Brushless doubly-fed electric machine

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Brushless doubly-fed electric machines (i.e., electric motors or electric generators) are Doubly-Fed Electric Machines, which are constructed by adjacently placing two multiphase winding sets with unlike pole-pairs and equal power rating on the stator body. 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, and Doubly-Fed Electric Machines.