Energy demands are steadily increasing as the world's population continues to grow. Automobiles are primary transportation means in a large portion of the world. The combination of fuel consumption by automobiles along with the shrinking fossil fuel reserves makes the development of new more energy efficient technologies crucial. Electric vehicle technologies have been studied and are still being studied today as a means of improving fuel efficiency. To that end, this work studies the effect of switching frequency on the efficiency of a hybrid electric vehicle traction drive, which contains both an internal combustion engine as well as electric motor. Therefore improving the efficiency of the electric motor and its drive will help improve the viability of alternative vehicle technologies. Automobiles spend the majority of their operational time in the lower speed, lower torque region. This work focuses on efficiency improvements in that region. To estimate the efficiency trend, the system is modeled and then tested both electrically and thermally. The efficiency is shown to increase at lower switching frequencies. The experimental results show that there are some exceptions, but the basic trend is the same. / Master of Science

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This thesis deals with a new solution for medium voltage drives. Load Commutated Inverter (LCI) fed synchronous motor drive is a popular solution for high power drive applications. Though the induction machine is more rugged and cheaper compared to the synchronous machine, LCI fed induction motor drive solution is not available. The basic advantage of a synchronous machine over an induction machine is the fact that the synchronous machine can operate at leading power factor. Due to this property load commutation of SCR switches of the LCI is achievable for synchronous machine. On the contrary an induction machine always draws lagging power factor current; this makes it unsuitable as a drive motor for LCI technology. In this thesis a new LCI fed induction motor drive configuration is developed as an alternative for synchronous motor drives. A new variant of six phase induction motor is proposed in this context. The machine is named as Active Reactive Induction Machine (ARIM). The ARIM contains two sets of three-phase windings with isolated neutral. Both the windings have a common axis. One winding carries the active power and can be wound for higher voltage (say 11kV). The other winding supplies the total reactive power of the machine and can be wound for lower voltage (say 2.2 kV). The rotor is a standard squirrel cage. High power induction machines usually demand lesser magnitude of reactive power compared to the total power rating of the machine ( 20% ). Therefore excitation winding has a smaller fraction of the total machine rating compared to the power winding. A VSI with an LC filter supplies reactive power to the ARIM through the excitation winding and ensures leading power factor at the power winding. This is similar to the excitation control of the LCI fed synchronous machine. The direct VSI connection is possible due to the lower voltage rating for the excitation winding. In this way, the VSI voltage rating does not limit the highest motor voltage that can be handled. An LCI supplies the real power into the ARIM from the power winding. The LCI currents are quasi square wave in shape. Therefore they have rich low order harmonic content. They cause 6th and 12th harmonic torque pulsations in the machine. This is a problem for the LCI fed synchronous machine drive. In the proposed drive, the VSI can compensate these low frequency m.m.f. harmonics inside the machine air gap to remove torque pulsation and rotor harmonic losses. The advantage of the proposed topology is that no transformer is required to drive an 11kV machine. It is always desirable to feed sinusoidal voltage and current to both the power winding and the excitation winding. To address this problem, a second configuration is proposed. A low power three-level VSI is connected in shunt at the power winding with the proposed ARIM drive as discussed above. This VSI compensates the low frequency harmonic currents to achieve sinusoidal motor currents at the motor winding. This VSI acts as a shunt active filter and compensates for the lower order harmonics injected by the LCI. The proposed topologies have LC filters to maintain sinusoidal motor voltages and currents by absorbing the VSI switching frequency components. But the motor terminal voltage oscillates at system resonant frequency due to the presence of LC filters. These resonant components in the terminal voltages are required to be eliminated for smooth terminal voltages and safe load commutation of the thyristors. In this thesis a simple active damping method is proposed to mitigate these issues. The proposed topologies are experimentally verified with an ARIM with 415 V power winding and 220 V excitation winding. The control is carried out on a digital platform having a TMS 320LF 2407A DSP processor and an ALTERA CYCLONE FPGA processor. Results from the prototype experimental drive are presented to show the feasibility and performance of the proposed drive configurations.

