A Highly-efficiency Position Sensorless Electric Vehicle Synchronous Reluctance Motor Drive
Ganisetti Vijay Kumar , Min-Ze Lu
, Chang-Ming Liaw *
Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan
* Correspondence: Chang-Ming Liaw
Academic Editors: Marco Bortolini and Francesco Gabriele Galizia
Special Issue: Energy Efficiency in Flexible and Reconfigurable Manufacturing: Emerging Trends, Models and Applications in the Industry 4.0 Era
Received: May 02, 2021 | Accepted: July 27, 2021 | Published: August 11, 2021
Journal of Energy and Power Technology 2021, Volume 3, Issue 3, doi:10.21926/jept.2103037
Recommended citation: Kumar GV, Lu MZ, Liaw CM. A Highly-efficiency Position Sensorless Electric Vehicle Synchronous Reluctance Motor Drive. Journal of Energy and Power Technology 2021; 3(3): 037; doi:10.21926/jept.2103037.
© 2021 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.
Abstract
The development of high-efficiency motor drives for various applications is important in the industry 4.0 era, especially for their extensive application to electric vehicles (EVs). In this study, a position sensorless EV synchronous reluctance motor (SynRM) drive has been developed, which exhibits good driving performance and efficiency over a wide speed and load range. To solve the key problems popularly encountered in the existing approaches, the high-frequency injection (HFI) scheme based on q-axis injection has been proposed. In addition, the changed-frequency injection has been adopted considering the effects of speed/load dependent slotting ripple current. Robust observed speed and position controllers have been added to enhance the sensorless control performance. For the SynRM basic driving control scheme, robust current control and adaptive commutation with minimized motor losses have been achieved that yield satisfactory driving performance up to the rated speed and load. Good EV driving performance has been demonstrated experimentally, including starting, dynamic, acceleration/deceleration, and reversible operations. In addition, the steady-state characteristics have been assessed, and the high efficiencies have been observed to be comparable to the standard drive.
Keywords
Electric vehicle (EV); synchronous reluctance motor (SynRM); control; sensorless; loss minimization; efficiency
1. Introduction
Electric motors [1] are the most popularly employed actuators in the industry. Improvements in their reliability and efficiencies are very important in the internet-of-things (IOT)- based automatic industry 4.0 era. Each type of motor possesses its own critical issues that need to be properly treated for achieving these goals. The synchronous reluctance motor (SynRM) [1,2,3] is a new type of motor compared to the commonly used induction motor and has high application potential, including EV driving. SynRM possesses two major features: (i) It has a simple and rigid rotor structure without permanent magnets and conductors and thus is suitable for high-speed driving applications; (ii) It has a highly developed torque similar to a series DC motor. However, an adequate commutation instant setting and tuning are recognized as the most challenging issues in obtaining a high-efficiency SynRM drive [4,5]. In particular, the inherent slotting effect [5,6,7,8,9] must also be considered, especially for a sensorless SynRM drive. A few advanced current control approaches [5,10,11] have been developed for achieving better performance considering the nonlinear behavior of SynRM.
For a standard SynRM drive, the absolute position of the rotor is required for implementing the vector control. Nevertheless, in harsh environments, a position sensorless controlled motor drive is preferable. Furthermore, the maloperation due to the failure of the position sensor can be avoided. The approaches developed for the permanent magnet synchronous motors (PMSMs) are also applicable to SynRM with suitable modifications. To date, a large number of position sensorless approaches have been developed, and their surveys can be found in the literature [12,13,14]. The key bottlenecks in the existing approaches and the future development trends have been commented on in detail [13]. The stable operation of a motor under heavier load and high speed is a key issue that needs to be solved, and accordingly, its successful application to electric vehicle (EV) driving is the major goal that is being pursued. The sensorless control approaches can be generally categorized into (i) observer-based methods, such as adaptive observers [15,16] and Kalman filter observers [17], (ii) extended back-electromotive force (back-EMF) methods [18,19,20], and (iii) methods based on rotor saliency [21,22,23,24,25,26,27], such as the high-frequency injection (HFI) approach, which is a typical method of this type.
The observed back-EMF-based approaches can only possess satisfactory running characteristics above a certain speed. In contrast, the HFI approach can be operated effectively in a standstill and low-speed mode, and thus this approach is applicable to EVs requiring frequent starting with a highly developed torque. However, the current ripples caused by its inherent slotting effect must be considered when making the current control and the HFI rotor position estimation.
