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kV Rating

Old 03-15-2008, 09:39 PM
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Default kV Rating

Evening folks have a questian on electric brushless motors here goes the KV rateing how do you pick a brushless motor by it KV rateing and can size say a slow flyer/trainer what kv will i be looking for a large kv number or less number i know some site label there motors by the size of the KV rateing and size of plane thank you
Old 03-15-2008, 10:21 PM
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Default RE: kV Rating

kV is a motor constant whose units are RPM/volt (RPM per volt). This tells you how fast the motor will spin for a given voltage with no load. That is, without a prop. The no-load RPM is simply the kV multiplied by the voltage.

For example, a motor with a kV of 1250 will spin at:
1250 RPM on 1 volt
2500 RPM on 2 volts
6250 RPM on 5 volts
12500 RPM on 10 volts
etc.

In general terms, we can say the following about kV.

A low kV motor is well suited to spinning a large prop at high torque but relatively low RPM. This would be good for a slow flyer or a 3D plane where thrust is more important than speed.

A high kV motor is well suited to spinning a small prop at low torque but relatively high RPM. This would be good for a fast-flying plane where speed is more important than thrust. At the extreme, the motors used in ducted fan applications have a very high kV. High kV motors are also used in gearbox applications, including helicopters.

- Jeff
Old 03-15-2008, 10:31 PM
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Default RE: kV Rating

yes jeff thats what i needed to know thank you
Old 03-15-2008, 11:02 PM
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Default RE: kV Rating

Well said Jeff, that should be the standard answer to the Kv question. I would only add that it is the voltage constant and the units are RPM/Volt.

Dave
Old 03-16-2008, 11:43 AM
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Default RE: kV Rating

Hi Dave -

I edited my post above to add the info you suggested.

- Jeff
Old 03-16-2008, 04:32 PM
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Default RE: kV Rating

And Kv says nothing about power, RPM and torque. Ditto Kt, torque/Ampère.
Kv is a motorconstant, Kv = xyz, not 'motor has xyx Kv'.
Old 03-16-2008, 07:17 PM
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Default RE: kV Rating

Have been looking at a gp brushless 35-48-700 for my cub i needed to know what if that motor with a KV rateing off 700 would be ok for slow flying/ lol
Old 03-16-2008, 07:27 PM
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Default RE: kV Rating

It would not be suited with a a 10s lipo pack, rpm would be off the scale.
Old 03-16-2008, 10:08 PM
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Default RE: kV Rating

10s lipo no i was thinking i have two GP 3600s nimh packs for such a plane my cub
Old 03-17-2008, 08:45 AM
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Default RE: kV Rating

Kv = (your props rpm) divided by (your battery voltage)
Old 03-17-2008, 09:18 AM
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Default RE: kV Rating

KV ratings are about as useless as HP ratings. The HP is come by on a dyno at an RPM that is way to high for picking a particular prop. Hp should be determined at the RPM that the engine will be proped for. That way you can put a tach on and know the engine is not going to be hurt by over revving or overloading. Same with the KV. I was trying to determine what gear ratio to choose for a heli. The unloaden RPM, or KV tells me nothing about what RPM the motor needs to run under load, It ended up as mostly guesswork. Loaded RPM would be much more usefull as then I'd have something to shoot for.

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