Efficiency and engine size

[extreme]

Hello Diy'ers. For some time now i'm in building my own brushless motors. I've made about 30 motors till now. Now I want to make a good motor for myself (2s 10 amps max)for a shockflyer. When I say a good motor, I mean light with good eficiency. My latest small motors make 280 grams of thrust from about 35 wats (indor models, 2s lipo). The motor weights 18 grams. I think i'ts i good result. I dont know how to measure motors efficiency, and would like someone to teach me . Anyway, back to the topic. If I will double engine size, would i get twice as much thrust from double power ? ? From my experience- I don't . Could someone explain me why ?

Cheers,

Robert

[swatson144]

if it's any condolence I don't either. :? I have a vague idea of why..but I don't know how to explain it. I can give you my perceptions. LBmiller could probably answer this with one of his excellent posts. Lucien's kung foo very good...my kung foo sucks :) .

It seems that you double the stack size of the motor you don't get near double current or double the thrust. Guessing at it I'd say by experience if you leave off about 1/3 of the turns or go from wye to delta you start getting closer. Does this jibe with what you have seen?

ie you have an 18t 12p WYE at ~ 10oz 4amps and you'd like to double it so you stack another stator on and get like 14oz at 6.5 amps roughly. After changing prop size.

[ScubaSteve]

Well with these small outrunners, when you're stacking stators the performance increase is definitely not linear... it's more of a curve until you reach that point of diminishing returns. To get significantly noticeable more power, you need to increase the diameter rather than the length. :wink:

[extreme]

Thanks for input guys.
Another question then. If i go from 20 mm stator to a 25 mm how much more powerful would the motor make (assume the thickness is the same )? I want to squeze as much efficiency as posible from 70 w motor that i'm planing to make. Any sugestions on this one would be nice. I want to get as much of thurst as posible from 2 cell lipo and 10 amps (prop: 8-10" diameter gws apc ...any available). Of course motor must be light as posible (as always). Now i have 25 mm and 20.2 mm 9 arm stators. I also have 36 mm drill rotor tha has 12 teth but its kind of heavy. 10 mm thick and 36 mm in diameter weights 36 grams, after removing some spare metal it would weight about 28 grams. i can get it thinner, lets say 4 mm ?
Some time ago, I was making an ultra light motor for an indor model, it was 25 mm i diameter and 3mm thick. It didn't have efficiency i needed so it was'nt in use. Any thought's how a 36x4 mm would work ? I'm too tired of trying new designs of motors that are no good. Maybe you can tell something on this topic from your experience?

[swatson144]

Going from 20mm to 25mm my experience shows you'll get close to 20% more. Pretty much for free as current will remain about the same. one of my motors would be a good candidate for your use just wind 15-16t instead.
http://www.gobrushless.com/testing/motor_test_details.php?Motor_ID=69&email=&password=&sort_dir=ASC&old_order_by=&ORDER_BY=2

If you have a lathe you can easily make it a 25mm without the segmented ring magnet. I use that motor because it is easy to make (well less time consuming) using gbx parts and not having to make the parts. works great on park flyers.

Don't get too twisted around the axles about motor weight. You pretty much need magnetic mass to make good power. One of my 10p 25mm motors weighs 65grams which sounds heavy compared to a 45gram gbx2 and is the same weight as a ips 350 rig...but I'can get 34oz thrust from it. :D so what is an extra ounce of motor?

If you are really married to a prop size and don't want to build eperimental motors, go through the data base and look at what people are getting from the same prop and how they are getting it. Decide which will work best for your use. Remember that the most thrust is not necesarily the best (most often) compare a motor that gives 15oz at 12amps to one that gives 14oz at 6amps I'd generally give up the 7% thrust increase for double flight time. Or if I had to have 15oz for a 3d I'd go with a heavier motor that gives more thrust for less current ie a ounce heavier motor that would give me 17 at 8a...I'd still be an ounce ahead and have 50% more flight time.

