Radio Frequency Measurement Method for Detecting Bearing Currents in Induction Motors

Induction motors are widely used in industry, and they are generally considered very reliable. They often have a critical role in industrial processes, and their failure can lead to significant losses as a result of shutdown times. Typical failures of induction motors can be classified into stator, rotor, and bearing failures. One of the reasons for a bearing damage and eventually a bearing failure is bearing currents. Bearing currents in induction motors can be divided into two main categories; classical bearing currents and inverter-induced bearing currents. A bearing damage caused by bearing currents results, for instance, from electrical discharges that take place through the lubricant film between the raceways of the inner and the outer ring and the rolling elements of a bearing. This phenomenon can be considered similar to the one of electrical discharge machining, where material is removed by a series of rapidly recurring electrical arcing discharges between an electrode and a workpiece. This thesis concentrates on bearing currents with a special reference to bearing current detection in induction motors. A bearing current detection method based on radio frequency impulse reception and detection is studied. The thesis describes how a motor can work as a “spark gap” transmitter and discusses a discharge in a bearing as a source of radio frequency impulse. It is shown that a discharge, occurring due to bearing currents, can be detected at a distance of several meters from the motor. The issues of interference, detection, and location techniques are discussed. The applicability of the method is shown with a series of measurements with a specially constructed test motor and an unmodified frequency-converter-driven motor. The radio frequency method studied provides a nonintrusive method to detect harmful bearing currents in the drive system. If bearing current mitigation techniques are applied, their effectiveness can be immediately verified with the proposed method. The method also gives a tool to estimate the harmfulness of the bearing currents by making it possible to detect and locate individual discharges inside the bearings of electric motors.

Hybrid LC Filter for Power Electronic Drives: Theory and Implementation

Power electronic converter drives use, for the sake of high efficiency, pulse-width modulation that results in sequences of high-voltage high-frequency steep-edged pulses. Such a signal contains a set of high harmonics not required for control purposes. Harmonics cause reflections in the cable between the motor and the inverter leading to faster winding insulation ageing. Bearing failures and problems with electromagnetic compatibility may also result. Electrical du/dt filters provide an effective solution to problems caused by pulse-width modulation, thereby increasing the performance and service life of the electrical machines. It is shown that RLC filters effectively decrease the reflection phenomena in the cable. Improved (simple, but effective) solutions are found for both differential- and common-mode signals; these solutions use a galvanic connection between the RLC filter star point and the converter DC link. Foil chokes and film capacitors are among the most widely used components in high-power applications. In actual applications they can be placed in different parts of the cabinet. This fact complicates the arrangement of the cabinet and decreases the reliability of the system. In addition, the inductances of connection wires may prevent filtration at high frequencies. This thesis introduces a new hybrid LC filter that uses a natural capacitance between the turns of the foil choke based on integration of an auxiliary layer into it. The main idea of the hybrid LC filter results from the fact that both the foil choke and the film capacitors have the same roll structure. Moreover, the capacitance between the turns (“intra capacitance”) of the foil inductors is the reason for the deterioration of their properties at high frequencies. It is shown that the proposed filter has a natural cancellation of the intra capacitance. A hybrid LC filter may contain two or more foil layers isolated from each other and coiled on a core. The core material can be iron or even air as in the filter considered in this work. One of the foils, called the main foil, can be placed between the inverter and the motor cable. Other ones, called auxiliary foils, may be connected in star to create differential-mode noise paths, and then coupled to the DC link midpoint to guarantee a traveling path, especially for the common-mode currents. This way, there is a remarkable capacitance between the main foil and the auxiliary foil. Investigations showed that such a system can be described by a simple equivalent LC filter in a wide range of frequencies. Because of its simple hybrid construction, the proposed LC filter can be a cost-effective and competitive solution for modern power drives. In the thesis, the application field of the proposed filter is considered and determined. The basics of hybrid LC filter design are developed further. High-frequency behaviour of the proposed filter is analysed by simulations. Finally, the thesis presents experimental data proving that the hybrid LC filter can be used for du/dt of PWM pulses and reduction of common-mode currents.

