GB2615762A

GB2615762A - A method for controlling a six-phase electric motor by a control system

Info

Publication number: GB2615762A
Application number: GB2202059.8A
Authority: GB
Inventors: Huang Yang; Bai Hua; Cheng Bing; Jin Fanning
Original assignee: Mercedes Benz Group AG; University of Tennessee Research Foundation
Current assignee: Mercedes Benz Group AG; University of Tennessee Research Foundation
Priority date: 2022-02-16
Filing date: 2022-02-16
Publication date: 2023-08-23
Also published as: GB202202059D0; WO2023156347A1

Abstract

A method for controlling a six-phase electric motor, particularly in an automobile, by a control system 10 of the automobile uses PWM control to eliminate common mode voltage. The method comprises the steps of: obtaining a modulation index and calculating six-phase duty cycle values with a sinusoidal pulse width modulation generator S1-S3; detecting a zone out of a plurality of zones (1-6) in which different stationary phases (A,B,C) of the sinusoidal pulse width modulation occur S4-S9; applying corresponding pulse shifting/splitting rules based on the detected zone S10; determining updated duty cycle values S11; generating a mathematical model for controlling the six-phase electric motor S12; and generating a six-phase pulse width modulation signal (12) depending on the mathematical model for controlling the six-phase electric motor S13.

Description

A METHOD FOR CONTROLLING A SIX-PHASE ELECTRIC MOTOR BY A CONTROL

SYSTEM

FIELD OF THE INVENTION

[0001] The invention relates to the field of electric motors. More specifically, the invention relates to a method for controlling a six-phase electric motor by a control system of an electric or hybrid automobile according to claim 1. Furthermore, the invention relates to a corresponding control system.

BACKGROUND INFORMATION

[0002] Common-mode voltage (CMV) is one of the major concerns in modern motor drive techniques. To alleviate the CMV issue, a series of different pulse width modulation (PWM) methods have been proposed, which are particularly effective when the motor phases have even numbers, such as six phases. Conventional modulation schemes, such as space vector pulse width modulation, discontinuous pulse width modulation, or active zero state pulse width modulation are widely adopted in the three-phase motor drive system. One of the most concerning issues is the common-mode voltage. These PWMs yield CMV varies between ±Vdc/6 or ±Vd12. Due to the physical limitation of the three-phase two-level inverter, it can never eliminate the CMV. Such CMV results in a large CM choke as the must to pass CM standards, such as CISPR25 in electric vehicles, which however adds the weight for the inverter system.

SUMMARY OF THE INVENTION

[0003] It is an object of the invention to provide a method as well as a control system, by which a common-mode voltage in an electric motor is minimized.

[0004] This object is solved by a method as well as a corresponding control system according to the independent claims. Advantageous forms of configuration are presented in the dependent claims.

[0005] One aspect of the invention relates to a method for controlling a six-phase electric motor by a control system of the automobile. The control system of the electric or hybrid automobile may reside in the motor control unit or another control unit. A modulation index is obtained and six-phase duty cycle values by a sinusoidal pulse width modulation generator are determined. A zone out of a plurality of zones, in which different stationary phases of the sinusoidal pulse width modulation occur, is detected. Corresponding pulse shifting/splitting rules based on the detected zone are applied. Updated duty cycle values are determined. A mathematical module for controlling the six-phase electric motor is generated and a six-phase pulse width modulation signal depending on the mathematical model for controlling the six-phase electric motor is generated.

[0006] Therefore, a modulation method based on the traditional sinusoidal pulse width modulation is proposed to fully eliminate the common-mode voltage (CMV) in a dual-three-phase motor drive system, thereby helps the sizing shrinking of the common-mode choke.

[0007] According to an embodiment the six-phase pulse width modulation signal is transmitted to corresponding gate drivers of the six-phase electric motor.

[0008] In another embodiment at least six different zones are detected.

[0009] In another embodiment a zone is detected every 60 degrees in the sinusoidal pulse width modulation.

[0010] In particular, the method is a computer-implemented method. Therefore, another aspect of the invention relates to a computer program product comprising means for performing the method.

[0011] A still further aspect of the invention relates to a control system for controlling a six-phase electric motor, comprising at least one electronic computing device, wherein the control system is configured to perform a method according to the preceding aspect. In particular, the control system is performing the method.

[0012] Further advantages, features, and details of the invention derive from the following description of preferred embodiments as well as from the drawings. The features and feature combinations previously mentioned in the description as well as the features and feature combinations mentioned in the following description of the figures and/or shown in the figures alone can be employed not only in the respectively indicated combination but also in any other combination or taken alone without leaving the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The novel features and characteristic of the disclosure are set forth in the appended claims. The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate exemplary embodiments and together with the description, serve to explain the disclosed principles. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system and/or methods in accordance with embodiments of the present subject matter are now described below, by way of example only, and with reference to the accompanying figures.

