Electric drive assembly with dynamic control of pulse width modulation switching

Abstract

The present invention relates to an electric drive assembly with dynamic control of pulse width modulation switching. An electric drive system includes a chargeable energy storage unit, a power inverter, an electric motor, and a controller having a processor and tangible, non-transient memory having instructions recorded thereon. The transfer of power between the chargeable energy storage unit and the electric motor is governed by a pulse width modulation (PWM) switching frequency. The controller is configured to determine a current switching frequency based in part on a PWM type, a PWM switching frequency regime, and an inverter direct current voltage. A PWM scalar is determined based in part on the current switching frequency and a control reference frequency maximum. The controller is configured to transmit a command signal to adjust the transfer of power based in part on the PWM scalar, the PWM switching frequency being proportional to a product of the PWM scalar and the control reference frequency.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to control of pulse width modulation switching in electric drive systems and electric vehicles including the same. BACKGROUND

[0002] In recent years, the use of pure electric vehicles and hybrid electric vehicles has increased, such as, for example, battery electric vehicles, extended-range electric vehicles, plug-in hybrid electric vehicles, and fuel cell unit hybrid electric vehicles. Many electric vehicles employ a rechargeable traction battery pack to store and supply power necessary for operating one or more traction motors in the vehicle's powertrain. The operation and control of each traction motor, which can have the nature of a multi-phase alternating current (AC) electric motor-generator unit, can be achieved by employing a power inverter module that converts battery-generated direct current (DC) power into AC power to drive the motor using pulse width modulation (PWM) control signals output from a control unit in the electric vehicle. The motor windings of each multi-phase AC motor-generator unit can be coupled to an inverter sub-module of the power inverter module, each inverter sub-module employing a pair of switches, and the pair of switches open and close in a complementary manner to perform a fast switching function to convert the DC power into AC power and vice versa. SUMMARY

[0003] Disclosed herein is an electric drive system including a rechargeable energy storage unit, a power inverter operably connected to the rechargeable energy storage unit, and an electric motor operably connected to the power inverter. A controller is in communication with the power inverter. The transfer of power between the rechargeable energy storage unit and the electric motor is governed by a pulse width modulation (PWM) switching frequency. The controller has a processor and a tangible, non-transitory memory on which instructions are recorded.

[0004] Execution of the instructions by the processor causes the controller to determine a current switching frequency based in part on a PWM type, a PWM switching frequency pattern, and an inverter direct current voltage. A PWM scalar is determined based in part on the current switching frequency and a control reference frequency maximum. Operation of the controller is defined at least in part by the control reference frequency. The controller is configured to communicate a command signal to adjust the transfer of power based in part on the PWM scalar. The PWM switching frequency is proportional to a product of the PWM scalar and the control reference frequency.

[0005] The current switching frequency can be based in part on an inverter coolant temperature and whether a drive torque or a regeneration torque is in operation. The instructions can be executed dynamically such that the PWM scalar varies over time. Obtaining the PWM scalar can include obtaining an extended PWM switching frequency that is a sum of the current switching frequency and a frequency span. A ratio of the extended PWM switching frequency is obtained by dividing by a control reference frequency maximum. The PWM scalar is set to a minimum integer greater than or equal to the ratio such that the PWM scalar is equal to CEILING (ratio).

[0006] Determining the current switching frequency can include determining, via the controller, a PWM region layout having a plurality of PWM regions arranged in a torque-speed curve calibrated for the electric motor. The specified PWM region is obtained, via the controller, from the PWM region layout in the torque-speed curve based on a speed and a torque of the electric motor, the speed and the torque of the electric motor being based in part on the torque command. Determining the current switching frequency can include selecting, via the controller, a PWM type according to the specified PWM region and selecting, via the controller, a PWM switching frequency manner according to the specified PWM region.

[0007] The PWM switching frequency manner can be selected from a predefined list of PWM switching frequency manners, the predefined list of PWM switching frequency manners including a constant pulse ratio switching manner, a constant switching frequency switching manner, and a look-up table (LUT) with a selectable switching frequency array. The PWM type can be selected from a predefined list of PWM types, the predefined list of PWM types including a discontinuous PWM (DPWM) technique, a zero vector modulation (ZVM) technique, and a space vector pulse width modulation (SVPWM) technique. When the PWM type is the space vector pulse width modulation (SVPWM) technique and the current switching frequency is greater than a threshold SVPWM switching frequency, the controller can be programmed to set the current switching frequency to the threshold SVPWM switching frequency before updating the PWM type to the SVPWM technique.

[0008] When the PWM type is the discontinuous PWM (DPWM) technique, the controller can be programmed to prevent switching from the DPWM technique to the space vector pulse width modulation (SVPWM) technique until the current switching frequency is below a threshold SVPWM switching frequency. When a six-step operation is activated, the controller can be programmed to override the control reference frequency and the PWM switching frequency. The controller can be programmed to incorporate a hysteresis at the control reference frequency maximum.

[0009] A method of operating an electric drive system having a chargeable energy storage unit, an electric motor, a power inverter, and a controller having a processor and a tangible, non-transitory memory is disclosed herein. The method includes regulating, via the power inverter, a transfer of electrical power between the chargeable energy storage unit and the electric motor by a pulse width modulation (PWM) switching frequency. A current switching frequency is determined, via the controller, based in part on a PWM type, a PWM switching frequency regime, and an inverter direct current voltage, the operation of the controller defined at least in part by a control reference frequency. A PWM scalar is obtained, via the controller, based in part on the current switching frequency and a control reference frequency maximum. The method includes transmitting, via the controller, a command signal to the power inverter to regulate the transfer of electrical power based in part on the PWM scalar, the PWM switching frequency proportional to a product of the PWM scalar and the control reference frequency.

[0010] When the PWM type is the DPWM technique, the method can include preventing a switch from the DPWM technique to the SVPWM technique until the current switching frequency is below a predetermined threshold. A hysteresis can be applied to the current switching frequency, the hysteresis extending to the control reference frequency maximum.

[0011] An electric vehicle having a traction motor adapted to output torque for propulsion, a traction battery pack adapted to power the traction motor, and a power inverter electrically connecting the traction battery pack to the traction motor is disclosed herein. A controller is in communication with the power inverter, the operation of the controller defined by a control reference frequency. The power inverter is operable to convert direct current power output by the traction battery pack to alternating current power and transfer the alternating current power to the traction motor based in part on a pulse width modulation (PWM) switching frequency. The controller has a processor and a tangible, non-transitory memory having instructions recorded thereon, execution of the instructions by the processor causing the controller to determine a current switching frequency based in part on a PWM type, a PWM switching frequency regime, and an inverter direct current voltage. A PWM scalar is obtained based in part on the current switching frequency and a control reference frequency maximum. The controller is configured to transmit a command signal to the power inverter to regulate the transfer of electrical power based in part on the PWM scalar, the PWM switching frequency proportional to a product of the PWM scalar and the control reference frequency.

