Optimization of switching frequency and pulse width modulation techniques for traction power inverters

Abstract

A method of operating a power inverter for an electric machine includes selecting an inverter switching technique; determining a frequency range of an inverter switch; determining a direct current (DC) voltage range of the inverter switch; determining an optimal switching map including inverter switching techniques and frequency selections for each value of torque-rate on the torque-rate curve; and controlling switches included in the power inverter using the optimal switching map.

Description

Technical Field

[0001] The present application relates to control systems and, more particularly, to control systems for use with electric machines. Background Art

[0002] Vehicles are increasingly dependent on one or more motors for all or at least part of the vehicle's propulsion. An electric vehicle (EV) typically includes a vehicle battery, an electric motor (sometimes referred to as a rotary electric motor), a power inverter, and a control system that selectively supplies the electrical energy stored in the vehicle battery to the electric motor through the power inverter. The power inverter can receive direct current (DC) current from the vehicle battery and convert it into alternating current (AC) current to supply the electric motor. The power inverter can be controlled using a control system that implements a switching program that can maximize efficiency during some operating conditions, but during other operating conditions, the program may operate relatively inefficiently. It would be helpful to implement a flexible control system for a power inverter that optimizes the efficiency of the electric motor depending on the operating conditions. Summary of the invention

[0003] In one implementation, a method of operating a power inverter for an electric motor includes: selecting an inverter switching technology; determining a frequency range for the inverter switches; determining a direct current (DC) voltage range for the inverter switches; determining an optimal switching diagram, the optimal switching diagram including inverter switching technology and frequency selection for each value of torque-rate on a torque-rate curve; and using the optimal switching diagram to control switches included in the inverter.

[0004] In another implementation, a method of operating a power inverter for an electric motor includes: selecting an inverter switching technology; determining a frequency range for the inverter switches; determining a direct current (DC) voltage range for the inverter switches; measuring performance of the electric motor at the selected inverter switching technology and the determined frequency range; determining an optimal switching diagram based on the measured performance, the optimal switching diagram including inverter switching technology and frequency selection for each value of torque-rate on a torque-rate curve; and using the optimal switching diagram to control switches included in the power inverter.

[0005] In yet another implementation, a power inverter for controlling an electric motor includes: a control system configured to: select an inverter switching technology; determine a frequency range for the inverter switches; determine a direct current (DC) voltage range for the inverter switches; determine an optimal switching diagram, the optimal switching diagram including inverter switching technology and frequency selection for each value of torque-rate on a torque-rate curve; and control switches included in the inverter, the switches configured to be electrically connected to the motor and a battery using the optimal switching diagram. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a block diagram depicting an implementation of a battery electric vehicle (BEV) including a control system;

[0007] Figure 2 is a block diagram depicting an implementation of a control system capable of controlling a motor;

[0008] Figure 3 is a block diagram depicting a portion of an implementation of a control system capable of controlling an electric motor;

[0009] Figure 4 is another block diagram depicting a portion of an implementation of a control system capable of controlling an electric machine;

[0010] Figure 5 is a graph depicting an implementation of an optimal switching frequency map used by a control system to control a motor;

[0011] Figure 6 is a graph depicting an implementation of an optimal switching frequency map used by a control system to control a motor;

[0012] Figure 7 is a graph depicting an implementation of an optimal switching frequency map used by a control system to control a motor;

[0013] Figure 8 is a diagram depicting an implementation of an optimal switching frequency and pulse width modulation (PWM) technique diagram used by a control system to control a motor; and

[0014] Fig. 9 is a diagram depicting an implementation of an optimal switching frequency and pulse width modulation (PWM) technique diagram used by a control system to control a motor. DETAILED DESCRIPTION

[0015] The control system controls a power inverter that converts the DC voltage provided by the vehicle battery into an AC current supplied to the motor or rotating motor. The control system selects a specific pulse width modulation (PWM) switching technology and switching frequency for each operating point on the feasible torque-rate (T-ω) curve of the drive system based on system limits. For each torque-rate (T-ω) value that the motor can reach within the DC voltage range, the optimal PWM switching technology can be selected, and the switching frequency at which the power inverter is operated can also be selected. The optimal PWM switching technology and switching frequency for each torque-rate (T-ω) value can be determined and stored by the control system to be accessed and used during the operation of the motor. In this way, the control system can be combined with the motor without prior knowledge of the environment in which the three-phase AC motor is installed or the technical specifications or characteristics of the motor. After the control system and the motor are combined, the control system can perform a process so that it determines the optimal PWM switching technology and switching frequency for each torque-rate (T-ω) value based on the voltage range and torque-rate (T-ω) curve of the motor. In addition, the control system can select an economic mode or a performance mode depending on what the operator values. The control system can reduce power losses in the power inverter and ripple in the AC current supplied to the motor.

