132.

R. Wegener, S. Gruber, K. Nötzold, F. Senicar, C. Junge and S. Soter, "Development and Test of a High Force Tubular Linear Drive Concept with Discrete Wound Coils for Industrial Applications" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1--5.

Abstract:
This paper deals with the development of a tubular permanent magnet linear drive with radial magnetized armature and discrete wound coils mounted on a star-shaped stator part. The rated force of the developed machine is 500 N per segment. This presented particular design results in a very economic product because all primary parts, except of the permanent magnets and coils, are made of standard non-laminated steel and are optimized for easy production and assembly. The control of this machine with a specially built low cost linear sensor based on the Hall-effect is also presented. The suitability of the design is proven by the demonstration of a prototype with measurements of thrust and cogging force.

131.

J. Holtz and J. Quan, "Drift and parameter compensated flux estimator for persistent zero stator frequency operation of sensorless controlled induction motors" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1687-1694 vol.3.

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 PWM 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.

130.

J. Holtz and H. Pan, "Elimination of saturation effects in sensorless position controlled induction motors" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1695-1702 vol.3.

Abstract:
Owing to the variable magnetic coupling between the stator windings and the discrete rotor bars of an induction motor, a quasi-continuous rotor position signal can be acquired by instantaneous measurement of the total leakage inductance of the three stator phases. The signals are sampled in synchronism with the regular commutations of the PWM process, thus making the injection of additional high-frequency carriers obsolete. The acquired position signal exhibits high spatial resolution and high dynamic bandwidth. Magnetic saturation also influences upon the total leakage inductances and hence constitutes a disturbance for position identification. The paper presents a detailed analysis of the saturation effects and proposes methods to eliminate their undesired impact on the position signal. Experimental results of closed loop sensorless position control at full load and high dynamic performance are presented.

129.

J. Holtz and N. Oikonomou, "Estimation of the Fundamental Current in Low Switching Frequency High-Dynamic Medium Voltage Drives" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 993--1000.

Abstract:
The switching frequency of medium voltage ac drives is limited to low values to restrain the dynamic losses of the power devices. This favors the use of synchronous optimal pulsewidth modulation schemes that minimize the harmonic current. It is a drawback, though, that optimal algorithms do not have a means to extract the fundamental component of the load current. High-performance torque control is therefore difficult to obtain. The paper proposes a method to identify the instantaneous fundamental component of the stator currents. A novel observer is developed for this purpose. The approach enables fast torque control at very low switching frequency. Experimental results from a 30-kW induction motor drive are presented.

128.

J. Holtz and R. Rosner, "Gate drive power recovery and regenerative snubber scheme for series-connected GTOs in high voltage inverters" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1535-1540 vol.3.

Abstract:
Pulsewidth modulated (PWM) inverters for power levels above 10 MW are presently the target of ongoing research. The implementation implies the series-connection of semiconductor devices, like GTOs or IGCTs, even if multilevel topologies are considered. Areas of application are flexible AC transmission systems (FACTS), transformerless reactive series compensators, HVDC transmission, static VAr compensators, and high-power AC drives. Existing problems are dynamic voltage balancing between the series-connected devices, snubber energy recovery and the transfer of gate drive power across potential barriers of more than 10 kV. The paper proposes an integrated solution for these problems. Experimental results from an inverter half-bridge composed of four IGCTs are presented. The respective device ratings are 4.5 kV and 4 kA. The DC-link voltage is 10 kV.

127.

J. O. Krah and J. Holtz, "High-performance current regulation for low inductance servo motors" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 490-499 vol.1.

Abstract:
The 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 overmodulation, dynamic overmodulation, anti-windup, and reduction of switching frequency at thermal overload are included.

126.

J. Holtz, M. Stamm, J. Thur and A. Linder, "High-power pulsewidth controlled current source GTO inverter for high switching frequency" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1330-1335 vol.2.

Abstract:
High-power GTO inverters require regenerative snubbers to ensure operation at predefined values of di/dt and du/dt. The use of regenerative snubbers for voltage source inverters is a well established technology. However, the proliferation of high-power PWM current source inverters is impeded by the nonavailability of an appropriate regenerative snubber scheme. The paper proposes a regenerative snubber for current source GTO inverters and reports experimental results of a 1.2 MVA prototype inverter. Energy recovery from the snubbers enables operation at extreme high switching frequency of 1 kHz and reduces the total filter kVA from 70{%} to 20{%} of the inverter power.

125.

J. Holtz, "Initial Rotor Polarity Detection and Sensorless Control of PM Synchronous Machines" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 2040--2047.

Abstract:
Sensorless control of a PM synchronous machine at low and zero speed is based on the injection of an oscillating high-frequency carrier signal. A particular demodulation technique serves to eliminate the estimation error introduced by the nonlinear characteristics of the inverter. The initial rotor position and the magnet polarity is detected before the drive is started. The initialization is performed by injecting an ac carrier and two short current pulses in a sequence

124.

J. Holtz and N. Oikonomou, "Neutral point potential balancing algorithm at low modulation index for three-level inverter medium voltage drives" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1246-1252 Vol. 2.

Abstract:
Three-level inverters produce low harmonic distortion of the ac currents even when operated at moderate switching frequency. This makes them the preferred candidates for high-power, medium voltage applications. To improve the utilization of the semiconductor devices, synchronous optimal pulsewidth modulation is employed. This permits reducing the switching frequency to very low values. Carrier modulation is maintained in the lower range of the modulation index. Operation at very low switching frequency increases the steady-state ripple of the neutral point potential. An intrinsic natural balancing mechanism of the neutral point clamped inverter topology eliminates long-term neutral point potential offsets. Transient conditions, however, may create successive increments of the offset to high values, which requires fast compensation. The novel method of selecting the appropriate redundant inverter subbridge meets this requirement without incurring additional penalties. The effectiveness of the approach is documented by experiments obtained from a medium voltage motor drive fed by a 1-MVA three-level inverter.

123.

J. Holtz and N. Oikonomou, "Optimal Control of a Dual Three-Level Inverter System for Medium-Voltage Drives" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1--8.

Abstract:
Actual developments in medium-voltage drives aim at increasing the power capability of the PWM inverter. Parallel connection of power semiconductor devices permits increasing the output current, while series connected devices increase the output voltage. In either case are additional means required for balancing the current or voltage stress of the devices. The three-level neutral-point clamped (NPC) inverter topology offers an inherent means to ensure dynamic voltage balancing of a maximum of two series connected devices. Higher operating voltages can be achieved by series connection of entire inverter topologies. Two circuit topologies are considered that behave as five-level units. Synchronous optimal pulsewidth modulation is applied for their control. This permits reducing the switching frequency to the 100-Hz region without sacrificing on harmonic distortion.