Wound-rotor doubly-fed electric machine
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Wound-Rotor Doubly-Fed Electric Machines (i.e., electric motors or electric generators) are a type of Doubly-Fed Electric Machines.
[edit] 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 are constantly being researched (example: Brushless Doubly-Fed Electric Machines and 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.
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.
[edit] 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. Also see Brushless Doubly-Fed Electric Machine, Brushless Wound-Rotor Doubly-Fed Electric Machine, and Doubly-Fed Electric Machines. Fklatt 23:45, 14 November 2006 (UTC)
