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.

Power quality improving with virtual flux-based voltage source line converter

Line converters have become an attractive AC/DC power conversion solution in industrial applications. Line converters are based on controllable semiconductor switches, typically insulated gate bipolar transistors. Compared to the traditional diode bridge-based power converters line converters have many advantageous characteristics, including bidirectional power flow, controllable de-link voltage and power factor and sinusoidal line current. This thesis considers the control of the lineconverter and its application to power quality improving. The line converter control system studied is based on the virtual flux linkage orientation and the direct torque control (DTC) principle. A new DTC-based current control scheme is introduced and analyzed. The overmodulation characteristics of the DTC converter are considered and an analytical equation for the maximum modulation index is derived. The integration of the active filtering features to the line converter isconsidered. Three different active filtering methods are implemented. A frequency-domain method, which is based on selective harmonic sequence elimination, anda time-domain method, which is effective in a wider frequency band, are used inharmonic current compensation. Also, a voltage feedback active filtering method, which mitigates harmonic sequences of the grid voltage, is implemented. The frequency-domain and the voltage feedback active filtering control systems are analyzed and controllers are designed. The designs are verified with practical measurements. The performance and the characteristics of the implemented active filtering methods are compared and the effect of the L- and the LCL-type line filteris discussed. The importance of the correct grid impedance estimate in the voltage feedback active filter control system is discussed and a new measurement-based method to obtain it is proposed. Also, a power conditioning system (PCS) application of the line converter is considered. A new method for correcting the voltage unbalance of the PCS-fed island network is proposed and experimentally validated.

Study of Control Methods of a High-Frequency Cycloconverter for Domestic Induction Heating Appliances

A high-frequency cyclonverter acts as a direct ac-to-ac power converter circuit that does not require a diode bidge rectifier. Bridgeless topology makes it possible to remove forward voltage drop losses that are present in a diode bridge. In addition, the on-state losses can be reduced to 1.5 times the on-state resistance of switches in half-bridge operation of the cycloconverter. A high-frequency cycloconverter is reviewed and the charging effect of the dc-capacitors in ``back-to-back'' or synchronous mode operation operation is analyzed. In addition, a control method is introduced for regulating dc-voltage of the ac-side capacitors in synchronous operation mode. The controller regulates the dc-capacitors and prevents switches from reaching overvoltage level. This can be accomplished by variating phase-shift between the upper and the lower gate signals. By adding phase-shift between the gate signal pairs, the charge stored in the energy storage capacitors can be discharged through the resonant load and substantially, the output resonant current amplitude can be improved. The above goals are analyzed and illustrated with simulation. Theory is supported with practical measurements where the proposed control method is implemented in an FPGA device and tested with a high-frequency cycloconverter using super-junction power MOSFETs as switching devices.

Analysis of torque and speed ripple producing non-idealities of frequency converters in electric drives

Electric motors driven by adjustable-frequency converters may produce periodic excitation forces that can cause torque and speed ripple. Interaction with the driven mechanical system may cause undesirable vibrations that affect the system performance and lifetime. Direct drives in sensitive applications, such as elevators or paper machines, emphasize the importance of smooth torque production. This thesis analyses the non-idealities of frequencyconverters that produce speed and torque ripple in electric drives. The origin of low order harmonics in speed and torque is examined. It is shown how different current measurement error types affect the torque. As the application environment, direct torque control (DTC) method is applied to permanent magnet synchronous machines (PMSM). A simulation model to analyse the effect of the frequency converter non-idealities on the performance of the electric drives is created. Themodel enables to identify potential problems causing torque vibrations and possibly damaging oscillations in electrically driven machine systems. The model is capable of coupling with separate simulation software of complex mechanical loads. Furthermore, the simulation model of the frequency converter's control algorithm can be applied to control a real frequency converter. A commercial frequencyconverter with standard software, a permanent magnet axial flux synchronous motor and a DC motor as the load are used to detect the effect of current measurement errors on load torque. A method to reduce the speed and torque ripple by compensating the current measurement errors is introduced. The method is based on analysing the amplitude of a selected harmonic component of speed as a function oftime and selecting a suitable compensation alternative for the current error. The speed can be either measured or estimated, so the compensation method is applicable also for speed sensorless drives. The proposed compensation method is tested with a laboratory drive, which consists of commercial frequency converter hardware with self-made software and a prototype PMSM. The speed and torque rippleof the test drive are reduced by applying the compensation method. In addition to the direct torque controlled PMSM drives, the compensation method can also beapplied to other motor types and control methods.

Tehokerrointa korjaava skalaariohjaus puoliohjatussa ja kuusikytkintasasuuntaajassa

Verkon harmonisvirtoja rajoittavien standardien tiukentuessa joudutaan etenkin suuritehoisissa tehoelektronisissa laitteissa siirtymään aktiivisiin transistoriohjattuihin tasasuuntaajiin, jotka korjaavat tehokerrointa ja siten pienentävät verkkoon kytkeytyviä häiriövirtoja. Tässä diplomityössä esitellään yleisimpien kolmivaiheisten tasasuuntaajatopologioiden eroja ja vertaillaan puoliohjatun kolmikytkintopologian ja kuusikytkintopologian suorituskykyä tehokertoimen ja harmonissärön osalta, 16 kilowatin teholuokan taajuusmuuttajassa. Tasasuuntaajille tehtiin skalaariohjaukseen perustuva simulointimalli. Työn tavoitteena esitellään simulointitulokset harmonistason sekä tehokertoimen osalta. Työ liittyy Lappeenrannan teknillisen yliopiston sovelletun elektroniikan laboratorion ja Vacon Oyj:n yhteiseen hankkeeseen.

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.

Vibrations in Rotating Machinery Arising from Minor Imperfections in Component Geometries

Vibrations in machines can cause noise, decrease the performance, or even damage the machine. Vibrations appear if there is a source of vibration that excites the system. In the worst case scenario, the excitation frequency coincides with the natural frequency of the machine causing resonance. Rotating machines are a machine type, where the excitation arises from the machine itself. The excitation originates from the mass imbalance in the rotating shaft, which always exists in machines that are manufactured using conventional methods. The excitation has a frequency that is dependent on the rotational speed of the machine. The rotating machines in industrial use are usually designed to rotate at a constant rotational speed, the case where the resonances can be easily avoided. However, the machines that have a varying operational speed are more problematic due to a wider range of frequencies that have to be avoided. Vibrations, which frequencies equal to rotational speed frequency of the machine are widely studied and considered in the typical machine design process. This study concentrates on vibrations, which arise from the excitations having frequencies that are multiples of the rotational speed frequency. These vibrations take place when there are two or more excitation components in a revolution of a rotating shaft. The dissertation introduces four studies where three kinds of machines are experiencing vibrations caused by different excitations. The first studied case is a directly driven permanent magnet generator used in a wind power plant. The electromagnetic properties of the generator cause harmonic excitations in the system. The dynamic responses of the generator are studied using the multibody dynamics formulation. In another study, the finite element method is used to study the vibrations of a magnetic gear due to excitations, which frequencies equal to the rotational speed frequency. The objective is to study the effects of manufacturing and assembling inaccuracies. Particularly, the eccentricity of the rotating part with respect to non-rotating part is studied since the eccentric operation causes a force component in the direction of the shortest air gap. The third machine type is a tube roll of a paper machine, which is studied while the tube roll is supported using two different structures. These cases are studied using different formulations. In the first case, the tube roll is supported by spherical roller bearings, which have some wavinesses on the rolling surfaces. Wavinesses cause excitations to the tube roll, which starts to resonate at the frequency that is a half of the first natural frequency. The frequency is in the range where the machine normally operates. The tube roll is modeled using the finite element method and the bearings are modeled as nonlinear forces between the tube roll and the pedestals. In the second case studied, the tube roll is supported by freely rotating discs, which wavinesses are also measured. The above described phenomenon is captured as well in this case, but the simulation methodology is based on the flexible multibody dynamics formulation. The simulation models that are used in both of the last two cases studied are verified by measuring the actual devices and comparing the simulated and measured results. The results show good agreement.

Photocatalysis Studies under Visible Light Irradiation with TiO2 Obtained by Pulsed Electric Field Assisted Sol-Gel Method

The objective of this thesis was to study the effect of pulsed electric field on the preparation of TiO2 nanoparticles via sol-gel method under the visible light irradiation. The literature part introduces properties of different TiO2 crystal forms and principle of photocatalysis. It was expected that pulsed electric field would have an influence on degradation for oxalic acid and formic acid. TiO2 samples were prepared by using three frequencies (50Hz, 294Hz, and 963Hz) and two treatment times (12 minutes and 24 minutes) of pulsed electric field. The photocatalytic activities of TiO2 samples produced with sol-gel method were also compared with the TiO2 particles made by previous study and with the commercial TiO2 powder Aeroxide® (Evonic Degussa GmbH) at the same condition. Results show that pulsed electric field does have an effect on degradation for oxalic acid and formic acid. Generally, higher photocatalytic activities for oxalic acid and formic acid were obtained with lower frequency and longer treatment time of pulsed electric field.

Comparison of Model-Based Flow Rate Estimation Methods in Frequency-Converter-Driven Pumps and Fans

Fluid handling systems such as pump and fan systems are found to have a significant potential for energy efficiency improvements. To deliver the energy saving potential, there is a need for easily implementable methods to monitor the system output. This is because information is needed to identify inefficient operation of the fluid handling system and to control the output of the pumping system according to process needs. Model-based pump or fan monitoring methods implemented in variable speed drives have proven to be able to give information on the system output without additional metering; however, the current model-based methods may not be usable or sufficiently accurate in the whole operation range of the fluid handling device. To apply model-based system monitoring in a wider selection of systems and to improve the accuracy of the monitoring, this paper proposes a new method for pump and fan output monitoring with variable-speed drives. The method uses a combination of already known operating point estimation methods. Laboratory measurements are used to verify the benefits and applicability of the improved estimation method, and the new method is compared with five previously introduced model-based estimation methods. According to the laboratory measurements, the new estimation method is the most accurate and reliable of the model-based estimation methods.

On site measurements of DC link electrolytic capacitor bank of frequency converter using constant DC magnetization of the motor

Preventive maintenance of frequency converters has been based on pre-planned replace-ment of wearing or ageing components. Exchange intervals follow component life-time expectations which are based on empirical knowledge or schedules defined by manufac-turer. However, the lifetime of a component can vary significantly, because drives are used in very different operating environments and applications. The main objective of the research was to provide information on methods, i.e. how in-verter's operating condition can be measured reliably under field conditions. At first, the research focused on critical components such as current transducers, IGBTs and DC link capacitor bank, because these aging have already been identified. Of these, the DC link capacitor measurement method was selected for closer examination. With this method, the total capacitance and its total series resistance can be measured. The suitability of the measuring procedure was estimated on the basis of practical measurements. The research was made by using so called triangulation method, including a literature review, simulations and practical measurements. Based on the results, the new measu-rement method seems suitable with some reservations to practical measurements. How-ever, the measuring method should be further developed in order to improve its reliability.