New Modulation Technique to Mitigate Common Mode Voltage Effects in Star-Connected Five-Phase AC Drives

Star-connected multiphase AC drives are being considered for electromovility applications such as electromechanical actuators (EMA), where high power density and fault tolerance is demanded. As for three-phase systems, common-mode voltage (CMV) is an issue for multiphase drives. CMV leads to shaft voltages between rotor and stator windings, generating bearing currents which accelerate bearing degradation and produce high electromagnetic interferences (EMI). CMV effects can be mitigated by using appropriate modulation techniques. Thus, this work proposes a new Hybrid PWM algorithm that effectively reduces CMV in five-phase AC electric drives, improving their reliability. All the mathematical background required to understand the proposal, i.e., vector transformations, vector sequences and calculation of analytical expressions for duty cycle determination are detailed. Additionally, practical details that simplify the implementation of the proposal in an FPGA are also included. This technique, HAZSL5M5-PWM, extends the linear range of the AZSL5M5-PWM modulation, providing a full linear range. Simulation results obtained in an accurate multiphase EMA model are provided, showing the validity of the proposed modulation approach.This work has been supported in part by the Government of the Basque Country within the fund for research groups of the Basque University system IT978-16 and in part by the Government of the Basque Country within the research program ELKARTEK as the project ENSOL (KK-2018/00040)

Reduction of output common mode voltage using a novel SVM implementation in matrix converters for improved motor lifetime

This paper presents the study of an alternative Space Vector Modulation (SVM) implementation for Matrix Converters (MC) which reduces the output Common Mode (CM) voltage. The strategy is based on replacing the MC zero vectors by the rotating ones. In doing this, the CM voltage can be reduced which in-turn reduces the CM leakage current. By reducing the CM current, which flows inside the motor through the bearings and windings, the Induction Motor (IM) deterioration can be slowed down. The paper describes the SVM pattern and analyses the CM voltage and the leakage current paths. Simulation and experimental results based on a MC-IM drive are provided to corroborate the presented approach

Special Power Electronics Converters and Machine Drives with Wide Band-Gap Devices

Power electronic converters play a key role in power generation, storage, and consumption. The major portion of power losses in the converters is dissipated in the semiconductor switching devices. In recent years, new power semiconductors based on wide band-gap (WBG) devices have been increasingly developed and employed in terms of promising merits including the lower on-state resistance, lower turn-on/off energy, higher capable switching frequency, higher temperature tolerance than conventional Si devices. However, WBG devices also brought new challenges including lower fault tolerance, higher system cost, gate driver challenges, and high dv/dt and resulting increased bearing current in electric machines. This work first proposed a hybrid Si IGBTs + SiC MOSFETs five-level transistor clamped H-bridge (TCHB) inverter which required significantly fewer number of semiconductor switches and fewer isolated DC sources than the conventional cascaded H-bridge inverter. As a result, system cost was largely reduced considering the high price of WBG devices in the present market. The semiconductor switches operated at carrier frequency were configured as Silicon Carbide (SiC) devices to improve the inverter efficiency, while the switches operated at fundamental output frequency (i.e., grid frequency) were constituted by Silicon (Si) IGBT devices. Different modulation strategies and control methods were developed and compared. In other words, this proposed SiC+Si hybrid TCHB inverter provided a solution to ride through a load short-circuit fault. Another special power electronic, multiport converter, was designed for EV charging station integrated with PV power generation and battery energy storage system. The control scheme for different charging modes was carefully developed to improve stabilization including power gap balancing, peak shaving, and valley filling, and voltage sag compensation. As a result, the influence on the power grid was reduced due to the matching between daily charging demand and adequate daytime PV generation. For special machine drives, such as slotless and coreless machines with low inductance, low core losses, typical drive implementations using conventional silicon-based devices are performance limited and also produce large current and torque ripples. In this research, WBG devices were employed to increase inverter switching frequency, reduce current ripple, reduce filter size, and as a result reduce drive system cost. Two inverter drive configurations were proposed and implemented with WBG devices in order to mitigate such issues for 2-phase very low inductance machines. Two inverter topologies, i.e., a dual H-bridge inverter with maximum redundancy and survivability and a 3-leg inverter for reduced cost, were considered. Simulation and experimental results validated the drive configurations in this dissertation. An integrated AC/AC converter was developed for 2-phase motor drives. Additionally, the proposed integrated AC/AC converter was systematically compared with commonly used topologies including AC/DC/AC converter and matrix converters, in terms of the output voltage/current capability, total harmonics distortion (THD), and system cost. Furthermore, closed-loop speed controllers were developed for the three topologies, and the maximum operating range and output phase currents were investigated. The proposed integrated AC/AC converter with a single-phase input and a 2-phase output reduced the switch count to six and resulting in minimized system cost and size for low power applications. In contrast, AC/DC/AC pulse width modulation (PWM) converters contained twelve active power semiconductor switches and a common DC link. Furthermore, a modulation scheme and filters for the proposed converter were developed and modeled in detail. For the significantly increased bearing current caused by the transition from Si devices to WBG devices, advanced modeling and analysis approach was proposed by using coupled field-circuit electromagnetic finite element analysis (FEA) to model bearing voltage and current in electric machines, which took into account the influence of distributed winding conductors and frequency-dependent winding RL parameters. Possible bearing current issues in axial-flux machines, and possibilities of computation time reduction, were also discussed. Two experimental validation approaches were proposed: the time-domain analysis approach to accurately capture the time transient, the stationary testing approach to measure bearing capacitance without complex control development or loading condition limitations. In addition, two types of motors were employed for experimental validation: an inside-out N-type PMSM was used for rotating testing and stationary testing, and an N-type BLDC was used for stationary testing. Possible solutions for the increased CMV and bearing currents caused by the implementation of WGB devices were discussed and developed in simulation validation, including multi-carrier SPWM modulation and H-8 converter topology