[extreme]

I kind of agree with motor size weight and magnet quantity theory. But i would like litle more than this. Say 19-20 oz... Is that posible ? I'm now thinking that maybe a 25x6 mm stator, with 4x6x2 mags would do the trick ? I think that maybe posible, because my tiny gem ( http://public.fotki.com/extremeflying/belekas/pict0027.html )produces 10 oz of thrust @35 wats of power. I'd say, motor weight uder an ounce would be nice. Is that posible ? Also i'd like to try an LRK type motor, but shiping from gobrushless cost a lot (for me atleast, I'm from lithuania). I buy my magnets and stators from aircraft-world (shiping only 2.5 $).
Thanks for help.

Robert

[swatson144]

I made a 25x12mm mainly because the qnty break on those magnets was 29 vice 49. 30 75 cent mags easier for me to justify than 50 10x5x2mm .65$as I might make 3 motors this big but 5?

anyway that motor is here.

http://www.gobrushless.com/testing/motor_test_details.php?Motor_ID=71&email=&password=&sort_dir=ASC&old_order_by=&ORDER_BY=2

Just guessing I'd say if you made it a 6mm stack and dropped it down to 14t to swing a smaller prop. it might do it. It'd weigh a bit over an ounce though. If you are gonna stay on 2 cells maybe drop down to 10-12t.
If you left it the same but with a 6mm stack I'd guess you'd get about 65-70% of what I listed changing torque/speed with lower turns would get you closer to the prop size you want and make a more effecient motor in that power range and get some of it back. I really don't have enough data to go on...I played with the motor sim and can't get it to say more than 8oz 2s lipo 8x4 but I'm probably under guessing the KV of the smaller motor. My gut tells me you'd be close.

[extreme]

swatson144, I would like to stay with 2s. I could go for 3s but i don't want to...:) What prop would do for that engine ? (I will be making an order today ) . And the last question: what motor sim are you using ? I even did'nt know they are available .
Here is the info on that litlle one : Stator 20.2x6, mags 24 6x3x1, 34 delta, 8x4 gws hyperdrive or 8.43 sf (similar results). Don't have a tach (so no Kv), Io- not measured, weight 18 grams with prop adapter.

Cheers

Robert

[swatson144]

Open up a motor in the database and if they filled in the specs you can click on " open motor sim with these specs" Well you can even if they didn't but it would be blank.

I was trying to get you down to an 8x4 ish prop. If you want to get up around 20oz you are really starting to want 3s. I stayed at 2s for as long as I could and then broke down and paid me money.

It seems you can get 15 or so from 2s but getting 20 doesn't seem very easy. In fact there only seems to be 1 and judging by the specs I'd guess it to be a typo. motor #10

Are you sure it isn't a 9 arm? something about the specs don't add up to a running motor. I assume the 24mags are 12 pole nnssnnss etc. a 6 arm won't run with 12p or 24p.

[extreme]

20.2x6 means stator diameter X thicknes. It has 9 arms and yes, magnets are nnssnnss,,,,. The fill all of the space in the can (i know about 80%.. ) , you dont need to align them, just put em in and glue. I dont kinda agree with the sim. It does not simulate my motors even close. Now i have a motor that is a bit poor im quality that turns a 12x6 prop thru 6.6 reduction on a gws gears and produces 20 oz. That motor had thin mags, there was stil space for more copper and the shaft was hand sanded for the gear to fit. Also the gears were to tight.
I would like to learn to calculate motors efficiency. Could you teach me or post a link ?

Thanks Robert

[swatson144]

Ah my bad! I was posting the same way 25x12mm 12a etc. I just jumped to the wrong conclusion.

I just had another realiztion...that is the same motor that started all this for me. I ordered it and a speed controller that came in from DAVID,AC or something like. I guess I have LRK on the brain now.