[0014] The drawings show in: [0015] Fig. 1 a schematic flow chart according to an embodiment of the invention; and [0016] Fig. 2 a schematic diagram for detection the stationary phases.

[0017] In the figures the same elements or elements having the same function are indicated by the same reference signs.

DETAILED DESCRIPTION

[0018] In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration". Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

[0019] While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawing and will be described in detail below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure.

[0020] The terms "comprises', "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion so that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus preceded by "comprises" or "comprise" does not or do not, without more constraints, preclude the existence of other elements or additional elements in the system or method.

[0021] In the following detailed description of the embodiment of the disclosure, reference is made to the accompanying drawing that forms part hereof, and in which is shown by way of illustration a specific embodiment in which the disclosure may be practiced. This embodiment is described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following description is, therefore, not to be taken in a limiting sense.

[0022] Fig. 1 shows a schematic flow chart according to an embodiment of the invention. In particular, Fig. 1 shows a method for controlling a six-phase electric motor by a control system 10 of the automobile. In a first step Si, a modulation index is obtained, and in a second step S2, six-phase duty cycle values are determined by a sinusoidal pulse width modulation generator. In a third step S3, the duty cycle values are obtained. In a fourth step 54, a zone related to a stationary phase A, B. or C is detected out of a plurality of zones 1, 2, 3, 4, 5, or 6, in which different stationary phases of the sinusoidal pulse width modulation occur. In a fifth step S5, it is determined, in which zone 1, 2, 3, 4, 5, or 6 the pulse width modulation signal 12 is located. If it is in a first zone 1 or in a fourth zone 4 out of six zones 1, 2, 3, 4, 5, or 6, a sixth step S6 is performed and it is determined that it is located in a first phase A. If the zone is not the first zone 1 or the fourth zone 4, it is determined in a seventh step S7, if the zone is a second zone 2 or a fifth zone 5. If it is the second 2 or the fifth zone 5, in an eighth step S8 it is decided that it is a stationary phase B. If it is not in the second zone 2 or the fifth zone 5, it is decided in a ninth step S9 that it is a stationary phase C. After the third step S3, the sixth step S6, the eighth step S8, and the ninth step 39, a tenth step 310 is performed, wherein corresponding pulse shifting/splitting rules based on the detected zone are applied. In an eleventh step S11, updated duty cycle values are obtained. In a twelfth step 312, a mathematical model for controlling the six-phase electric motor is generated and in a thirteenth step 313, a six-phase pulse width modulation signal 12 depending on the mathematical model for controlling the six-phase electric motor is generated.

[0023] In particular, as multi-phase electric motor becomes more popular in the automotive industry, the common-mode voltage CMV elimination is possible. A six-phase motor shows outstanding advantages of the conventional three-phase motors in terms of reliability, fault-tolerance, torque pulsations, and current stress. In addition, there is now a possibility to eliminate the common-mode voltage. The following equation formulates the common-mode voltage Vem for a six-phase inverter. As the phases are an even number, there exists possibility to reduce the common-mode voltage to zero: lick! = (7,0 +17b0 Vco ± lido +17,0 ± 17f 0)16 (1) Wherein Vao -Vic) denote the voltage between the phase leg output and the DC-link middle point, and the value can only vary from ±Vdc/2.

[0024] For a conventional space vector pulse width modulation (SCPWM), the pulse width modulations (PWMs) for each phase are center-aligned. The corresponding CMV exhibits a seven-level profile with the peak value equal to ±Vd12. Every switching action results in a step change of the CMV. A switch-on action results in a +Vdc/2 at the corresponding phase leg output, and a switch-off action generates -Vdc/2. Most of the time during one switching period, the switch-on and -off actions of two phases cannot happen simultaneously due to the center-aligned modulation. However, when eliminating such constraint, the pulse width modulation for all six phases can be shifted or split to align each switch-on/-off pair, which has the chance to eliminate the CMV as the ±Vdc/2 step changes neutralize each other. This approach may be named zero CMV modulation (ZCMVM).

[0025] Aligning the switch-on/off edges to cancel the CMV is easy to comprehend. The crucial point is whether this alignment can be applied in the whole switching period and extended to the whole modulation range. Therefore, a mathematical model is important. For CSPWM, the duty cycle for each phase can be express as (2), where M is the modulation index, x represents the phase index and 0=0, 2r/3, 4.7/3, r16, 5r/6, 3-7/2, for phase A-F, respectively. Note the two winding sets, ABC and DEF have 30° phase difference: D, = + -2M COS (tE t + 0)(2) [0026] To clarify the PWM shift and split rule, assume the PWM of phase-A follows CSPWM (conventional sinusoidal pulse width modulation) without any shift or split. Angle shifting happens to phase-B/C and pulse splitting happens to phase-D/E/F. After the splitting, the pulses at the beginning of the switching period denote as d1/e1/f1, the pulse at the end of the switching period denotes as d2/e2/f2. Following this, Eqns. (3)-(8) could be derived. If all the switch-on/off pairs can be aligned, De2 must be equal to Dfl.

