Doing the Math

[Kingfish]

It sounds to me like we have slightly different 2W classes to consider:

• Fast Bicycle, lightweight, distance, efficiency
• Fast Motorcycle, street-legal, speed, distance, efficiency
• Drag-cycle, lightning torque, short runs
• Bonneville, speed, aero

The first two are excellent candidates for hub motors, natch. Conversely the second two would have to be drive train: IMM I think BMW; opposing jugs to offset and balance the inertia. With AF, this can be envisioned as compact counter-rotating twin-shafts, either co-centric or offset. Man what a tear-arse blast it would be to build!

Hey, what about hydro? Electric Hydro @ Seafair!

Lotto: I still have time to buy more tickets…
Mha-ha-ha! KF

[Arlo1]

Yup keep buying those tickets
So if there was a stator on both sides of the magnet would you still need iron core? I'm talking AF motor design.
I want the whole motor as light as possible but any time you can reduce rotational weight you increase HP on an inertia Dyno! This is important because although most will argue you should mesure HP on a load type Dyno, we are trying to build engines that accelerate not steady load like tractors!

[Biff]

So if there was a stator on both sides of the magnet would you still need iron core? I'm talking AF motor design.

Iron in the stator, no iron needed on the rotor, just magnets and something to keep them in position. You need the flux path to return to the magnet with the least amount of resistance (magnetic resistance called reluctance). So once the flux travels through the coils, it needs to make a Uturn and travel back through the coils then to the rotor. I have seem designs with a rotor sandwitched between to stators, but I don't see how it can be down without iron at least in the stator. The Hallbach array 2 rotor coreless design is the only one with no iron in it at all.

But you are right, with just magnets rotating on the rotor, it would make the rotor as light as possible. It might be worth calculating how much energy is stored in a rotor that weights 5kg spinning at 8 or 10 thousand RPM (assume it has about a 6 inch moment of inertia) and a 400kg vehicle driving at 177kph. I think the rotor energy will be insignificant but I could be wrong.

-ryan

[Kingfish]

A short rod & piston ala rice-rocket will spin faster than a long-rod ala Diesel. Unless we're designing custom magnets, the choices are similar: 14-inch OD or 8-inch OD. One model I studied with FEMM utilized thick magnets with fewer rotors verses the reverse - thin and many. Guess which had better utilization? Many thin, though there was a resistance penalty by having extra stators. Maybe I should post those results. Also the value of HA verses NHA is lessened with each additional rotor-stator pairing. On the flip-side, there is an equation that defines the limit of the number of stators per rotor diameter sort of like the relationship of additional magnet height limits out to be impractical for the cost of the gain.

BTW - I forgot to mention that a drive train motor can be designed with a shaft parallel to the axle which does not have to be twined; forgive me - I have shaft-drive on the brain for some reason.

Ex-Navy Pit-Snipe, KF

[Arlo1]

So if there was a stator on both sides of the magnet would you still need iron core? I'm talking AF motor design.

Iron in the stator, no iron needed on the rotor, just magnets and something to keep them in position. You need the flux path to return to the magnet with the least amount of resistance (magnetic resistance called reluctance). So once the flux travels through the coils, it needs to make a Uturn and travel back through the coils then to the rotor. I have seem designs with a rotor sandwitched between to stators, but I don't see how it can be down without iron at least in the stator. The Hallbach array 2 rotor coreless design is the only one with no iron in it at all.

But you are right, with just magnets rotating on the rotor, it would make the rotor as light as possible. It might be worth calculating how much energy is stored in a rotor that weights 5kg spinning at 8 or 10 thousand RPM (assume it has about a 6 inch moment of inertia) and a 400kg vehicle driving at 177kph. I think the rotor energy will be insignificant but I could be wrong.

-ryan

The therory with iron in the stator is it creates eddy losses and hurts efficiency in the end. Although it helps make a motor easier to build realativly good if you want great you need an ironless stator!

[Arlo1]

A short rod & piston ala rice-rocket will spin faster than a long-rod ala Diesel. Unless we're designing custom magnets, the choices are similar: 14-inch OD or 8-inch OD. One model I studied with FEMM utilized thick magnets with fewer rotors verses the reverse - thin and many. Guess which had better utilization? Many thin, though there was a resistance penalty by having extra stators. Maybe I should post those results. Also the value of HA verses NHA is lessened with each additional rotor-stator pairing. On the flip-side, there is an equation that defines the limit of the number of stators per rotor diameter sort of like the relationship of additional magnet height limits out to be impractical for the cost of the gain.