JayC has been working on a motor calculator that currently only works with 22.7 9arm.

http://www.gobrushless.com/testing/brushless_calcutort.php

I'd sure like to find a good calculator with 6/12 arms or maybe as I get more motors built I'll be able to kludge a reasonable factor for my motors. to compare "what ifs" Currently on the 25mm 12 arms I just go with gut feelings (I assume as does every one else)

Funny about the reduction gear I just tested my 1st between posts while witing for the garage to warm for glueing.. I wouldn't call it a success. I was running it with a 22.7 x 5mm 9 arm 14t Wye. I know I can get better results with the motor direct drive. I'll get about 8oz from a motor like that with 18t on a 2s at 4A. I had all them ips 350s laying around and it took about 10 mins with the lathe to kludge it together. I think the best I got out of it was 150g at 3.4A 40w on a 3s gws 12x6.

[extreme]

What was the reduction ? I had that engine from ac (diy with 20mm stator). It's a nice engine, it puled on tu cells and 5 amps , gws 4.5:1 gear and gws 11x8 400 grams of thurst (i think). Anyway i liked it (just too heavy).
I dont like gears because of the sound they produce. Direct drive is just like quiet music :)

[swatson144]

spur had a "C" on it. I usually just toss the ones that came on kits in the corner...so I don't know much about them. I agree heavy and load and no real help.

[extreme]

You were so right about that motor ! Today I made 24mm x 6mm thick (it was'nt 25 afterall). As you predicted, it made a bit less than 400 grams...
I realy hate to go to 3s... I'll make some test with a 20.2 x 12mm stator motor (12 poles), and then make my mind. As i was playing a bit with the sim, I realized that now it is wound (20x12mm) for 5000 rpm/v (it might be that I misunderstood). From prop thrust calc i get 3900 rpm/v under load. Any guess what prop 20x12mm stator would spin with good efficiency without gears (rewound ofcourse) ? I still get 550 grams from it @ 70w input :) But it's kind of heavy ... (abot 60 grams with gearbox). I also tried same setup with 10 volts, and @ 10 amps it gets me 4 ounces more ... not much to change my mind.... Man it's hard to decide where to go when budget is'nt as big as Bill Gates.

Your suggestions are very welcome.

Robert

[swatson144]

Best suggestion I can give is put your weight in diameter and not length, wherever practicle. I think you'll be kinda underwhelmed by the 20x12mm. You didn't mention # of turns to guess at a prop. Though a high turn (18t) delta termed, double for 2s, is a very expedient motor to build and gives a very efficient motor for 3/4 pound park fliers using a 8x4 to 9x7 prop (in 22.7 mm anyway). I don't really try to build the baddest motor I can (too expensive) I decide what my minimum acceptable standard are for the job and try to build to meet or slightly exceed them. For instance I like park fliers to have > 1/2 thrust to weight so I'll build very few in the >1lb catagory. I'd have to build a time consuming/expensive motor for each...or draw so much current it won't stay in the air for 10 mins. I think you are in agreement with me.

well the epoxy should be hard enough to finish mounting the gear in my formosa so I'd best get that done.

[extreme]

Man I'm geting disapointed. I've just teste speed 400 with 3.9 reduction and 11x8 prop. Guess what, @ 7volts and 8 amps it gives 450 grams of thrust. Mine gets me 550 grams @ 10amps. The difference is just in setup weight (twice heavier). Ofcourse, with 3 cells mine blows 1 kg, but my target is efficiency.

[extreme]

I'm finaly getting somewhere... I just tuned up a bit the motor from aircraft world (the purple one;they dont sell it now). Original stator is 4.7 mmin height, i added more statorplates to get 6 mm. Wound it 6 delta (20.2 mm diameter) as I needed rpm. Funny thing, the controller spun it only to half of throtle and then lost sync. I think it's because i'm exeding 100k magnetic rpm :) . So I went for wye connection. It went fine. I pressed 6.6 reduction pinion, put in a gearbox, and with 12x6 prop i'm getting 450 grms (7v,7A). Went a volt up and get 570 grams (Makes difference for 4000-5000 Kv motor), so It was 8 volts and 9 amps = 72W, just what im looking for. These litle motors are super, weight 30 grams, price 30 $ (I got mine for free), superior quality and so on... Anyway, i realy want to thank you for your help, I will use this info for future projects. Now I have a nice powerplant and 4 lipo pack's, lets do some flying ! (3d flying that is 8) if i didn't mention...)

