Comparative Turn-off Safe Modes of Ferrite- and NdFeB- Interior PMSMs

This paper deals with the analysis of the safe turnoff strategies recommended for permanent magnet synchronous machines. Two common safe state modes are considered: active short circuit, useful to avoid dangerous levels of uncontrolled generator voltages; and active open circuit, which is preferred to avoid abrupt torque transient and the risk of irreversible demagnetization. Two permanent magnet traction motors, one with NdFeB magnets and the other with ferrite magnets, are considered, highlighting the pros and cons with reference to the safe state modes. Steady-state and transient operation under active short- and open-circuit conditions are swiftly evaluated for the existing designs using offline flux maps manipulation. Also, the influence of the magnet temperature on the safe turnoff is investigated. Finally, novel indexes are introduced and implemented on the (x, b) design plane, for the generalization of the results and the early evaluation of the safe state modes during the preliminary design of the machine.