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Old Dec 30, 2002, 10:40 PM
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Turns on a motor????

Hi all. Dumb question no 17. I found a thread talking about eletrifying the sureflight Spitfire which said to use an "endo" motor. Went over to Hobby Lobby found the motor in different numbers of turns. Searched hi and low on this site and the web and couldn't find a simple answer (one not having a diffequ.in it) as to what do the different numbers of "turns" on a dc motor do for its performance. So what do the different numbers of "turns" on a dc motor do for its performance?
TIA

Tom S
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Old Dec 30, 2002, 10:49 PM
Purple power
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lower number of turns=higher kv=hotter motor=lower cell count or higher gearing if geared.....
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Old Dec 31, 2002, 06:23 AM
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Ok thanks for the try but I still don't get it
Tower list the following motors and specs.

Kyosho Terminal Velocity 11x2 Motor
Kyosho Atom Smasher 12x2 Motor
Kyosho Krypton Krystal 14x2 Motor
Kyosho EndoPlasma 16x2 Motor
Kyosho Atomic Force 17x2 Motor

For a given propeller say a 10 X6 and a 1800 6 cell pack attached to each of the above motors what performance differences will I see? What peak RPM and how much duration.
TIA
Tom, I haven't a Clue.
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Old Dec 31, 2002, 07:30 AM
jrb
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Here's the Kv for those motors; that is rpm/volt -- approx no-load (when Rm is ignored) rpm for each volt applied.

TV = 5930
AS = 5233
KK = 4399
EP = 3785
AF = 3531

TV is hottest, it has the lowest Rm (internal resistance) and highest Io (no-load current).

For same pack and prop, TV will have highest rpm and amps (most watts) with shortest duration.

Checkout www.motocalc.com a try the program free for 30 days.
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Old Dec 31, 2002, 09:44 AM
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For a start none of those motors will run a 10x6 prop without a gearbox.

But as everyone has said the lower the turns the more revs you will get for a given prop and battery voltage. But a lower turn motor will also take more current (amps) and so the duration will be less. If you try to use too big a prop the current will be so high it will burn the motor up.

BTW only the 16 turn Endoplasma is called an "Endo". The other motors are not "Endos" they are different Kyosho car motors. People use that specific motor a lot because it's a well built motor with nice balance between revs and current (using a fairly high ratio gearbox).

Steve
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Old Dec 31, 2002, 10:49 AM
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Okay, you are aware that a motor is basically a chunk of metal (armature) with wire wound around it, spinning inside a magnet (field). The number of turns indicates how many times the wire is wound around the armature. A 16-turn motor has the wire wrapped around the armature 16 times. The second number indicates how many strands the wire has. A 16x2 motor has a 2-strand wire wrapped around the armature 16 times. It's also referred to as a "double wind" motor.

Assuming the same battery and propeller:
The more turns a motor has, the longer the wire is. The longer the wire, the more resistance it has. The more resistance it has, the less electricity can flow through it at a given voltage. The less the electicity flows, the slower the motor turns. In a nutshell, the more turns a motor has, the less current it draws with a given propeller, and the slower it turns.

You can make a motor draw more current by increasing the size of the propeller. So a motor with a high number of turns needs a larger propeller in order to draw the same amount of Amps as a motor with a low number of turns. In a nutshell, a motor with a high number of turns can spin a big prop slowly, while a motor with a low number of turns can spin a small prop quickly.

Another factor to consider is efficiency. The longer the wires are, the more energy is lost, because the natural resistance in wire converts electrical energy into heat. A motor with fewer turns of wire on the armature will be more efficient, provided you don't overload it. That's where gearboxes come into play. Gearboxes allow you the flexibility to run your motor in the conditions where it's most efficient, and where the propeller size, pitch and RPM match the flying qualities of the plane.

The lower the number of turns a motor has, the "hotter" it is. "Hot" in this case is similar to a "hot car" in that it goes really really fast, not a "hot car" as in stolen.
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Old Dec 31, 2002, 06:50 PM
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Thanks troops! That helps a bunch!!

