I am controlling a 3-phase motor with 3 120° phase-shifted currents, which is done blindly, i.e without any sensor feedback, at the moment.

I have a Magnetic rotary position sensor which i communicate via SPI and want to integrate it to my control scheme. I already checked the related decomposition techniques, namely Clarke transformation, to find the corresponding values for the optimal driving (rotor angle + 90 °).

My question is: The sensor has 14 bit resolution and i tried and validated that it works properly. The confusing part is, 14 bit resolution apparently stands for 1 mechanical revolution (360° mechanical degrees) and since i have a 7 pole-pair motor, the resolution for 1 electrical cycle drops to (2^14 / 7) and as far as i have understood it is the electrical degrees that i should apply the inverse-clark transform on.

This resolution is still pretty acceptable but i am not really sure if my way of thinking is correct. It didnt make sense to me that the sensor resolution depends on the number of poles. Any clarification is much appreciated.

Link to the mentioned sensor: http://ams.com/eng/Products/Magnetic-Position-Sensors/Angle-Position-On-Axis/AS5147

  • 2
    \$\begingroup\$ makes sense to me. Energise one motor phase with a small amount of DC. Turn the rotor by hand and it should 'cog' 7 times per mechanical revolution. The sensor doesn't 'know' how many poles you have, it only works in mechanical degrees. If you want to interpret that as 'pole pair' degrees (a better term I think than 'electrical degrees') then you will have to scale by the number of pole pairs. \$\endgroup\$
    – Neil_UK
    Aug 31, 2017 at 11:50

1 Answer 1


You are correct. Here is a way to think of it: You have 14 magnets with alternating polarities. If you start with a magnet with the north pole starting up, and rotate, after one-seventh of a turn, another magnet will be in the same position with the north pole pointing up. All of the other north and south poles will also be in the same position as before. The motor phase current at this position must be the same as it was when the previous north pole had passed. Therefore, the motor most go through its complete phasing seven times per revolution.


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