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Doubly-fed electric machines (i.e., electric motors or electric generators) belong to a category of electric machines that incorporate two multiphase winding sets of similar power rating that have independent means of excitation. As a result, doubly-fed electric machines are synchronous electric machines by nature with both winding sets actively participating in the energy conversion process (i.e., doubly-fed).
All electric machines are categorized as either Singly-Fed or Doubly-Fed. Although sometimes described as doubly-fed, the wound-rotor induction machine (slip-energy recovery) and the field-excited synchronous machine are singly-fed machines because only one winding set actively participates in the energy conversion process.
Operation
An electronic controller conditions bi-directional, speed synchronized, and multiphase electrical power to at least one of the winding sets. The sum of the power ratings of the multiphase winding sets determine the total electro-mechanical conversion power rating of the machine.
Features of doubly fed machines
Uniquely, doubly-fed electric machines can operate at constant torque to twice synchronous speed for a given frequency of excitation with each active winding set rated at half the total power of the machine (i.e., contiguous operation between sub-synchronous through super-synchronous speed range).
Electronic control
Doubly-Fed Electric Machines are very sensitive to the synchronous relationship between speed and excitation frequency and as a result, are susceptible to instability without introducing extraordinary means of control. Like any synchronous machine, losing synchronism will result in alternating torque pulsation and other related consequences. The Wound-Rotor Doubly-Fed Electric Machines, the Brushless Wound-Rotor Doubly-Fed Electric Machine, and the so-called Brushless Doubly-Fed Electric Machines are the only examples of doubly-fed electric machines.
Like all synchronous electric machines or any state-of-art efficient electric machines, including the brushless DC or Permanent Magnet electric machines, doubly-fed electric machines require electronic control for practical operation and should be considered an electric machine system or more appropriately, an adjustable speed drive.
Theoretically, the electronic controller is comparably less expensive, more efficient, and more compact than electronic controllers of other electric machine because only the power of the rotating (or moving) active winding set is controlled, which is half the total power output (or less) of the electric machine.
Double fed induction generator
DFIG is an abbreviation for Double Fed Induction Generator, a generating principle widely used in wind turbines. It is based on an induction generator with wound rotor and brushes for access to the rotor.
The principle is that it connects to the grid with a back-to-back voltage source converter which controls the excitation system. This is in order to decouple the mechanical and electrical rotor frequency. By controlling the frequency delivered to the rotor it is possible to keep the frequency out of the generator on a stable level independently of the generators turning speed.