[0012] According to the present application, it further includes the following technical solutions:

[0013] 1. An electric drive system comprising:

[0014] a chargeable energy storage unit and a power inverter operably connected to the chargeable energy storage unit;

[0015] an electric motor operably connected to the power inverter;

[0016] a controller in communication with the power inverter, operation of the controller defined at least in part by a control reference frequency;

[0017] wherein power transfer between the rechargeable energy storage unit and the electric motor is governed by a pulse width modulation (PWM) switching frequency;

[0018] wherein the controller has a processor and a tangible, non-transitory memory having instructions recorded thereon, execution of the instructions by the processor causing the controller to:

[0019] determine a current switching frequency based in part on a PWM type, a PWM switching frequency regime, and an inverter DC voltage;

[0020] obtain a PWM scalar based in part on the current switching frequency and the control reference frequency maximum; and

[0021] communicate a command signal to the power inverter to adjust power transfer based in part on the PWM scalar, the PWM switching frequency being proportional to a product of the PWM scalar and the control reference frequency.

[0022] 2. The electric drive system of technical solution 1, wherein:

[0023] the current switching frequency is based in part on an inverter coolant temperature and whether drive torque or regenerative torque is in operation; and

[0024] the instructions are executed dynamically such that the PWM scalar varies over time.

[0025] 3. The electric drive system of technical solution 1, wherein obtaining the PWM scalar comprises:

[0026] obtaining an extended PWM switching frequency that is a sum of the current switching frequency and a frequency span;

[0027] obtaining a ratio of the extended PWM switching frequency divided by the control reference frequency maximum; and

[0028] setting the PWM scalar to a minimum integer greater than or equal to the ratio such that the PWM scalar is equal to CEILING (ratio).

[0029] 4. The electric drive system of technical solution 1, wherein determining the current switching frequency comprises:

[0030] determining, via the controller, a PWM region layout having a plurality of PWM regions arranged in a torque-speed curve calibrated for the electric motor; and

[0031] via the controller, selecting the PWM type according to the specified PWM region; and

[0032] 5. The electric drive system of technical solution 4, wherein determining the current switching frequency comprises:

[0033] via the controller, selecting the PWM type according to the specified PWM region; and

[0034] via the controller, selecting the PWM switching frequency approach according to the specified PWM region.

[0035] 6. The electric drive system of technical solution 5, wherein:

[0036] the PWM switching frequency approach is selected from a predefined list of PWM switching frequency approaches, the predefined list of PWM switching frequency approaches including a constant pulse ratio switching approach, a constant switching frequency switching approach, and a look-up table (LUT) with a selectable switching frequency array.

[0037] 7. The electric drive system of technical solution 1, wherein:

[0038] the PWM type is selected from a predefined list of PWM types, the predefined list of PWM types including a discontinuous PWM (DPWM) technique, a zero vector modulation (ZVM) technique, and a space vector pulse width modulation (SVPWM) technique.

[0039] 8. The electric drive system of technical solution 1, wherein:

[0040] when the PWM type is a space vector pulse width modulation (SVPWM) technique and the current switching frequency is greater than a threshold SVPWM switching frequency, the controller is programmed to set the current switching frequency to the threshold SVPWM switching frequency before updating the PWM type to the SVPWM technique.

[0041] 9. The electric drive system of technical solution 1, wherein:

[0042] when the PWM type is a discontinuous PWM (DPWM) technique, the controller is programmed to prevent switching from the DPWM technique to a space vector pulse width modulation (SVPWM) technique until the current switching frequency is below a threshold SVPWM switching frequency.

[0043] 10. The electric drive system of technical solution 1, wherein:

[0044] The controller is programmed to override the control reference frequency and the PWM switching frequency when a six-step operation is activated.

[0045] 11. The electric drive system of technical solution 10, wherein:

[0046] The controller is programmed to incorporate a hysteresis at the control reference frequency maximum.

[0047] 2. A method of operating an electric drive system having a chargeable energy storage unit, an electric motor, a power inverter, and a controller having a processor and a tangible, non-transitory memory, the method comprising:

[0048] regulating, via the power inverter, transfer of power between the chargeable energy storage unit and the electric motor by a pulse width modulation (PWM) switching frequency;

[0049] determining, via the controller, a current switching frequency based in part on a PWM type, a PWM switching frequency regime, and an inverter DC voltage, operation of the controller being defined at least in part by a control reference frequency;

[0050] obtaining, via the controller, a PWM scalar based in part on the current switching frequency and a control reference frequency maximum; and

[0051] communicating, via the controller, a command signal to the power inverter to regulate transfer of power based in part on the PWM scalar, the PWM switching frequency being proportional to a product of the PWM scalar and the control reference frequency.

[0052] 13. The method of technical solution 12, wherein obtaining the PWM scalar comprises:

[0053] obtaining an extended PWM switching frequency as a sum of the current switching frequency and a dithering frequency span;

[0054] obtaining a ratio of the extended PWM switching frequency divided by the control reference frequency maximum; and

[0055] obtaining the PWM scalar based in part on the ratio.

[0056] 14. The method of technical solution 13, wherein obtaining the PWM scalar comprises:

[0057] setting the PWM scalar to a minimum integer greater than or equal to the ratio such that the PWM scalar is equal to a CEILING function of the ratio.

[0058] 15. The method of claim 12, wherein obtaining the current switching frequency comprises:

[0059] selecting the PWM type from a predefined list of PWM types, including discontinuous PWM (DPWM) techniques, zero vector modulation (ZVM) techniques, and space vector pulse width modulation (SVPWM) techniques; and

[0060] when the PWM type is SVPWM techniques and the current switching frequency is greater than a threshold SVPWM switching frequency, setting the current switching frequency to the threshold SVPWM switching frequency prior to communicating the command signal to the power inverter.

[0061] 16. The method of claim 15, when the PWM type is DPWM techniques, the method further comprising:

[0062] preventing a switch from the DPWM techniques to the SVPWM techniques until the current switching frequency is below a predetermined threshold.

[0063] 17. The method of claim 12, further comprising:

[0064] applying a hysteresis to the current switching frequency, the hysteresis extending to a control reference frequency maximum.

[0065] 18. An electric vehicle, comprising:

[0066] a traction motor adapted to output torque for propulsion;

[0067] a traction battery pack adapted to power the traction motor;

[0068] a power inverter electrically connecting the traction battery pack to the traction motor;

[0069] a controller in communication with the power inverter, the operation of the controller defined by a control reference frequency;

[0070] wherein the power inverter is operable to convert direct current power output by the traction battery pack to alternating current power based in part on a pulse width modulation (PWM) switching frequency, and transmit the alternating current power to the traction motor;

[0071] wherein the controller has a processor and a tangible, non-transitory memory having instructions recorded thereon, execution of the instructions by the processor causing the controller to:

[0072] determine the current switching frequency based in part on a PWM type, a PWM switching frequency regime, and an inverter direct current voltage;

[0073] obtaining a PWM scalar based in part on the current switching frequency and the control reference frequency maximum; and

[0074] communicating a command signal to the power inverter to adjust transmission of power based in part on the PWM scalar, the PWM switching frequency being proportional to a product of the PWM scalar and the control reference frequency.

