David A. Torrey

Switched reluctance generators and their control

This paper discusses how the switched reluctance generator (SRG) converts energy as directed by a controller. Beginning with a review of the electromechanics of generation, the paper identifies the implications of the energy conversion process on how the SRG is controlled. The structure of the SRG controller for speed-control and power-control applications is discussed. Practical implementation details for commutation of the SRG are reviewed. Concepts are illustrated with a 6-kW SRG designed to serve as a starter/alternator in automotive applications.

Magnetic circuit model for the mutually coupled switched-reluctance machine

The mutually coupled switched-reluctance motor (SRM) appears to have several performance advantages over other motor technologies. The existence of strong coupling between phases, however, makes the analysis of this machine quite complicated. Preliminary design of this machine can be greatly accelerated by the ability to evaluate potential motor geometries quickly. This paper introduces a general magnetic circuit model of the mutually coupled SRM that adapts to any geometry, unlike existing geometry-dependent approaches (such as finite elements), which are numerically intensive and require excessive computation time. The model uniquely implements the magneto-motive force (mmf) sources necessary to accommodate complex flux paths through the machine and includes the effects of magnetic saturation. The results are compared to those of a finite element solver to demonstrate the performance of this method as a first-step to evaluating candidate designs.

The design and implementation of a three-phase active power filter based on sliding mode control

This paper presents an active power filter for three-phase power systems. The work is motivated by the need for active filtration in a current-source excitation system for a variable-reluctance generator. The active filter is comprised of a six-switch three-phase inverter, a DC bus capacitor, and an isolation transformer. The isolation transformer is required by the application. The leakage inductance associated with each phase of the isolation transformer is used as the series impedance with each phase, by which the inverter is able to actively shape the phase currents in order to compensate for the nonlinearities of all loads within the point of common coupling. The active filter is controlled through two control loops. The inner current regulation loop uses sliding-mode control by virtue of its ease of implementation. The outer voltage loop regulates the average voltage on the DC bus capacitor. The outer voltage loop is responsible for correctly setting the commanded magnitude of the phase currents. This paper presents the analysis, design, and operation of the active filter. Experimental results are provided for the active filter compensating a phase-controlled rectifier which is drawing 10.4 kW. >

Soft switching active snubbers for dc/ac converters

A soft-switching active snubber is proposed to reduce the turn-off losses of the insulated gate bipolar transistor (IGBT) in a buck power converter. The soft-switching snubber provides zero-voltage switching for the IGBT, thereby reducing its high turn-off losses due to the current tailing. The proposed snubber uses an auxiliary switch to discharge the snubber capacitor. This auxiliary switch also operates at zero-voltage and zero-current switching. The size of the auxiliary switch compared to the main switch makes this snubber a good alternative to the conventional snubber or even to passive low-loss snubbers. The use of the soft-switching active snubber permits the IGBT to operate at high frequencies with an improved RBSOA. In the experimental results reported for a 1 kW, 40 kHz prototype, combined switching/snubbing losses are reduced by 36% through the use of the active snubber compared to a conventional RCD snubber. The use of an active snubber allows recovery of part of the energy stored in the snubber capacitor during turn-off. The generic snubber cell for the buck power converter is generalized to support the common nonisolated DC/DC power converters (buck, boost, buck-boost, Cuk, sepic, zeta) as well as isolated DC/DC power converters (forward, flyback, Cuk, and sepic).

An approach for sensorless position estimation for switched reluctance motors using artifical neural networks

This paper presents a new approach to the sensorless control of the switched-reluctance motor (SRM). The basic premise of the method is that an artificial neural network (ANN) forms a very efficient mapping structure for the nonlinear SRM. Through measurement of the phase flux linkages and phase currents the neural network is able to estimate the rotor position, thereby facilitating elimination of the rotor position sensor. The ANN training data set is comprised of magnetization data for the SRM with flux linkage (/spl lambda/) and current (i) as inputs and the corresponding position (/spl theta/) as output in this set. Given a sufficiently large training data set, the ANN can build up a correlation among /spl lambda/, i and /spl theta/ for an appropriate network architecture. This paper presents the development, implementation, and operation of an ANN-based position estimator for a three-phase SRM.