Active du/dt Filtering for Variable-Speed AC Drives

An oscillating overvoltage has become a common phenomenon at the motor terminal in inverter-fed variable-speed drives. The problem has emerged since modern insulated gate bipolar transistors have become the standard choice as the power switch component in lowvoltage frequency converter drives. Theovervoltage phenomenon is a consequence of the pulse shape of inverter output voltage and impedance mismatches between the inverter, motor cable, and motor. The overvoltages are harmful to the electric motor, and may cause, for instance, insulation failure in the motor. Several methods have been developed to mitigate the problem. However, most of them are based on filtering with lossy passive components, the drawbacks of which are typically their cost and size. In this doctoral dissertation, application of a new active du/dt filtering method based on a low-loss LC circuit and active control to eliminate the motor overvoltages is discussed. The main benefits of the method are the controllability of the output voltage du/dt within certain limits, considerably smaller inductances in the filter circuit resulting in a smaller physical component size, and excellent filtering performance when compared with typical traditional du/dt filtering solutions. Moreover, no additional components are required, since the active control of the filter circuit takes place in the process of the upper-level PWM modulation using the same power switches as the inverter output stage. Further, the active du/dt method will benefit from the development of semiconductor power switch modules, as new technologies and materials emerge, because the method requires additional switching in the output stage of the inverter and generation of narrow voltage pulses. Since additional switching is required in the output stage, additional losses are generated in the inverter as a result of the application of the method. Considerations on the application of the active du/dt filtering method in electric drives are presented together with experimental data in order to verify the potential of the method.

Tasasähkönjakeluverkon vaihtosuuntaaja galvaanisella erotuksella

Suomessa sähkönjakeluverkko koostuu pääasiassa 20 kV ja 400 V jännitetasoista. Tällöin sähkö viedään lähelle kuluttajia suuremmalla jännitetasolla ja muunnetaan alhaisemmaksi lähellä asiakkaita. Haittapuolena on se, että haja-asutusalueilla jakelumuuntajien määrä kasvaa suureksi, koska ne täytyy sijoittaa lähelle kuluttajaa. Vaihtoehtona on toteuttaa osa sähkönjakelusta tasajännitteellä, jolloin tehollinen jännite olisi suurempi. Tällöin sähköä voitaisiin siirtää pidempiä matkoja ilman, että asiakaskohtaisia tai muutaman asiakkaan kattavia 20 kV siirtolinjoja tarvitsisi käyttää. Tämä taasen edellyttää asiakaskohtaisien vaihtosuuntaajien käyttöä. Tässä työssä esiteltävällä 1 kVA vaihtosuuntaajalla muodostetaan tasasähköjakeluverkosta saatavasta 750 V tasasähköstä yksivaiheista (230 VRMS, 50 Hz) verkkojännitettä. Laite on suunniteltu toteuttamaan galvaaninen erotus mahdollisimman hyvän hyötysuhteen puitteissa. Vaihtosuuntaaja on toteutettu käyttämällä resonanssikonvertteria, joka vaihtosuuntaa jakelujännitteen korkeataajuiseksi vaihtojännitteeksi. Tämän jälkeen toteutetaan galvaaninen erotus käyttäen suurtaajuusmuuntajaa. Tätä seuraa syklokonvertteri, joka pulssintiheysmodulaatiota soveltaen muodostaa lähtöjännitteen. Tämä suodatetaan lopuksi LC- alipäästösuotimella säröltään standardin mukaiseksi. Laite on jaettu työssä kolmeen osaan, joista jokaisen toiminta on selitetty ja simuloitu itsenäisesti. Lopussa koko järjestelmä on simuloitu yhtenä kokonaisuutena. Hyötysuhteeksi arvioitiin karkeasti 94 % ja lopullisista tuloksista voidaan päätellä, että laitteen toteuttaminen prototyypiksi voisi olla kannattavaa.

Frequency converter protection against supply network phenomena

This master’s thesis is focused on power supply network disturbances and their effects on the frequency converter – one of the components of modern process crane electrical system. The most critical disturbance types, their causes and the effect they bring to crane electrical system was determined in this work. Variety of protective devices used for mitigation of disturbances is investigated. Device protection solutions for the frequency converter used in crane applications are presented. Analyses of the power supply requirements for frequency converter and various components of crane electrical system were carried out and as a result the crane power supply requirements list that guarantees normal crane operation was built. This list is to be included in crane projects for troubleshooting purposes in order to determine potentially dangerous network.

Pronssinen Kairamo-mitali FM Juha Valsteelle

Kairamo-mitalit 2008.

