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I am little bit curious about, is hall sensor also used in PMSM motor drive? Since in PMSM motor the voltage applied is sinusoidal, all 3 phase are active, so a rotating flux is introduced, which is not in BLDC motor. In BLDC motor only two phase are active at any instant. So it is necessary in switching in BLDC motor. But due to rotating flux in PMSM there is no any requirement of position sensor. Is my view correct or not? Please help me.

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  • $\begingroup$ I have personally used both a PMSM motor with embedded Hall-Effect Sensors, and a BLDC motor without any sensing. The requirement doesn't come from the drive technology, but rather the application - is it variable load, for example? $\endgroup$ May 5 '18 at 16:46
  • $\begingroup$ You really should spell out acronyms, at least the first time you use them. I would have +1 to this question if you had. $\endgroup$ May 6 '18 at 12:55
  • $\begingroup$ I am asking that as like BLDC hall sensor is used for mosfet/ switch selection the same case with PMSM? $\endgroup$
    – aman2909
    May 6 '18 at 18:17
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Short answer: neither BLDC nor PMSM requires a hall sensor, and both may need need it for some applications.

BLDC (brush-Less Direct Current) motors and PMSM (Permanent Magnet Synchronous Motor) are basically the same type of motor: three-phase synchronous motors, with permanent magnets to generate the rotor's magnetic field (it would work with other numbers than 3 phases, but it's analog to AC motors and allow 6-steps control). PMSM and BLDC acronyms are used to indicate for which control type the motor has been optimized. On PMSM, we try to get no magnetic saturation to get a sinusoidal back-EMF (electromotive force) and therefore an optimal control with sinusoidal current, while the magnetic saturation tends to compensate torque ripple on BLDC. The principle of both motor is still the one of a three-phase synchronous motor.

So, what is the principle of a three-phase synchronous motor ? The stator produces a fixed amplitude (at a given speed and load) rotating magnetic field, and the rotor produces a fixed magnetic field that will align on the rotating field from the stator. To get the best efficiency, the stator's field should stay at 90° from the rotor's field. To adjust the stator field intensity, it is therefore required to know where the rotor is. Using a sensor (such as hall effect sensor) is always an option, but for cost reduction, and to mitigate the risk of sensor malfunction, sensor-less solutions exist:

  • on BLDC, with a 6-step control, there always is a non-driven phase, on which reading the voltage and detecting zero-crossings is enough to know when to commute steps.
  • on PMSM, with either sinusoidal or vector control, the three phases are driven, and it's a bit more tricky to rebuild the position of the rotor from the current measurement in phases

Those sensor-less solutions use the fact that the motor produces a back-EMF which depends on the position of the rotor. The issue is that the motor produces back-EMF proportionally to its speed. Those methods don't work for position control. Also, the position computation from the phase currents is computationally more demanding than Zero-Crossing detection, and may be impossible to run at high speed on limited hardwares. Last but not least, sensor-less solutions start in open-loop, with no-feedback from the motor, to make it reach the require speed to apply sensor-less control. This requires to know roughly the load applied on the motor when it starts.

Long story short, the choice to use a sensor depends more on your application than on your control method.

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Yes, Hall sensors can be used as position feedback with PMSMs (permanent magnet synchronous motors).

In many applications you can't just blindly apply a frequency ramp and know that the motor will follow. Most of the time you need some position feedback to know where the rotor is within the magnetic cycle. This then dictates the phase of the drive signals. Hall sensors are one means to provide this position feedback.

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