Velocity Measurement Technique for Permanent Magnet Synchronous Motors Through External Stray Magnetic Field Sensing

This paper presents a technique to measure the velocity of permanent magnet synchronous motors (PMSMs) by sensing the external stray magnetic field outside the stator yoke. A magnetic- equivalent circuit model for the surface-mounted PMSM was developed to analyze the behavior of the stray magnetic field. The external stray magnetic field can provide the position and the speed information of motor rotor. Simulation by means of a 3-D finite-element-method modeling and experimental results over a wide speed range are presented to substantiate the successful application of the proposed technique. A sensitive triaxial tunneling magnetoresistive sensor was employed to measure the stray magnetic field outside the motor stator yoke. The proposed method was validated in both steady and transient operating conditions, and it demonstrated small measurement errors in the range between -0.17% and 0.55% over the speed range from 50 to 1000 rpm. Therefore, it verifies that the proposed technique can provide an easy-to-operate, low-cost, and high-accuracy alternative to the velocity measurement for PMSMs.