The inverter dead-time is integral to the safety of a voltage source inverter (VSI). Dead-time is introduced between the complementary gating signals of the top and bottom switches in each VSI leg to prevent shoot-through fault. This thesis reports and investigates dead-time induced sub-harmonic oscillations in open-loop induction motor drives of different power levels, under light-load conditions. The thesis develops mathematical models that help understand and predict the oscillatory behaviour of such motor drives due to dead-time act. Models are also developed to study the impact of under-compensation and over-compensation of dead-time act on stability. The various models are validated through extensive simulations and experimental results. The thesis also proposes and validates active damping schemes for mitigation of such sub-harmonic oscillations. The thesis reports high-amplitude sub-harmonic oscillations in the stator current, torque and speed of a 100-kW open-loop induction motor drive in the laboratory, operating under no-load. Experimental studies, carried out on 22-kW, 11-kW, 7.5-kW and 3.7-kW open-loop induction motor drives, establish the prevalence of dead-time induced sub-harmonic oscillations in open-loop motor drives of different power levels. An experimental procedure is established for systematic study of this phenomenon in industrial drives. This procedure yields the operating region, if any, where the motor drive is oscillatory. As a first step towards understanding the oscillatory behaviour of the motor drive, a mathematical model of the VSI is derived in a synchronously revolving reference frame (SRF), incorporating the of dead-time on the inverter output voltage. This leads to a modified dynamic model of the inverter-fed induction motor in the SRF, inclusive of the dead-time act. While the rotor dynamic equations are already non-linear, dead-time is found to introduce nonlinearities in the stator dynamic equations as well. The nonlinearities in the modified dynamic model make even the steady solution non-trivial. Under steady conditions, the dead-time can be modelled as the drop across an equivalent resistance (Req0) in the stator circuit. A precise method to evaluate the equivalent resistance Req0 and a simple method to arrive at the steady solution are proposed and validated. For the purpose of stability analysis, a small-signal model of the drive is then derived by linearizing the non-linear dynamic equations of the motor drive, about a steady-state operating point. The proposed small-signal model shows that dead-time contributes to different values of equivalent resistances along the q-axis and d-axis and also to equivalent cross-coupling reactance’s that appear in series with the stator windings. Stability analysis performed using the proposed model brings out the region of oscillatory behaviour (or region of small-signal instability) of the 100-kW motor drive on the voltage versus frequency (V- f) plane, considering no-load. The oscillatory region predicted by the small-signal analysis is in good agreement with simulations and practical observations for the 100-kW motor drive. The small-signal analysis is also able to predict the region of oscillatory behaviour of an 11-kW motor drive, which is con consumed by simulations and experiments. The analysis also predicts the frequencies of sub-harmonic oscillations at different operating points quite well for both the drives. Having the validity of the small-signal analysis at different power levels, this analytical procedure is used to predict the regions of oscillatory behaviour of 2-pole, 4-pole, 6-pole and 8-pole induction motors rated 55 kW and 110 kW. The impact of dead-time on inverter output voltage has been studied widely in literature. This thesis studies the influence of dead-time on the inverter input current as well. Based on this study, the dynamic model of the inverter fed induction motor is extended to include the dc-link dynamics as well. Simulation results based on this extended model tally well with the experimentally measured dc-link voltage and stator current waveforms in the 100-kW drive. Dead-time compensation may be employed to mitigate the dead-time and oscillatory behaviour of the drive. However, accurate dead-time compensation is challenging to achieve due to various factors such as delays in gate drivers, device switching characteristics, etc. Effects of under-compensation and over-compensation of dead time are investigated in this thesis. Under-compensation is shown to result in the same kind of oscillatory behaviour as observed with dead-time, but the fundamental frequency range over which such oscillations occur is reduced. On the other hand, over-compensation of dead-time effect is shown to result in a different kind of oscillatory behaviour. These two types of oscillatory behaviour due to under- and over-compensation, respectively, are distinguished and demonstrated by analyses, simulations and experiments on the 100-kW drive. To mitigate the oscillatory behaviour of the drive, an active damping scheme is proposed. This scheme emulates the effect of an external inductor in series with the stator winding. A small-signal model is proposed for an induction motor drive with the proposed active damping scheme. Simulations and experiments on the 100-kW drive demonstrate effective mitigation of light-load instability with this active damping scheme. In the above inductance emulation scheme, the emulated inductance is seen by the sub-harmonic components, fundamental component as well as low-order harmonic components of the motor current. Since the emulated inductance is also seen by the fundamental component, there is a fundamental voltage drop across the emulated inductance, leading to reduced co-operation of the induction motor. Hence, an improved active damping scheme is proposed wherein the emulated inductance is seen only by the sub-harmonic and low-order harmonic components. This is achieved through appropriate altering in the synchronously revolving domain. The proposed improved active damping scheme is shown to mitigate the sub-harmonic oscillation effectively without any reduction in flux.

