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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).
Explanation
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.
Features of doubly fed machines
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.
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.
Brushless versions
Brushless doubly-fed electric machines (i.e., electric motors or electric generators) are machines, which are constructed by adjacently placing two multiphase winding sets with unlike pole-pairs on the stator body. One of the stator winding (power winding) is connected to the grid and the other one (control winding) is supplied from a frequency converter. The shaft speed is adjusted by varying the frequency of the control winding. The rating of the frequency converter need only be fraction of the machine rating.
This does not utilize core real-estate efficiently and makes the stator assembly physically larger than other electric machines of comparable power rating. In addition, a specially designed rotor assembly tries to focus most of the magnetic field to follow an indirect path across the air-gap and through the rotor assembly for inductive coupling (i.e., brushless) between the two adjacent winding sets. As a result, the adjacent winding sets are excited independently and actively participate in the electro-mechanical energy conversion process, which is a criteria of doubly-fed electric machines.
The type of rotor assembly determines if the machine is a reluctance or induction doubly-fed electric machine. The constant torque speed range is always less than 1800 rpm @ 60 Hz because the effective pole count is the average of the unlike pole-pairs of the two active winding sets. Brushless doubly-fed electric machines incorporate a poor electromagnetic design that compromises physical size, cost, and electrical efficiency, to chiefly avoid a multiphase slip ring assembly. Although brushless doubly-fed electric machines have not seen commercial success since their conception in the early 1970's, the promise of a low cost, highly efficient electronic controller keeps the concept under perpetual study, research, and development. Also see Wound-Rotor Doubly-Fed Electric Machines, Brushless Wound-Rotor Doubly-Fed 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.
A doubly fed induction machine has several advantages over a conventional induction machine in wind power applications. Firstly, as the rotor voltage is controlled by a power electronics converter, the induction generator is able both import and export reactive power. This has important consequences for power system stability and allows the machine to remain connected to the system during severe voltage disturbances. Secondly, the control of the rotor voltage enables the induction machine to remain with the grid while the wind turbine varies in speed. A variable speed wind turbine utilises the available wind resource more efficiently than a fixed speed wind turbine, especially during light wind conditions.