29.

J. Holtz, "Pulsewidth modulation for electronic power conversion", Proceedings of the IEEE, vol. 82, no. 8, pp. 1194--1214, 1994.

Abstract:
The efficient and fast control of electric power forms part of the key technologies of modern automated production. It is performed using electronic power converters. The converters transfer energy from a source to a controlled process in a quantized fashion, using semiconductor switches which are turned on and off at fast repetition rates. The algorithms which generate the switching functions-pulsewidth-modulation techniques-are manifold. They range from simple averaging schemes to involved methods of real-time optimization. This paper gives an overview.{\textless}{\textgreater}

28.

J. Holtz and B. Beyer, "Optimal pulsewidth modulation for AC servos and low-cost industrial drives", IEEE Transactions on Industry Applications, vol. 30, no. 4, pp. 1039--1047, 1994.

Abstract:
An optimal pulsewidth modulation method for inverters is described in which the durations of switching subcycles are considered as optimization variables, a subcycle being the time sequence of three consecutive switching state vectors. Operation at variable subcycle duration entails a prediction of the controlling reference voltage vector. It is a special advantage that the optimization and prediction can be carried out off line. The optimal technique exhibits high dynamic performance. It can be used for synchronized and asynchronous modulation in a wide range of switching frequencies. The optimization reduces the harmonic currents at a given switching frequency. The Fourier spectrum lacks dominant carrier frequencies. Hardware implementation cost compares with existing nonoptimal modulation methods.{\textless}{\textgreater}

27.

J. Holtz, W. Lotzkat and S. Stadtfeld, "Controlled AC drives with ride-through capability at power interruption", IEEE Transactions on Industry Applications, vol. 30, no. 5, pp. 1275--1283, 1994.

Abstract:
General purpose PWM inverter drives are equipped with an undervoltage protection mechanism, causing the system to shut down within a few milliseconds after a power interruption in the mains. This may entail loss or damage of material in such critical applications as the production of textile fibers, paper, or with extruder drives. The proposed solution to this problem is to recover some of the mechanical energy stored in the rotating masses. When a power interruption occurs, a sequence of fast feedforward commands is applied to force an immediate transition into the regeneration mode. During the interrupt interval, the drive system continues to operate at almost zero torque, just regenerating a minor amount of power to cover the electrical losses in the inverter. The method is implemented in an additional software package to be used with general-purpose inverter drives of limited dynamic performance. Experimental results are presented.{\textless}{\textgreater}

26.

H. Groschwitz and J. Holtz, "An expert system on transformer design for switched mode power supplies" in Proceedings of IECON'94 - 20th Annual Conference of IEEE Industrial Electronics, 1994, pp. 1413-1418 vol.2.

Abstract:
Switched mode power supplies (SMPS) have become an important subassembly for any type of electronic equipment. Operation in the switched mode decreases the volume of the magnetic core material. The choice of the magnetic components depends on the circuit topology. Given the large number of converter topologies available, the design of the magnetic components constitutes a substantial problem. A wide variety of materials, core geometries, and commercial core products must be considered for an optimal transformer design. An expert system was developed to support this task. It selects the optimum core for a specified SMPS from a given database. Criteria like power rating, DC output voltage, temperature rating, importance of a low-profile core and isolation class can be preselected. A fully interactive graphic interface is realized. The expert system is based on Common-LISP. It is executable on UNIX graphic workstations using the X11R4 window system and the OSF/Motif library.{\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}

22.

J. Holtz, W. Lotzkat and A. M. Khambadkone, "On continuous control of PWM inverters in the overmodulation range including the six-step mode" in Proceedings of the 1992 International Conference on Industrial Electronics, Control, Instrumentation, and Automation, 1992, pp. 307-312 vol.1.

Abstract:
The power output and the dynamic performance of pulse width modulation (PWM)-controlled AC motor drives can be improved by increasing the inverter output voltage through overmodulation. Two different solutions are proposed to increase the output voltage in a continuously controllable fashion up to the maximum possible value, which is reached in the six-step mode. The solutions differ in their approach. A space vector strategy is used for high dynamics performance high switching frequency drives, while a field-oriented PWM method is used for low switching frequency high power inverters. The methods are described and analyzed, and experimental results are presented.{\textless}{\textgreater}

21.

J. Holtz and B. Beyer, "Optimal synchronous pulsewidth modulation with a trajectory tracking scheme for high dynamic performance (invertor control)" in [Proceedings] APEC '92 Seventh Annual Applied Power Electronics Conference and Exposition, 1992, pp. 147--154.

Abstract:
It is noted that synchronous pulsewidth modulation 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 feedforward control technique eliminates this 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 pulsewidth modulator are avoided. Experimental results obtained from a 30 kW AC drive are presented.{\textless}{\textgreater}

20.

J. Holtz, "Pulsewidth Modulation in Motion Control" in Singapore International Conference on Intelligent Control and Instrumentation [Proceedings 1992], 1992, pp. 115--120.

Abstract:
University of Wuppertal, Germany