Direct Torque Control Techniques of Three-Level H-Bridge Inverter Fed Induction Motor for Torque Ripple Reduction at Low Speed Operations

In this article, two constant switching frequency direct torque control (CSF-DTC) techniques are proposed for H-bridge three-level inverter (HB-TLI) fed induction motor (IM) for reducing torque ripple at low-speed operation. Both the methods use modulators and characteristics of the pulsewidth modulation (PWM) mainly determine the motor drive performance. The first method uses two level-shifted carriers signals, whereas the second method uses carrier signals with appropriate offset voltages. Detailed analytical expressions describing complete harmonic characteristics of HB-TLI are also presented using double Fourier integral approach. Superior harmonic performance is obtained with the proposed PWM methods compared to the existing ones. Drawback of one of the envisaged CSF-DTC techniques entering into overmodulation is rectified at higher motor speeds and a smooth slide-over using zero offset voltage to carrier signals is recommended to overcome the issue. The proposed CSF-DTC techniques are simulated and experimentally validated. The dynamic and steady-state torque performance obtained with the proposed CSF-DTC techniques for IM are compared with the existing DTC methods, which show a significant reduction in the steady-state torque and current ripple, whereas the torque dynamic performance is almost preserved thus, demonstrating its usefulness especially at lower motor speeds.