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Corona resistant enameled wires as solution for electrical motor winding systems supplied by high du/dt inverter voltage: An insulation life-time estimation

Prof. Dr.-Ing. habil. Dr. h. c. Kay Hameyer

 Institute of Electrical Machines (IEM)

RWTH Aachen University, Aachen, Germany

 High slew rate inverters and an increasing DC-link voltage move electrical ageing of Insulation systems of low voltage traction drives into the focus. While state of the art machines are designed to be partial discharge (PD) free, corona resistant materials can withstand partial discharge significantly longer than standard materials. This makes tolerating PD during operation a future option in low voltage drives. While the effects of electrical ageing are well known for high voltage machines, there is no experience with PD in the insulation systems of low voltage drives. Therefore, lifetime tests which characterize the electrical ageing of corona resistant wires in different operating points need to be performed.

Lower losses and a better overcurrent capability of SiC-MOSFETs compared to Si-IGBTs lead to increasing voltage slew rates (du/dt) of the inverter. At the same time the DC-link voltages in electric vehicles are increasing to achieve a faster charging process. Both of these aspects lead to a higher electric stress in the insulation system.

One option to meet the higher demands is to employ thicker insulation layers. However, this leads to reduction of the copper fill factor. In a parametrization of the Paschen-curve for polyamide imide is given, which can be used alongside with the geometry of the insulation system to calculate the minimum voltage at which PD can occur. The calculated partial discharge inception voltages (PDIV) largely depend on the permittivity and the thickness of the insulation.

Therefore, another option is to decrease the permittivity of the materials which are employed. However, there are only a few low permittivity materials which offer the same thermal and mechanical properties of polyamide imide. A third way is to develop an insulation system which can withstand partial discharge significantly longer than standard materials. While insulation materials degrade – depending on the load point – within 10 minutes to 4 hours when PD is present, manufacturer data suggests, that corona resistant wires can last more than 100 times longer. PD-endurance tests of polyimide films with and without inorganic nanoparticles show an increased lifetime by approximately the same factor. However, these tests have been conducted at fixed operating points, while the insulation system of electric vehicles is subjected to a large range of operating points.

As a result, it can be stated that by increasing the insulation thickness and decreasing the permittivity of the insulation material only yield a little increase of PDIV in the insulation system of low voltage drives. Employing PD-resistant materials offers the possibility of using higher DC-link voltages even while PDs are present. First measurements show that the lifetime of such insulating materials is increased significantly when compared to standard materials. Further measurements have to be performed for a more detailed understanding of the lifetime behavior of PD-resistant enameled wires.

 

 

Professor Kay Hameyer

Dr. Kay Hameyer (FIET, SMIEEE) received his M.Sc. degree in electrical engineering from the University of Hannover and his Ph.D. degree from the Berlin University of Technology, Germany. After his university studies he worked with the Robert Bosch GmbH in Stuttgart, Germany as a Design Engineer for permanent magnet servo motors and vehicle board net components. Until 2004 Dr. Hameyer was a full Professor for Numerical Field Computations and Electrical Machines with the KU Leuven in Belgium. Since 2004, he is full professor and the director of the Institute of Electrical Machines (IEM) at RWTH Aachen University in Germany. From 2006 on he served as vice dean and from 2007 to 2009 he was elected dean of the faculty of Electrical Engineering and Information Technology of RWTH Aachen University. His research interests are numerical field computation and optimization, the design and controls of electrical machines, in particular permanent magnet excited machines and induction machines. Since several years Dr. Hameyer’s work is concerned with the characterization and modelling of soft- and hard-magnetic materials to enhance the performance of electric drive systems. Own developments of measurement technology for winding insulation systems for high du/dt inverter fed operated low voltage electrical machines leads to models for the life-time estimation of insulation systems. Magnetically excited audible noise in electrical machines are further topics of interest. Dr. Hameyer is author of more than 350 peer review journal publications, more than 700 international conference publications and author of 4 books. Dr. Hameyer is a member of VDE, IEEE senior member, fellow of the IET.

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