# Axial Flux Motor Gung-ho Design/Build



## Duncan (Dec 8, 2008)

Hi Modern

_*This is easy really. Fit as many poles/teeth/windings as possible within your desired rotor diameter using the strongest magnets you can afford and the thickest wire you can wind.*_

You need to think about cooling as well - as far as I can tell you can easily find a motor that can deliver oodles of power - the issue is for how long!

Permanent magnets have tight temperature limits - get too hot and all the magnetism runs away

Design a cooling system - then build the motor around it -


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## modern_messiah (Dec 8, 2010)

Yeah that's been in the back of my mind constantly. The coils are what heats up (not the magnets themselves) so that makes it easier because the coils are obviously stationary.

The basic design is the same irrespective of how I cool the stator. So all I need to do is design some kind of housing for the stator that keeps it as thin as possible - the smaller the air gap between rotors the greater the flux density which = more pow-ah.

If I fry a few hundred dollars worth of magnets though I'll be mega pissed lol.


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## Duncan (Dec 8, 2008)

Hi Modern
*The coils are what heats up (not the magnets themselves)*

Not sure that is true - you will be using the coils to change the magnetic fields the magnets live in - I think you will be dumping heat from eddy current effects into your magnets

If you have a 300 Kw motor that is 90% efficient 30 Kw of heat is being wasted in the motor - 
10 off 3 kw fires

that will raise 100kg of steel 90C in 1 minute


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## modern_messiah (Dec 8, 2010)

Ok....bad assumption 

_*However*_ a single stator AF Motor producing 300kW is highly unlikely in any size that would fit in a car. I will most likely be going for a 3 stator design (around 75kW per stator) in series or parallel.

Further more I can do no more than cool the rotors with air seen as they will be spinning at a few thousand rpm, and bathing them in a tank of coolant will cause way too much drag -especially as they will a high number of magnets attached to them, that even when flush mounted will add to the drag dramatically.

I will place a impeller ring between the rotors (and above the stator) in order to suck air in and over the coils/surface of the magnets.

So basically cooling will come down to simply not using too much power per stator! I hope.


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## crap (Dec 5, 2009)

You should also choose a controller to match before making the motor. The amp and voltage ratings shouldn't be a problem, especially since you could power each stator with it's own controller if necessary, and change between y/d and series/parallell. The electric rpm limitation however is a more relevant figure. A high amount of motor poles combined with a slow controller may limit you to dissapointing outputs.


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## modern_messiah (Dec 8, 2010)

So this is where I am yet to do some serious research...

What exactly do you mean by electric rpm? I was under the impression your voltage was tied to the rpm of the motor, or are you inferring the PWM frequency capability of the controller?

You are correct though - I'm looking at around 10 pole pairs per motor so if I go with 3 motors that's 30 poles... separate controllers may definitely be required.


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## crap (Dec 5, 2009)

The electric rpm is connected to the maximum AC frequency the controller is able to produce. Indirectly, this is affected by the pwm frequency, but it is not the same thing.

The Curtis AC controllers for an example will only handle 300Hz, meaning 18 000 electric rpm. Since you have 10 pole pairs, this controller would limit you to only 1800 motor rpm. The standard kelly controllers can handle 40 000 electric rpm, still only 4000 motor rpm. Kelly has upgrades to deliver 70 000 and 100 000 electric rpm, which should suffice. But kelly isn't exactly the finest brand, the quality is considered quite low.

The pole pairs need not be added with multiple rotors, 10 pairs each at four rotors (three stators surrounded by rotors) still adds up to 10 pole pairs if properly synchronized.

The rpm also depends on voltage, but since you are making the motor yourself you will be able to choose the voltage dependecy freely. You will wind as many turns as necessary to get the decided max rpm at the max available voltage. This is another reason to choose controller first, so the max available voltage is known.


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## modern_messiah (Dec 8, 2010)

I’ve been reading on the theory and design of motor controllers this morning, so your reply does make sense to me lol.

I’ll have to look around for an appropriate controller, because at this stage I believe building a controller, while not beyond my technical skill or knowledge, is a pain I don’t really want to deal with.

I see your point on the rotors and their magnet pairs. So long as the rotors are all aligned perfectly, the one controller can be used to control the AC-phase in all 3 stators.

Some work to be done just to do that though.