[0075] 19. The electric vehicle of clause 18, wherein obtaining the PWM scalar comprises:

[0076] obtaining an extended PWM switching frequency as a sum of the current switching frequency and a dithering frequency span;

[0077] obtaining a ratio of the extended PWM switching frequency divided by the control reference frequency maximum; and

[0078] obtaining the PWM scalar based in part on the ratio.

[0079] 20. The electric vehicle of clause 19, wherein obtaining the PWM scalar comprises:

[0080] setting the PWM scalar to a minimum integer greater than or equal to the ratio such that the PWM scalar is equal to a CEILING function of the ratio.

[0081] The above-mentioned features and advantages of the present disclosure, as well as other features and advantages of the present disclosure, are readily apparent to one having ordinary skill in the art from the following detailed description, when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0082] Figure 1 is a schematic partial illustration of an electric drive system having an electric motor, a power inverter, and a controller;

[0083] Figure 2 illustrates switching frequency regions of an example electric motor, where vertical axis is motoring and regenerative torque, and horizontal axis is electric motor speed;

[0084] Figure 3 is a flowchart of a method of operating Figure 1 the electric drive system;

[0085] Figure 4 is a flowchart detailing a portion of Figure 3 the method;

[0086] Figure 5 is a flowchart detailing another portion of Figure 3 the method;

[0087] Figure 6 is a flowchart detailing another portion of the method of Figure 3 ;

[0088] Figures 7A-7C is an example representative PWM region layout arranged within a torque-speed curve of motor speed (R) versus motor torque (T), calibrated for an example electric motor and power inverter of an electric vehicle, to determine a coolant temperature based PWM region layout; and

[0089] Figures 8A-8D illustrates selected ones of the PWM region layouts within the torque-speed curve of Figures 7A-7C for determining PWM regions ( Figure 7A ), PWM types ( Figure 7B ), switching frequency modes ( Figure 7C ), and switching frequencies.

[0090] Representative embodiments of the present disclosure are shown, by way of non-limiting example, in the drawings and described in additional detail below. It should be understood, however, that the novel aspects of the present disclosure are not limited to the particular forms illustrated in the above-enumerated drawings. Rather, the present disclosure is to cover modifications, equivalents, combinations, sub-combinations, permutations, and alternatives falling within the scope of the present disclosure, for example, as encompassed in the appended claims. DETAILED DESCRIPTION

[0091] Referring to the drawings, wherein like reference numbers refer to like components, Figure 1 schematically illustrates an electric drive system 10. The electric drive system 10 can be located in a device 12. The device 12 can be an electric vehicle 14, which can be fully electric or hybrid / partially electric. The device 12 can include, but is not limited to, a passenger vehicle, a sport utility vehicle, a light truck, a heavy vehicle, a minivan, a bus, a transport vehicle, a bicycle, a mobile robot, an agricultural implement (e.g., a tractor), a sports-related apparatus (e.g., a golf cart), a boat, an airplane, and a train. The device 12 can include manufacturing apparatuses and other electrical apparatuses. It will be appreciated that the device 12 can take many different forms and have additional components.

[0092] Referring to Figure 1The electric drive system 10 includes a DC power source, such as a rechargeable energy storage unit 20. The electric drive system 10 includes a power inverter 22 and an electric motor 24. The rechargeable energy storage unit 20 can be a traction battery pack to generate high voltage power that can be directed to a propulsion unit 26 via the electric motor 24 and used to operate other electrical systems 28 in the device 12. The rechargeable energy storage unit 20 can include battery cells of different chemistries. In one example, the power inverter 22 is a three-phase, three-wire voltage source inverter. In some embodiments, the power inverter 22 can be a TPIM unit of the electric vehicle 14. To generate traction power with sufficient vehicle range and speed, the rechargeable energy storage unit 20 in the electric vehicle 14 can be larger and higher in capacity than a standard 12-volt starting, lighting, and ignition battery. In such cases, the electric drive system 10 is a high voltage electric drive system 10.

[0093] Referring to Figure 1 The electric drive system 10 includes a controller C that is adapted to regulate the operation of various onboard systems and components in the device 12. For example, the controller C can be an electronic control unit (ECU) of the electric vehicle 14. The controller C is communicatively connected with the electric motor 24 to control, for example, the bidirectional transfer of energy between the rechargeable energy storage unit 20 and the electric motor 24. The electric motor 24 can operate using three-phase AC current. In such cases, the power inverter 22 is governed by the controller C to convert DC voltage (provided by the rechargeable energy storage unit 20) into three-phase AC voltage for use by the electric motor 24. In a regenerative mode in which the electric motor 16 is configured as a motor / generator, the power inverter 22 converts AC power from the electric motor 24 into DC power compatible with the rechargeable energy storage unit 20. It will be understood that the electric drive system 10 can include additional components not shown.

[0094] Referring to Figure 1 The components of the electric drive system 10 can communicate with the controller C (and with each other) via a wireless network 30, which can be a short-range network or a long-range network. In addition, the components of the electric drive system 10 can include physical wired connections. The wireless network 30 can be a communication BUS, which can be in the form of a serial controller area network (CAN-BUS). The wireless network 30 can include Bluetooth TM connections, a wireless local area network (LAN) that links multiple devices using a wireless distribution method, a wireless metropolitan area network (MAN) that connects several wireless LANs, or a wireless wide area network (WAN). Other types of connections can be employed.

[0095] The controller C is programmed to receive a torque command in response to an operator input (e.g., through an accelerator pedal or brake pedal) or an automatic transmission input condition monitored by the controller C. Once the torque command is received, the controller C is programmed to transmit a command signal to the power inverter 22 to regulate the power transfer between the rechargeable energy storage unit 20 and the electric motor 24. One technique for obtaining variable frequency, variable voltage, or variable power from a power inverter 22 operating off of a fixed voltage DC power source, such as the rechargeable energy storage unit 20, is a pulse width modulation (hereinafter “PWM”) technique. The “switching frequency” of a PWM signal determines how quickly the PWM completes one cycle (i.e., 500 Hz would be 500 cycles per second), and thus how quickly it switches between a high state and a low state. In other words, the PWM switching frequency corresponds to the rate at which the DC voltage is turned on and off during the PWM process in a switched power supply. There are various PWM techniques that can be implemented, such as, for example, sinusoidal PWM (SPWM), space vector PWM (SVPWM), zero vector modulation (ZVM), and discontinuous PWM (DPWM). These techniques differ in their voltage linearity range, ripple voltage / current, switching losses, and high frequency common mode voltage or current characteristics.

[0096] Referring now to Figure 2 , an example graph of various switching frequencies that can be employed at different combinations of speed and torque of the example electric motor 24 is shown. Figure 2 The horizontal axis 50 of the graph of FIG. 5 indicates the electric motor speed. The first portion 52 of the vertical axis corresponds to the drive mode (or drive torque), while the second portion 54 of the vertical axis corresponds to the regeneration mode. Figure 2 The first region 56 of the graph of FIG. 5 represents a first switching frequency, the second region 58 represents a second switching frequency, and the third region 60 represents a third switching frequency. In one example, the first switching frequency, the second switching frequency, and the third switching frequency are 10 kilohertz (kHz), 15 kilohertz (kHz), and 20 kilohertz (kHz), respectively.