In the HFI sensorless mechanism with sinusoidal signal injection, low-pass filters (LPFs) are used for extracting the current signal containing the estimated position error. The inherent time delay of LPFs limits the dynamic performances of the current and speed loops. To overcome these difficulties, the square-wave HFI sensorless schemes have been proposed [28,29,30,31,32,33]. By increasing the injection frequency, the effect of slot harmonics can be reduced, and fixed-frequency injection can be adopted. The number of LPFs used in the estimation scheme is reduced, and thus the bandwidths of the current, speed, and position estimation schemes are improved. In recent studies [32,33], the problem of instability under heavy loads has been pointed out, and to address it, improved square-wave HFI schemes have been proposed in order to enhance the performance of the motor under heavy load. However, only the ones at low speed are treated. At present, methods for achieving satisfactory driving characteristics under wide speed and load ranges for EV application are being pursued. As the speed and load are increased, the difference between d- and q-axis inductances (
As far as the single-axis HFI scheme is concerned, for the surface PMSM (SPMSM),
2. System Configuration
2.1 The Developed EV SynRM Drive
The developed EV SynRM drive is shown in Figure 1. The direct current (DC)-link is powered from the battery via a bidirectional interface converter with boosted voltage to achieve better driving performance over a wide speed range. A dynamic brake leg has been incorporated to avoid the over-voltage caused by the failure of regenerative braking.
Figure 1 System configuration of the developed EV SynRM drive.
2.1.1 Governing Equations of SynRM
The voltage and developed torque of a SynRM in the d-q domain of the rotor frame can be expressed as [1,2]
where
2.1.2 Observations
(a) From Equation (2), it can be seen that the commutation angle for a SynRM drive must be set to
(b) Regenerative braking of a SynRM drive must be achieved by reversing the developed torque. For reversing the torque, two approaches are suggested. In the first approach, the q-axis current command is set to be negative when the motor speed starts decelerating and again set to be positive when the speed command reaches zero. In the second approach, the reversing of torque can be achieved by reversing the
The EV SynRM drive system components are as follows :
(1) SynRM drive
• SynRM: 3P, 4-pole, 3.7 kW, 11.5 A, 2000 rpm.
• Load PMSG: 3P Y-connected, 8-pole, 5 kW, 2000 rpm.
• Inverter: It was constructed using three insulated-gate bipolar transistor (IGBT) modules, CM100DY-12H. Sinusoidal pulse-width modulator (PWM) switching frequency
(2) Battery interface converter
• Battery voltage:
• DC-link voltage:
• Power rating:
• Switching frequency:
(3) Dynamic brake: Braking resistance
2.2 Control Schemes
The control schemes of the battery interface converter, the dynamic brake, and the EV SynRM drive are shown in Figures 2(a) to 2(c), respectively. Details of the design procedures and all the designed constituted controllers can be found in a previous study [5], which were simplified as follows:
Figure 2 Control schemes of the developed sensorless EV SynRM drive. (a) battery interface converter, (b) dynamic brake, and (c) the motor drive.
2.2.1 Battery Interface Converter
Current controller. Using a large-signal stability criterion for a ramp-comparison current control pulse-width modulator (RC-CCPWM), i.e.,
Voltage controller. The controller
Voltage robust controller.
Robustness analysis. From the derivation corresponding to the voltage loop shown in Figure 2(a), it can be observed that the voltage tracking error,
2.2.2 SynRM Drive
For the SynRM drive, the critical controllers include the following: (i) An adaptive commutation scheme (ACS), which was used for determining the commutation angle
Current controller. Similar to the current control of a battery interface, the corresponding controller of the SynRM drive is expressed as:
Speed controller. The dynamic model of the SynRM was estimated under a small step command change. By defining the speed command tracking response, the designed speed controller was derived as
Robust controllers.
Similar to the robustness analysis made above, if the weighting factor,
The detailed analysis and design description of the proposed HFI sensorless control scheme will be presented in the following sections.
3. Determination of the Key Attributes for the HFI Scheme
For a pulsating HFI scheme [21,22,23], the high-frequency (HF) voltage can be injected into the q-axis (or d-axis), and the detected d-axis (or q-axis) current is processed to yield the observed rotor angle. In determining the adequate injected axis, the current levels in both axes must be observed. Hence, the back-EMF coupling effects were considered in the derivations.
For the injected high-frequency signal voltages,
where
The relationship between the estimated rotor reference frame with the estimated rotor angle,
Figure 3 Schematic showing the relationship between the estimated and the actual rotor reference frames.
