E. Levi

Liverpool John Moores University, Liverpool, England, United Kingdom

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Publications (189)300.65 Total impact

  • [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.
    IEEE Transactions on Industrial Electronics 12/2014; 61(12):6603-6614. · 6.50 Impact Factor
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    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.
    Energy Conversion Congress and Exposition (ECCE), Pittsburgh, PA, USA; 09/2014
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    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.
    IEEE Transactions on Industrial Electronics 08/2014; 61(8):3867-3878. · 6.50 Impact Factor
  • 7th IET International Conference on Power Electronics, Machines and Drives,, Manchester; 05/2014
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    ABSTRACT: Open-end winding three-phase drive topologies have been extensively investigated in the last two decades. In the majority of cases supply of the inverters at the two sides of the winding is provided from isolated dc sources. Recently, studies related to multiphase open-end winding drives have also been conducted, using isolated dc sources at the two winding sides. This paper investigates for the first time a five-phase open-end winding configuration, which is obtained by connecting a two-level five-phase inverter at each side of the stator winding, with both inverters supplied from a common dc source. In such a configuration it is essential to eliminate the common-mode voltage (CMV) that is inevitably created by usual PWM techniques. Based on the vector space decomposition (VSD), the switching states that create zero CMV are identified and plotted. A space vector pattern with large redundancy of switching states is obtained. Suitable space vectors are then selected to realize the required voltage reference at the machine terminals with zero CMV. The large number of redundant states enables some freedom in the choice of switching states to impress these space vectors. Out of numerous possibilities, two particular switching sequences are chosen for further investigation. Both are implemented in an experimental setup, and the results are presented and discussed.
    IEEE Transactions on Industrial Electronics 05/2014; 61(5):2197-2207. · 6.50 Impact Factor
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    ABSTRACT: This paper proposes a noninvasive temperature measurement technique of indirect type, applicable in the permanent magnet synchronous motor (PMSM) drives. The motor temperature is required for protection and monitoring purposes, as well as for updating temperature-dependent control parameters. Direct measurement with dedicated sensors requires peripherals and cabling; hence, it is quite involved and often avoided. Temperature estimation based on test signal injection contributes to torque ripple and often relies on other motor parameters. A solution is proposed here, which makes the use of intrinsic pulse-width modulation excitation and does not use electrical or thermal parameters of the considered PMSM. The temperature of the stator winding is estimated from the motor input impedance $bm Z_{bf IN}$(ω) calculated over the range of frequencies starting at and going well beyond $bm f_{bf PWM}$. The paper includes analytical considerations, implementation details and experimental verification obtained with a 4.5-kW PMSM used in battery-supplied propulsion systems.
    IEEE Transactions on Energy Conversion 03/2014; 29(1):215-223. · 3.35 Impact Factor
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    ABSTRACT: This paper presents an investigation of the finite-control-set model predictive control (FCS-MPC) of a five-phase induction motor drive. Specifically, performance with regard to different selections of inverter switching states is investigated. The motor is operated under rotor flux orientation, and both flux/torque producing (d-q) and nonflux/torque producing (x-y) currents are included into the quadratic cost function. The performance is evaluated on the basis of the primary plane, secondary plane, and phase (average) current ripples, across the full inverter's linear operating region under constant flux-torque operation. A secondary plane current ripple weighting factor is added in the cost function, and its impact on all the studied schemes is evaluated. Guidelines for the best switching state set and weighting factor selections are thus established. All the considerations are accompanied with both simulation and experimental results, which are further compared with the steady-state and transient performance of a proportional-integral pulsewidth modulation (PI-PWM)-based current control scheme. While a better transient performance is obtained with FCS-MPC, steady-state performance is always superior with PI-PWM control. It is argued that this is inevitable in multiphase drives in general, due to the existence of nonflux/torque producing current components.
    IEEE Transactions on Industrial Electronics 01/2014; 61(1):149-163. · 6.50 Impact Factor
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    ABSTRACT: This paper discusses the operation of a multiphase system, which is aimed at both variable-speed drive and generating (e.g., wind energy) applications, using back-to-back converter structure with dual three-phase machine-side converters. In the studied topology, an asymmetrical six-phase induction machine is controlled using two three-phase two-level voltage source converters connected in series to form a cascaded dc link. The suggested configuration is analyzed, and a method for dc-link midpoint voltage balancing is developed. Voltage balancing is based on the use of additional degrees of freedom that exist in multiphase machines and represents entirely new utilization of these degrees. The validity of the topology and its control is verified by simulation and experimental results on a laboratory-scale prototype, thus proving that it is possible to achieve satisfactory dc-link voltage control under various operating scenarios.
