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Person# Frédéric Michael Maurer

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Related research domains (3)

Related publications (6)

Electric machine

In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating (rotating machines) or linear (linear machines).

Magnetic field

A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron, and attracts or repels other magnets.

Magnetic vector potential

In classical electromagnetism, magnetic vector potential (often called A) is the vector quantity defined so that its curl is equal to the magnetic field: . Together with the electric potential φ, the magnetic vector potential can be used to specify the electric field E as well. Therefore, many equations of electromagnetism can be written either in terms of the fields E and B, or equivalently in terms of the potentials φ and A. In more advanced theories such as quantum mechanics, most equations use potentials rather than fields.

To meet the electrical grid's demand on pump power variation, GE Renewable Energy develops since 10 years variable speed machines (also called Doubly Fed Induction Generators or DFIG). Within turbogenerators, the phenomenon of circulating currents in Roebel bars is well known, while is it has not yet been studied for DFIGs. The main goal of this study is to calculate the circulating currents in the stator and rotor bars of DFIG under different operating points with a theoretical and practical precision of around 1%.
This study starts with an overview of the current situation in circulating current calculation and presentation of the characteristic circulating current curves for a hydrogenerator, study that led to a patent application. After a presentation of the possible calculation methods and models based on a deep and broad literature review, this study performs with a deep review of the slot inductance model analysing its precision and limitations. Based on these finds, two novel analytical models are proposed to enhance the taking into account of the strand dimensions. Only the last slot inductance model developed, based on a slot differential inductance model, permits to take the strand dimensions and the saturation into account. This model is validated experimentally using a small-scale slot/strand-model, while all slots models are compared to each other to highlight their differences.
The winding overhang model and novel analytical expressions are presented in a later chapter as well as the analytical treatment of the rotor overhang made of non-linear steel. The winding overhang model uses analytical expressions to determine the magnetic field and vector potential in the winding overhang, which have the advantage of additional knowledge compared to the results of a finite-element computation.
\In another chapter, novel exact transient current and torques expressions are derived for a DFIG experiencing a 3-phase and a 2-phase short-circuit. Then the winding overhang force computation and the circulating current calculation results are presented in two crowing chapters. In these chapters, the influence of the approximation and boundary on the end winding forces as well as the origin of the end winding forces are shown. The last result chapter is dedicated to the circulating current calculation, where several original results are presented to detail the circulating current losses reduction potential and the impact of well-known classical special transitions on the circulating currents in the case of a DFIG. The influence of the operating point and the boundary are also shown for the stator and the rotor.
This study presents many original contributions on several domains. It presented a novel slot inductance model, which was validated using a specially designed small-scale model of a slot. This small-scale model concept can certainly be extended to other parts of an electrical machine, which could help to study these effects in a laboratory instead of a power plant. This study could quantify the circulating current losses in the stator winding of a DFIG, losses that can easily be reduced to increase the efficiency of this machine. This study also presented several original fundamental contributions in the field of analytical expressions for the transient expression of current and torque in the case of a 3-phase and 2-phase short-circuit.

Basile Kawkabani, Frédéric Michael Maurer

The computation of 3-D magnetic fields is a demanding task in the analysis of electrical machines and other electromagnetic devices. In this context, integral field calculation provides a smooth solution, high precision and resolution, "on-demand"-calculation, and an origin-based formulation of the magnetic field and the magnetic vector potential. However, conventional elliptic methods lead to huge parallelizable computing efforts and significant errors. In this article, a 3-D generic current-carrying arc segment with rectangular cross section is studied. A new analytic formulation is proposed to speed up the computation of magnetic fields and reduce the error by more than three orders of magnitude. In addition, the proposed magnetic vector potential expression has a similar accuracy as numerical integration. In fact, a significant reduction of the error level has been showcased clearly with respect to the existing approaches. This article is promising for improving the design methodology and optimization of large superconducting dipole magnets or arched end-winding geometries of large electrical machines.

Basile Kawkabani, Frédéric Michael Maurer

Arch-shaped axially magnetized bodies tend to appear frequently in electrical machine analysis such as in the overhang parts of the classical radial-flux machines as well as in the main parts of the axial-flux machines. The calculation of the 3-D fields originating from these bodies is demanding. The 3-D FEA suffers from high computational burden as well as no knowledge of the field origin. Analytic techniques involve the use of elliptic integrals and complex numbers for numerical evaluation, which makes them significantly slower than the 3-D FEA. In this article, a new analytical technique is proposed to speed up the computation by a factor of 20 for the global magnetic field created by a generic magnetized body by removing complex numbers and reducing the analytic equations significantly without any loss of precision. As a result, it can compete with the conventional 3-D FEA. In addition, integral methods may contribute to the wider use of parallel processing techniques. The original expressions for the vector potential are also provided, which has its own benefits and applications. Finally, the showcased magnetized body is assessed against 3-D FEA and discussed in terms of practical applications.