Using
where
3.1 D-axis Injection
The normally employed d-axis injected mechanism is analyzed first. By applying the input voltage vector
into Equation (13) and rearranging, the resultant q-axis current can be expressed as
3.1.1 Observations
The current expressed in Equation (17) contains the speed-dependent bias as well as the small-signal terms with the information
Speed-dependent biased current. Generally,
Small-signal current (˜θr). By neglecting the
3.2 Q-axis Injection
The following q-axis injected vector was adopted:
Similarly, substituting Equation (20) into Equation (13), we get,
Following the same procedure as above, the following current components can be obtained from Equation (21):
(i) Speed-dependent biased current
(ii) Small-signal current (˜θr)
3.2.1 Comments
(1) From Equations (18), (19), (22), and (23), it is observed the small-signal currents,
(2) Hence, the q-axis injection scheme is suitable for a SynRM drive over a wide speed and load range, especially for EV drives. The numerical analysis of the experimental results will be presented in the subsection below, following the discussion on the experimental verification. The developed q-axis injected HFI EV SynRM drive will be presented in Section 4.
3.3 Experimental Verification
Let the established SynRM drive shown in Figure 1 be operated under the standard mode at
with the center frequency being 600 Hz.
3.3.1 D-axis Injection
First, the HF voltage was injected into the d-axis. The measured steady-state ω′r,(i′as,i′ds,i′qs), and the spectrum of i′as are shown in Figures 4(a) and 4(b). The corresponding q -axis current, i′qs was sensed to yield the band-pass filtered iˆrqsh, plotted in Figure 5(a), and its spectrum is shown in Figure 5(b).
Figure 4 Measured steady-state characteristics of the developed standard EV SynRM drive at
Figure 5 Measured steady-state characteristics of the developed standard EV SynRM drive at ω∗r=2000rpm,RL=16.67Ω,and β=βo1=45∘ with the high-frequency voltage injected in the d-axis. (a) ω′r and iˆrqsh, and (b) the spectrum of iˆrqsh.
3.3.2 Q-axis Injection
Next, the injection was applied to the q-axis. The measured steady-state characteristics are plotted in Figure 6 and Figure 7.
Figure 6 Measured steady-state characteristics of the developed standard EV SynRM drive at
Figure 7 Measured steady-state characteristics of the deve loped standard EV SynRM drive at
3.3.3 Comments
(i) From the measured results shown in Figures 5 and 7, it can be seen that the ratio of the magnitude of the q-axis current
(ii) For theoretical proof, the motor is running at ω′r=2000rpm,
Hence, analytically, from Equations (24) and (25),
3.4 Slotting Effect of the HFI Operation
3.4.1 Current Harmonics by Slotting Effect
The inverter-powered stationary armature winding facing the rotating slotted rotor can yield ripple currents. The dominant low-order current harmonics of the employed SynRM are
3.4.2 Spectral Characteristics
During real operation, the injected frequency is fixed, whereas the d-q domain slot harmonic frequency
3.5 Injected Axis Choice for Pulsating Voltage Injection
From the previous exploration, the following distinct facts for an HFI SynRM drive can be noted: (i) The d-q slot harmonic currents increase with the load at the frequency
4. The Developed HFI Position Sensorless EV SynRM Drive
4.1 System Configuration and Operation Principle
The control scheme of the developed q-axis injected HFI sensorless SynRM drive is shown in Figure 2(c), and the proposed changed-frequency injection mechanism and the frequency distributions are shown in Figures 8(a) and 8(b). The HF voltage,
Figure 8 Frequency distribution in the proposed changed frequencies injection mechanism for (a)
The small-signal current, as a function of
Generally,
Based on Equation (27), the rotor position and speed estimation scheme are shown in Figure 9(a). In order to yield the rotor position estimation error from Equation (27), a BPF was adopted to extract the high-frequency current,
The signal in Equation (28) is composed of a DC and a 2nd order harmonic component. Using an LPF, the DC component, i˜θr, can be extracted as follows:
Further, applying linearization to Equation (29), we get,
Equation (30) indicates that i˜θr is proportional to the rotor position error, ˜θr, by a sensitivity factor, Kerr. In the rotor position and speed estimation scheme shown in Figure 9(a), the disturbance signal is eliminated by a PI controller,
to yield the estimated rotor speed. Then, the estimated rotor position is obtained by the integration process.
Figure 9 Rotor position and speed estimation scheme based on the q-axis injection scheme. (a) the estimation mechanism, (b) equivalent estimated rotor position tracking control block, and (c) equivalent block of (b) after adding PRECC.
