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Dual Rotor Alt - with laminations??
By jimovonz, Section Homebrewed Electricity Posted on Thu Feb 03, 2005 at 09:05:16 AM MST
Why wouldn't laminations give the same benefit in a dual rotor?

I my quest to get the most from my lowly ceramic magnets (http://www.fieldlines.com/story/2005/1/21/20579/5871), I was pondering on motor conversions we have seen lately. You have to admit, the motor conversions get pretty impressive output for a given size/magnet mass. After some thought, it became apparent that the reason for the increased output was due to the superrior magnetic path offered by the laminations. Your typical axial flux dual rotor has an air gap around 3/4" while your typical motor has an air gap of less than 1/16" inch allowing for a greater magnetic field using less magnet. Sure, laminations also have their drawbacks (cogging/saturation...) but there seems to be ways to successfully deal with these (skewed coils/magnets etc) I then wondered why laminations were not utilized in dual rotor alts to much the same effect. After trauling through the board archives I found numerous references to folk who talked about why it was not a good idea, but no examples of it being tried. The arguments against could all be equally applied to any motor conversion and these are obviously proving successful - so why not? I thought that a reasonable approach would be to use 'fingers' made up from laminated steel in between the coil legs. These could be cast in place using resin in the normal fashion. This would most probably work best for overlapped coil arrangements. One possible drawback with axial flux alts may be the lack of room towards the inner part of the rotor but in my case (dual rotor radial flux alt) this is not a problem. I decided to make up a couple of coils to test this out. By this stage I have many, many microwave transformers at my disposal so I have used the primary windings from two for my test. I have also used the laminated core from one transformer as the doner material for my laminations. Both coils are made up of approx 220 windings of AWG#18 enameled copper(yes it is copper, not aluminium!) wire. The coils are just the right height, but the hole in the middle is a little smaller than is ideal for my micromags. For testing purposes I used one row of single micromags on each of the rotors (24 mags per rotor, 48 mags total). The air gap between the magnets is a little over 1 1/2". I needed this large gap to give me suitable clearance for the flimsey fixing I came up with for the coils. The middle point between the two rotors is approx 20" in diameter (this is the size that the stator will be when complete). The laminations I have used are approx 1/2" wide, 3/4" tall and 2 1/2" long. There is one either side of each coil leg. These laminations were a bit of a pain to put together as, to avoid eddy currents, the laminations are accross the 'finger' made up of many 1/2" x 3/4" pieces stacked to make up the 2 1/2" length. The two coils were fixed in between the two rotors with good separation to ensure no interference. The output from the two coils was monitored using both multimeters and logging PC scope software on a PC via the sound card. The rotors were turned by hand, and with a rotating mass close to 350lb, it is hard work getting beyond 100rpm! (though the rotors turn very 'freely' it is difficult to apply additional force to the rotor because of its large diameter).
Here are the two coils:

Here is the transformer after I cut off the laminations:

Here are the results of the test:

As you can see, the coil with the laminations produces around 1.8 times the voltage of the equivalent coil without laminations at any given rpm.
If I were to use laminations for this alt, I would look at 3 phase overlapped coils - either 3 per magnet or 3 per pair of magnets (72 or 36 coils total) with say, 5mm (1/5") wide laminations either side of every coil leg going right through the stator.
Is there anyone out there that can tell me why building such a stator would not be a good idea? Would this not allow me to get the most from my magnets? Perhaps the efficiency suffers even though the overall output increases? Perhaps there is something I'm overlooking? Comments welcome!

Dual Rotor Alt - with laminations?? | 13 comments (13 topical)

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#1)
by Flux on Thu Feb 03, 2005 at 02:35:22 AM MST

There is no doubt that using laminations will give you more output for a given amount of magnet material. This is more true for ceramic than for neo.

If you are looking for maximum output at the top end and are not worried about low wind performance that is the way to go.

I don't know of anyone who has tried using bits of lamination in a dual rotor, it certainly has serious mechanical difficulties as the magnets will be trying to pull the cores out. The single rotor arrangement with slotted laminated core can be done quite successfully. for more power you could put 2 back to back with magnets on both sides of the rotor plate.