This diploma thesis deals with the possibility of using a concentrated winding in an induction motor and the use of copper bars in a rotor cage. The first part of the thesis explains the basic theoretical knowledge of the operation and principle of the induction machine. In this theoretical part we will learn what is actually a concentrated winding and how it is placed on the stator. The second part is focused on the analytical design of the machine with classical distributed winding and with concentrated winding. The third theoretical part is an introduction to the issue of influencing the motor harmonics. The last final part of the thesis shows differences in operational properties of designed machines.

The thesis Diagnostic of induction motor with regard to its maintenance and reliability aims to be a theoretical preparation and application of online diagnostics. It is trying to map correlations between electromechanical system failures driven by induction motor and output information of the diagnostic method. It mainly focuses on spectrum analysis of stator current which carries information on the induction motor status in the form of characteristic signatures in the spectrum (Motor Current Signature Analysis). The theoretical part deals with the concept of diagnostics and mainly with the operational diagnostics, it classifies and characterises types of induction motor failure and it accordingly processes diagnostic methods. This all is done with regard to the application of the maintenance and inspection of machines in a manufacturing company. The practical part develops methodology and measurement procedure. Several measurements demonstrate, validate and apply the MCSA method. It simulates abnormalities and searches for their correlations in the spectrum stream. First, a motor powered from electric distribution network is measured and then the MCSA method is applied to motors powered from frequency converters.

Multilevel inverters have advantages over two-level inverters such as reduced THD, ability to operate at low switching frequencies, reduced switching losses etc. Moreover, higher voltage levels can be handled with devices of lower voltage rating. The main disadvantage with the multilevel configurations compared to the two-level inverter configuration is the increase in the number of power devices required and the circuit complexity, which necessitates complex control schemes that add to the cost. Also, the reliability of the converters comes down as the number of devices increases. Reduction in complexity and modularity are desirable characteristics for the multilevel inverters. Open-end winding Induction Motor (IM) drive configurations are shown to have advantages over the motor drive schemes with isolated neutral. The DC-link requirement in case of open-end winding structures comes down to half the voltage rating of the conventional NPC inverters. The DC- link requirement in case of open-end winding structures comes down to half compared to that of the conventional NPC inverters. The number of switching states is higher in the case of open-end winding configuration compared to multiplicity of switching states of conventional NPC inverters, which gives a control flexibility that can be used for optimizing the hardware requirements. Taking advantage of the flexibility given by open-end winding configuration, this thesis proposes schemes which have reduced power circuit complexity. Non-sinusoidal voltage fed IM drives suffer from the problems related to the common mode voltage (CMV) generated by the inverters. This CMV causes bearing currents and shaft voltages which in turn cause increased conducted EMI, ground loop currents and premature bearing failure. A three-level scheme was proposed for an open-end winding Induction machine in the literature, which completely eliminate the CMV variation from the pole voltages as well as the phase voltages. This configuration uses 24 controlled switches and two isolated DC-sources. In this thesis, three-level inverter schemes with CMV elimination and reduced power device count for an open-end winding IM drive are proposed. The first scheme gets the reduction in switch count by sharing the top inverter of the three-level scheme and the second scheme achieves the same by sharing the bottom inverter. This way, the number of controlled switches comes down to 18 from 24. Another problem with multilevel inverters is the large number of isolated DC-sources required to achieve the multilevel inversion. Reducing the number of isolated supplies and using capacitors to split the voltage levels poses the problem of capacitor voltage balancing. A four-level inverter with both CMV elimination and capacitor voltage balancing for an open-end winding IM drive is proposed in this thesis. The motor is fed by two four-level inverters from both the sides. A closed loop capacitor voltage balancing scheme is implemented and the redundancies in the switching states are used for achieving the capacitor voltage balancing and thereby reducing the total number of DC-link to two. The control scheme is independent of the load power factor and maintains the balance in the entire modulation range. A five-level inverter scheme is proposed for an open-end winding IM drive in this thesis. It requires only two isolated DC-sources to achieve the five-level inversion. The motor is fed by one NPC three-level inverter from one side and a two-level inverter from the other. The inverters on either side share the DC-sources. Common mode voltage in the phases are made zero in an average sense using sine-triangle modulation in the proposed scheme so that the common mode currents through the phases are suppressed. The maximum fundamental voltage that can be obtained at the phase is limited to 0.5Vdc. DC-link requirement of the inverter scheme is half of that of conventional five-level inverter scheme because of the open-end winding structure. The two-level inverter, which should withstand half the DC-link voltage, is always in square wave operation and hence the switching losses are very less. All the schemes are simulated extensively in MATLAB/Simulink and experimentally verified on laboratory prototypes under V/f control. TI Motor control DSP and Xilinx CPLD/FPGA are used for generation of the PWM signals for the schemes. The inverters are switched at around 1.25 kHz to keep the switching losses low. Due to laboratory constraints, the experimental verification is done on low power prototypes. Nonetheless, the generality of the schemes allow them to be used for medium voltage high power applications.

