Reliability Analysis of a Fault-Tolerant Integrated Modular Motor Drive for an Urban Air Mobility Aircraft Using Markov Chains
James Swanke, Thomas M. Jahns
Abstract
Electrified urban air mobility (UAM) aircraft requires catastrophic failure rates for their propulsion systems that are lower than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-9}$ </tex-math></inline-formula> failures per flight hour. Electric machines and drives have been identified as limiting propulsion subsystems with failure rates in the range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-5}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-6}$ </tex-math></inline-formula> failures/h. Fault-tolerant (FT) machine drives offer a promising approach to improve reliability using existing technology. An FT integrated modular motor drive (IMMD) design is proposed for a quadrotor UAM application, and its estimated reliability is studied using Markov chain analysis. Quantitative examples show that raising the repair rate for the motor drive modules significantly reduces the IMMD’s catastrophic failure rate. However, the lower bound on its failure rate with high repair rates is determined by its single-point failure (SPF) rate (e.g., motor bearings), highlighting the importance of aggressively suppressing as many of the IMMD’s SPFs as possible. Markov chain analysis shows that SPF suppression can result in major reductions in the IMMD catastrophic failure rate by factors exceeding <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{3}$ </tex-math></inline-formula> as the SPF rate is driven toward its ideal limit value of zero.