[0097] Referring to Figure 1 , the controller C includes at least one processor P and at least one memory M (or non-transitory, tangible computer-readable storage medium) having instructions recorded thereon for performing a method 100 of operating the electric drive system 10. The method 100 is described below with reference to Figures 3-6 . The memory M is capable of storing a set of controller-executable instructions, and the processor P is capable of executing the set of controller-executable instructions stored in the memory M.

[0098] The operation of controller C is defined at least in part by the closed-loop control reference frequency, referred to herein as the control reference frequency. The control reference frequency (sometimes referred to as the "Task0" frequency) is limited by the throughput and other capabilities (e.g., processing) of controller C and its components. The control reference frequency may be the frequency at which input variables are sampled. Figure 1 The controller C is specifically programmed to execute the block of method 100 (see below for reference). Figures 3-6 (Discussed in detail), and capable of receiving input from various sensors, each capable of measuring corresponding physical factors and sending corresponding signals to controller C. For example, refer to... Figure 1 The electric drive system 10 may include methods for determining the inverter coolant temperature (T). W The controller C includes a temperature sensor 32 and a motor speed sensor 34 for determining the motor speed. Additionally, the controller C can be programmed to determine the corresponding physical factors through modeling or other estimation techniques available to those skilled in the art. It should be understood that portions of method 100 can operate at a rate slower than the control reference frequency. For example, the temperature sensors 32 can be read at a slower rate to free up processor resources because they change slowly, while the motor speed / position sensors can continue to read at a faster rate.

[0099] Method 100 improves the range and functionality of the electric vehicle 14 by increasing the span of the PWM switching frequency. The increased span is achieved by dynamically changing the PWM scalar, thus allowing for a higher PWM switching frequency. The PWM scalar is the number of PWM cycles per reference cycle or control loop of the controller C, as can be seen from the equation below. Furthermore, at higher switching frequencies, current ripple is reduced.

[0100] PWM switching frequency = PWM scalar * control reference frequency

[0101] Now for reference Figure 3 A flowchart of method 100 is shown. Method 100 can be implemented by storing... Figure 1 The method comprises computer-readable code or instructions that are executed on and can be executed by the controller C. The method may be executed in real time, continuously, systematically, sporadically, and / or at regular intervals, such as once every 10 milliseconds during normal and ongoing operation of the electric vehicle 14.

[0102] Figure 3 Method 100 begins at block 101 and ends at block 103, and includes subroutines or modules 102, 202, 302, 402, 502, and 602. Method 100 does not need to be applied in the specific order described herein. Furthermore, it should be understood that some modules (or some blocks within modules) may be omitted.

[0103] As will be described below, module 102 includes selecting the optimal PWM region. For example, Figures 7A-7C The diagram illustrates three representative PWM region layouts for the torque-speed curve 702 of the example electric motor: low-temperature PWM region limitation ( Figure 7A ), Hybrid PWM region limitation ( Figure 7B ) and high-temperature PWM region limitation ( Figure 7C ). Figures 7A-7C This shows the motor speed (R) on the X-axis in revolutions per minute versus the motor torque (T) on the Y-axis. Torque-speed curve 702 (which is in...) Figure 7A , Figure 7B and Figure 7C (The values ​​in the original text are the same.) The torque-speed curve can be derived by directly connecting a force gauge or chassis force gauge. Generally, the torque-speed curve originates at the intersection point on the Y-axis where the torque is maximum and the speed is zero. This intersection point is the "stall torque" of the electric motor, i.e., the maximum available motor torque when the motor is running at zero speed. The torque-speed curve slopes downwards until it intersects the X-axis at the point where there is zero torque and maximum speed. Figures 7A-7C The “drive” operating region, where both torque and speed are positive, is depicted; however, the disclosed method is equally applicable to the remaining regenerative regions (not depicted) along both the negative drive and negative regions.

[0104] Module 102 further includes selecting the PWM type based on the selected PWM region. Figure 8B The illustration shows three example PWM schemes available: discontinuous PWM (DPWM) technology, zero vector modulation (ZVM) technology, and continuous PWM (CPWM) technology, such as space vector PWM (SVPWM). Figure 8A Each of the PWM regions (1) to (7) is assigned Figure 8B One of the available PWM types is presented in the figure. Based on the example curves shown in the attached figure, for the corresponding motor torque and speed falling within this region, it can be obtained from... Figure 8A In the PWM region layout, select PWM region six (6); such as Figure 8B As seen in the document, PWM region six (6) is assigned to perform DPWM technology. Although three examples of alternative PWM schemes are described herein, it should be understood that method 100 may employ more or fewer than three PWM types, which may include additional or alternative PWM principles.

[0105] As will be described below, module 302 includes selecting a PWM switching frequency regime and determining a PWM switching frequency (according to the PWM region and PWM type selected in module 102). Module 402 relates to a six-step reference frequency override mechanism. Modules 202 and 502 provide multiple frequency limit mechanisms for hardware protection. Finally, module 602 relates to using an extended value of the PWM switching frequency to calculate the PWM scalar, which extended value includes a frequency span from dither (to avoid aliasing) and incorporates a hysteresis at the control reference frequency maximum.

[0106] Reference is now made to Figure 4 wherein module 102 is shown in detail, module 102 begins with decision block 104, wherein it is determined whether the inverter coolant temperature ( T W ) is less than a first (low) threshold coolant temperature. If so (block 104 = yes), module 102 proceeds to process block 106 and sets the PWM region layout to a low coolant temperature PWM region layout (see, e.g., Figure 7A ). If the inverter coolant temperature is greater than the first (low) threshold coolant temperature (block 104 = no), module 102 proceeds to decision block 108 of Figure 4 and determines whether the inverter coolant temperature ( T W ) is greater than a second (high) threshold coolant temperature. If so (block 108 = yes), module 102 proceeds to process block 110 and sets the PWM region layout to a high coolant temperature PWM region layout (see, e.g., Figure 7C ). If the coolant temperature ( T W ) is both greater than the low threshold coolant temperature and less than the high threshold coolant temperature (block 108 = no), module 102 performs the control operation of process block 112 and sets the PWM region layout to a linearly blended region layout (see, e.g., Figure 7B ).

[0107] The PWM region definition curves can be defined by calibration to low and high inverter coolant temperatures. To avoid predefining a large number of PWM region definition curves for each temperature between high and low temperatures, thereby reducing the memory storage space necessary for these curves, an "on-the-fly" estimation procedure can be implemented to linearly interpolate between the high (temperature) and low (temperature) region definition curves to estimate a new torque-speed based PWM region curve definition for the current "mid-range" coolant temperature. It will be appreciated that, Figure 7B the blended PWM region definition is a linear blend of the low temperature PWM region definition ( Figure 7A ) and the high temperature PWM region definition ( Figure 7CAn example at one temperature between the upper and lower temperature thresholds; the region definition changes at different temperatures between the upper and lower temperature thresholds.