[0016] Go to Figure 1 , shows an implementation of an electrical system 10, which includes an electric power grid 12 and a battery electric vehicle (BEV) 14, which can receive electric power from the grid 12 or provide electric power to the grid 12. Although the control system and the method of operating the control system are described for BEVs, the method / system can be used in other environments. The power grid 12 can include any of a number of electric power generators and power transmission mechanisms. The generator (not shown) produces AC electric power, which can then be transmitted a considerable distance from the generator for residential and commercial use. The generator can be coupled to the power grid 12, which transmits the AC electric power from the generator to an end user, such as a residence or business. When the AC electric power is provided to the power grid 12, the electric power can exist at a relatively high voltage to transmit a relatively long distance. Once the electric power reaches the location where it is intended to be used, a power transformer (not shown) can be used to reduce the voltage level (before it is ultimately provided to a residence or business).

[0017] The BEV 14 may include one or more motors 16, also referred to as rotary motors, that propel the vehicle in whole or in part. In the described implementation, the motor 16 is a permanent magnet motor, but other implementations using the control system and functionality described herein are possible. A three-phase power inverter 18 may be electrically coupled to the BEV battery 20 and the motor 16. The power inverter may receive a DC voltage from the BEV battery 20 and convert the DC electric power into a three-phase AC current before supplying the AC current to the motor 16. The power inverter 18 may be implemented in a variety of ways. For example, the power inverter 18 may include six MOSFETs that selectively become conductive or are turned on and off via a gate that is electrically in communication with a control system or controller. The voltage supplied to the motor 16 by the BEV battery 20 may vary depending on the application. The term "battery electric vehicle" or "BEV" may refer to a vehicle that is propelled in whole or in part by a motor. BEV may refer to an electric vehicle, a plug-in electric vehicle, a hybrid electric vehicle, and a battery-powered vehicle.

[0018] The BEV charging station (also referred to as a DC fast charger 22) may receive AC electric power from the grid 12, rectify the AC voltage into DC electric power, and provide the DC electric power to the BEV 14. The DC fast charger 22 may be geographically fixed, such as a charging station located in a vehicle garage or in a vehicle parking lot. The DC fast charger 22 may include input terminals that receive AC electric power from the grid 12 and directly transfer the AC electric power to the BEV battery 20, thereby bypassing the onboard vehicle battery charger 24 included on the BEV 14. The cable 26 may be removably connected to an electrical socket on the BEV 14 and electrically link the DC fast charger 22 with the BEV 14 so that DC electric power can be transferred between the DC fast charger 22 and the BEV battery 20. One type of DC fast charging may be referred to as Level 3 BEV charging, which is considered to be 60-350kW. However, other charging standards and power levels having the structure and functionality disclosed herein are possible. The BEV battery 20 can supply DC electric power controlled by power electronics to the motor 16 that propels the BEV 14. One or more BEV batteries 20 are rechargeable. Examples of batteries include lead-acid batteries, nickel-cadmium (NiCd), nickel-metal hydride, lithium-ion, and lithium-polymer batteries. However, battery technology is evolving, and other chemical compositions and / or voltages may be used. A typical range of vehicle battery voltage may vary within a range of 100 to 1000V DC electric power (VDC). A control system 28 (also referred to as a controller) implemented as computer-readable instructions executable by a microprocessor may be stored in a non-volatile memory and called to monitor a current sensor and generate a control signal including a gate signal for the power inverter 18. This will be discussed in more detail below.

[0019] Go to Figure 2 , a method 200 for controlling a power inverter 18 electrically connected to an electric machine 16 is shown. The method 200 may be implemented using a control system 28 and includes: selecting a particular pulse width modulation (PWM) switching technique at box 202; determining a switching frequency range at box 204; and selecting a DC voltage range at box 206. The outputs from boxes 202-206 may be used at a performance measurement analysis box 208 to analyze the electric machine 16 using different PWM switching techniques at voltage values ​​within a voltage range and different frequency values ​​within a frequency range. The power loss of the power inverter 18 may be determined at each torque-rate (T-ω) value along an operating torque-rate (T-ω) curve at each possible switching frequency based on each selected PWM switching technique. It is also possible to determine the DC voltage ripple and the AC current ripple.