    IEEE Transactions on Industrial Electronics 01/2014; 61(1):164-176. · 6.50 Impact Factor
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    ABSTRACT: Using the vector space decomposition approach, the currents in a multiphase machine with distributed winding can be decoupled into the flux and torque producing α-β components, and the loss-producing x-y and zero-sequence components. While the control of α-β currents is crucial for flux and torque regulation, control of x-y currents is important for machine/converter asymmetry and dead-time effect compensation. In this paper, an attempt is made to provide a physically meaningful insight into current control of a six-phase machine, by showing that the fictitious x-y currents can be physically interpreted as the circulating currents between the two three-phase windings. Using this interpretation, the characteristics of x-y currents due to the machine/converter asymmetry can be analyzed. The use of different types of x-y current controllers for asymmetry compensation and suppression of dead-time-induced harmonics is then discussed. Experimental results are provided throughout the paper, to underpin the theoretical considerations, using tests on a prototype asymmetrical six-phase induction machine.
    IEEE Transactions on Power Electronics 01/2014; 29(1):407-417. · 5.73 Impact Factor
  • E. Levi, M. Jones, O. Dordevic
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    ABSTRACT: In this paper derivation of an analytical formula for the leg voltage THD is presented. The considered system is a leg of a multilevel pulse width modulated (PWM) voltage source inverter (VSI). The solution is based on the Parseval's theorem. The assumption throughout the derivations is that the ratio of the switching to the 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 an ideal sinusoidal reference leg voltage is analysed. Analytical expression for the leg voltage THD is given for any number of levels. Validity of the derived analytical equations is confirmed by simulations and experiments.
    7th IET International Conference on Power Electronics, Machines and Drives (PEMD 2014); 01/2014
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    ABSTRACT: Multiphase electric drives have been recently proposed for applications where the highest overall system reliability and a reduction in the total power per phase are required. Strategies derived from the conventional field-oriented control have been traditionally used in high-performance speed-control applications of multiphase drives. The well-known direct torque control (DTC) technique has been also applied in the multiphase case, but the achieved performance with a hysteresis-control-based approach is far from that obtainable with the three-phase drive, although the control structure is actually more complex. In this paper, a predictive-torque-control method is introduced as an alternative to the DTC technique for the high-performance variable-speed operation of multiphase drives. Simulation and experimental results are provided to illustrate the properties of the developed method.
    IEEE Transactions on Industrial Electronics 08/2013; 60(8):2957-2968. · 6.50 Impact Factor
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    ABSTRACT: The study discusses development, implementation and performance of carrier-based pulse-width modulation (PWM) techniques for an asymmetrical six-phase open-end winding drive supplied by two two-level six-phase voltage source inverters (VSIs). The VSIs are fed from two equal and mutually isolated dc voltage sources, so that the same performance as with a three-level VSI in single-sided supply mode can be achieved. The considered modulation techniques are the in-phase disposition (PD-PWM) and the alternate phase opposition disposition, which are both classified as level-shifted PWM methods, and the phase-shifted PWM method. The performance is studied and compared using computer simulations and experiments undertaken on a laboratory prototype. It is shown that, in contrast to the three-phase and five-phase cases, the PD-PWM method does not offer the best performance.
    IET Electric Power Applications 07/2013; 7(6):441-452. · 1.31 Impact Factor
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    ABSTRACT: Multilevel inverter supplied multiphase variable-speed drive systems have in recent times started attracting more attention, due to various advantages that they offer when compared to the standard three-phase two-level drives. For proper functioning of such systems good pulsewidth modulation (PWM) strategy is of crucial importance. Control complexity of multiphase multilevel inverters increases rapidly with an increase in the number of phases and the number of levels. This paper deals with a three-level neutral point clamped (NPC) inverter supplied five-phase induction motor drive and analyses five PWM strategies: three are carrier-based (CBPWM) and two are space vector based (SVPWM). The aim is to provide a detailed comparison and thus conclude on pros and cons of each solution, providing a guideline for the selection of the most appropriate PWM technique. Experimental results are provided for all analysed PWM methods. The comparison of the PWM techniques is given in terms of the voltage and current waveforms and spectra, as well as the total harmonic distortion (THD) in a whole linear modulation index range, which is used as the global figure of merit. Properties of the common mode voltage (CMV) are also investigated. Complexity of the algorithms, in terms of the computational time requirements and memory consumption, is addressed as well. It is shown that the performance of the PWM techniques is very similar and that one CBPWM and one SVPWM technique are characterised with identical performance. However, using the algorithm complexity as the main criterion, space vector techniques are more involved.