[0108] Module 102 proceeds from process blocks 106, 110, and 112 to process block 114. According to... Figure 4 In process block 114, controller C is adapted to calculate a hybrid torque-speed region corresponding to the current inverter coolant temperature based on coolant temperature and mechanical power state, which can be positive for drive and negative for regenerative braking. A regenerative flag (in the form of a software flag) can be used to indicate that the electric vehicle 14 is currently operating in regenerative braking mode rather than drive mode. In this regard, the use / selection of PWM region definition curves, PWM type, and PWM switching frequency can differ based on whether the electric vehicle 14 is operating in drive mode or regenerative mode. Therefore, module 102 can define different curves for drive mode and different curves for regenerative mode.

[0109] Module 102 in Figure 4 The process continues at process block 116, and based on the selected coolant temperature-based PWM region layout and the current operating mode of the electric drive system 10 (e.g., drive operating mode versus regenerative operating mode in automotive applications), a corresponding first (upper) torque threshold and a corresponding second (lower) torque threshold are determined for each PWM region at the current motor speed (absolute value). For example, Figure 1 The controller C can prompt the motor speed sensor 34 to acquire sensor data indicating the real-time motor speed. Using this information, the controller C can... Figure 8A Each of regions (1) through (7) defines a set of torque limits at the current motor speed. In a non-limiting example, at very low speed operating points, the first (upper) torque threshold could be approximately 0.4 per unit, and the second (lower) threshold could be approximately zero (0) per unit. In contrast, at medium speed operating points, region (1) could have a first (upper) torque threshold of approximately 0.95 per unit and a second (lower) threshold of approximately 0.45 per unit.

[0110] After determining the torque limit for each PWM region in process block 116, module 102 proceeds to decision block 118. According to... Figure 2 Decision block 118, controller C is adapted to determine motor torque ( T rq (1) Whether it is within the torque limit of the first region: T rq >Torque threshold in region 1; and (2) whether T rq<Zone 1 upper torque threshold. If so (block 118 = yes), the module 102 proceeds to process block 120 and sets the PWM zone to PWM zone (1). If the motor torque is outside the torque limits of the first zone (block 118 = no), the module 102 moves to decision block 122 and determines whether the motor torque is within the torque limits of the second zone: (1) whether T rq ) whether T rq > Zone 2 lower torque threshold; and (2) whether T rq <Zone 2 upper torque threshold. If so (block 122 = yes), the module 102 proceeds to process block 124 and sets the PWM zone to PWM zone (2) ( Figure 7A ). However, if the motor torque is outside the torque limits of the second zone (block 122 = no), the controller C proceeds to decision block 126 and process block 128 and repeats the above inquiry for each remaining PWM zone until the corresponding zone is identified.

[0111] Referring to Figure 4 , the module 102 proceeds from process blocks 120, 124 and 128 to process block 130. Figure 4 Process block 130 of the method 100 includes setting the PWM type and PWM frequency regime in accordance with the identified or selected PWM torque-velocity zone. Figure 3 The method 100 of

[0112] Module 202 inquires whether the motor torque is within the torque limits of the third zone: (1) whether Figure 3is set to the PWM type determined in the previous control iteration and the SVPWM request flag is set to true. If not (block 204 = false), the module 102 proceeds to process block 208 in which the SVPWM request flag is set to false. If block 204 = false, the SVPWM request flag is set to false and a "determine PWM_type" request is sent through (which can be any PWM type, including SVPWM). Block 204 can include a check to see if the switching frequency is currently too high for SVPWM to be used. If so, the controller C can be programmed not to switch to SVPWM until the switching frequency is within a predefined threshold. In this case, the previous PWM type will continue and a flag is sent to indicate that SVPWM is being requested. The module 202 proceeds to process block 210 (from process blocks 206 and 208) in which the previous PWM type is set to the current PWM type for use in subsequent control iterations. Figure 3 The method 100 then moves from module 202 to module 302.

[0113] Module 302 is shown in detail in Figure 5 Module 302 includes selecting a PWM switching frequency approach. As a non-limiting example, Figure 8C Three PWM switching frequency approaches are illustrated: a constant pulse ratio (cPR) switching approach, a constant switching frequency (cFsw) switching approach, and a look-up table (LUT) having an array of selectable switching frequencies associated with various inputs such as motor speed and / or torque. Similar to Figure 8B the PWM type, Figure 8A Each PWM region of Figure 8C is assigned to one of the available PWM switching frequency approaches of Figure 8A Additionally, PWM region six (6) can be selected from the PWM region layout of Figure 8C As seen in Figure 3 the method 100 of may employ more or less than three switching approaches, which can include additional or alternative available approaches.

[0114] Reference is now made to Figure 5, module 302 proceeds from decision block 304 to evaluate whether the PWM switching frequency regime corresponding to the selected PWM region and the current operating mode of the motor is a constant switching frequency switching regime ("cFsw"). If so (block 304 = yes), module 302 proceeds to process block 306 to determine the desired switching frequency based on a lookup table for cFsw, and proceeds to module 402 via line 315. If not (block 304 = no), method 100 proceeds to decision block 308 to determine whether the PWM switching frequency regime corresponding to the selected PWM torque-speed region and the current operating mode of the motor is a constant pulse ratio switching regime ("cPR"). If so (block 308 = yes), module 302 proceeds to process block 310 to determine the desired switching frequency based on a lookup table for cPR, and proceeds to module 402 via line 315. With reference to Figure 5 If the PWM switching frequency regime is neither the cPR nor the cFsw switching regime (block 308 = no), module 302 of method 100 proceeds to process blocks 312-326 to determine the desired PWM switching frequency prior to proceeding to module 402 via line 315. Figure 5

[0115] The switching frequency is retrieved from one of a plurality of tables based on a combination of the following parameters: PWM type (e.g., DPWM / ZVM vs. SVPWM); operating mode (e.g., drive operating mode vs. regeneration operating mode); and magnitude of inverter voltage (e.g., relative to high, medium, low, medium-low, and medium-high thresholds). According to decision block 314 of method 100, Figure 5 Controller C is programmed to determine: (1) whether the PWM type is DPWM / ZVM; and (2) whether the selected PWM type is SVPWM. If so (block 314 = yes), method 100 proceeds to decision block 316 to ascertain whether the operating mode is the drive operating mode (versus the regeneration operating mode). If not (block 314 = no), method 100 proceeds to decision block 322.

[0116] If the mode is the drive operating mode (block 316 = yes), method 100 proceeds to process block 318. If the mode is the regeneration operating mode (block 316 = no), method 100 proceeds to process block 320. Similarly, decision block 322 is executed to ascertain whether the operating mode is the drive operating mode (versus the regeneration operating mode), and proceeds to process block 324 (block 322 = yes) and process block 326 (block 322 = no).

