Performance of a direct torque controlled IPM drive system in the low speed region

High power density, high speed operation, high efficiency and wide speed range made the interior permanent magnet (IPM) synchronous motors an interesting choice for ac drive systems. Different control strategies have been proposed to reach a high performance drive system. Direct torque control (DTC) is one of these widely used methods which has fast torque dynamic and a simple structure. Different motor, inverter and controller parameters affect the drive system performance in this scheme. The drive system performance is investigated for four possible inverter switching patterns in terms of the torque ripple, stator current ripple, flux ripple and inverter switching frequency in the low speed region. The results show that the switching pattern in which zero voltage is applied to reduce the torque has better performance compared to the other switching patterns. The analytic solution is provided to quantify the effects of the inverter zero voltage vector on the flux and torque of the machine and how they change when the speed varies.