84.

E. P. Wiechmann, R. P. Burgos and J. Holtz, "Active front-end converter for medium-voltage current-source drives using sequential-sampling synchronous space-vector modulation", IEEE Transactions on Industrial Electronics, vol. 50, no. 6, pp. 1275--1289, 2003.

Abstract:
This paper presents an active front-end converter for medium-voltage current-source drives. It comprises two series-connected pulsewidth-modulation (PWM) rectifiers fed from a delta-wye isolation transformer. These employ sequential-sampling synchronous space-vector modulation (SVM), and operate at a switching frequency of 100 Hz, thus reducing the switching losses by 66{%} compared to previous PWM approaches. Digital processing requirements of this technique are minimized by using decision-making SVM. This online modulation technique ensures full harmonic current cancellation through the magnetic interface, and minimizes the sampling/control-action delay. Therefore it enables the use of advanced control strategies notwithstanding the low sampling frequency of the control system. Exploiting this fact, the proposed converter is controlled using a nonlinear strategy in the synchronous frame. The strategy employs input-output and feedback linearization, and is derived from the complex-signal flow diagram of the converter. The proposed strategy linearizes and partially decouples the converter d-q-axes dynamics, ensuring a dynamic response totally independent from the operating point, even under regeneration. This is a highly desirable feature for high-performance drives presenting continuous transitions from motoring to regeneration. Moreover, the converter presents an independent active and reactive power flow, which enables it to operate as a reactive power compensator if desired. Computer analysis and experimental work with a TMS320C32 digital-signal-processor-based 5-kVA laboratory prototype validate the excellent results attained by the proposed converter.

83.

D. Hansen, J. Holtz and R. Kennel, "Close contact [cross cutters, knives]", IEEE Industry Applications Magazine, vol. 9, no. 5, pp. 33--39, 2003.

Abstract:
This article presents a cutter knife for synchronous and asynchronous cutters with scissors-cutting technique. A well-known design is using mechanically coupled cutters to provide synchronous motion of cutter cylinders, The cutting cylinders are braced by torsional force. The contact force between the knives has to be adjusted by a couple of adjusting devices (e.g., screws). It is necessary to readjust the equipment rather often for maintenance reasons. An improvement of this situation would be appreciated by the printing and foil production industries. A cutter distance sensor with good performance is required to introduce separately controlled servo drives in cutter knives. The test results obtained by test samples of the proposed sensor show that all information necessary for adapting the control structure and control parameters can be obtained by the proposed sensor concept. This type of sensor enables further progress in industrial drive applications, which leads to more replacement of mechanical designs by servo drives with flexible control.

82.

J. Holtz and J. Quan, "Drift- and parameter-compensated flux estimator for persistent zero-stator-frequency operation of sensorless-controlled induction motors", IEEE Transactions on Industry Applications, vol. 39, no. 4, pp. 1052--1060, 2003.

Abstract:
The performance of sensorless-controlled induction motors is poor at very low speed. The reasons are the limited accuracy of stator voltage acquisition and the presence of offset and drift components in the acquired signals. To overcome these problems, a pure integrator is employed for stator flux estimation. The time-variable DC offset voltage is estimated from the flux drift in a parallel stator model and used to eliminate the offset by feedforward control. Residual high-frequency disturbances are compensated by feedback flux amplitude control. A linearization of the pulsewidth-modulation inverter transfer function and an improved stator resistance estimation scheme further enhance the system performance. Experiments demonstrate high dynamic performance of sensorless control at extreme low speed and zero stator frequency.

81.

J. Holtz, J. Quan, G. Schmittt, J. Pontt, J. Rodriguez, P. Newman and H. Miranda, "Design of fast and robust current regulators for high power drives based on complex state variables" in 38th IAS Annual Meeting on Conference Record of the Industry Applications Conference, 2003, 2003, pp. 1997-2004 vol.3.

Abstract:
High-power PWM inverters for medium voltage applications operate at switching frequencies below 1 kHz to keep the dynamic losses of the power devices at permitted level. Also the sampling rate of the digital signal processing system is then low, which introduces considerable signal delays. These have adverse effects on the dynamics of the current control system and introduce undesired cross-coupling between the current components i/sub d/ and i/sub q/. To overcome this problem, complex state variables are used to derive more accurate models of the machine and the inverter. From these, a novel current controller structure employing single-complex zeroes is synthesized. Experimental results demonstrate that high dynamic performance and zero cross-coupling is achieved even at very low switching frequency.

