Doubly-fed electric machine
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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 but with both winding sets actively participating in the energy conversion process (i.e., doubly-fed or dual armature).
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[edit] Explanation
All electric machines are categorized as either Singly-Fed with one winding set that actively participates in the energy conversion process 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.
[edit] 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).
The sum of the power ratings of the multiphase winding sets determine the total electro-mechanical conversion power rating of the machine.
[edit] Electronic control
The electronic controller conditions bi-directional (i.e., four quadrant), speed synchronized, and multiphase electrical power to at least one of the winding sets (generally, the rotor winding set). Using four quadrant control, which must be continuously stable throughout the speed range, 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.
The electronic controller is smaller, less expensive, more efficient, and more compact than electronic controllers of singly-fed electric machine because in the simplest configuration, 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.
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 control means. 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.
Like any synchronous electric machine, 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.
[edit] Wound-rotor doubly-fed
[edit] Construction
Two multiphase winding sets with similar pole-pairs and equal power rating are placed on the rotor and stator bodies, respectively. The wound-rotor doubly-fed electric machine is the only electric machine with two independent active winding sets, the rotor and stator winding sets, occupying the same core volume as other electric machines. Since the rotor winding set actively participates in the energy conversion process with the stator winding set, utilization of the magnetic core real estate is optimized.
A multiphase slip ring assembly (i.e., sliding electrical contacts) is traditionally used to transfer power to the rotating (moving) winding set and to allow independent control of the rotor winding set. The slip ring assembly requires maintenance and compromises system reliability, cost and efficiency. Attempts to avoid the slip ring assembly are constantly being researched with limited success (see brushless wound-rotor doubly-fed electric machines).
[edit] 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 with the electronic controller controlling half (or less) of the power capacity of the electric machine for full control of the machine.
Controlling the wound-rotor doubly-fed electric machine is particularly sensitive about synchronous speed (i.e., 3600 rpm @ 60 Hz with 2 Poles) where loss of synchronism is imminent without extraodinary control means because excitation frequency and amplitude levels elude electronic measurement or excitation synthesis. Like any synchronous machine, loss of synchronism will result in alternating torque pulsation and overall failure.
[edit] Efficiency
Neglecting the slip ring assembly or potential control instability, the theoretical electrical loss of the wound-rotor doubly-fed machine is comparable to the most efficient electric machine systems available (i.e., the synchronous electric machine with permanent magnet assembly) with similar operating metrics because the total torque current is split evenly between the rotor and stator winding sets (i.e., half the current in each winding set) while the electrical loss of the winding set is proportional to the square product of the current flowing through the winding set. Further considering the electronic controller conditions less than 50% of the power of the machine, the wound-rotor doubly-fed electric motor or generator (without brushes and with stable control at any speed) theoretically shows nearly half the electrical loss (i.e., winding set loss) of other electric motor or generator systems of similar rating.
[edit] Power density
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. In all other electric machines, the rotor assembly is passive real estate that does not actively contribute to power production. The potential of higher speed for a given frequency of excitation, alone, is an indication of higher power density potential. The constant-torque speed range is up to 7200 rpm @ 60 Hz with 2 Poles compared to 3600 rpm @ 60Hz with 2 Poles for other electric machines. In theory, the core volume is nearly half the physical size(i.e., winding set loss) of other electric motor or generator systems of similar rating.
[edit] Cost
Neglecting the slip ring assembly or the extraodinary means to overcome control instability, the theoretical system cost is nearly 50% less than other machines of similar rating because the power rating of the electronic controller, which is the significant cost of any electric machine system, is 50% (or less) than other electric motor or generator systems of similar rating.
[edit] Conclusion
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 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.
[edit] 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 a multiphase wound rotor and a multiphase slipring assembly with brushes for access to the rotor (see wound-rotor doubly-fed). Only recently could the multiphase slipring assembly with brushes be avoided, which is the ideal configuration (see brushless wound-rotor doubly-fed electric machines).
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.
As described, a doubly fed induction machine is a wound-rotor doubly-fed electric machine (see wound-rotor doubly-fed electric machine) and 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 [synchronized] 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.
[edit] Brushless doubly-fed versions
[edit] Brushless doubly-fed electric machines
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. With unlike pole-pairs between the two winding sets, low frequency magnetic induction is assured over the speed range. One of the stator winding sets (power winding) is connected to the grid and the other winding set (control winding) is supplied from a frequency converter. The shaft speed is adjusted by varying the frequency of the control winding. As a doubly-fed electric machine, the rating of the frequency converter need only be fraction of the machine rating.
The brushless doubly-fed electric machine does not utilize core real-estate efficiently and the dual winding set stator assembly is physically larger than other electric machines of comparable power rating. In addition, a specially designed rotor assembly tries to focus most of the mutual 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 criterion 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.
[edit] Brushless wound-rotor doubly-fed electric machines
Brushless wound-rotor doubly-fed electric machines (i.e., electric motors or electric generators) incorporate the core structure of the wound-rotor doubly-fed electric machine, but replaces the traditional multiphase slip ring assembly with a brushless means to independently power the rotor winding set (i.e., doubly-fed) with multiphase AC power. For stable operation, the frequency and phase of the multiphase AC power must be sychronized to the speed and position of the shaft, which is not trivial about synchronous speed where induction no longer exists. All the attractive attributes of the wound-rotor doubly-fed electric machine, such as high power density, low cost, ultra-high efficiency, are realized without the traditional slip-ring assembly and instability problems. Only one company, Best Electric Machine, successfully developed a brushless, fully stable, synchronous wound-rotor doubly-fed electric machine with symmetric quality of motoring or generating.
