A doubly fed induction machine based solution to medium frequency power supply

The 400Hz medium frequency power supply is a key component feeding the critical loads on ships and airplanes. In order to solve some problems met in the existed system, this paper developed a complete mathematical model and operation strategy for the doubly fed induction machine (DFIM) based motor-generator frequency conversion system, including the process of starting, grid synchronization and vector-controlled speed regulation of the system. A unique feature of the proposed system is that the two synchronous speeds of the DFIM and the generator are different, therefore, only a standard one quadrant AC-DC-AC converter with about 1/6 power rating of the DFIM is needed, which benefits from low cost and simple structure. Furthermore, the rotor inverter is also used as the starter to limit the starting current in the DFIM. A new grid synchronization scheme is also proposed to achieve the soft connection by actively adjusting the DFIM stator voltage angle and amplitude. Experimental results with a scaled 7.5kW system are presented to show the validity of the proposed method.