Bearings Fault Detection Using Inference Tools

The most used electric machine in the industry is the Induction Motor (IM), due to its simplicity and reduced cost. The analysis of the origin of IMs failures exhibits that the bearings are the major source of fault, and even a common cause of degradation in other kinds of motors as Permanent Magnet Synchronous Machines.Peer ReviewedPostprint (published version

Modeling of Bearing Voltage in Electric Machines Based on Electromagnetic FEA and Measured Bearing Capacitance

Bearing voltages and associated bearing currents in electric machines driven by pulsewidth modulation converters with high switching frequencies and high dv/dt can cause premature bearing failures. This article proposes a new modeling approach for the prediction of steady-state and transient bearing voltages based on two-dimensional (2-D) electromagnetic finite element analysis with coupled external circuits using measured bearing capacitance values. The distributed-element external circuit was employed mainly to take into account the influence of wire distribution and frequency dependency, which are typically neglected by traditional equivalent circuits. The developed model was then used to simulate bearing voltages for various scenarios and evaluate the effectiveness of several easy-to-implement bearing voltage reduction methods from the perspective of machine design and manufacturing, such as using the insulated shaft and/or bearings, introducing additional insulation in the rotor, and changing the material of machine components. Experimental measurements are also provided to facilitate the analysis and validate the proposed approach

Laakerivirtojen ja -jännitteiden mallintaminen taajuusmuuttajaohjatuissa sähkömoottoreissa

