Grigore A. Cividjian

Permeance of Fringing Flux

Field modeling becomes simpler if strong field nonuniformities are solved theoretically. The early obtained formulas for 2-D ldquocornerrdquo and ldquoconstrictionrdquo permeances pertain to right angle configurations. In this paper, using a conformal mapping, the field in the vicinity of arbitrary angle vertex is studied and the fringing permeance determined. The results are applied to permeance evaluation of lunate cross section grooves from the rotors of certain types of reluctance motors. Closed-form expressions are obtained, in good agreement with finite element analysis. Combined with the finite element method, obtained results offer the possibility to make simpler and more accurate analytical calculations of motors and other magnetic devices and to better evaluate magnetic forces that act on them.

Inductance of a plunger-type magnet

A new method, the power approximation of boundary conditions in connected regions, is used to solve the two-dimensional magnetic field problem within the window and the gap of the magnet, considering the core uniform saturated. Expressions of inductance and magnetic flux are given.

Solutions for quality pre-control of crimp contacts used in electric power systems and electrical machines

Crimping is the most widely used method of pressure connection for permanent electrical contact between a wire conductor and a wire conductor terminating device. In this paper we present two solutions for connections quality pre-control of crimp contacts used in electric power systems and electrical machines and for early identification of the improperly executed connections, before final assembly. Thus can be avoid any damage that may occur during operation. The first solution supposes an experimental determination of specific losses by calculating the initial rate of temperature rise. This can be made drawing the tangent at the origin at heating curve. The method can be used to identify bad connections by passing a current through the winding at ambient temperature and simultaneously recording connections temperatures in the first few minutes since the current is setting. The second proposed solution is to apply to each element crimping an one level thermal indicator and making a test heating with a heating current corresponding to indicator critical temperature.

Corner electrodynamic force

The electrodynamic force acting on current carrying straight filiform conductors can be easily calculated by using the formulas based on the work of Laplace. It is much more difficult to evaluate the force acting on the corner part of bent conductors or in contacts, where the distance between the conductors becomes zero and the above-mentioned formulas cannot be used any more. In the paper, by using a conformal mapping and a formula for inductance calculation, we analyze two typical cases: the bent broad flat conductor (two-dimensional model) and the bent round conductor (three-dimensional model). We propose practical formulas to evaluate the force acting on the corner part of the conductors, considering the nonuniform distribution of direct current density.

Transient thermal field in cylindrical item and shaft

For easy removal of cylindrical item from the shaft the item is heated on its external surface for thermal expansion. The best result is obtained if the temperature difference between the internal surface of the item and external surface of the shaft is highest. Since the item can be heated only on its external surfaces, in the paper the transient thermal field is analyzed inside the item and the shaft, when a constant high temperature suddenly occurs on the external cylindrical surface of the homogenous item and the best time for item removal is determined. The maximal temperature drop on the air gap is also calculated. An example of application is given.

Current distribution in crimped connections

In the paper, the direct current distribution in the rectangular cross-section strands of stranded conductor, in the case of crimped connection of two identical stranded conductors is studied. The influence of relative contact resistance and of the number of strands in conductor is analyzed. Two configurations are considered: interleaved and superposed strands. The contact resistances between adjacent strands are considered equal to each other.

Distorted constriction contact resistance between clamped slabs

Purpose – The purpose of this paper is the more exact evaluation of distorted constriction contact resistance between two clamped slabs or thin films, having a bi-dimensional current lines structure. Design/methodology/approach – Mathematical modeling using conformal mappings. Findings – The influence of the tarnish film on the distorted constriction resistance is clarified and three new exact formulas are proposed for the distorted constriction resistance between clamped slabs with rectangular contact spot. Comparisons with early proposed formula for constriction resistance of slab narrowing and with finite element analysis results are presented. Research limitations/implications – The research is limited to direct current and homogeneous and isotropic media and the results can be extended at alternate current when the skin effect is negligible. Practical implications – Exact evaluation of 2D constriction contact resistance which appears in macro-scale contacts electrical equipment and in MEMS devices, p...

Modeling the heating of cylindrical items for removal from the shaft

For easy removal of cylindrical items from the shaft, they are heated on their external cylindrical surface for thermal expansion. The best result is obtained if the temperature drop is highest on the separation from shaft air layer. In the paper, considering a suddenly occurring constant high temperature on the external cylindrical surface of the homogeneous item and the shaft with the same thermal diffusivity as the item, the transient thermal field is analyzed and the optimal heating time for item removal and maximum temperature drop on separation layer are determined. Example is given as well.

Technological solution for increasing the quality of crimped connections

In this paper we propose a new technological solution for crimping, to increase the quality of crimped connections by reducing the contact resistance and increasing the connection reliability. The method use two adjacent crimp indents in opposite sides. To evaluate the quality of crimped connections we make experimental determinations of contact resistance corresponding to two crimp method: the first method uses one crimp indents and the second is a proposed method with two crimp indents. All the used samples are with 8 copper wires 7.1 × 3 mm2. After we measure the contact resistance, we compare the obtained values for the two crimp method and present the conclusions.

Optimal shading coil cross-section for minimal ripple factor of AC electromagnet attracting force

For the sake of armature vibration reduction, the AC electromagnets are equipped with end of pole shading coils which split the magnetic flux in two phase-shifted fluxes. As result the magnet attracting force varies periodically between a minimum and maximum values. When the opposing force is about the minimal attracting force, the amplitude of vibration become proportional to the attracting force ripple factor, which consequently should be as low as possible. In the paper this minimal value of magnetic force ripple factor is determined as function of shading coil parameters. The given equations can be used for the determination of optimal shading coil cross-section. An example of application is given.