EDIT: Question – this is a bit embarrassing but what numbers am I looking for to determine the electric rpm? The only numbers I can see are frequency of operations and at 16.6kHz this is someway off 40,000…

Another embarrassing question; the power from the batteries runs through the controller yes? So if you have 3 motors you are limited to powering each motor by the total power the controller can handle divided by 3. So if the controller can handle 144v and 900A, the most I can feed to each motor at any one time is 48V and 300A or 14.4kW. Kinda lame…The other option is a controller per motor. Provided the rotors are all aligned correctly this would still work fine…

I could also have my understanding of how it’d work totally wrong, so please correct me if this is the case. Bear in mind I started this a week ago and have no prior knowledge on the subject….and I’m trying to build from scratch!


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## Anaerin (Feb 4, 2009)

modern_messiah said:


> Another embarrassing question; the power from the batteries runs through the controller yes? So if you have 3 motors you are limited to powering each motor by the total power the controller can handle divided by 3. So if the controller can handle 144v and 900A, the most I can feed to each motor at any one time is 48V and 300A or 14.4kW. Kinda lame…The other option is a controller per motor. Provided the rotors are all aligned correctly this would still work fine…


The question is, are you connecting the motors in series, or in parallel.

If they are connected in series, then using the controller listed each motor would see 48V and 900A.
If they are connected in parallel, then each motor would see 144V and 300A.

Some advanced controllers also have series-parallel switching, so they start the motors in Series mode, to get the most out of the high amperage, then when the back EMF starts decreasing the acceleration, the controller switches to Parallel mode, to increase speed higher at the cost of decreased acceleration.


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## modern_messiah (Dec 8, 2010)

> If they are connected in series, then using the controller listed each motor would see 48V and 900A. If they are connected in parallel, then each motor would see 144V and 300A.


 
I’m assuming in these examples only the one controller is being used for all three motors? Is one controller per motor an issue other than increased complexity cost and battery drain? lol “other than” – I think that’s everything that could change!


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## Anaerin (Feb 4, 2009)

modern_messiah said:


> I’m assuming in these examples only the one controller is being used for all three motors? Is one controller per motor an issue other than increased complexity cost and battery drain? lol “other than” – I think that’s everything that could change!


I think the main complexity would be keeping all the motors in perfect sync. If any of the motors is out of sync, you'll get "Transmission wind-up", which may break your connecting shafts at best or trash the motor entirely at worst.

Other than that, however, you would get full voltage and full amperage to all three motors.

Of course, another option would be to have 2 of the motors on the rear wheels, one per wheel, and the last motor on a differential to the front. It would give you 4-wheel drive, a 33/66 power split (Best for handling without the typical 4WD bogging down and torque-steer), and would have no problems at all using separate controllers per motor, though if they were linked in series it would act as a limited-slip differential, and if they were connected in parallel, it would act as an open differential.


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## Coulomb (Apr 22, 2009)

crap said:


> The Curtis AC controllers for an example will only handle 300Hz ...


These things seem to want to run at high frequencies. That becomes possible with an ironless design.

However, it seems that at these higher frequencies you need to consider using litz wire (a sort of wire woven from many thin strands of wire all electrically insulated from each other). There are few places in the world any more where such wire can be sourced. The idea of course is to limit the effect of the skin effect; even at 50 Hz, there is a fair bit more current at the surface of a thick conductor than at the middle.

Litz wire used to be common in intermediate and high frequency radio frequency coils in radios and TVs.

I suppose you could ignore the skin effect, and use ordinary thick round or rectangular cross section wire, and put up with slightly lower efficiency (due to the higher effective resistance).


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## modern_messiah (Dec 8, 2010)

> you need to consider using litz wire


If I was rich and able to afford it I would be using Litz wire, a Halbach array and N52 magnets. Unfortunately I don't have the cash lying around to invest that sort of coin 

At this stage I am using 1mm (18AWG) copper for my conductor, and will be air cooling it (split across 3 stators it can be run cool enough) however I will be leaving room in my design for liquid cooling just in case I want to push the system harder.

Still early days though - hell, I'm still trying to figure out what size rotor and hence how many magnets I'll be using.


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## crap (Dec 5, 2009)

litz wire is probably not needed, and can actually lower your efficiency. The fill factor decreases due to increased "insulation thickness to conducting area ratio" and decreased order, when you decrease individual strand thickness. Twisted litz, which is quite common, is even worse and should never be used. Just make your own bundle of, for an example, 0.3mm wires. Depending on the motor design it may actually be better.