Viral and cellular modulation of virus-induced apoptosis in experimental infection models

The aim of this study was to investigate herpes simplex virus type 1 (HSV-1)- and measles virus (MV)-induced cell death. HSV-1 with deletion in genes encoding infected cell protein (ICP)4 and protein kinase Us3 (d120) induced apoptosis and cathepsin activation in epithelial (HEp-2) and monocytic (U937) cells. Inhibition of cathepsin activity decreased the amount of d120-induced apoptosis indicating that d120-induced apoptosis could be cathepsin-mediated. Also, HSV-1 infection increased caspase activation suggesting that d120-induced apoptosis is probably caspase-mediated. Cystatin treatment decreased the activity of cathepsins and the replication of HSV-1 indicating that cathepsins contribute to HSV-1 infection. Interestingly, d120 induced also necroptosis in monocytic cells. This is the first report on necroptosis in HSV-1- infected cells. MV induced apoptosis in uninfected bystander T lymphocytes, probably via interaction of MV-infected monocytes with uninfected lymphocytes. The expression of death receptor Fas was clearly increased on the surface of lymphocytes. The number of apoptotic cells and the activation of cathepsins and caspases were increased in MVinfected U937 cells suggesting that MV-induced apoptosis could be cathepsin- and caspase-mediated. Cystatin treatment inhibited cathepsin activities but not MV-induced apoptosis. Besides HSV-1-induced apoptosis, innate immune responses were studied in HSV-1-infection. HSV-1 viruses with either ICP4 and Us3, or Us3 deletion only, increased the expression of Toll-like receptor (TLR)3 and stimulated its downstream pathways leading to increased expression of type I interferon gene and to functional interferons. These findings suggest that besides controlling apoptosis, HSV-1 ICP4 and Us3 genes are involved in the control of TLR3 response in infected cell.

Monitoring of centrifugal pump operation by a frequency converter

Centrifugal pumps are widely used in industrial and municipal applications, and they are an important end-use application of electric energy. However, in many cases centrifugal pumps operate with a significantly lower energy efficiency than they actually could, which typically has an increasing effect on the pump energy consumption and the resulting energy costs. Typical reasons for this are the incorrect dimensioning of the pumping system components and inefficiency of the applied pump control method. Besides the increase in energy costs, an inefficient operation may increase the risk of a pump failure and thereby the maintenance costs. In the worst case, a pump failure may lead to a process shutdown accruing additional costs. Nowadays, centrifugal pumps are often controlled by adjusting their rotational speed, which affects the resulting flow rate and output pressure of the pumped fluid. Typically, the speed control is realised with a frequency converter that allows the control of the rotational speed of an induction motor. Since a frequency converter can estimate the motor rotational speed and shaft torque without external measurement sensors on the motor shaft, it also allows the development and use of sensorless methods for the estimation of the pump operation. Still today, the monitoring of pump operation is based on additional measurements and visual check-ups, which may not be applicable to determine the energy efficiency of the pump operation. This doctoral thesis concentrates on the methods that allow the use of a frequency converter as a monitoring and analysis device for a centrifugal pump. Firstly, the determination of energy-efficiency- and reliability-based limits for the recommendable operating region of a variable-speed-driven centrifugal pump is discussed with a case study for the laboratory pumping system. Then, three model-based estimation methods for the pump operating location are studied, and their accuracy is determined by laboratory tests. In addition, a novel method to detect the occurrence of cavitation or flow recirculation in a centrifugal pump by a frequency converter is introduced. Its sensitivity compared with known cavitation detection methods is evaluated, and its applicability is verified by laboratory measurements for three different pumps and by using two different frequency converters. The main focus of this thesis is on the radial flow end-suction centrifugal pumps, but the studied methods can also be feasible with mixed and axial flow centrifugal pumps, if allowed by their characteristics.

Galvaanisen erotuksen toteuttaminen tasavirtasähkönjakelun asiakaspäätelaitteessa

Suomessa sähkönjakeluverkon pääasialliset jännitetasot ovat 20 kV ja 400 V. 20 kV jännitetasolla sähkö viedään lähelle kuluttajia ja muunnetaan pienemmäksi lähellä asiakkaita. Haittapuolena on se, että 20 kV avojohtosähkönsiirtoverkko on hyvin vika-altis ja usein yhden haaran vikaantuessa monta muutakin jää ilman sähköä. Lisäksi hintavien ja suurien jakelumuuntajien määrä on suuri. Vaihtoehtona on toteuttaa osa sähkönjakelusta tasajännitteellä, jolloin tehollinen pienjännite olisi 400 V:a suurempi. Tällöin sähköä voitaisiin siirtää pidempiä matkoja ilman, että asiakaskohtaisia tai muutaman asiakkaan kattavia 20 kV siirtolinjoja tarvitsisi käyttää. Tämä edellyttää vaihtosuuntauksen toteuttamista kuluttajan päässä. Tässä työssä esiteltävällä 1 kVA:n tehoisella vaihtosuuntaajalla muodostetaan tasasähköjakeluverkosta saatavasta 750 V tasasähköstä yksivaiheista (230 VRMS, 50 Hz) verkkojännitettä. Laite on suunniteltu toteuttamaan galvaaninen erotus mahdollisimman hyvällä hyötysuhteella. Suurtaajuusmuuntajan mitoitus mahdollisimman hyvälle hyötysuhteelle on haastava tehtävä, koska vaatimuksia sille asettavat sekä syöttävä resonanssikonvertteri että syötettävä syklokonvertteri. Mitoituksessa on pyrittävä löytämään mahdollisimman hyvä hyötysuhde kustannusten ja toteutettavuuden suhteen.

