Sérgio Cruz

• PhD
• Electric Machines Laboratory Head at University of Coimbra

This paper introduces a novel finite-element-method-based model designed to analyze the electromagnetic–thermal dynamics of resistance spot welding (RSW) transformers used in battery manufacturing. The RSW process, inherently multiphysics and sensitive to temperature fluctuations, involves phase changes within the metal materials. This complexity,...

This article proposes a novel approach to design a robust estimator that is able to keep its consistency in system state estimation when system process model mismatch occurs. To successfully develop such an estimator, not only the estimation strategy proposed but also the designer's knowledge and experience about the system behavior are crucial and...

In the context of inductive power transfer (IPT) for electric vehicle (EV) charging, the precise determination of the mutual inductance between the magnetic pads is of critical importance. The value of this inductance varies depending on the EV positioning, affecting the power transfer capability. Therefore, the precise determination of its value y...

In the above article [1] , several projects are missing in the funding information. Thus, the correct information should be:

This article introduces an impulse frequency response analysis (IFRA) technique to detect stator winding earth faults in salient-pole synchronous generators during startup. The proposed technique involves injecting a square pulse with an appropriate frequency spectrum into the neutral point of the stator winding, which is grounded through a resista...

This paper presents a new minimum-loss control strategy for vector-controlled dual-VSI DFIG-dc systems. The optimization is based on a simplified formalism using Lagrange multipliers to determine the optimal stator frequency, stator/rotor magnetizing current split, and the magnetizing flux magnitude as a function of the machine operating conditions...

This chapter provides a general overview of the existing control systems and diagnostic methods available for diagnosing faults in multiphase permanent magnet synchronous generator (PMSG) drives that can be used in wind energy conversion systems. It also provides a general description of the modeling of multiphase PMSGs. The chapter discusses the m...

This paper presents a new offline method based on the analysis of the magnetic flux to determine the location of stator ground faults in synchronous machines. First, the defective phase is identified via the insulation resistance (Megger) test, and then the longitudinal axis of the rotor is aligned with the faulty phase winding. During two measurin...

Six-phase machines are increasingly used in safety-critical applications due to their inherent fault-tolerant capabilities. Due to the greater complexity of controlling six-phase machines and the fast dynamics required in safety-critical applications, finite control set model predictive control (FCS-MPC) emerged as an ideal candidate for the contro...

By manipulating the additional degrees of freedom of multiphase machines, inherent to this type of machine, fault-tolerant control strategies are able to keep multiphase drives in service after one or more open-phase faults (OPFs). Therefore, multiphase machines controlled by finite control set model predictive control (FCS-MPC) strategies are an e...

Wind turbines are increasingly expanding worldwide and Doubly‐Fed Induction Generator (DFIG) is a key component of most of them. Stator winding fault is a major fault in this equipment and its incipient detection is of vital importance. However, there is a paucity of research in this field. In this study, a novel machine learning‐based method is pr...

This paper proposes a non-invasive negative sequence impedance-based technique to detect stator turn-to-turn faults (STTFs) and dentify the related faulty phase at early stages based on the tracking the magnitude and angle variations of the negative sequence current component generated due to STTFs. To extract these indicators, a simplified steady-...

This paper proposes a non-invasive negative sequence impedance-based technique to detect stator turn-to-turn faults (STTFs) and dentify the related faulty phase at early stages based on the tracking the magnitude and angle variations of the negative sequence current component generated due to STTFs. To extract these indicators, a simplified steady-...

Multiphase machines, associated to state-of-the-art control strategies like finite control set model predictive control (FCS-MPC) are suitable for applications where reliability and disturbance-free operation are key factors. Although FCS-MPC provides good transient performance and straightforward design, the performance of the drive system is affe...

Even though multiphase machines have inherent fault tolerant capabilities due to their additional degrees of freedom, fault diagnosis still plays an important role to continuously monitor these machines and thus prevent critical failures. High resistance connections (HRCs) are one of the most common electric faults in electrical machines, being cau...

Uninterruptible Power Supplies (UPS) represent the key technology to continuously feed and protect a wide range of critical applications in electric power systems. Due to their continuous operation, UPS components degrade over time, including their filtering elements, leading to decreased filtering capabilities. With these deviations, UPS performan...

The most recent finite control set model predictive control (FCS-MPC) strategies for six-phase electric drives avoid the excitation of the secondary subspace by using virtual vectors to minimize the current ripple. However, these strategies can only control the fundamental current components, which adjust the machine flux and torque and leave the s...

