Review my assumptions about brushless motors - RC Groups
Jan 05, 2005, 07:02 AM
Registered User

# Review my assumptions about brushless motors

Hello!

I'm a beginner to RC-flying and total newbie to brushless motors. I've done some reading and came to the following assumptions, please correct them if they are wrong.

1) The ampere which a brushless motor needs depends on two factors, the propeller and the number of windings.

2) The bigger the propeller, the more ampere are needed to turn it at the RPM which the motor would like to turn it.

3) More windings mean that the current takes longer to flow through the windings, the poles change slower and thus we get less RPM

4) Thicker windings mean that the more ampere can flow through the windings at the same time. So if you want to turn your motor fast (low number of turns) and/or with a big propeller, then you need sufficiently thick windings in order to withstand the needed current flow.

Additionally I also have a question. I'm considering whether to get a Phoenix 10 or Phoenix 25 from gobrushless.com with my kit. Given the above assumptions (and given that they are not completely wrong) I can calculate the current needed by my motor/propeller configuration. But what about the blocking current (i.e. the current which flows when my propeller gets stuck due to a crash or something). Is there a way to calculate this?

Many thanks and sorry for the lengthy post,
Michael
 Jan 05, 2005, 07:35 AM Crash Master Your summary is close enough to the theory to work I wouln't bother with the 25A ESC for a single stator motor. Unless you are going for a very high speed, small prop, like a ducted fan or if you plan to use a geared drive, there's no way you will be able to run an efficient single over 10A. The winding resistance is just too high on a 15 or so turn motor or up to be able to withstand >10A without getting too hot. The current limit on the ESC should protect itself from a stalled motor.
 Jan 05, 2005, 04:35 PM Registered User Some math facts about proppeler power absorption... -doubling diameter requires 16 times more power (spinning a 12X6 at 10000 rpm requires 16 times more power than a 6X6 at the same rpm) -doubling rpm requires 8 times the power (it takes 8 times as much power to turn 12X8 prop at 10000 rpm than it does to spin it at 5000 rpm) -doubling pitch requires 2 time the power (12X12 prop requires twice as much power to spin at a given rpm than a 12X6 prop does) Remember basic formula: watts = volts X amps (P = U x I) Regards, Hrvoje Last edited by hrvojekl; Jan 05, 2005 at 04:38 PM. Reason: error
Jan 06, 2005, 09:44 AM
Registered User
Quote:
 Originally Posted by Daneel Hello! I'm a beginner to RC-flying and total newbie to brushless motors. I've done some reading and came to the following assumptions, please correct them if they are wrong. 1) The ampere which a brushless motor needs depends on two factors, the propeller and the number of windings.
Mostly wrong. But in teh case of a foxd input voltage and fixed magnet type, ,ore or less true.
Quote:
 2) The bigger the propeller, the more ampere are needed to turn it at the RPM which the motor would like to turn it.
True.
Quote:
 3) More windings mean that the current takes longer to flow through the windings, the poles change slower and thus we get less RPM
Wrong reasoning, right result. The back EMF teh motr generates is proportional to the flux density (magnet strength) times the speed (RPM) times the number of windings. More windings generates more back EMF and therefor the motor will reach its voltage limited speed at a lower RPM.
Quote:
 4) Thicker windings mean that the more ampere can flow through the windings at the same time. So if you want to turn your motor fast (low number of turns) and/or with a big propeller, then you need sufficiently thick windings in order to withstand the needed current flow.
As above. Thick windings havbe less resistance and run cooler, allowing ore power into the motor for the same heating.

Quote:
 Additionally I also have a question. I'm considering whether to get a Phoenix 10 or Phoenix 25 from gobrushless.com with my kit. Given the above assumptions (and given that they are not completely wrong) I can calculate the current needed by my motor/propeller configuration. But what about the blocking current (i.e. the current which flows when my propeller gets stuck due to a crash or something). Is there a way to calculate this? Many thanks and sorry for the lengthy post, Michael
Stall current is pack voltage over the winding resistace, plus pack resistance, plus ESC resistance.

Generally enough to blow any ESC if you leave it full throttle with the prop jammed, (and the wiring, and the pack, and the motor)

Short answer. Yank the stick back before you crash or risk a fire.
Jan 06, 2005, 10:26 AM
Registered User
Quote:
 Originally Posted by Gene Bond Your summary is close enough to the theory to work I wouln't bother with the 25A ESC for a single stator motor. Unless you are going for a very high speed, small prop, like a ducted fan or if you plan to use a geared drive, there's no way you will be able to run an efficient single over 10A. The winding resistance is just too high on a 15 or so turn motor or up to be able to withstand >10A without getting too hot. The current limit on the ESC should protect itself from a stalled motor.

