An Improved Dual DTC of Double-Inverter-fed WRIM Drive with Reduced Torque Ripple by Emulating Equivalent 3L NPC VSC

Wound Rotor Induction Machine (WRIM) connected to stator and rotor Voltage Source Converters (VSCs), known as the Double-Inverter-fed WRIM drive, can be viewed as two virtual Squirrel Cage Induction Machine (SQIM) drives, when seen from air-gap. In this article, this feature of WRIM is explored to develop an improved Dual Direct Torque Control (DDTC) technique such that, two individual and independent conventional DTC controllers can be directly employed to control the stator and rotor VSCs. The improved technique is inculcated with all the benefits of conventional DTC controller, especially the benefit of zero dependency on rotor position. Furthermore, the increased torque ripple with the conventional DTC controllers is mitigated by modifying the Switching Tables (STs) such that, the Voltage Vectors (VVs) emulated in the air-gap are equivalent to a 3-Level Neutral Point Clamped (3L-NPC) VSC. The STs are developed by analyzing the WRIM as an Open-End Winding Induction Machine (OEWIM). To demonstrate the performance of the proposed DDTC technique, the experimental results obtained from 3 kW WRIM are presented, under various transients and steady state conditions. The reduction in the torque ripple is demonstrated by estimated torque ripple waveforms, at different speeds.