Abstract
Designing the insulation system for motors to be used in electrical aircraft requires efforts for maximizing specific power, but, in parallel, particular attention to achieve high reliability. As a major harm for organic insulation systems is partial discharges, design must be able to infer their likelihood during any operation stage and handle their potential inception. This paper proposes a new approach to carry out optimized or conservative insulation system designs which can provide the specified life at the chosen failure probability as well as look at the option of possibly reducing the risk of partial discharges to zero, at any altitude. Examples of designing turn, phase to ground and phase-to-phase insulation systems are reported, with cases where the design can be optimized and other cases where the optimized design does not pass IEC testing standard. Therefore, the limits for design feasibility as a function of the required level of safety and reliability are discussed, showing that the presence of partial discharges cannot be always avoided even through conservative design criteria. Therefore, the use of advanced, corona-resistant materials must be considered, in order to reach a higher, sometimes redundant, level of reliability.
Highlights
In order to show how the innovative design approach proposed here works and how this interlaces with the selection of insulating materials, a case based on a recent DOE Advanced Research Projects Agency-Energy (ARPA-E) project, Aviation-Class Synergistically Cooled Electric-Motors with Integrated Drives (ASCEND), is dealt with here
Design quantities bringing to the choice if insulation thickness are discussed in terms of reliability and likelihood of PD inception, at different pressure as the aircraft can experience during take-off/landing and cruise
Since the major cause of accelerated degradation and premature failure of insulation systems under electrothermal stress is partial discharges, the design should be PD free. It must be PD free at nominal operating voltage, and at the test voltage indicated by IEC 60034-18-41
Summary
In order to show how the innovative design approach proposed here works and how this interlaces with the selection of insulating materials, a case based on a recent DOE ARPA-E project, ASCEND, is dealt with here. The project has the aim to design and manufacture a PWM-controlled motor for electrical aircraft, at a voltage up to 1 kV and power larger than 250 kW. The driving ideas is to have a motor divided in sections, each driven by its PWM control with 166 V. The insulation design should prove to be PD free up to test voltage levels that depend on the expected converter switching overshoots (encompassing stress categories from benign to extreme), aging and temperature effects [3,4,5,6,7,8,9,10,11]. In the specific case of this project, the very short distance between inverter and machine runs into the benign category, even if switching rise time is some tens of nanoseconds. Design quantities bringing to the choice if insulation thickness are discussed in terms of reliability and likelihood of PD inception, at different pressure as the aircraft can experience during take-off/landing and cruise
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