase voltage reference vector to the AC electric motor.

[0014] The first subset of the plurality of non-zero vectors and the second subset of the plurality of non-zero vectors may not have any common members. Each non-zero vector in the first subset of the plurality of non-zero vectors may result in a first resultant voltage at a neutral point of the AC electric motor, and each non-zero vector in the second subset of the plurality of non-zero vectors may result in a second resultant voltage at the neutral point of the AC electric motor. The first resultant voltage may be different from the second resultant voltage.

[0015] The one or more controllers may be further programmed to change from using the first subset of non-zero vectors to using the second subset of the plurality of non-zero vectors in response to a voltage of a shaft of a rotor of the AC electric motor.

[0016] The above summary does not represent every embodiment or aspect of the present disclosure. The above features and advantages of the present disclosure, as well as other possible features and advantages, will be readily apparent from the following detailed description of the embodiments and best modes for carrying out the disclosure, when considered in conjunction with the accompanying drawings and appended claims. Furthermore, this disclosure expressly contemplates combinations and subcombinations of the elements and features presented above and below. Brief description of the drawings Fig. 1 shows a motor vehicle and a control system for an electric motor. Fig. Figure 2 is a vector diagram illustrating a control mode for an electric motor. Fig. 3 shows a cross section of the electric motor of Fig. 1 and certain electrical currents associated with the electric motor. Fig. Figure 4 shows a vector diagram illustrating an alternative control mode for an electric motor. Fig. Figure 5 shows a vector diagram illustrating another alternative control method for an electric motor. Fig. Figure 6 shows the switching voltage of a voltage inverter and the resulting waveform of the induced voltage on the shaft of an electric motor connected to the voltage inverter. Fig. 7 shows a multi-level inverter. Fig. Figure 8 shows vectors for the switching elements of the multi-level inverter of Fig. 7. Fig. Figure 9 shows a strategy for controlling an electric motor using a multi-level inverter, such as the inverter of Fig. 7. Detailed description

[0017] The present disclosure may be embodied in many different forms. Representative examples of the disclosure are illustrated in the drawings and are described in detail herein as non-limiting examples of the disclosed principles. To this end, elements and limitations described in the "Summary," "Introduction," "Summary," and "Detailed Description" sections, but not expressly set forth in the claims, should not be incorporated into the claims, either individually or collectively, by implication, inference, or otherwise.

[0018] For the purposes of this description, the use of the singular includes the plural and vice versa, unless expressly excluded; the terms "and" and "or" apply both in the subjunctive and disjunctive moods; "each" and "all" mean "each and all," and the words "including," "including," "comprising," "with," and the like mean "including without limitation." Furthermore, words of approximation such as "approximately," "almost," "substantially," "generally," "about," etc., may be used herein to mean "at, near, or almost at," or "within 0-5% of," or "within acceptable manufacturing tolerances," or logical combinations thereof.

[0019] In Fig. 1, a motor control system 12 is first shown. The motor control system 12 may be used for an electric vehicle 10. The motor control system 12 may include a traction battery 20, a power inverter module (PIM) 22, an electronic control unit (ECU) 24, and an electric motor 26. The traction battery 20 may consist of one or more batteries or battery cells. The electric vehicle 10 may be a vehicle that uses an electric motor for at least part of the vehicle's propulsion, and it may be an all-electric or hybrid-electric vehicle. Furthermore, the electric vehicle 10 may be any vehicle, for example, a car, a truck, a sport utility vehicle, a motorcycle, a bicycle, an off-road vehicle, etc. The electric motor 26 may be an engine that provides the tractive power for propelling the electric vehicle 10.

[0020] The inverter module 22 may include six switching elements: switching element 30, switching element 31, switching element 32, switching element 33, switching element 34, and switching element 35. The switching elements may be various technologies within the meaning of this disclosure, including insulated-gate bipolar transistors (IGBTs) and field-effect transistors (FETs). Switching element 30, switching element 31, switching element 32, switching element 33, switching element 34, and switching element 35 may also be collectively referred to as switching elements 30-35 below.

