What is a Doubly feed Electrical Machine or DFIG?

Electric machines are either Singly Fed with one winding set that actively participates in the energy conversion process or Doubly Fed with two active winding sets. Having two electrical ports, many confuse the singly-fed slip-energy recovery induction and the field-excited synchronous electric machines as doubly-fed; however, the port of only one winding set is actively excited while the port of the other winding set passes dissipative power for passive participation in the energy conversion process

Only practical with the evolution of control technology, there are now three varieties of doubly fed electric machine systems: 1) the Doubly Fed Induction Machine (DFIM), which is the conventional wound-rotor doubly fed electric machine with an active winding set on the rotor and stator, respectively, and flux vector controlled rotor excitation through a multiphase slip-ring assembly; 2) the Brushless Doubly-Fed Induction Machine (BDFIM), which is the brushless doubly fed induction (or reluctance) electric machine with cascaded active winding sets of unlike pole-pairs on the stator assembly of which one is flux vector controlled and a flux focusing rotor assembly; and 3) the Brushless Doubly-Fed Synchronous Machine (BDFSM), which has the traditional DFIM circuit topology with a rotor and stator active winding set but with a brushless real time control method replacing the slip ring assembly and rotor flux vector controller.

The symmetrical circuit topology and operational relationships of the wound-rotor doubly-fed electric machine core with active winding sets on the rotor and stator, respectively, become the classic study for all other electric machines by de-optimizing their symmetry with asymmetry; for instance, by replacing the symmetrical circuit topology provided by the rotor active winding set with the asymmetrical circuit topology provided by a passive permanent magnet assembly, which has no active power port and as a result, cannot actively participate in the energy conversion process. A true doubly-fed electric machine must have two active winding sets (ports) excited with bi-directional power for practical operation from sub-synchronous to super-synchronous speed without regions of discontinuity, such as about synchronous speed.

Doubly fed electrical machines are electric motors or electric generators that have windings on both stationary and rotating parts, where both windings transfer significant power between shaft and electrical system. Usually the stator winding is directly connected to the three-phase grid and the three-phase rotor winding is fed from the grid through a rotating or static frequency converter or AC to AC cycloconverter.

Doubly fed machines are typically used in applications that require varying speed of the machine's shaft in a limited range around the synchronous speed, for example ± 30%, because the power rating of the frequency converter is reduced similarly. Today doubly fed drives are the most common variable speed wind turbine concept.

The DFIM and BDFIM rely on speed-based asynchronism (or slip) between the rotor and stator windings to induce speed-synchronized current onto the rotor winding set. However at the low slip experienced about synchronous speed, the time critical measurement or excitation synthesis of shallow time-differential signals makes stability increasingly elusive. The BDFIM has eliminated the multiphase slip-ring assembly and partially improved stability by sacrificing size, cost, and efficiency. In contrast, the BDFSM without brushes propagates instantaneously derived speed-synchronized multiphase excitation to the rotor winding set without discontinuity and without relying on slip induction, although slip-induction is experience beyond synchronous speed as in all doubly-fed electric machines.