Brushless motors-how many poles?
Steve Conner, Sat Aug 02 2014, 09:15AMHi 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?
Re: Brushless motors-how many poles?
Marko, Sat Aug 02 2014, 01:30PM
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.
Marko
Marko, Sat Aug 02 2014, 01:30PM
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.
Marko
Re: Brushless motors-how many poles?
Erlend^SE, Sun Aug 03 2014, 09:51PM
I would expect there to be a 2:3 ratio.
2 due to two possible poles, and 3 due to 3 phase.
Anyone got some more specific details?
Erlend^SE, Sun Aug 03 2014, 09:51PM
I would expect there to be a 2:3 ratio.
2 due to two possible poles, and 3 due to 3 phase.
Anyone got some more specific details?
Re: Brushless motors-how many poles?
Electroholic, Mon Aug 04 2014, 03:37PM
There are also different winding arrangements, RLK and DRLK.
Edit
Wiki links to this page
,could make for good bed time reading.
Electroholic, Mon Aug 04 2014, 03:37PM
There are also different winding arrangements, RLK and DRLK.
Edit
Wiki links to this page
,could make for good bed time reading.Re: Brushless motors-how many poles?
BigBad, Tue Aug 05 2014, 02:11AM
I think the two patterns spatially form a Moire fringe, so that it would (off-hand) be 14/12 different frequency.
Moiré pattern
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.
BigBad, Tue Aug 05 2014, 02:11AM
I think the two patterns spatially form a Moire fringe, so that it would (off-hand) be 14/12 different frequency.
Moiré pattern
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.
Re: Brushless motors-how many poles?
Steve Conner, Tue Aug 05 2014, 08:55AM
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.
Steve Conner, Tue Aug 05 2014, 08:55AM
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.
Re: Brushless motors-how many poles?
Ash Small, Tue Aug 05 2014, 09:49AM
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)
Ash Small, Tue Aug 05 2014, 09:49AM
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)
Re: Brushless motors-how many poles?
Dr. Slack, Tue Aug 05 2014, 10:33AM
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.
Dr. Slack, Tue Aug 05 2014, 10:33AM
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.
Re: Brushless motors-how many poles?
Steve Conner, Tue Aug 05 2014, 01:11PM
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.
Steve Conner, Tue Aug 05 2014, 01:11PM
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.
Re: Brushless motors-how many poles?
Dr. Slack, Tue Aug 05 2014, 02:39PM
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?
Dr. Slack, Tue Aug 05 2014, 02:39PM
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?
Re: Brushless motors-how many poles?
Ash Small, Tue Aug 05 2014, 02:58PM
If you compare it to a stepper motor, which is effectively two phase, and, depending on which 'pair' changes determines the direction that it 'cogs', ie 1st phase positive, 2nd phase positive, 1st phase negative second phase negative produces 4 'cogs' in one direction, which moves one stator pole to the next magnet of the same polarity.
In the three phase system, six 'cogging events' are required to move one stator pole onto the next magnet of the same polarity. There are 7 magnets of each polarity in your motor, which requires 42 cogging events (7 x 6) for one complete rotation. There are 6 cogging events per three phase cycle, so it takes 7 cycles (42/6) for one complete rotation.
Ash Small, Tue Aug 05 2014, 02:58PM
Steve Conner wrote ...
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.
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.
If you compare it to a stepper motor, which is effectively two phase, and, depending on which 'pair' changes determines the direction that it 'cogs', ie 1st phase positive, 2nd phase positive, 1st phase negative second phase negative produces 4 'cogs' in one direction, which moves one stator pole to the next magnet of the same polarity.
In the three phase system, six 'cogging events' are required to move one stator pole onto the next magnet of the same polarity. There are 7 magnets of each polarity in your motor, which requires 42 cogging events (7 x 6) for one complete rotation. There are 6 cogging events per three phase cycle, so it takes 7 cycles (42/6) for one complete rotation.
Re: Brushless motors-how many poles?
BigBad, Tue Aug 05 2014, 05:07PM
No, it is actually a rotating field; in the animated diagram they're only showing you the moments when the fields line up for simplicity.
It says:
"Praxis:
The 3-phase field is rotating at 42,000 rpm, the magnets bell and he propeller at 42,000/7=6,000rpm."
They've also got diagrams of the driving voltages above, and they're fairly normal (non sinusoidal) 3-phase waveforms. It's not pulsed.
edit: cool related Moire gif:
Notice how much faster the banded pattern moves; that's the drive frequency.
BigBad, Tue Aug 05 2014, 05:07PM
Steve Conner wrote ...
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.
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.
No, it is actually a rotating field; in the animated diagram they're only showing you the moments when the fields line up for simplicity.
It says:
"Praxis:
The 3-phase field is rotating at 42,000 rpm, the magnets bell and he propeller at 42,000/7=6,000rpm."
They've also got diagrams of the driving voltages above, and they're fairly normal (non sinusoidal) 3-phase waveforms. It's not pulsed.
edit: cool related Moire gif:
Notice how much faster the banded pattern moves; that's the drive frequency.
Re: Brushless motors-how many poles?
Steve Conner, Tue Aug 05 2014, 05:09PM
I'm not trying to "design a motor starting from hints", I'm trying to understand the general principle behind the "outrunner" brushless motors that have recently become very popular for quadcopters and the like.
I am more or less satisfied by Dr. Slack's answer, that the number of stator poles doesn't figure in the equation, because the motor simply won't work unless it bears the correct relation to the number of rotor poles. However, the literature on outrunner motors says that a few other ratios work besides 14:12.