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## modern_messiah (Dec 8, 2010)

I'm going to use the thickest wire I can reasonably work with - less resistance and greater heat capacity.

I do have something I need to know a little bit more about before I can continue with my design...

Does it matter if the controller can output more current/voltage than the motor can handle? Inversely does it matter if the motor can handle more than the controller can supply?

So if I just go ahead and build an “as powerful as possible” motor and hook it up to whatever controller I can afford, other than giving the motor too much juice and blowing it up, are there any other reasons to avoid this route?

I am of course assuming motor controllers give you a decent level of control over the current and voltage draw.

The way I see it I can build my motor, buy the controller and make sure the temperature of the stator core is closely monitored. If it gets too hot then I know I need to back off. Through trial and error I could deduce the motors continuous and peak power ratings.

It’s obviously the wrong way to go about things, but if there is no overly hazardous reason to do this then I’m going to plough on.


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## Coulomb (Apr 22, 2009)

modern_messiah said:


> Does it matter if the controller can output more current/voltage than the motor can handle?



Motors often don't have a very hard limit; they can usually take what the controller can dish out until they get too hot. If this happens routinely, then it's a shame about the cost of the silicon that rarely gets used, that's all.



> Inversely does it matter if the motor can handle more than the controller can supply?


It means you could be frustrated by the lack of performance, knowing that if the controller could only push out a bit more, the motor is ready to take it.

So as far as I can see, there is no huge disaster if the controller and motor are not matched well.

One thing though - controllers need a certain minimum of inductance. If there is not enough inductance, that's really bad news; the silicon can't protect itself fast enough. That's not usually a problem with iron cored motors; they have a factor of ten or a hundred more inductance than the minimum the controller needs.

But if your design is ironless, you may actually need external inductances. This is the case for the Ultramotive Carbon motors, for example, which are axial flux and ironless.


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## modern_messiah (Dec 8, 2010)

> One thing though - controllers need a certain minimum of inductance. If there is not enough inductance, that's really bad news; the silicon can't protect itself fast enough. That's not usually a problem with iron cored motors; they have a factor of ten or a hundred more inductance than the minimum the controller needs.
> 
> But if your design is ironless, you may actually need external inductances. This is the case for the Ultramotive Carbon motors, for example, which are axial flux and ironless.


My design is core-less, but the magnets are attached to magnetic steel to focus the field properly. Or is this exactly what you are talking about?


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## Coulomb (Apr 22, 2009)

modern_messiah said:


> My design is core-less, but the magnets are attached to magnetic steel to focus the field properly. Or is this exactly what you are talking about?


I'm not a motor design guru, but it seems to me that steel in the rotor won't increase the inductance in the stator much, if at all. So you may need to measure the inductance once you've made your creation.


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## modern_messiah (Dec 8, 2010)

Most axial flux motors place an iron backing plate behind the magnets to complete the magnetic circuit. This seems to work fine for every motor I've managed to lay my eyes on over the last few days.

Nothing quite as powerful as what I am attempting to make, but considering this one has nothing but wood backing (a _poor _design choice) and manages a continuous power rating of over 10kW (according to the designer it has a peak of around 18-20kW) I think the inductance may not be too much of an issue.

http://www.youtube.com/watch?v=j53FIHP3bPw

I should point out my design is quite a bit more professional than the above job...

However inductance is something I will keep mindful of and will definitely test at some point before hooking a controller up.


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## jk1981 (Nov 12, 2010)

http://www.femm.info/wiki/HomePage Might prove useful for saving you some money or design/build optimisation iterations.


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## modern_messiah (Dec 8, 2010)

jk1981 said:


> http://www.femm.info/wiki/HomePage Might prove useful for saving you some money or design/build optimisation iterations.


Yeah I've seen it used numerous times in discussions on motor builds. I've been avoiding it because it's another program to learn but I guess it'd be best lol.

Thanks for the link.


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## nfisher (Dec 14, 2010)

i'm just starting on a ground up electric vehicle. the purpose is as a test mule for a whole pile of ideas i have regarding evs. one of which has to do with multi phase axial flux motors. i'm interested in building one myself. can you share some of the sources you have found?

thanks
/n


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## modern_messiah (Dec 8, 2010)

I’ve been thinking about the control issue.