Abstract Here a new sensitive fault detection criterion is proposed, based on the stationary wavelet transform (SWT) decomposition components of the stator currents, for the online detection of stator winding turn‐to‐turn faults at an early stage of development. The energies of the desired detail components are calculated and the normalized Euclide...

Uninterruptible Power Supplies (UPS) have been demonstrated to be the key technology in feeding either single- and three-phase loads in a wide range of critical applications, such as high-tier datacenters and medical facilities. To increase the overall system power capacity and resilience, UPS systems are usually connected in parallel. When UPS sys...

An efficient online flux‐based diagnostic approach is proposed to detect turn‐to‐turn faults in the stator and rotor windings of wound rotor induction machines, at an early stage of development, which can be applied in the online mode. Some flux sensors are installed in the stator slots to detect the rotational magnetic field at various positions a...

The application of finite control set model predictive control (FCS-MPC) to multiphase electric drives has been widely studied in recent years due to its enhanced transient performance, simple structure, and low tuning requirements. However, the selection of the optimal control actuation by FCS-MPC strategies relies on the accuracy of the system mo...

Multiphase machines are ideal candidates for application in electric drives for critical applications. Since they can operate with open-phase faults (OPFs) without requiring extra hardware, the fault detection is important to reconfigure the control strategy. Hence, this paper proposes a new signal-based diagnosis method to detect OPFs in six-phase...

Due to their fault-tolerant capabilities, multiphase machines are a good candidate for application in electric drives where the reliability is a key factor. Recently, finite control set model predictive control (FCS-MPC) has been applied to multiphase drives as it provides good dynamic performance, however it cannot provide zero steady-state errors...

Although multiphase machines can operate with one or more disabled phases, the diagnosis of an open-phase fault (OPF) is important in order to reconfigure the control strategy. High resistance connection faults (HRCFs) are created by unsecured and damaged connections and if not detected early on can evolve to OPFs. In the case of six-phase machines...

Nowadays, uninterruptible power supplies (UPS) play an important role in feeding critical loads in the electric power systems such as data centers or large communication hubs. Due to the increasing power of these loads and frequent need for expansion or redundancy, UPS systems are frequently connected in parallel. However, when UPS systems are para...

An invasive diagnostic technique is proposed for the detection and discrimination of turn-to-turn faults in the stator and rotor windings of synchronous machines during the startup period. For this purpose, search coils are installed in the machine to sense the rotational magnetic field along the air-gap circumference, while the difference in the r...

Recently, the control of multiphase electric drives has been a hot research topic due to the advantages of multiphase machines, namely the reduced phase ratings, improved fault tolerance and lesser torque harmonics. Finite control set model predictive control (FCS-MPC) is one of the most promising high performance control strategies due to its good...

This paper addresses the minimum-loss control of the Dual-VSI DFIG system connected to a dc link. The minimum-loss operating conditions for field-oriented control based on the airgap flux are obtained analytically using Lagrange multipliers and validated with numerical optimization. As the main contribution of the paper, the analysis accounts for c...

A flux-based technique is proposed for the on-line detection, localization and discrimination of stator and rotor turn-to-turn faults in synchronous generators, using the measured air-gap rotational magnetic field. Simple magnetic field sensors (MFSs) are installed in the stator slots at different regions, and the corresponding induced voltages are...

This paper presents a novel predictive current control strategy with modulated virtual voltage vectors for six-phase drives based on the asymmetrical permanent magnet synchronous machine (PMSMs) with isolated neutrals. The proposed strategy combines two virtual vectors and two zero vectors over a sampling period, whose duty cycles are computed to m...

This paper presents an innovative predictive current control (PCC) strategy based on virtual voltage vectors with optimal amplitude and phase for six-phase asymmetrical permanent magnet synchronous machines (PMSMs) with isolated neutrals. This strategy combines two virtual vectors and two zero vectors over a sampling period in order to reduce the t...

This paper proposes a 3-level double conversion uninterruptible power supply with a new cooperative and dynamically weighted model predictive control technique. A 3-level DC-DC converter is used to improve the DC bus balance and the overall response of the UPS. A new cooperative model predictive control principle is proposed to enhance the response...

Owing to the low equivalent impedance of six‐phase machines in the secondary subspace, in which currents do not contribute to torque production, low‐order current harmonics with high amplitude are likely to appear in the stator currents, increasing the harmonic distortion and the losses in the machine. In the case of permanent magnet synchronous ma...