.... blurch ....

The suggest statement it quite missleading ... it is correct only for very light airplane ... to be flown in almost no wind situation ....

There are a hell of parmeter related to the power involved in a brushless motor turning a propeller ....

Brushless ESC and motor are not 100% cheap, and if you want the best you have to match with the right LiPo .... so lot of parameter are involved ... and there are high risk for make mistake on set up the power gear for your airplane ...

For be safe, I would suggest to download Motorcalc , update all the Motorcal DB also with contributed data ... and then start to make experiment with motor and propeller ... and you will see how much Amp are drawn for different environment and set-up.

e_lm_70

P.S: Personally I like simple approach ... so I like direct propeller connection ... this avoid warring about the gearbox and gear ratio ... anyhow a gearbox add some weight ... and the new high torque BL motor work fine without any gear reduction. A lot depends by your airplane ... 200g or 2000g airplane ? ... In general up to 300Watt BL set is relative cheap ... HiMark BL motor, and Align BL ESC will cost less then 100\$ (40+55) ... considering that 3S Lipo is almost the most common high power high efficiency approach .. I will got for a 30-35 Amp rated ESC .... but this assume a airplane in the 600-1000g ... and very aerobatics 2 3D fly stilye ....
 Jan 06, 2005, 03:02 PM homo ludens modellisticus @Michael www.rclineforum.de ??? Cu, Ron van Sommeren
 Jan 06, 2005, 03:02 PM Registered User ... About propeller blocked due to a crash or something else ... most of the BL esc will not been able to init the start up procedure ... and nothing will happen .... no damage ... no overload of the motor or so ... e_lm_70 Last edited by e_lm_70; Jan 06, 2005 at 03:22 PM.
Jan 06, 2005, 03:06 PM
Registered User
Hi Ron,

Quote:
 Originally Posted by Ron van Sommeren @Michael www.rclineforum.de ??? Cu, Ron van Sommeren
I'm not sure what you are refereing to? Please enlight me.

All the best,
Michael
Jan 06, 2005, 03:31 PM
Registered User
Quote:
 Originally Posted by Daneel All the best, Michael
Hi Michael,

I didn't see that you are from Austria as well ... servus ...

Being around Eisenstadt I'm quite far from you ...

About BL ... I will suggest you to get the X-Power (HiMark BL motor) from www.Lindinger.at ... they are cheap , and Lindinger is the big and more efficient eShop in austria ... vielen besser then DerSchweigofer ... for the ESC ... the Jeti are the cheaper locally available ... and they work relative fine.

I will suggest you to go on www.motocal.com .. and download the free trial of the software ... it is very accurate and you will learn a lot about powering an airplane ... better then know a RC guru ...

e_lm_70
 Jan 06, 2005, 03:50 PM Registered User Servus :-) I will check them out. But I'm really keen to try to build my own ... so I'll probably go with a gobrushless kit for starters. As for the ESC, I might order a castlecreations 25 from gobrushless as well, at the current dollar/euro ratio it's cheaper than anything similiar over here. Just have to take care to have the value below a certain value to not have to pay taxes ... I think it's below 50 Euros, so it's close ;-) Edit: It's below 22 Euros ;-( Last edited by Daneel; Jan 06, 2005 at 03:53 PM.
Jan 06, 2005, 05:43 PM
Registered User
Quote:
 Originally Posted by Daneel Servus :-) I will check them out. But I'm really keen to try to build my own ... so I'll probably go with a gobrushless kit for starters. As for the ESC, I might order a castlecreations 25 from gobrushless as well, at the current dollar/euro ratio it's cheaper than anything similiar over here. Just have to take care to have the value below a certain value to not have to pay taxes ... I think it's below 50 Euros, so it's close ;-) Edit: It's below 22 Euros ;-(

I know quite well the story about taxes ... it is below 25-30\$ that there is no taxes to pay ... Already with 36\$ from Japan I had to pay 7 Euro for custom taxes ... of course you could ask to the shop in US to make a fake invoice ...

My experience tell me that ordering from US to Austria it could be a long pain ... so I would avoid it !

A Jeti 30Amp cost just 61 Euro in Lindinger + 2 Euro shipping ... in front of your door in 2 days !

A X-Power motor cost from 35 to 69 Euro ... from 130 to 700 watt power ... from 40g to 250g weight ... I will not warry to build one .... since could be a long, expensive and risky process ... the XPower or HiMark are quite good clone motor (mainly copied from hacker) ... and offer efficiency between 75% to 90% ... the 2815 and 2825 series are quite light and can offer around 85% efficency .... just some little % less then equivalent hackers ...

