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I want to know if it is physically possible for a BLDC motor to be controlled in speed and position at the same time.

Let me clarify : while the rotor is spinning (speed control), is it possible to rotate the stator backward or forward (position control).

I don't know if I made myself clear enough...

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  • \$\begingroup\$ If the rotor is spinning, what makes the stator stay stationary is that it is tied to its place, right? So, stator is stationary at all times. How would you change the position of the stator without letting it free spin because of the rotor? \$\endgroup\$
    – user132236
    Commented Dec 19, 2016 at 16:05
  • \$\begingroup\$ But if the stator is free to move, is it possible ? \$\endgroup\$
    – Tagadac
    Commented Dec 19, 2016 at 16:12
  • \$\begingroup\$ BLDC are siblings to steppers. Both are position-able at any point. But positioning between steps accurately likely requires additional position feedback - these motors are not well controlled open-loop. \$\endgroup\$
    – glen_geek
    Commented Dec 19, 2016 at 16:27
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    \$\begingroup\$ @Tagadac Yes, it is possible. When the rotor is turning, and there is enough friction to keep the stator stationary, your system is in equilibrium. What you must do is to accelerate or decelerate the rotor momentarily to let stator defeat the static friction for a moment and change its position. After that, you need to make sure that the stator is stable again, by static friction. Charles Cowie, explains it well below. \$\endgroup\$
    – user132236
    Commented Dec 19, 2016 at 18:59
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    \$\begingroup\$ "But if the stator is free to move, is it possible?" - Generally the stator is not free to move, because its wires are connected to a controller and power supply etc. That's why it's called a stator. But sure, if the stator is not fixed in place then any net torque between rotor and stator (even due to the rotor's own inertia) will cause the stator to twist in the opposite direction to the rotor. However accurately controlling both rotor speed and stator position at the same time could be tricky. \$\endgroup\$ Commented Dec 20, 2016 at 17:48

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It is possible to control the position of a spinning BLDC or any other type of motor while it is turning. The motor shaft position could only be controlled relative to another spinning shaft. In order to make position corrections, it would be necessary to momentarily increase or decrease the motor speed by a small amount. The average speed would need to match the average speed of the reference shaft.

With a BLDC motor, the rotor could maintain synchronism with the stator field rotation and maintain a fixed angular relationship with the stator field. The stator could be turned forward or backward to accomplish position corrections.

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