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In the application of electric railway traction, induction machines may suffer momentary power interruptions. Restarting control while machine running is a critical part of such applications, but residual flux in the free-running rotor deteriorates conventional controller performances, leading to inrush current and torque impact. Thus, this article proposes an improved feed-forward current controller that compensates voltage variations in the restart process. The residual rotor flux, as well as the induced back electromotive force, is estimated only with the measured speed in the free-running stage. The exact electromotive force is compensated at the restart moment, and the following current re-establishing stage. Based on the feed-forward compensation, the restart impact is eliminated and the restart current is controlled accurately in the predefined trajectories. Verified by experiments, the restart controller provides flexible restart manipulations. [ABSTRACT FROM AUTHOR]

This paper presents a load torque estimation method for self-sensing brushless dc drives. Torque ripples in brushless dc machines can be reduced using load torque information. This method uses the terminal voltage, the virtual neutral point voltage, and the dc bus current of the machine. The algorithm uses the variation of successive back electromotive force (back EMF) samples to estimate the rotor speed. The rotor position is estimated by defining an intermediate function of estimated speed and back EMF samples. An estimate of acceleration is used to estimate load torque. The load torque information is used for increasing the dynamic stiffness of the drive. The mathematical background is given and discussed, and the simulations and the experimental results prove the performance of the proposed method. [ABSTRACT FROM PUBLISHER]