[0020] There are many PWM switching techniques from which one may be selected at block 202. For example, such switching techniques include space vector pulse width modulation (SVPWM), discontinuous pulse width modulation (DPWM), third harmonic injected sinusoidal pulse width modulation (THIPWM), and generalized discontinuous pulse width modulation (GDPWM), to name a few different types of PWM switching techniques. It should be understood that other PWM switching techniques not explicitly set forth herein are possible. Given the selected switching technique (such as SVPWM) at block 202, a specific switching frequency range (f s =[f 最小 :f 最大 At block 206, a DC voltage range ([V DC最小 :V Dc最大 ]). For example, SVPWM may be selected, a switching frequency between 2-16 kHz may be selected, and a voltage range of 700 V (-350 V to +350 V) may be selected in block 206.

[0021] With these selections in mind, method 200 proceeds to block 208 where motor performance measurements may be performed. Figure 3208, for each torque-rate (T-ω) point along the torque-rate (T-ω) curve, method 200 may determine the conduction power loss of power inverter 18, the switching power loss of power inverter 18, the total power loss of power inverter 18, DC voltage ripple, AC current ripple and / or the switching temperature of MOSFET. The result may be provided to a torque-rate (T-ω) diagram, which includes a data table for each torque-rate (T-ω) point on the torque-rate (T-ω) curve. Each point may be associated with a determined power loss and voltage / current ripple. The steps detailed for boxes 202-206 may be repeated for different selected PWM switching techniques (PWM 1, PWM2, ... PWMx), while keeping the switching frequency and voltage range fixed. Power loss and current / voltage ripple may be determined using each switching technique. Method 200 proceeds to step 210.

[0022] Method 200 may use the output of block 208 to generate an optimal switching diagram for power inverter 18 at block 210. The optimal switching diagram may include each torque-rate (T-ω) point along the torque-rate (T-ω) curve such that an optimal PWM switching technique and an optimal switching frequency are specified at that point. As part of generating the optimal switching diagram, for peak-to-peak DC voltage ripple limits (|V DC pk-pk|≤V 极限 ) within the specified voltage range ([V DC最小 :V DC最大 ]) and greater than or equal to the pulse ratio limit (PR 极限 ) pulse ratio, the control system 28 can identify the power inverter loss (P 损耗 ) of the minimum switching frequency (f s ). The switching frequency that minimizes the losses at each torque-rate (T-ω) value can be stored in memory. This can be repeated for other PWM switching techniques (PWM 1, PWM 2, ..., PWMx). The control system 28 can repeat the switching frequency that causes the power inverter power loss (P 损耗 ) of the minimum switching frequency (f s ) to select the power inverter loss (P 损耗 ) and / or minimize the AC current ripple (I AC ) is the switching technology and switching frequency at each torque-rate (T-ω) value on the torque-rate curve that is minimized.

[0023] The method 200 may include other steps to determine an optimal switching diagram, including the selected switching technology and switching frequency at each torque-rate (T-ω) position along the torque-rate (T-ω) diagram. For example, at box 212, the control system 28 may access a setting indicating whether maximum efficiency or maximum performance is preferred. At step 214, additional variables may be considered, such as a voltage ripple limit and / or a pulse ratio limit. The method 200 then ends. Figure 3 The implementation of this method is shown in FIG.

[0024] During operation of the electric machine 16, the control system 28 may receive an input indicative of a desired operating torque-rate input to the electric machine 16 and select a PWM switching technique and a switching frequency for that input based on a stored optimal switching map. It is possible that the optimal switching map is stored in a memory at the BEV 14. Figure 4-8 A graphical representation of an optimal switching diagram including a torque-rate (T-ω) curve resulting from the output of block 210 is shown in FIG. Figure 4 The stored optimal switching diagrams depicting torque (T) and angular velocity (ω) for the control system 28 selecting a switching frequency of 4 kHz, 6 kHz, 8 kHz, 10 kHz, 12 kHz, or 14 kHz at 700 V using SVPWM are depicted. Figure 5 The stored optimal switching diagrams depicting torque (T) and angular velocity (ω) for the control system 28 selecting a switching frequency of 4 kHz, 6 kHz, 8 kHz, 10 kHz, 12 kHz, or 14 kHz at 700 V using GDPWM are depicted. Figure 6 The stored optimal switching diagrams depicting torque (T) and angular velocity (ω) for the control system 28 selecting a switching frequency of 4 kHz, 6 kHz, 8 kHz, 10 kHz, 12 kHz, or 14 kHz at 700V using THIPWM are depicted.

[0025] Figure 7 The stored optimal switching diagram depicting the control system 28 selecting a switching frequency of 4kHz, 6kHz, 8kHz, 10kHz, 12kHz or 14kHz and torque (T) and angular rate (ω) for different PWM selections (SVPWM, GDPWM or THIPWM) based on minimized AC current ripple at a DC voltage of 700V is depicted. Figure 8 The stored optimal switching diagrams depicting the control system 28 selecting a switching frequency of 4kHz, 6kHz, 8kHz, 10kHz, 12kHz or 14kHz and torque (T) and angular velocity (ω) for different PWM selections (SVPWM, GDPWM or THIPWM) at a DC voltage of 700V are depicted.