[0117] Process blocks 318, 320, 324, and 326 include determining the desired switching frequency based on the inverter DC bus voltage (referred to herein as the inverter voltage (V dc ​multiple look-up tables (LUTs) of values of the magnitude of V dc above a high threshold, a respective high (voltage) table is used; (2) if V dc is at or below a low threshold, a respective low (voltage) table is used; (3) if V dc is at or between a mid-low and a mid-high threshold, a respective mid (voltage) table is used; (4) if V dc is between the mid-low threshold and the low threshold, a linear interpolation is performed between the respective mid (voltage) table and the low (voltage) table; and (5) if V dc is between the mid-high threshold and the high threshold, a linear interpolation is performed between the respective mid (voltage) table and the high (voltage) table. As a non-limiting example (which can change from one vehicle model to another), the low threshold, mid-low threshold, mid-high threshold, and high threshold voltages can be 250 V, 300 V, 350 V, and 400 V, respectively. It will be appreciated that the stratification of values of the magnitude of V dc SW may vary based on the application. For example, instead of the five categories described above with 20 look-up tables, the method 100 can include ten narrower categories implemented by 40 look-up tables. With reference to Figure 5 , the method 100 passes from the process blocks 318, 320, 324, and 326 to the module 402 via line 315.

[0118] It can be desirable to combine PWM type and PWM switching frequency selection to optimize performance and range of the electric vehicle 14 and reduce noise. As a non-limiting example, a switching frequency (F SW ) of 2 kHz can be combined with ZVM techniques for low motor speed and high torque demand, for example, to enable rock-climbing maneuvers of an electrified sport utility vehicle (SUV), ATV, or industrial vehicle and reduce thermal wear on the inverter and electrical units. In contrast, Figure 8D shows that a switching frequency of 10 kHz can be combined with SVPWM or DPWM techniques for low motor speed and low motor torque, for example, to reduce NVH of the powertrain. DPWM techniques can be combined with switching frequencies of 10 kHz, 15 kHz, and 20 kHz for low torque and low speed, medium speed, and high speed, respectively, to increase EV range. In another example, a switching frequency of 10 kHz can be combined with SVPWM techniques for medium range torque to reduce NVH. DPWM techniques can be combined with constant duty cycle switching to achieve higher torque or acceleration, both to increase performance and manage thermal loading.

[0119] Reference is now made to Figure 3, module 402 relates to the overriding of control reference frequencies during six-step operation. As understood by those skilled in the art, six-step operation is a mode of operation in which voltage vectors are applied in six intervals (for a three-phase inverter) during one fundamental cycle (i.e., electrical speed regulation). Because six-step operation does not tie into PWM switching, a higher control reference frequency will generally provide better control. Module 402 begins with decision block 404, in which controller C is configured to determine whether six-step operation is in place or active. If so (block 404 = yes), method 100 proceeds to process block 406, in which the control reference frequency determined up to this point in previous modules is overridden or ignored, and a different six-step reference frequency is set as the current control reference frequency. This ensures optimal control independent of the current operation when transitioning to six-step operation. If not (block 404 = no), module 402 transitions to module 502.

[0120] Module 502 is shown in detail in Figure 3 , and relates to a second set of frequency limits or controls. Module 502 begins with decision block 504, in which it is determined: (1) whether the PWM type is SVPWM; and (2) whether the current switching frequency (F SW ) is greater than a threshold SVPWM switching frequency. If so (block 504 = yes), method 100 proceeds to process block 506, in which the current switching frequency (F SW ) is set to the threshold SVPWM switching frequency. If not (block 504 = no), method 100 proceeds to decision block 508 to determine whether the current switching frequency (F SW ) is greater than a predefined maximum switching frequency (max F SW ). If so (block 508 = yes), method 100 proceeds to process block 512 to set or maintain the current switching frequency (F SW ) to the predefined maximum switching frequency (max F SW ). If not (block 508 = no), method 100 proceeds to process block 510 to determine whether the current switching frequency (F SW ) is less than a predefined minimum switching frequency (min F SW ). If so (block 510 = yes), module 502 proceeds to process block 514 to set or maintain the current switching frequency (F SW ) to the predefined minimum switching frequency (min F SW ). If not (block 510 = no), method 100 proceeds to module 602.

[0121] The predefined threshold described herein can be obtained through calibration in a controlled setting. The terms "calibration," "calibrated," and related terms refer to the result or process of comparing actual or standard measurements associated with a device or system to a measurement or commanded position perceived or observed by the device or system. Calibration as described herein can be reduced to a table of storable parameters, a plurality of executable equations, or other suitable form that can be used as part of a measurement or control routine. A parameter is defined as a measurable quantity that represents a physical characteristic of a device or other element that is discernible through the use of one or more sensors and / or physical models.

[0122] The module 602 is shown in greater detail in Figure 6 Referring to Figure 6 , the module 602 performs a decision block 604 to ascertain whether the dither function is enabled. If so (block 604 = YES), the method 100 proceeds to a process block 608 in which a switching frequency span (F SW Span) related to the dither function is calculated. For example, the frequency span can be 2% of the switching frequency. If not (block 604 = NO), the method 100 proceeds to a process block 606 in which the switching frequency span (F SW Span = 0) is set to zero.

[0123] The method 100 proceeds from the process block 606 and the process block 608 to a process block 610 in which an interim (or "expanded") switching frequency is set to the sum of the current switching frequency and one-half of the switching frequency span, as follows:

[0124] Expanded F SW = (Intermediate F SW + ½ F SW DitherSpan).

[0125] In other words, the expanded switching frequency (Expanded F SW ) includes the dither switching frequency span. The method 100 proceeds from the process block 610 to a process block 612.

[0126] In the process block 612, the controller C is programmed to determine a PWM scalar in the following manner: (1) obtain a ratio of the expanded switching frequency divided by the control reference frequency maximum; and (2) obtain a minimum integer greater than or equal to the ratio (e.g., round up). For example, the PWM scalar can be obtained through a CEILING function as follows: PWM scalar = CEILING[ratio]; ratio = (Expanded F SW(maximum reference frequency). For example, if the ratio is 5.3, the ceiling value [CEILING (5.3)] as described above would be 6. The method 100 proceeds to decision block 614. The maximum reference frequency is constrained by the microprocessor speed and the control and throughput requirements of the electric drive system 10.

[0127] According to decision block 614, the method 100 determines whether the PWM scalar is greater than the predefined maximum scalar value. If yes (block 614 = yes), the method 100 proceeds to process block 616, where the PWM scalar is set to the predefined maximum scalar value. Process block 616 then proceeds to process block 618. If no (block 614 = no), the method 100 proceeds to process block 618, where the controller C is programmed to first set the high threshold frequency as follows:

[0128] High threshold frequency = PWM scalar * maximum reference frequency.

[0129] The method 100 proceeds from process block 618 to decision block 620 of Figure 6 According to decision block 620, the controller C determines: (1) whether the previous PWM scalar (from the previous iteration) is greater than the PWM scalar (from the current iteration); (2) whether the Expanded switching frequency (Expanded F SW ) is within a predefined hysteresis; and (3) whether the Expanded switching frequency (Expanded F SW ) is less than or equal to the high threshold frequency.

