Daniel Roger

A New Method for AC Machine Turn Insulation Diagnostic Based on High Frequency Resonances

The stator insulation breakdown is a major cause of ac machine failures. Ground insulation defaults are easily detected by classical systems based on leakage current measurements, however the turn-to-turn insulation degradations are more difficult to detect. For large machines, on-line methods, based on partial discharge detection and analysis, give good results but they cannot be used for low-voltage machines fed by adjustable speed drives (ASD). This paper presents a new monitoring system able to detect slight variations of high frequency resonances in the winding of a working machine fed by an industrial inverter. The new system works in association with statistical data on accelerated aging of the magnet wire used to build the machine.

A simulation method to predict the turn-to-turn voltage spikes in a PWM fed motor winding

This paper proposes a numerical simulation method dedicated to design assistance of high reliability machines fed by PWM inverters. It allows the computation of winding turn-to-turn maximum voltage stress for any combination of the wire positions in the stator slot. With such a tool, it is possible to design coils with the best wire arrangement for any fast-fronted pulses. An equivalent circuit is used to simulate the voltage distribution among the turns of the winding. A finite-element analysis simulation package is used to estimate the high-frequency distributed-circuit parameters of the winding. In order to validate the simulation results, they are compared with experimental results.

Turn-to-turn capacitance variations correlated to PDIV for AC motors monitoring

The paper deals with a new indicator for monitoring the turn-to-turn insulation aging of ac motors. Measurements of capacitance and Partial Discharge Inception Voltage (PDIV) were performed on a large number of twisted pairs of enameled wires between standard thermal accelerated aging cycles. A significant increase of the capacitances and a reduction of the PDIV were noted on all the samples for three different diameters of enameled wires. The correlation of the capacitance increase with the degradation of the insulation quality opens perspectives for preventive maintenance applications.

Experimental Characterization of Interlamination Shorts in Transformer Cores

Burr-induced interlamination shorts in magnetic circuits increase eddy current loss. In large transformer cores, the short-circuit currents can become so high as to even damage the core. This paper presents contributions to modeling an individual short circuit between two transformer sheets. The aim of the model is to predict the additional eddy current loss caused, depending on the excitation conditions, the placement of the contact points, and the contact resistances. Measurements have been conducted on artificial short circuits between single pairs of transformer sheets. Findings prove that the interlaminar short can be modeled with good approximation by a resistive equivalent circuit, even when the core is saturated. The influence of different parameters on the elements of the equivalent circuit is discussed.

Prediction of the turn-to-turn voltages in parallel connected wires configuration for motor coils fed by steep fronted pulses

This paper proposes an original method for the design assistance of high reliability windings for machines fed by PWM inverters. The method is able to calculate the winding turn-to-turn maximum voltage stress for any combination of the positions of the wires in the stator slot. This kind of tool makes it possible the computation of an optimized coil with the best wire arrangement in the slot. The developed tool is used to compare many configurations for the coils by changing the number of parallel connected wires while preserving a constant copper section.

Attenuation of magnetic field components through an AC machine stator

Purpose – Non‐intrusive magnetic measurements in AC machines are possible with small flat coils stuck on the external surface of the housing of a running motor. The aim of the paper consists in determining transmission coefficients able to give a direct relationship between the weak external flux density and the airgap one.Design/methodology/approach – An experimental approach shows that the decoupling principle can be applied. Transmission coefficients are determined separately for the stator yoke, the motor housing and the external air.Findings – For low frequencies and a housing made of steel, eddy current can be neglected. The transmission coefficient depends strongly of the mode (number of poles) of the rotating field. Conversely, for higher harmonic ranks, the additional attenuation caused by eddy currents in the housing does not practically depend on the mode but is strongly dependant on the frequency.Research limitations/implications – The transmission coefficients are determined considering a 2D ...

Eddy current influence on electromagnetic emissions of laminated cores

Purpose – A new model able to describe the high frequency (HF) behaviour of the laminated cores of AC machines is proposed. The aim is to compute the external flux density of machine cores, corresponding to electromagnetic emissions in the HF range when the skin effect is predominant.Design/methodology/approach – For high frequencies, the skin depth is much lower than the thickness of a lamination and the external flux density is determined using a new analytical model. The validity of this model is confirmed by measurements performed on a magnetic core representing a small part of a large machine and a finite element 3D simulation.Findings – For high frequencies, the external flux density is computed considering an equivalent current layer flowing on the laminated core external surface. Eddy currents in the laminated core have a large influence on the current density in this current layer.Research limitations/implications – The new model proposed is valid when the skin depth is lower than half the thickn...

Short Circuit Current Measurements Between Transformer Sheets

In large transformer cores, burr-induced insulation faults between laminations can result in short-circuit currents which may be high enough to damage the core. This study presents measurements of the actual worst-case short circuit currents encountered in artificial interlamination shorts. A procedure using an adapted miniaturized Rogowski coil is described. Results show that the maximum short circuit current is not proportional to the core width, but, except for extremely narrow cores, it increases with good approximation linearly with the core width. Peak currents can in large cores reach several amperes even when only two laminations are involved.

Contribution to the Study of Losses Generated by Interlaminar Short-Circuits

The paper proposes an improved analytical model, valid for a wide frequency range, of interlaminar short-circuits on the edges of the sheets of high power transformers working at 50-60 Hz. This model is able to determine the current flowing in the microscopic interlaminar contacts formed by burrs, in order to estimate the maximum power dissipated in such hot spots. With this theoretical approach, it is also possible to get an interpretation of impedance measurements on a test coil wound around the core, in order to define a new index able to characterize the part of the shearing burrs in the manufacturing factor of large transformer cores. This new index will help the transformer manufacturers to improve their processes.

Aging effects on the AC motor windings: A correlation between the variation of turn-to-turn capacitance and the PDIV

The presented paper focuses on monitoring of turn-to-turn insulation quality. It is based on the measure of the variation of the winding turn-to-turn capacitance and the Partial Discharge Inception Voltage (PDIV). The test methods is defined and described. Twisted pairs of three different diameters of enameled wires were tested. The PDIV is measured with an adequate system properly calibrated to minimize background noise to an acceptable value. Finally, it is shown an increased capacitance and a reduction PDIV between aging cycles. The correlation of these two parameters opens perspectives for preventive maintenance applications.