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BLDC MOTOR and hall sensor .some basic question

YIANNIS PANAIS
Prodigy 120 points

Hi to all

i have some question:

1) for edication porpuse can anybody  teel to us any side with picture to see the diference between BLDC and PMSM windings

2) when i have a BLDC motor with a encoder 2000 resolution is less accuracy (position control use) from a PMSM with the same encoder?

3) what is better to use for position control a BLDC or brush DC?

4) what is the difference between space vector modulation and field oriented control?

5) i have a BLDC motor with 8 pole magnet rotor and use a magnet disk (8 pole) with 3 hall sensor in 60 mechanical
degrees(as i see) configuration between them, a total arc 120 degrees.The manual say the hall is in 120 electrical deg,
i do the simple convert the mechanical to electrical deg: 360el-deg/4=90
that mean 360 ele-deg of stator is 90 mechanical deg of rotor so that mean:
with 30 mech-deg i have 120 el-deg, so the hall must to be in 30 mech-deg configuration
where i am wrong ?because the motor work fine with my controller (is accept only hall in 120 configuration)

thanks!

Best regards, Yiannis

  • 0
    Prodigy 135 points

    Hi Yianis,

    The main difference between BLDC and PMSM is the commutation scheme. In BLDC you use trapezoidal commutation, whereas in PMSM you use sine wave commutation. In theory, a PMSM is wired to optimize the sine wave commutation, but as it turns out pretty much any BLDC can be sine wave commutated. With sine wave commutation you can get better motion quality that with trapezoidal commutation as the winding energization is softer. Hence, lower torque ripple.

    As far as I know, a BLDC should be as accurate as the same PMSM, but you will get larger torque ripple with the BLDC. It is possible, however, that you get a "microstepping" component on a PMSM scheme which you can't get with trapezoidal commutation, so anybody could argue you will get better accuracy with PMSM than with BLDC. However, I bet there are too many other factors such as motor construction, rotor stiction and motion control algorithm which will all play a final role on accuracy.

    Once you add the commutation circuitry to a BLDC, it pretty much becomes a brushed DC. Again, this is quite the coarse statement as at the end you will need to take into consideration motor construction as well as other variables. However, I believe a decent BDC will give you similar performance than a BLDC when it comes to position control. Where the BLDC excels is on extended life, better efficiency ratio and higher speed capabilities.

    As far as I know FOC and space vector modulation are the same thing with different fancy names. I could be wrong, though.

    I wouldn't fixate on where the hall effect sensors are located in order to understand where the 120 degrees comes from. The 120 degrees come from the fact that you have three phases that are 120 degrees out of phase. Where you position the hall effect sensors will be defined on how the revolving magnet was encoded to extract the rotor position information. I have never been into designing motors, so do not know how to answer this question, but according to what you say, the motor will move a quarter of a revolution every time a full electrical revolution is generated. This is akin to having a "24 steps" motor (every 360 electrical degrees are made of 6 commutation steps). We can then deduce that there are 8 magnets per phase (4 magnetic pairs).