[Show abstract][Hide abstract] ABSTRACT: Although the concept of a multiphase drive system dates back to the middle of the 20th century, the initial pace of development was rather slow, as witnessed by the first two surveys of the area published in the beginning of this century. However, considerably new developments have resulted in the last decade of the 20th century and the beginning of this century, leading to an authoritative survey of the asymmetrical six-phase drive control and subsequently of the review of the complete area. This also initiated the organization and subsequent publication of the first IEEE Transactions on Industrial Electronics "Special Section on Multiphase Machines and Drives" in May 2008, which commenced with another survey paper, and that contained 12 original research papers. Since the publication of this Special Section in May 2008, the level of interest and pace of developments in the area have further accelerated and substantial new knowledge has been generatedwith an ever-increasing number of published research papers and reported new industrial applications. Such a trend has been emphasized in a recent paper. It therefore seemed appropriate to revisit the area and organize this Special Section as a sequel to the first one. The call for the Special Section papers resulted in 51 submissions, almost twice as many as the total back in 2008, thus confirming a substantial growth of the area. Indeed, the amount of new knowledge acquired since the publication of the first Special Section in 2008 has meant that it was not possible to provide a complete and thorough survey of the field in a single review paper.
Full-text · Article · Jan 2016 · IEEE Transactions on Industrial Electronics
[Show abstract][Hide abstract] ABSTRACT: In this paper, we introduce an improved ac current controller with robustness against the noise in the feedback path. This three-phase current controller is suited for inverter supplied ac machines and grid-connected power converters. Conventional solutions make use of symmetric pulse-width modulation (PWM) techniques with feedback sampling in the middle of the voltage pulses. Single-sample-based feedback acquisition gets affected by the noise and parasitic phenomena. We perform a thorough analytical and experimental study on feedback errors with conventional sampling, and we consider the impact of the lockout time, motor cables, winding capacitance, and anti-aliasing filters. In order to suppress a significant spectral content in the area of low-order harmonics, we apply an improved acquisition technique, which uses a period-average and removes any PWM noise from the feedback signals. Both anti-aliasing filter and proposed period-average filter introduce delays, which impair the control-loop performance. The conventional current controller is then extended to suppress the effects of the delays. Parameter setting procedure is devised to achieve both the bandwidth and the robustness against the parameter changes. Analytical and experimental studies prove that the proposed feedback acquisition technique improves the robustness in the presence of variable delays and switching transients. Experimental tests show that the extended current controller with period-average feedback acquisition reaches the same bandwidth and robustness as the state-of-the-art controller.
No preview · Article · Oct 2015 · IEEE Transactions on Energy Conversion
[Show abstract][Hide abstract] ABSTRACT: Two four-level five-phase open-end winding drives are analysed in this paper, and their performances compared. The first topology consists of two two-level voltage source inverters (VSI) supplying a five-phase machine with open-end windings (OeW). The second topology comprises one three-level and one two-level VSI to supply the five-phase OeW machine. In both cases, two VSIs are supplied from isolated dc-sources, with voltages in a ratio 21. As a consequence, the output phase-voltage waveforms are equivalent to those produced by a four-level five-phase VSI. The paper shows that the considered topologies exhibit significantly different operating characteristics.
[Show abstract][Hide abstract] ABSTRACT: This study discusses the space vector pulse-width modulation (SVPWM) scheme for an eleven-phase two-level voltage source inverter, aimed at producing a sinusoidal output voltage waveform. Generalised space vector theory is used to realise the SVPWM. As per the general inverter switching theory, there are 211 = 2048 switching states that yield 2046 active voltage space vectors and one zero voltage vector, which results with two switching states. Out of the total of 2046 active voltage vectors, the most suitable set comprising 110 active voltage vectors is identified and is utilised in the implementation of the SVPWM. The sinusoidal voltage is obtained by controlling the duty cycles of the applied voltage space vectors in such a way that the non-zero reference voltage in the first (d-q) plane is achieved, while simultaneously zeroing the average voltage in the other four (x-y) planes in accordance with the zero references. The theoretical results are verified by experimentation using a passive resistive-inductive load. Finally, experimentally obtained total harmonic distortion values of the phase voltage and current for the eleven-phase SVPWM are compared with the corresponding values obtained using SVPWM for other odd phase numbers.