Once you start introducing iron into the core you start to bring uncertainties into the design with regard to leakage reactance and how the thing reacts under load. This is more of a problem with ceramics and their low flux density as you need more turns.

If you have a leakage reactance greater than the resistance it will tend to flatten out to constant current since the reactance rises with frequency. I think your idea with bits of iron in a dual rotor will be less prone to this problem as the armature reaction flux will have a hard job finding a leakage path.

It will cog like nothing else but the iron loss should be modest.

Give it a try and let us know how you get on .
Flux

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#5)
by johnlm on Thu Feb 03, 2005 at 10:28:43 AM MST

Sorry I dont mean to be off topic here too much but a Question for Flux on his above response. You wrote

"If you are looking for maximum output at the top end and are not worried about low wind performance that is the way to go."

I have seen this statement mentioned several times in various posts. Exaxtly what do you mean? The implication in most of these posts is that if you have iron in the stator then one cannot get any performance at low wind speeds.

Is it because of cogging and the force needed to startup the mill in low winds? Or is there something else going on that Im missing? I have convertsed several small shaded pole motors (20 inch breeze box fan motors in the 1/25th to 1/15th HP range) using both ceramic and neo magnets, using the stock windings in some and rewinding others, and have been able to get the cogging down to practically nothing (1 to 2 ft oz), and have designed bladesets for these motors in the range of 2.5 to 4 ft diameter. In most of the cases I design the prop and the alternator to hit cutin at anywhere from 5 mph to 7 mph wind speeds. Granted, these are not big machines and the best of them will only put out about 0.5 amp at 7-8 mph and 2 amps into a battery at around 15 mph, (and the reactance does start coming into play and limiting the output current pretty significantly above 50 to 60 HZ output frequency) but between 5 mph and 15 mph (which I consider low wind speed) the alternator output pretty much follows the power curve of the prop. And the prop will start usually at wind speed from 3 to 4 mph. The latest one I have attached an electronic bike spedometer / odometer to the rotor calibrated to the prop size and TSR to get more accurate measurements. Will start testing in a few days.
Johnlm

[ Parent ]

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#6)
by Flux on Thu Feb 03, 2005 at 11:44:21 AM MST

Johnlm
This is not an easy question to answer as it depends a lot on what you want.

Any design is always a trade off between conflicting requirements. The post above was about a large machine and I think the idea is to screw the maximum amount of power from reasonable winds. Others have a requirement to extract a modest power in the lowest winds to have a steady reliable source. Also much depends on the local winds.

It is a fact that in low winds there is little power anyway and to get anything you have to keep losses to a minimum. Bearings are a necessary evil and even here I have noticed that tapered rollers waste more power than ball bearings.

If you have iron you have to have iron loss it is a question of how much. Your small motors probably score well on bearing loss and they won't have much iron so the iron loss can't be that great. You may find that the ceramics do better in low wind than neo as the iron loss ought to be less.

Even so if you get the outputs you claim you are doing pretty well at 7 mph.

When you try for bigger output in high winds you have to keep flux densities higher or you have to increase speed or both. If you use good grade iron and keep reasonable air gaps with the slotless designs the iron loss stays quite reasonable.

If you use large magnets on slotted iron cores with small air gaps the iron loss becomes considerable. It will be a very low percentage of the total output but it will be significant at very low winds. It will delay starting and I consider it to be more of a problem than cogging ( unless you have an F & P or something).

Personally I have no objection to using iron to save on the cost of magnets if done carefully. Using radial designs with magnets rotating round a motor ( brake drum type) I find the losses are not an issue. Using iron laminates on a single rotor with no slots requires good core material which is either expensive or requires cutting up transformer cores or something which I just couldn't be bothered to do.

To use slotted cores for maximum output usually means a trade off at the low wind end.

