INVESTIGATION OF PERMANENT MAGNET SYNCHRONOUS MACHINES FOR DIRECT-DRIVE AND INTEGRATED CHARGING APPLICATIONS IN ELECTRIC VEHICLES

Electrified vehicles have proven to be potential candidates in the future for disrupting the automotive industry which is dominated by conventional gasoline vehicles. Electric vehicle (EV) technology has evolved rapidly over the last decade with new designs of EV drivetrain systems and components but no specific design has been able to serve as a solution that is affordable, reliable and performance-wise similar to existing gasoline vehicle equivalent. Extended driving range and overall cost of the vehicle still remain major bottlenecks. Understanding the state-of-the-art technologies and challenges in existing electric vehicle powertrain and charging systems, with major focus on permanent magnet synchronous machines & drives, this dissertation presents the following

Three-level neutral point-clamped (NPC) traction inverter drive for electric vehicles

The motivation of this project was to develop a three level neutral point clamped (NPC) traction inverter for a permanent magnet synchronous machine drive. The three-level inverter helps to reduce the total inverter losses at higher switching frequencies, compared to a two-level inverter for electric vehicle applications. The three-level inverter has also more power switches compared to the two-level inverter. This helps to reduce the voltage stress across the switches and the machine winding. In addition, it also allows an increase in the DC-link voltage, which in turn helps to reduce the DC-link current, phase conductor size and the associated losses. Moreover, at higher DC-bus voltages the power switches will have lower thermal stress when compared to the 2-level. However, the NPC inverter topologies have an inherent problem of DC-link voltage balancing. In the initial part of this thesis, a novel space vector based DC-link voltage balancing strategy is proposed. This strategy can keep the two DC-link capacitor voltages balanced during transient changes in both speed and torque. The performance of the three-level inverter system is then compared with a two-level inverter based drive to validate its performance improvement. The results showed a significant reduction in total voltage and current harmonic distortions, reduced total inverter losses (by 2/3rd) and was even was able to keep the neutral point fluctuation low at all operating load power factor conditions. The second motivation of this thesis was to reduce the computational time in the real-time implementation of the control logic. For this purpose, a modified carrier and hybrid-carrier based PWM strategy was proposed, which also kept the DC-link capacitor voltages balanced. The modified carrier based strategy was able to reduce the switching losses compared to the conventional strategies, while the hybrid-carrier based strategy kept the advantages of both carrier and the space vector techniques. Finally, a performance comparison study was carried out to compare the total harmonic distortion, switching loss distribution, and total inverter loss of all the four proposed strategies

Fuzzy logic based online adaptation of current and speed controllers for improved performance of IPMSM drive

Precise torque and speed control of electric motors is a key issue in industries for variable speed drives (VSD). Over the years the induction motors have been widely utilized in industries for VSD applications. However, induction motor has some significant drawbacks like low efficiency, lagging power factor, asynchronous speed, low torque density etc. Nowadays the interior permanent magnet synchronous motor (IPMSM) is becoming popular for high performance variable speed drive (HPVSD) due to its high torque-current ratio, large power-weight ratio, high efficiency, high power factor, low noise and robustness as compared to conventional induction and other ac motors. Smooth torque response, fast and precise speed response, quick recovery of torque and speed from any disturbance and parameter insensitivity, robustness in variable speed domain and maintenance free operations are the main concerns for HPVSD. This work proposes a closed loop vector control of an IPMSM drive incorporating two separate fuzzy logic controllers (FLCs). Among them one FLC is designed. to minimize the developed torque ripple by varying online the hysteresis band of the PWM current controller. Another Sugeno type FLC is used to tune the gains of a proportional-integral (PI) controller where the PI controller actually serves as the primary speed controller. Thus, the limitations of traditional PI controllers will be avoided and the performance of the drive system can be improved. A flux controller is also incorporated in such a way that both torque and flux of the motor can be controlled while maintaining current and voltage constraints. The flux controller is designed based on maximum-torque- per-ampere (MTPA) operation below the rated speed and flux weakening operation above the rated speed. Thus, the proposed drive extends the operating speed limits for the motor and enables the effective use of the reluctance torque. In order to verify the performance of the proposed IPMSM drive, first a simulation model is developed using Matlab/Simulink. Then the complete IPMSM drive has been implemented in real-time using digital signal processor (DSP) controller board DS1104 for a laboratory 5 HP motor. The effectiveness of the proposed drive is verified both in simulation and experiment at different operating conditions. In this regard, a performance comparison of the proposed FLC based tuned PI and adapted hysteresis controllers based drive with the conventional PI and fixed bandwidth hysteresis controllers based drive is provided. These comparison results demonstrate the better dynamic response in torque and speed for the proposed IPMSM drive over a wide speed range

