32.

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}

31.

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}

30.

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}

29.

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

28.

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}

27.

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}

26.

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}

25.

J. Holtz and B. Beyer, "Optimal synchronous pulsewidth modulation with a trajectory-tracking scheme for high-dynamic performance", IEEE Transactions on Industry Applications, vol. 29, no. 6, pp. 1098--1105, 1993.

Abstract:
Synchronous pulse-width modulation (PWM) based on precalculated and stored optimal pulse patterns could be a superior method for the control of high-power inverters operated at low switching frequency. The technique has rarely been applied in practice owing to its poor dynamic performance. A novel feedforward control technique eliminates this decisive drawback: the space vector of the machine currents is forced to track a precalculated optimal trajectory in the steady-state and under the transient operating conditions commanded by the drive control system. Undesired transients caused by the pulse-width modulator are avoided. Experimental results obtained from a 30 kW AC drive are presented.{\textless}{\textgreater}

24.

J. Holtz and L. Springob, "Reduced harmonics PWM controlled line-side converter for electric drives", IEEE Transactions on Industry Applications, vol. 29, no. 4, pp. 814--819, 1993.

Abstract:
The concept for a reduced-harmonics PWM modulator, as applied for the control of a line-side power converter for a variable speed AC motor drive, is described. The PWM algorithm determines the on-state duration of each switching vector based on the observation of the time-variable voltage reference vector. Since there is no reference made to a constant-frequency carrier signal, the generated pulse patterns become asynchronous. It is the essential property of this method to produce a quasicontinuous harmonic spectrum in which all frequency components have more or less equal magnitudes. This is an advantage as compared with carrier-based PWM control schemes that exhibit high-amplitude carrier and side-band components in their harmonic spectra. The emission of acoustic noise radiated from the AC filter inductor is reduced.{\textless}{\textgreater}

23.

J. Holtz, "Speed estimation and sensorless control of AC drives" in Proceedings of IECON '93 - 19th Annual Conference of IEEE Industrial Electronics, 1993, pp. 649-654 vol.2.

Abstract:
The operation of speed controlled AC 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 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 description of the physical and mathematical systems used in sensorless control.{\textless}{\textgreater}