    IEEE Transactions on Industrial Informatics 05/2013; 9(2):609-619. · 8.79 Impact Factor
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    ABSTRACT: This paper discusses the implementation of level-shifted and phase-shifted carrier-based modulation methods, in conjunction with a multiphase open-end winding drive topology. The considered drive is supplied using two five-phase two-level voltage source inverters (VSIs), with input provided from two isolated supplies of equal dc voltages. The topology is known to yield the same space-vector pattern as a corresponding three-level inverter in single-sided supply mode. It is shown in this paper that, with the application of a simple logic, the same phase voltage waveforms result as those obtainable with the appropriate carrier-based modulation scheme applied to the three-level VSI in single-sided supply mode. While the outcomes of the modulation techniques are the same, the open-end winding topology offers certain advantages, such as modularity and absence of capacitor voltage balancing requirements. The analysis is conducted for selected modulation methods using voltage and current waveforms, spectra Fast Fourier Transform (FFT), and total harmonic distortion as figures of merit. Theoretical considerations are verified by means of simulation and experimental results.
    IEEE Transactions on Industrial Electronics 05/2013; 60(5):2054-2065. · 6.50 Impact Factor
  • N. Bodo, M. Jones, E. Levi
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    ABSTRACT: Although three-phase open-end winding drives have been under investigation for two decades, there is hardly any work available related to multiphase (more than three phases) open-end winding drive structures. This paper extends one possible PWM technique, recently introduced in conjunction with five-phase drives, to seven- and nine-phase open-end winding structures. Both multiphase inverters are two-level and require two isolated dc sources of equal voltage values. The technique, termed unequal reference sharing (URS), is explained and subsequently applied in simulations to examine the performance of the drive systems. Benchmarking is performed with respect to the equivalent single-sided supply with a two-level inverter and with respect to another PWM method recently developed for the dual-inverter supply structure. This enables a comparison of the achievable performance in single-sided and dual-inverter supply modes. Simulation results are given for both phase numbers. The THDs, obtained in this manner, are compared throughout the modulation index range for both phase numbers as well. The simulation results are verified experimentally on a seven-phase laboratory set-up in the whole linear modulation index range.
    Mathematics and Computers in Simulation 04/2013; 90:15–27. · 0.86 Impact Factor
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    ABSTRACT: The paper considers on-board battery charging of electrical vehicles (EV) using a three-phase voltage source and a nine-phase propulsion motor. The nine-phase inverter and induction machine are fully integrated into the charging process. The proposed integrated on-board battery charger has an advantage of unity power factor operation with no torque production in the machine during the charging mode. Moreover, there is no need for any hardware reconfiguration between the charging and propulsion mode. The principle of the charging mode operation is based on the additional degrees of freedom that exist in nine-phase machines and that can be conveniently utilized to achieve charging through the machine's stator windings with zero electromagnetic torque. Detailed theoretical analysis is reported for asymmetrical and symmetrical nine-phase systems. For both systems control in the charging mode is discussed, and the theoretical considerations are validated by simulations.
    Electric Machines & Drives Conference (IEMDC), 2013 IEEE International; 01/2013
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    ABSTRACT: This paper presents a flux and torque control scheme, based on finite-control-set model predictive control (FCS-MPC), for two three-phase induction motors supplied by a five-leg two-level inverter. The reduced-switch-count topology with leg sharing inherently imposes an additional constraint on the voltages in the system. In the best available PWM-based control scheme for this topology, the constraint means that, in simple terms, the sum of speeds of two machines cannot exceed the rated speed of one machine, in order to avoid over-modulation and large torque oscillations. In essence, no provision exists to account for the additional voltage limit of the topology. It will be shown here that the FCS-MPC can consider the voltage constraint dynamically in the control loop, and hence, apart from preserving the independent control of the two machines, it can significantly widen the speed operating range. Three different cost functions, corresponding to three operating modes, are considered. The unique way in which the MPC handles tracking errors allows the motors to operate dynamically in the base speed region with field weakening, without requiring any external change of the flux references. Simulation and preliminary experimental results verify the theory.