[0021] The electronic controller 24 may control the switching of the switching elements 30-35 to control and supply voltage to the electric motor 26. The electronic controller 24 is assumed to have suitable electronic resources (microprocessor, software, memory, inputs, outputs, and the like) to perform the functions attributed to the electronic controller 24 herein. The electronic control unit 24 may be a stand-alone control unit, it may be integrated into the inverter module 22, or it may be integrated with other control units of the electric vehicle 10, which includes the motor control system 12. The electronic control unit 24 may also be networked with other control units and share responsibility for performing the functions described herein.

[0022] The electric motor 26 may be an alternating current (AC) electric motor. For example, the electric motor 26 may be a permanent magnet motor (and furthermore, a permanent magnet synchronous motor) or an induction motor. For the sake of simplicity, the electric motor 26 is Fig. 1 by its stator windings, the stator winding 40, the stator winding 42 and the stator winding 44, wherein the rotor of the electric motor 26 in Fig. 1 is omitted. The stator windings may be connected to each other at a neutral point 48. The electric motor 26 may have a conductive housing 46.

[0023] In the Fig. 1, various stray capacitances may be present, including stray capacitance C S1 between the negative rail N of the inverter module 22 and ground 49 (GND), the stray capacitance C S2between the housing 46 of the electric motor 26 and ground 49 and the stray capacitance C S3 between the neutral point 48 of the stator windings of the electric motor 26 and the housing 46 of the electric motor 26.

[0024] The electric motor 26 can be controlled by the so-called field-oriented control (FOC). In FOC, the currents in the stator windings of the electric motor 26 are resolved into two orthogonal currents, a flux-inducing component i d and a torque-inducing component i q , which form the basis for the signals controlling the electric motor 26. In connection with the FOC and elsewhere, the so-called Clarke transformation can be used to simplify multiphase quantities in a symmetrical three-phase system into two-dimensional quantities in the α-β range.

[0025] In one implementation of FOC control, a three-phase voltage reference vector in the α-β domain can be calculated at high speed (i.e., much higher than the speed at which the electric motor is operating) to supply an electric motor with a three-phase voltage and thus generate the desired mechanical power (torque / speed) of the electric motor. This three-phase voltage reference can be applied from the inverter module 22 to the electric motor 26, for example, using pulse width modulation (PWM), including state vector PWM (SVPWM), where the three-phase voltage reference vector can be referred to as the "state vector."

[0026] Fig. Figure 2 describes an implementation of SVPWM. SVPWM can operate under the assumption that the switching elements in any branch of the inverter module 22 (for example, the leftmost branch, which includes switching element 30 and switching element 31, the middle branch, which includes switching element 32 and switching element 33, or the rightmost branch, which includes switching element 34 and switching element 35) have complementary states (i.e., one switching element closed and one switching element open). Under this assumption, there are eight states that define eight vectors in which the switching elements 30-35 can be together.

[0027] For simplicity, the eight vectors can be denoted by three binary digits. The first digit can indicate the state ("1" = closed, "0" = open) of switching element 30, the upper switching element in the leftmost branch of the inverter module 22. Therefore, when switching element 30 is closed, switching element 31 is open. The second binary digit can indicate the state ("1" = closed, "0" = open) of switching element 32, the upper switching element in the middle branch of the inverter module 22. Therefore, when switching element 32 is closed, switching element 33 is open. The third digit can indicate the state of switching element 34, the upper switching element in the rightmost leg of the inverter module 22. When switching element 34 is closed, switching element 35 is open (any two switching elements in a branch of the inverter module have complementary states).

[0028] The eight different vectors of the switching elements 30-35 of the inverter module 22 are shown accordingly in the following table: Vektor Wechselrichter-Schaltzustände CMV-Wert M0 000 0 M1 100 V / 3 PN M2 110 2V PN / 3 M3 010 V / 3 PN M4 011 2 V PN / 3 M5 001 V / 3 PN M6 101 2 V PN / 3 M7 111 V PN

[0029] Vectors M0 to M7 can be called "basis" vectors. Vectors M1 to M6 can be called "non-zero" vectors because they have magnitudes greater than zero. Vectors M0 and M7 can be called "zero" vectors because they have zero magnitude (i.e., they lie at the origin of the α-β plane). Vectors M1-M6 can be called "angularly arranged" around a common origin 51 in the α-β plane.

