Full-Torque Starting and Low-Speed Operation of an LCI-Fed Active-Reactive Induction Motor Drive

Load commutated inverter (LCI)-based drives are quite popular in high-power, medium-voltage (MV) applications. However, the limited torque capability during starting and low-speed operation is a major drawback of these drives. This is due to the small back electromotive force (EMF) in the drive motor at low speeds, which is usually insufficient to commutate the silicon-controlled rectifiers in the LCI. A new technique to overcome this drawback for an LCI-fed active-reactive induction motor (ARIM) drive is proposed in this article. The ARIM is a dual-stator induction machine reported in literature, with one winding rated for MV and the other rated for low voltage (LV). The MV winding is fed with active power from the LCI, while the LV winding is supplied with reactive power from a lower rated LV voltage source inverter (VSI). In the proposed method, the VSI is used to apply a preselected voltage vector for a short time during each turn-
off
instant, such that a commutation voltage of sufficient magnitude is induced in the MV winding, providing safe commutation irrespective of the magnitude of the back EMF. Thus, the drawback of the low torque capability at starting and low speeds is overcome in the proposed method, which is also experimentally validated using a 75-kW, 3.3-kV prototype.