Robert

[DadHav]

Hi guys, I just breezed through the post. I wanted to add that I've tried my hardest to get something worth while out of the small 20mm stators. I have a triple that I tried jhust about every combination on, and I gave up on it as soon as I tried something with a larger diameter. I'm in agreement with Swatson about going up in diameter. I have never been overwhelmed by stacking. I do have a 25mm triple LRK that gives me 37 oz of thrust on a three cell lithium. I have two single 25mm LRKs and a 25mm single 9 tooth. I'm testing and plan to use them with three cells. If I get caught up on things a little, I'll do some testing with two cells and get the info posted for you. Here's something that I'm seeing though: Going up in diameter really gives some power, but keep in mind the style plane you're flying. If I'm not wrong: You will have much less air speed. This is fine though for park flyers and shock flyers, etc.
John

[swatson144]

I'm in the planning stage of my next motor. I want something smaller for a small 3d. So as soon as I can get some garage time I'm making a 25x5mm 12 arm 8pole 10t Wye should get some speed up from it. If not I'll know what I have after I test it and build a plane for it.

[DadHav]

Hi Swatson, I can't wait to see how that turns out for you if you go ahead with it. I tried almost the same combination and then tried to get answers as to why it vibrated so much until 1/3 throttle. (Post: Why the vibrtation?) some thought the can being made for 10mm long magnets may be deflecting and causing some sort of harmonic that fixed itself as the speed increased. This motor had 8 poles made from 16 magnets. I hope you have great results, because that would mean I just did something wrong again, and would have a reason to try the combination over again.
C YA

[swatson144]

I don't think you did anything wrong building that motor..I'm gonna go 8p with 8 5x5x1.5 I have about 40 left over from another motor. I think you have magnets spanning poles. I had the same symptoms on a 14 pole but it was probably that I needed wider than 10x4x2 I used because I had them left over. I'm hoping to get another piece of the puzzle with this motor...puzzle being why do some motors run so darn smooth at low rpms, while others clack and jitter. I'd like to build another 14p just like the other but use some of the segmented ring or 12x5x2 and see if it smooths out any. with 2 motors I can switch cans and see if the problem moves with the can or stays with the windings. Unfortunatly I have no use for a motor that needs a 13x8 prop, so It's not on my priority list :)

Hmm If what I said makes any sense you should realize I am thinking that there is more to magnet selection than just trying to fill the can. Too narrow and they just want to lock over the stator arms (almost but not quite like 12a 12p), too wide and you have a chance for them to get in a position where some arms are attracting/repelling axially vice rotationally.

If the little motor runs rough at low rpms I'll have enough use for them that I'll experiment with NNSSNN or wider/narrower mags to smooth it out. At least with the smaller motor I think I can assume that it's not shaft flex etc. All I gotta do is build it and get it sorted out for the power range I want then worry about proving I'm wrong.

I may find out that what I want is a very low turn 14p after all. I'll have a nice grasp on this within the next 20 or so motors. :lol:

[latrans]

Swatson,

Good thinking! That had never occurred to me, I'll be looking forward to seeing your results.

latrans

[extreme]

