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I'm familiar with regular DC motors, which is what brushed motors appear to be. They have fixed stator magnets, rotor coils connected to the shaft and two brushes that power the coils at the right time via a commutator. By varying the amount of power via the two leads connected to the brushes I can vary the speed the motor spins at (more or less).

Yet brushless motors typically have 3 input wires. I can see an arrangement of coils and magnets still, but why does it need 3 wires and a special ESC to drive it?

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  • $\begingroup$ This question should be asked on the electronics SE, electronics.stackexchange.com $\endgroup$
    – Kenn Sebesta
    Commented Apr 15, 2020 at 18:34
  • $\begingroup$ @KennSebesta Do you think it would be more applicable here if I reworked the question into the advantages of brushless motors motors for RC? $\endgroup$
    – Daniel Ballinger
    Commented Apr 15, 2020 at 19:57
  • $\begingroup$ Yeah! If you can relate it to drones in particular, then it becomes highly relevant. As it is right now, the best people to answer your question are experts in electronics. $\endgroup$
    – Kenn Sebesta
    Commented Apr 15, 2020 at 20:26

2 Answers 2

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TL;DR:

Brushed DC motors have two wires because they require DC current (+VCC and Ground), while brushless motors require more complex driving circuitry which controls the AC current required to drive them.

Despite sometimes looking similar on the outside, the functional principles of brushed DC motors and brushless DC motors (aka. BLDC motors) are quite different.

Brushed Motors

These function by physical commutation, where the rotation of the armature breaks and establishes contact with pads which continue reversing the direction of current flow through the coil. This mechanical process perpetuates the cycle of magnetic attraction and repulsion which turns the motor.

enter image description here

Brushless Motors

BLDCs use three wires which are driven by the ESC with a phase-shifted AC waveform. Each wire's waveform is shifted by 120 degrees from the other two.

enter image description here

This is because BLDC motors are indeed three-phase AC motors, each with usually more than one actual coil per phase. Inside the motor, phases are wired up in ascending order, e.g. 1 2 3 1 2 3 ...

enter image description here

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  • $\begingroup$ what about the BLDC with 4 wires instead like those in HDD's? What does the 4th wire do? $\endgroup$
    – user0193
    Commented Jul 23, 2022 at 15:06
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    $\begingroup$ @user0193 It would seem (electroschematics.com/hdd-bldc-motor/…) that such motors have three phase wires and the center tap (of the wye configuration) on the fourth wire. This additional terminal is likely used for sensing the location of the rotor. $\endgroup$
    – ifconfig ♦
    Commented Jul 24, 2022 at 16:42
  • $\begingroup$ this makes so much sense now! Thanks $\endgroup$
    – user0193
    Commented Jul 24, 2022 at 17:53
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A standard brushed motor has a positive, negative and a commutator that switches the polarity of the rotor.

Conversely, a brushless motor has a multiple of three sets of coils. These coils are energized in a particular order to create a series of electromagnetic forces that ‘pull’ the bell (which contains permanent magnets) around.

As there are three sets of coils, there are three wires - one for each set of coils so they can be individually energized.

The ESC is what controls the sequence in which the coils are energized.This is an image I sourced from Pinterest to help explain

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    $\begingroup$ Source for the diagram: pin.it/QazfFbA $\endgroup$
    – Drones and Whatnot
    Commented Apr 14, 2020 at 21:50
  • $\begingroup$ Essentially, a brushless DC motor is a three-phase AC motor. $\endgroup$
    – gparyani
    Commented Apr 14, 2020 at 21:52
  • $\begingroup$ @gparyani yes, excellent summary! Thanks. $\endgroup$
    – Drones and Whatnot
    Commented Apr 14, 2020 at 21:54

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