A Method to Compute the Irreversible Demagnetization Temperature in Permanent Magnets

Permanent magnets (PM) containing rare earth elements are critical in developing traction drives for electric and hybrid vehicles. The demanding driving conditions heat the PMs and could lead to a degraded vehicle performance if the temperature exceeds the irreversible demagnetization boundary. Moreover, developing novel machine protection strategies, such as sensorless PM temperature estimation, relies in anticipating the temperature limit under different electromagnetic loads. Although this limit can be computed analytically from the temperature-dependent demagnetization model, its derivation is not evident. Therefore, the present work discusses a simplified method to compute the magnet temperature before irreversible demagnetization. This technique consists in evaluating the minimum flux density in open circuit and a severe working condition for a single magnet grade, then evaluating the temperature limit and extrapolating the results for other grades. The method was proven in twelve PM grades in a ten-pole nine-slot surface-mounted PM synchronous motor with a subsequent discussion on the effect of magnetic parameters on temperature limit. Results validated the temperature behavior of all evaluated PM grades. Furthermore, magnet grades with a lower remanence could withstand higher temperatures. Therefore, they are more suitable for extended transient operation and higher nominal currents.