Efficiency Optimization and Control of Permanent Magnet Synchronous Brushless Motors in Three-Phase Pulse Width Modulated Voltage Source Inverter Drives

In high performance drives where it is desirable to exploit the usefulness of reluctance torque and machine saliency, permanent magnet synchronous brushless motors are machines of choice. However, speed control of these machines especially in the flux weakening region becomes more complex due to the non-linear coupling among the winding currents as well as the nonlinearity present in the torque. While numerous research efforts in the past have considered control and efficiency improvements of induction motors, and synchronous motors with field windings, research efforts in developing an efficiency optimization and control strategy applicable to all salient-type permanent magnet synchronous brushless motors are still in their infancy.;A traditional control technique that has commonly been employed in efficiency improvement efforts is the stator\u27s zero d-axis current (i ds=0) technique. In this method, the rotor flux is aligned with the direct-axis so that the stator\u27s direct-axis current is zero and the torque becomes a linear function of the stator\u27s quadrature-axis current. Although this method achieves decoupling of winding currents and simplicity of control, it does not fully exploit the use of the machine\u27s saliency and reluctance torque, and is also not well-suited for wide-range load operations. The maximum torque per ampere (MTPA) technique is another less complex technique that has been considered which fully exploits the use of machine saliency with motor torque selected along the geometric curve of minimum-amplitude current space vectors for minimum loss operation. The drawback of the MTPA technique is that it does not provide high efficiency performance for synchronous reluctance motors running at low fractional loads.;In this work, the problem of efficiency optimization in the salient-type permanent magnet synchronous brushless motors is investigated. A machine model which includes the effect of core losses is proposed for developing a loss minimization algorithm that dynamically determines the optimal reference currents and voltages required for minimizing the total electrical losses (copper losses and core losses) within the feasible operating regions imposed by the motor and inverter capacities. The loss minimization strategy is implemented within a speed control loop for a synchronous reluctance motor drive and the effectiveness of the proposed scheme is validated by comparing performances with that of the traditional maximum torque per ampere and stator\u27s zero d-axis current vector control methods. It is shown that the proposed scheme offers the advantages of simplicity and superior performance throughout the entire operating range, and also improves motor efficiency to 96% at full load and full-speed operating condition

Multi-Phase Fault Tolerant PMSM Drive Systems

The drive to develop electric machines with a wide constant power-speed range (CPSR), high torque capabilities, excellent efficiency, superior reliability, and a reduced environmental footprint for EV traction and ship propulsion systems has led to research interest in various Permanent Magnet Synchronous Motors (PMSM). One particular area of interest is multi-phase fault tolerant PMSM drive systems, which are integral to the development of electric traction systems with all-inclusive motors that include sensors, inverter modules, and a cooling system, much like an automobile engine. Furthermore, these designs simplify fault condition maintenance because their independent single-phase structure allows them to be used with replaceable modular inverter units which have one H-bridge for each phase. In order to provide high reliability for the PMSM drive systems, even in a fault condition, simple but effective current control methods are necessary. An interior PMSM configuration with 5 independent phases is presented for electric vehicle (EV) traction and ship propulsion applications along with the proposed design procedure as well as an associated inverter design and current control methods. The proposed design process is verified using finite element analysis (FEA). An existing 5-phase 15-slot 4-pole Interior PMSM was modified to remove the neutral point, thus allowing for independent control of the 5 phases with 5 H-bridge inverters through a fabricated custom-made control board. Bipolar and unipolar switching methods were evaluated and an effective switching method was proposed to drive the motor. Closed loop speed control was implemented using Step VSI control, SPWM control, and hysteresis control methods. Finally, the 5-phase 10-lead PMSM systems were evaluated under the various control methods using simulated and experimental data after fabricating a new inverter interface board with TI floating point DSP, Delfino (F28335). The results suggest that multi-phase fault tolerant PMSM drive systems could play a key role in the future of EV traction and ship propulsion systems. An interior PMSM configuration with 5 independent phases is presented for electric vehicle (EV) traction and ship propulsion applications along with the proposed design procedure as well as an associated inverter design and current control methods. The proposed design process is verified using finite element analysis (FEA). An existing 5-phase 15-slot 4-pole Interior PMSM was modified to remove the neutral point, thus allowing for independent control of the 5 phases with 5 H-bridge inverters though a fabricated custom-made control board. Bipolar and unipolar switching methods were evaluated and an effective switching method was proposed to drive the motor. Closed loop speed control was implemented using Step VSI control, SPWM control, and hysteresis control methods. Finally, the 5-phase 10-lead PMSM systems were evaluated under the various control methods using simulated and experimental data after fabricating a new inverter interface board with TI floating point DSP, Delfino (F28335). The results suggest that multi-phase fault tolerant PMSM drive systems could play a key role in the future of EV traction and ship propulsion systems