Can someone please clarify this statement for me: 


> The Curtis AC controllers for an example will only handle 300Hz, meaning 18 000 electric rpm. Since you have 10 pole pairs, this controller would limit you to only 1800 motor rpm.


Is this because for each full rotation of the rotor, the controller would have to update the position of the rotor 10 times (10 pole pairs), and because it is only capable of 300Hz, this limits the speed to 1800 electric rpm?

So the more pole pairs I use, the greater the frequency the controller needs to handle. This is a problem because I _was_ looking at 18 poles!

Back to the drawing board.


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## modern_messiah (Dec 8, 2010)

nfisher said:


> i'm just starting on a ground up electric vehicle. the purpose is as a test mule for a whole pile of ideas i have regarding evs. one of which has to do with multi phase axial flux motors. i'm interested in building one myself. can you share some of the sources you have found?
> 
> thanks
> /n


Hey, this is by far the best thus far: http://endless-sphere.com/forums/viewtopic.php?f=30&t=21127

Though it gets pretty messy and seems to be more focused on hub motors, but the principles are the same. Plenty of links to other resources in there including an excellent thesis on the subject, but I've misplaced the link.

Hope that helps!


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## nfisher (Dec 14, 2010)

modern_messiah said:


> I’ve been thinking about the control issue.
> 
> Can someone please clarify this statement for me:
> 
> ...


i'll take a stab at this: 300 hz (cycles/sec)=18,000 cycles/min. divide this by the number of poles to determine maximum rpm. therefore 18 poles = 1000 rpm. make it small and thin enough it might make a good wheel motor


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## nfisher (Dec 14, 2010)

i just had an idea about your 18 pole issue. you could wire your poles in groups (parallel), say in groups of 6. then use a 3 phase controller.


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## Coulomb (Apr 22, 2009)

nfisher said:


> I just had an idea about your 18 pole issue. you could wire your poles in groups (parallel), say in groups of 6. then use a 3 phase controller.


You don't need an 18 phase controller for an 18 pole motor, any more than you need a 4 phase controller for a common 4-pole motor. You just use three phases, and every mechanical cycle is divided into 9 electrical cycles (18 poles = 9 pole _pairs_) (or 2 cycles for a 4-pole motor). You do this for each phase; the other 2 phases are doing the same thing 120 electrical degrees different.


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## nfisher (Dec 14, 2010)

Coulomb said:


> You don't need an 18 phase controller for an 18 pole motor, any more than you need a 4 phase controller for a common 4-pole motor. You just use three phases, and every mechanical cycle is divided into 9 electrical cycles (18 poles = 9 pole _pairs_) (or 2 cycles for a 4-pole motor). You do this for each phase; the other 2 phases are doing the same thing 120 electrical degrees different.


i have the same understanding. however, then the hz/n poles*60 = rpm doesn't work does it? it really should be hz/n discrete poles*60 =rpm. or am i missing something fundamental?


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## Coulomb (Apr 22, 2009)

modern_messiah said:


> Is this because for each full rotation of the rotor, the controller would have to update the position of the rotor 10 times (10 pole pairs), and because it is only capable of 300Hz, this limits the speed to 1800 electric rpm?


The controller is updating the voltage to the three phases every PWM cycle, i.e. 3000-15000 times per second.

You add many poles to a motor to effectively "gear it down" for free (more torque, less speed, compared to a similar motor with fewer poles). With a large number of poles (e.g. 24), you might be able to eliminate all gearing, i.e. drive the wheels direct (not even a differential). But then you'll want a wide range of speeds, so the controller will need to be able to provide a wide range of frequencies.

If you keep a gearbox, so your motor only needs to do say 1000 RPM for 80 mph, then even with many poles, you may not need a very high frequency from the controller. It's the combination of many poles in the motor and a high motor (mechanical) speed requirement that necessitates a high frequency from the controller. So in a sense, it's the elimination of gearing that necessitates the high controller frequency. So a high controller frequency may be unavoidable, if you want to eliminate the gearbox and/or differential.

I hope I got that right; I've not thought about it in detail before.


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## Coulomb (Apr 22, 2009)

nfisher said:


> However, then the hz/n poles*60 = rpm doesn't work does it? it really should be hz/n discrete poles*60 =rpm.


It should be RPM = (electrical frequency / number of pole pairs) * 60, where electrical frequency is in cycles per second, and RPM is of course in mechanical revolutions per minute.