[0030] A vector labeled M0 (000) can represent that all upper switching elements, i.e., switching element 30, switching element 32, and switching element 34, are open. A vector labeled M7 (111) can represent that all upper switching elements, i.e., switching element 30, switching element 32, and switching element 34, are closed. Vectors M1 to M6 can represent vectors in which one or two of the switching elements 30, 32, and 34, i.e., the upper switching elements in inverter module 22, are closed.

[0031] The table above shows the resulting common-mode voltages (CMVs) at the neutral point 48 of the electric motor 26, which are connected to vectors M0 through M7. For vector M0, the resulting common-mode voltage at the neutral point 48 is zero because all switching elements 30, 32, and 34 connected to the positive rail P are open and all switching elements 31, 33, and 35 connected to the negative rail N are closed. For vector M7, the resulting CMV at the neutral point is 48 V. PN , the voltage between the positive rail P and the negative rail N, since all upper switching elements are closed (i.e. connected to the positive rail P) and all lower switching elements are open.

[0032] For vector M1, switching element 30, switching element 33, and switching element 35 are open. Accordingly, the resulting CMV at the neutral point 48 of the electric motor is 26 V PN / 3. where V PNis the DC voltage between the positive rail P and the negative rail N of the traction battery 20 and the inverter module 22. The resulting CMV at the neutral point 48 is also V PN / 3 for the vectors M3 and M5, in each of which an upper switching element (switching element 32 and switching element 34) is closed.

[0033] For vector M2, switching element 30, switching element 32 and switching element 35 are closed. Accordingly, the resulting CMV at the neutral point 48 of the electric motor 26 is 2V PN / 3. The same applies to the vector M4 and the vector M6.

[0034] The unintentional leakage current generated in the electric motor 26 may be due to and dependent on the time rate of change of the CMV of the electric motor 26. This can be illustrated by the fact that for a capacitor, the current i through the capacitor is i = Cdv / dt, where C is the capacitance and dv / dt is the time rate of change of the voltage across the capacitance. A decrease in the time rate of change of the voltage across a capacitor therefore results in a decrease in the current through the capacitor.

[0035] As an example of SVPWM modulation, assume that V ref , the three-phase voltage reference vector to be applied to the electric motor 26 by means of the inverter module 22 is in the Fig. 2 in the α-β plane. To generate this voltage vector, a combination of vector M1, vector M2, and vector M0 and / or vector M7 can be used. During the PWM duty cycle, sufficient fractions of the period of the carrier signal M1, M2, and M0 and / or M7 can be applied so that the vector sum (magnitude and direction) determines the vector V ref In this case, however, the resulting CMV at the neutral point 48 of the electric motor 26 between the vectors M1 and M2 will change considerably, namely between V PN / 3 and 2V PN / 3. This tends to result in a dv / dt in the CMV, which leads to a leakage current.

[0036] See also Fig. 3. There, the stator 50 and a rotor 56 are shown, together with the housing 46 of the electric motor 26 and the bearings 52 and 54 of the electric motor 26. The rotor 56 is rotatably mounted on a shaft 58. Also schematically shown is a gear housing 70, the gears of which can be driven by the electric motor 26. Various currents are shown that result from dv / dt in CMV, including the leakage current 60, the leakage current 62 and the leakage current 64 (which together form a leakage current ileakage result), as well as the circulating current 66.

[0037] However, let us consider an alternative control strategy, as described in Fig. 4. With this control strategy, the use of vectors M2, M4, and M6 is not necessary. The voltage reference vector V refcan be generated as a vector sum using only vector MI, vector M3, and vector M5. From the table above, it can be seen that in each of the vectors M1, M3, and M5, the resulting common-mode voltage (CMV) at the neutral point 48 of the electric motor is 26 V. PN / 3. Using only vector M1, vector M3, and vector M5 reduces or eliminates the rate of change of the CMV of the electric motor 26 and the resulting current i = Cdv / dt, which is related to the time rate of change of the CMV. Fig. 4 shows that vector M1, vector M3, and vector M5 are non-adjacent non-zero vectors due to the elimination of vector M2, vector M4, and vector M6. In other words, no adjacent non-zero vectors are used.

