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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi all
You probably know that the brushless motors used in RC models have a different number of poles on the rotor than the stator. For example, one motor has 12 coils and 12 pole pieces on the stator, but the rotor has 14 magnets. The purpose of this is to make the torque smoother. If every pole piece could align with a magnet at once, this would cause a huge torque ripple (also referred to as detent torque or cogging)
My question is, since the rotor and stator have different pole numbers, which one do you use to calculate the speed? Put another way, if I removed 2 magnets from the above motor (and redistributed the remaining ones evenly) would the synchronous speed stay the same, or would it increase by a factor of 14/12?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Steve
To my understanding it is the stator pole number that determines the speed of rotating magnetic field, and hence synchronous speed and you shouldn't be able to change this by adding magnets to rotor. You could get a rotor with just 2 magnets rotate at the same synchronous speed; however it's torque would be low and ripple atrocious.
As you can see in some of the diagrams, Moire patterns have a magnifying effect, and that's also why they're used here.
In Moire patterns it's more to do with the difference in the number of poles; if one is 14 and the other is 12, they will line up at 2 places (or is it 4, I need to think about it more carefully, I think it's 4 because they're dipoles); it's where they line up and around that they generate the most torque. In effect by having different numbers of poles you're generating pseudo poles that are much bigger and that reduces the torque ripple.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
This article suggests that with 12 stator poles and 14 magnets, the rotor turns at 1/7 of the stator frequency, not 12/14. For each complete revolution of the stator field, the rotor moves on by one magnet pair.
This is quite different to a classical synchronous machine, it reminds me more of a stepper motor.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
If you get a double pole, double throw switch, (or is it three phase?...you'll need three) you can connect it (them) up and, by toggling, you can simulate the driver at a speed you can watch.
I used two for playing around with stepper motors to check I had everything correct.
(I'm still on my first coffee this morning (late night), hope this makes sense)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Look at it this way.
After one complete electrical cycle of winding drive, the armature will be in the same magnetic position again, by symmetry.
As there are 7 pole pairs, the armature must have advanced n/7ths of a turn.
If each coil is driven with a waveform whose cycle consists of two opposite peaks with a monotonic change between each (striving to keep the drive waveform no more specified than is absolutely necessary) then n cannot be other than one.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I can't argue with that, but I also find it non-intuitive that the answer is independent of the number of poles in the stator winding.
The article said that due to a different winding configuration, the stator doesn't generate a rotating magnetic field like in a classical polyphase motor. It is a pulsating magnetic field and the 3 pole pairs act like 3 independent single phase machines. So adding pole pairs (triplets, sextuplets, whatever) to the stator doesn't change the speed, it just increases the torque.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Consider a 14 pole rotor, with just 2 coils spaced a half pole-pitch (quarter magnetic cycle) apart somewhere on the circumference. Quadrature drive to these coils will rotate the rotor 1/7th turn per cycle.
Add another pair of coils somewhere else round the circle. If they're n/7ths the way round, you can drive them with the same phases, and get double the torque, but it doesn't improve the cogging. Put them other than 1/7ths round, cogging is reduced, but they need to be driven with a different phase for best efficiency.
Whether these coil pairs are pulsing, or creating a sparsely sampled rotating magnetic field is a question of semantics. As you add more coils pairs to populate the whole circle, more samples of this rotating field make it look more and more just like a rotating field.
The answer is independent of the number of coils, because the fixed rotor magnet configuration determines the shape of the field the coils and their appropriately phased drive waveforms has to produce.
Of course all this above is just designing a motor starting from hints. Have you got a picture of the offending parts, and scope traces of the drive waveforms?
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Brushless motors-how many poles?