Frequency-Domain and Wide-Pulse Time-Domain Measurements of Lanthanide Luminescence and Lanthanide-Based Resonance Energy Transfer

Resonance energy transfer (RET) is a non-radiative transfer of the excitation energy from the initially excited luminescent donor to an acceptor. The requirements for the resonance energy transfer are: i) the spectral overlap between the donor emission spectrum and the acceptor absorption spectrum, ii) the close proximity of the donor and the acceptor, and iii) the suitable relative orientations of the donor emission and the acceptor absorption transition dipoles. As a result of the RET process the donor luminescence intensity and the donor lifetime are decreased. If the acceptor is luminescent, a sensitized acceptor emission appears. The rate of RET depends strongly on the donor–acceptor distance (r) and is inversely proportional to r6. The distance dependence of RET is utilized in binding assays. The proximity requirement and the selective detection of the RET-modified emission signal allow homogeneous separation free assays. The term lanthanide-based RET is used when luminescent lanthanide compounds are used as donors. The long luminescence lifetimes, the large Stokes’ shifts and the intense, sharply-spiked emission spectra of the lanthanide donors offer advantages over the conventional organic donor molecules. Both the organic lanthanide chelates and the inorganic up-converting phosphor (UCP) particles have been used as donor labels in the RET based binding assays. In the present work lanthanide luminescence and lanthanide-based resonance energy transfer phenomena were studied. Luminescence lifetime measurements had an essential role in the research. Modular frequency-domain and time-domain luminometers were assembled and used successfully in the lifetime measurements. The frequency-domain luminometer operated in the low frequency domain ( 100 kHz) and utilized a novel dual-phase lock-in detection of the luminescence. One of the studied phenomena was the recently discovered non-overlapping fluorescence resonance energy transfer (nFRET). The studied properties were the distance and temperature dependences of nFRET. The distance dependence was found to deviate from the Förster theory and a clear temperature dependence was observed whereas conventional RET was completely independent of the temperature. Based on the experimental results two thermally activated mechanisms were proposed for the nFRET process. The work with the UCP particles involved the measurement of the luminescence properties of the UCP particles synthesized in our laboratory. The goal of the UCP particle research is to develop UCP donor labels for binding assays. In the present work the effect of the dopant concentrations and the core–shell structure on the total up-conversion luminescence intensity, the red–green emission ratio, and the luminescence lifetime was studied. Also the non-radiative nature of the energy transfer from the UCP particle donors to organic acceptors was demonstrated for the first time in aqueous environment and with a controlled donor–acceptor distance.

Loss Modelling of Three-Level Inverters controlled with Space Vector Modulation Technique

The objective of this master’s thesis is to investigate the loss behavior of three-level ANPC inverter and compare it with conventional NPC inverter. The both inverters are controlled with mature space vector modulation strategy. In order to provide the comparison both accurate and detailed enough NPC and ANPC simulation models should be obtained. The similar control model of SVM is utilized for both NPC and ANPC inverter models. The principles of control algorithms, the structure and description of models are clarified. The power loss calculation model is based on practical calculation approaches with certain assumptions. The comparison between NPC and ANPC topologies is presented based on results obtained for each semiconductor device, their switching and conduction losses and efficiency of the inverters. Alternative switching states of ANPC topology allow distributing losses among the switches more evenly, than in NPC inverter. Obviously, the losses of a switching device depend on its position in the topology. Losses distribution among the components in ANPC topology allows reducing the stress on certain switches, thus losses are equally distributed among the semiconductors, however the efficiency of the inverters is the same. As a new contribution to earlier studies, the obtained models of SVM control, NPC and ANPC inverters have been built. Thus, this thesis can be used in further more complicated modelling of full-power converters for modern multi-megawatt wind energy conversion systems.

Active inverter output filtering methods

Frequency converters are widely used in the industry to enable better controllability and efficiency of variable speed AC motor drives. Despite these advantages, certain challenges concerning the inverter and motor interfacing have been present for decades. As insulated gate bipolar transistors entered the market, the inverter output voltage transition rate significantly increased compared with their predecessors. Inverters operate based on pulse width modulation of the output voltage, and the steep voltage edge fed by the inverter produces a motor terminal overvoltage. The overvoltage causes extra stress to the motor insulation, which may lead to a prematuremotor failure. The overvoltage is not generated by the inverter alone, but also by the sum effect of the motor cable length and the impedance mismatch between the cable and the motor. Many solutions have been shown to limit the overvoltage, and the mainstream products focus on passive filters. This doctoral thesis studies an alternative methodology for motor overvoltage reduction. The focus is on minimization of the passive filter dimensions, physical and electrical, or better yet, on operation without any filter. This is achieved by additional inverter control and modulation. The studied methods are implemented on different inverter topologies, varying in nominal voltage and current.For two-level inverters, the studied method is termed active du/dt. It consists of a small output LC filter, which is controlled by an independent modulator. The overvoltage is limited by a reduced voltage transition rate. For multilevel inverters, an overvoltage mitigation method operating without a passive filter, called edge modulation, is implemented. The method uses the capability of the inverter to produce two switching operations in the same direction to cancel the oscillating voltages of opposite phases. For parallel inverters, two methods are studied. They are both intended for two-level inverters, but the first uses individual motor cables from each inverter while the other topology applies output inductors. The overvoltage is reduced by interleaving the switching operations to produce a similar oscillation accumulation as with the edge modulation. The implementation of these methods is discussed in detail, and the necessary modifications to the control system of the inverter are presented. Each method is experimentally verified by operating industrial frequency converters with the modified control. All the methods are found feasible, and they provide sufficient overvoltage protection. The limitations and challenges brought about by the methods are discussed.

Tasasähkönjakeluverkon vaihtosuuntaajan pulssintiheysmoduloidun pääteasteen ja prototyypin toteutus

Työssä esiteltävä laite on osa DC-AC hakkuria, jolla muodostetaan 750 V tasajännitteestä yksivaiheista (230 VRMS, 50 Hz) galvaanisesti erotettua verkkojännitettä. Tasajännite muunnetaan resonanssikonvertterilla korkeataajuiseksi vaihtojännitteeksi, joka johdetaan erotusmuuntajaan. Galvaanisen erotuksen jälkeen korkeataajuisesta vaihtojännitteestä muodostetaan suoraan verkkotaajuista vaihtojännitettä työssä esiteltävällä syklokonvertterilla. Suunnittelussa on pyritty minimoimaan häviöt mahdollisimman tarkkaan, jotta laite olisi kilpailukykyinen ei-galvaanisesti erottavien konverttereiden kanssa. Tämä on toteutettu käyttämällä mahdollisimman vähän komponentteja virran kulkureitillä sekä soveltamalla pehmeää kytkentää kaikissa tilanteissa. Lopuksi esitellään prototyyppi, jonka tarkoitus oli selvittää laitteen toiminta ja ongelmakohdat käytännössä.

Sähköavusteisen polkupyörän vääntömomenttisäädön toteuttaminen amplitudimodulaatiolla

Sähköavusteinen polkupyörä on hieman tavallista polkupyörää vahvarakenteisempi ja painavampi. Lisäpainoa tuovat muun muassa akut ja sähkömoottori, joka avustaa polke-mista. Moottori asennetaan yleensä eturenkaan keskiöön. Suurimmassa osassa, jollei kaikissa, kaupallisista sähköavusteisissa polkupyörissä moottori on hiiliharjaton tasavirta-moottori. Hiiliharjattoman tasavirtamoottorin pyörimisnopeutta ja vääntömomenttia voidaan säätää usealla eri tavalla. Markkinoilla olevat säätimet perustuvat poikkeuksetta pulssinleveys-modulaatioon. Tämän työn tarkoituksena on tutkia, onko säätöjärjestelmä mahdollista to-teuttaa muulla tavalla kuin pulssinleveysmodulaattorilla. Tutkimuksessa päädytään käyt-tämään amplitudimodulointia, jolloin taajuus voidaan pitää vakiona. Tämän tutkimuksen rinnalla samaan polkupyörään rakennetaan polkemisenergian avulla akkuja lataava la-tausjärjestelmä. Tämän kandityön puitteissa tutkimus jää kesken, mutta toiminta osoitetaan teoreettisesti ja simuloimalla. Suurin syy kokeellisen testauksen epäonnistumiseen on käytettyjen kom-ponenttien sopimattomuus sovellukseen.