A modified hysteresis-based DTC strategy for synchronous reluctance machines in high speed range
Date of Issue2017-05-05
School of Electrical and Electronic Engineering
Recently, synchronous reluctance machines (SynRMs) have gained popularity in certain variablespeed applications. In terms of torque and efficiency these machines demonstrate similar properties to induction machines. However, they have lower rotor losses than induction machines. Because of its ability to regulate flux levels, "spinning" core losses can be eliminated. This makes them preferable to permanent-magnet machines in scenarios where machines rotate without load torque. Furthermore, they are able to be applied to extremely high speeds. A flywheel energy storage system is a typical application which utilizes above-mentioned advantages. During high speed range, torque capability is of major concern. For classical hysteresis-based DTC, torque capability is reduced due to the fact that stator flux is weakened. In other words, the constant torque range is limited. In this thesis, a modified hysteresis-based direct torque control is proposed to extend the constant torque region. Consequently, torque capability is enhanced in high speed range and hence speed response time is lessened. The proposed method is based on the classical hysteresisbased DTC structure. Like the classical hysteresis-based DTC, the proposed method is implemented based on the measured currents and machine parameters. The modified hysteresis-based DTC is established by combining the classical hysteresis-based DTC with a flux error status modifying block. The block features a simple structure and an easily-executed algorithm, which renders it possible to fulfill even in a logic circuit. The fl~x error status block adjusts the flux error status produced by the flux hysteresis controller. After this, the new flux error status is fed into the switching table to produce the optimum voltage vector. It is worth mentioning that the modified flux error status is determined by flux position in the stationary reference frame a-~, which is fixed in space. Also, the moment to activate this block is a major factor to consider. Since the objective of this incorporated block is to increase torque capability in dynamic conditions, this flux error status modifying block should be deactivated under steady states. In this thesis, the torque error is analyzed continuously to decide whether the incorporated block should be triggered or not. A simulation is carried out by SIMULINK/MATLAB to confirm the suggested theory. A further comparison is made between the classical and the proposed hysteresis-based DTC methods. However, the modified DTC has relatively large torque ripples. Therefore, the torque hysteresis controller is replaced with a PI controller to address intrinsic disadvantages of hysteresis-based DTC.