A Converter-Based Starting Method and Speed Control of Doubly Fed Induction Machine With Centrifugal Loads

The adjustable-speed drive is attractive for pump, compressor, and other centrifugal load applications due to its flexibility and high operating efficiency, compared with the conventional constant-speed drive system. This paper has developed a converter-based starting method for a doubly fed induction machine (DFIM) with centrifugal loads by constant volts/hertz control, instead of using the additional starting resistors or autotransformer. A grid synchronization scheme is then developed to complete the transition from motor starting to vector-controlled speed regulation in the high-speed region. Therefore, the machine can successfully operate in the full speed range by combining constant volts/hertz control and vector control. Since the centrifugal load torque reduces quickly with speed (square relationship), the partially rated converter may provide enough torque at low speeds. Two starting alternatives can be used according to different turns ratios between the machine stator and rotor windings. The proposed control scheme can be simply implemented with a one-quadrant partially rated power converter. The voltage and current ratings of the converter are carefully designed considering the centrifugal load characteristic and the operating speed range. Some important application issues such as “soft grid connection” and rotor position estimation are also discussed. The proposed control scheme has been validated by simulation and experimentally verified through a DSP-controlled 30-kW DFIM system by starting from the rotor side. The drive system can successfully run in the full speed range with the proposed control method and control sequences.