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In this study, three different programs are developed for the analysis of the three-phase induction motor, single-phase capacitor type induction motor and switched reluctance motor. The programs are developed by using Pascal and C++ programming languages. In the performance calculations of motors, analytical methods are used and these methods are tested for accuracy. These programs have also capabilities to design an optimum motor, which meets a set of performance, material and manufacturing constraints while minimizing the weight or any other defined objective function. In addition, in this study, an optimization tool is used to obtain an appropriate optimization method for the design of different types of motors. The software is tested over different commercial motors. The results illustrates that the performance calculations and optimization approach of the programs lead to good results.

O motor de indução é, talvez, o mais robusto e, certamente, um dos motores mais comumente usados. Graças à simplicidade de sua construção, do seu baixo custo, confiabilidade e rendimento relativamente alto com carga nominal é provável que ele permaneça sendo a principal fonte de transformação de energia elétrica em energia mecânica nas aplicações industriais e comerciais por um futuro previsível. O estudo de economia de energia pela redução de perdas é o objetivo do presente estudo. Os motores de indução operam regularmente com fluxo quase constante no entreferro e, portanto, com perdas magnéticas quase constantes. A utilização de um circuito gradador de tensão em série com a alimentação do motor para reduzir o fluxo no entreferro pela redução da tensão aplicada quando a carga não requer fluxo total é o objeto de análise neste estudo. Com a redução da tensão, para manter o conjugado de operação, a velocidade de rotação diminui, ou seja, há um aumento no escorregamento até um valor ótimo para reduzir as perdas totais. Então, esperaria-se que com a redução da tensão aplicada, as perdas magnéticas decresceriam e a eficiência total cresceria. Via de regra, no motor de indução, dada a característica mergulhante de sua curva Conjugado x Velocidade na região em torno da velocidade nominal, o conjugado varia muito mais que a velocidade. Para operação eficiente, a sua tensão aplicada deve ser função da carga que traciona. É objetivo desse trabalho estudar um dispositivo que, colocado em série com a fonte de alimentação de um motor de indução de CA, promove a redução de potência fornecida ao motor, quando a carga aplicada ao motor é menor que a carga nominal. Uma análise da redução da tensão senoidal aplicada através de um auto-transformador de tensão variável é comparada com a redução da tensão através de um circuito gradador baseado em tiristores. A melhor tensão a ser obtida é a que reduz as perdas magnéticas ao mínimo, para cargas que não requerem o fluxo total no entreferro como quando da tensão nominal aplicada. As limitações do processo são estudadas e apresentadas. / The induction motor is perhaps the most rugged, and certainly one of the most commonly used motors. With simply construction, low cost, reliability and efficiency relatively high with rated-load it seems to be a good way to convert electric energy into mechanical energy for industrial and domestic applications for a predictable future. An economy in energy consumption by loss reduction is the goal of this study. The induction motors as normal operate with constant flux in the air-gap and, hence with almost constant losses in the core. The use of a voltage gradador circuit in series with the voltage source applied to the motor for flux reduction in the air-gap by reduction of the voltage when the load do not need full-flux is this study proposal. Hence, with applied voltage reduction, to keep the operational torque, the motor rotation decreases or the slip increases to an optimum value to reduce the total core losses and increase the efficiency. As a rule, the induction motor, by the dip characteristic of the torque x speed curve in the region near the nominal speed, the torque changes much more then the speed. For efficient operation, the applied voltage should be a function of the load. It is the goal or aim of this work to study a device which, when placed in series with the power input of an alternating current induction motor, will produce a reduction in power normally provided to the motor when operated in either a condition where motor loading is less than a rated load. An analysis of voltage ideal sine wave supply reduction applied by an autotransformer with variable voltage is compared with the voltage reduction using a gradador circuit based on thyristors. The optimal voltage operation is the one that decrease the iron losses to minimum, for partial-load that do not need full-flux in the air-gap as when the full voltage is applied. Limitations in the process are investigated and will be showed.