Commercial manufacturers go for high outputs in high wind to have nice impressive sales figures but unless you live in a high wind area they are not likely to produce more than a good home built machine and most of them tend to be fast and noisy.

If you want good power in high winds then iron is probably best.

For a reasonable all round performance iron can still be good but I tend to avoid slots.

For best results in low winds & ease of manufacture dual rotor is the way to go but you will pay more for your magnets.

In the end everyone has their own ideas depending on what they have in the way of bits and pieces and facilities and how they value financial cost against the cost of time and effort.

Flux

[ Parent ]

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#7)
by jimovonz on Thu Feb 03, 2005 at 12:29:12 PM MST

Thanks for your input Flux - I appreciate it. With regard to the magnets trying to 'pull the cores out', the cores are being pulled from both sides so would not the forces on the cores cancel out the closer they are to the centre of the magnets? Even if it is not possible to exactly centre the stator, the net force on the cores should be minimal. Assembly of the machine may pose a problem, I haven't yet come up with a way to keep the rotors/stator apart during assembly. As for the cogging, I was hoping that this would be minimised by the fact that in my proposed setup, each magnet covers more than one lamination with only one leaving/entering the influence of the magnet at any one time. This coupled with a slight offset/skew to the magnets and the round shape of the magnets themselves (no abrupt leading edge) should hopefully keep cogging to a minimum.

[ Parent ]

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#10)
by Flux on Fri Feb 04, 2005 at 01:38:47 AM MST

jimovonz
The forces will be a lot less when centred but still considerable once there is any deviation from centre. You also have the problem of the resin loosing strength at higher temperatures.

With overlapped coils you can bind the ends together to form a self supporting structure to act as a support to the resin and you can make a strong support ring on the outside. If you can find some way to support the laminations you may do it.

As you say assembly will be a nightmare but you only have to face that once.

With only one shot at it cogging may be high although much fiddling with the core placing could reduce it if you tried several, but I don't think it will be a major issue. If you can solve the other problems I think it will fly.

This is a big machine to try an un-proven design, I would be tempted to try a prototype at about 1 kW and see what the problems are.

Flux

[ Parent ]

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#2)
by PaulJ on Thu Feb 03, 2005 at 03:45:51 AM MST

My big worry would be the structural integrity of the stator with that much steel in it. As I understand it, coils under load experience a force trying to pull them in the direction of rotation of the magnets; the steel, unless perfectly centred between the magnets, will experience a force at right angles to this.

I did some single coil tests a while ago using powdered iron (brake machining residue) mixed with the resin in a dual rotor axial flux machine, got about a 20% gain in open volts. Cogging was significant, although as you have mentioned could be overcome with your setup. The big problem, as I saw it, was that the stator wouldn't centre between the magnet rotors, the steel in it wanted to pull one way ot the other, which translated into quite a nasty vibration when the rotors were turning. This may have cancelled out somewhat if I had cast a complete stator this way, but I was sufficiently concerned about possible fatigue failure of the stator, along with eddy current heating, that I didn't pursue it any further.

This is just my 2 cents worth based on a couple of experiments and a gut feeling, I'm not an engineer and I may have underestimated the strength of fibreglass resin.

Paul.

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#3)
by DanB on Thu Feb 03, 2005 at 06:30:34 AM MST

yes.. like flux said.

The advantages of the dual rotor is -

ease of construction
no cogging
no iron losses.

The drawback is that it takes a lot more magnet to get the job done. You'd solve the drawback by doing this, but you'd probably kill our main 3 benifits, vibration and cogging would become a real problem, especially if it were for a wind turbine. I think before I went putting iron into the stator on one of these I'd look to a single rotor design.. or - start messing with converting existing induction motors which makes for real efficient use of our magnets. Just my opinion ;=)

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#8)
by jimovonz on Thu Feb 03, 2005 at 12:54:46 PM MST