Finished... Wound it 5 turns wye, 5.4 reduction and 12x6 sf prop. @ 7volts
and 13 amps it gives me 630 grams of thurst. Not bad. In reality performance will be lower, because battery voltage drops below it's nominals (dono how much). I'll try it with charged battery tomorow (test were made with power suply). If current will be to hi, I'll just prop down.
On your topic, the more are you fill with magnets, the smother the motor runs. In my mini motors its filled 100% (maybe 98% :) ), and the work nicely with all rpm range. Also I have noticed, if the area is 100 % filled with mags (i.e. they touch eachother) coging is bigger (much bigger), but runs smooth. I also depends a lot from motor (teeth, poles, magnets).
DadHav, if your motor vibrates at certain rpm, then it a resonance. I could be solved, as i think by making bell wall a bit thicker (to shift resonance frequency somwhere else) or fill the gabs between magnets with some epoxy. I assume dissbalance is out of question. Also, as swatson144 said, it may be from linking magnets (NNSS) which is most likely, especialy on lrk winding. (sorry if it kind of hard to read...)

Robert

[LBMiller5]

Extreme,

I have been putting together some info on motor efficiency, pole saturation, flux density, and other parameters that control the output of our motors. I have also put together some charts and graphs to illustrate the information and make it easier to understand. This will probably end up being one of my longest posts yet, so it will take a while to put everything together.

The info will most likely deserve a new topic, so I will post a link in this thread when I post it.

Lucien

[extreme]

That would be, and will be great LBMiller5.
Thanks

Robert

[swatson144]

Extreme,

I have been putting together some info on motor efficiency, pole
Lucien

I look forward to it! It'll definatly deserve a new thread.

[extreme]

WOW, it's awesome, airplane weights 300 grams, thrust is about 600, vertical is awesome. It performs like it would run on three cels. It's realy a blast !

[swatson144]

Well I built my little motor 1 of 3? Preliminaries look pretty good. it runs smoothly at 2 clicks on the transmitter (I still need to ditch the clicks) 3k with 2s 5.5k on 3s. 1st click it don't even seem to trigger.

It's potted now and cureing. the no load stuff can be found #74 in the database. Note the magnets are 1.5 thick not 2mm thick.

One of the small strips of electrical tape flew off at about 25k. it got out of balance and near scared the doo doo out of me :)

I tried a 6035 prop and had to quit at about 1/4 throttle as there ain't enough magnet to hold it together, and the can was 2/3 off when I noticed it. It's now locked on so I'll test when the potting cures.

[swatson144]

John, if you still watching I got 22.2 thrust at better than 51mph ps 13.8 amp 6x3.5 gws hd. Idled to almost "second hand" speed. ran for a full min wot and never got warm (ir at 85F). Switched to the next size up on hand 7x5 apc TE and noticed I got 19.5 oz couldn't tach it because of the smoke, cooled it down and will re wind 12t. sure was a fast little motor. you should be able to see #74 now that it ain't blank and may help you in your pursuits. I'll rewind it and try it 8 pole 8 mags again, and then 8/16 as planned. I've got the parts turned for another motor...GAWD what an ordeal...plaqued by dropsi, lossi, etc I was like all 3 stouges in 1. I'll wind that one 13t wye. Well atleast I know not much more than 150W pi for that motor.

[DadHav]

Hi Swatson, I'm still around. That looks promising. That might work for me if I can get a battery to put out that amperage and still fit in the small plane, Here's the thing that I'm having trouble comprehending: If I'm lucky, I might be able to get the racer in the air with a combo like we are talking about. On the other hand I'm flying a Vermont Bell weighing in at about 24 ounces with a motor that uses no more than 13 amps but delivers unlimited vertical flight with 34 ounces of thrust, and a 15 minute flying time. Maybe it's more practical to fly the slightly larger planes. I'm so confused. I'm gonna build another airplane.
C YA
John

[swatson144]

Yeah I think it was a bit too few turns on a 5mm stack probably would have been right on with a 10mm stack...At least I got to check that configuration at low rpm which is what I set out to do. I'm hoping 12t will tame the current thirst and still give plenty of speed. It's gonna stay a 5mm stack since I can make another identical one from parts on hand and play with magnet configuration. I think 12t would be more practical. It may have done a little better with 16 mags 8 pole, but I can still test that at 12t and maybe back up later.