[0130] The technical advantage of employing hysteresis is to reduce toggling between individual values of the PWM scalar. In a non-limiting example, the maximum reference frequency of the electric drive system 10 can be 10000 Hz, and the hysteresis can be 100 Hz (9900 Hz to 10000 Hz). Thus, if the change in switching frequency is within the hysteresis, the previous PWM scalar is retained in order to avoid toggling. When the change in switching frequency exceeds the hysteresis, the newly calculated PWM scalar is employed. Hysteresis provides a form of instantaneous feedback control in which the deviation from the hysteresis is continuously tracked.

[0131] If so (block 620 = yes), the method 100 proceeds to process block 622 of Figure 6 where the PWM scalar (from the current iteration) is set to the previous PWM scalar (from the previous iteration). The method 100 proceeds from process block 622 to process block 624. If not (block 620 = no), the method 100 proceeds directly to process block 624. In process block 624, the previous PWM scalar is set to the PWM scalar, and the control reference frequency is set to the current switching frequency divided by the PWM scalar, as follows:

[0132] Control reference frequency = (Current F SW / PWM scalar).

[0133] From Figure 6 process block 624, the method 100 proceeds to process block 626, in which the following parameters are output: control reference frequency, PWM scalar, and current switching frequency. Instead of the control reference frequency, the control reference period (PWM scalar / current F SW ) can be output. Figure 6 The module 602 of FIG. 6 ends at process block 626. The method 100 can be executed dynamically to obtain a dynamic PWM scalar. As used herein, the terms “dynamic” and “dynamically” describe steps or processes that are executed in real-time, and are characterized by monitoring or otherwise determining the state of a parameter, and updating the state of the parameter periodically or periodically during or between iterations of the routine execution. The controller C is programmed to transmit command signals to the power inverter 22 to adjust the transfer of power between the chargeable energy storage unit 20 and the electric motor 24 based in part on the PWM scalar.

[0134] In summary, the electric drive system 10 improves the range and functionality of the electric vehicle 14 by executing the PWM scalar to increase the operable range of the PWM switching frequency. As a result, the control reference frequency and the PWM switching frequency are decoupled. The electric drive system 10 can reduce noise and vibration by forcing the PWM switching frequency outside of the audible range. Furthermore, the controller C can be programmed to enable a switching frequency deadband in accordance with the current and inverter DC bus voltage to avoid the occurrence of resonance at relatively high currents. As understood by those skilled in the art, a deadband refers to a discontinuity intentionally introduced into a reference waveform for better performance. For example, the PWM switching frequency can be reduced by avoiding the crossing of the reference modulation wave and the carrier wave in a range of frequencies. For example, if the switching frequency is between 2 kHz and 20 kHz, and the deadband is 12-14 kHz, then the operable frequency would be 2-12 kHz and 14-20 kHz.

[0135] Figure 1 The controller C of FIG. 6 can be an integral part of or a separate module operably connected to other controllers of the device 12. Figure 1The controller C of the system 100 includes computer-readable media (also referred to as processor-readable media), including non-transitory (e.g., tangible) media involved with providing data (e.g., instructions) that can be read by a computer (e.g., a processor of a computer). Such media can take many forms, including but not limited to non-volatile media and volatile media. Non-volatile media can include, for example, optical or magnetic disks and other persistent memory. Volatile media can include, for example, dynamic random access memory (DRAM), which can constitute a main memory. Such instructions can be conveyed by one or more transmission media including coaxial cables, copper wire, and fiber optics, including the wires that comprise a system bus coupled to a computer processor. Some forms of computer-readable media include the following: a floppy disk, a flexible disk, a hard disk, magnetic tape, other magnetic media, a CD-ROM, DVDs, other optical media, punch cards, paper tape, other physical media with patterns of holes, a RAM, a PROM, an EPROM, a FLASH- EPROM, other memory chips or cartridges, a magnetic ink cartridge, or any other medium from which a computer is able to read.

[0136] The lookup tables, databases, data repositories, or other data stores described herein can include various kinds of mechanisms for storing, accessing, and retrieving various kinds of data, including a hierarchical database, a set of files in a file system, an application database in a proprietary format, a relational database management system (RDBMS), etc. Each such data store can be included within a computing device employing a computer operating system such as one of those mentioned above, and can be accessed via a network in one or more manners such as those described above. A file system can be accessed from a computer operating a rechargeable energy storage system, and can include files stored in various formats. An RDBMS can employ, in addition to the language for creating, storing, editing, and executing stored procedures, a structured query language (SQL), such as the PL / SQL language mentioned above.

[0137] Figures 3-6The flow diagrams herein illustrate the architecture, functionality, and operations of possible implementations of systems, methods and computer program products according to various embodiments of the disclosure. In this regard, each block in the flow diagrams or block diagrams can represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical functions (“instructions”). It should also be noted that each block of the block diagrams and / or flow diagrams illustrations, and combinations of blocks in the block diagrams and / or flow diagrams illustrations, can be implemented by special-purpose hardware-based systems that perform the specified functions or acts, or combinations of special-purpose hardware and computer instructions. These computer program instructions can also be stored in a computer-readable medium that can direct a controller or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instructions which implement the functions / acts specified in the block diagrams and / or flow diagrams.

[0138] The numerical values of parameters (e.g., quantities or conditions) in this specification (including the appended claims) are to be understood as being modified in each instance by the term “about” whether or not “about” actually appears before the numerical value. “About” means that the described value allows some slight imprecision (and that is near the stated value; approximately or reasonably close to the value; nearly). If an imprecision is provided in a value in this specification, unless otherwise indicated, the imprecision is at least + / - 10% around the stated value, preferably + / - 5%, more preferably + / - 1%, and even more preferably + / - 0.1%. If an imprecision is not otherwise indicated, “about” indicates at least the imprecision that would be expected in the ordinary use of such a parameter. Additionally, disclosure of a range includes disclosure of each individual value within the range and disclosure of the entire range is also expressly disclosed. Therefore, every individual value and range-forming endpoint within the range is hereby incorporated into the disclosure as an independent embodiment.

[0139] The summary and abstract are provided herein for technical employees of the Office, and are not intended to limit the scope, or to describe essential elements of the disclosure. The specific embodiments and examples of the disclosure are described herein for illustrative purposes, and various modifications are possible within the scope of the appended claims. Additionally, the features of the embodiments shown in the drawings or mentioned in the specification may not necessarily be understood as independent embodiments. Rather, it is possible that each feature described in one example of an embodiment can be combined with one or more other desired features from other embodiments, resulting in other embodiments not described with words or referenced in the drawings. Therefore, such other embodiments also fall within the framework of the scope of the appended claims.

Claims

1. An electric drive system, comprising: a chargeable energy storage unit and a power inverter operably connected to the chargeable energy storage unit; an electric motor operably connected to the power inverter; a controller in communication with the power inverter, operation of the controller defined at least in part by a control reference frequency; wherein power transfer between the chargeable energy storage unit and the electric motor is governed by a pulse width modulation (PWM) switching frequency; wherein the controller has a processor and a tangible, non-transitory memory having instructions recorded thereon, execution of the instructions by the processor causing the controller to: determine a current switching frequency based in part on a pulse width modulation type, a pulse width modulation switching frequency regime, and an inverter direct current voltage; obtain a pulse width modulation scalar based in part on a maximum value of the control reference frequency and the current switching frequency; and communicate a command signal to the power inverter to adjust power transfer based in part on the pulse width modulation scalar, the pulse width modulation switching frequency proportional to a product of the pulse width modulation scalar and the control reference frequency.

