Veröffentlichungen
- 120.N. Oikonomou and J. Holtz, "Closed-Loop Control of Medium-Voltage Drives Operated With Synchronous Optimal Pulsewidth Modulation", IEEE Transactions on Industry Applications, vol. 44, no. 1, pp. 115--123, 2008.
Abstract:
Inverters for medium voltage drives operate at reduced switching frequency so as to restrain the dynamic losses of the power semiconductor devices. The resulting current harmonics can be reduced by synchronous optimal pulsewidth modulation (PWM), provided that steady-state conditions prevail. Transient conditions, however, interfere adversely with the optimal modulation patterns. Such conditions necessarily occur when the modulator forms part of a conventional closed-loop control scheme. Trajectory tracking control is employed to achieve high dynamic control in conjunction with synchronous optimal PWM. An optimal trajectory of the stator flux linkage vector is derived from the pulse pattern in actual use. The stator flux linkage vector is forced to follow this target trajectory. Modifying the target trajectory in transient conditions enables closed-loop torque control in a deadbeat fashion while conserving optimal modulation. Experimental results obtained from a 30-kW prototype drive operated at only 200 Hz switching frequency demonstrate the effectiveness of the approach. - 119.R. Wegener,Zylindrischer Linearmotor mit konzentrierten Wicklungen für hohe Kräfte: Dortmund, Techn. Univ., Diss., 2008. .... Norderstedt: {Books on Demand}, 2008.
- 118.J. Jiang and J. Holtz, "An efficient braking method for controlled AC drives with a diode rectifier front end" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1446-1453 vol.3.
ISBN: 0197-2618
Abstract:
Standard PWM inverter fed induction motor drives employ a diode rectifier bridge to supply AC power from the utility to the DC link. Although a diode rectifier is the most cost-effective solution, it does not permit reversing the power flow. This prohibits operating the machine in the regenerative braking mode for active deceleration. An innovative control method substitutes conventional hardware, such as an active front-end rectifier or a chopper controlled braking resistor in the DC link circuit, by additional software that is implemented in the standard microprocessor control. The control algorithm maximizes the power losses in the machine and in the inverter. It enables regenerative braking operation of the induction motor at high torque. The algorithm conserves the high dynamic performance of a vector controlled drive system. - 117.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.
ISBN: 0197-2618
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. - 116.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.
ISBN: 0197-2618
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. - 115.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.
ISBN: 0197-2618
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. - 114.J. Holtz and N. Oikonomou, "Neutral Point Potential Balancing Algorithm at Low Modulation Index for Three-Level Inverter Medium-Voltage Drives", IEEE Transactions on Industry Applications, vol. 43, no. 3, pp. 761--768, 2007.
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 sub-bridge 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 - 113.J. Holtz and N. Oikonomou, "Synchronous Optimal Pulsewidth Modulation and Stator Flux Trajectory Control for Medium-Voltage Drives", IEEE Transactions on Industry Applications, vol. 43, no. 2, pp. 600--608, 2007.
Abstract:
Employing synchronous optimal pulsewidth modulation (PWM) techniques permits operating the PWM inverter of medium-voltage drives at very low switching frequency. The switching losses of the power semiconductor devices are thus reduced. The benefit is that a given inverter produces higher fundamental power. The optimal pulse patterns are determined by offline calculation, assuming steady-state operation of the drive machine. Dynamic modulation errors and high overcurrents, as a consequence, are therefore encountered when the operating conditions change. To overcome this problem, the harmonic components of the stator flux linkage vector are subjected to closed-loop control. The target trajectory is derived from the respective pulse pattern in use, while the actual stator flux trajectory is estimated. The approach is insensitive to parameter variations. It eliminates excessive transients when the operating conditions change. Experimental results obtained from an industrial 1-MVA 4.16-kV three-level inverter ac drive are presented - 112.S. Soter and R. Wegener, "Development of Induction Machines in Wind Power Technology" in 2007 IEEE International Electric Machines & Drives Conference, 2007, pp. 1490--1495.
Abstract:
This paper gives an overview of the development of the induction machine as a generator for wind turbines from the middle of the 20th century up to now. After a short history chapter the different energy conversion concepts are presented. At first a squirrel cage machine is direct coupled to the grid (Danish concept). To adjust the machine speed the second step is a wound rotor machine with collector rings to change rotor resistance. This concept allows the first adaption of the operating point of the wind turbine and the induction machine. Another concept is to connect the stator of a squirrel cage machine with a full inline voltage source converter to the grid to get the flexibility in rotational speed and to control the reactive power. The doubly fed induction machine is the latest development. This concept uses a bidirectional voltage source converter in the rotor circuit with a rated power of only 30{%} of the rated generator power. Now it is possible to change the rotational speed and the reactive power independently and in a wide range. A generalized control scheme of a modern doubly fed induction machine (DFIG) is shown in the last part to explain the variability and the range of application. Today over 70{%} of the wind turbines are build up with DFIG. - 111.R. Wegener, F. Senicar, C. Junge and S. Soter, "Low Cost Position Sensor for Permanent Magnet Linear Drive" in 2007 IEEE 2nd International Conference on Power Electronics and Drive Systems (PEDS), 2007, pp. 1367--1371.
ISBN: 2164-5264
Abstract:
This paper deals with a custom made low cost sensor for measuring the position of a permanent magnet linear motor. The principle how to measure position and movement direction with two analog hall sensor elements is described. The following simulated and detailed error and failure treatment is very important to know exactly the performance and the possibilities of this low cost sensor element. Afterwards this position sensor is build and some measurements with a linear machine is done. After filtering, the accuracy of the two signals is high enough to be an input of a converter control to determine the correct current which has to be injected. If there is another higher ranking closed-loop control, e.g. pressure, flow or force, in the control system this low cost sensor is sufficient and works very well. It is possible to implement the very small sensor in the housing of the linear drive. This sensor costs less than 15 dollar and can not be compared to a very precise working linear senor for some hundred dollar in order to position the linear drive very exact but the accuracy is high enough to build a lower ranking closed-loop control and to stabilize a complex control system of converter, linear drive and load.
