Olorunfemi Ojo

PWM-VSI inverter assisted stand-alone dual stator winding induction generator

This paper presents a novel usage of a dual stator winding, three-phase induction machine as a stand-alone generator with both controlled output load voltage magnitude and frequency. This generator, with both three-phase power and control windings housed in the stator structure, has the load connected to the power winding and a three-phase PWM voltage source inverter sourcing the control winding. The input to the PWM inverter is either a battery source or a charged DC capacitor. The operational characteristics of these generator schemes with either of the two inverter sources is investigated and shown to have desirable performance. How the load voltage magnitude depends on the various control and design parameters such as rotor speed, compensating capacitance and load impedance are determined using a detailed mathematical model of the system which is confirmed to accurately predict dynamic and steady-state performance characteristics.

The generalized discontinuous PWM scheme for three-phase voltage source inverters

This paper presents analytical techniques 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, three-phase voltage source inverters. The proposed modulation schemes are applicable to inverters generating balanced or unbalanced phase voltages. Some results presented in this paper analytically generalize the several expressions for the modulation signals already reported in the literature and new ones are set forth for generating unbalanced three-phase voltages. Confirmatory experimental and simulation results are provided to illustrate the analyses.

Current Regulation in Four-Leg Voltage-Source Converters

Three-phase four-leg voltage-source converters by virtue of the fourth leg connected to the neutral points of the wye-connected loads effectively handle the zero-sequence currents and voltages occasioned by connected load impedance imbalances and/or nonlinearity of loads. This paper develops a generalized carrier-based pulsewidth-modulation scheme for this converter which with a natural reference-frame control scheme that directly controls the actual load currents ensures current regulation irrespectively of the nature of the load. The proposed modulation and control schemes are effectively validated by computer simulation and experimental results for the converter operation in the linear and overmodulation regions.

Generalized discontinuous carrier-based PWM modulation scheme for multi-phase converter-machine systems

Using an expanded reference frame transformation expressed in the complex-variable form, the possibilities offered by multi-phase converters, using carrier-based, sine-triangle pulse-width modulation to generate voltages of different frequencies to actuate multi-phase electric machines are set forth. The generalized expressions for the modulation signals, including the neutral voltage component to be compared with the high frequency triangle waveform to synthesize reference voltages for both five and six-phase converter-electric machines are determined. How the inclusion of the neutral voltage affects the modulation signals and the voltage gain and waveform quality is discussed. Some experimental results of synthesized five phase converter voltage waveforms are provided to validate the voltage waveform synthesis using carrier-based generalized (dis)continuous pulse-width modulation for operation in both the under and over- modulation regimes.

Efficiency Optimizing Control of Induction Motor Using Natural Variables

This paper presents a new approach of optimizing the efficiency of induction-motor drives through minimizing the copper and core losses. The induction-machine model, which accounts for the varying core-loss resistance and saturation dependent magnetizing inductance, uses natural and reference frame independent quantities as state variables. Utilization of the nonlinear geometric control methodology of input-output linearization with decoupling permits the implementation of the control in the stationary reference frame. This approach eliminates the need of synchronous reference transformation and flux alignment required in classical vector control schemes. The new efficiency optimizing formulation yields a reference rotor flux, which ensures a minimum loss and yields an improved efficiency of the drive system especially when driving part load. The proposed scheme and its advantages are demonstrated both by computer simulations and some experimental results for motor speed control

An analysis of single-phase self-excited induction generators: model development and steady-state calculations

The modeling and steady-state performance of single-phase induction generators based on the principles of harmonic balance is set forth in this paper. Magnetizing flux linkage saturation and flux dependent core loss resistances are included. Experimental results are provided to justify the analytical approach and steady-state calculations. >

Performance of self-excited single-phase induction generators with shunt, short-shunt and long-shunt excitation connections

The influence of three capacitor excitation topologies (shunt, short-shunt and long-shunt) on the steady-state and dynamic performance of a single-phase, self-excited induction generator is explored in this paper. Attention is focused on the influence of the different capacitor connections on the generator overloaded and output power capabilities. The generator voltage with shunt excitation connection collapses when overloaded, while with either the long or short-shunt excitation connection the generator is able to sustain the load at a lower operating voltage and larger load current.

The operation of an inverter-assisted single-phase induction generator

The enhancement of the operability range of a stand-alone single-phase induction generator scheme with an inverter-battery system connected to the auxiliary winding is the theme of this paper. The reactive power requirement of the load connected to the main winding is met by the inverter system operating with either of two proposed pulsewidth modulation techniques, in the process of which the load voltage and frequency are regulated. The proposed schemes have potential application as power sources for autonomous systems and for utility interface to single-phase power system grids. Simulation and calculation results of the generator system connected to an impedance and motor loads, confirmed with some experimental results, are also set forth to validate some of the conclusions of the paper.

Minimum airgap flux linkage requirement for self-excitation in stand-alone induction generators

A minimum airgap flux linkage is required for the self-excitation and stable operation of an isolated induction generator feeding an impedance load. With the aid of bifurcation theory, it is shown that the minimum airgap flux linkage requirement is the value at which the derivative of the magnetizing inductance with respect to the airgap flux linkage is zero. This minimum airgap flux linkage determines the minimum or maximum load impedance and minimum excitation capacitance requirements. This result is demonstrated using single-phase and three-phase induction generators. >

The transient and qualitative performance of a self-excited single-phase induction generator

The modeling and transient performance of a single-phase induction generator with series or parallel connected load is the theme of this paper. The system of equations are expressed in terms of flux linkages and includes the effect of magnetizing flux linkage saturation. Generator self-excitation and voltage collapse phenomena are simulated. The balance of the paper deals with the qualitative behavior of the generator using concepts of harmonic balance and system bifurcation. >