2. The electric drive system of claim 1, wherein: the current switching frequency is based in part on an inverter coolant temperature and whether a drive torque or a regenerative torque is in operation; and the instructions are executed dynamically such that the pulse width modulation scalar varies over time.

3. The electric drive system of claim 1, wherein obtaining the pulse width modulation scalar comprises: obtaining an extended pulse width modulation switching frequency as a sum of the current switching frequency and a frequency span; obtaining a ratio of the extended pulse width modulation switching frequency divided by the maximum value of the control reference frequency; and setting the pulse width modulation scalar to a minimum integer greater than or equal to the ratio.

4. The electric drive system of claim 1, wherein determining the current switching frequency comprises: determining, via the controller, a pulse width modulation region layout having a plurality of pulse width modulation regions arranged in a torque-speed curve calibrated for the electric motor; and obtaining, via the controller, a specified pulse width modulation region from the pulse width modulation region layout in the torque-speed curve based on a speed and a torque of the electric motor, the speed and the torque of the electric motor based in part on a torque command. determining the current switching frequency comprises: selecting, via the controller, the pulse width modulation type according to the specified pulse width modulation region; and selecting, via the controller, the pulse width modulation switching frequency regime according to the specified pulse width modulation region.

5. The electric drive system of claim 4, wherein, 6. The electric drive system of claim 5, wherein: the pulse width modulation switching frequency regime is selected from a predefined list of pulse width modulation switching frequency regimes, the predefined list of pulse width modulation switching frequency regimes including a constant pulse duty cycle switching regime, a constant switching frequency switching regime, and a look-up table with a selectable switching frequency array.

7. The electric drive system of claim 1, wherein: ​ ​ ​ The pulse width modulation type is selected from a predefined list of pulse width modulation types, the predefined list of pulse width modulation types including discontinuous pulse width modulation (DPWM) techniques, zero vector modulation (ZVM) techniques, and space vector pulse width modulation (SVPWM) techniques.

8. The electric drive system of claim 1, wherein: when the pulse width modulation type is space vector pulse width modulation (SVPWM) techniques and the current switching frequency is greater than a threshold space vector pulse width modulation switching frequency, the controller is programmed to set the current switching frequency to the threshold space vector pulse width modulation switching frequency before updating the pulse width modulation type to space vector pulse width modulation techniques.

9. The electric drive system of claim 1, wherein: when the pulse width modulation type is discontinuous pulse width modulation (DPWM) techniques, the controller is programmed to prevent switching from discontinuous pulse width modulation techniques to space vector pulse width modulation (SVPWM) techniques until the current switching frequency is less than a threshold space vector pulse width modulation switching frequency.

10. The electric drive system of claim 1, wherein: the controller is programmed to override the control reference frequency and the pulse width modulation switching frequency when a six-step operation is activated.

11. The electric drive system of claim 10, wherein: the controller is programmed to incorporate a hysteresis at the control reference frequency maximum.

12. A method of operating an electric drive system having a chargeable energy storage unit, an electric motor, a power inverter, and a controller having a processor and a tangible, non-transitory memory, the method comprising: regulating, via the power inverter, transfer of electrical power between the chargeable energy storage unit and the electric motor by a pulse width modulation (PWM) switching frequency; determining, via the controller, a current switching frequency based in part on a pulse width modulation type, a pulse width modulation switching frequency regime, and an inverter direct current voltage, the operation of the controller defined at least in part by a control reference frequency; obtaining, via the controller, a pulse width modulation scalar based in part on the current switching frequency and the control reference frequency maximum; and communicating, via the controller, a command signal to the power inverter to regulate the transfer of electrical power based in part on the pulse width modulation scalar, the pulse width modulation switching frequency proportional to a product of the pulse width modulation scalar and the control reference frequency.

13. The method of claim 12, wherein, obtaining the pulse width modulation scalar includes: obtaining an extended pulse width modulation switching frequency as a sum of the current switching frequency and a dithering frequency span; obtaining a ratio of the extended pulse width modulation switching frequency divided by the control reference frequency maximum; and obtaining the pulse width modulation scalar based in part on the ratio.

14. The method of claim 13, wherein, obtaining the pulse width modulation scalar includes: setting the pulse width modulation scalar to a minimum integer greater than or equal to the ratio such that the pulse width modulation scalar is equal to a CEILING function of the ratio.

15. The method of claim 12, wherein, obtaining the current switching frequency includes: selecting the pulse width modulation type from a predefined list of pulse width modulation types, including discontinuous pulse width modulation (DPWM) techniques, zero vector modulation (ZVM) techniques, and space vector pulse width modulation (SVPWM) techniques; and when the pulse width modulation type is a space vector pulse width modulation technique and the current switching frequency is greater than a threshold space vector pulse width modulation switching frequency, setting the current switching frequency to the threshold space vector pulse width modulation switching frequency prior to communicating the command signal to the power inverter.

16. The method of claim 15, when the pulse width modulation type is a discontinuous pulse width modulation technique, the method further comprising: preventing a switch from the discontinuous pulse width modulation technique to the space vector pulse width modulation technique until the current switching frequency is below a predetermined threshold.

17. The method of claim 12, further comprising: applying a hysteresis to the current switching frequency, the hysteresis extending to a control reference frequency maximum.

18. An electric vehicle, comprising: a traction motor adapted to output torque for propulsion; a traction battery pack adapted to power the traction motor; a power inverter electrically connecting the traction battery pack to the traction motor; a controller in communication with the power inverter, the operation of the controller defined by a control reference frequency; wherein the power inverter is operable to convert direct current power output by the traction battery pack to alternating current power and transmit the alternating current power to the traction motor based in part on a pulse width modulation (PWM) switching frequency; wherein the controller has a processor and a tangible, non-transitory memory having instructions recorded thereon, execution of the instructions by the processor causing the controller to: determine a current switching frequency based in part on a pulse width modulation type, a pulse width modulation switching frequency regime, and an inverter direct current voltage; obtain a pulse width modulation scalar based in part on the current switching frequency and a control reference frequency maximum; and communicate a command signal to the power inverter to adjust transmission of power based in part on the pulse width modulation scalar, the pulse width modulation switching frequency proportional to a product of the pulse width modulation scalar and the control reference frequency. obtaining the pulse width modulation scalar includes:

19. The electric vehicle of claim 18, wherein, obtaining an extended pulse width modulation switching frequency as a sum of the current switching frequency and a dithering frequency span; obtaining a ratio of the extended pulse width modulation switching frequency divided by the control reference frequency maximum; and obtaining the pulse width modulation scalar based in part on the ratio. obtaining the pulse width modulation scalar includes:

20. The electric vehicle of claim 19, wherein, setting the pulse width modulation scalar to a minimum integer greater than or equal to the ratio such that the pulse width modulation scalar is equal to a CEILING function of the ratio. ​

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