Thanks Dan. I think my $$ situation coupled with my ambition have edged me out of the 'easy route' :) Unfortunately the magnets to do the job properly would cost me literally $1000's - with freight to NZ another significant chunk (the only local supplier of neos I've found charges close on 10 x the going 'international' rate...Hmmm - maybe an opening??). I'm sure there are a few folks that would try for something 'a little bigger' if only the cost were a bit cheaper (no dig at you guys - you offer some of the best prices around!). In my case a single rotor design would require a massive laminated backing that would hard to make and very heavy (as if the thing isn't heavy enough!). I could go to an axial flux setup to aid construction but it would all be additional weight hanging off the front. I'm already worried about the weight distribution - to the point of considering a tail/counter weight where I was considering a mechanical drive for yawing. An induction conversion may work - if I could find a doner motor larger enough - however I think you'd be really pressed to get the kind of direct drive output I require at the low rpm's I will have (10kW @ 200rpm target) with anything smaller than the setup I already have. Output shaft size with such a conversion may be an issue too...

[ Parent ]

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#4)
by wooferhound on Thu Feb 03, 2005 at 10:15:00 AM MST

I see that you measured the power output from your coils
but I don't see where you measured the power input to the rotors with the magnets
I'm sure you will find that the gain you get in the output voltage, will be more than canceled out by the amount of extra torque needed to spin the roters. Not to mention that startup will be MUCH slower & harder.

)}=- W o o f -={(

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#9)
by jimovonz on Thu Feb 03, 2005 at 01:04:57 PM MST

Hey Wooferhound. For the flimsy setup I had, I was barely able to keep the laminated coil still without any load. If I had loaded it up at all she would have all been over quick smart! As you can see from my graph, I measured open circuit volts only - no power measurements. The efficiency of the laminated setup was one of the uncertainties I had - though I don't think it would be quite as bad as you make out, after all, laminations are used in highly efficient motors all the time! I don't expect that I could elimate cogging totally, but I'm confident that it can be much reduced with the appropriate design. I am leaning towards using a pitch control mechanism for my blades so with the appropriate startup pitch, 25+ft diameter blades should provide a fairly impressive torque.

[ Parent ]

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#11)
by tecker on Fri Feb 04, 2005 at 03:57:44 AM MST

Laminated cores under load will stall faster .You talk more about horsepower requirements with iron cores .The voltage of your load has to be balanced with the voltage output of the coil set (Single two phases or three phase) . Low wind or intermittent wind patterns stall easier with a large rotors . The coils with air core will hit peak charge with the magnetic moment and and bounce the rotor along without the drag from the iron reluctance . So you can convert higher induced voltages to current with less drag. So using more copper and more phases is the trick with large rotors and the inertia of the rotor and prop remain longer and can do more work through different wind patterns . drop down the size of your rotor and the mechanical advantage (vector angle) to tdc of the laminates is easier to maintain .

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#12)
by Running Blue on Fri Feb 04, 2005 at 12:23:09 PM MST

I had read awhile back about a new material being used inside coils in generating applications that had some interesting properties. The material was described as being "amorphous" in that the molecules weren't aligned like iron is but still had magnetic properties. Probably similar to the iron filings that Paul was using. The advantage was less attraction to the magnets but it still showed good saturation. Heat issues may have been decreased as well. The amorphous material (seems like it was iron impregnated glass?) was still, of course, solid which would have the same tendancy to vibrate, but to a lesser degree.

Now, with the vibration problems of a solid laminated or amorphic coil filler, would it make sense to use a ferrofluid filler in the coils? I can see it jiggling around loose inside a sturdy plastic pouch or case, but it would be a solid fixture mounted to the stator. The ferrofluid would still become saturated by the magnets, but any cogging shouldn't be strong enough to bog things as much as laminations would. Heat might become issue as the fluid would be in a state constant motion.

Dunno, might worth experimenting with!
-Blue

Re: Dual Rotor Alt - with laminations?? (3.00 / 0) (#13)
by electrondady1 on Thu Feb 24, 2005 at 10:49:26 PM MST

jimovonz, don't know if you'll see this so long after this thread but an aussie has your mags. check out the free mag posting

Dual Rotor Alt - with laminations?? | 13 comments (13 topical)

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