Cage rotor structure and its effect on magnetic field modulation of brushless doubly-fed machine

As a new type of induction machine, brushless doubly-fed machine (BDFM) has excellent speed regulation performance and can run in the condition of variable-speed-constant-frequency power generation. It has good applicable potentials on energy-saving-speed-adjusting motors and wind power generation system, but its structure and operating principle are more complex. To study the effect of different cage rotor structures on the magnetic field modulation of BDFM, the basic electromagnetic theory of AC machine is used to analyze the MMFs and air gap magnetic flux densities of the stator power winding and control winding. And then, the induced EMF, current and MMF of the cage rotor bars are deduced respectively in this paper. Furthermore, the characteristics of the cage rotor MMF and its effect on the magnetic field modulation are discussed by analyzing a prototype BDFM. At last, the theoretical results are verified by the finite element numerical computation. The results show that the rotor short-circuit loops near the common cage bars play the main role on the effect of the magnetic field modulation. Suitable distribution of the rotor short-circuit loops may reinforce the effective harmonics, as well as weaken the unuseful harmonics. Cage rotor structure of BDFM may be optimized according to the results.