Position Sensorless Control of multiphase PMSMs

Performance improvement in electrical motor drives is today constant task in many research centres around the world. Following from stated, my thesis investigates sensorless field-oriented control (FOC) of multiphase machine (n > 3). To measure rotors position and compute shaft’s speed, typically a position sensor is mounted on rotor’s shaft, but various problems may arise (i.e. higher drive cost, electromagnetic interference). In many applications these sensors increase the mounting and maintenance cost i.e. reduce the reliability of the whole system, hence are not used. Practical and attractive alternative nowadays to the motor control with mechanical sensors is cost-saving sensorless motor control. It is common to categorize the position estimation of permanent magnet synchronous machine (PMSM) in two main groups: back-electromotive force (EMF) methods and saliency-based methods. In case where the production cost is very important and low-speed operation is not required, the EMF-methods are preferred. The motor considered in this work for the development of the sensorless strategy, based on EMF-method, is a nine-phase surface PMSM. The objective is to investigate sensorless control possibilities i.e. implement control without usage of standard and well-known three-phase observers/schemes and to derive adequate sensorless FOC algorithm. Magnets on rotor are shortened which causes production of highly non-sinusoidal EMF in the stator windings. The FFT analysis of the EMF in real machine reveals a high-magnitude low-order harmonic spectrum, with third-harmonic magnitude almost equal to the fundamental. However, the third EMF harmonic plays a key role in the rotor angular position/speed estimation. It is believed that an I-Hz starting method for smooth and fast transition from open-loop frequency to sensorless FOC based on phase-locked-loop can also be implemented for studied multiphase surface PMSM. Considering the machine model in vector space decomposition variables, into third sub-plane only the third harmonic is mapped (from the fundamental to thirteenth harmonic are considered). The stator current cannot be evaluated because the harmonic currents, induced by EMF harmonics, are eliminated using the vector proportional integral controllers to reduce losses. Following from stated, third sub-plane stator voltage is just related to back-EMF, without current contributions. It is obvious that after reconstructing phase voltages in output to the control system, rotor position can easily be estimated and the current observer is not necessary nor achievable as in observers. Derived hybrid control composed by the I-Hz control for low speed and sensorless FOC control for medium-high speed based on the third EMF harmonic, Matlab/Simulink algorithm has been performed. The simulation results have been experimentally validated for symmetrical nine-phase surface PMSM highly non-sinusoidal. The digital controller is the dSpace DS1006 development board. Experimental verification are obtained in speed control mode under hybrid control. Good agreement in simulation and experimental results was recorded. This confirms validity of hybrid control algorithm derived in my thesis. As it will be concluded, the main advantage of the new proposed method is related to the computation time and algorithm complexity to define the rotor position, which are significantly reduced.