62.

J. Holtz, "Is sensorless position control of standard induction motors a feasible technology?" in Proceedings IPEMC 2000. Third International Power Electronics and Motion Control Conference (IEEE Cat. No.00EX435), 2000, pp. 21-32 vol.1.

Abstract:
Concepts for sensorless position control of induction motor drives rely on anisotropic properties of the machine rotor. The built-in spatial anisotropy is detected by injecting a high-frequency flux wave into the stator. The resulting stator current harmonics contain frequency components that depend on the rotor position. Models of the rotor saliency serve to extract the rotor position signal using phase-locked loop techniques. A different approach makes use of the parasitic effects that originate from the discrete winding structure of a cage rotor. It has the merit of providing high spatial resolution for incremental positioning without a sensor. The practical implementation of sensorless position identification, and of a high-accuracy position control system are reported.

61.

J. Holtz, "Sensorless control of induction motors-performance and limitations" in ISIE'2000. Proceedings of the 2000 IEEE International Symposium on Industrial Electronics (Cat. No.00TH8543), 2000, pp. PL12-PL20 vol.1.

Abstract:
Controlled induction motor drives without mechanical sensors for speed or motor shaft position have the attraction of low cost and high reliability. The identification of rotor speed is generally based on measured terminal voltages and currents. Dynamic models are used to estimate the magnitude and spatial orientation of the fundamental magnetic flux waves in the stator or in the rotor. Open loop estimators and closed loop observers differ with respect to accuracy, robustness, and sensitivity against model parameter variations. Dynamic performance and steady-state speed accuracy in the low speed range is achieved by exploiting parasitic effects of the machine. The overview in this paper uses signal flow graphs of complex space vector quantities to provide an insightful description of the systems used in sensorless control of induction motors.

60.

F. Joswig and S. Soter, "Windenergienutzung - Ein Vergleich von Technik und Kosten" in European Wind Energy Conferences and Exhibitions, 2000.

59.

S. Soter and S. Buchhold, "Wirkungsgrad- und Kostenvergleich unterschiedlicher Schaltungskonzepte zur Netzeinspeisung von Leistungen bis 10 Kilowatt und kleinen Gleichspannungen (20 bis 80 Volt)" in 15. Symposium Photovoltaische Solarenergie, 2000.

58.

J. Holtz and M. Stamm, "Gate-assisted reverse and forward recovery of high-power GTOs in series resonant DC-link inverters", IEEE Transactions on Power Electronics, vol. 14, no. 2, pp. 227--232, 1999.

Abstract:
The series resonant DC-link inverter is an attractive circuit topology for interfacing a DC current with a three-phase AC system. It uses gate turn-off thyristors (GTOs) as semiconductor switches. The conventional solution requires an additional series diode to perform the turn off process and to enable forward recovery of the GTO. This paper uses a single GTO along with a special gate drive to provide reverse and forward recovery. A new device testing circuit was designed to create the same electrical and thermal stresses as in a series resonant DC-link inverter. Experimental results using 2000 A GTOs at 26 kHz switching frequency demonstrate that the total device losses are reduced, while the hold-off time is slightly increased. The new single-device solution makes resonant switching attractive for very high-power applications.

57.

J. O. Krah and J. Holtz, "High-performance current regulation and efficient PWM implementation for low-inductance servo motors", IEEE Transactions on Industry Applications, vol. 35, no. 5, pp. 1039--1049, 1999.

Abstract:
This paper reports on a standard microcontroller implementation of a pulsewidth modulator and near-deadbeat current regulator for high switching frequency. The application is in high-performance positioning systems. The control strategy relies on a simplified machine model without incurring performance degradations. Changes between different modulation strategies are programmed depending on the modulation index. The values of switching time durations are obtained exclusively by decision making, thus minimizing computational load. Features like over-modulation (OVM), dynamic OVM, anti-windup, and reduction of switching frequency at thermal overload are included.

56.

