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Revision as of 14:58, 10 September 2007 by Fklatt (talk | contribs) (→Brushless versions)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)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 but 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 (generally, the rotor winding set). 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). With four quadrant control, a wound-rotor doubly-fed electric machine with two Poles (i.e., one pole-pair) has a constant torque speed range of 7200 rpm when operating at 60 Hz.
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 Machine, 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.
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.
The electronic controller is smaller, less expensive, more efficient, and more compact than electronic controllers of singly-fed electric machine because only the power of the rotating (or moving) active winding set is controlled, which is less than half the total power output of the electric machine.
Wound rotor
Construction
Two multiphase winding sets with similar pole-pairs and equal power rating are placed on the rotor and stator bodies. This configuration optimizes utilization of the magnetic core real estate.
A multiphase slip ring assembly (i.e., sliding electrical contacts) is traditionally used to transfer power to the rotating (moving) winding set. However, attempts to avoid the slip ring assembly have been developed (see Brushless Wound-Rotor Doubly-Fed Electric Machine) or are constantly being researched.
Control
Although the multiphase slip ring assembly compromises core real estate, reliability, cost, and efficiency, it allows independent electronic control of the rotor (moving) winding set so both multiphase winding sets actively participate in the energy conversion process. In effect, the electronic controller controls half the power capacity of the electric machine for full control of the machine, which equates to a low cost and highly efficient electronic controller.
Controlling the wound-rotor doubly-fed electric machine is particularly sensitive about synchronous speed (i.e., 3600 rpm @ 60 Hz with 2 Poles) where excitation frequency and voltage eludes electronic measurement or excitation synthesis.
Like any synchronous machine, losing synchronization will result in alternating torque pulsation.
Efficiency
Neglecting the slip ring assembly, the theoretical electrical loss of the wound-rotor doubly-fed machine is comparable to the most efficient electric machines available with similar operating metrics. This is because the total torque current is split evenly between the rotor and stator winding sets, whereas loss due to electrical heating of a conductor is proportional to the square of the current flowing through that conducter.
Neglecting the slip ring assembly and considering similar air-gap flux density, the physical size of the magnetic core of the wound-rotor doubly-fed electric machine is smaller than other electric machines because the two active winding sets are individually placed on the rotor and stator bodies, respectively, with virtually no real-estate penalty and the physical size of each of the two half power rated active winding sets is proportionally smaller than a full power rated active winding set of a singly-fed machine.
The constant-torque speed range is up to 7200 rpm @ 60 Hz with 2 Poles.
Overall, the wound-rotor doubly-fed electric machine incorporates the most optimum electromagnetic design of any electric machine but requires a slip ring assembly and very responsive electronic control, which together is its Achilles' heel; otherwise, the wound-rotor doubly-fed electric machine (including electronic control) would surpass all electric machine systems, if efficiency, cost, and size of the system were the combined issue. The wound-rotor doubly-fed electric machine has found some commercial success in very large applications with a limited speed range, such as Windmills, where efficiency and low cost power electronics outweigh the cost and reliability issues associated with the slip ring assembly and the control complexity. Still, the Achilles' heel has not been satisfactorily remedied by traditional research and development, which has kept the wound-rotor doubly-fed electric machine from achieving "truly" doubly-fed status.
Brushless versions
Brushless doubly-fed electric machines (i.e., electric motors or electric generators) 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 1970s, the promise of a low cost, highly efficient electronic controller keeps the concept under perpetual study, research, and development.
Brushless wound-rotor doubly-fed electric machines (i.e., electric motors or electric generators) incorporate the most optimum electromagnetic core structure of any electric machine, which is the core structure of the wound-rotor doubly-fed electric machine, but without the traditional Achilles' Heel of the wound-rotor doubly-fed electric machine, which are the multiphase slip ring assembly and control instability. Development of the brushless wound-rotor doubly-fed electric machine is limited. One such company is Best Electric Machines (http://www.bestelectricmachine.com). See also Wound-Rotor Doubly-Fed Electric Machine, Brushless Doubly-Fed Electric Machine, and 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.