BLDC motors - cogging

Discussion in 'Physics' started by Skeebopstop, Feb 9, 2009.

  1. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Anyone know where the cogging in BLDC motors comes from?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    1,786
    It comes from applying current to a phase before the rotor reachs the point where maximum torque is applied, so the current in that phase acts like a brake until the rotor passes a certain point and then it accelerates again.

    The same thing happens in an internal combustion engine if you fire the spark too many degrees BTDC. It goes by many names, and it is really bad for an engine. Don't think the BLDC motor will be permanently damaged from "cogging"
     
  3. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    So assuming infinite resolution feedback and instantaneous current loop refresh times, there should essentially be 0 cogging?

    Somehow my gut tells me there would still be something mechanical or physical that might cause it?
     
  4. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    Hey mate,

    the reason I feel there is more to this is that someone recently told me that some motor manufacturers will slightly 'skew' their permanent magnets in PMSM, such that cogging is reduced. This leads me to believe it has more to do with the affinity of the permanent magnets to the stator fields, than actual control non-linearities.

    Mind you I do agree that control non-linearities will certainly have some impact, I only argue as to whether they are the dominant cogging source.
     
  5. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    Any more input into this anyone?

    I thought it had to do with the alignment of a rotor pole to a stator coil, giving that stator a higher flux affinity to the rotor than the other phases, when considering the rotation flux vectors being generated.

    I think this is what 'cogging' compensation in more sophisticated controls are doing, having pre-determined knowledge of the rotor to stator alignment points and adjusting your control loops accordingly.
     
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