Temperature Influence of NiFe Steel Laminations on the Characteristics of Small Slotless Permanent Magnet Machines

High performance electrical machines can operate at temperatures of 100°C and beyond in rotor and stator cores. However, magnetic properties are generally measured at room temperatures around 23°C to 25°C according to the standards, even if it is known that the magnetization of some materials is substantially influenced by increasing temperatures. This paper investigates the thermal influence on the magnetic properties and iron losses in the stator cores of small slotless permanent magnet synchronous machines (PMSMs). The stator stack is made of thin nickel iron (NiFe) lamination sheets. Magnetic measurements of the stator core are conducted for different frequencies and flux densities at several temperatures between 25°C and 105°C. The obtained measurement data is afterwards used in finite element method (FEM) simulations to investigate the influence of the magnetic property change on the machine performance. For the PMSM in consideration, the FEM simulations show that an increased stator core temperature reduces the electromagnetic torque considerably; approximately 1/3 of the torque reduction due to increased rotor magnet and stator core temperatures (from 25°C to 100°C) can be attributed to the increased stator core temperature.