A Sensorless I-F Start-up Strategy for Flying Start of Ultra-High-Speed Permanent Magnet Motors based on Short-Circuit Transient Differentiation

Abstract

Conventional current-to-frequency (I-F) start-up strategies apply only to zero-speed conditions. When a nonzero initial speed (referred to as a flying start) exists, loss of synchronism is likely to occur because the virtual synchronous reference frame fails to align with the rotor flux linkage direction. To meet the flying start requirements of ultra-high-speed permanent magnet motors, this paper proposes a sensorless I-F start-up strategy for flying starts based on differential analysis of short-circuit transients. Upon power-up, the inverter places the three-phase stator windings in a controlled short-circuit state and applies two short-duration short-circuit pulses at different instants. By sampling the phase variation of the stator current vectors during the two short-circuit transients, differential current characteristics are extracted to rapidly estimate the initial rotor position and speed, thereby facilitating smooth I-F start-up. The proposed method overcomes the limitations of conventional I-F strategies confined to zero-speed start-up and is applicable to oil-film and air-bearing supported motor systems. Experimental results validate a smooth flying start featuring continuous and impact-free stator currents, effective suppression of mechanical friction prior to air-film formation, and the enhanced stability and robustness of I-F start-up.