How Do Rotor and Stator Pole Numbers Affect BLDC Motor Efficiency?

[jainabhs]

hi,
There is one thing that I do not understand and nor do I find any direct explanations for it.
Is there any relationship between no of rotor poles and no of stator poles in BLDC motor (or PM Synchronous Motor)? I have read somewhere there doesn't have to be. I can imagine if no of poles are same then each rotor pole would interact with stator pole. But it is difficult for me to imagine if the numbers are unequal.
Can somebody explain or point to any resource...

Thanks in advance.
aj

 

[meBigGuy]

Perhaps this will help
http://www.edn.com/design/sensors/4...Part-I--Construction-and-Operating-Principles

 

[jainabhs]

Thanks meBigGuy for pointing to this page.
Although I have been to this page before, some how when I read it this time, I got slightly better idea what is going on. So it looks like if you have more rotor poles, you have to change the stator winding firing more rapidly than for less no of poles (feedback from hall/bEmf) so that rotor and stator magnetic field vectors have certain angle (I think is 90 degree) maintained between them, which produces the required torque.
But half part of my question is still open, let me rephrase, if we have six stator poles (2 poles per phase for 3phase motor) then we will have one stator magnetic field vector. What is we have more stator poles like 12, 18 will we have more than one mag field vectors to align with rotor's field.

Let me know what you think or have some other resource on this.

 

[meBigGuy]

I'm not very strong in this area.

My assumption would be that you can have any relationship as long as there is proper alignment between the stator and rotor when the stator(s) is/are fired. (I'm thinking like a many legged Y or multiple Y's). I have not researched what is available.