The doubly-fed induction generator as part of the electrical machines curriculum

The doubly-fed induction machine has long been reputed as a curiosity with few practical applications. Consequently, very often there was not enough space to treat this machine in undergraduate electrical machines curricula in the past. But times changed and today this machine is often an integral part of wind turbines as electric generator. This way, getting familiar with the relevant operating principles becomes a desirable task for undergraduate students. This paper proposes a circuit-based theoretical approach which may serve as a template for an undergraduate lecture. To this end, firstly the basic characteristics of the wind turbine are introduced. Afterward, the operating principles of the doubly-fed induction machine are explained with reference to the single-phase equivalent circuit. A method based on the superposition principle is proposed for circuit solving, exploiting the concept of 'virtual' synchronous speed. An example given in the paper further illustrates the principle of operation and shed light on the power flows.