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QA:More questions about BLDC motor control

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Q) Which method is more power efficient? Six step or sinusoidal?
A) Sinusoidal control will be more power efficient as it will directly control the current flowing through the motor.


Q) Does sinusoidal control necessitate FOC (field-oriented control)?
A) There are methods of using sinusoidal control combined with six-step, but FOC has many advantages.


Q) When the rotor has multiple poles, how do you define the direction of the rotor magnetic field?  Doesn't it have multiple magnetic field directions?
A) In the examples we were only talking about a single pole-pair motor. In motors with multiple pairs, there will be additional instances of the 3 motor phases as well.


Q) Is FOC a form of sensorless control?  Or are they two different things?
A) FOC could be sensored or sensorless. FOC is the algorithm used to control the motor; it could use external sensors or not.


Q) How do you judge which type of control (six-step or field-oriented) to implement for BLDC motors?
A) It really depends on your application requirements. FOC is more complex but has many advantages for dynamic performance, while Trap is simpler and works well for basic applications.
 
Q) Does sinusoidal control assumes sinusoidal back-EMF?
A) It does, but the vast majority of BLDC motors have sinusoidal back EMF.


Q) What makes the Back EMF Trapezoidal or Sinusoidal?
A) The shape of the magnets and the shape of the stator windings.  Their interaction will dictate the shape of the back emf.


Q) Is there a type of motor that would be more efficient with a six-step control than a sinusoidal control?
A)  A motor with extremely trapezoidal back-EMF could be more efficient with six-step, but that is a rare case.  Most motors will be more efficient with sinusoidal control.


Q) Can you guarantee start rotation with FOC sinusoidal?
A) When using a sensor you can guarantee startup.  When using sensorless you can't guarantee startup but there are many ways to help ensure the motor starts.  One of the ways is to use additional current or a more advanced algorithm such as HFI.


Q) What is the difference between a BLDC motor and PMSM?
A) The biggest difference between a BLDC and a PMSM is that BLDC motors have concentrated windings that tend to produce more trapezoidal back-EMF but they both can be controlled via the same methods.

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