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Old Dec 29, 2012, 07:42 AM
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Testing motors with no supplier data except kv

Hi again,

I've been into electric flight for some years now and own and use a watt meter and test rig on newly purchased motors. Usually I'm looking for a usable prop size that keeps me within the stated maximum amps/power for the motor. I start with the suppliers prop recommendation and check from there.

For the first time I've acquired a motor for which no maximums are given (for interest a KD 2217 1600kv, but my question is more general). So I now have to determine an appropriate prop size from scratch.

Lets assume I never exceed the max motor RPM for this question.
I'm using 3s Lipos for all tests and an adequate ESC.

Q1 - Is it the case that I can start with a small propellor and work up, testing for, say, 30 seconds at a time until I reach the point where the motor gets quite warm to the touch, then assume the previous prop size is a good start for "in the air" testing? If not 30 seconds, how long?

Q2 - Is it the case that different wind/different Kv motors from the same maker and identical in external size and appearance can/will have radically different maximum current/power limits? (In this case, another KD 2217 has a power limit of 220w and I was worried that my initial testing of the new motor was giving a much higher reading without seeming too "stressed".)

Q3 - If I was to try different lipo configurations (say 4s) in this kind of testing, is the impotant maximum figure power (watts) or current (amps) - I'm assuming power. Is that correct?

Thanks for all answers.
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Old Dec 29, 2012, 08:24 AM
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Most important thing is HEAT.

Testing on a bench CAN BE USELESS.........Why ??............More than enough airflow in a cool basement or air condtion room.

Crammed into a plane with not close to enough air flow on a hot day is a fast & totally unexpected death.

The really limiting value to me is ...........under ideal cooling conditions............Wire size maximum current of the DEEPEST windings. The ones at the bottom of the winding slots.

Only way to measure a motor being pushed. Is your nose. A chemical smell after 15 seconds of " ideal " cooling at WOT tells me everything............The first signs of overheating will be from those windings at the bottom of each slot. I keep increasing the run time by 15 second steps.

Dr Kiwi does have some excellent watts per motor weights.

Good CAREFULLY hand rewound motors can produce scarry powers for their size
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Old Dec 29, 2012, 09:04 AM
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[QUOTE=deustace

Q3 - If I was to try different lipo configurations (say 4s) in this kind of testing, is the impotant maximum figure power (watts) or current (amps) - I'm assuming power. Is that correct?

[/QUOTE]

No. Not correct.

The genuine limit is TEMPERATURE.
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Old Dec 29, 2012, 10:12 AM
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No data except Kv ?

The suppliers do provide more useful information. Namely, motor weight and recommended voltage.

These three items [wt, V, and Kv] are sufficient to make a fair estimate of......... usable power-in..........rpm at WOT..............prop size.
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Old Dec 29, 2012, 10:30 AM
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You can also only buy from a retailer who HAS & POSTS motor data very completly.

I use www.bphobbies.com Look up their own brand of motors specs. Piece of cake to pick the motor to do the jobs.

I have locked into 3 motors for all my flying.

Rick
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Old Dec 29, 2012, 04:48 PM
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Quote:
Originally Posted by peterangus View Post
No data except Kv ?

The suppliers do provide more useful information. Namely, motor weight and recommended voltage.

These three items [wt, V, and Kv] are sufficient to make a fair estimate of......... usable power-in..........rpm at WOT..............prop size.
Peter,

I do have this info but I don't know how to use it to project prop size, etc

* assume 3s lipo (not actually stated but I think a safe assumption in this case)
* Kv: 1600
* Dimensions: 27.6 x 36mm
* Weight: 71g
* Shaft Size: 3.175mm

Dougie
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Old Dec 29, 2012, 04:51 PM
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Quote:
Originally Posted by cyclops2 View Post
You can also only buy from a retailer who HAS & POSTS motor data very completly.

I use www.bphobbies.com Look up their own brand of motors specs. Piece of cake to pick the motor to do the jobs.

I have locked into 3 motors for all my flying.

Rick
Rick,

I'm based in Scotland and shipping/customs make buying from the USA difficult.

We do have a number of suppliers that specify prop sizes and power limits. It's just that I didn';t use them on this occassion.

Dougie
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Old Dec 29, 2012, 05:45 PM
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Hi again,

After another huge bout of searching, I've found an Ampeer article by Ken Myers saying that you can approximate the power capability of a motor by multiplying the weight in grams by 200 - 300 depending on how consevative you want to be Or 400 if you don't care too much about the longevity of the motor!

Is this really it? If so, I'm amazed I haven't come across it till now.

So my 70g motor will do between 140 watts and 280 watts......that leaves quite a wide area for confusion.

Still interested in views on the temperature method please. Dr Kiwi seems to use a version of this in his testing of new motors?

Dougie
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Old Dec 29, 2012, 05:46 PM
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Start with the basics....you can assume most motors are good for about 3W/g so a 71g motor should be o.k. for about 210W.