BTW - I forgot to mention that a drive train motor can be designed with a shaft parallel to the axle which does not have to be twined; forgive me - I have shaft-drive on the brain for some reason.

Ex-Navy Pit-Snipe, KF

You do what you want with your shaft
I want to build a small scaled down prototype and because it will be cheeper and smaller I might use it in a bicycle frame!

[Kingfish]

...there is an equation that defines the limit of the number of stators per rotor diameter sort of like the relationship of additional magnet height limits out to be impractical for the cost of the gain.

OK, I found the reference where this is discussed in the Axial Flux Permanent Magnet Brushless Machines book on Page 34, section 2.1.6 Multidisc Machines.

Essentially there is no formula, rather instead a set of guidelines that suggests that there is a limit to the benefits of increasing the motor diameter, taking into account relative rotor stiffness, axial joint, and rotor bearings. Then the book goes on to suggest it is more practical to elect multiple disc solutions for rapid torque and speed development, citing several examples.

That makes sense.
~KF

[Biff]

I've been very curious to know some figures on no-load for the CISRO motor if you happen to have them handy.

I finally got them, I didn't do the tests myself so I am not entirely certain of their accuracy.

Basically at no load, the torque is always the same (at 0.09Nm), the highest no-load power loss measured was at around 815 RPM which used 8W. That is just the power needed to spin the motor as measured by the torque sensor that the motor was mounted on, it does not include the loss in the motor controller needed to actually spin the motor. In the setup there were 2 motors, one was spinning the test motor at a fixed RPM, the test motor had no load, and was attached to the mount that had the torque sensor.

So essentially the only loss worth modelling in the motor is the copper loss. But since external inductors are required to actually operate the motor, those would need to be included in a system model. I don't have the no-load power required by the system (ie battery voltage and current required to spin a free-wheeling motor)

-ryan

[Arlo1]

OK so I have a new spin on the motor controler concept thought process!
As batteries get better and are able to produce more and more amps I belive this may become a problem for the fets turning on and off!
I think of the inside of a fet like a set of points on a ignition system and the more amps flowing through them the bigger the arc when lifted to shut that phase of the motor off. I am comparing the 100 volt 24s 2p 10ah 20c lipo pack I have that 1 year ago was prety damb cool being able to produce 300 amps! Now the pack I plan to build is 24s 3p 100v 13.5 ah 90c
so capable of 1215 amps and as luke pointed out they produce way over their ratings my pack will likly be able to push 2400 amps for a short period!

The reason I am bringing this up is maybe we need to start running a properly sized non polerized cap across each set of phase wires to help take the internal arcing away in the fets as we turn them on and off!

[Kingfish]

Arlo, is this related to the Brushless Drag Bike! thread? If so then let’s track it there. The goal of this thread is the study of AF configurations prior to build. Make sense friend?

Best, KF

[Arlo1]

Arlo, is this related to the Brushless Drag Bike! thread? If so then let’s track it there. The goal of this thread is the study of AF configurations prior to build. Make sense friend?

Best, KF

Actualy its more of a general controler/motor idea. I dont care what hi power motor controler system you build its something that could use looking at!

[modern_messiah]

Hi all,

First post in these forums after studying this thread back to front for the better part of a week!

Great source of information – unfortunately it seems to go around in circles and break off into tangents constantly, so I have had a great deal of trouble actually putting together a set of “Design restraints”.

For the record I am attempting to design and build a multi-stator (3 is the plan) axial flux motor for powering a car. I’m not trying to build something that just ‘runs’. I am aiming to build a performance vehicle. The motor at this stage will be connected to a clutch and transmission, so gearing will be in use.

I plan to build the motor by basically filling the motor with as much ‘good stuff’ as I can within a pre-determined size. So the thickest copper winding with as many turns as possible, with the most number of coils possible (multiples of 3 naturally), and using the strongest magnets I can afford with as little air gap as possible.

I plan to use liquid cooling on the stators.

My problem that I have is what number of poles to use. I seem to recall (I cannot for the life of me find the section in this thread!) reading that as close to 1:1, but not actually 1:1, was the best bet. Why was this again?

So if I had a 15-coil stator, then 16 would be the best number of pole-pairs?