An improved direct torque controlled interior permanent magnet synchronous machine drive without a speed sensor

Some essential and important improvements of the direct torque controlled interior permanent magnet (IPM) synchronous machine drive are presented in this thesis. These studies, including analysis, modeling and experimental implementations confirm the possibility of a high performance direct torque controlled IPM synchronous motor drive without any continuous rotor position and speed sensor and without any current controller. The direct torque control technique, the comparison between DTC and FOC, and compensation methods for the problems/limitations associated with DTC have been investigated in this thesis. A number of important problems that affect the accuracy of the estimated machine flux linkage on which the DTC technique is built are thoroughly examined. Estimation of stator resistance variation, analysis and compensation of the non-linear effects of the inverter such as forward voltage drop and dead-time, speed sensorless control, and torque and flux ripple minimization for a direct torque controlled IPM motor drive are of major concern in this thesis. A Proportional-Integral stator resistance estimator based on stator current has been investigated for the compensation of any variation in stator resistance. It is shown that the estimator can track the variation of the stator resistance adequately. The scheme utilizes the error between the actual current and the reference current and requires no position signal. Modeling and experimental results will be shown. The non-linear effects of the inverter affect flux estimation greatly, especially at low speed. The effects such as forward voltage drop, dead-time and switching delay is analyzed, they degrade the system performance by introducing error between the estimated values and the actual values. The effects of the forward voltage drop and deadtime can be compensated by using a look-up table. The performance improvement of the drive has been shown in experiments. A speed estimation scheme based on stator flux linkage estimation is adopted and investigated experimentally. Furthermore, the possibility of fielding-weakening operation of the speed sensorless control is also investigated by modeling. The torque and flux ripples are significant problems of the DTC, and are mentioned widely. In order to solve this problem, the changes of torque and flux linkage over a sampling period are derived. Based on the analysis, a modified DTC is proposed to overcome these significant problems. Modeling and experimental results confirm the effectiveness of the proposed scheme. The field weakening control and speed sensorless control scheme is also combined with the proposed scheme. The experimental results show the new DTC scheme can achieve wider range operation and speed sensorless control successfully. The torque and flux ripples are reduced greatly under the new scheme in all experimental results. These abovementioned studies have clearly established that the DTC technique for the IPM machine is now much closer to being a viable and cost-effective candidate for a sensorless PM synchronous motor drive