A Safe and happy new year to all

Tom
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Old Jan 01, 2003, 08:19 AM
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Puzzled

I must admit continuing confusion on this same subject. What seems to be the case is low turns = high rpm/volt = needs gearing. For example, if I compare an Aveox to Jeti Phasor, former low turns / high rpm, so gear reduction is used, latter is low rpm, so direct-drive is used. What is very unclear is what choice to make? Presumably if I need 200 watts to fly a plane as determined by watts/sq in wing area or watts/ oz, or other "rules of thumb", prop I need and rpm needed will be determined by the type of plane ( low/hi wing loading ). Having established that, seems I could either gear a low turn motor or run hi turn motor direct. Given comments about electrical efficiency of low turn motor ( and losses in gear reduction unit ), seems geared low turn motor is the way to go ?

Only way I know to approach this now is to use electricalc and motocalc to perform the calculations. However, seems the e-zone experts have feel for these things that would be useful for us neophytes to understand.
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Old Jan 01, 2003, 11:29 AM
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> seems I could either gear a low turn motor or run hi turn motor direct.

A lot depends upon the motor and the model. Some motors (eg S400 4.8V) have such a high Kv (RPM/Volt) that you practically always have to gear them, so then the question comes down to what gear ratio.

However generally it depends if you want to go fast (direct drive) or climb fast (high ratio gearbox), or have reasobable performance for a long time (low Kv direct drive, or lower ratio gearbox).

The other consideration is that you ideally want to choose a motor whose max efficiency current is close to your model's average current. If you want to fly for 8 minutes from Hecel 1100 cells, you will actually get about 0.9Ah out of them = 54 amp minutes. So average current will be just 6.75A. Thus you want to choose a motor whose max efficiency is close 6.75A.

> Given comments about electrical efficiency of low turn motor ( and losses in gear reduction unit ), seems geared low turn motor is the way to go?

Not necessarily. In fact the comment that low turn motors are more efficient because they have a lower resistance is incorrect. So long as the high turn motor is packed with the same amount of copper as the low turn motor it will have the same efficiency, but at a lower current (and lower rpm). The reason why low turn motors aren't more efficient is because their no load current is higher.

Take for example S400 motors on 8 cells:
S400 4.8V
Kv = 4681 rpm/V
Rm = 0.10R
Io = 1.2A
Max efficiency = 72% at 7.5A

S400 6V
Kv = 3097 rpm/V
Rm = 0.269R
Io = 0.7A
Max efficiency = 72% at 5.0A

S400 7.2V
Kv = 2332 rpm/V
Rm = 0.338R
Io = 0.6A
Max efficiency = 72% at 3.9A

(I doctored the figures slightly to illustrate the point and match my experience, you may get slightly different numbers if you try it in MotoCalc.)

Thus from the above example you could choose either a 4.8V motor or a 6V motor as either of their max efficiencyes is close to 6.75A. However you would have to gear the 4.8V motor to be able to use a reasonably sized prop, whereas the 6V motor could be run direct drive. Note if I had used 0.5Ah cells in the example the average current would have been reduced, which is why 6V and 7.2V motors have been more popular in the past.

Hope this makes some sense.

Neil.
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Old Jan 01, 2003, 01:40 PM
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Ok, given numbers mentioned, I can run a 4.8 v at 7.5 amps ( geared ) getting ~ 40 WATTS TO PROP ( 7.5*9.6*50%) OR run a 6 volt at 5 amps direct, getting 75% of 40 or 30 watts to prop. As uninitiated, I would conclude that the former would always be preferred. So why am I usung a 6 v Graupner 3321 instead of the 4.8 volt through a gearbox for my Sport 400? I actually draw (and motocalc predicts) more like 9 amps on 8 cells with a 5x4.
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Old Jan 01, 2003, 02:02 PM
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You don't prop for the max efficiency current, you prop for the motor run you want, and the max current the motor can take - about 12A for a Speed 400.

Lets say you are doing 4 min flights and getting .5ah out of 600AE cells, the actual average current is 7.5A. In which case the 4.8V motor would give you its best 72% efficiency, though you would lose up to 10% through the gearbox, and the model would weigh 7% more. OTOH the 6V motor's efficiency is down to 67% at 7.5A, and you would have the simplicity, lightweight, and low cost of a direct drive setup - so at these currents it is a wash. If in fact you were averaging 10A in flight a geared 4.8V motor would be better.

Neil.
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