This paper is focused on the reliability evaluation of an electric vehicle (EV) motor based on hairpin technology. Besides bearings (not discussed here), the weak point of an electric motor is its insulating system. For inverter-fed motors, the inception of partial discharges might lead to failure in a matter of days, and thus, deserves particular attention. Qualification and lifetime evaluation of inverter-fed machines are described in IEC 60034-18-41, which specifies accelerated aging procedures and considers partial discharge inception as the end-of-life criterion. This standard was used as a reference for this paper. The only exception is that insulation models were subjected to the mechanical stress profile reported in the ISO 16750-3-2012 standard since an automotive motor is subject to significant vibrations during its operation. From the tests performed, we observed that, with the hairpin technology, turn-to-turn is the weakest link in the insulation system. All the insulation models were partial discharge (PD) free from the beginning of the tests to breakdown and the root cause for breakdown was, in all cases, traced back to cracks on the surface of the insulation. This suggests that, depending on insulating enamel thickness and conductor geometry, some insulation systems are intrinsically PD free by design, despite the effect of aging. Considering the severe vibration profiles typical of EVs, and the principal breakdown mechanism (cracking of the insulation), mechanical stress coupled with thermal stress appears as the main aging driver. Therefore, this paper spotlights the lack of proper standards for the qualification of automotive electric motors and hints at the possibility that IEC 60034-18-41 considers dealing with motors that might be intrinsically partial discharge free even after long-term exposure to operational stresses.

The electrical properties of epoxy based composites modified by low amounts of graphite oxide, below the conduction threshold, have been investigated. The composites have been prepared without the use of solvents by direct sonication of graphite oxide (GO) powders and of chemically modified and partially reduced GO powders in the based epoxy monomer. Through a mild thermal treatment, in situ reduction of the previously dispersed GO has been obtained directly inside the epoxy resins. The changes in the electrical response of the materials thus obtained have been compared to that of pristine unmodified epoxy resin. Data so far collected underline the possibility to tune the electrical conductivity of the composites within two orders of magnitude and to increase the values of permittivity without significantly worsening dielectric losses. POLYM. COMPOS., 36:294–301, 2015. © 2014 Society of Plastics Engineers

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This work assesses the initial and crucial part of electrical treeing degradation, the inception stage, focusing on its dependence on applied voltage waveform and frequency. Tests have been performed on needle-plane configuration samples in solids and gels. A physical model has been formulated through an adaptation of an established theory for solids in which electrical tree inception is related to damage-producing injection currents. The voltage rise time appeared to be the most important parameter influencing the tree inception in the gel, while in the solid material the frequency is more relevant. The analysis leads to the conclusion that tree inception in gels is due to a single highenergy event, in contrast to what is commonly known for solids where damage accumulation takes place. A tree inception model is proposed for the gel, in which initiation is driven by a pressure wave generated by the electric field and the space charge injected into the sample. The model fits the experimental data and may be used to predict the tree initiation for different waveforms and voltage values.

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This article deals with the investigation of electrical properties of epoxy-based nanocomposites containing graphene oxide nanofillers dispersed in the polymer matrix through two-phase extraction. Broadband dielectric spectroscopy and dc electrical conductivity as a function of electric field have been evaluated in specimens containing up to 0.5 wt % of nanofiller. Nanocomposites containing pristine graphene oxide do not show significant changes of electrical properties. On the contrary, the same materials after a proper thermal treatment at 135 C, able to provoke the in situ reduction of graphene oxide, exhibit higher permittivity and electrical conductivity, without showing large decrease of breakdown voltage. Moreover, a nonlinear behavior of the electrical conductivity is observed in the range of electric fields investigated, i.e. 2-30 kV mm 21 . A new relaxation phenomenon with a very low temperature dependence is also evidenced at high frequency in reduced graphene oxide composites, likely associated to induced polarization of electrically conductive nanoparticles.

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