Analysis and Neutral Voltage-Based Detection of Interturn Faults in High-Speed Permanent-Magnet Machines With Parallel Strands

This paper analyzes short circuits in high-speed permanent-magnet (PM) machines. Analytical models are used to estimate the consequences of single-strand, parallel-strand, and phase-to-phase turn-to-turn short circuits, as well as open-circuit faults. The parameters for the analytical models are obtained from 3-D finite-element simulations and consider the actual position of each strand inside the slot. It is shown that parallel-strand short circuits are less severe than single-strand short circuits, both before and after fault mitigation actions. The potential for large fault current densities in high-speed PM machines is also demonstrated, with measured rms current densities up to 450 A/mm
²
. A simple fault detector based on the synchronous detection of the machine and inverter neutral voltage difference is proposed, which is suitable for hardware implementation and can function as an independent observer of a drive system. The dependence of the detectability on the short circuit location and the effects of natural imbalances on the detector output are investigated. Experiments are performed to demonstrate the concept, showing that faults of at least 0.4% can be detected.