Permanent magnet savings in high speed electrical motors

For high speed permanent magnet (PM) synchronous machines with a moderate over-speed limit of up to typically 150 m/s surface velocity, the buried magnet rotor concept, utilizing the steel sheet strength for fixing the magnets, is applicable. It allows the utilization of an additional reluctance torque via a negative d-current operation. A reduction of the PM volume by 50% in comparison to surface mounted magnets with carbon or glass fiber bandages is possible. For that a small magnetic air gap is necessary, which may lead to increased magnetically excited acoustic noise production. A finite element comparison of motor utilization, mechanical strength, noise production and magnet mass saving of three different rotors for identical stator geometry is given.