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Motor ratings (KV or watts)

Old 05-23-2011, 01:21 AM
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Bald Paul
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Default Motor ratings (KV or watts)

I'm looking around at various brushless motors. Some of them are rated in KV, others in watts. Is there any conversion formula to start comparing apples to apples here?
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Old 05-23-2011, 01:27 AM
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gramps2361
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Here you go Paul every thing is right in this thread give you some good reading to get your head spinning with numbers.

http://www.wattflyer.com/forums/showthread.php?t=31071
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Old 05-23-2011, 02:00 AM
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Bald Paul
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Okay, so according to the link to the Himax motors, if I'm looking for something in the 100 watt range for my Spacewalker kit, I can choose the 860 Kv, 980 Kv, or 1160 Kv motors swinging anywhere from a 7X4 to a 12X6 prop, all of which is highly dependent on the battery type.

I think I'll just go to bed.
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Old 05-23-2011, 02:12 AM
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It only gets better Paul best thing to do is post up and ask what folks are using for a power setup for that plane. My mind was whirling when I first started now it just spins.
You need to figure the watts you need to power it say 100 watts to a pound for starters. What size prop you can use say a 10x6 on this plane so you would want something in the 1000 kv area rough estimate. the higher the kv the smaller the prop. A low kv you can swing a larger prop. It is confusing. I still drop back and reread that thread over again. There is a lot to it,but you will get the hang of it in about 5 years I hope I do myself.

So figure what size prop you can fit on that plane then how much the plane will weigh. What do you expect for performance say mild arobatics thats where 100 watts per pound comes in so a three pound plane you want at least 300 watts a little more won't hurt. You will get it just don't get hung up in the number game hurts the brain after a while.
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Old 05-23-2011, 02:14 AM
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kyleservicetech
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Originally Posted by Bald Paul View Post
I'm looking around at various brushless motors. Some of them are rated in KV, others in watts. Is there any conversion formula to start comparing apples to apples here?
Ooops
The KV and Watts are different specifications on the motor. In fact, both are required to fully define a motor.

Add to these two figures, the motors no load current in amps, and the motors winding resistance. And, last but not least, the maximum amperes allowed, and the motors weight in ounces.

So, when you've got the KV, Watts, No Load Amps, Winding resistance, you can predict how the motor will run with mathematical calculations. Some of the lower cost motors don't provide these four specifications. The more expensive higher quality motors almost always do.

The KV specification suggests how many RPM the motor will turn with a given battery voltage, and with no prop.

The Wattage specification is "horsepower", or what maximum power level the motor is rated for without melting into a puddle of copper.

The Motors No Load Amps is a suggestion of power loss in the motor while its running. That power loss is heat.

The Winding Resistance is the resistance of the motors winding resistance of the copper windings. Total power loss is approximately the addition of no load power loss plus the power lost in the windings under load.

The maximum amperes is how much current the windings can handle without overheating. Multiply that maximum current times the voltage of the battery at the motors full load with a propeller, and you get Amps X Volts = Watts. And it takes 746 watts to get exactly one horsepower.

The motor weight is not really anything, but a crude rule of thumb. IMHO, be wary of any motor claiming to put out much more than 100 watts per ounce of motor weight. It might run pretty hot, and have pretty low efficiency.

This stuff can be calculated with someone very knowledgeable in this stuff. Or go to those computer programs such as www.motocalc.com that does it all for you. (Free for 30 days, then $39)

Hope this helps.
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Old 05-23-2011, 03:03 AM
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Good stuff Denny I never was good at getting into the technical of the motor and windings and so forth just confused the heck out of me.

Paul confused yet?
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Old 05-23-2011, 05:14 AM
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CrimzonRider
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thanks denny What a plethora of info you have ....lol but it did help in reinforcing what I "think" I know about motors!

have a good one
cr
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Old 05-23-2011, 05:25 AM
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kyleservicetech
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Originally Posted by CrimzonRider View Post
thanks denny What a plethora of info you have ....lol but it did help in reinforcing what I "think" I know about motors!

have a good one
cr
Thanks guys
I learned some of this stuff the hard way when taking motors from work, rewinding them, made new silver type brushes for them, gear boxes for them, and was really happy when they got my 90 inch sailplane up to about 200 feet before the comutator burned up in the motor. Those motors ran at about 15 Amps on 12 Nicad cells. And they were HEAVY.

