Temperature Influence on Hard Magnetic Ferrites for PM Electric Machines

Current research on increasing energy efficiency in electric machines with permanent magnets (PM), stimulated by the actual policy on reducing energy consumption and limiting environmental pollution, is focused on optimizing the machines design, in which materials for permanent magnets playa key role. Hard magnetic materials such as hard ferrites, AlNiCo alloys and rare earth magnetic materials have gained ground due to their advantages, such as ease of integration into a compact structure, small size and relatively high stability at operating speeds. For hard ferrites, the advantages related to the price and the manufacturing facilities of different PM geometries are pleading. However, this class of hard magnetic materials requires specific technical data as accurately as possible to take into account the influence of various factors on the magnetic characteristics, in order to enhance their effectiveness and efficiency. This paper analyzes the temperature influence on the performances of hard magnetic ferrites used in PM electrical machines, in correlation with other factors that modify the PM optimal operating point such as the load line and the external demagnetizing magnetic fields. As a case study, the influence of thermal stress on the main parameters of strontium ferrite samples is experimentally investigated. The families of demagnetization curves and hysteresis cycles for magnetic polarization <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$J_{B}=\mathrm{f}(H)$</tex> and magnetic induction <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B=\mathrm{f}(H)$</tex> for cylindrical samples of sintered strontium ferrite magnets are obtained using the hysteresisgraph method, in the temperature range 20°C-200°C.