Steve Conner, Tue Aug 05 2014, 05:09PM
I'm not trying to "design a motor starting from hints", I'm trying to understand the general principle behind the "outrunner" brushless motors that have recently become very popular for quadcopters and the like.
I am more or less satisfied by Dr. Slack's answer, that the number of stator poles doesn't figure in the equation, because the motor simply won't work unless it bears the correct relation to the number of rotor poles. However, the literature on outrunner motors says that a few other ratios work besides 14:12.
Re: Brushless motors-how many poles?
BigBad, Tue Aug 05 2014, 05:22PM
It's basically just sampling theory.
Any ratio of poles should work pretty well to reduce cogging provided they have a coprime relationship; in this case 7:6 because at any instant that averages out the different relative positions of the stator poles to the magnet poles around the circle.
Also: of course more poles are better, less poles are worse.
BigBad, Tue Aug 05 2014, 05:22PM
It's basically just sampling theory.
Any ratio of poles should work pretty well to reduce cogging provided they have a coprime relationship; in this case 7:6 because at any instant that averages out the different relative positions of the stator poles to the magnet poles around the circle.
Also: of course more poles are better, less poles are worse.
Re: Brushless motors-how many poles?
Steve Conner, Tue Aug 05 2014, 05:50PM
OK, I think I get it. In the particular case of the motor with 14 magnets and 12 coils: The stator field rotates at 7f, but it is sampled spatially in 12 places, so the Nyquist frequency is 6f, and there will be an "aliased" field rotating at 1f, which is the speed of the rotor.
(Edit: Maybe I meant 6 places rather than 12, but either way the argument seems reasonable.)
I just need to prove that the rotor magnets will follow this particular alias frequency in preference to the others. I guess by the principle of least action, you could show that the lowest alias frequency will generate the greatest torque.
Steve Conner, Tue Aug 05 2014, 05:50PM
OK, I think I get it. In the particular case of the motor with 14 magnets and 12 coils: The stator field rotates at 7f, but it is sampled spatially in 12 places, so the Nyquist frequency is 6f, and there will be an "aliased" field rotating at 1f, which is the speed of the rotor.
(Edit: Maybe I meant 6 places rather than 12, but either way the argument seems reasonable.)
I just need to prove that the rotor magnets will follow this particular alias frequency in preference to the others. I guess by the principle of least action, you could show that the lowest alias frequency will generate the greatest torque.
Re: Brushless motors-how many poles?
Ash Small, Tue Aug 05 2014, 06:02PM
It's exactly the same principle as stepper motors, except this has three phases instead of two (to reduce the cogging effect, which is utilised in stepper motors) and is driven with a pretty constant waveform (apart from speed control). The last stepper motor I took apart had 48 poles and 50 magnets, if I recall correctly.
Ash Small, Tue Aug 05 2014, 06:02PM
BigBad wrote ...
They've also got diagrams of the driving voltages above, and they're fairly normal (non sinusoidal) 3-phase waveforms. It's not pulsed.
They've also got diagrams of the driving voltages above, and they're fairly normal (non sinusoidal) 3-phase waveforms. It's not pulsed.
It's exactly the same principle as stepper motors, except this has three phases instead of two (to reduce the cogging effect, which is utilised in stepper motors) and is driven with a pretty constant waveform (apart from speed control). The last stepper motor I took apart had 48 poles and 50 magnets, if I recall correctly.
Re: Brushless motors-how many poles?
radiotech, Sun Aug 31 2014, 10:33PM
My answer to the original question:
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?
Is this:
In a poly phase induction motor, the synchronous speed is f * (n / 2) f= Hz/sec, n = number of poles on the stator This yields revolutions/ second. A 2 pole machine on 50 Hz. runs at 50 revs/second or 3000 revs per min.
The speed is a function of the number of poles on the stator.
This has been the case ever since Tesla made the first practical one.
radiotech, Sun Aug 31 2014, 10:33PM
My answer to the original question:
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?
Is this:
In a poly phase induction motor, the synchronous speed is f * (n / 2) f= Hz/sec, n = number of poles on the stator This yields revolutions/ second. A 2 pole machine on 50 Hz. runs at 50 revs/second or 3000 revs per min.
The speed is a function of the number of poles on the stator.
This has been the case ever since Tesla made the first practical one.
Re: Brushless motors-how many poles?
Steve Conner, Mon Sept 01 2014, 07:27AM
Way to go Radiotech, you just missed the entire point of the thread. The example motor we are using, with a 12 pole stator and 14 pole rotor, spins at one-seventh the speed predicted by that equation.
Steve Conner, Mon Sept 01 2014, 07:27AM
Way to go Radiotech, you just missed the entire point of the thread. The example motor we are using, with a 12 pole stator and 14 pole rotor, spins at one-seventh the speed predicted by that equation.
Re: Brushless motors-how many poles?
radiotech, Tue Sept 02 2014, 05:21PM
Way to go Radiotech, you just missed the entire point of the thread. The example motor we are using, with a 12 pole stator and 14 pole rotor, spins at one-seventh the speed predicted by that equation.
I agree with missing the point and later got this information from a book that I used
when stepper motors was a concern. And the page of interest was marked !.
m is number of phases.
For the case where the teeth(n) on stator exceeds rotor, and vice versa, the steps
per revolution(S) is different:
radiotech, Tue Sept 02 2014, 05:21PM
Way to go Radiotech, you just missed the entire point of the thread. The example motor we are using, with a 12 pole stator and 14 pole rotor, spins at one-seventh the speed predicted by that equation.
I agree with missing the point and later got this information from a book that I used
when stepper motors was a concern. And the page of interest was marked !.
m is number of phases.
For the case where the teeth(n) on stator exceeds rotor, and vice versa, the steps
per revolution(S) is different:
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