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PMSM - number of pole pairs

Kurt Kloesel
Expert 1010 points
Other Parts Discussed in Thread: DRV8312, MOTORWARE, DRV8301

I am working through the InstaSPIN labs.   At some point I switched from using the supplied Anaheim NEMA17 in the DRV8312 kit to some lower inductance outrunner RC PMSM motor.  I am able to count the number of stator windings/teeth and this number is 12.  A literature search indicates that the number of magnets for this type of motor should be 14.  After much searching and reading, I have concluded that  the user.h file should have 7 pole pairs based the number of magnets and not the number of stator poles/teeth.

 #define USER_MOTOR_NUM_POLE_PAIRS       (7)

Therefore is the number of pole pairs in a PMSM based on the windings/teeth or the number of magnets?

Thanks,

Kurt 

  • 0
    TI__Guru** 118350 points
    this is really common with Hobby Motors. Many of them will have say 12 stator coils and 14 rotor magnets. I always use the rotor magnets as the number of poles, so /2 for pairs.
  • 0 in reply to ChrisClearman
    TI__Guru** 118350 points
    cool, found this on wiki

    en.wikipedia.org/.../Outrunner

    The number of permanent magnets in the rotor does not match the number of stator poles, however. This is to reduce Cogging torque and create a sinusoidal back emf. The number of magnet poles divided by 2 gives the ratio of magnetic field frequency to motor rotation frequency.
  • 0 in reply to ChrisClearman
    Expert 1010 points

    Yes, I found that one. I used this and an amalgamation of information to arrive at the 14 magnet pole, 7 pole pair decision. The really informative one is this one:

    Go to
    <corrupted link removed>

    Click on “Articles from Southeasters” then “Electric Motors – Part 3”

     This is a discussion of the ratio of #magnets to #windings affects the torque/RPM response. I would label it as "power transmission tuning motor to load".

    Now, I believe that the InstaSPIN user.h and motorwareGUI need the "number of poles" because of Equations 20 and 21 found in the document "C:\ti\motorware\motorware_1_01_00_13\docs\labsinstaspin_labs.pdf"

    Thus, the GUI and supporting documentation could possibly clarify the #polepair difference between permanent magnet motors and induction motors.

    Anyways, thanks for the quick response.

    Kurt Kloesel

  • 0 in reply to Kurt Kloesel
    Expert 1010 points
    I want to clarify to anyone who reads this particular thread. When I switched to a lower inductance motor, I also switched to the DRV8301 kit. If I used the DRV8312 kit for any old motor I found on the streets, I would run the risk of burning the driver chip during experimentation. Notice the DRV8312 kit does not possess individually packaged MOSFET/IGBT's.