It's tricky getting the concept of many poles straight in your head; I'm having trouble describing it myself. So I won't attempt it; I'd likely confuse all readers.

[ Edit: removed some brackets (effectively bracketing the division) to make it clear that the 60 is on the top line. ]


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## Coulomb (Apr 22, 2009)

Coulomb said:


> It's tricky getting the concept of many poles straight in your head; I'm having trouble describing it myself. So I won't attempt it; I'd likely confuse all readers.


Ok, can't help myself. With 18 poles, you have 9 pole pairs; so the same thing that is happening in a 2-pole (1 pole pair) motor is happening nine times in one revolution, or equivalently, one mechanical cycle (of a 2 pole motor) is compressed into one ninth of a revolution (40 mechanical degrees). So in that 40 degrees, you have three windings, each getting the power from one of the three phases. The other 8 pole pairs for each phase can be connected in series or parallel or any sensible series/parallel arrangement, such that they all get the same voltage and current.

Because your magnetic interactions affect only 1/9th of a revolution, you end up with 1/9th of the speed (compared to a 2-pole motor), but 9 times the torque (again, compared to a 2-pole motor).


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## nfisher (Dec 14, 2010)

Coulomb said:


> Ok, can't help myself. With 18 poles, you have 9 pole pairs; so the same thing that is happening in a 2-pole (1 pole pair) motor is happening nine times in one revolution, or equivalently, one mechanical cycle (of a 2 pole motor) is compressed into one ninth of a revolution (40 mechanical degrees). So in that 40 degrees, you have three windings, each getting the power from one of the three phases. The other 8 pole pairs for each phase can be connected in series or parallel or any sensible series/parallel arrangement, such that they all get the same voltage and current.
> 
> Because your magnetic interactions affect only 1/9th of a revolution, you end up with 1/9th of the speed (compared to a 2-pole motor), but 9 times the torque (again, compared to a 2-pole motor).


works for me  i think i got it. thanks


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## Tesseract (Sep 27, 2008)

Coulomb said:


> It should be RPM = (electrical frequency) / (number of pole pairs) * 60, where electrical frequency is in cycles per second, and RPM is of course in mechanical revolutions per minute.


Ummm... the synchronous speed of an induction motor in RPM is (Hz * 120)/poles.

E.g. - a 2 pole motor at 50Hz has a synchronous speed of 3000 rpm.


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## Coulomb (Apr 22, 2009)

Tesseract said:


> Ummm... the synchronous speed of an induction motor in RPM is (Hz * 120)/poles.


Sure. Comes to the same thing I said, though perhaps I could have made it clearer by putting the division in brackets. So the 60 is on the top line. Number of pole pairs is half the number of poles; since the number of poles is on the bottom, that puts a 2 on the top line. 60 * 2 = 120.

Edit: 2 pole (1 pole pair) example. I say (50 / 1) * 60 = 3000 RPM.


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## modern_messiah (Dec 8, 2010)

OK so what are we saying here?

Is the original statement about the more poles I have, the slower the motor turns, correct or is this not the case? It would appear so but I can't figure out why...



> the synchronous speed of an induction motor in RPM is (Hz * 120)/poles


What is the Hz representing here - the controller frequency? ie: how fast it can update the position of the rotor? I'm a little lost.

Hence a 10 pole motor will not spin as fast as an 8 pole motor using the same controller because the 10 pole motor needs to be updated more per full rotation. Or something like that.


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## Coulomb (Apr 22, 2009)

modern_messiah said:


> Is the original statement about the more poles I have, the slower the motor turns, correct


Yes, it is. You divide by the number of poles (or pole pairs for my equation).



> It would appear so but I can't figure out why...


Well, essentially, because one electrical cycle (low frequency cycle, not PWM cycle, e.g. 50 Hz not 8 kHz) only moves the rotor a fraction of a turn. Considering an 18 pole motor, that's 9 pole pairs; it's like the whole motor is replicated 9 times around the circumference of the stator. So the whole machinery of torque producing and moving the rotor along happens nine times every revolution. So now there is nine times as much torque available, but it happens in a ninth of a revolution. So it's like a two pole machine with a 9:1 lossless, massless gearbox attached to it. Well, the extra windings have their losses and mass, but in an axial flux machine, these are both small, so why not have them and get rid of the gearbox.