Slightly confused,

- Matt

EDIT:

On another note a question came up in another forum regarding control issues. The 'problem' is as follows:

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.

Sorry if it has been explained already but how do you plan on tackling this problem? Is there something I am missing?

[Kingfish]

Hello MM
We are honored by you first post; welcome to ES!

Briefly,
1:1 appears to be nearly optimum, generally speaking. Advanced engineering analysis will likely determine the best ratio for any particular load – however that is a little beyond the scope of this thread; we are merely simple bright sparks with pcs without access to PhDs with super computers working for The Man.

With that said, magnetic poles will be even-numbered (plus & minus in pairs), and windings can be even or odd depending on how many phases are in the design. Generally we deal with 3-Phase with EV and ebikes, and that would be in odd numbers. There was a link provided someplace that is great for sorting out the groupings, let me see… Ah! Here you go:

Motor Winding Scheme Calculator V1.0

There are some ratios to avoid. Generally speaking - the lower the pole counts the faster the axle will spin. I suggest reading books referenced in this thread; they do a good job of discussing the issues.

15:16 ratio: Yes. Using the calculator, a favorable winding scheme is AaAaABbBbBCcCcC.

Another question et al: This thread is about doing the math for an AF motor in preparation for prototyping. RF motors can be discussed, and should be at some point for compare/contrast of proposed configurations for rhetorical purposes.

I have little interest (other than cursory attention) in discussing controllers and control components; items that are not germane to AF or RF motor construction. As you can see this thread already has plenty on its’ plate and we should stay focused. The AF motors that I have considered are ironless, thus I seek a sinusoidal controller: That’s as close as I’ll get to the topic of controllers; no need to go farther. Other dedicated threads serve it better, and there are two or three floating around right now that have my avid lurker’s attention.

Again welcome, KF

[modern_messiah]

Hey kingfish, thanks for the reply. You have cleared up the one issue holding me back. This thread rules as apart from the actual application your considerations and design choices are almost exactly the same as mine.

In regards to controllers I should give my apologies as I found another thread of yours on controllers about 5mins after I added the last part of my post. I simply forgot to remove the 'edit'

[Kingfish]

Have no worries friend. We are all just visitors here of our own device ♪ ♪
Checking-in, KF

[Kingfish]

ADDENDUM:
My bad; you asked had asked about windings. The topic is discussed here though it is diverse. Rather than create a large rabbit hole, I branched a specific conversation here: Is all magnet wire the same?

For completeness of the record, I sponsored a “controller” conversation that is loosely scoped to include the two common types of waveforms; trapezoidal & sinusoidal: Building the Best Controller

Cooling:
At present, I am considering the three options to various degrees (no pun intended):

• Nothing: Radiant. Smooth or veined surfaces.
• Air-cooled through vias: Radiant & Convection. Fixed, adjustable, dynamic, forced.
• Liquid-cooled: Radiant, Convection, & Conduction. Fluid-types: Mineral Oil, Water, Alcohol, Glycol, possibly others (preferential on biodegradable). Internally or externally pumped. Constant flow or thermally throttled.

I haven’t written a whole lot about a solution because the primary issue for AF is the magnets. We can touch however briefly on the thermal constraints just to be clear:

All theoretical calculations are based upon NdFeB 52 MGOe materials – being the best that are commonly available. The reality is that they are extremely temperature sensitive with an operating range of 60-80°C (depending on the manufacturer’s rating). K&J Magnetics provides a really decent matrix of the material performance and availability.

The problem with the N-type materials is that it becomes impractical to manage cooling unless they are used above the Arctic Circle. The reality is that 120-150°C is going to be a better bet for an optimistic motor designer, which means that we need to constrain the material specifications to NdFeB 48-42.

Then we need to source the magnets at a reasonable price. Biff forwarded some information to me about an alternate supplier; I have not acted upon that because I have a full plate trying to get my ebike winterized, along with balancing the rest of Life’s needs (and not forgetting that we are also in the holidaze). As we close towards the New Year I’m lucky if I can answer two posts a day. Hopefully January will allow more time.

Don’t let that inhibit you: Keep digging away as there’s lots of people here that are chock-full of knowledge.
Cheers, KF

[Arlo1]

As far as i am concerend if we get it efficient enough cooling is not an issue!

[modern_messiah]

15:16 ratio: Yes. Using the calculator, a favorable winding scheme is AaAaABbBbBCcCcC.

