Stray Magnetic Flux Based Condition Monitoring Techniques for Permanent Magnet Synchronous Motors

Permanent Magnet Synchronous Motors (PMSM) are widely used in various applications from home appliances to electric vehicles due to its advantages like high efficiency, high power density etc. The market share of this type of motor is increasing significantly in the recent decade. In order to ensure their reliable operation in safety critical system, condition monitoring tools are essential. The effective fault diagnosis and condition monitoring system reduces downtime and unplanned maintenance which saves both time and money. Though several motor attributes like current, vibration, temperature etc. can be used for diagnosis of health condition of an electric motor, stray magnetic flux around the motor housing has rich information about the health status of PM motors due to the use of high energy magnets. This dissertation presents condition monitoring techniques which are developed to address three major issues in PMSM. Stator fault is the most common electrical fault which begins as an inter-turn failure. It is essential to identify this failure at its initial stage to prevent the catastrophic damage caused by stator fault. It is shown that the fundamental component of stray magnetic flux may result in fault negative error under certain operating conditions. To overcome this problem, third harmonic component is analyzed comprehensively and proposed as a reliable fault metric throughout the entire range of operation to detect and locate the fault. Secondly, the common mechanical fault in electrical motors, bearing damage is studied. It is shown that the magnetic field of PM motors exhibit asymmetry due to the manufacturing imperfections of magnet. A stray magnetic flux-based bearing fault detection method is proposed by leveraging this asymmetry. The proposed method is analyzed extensively in both simulations and experiments. A comparative study between motor current based detection and the proposed method is also performed. In the last part of the research, a stray magnetic flux based the temperature estimation of permanent magnet is proposed. The stray magnetic flux around the motor can be correlated to the magnet temperature. However, it also is affected by the permeability variation of iron core and magnetic field generated by the stator current. A compensation co-efficient is proposed to compensate the mentioned effects and obtain the permanent magnet component of stray magnetic flux from the measured data. Then the proposed scheme is adopted to estimate the temperature of magnets online under dynamic operating conditions.