Comparative Study on a Composite Coil Rotor of a Novel Soft-Start Brushless Doubly-Fed Machine

As a type of asynchronous motor which could be specially regarded as a synchronous motor when running, the brushless double-fed machine has several disadvantages including soft mechanical property and large impact on the power grid which not affordable for heavy load starting and similar industrial applications. Considering these problems, this paper has proposed a soft-start method which could improve the starting characteristics of the motor by adjusting the winding structures of both stator and rotor, combining the reliability of startup with running state. The paper focused on the winding structures of BDFM especially the rotor which designed by the principle of "composite coil", then analyzed the design scheme of BDFM windings, illustrated the work principle of the rotor when motor startup. In order to prove the optimizing of this method, several other methods with different turn ratios are designed for comparing. Finally, the starting torque and starting current are analyzed by time step finite element method when the new prototype starts by this method and works in different mode including no-load and load states, then the prototype experiment results are given to verify the mentioned characteristics of the prototype. The results show that the new kind of BDFM and the proposed new kind of soft-start method have the superiorities of hard mechanical characteristics, large starting torque and small impact of the starting current on the power grid.