[0026] It is to be understood that the foregoing is a description of one or more embodiments of the present invention. The present invention is not limited to the (one or more) specific embodiments disclosed herein, but is defined only by the claims hereinafter. In addition, except for the case where the terms or phrases are clearly defined above, the statements contained in the foregoing description relate to specific embodiments and are not to be interpreted as limitations on the scope of the present invention or the definitions of the terms used in the claims. Various other embodiments and various changes and modifications to the (one or more) disclosed embodiments will become apparent to those skilled in the art. All such other embodiments, changes and modifications are intended to fall within the scope of the appended claims.

[0027] As used in this specification and claims, when used in conjunction with a list of one or more components or other items, the terms "such as," "for example," "for example," "such as," and "similar to," and the verbs "comprising," "including," "having," and their other verb forms are each to be interpreted as open-ended, meaning that the list is not to be construed to exclude other additional components or items. Other terms are to be interpreted using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.

Claims

1. A method of operating a power inverter for an electric machine, comprising the steps of: (a) Select inverter switching technology; (b) Determine the frequency range of inverter switching; (c) Determine the direct current (DC) voltage range of the inverter switches; (d) determining an optimal switching diagram including inverter switching techniques and frequency selection for each value of torque-rate on a torque-rate curve; and (e) controlling switches included in the inverter using the optimal switching map.

2. The method of claim 1, further comprising the step of determining a maximum inverter efficiency at each torque-rate value along the torque-rate curve based on inverter switching technology and switching frequency.

3. The method as described in claim 1 further includes the steps of determining the inverter power loss at the torque-rate value along the torque-rate curve, and determining the maximum inverter efficiency at each torque-rate value along the torque-rate curve based on the inverter switching technology and the switching frequency.

4. The method of claim 1, further comprising the step of determining a maximum efficiency at each torque-speed value along the torque-speed curve based on an alternating current (AC) ripple.

5. The method of claim 1, further comprising the step of determining whether the selected pulse ratio is greater than a pulse ratio maximum value.

6. A method of operating an inverter for an electric machine, comprising the steps of: (a) Select inverter switching technology; (b) Determine the frequency range of inverter switching; (c) Determine the direct current (DC) voltage range of the inverter switches; (d) measuring the performance of the motor according to the selected inverter switching technology and the determined frequency range; (e) determining an optimal switching diagram based on step (d), the optimal switching diagram including inverter switching techniques and frequency selections for each value of torque-rate on the torque-rate curve; and (e) controlling switches included in the inverter using the optimal switching map.

7. The method of claim 6, further comprising the step of determining a maximum efficiency at each torque-rate value along the torque-rate curve based on inverter switching technology and switching frequency.

8. The method as claimed in claim 6 further comprising the steps of determining inverter power losses at torque-rate values ​​along a torque-rate curve, and determining a maximum efficiency at each torque-rate value along the torque-rate curve based on inverter switching technology and switching frequency.

9. The method of claim 6, further comprising the step of determining a maximum efficiency at each torque-speed value along the torque-speed curve based on an alternating current (AC) ripple.

10. A power inverter for controlling a motor, comprising: A control system configured to: select an inverter switching technology; determine a frequency range for the power inverter switches; determine a direct current (DC) voltage range for the power inverter switches; determine an optimal switching map, the optimal switching map including inverter switching technology and frequency selection for each value of torque-rate on a torque-rate curve; and control switches included in the power inverter, the switches configured to be electrically connected to the motor and the battery using the optimal switching map.

11. The power inverter as claimed in claim 10, wherein: The power inverter is configured to measure performance of the electric machine at a selected inverter switching technique and a determined frequency range.

12. The power inverter as claimed in claim 10, wherein: The power inverter is configured to determine a maximum efficiency at each torque-rate value along the torque-rate curve based on an inverter switching technique and a switching frequency.

13. The power inverter as claimed in claim 10, wherein: The power inverter is configured to determine a maximum efficiency at each torque-rate value along the torque-rate curve based on an inverter switching technique and a switching frequency.

14. The power inverter as claimed in claim 10, wherein: The power inverter is configured to determine inverter power losses at torque-rate values ​​along a torque-rate curve and to determine a maximum efficiency at each torque-rate value along the torque-rate curve based on an inverter switching technique and a switching frequency.

15. The power inverter as claimed in claim 10, wherein: The power inverter is configured to determine a maximum efficiency at each torque-rate value along the torque-rate curve based on an alternating current (AC) ripple.

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