Richard G. Hoft

Generalized Techniques of Harmonic Elimination and Voltage Control in Thyristor Inverters: Part I--Harmonic Elimination

This paper considers the theoretical problem of eliminating harmonics in inverter-output waveforms. Generalized methods are developed for eliminating a fixed number of harmonics in the half-bridge and full-bridge inverter-output waveforms, and solutions are presented for eliminating up to five harmonics. Numerical techniques are applied to solve the nonlinear equations of the problem on the computer. The uneliminated higher order harmonics can be easily attenuated by using filter circuits in the output stage of the inverter. The results show the feasibility of obtaining practically sinusoidal output waveforms, which are highly desirable in most inverter applications.

Generalized Techniques of Harmonic Elimination and Voltage Control in Thyristor Inverters: Part II --- Voltage Control Techniques

Theoretical techniques of voltage control for the half-bridge and full-bridge inverters are derived based on the results in [1]. Detailed analytical results for the symmetrical pulsewidth modulation method of voltage control are also presented. Voltage control techniques are derived whereby harmonic elimination is possible in variable-frequency variable-voltage three-phase inverter circuits. The technique for the half-bridge inverter is optimized subject to the constraint of switching frequency of the SCRs, using the concepts of modern control theory. Variable-frequency variable-voltage sinusoidal output in three-phase inverters is possible by employing the techniques developed. The methods show great promise in application to variable-speed ac motor drive systems.

Dead Beat Microprocessor Control of PWM Inverter for Sinusoidal Output Waveform Synthesis

A new control technique based on dead beat control theory to obtain a nearly sinusoidal PWM inverter output voltage is described. The closed loop digital feedback system measures the output and controls the inverter switches to generate the required PWM pattern to produce low total harmonic distortion (THD) sinusoidal output voltage. This scheme inherently provides very good voltage regulation, phase positioning and compensation for load disturbances and nonlinear loads. This paper presents a theoretical analysis, computer simulation and experimental results for a single-phase bridge inverter controlled by an Intel 8086 microprocessor based system.

Waveform compensation of PWM inverter with cyclic fluctuating loads

A repetitive voltage compensation technique that generates a high-quality sinusoidal output voltage from a single-phase pulse width modulation (PWM) inverter used for uninterruptible power supplies is described. A repetitive control technique eliminates the steady-state error in the distorted output voltage caused by cyclic loads. The proposed PWM inverter system uses microprocessor-based closed-loop digital feedback with a sinusoidal reference. The PWM pattern is determined at every sampling instant by the proposed algorithm, implemented by a microprocessor using a set of detected output voltages and the reference signals through one cycle. The system has low distortion and very fast response for AC phase-controlled loads. >

Inverter harmonic reduction using Walsh function harmonic elimination method

A pulse-width-modulated (PWM) inverter using the Walsh function harmonic elimination method is proposed in this paper. By using the Walsh domain waveform analytic technique, the harmonic amplitudes of the inverter output voltage can be expressed as functions of switching angles. Thus, the switching angles are optimized by solving linear algebraic equations instead of solving nonlinear transcendental equations. The local piecewise linear relations between the switching angles and the fundamental amplitude can be obtained under an appropriate initial condition. By searching all feasible initial conditions, the global solutions are obtained. The relations between switching angles and fundamental amplitude can be approximated by straight-line curve fitting. Thus, on-line control of fundamental amplitude and frequency is possible for the microcomputer-based implementation. The developed algorithm can be applied to both bipolar and unipolar switching schemes. The theoretical predictions are confirmed by computer simulations and DSP-based hardware implementation.

Instantaneous Feedback Controlled PWM Inverter with Adaptive Hysteresis

A new control strategy for a PWM inverter controlled through adaptive hysteresis in an instantaneous feedback loop is theoretically analyzed and verified through simulations and a low-power experimental circuit. This control gives excellent performance under various load conditions, and it is especially effective in reducing load injected harmonics.

Adaptive frequency domain control of PWM switched power line conditioner

An active power line conditioner (PLC) which minimizes harmonic current in the AC line and improves the power factor to unity is presented. A six-switch pulse-width-modulated (PWM) current source inverter implements the active PLC. Simulation results of the PLC adaptive frequency-domain control using an innovative PWM switching algorithm are presented.<<ETX>>

A high frequency AC/DC converter with unity power factor and minimum harmonic distortion

A new forced commutated ac/dc converter and control strategy is proposed that is able to draw nearly sinusoidal currents at unity power factor from three-phase power lines. The power factor is controlled by adjusting the relative position of the fundamental component of an optimized PWM type voltage with respect to the supply voltage. Current harmonic distortion is minimized by the use of optimized firing angles for the converter at a frequency where GTOs can be used. This feature makes this approach very attractive at power levels of 100 kW to 600 kW. An 8096 microcontroller is used to minimize the interface hardware requirements. This paper presents the theoretical analysis of the converter, the control strategy and experimental results for a low power prototype.

Dead beat microprocessor control of PWM inverter for sinusoidal output waveform synthesis

A new control technique based on dead beat control theory to obtain a nearly sinusoidal PWM inverter output voltage is described. The closed loop digital feedback system measures the output and controls the inverter switches to generate the required PWM pattern to produce low total harmonic distortion (THD) sinusoidal output voltage. This scheme inherently provides very good voltage regulation, phase positioning and compensation for load disturbances and nonlinear loads. This paper presents a theoretical analysis, computer simulation and experimental results for a single-phase bridge inverter controlled by an Intel 8086 microprocessor based system.

Performance of a 10-Hp switched reluctance motor and comparison with induction motors

A comparison of computed and measured performances of a 10 hp Oulton switched reluctance motor (SRM) and a comparison with the performance of induction motors are presented. The instantaneous current and torque waveforms are computed for the SRM using the results of finite element method (FEM) analysis. The SRM calculated and experimental locked rotor torque, instantaneous current, efficiency, and losses when pulsed voltage excitation is applied to the motor windings were compared. In all cases, there is quite good agreement between the theoretical and experimental results. In addition, the measured total losses, efficiency, and temperature rise for a 10 hp SRM and different induction motors of the same rating are presented. The SRM has a higher efficiency and a lower temperature rise than the induction motors tested. >