Is that calculator correct? I'm assuming it is, in which case is my understanding of the wiring scheme correct?

It states in the small notation at the bottom that the first 5 teeth are all phase 1, then the next 5 are all phase 2 while the last 5 are all phase 3. This contradicts everything I have learnt (and indeed some wiring diagrams on that very site) that the phase should cycle between the phases from one tooth to the next: P1,P2,P3,P1,P2 and so on - like here:



But if you put in 9 teeth and 12 poles into the calculator you get ABCABCABC and going off the notation at the bottom, according to them the first 3 teeth from the top dead centre, moving clockwise around the rotor should all be phase 1 wound clockwise. This is clearly wrong as it will result (among many other problems) in an unbalanced rotor.

What's going on

[Kingfish]

MM,
I have some references for you to read: http://endless-sphere.com/forums/viewtopic.php?f=30&t=22070&p=321611

They are great books that go a long way to explain in better detail than I could possibly do, and they are written by instructors and serious researchers. I encourage you to seek them out, read them, cover to cover, twice & thrice. After that, you might be ready for Shane Colton.

Otherwise we’ll end up taking this thread backwards to a humble beginning, and that’s not where I am at in this evolutionary process. I’ve parked myself at this point in the thread for a reason: I’ve reached a plateau and realized I have more questions as well, but they are very specific with many forks – and I need to read about how to ask them correctly before moving to the next level of understanding.

I could (and have) so give you the answers. It is better to direct you to the sources I have used and have you drink the same well of knowledge. References are of great value! Like the old saying:

“You can give a man a fish and he will eat for a day, or you can teach a man to fish and he will feed himself and his family for the rest of his life.”

Trolling for salmon - with crab pots set, KF
Ohh I think I’ve got a bite!

[modern_messiah]

Apologies are in order...again. Bit embarrassing really.

I will delete my above posts to try and keep the thread a little bit cleaner. After they're gone I'll get rid of this as well.

For the record I already have that paper and have read it a fair bit, though I must be missing something because while I understand the principles of AC control, I can't quite figure out how to correctly wind my stator. Ah well - i'll keep looking....and thinking

[Kingfish]

Gosh MM, I wouldn’t delete a thing. And don’t feel embarrassed.

My refs are well-thumbed. I seek enlightenment by other means, solo mainly because that is my M.O., and when I get to a point (other than panic, and… ok ok – when I get that happy-puppy feeling) where I need direction, then I ask.

I also find that reading multiple sources has an advantage in that each author (or team of collaborators) has a differing perspective; when combined together - a larger more complex canvas is rendered, sometimes igniting an epiphany, a eureka moment which may unlock other doors of opportunity or paths to ponder.

Winding Question:
Try this resource; I found it useful when first starting out; in fact – it’s near the top of my Favorites list: Motor Building Info

Want more?
Brushless 101 - Chapter 1
Motor Formulas
An introduction to the air cored coil
Simulink Simulator for a Brushless DC Motor

That’s a lot of wood for making fishing poles friend
Keep plugging away. KF

[modern_messiah]

Those links are gold. Thanks a bunch!

Any advertisement you see on this site can infect your computer. DO NOT CLICK ON ANY ADS

[wanders]

Just saw this thread last night for the first time. I've been working on an axial-flux motor design for quite a while. AFAIK, the best book on the subject is by Jacek Gieras et al. Here's a link to an on-line lecture of his that really is useful:

http://www.4shared.com/document/cvLjDFSW/07__Axial_Flux_PMBM.html

My simplest take on coreless axial-flux machines is that that have a ton of advantages for ebikes (compact, no cogging torque, etc), but the stator for the preferred configuration (internal stator with a dual external magnetic rotor, i.e. an axial outrunner) is challenging to fabricate. Plus, of course, if you want light weight you need to do a Halbach array.

wanders

PS - Sorry to be shipping the coal to Newcastle! I'm finally reading the thread and see that Gieras' book is mentioned (indeed, it was purchased by the OP). Nonetheless, the presentation referenced above captures an astonishing amount of the basic formulae found in the book. Regards, all.

[modern_messiah]

The link doesn't work for me - none of the Google doc ones seem to. Very annoying!

[Kingfish]

The link doesn't work for me - none of the Google doc ones seem to. Very annoying!

I think you need a Google account - of which I do not have need. However -

Wanders, could you investigate and see if there is another source to the information that does not require logging in please?

Best, KF