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My question has two parts on the same topic.

1) With BLDC motor control, not focused on position control, it seems like commercial controllers are split on control methodologies. The first group control the motor with two legs enabled at any one instant in time and held as close to a 90 degree phase difference as possible, the third leg is disconnected. The second group uses all three legs simultaneously, each leg subject to a sinusoidal input with a phase difference of 120 degrees between each leg. Is there a benefit to one method over the other? It would seem that the second group, by using all three legs, would offer higher possible torque output?

2) With regard to actual control, how does one reconcile the unknowns when designing a control system for BLDC motors?

For example, in my system I have a BLDC motor which is magnetically coupled to an encoder, the controller has current sense capability. My goal is position control. Required torque is an unknown at any given instant in time. Thus my control inputs are target position & current position, control outputs are phase angles (x3) and phase amplitude (which directly corresponds to output current / torque), required torque is an unknown.

So say I want to apply a relatively simple PI to bring the BLDC motor from point A to point B, neglecting acceleration curves and all that fun stuff. What would it look like since this system looks like a single input, multiple output?

I'm having trouble wrapping my head around this, but I know it has been done before, which makes it all the more irritating. Any advice or redirects to useful papers would be extremely appreciated!

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    $\begingroup$ The ‘legs’ as you call them are actually called phases U, V, W typically. $\endgroup$ – morbo Jan 24 at 21:45

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