Effect of eddy current loss of magnet steel on rotor temperature rise of permanent magnet motor
The rare earth permanent magnet synchronous motor (REPMSM) has the characteristics of small volume, light weight and high efficiency. Theoretically, the rotor has no fundamental wave loss, and the rotor temperature rise should be low, but the actual situation is not. Taking an increased-type rare earth permanent magnet synchronous motor developed by the author as an example, the rotor temperature rises to 125 °C during the test. If the rotor temperature is too high, it will cause demagnetization of the NdFeB permanent magnet and affect the normal operation of the motor. This paper analyzes the reasons that may cause the rotor temperature to rise too much, and proposes measures to reduce the temperature rise.
1 rotor structure:
The stator of the REPMSM takes the stator of an asynchronous machine, and its structure generally refers to the structure of the rotor. The rotor of the asynchronous start REPMMS consists of a squirrel cage, a rotating shaft, a rotor core and a permanent magnet. The rotor core is laminated by punching, and a neodymium iron boron permanent magnet is filled in the rotor core, and the cast aluminum is formed into a squirrel cage, such as Figure 1 shows. The starting process is the same as that of the asynchronous motor. When the three-phase symmetrical alternating current is passed through the stator armature winding, a circular rotating magnetic field is formed. At this time, the rotor is stationary, the rotor cage cuts the magnetic lines of force, and the alternating current is induced to form an alternating magnetic field, and the stator magnetic field. The rotor starts to rotate. When the rotor speed is close to the synchronous speed, the induced current is no longer generated in the squirrel cage, but the constant magnetic field formed by the permanent magnet rotates synchronously with the stator magnetic field to enter normal operation.
2 rotor temperature rise causes:
The heat generated during motor operation is derived from the loss of the motor. When REPMSM is running synchronously, rotor losses include permanent magnet losses and harmonic losses.
2.1 Permanent magnet loss: The resistivity of NdFeB is (1.44×10 ˉ) Ω•m, which has a certain conductivity and will generate eddy current loss in the alternating magnetic field. The thermal conductivity of NdFeB is 7.7 cal/m. h. °C, poor heat transfer. NdFeB magnets are prone to rust and oxidation, making it difficult for heat to conduct outward, which exacerbates the temperature rise of the rotor.
2.2 Harmonic loss: Due to the cogging effect, stator magnetic field and other factors, the harmonic magnetic field in the motor air gap is very complicated. The harmonic magnetic field in the air gap moves relative to the rotor at different speeds, inducing current in the rotor core and the squirrel cage, thereby generating harmonic losses and raising the rotor temperature.
