Magnetization State Selection Method for Uncontrolled Generator Fault Prevention on Variable Flux Memory Machines

Permanent magnet (PM) machine drive system is vulnerable to uncontrolled generator fault (UCGF) when all gating signals are removed from inverter during high-speed operation. Due to the PM flux controllability of variable flux memory machine (VFMM), this article proposes a novel magnetization state selection (MSS) method, which can not only extend the machine speed range effectively but also avoid the overvoltage issue during UCGF process. In the proposed MSS method, the VFMM operates at several different magnetization states (MSs) to reduce the MS changing frequency. The critical speed of each MS is determined by a steady-state UCGF model to restrict the maximum voltage within a safety margin. Consequently, the overvoltage issue of the VFMM can be eliminated during the entire UCGF process. On the other side, the proposed scheme can also realize highly efficient flux weakening for speed range extension with a reduced requirement of
d
-axis current excitation. Finally, the developed control strategy is verified by experimental measurements on a VFMM prototype.