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Morales Caporal

Dr.-Ing. Roberto Morales Caporal

"Encoderless predictive direct torque control of the synchronous reluctance machine at low and zero speed" (2007)

This thesis deals with a Predictive Direct Torque Control (PDTC) for the Synchronous Reluctance Machine (SynRM) with and without encoder at very low speeds including zero speed.

This method to control stator flux and electromagnetic torque of the machine combines the good dynamic of direct torque control (DTC) with the advantages of constant switching frequency and relative long sampling time. PDTC is a model-based, predictive control, which calculates the switching instants of two possible active voltage space phasors to build the demanding torque one step in advance. The switching instants are computed in such a way, that the mean value of the torque is reached as fast as possible. Next, the trajectory of the stator flux is predicted with these two pre-selected voltage space phasors and the optimum of both, which leads to the best trajectory of the stator flux at the end of the cycle, is applied to the machine.

At very low speeds a second active voltage space phasor is used to counteract the drop of the stator flux. The optimum switching instant of this second active voltage space phasor as well as the criteria for its appropriate selection are explained in detail.

As a further research, a method for estimating the rotor position in a direct torque controlled SynRM at very low and zero speed is proposed. In this range of operation, the natural anisotropy of the rotor is exploited for the detection of the rotor Position. By injecting during short time a test voltage signal (TVS) in certain control cycles and with determinate sequence. The injected TVS does not produce additional torque at the end of the cycle as a compensatory voltage signal is proved.

The identification of the rotor position is based on the transient stator current change due to the TVS, so that, the stator current derivatives can be estimated in digital way. These current derivatives contain the required rotor position information. The obtained signals are processed by means of a quadrature phase-locked loop (QPLL) observer to extract the rotor position signal without detriments in phase and frequency. Following this strategy no extra hardware, especial current transducers or connections are needed in comparison with a standard drive with encoder.

The proposed algorithm has been implemented in a digital signal processor (DSP) and a field programmable gate array (FPGA) embedded in the same board. Experimental results at very low and zero speed using a commercially available machine demonstrate high performance torque control with the proposed control scheme and the effectiveness of the investigated encoderless strategy.