The electromagnetic design of a six-phase permanent magnet synchronous generator (PMSG) for application in medium/high-speed wind energy conversion systems (WECS) is studied in this work. The dynamic model and the analytical design process of the six-phase PMSG are presented and some design aspects, such as the winding configuration and the geometr...

Doubly-fed induction generator (DFIG) is the dominant technology in the wind energy market. Rotor inter-turn short-circuits (RITSCs) and unbalanced rotor resistance (URR) are the main types of rotor electrical asymmetries in DFIG. The URR has already been considered as an electrical fault or asymmetry in the rotor of DFIG. Although the RITSC introd...

The severe torque ripple normally occurring in the DFIG-dc system can cause premature failure of mechanical components and shorten the life of the drive train. This paper addresses the torque ripple issue by proposing a predictive direct torque control strategy which delivers at the same time torque ripple suppression and minimization of losses. Th...

This paper proposes a novel fault-tolerant predictive power control strategy, with minimal hardware requirements,for wind energy conversion systems based on the doubly-fed induction generator with the rotor fed by three-level neutral-point clamped converters in a back-to-back configuration. The strategy presented is able to maintain the system in o...

This paper presents a comparative study between different finite control set model predictive control (FCS-MPC)strategies for electric drives based on the six-phase permanent magnet synchronous machine (PMSM) with surface mounted permanent magnets (PMs). Special emphasis is given to predictive current control (PCC) and predictive torque control (PT...

This paper presents a novel technique for the online detection of broken rotor bar(s) (BRBs) in induction motors, based on the analysis of the air-gap rotational magnetic field (RMF), being it indirectly sensed using the voltages induced in dual search coils installed in several stator regions. The induced voltages allow obtaining three differentia...

A new air-gap flux-based technique is proposed in this paper for turn-to-turn fault (TTF) detection and defective region identification which is implemented in line-connected and inverter-fed induction motors. Some search coils (SCs) are used to measure the magnetic flux in various air-gap regions of the mentioned motor. The induced voltages in the...

In this paper, new techniques are presented to improve the performance of Optimal Switching Vector Model Predictive Control. A well-defined analytic technique is proposed for the real-time adjustment of dynamic weighting factors, as well as a new objective function formulation. These techniques provide a dynamic controller behavior, automatically a...

The electromagnetic design of a six-phase permanent magnet synchronous generator (PMSG) for application in medium/high-speed wind energy conversion systems (WECS) is studied in this paper. The dynamic model and the analytical design process of the six-phase PMSG are presented and some design aspects, such as the winding configuration and the geomet...

This paper proposes a new fault-tolerant predictive control strategy, with minimal hardware requirements, for wind energy conversion systems based on the doubly-fed induction generator with the rotor fed by two three-level neutral-point clamped converters in a back-to-back configuration. The proposed strategy is capable of maintaining the system in...

This paper presents a new simplified finite control set model predictive control strategy for synchronous reluctance motors operating in the entire speed range. It is a predictive control scheme that regulates the stator flux and the load angle of the synchronous reluctance motor, incorporating the ability to operate the drive in the field-weakenin...

This study presents a new fault-tolerant predictive power control strategy for doubly-fed induction generators (DFIGs) used in wind energy applications, with the rotor fed by a three-level neutral point clamped converter. The proposed control strategy is able to maintain the system in operation with a good performance after the occurrence of either...

This paper presents a new approach for fault diagnosis in three-level neutral point clamped inverters. The proposed method is based on the average values of the positive and negative parts of normalized output currents. This method is capable of detecting and locating multiple open-circuit faults in the controlled power switches of converters in ha...

In this study, a wound rotor induction machine, as the core of a doubly-fed induction generator (DFIG) system, is modelled using the magnetic equivalent circuit approach. The entire DFIG system is implemented in Matlab/Simulink environment, including the machine model, power converters, control system and the grid. The model, which includes the abi...

A new real-time method able to diagnose multiple semiconductor open-circuit faults in a three-level neutral-point clamped inverter is introduced in this paper. The proposed diagnostic method is based on the evaluation of the output pole voltages and output currents of the inverter. The proposed method allows a real-time detection and localization o...

This paper investigates the performance of a wound rotor induction motor (WRIM) under rotor inter-turn short circuit (SC) fault. Magnetic equivalent circuit (MEC) and finite elements method (FEM) are used to model the machine. Impacts of the fault on the signals of the machine including rotor current, stator current, and developed torque of the mot...

