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KV Rating??

Old 01-28-2009, 11:58 PM
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Fishin Geezer
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Default KV Rating??

Can someone please define "KV" for me, please? I'm not comfortable with Wayne Miller's definition of "1000RPM per volt". I can believe it describes a motor's RPM as a function of applied voltage but I can't figure out the relationship. I guess applied voltage depends on ESC and throttle stick position.

I suppose my question is really "what does KV tell me about motor performance and how do I use the KV specification"?

Any ideas for me?

Frank

Old 01-29-2009, 08:17 AM
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Mr. Mugen
 
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Default RE: KV Rating??

Kv is rpms/volt. If you have a 5000kv motor and feed it 11.1v you have 55,500rpm unloaded. Obviously as you apply a load it will come down and the amp draw will go up.

Thats kv in a nut shell.

Kevin
Old 01-29-2009, 10:43 AM
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Fishin Geezer
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Default RE: KV Rating??

Thanks, Kevin. I had looked at numbers like that (55500) and couldn't believe them because I overlooked the fact of unloaded. Also, I suppose that spec is at WOT.

So, is there any way we can use that spec in choosing a motor for a given application? Is it useful in comparing different but similar motors? We don't know how much the motor slows down under load.

Darn, it's complicated!!

Frank
Old 01-29-2009, 10:59 AM
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Default RE: KV Rating??

it is part of the puzzle......

for me..one of the most important factors is the watts that the motor outputs...... among others....

and then for a basic breakdown you can use....

about 75w per lb for scale flying....
100 - 125w per lb for sport flying
150 and up watts per pound for 3d and unlimited aerobatics.....





Or you just ask on here what is a good setup for a specific plane... as someone has probably done it before!
Old 01-29-2009, 12:02 PM
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Walt Thyng
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Default RE: KV Rating??

FG,
KV does give a little hint as to how to use the motor. In general the higher the KV the smaller the prop and the lower the KV the larger the prop. Said another way high KV motors are for speed, edfs or gearboxes. Lower KV motors tend to give more thrust.

One other thing, a motor will always try to reach its rated KV and will draw as many amps as it needs to accomplish this. That is why a larger diameter or deeper pitched prop draws more amps. It is also why a stalled motor (one whihc is receiving power, but is prevented from turning) will destroy itself the battery or both.
Walt
Old 01-29-2009, 06:46 PM
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Fishin Geezer
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Default RE: KV Rating??

Thanks, All!! I have a little better understanding of what it's all about.

Now to remember all that.

Frank
Old 01-29-2009, 07:43 PM
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Dr Kiwi
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Default RE: KV Rating??

You can get an idea of how much your motor is loaded down by the prop, by determining Kv X V with that prop on - if the motor doesn't manage more than 75% of Kv x V, then probably the prop is too big for it.

Obviously tiny props put less of a load on the motor, so with a good quality high Kv motor you may get as much as 90% of Kv x V..... if you get an el cheapo Chinese motor, and use the manufacturer's guidelines for prop selection (designed to show how much thrust their motor produces, not caring that it will probably fry in the process) you may find you can manage only 60% of Kv x V! Overload = Heat
Old 01-30-2009, 11:09 AM
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Default RE: KV Rating??

WOW!! That means a high KV motor could run at 30 to 40 thousand RPM. I had no idea they could run so fast and have never put my little tachometer on an electrically driven prop. I'll have to try that.

I have noticed that the brushless outrunner on my GWS Slowstick gets pretty hot even tho I don't usually fly with much throttle.

Old 01-30-2009, 03:44 PM
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Default RE: KV Rating??

Yep. A guy at our field has a little electric pylon racer with an in-runner direct driving the prop. He said it spins around 33,000 - 35,000 rpms probably hits ~120mph.
Old 01-31-2009, 11:30 AM
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Default RE: KV Rating??


ORIGINAL: Fishin Geezer

WOW!! That means a high KV motor could run at 30 to 40 thousand RPM. ...
NO! It means that motor/battery combination WANTS to run at 30-40.000RPM. But maybe the motor will explode due to centrifugal forces. Or maybe the motor is overpropped: not that rpm but an excissive current.

Having said all this, motors can do +30.000rpm, yes outrunners too. But Kv says nothing about motors physical max. rpm.

Vriendelijke groeten Ron
Old 01-31-2009, 11:57 AM
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Dr Kiwi
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Default RE: KV Rating??

Just out of interest - I had a Turnigy 2836-2350 (actually ~2800Kv) on the stand last night - with a 3s pack and a 4.1x4.1 EMP prop - measured rpm = 26760rpm... Kv x V = 2800 x 10.7 = 29960rpm......thus, Kv/V = 89%. 4s pack and a 4.1x4.1 EMP prop - measured rpm = 30570rpm... Kv x V = 2800 x 13.3 = 37240rpm......thus, Kv/V = 82%.

Obviously the motor is struggling a bit at the high power input on 4s.

Let's load it up a bit more: a 3s pack and a 6x4 APC E prop - measured rpm = 21450rpm... Kv x V = 2800 x 10.7 = 29960rpm......thus, Kv/V = 72%.

Now it is really struggling!
Old 02-16-2009, 01:31 PM
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Default RE: KV Rating??

I'm lost. I've read all of this kv stuff and still don't understand. I have several power planes and just getting started in electric.I bought a foam pusher with a 400 brushed motor. Does not fly good,not enough power. Local hobby shop told me to buy a brushless in runner and they talked me into a 4200 Kv motor, 25 AMP speed control,and a 3 cell li po battery. The motor barely runs. WHATS WRONG I need HELP
Old 02-16-2009, 08:54 PM
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Default RE: KV Rating??

Your 3 cell Lipo is about 10 volts (11.2) so multiple that times the Kv and your motor wants to run at 42,000 RPM. If you prop it for that RPM it will run OK but I don't think that is what you want.
Kv is also a measure of how much torque the motor will generate, high Kv motors make less torque per amp. If you reduce the Kv by half the amount of torque per amp will double.
So your high Kv motor sucks at torque but makes high RPM.
You need a gearbox to turn some of the RPM into torque.
Old 02-16-2009, 09:05 PM
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Default RE: KV Rating??

Typical hobbyshop if they don't know you [:@]. Yeah, you'll need a gearbox depending on the plane of at leats 2:1 reduction. What plane is it going in? I use a himaxx 2025 4200 on 3s lipo for several applications from fast pusher jets to slowflyers to 3d planes. Geared 2:1 with a 5x5 prop in a small pusher it screams, but geared 10:1 and with a 10x6 prop I can slow fly for a loooong time. If it is a himax motor there are several gearboxes that work well. For large reductions I use gws gearboxes. Hope this helps! If you don't want to deal with gears, take it back and trade it in for something like a 1500-2100kv rated motor.
Old 02-16-2009, 09:21 PM
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Default RE: KV Rating??

Thanks for the help.

mjsas And CBM Racing

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