Sensorless direct torque and flux control of interior permanent magnet synchronous motors at very low speeds including standstill

摘要

In this thesis, several essential improvements of the sensorless direct torque control (DTC) strategy for interior permanent magnet (IPM) synchronous motor drives are presented. The studies comprising of analytical, modeling and experimental implementation clearly indicate the potential of a high-performance direct torque controlled IPM synchronous motor drive without a mechanical encoder. The DTC technique and its associated problems have been further investigated in this thesis. Its major drawback namely the high torque and flux ripples are remedied by the proposed direct torque and flux controller (DTFC). It utilizes two PI controllers to predict the most appropriate voltage vector adaptive to error amplitudes are predicted and synthesized. This results in reduced torque and flux ripples with a constant switching frequency. Several stator flux and speed estimators are proposed to achieve sensorless control. The simple open-loop estimator works well only in the medium to high speed region. This is because the other problems of the DTC such as non-linearities of the inverter, stator resistance variation and dc-offset effects are not addressed. Improved flux and speed estimation is achieved by employing closed-loop observers. A speed adaptive sliding mode observer for joint stator flux and rotor speed estimation is firstly proposed. To further improve the very low speed performance of the DTFC, another inherently speed sensorless sliding mode observer based on a novel concept called the extended rotor flux is proposed. This observer delivers satisfactory performance in the very low speed region. Nevertheless, continuous full-load operation at standstill is not possible. For persistent zero speed operation with full-load, a high frequency signal injection technique is proposed in this thesis. A carrier excitation signal was injected into the machine and the stator current response was used to extract the rotor position information. As the signal injection scheme becomes less effective at higher speeds, a hybrid signal injection and sliding mode observer is introduced to enable sensorless operation over a wide speed range. The abovementioned studies show that the problems of the DTC for IPM machines have now been significantly reduced compared to what have been achieved to date; making it a viable and cost-effective candidate for future wide speed sensorless IPM synchronous motor drives.