Laminated Stator Core Research Articles

An Iron Core Probe Based Inter-Laminar Core Fault Detection Technique for Generator Stator Cores

A new technique for detecting incipient inter-laminar insulation failure of laminated stator cores of large generators is proposed in this paper. The proposed scheme is a low flux induction method that employs a novel probe for core testing. The new probe configuration, which uses magnetic material and is scanned in the wedge depression area, significantly improves the sensitivity of fault detection as well as user convenience compared to existing methods. Experimental results from various test generators tested in factory, field, and lab environments under a number of fault conditions are presented to verify the sensitivity and reliability of the proposed scheme.

Localised losses in stator laminations of an induction motor under PWM excitation

Electronically generated voltage waveforms have increasingly been used to energise small induction motors for speed and torque control due to low cost. The most popular such waveform is a pulse width modulation (PWM), which is obtained by logically comparing a reference voltage waveform with a carrier wave of different frequency. When a small induction motor is driven by a typical PWM voltage supply, additional power losses occur in its stator core due to a magnetisation process, which is very different to standard sine wave excitation. In this investigation flux density and power loss distribution under PWM and sinusoidal voltage excitations were measured in a small induction motor stator core using a system which set up flux distribution as would occur in a rotating 3-phase stator core. The iron loss increase was between 15 and 20% under PWM excitation. Surprisingly, the increase in iron loss in a typical induction motor stator core lamination was 3% lower than that in Epstein strips of the same electrical indicating an effect of the magnetic circuit geometry as well as the voltage waveform.

Novel method for detecting interlamination short-circuits in a stator core based onmeasurement of eddy-current losses in power generators

A novel method based on the measurement of eddy-current losses for detectingshort-circuits in laminated stator cores in power generators is presented. The basictheoretical principle and its comparison with current methods are reviewed. The proposedtechnique set-up and the mathematical data processing are shown. Finally, the newtechnique is tested both in the laboratory and the field. The results of the application areshown and discussed.

Wide frequency range characterization of rotating machine stator-core laminations

In this paper a new method of stator-core lamination characterization, aimed at studying magnetic field in the vicinity of a machine, is proposed. This characterization, made with small signals over a wide frequency range, is useful to study high frequency magnetic behavior of motors fed by PWM static converters. Small minor hysteresis loops of magnetic steel are assimilated to ellipses and, neglecting anisotropy, any point of a stator-core lamination is characterized by two local parameters: the relative permeability and the resistivity. The proposed method consists in measuring the impedance of a test coil over a wide frequency range by an impedance analyzer and in comparing experimental results to theoretical predictions based on magnetic field diffusion equation. An algorithm yields values of local relative permeability and resistivity from inductance measurements and experimental values of resistances are used to control the result quality.

Measurements of power loss distribution in a typical stator core under PWM voltage excitation

The pulse width modulated (PWM) inverter is widely used to feed small induction motors for variable speed and torque control. When a laminated stator core is energised in this way additional iron losses occur due to localised distorted flux. Flux density and power loss distribution under PWM and sinusoidal voltage excitations were measured in a typical induction motor stator core lamination at 1.3 T, 50 Hz by using a computer-aided magnetising system to set up flux distribution as would occur in a practical three-phase stator core. The iron loss increased 15–20% under PWM excitation. The loss increase under PWM excitation in the stator core laminations was 3% lower than in Epstein strips of the same electrical steel under the same conditions showing an effect of the magnetic circuit geometry.

Noise problems of the induction motors

The audible noise produced by electrical machines is developed depending on the various energy sources that generally appear during the rotation of the rotor. Usually the main energy source in the electrical machines is the magnetic field. Maxwell stresses act on the stator core, armature, and stator laminations, and generally exhibit spectral distribution as a function of time. These forces may provide excitation on the mechanical resonances of the machine’s mechanical system that have a distributed mass, stiffness, and damping. The force waves and their possible contribution to overall machine noise can be predicted by analysis of the electrical design, which can be optimized to reduce their effect. Careful analysis and design are required, with perhaps additional magnetic core material to keep flux levels low, and air gap large, without compromising good performance. Behind the features of the magnetic and mechanical structure, the aerodynamic features and excitations provided by the air circulation and the movement can also gain significance when the similar interaction appears with the machine structure. Interaction might be amplified during the coincidence of the mechanical resonances and the provided excitation on similar frequencies.

Electric motor repair industry update

The concerns of the typical rewind shop, or electric motor service center, identified through various surveys are reviewed. The issues discussed include energy and environmental issues, insulation concerns, winding patterns, and a renewed interest in stator core laminations. >

An investigation of the axial flux in laminated stator cores with helical armature windings

An investigation of the axial flux in laminated stator cores with helical armature windings

Discussion on “Interlamination voltages in large turbogenerators”, “Influence of winding design on the axial flux in laminated-stator cores” and “Examination of flux distribution in segmented stator cores”

Discussion on “Interlamination voltages in large turbogenerators”, “Influence of winding design on the axial flux in laminated-stator cores” and “Examination of flux distribution in segmented stator cores”

Influence of winding design on the axial flux in laminated-stator cores

The magnetic field of an isolated winding of helical form is obtained both theoretically and by measurement on a small model. The axial and radial fields of that winding are compared with those of straight windings with conventional ends, both when the windings are in air, and inside a slotless, laminated core. It is found that a knowledge of the field in air of a winding is not of itself helpful in predicting the axial-field distribution when the winding is placed inside a laminated core. Experimental results of the axial-flux distribution inside the core are also presented. They demonstrate that axial flux is reduced when the winding radius and overhang are reduced. The axial flux continues to decrease even when the endwinding ceases to project from the core end.