    Electric Machines & Drives Conference (IEMDC), 2013 IEEE International; 01/2013
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    ABSTRACT: Model Based Predictive Current Control (MBPCC) techniques have been recently proposed as a viable control method for Voltage Source Converters (VSCs). Predictive torque and current control of electrical drives are drawing attention because of the good current tracking, flexible control design and reduced switching losses. The multiphase drive is an attractive proposition due to its usefulness in applications where high overall system reliability and reduction in the total power per phase are required. The predictive model of the drive is the core of the MBPCC technique and it depends on the knowledge of the parameters of the real system. Previous works assumed good agreement between parameters of the predictive model and the real machine, on the basis of the initial estimation of the electrical parameters using off-line procedures and the influence of the parameter variations on the drive performance has not been thoroughly investigated until now. This paper attempts to fill in this gap, by examining the impact of the variation of electrical parameter, used in the predictive model, on the performance of an asymmetrical six-phase induction motor drive.
    Industrial Electronics Society, IECON 2013 - 39th Annual Conference of the IEEE; 01/2013
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    ABSTRACT: Parameter identification of multiphase machines is a new and interesting topic in the development of multiphase drive systems. Regardless of the applied control technique, an accurate knowledge of the parameters is required to ensure high-performance operation of the machine. This also applies to the magnetizing inductance of the machine. Available identification schemes for multiphase induction machines utilize AC and time-domain methods, some of which require non-conventional winding arrangement or a combination of different procedures that need tests in the non-flux/torque producing plane(s). This paper introduces a simple magnetizing inductance identification technique, which relies on an induced DC voltage test. It is an extension of a procedure previously proposed for the three-phase case to the five-phase induction machine. Experimental results illustrate the reliability and validity of the technique using two different five-phase induction motor drives.
    Industrial Electronics Society, IECON 2013 - 39th Annual Conference of the IEEE; 01/2013
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    ABSTRACT: Model predictive control (MPC) for a two-motor drive, supplied from a five-leg inverter, is presented in this paper. As an alternative to existing methods, use of MPC in multimachine drives has the advantages of independent fast current control of the machines, elimination of the closed-loop system's cascaded structure, and a reduced number of microcontrollers. A vector control algorithm is required, necessitating state-space modeling, with each machine's direct- and quadrature-axis currents chosen as state variables. Prediction of future states is via a discrete-time model of the five-leg inverter and a piecewise-affine model of two permanent-magnet synchronous motors (PMSMs). A method which eliminates unfeasible switching states inherent in reduced-switch-count inverters while reducing computation and sampling times is proposed. The algorithm is implemented in a TMS320F28335 DSP microcontroller, which controls the five-leg inverter and the two PMSMs. Simulation and experimental results validate the presented control concept.
    IEEE Transactions on Industrial Electronics 01/2013; 60(1):54-65. · 6.50 Impact Factor

Publication Stats

4k Citations
300.65 Total Impact Points


  • 1994–2014
    • Liverpool John Moores University
      • Electrical Electronic Engineering Research Centre
      Liverpool, England, United Kingdom
  • 1999–2013
    • University of Liverpool
      • • School of Engineering
      • • Department of Electrical Engineering and Electronics
      Liverpool, England, United Kingdom
  • 2011
    • University of Vigo
      • Department of Electronics Technology
      Vigo, Galicia, Spain
  • 2010
    • Universidad de Sevilla
      • Electronical Engineering
      Hispalis, Andalusia, Spain
  • 2009
    • University of Nottingham
      • Department of Electrical and Electronic Engineering
      Nottingham, ENG, United Kingdom
  • 2003–2008
    • University of Belgrade
      • School of Electrical Engineering
      Belgrade, SE, Serbia
  • 2007
    • Indian Institute of Science
      • Centre for Electronic Design and Technology
      Bengalore, State of Karnataka, India
  • 2004–2006
    • Politecnico di Torino
      • DET - Department of Electronics and Telecommunications
      Torino, Piedmont, Italy
  • 1999–2004
    • University of Novi Sad
      • Faculty of Technical Sciences
      Varadinum Petri, Autonomna Pokrajina Vojvodina, Serbia