[JasonCampbell]

My understanding is that a 3 phase sinusoidal AC servo makes one turn(rotation) for each sine cycle. Now can there be a sinusoidal AC servo that makes half a turn for each sine cycle? (maybe a 6 phase sinusoidal servo..?)

I am asking this since I spent all day reading about anything brushless and came up with some BLDCs that have 6 or 9 or 12 and even 24 stators. And the rotation speed is half for 6-stator BLDC, 1/3 for 9-stator, 1/4 for 12 and 1/8 for 24... Now BLDCs and AC servos can take either wave, AC or DC... Then does this mean such as 12 phase AC servo is possible..?

12 stator means 4 sets of 3 stators... and this means each set of stators has to be fired in timed sequence to complete one turn (a parallel to this could be a 4-cylinder engine)... Then what controls that sequence? In other words what gives an order such as "Now is the time for the third set to fire"... It cannot be the drive(controller) since there are only 3 wires going into the motor... Only possible answer is that the motor itself has a circuitry that does this sequencing..?

What I am really curious about is --> What happens when sinusoidal wave is applied to 12-stator BLDC?

Maybe I am a prisner of a fundamental misunderstanding of something, which I can't seem to pinpoint out... Thanks again for your time.

[Al_The_Man]

My understanding is that a 3 phase sinusoidal AC servo makes one turn(rotation) for each sine cycle. .

When all three phases complete a cycle, that is 360 Electrical degree's, depending on how many pairs of poles the motor has decides how many 360deg Electrical cycles complete one mechanical 360deg rotation.
IOW if the motor only had two pairs of poles, there would be 2 Electrical cycle/mechanical cycle.
The Technical info on the Renco Encoder site, show the cycle arrangement for a BLDC commutated motor/poles etc.
Al.

[JasonCampbell]

Thank you. I just read some of the articles... but I just can't point out what I am not getting though I do feel that I am not getting something right... I feel that I will get it once I could answer this question --> "What happens when sinusoidal wave is applied to 12-stator BLDC?"

...
Can't sleep

[Al_The_Man]

What do you mean by stators? Do you mean poles? The main difference between a BLDC and AC servo apart from the fact that in a BLDC, only two of the three conductors are energized at one time Hence the DC in BLDC.
The AC servo has true 3 phase sinusoidal conduction.
The other difference is in the comutation, in both motors there has to be an armature positional device. In the BLDC,this is done with what are known as Hall effect sensors (3) or the equivalent on the encoder disc.
The AC motor type often use a resolver, this outputs a 2 phase sinusoidal voltage and the third phase is derived from these two in the drive.
There are also a few other methods used.

[JasonCampbell]

I have this weird habit to ponder upon something seemingly irrelevant but many a time it gives me a new perspective of things. I think the answer to my question "What happens when 3-phase sinusoidal wave is applied to 12-stator(12 phase) BLDC?" is --> It will behave the same as 3-stator(3 phase) ac servo except each completion of 3 phase cycle will result in 390 dgree rotation, not 360 degree.

And as to sinusoidal AC servo, would it be safe to say anything other than 3-stator(3 phase) design is meritless?

What is your verdict?
Thank you again Al.

[Al_The_Man]

Can you post detail (pic) as to what you mean by 12 stator BLDC, AFAIK most 3-lead BLDC's are constructed the same as the AC servo, just the commutation is completely different.
Al.

[dondeaida]

BTW, are there 12 pole BLDC servos? The 12 pole BLDC I saw was for R/C.. Anyways Assuming he meant 12 pole BLDC and such BLDC exists, it makes sense to me. I was always wondering too, most 6-and-beyond-pole BLDC motors got 3 wires, 2 wires are needed for simutaneous firing and the 3rd for EMF feedback, then how does the controller decide and command what pair of poles to fire next? IMHO, the firing sequence is embeded in the motor and the motor simply deliver the incoming trapezoid to the next pair according to the preset order. If so, for 12 pole motor, 390 degree argument makes sense. A 3 phase sinusoidal cycle signal will cause BLDC to make a full 360 degree turn and then the motor will automatically deliver the next incoming cycle to the pre-programmed next pair of poles, one of which will be the one right next to the pole that had started the just-finished-cycle, and these 2 poles are always 30 degrees apart. 360+30=390. Of course some momentum is needed to travel that extra 30 dgrees. Has anyone actually tried this?

I'm also curious too, are there 6 or 9 pole(phase) sinusoidal ac servos?

[JasonCampbell]

Sorry Al, yes I was talking 12 "pole" BLDC and my hypothetical question was loose at best since I was disregarding the feedback mechasim, a possible sequencing circuit in the BLDC as donde mentioned etc.

And now I need to rethink the whole thing since I find the 12+ pole BLDCs I was refering to such as Kollmorgen's were "pancake" types

I have many more questions, but I'll stop bugging you for now I'd better study more. I have to mention that the variety and ingenuity incorporated in these motors are fascinating.

Thank you again AL.