Modeling and Control of Utility Interactive Inverters

As alternative energy sources become more competitive with traditional energy sources, the proliferation of distributed generation sources that interface to the electric utility grid continues. Most of the alternative energy sources either produce dc directly (solar photovoltaics or fuel cells) or create dc before inversion to the utility (wind or hydroelectric turbines). The dc electric energy is injected into the ac utility through an inverter. The resulting ac electric energy has to be compatible with the energy within the ac utility system at the point where the inverter is connected to the utility system. The control, design, and operation of the inverter must meet the applicable standards. This paper provides an overview of modeling and control of the inverter system that interfaces with the utility grid. Recent advancements in the state of the art are presented along with practical implementations. Simulation and experimental test results are provided to emphasize concepts and illustrate issues. Embedded control of the inverter is assumed to be implemented through digital control techniques. Algorithms are given in general form for application to single- and three-phase inverters with any number of levels.

New inverter output filter topology for PWM motor drives

This paper presents a new inverter output filter topology for pulse width modulation (PWM) motor drives. It is shown that the proposed filter effectively reduces high frequency harmonics which can cause serious damage to the motor bearings and insulation. The proposed filter is comprised of a conventional resistance, inductance, capacitance (RLC) network cascaded with an LC trap filter. The LC trap, tuned to the inverter switching frequency, is very effective in reducing the switching harmonics. By using this new topology the need for high damping resistance and low RLC cut-off frequency is eliminated. This reduces the phase shift in the current regulation loop and increases the filter efficiency. Experimental verification of the filter topology is provided with a 180 V inverter and a 25 hp permanent magnet synchronous motor. Space-vector predictive current regulation is implemented as an inner-loop current regulator for the outer-loop speed control using a digital signal processor. The effectiveness of the filter at different motor speeds is presented.

A passive series, active shunt filter for high power applications

This paper presents a hybrid series passive/shunt active power filter system for high power nonlinear loads. This work is motivated by the fact that the ability of a converter to perform effectively as an active filter is limited by the power and the frequency distribution of the distortion for which it must compensate. This system is comprised of a three-phase shunt active filter and series AC line smoothing reactance installed in front of the target load. The proposed system significantly reduces the required shunt active filter bandwidth. The space-vector pulse width modulation (PWM) controller is based on a dead-beat control model. It is implemented digitally using a single 16-bit microcontroller. This controller requires only the supply current to be monitored, an approach different from conventional methods. The paper provides background on the operation of the filter, the details of the power circuit, the details of the control design, representative waveforms, and spectral performance for a filter which supports a 15 kVA phase controlled rectifier load. Experimental data indicate that the active filter typically consumes 2% or less of the average load power, suggesting that a parallel filter is an efficient compensation approach. The spectral performance shows that the active filter brings the system into compliance with IEEE519-1992 up to the 33rd harmonic for an AC line smoothing reactance of 0.13 p.u.

Series connection of IGBT's with active voltage balancing

This paper describes an active gate drive circuit for series-connected insulated gate bipolar transistors (IGBTs) with voltage balancing in high-voltage applications. The gate drive circuit not only amplifies the gate signal, but also actively limits the overvoltage during switching transients, while minimizing the switching transients and losses. In order to achieve the control objective, an analog closed-loop control scheme is adopted. The closed-loop control injects current to an IGBT gate as required to limit the IGBT collector-emitter voltage to a predefined level. The performance of the gate drive circuit is examined experimentally by the series connection of three IGBTs with conventional snubber circuits. The experimental results show the voltage balancing by an active control with wide variations in loads and imbalance conditions.

A Three-Phase Unity Power Factor Single-Stage AC–DC Converter Based on an Interleaved Flyback Topology

This paper presents the design and implementation of a three-phase unity-power-factor single-stage ac-dc converter based on an interleaved flyback topology. The primary market target of the converter is within the telecommunications industry where it supplies high-quality dc power to the telecom loads and performs high-capacity battery charging while providing zero harmonic emission and unity power factor to the utility grid. The main design objective is to produce the lowest cost within a small-size system. The study includes mathematical analysis and simulation steps where the optimum number of cells to be interleaved and the associated phase shifts among the cells are determined while the emphasis being on the design of a perfectly coupled flyback transformer. Design of a transformer with the lowest leakage inductance and selection of components providing the lowest parasitic effects are critical for obtaining high efficiency and good performance. After the design is verified through simulation studies that uses Simulink and piecewise linear electrical circuit simulation model (PLECS) of the converter, a full-scale prototype is implemented to evaluate the performance of the design. In conclusion, experimental results demonstrate that converter works successfully and meets the commercialization expectations.