40.

J. O. Krah and J. Holtz, "Total compensation of line-side switching harmonics in converter-fed AC locomotives", IEEE Transactions on Industry Applications, vol. 31, no. 6, pp. 1264--1273, 1995.

Abstract:
PWM controlled line-side power converters of modern AC traction locomotives inject harmonic currents into the feeding overhead line. This causes problems of electromagnetic interference. Passive or active filters are usually provided for a partial reduction of the line harmonics. A novel and superior approach employs a switched electronic compensator. It generates an exact replica of the harmonic current, feeding it to the high-voltage transformer to produce a harmonic counter MMF. The transformer main flux is then forced to be sinusoidal, and so is the induced voltage in the primary. The line current, being low-pass filtered by the transformer leakage inductance, assumes a pure sinusoidal waveform. The compensator operates in the low switching frequency range of the main power converters. Its installed power is only 1{%} of the traction power. Operation, control, and design considerations are described. Results from laboratory tests at full power level are presented.

39.

J. Jiang and J. Holtz, "Speed sensorless AC drive for high dynamic performance and steady state accuracy" in Proceedings of IECON '95 - 21st Annual Conference on IEEE Industrial Electronics, 1995, pp. 1029-1034 vol.2.

Abstract:
Controlled speed sensorless AC motor drives have reached a stage of development permitting good dynamic performance above 3{%} of rated speed. However, the accuracy of the rotor speed estimation under load remains sensitive to parameter errors of the internal machine model. This paper presents an approach that ensures high steady-state speed accuracy in addition to high dynamic performance. To eliminate the speed estimation error, the machine parameters are adapted online, based on the evaluation of rotor slot harmonic effects. A stator flux oriented control scheme is implemented in a digital signal-processor system to demonstrate the robustness of the speed estimation to parameter variations. Experimental results demonstrate that the control system advantageously combines high dynamic performance with accuracy of speed estimation.

38.

J. Holtz, "State of the art of controlled AC drives without speed sensor" in Proceedings of 1995 International Conference on Power Electronics and Drive Systems. PEDS 95, 1995, pp. 1-6 vol.1.

Abstract:
The operation of speed controlled AC motor drives without mechanical speed or position sensors requires the estimation of internal state variables of the machine. The assessment is based exclusively on measured terminal voltages and currents. Low cost, medium performance sensorless drives can be designed using simple algebraic speed estimators. High-performance systems rely on dynamic models for the estimation of the magnitude and spatial orientation of magnetic flux waves in the stator or in the rotor. Open loop estimators and closed loop observers differ with respect to accuracy, robustness, and limits of applicability. The overview in this paper uses signal flow graphs of complex space vector quantities to give an insightful description of the physical and mathematical systems used in sensorless control.{\textless}{\textgreater}

37.

J. Rodriguez, E. Wiechmann, J. Holtz, A. Suarez and M. Sepulveda, "IGBT inverter with vector modulation" in Proceedings of 1994 IEEE International Symposium on Industrial Electronics (ISIE'94), 1994, pp. 131--136.

Abstract:
This work presents the design of a three-phase inverter with insulated gate bipolar transistors (IGBTs). The pulse width modulation technique based on the space vector theory is analysed in detail. The modulation strategy considers the calculation of the on-times of the different voltage vectors that must be applied to the load, to generate the voltage vector required by the speed control system of the machine. These times are calculated previously and stored in a ROM, producing a simple, reliable, compact and low cost digital modulator. The inverter behaves as a voltage source with variable amplitude and frequency. By using the space vector theory, the number of commutations in the inverter is minimized, reaching a reduction in the switching frequency and in the current distortion.{\textless}{\textgreater}

36.

J. Holtz, "The induction motor-a dynamic system" in Proceedings of IECON'94 - 20th Annual Conference of IEEE Industrial Electronics, 1994, pp. P1-P6 vol.1.