80.

M. Linke, R. Kennel and J. Holtz, "Sensorless speed and position control of synchronous machines using alternating carrier injection" in IEEE International Electric Machines and Drives Conference, 2003. IEMDC'03, 2003, pp. 1211-1217 vol.2.

Abstract:
High frequency carrier injection is a promising approach solving high performance sensorless drive demands. Position control at low and zero speed is only possible using anisotropic effects considered in high-frequency models. The usually open loop carrier signal injection is impacted by nonlinear inverter properties like the dead-time effect. This paper discusses the influence of the dead time effect on the carrier signal excitation comparing alternating and revolving injection principles. To overcome disturbing effects an alternating injection procedure is proposed using a predefined injection angle. The approach reduces the effects of the inverter distortion voltages. As a result it is possible to track even small saliencies typical for surface mounted permanent magnet synchronous machines. For processing the high frequency current for position estimation, there is no additional hardware necessary within a standard drives with field oriented control. The paper presents theoretical analysis and experimental results.

79.

A. M. Khambadkone and J. Holtz, "Compensated synchronous PI current controller in overmodulation range and six-step operation of space-vector-modulation-based vector-controlled drives", IEEE Transactions on Industrial Electronics, vol. 49, no. 3, pp. 574--580, 2002.

Abstract:
Overmodulation enhances the power utilization of the installed capacity of a voltage-source inverter. A space-vector strategy is used for constant-switching-frequency inverters. In order to achieve the overmodulation, a modified reference signal with nonuniform angular velocity is generated using a preprocessor. Such a reference wave produces low-frequency harmonics in currents. The presence of current harmonics restricts the bandwidth of the synchronous proportional plus integral current controller in the overmodulation range. A compensating current control is presented to allow for high-bandwidth current control in synchronous coordinates during overmodulation and six-step. The proposed scheme allows for an easy upgrade of a conventional vector control scheme to include overmodulation and, thus, reduce the design-to-market time.

78.

J. Holtz, "Sensorless control of induction motor drives", Proceedings of the IEEE, vol. 90, no. 8, pp. 1359--1394, 2002.

Abstract:
Controlled induction motor drives without mechanical speed sensors at the motor shaft have the attractions of low cost and high reliability. To replace the sensor the information on the rotor speed is extracted from measured stator voltages and currents at the motor terminals. Vector-controlled drives require estimating the magnitude and spatial orientation of the fundamental magnetic flux waves in the stator or in the rotor. Open-loop estimators or closed-loop observers are used for this purpose. They differ with respect to accuracy, robustness, and sensitivity against model parameter variations. Dynamic performance and steady-state speed accuracy in the low-speed range can be 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.

77.

J. Holtz and J. Quan, "Sensorless vector control of induction motors at very low speed using a nonlinear inverter model and parameter identification", IEEE Transactions on Industry Applications, vol. 38, no. 4, pp. 1087--1095, 2002.

Abstract:
The performance of vector-controlled induction motor drives without a speed sensor is generally poor at very low speed. The reasons are offset and drift components in the acquired feedback signals, voltage distortions caused by the nonlinear behavior of the switching converter, and the increased sensitivity against model parameter mismatch. New modeling and identification techniques are proposed to overcome these problems. A pure integrator is employed for stator flux estimation which permits high-estimation bandwidth. Compensation of the drift components is done by offset identification. The nonlinear voltage distortions are corrected by a self-adjusting inverter model. A further improvement is a novel method for online adaptation of the stator resistance. Experiments demonstrate smooth steady-state operation and high dynamic performance at extremely low speed.

76.

S. Soter, "Windkraftnutzung an der Universität Dortmund", Mundo, vol. 2002, no. 2, 2002.

75.

S. Soter and S. Buchhold, "Adaptable inverter for injection of fuel cell and photovoltaic power" in Proceedings of the Power Conversion Conference-Osaka 2002 (Cat. No.02TH8579), 2002, pp. 1453-1455 vol.3.

Abstract:
Due to environmental conditions it is becoming more and more important to apply renewable energies and efficient technologies. For power injection of photovoltaic and fuel cell energy an inverter is needed. The presented modular and flexible prototype works with different energy sources such as photovoltaic, fuel cell and battery. It is possible to adjust active and reactive power as well as current shape separately within limits to improve power quality. The power electronics works very well and is able to convert 20 to 80 VDC into 110 VAC/60 Hz or 230 VAC/50 Hz with a high flexibility and efficiency.