RECENT TECHNIQUES ON OBSERVER DESIGN FOR DISTURBANCE ESTIMATION AND REJECTION IN PERMANENT MAGNET SYNCHRONOUS MOTORS

Permanent magnet synchronous machines (PMSMs) (either motor or generator) have attracted attention of research community comparing to other types of AC machines in the recent two decades. PMSMs are preferable than other AC machines in terms of large power-factor, broad speed of operation, compact proportions, and effective operation. Unfortunately, different sources of nonlinearities, model uncertainties, and external perturbations determine severity in a design of accurate speed control scheme for PMSMs. In the era of developing science and technologies, many advanced control solutions are proposed to control PMSMs. Although new solutions show their advantages comparing to traditional methods in terms of performance evaluation, practical realization of those algorithms could require expensive hardware with high computational capabilities. Furthermore, people in industry with less knowledge about the motor control may experience difficulties in using such advanced controllers on their own. Traditional PI/PID control schemes still work as a major control technique in modern industry, and in motor control as well. Numerous positive facts about the PI/PID schemes make such superiority of these control schemes. Firstly, the PI/PID can be implemented easily on most industrial software and hardware components. Secondly, while its scheme has clear mechanism of operation, most industrial processes could be controlled via the PI/PID scheme. These schemes are good in terms of small number of parameters to tune and tuning process itself could be very straightforward. Finally, implementation of the PI/PID controllers would require smaller time comparing to most proposed complex control solutions. It is studied that the traditional PI/PID controllers usually cannot deal with unpredictable disturbances, which in turn leads to degraded performance of an overall control system. Inspired by the advantages and widespread application of PI/PID control structure in industry, we propose a disturbance observer based composite control scheme which uses the PI-like controller for the feedback regulation and disturbance observer for estimation of lumped disturbances presented in a PMSM control system. Under this circumstance, this thesis work proposes three different control solutions for PMSM such as High-order disturbance observer-based composite control (HDOBCC), Disturbance rejection PI (DR-PI) control, and Hierarchical optimal disturbance observer-based control (HODOBC). Furthermore, to deeply understand the similarity and difference between the traditional disturbance observer-based control (DOBC) and active-disturbance rejection control (ADRC) schemes, this thesis also presents results of unification of these two control approaches in the speed control of a PMSM. The HDOBCC as the first method proposed in this thesis is designed to improve reference speed tracking performance of a PMSM under various operational conditions. A structure of the HDOBCC comprises a fuzzy-PI controller in a feedback stabilization part and novel high-order disturbance observer in a feedforward compensation part of the speed control system. The proposed controller is designed based on the research questions such as: firstly, although a fixed gain traditional PI controller is able to present satisfactory performance at some extent, still it does not guarantee such performance when sudden disturbances occur in a system; secondly, many disturbance observers designed for a PMSM in literature consider only a load torque as a disturbance, neglecting model uncertainties and parameter variations in design stage. Therefore, the HDOBCC is proposed such that it utilizes a fuzzy approach to determine parameters of the PI controller to overcome limitations of the fixed gain PI controller. Furthermore, the proposed scheme includes a high-order disturbance observer, which estimates not only the load torque, but also disturbances due to model uncertainties and parameter variations. Moreover, extended simulation and experimental studies are conducted to affirm performance of the HDOBCC under various form of the load torque. In addition to commonly tested step form of a load torque, severe forms of the load torque such as triangular form and sinusoidal form are tested with the proposed controller. Stability analysis of the closed-loop HDOBCC system is further provided. The next proposed method, DR-PI control, is designed by seeking answer for questions such as: firstly, although the traditional DOBC scheme applied for PMSM shows reasonable results in a PMSM control, its design can be limited to known actual parameters of the PMSM. In practice, actual parameters are usually not available, hence it could be hard to design the traditional DOBC in the absence of a plant information; secondly, for tuning a PI controller the traditional Ziegler-Nichols tuning approach still remains as one of the popular tuning approaches, however it does not give a rigorous explanation on selection of parameters during its design. Consequently, to answer these questions, the DR-PI control is designed for the PMSM speed control. The DR-PI control is designed such that it has a simple PI-like structure with intrinsic disturbance rejection mechanism determined by the parameters of a filtering element, desired plant model, and desired closed-loop system. Simulation and experimental validations are provided to validate the performance of the DR-PI. Furthermore, gain tuning mechanism and stability analysis of the closed-loop DR-PI-based speed control are also presented. The HODOBC scheme as a third proposed control scheme targets on the next research questions as: first, parameters of the traditional PI controller are mostly obtained by trial-and-error approach, which in turn may not guarantee satisfactory results; in a cascaded PMSM control, the outer speed loop performance highly depends on the performance of the inner current loop. The well-tuned speed control loop may degrade in performance, if the inner current loop is not tuned properly. To address these questions, we propose the HODOBC scheme, which consists of optimal PIlike controller in the feedback stabilization part and optimal extended-state observer (ESO) in the disturbance compensation part. The proposed HODOBC showed better performance when it is compared with other traditional controllers via experiments. Stability analysis is provided via the root locus approach. The study on unification of the DOBC and ADRC schemes has the following research question: the DOBC and ADRC are both used in estimation of total disturbance, but these two schemes are considered differently in literature. Hence, the study of both scheme is conducted to show the condition at which these two schemes show identical performance. The analysis of the traditional DOBC and ADRC schemes concludes that both scheme are equivalent in terms of performance characteristics if the dynamical delays of disturbance observers in each scheme are same. The results of analysis reveal that both scheme can be utilized to design a robust control system for PMSM, i.e. once the gains of disturbance observers can be calculated under the DOBC framework, further the disturbance rejection mechanism can be achieved via the ADRC framework. The results of PMSM control with the proposed control schemes have been tested on the Lucas-Nuelle DSP-based experimental setup