No preview · Article · Jun 2015 · IET Power Electronics
[Show abstract][Hide abstract] ABSTRACT: This paper considers an integrated onboard charger for electric vehicles that incorporates an asymmetrical nine-phase machine and an inverter into the charging process. The charging is from three-phase mains, and it employs exclusively the power electronic components that already exist on board the vehicle and that are mandatory for the propulsion. No new elements are introduced. Moreover, the charging is achieved without any hardware reconfiguration since the existing elements and their connections are not altered during the transfer from propulsion to the charging mode. Instead, the operating principle is based on additional degrees of freedom that exist in nine-phase machines. These degrees of freedom are employed to avoid electromagnetic torque production in the machine during the charging process, although currents flow through its stator windings. The configuration operates with a unity power factor and is capable of vehicle to grid (V2G) operation as well. A detailed theoretical analysis is given, and the control for the charging/V2G and propulsion modes is discussed. Theoretical analysis is validated by experiments for charging, V2G, and propulsion operating regimes.
No preview · Article · May 2015 · IEEE Transactions on Industrial Electronics
[Show abstract][Hide abstract] ABSTRACT: A four-level five-phase open-end winding (OeW) drive topology is introduced in the paper. The drive comprises a five-phase induction machine with open-end stator windings, supplied using two two-level voltage source inverters with isolated and unequal dc-link voltages, in the ratio 2:1. The topology offers the advantages of a modular structure with fewer semiconductor components and has a greater potential for fault tolerance, compared to an equivalent single-sided fourlevel drive. Due to the large number of switching states, development of a suitable space-vector pulse-width modulation (SVPWM) method can be challenging. Hence, this paper examines the implementation of two level-shifted carrier-based modulation (CBPWM) methods. The effect of dead time on the drive performance is discussed and it is shown that simultaneous PWM switching of both inverters can lead to degraded output phase voltage waveforms. Detailed analysis of this phenomenon is presented, a solution is proposed, and the modified modulation techniques are incorporated in an experimental set-up, at first in conjunction with V/f control. Once when the proof-of-concept has been provided, full field oriented control (FOC) is implemented in this OeW drive topology for the first time; detailed experimental testing is conducted and results are reported.
No preview · Article · Apr 2015 · IEEE Transactions on Industrial Electronics
[Show abstract][Hide abstract] ABSTRACT: The analysis and assessment of the pulsewidth modulation (PWM) techniques is commonly based on the comparison of the total harmonic distortion (THD) results. THD is usually calculated by application of the Fourier transformation and by taking a limited number of harmonics into the consideration. In this paper, derivation of analytical formulas for the phase voltage THD is presented. The considered system is a symmetrical multiphase star-connected load, supplied from a multilevel pulsewidth-modulated voltage-source inverter (VSI, three-phase case is also covered). The solution is based on the Parseval's theorem, which links frequency spectrum and time domain through the average power (i.e., root-mean-square (rms) squared value) of the signal. The assumption throughout the derivations is that the ratio of the switching to fundamental frequency is high. Derivations are based on the integration of the power of the PWM signal in a single switching period over the fundamental period of the signal. Only ideal sinusoidal reference voltages are analyzed, and no injection of any type is considered. Formulas for phase voltage THD for any number of phases are derived for two- and three-level cases, for the most commonly used carrier-based methods. Comparison of the analytically obtained curves with simulation and experimental results shows a high level of agreement and validates the analysis and derivations.
