Sosthenes Karugaba

Dynamic and steady-state operation of a five phase induction machine with an open stator phase

The analysis of an open phase fault for a five phase induction motor is presented. A circuit based model is developed which can help to predict not only the steady-state performance but also the dynamics including the pulsating torque and to evaluate the small-signal stability of the faulted machine. This is made possible by the utility of the harmonic balance technique on the model of the machine in the stationary reference frame. The resulting model is used in determining the small-signal stability of the five-phase induction motor under open-phase fault. At low speeds the machine exhibits instability on the rotor flux linkages but it is stable at higher speeds. Simulation results and steady state results are presented which also predict the speed harmonic components as well as the torque pulsations.

A Carrier-Based PWM Modulation Technique for Balanced and Unbalanced Reference Voltages in Multiphase Voltage-Source Inverters

The advancement in power electronics has made the use of any number of phases in ac machines possible since with the inverter, any number of phase voltages can be realized as long as the corresponding inverter exists. To supply multiphase drives requires multiphase voltage source inverters (VSIs) whose output voltages depend very much on the methods of generating the pulses to turn on and turn off the inverter devices appropriately. This paper presents an analytical technique for the determination of the expressions for the modulation signals used in the carrier-based sinusoidal and generalized discontinuous pulse width modulation schemes for two-level, five-phase voltage source converters. These expressions can be used for both balanced and unbalanced phase voltages generated by any two-level multiphase inverter, in generating the required reference signals appropriately. An example of a five-phase VSI is presented to illustrate the strategy. Simulation and experimental results are presented in which a close examination shows that both results are similar for the balanced and unbalanced load voltage cases.

Carrier based PWM scheme for a three-level diode-clamped five-phase voltage source inverter ensuring capacitor voltage balancing

A carrier-based modulation scheme for a three-level diode-clamped five-phase voltage source inverter is presented. This technique employs the generation of three average modulation signals for each inverter leg. These signals are compared with the same high frequency triangular carrier signal to produce the composite switching functions for each respective pair of the switching devices. It introduces natural balancing of the capacitor voltage and significantly reduces the harmonic content in both capacitor and output inverter voltages. The technique also ensures zero-neutral-point (NP) potential in the three-level voltage source inverter. Simulation results being compared to the phase disposition (PD) method as well as experimental results on a 10 kVA, three-level diode-clamped five-phase voltage source inverter are included in this paper to validate the presented approach.

Modulation schemes for five-phase to three-phase AC-AC matrix converters

The AC-AC matrix converters are pulse-width modulated using either a carrier-based or space vector modulation techniques. This paper presents these two approaches as they relate to the non-square 5 × 3 converter design targeted for a five-phase voltage source providing real and active powers for three-phase power systems found in high power applications such as on-board ship power systems. It is desirable to be able to synthesize the three-phase voltages with minimum harmonics and the highest voltage gain possible. Furthermore, the input power factor of the converter is to be variable, even to be unity. These goals are achieved with the determined modulation schemes. Computer simulations are given to validate the analytic modulation methods set forth, to demonstrate the waveform synthesis and the achievement of the unity input power factor.

Modeling of nine-phase interior permanent magnet machines (IPM) including harmonic effects

This paper presents the modeling of the nine-phase interior permanent magnet machine (IPM) with and without damper windings for the clear understanding of its advantages such as fault tolerant and torque ripple reduction. This includes the influence of the higher order magneto-motive force (MMF) harmonics of the stator windings and the buried magnets for the improvement of the torque capability of the machine through injection of corresponding stator harmonic currents. The inductances are determined using the concept of winding functions for exact computer simulation based calculation and using Fourier components of the stator windings and the magnet flux for achieving closed-form expressions for the fundamental, third, fifth and the seventh harmonic components. Further analysis of the q-d equations of the nine-phase machine also shows the presence of low impedance circuits which could be excited under fault conditions signaling the detrimental effects of fault conditions on the multi-phase machines which may be avoided/reduced when the machine is connected into various three-phase sets. Some inductances showing their existence in different harmonic representation as well as experimental waveforms have been included.

Carrier-based modulation of non-square multi-phase AC-AC matrix converters

This paper sets forth the process for the determination of the expressions for the modulation signals required to generate the switching pulses for non-square AC-AC matrix converters. Using the 5 × 3 matrix converter as an example, the converter model is set forth and computer simulation results are presented to demonstrate the possibility of unity input power factor operation and generation of desired output three-phase balanced voltages.

A five-phase interior permanent magnet generator-diode rectifier with a non-integer number of stator slots per phase as the front end of a wind generation system

This paper presents the models of a five-phase interior permanent magnet machine (IPM) feeding a rectifier resistive load. The model predicts the performance of a machine with a non-integer number of stator slots per pole. Using the stationary and rotor reference frames, the approximate diode switching functions are derived. Then harmonic balance technique is used to obtain the steady state average and ripple models for the prediction of the systems performance. The steady state model helps in determining the equivalent ac resistive load across the five-phase generator output capacitor bank. The obtained results reveal that despite the imbalance of the stator windings, the machine is still able to produce near balanced voltages at normal operation, and also the fraction of the rated output power with very little harmonic components in the output load voltage when a rectifier leg is open.

Determination of steady state control laws of doubly - fed induction generator using natural and power variables

A doubly - fed induction generator (DFIG) system is presented in terms of its natural and power variables as the state variables. The natural variables are the electromagnetic torque (T<inf>ℯ</inf>), the reactive torque (T<inf>r</inf>), the magnitude of rotor flux linkage (λ<inf>r</inf>), the magnitude of stator flux linkage (λ<inf>s</inf>), and the rotor speed (ω). These are used in modelling the induction machine and the machine side converter (MSC) of the system. The real power (P<inf>ƒ</inf>) and reactive power (Q<inf>ƒ</inf>) that the grid side converter (GSC) exchanges with the grid make up the power variables of the system and are used to model the GSC. Simulation of the dynamic natural variable model of the induction machine is carried out and the results are compared with a conventional induction machine simulation technique. Steady state analysis is presented for various power factor operations. A method of predicting an optimal stator power factor operation is proposed.

Switching function based modeling of flying capacitor DC-DC converters

This paper presents a new approach to the modeling and analysis of flying capacitor DC-DC converters using the switching function approach. This method helps to obtain the model dynamic equations which can be used in computer simulations to observe the response of the converter, voltages across the switching devices and the flying capacitors. The harmonic balance technique applied to the resulting model dynamic equations enables the determination of equations that predict the steady state variables and the peaks of the ripple quantities. The four-level DC-DC boost converter is analyzed to illustrate the utility of the proposed modeling approach which can be easily extended to other flying capacitor multilevel converter topologies.

Design and analysis of a five-phase interior permanent magnet generator with a non-integer number of stator slots per phase

Traditional electric machine design requiring the balancing of the phases both in phase voltage magnitudes and the angles between adjacent phases informs that the number of stator slots per phase must be an integer. There are situations however, where it is cheaper to use existing laminations for designs with non-integer number of slots per phase to build generators for specialized applications especially for DC power. In such cases a multi-phase design ensures a measure of fault tolerance and the unbalances in the phase angles between the winding phases have little effect on the quality of the output DC voltage. The methodology for the design of a five phase, 4-pole interior permanent magnet (IPM) generator using 36 slots as a source of DC power is outlined. Experimental results for the operation under healthy and faulty conditions (one and two phase windings open) are presented for verification of fault tolerance of multi-phase machines.