This paper presents the power converters topology and the corresponding control systems of a three-phase regenerative grid simulator. The proposed system consists in three identical single-phase units, each one consisting in two H-bridge power converters sharing a common dc-bus. The mathematical model of the two converters is developed and based on...

This paper presents an encoderless highperformance synchronous reluctance motor drive for traction applications. The control system is based on the active flux concept and a hybrid rotor position estimation algorithm is used, being this algorithm based on the injection of high-frequency signals at low speeds and on the position of the active flux v...

Purpose – The purpose of this paper is to present a comparative study of the thermal behavior and efficiency of an induction motor fed by a fault-tolerant Three-Level Neutral Point Clamped (3LNPC) inverter, under normal conditions as well as after a post-fault reconfiguration, following an open-circuit fault in the inverter. For this purpose, a Mat...

This paper presents a new online diagnostic approach for the detection of stator and rotor inter-turn short circuit (ITSC) faults in doubly-fed induction generators (DFIGs) for wind power applications. The proposed technique is based on the Fast Fourier Transform (FFT) of the instantaneous stator reactive power. It is shown that stator and rotor IT...

This paper proposes a high-efficiency synchronous reluctance motor drive for the traction system of an electric vehicle. A vector control system, along with a maximum torque per ampere strategy was implemented taking into account the cross-magnetic saturation in the motor, to achieve a high- efficiency drive with a good dynamic behavior. Experiment...

This paper proposes a new technique for the detection of inter-turn short circuit faults in the stator and rotor windings of doubly-fed induction generators. The proposed technique is based on the Fast Fourier Transform of the stator instantaneous reactive power. The effectiveness of the proposed diagnostic method to detect these faults is evaluate...

Finite control set model predictive control requires a considerable computational burden and it is inevitable to have a time delay between the measurement and the actuation instants in an electric drive based on this control strategy. In this work, it is proposed a new method to compensate the calculation delays in an induction motor drive using pr...

This study presents a new fault diagnostic technique for a semiconductor open-circuit fault in a neutral-point clamped inverter, based on the average current Park's vector approach. This method relies on the measurement of two motor line currents and is able to perform a complete diagnosis in less than half of the inverter output current period. Th...

In this paper, the fault-tolerant capabilities of a back-to-back three-level Neutral-Point-Clamped induction motor drive are analysed. A strategy is proposed to manage fault-tolerant operation and the reconfiguration procedure, using minimal hardware expansion. This strategy includes diagnosis and tolerance solutions for IGBT open-circuit faults in...

This paper presents the results of an experimental investigation regarding the performance of a three-phase induction motor fed by a three-level neutral-point-clamped converter with fault tolerant control strategies. A discussion about the fault diagnostic method used in this work, together with the used fault tolerant control strategy is presented...

This paper presents a diagnosis method for the detection and location of open-circuit faults in 3-Level NPC inverters with SVPWM modulation scheme based on even-order harmonic elimination. The performance of the NPC inverter under normal and faulty conditions is firstly investigated both by simulation and experiments. Afterwards, a method for inver...

This paper presents a new method to diagnose rotor faults in operating three-phase induction motors under the presence of time-varying loads. The proposed diagnostic strategy relies on a combined analysis of the amplitude and phase spectra of the instantaneous active and reactive powers of the motor, and allows to discriminate the effects introduce...

Despite the emergence of new types of electric machines, the squirrel-cage induction motor remains the dominant one. A motor failure can result in large lost revenues. One of types of faults that can occur in these motors is airgap eccentricity. This paper presents results, based on a Finite Element Analysis. It is shown that static eccentricity le...

This paper presents a new diagnostic technique for the diagnosis of open-circuit faults in matrix converters operating with an Optimum Alesina-Venturini modulation scheme. Compared to previous diagnostic techniques developed for matrix converters, the technique proposed in this paper has the major advantage of not requiring the installation of any...

Despite the emergence of new types of electric machines, the squirrel-cage induction motor remains the dominant one. A motor failure can result in large lost revenues. One of types of faults that can occur in these motors is airgap eccentricity. This paper presents results, based on a Finite Element Analysis. It is shown that static eccentricity le...

This paper presents a new method to diagnose rotor faults in operating three-phase induction motors coupled to time-varying loads. The method relies on the simultaneous analysis of the amplitude and phase spectra of the instantaneous active and reactive powers of the motor, and allows to effectively distinguishing a true rotor fault from an oscilla...