Abstract:
Induction motors are nonlinear higher-order dynamic systems of considerable complexity. They are well amenable to a formal mathematical analysis. It has proved more difficult, however, to comprehend and penetrate in an imaginative way the principles of their operation under transient conditions. Complex graphic signal flow graphs are suited to visualize the dynamic performance of AC machines. They can assist to form an understanding of the internal dynamic processes of a machine and their interactions with external controls.{\textless}{\textgreater}

35.

J. Holtz, "The dynamic representation of AC drive systems by complex signal flow graphs" in Proceedings of 1994 IEEE International Symposium on Industrial Electronics (ISIE'94), 1994, pp. 1--6.

Abstract:
Induction motors represent nonlinear higher-order dynamic systems of considerable complexity. They are well amenable to a formal mathematical analysis. It has proved more difficult, however, to comprehend and penetrate in an imaginative way the principles of their operation under transient conditions. Complex graphic signal flow graphs are suited to visualize the dynamic performance of AC machines. They can assist to form an understanding of the internal dynamic processes of a machine and their interactions with external controls.{\textless}{\textgreater}

34.

S. Soter, "Systemtechnik für photovoltaisch gespeiste Asynchronmaschinen zum Antrieb von Wasserpumpen" in Programm zur Einführung photovoltaischer Pumpen, GTZ, Eds. 1994.

33.

Y. Lou and J. Holtz, "Improving the performance of industrial robot manipulators by neural networks" in Proceedings of IECON'94 - 20th Annual Conference of IEEE Industrial Electronics, 1994, pp. 1265-1270 vol.2.

Abstract:
Robot manipulators are nonlinear systems. Centrifugal and Coriolis forces as well as the influence of gravitation and friction depend on the state variables of the system. In the presence of strong nonlinearities, linear PID controllers for the individual joint axis drives, usually employed in industrial applications, cannot provide satisfactory performance due to their inherent limitations. Model-based schemes have the disadvantage that they require accurate system models, which are difficult to obtain. The problem is solved by using multilayer feedforward neural networks, which do not rely on a system model. They are used as an addition to the existing linear individual joint control structure. The convergence of the system is proved using Lyapunov's stability theory. Experiments obtained on a two-degree-of-freedom manipulator demonstrate the effectiveness of the proposed technique.{\textless}{\textgreater}

32.

J. Holtz, M. Stamm and O. Stielau, "The performance of high-power GTOs operated as fast turn-off thyristors" in Proceedings of IECON'94 - 20th Annual Conference of IEEE Industrial Electronics, 1994, pp. 112-117 vol.1.

Abstract:
The series resonant DC link inverter is an attractive circuit topology for interfacing a superconducting magnetic energy storage (SMES) inductance with the electric utility. The principle of zero current switching (ZCS) ensures low switching losses at high switching frequency, provides power factor correction capability, and reduces the harmonic line currents. For applications at high power level, the use of GTOs as semiconductor switches is mandatory. High-power devices behave differently at switching than the smaller GTOs treated in previous publications. The device losses in the kiloampere range are reduced by gate-assisted forward recovery. The dynamic performance of different power switch topologies is experimentally evaluated.{\textless}{\textgreater}

31.

J. Holtz and B. Beyer, "Fast current trajectory tracking control based on synchronous optimal pulsewidth modulation" in Proceedings of 1994 IEEE Industry Applications Society Annual Meeting, 1994, pp. 734-741 vol.1.

Abstract:
State-of-the-art pulsewidth modulation techniques fail to give satisfactory results when high-bandwidth torque control is required at the low switching frequency of modern high-power AC machine drives. A novel current control method refers to precalculated optimal synchronous pulse patterns. These are used to generate specific current reference trajectories to be adapted on line to the actual dynamic torque command. A fast tracking controller, operated in parallel to a conventional PI controller, minimizes the trajectory tracking error. The optimized current trajectories ensure fast response and minimum harmonic distortion.{\textless}{\textgreater}