[0038] A variation of the Fig. The control strategy shown in Figure 4 is Fig. 5. There, a voltage reference vector V refcan only be generated with vectors M2, M4 and M6. From the table above, it can be seen that in each of the vectors M2, M4 and M6, the resulting common-mode voltage at the neutral point is 48 2 V PN / 3. The use of only vector M2, vector M4, and vector M6 reduces or eliminates the time rate of change of the common-mode voltage of the electric motor 26 and the resulting current i = Cdv / dt, which is related to the time rate of change of the common-mode voltage between the neutral point 48 and ground 49. From Fig. 4 shows that vector M2, vector M4, and vector M6, non-adjacent non-zero vectors, are used due to the elimination of vector M1, vector M3, and vector M5. In other words, no adjacent non-zero vectors are used.

[0039] As mentioned above, the voltage reference vector Vref is a rotating vector. However, regardless of which section of the α-β plane Vref is located in, Vref can be synthesized using the vector sum of vector M1, vector M3, and vector M5 (at appropriate PWM duty cycles of the three vectors) or using the vector sum of vector M2, vector M4, and vector M6 (again at appropriate PWM duty cycles of the three vectors).

[0040] It should also be noted that the switching of the switching elements 30-35 may induce a common-mode voltage on the shaft 58 of the electric motor 26. An example is shown in Fig. 6, which shows that the common-mode voltage (CMV) associated with the switching of the switching elements 30-35 of the inverter module 22 can induce a common-mode voltage at the shaft 58 of the electric motor 26. A waveform 80 associated with the switching of the switching elements 30-35 can be reflected in a waveform 82 that represents the voltage V measured at the shaft 58 of the electric motor 26. sft The induced common-mode voltage on shaft 58 can accumulate over time and become sufficiently large to exceed the breakdown voltage of the oil film in bearing 52. Such an induced voltage can flow along a path 72 through bearing 52, presenting a significant potential for fretting due to electrical discharge machining (EDM). The current flowing along path 72 can also flow through gear housing 70, causing damage there as well.

[0041] However, the absolute voltage level at the shaft 58 of the electric motor 26 can be measured, for example, with a conductive brush resting on the shaft 58 of the electric motor 26. If the voltage at the shaft 58 of the electric motor 26 is observed to increase, the Fig. 5 shown control strategy (i.e. the use of vectors M2, M4 and M6) to the Fig. 4 (i.e., the use of vectors M1, M3, and M5) to counteract the accumulation of the common-mode voltage on the shaft of the electric motor 26. The opposite approach can also be taken (transition from the control strategy Fig. 4 on tax strategy Fig. 5).

[0042] The motor control method and system thus described may be useful when the electric motor 26 is operated at a relatively low or medium speed at which the full voltage capacity of the inverter module 22 is not required.

[0043] See now Fig. 7. An alternative inverter is shown here. The 300 inverter can be installed between two rails V DC / 2 and -V DC / 2. Here, the inverter 300 comprises three legs, leg 302, leg 304, and leg 306, as well as three phase outputs, output 308, output 310, and output 312, which can be connected to the stator windings of the electric motor 26. In the multi-stage configuration shown for the inverter 300, each of the outputs 308, 310, and 312 can assume three different voltages. To illustrate, assume that the switching element 320, the switching element 322, the switching element 324, and the switching element 326 are turned off. In this case, the output 308 assumes the voltage V DC / 2. When the switching element 324 and the switching element 326 are turned on and the switching element 320 and the switching element 322 are turned off, the output 308 assumes the voltage - V DC / 2. When switching element 322 and switching element 324 are turned on and switching element 320 and switching element 326 are turned off, output 308 assumes a voltage of 0 V (zero volts).

[0044] The vector diagram for the vectors of the inverter 300 is shown in Fig. 8. Here, the voltage applied to the electric motor 26 can have 27 vectors, compared to the eight vectors in Fig. 3. In every Fig. The vector shown in Figure 8 is the voltage state (P = V DC / 2, N = - V DC / 2 and O = 0V). For vector 300a, vector 300b, vector 300c, vector 300d, vector 300e and vector 300f the resulting voltage at the neutral point is 48 + / -V DC / 2. For vector 302a, vector 302b, vector 302c, vector 302d, vector 302e and vector 302f the resulting voltage at the neutral point is 48 + / -V DC / 3. For vector 304a, vector 304b, vector 304c, vector 304d, vector 304e and vector 304f the resulting voltage at the neutral point is 48 + / - V DC / 6. Finally, for vector 307, the resulting voltage at the neutral point 48 is zero volts. It can be seen that Fig. 8 shows 24 non-zero vectors.