Now, you can buy a brushless motor far more powerful than those I made back in the early 1980's for $15.00.
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Old 05-23-2011, 06:46 AM
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Paul,

Regarding all the confusion about kv rating and different prop sizes...

Larger props are more efficient than small ones so 'watt for watt' a large prop will give you more thrust than a small one. Because of this, as a rule, you usually want to pick the largest diameter prop that your model can physically accommodate. On planes with wheels that's usually set by the ground clearance. On planes with pylon mounted motors or twin engines it's fuselage clearance that usually sets max prop diameter.

Once you know your prop diameter and you have an idea of the watt rating you can start looking through the motor tables and specs to find a motor that meets the criteria.

Prop pitch is another variable that makes a difference. Pitch is quite complex to calculate properly as you need an idea of the planes flying speed and the motor rpm.. But if you base your initial selection on a prop pitch that is in the region approx 0.66 x diameter you will be in the right ballpark for the average model.

Steve
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Old 12-17-2016, 02:42 PM
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Fudi50
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Default Power Source for Arcoupe

This not a reply but a cry for help!

I am building a BalsaUS 3rd scale Aercoupe model span 10ft weight around 30 lbs prop clearance 2"would give 24" prop suggested motor 45 55cc gas

I need to power with electric because I want to add sound Hobby King have a motor I purchased and now returning way over weight at 6 lbs and it appears to be a return item. ''I will fly at scale speed no aerobatics however I would like to aero tow a 5.3 meter sailplane weight around 18 lbs

What if anything can you suggest will fit the bill
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Old 12-17-2016, 02:48 PM
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birdDog
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Originally Posted by Bald Paul View Post
Okay, so according to the link to the Himax motors, if I'm looking for something in the 100 watt range for my Spacewalker kit, I can choose the 860 Kv, 980 Kv, or 1160 Kv motors swinging anywhere from a 7X4 to a 12X6 prop, all of which is highly dependent on the battery type.

I think I'll just go to bed.
It's cool. Go to bed. There is always tomorrow. A little research and in time it just "clicks", like a light just turned on. Then you can be that one guy at your club who understands all this "Black Magic" LOL.
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Old 12-17-2016, 04:55 PM
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kyleservicetech
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Originally Posted by Fudi50 View Post
This not a reply but a cry for help!

I am building a BalsaUS 3rd scale Aercoupe model span 10ft weight around 30 lbs prop clearance 2"would give 24" prop suggested motor 45 55cc gas

I need to power with electric because I want to add sound Hobby King have a motor I purchased and now returning way over weight at 6 lbs and it appears to be a return item. ''I will fly at scale speed no aerobatics however I would like to aero tow a 5.3 meter sailplane weight around 18 lbs

What if anything can you suggest will fit the bill
I am using a Rimfire 50 cc motor that turns a 22x10 prop at 7100 RPM, pulling 4500 Watts on a 12S3P A123 pack. That power system hauls a 20 pound model straight up out of sight.

After 105 flights, it's been flawless. So, Take a look at the Rimfire 65cc motor.

Here is my thread on the model used with the Rimfire 50cc motor.

Giant Peak Model
http://www.rcgroups.com/forums/showthread.php?t=2563126

I've no clue on what it will take to also haul up a giant scale 18 pound sailplane though.

Last edited by kyleservicetech; 12-17-2016 at 08:54 PM.
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Old 12-18-2016, 03:11 AM
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Denny is the best on this stuff ....

My quick way around the houses is to use eCalc ......

I can put in weights, ESC, battery, motor, prop ..... all the relevant data ... the motor selection is by brand and model so directly related to what you are looking at etc.

Click on calculate and you get all the info you need.

All for a couple of $ registration.

Nigel
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