Sensorless Control of a Nine-phase Surface Mounted Permanent Magnet Synchronous Machine with Highly Non-Sinusoidal Back-EMF

The paper investigates sensorless field-oriented control of a two-pole surface mounted permanent magnet synchronous machine (PMSM) with a symmetrical nine-phase winding configuration. Reduced magnet span on rotor causes production of highly non-sinusoidal back-electromotive force (EMF) in the stator windings. FFT analysis of the EMF reveals a high-magnitude low-order harmonic spectrum, with third-harmonic magnitude almost equal to the fundamental. Harmonics with such magnitudes have been used in the past to increase electromagnetic torque of the multiphase machines. An alternative use of such harmonics is for speed/position estimation, as discussed here. It is believed that an I-f starting method for smooth and fast transition from open-loop frequency to sensorless field-oriented control (SFOC) based on phase-locked-loop (PLL) can also be implemented for studied PMSM, employing the same low-order harmonic spectrum. The objective of this work is therefore to investigate sensorless control possibilities, i.e. to implement closed loop control without use of either the encoder/resolver or standard three-phase observers/schemes (e.g. Luenberger, extended Kalman filter etc.). As it will be shown, the main advantage of the developed method is related to the significantly reduced computation time and algorithm complexity. Derived hybrid control schematic and corresponding simulation results are also shown. Some simulation results are validated using real machine in an experimental setup.