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I found a quote which states that:

"Brushless DC" is a marketing term for "AC induction motor with built-in speed feedback."

Is it correct? What is "built-in speed feedback?"

I found a decent discussion, but that doesn't clarify this question.

JRE
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hkjz
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    Is brushless DC used to refer to induction motors? I thought it only referred to synchronous motors (even though induction motors are also brushless). – DKNguyen Mar 16 '20 at 20:49
  • Built in speed feedback just means there is an encoder or some other sensory device on there that outputs the motor speed (or more likely in this case, the actual rotor position). I am pretty sure they meant to say position feedback, not speed feedback. But I think there are sensorless methods too so I that may also be false. – DKNguyen Mar 16 '20 at 20:51
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    It tells you more about marketing than about motors. It's flat wrong. –  Mar 16 '20 at 20:52

2 Answers2

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It means the poster doesn't know the difference between an induction machine and a synchronous machine, nor do they know the range of DC brushless motors out there.

Having waxed cynical: the term "Brushless DC motor" is imprecise. The best-fitting explanation for what people usually mean is that it's a brushless motor that's got a magnetic design that makes it tend to have a trapezoidal back-EMF profile. This, in turn, means that the drive electronics can be simplified, in the sense that the excitation doesn't need to be sinusoidal.

Induction Machine: A motor (or generator) that works by a rotating magnetic field that induces current in the rotor, which in turn generates a magnetic field that in turn generates torque. (Alternately, you can think of the rotor as a magnetic brake trying to stop the rotating field, which generates torque, but that's kind of a wacky way to think of it).

Synchronous Machine: A motor (or generator) that works by a rotating magnetic field, and a fixed field in the rotor. This forces the rotor to turn at some exact fraction of the excitation frequency of the field (hence, synchronous).

A brushless motor is constructed to be a synchronous machine, but the rotating field is timed to match the position of the rotor to generate torque.

Contrast these with a conventional DC brushed motor where the rotor magnetic field is switched, and there's an unchanging field that's fixed to the frame. The rotor switching is called "commutating"; that term is carried out to the method used to make brushless motors ("AC" or "DC", although as I mention above the terms are vague) stay synchronized.

TimWescott
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  • It seems that there is close distance between (brushed) Universal Motor used both in "AC" and "DC"

  • It seems as well that it is worth to clear how in Synchronus Machine, kind of voltage connection to rotor is established, which makes that field from rotor and stator are Synchronized

  • "Tend to have a trapezoidal back-EMF profile", means that we can use PWM to control rotation?

  • 'Rotating field is timed to match..." means that we can control Brushless motor quite precise with computer like RaspberryPi + python, right?

  • i like the rotor == magnetic_break -> torque image

    • – hkjz Mar 16 '20 at 23:51
  • From your comment, it looks like you're asking me to please expand my answer to go into detail on every possible motor type -- that's a book-length subject (the title of the book on my shelf is "Electric Machines", and was taught in my junior year of college). – TimWescott Mar 17 '20 at 20:11