Grain-Boundary-Diffused Magnets: The challenges in obtaining reliable and representative BH curves for electromagnetic motor design

Hybrid electric vehicles (HEVs) and electric vehicles (EVs) frequently have interior permanent magnet-type motors where the permanent magnets are embedded inside the rotor in one or more layers with V-shaped poles. In general, the permanent magnets in an EV motor are subjected to high levels of a demagnetizing field (large motor current with high control angle). To protect the magnets from being permanently demagnetized due to this high demagnetizing field, it is essential that they exhibit high coercivity. Thus, the permanent magnets most commonly used in these cases are neodymium iron boron (main phase: Nd
₂
Fe
₁₄
B), as these provide a superior combination of high remanence (B
_r
) and high coercivity (H
_cj
) under typical motor operating conditions. However, to ensure high magnetic properties at high temperatures, these magnets traditionally need to be heavily alloyed with heavy rare-earth (HRE) elements such as dysprosium (Dy) or terbium (Tb). Depending on the application, as much as 10% of Dy and Tb by weight can be required to ensure high magnetic properties at the desired temperature. It is desirable to reduce and even eliminate the use of HRE elements in NdFeB magnets to decrease cost and avoid price fluctuations or supply uncertainties, which are of major concern to the automotive industry.