Design and implementation of a loss optimization control for electric vehicle in-wheel permanent-magnet synchronous motor direct drive system

As a main driving force of electric vehicles (EVs), the losses of in-wheel permanent-magnet synchronous motor (PMSM) direct drive system can seriously affect the energy consumption of EVs. This paper proposes a loss optimization control strategy for in-wheel PMSM direct drive system of EVs which optimizes the losses of both the PMSM and the inverter. The proposed method adjusts the copper losses and iron losses by identifying the optimal flux-weakening current, which results in the PMSM achieving the lower losses in the whole operational range. Moreover there are strongly nonlinear characteristics for the power devices, this paper creates a nonlinear loss model for three-phase half-bridge inverters to obtain accurate inverter losses under space vector pulse width modulation (SVPWM). Based on the inverter loss model and double Fourier integral analysis theory, the PWM frequency is optimized by the control strategy in order to maximize the inverter efficiency without affecting the operational stability of the drive. The proposed loss optimization control strategy can quickly find the optimum flux-weakening current and PWM frequency, and as a result, significantly broaden the high efficiency area of the PMSM direct drive system. The effects of the aforementioned strategy are verified by both theoretical analysis and experimental results

Sensorless position estimation in fault-tolerant permanent magnet AC motor drives with redundancy.

Safety critical applications are heavily dependent on fault-tolerant motor drives being capable of continuing to operate satisfactorily under faults. This research utilizes a fault-tolerant PMAC motor drive with redundancy involving dual drives to provide parallel redundancy where each drive has electrically, magnetically, thermally and physically independent phases to improve its fault-tolerant capabilities. PMAC motor drives can offer high power and torque densities which are essential in high performance applications, for example, more-electric airplanes. In this thesis, two sensorless algorithms are proposed to estimate the rotor position in a fault-tolerant three-phase surface-mounted sinusoidal PMAC motor drive with redundancy under normal and faulted operating conditions. The key aims are to improve the reliability by eliminating the use of a position sensor which is one of major sources of failures, as well as by offering fault-tolerant position estimation. The algorithms utilize measurements of the winding currents and phase voltages, to compute flux linkage increments without integration, hence producing the predicted position values. Estimation errors due measurements are compensated for by a modified phase-locked loop technique which forces the predicted positions to track the flux linkage increments, finally generating the rotor position estimate. The fault-tolerant three-phase sensorless position estimation method utilizes the measured data from the three phase windings in each drive, consequently obtaining a total of two position estimates. However, the fault-tolerant two-phase sensorless position estimation method uses measurements from pairs of phases and produces three position estimates for each drive. Therefore, six position estimates are available in the dual drive system. In normal operation, all of these position estimates can be averaged to achieve a final rotor angle estimate in both schemes. Under faulted operating conditions, on the other hand, a final position estimate should be achieved by averaging position estimates obtained with measurements from healthy phases since unacceptable estimation errors can be created by making use of measured values from phases with failures. In order to validate the effectiveness of the proposed fault-tolerant sensorless position estimation schemes, the algorithms were tested using both simulated data and offline measured data from an experimental fault-tolerant PMAC motor drive system. In the healthy condition, both techniques presented good performance with acceptable accuracies under low and high steady-state speeds, starting from standstill and step load changes. In addition, they had robustness against parameter variations and measurement errors, as well as the ability to recover quickly from large incorrect initial position information. Under faulted operating conditions such as sensor failures, however, the two-phase sensorless method was more reliable than the threephase sensorless method since it could operate even with a faulty phase.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 201