This paper presents the application of the on-load exciting current Extended Park's Vector Approach to diagnose incipient turn-to-turn winding faults in operating power transformers. Experimental and simulation test results demonstrate the effectiveness of the proposed technique, which is based on the spectral analysis of the AC component of the on...

The monitoring of power converters is critical for obtaining systems with self-diagnostic capabilities and fault-tolerant features. This paper presents an analytical work concerning the operation of matrix converters (MCs) under the presence of open-circuit faults and discusses the applicability of a method for the diagnosis of this type of failure...

This paper presents a new method for the diagnosis of faults in matrix converters. The method is based on the monitoring of nine modulated error signals, each one associated to a bidirectional power switch of the converter. Using the proposed technique, an open-circuit in a given switch is detected whenever the associated modulated error voltage si...

This paper presents a new diagnostic technique, known as the modulated error voltages (MEV), for the diagnosis of open-circuits faults in the power switches of matrix converters. The proposed technique is based on the monitoring of nine modulated output error voltages, each one associated to a bidirectional switch of the converter. For each output...

This paper presents a comparison study between the operation of a back-to-back and a matrix converter induction motor drive under the presence of faults in the power switches of these converters. These two topologies are functionally equivalent in terms of input power quality and energy regeneration capabilities but need to be compared in terms of...

This paper presents a new method to diagnose open-circuit faults in matrix converters. The proposed method consists in the measurement and estimation, simultaneously, of the three output phase voltages of the matrix converter. Afterwards, three error voltage signals are generated, which will be subsequently modulated using the control signals of ea...

This paper proposes a new model-based diagnostic technique, which is the so-called virtual current technique (VCT), for the diagnosis of rotor faults in direct rotor field oriented controlled (DRFOC) induction motor drives. By measuring the oscillations at twice the slip frequency found in the rotor flux of the machine, and by conjugating this info...

This paper proposes a first diagnostic technique for the detection and location of open-circuit faults in the power switches of matrix converter drives. The proposed method relies on the continuous monitoring of the amplitude of the three output error voltages and works in a two-stage approach. In a first stage, if an open-circuit fault arises in t...

This paper presents the virtual current technique as a new model-based diagnostic procedure for the diagnosis of rotor faults in traction drives for railway applications. The occurrence of a rotor fault in a traction drive based on induction motors introduces some oscillations at twice the slip frequency in the rotor flux as well as in the magnetiz...

This paper presents the development of a simulation model of a matrix converter drive with the aim to diagnose faults in the converter. Using a model developed in Simulink environment, simulation results were obtained demonstrating that the occurrence of an open-circuit fault in one of the power switches of the matrix converter affects heavily the...

Metallized polypropylene film capacitors (MPPF) are a very interesting alternative to electrolytic capacitors, because they can withstand much higher levels of surge voltage, very high RMS currents, and offer longer lifetimes. This paper analyses the lifetime of MPPF capacitors present in the DC link of a single-phase regenerative induction motor (...

This paper is devoted to the diagnosis of rotor faults in direct and indirect rotor flux oriented controlled drives. Different diagnostic techniques are proposed and analyzed with this aim, putting in evidence the advantages and limitations of each one. It is demonstrated that depending on the structure of the control system, some diagnostic techni...

In this paper, different diagnostic techniques are proposed for the diagnosis of rotor faults in field oriented controlled (FOC) induction motor drives. In particular, the applicability of the estimated rotor flux, the current error signals and the outputs of the current controllers of a FOC drive are analyzed. Some block diagrams are presented whi...

This paper is devoted to the diagnosis of rotor faults in closed-loop induction motor drives. Different control strategies are studied, namely the direct torque control and the indirect rotor field oriented control. A detailed analysis of different motor/controller variables enlightens about the behavior of these systems under the presence of rotor...

This paper proposes the use of the multiple reference frames theory for the diagnosis of stator faults in three-phase induction motors. The development of a simplified mathematical motor model allowed the establishment of the equivalent circuits of the motor, in $d!!-!!q!!-!!0$ axes, in the presence of stator interturn short circuits. The use of th...

This paper deals with the use of a new diagnostic technique based on the multiple reference frames theory for the diagnosis of stator winding faults in a direct-torque-controlled (DTC) induction motor drive. The theoretical aspects underlying the use of this diagnostic technique are presented but a major emphasis is given to the integration of the...