> What is the Hz representing here - the controller frequency?


Yes.



> ie: how fast it can update the position of the rotor? I'm a little lost.


Not how fast the controller can "update the position", no. The three phase AC (at say 50 Hz) establishes a rotating magnetic field. In one electrical cycle (1/50th of a second for the 50 Hz example), each phase goes from positive to negative back to positive again (but they are 120 degrees apart, so one of the other phases is actually going from part negative to positive to full negative and back to part negative again). In a two pole machine, this magnetic field will drag the rotor of a synchronous machine one complete mechanical revolution. In an induction machine with 2% slip, it will drag the rotor 98% of a revolution. In an 18 pole synchronous machine, the magnetic field will drag the rotor a ninth of a revolution (40 mechanical degrees).

The controller has to know what the speed of the rotor is, so it can adjust the frequency and phase of the three phase signals correctly. It usually does this with a position sensor that provides something like 256 pulses per revolution, possibly as many as 1024 pulses per rev. So in that sense, the position of the rotor is potentially updated 1024 times per revolution. In practice, the controller only has to know the position of the rotor once for every PWM cycle. So it probably only bothers to work out the current position 8000 times per second, if the PWM frequency is 8 kHz. If the motor is spinning at 3000 RPM and has a 1024 pulse per revolution encoder, that means 3000/60 * 1024 = 51,200 pulses per revolution. Some small piece of electronics has to count those pulses to keep track of the position, but the main controller software is only going to access the results of that count about 8,000 times per second.



> Hence a 10 pole motor will not spin as fast as an 8 pole motor using the same controller because the 10 pole motor needs to be updated more per full rotation.


The 10-pole is slower because each electrical cycle only moves the rotor 1/5th of a revolution (72 degrees) compared with the 8-pole, which moves the rotor 1/4 of a revolution (90 degrees). But the 10-pole motor has 5 pairs of windings contributing to the torque, compared to the 8-pole motor that has only 4 pairs of windings producing torque. So the motors have about the same power, just delivered at different torque/speed ratios.

If you could easily switch from say a 12-pole motor to a 6-pole motor, you'd have the same effect as a 2:1/1:1 gearbox. Unfortunately, you'd need many many wires and a contactor with many many poles (36 perhaps?) to do this. Some motors do manage to switch the number of effective poles (e.g. Dahlander machines), but they are not very efficient.

You can do much the same thing with fewer wires by having each winding able to be switched into series or parallel; that's what the Ultramotive Carbon motor does with 9 high power wires.


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## modern_messiah (Dec 8, 2010)

I'm learning more and more every day....feels great lol.

I'm going to build a small 5" motor as a test run to practice on before I design the larger motor. This will most definitely help me as making mistakes on a cheap build is more preferable


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## nfisher (Dec 14, 2010)

ok, so now its my turn for a question: why pairs? why not 3 or even 4 or more poles wired together. i assume the poles are wired in parallel. so why not 3 or more in parallel?


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## modern_messiah (Dec 8, 2010)

Poles are the magnets....you can't 'wire them together'. These must be in multiples of 2 because each magnetic pole must have a magnetic opposite either side of it.

The teeth (coils) on the other hand must be in multiples of 3 (for 3-phase AC) and can be wired in series or parallel (delta/wye or whatever it is - I'm not up to that part yet lol)

AF Motors work in exactly the opposite way AF Generators work - if you can understand how an AF generator works (and there is a lot more information on the net about the generators than the motors) it helps understand the way the motors work.


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## Coulomb (Apr 22, 2009)

nfisher said:


> ok, so now its my turn for a question: why pairs?


If you're asking "why pole pairs" and not pole triples or quads, it's because with any pair of poles, one will be a north and one will be a south, and the two will act together to create one electromagnet, or otherwise the motor doesn't work well.

It turns out that if you have an odd number of poles, you can actually "induce" an opposite pole. The induced pole is not quite as effective as a real pole. I don't have a good feel for how this works.


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## nfisher (Dec 14, 2010)

ok, i thought the poles were the coils on the stator, not the magnets on the rotor.

for 3 phase power, delta and wye are how the 3 phases are arranged electrically. either as a triangle (delta) or a Y (wye). imagine each 'hot' wire goes to a point on the Y or delta. each configuration has its advantages and disadvantages (one is better for low speed the other is better for high speed, though i can never remember which, that's what google is for ) 

so my question still stands, but perhaps my terminology is wrong. why can't the coils be wired in multiples of greater than 2? why not 3 or 6, or 9 or whatever?