If you want a nice BL ESC .... then try to get a Align 35Amp BL ESC ... it should cost around 55\$+4\$ shipping ... a good alternative from the Jeti .... anyhow Lindinger is going to stock also these china made ESC for a reasonable price ...

Happy fly

e_lm_70
 Jan 06, 2005, 05:59 PM Registered User Michael, ... since you are a beginner ... I would suggest you to start with an EPP airplane ... since every beginner will crash ... and most of the airplane after a crash need hours for being fixed again .... but the new EPP airplane will bounce without a single scratch .... again from lindinger you can get the EleBee from Windrider for just 59 Euro ! My current most flown plane now it is: EleBee + Himark 2825-2700 + 6x3 propeller (5.5x4.5 when I want high speed) + Jeti 30 Advance + 3s2p LiPo/LiIon (Kokam 910 for ultra power or Konion 1100 or XCell 1400 for more fly time) ... man it is a super set up ... with Xcell I can fly around 12minutes ... with lot of loop and oder acrobatics stuffs.... I have also added in my plane the Fuzzy Gyro (59€) for help my still little skills ... and even with strong wind (today was over 60km/h wind in Burgenland) ... I can fly stable without any problem ... I have also to make my first fly with my new ACT FoxBat (34€ airplane) ... it is not an EPP, so I'm waiting a calm day ... but it is should be funny with my HiMark 2815-2000 + 6x4 propeller + 2S1P Konion 1100 (as a starting point) e_lm_70 P.S: About ESC sometime on eBay you can get for a good price the Schulze BL ESC ... they are the best possible ESC, the only one that can be used in all environment (from Car to Airplane via Boat & Helicopters) ... I got my 11.30 ESC for around 50 Euro ... not a bad deal for a almost new ESC with the latest software inside (just few months old ESC) ...
Jan 06, 2005, 06:12 PM
Registered User
Quote:
 Originally Posted by Daneel Additionally I also have a question. I'm considering whether to get a Phoenix 10 or Phoenix 25 from gobrushless.com with my kit.
Hi Michael,

I have checked the kit ... clearly these CD-ROM bruhless motor will smoke with more then 10Amp ... so the Phoenix 25 is useless ... but don't expect magic out of these little BL ... they are very nice for very little airplane ... but they don't pump more power then a Stock 400 brushed motor ... with not so different efficiency ...

As well what the sense to save some little \$\$ in the motor, if then you have to pay no less the 58\$ in a BL ESC ?

If you tell us which airplane you have in mind, motocal or us can help you on finding the right BL set-up

e_lm_70

P.S: For every absolute beginner I will suggest to start with a 48" wing made of EPP with a pusher motor ... and as well I will learn to fly first with a brushed motor ... for then go to a brushless ...
Jan 07, 2005, 06:42 AM
Registered User
Quote:
 Originally Posted by Daneel Hi e_lm_70, I'm currently flying an EasyStar, I've succesfully flown it a few times and I think I can handle it to some extend now, even did some loops already :-) ... Your note about lindinger stocking the Align ESCs is very interesting. I've read the thread about them. Do you know when lindinger will have them?
Hi Michael,

The CD-Rom brushless motor in a kit are very light, bust as visible in their site will not deliver more then 50watt, and with an efficiency never more then 60% ... so they are comparable as performance as a stock 400 motor ... you don"t gain nothign more then 30-40 gram ... nothing in a medium size foammy plane.
As well if you see who is using these motors, you can see that maily are Micro-Helicopter people ... with some sub-micro indoor airplane ... I will personally stay away from this unefficient micro motor.

About performance ... man my EleBee does consecutive loops with easy, fly inverted and almost ulimited vertical ... with torque rolls ... top spead around 100km/h and great gliding capabilily .... so I can do all the aerobatics stuffs with it .. even without crash at all in the recent time .. not bad for my crappy RC skills ... thanks to a real brushless motor and my 3S Lipo !

About Lindinger ... it look like that at the moment (and as well yesterday) they have technical problem ... but in the past I have seen that was listed a sequence of BL ESC from china with similar specs as Align ESC ... they are still not available and the only comment available was that this was part of the new thing from the 2005 catalog.

e_lm_70

P.S: I'm in process to get a Align 35 ESC for 53\$ from HK (plus 6\$ shipping + 2x30\$ 1800LiPo 3S, not a bad deal) ... since all my BL ESC with BEC are just capable at best of 30Amp (I gain a bit here) , and 10cells (so up to 3S), while the align is capable 4S Lipo ... so I can enter in a little new area here.