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Electrical machines for transportation applications need to be highly reliable, particularly if they drive safety-critical systems. At the same time, another main requirement is represented by the significant torque density, especially for aerospace, where weight constraints are extremely stringent. For achieving high peak torque, an effective strategy consists in supplying the windings with a current greater than the rated value; thus, thermally overloading the machine for limited time periods. However, if the insulation is overheated, the machine lifetime is shortened and reliability issues can arise. This paper experimentally investigates the influence of short-time thermal overload on the insulation lifetime for low voltage, random wound electrical machines. The analysis is performed on round enamelled magnet wire coils, which are aged by accelerated thermal cycles. The obtained results are statistically processed through a two parameter Weibull distribution. According to the findings of the experimental data post-processing, a lifetime prediction model is built. This model is employed for predicting the lifetime consumption of a motor embedded into an electromechanical actuator for aerospace application.

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This paper focuses on the inception of partial discharges within the insulation system of electrical actuators used for the More Electrical Aircraft (MEA). Since these machines should operate in the absence of Partial Discharges (PDs), the dependence of the PD Inception Voltage (PDIV) on voltage impulses typical of wide bandgap (SiC) devices is investigated at both 1 bar, reduced pressures close to those typical of aircraft cruising altitudes (150 mbar-200 mbar) or lower (down to 5 mbar). Propagation issues are not dealt with here as results were obtained working on insulation models consisting of couples of wires twisted together (twisted pairs), thus knowing exactly the potential differences between all points of the insulation model. The results show that the rise times and the switching frequencies associated with wide bandgap devices have little impact on the PDIV. A model able to predict the PDIV of the turn/turn insulation of random wound motors (the most vulnerable part of the insulation) at different pressures is proposed. The model is also able to deal with temperature changes, with limitations that depend on the type on insulation systems. INDEX TERMS Partial discharges, More Electrical Aircraft, MEA, wide bandgap devices, SiC inverter, Qualification. I.

Electric motors for aerospace applications are often characterised by short-time duty rather than continuous-duty operations. Some of the main requirements for these machines are high torque density and reliability. Considerable torque density improvement can be achieved by supplying the windings with current greater than the rated value and thus thermally overloading the machine for short periods of time. However, if the insulation is overheated, the machine lifetime is shortened, and reliability issues can arise. This paper experimentally investigates the influence of short-time thermal overload on the insulation lifetime of low voltage, random wound electrical machines. The analysis is performed on round enamelled magnet wires coils aged by accelerated thermal cycles, and the results are statistically processed by the two-parameter Weibull distribution. The experimental data is also used for building a lifetime estimation model, which is employed for predicting the lifetime of a motor embedded into an electromechanical actuator for landing gear extension/retraction.

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In this paper, the results obtained from lab tests on twisted pairs subjected to different voltage waveforms and atmospheric conditions are used to propose how to modify the IEC Std. 60034-18-41. The goal is to make the standard suitable for the More Electrical Aircraft (MEA). The results show that it is initially necessary to screen out materials through simple tests. The enhancement factors for temperature can be modified to consider reduced pressures and temperatures using a simple model. The aging enhancement factor can be reduced considering the reduced sensitivity of the partial discharge inception voltage (PDIV) at low pressures on the enamel thickness. Eventually, reference will be made to the drive discussed in Part I of this series to draw conclusions about the likelihood of partial discharge inception in a random wound stator and how to reduce it by modifying either the inverter or the stator insulation. Reference to a random wound motor is made throughout the paper.

Current standards base the evaluation of partial discharge (PD) harmfulness on their amplitude and repetition rate. This work shows how, on the contrary, the non conventional environmental conditions introduced by aerospace applications (i.e. low-pressure atmosphere, higher frequency supplies) lead respectively to lower or unchanged PD apparent charges, which are however characterized by different mean energies, hence destructive potential, in a counter intuitive manner.