E. P. Wiechmann, R. P. Burgos and J. Holtz, "Optimized sequential control for electrowinning high-current phase-controlled rectifiers" in IECON'99. Conference Proceedings. 25th Annual Conference of the IEEE Industrial Electronics Society (Cat. No.99CH37029), 1999, pp. 241-246 vol.1.

Abstract:
This paper proposes an optimized sequential control technique for electrowinning 12-pulse high-current rectifiers. The converter comprises two series connected 6-pulse double-wye rectifiers, a step-down transformer and a tuned input filter. The 6-pulse rectifiers are fed from delta and polygon primary windings with different turns ratio and phase shifted by 5/spl deg/. Under the proposed control scheme, one rectifier is kept at nominal output voltage and the other one is phase controlled to control the load's current. The proposed strategy greatly improves the rectifier's performance, reducing Its reactive power maximum demand by 62{%} compared to conventional rectifiers. Therefore reduces the input filter's power rating also by 62{%}. All this while maintaining an input power factor above 0.95. Further, the converter's reactive power consumption presents a low varying characteristic, allowing it to use a fixed filter even when operating from a power system not capable of withstanding large reactive power variations. Finally, it presents a harmonic current distortion comparable to conventional 12-pulse rectifiers. This paper presents the design and optimization procedure of the rectifying system. Simulation results are used to evaluate the converter in a 10.5 MVA electrowinning facility, and in conjunction with results obtained with a 2.5 kVA experimental prototype are used to validate the proposed converter and control strategy.

55.

A. M. Khambadkone and J. Holtz, "Fast current control for low harmonic distortion at low switching frequency", IEEE Transactions on Industrial Electronics, vol. 45, no. 5, pp. 745--751, 1998.

Abstract:
The structure of the current control loop of an induction machine drive determines decisively the dynamic performance of the overall system. Fast current control is a prerequisite for dynamic decoupling between the torque and the flux commands. Standard solutions are well established for drives in the low- and medium-power ranges. The low switching frequency of high-power pulsewidth modulation inverters calls for a tradeoff in controller design between the low harmonic losses and torque ripple in the steady state on one hand, and fast dynamic response during the transients on the other. The problem is developed in detail. A variable-structure approach is proposed as the solution.

54.

L. Springob and J. Holtz, "High-bandwidth current control for torque-ripple compensation in PM synchronous machines", IEEE Transactions on Industrial Electronics, vol. 45, no. 5, pp. 713--721, 1998.

Abstract:
Active compensation of torque harmonics in high-performance synchronous permanent magnet (PM) motor drives requires high-bandwidth current control. It is demonstrated that proportional integral (PI) current control exhibits performance limits, even when feedforward compensation of the rotor induced voltage and the stator inductance drop is used. High bandwidth requirements are satisfied using a digital deadbeat current controller. Sampling time delays are eliminated to the extent possible by means of a current predictor. The current controller and the predictor refer to a model of the parasitic effects of the PM synchronous machine that is acquired and adapted to parameter changes in real time. Stator current distortions due to deviations from the sinusoidal flux linkage distribution are thus eliminated. The control system facilitates compensation of high-frequency torque ripple of the machine.

53.

Y. Lou, J. Holtz and T. H. Lee, "Critical implementation issues in compensation for nonlinearities in industrial robot manipulators by adaptive multilayer neural networks" in Proceedings of the 1998 American Control Conference. ACC (IEEE Cat. No.98CH36207), 1998, pp. 2200-2202 vol.4.

Abstract:
To improve the performance of an industrial robot manipulator with linear individual-joint controllers, an adaptive feedforward multilayer neural network (MNN) is proposed as an addition to the existing linear control structure at each joint to compensate the nonlinearity. System stability is guaranteed by three measures: the initialization of the MNN, which ensures that the MNN learning start from a reasonable point; a Lyapunov-based adaptive law in which the MNN is linearized and the residual error is tolerated by a dead-zone or a leakage term; and a contribution function which manipulates the contribution of the MNN to the system. The MNN and the control algorithm are implemented on a TMS320C30 digital signal processor. The realization on a two-link manipulator demonstrates the effectiveness of the proposed scheme.