[0045] When modulated with the inverter 300, the operation of the electric motor 26 can be carried out in different torque-speed ranges of the electric motor 26 (shown in Fig.9) by selecting vectors that result in the same common-mode voltage at the neutral point 48 of the electric motor 26. For example, operation in ranges 320a, 320b, and 320c can be performed by selecting vector 300a, vector 300b, vector 300c, vector 300d, vector 300e, and vector 300f, while excluding vectors 302a-302f and vectors 304a-304f. Furthermore, operation in ranges 322a and 322b can be performed by selecting vector 304a, vector 304b, vector 304c, vector 304d, vector 304e, and vector 304f. Finally, the operation in area 324 can be performed by selecting vector 302a, vector 302b, vector 302c, vector 302d, vector 302e and vector 302f.

[0046] In order to increase the voltage available for controlling the electric motor 26, the so-called third harmonic injection can be used.

[0047] The motor control method and system disclosed in this disclosure can reduce leakage current in the electric motor 26 by reducing the time rate of change of the common-mode voltage at the neutral point 48 of the electric motor 26. It can also reduce electrical discharge machining (EDM) current, which can damage the bearings of the electric motor 26 and the gears in the transmission housing 70. The motor control method and system described herein can reduce the need for a common-mode choke that might otherwise be used to suppress common-mode currents.

Claims

[1] A method for controlling an AC electric motor by means of a voltage inverter having a plurality of electrical switches with on-off states which together define a plurality of non-zero vectors arranged angularly around a common origin, the method comprising: by means of one or more controllers, calculating a three-phase voltage reference vector to be applied to the AC electric motor by means of the voltage inverter; by means of one or more controllers, calculating switching times of the voltage inverter using a first subset of the plurality of non-zero vectors, the first subset comprising fewer than all of the plurality of non-zero vectors and not comprising any adjacent non-zero vectors, to synthesize the three-phase voltage reference vector; and by means of the voltage inverter, applying the three-phase voltage reference vector to the AC electric motor. [2] The method of claim 1, further comprising: by means of one or more controllers, calculating switching times of the voltage inverter using a second subset of the plurality of non-zero vectors, the second subset comprising fewer than all of the plurality of non-zero vectors and not comprising any adjacent non-zero vectors to synthesize the three-phase voltage reference vector, the second subset of the plurality of non-zero vectors being different from the first subset of non-zero vectors; and by means of the voltage inverter, applying the three-phase voltage reference vector to the AC electric motor. [3] The method of claim 2, wherein the first subset of the plurality of non-zero vectors and the second subset of the plurality of non-zero vectors have no common members. [4] The method of claim 1, wherein: the AC electric motor is a three-phase electric motor; and the voltage inverter is an H-bridge two-level voltage inverter with six electrical switches and six non-zero vectors. [5] The method of claim 1, wherein: the AC electric motor is a three-phase electric motor; and the voltage inverter is a multi-level voltage inverter with 24 non-zero vectors. [6] The method of claim 1, wherein the step of applying the three-phase voltage reference vector to the AC electric motor comprises applying the voltage reference vector by means of pulse width modulation. [7] The method of claim 6, wherein the pulse width modulation is a state vector pulse width modulation. [8] The method of claim 2, wherein each non-zero vector in the first subset of the plurality of non-zero vectors results in a first resultant voltage at a neutral point of the AC electric motor. [9] The method of claim 8, wherein each non-zero vector in the second subset of the plurality of non-zero vectors results in a second resultant voltage at the neutral point of the AC electric motor, the first resultant voltage being different from the second resultant voltage. [10] The method of claim 2, comprising switching from using the first subset of non-zero vectors to using the second subset of the plurality of non-zero vectors in response to a voltage of a shaft of a rotor of the AC electric motor.

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