No preview · Article · Mar 2015 · IEEE Transactions on Power Electronics
[Show abstract][Hide abstract] ABSTRACT: An obvious obstacle to a wider utilization of electric vehicles (EVs) is the absence of a widely available charging network. Direct charging of EV batteries with dc voltage in charging stations requires deployment of expensive power electronics equipment. On the other hand, placing the same equipment on-board an EV would enable access to any three-phase grid outlet, but would substantially increase the vehicle cost, weight and volume. A multitude of solutions have been proposed to integrate the charger using the already existing equipment, which normally serves the propulsion operating mode of the vehicle. However, only a few of them have the advantage of torque-free fast charging that is accomplished without any hardware reconfiguration. The primary aim of these integrated on-board chargers is to provide fast charging by enabling connection to a three-phase grid. These structures also enable slower charging by connection to a single-phase grid. This paper examines single-phase charging capabilities of an integrated charger using a nine-phase machine. The charger does not require any reconfiguration or additional elements other than those used in propulsion mode and it does not produce torque during the charging. Additionally, the charger can provide energy recuperation to the grid. The operating principle relies on additional degrees of freedom of multiphase machines, to draw currents through non-flux and non-torque producing planes of the machines.
[Show abstract][Hide abstract] ABSTRACT: The fault tolerance provided by multiphase machines is one of the most attractive features for industry applications where a high degree of reliability is required. Aiming to take advantage of such post-fault operating capability, some newly designed full-power energy conversion systems are selecting machines with more than three phases. Although the use of parallel converters is usual in high-power three-phase electrical drives, the fault tolerance of multiphase machines has been mainly considered with single supply from a multiphase converter. This work addresses the fault-tolerant capability of six-phase energy conversion systems supplied with parallel converters, deriving the current references and control strategy that need to be utilized to maximize torque/power production. Experimental results show that it is possible to increase the post-fault rating of the system if some degree of imbalance in the current sharing between the two sets of three-phase windings is permitted.
Full-text · Article · Jan 2015 · IEEE Transactions on Power Electronics
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a new class of on-board chargers for electric vehicles (EVs). Instead of being placed on-board as a separate unit, the three-phase (fast) chargers reutilize the existing components in EVs, which are already used for the propulsion. These are primarily the inverter and the machine, which however have to be multiphase (with more than three phases). The concept is valid for all multiphase propulsion drives with a prime number of phases higher than three and a single neutral point in motoring and is illustrated in detail for the five-phase inverter/five-phase machine configuration. During the charging mode, electromagnetic torque is not produced in the machine so that the rotor does not require mechanical locking. Hardware reconfiguration between propulsion and fast charging is required, but it is achieved with only two switches, which are the only two nonintegrated elements. The integrated topology is explained in this paper, together with the control scheme, and extension from five phases to higher phase numbers is illustrated using the seven-phase system as an example. Finally, the propulsion-mode operation with complete suppression of low-order harmonics, which map into the second plane, is achieved for the five-phase machine. Experimental verification of theoretical results and proposed control is provided for both charging and vehicle-to-grid mode of operation, as well as for propulsion.
No preview · Article · Jan 2015 · IEEE Transactions on Power Electronics
[Show abstract][Hide abstract] ABSTRACT: Independently controlled multimotor drives are typically realized by using a common dc link and independent sets of three-phase inverters and motors. In the case of an open-circuit fault in an inverter leg, one motor becomes single phase. To enable continued controllable operation by eliminating single phasing, the supply for the motor phase with the faulted inverter leg can be paralleled to a healthy leg of another inverter using hardware reconfiguration. Hence, the two motors are now supplied from a five-leg inverter, which has inherent voltage and current limitations. Theoretically, violating the voltage limit leads to inverter overmodulation and large torque oscillations. It is shown here that the finite-control-set model predictive control, designed to control the machines' stator flux and torque, can consider the inherent voltage limit dynamically in the control loop. Apart from preserving the independent control of the two machines, the additional constraint consideration significantly widens the operating speed ranges of the machines. In particular, it is shown that, whenever the voltage limit is entered, the controller reduces the stator flux level automatically, without requiring external flux reference change. The obtained performance is illustrated using experimental results and is also compared to the conventional two-motor field-oriented control scheme. The control concept is thus fully experimentally verified.