To enhance the rotor position estimation performance, an estimated position robust error cancellation controller (PRECC) with the following weighting function was added:
Similarly, the LPF time-constant,
The equivalent tracking control block representing the HFI sensorless control scheme can be presented by the block diagram shown in Figure 9(b), and its equivalent block obtained through derivation after applying the robust control is presented in Figure 9(c). The closed-loop tracking transfer function is found to be
with
From Equations (33) and (34), the following facts can be noted: (i) Since
4.2 HFI Scheme System Parameters
(1) Injected voltage:
(2) Rotor position estimation regulator:
Robust rotor position error cancellation controller:
(3) Signal processing filters:
LPF of
LPF of
BPFs of
4.3 Comparative Evaluation of the d- axis and q- axis Injected SynRM Drives
4.3.1 d-axis Injection Scheme
As explored above, for d-axis injection, the resultant current level of
(1) Injected voltage:
(2) Rotor position estimation regulator:
(3) BPFs of
4.3.2 Comparative Evaluation
At
Figure 10 Measured time variation of
Figures 11(a) and 11(b) show the measured values of
Figure 11 Measured time variation of
The results of d-axis injection when the load is further increased to
Figure 12 Measured time variation of
Figures 13(a) and 13(b) show the measured time variation of
Figure 13 Measured time variation of
4.4 Operation Characteristics of the Developed q- axis Injected EV SynRM Drive
4.4.1 Starting Process
Figure 14 shows the measured time variation of
Figure 14 Measured time variation of
4.4.2 Steady- state Characteristics
The measured time variation of (
Figure 15 Measured steady-state characteristics of the developed sensorless EV SynRM drive for
Figure 16 Measured steady-state characteristics of the developed sensorless EV SynRM drive for
4.4.3 Dynamic Response
Figure 17(a) show the measured speed tracking response of the developed EV drive by PI control without and with SRECC for a speed command change of
Figure 17 Measured values of
4.4.4 Acceleration/Deceleration and Reversible Operations
Initially, the motor is stably operated at
Figure 18 Measured results of the established sensorless EV SynRM drive at
Let the reversible speed command be set to
Figure 19 Measured values of
4.4.5 Regenerative Braking
Since the total moment of inertia of the established SynRM drive is not sufficiently large, the maximum speed is increased to 2500 rpm when applying regenerative braking. The SynRM drive is stably operated at
Figure 20 Measured results of the sensorless EV SynRM drive for a ramp speed command change from 2500 rpm with the deceleration rate of 500 rpm/s at
4.4.6 ECE15 Speed Pattern
The measured results of the developed sensorless EV SynRM drive under the programmed ECE15 speed pattern for
Figure 21 Efficiency assessment for the developed q-axis infected HFI sensorless SynRM drive. (a) measured values of ω∗r,ω′r,vdc,vb,ib, and Pb for a programmed ECE15 speed pattern with a changing rate of 100 rpm/s at
4.4.7 Efficiency Assessment
For the same condition, i.e.,
Table 1 Measured powers and efficiencies of the HFI position sensorless SynRM drive and the standard drive at
5. Conclusions
This study presents a high-efficiency sensorless EV SynRM drive using a pulsating sinewave HF voltage injection. Through analytic and experimental explorations and by comparing the characteristics of the d-axis and q-axis injected HFI sensorless controlled SynRM drives, the q-axis injection was adopted. Experimental observations indicated that the d-axis injected mechanism could not be operated stably above a certain load level. Hence, only the q-axis injection mechanism was tested further. Because of the influence of speed-and load-dependent slot harmonic effects on the sensorless control behavior, the changed injection frequency with rotor speed was proposed. The slotting effects were also considered in the current controller design. It was shown that the EV SynRM drive can be operated under wider speed and heavier load ranges with the designed feedback and robust controllers. To achieve maximum efficiency, ACS was applied to conduct a proper commutation shift of SynRM. The measured results indicated that the developed sensorless EV drive exhibits stable operation over a wide speed and load range, and the EV drive exhibits good driving performance in terms of acceleration/deceleration, reversible, and regenerative braking operations. The experimental demonstration of the high energy conversion efficiencies being comparable to the standard drive was also presented in this study.
Author Contributions
G. Vijay Kumar: Main author involving the measurements and the paper writing; Min-Ze Lu: Assisting the author and doing the proofreading; Chang-Ming Liaw: Advisor, giving suggestion and revisions for research results and writing.
Funding
No organization or foundation that funded this research.
Competing Interests
The authors have declared that no competing interests exist.
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