rules of brushless commutation

[dirty_d]

is there any defined set of rules or equations for the sequence the phases are to be commutated for a given pole/tooth count? for a stator with 3 teeth and 2 poles, i found the sequence i see everywhere to work, AB, AC, BC, BA, CA, CB, with the magnetic field rotating the same direction as the rotor. but for the same stator and a 4 pole rotor, the sequence was the opposite, CB, CA, BA, BC, AC, AB, with the magnetic field rotating the opposite direction as the rotor. i found 3 teeth and 6 poles to not work at all, and 3 teeth and 8 poles to maybe work but probably not good, i did all this stuff on paper with little cutouts i could rotate as the rotor with the magnetic potential vectors drawn on it.

so what are the rules? an odd number of pole pairs and the magnetic field rotates the same way as the rotor, and an even number of pole pairs and the magnetic field rotates the opposite way as the rotor?

and how about tooth/pole counts, is there some math to figure out what combinations work good and what ones don't work at all? that stuff is pretty confusing.

 

[dirty_d]

seems like 6 teeth/8 poles, follows the same sequence as 3 teeth 2 poles.

 

[Miles]

Have you explored this site?

http://translate.google.com/translate?u=http%3A%2F%2Fwww.powercroco.de%2Ftheorie.html&sl=de&tl=en&hl=en&ie=UTF-8
[ http://www.powercroco.de/ ]

 

[dirty_d]

yea ive looked at similar stuff, but it doesn't really explain the math behind figuring out what combinations work.

 

[dirty_d]

aha!, i think i figured something out as far as poles/slot numbers are concerned, for the motor to work best the following should be true:

slots / poles = a multiple of 0.75

3 slots 2 poles = 1.5
3 slots 4 poles = 0.75
6 slots 4 poles = 1.5
6 slots 8 poles = 0.75

3 slots 6 poles = 0.5, no good
6 slots 6 poles = 1.0, no good

i still don't understand why for both 4 pole motors the commutation is in the opposite order as the others, there must be some formula to show it.

 

[dirty_d]

ok i figured a lot more stuff out, the commutation sequence is always AB-AC-BC-BA-CA-CB or backward for the opposite direction of rotation. and the winding sequence is changed depending on the slot/pole count, heres what ive found so far.

slot/pole winding sequence:
0.75 ACBACB...
1.125 AaABbBCcC... (untested, i cant figure out how to draw this asymmetrical piece of crap, 2.666 commutation steps between slots, this is how the MAC-BMC motor is)
1.5 ABCABC...
3.0 AcBaCb... (crystalyte 48 slot/16 pole)

there must be a bunch more combinations. anyone see a pattern or formula to take the number(0.75, 1.5...) and figure out the sequence by math?

 

[The7]

so what are the rules? an odd number of pole pairs and the magnetic field rotates the same way as the rotor, and an even number of pole pairs and the magnetic field rotates the opposite way as the rotor?

This is no correct.

 

[dirty_d]

yes i know, its dependent on slots/poles, the commutation stays the same the winding scheme just changes.

 

[The7]

Nearly all BLDC motors have 3-phase winding.
1 pole-pair = 2 magnet poles

Rule 1: No of magnet poles must be even (to have pole-pairs).
Rule 2: No of slots (or teeth) must be multiple of 3 ( for 3-phase winding).

There are two common types:

(a) No of slots = 3 X No of magnet poles
e.g. Crystal motors.

(b) "No of slots" is NOT equal to "3 x No of magnet poles"
e.g. PUMA, BMC

 

[dirty_d]

yes it is the exact ratios and the resulting winding patterns that i am trying to figure out. i've solved the most common ones so far.

 

[rkosiorek]

http://translate.google.com/transla...co.de/theorie.html&sl=de&tl=en&hl=en&ie=UTF-8

has a chart and a calculator for "Concentrated" windings. these are coils that are tightly wound around each tooth. these types of windings are used on the PUMA/BMC motors for example.

another winding scheme is called "Distributed" winding. an example would be the Crystalyte motors. here the winding is wound on a buck or a form. the preformed coil is large enough to fit over 3 stator teeth. once wound the loose coil if fit into the stator.

a third possible scheme would be a "Wave" winding. this is common on many motor alternators.

all of this is done just to make your life as complicated as possible.

rick

 

[dirty_d]

the distributed winding seems a little confusing, would the winding sequence still be the same for a concentrated winding with a slot/pole ratio of 3:1? since each coil goes around 3 teeth when the coil is on it seems they act as a single big stator pole, but the next coil in another phase is also wrapped around two of those teeth, its kinda hard to picture whats going on there. anyone have a better understanding of the resulting magnetic fields in the stator?

 

[fechter]

There are many pole configurations that can work. Here is a paper that describes many of them. Take with a grain of salt, I cannot guarantee the accuracy of the contents: