Turn-around-ratio

Microbatman

Microbatman

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What does anyone know about
"turn around ratio" ??

Read about it here and now am trying to figure out the best combination of poles for designing axial flux halbach array direct drive motor.


http://images.google.com/imgres?imgurl=http://home.solcon.nl/gjkool/brushless5/W-schms-3p-E.JPG&imgrefurl=http://home.solcon.nl/gjkool/brushlessE5.html&usg=__H_MjPutwzoIVWAUA_GjRzyQKBUo=&h=298&w=758&sz=51&hl=en&start=39&um=1&itbs=1&tbnid=rek27djW_e6_PM:&tbnh=56&tbnw=142&prev=/images%3Fq%3D4%2Bphase%2Bbrushless%2Bmotors%26start%3D20%26um%3D1%26hl%3Den%26sa%3DN%26rlz%3D1W1DKUS_en%26ndsp%3D20%26tbs%3Disch:1
 
From your link:
The three-phase synchronous motor and "turn-around-ratio".
The electric alternating current provided to a three-phase motor, creates
a rotating magnetic field inside, or outside this motor, where the rotor is positioned.

The rotor from our "brushless" motor contains magnets, which are our rotor poles.
In the basic design, those rotor-magnet-poles move at precise the same speed and position as this rotating magnetic field.
This is done not just by pushing the magnets around, but by switching from one magnet to the other.
That's why such a motor is called a three-phase synchronous motor.
A nice animation of this can be seen here: 2-pole motor 4-pole motor.

In its basic shape, this motor does have a 3-pole stator, and a 2-pole rotor.
Or with a 6-pole stator, a 4-pole rotor.
In this case, when the rotating magnetic field goes around once, the rotor also goes around once.
The "turn-around-ratio" of this motor is: 1:1.

In the case of a 3-pole stator, you can also use a 4-pole rotor instead of the 2-pole one.
Because the stator field switches from on magnet-pole to the other, and there are 4-poles instead of two,
the rotor will only make half a turn for one whole magnetic rotation of the stator.
The turn-around-ratio of this motor is: 2:1.
(This is a somewhat simplified explanation, in detail not completely correct.)

The total number of stator-poles always is 3 or a multiple of 3. (counts only for three phase motors)
The total number of rotor-poles always is 2 or a multiple of 2. (because of the 2 poles of a basic magnetic field)

So when using a 6-pole stator, and an 8-pole rotor, there also is a turn-around-ratio of 2:1.

On a 6-pole stator, theoretically, you can also use a 10-pole rotor. The turn-around-ratio should then be 5:1.

If you compare a 5:1 turn-around ratio motor with a 1:1 turn-around ratio motor,
The 5:1 will be capable of a higher torque at lower speeds.
Also the maximum efficiency of that 5:1 turn-around ratio motor can be achieved at lower speeds.
So you can use a larger prop.
Still these high turn-around ratio motors are capable of high speeds (high rpm's).
Click to expand...
Seems to be referring to the ratio between the mechanical rotation and the rotation of the field.
I'm not sure where the higher torque comes from................

See also here: http://translate.googleusercontent.com/translate_c?hl=en&ie=UTF-8&sl=de&tl=en&u=http://www.powercroco.de/Konstruktionsregeln.html&rurl=translate.google.com&twu=1&usg=ALkJrhiGAvsXSg6O5aRmBLuXyxbUsNaQ6g
 
I'm not sure where the higher torque comes from................
Click to expand...
Are you kidding? :)
The revolution of the outrunner BLDC in RC world was, that it had an inbuildt 7:1 electric gear ratio.
With the same amount of electric energy, the motor rev'ed 7 times slower but also 7 times stronger than the usual 1 pole brushed DC motor.
I can't explain it in formulas but its related to the path a magnet has to travel from one active stator tooth to the next.

And to aswer the question: There is no perfect combination of poles. It's always depending on the environment/ the task a motor is designed for. And even then: many ways lead to Rome...

-Olaf
 
olaf-lampe said:
I'm not sure where the higher torque comes from................
Click to expand...
Are you kidding? :)
The revolution of the outrunner BLDC in RC world was, that it had an inbuildt 7:1 electric gear ratio.
With the same amount of electric energy, the motor rev'ed 7 times slower but also 7 times stronger than the usual 1 pole brushed DC motor.
I can't explain it in formulas but its related to the path a magnet has to travel from one active stator tooth to the next.
Click to expand...
Electric gear ratios........... yuk! :)

http://forumrc.alexba.eu/nutpol_e.htm
 
Here is more info on suposedly on Magnetic gearing. (or comutational turns per rpm)

3/4 down the page they discuss the effects fo 10 magnets vs 14 magnets.
http://www.gobrushless.com/kb/index.php?title=Brushless_101_-_Chapter_3

From my point of veiw this is all just theroetical bunk. Tourqe produced is a function of EM fields & pm fields along with angular constants of the motor's size-PMBLDC speaking.

Obviously Less poles=higher rpm potentials (comutational speed advantage) VS more poles= smaller angles of force on the rotor(more leverage/tourque)

The term "gearing" is quite a misnomer in this instance.
Maybe I am too simple.
 
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