Brushless Wound-rotor Doubly Fed Electric Machine
The brushless wound-rotor doubly fed electric machine incorporates the electromagnetic 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 with multiphase AC power. Since it does not rely on slip for operation, it differs from brushless doubly fed induction electric machines, which rely on very different principles of unlike pole-pair induction for operation.
Without an independent means of exciting the rotor winding set, the torque of the wound-rotor doubly fed electric machine is dependent on both slip and position, which is a classic condition for instability, and cannot produce torque at synchronous speed. For stable operation, the frequency and phase of the multiphase AC power must be synchronized and fixed instantaneously to the stator excitation frequency and the speed and position of the shaft, which is not trivial at any speed and particularly difficult about synchronous speed where induction no longer exists. If these conditions are met without relying on induction, all the attractive attributes of the synchronous wound-rotor doubly fed electric machine, such as high power density, low cost, ultra-high efficiency, and ultra-high torque potential are realized without the traditional slip-ring assembly and instability problems. One company, has patented and is selling a wound-rotor doubly fed electric machine with a brushless means of independently exciting the wound rotor without relying on slip (induction) with the stator excitation and as a result, is a truly synchronous brushless wound-rotor doubly fed electric machine with symmetrical quality of fully stable motoring or generating, even at synchronous speed where induction no longer exists. Another brushless wound-rotor construction invented by Lars Gertmar has been described in the patent application.
Read more about this topic: Doubly Fed Electric Machine, Brushless Doubly Fed Versions
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