No preview · Article · Dec 2014 · IEEE Transactions on Industrial Electronics
[Show abstract][Hide abstract] ABSTRACT: The paper presents a study of postfault control for an asymmetrical six-phase induction machine with single and two isolated neutral points, during single open-phase fault. Postfault control is based on the normal decoupling (Clarke) transformation, so that reconfiguration of the controller is minimized. Effect of the single open-phase fault on the machine equations under this control structure is discussed. Different modes of postfault operation are analyzed and are further compared in terms of the achievable torque and stator winding losses. Validity of the analysis is verified using experimental results obtained from a six-phase induction motor drive prototype.
Full-text · Article · Oct 2014 · IEEE Transactions on Power Electronics
[Show abstract][Hide abstract] ABSTRACT: The paper studies pulse width modulation (PWM) techniques for a five-phase multilevel open-end winding drive with two inverters supplied with unequal dc-link voltages, which are in the ratio 2:1. It is shown in the paper that application of in-phase disposition modulation (PD-PWM), often used in multiphase multilevel converters, results in overcharging of the capacitor in the dc-link of the converter intended to operate at the lower dc voltage. The voltage space vector combinations which lead to the overcharging are identified and two decoupled modulation techniques which do not activate the troublesome vector combinations are proposed. The performance of the developed modulation techniques is investigated using simulations and an experimental prototype, and the results are presented in the paper.
[Show abstract][Hide abstract] ABSTRACT: A novel type of fast on-board battery charger, applicable for four-motor electric vehicles (EVs), is presented in the paper. The charger consists exclusively of components that are already present on-board the vehicle. Having no new elements, it has a positive impact on the cost, weight and space saving in the vehicle. A three-phase grid is directly attached to the neutral points of three propulsion machines, so that hardware reconfiguration with respect to the propulsion mode of operation is not required. The charger operates with unity power factor and both charging and vehicle-to-grid (V2G) modes are feasible. The torque is not produced in the machines during the charging/V2G process. The charger is particularly suited for the interleaving process, which is used to improve the quality of the current that is taken/injected into the grid. A complete control algorithm for the charging/V2G operation is given, and performance of the charger, including torque-free operation feature, is validated by simulations.
[Show abstract][Hide abstract] ABSTRACT: A two-motor drive, supplied by a five-leg inverter, is considered in this paper. The independent control of machines with full dc-bus voltage utilization is typically achieved using an existing pulsewidth modulation (PWM) technique in conjunction with field-oriented control, based on PI current control. However, model predictive control (MPC), based on a finite number of control inputs [finite-control-set MPC (FCS-MPC)], does not utilize a pulsewidth modulator. This paper introduces three FCS-MPC schemes for synchronous current control in this drive system. The first scheme uses all of the available switching states. The second and third schemes are aimed at reducing the computational burden and utilize a reduced set of voltage vectors and a duty ratio partitioning principle, respectively. Steady-state and transient performances are analyzed and compared both against each other and with respect to the field-oriented control based on PI controllers and PWM. All analyses are experimental and use the same experimental rig and test conditions. Comparison of the predictive schemes leads to the conclusion that the first two schemes have the fastest transient response. The third scheme has a much smaller current ripple while achieving perfect control decoupling between the machines and is of low computational complexity. Nevertheless, at approximately the same switching loss, the PI-PWM control yields the lowest current ripple but with slower electrical transient response.
No preview · Article · Aug 2014 · IEEE Transactions on Industrial Electronics
[Show abstract][Hide abstract] ABSTRACT: Control techniques for electrical machines have been applied for many years to what are called electrical drives (EDs) or adjustable speed drives (ASDs), which are part of almost 25% of applications of electrical machines in the industry. Therefore, this technology has followed the evolution of performances of both power converters and electrical actuators or generators for more than 50 years and the invention of silicon-controlled rectifiers. The aim of this article is not to cover a survey of EDs or ASDs since it has already been covered recently but rather to present trends of the control part related to this technology. Of course, the control part is fully dependent of the power part and cannot be treated as a stand-alone technique.