If you want to run it on a 3S you're aiming for about 80% of KV x V so that's 11V x 1600 x 0.8 = roughly 14000 rpm.

Now you need some experience or to play about with some of the "calc" programs like DriveCalc, Motocalc etc. Find a motor of similar size and Kv and see what prop gets it to around 200W. I'd guess that a 6" or 7" prop would be about all you could get away with on 3S. With that relatively high Kv you might be better off running it on 2S.

Steve
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Old Dec 29, 2012, 05:55 PM
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Thanks Slipstick.

This is exactly the kind of pragmatic approach I'm looking for.

So, in this case, would you limit prop at 210w, or would you, for instance, be happy to higher if the motor was running cool?

My guess is that, if pushed, this motor will run about 270 watts without seming to overheat but I've not fully tested yet till I had this discussion.

Cheers

Dougie
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Old Dec 29, 2012, 06:04 PM
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If i use 3w/g as a rule of thumb, I don't know what to make of specs like this below for three motors of the same weight (90g, therefore theoretically 270w) but with supplier ratings between 410 and 480 watts.

N3530/09-KV1400 Outrunner
Motor KV Power (W) Dia(mm) L(mm) Shaft(mm) Weight(g) Cells Li-Po Cells Ni-cd/Ni-MH
N3530/07 1700 410 35 32.7 4 90 2-4 6-12
N3530/09 1400 450 35 32.7 4 90 2-4 6-12
N3530/13 1100 480 35 32.7 4 90 2-4 6-12

Opinion sought.

Dougie
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Old Dec 29, 2012, 06:46 PM
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Dougie

I guess you have been using the Giantcod site.

The RobotBirds site clearly states "3S"
-----------------------------------------------------------------------------------------------

Initial data..........weight 71g...........3S............1600 Kv....

typical usable power-in = 71g x 3watts per gram = 213 watts

typical best rpm = 75% of no-load speed = .75 x 1600rpm per volt x 10.5v = 12,600 rpm
-----------------------------------------------------------------------------------------------

For prop estimate, refer to attached graph. This was published in RCGroups July 2009. http://www.rcgroups.com/forums/showt...2#post12628540 post # 9

typical motor effiency at 75%NLS is 68%

Typical usable power-out = .68 x 213w = 145w

prop power = B [rpm/1000]^3
145w = B [12600/1000]^3
B = .072
from graph, prop dia =7.4"
Pitch is .67 x 7.4 = 5"
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Old Dec 29, 2012, 07:07 PM
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Dougie

I guess you have been using the Giantcod site.

The RobotBirds site clearly states "3S"
-----------------------------------------------------------------------------------------------

Initial data..........weight 71g...........3S............1600 Kv....

typical usable power-in = 71g x 3watts per gram = 213 watts

typical best rpm = 75% of no-load speed = .75 x 1600rpm per volt x 10.5v = 12,600 rpm
-----------------------------------------------------------------------------------------------

For prop estimate, refer to attached graph. This was published in RCGroups July 2009. http://www.rcgroups.com/forums/showt...2#post12628540 post # 9

typical motor efficiency at 75%NLS is 68%

Typical usable power-out = .68 x 213w = 145w

prop power = B [rpm/1000]^3
145w = B [12600/1000]^3
B = .072
from graph, prop dia =7.4"
Pitch is .67 x 7.4 = 5"
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Old Dec 29, 2012, 08:14 PM
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I'm using this EXcel, To test within an acceptable load.
Volts and propeller.

Post 24

http://www.rcgroups.com/forums/showt...1123511&page=2
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Old Dec 29, 2012, 09:56 PM
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Or just go to WebOcalc, start with some strawman numbers, and adjust.

You know the Kv. You know the motor's nothing special, so select average outrunner efficiency. You know the weight, so you can take a stab at power based on 3W/g, say 210-240W max. On 3S, that means about 20-24A. I started at 20A, but a rather low pitch speed, and it told me I was dreaming - should have seen that as 1600 is a highish Kv for 3S. Upped it to 60mph, and it offered a potential prop solution of APC 7x5E, at around 18A. Jumping up to 27A, which is likely to be at the top end for a 70g motor and dropping back a bit on pitch speed offered up an APC 8x4E. Coming back in current and upping the pitch speed some more suggested an APC 6x5.5E at about 10A.

WebOCalc isn't a detailed motor calculator in the same vein as Drivecalc for example, but it does allow you to start with only some basic info about a motor, make a couple of assumptions and get some likely options before proceeding to the LHS for a handful of props. It looks to me that it's using similar methodology to peterangus, but is does the calculations for you rather than manually.

ps - there's a tutorial on the parent site, but cutting to the chase - WebOcalc assumes gear ratios that would make the props it offers work at the set current and set pitch speed. For direct drive, adjust the current and pitch speed by trial and error to get a gear ratio close to 1.0. For a given prop, if gear ratio is > 1, increase current, and maybe increase pitch speed.
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