42.

H. Pan, J. Jiang and J. Holtz, "Decoupling control and parameter identification of field-oriented induction motor with saturation" in Proceedings of the IEEE International Conference on Industrial Technology (ICIT'96), 1996, pp. 757--761.

Abstract:
A new decoupling control and self-commissioning parameter identification scheme for the PWM-inverter-fed asynchronous motor with saturation is presented. The modification of the field-oriented control model is considered with respect to rotor-flux-oriented control, while the nonlinear magnetizing curve is expressed as a function of the magnetizing current of the main flux path. The control model and the magnetizing curve are bridged through the inherent relationship between the rotor flux and the airgap flux. The complex signal flow graphs indicate that the dynamic structure of AC machines are not changed with magnetic saturation effects. The nonlinear magnetizing curve is estimated from a transient voltage caused by an ejected constant stator d-axis current. Experiment results indicate that the vector control of induction motor with saturation can be finely realized with the proposed model, on the basis of the recognized magnetizing characteristics.

41.

J. Holtz, "The representation of AC machine dynamics by complex signal flow graphs", IEEE Transactions on Industrial Electronics, vol. 42, no. 3, pp. 263--271, 1995.

Abstract:
Induction motors are modeled by nonlinear higher-order dynamic systems of considerable complexity. The dynamic analysis based on the complex notation exhibits a formal correspondence to the description using matrices of axes-oriented components; yet differences exist. The complex notation appears superior in that it allows the distinguishing between the system eigenfrequencies and the angular velocity of a reference frame which serves as the observation platform. The approach leads to the definition of single complex eigenvalues that do not have conjugate values associated with them. The use of complex state variables further permits the visualization of AC machine dynamics by complex signal flow graphs. These simple structures assist to form an understanding of the internal dynamic processes of a machine and their interactions with external controls.{\textless}{\textgreater}

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. 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}

36.

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}

35.

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}

34.

J. Holtz and B. Beyer, "The trajectory tracking approach-a new method for minimum distortion PWM in dynamic high-power drives", IEEE Transactions on Industry Applications, vol. 30, no. 4, pp. 1048--1057, 1994.

Abstract:
Feedforward pulsewidth modulators generate volt/spl middot/second differences between the controlling reference signal and the switched output waveform during transient operation. This dynamic modulation error is particularly pronounced at low switching frequency, e.g. with modern high-power GTO inverters. The compensation of this error is a key requirement when designing an optimal pulsewidth modulator for these applications. The trajectory tracking approach uses the steady-state trajectories of the current vector, computed from the optimal switching sequences, as templates. A tracking controller counteracts if the actual current vector deviates from this given path at transient operation. A decomposition of the current vector trajectory into its components enables, in addition, the identification of the instantaneous fundamental current, permitting the implementation of a fast current control system. Oscillograms from a 30 kW model induction motor drive demonstrate the high quality in terms of dynamic performance and minimum harmonic distortion obtained with a three-level inverter at only 200 Hz switching frequency.{\textless}{\textgreater}

33.

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}