Comparison of PM-machines with ferrite and NdFeB magnets in terms of machine performance and sensorless start-up control

The costs of rare-earth permanent magnets such as NdFeB increased rapidly in the last few years. Therefore, great attention is paid for alternative magnet materials. This paper compares two permanent magnet synchronous machines using external rotors with different magnet materials - one with NdFeB and the other with much cheaper ferrite magnets. The first section examines the risk of irreversible demagnetization of the ferrite magnets at inappropriate operating points. Furthermore, measurements and some numerical-simulation results contrast characteristics of the machines in terms of no-load voltage, short-circuit behavior, torque output and efficiency. Different magnet materials influence position-sensorless techniques based on anisotropic effects especially for start-up control. The last part of this study compares two methods to estimate the absolute electrical rotor position at standstill. Reliability considerations show their quality at the respective machine.