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## Coulomb (Apr 22, 2009)

nfisher said:


> ok, i thought the poles were the coils on the stator, not the magnets on the rotor.


They are.



> so my question still stands, but perhaps my terminology is wrong. why can't the coils be wired in multiples of greater than 2? why not 3 or 6, or 9 or whatever?


They are. In a 10-pole machine, 10 sets of poles are wired in some combination of series-parallel, so that 1/3 of all the coils (30 total. 10 for each phase) get the same power at the same instant. It's just that the speed and torque of the whole motor depends on the number of pairs of poles, since you can't have a north pole by itself; there has to be a south pole for it to work with.


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## modern_messiah (Dec 8, 2010)

> so my question still stands, but perhaps my terminology is wrong. why can't the coils be wired in multiples of greater than 2? why not 3 or 6, or 9 or whatever


Not totally sure what you're on about tbh!

If you have a 3-Phase motor with 18 coils, you will be wiring the 3-phases in multiples of 6. Phase A will have 6 coils, Phase B will have 6, Phase C will have 6.

Look here - http://www.bavaria-direct.co.za/models/motor_info.htm


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## nfisher (Dec 14, 2010)

thanks for the link. i guess my question is why not 4 (in this case of 18 it doesn't work). if you had 20 coils you could do 4 phases, 90 degrees apart, instead of 120 degrees - if you are making a controller, and converting dc to multi-phase power, why stick with just 3?. my understanding of 3 phase, is that it is more efficient because the sum of the phases adds up to higher more consistent flow of electrons. if this is the case, more phases should be even better right? is it just a case of diminishing returns?

(sorry if i'm a little slow on the uptake, craze time of year, at work and home)


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## aeroscott (Jan 5, 2008)

yes you are right ,


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## modern_messiah (Dec 8, 2010)

I think your assumption about more phases is correct - but most off the shelf controllers, and most OTS IC's etc (for building you're own controllers) are all 3-phase based.

So yeah you could design and build you're own controller to run as many phases as possible, but then it comes down to cost and effort. Will the greatly increased expenditure be worth the returns? I doubt it seen as 3-phase AC motors are pretty good at what they do already.

I've started building a smaller AF motor for pure testing purposes - to put theory in practice. Hoenstly the hardest part so far? Winding the damn coils - and I'm only using 9 for this motor 

But the point is to test control stuff, how easy it is to have sensored control of the motor, different winding patterns etc. Tricky stuff.


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## nfisher (Dec 14, 2010)

well good luck with that. i'm very interested in how it turns out.


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## Coulomb (Apr 22, 2009)

nfisher said:


> I guess my question is why not 4 (in this case of 18 it doesn't work). if you had 20 coils you could do 4 phases, 90 degrees apart, instead of 120 degrees - if you are making a controller, and converting dc to multi-phase power, why stick with just 3?.


Oh, why three phases? All high power AC is three phase, because 
(a) that's the most efficient way to use your conductors; you get the most power delivered per kg of copper
(b) three phases is the minimum you need to establish a smoothly rotating magnetic field. Well, I think you can do it with two phases, but that's four wires; three phase is three wires. Or you could do two phase with three wires, but then one of the conductors carries more current than the others, and it's less efficient. So you are better off using three phases with 3 uniform diameter conductors.

Running four phases means at least 8 switches as well; three phases needs six switches (banks of MOSFETs or IGBTs). These are the most expensive part of the controller, so minimising the cost of the switches is important.

Well over a hundred years ago, the engineers of the time investigated the number of phases question, and it was demonstrated that three phases is optimal. Nothing has changed that situation. That's the main reason that three phase dominates.

Having said all that, I vaguely recall some Chinese motor and controller manufacturers coming up with more than three phases; I can't remember why they considered that. I think it was some new motor design. I suppose if the controller is near the motor (as it should be), the amount of copper in the controller wires is not too expensive, and it may be that 8 300 A devices works out cheaper than 6 500 A devices, for example (maybe 400 A devices are not as readily available and inexpensive).

With 5 or more phases, any possible benefit is quickly eaten up by the cost of the extra conductors and switches, so 5 or more phases are even more unlikely than 4 phases.


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