Eon =1-2° a (3) Dr2 = Di -D11 (4) Ddl 1 -Db -D12(5) Da2 = Da -Dai(6) Del = 1 -Dc_Da2 (7) De2 = De -Del (8) By substituting (2) to (3)-(8), the expression of De2 and D11 could be found as (9): De2 =D11 = 71; -cos(ffit) (9) [0027] This validates that shifting/splitting the PWMs to align all switch-on/off pairs is theoretically feasible regardless of the modulation index M. The challenge is, however, overmodulation happens when the modulation index is too large if such rule still applies. For instance, the modulation index may be 0.5, which is less than the maximum undistorted linearity limit of SPWM (0.785), nevertheless, the overmodulation still happens and cause phase current distortion because some of the modulation waves have negative part (the carrier signal varies from 0 to 1). The root cause is, in some cases, the ZCMV rule cuts the original duty cycle length to force the edges to align. If always fixing Phase-A as the reference phase, as the duty cycle of Phase-B/C becomes too large, the PWM shift is hard to realize as there is very limited room to move. Therefore, when the duty cycle of a certain phase is too high or low, the related phase is not shifted but fixed at the center of the period, which is termed as the stationary phase. To pick a stationary phase properly, specifying the zone related to the stationary phase A, B, or C as the indicator is necessary.

[0028] Fig. 2 then introduces the zone division rule. By solving (10), a fundamental period could be divided into six zones related to the phase A, B, or C by every 600. The zone related to the stationary phase A. B, or C are given in Fig. 2 as well.

Elmo, = 1-Dmin (10) As the stationary phase varies from zone A, B, C to zone A, B, C, the ZCMV requirement also changes. The rule is given in the following table: Zone Stationary phase ZCMV requirement 1,4 A De2=Dfl 2, 5 B Ddi =Df2 3, 6 C Del =Dd2 [0029] Generally, to realize the PWM shift and split, changing the carrier wave is the direct method. However, for ZCMVM, the PWM shift/split changes rapidly, causing significant difficulty in carrier wave modification. In this method, another efficient PWM manipulation method is proposed. By calculating the ON/OFF duty cycle for each phase, it can be compared with the carrier and can be calculated the intermediate part with "AND" logic to obtain the duty cycle. Therefore, by simply moving and splitting the ON/OFF duty-cycle, the PWM manipulation can be realized. The ON/OFF duty cycle equations are given through (11)-(13): 1 1(2M Don_, = 7 cos(wt + 0) + (11) 2 21 1(2M Doff = 7 + 7 7 cos (cot + 0) + 75 (12) D, = Doff x -D" (13) [0030] Such method is very easy to implement in FPGA. In the electric vehicle domain, the high-resolution microcontrollers, such as FPGAs draw more attention. With their unique parallel computation capability, FPGAs can update the duty cycle in every switching period.

S

[0031] This method proposes an SPWM based CMV elimination modulation method. A comprehensive mathematical derivation is performed to validate its feasibility. An efficient implementation approach in FPGA for PWM manipulation is also introduced.

Reference Signs control system 12 pulse width modulation S1-S13 steps of the method

Claims

CLAIMS1. A method for controlling a six-phase electric motor by a control system (10) of the automobile, comprising the steps of: - obtaining a modulation index; (Si) - calculating six-phase duty cycle values by a sinusoidal pulse width modulation generator; (S2 and S3) - detecting a zone (1, 2, 3, 4, 5, 6) out of a plurality of zones (1, 2, 3, 4, 5, 6), in which different stationary phases (A, B, C) of the sinusoidal pulse width modulation occur; (54-59) - applying corresponding pulse shifting/splitting rules based on the detected zone (1, 2, 3, 4, 5, 6); (S10) - determining updated duty cycle values; (Si 1) - generating a mathematical model for controlling the six-phase electric motor; (S12) and - generating a six-phase pulse width modulation signal (12) depending on the mathematical model for controlling the six-phase electric motor. (S13) 2. The method according to claim 1, characterized in that the six-phase pulse width modulation signal (12) is transmitted to corresponding gate drivers of the six-phase electric motor.3. The method according to claim 1 or 2, characterized in that at least six different zones (1, 2, 3, 4, 5, 6) are detected.4. The method according to claim 3, characterized in that a zone (1, 2, 3, 4, 5, 6) is detected every 60 degrees in the sinusoidal pulse width modulation signal (12).5. A control system (10) for controlling a six-phase electric engine, comprising at least one electronic computing device (14), wherein the control system (10) is configured to perform a method according to any of the claims 1 to 4.