With electric vehicles (EVs) and hybrid electric vehicles (HEVs)
set to grow in the coming years, design optimizations of electric
motors for automotive applications are receiving more attention.
Under demanding duty cycles, the moving part within a motor, the
rotor, may experience high and varying stresses, which may lead to
fatigue failure. Therefore, engineers are facing challenges in
designing efficient and durable motors, especially for interior
permanent-magnet (IPM) motors, in which the rotors have embedded
magnets with small "bridges" of laminated electrical
steel to keep the magnets in place.
Cost-effective stators and rotors are made from electrical
steels, with high magnetic permeability and low power losses.
However, national and international standards for electrical steels
do not specify mechanical properties. Steel producers would
normally state typical mechanical properties only, and no fatigue
properties are available in published literature.
This paper describes typical fatigue performance of an
electrical steel grade SURA® M270-35A for EV/HEV motor
applications. Focus will then turn to finite element (FE)
structural and magnetic evaluations of various IPM rotor designs.
Particular attention will be paid to understanding the influence of
the magnet pole arrangements, the magnet slot geometry and
"bridge" dimensions on rotor fatigue lives and
rotor-stator air-gap flux densities.
By generating and applying fatigue data for electrical steels,
in conjunction with FE-based magnetic, structural, and fatigue
modeling, the paper summarizes how various magnet-slot schemes and
rotor profiles can be assessed under a typical HEV motor duty
cycle. Such understanding provides good pointers to optimized motor
rotor profiles with balanced durability and magnetic flux density
in the air-gap. The paper demonstrates that, using the electrical
steel property data and the proposed design procedure, it is
possible to design durable IPM motor rotors that can last much
longer than the expected lifetime of an EV/HEV vehicle.