No. 2, 2006


Ludek Schreier, Jiri Bendl, Miroslav Chomat, Jiri Klima : Possibility of reducing stray losses and parasitic torques in two-phase emergency feeding of induction motors, pp.109-131

The paper deals with emergency feeding of a three-phase induction motor by injected currents after disconnection of the faulty leg of the feeding converter and subsequent reconfiguration of the converter. At reconfiguration, the winding fed by the damaged leg is disconnected and the stator winding neutral is connected to the midpoint of the capacitor bank in the dc link. It gives rise to the zero-sequence component of stator currents and to the third space harmonic of current layer, magnetic flux density, and yoke flux. The rise of stray losses and parasitic torques due to the zero- and negative-sequence components in the currents is discussed. A possibility to reduce these losses and torques by a proper choice of the phase shift of stator currents is shown.

Jiri Büllow : Determination of numerical step for reliable numerical integration of parabolic partial differential equations, pp.133-148

Finding of the step of numerical integration has radical impact on the accuracy of numerical solution of ordinary differential equations (ODEs). This paper suggests a procedure for determination of its maximal length ensuring that the numerical solution will satisfy the ``important features'' of the analytically unknown solution of ODEs. Thus, the goal of this paper is to find conditions for stability of the numerical solution (that keeps its error in a reasonable range), and for geometrical convergence of results guaranteeing good accordance of the numerical solution with physical reality.

Lubomir Sumichrast : Numerical phase and group velocities in the ADI-FDTD and CNSS-FDTD method, pp.149-163

The phase and group velocity of the simulated wave propagation using the ``Alternating Directions Implicit'' (ADI) Finite-Difference in Time Domain (FDTD) and the ``Crank-Nicolson Split Step'' (CNSS) FDTD method are investigated and thoroughly discussed.

Daniel Mayer, Bohus Ulrych : Optimal design of electromagnetic actuator with permanent magnets, pp.165-177

The paper deals with the design of a new arrangement of an electromagnetic actuator with permanent magnets. The principal advantage of such actuators is relatively flat static characteristic.

Jiri Matejicek, Vladimir Weinzettl, Edita Dufkova, Vojtech Piffl, Vratislav Perina : Plasma sprayed tungsten-based coatings and their usage in edge plasma region of tokamaks, pp.179-191

Tungsten is a candidate material for plasma facing components for ITER and other fusion devices. Plasma spraying is among prospective fabrication technologies, thanks to its ability to coat large areas and the possibility of in-situ repair. Several types of tungsten-based coatings were produced at IPP Prague, using water-stabilized plasma spraying. Their structure, porosity, oxide content and mechanical and thermal properties were characterized. Several stages of the spraying process optimization towards the foreseen application were performed.

The usage of these coatings as plasma facing materials (covering different diagnostic tools) in the edge plasma region was tested at the CASTOR tokamak at IPP. Behavior of 0.4-2mm thick plasma-sprayed surfaces and their influence on the discharge quality was studied. Plasma sprayed tungsten-based samples, solid tungsten and graphite were inserted into the plasma at various radii and exposed to tokamak discharges (30ms pulse length, 30kW ohmic heating). Moreover, the samples were biased by positive and negative voltages to increase electron and ion interaction with the sample surface. Broad scans over the biasing voltages (from -300V to +200V) and sample surface locations (from the plasma edge up to the core region) were performed. Afterwards, the sample surfaces were imaged by electron microscopy. Only a moderate surface modification was observed - narrow bright tracks, probably caused by unipolar arcing. Compositional changes in sprayed surfaces were also measured by RBS and ERDA methods. Hydrogen and carbon deposition, and oxygen content decrease were observed. Generally, pure tungsten coatings are suitable for covering different edge plasma diagnostics because of their low surface erosion and negligible influence on the discharge parameters.

Phan Tu Vu, Josef Tlusty : Non-uniform finite difference grid approach for modeling grounding systems, pp.193-202

This paper deals with a new way of modeling grounding systems using Higher-Order 2D and 3D Finite Difference Methods (HO-FDM) with non-uniform grid approach. In particular, the non-uniform finite difference grids used for modeling the geometry of grounded elements are presented. This numerical method examined to the rod electrode and grounding grid buried in the uniform and two-layer soils. Their solutions obtained by the iterative methods of Jacobi, Gauss-Seidel and Successive-Over-Relaxation (SOR) are presented and discussed. The results show that the proposed method is acceptable and suitable for modeling and calculating any shape of grounding systems buried in uniform, two-layer and multilayer soils. Specially, the solution obtained by the non-uniform FD grid approach has high accuracy.

Pavel Zaskalicky, Maria Zaskalicka : Torque ripple calculation of a DC series wound motor supplied by a semi-controlled rectifier, pp.203-211

An DC-series wound motor is one of most used electrical machines in a small electrical instruments and home equipments by reason, that its speed is easily controlled by a supply voltage. The present paper shows analytical calculation of the torque ripple and current waveform of the motor, which is supplied by a thyristor semi-controlled rectifier. The rectifier output voltage waveform is formulated by the Fourier series. The armature reaction of the motor is included in the calculus. The motor performance is computed using the circuit parameters determined by measurements.