Modulation of Power Electronic Converter Fed Split-phase Induction Machine Drive

Abstract

Induction machine (IM) is the workhorse of several industries due to its low cost and minimal maintenance. Power electronic converters play a major role in driving IMs which give better flexibility in these applications. With the advancement of production levels and effi ciency, the power-level demands of the industries are going up day by day. To meet this increased power-demand, the required ratings of the power converter components are also increased which sometimes can't be realized with the existing technologies. Due to this limitation, induction machines with more than three phases are becoming popular where the power handled by each phase is reduced compared to three-phase machine for the same power rating of IM. These machines are called multi-phase machine and six-phase machine is one of the most popular multi-phase machines. Six-phase machine is of two types: Symmetrical and Asymmetrical. In symmetrical six-phase machine, all the six stator windings are spatially displaced by 60 electrical. Asymmetrical six-phase machine, also known as split-phase induction machine (SPIM), has two sets of balanced three-phase windings which are displaced by 30 degree electrical. The later confi guration has become popular due to its less susceptibility to the time-harmonic components present in the excitation waveform. This thesis is aimed at studying the modulation strategies of SPIM driven by the power-electronic converter. There are two types of power-converters to drive SPIM: DC-AC (Inverter) and AC-AC (matrix converter). The thesis provides detailed discussions on winding structure, nature of excitation, dynamic modeling and steady-state equivalent circuit of SPIM which are required to investigate the modulation strategies of power-converter fed SPIM drive. The dynamic model of SPIM reveals that the fundamental component and a selective group of odd harmonics can contribute to the air-gap flux and hence participate in electromagnetic energy transfer and torque production. The equivalent circuit seen by the fundamental component and the above group of harmonics is similar to the equivalent circuit of three-phase IM. There is another group of odd harmonics which doesn't contribute to the air-gap flux and the equivalent circuit seen by this group consists of stator resistance and leakage inductance. So the excitation of SPIM with these harmonics will cause a large amount of harmonic currents due to low impedance of the equivalent circuit. These harmonic currents don't contribute to the air-gap flux or torque ripple but cause copper losses. So the objective of SPIM modulation is to excite the machine with fundamental component and zero or minimal injection of harmonics belonging to the first or second group. Vector Space Decomposition (VSD) technique, which exists in the literature, modulates the SPIM without injecting any harmonics in the line-neutral voltage. In this thesis, the modulation index of inverter fed SPIM drive has been de fined as the ratio of peak fundamental line-neutral voltage and DC-bus voltage. This thesis provides the derivation of the maximum modulation index achieved by VSD technique. This work tries to unify the understandings behind the existing modulation techniques by proposing a new way of modeling of six-phase inverter. The existing Conventional Space Vector PWM (CSVPWM) technique modulates the SPIM by keeping air-gap flux sinusoidal and it attains the modulation index higher than the maximum modulation index achieved by VSD technique by injecting signifi cant amount of second group of harmonics. Although this group of harmonics doesn't create any torque ripple in the machine, higher injection results into reduction in the efficiency of the SPIM drive modulated by CSVPWM technique. To overcome this, two novel modulation techniques have been proposed in this thesis for the modulation index higher than the maximum modulation index obtained by VSD technique and these two techniques have much-reduced injections of the second group of harmonics compared to CSVPWM. One of these two techniques is able to attain the complete range of modulation index as can be achieved by the CSVPWM and this technique solves a constraint optimization problem in order to minimize the second group of harmonics injection. Another technique is easy to implement but it doesn't attain the whole range of modulation index as achieved by CSVPWM. Within the sub-range, the performance of the second technique is close to the performance of the first technique. The thesis also explores the modulation of matrix converter (MC) fed SPIM drive. Although MC is a promising candidate for drive applications due to high power density and extended lifetime in absence of DC-link electrolytic capacitor, modulation technique of MC fed SPIM drive doesn't exist in the literature. A modulation technique of MC fed SPIM drive has been proposed in this work and this technique keeps the air-gap flux sinusoidal without injecting any harmonics in the line neutral voltage. The proposed modulation techniques have been veri fied by the experiments performed on laboratory prototype hardware built and tested in the lab. The design details of these hardware prototypes are included in this thesis.