The possible bearing damages in inverter-fed AC motors are nowadays an important issue in the industrial scope. These motors are known to experience bearing voltages and currents. The root causes for the bearing voltages and currents are common-mode voltages produced by the inverter. However, no cost-effective solution for large AC motors, especially in Azipod systems, has been found to mitigate these problematic voltages and currents. The existing solution is to insulate the non-drive end bearing shield, which takes a large amount of space, is expensive and challenging to manufacture. In addition, no high-frequency model of large AC motors has been created for the purpose of the analysis of bearing voltages and currents. The work presented in this thesis identifies the common factors that lead to these harmful voltages and currents, and from a consideration of these factors, a high-frequency model of the AC motor is created. This model is then adapted to the Azipod system. Simulations are carried out showing the interdependence of different components in the system. In order to complete the simulations, extraction of model parameters that involves analytical calculations and measurements are shown in detail. The model presented in this work can be connected to an appropriate electric drive in order to predict the bearing voltages and currents with a simulator such as OrCAD. The results of the simulations show that the present mitigation techniques can be simplified, with certain precautions, without increasing the bearing voltages and currents to dangerous levels.Mahdolliset laakerivauriot taajuusmuuttajasyötetyissä vaihtovirtamoottoreissa ovat merkittävä haaste teollisuudessa. Kyseisten moottorien tiedetään altistuvan laakerijännitteille ja -virroille, joiden on havaittu aiheutuvan taajusmuuttajan tuottamasta yhteismuotoisesta jännitteestä. Toistaiseksi ei ole löydetty kustannustehokasta ratkaisua pienentämään näitä ongelmallisia jännitteitä ja sähkövirtoja suurissa vaihtovirtamoottoreissa, varsinkaan Azipod-järjestelmissä. Nykyinen ratkaisu on trustipään laakerikilven eristäminen, mikä vie paljon tilaa ja on kallista sekä haastavaa toteuttaa. Toistaiseksi ei ole kehitetty suurtaajuusmallia isoille vaihtovirtamoottoreille laakerijännitteiden ja –virtojen analysointia varten. Tässä työssä on esitetty yleiset tunnistetut tekijät, jotka johtavat näihin haitallisiin jännitteisiin ja virtoihin vaihtovirtamoottoreissa. Työssä on luotu nämä tekijät huomioon ottaen suurtaajuusmalli vaihtovirtamoottoreille. Mallia on kehitetty niin, että sitä voidaan käyttää Azipod-järjestelmässä. Simulaatioiden perusteella voidaan osoittaa eri komponenttien keskinäiset riippuvuudet järjestelmässä. Simulaatioiden suorittamiseksi työssä on esitetty yksityiskohtaisesti mallin parametrien ekstraktoiminen, mikä sisältää analyyttista tarkastelua ja mittauksia. Työssä esitetyn mallin voi yhdistää sopivaan sähkökäyttöön, jolloin laakerijännite ja –virta ovat ennustettavissa piirisimulaattorilla, kuten OrCAD:lla. Työn simuloinnit osoittavat, että nykyisiä käytettyjä laakerivirtojen lieventämistekniikoita voidaan yksinkertaistaa tietyllä varauksella ilman laakerivirtojen ja -jännitteiden nousua vaarallisille tasoille

Evaluation of Bearing Voltage Reduction in Electric Machines by Using Insulated Shaft and Bearings

Bearing voltages and corresponding currents in electric machines driven by pulse width modulation (PWM) converters with fast switching and high dv/dt can cause premature bearing failures. This paper evaluates the bearing voltage reduction by using insulated shafts and bearings. An equivalent circuit representation of electric machines taking into account high-frequency effects is developed to show the production mechanism of bearing voltage, based on which simulations are performed with detailed finite element models for transient and simplified equivalent circuit for steady-state analysis. The steady-state equivalent circuit is then calibrated, by combined numerical calculations and experimental measurements, and used to predict the bearing voltage for various scenarios, proving the effectiveness of using insulated shaft and bearings in reducing the steady-state bearing voltage

Analysis and Methodology for Determining the Parasitic Capacitances in VSI-fed IM Drives Based on PWM Technique

Three-phase induction motors present stray capacitances. The aim of this chapter is to present a methodology to experimentally determine these capacitances and also evaluate the effects of electromagnetic interference on motors in common mode. The proposed procedures for this methodology consist of: a) identifying the motor equivalent electrical circuit parameters through characteristic tests performed in the laboratory; b) setting up configurations between the PWM inverter and the motor for voltage and current measurements: common mode and shaft voltages, leakage and shaft (bearing) currents by using a dedicated measuring circuit; c) calculating the parasitic capacitance values between stator and frame, stator and rotor, rotor and frame and bearings of the motor using the capacitance characteristic equation; d) using the dedicated software Pspice to simulate the system composed by the three-phase induction motor fed by PWM inverter with the equivalent electrical circuit parameters; e) determining the characteristic waveforms involved in the common mode phenomenon

A review of modeling and mitigation techniques for bearing currents in electrical machines with variable-frequency drives

The converter switching in variable-frequency drives can generate high frequency common mode voltage between the machine winding and the converter ground, leading to high frequency parasitic currents which can flow through the machine bearings unless precautions are taken in design and installation. These parasitic and unintended currents in the bearings cause deterioration of the lubrication film and surface damage to the rolling elements of the bearings. These problems will be exacerbated as wide-bandgap semiconductors with faster switching rise times start becoming more widespread in variable-frequency drives. This paper reviews the modelling and mitigation techniques of high frequency bearing currents in inverter fed AC drives. It aims to provide a solid base for the research community to further understanding the bearing currents phenomenon and helping to find novel improved technique for their mitigation and measurement