Electronically Commutated Fans

Efficiency is better than 30-50%. Where did you get your information?

Operating principle is same as a brush-commutated motor with rotor and stator reversed. Some BDC motors use a synchronous method of electronic commutation, much like a stepper motor or AC synchronous, whereby the stator windings are switched on and off at some desired rate, but such a method is not particularly suited to variable loads, but fans have a load that is fairly constant.

Brushless DC servo motor that may be subjected to widely changing loads, speed variations and accelerations will typically have rotor position sensors giving feedback to the controller.

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