It is a difficult to accurately estimate the rotor position in a surface permanent magnet synchronous motor (SPMSM), especially for low speed conditions because the back electromotive force (EMF) is almost zero. In this paper, an improved sliding mode observer (SMO) with a continuous sliding mode switching function (which could reduce the chattering effect) is proposed to estimate the real-time rotor position at a low speed. Through the introduction of an intermediate variable H , the back EMF is extended at low speed, which could improve the estimation accuracy for the rotor position. In addition, the chattering phenomenon is reduced by redesigning the sliding switching function. On the other hand, inverter nonlinearity also results in torque ripple and deteriorates the drive performance because of the harmonic components in the three-phase stator current. Here, a generalized sliding discrete Fourier transform (GSDFT) strategy is studied to extract the harmonic components, which can then be compensated with the corresponding compensation voltage calculated from the motor mathematical model. With the proposed strategy, the estimation accuracy can be apparently restricted when the rotor speed is lower than 50 rpm, while the chattering effect is also improved. With the nonlinearity compensation, the total harmonic distortion obviously decreases, which can improve the rotor estimation and the torque performance. In addition, the GSDFT algorithm executes in approximately half the time of the SDFT compensation method. INDEX TERMS Surface permanent magnet synchronous motor (SPMSM), low speed, sliding mode observer (SMO), inverter nonlinearity compensation, generalized sliding discrete fourier transform (GSDFT).
Dc-link capacitor voltage unbalance would affect the performance of the neutral-point clamped (NPC) three-level inverter. With the traditional virtual space vector modulation (VSVPWM) method, the dc-link capacitor voltage could be kept balance under the different modulation indexes or power factors. With the virtual medium vector being set as an adjustable one, the voltage balance could be better guaranteed no matter there might have some non-linear factors However, whether the traditional or improved VSVPWM method has a complicated trigonometric calculation, thus, a newly MPC-based virtual vector modulation has been proposed in this paper. With the 19 virtual space vectors being in roll optimisation, a nice current trajectory, dc-link capacitor voltage balance, even a relative low switch losses have been implemented. Considering that the optimised virtual space vector could not be taken as a direct output, the distribution of the basic vectors has been generated to make it being favourable for modulation. The effectiveness of this MPC-based virtual vector modulation is verified in comparisons with other methods, which can save over 67.3% execution time of the traditional SVPWM method, and over 46.6% of the improved VSVPWM way while provide a better dc-link capacitor voltage balance and output current quality.
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