Stéphane Duchesne

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.

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.

Voltage stress predetermination for long-life design of windings for electric actuators in aircrafts

The more electric aircrafts require high specific power motors; on the other hand, the design must guarantee long lifetimes. Therefore, most of these motors are synchronous machines, working at high frequencies and made with a large number of coils placed on stator teeth. The Electrical Insulation System (EIS) must be designed for supporting repetitive voltage spikes imposed by modern PWM inverters. Today, most of inverters are built with standard silicon IGBT but, in the near future, faster electronic switches based on silicon-carbide will be used. Consequently the voltage slopes imposed to the motor winding will be much steeper. The paper proposes a method based on equivalent circuits able to predict the voltage spikes distribution in each coil of the motor during the fast transients that follow fast voltage fronts.

A simulation method for the design of windings of high reliability machines fed by PWM inverters

This paper proposes a numeric 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.

Experimental characterization of the maximum turnto-turn voltage for inorganic high temperature motor

Ceramic-insulated wires have very good thermal characteristics. With an adapted motor design, such inorganic wires could be used for building new electric motors able to work at very high internal temperatures, up to 500°C, which opens interesting perspectives for very high power density electric motors. After explaining the advantages of high internal temperatures, electrical characteristics of inorganic coils are compared to conventional enameled wires associated with a polymer impregnation. For classical organic wire, the Partial Discharge Inception Voltage (PDIV) is an important parameter; for ceramic wires, physical phenomena are very different and this concept is not still completely defined. However, the tests performed on ceramic-coated wires show that fast current pulses appears for voltages over a threshold measured for temperatures up to 500°C. This threshold voltage is an interesting indicator for inorganic coil designs: the turn-to-turn voltage must remain below this threshold at any point of the coil.

High temperature motors: Investigations on the voltage distribution in windings at a short scale times for a PWM supply

Most of electrical machines are wound with enameled copper wires insulated by a thin polymer layer, which has excellent mechanical and insulation properties. However, the organic nature of these insulating layers limit their operating temperatures to more or less 280° C for the best polymers. It is possible to operate at much higher internal temperatures with an inorganic Electrical Insulation System (EIS) made without any polymer. Unfortunately, the available inorganic wires made with a thin ceramic insulating layer have poor electrical and mechanical performances. Therefore, the design of High Temperature (HT°) coils, able to work with standard PWM inverters is more difficult. The paper presents a method for computing the voltage spikes that follow each voltage fast edge with a high frequency (HF) equivalent circuits.

Assessment and improvements of inorganic insulation for high temperature low voltage motors

The increase of internal temperature capability of electrical machines is a major challenge for building high power density actuators for many applications in transport systems and in aerospace industry. Today, the main limit is the thermal class of the organic Electrical Insulation System (EIS), but a breakthrough toward very high temperature is possible with inorganic insulation technologies. An inorganic EIS is proposed and tested for designing motors able to work permanently up to 500 °C in the heart of windings. After a review of several high temperature (HT°) insulation solutions, the paper focuses on currently available ceramic-coated wires insulated with a very thin inorganic layer, which is a favorable solution for high current densities in machines of small sizes. The electrical limits of this technology are investigated, the weak points of the HT° wire are circumvented using impregnating cements able to provide adequate mechanical and electrical properties up to 500°C for designing HT ° electrical machines fed by a standard PWM inverter connected to the HVDC bus of the more electric aircraft, for example.