Interlaminar Insulation Faults Detection and Quality Assessment of Magnetic Cores Using Flux Injection Probe

Interlaminar faults between laminations of the magnetic cores increase core losses and could cause major damage to electrical machines. A system has been developed to detect an interlaminar fault between the laminations of the magnetic cores by means of the flux injection probe (FIP). Experimental work was carried out to calibrate the measuring system and qualify its accuracy over a wide measurement range. Application of a prototype FIP to quality assessment of transformer laminations was investigated in two stages: 1) inspection of stack of laminations with known interlaminar faults applied by artificial shorts and 2) inspection of stack of laminations with unknown quality. The experimental results show that the developed system is capable of detecting an interlaminar fault between as few as two laminations.

I. Introduction

Magnetic cores of the electrical machines and other magnetic devices are constructed from stacks of electrical steel laminations, typically 0.23–0.5 mm thick. Since the magnetic cores are exposed to time-varying magnetic fields, eddy currents are induced in the individual laminations and, consequently, energy is converted into heat in the resistance of the eddy current path [1]. The laminations are insulated from each other by means of insulating varnish or other materials to prevent electrical conduction between the laminations and limit the induced eddy currents to the individual laminations, rather than the entire core [2], [3]. However, since the materials used for interlaminar insulation are susceptible to decline and damage, short circuits between the laminations due to electrical failure could occur due to a number of reasons listed in the following [4]–[6] items: •

manufacturing defects in laminations, known as burrs;

•

mechanical damage on sides of the stacks during assembly, winding, and inspections;

•

foreign particles introduced during assembly, inspection, and repair, for example, nut, bolt and broken lamination;

•

heat and chemical factors or mechanical forces applied when stripping the winding during rewind;

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stator-rotor rubs during assembly and operation;

•

vibration of loose windings and laminations;

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arcing from the winding failure.