Analytical and Offline Approach to Select Optimal Hysteresis Bands of DTC for PMSM

The switching frequency of an inverter under direct torque control is mainly decided by the flux- and torque-controller output switching counts. It is known that, with fixed hysteresis bands, the switching frequency varies as the operating point changes. It is possible, however, to keep the switching frequency constant by the adaptation of hysteresis bands. The switching frequency is a function of both torque and flux bands. Different combinations of flux and torque bands can be used to achieve a desired switching frequency. However, for optimal control, one among the many available combinations has to be chosen. In this contribution, it is proposed to select the hysteresis bands based on the criterion of the minimum harmonic distortion in the motor currents for the required switching frequency. An analytical procedure is presented to calculate these optimal bands for the permanent-magnet synchronous motor. The analytical results were cross checked with simulation studies and validated by detailed experimentation. In order not to degrade the control performance by computational delay, a field-programmable-gate-array-based control platform is being used for implementation.