Effect of Magnets Asymmetry on Stray Magnetic Flux Based Bearing Damage Detection in PMSM

In this paper, it is shown that stray magnetic flux is a highly reliable condition monitoring variable which is rich in motor health related information and it is superior to motor current. It is well-known that the magnetic field distribution in permanent magnet (PM) rotors exhibit asymmetry due to magnet manufacturing process. It is shown that, in addition to the clear characteristic bearing signatures, the magnet imperfections induce multiple new bearing fault signatures in the stray magnetic flux. Different than current, the signatures in stray flux are more immune to noise, consistent and independent of load and speed. It is also shown that the stray flux-based detection method can locate the faulty bearing. To justify the proposed approach, comparative simulations and experiments are carried out on a surface mounted PM synchronous motor (PMSM) and an interior permanent magnet motor (IPM) with outer raceway bearing fault and different degree of magnet field asymmetry.