Jacobus Daniel van Wyk

A Comparative Investigation of a Three-Phase Induction Machine Drive with a Component Minimized Voltage-Fed Inverter under Different Control Options

Possibilities for minimizing component cost in variable- speed drives with frequency control by solid-state converter are investigated. It is shown that in a three-phase system voltage and frequency control of a three-phase squirrel-cage machine may be attained by PWM of the output of a simplified bridge inverter with only four switches. As the phase voltages are in principle asymmetrical, this technique is called phase asymmetric PWM (PAPWM). The operational region of PAPWM control is investigated theoretically in the frequency plane, compared to PWM control of a six-element bridge, and shown to have application possibilities. The system was experimentally tested on a 1O-kVA four-transistor inverter and experimentally compared to the operation of a six-transistor inverter of the same rating. The entire signal electronics subsystem was also minimized regarding components by using a specially dedicated IC circuit developed for PWM applications in three-phase systems. These results prove that a PAPWM system with a B4 bridge can be operated to give the same operational drive characteristics as a B6 bridge with PWM. The price to be paid for using a bridge with only four switches lies in the higher pulse frequencies required to give the PAPWM the same characteristics as a conventional system. Since transistor switches may be operated at high switching frequencies with low switching losses, the system is well suited to transistor inverter control of induction machine drives.

Simulation and Experimental Study of a Reactively Loaded PWM Converter as a Fast Source of Reactive Power

The question of eliminating distorted current waveforms from power networks by active power filters is investigated. If the active current is subtracted from the distorted current, the compensating equipment must be capable of generating the inverse of this difference. Addition of this compensating current to the distorted current then eliminates the distortion. This requires the capability to generate these currents with response times many times less than the fundamental power frequency cycle. This capability is presented by reactively loaded PWM converters. A system configuration with three single-phase PWM converters building a three-phase unit is investigated. The single-phase converter with PWM operation, inductive reactance as load, low-pass input filter, and representative system losses is simulated by state-space techniques. An experimental system of 1 kVA is also constructed for verification. Good correspondence between the simulation and experiment s shown. Starting up the system, compensating the system losses from the supply, and the influence of the control system on the compensating capabilities are investigated. It is concluded that a PWM converter with a reactive load can be used as a fast distortion compensator and that the present state of the art and future development of PWM converter technology will make this technique applicable at least up to 1 Mvar.

Study and Application of Nonlinear Turn-Off Snubber for Power Electronic Switches

A nonlinear capacitive turn-off snubber is discussed. Customarily, only linear capacitors are used in high-frequency power electronic switches. However, ceramic ferro-electric materials, with ratios of initial to final capacitance of nearly 30, are found suitable and are investigated. The energy stored in these nonlinear capacitors, with an initial capacitance of 300 nF, amounts to only about 1.1 mJ at 300 V compared to 13.5 mJ in a 300-nF linear capacitor. As far as the power switch is concerned, the difference in switching behavior and switch turnoff loss between these two cases is negligible. This nonlinear snubber is analyzed approximately by assuming an exponential relation between capacitance and voltage, as well as with a more accurate analysis by means of a computer program. The latter uses an exact capacitance-voltage curve stored on a data file. Computed values correlated well with experimental measurements. As the dielectric losses in the ceramic material are very important, careful calorimetric measurements in situ revealed an energy loss in the capacitor of 135 ¿J per turn-off cycle, taken at 500 Hz, 30 A, and 300 V.

Transistor Inverter Design Optimization in the Frequency Range Above 5 kHz Up to 50 kVA

Different aspects of the design of inverters for industrial applications including static power supplies, light welding equipment, and high-speed motor control at input voltages from 30 to 300 V are described. High frequencies (5-30 kHz) are used at power levels between 1 and 50 kW, and emphasis is placed on minimization of losses in order to obtain efficient and compact systems. Attention is given to optimization of snubber technology for low switching losses and feedback of the snubbing power to the supply at the larger powers, design aspects of large ferrite transformers, as well as protection of the equipment. The on-voltage of the Darlington switches is reduced by adding a compensation voltage source to the base of the main transistor, thus lowering on-losses drastically. Experimental results obtained from 1-kVA, 6-kVA, and 50-kVA single-phase units are used to highlight certain aspects of the inverter design principles.

On-Line Harmonic Analysis as a Diagnostic Design and Control Tool for Power Systems Feeding Arc Furnaces, Thyristor-Controlled Mill Drives, and Power Factor Correction Equipment

Microprocessor-based signal processing equipment for frequencies up to 20 kHz have become compact and fast enough to allow direct on-line real-time analysis on power systems to determine behavior. A case study is presented for a small steelworks operating a 25-MVA open arc furnace, installing 6.5 MVA of thyristor control equipment and further power factor correcting equipment and harmonic filters on a system with ill-defined parameters, low short- circuit capacity, and already containing power factor correcting capacitors tuned at the third harmonic. Discrete spectrum analysis equipment, microprocessor-based and coupled via IEEE-bus to a microcomputer system, was used to do on-line analysis at various points in the system. The high-speed system enabled sophisticated signal processing to yield even and uneven harmonics, stochastic components, system parameters and resonances and mutual interference. It is shown how this is used to rectify problems in the system, adapt equipment, and design power factor and filter equipment.